Solar Energy

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1 INTRODUCTION SOLAR ENERGY Solar energy is the utilization of the radiant energy from the Sun. Solar power is often used interchangeably with solar energy but refers more specifically to the conversion of sunlight into electricity, either by photovoltaics and concentrating solar thermal devices, or by one of several experimental technologies such as thermoelectric converters, solar chimneys and solar ponds. Solar energy and shading are important considerations in building design. Thermal mass is used to conserve the heat that sunshine delivers to all buildings. Daylighting techniques optimize the use of light in buildings. Solar water heaters heat swimming pools and provide domestic hot water. In agriculture, greenhouses expand growing seasons and pumps powered by solar cells (also known as photovoltaics) provide water for grazing animals. Evaporation ponds are used to harvest salt and clean waste streams of contaminants. Solar energy is the fastest growing form of energy production. Solar distillation and disinfection techniques produce potable water for millions of people worldwide. Family-scale solar cookers and larger solar kitchens concentrate sunlight for cooking, drying and pasteurization. Clotheslines are a common application of solar energy. More sophisticated concentrating technologies magnify the rays of the Sun for high-temperature material testing, metal smelting and industrial chemical production. A range of prototype solar vehicles provide ground, air and sea transportation.

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Solar Energy and its Uses

ENERGY FROM THE SUN Earth receives 174 petawatts (PW) of incoming solar radiation (insolation) at the upper atmosphere. Approximately 30% is reflected back to space while the rest is absorbed by the clouds, oceans and land masses. The spectrum of the solar light at the surface of Earth is mostly split between the visible and nearinfrared ranges with a small part in the near-ultraviolet. The absorbed solar light heats the land surface, oceans and atmosphere. The warm air containing evaporated water from the oceans rises driving atmospheric circulation or convection. When the ascending air reaches a high altitude, where the temperature is low, the water vapoir condenses forming various types of clouds. Eventually all evaporated water rains down on to the surface closing what is known as the water cycle. The latent heat of the water condensations amplifies the convection producing such atmospheric phenomena as cyclones and anti-cyclones. The winds are observational manifistation of the atmospheric circulation. Sunlight absorbed by the oceans and land masses keeps the surface at an average temperature of 14 °C. The conversion of solar energy into chemical energy via photosynthesis produces food, wood and the biomass from which fossil fuels are derived. Solar radiation along with secondary solar resources such as wind and wave power, hydroelectricity and biomass account for over 99.9% of the available flow of renewable energy on Earth. The flows and stores of solar energy in the environment are vast in comparison to current human energy needs. The total solar energy absorbed by Earth's atmosphere, oceans and land masses is approximately 3,850 zettajoules (ZJ) per year, while global wind energy at 80 m, the minimum height of modern large wind turbines, is estimated at 2.25 ZJ per year. Photosynthesis captures approximately 3 ZJ per year in biomass. In contrast, worldwide electricity consumption was approximately 0.0567 ZJ in 2005, and total worldwide primary energy consumption was 0.487 ZJ in the same year. APPLICATIONS OF SOLAR ENERGY TECHNOLOGY Solar energy technologies use solar radiation for practical ends. Technologies that use secondary solar resources such as

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biomass, wind, waves and ocean thermal gradients can be included in a broader description of solar energy but only primary resource applications are discussed here. Because the performance of solar technologies varies widely between regions, solar technologies should be deployed in a way that carefully considers these variations. Solar technologies such as photovoltaics and water heaters increase the supply of energy and may be characterized as supply side technologies. Technologies such as passive design and shading devices reduce the need for alternate resources and may be characterized as demand side. Optimizing the performance of solar technologies is often a matter of controlling the resource rather than simply maximizing its collection. ARCHITECTURE AND URBAN PLANNING Darmstadt University of Technology won the 2007 Solar Decathlon with this passive house designed specifically for the humid and hot subtropical climate in Washington, D.C. Sunlight has influenced building design since the beginning of architectural history. Fully developed solar architecture and urban planning methods were first employed by the Greeks and Chinese who oriented their buildings toward the south to provide light and warmth. The elemental features of passive solar architecture are Sun orientation, compact proportion (a low surface area to volume ratio), selective shading (overhangs) and thermal mass. When these features are tailored to the local climate and environment they can produce well-lit spaces that stay in a comfortable temperature range. Socrates' Megaron House is a classic example of passive solar design. The most recent approaches to solar design use computer modeling to tie together solar lighting, heating and ventilation systems in an integrated solar design package. Active solar equipment such as pumps, fans and switchable windows can also complement passive design and improve system performance. Urban heat islands (UHI) are metropolitan areas with higher temperatures than the surrounding environment. These higher temperatures are the result of urban materials such as asphalt and

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concrete that have lower albedos and higher heat capacities than the natural environment. A straightforward method of counteracting the UHI effect is to paint buildings and roads white and plant trees. Using these methods, a hypothetical "cool communities" program in Los Angeles has projected that urban temperatures could be reduced by approximately 3 °C at an estimated cost of US$1 billion, giving estimated total annual benefits of US$530 million from reduced air-conditioning costs and healthcare savings. AGRICULTURE AND HORTICULTURE Agriculture inherently seeks to optimize the capture of solar energy, and thereby plant productivity. Techniques such as timed planting cycles, tailored row orientation, staggered heights between rows and the mixing of plant varieties can improve crop yields. While sunlight is generally considered a plentiful resource, there are exceptions which highlight the importance of solar energy to agriculture. During the short growing seasons of the Little Ice Age, French and English farmers employed fruit walls to maximize the collection of solar energy. These walls acted as thermal masses and accelerated ripening by keeping plants warm. Early fruit walls were built perpendicular to the ground with a south facing orientation but over time sloping walls were developed to make better use of sunlight. In 1699, Nicolas Fatio de Duillier even suggested using a tracking mechanism, which could pivot to follow the Sun. Solar energy applications in agriculture, aside from growing crops, include pumping water, drying crops, brooding chicks and drying chicken manure. Greenhouses convert solar light to heat enabling year-round production and the growth (in enclosed environments) of specialty crops and other plants not naturally suited to the local climate. Primitive greenhouses were first used during Roman times to produce cucumbers year-round for the Roman emperor Tiberius. The first modern greenhouses were built in Europe in the 16th century to keep exotic plants brought back from explorations abroad. Greenhouses remain an important part of horticulture today, while plastic transparent materials have also been used to similar effect in polytunnels and row covers.

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SOLAR LIGHTING The history of lighting is dominated by the use of natural light. The Romans recognized a right to light as early as the 6th century and English law echoed these judgments with the Prescription Act of 1832. In the 20th century artificial lighting became the main source of interior illumination. Daylighting systems collect and distribute sunlight to provide interior illumination; they are passive systems. These systems directly offset energy use by replacing artificial lighting, and indirectly offset non-solar energy use by reducing the need for airconditioning. The use of natural lighting also offers physiological and psychological benefits compared to artificial lighting, Although difficult to quantify. Daylighting design implies careful selection of window types, sizes and orientation; exterior shading devices may also be considered. Individual features include sawtooth roofs, clerestory windows, light shelves, skylights and light tubes. These features may be incorporated into existing structures, but are most effective when integrated into a solar design package that accounts for factors such as glare, heat flux and time-of-use. When daylighting features are properly implemented they can reduce lighting-related energy requirements by 25%. The most important of the active solar lighting methods is the hybrid solar lighting (HSL). HSL systems collect sunlight using focusing mirrors that track the Sun and use optical fibers to transmit the light into a building's interior to supplement conventional lighting. In single-story applications, these systems are able to transmit 50% of the direct sunlight received. Although daylight saving time is promoted as a way to use sunlight to save energy, recent research has been limited and reports contradictory results: several studies report savings, but just as many suggest no effect or even a net loss, particularly when gasoline consumption is taken into account. Electricity use is greatly affected by geography, climate and economics, making it hard to generalize from single studies. SOLAR THERMAL Solar thermal technologies can be used for water heating, space heating, space cooling and process heat generation.

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WATER HEATING Solar hot water systems use sunlight to heat water. In low geographical latitudes (below 40 degrees) solar heating system can provide from 60 to 70% of domestic hot water use with temperatures up to 60 °C. The most common types of solar water heaters are evacuated tube collectors (44%) and glazed flat plate collectors (34%) generally used for domestic hot water; and unglazed plastic collectors (21%) used mainly to heat swimming pools. As of 2007, the total installed capacity of solar hot water systems is approximately 154 GW. China is the world leader in the deployment of solar hot water with 70 GW installed as of 2006 and a long term goal of 210 GW by 2020. Israel is the per capita leader in the use of solar hot water with 90% of homes using this technology. In the United States, Canada and Australia, heating swimming pools is the dominant application of solar hot water, with an installed capacity of 18 GW as of 2005. HEATING, COOLING AND VENTILATION In the United States, heating, ventilation and air conditioning (HVAC) systems account for 30% (4.65 EJ) of the energy used in commercial buildings and nearly 50% (10.1 EJ) of the energy used in residential buildings. Solar heating, cooling and ventilation technologies can be used to offset a portion of this energy. Thermal mass, in the most general sense, is any material that has the capacity to store heat. In the context of solar energy, thermal mass materials are used to store heat from the Sun. Common thermal mass materials include stone, cement and water. These materials have historically been used in arid climates or warm temperate regions to keep buildings cool by absorbing solar energy during the day and radiating stored heat to the cooler atmosphere at night, but they can also be used in cold temperate areas to maintain warmth. The size and placement of thermal mass should consider several factors such as climate, daylighting and shading conditions. When properly incorporated, thermal mass maintains space temperatures in a comfortable range and reduces the need for auxiliary heating and cooling equipment.

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A solar chimney (or thermal chimney) is a passive solar ventilation system composed of a vertical shaft connecting the interior and exterior of a building. As the chimney warms, the air inside is heated causing an updraft that pulls air through the building. Performance can be improved by using glazing and thermal mass materials in a way that mimics greenhouses. These systems have been in use since Roman times and remain common in the Middle East. Deciduous trees and plants have often been promoted as a means of controlling solar heating and cooling. When planted on the southern side of a building, the leaves provide shade during the summer while the bare limbs allow light and warmth to pass during the winter. Since bare, leafless trees shade 1/3 to 1/2 of incident solar radiation, there is a balance between the benefits of summer shading and the corresponding loss of winter heating. In climates with significant heating loads, deciduous trees should not be planted on the southern side of a building because they will interfere with winter solar availability but they can be used on the east and west sides to provide a degree of summer shading without appreciably affecting winter solar gain. DESALINATION AND DISINFECTION The production of potable water from saline or brackish water using solar energy is called the solar distillation. The first recorded use was by 16th century Arab alchemists. The first large-scale solar distillation project was constructed in 1872 in the Chilean mining town of Las Salinas. This plant, which had solar collection area of 4,700 m², still could produce up to 22,700 L per day and operated for 40 years. Individual still designs include single-slope, double-slope (or greenhouse type), vertical, conical, inverted absorber, multi-wick and multiple effect. These stills can operate in passive, active or hybrid modes. Double slope stills are the most economical for decentralized domestic purposes while active multiple effect units are more suitable for large-scale applications. Solar water disinfection (SODIS) is a method of disinfecting water by exposing water-filled plastic polyethylene terephthalate (PET) bottles to several hours of sunlight. Exposure times vary depending on weather and climate from a minimum of six hours

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to two days during fully overcast conditions. SODIS is recommended by the World Health Organization as a viable method for household water treatment and safe storage. Over two million people in developing countries use SODIS for their daily drinking water needs. COOKING Solar cookers use sunlight for cooking, drying and pasteurization. These devices can be grouped into three broad categories: box cookers, panel cookers and reflector cookers. The simplest type of solar cooker is the box cooker first built by Horace de Saussure in 1767. A basic box cooker consists of an insulated container with a transparent lid. These cookers can be used effectively with partially overcast skies and will typically reach temperatures of 90-150 °C. Panel cookers use a reflective panel to direct sunlight onto an insulated container and reach temperatures comparable to box cookers. Reflector cookers use various concentrating geometries (dish, trough, Fresnel mirrors) to focus light on a cooking container. These cookers reach temperatures of 315 °C and above but require direct light to function properly and must be repositioned to track the Sun. The solar bowl is a unique concentrating technology employed by the Solar Kitchen in Auroville, India. The solar bowl is a stationary spherical reflector that focuses light along a line perpendicular to the sphere's interior surface and a computer control system moves the receiver to intersect this line. Steam is produced in the receiver at temperatures reaching 150 °C and then used for process heat in the kitchen. A reflector developed by Wolfgang Scheffler in 1986 is used in many solar kitchens. Scheffler reflectors are flexible parabolic dishes that combine aspects of trough and power tower concentrators. Polar tracking is used to follow the Sun's daily course and the curvature of the reflector is adjusted for seasonal variations in the incident angle of sunlight. These reflectors can reach temperatures of 450-650 °C and have a fixed focal point which improves the ease of cooking. The world's largest Scheffler reflector system in Abu Road, Rajasthan, India is capable of cooking up to 35,000 meals a day. As of 2008, over 2,000 large Scheffler cookers had been built worldwide.

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PROCESS HEAT Solar concentrating technologies such as parabolic dish, trough and Scheffler reflectors can provide process heat for commercial and industrial applications. The first commercial system was the Solar Total Energy Project (STEP) in Shenandoah, Georgia where a field of 114 parabolic dishes provided 50% of the process heating, air conditioning and electrical requirements for a clothing factory. This grid-connected cogeneration system provided 400 kW of electricity plus thermal energy in the form of 401 kW steam and 468 kW chilled water, and had a one hour peak load thermal storage. Evaporation ponds are shallow pools that concentrate dissolved solids through evaporation. The use of evaporation ponds to obtain salt from sea water is one of the oldest applications of solar energy. Modern uses include concentrating brine solutions used in leach mining and removing dissolved solids from waste streams. Clothes lines, clotheshorses, and clothes racks dry clothes through evaporation. These devices use wind and sunlight instead of electricity or natural gas. Florida legislation specifically protects the 'right to dry' and similar solar rights legislation has been passed in Utah and Hawaii. Unglazed transpired collectors (UTC) are perforated sun-facing walls used for preheating ventilation air. UTCs can raise the incoming air temperature up to 22 °C and deliver outlet temperatures of 45-60 °C. The short payback period of transpired collectors (3 to 12 years) makes them a more cost-effective alternative than glazed collection systems. As of 2003, over 80 systems with a combined collector area of 35,000 m² had been installed worldwide, including an 860 m² collector in Costa Rica used for drying coffee beans and a 1,300 m² collector in Coimbatore, India used for drying marigolds. SOLAR ELECTRICITY Sunlight can be converted into electricity using photovoltaics (PV), concentrating solar power (CSP), and various experimental technologies. PV has mainly been used to power small and medium-sized applications, from the calculator powered by a

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single solar cell to off-grid homes powered by a photovoltaic array. For large-scale generation, CSP plants like SEGS have been the norm but recently multi-megawatt PV plants are becoming common. Completed in 2007, the 14 MW power station in Clark County, Nevada and the 20 MW site in Beneixama, Spain are characteristic of the trend toward larger photovoltaic power stations in the US and Europe.

Economies of scale which resulted from increasing production along with improvements in system performance brought the price of PV down from 100 USD/watt in 1971 to 7 USD/watt in 1985. Steadily falling oil prices during the early 1980s led to a reduction in funding for photovoltaic R&D and a discontinuation of the tax crs associated with the Energy Tax Act of 1978. These factors moderated growth to approximately 15% per year from 1984 through 1996.

PHOTOVOLTAICS

Since the mid-1990s, leadership in the PV sector has shifted from the US to Japan and Germany. Between 1992 and 1994 Japan increased R&D funding, established net metering guidelines, and introduced a subsidy program to encourage the installation of residential PV systems. As a result, PV installations in the country climbed from 31.2 MW in 1994 to 318 MW in 1999, and worldwide production growth increased to 30% in the late 1990s.

A solar cell (or photovoltaic cell) is a device that converts light into direct current using the photoelectric effect. The first solar cell was constructed by Charles Fritts in the 1880s. Although the prototype selenium cells converted less than 1% of incident light into electricity, both Ernst Werner von Siemens and James Clerk Maxwell recognized the importance of this discovery. Following the fundamental work of Russell Ohl in the 1940s, researchers Gerald Pearson, Calvin Fuller and Daryl Chapin created the silicon solar cell in 1954. These early solar cells cost 286 USD/watt and reached efficiencies of 4.5-6%. The earliest significant application of solar cells was as a backup power source to the Vanguard I satellite, which allowed the satellite to continue transmitting for over a year after its chemical battery was exhausted. The successful operation of solar cells on this mission was duplicated in many other Soviet and American satellites, and by the late 1960s PV had become the established source of power for satellites. Photovoltaics went on to play an essential part in the success of early commercial satellites such as Telstar and continue to remain vital to the telecommunications infrastructure today. The high cost of solar cells limited terrestrial uses throughout the 1960s. This changed in the early 1970s when prices reached levels that made PV generation competitive in remote areas without grid access. Early terrestrial uses included powering telecommunication stations, off-shore oil rigs, navigational buoys and railroad crossings. These and other off-grid applications have proven very successful and accounted for over half of worldwide installed capacity until 2004. The 1973 oil crisis stimulated a rapid rise in the production of PV during the 1970s and early 1980s.

Germany has become the leading PV market worldwide since revising its Feed-in tariff system as part of the Renewable Energy Sources Act. Installed PV capacity has risen from 100 MW in 2000 to approximately 4,150 MW at the end of 2007. Spain has become the third largest PV market after adopting a similar feed-in tariff structure in 2004, while France, Italy, South Korea and the US have also seen rapid growth recently due to various incentive programs and local market conditions. CONCENTRATING SOLAR POWER Concentrating Solar Power (CSP) systems is divided into Concentrating solar thermal (CST) and Concentrating PV (CPV). CSP use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. The concentrated light is then used as a heat source for a conventional power plant. A wide range of concentrating technologies exist; the most developed are the solar trough, parabolic dish and solar power tower. These methods vary in the way they track the Sun and focus light. In all these systems a working fluid is heated by the concentrated sunlight, and is then used for power generation or energy storage. A solar trough consists of a linear parabolic reflector that concentrates light onto a receiver positioned along the reflector's focal line. The reflector is made to follow the Sun during the

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daylight hours by tracking along a single axis. Trough systems are the most mature CSP technology. The SEGS plants in California and Acciona's Nevada Solar One near Boulder City, Nevada are representatives of this technology. EXPERIMENTAL SOLAR POWER A solar updraft tower (also known as a solar chimney or solar tower) consists of a large greenhouse that funnels into a central tower. As sunlight shines on the greenhouse, the air inside is heated and expands. The expanding air flows toward the central tower where a turbine converts the air flow into electricity. A 50 kW prototype was constructed in Ciudad Real, Spain and operated for eight years before decommissioning in 1989.

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the form of microwave radiation to receivers (rectennas) on Earth for distribution. This concept was first proposed by Dr. Peter Glaser in 1968 and since then a wide variety of systems have been studied with both photovoltaic and concentrating solar thermal technologies being proposed. Although still in the concept stage, these systems offer the possibility of delivering power approximately 96% of the time. SOLAR CHEMICAL

A solar pond is a pool of salt water (usually 1-2 m deep) that collects and stores solar energy. Solar ponds were first proposed by Dr. Rudolph Bloch in 1948 after he came across reports of a lake in Hungary in which the temperature increased with depth. This effect was due to salts in the lake's water, which created a "density gradient" that prevented convection currents. A prototype was constructed in 1958 on the shores of the Dead Sea near Jerusalem. The pond consisted of layers of water that successively increased from a weak salt solution at the top to a high salt solution at the bottom. This solar pond was capable of producing temperatures of 90 °C in its bottom layer and had an estimated solar-to-electric efficiency of two percent.

Solar radiation stimulated chemical processes use solar energy to drive chemical reactions. These processes offset energy that would otherwise be required from an alternate source and can convert solar energy into a storable and transportable fuel. Solar induced chemical reactions are diverse, but can be divided into thermochemical or photochemical. Hydrogen production production technologies involving the use of solar light have been a significant area of research since the 1970s. Aside from electrolysis driven by photovoltaic or photochemical cells, several thermochemical processes have also been explored. The seemingly most direct of these routes uses concentrators to split water at high temperatures (2300-2600 °C), but this process has been limited by complexity and low solar-to-hydrogen efficiency (1-2%). A more conventional approach uses the heat from solar concentrators to drive the steam reformation of natural gas thereby increasing the overall hydrogen yield.

Thermoelectric devices convert a temperature difference between dissimilar materials into an electric current. First proposed as a method to store solar energy by solar pioneer Mouchout in the 1800s, thermoelectrics reemerged in the Soviet Union during the 1930s. Under the direction of Soviet scientist Abram Ioffe a concentrating system was used to thermoelectrically generate power for a 1 hp engine. Thermogenerators were later used in the US space program as an energy conversion technology for powering deep space missions such as Cassini, Galileo and Viking. Research in this area is focused on raising the efficiency of these devices from 7-8% to 15-20%.

Thermochemical cycles characterized by the decomposition and regeneration of reactants present another avenue for hydrogen production. The Solzinc process under development at the Weizmann Institute uses a 1 MW solar furnace to decompose zinc oxide (ZnO) at temperatures above 1200 °C. This initial reaction produces pure zinc, which can subsequently be reacted with water to produce hydrogen. Sandia's Sunshine to Petrol (S2P) technology uses the high temperatures generated by concentrating sunlight along with a zirconia/ferrite catalyst to break down atmospheric carbon dioxide into oxygen and carbon monoxide (CO). The CO may then be used to synthesize methanol, gasoline and jet fuel.

Space solar power systems use a large solar array in geosynchronous orbit to collect sunlight and beam this energy in

Photoelectrochemical cells or PECs consist of a semiconductor, typically titanium dioxide or related titanates, immersed in an

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electrolyte. When the semiconductor is illuminated an electrical potential develops. There are two types of photoelectrochemical cells: photoelectric cells that convert light into electricity and photochemical cells that use light to drive chemical reactions such as electrolysis. A photogalvanic device is a type of battery in which the cell solution (or equivalent) forms energy-rich chemical intermediates when illuminated. These chemical intermediates then react at the electrodes to produce an electric potential. The ferric-thionine chemical cell is an example of this technology. SOLAR VEHICLES Development of a solar powered car has been an engineering goal since the 1980s. The World Solar Challenge is a biannual solar-powered car race, in which teams from universities and enterprises compete over 3,021 kilometres (1,877 mi) across central Australia from Darwin to Adelaide. In 1987, when it was founded, the winner's average speed was 67 kilometres per hour (42 mph). The 2007 race included a new challenge class using cars which could be a practical proposition for sustainable transport with little modification. The winning car averaged 90.87 kilometres per hour (56.46 mph). The North American Solar Challenge and the planned South African Solar Challenge are comparable competitions that reflect an international interest in the engineering and development of solar powered vehicles. In 1975, the first practical solar boat was constructed in England. By 1995, passenger boats incorporating PV panels began appearing and are now used extensively. In 1996, Kenichi Horie made the first solar powered crossing of the Pacific Ocean, and the sun catamaran made the first solar powered crossing of the Atlantic Ocean in the winter of 2006-2007. Plans to circumnavigate the globe in 2009 are indicative of the progress solar boats have made. In 1974, the unmanned Sunrise II inaugurated the era of solar flight. In 1980, the Gossamer Penguin made the first piloted flights powered solely by photovoltaics. This was quickly followed by the Solar Challenger which demonstrated a more airworthy design with its crossing of the English Channel in July 1981. Developments then turned back to unmanned aerial vehicles (UAV) with the

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Pathfinder (1997) and subsequent designs, culminating in the Helios which set the altitude record for a non-rocket-propelled aircraft at 29,524 metres (96,860 ft) in 2001. The Zephyr, developed by BAE Systems, is the latest in a line of record-breaking solar aircraft, making a 54-hour flight in 2007, and month-long flights are envisioned by 2010. A solar balloon is a black balloon that is filled with ordinary air. As sunlight shines on the balloon, the air inside is heated and expands, causing an upward buoyancy force, much like an artificially-heated hot air balloon. Some solar balloons are large enough for human flight, but usage is limited to the toy market as the surface-area to payload-weight ratio is relatively high. Solar sails are a proposed form of spacecraft propulsion using large membrane mirrors to exploit radiation pressure from the sun. Unlike rockets, solar sails require no fuel. Although the thrust is small compared to rockets, it continues as long as the Sun shines onto the deployed sail and in the frictionless vacuum of space significant speeds can eventually be achieved.

ENERGY STORAGE METHODS Storage is an important issue in the development of solar energy because modern energy systems usually assume continuous availability of energy. Solar energy is not available at night, and the performance of solar power systems is affected by unpredictable weather patterns; therefore, storage media or back-up power systems must be used. Thermal mass systems can store solar energy in the form of heat at domestically useful temperatures for daily or seasonal durations. Thermal storage systems generally use readily available materials with high specific heat capacities such as water, earth and stone. Well-designed systems can lower peak demand, shift time-of-use to off-peak hours and reduce overall heating and cooling requirements. Phase change materials such as paraffin wax and Glauber's salt are another thermal storage media. These materials are inexpensive, readily available, and can deliver domestically useful temperatures (approximately 64 °C). The "Dover House" (in Dover, Massachusetts) was the first to use a Glauber's salt heating system,

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in 1948. Solar energy can be stored at high temperatures using molten salts. Salts are an effective storage medium because they are low-cost, have a high specific heat capacity and can deliver heat at temperatures compatible with conventional power systems. The Solar Two used this method of energy storage, allowing it to store 1.44 TJ in its 68 m³ storage tank with an annual storage efficiency of about 99%. Off-grid PV systems have traditionally used rechargeable batteries to store excess electricity. With grid-tied systems, excess electricity can be sent to the transmission grid. Net metering programs give these systems a cr for the electricity they deliver to the grid. This cr offsets electricity provided from the grid when the system cannot meet demand, effectively using the grid as a storage mechanism. Pumped-storage hydroelectricity stores energy in the form of water pumped when energy is available from a lower elevation reservoir to a higher elevation one. The energy is recovered when demand is high by releasing the water to run through a hydroelectric power generator. DEVELOPMENT, DEPLOYMENT AND ECONOMICS Beginning with the surge in coal use which accompanied the Industrial Revolution, energy consumption has steadily transitioned from wood and biomass to fossil fuels. The early development of solar technologies starting in the 1860s was driven by an expectation that coal would soon become scarce, but solar development stagnated in the early 20th century in the face of the increasing availability, economy, and utility of fossil fuels such as coal and petroleum. The 1973 oil embargo and 1979 energy crisis caused a reorganization of energy policies around the world and brought renewed attention to developing solar technologies. Deployment strategies focused on incentive programs such as the Federal Photovoltaic Utilization Program in the US and the Sunshine Program in Japan. Other efforts included the formation of research facilities in the US (SERI, now NREL), Japan (NEDO), and Germany (Fraunhofer Institute for Solar Energy Systems ISE). Between 1970 and 1983, photovoltaic installations grew rapidly, but falling oil

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prices in the early 1980s moderated the growth of PV from 1984 to 1996. Since 1997, PV development has accelerated due to supply issues with oil and natural gas, global warming concerns (see Kyoto Protocol), and the improving economic position of PV relative to other energy technologies. Photovoltaic production growth has averaged 40% per year since 2000 and installed capacity reached 10.6 GW at the end of 2007. Since 2006 it has been economical for investors to install photovoltaics for free in return for a long term power purchase agreement. 50% of commercial systems were installed in this manner in 2007 and it is expected that 90% will by 2009. Nellis Air Force Base is receiving photoelectric power for about 2.2 ¢/kWh and grid power for 9 ¢/kWh. Commercial solar water heaters began appearing in the United States in the 1890s. These systems saw increasing use until the 1920s but were gradually replaced by cheaper and more reliable heating fuels. As with photovoltaics, solar water heating attracted renewed attention as a result of the oil crises in the 1970s but interest subsided in the 1980s due to falling petroleum prices. Development in the solar water heating sector progressed steadily throughout the 1990s and growth rates have averaged 20% per year since 1999. Although generally underestimated, solar water heating is by far the most widely deployed solar technology with an estimated capacity of 154 GW as of 2007. Commercial concentrating solar power (CSP) plants were first developed in the 1980s. CSP plants such as SEGS project in the United States have a LEC of 12-14 ¢/kWh. The 11 MW PS10 power tower in Spain, completed in late 2005, is Europe's first commercial CSP system and a total capacity of 300 MW is expected to be installed in the same area by 2013.

CARBON NANOTUBES IN PHOTOVOLTAICS Organic photovoltaic devices (OPVs) are fabricated from thin films of organic semiconductors, such as polymers and smallmolecule compounds, and are typically on the order of 100 nm thick. Because polymer based OPVs can be made using a coating process such as spin coating or inkjet printing, they are an attractive option for inexpensively covering large areas as well as flexible

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plastic surfaces. A promising low cost alternative to silicon solar cells, there is a large amount of research being dedicated throughout industry and academia towards developing OPVs and increasing their power conversion efficiency.

CARBON NANOTUBE COMPOSITES IN THE PHOTOACTIVE LAYER Combining the physical and chemical characteristics of conjugated polymers with the high conductivity along the tube axis of carbon nanotubes (CNTs) provides a great deal of incentive to disperse CNTs into the photoactive layer in order to obtain more efficient OPV devices. The interpenetrating bulk donoracceptor heterojunction in these devices can achieve charge separation and collection because of the existence of a bicontinuous network. Along this network, electrons and holes can travel toward their respective contacts through the electron acceptor and the polymer hole donor. Photovoltaic efficiency enhancement is proposed to be due to the introduction of internal polymer/ nanotube junctions within the polymer matrix. The high electric field at these junctions can split up the excitons, while the SWNT can act as a pathway for the electrons. The dispersion of CNTs in a solution of an electron donating conjugated polymer is perhaps the most common strategy to implement CNT materials into OPVs. Generally poly(3hexylthiophene) (P3HT) or poly(3-octylthiophene) (P3OT) are used for this purpose. These blends are then spin coated onto a transparent conductive electrode with thicknesses that vary from 60 to 120 nm. These conductive electrodes are usually glass covered with indium tin oxide (ITO) and a 40 nm sublayer of (poly (3,4ethylenedioxythiophene) (PEDOT) and poly(styrenesulfonate) (PSS). PEDOT and PSS help to smooth the ITO surface, decreasing the density of pinholes and stifling current leakage that occurs along shunting paths. Through thermal evaporation or sputter coating, a 20 to 70 nm thick layer of aluminum and sometimes an intermediate layer of lithium fluoride are then applied onto the photoactive material. Multiple research investigations with both multi-walled carbon nanotubes (MWCNTs) and single-walled carbon nanotubes (SWCNTs) integrated into the photoactive

Introduction

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material have been completed. Enhancements of more than two orders of magnitude have been observed in the photocurrent from adding SWCNTs to the P3OT matrix. Improvements were speculated to be due to charge separation at polymer-SWCNT connections and more efficient electron transport through the SWCNTs. However, a rather low power conversion efficiency of 0.04% under 100 mW cm-2 white illumination was observed for the device suggesting incomplete exciton dissociation at low CNT concentrations of 1.0% wt. Because the lengths of the SWCNTs were similar to the thickness of photovoltaic films, doping a higher percentage of SWCNTs into the polymer matrix was believed to cause short circuits. To supply additional dissociation sites, other researchers have physically blended functionalized MWCNTs into P3HT polymer to create a P3HT-MWNT with fullerene C60 doublelayered device. However, the power efficiency was still relatively low at 0.01% under 100 mW cm-2 white illumination. Weak exciton diffusion toward the donor-acceptor interface in the bilayer structure may have been the cause in addition to the fullerene C60 layer possibly experiencing poor electron transport. More recently, a polymer photovoltaic device from C60modified SWCNTs and P3HT has been fabricated. Microwave irradiating a mixture of SWCNT-water solution and C60 solution in toluene was the first step in making these polymer-SWCNT composites. Conjugated polymer P3HT was then added resulting in a power conversion efficiency of 0.57% under simulated solar irradiation (95 mW cm-2). It was concluded that improved short circuit current density was a direct result of the addition of SWCNTs into the composite causing faster electron transport via the network of SWCNTs. It was also concluded that the morphology change led to an improved the fill factor. Overall, the main result was improved power conversion efficiency with the addition of SWCNTs, compared to cells without SWCNTs; however, further optimization was thought to be possible. Additionally, it has been found that heating to the point beyond the glass transition temperature of either P3HT or P3OT after construction can be beneficial for manipulating the phase separation of the blend. This heating also affects the ordering of the polymeric chains because the polymers are microcrystalline

20


systems and it improves charge transfer, charge transport, and charge collection throughout the OPV device. The hole mobility and power efficiency of the polymer-CNT device also increased significantly as a result of this ordering. Emerging as another valuable approach for deposition, the use of tetraoctylammonium bromide in tetrahydrofuran has also been the subject of investigation to assist in suspension by exposing SWCNTs to an electrophoretic field. In fact, photoconversion efficiencies of 1.5% and 1.3% were achieved when SWCNTs were deposited in combination with light harvesting CdS quantum dots and porphyrins, respectively. Among the best power conversions achieved to date using CNTs were obtained by depositing a SWCNT layer between the ITO and the PEDOT : PSS or between the PEDOT : PSS and the photoactive blend in a modified ITO/PEDOT : PSS/ P3HT : (6,6)phenyl-C61-butyric acid methyl ester (PCBM)/Al solar cell. By dip-coating from a hydrophilic suspension, SWCNT were deposited after an initially exposing the surface to an argon plasma to achieve a power conversion efficiency of 4.9%, compared to 4% without CNTs. However, even though CNTs have shown potential in the photoactive layer, they have not resulted in a solar cell with a power conversion efficiency greater than the best tandem organic cells (6.5% efficiency). But, it has been shown in most of the previous investigations that the control over a uniform blending of the electron donating conjugated polymer and the electron accepting CNT is one of the most difficult as well as crucial aspects in creating efficient photocurrent collection in CNT-based OPV devices. Therefore, using CNTs in the photoactive layer of OPV devices is still in the initial research stages and there is still room for novel methods to better take advantage of the beneficial properties of CNTs.

CARBON NANOTUBES AS A TRANSPARENT ELECTRODE ITO is currently the most popular material used for the transparent electrodes in OPV devices; however, it has a number of deficiencies. For one, it is not very compatible with polymeric substrates due to its high deposition temperature of around 600oC.

Introduction

21

Traditional ITO also has unfavorable mechanical properties such as being relatively fragile. In addition, the combination of costly layer deposition in vacuum and a limited supply of indium results in high quality ITO transparent electrodes being very expensive. Therefore, developing and commercializing a replacement for ITO is a major focus of OPV research and development. Conductive CNT coatings have recently become a prospective substitute based on wide range of methods including spraying, spin coating, casting, layer-by-layer, and Langmuir-Blodgett deposition. The transfer from a filter membrane to the transparent support using a solvent or in the form of an adhesive film is another method for attaining flexible and optically transparent CNT films. Other research efforts have shown that films made of arc-discharge CNT can result in a high conductivity and transparency. Furthermore, the work function of SWCNT networks is in the 4.8 to 4.9 eV range (compared to ITO which has a lower work function of 4.7 eV) leading to the expectation that the SWCNT work function should be high enough to assure efficient hole collection. Another benefit is that SWCNT films exhibit a high optical transparency in a broad spectral range from the UV-visual far into the near IR range. Only a few materials retain reasonable transparency in the infrared spectrum while maintaining transparency in the visible part of the spectrum as well as acceptable overall electrical conductivity. SWCNT films are highly flexible, do not creep, do not crack after bending, theoretically have high thermal conductivities to tolerate heat dissipation, and have high radiation resistance. However, the electrical sheet resistance of ITO is an order of magnitude less than the sheet resistance measured for SWCNT films. Nonetheless, initial research studies demonstrate SWCNT thin films can be used as conducting, transparent electrodes for hole collection in OPV devices with efficiencies between 1% and 2.5% confirming that they are comparable to devices fabricated using ITO. Thus, possibilities exist for advancing this research to develop CNT-based transparent electrodes that exceed the performance of traditional ITO materials.

CNTS IN DYE-SENSITIZED SOLAR CELLS Due to the simple fabrication process, low production cost, and high efficiency, there is significant interest in dye-sensitized

22


solar cells (DSSCs). Thus, improving DSSC efficiency has been the subject of a variety of research investigations because it has the potential to be manufactured economically enough to compete with other solar cell technologies. Titanium dioxide nanoparticles have been widely used as a working electrode for DSSCs because they provide a high efficiency, more than any other metal oxide semiconductor investigated. Yet the highest conversion efficiency under air mass (AM) 1.5 (100mWcm2) irradiation reported for this device to date is about 11%. Despite this initial success, the effort to further enhance efficiency has not produced any major results. The transport of electrons across the particle network has been a key problem in achieving higher photoconversion efficiency in nanostructured electrodes. Because electrons encounter many grain boundaries during the transit and experience a random path, the probability of their recombination with oxidized sensitizer is increased. Therefore, it is not adequate to enlarge the oxide electrode surface area to increase efficiency because photo-generated charge recombination should be prevented. Promoting electron transfer through film electrodes and blocking interface states lying below the edge of the conduction band are some of the non-CNT based strategies to enhance efficiency that have been employed. With recent progress in CNT development and fabrication, there is promise to use various CNT based nanocomposites and nanostructures to direct the flow of photogenerated electrons and assist in charge injection and extraction. To assist the electron transport to the collecting electrode surface in a DSSC, a popular concept is to utilize CNT networks as support to anchor light harvesting semiconductor particles. Research efforts along these lines include organizing CdS quantum dots on SWCNTs. Charge injection from excited CdS into SWCNTs was documented upon excitation of CdS nanoparticles. Other varieties of semiconductor particles including CdSe and CdTe can induce charge-transfer processes under visible light irradiation when attached to CNTs. Including porphyrin and C60 fullerene, organization of photoactive donor polymer and acceptor fullerene on electrode surfaces has also been shown to offer considerable improvement in the photoconversion efficiency of solar cells. Therefore, there is an

Introduction

23

opportunity to facilitate electron transport and increase the photoconversion efficiency of DSSCs utilizing the electronaccepting ability of semiconducting SWCNTs. Other researchers fabricated DSSCs using the sol-gel method to obtain titanium dioxide coated MWCNTs for use as an electrode. Because pristine MWCNTs have a hydrophobic surface and poor dispersion stability, pretreatment was necessary for this application. A relatively low destruction method for removing impurities, H2O2 treatment was used to generate carboxylic acid groups by oxidation of MWCNTs. Another positive aspect was the fact that the reaction gases including CO2 and H2O were nontoxic and could be released safely during the oxidation process. As a result of treatment, H 2O2 exposed MWCNTs have a hydrophilic surface and the carboxylic acid groups on the surface have polar covalent bonding. Also, the negatively charged surface of the MWCNTs improved the stability of dispersion. By then entirely surrounding the MWCNTs with titanium dioxide nanoparticles using the sol-gel method, an increase in the conversion efficiency of about 50% compared to a conventional titanium dioxide cell was achieved. The enhanced interconnectivity between the titanium dioxide particles and the MWCNTs in the porous titanium dioxide film was concluded to be the cause of the improvement in short circuit current density. Here again, the addition of MWCNTs was thought to provide more efficient electron transfer through film in the DSSC. ENERGY STORAGE Energy storage is the storing of some form of energy that can be drawn upon at a later time to perform some useful operation. A device that stores energy is sometimes called an accumulator. All forms of energy are either potential energy (eg. chemical, gravitational or electrical energy) or kinetic energy (eg. thermal energy). A wind up clock stores potential energy (in this case mechanical, in the spring tension), a battery stores readily convertible chemical energy to keep a clock chip in a computer running (electrically) even when the computer is turned off, and a hydroelectric dam stores power in a reservoir as gravitational potential energy. Even food is a form of energy storage, chemical in this case.

24


HISTORY Energy storage as a natural process is as old as the universe itself - the energy present at the initial creation of the Universe has been stored in stars such as the Sun, and is now being used by humans directly (e.g. through solar heating), or indirectly (e.g. by growing crops or conversion into electricity in solar cells). Energy storage systems in commercial use today can be broadly categorized as mechanical, electrical, chemical, biological, thermal and nuclear. As a purposeful activity, energy storage has existed since pre-history, though it was often not explicitly recognized as such. An example of deliberate mechanical energy storage is the use of logs or boulders as defensive measures in ancient forts the logs or boulders were collected at the top of a hill or wall, and the energy thus stored used to attack invaders who came within range. A more recent application is the control of waterways to drive water mills for processing grain or powering machinery. Complex systems of reservoirs and dams were constructed to store and release water (and the potential energy it contained) when required. Energy storage became a dominant factor in economic development with the widespread introduction of electricity and refined chemical fuels, such as gasoline, kerosene and natural gas in the late 1800s. Unlike other common energy storage used in prior use, such as wood or coal, electricity must be used as it is generated and cannot be stored on anything other than a minor scale. Electricity is transmitted in a closed circuit, and for essentially any practical purpose cannot be stored as electrical energy. This meant that changes in demand could not be accommodated without either cutting supplies (eg, via brownouts or blackouts) or arranging for a storage technique. An early solution to the problem of storing energy for electrical purposes was the development of the battery, an electrochemical storage device. It has been of limited use in electric power systems due to small capacity and high cost. A similar possible solution with the same type of problems is the capacitor. Chemical fuels have become the dominant form of energy storage, both in electrical generation and energy transportation. Chemical fuels in common use are processed coal, gasoline, diesel

Introduction

25

fuel, natural gas, liquefied petroleum gas (LPG), propane, butane, ethanol, biodiesel and hydrogen. All of these chemicals are readily converted to mechanical energy and then to electrical energy using heat engines (turbines or other internal combustion engines, or boilers or other external combustion engines) used for electrical power generation. Heat engine powered generators are nearly universal, ranging from small engines producing only a few kilowatts to utility-scale generators with ratings up to 800 megawatts. Electrochemical devices called fuel cells were invented about the same time as the battery. However, for many reasons, fuel cells were not well developed until the advent of manned spaceflight (the Gemini Program) when lightweight, non-thermal (ie, efficient) sources of electricity were required in spacecraft. Fuel cell development has increased in recent years to an attempt to increase conversion efficiency of chemical energy stored in hydrocarbon or hydrogen fuels into electricity. At this time, liquid hydrocarbon fuels are the dominant forms of energy storage for use in transportation. However, these produce greenhouse gases when used to power cars, trucks, trains, ships and aircraft. Carbon-free energy carriers, such as hydrogen, or carbon-neutral energy carriers, such as some forms of ethanol or biodiesel, are being sought in response to concerns about the possible consequences of greenhouse gas emissions. Some areas of the world (Washington and Oregon in the USA, and Wales in the United Kingdom are examples) have used geographic features to store large quantities of water in elevated reservoirs, using excess electricity at times of low demand to pump water up to the reservoirs, then letting the water fall through turbine generators to retrieve the energy when demand peaks. Several other technologies have also been investigated, such as flywheels or compressed air storage in underground caverns, but to date no widely available solution to the challenge of mass energy storage has been deployed commercially.

GRID ENERGY STORAGE Grid energy storage lets energy producers send excess electricity over the electricity transmission grid to temporary

26


electricity storage sites that become energy producers when electricity demand is greater. Grid energy storage is particularly important in matching supply and demand over a 24 hour period of time.

STORAGE METHODS • Chemical o Hydrogen o Biofuels • Electrochemical o Batteries o Flow batteries o Fuel cells • Electrical o Capacitor o Supercapacitor o Superconducting magnetic energy storage (SMES) • Mechanical o Compressed air energy storage (CAES) o Flywheel energy storage o Hydraulic accumulator o Hydroelectric energy storage o Spring • Thermal o Molten salt o Cryogenic liquid air or nitrogen o Seasonal thermal store o Solar pond o Hot bricks o Steam accumulator o Fireless locomotive HYDROGEN Hydrogen is a chemical energy carrier, just like gasoline, ethanol or natural gas. The unique characteristic of hydrogen is

Introduction

27

that it is the only carbon-free or zero-emission chemical energy carrier. Hydrogen is a widely used industrial chemical that can be produced from any primary energy source. Most of the world's production is by the thermal reformation of natural gas (methane) into hydrogen that is used immediately to refine petroleum into gasoline, diesel fuel and other petrochemicals. The carbon dioxide produced by the reforming process is either captured and processed into liquid carbon dioxide or vented to the atmosphere. Because hydrogen is produced and distributed in such huge quantities, the technology needed to build infrastructure to serve wholesale and retail energy markets is proven, reliable and commercially available. Hydrogen can be used as a fuel for all types of internal and external combustion heat engines and turbines (with adjustments to compensate for the difference between, say, diesel fluid and hydrogen gas). Hydrogen fueled heat engines can be optimized to operate at higher thermal efficiencies than traditional heat engines using traditional hydrocarbon fuels. The increased thermodynamic efficiency, and reduced pollution, would be beneficial, but they are not produced in quantity largely because hydrogen is not industrially available. Sufficiently purified hydrogen can also be used to power electrochemical engines, such as the proton exchange membrane (PEM) fuel cell. Hydrogen fuel cells can be more efficient than hydrogen fueled heat engines, and thus much more efficient than hydrocarbon fuel heat engines. They are also less polluting. Several companies are attempting to develop reliable, inexpensive PEM fuel cells. However, designs are not sufficiently developed to be routinely mass produced. The limited quantities available for purchase are hand made and much more expensive than conventional heat engines. Hydrogen production in quantities sufficient to replace existing hydrocarbon fuels is not possible. Such production will require more energy than is currently being used, and require large capital investment in hydrogen production plants. Because of the increased costs, hydrogen is not yet in widespread use. If the cost of greenhouse gas production is fully included into the market price of hydrocarbon fuels, hydrogen fuels may become more attractive

28


commercially, providing clean, efficient power for our homes, businesses and vehicles. Disadvantages of hydrogen include a low energy density per volume (even when highly compressed) compared to traditional hydrocarbon fuels, changing such things as the volumes of fuel required for equivalent performance. And, for many hydrogen production methods, there is a significant loss of energy during the conversion. Some production methods, for instance, electrolytic generation from water, are more efficient.

BIOFUELS Various biofuels such as biodiesel, straight vegetable oil, alcohol fuels, or biomass can be used to replace hydrocarbon fuels. Various chemical processes can convert the carbon and hydrogen in coal, natural gas, plant and animal biomass, and organic wastes into short hydrocarbons suitable as replacements for existing hydrocarbon fuels. Examples are Fischer-Tropsch diesel, methanol, dimethyl ether, or syngas. This diesel source was used extensively in World War II in Germany, with limited access to crude oil supplies. Today South Africa produces most of country's diesel from coal for similar reasons. A long term oil price above 35 USD may make such synthetic liquid fuels economical on a large scale (See coal). Some of the energy in the original source is lost in the conversion process. Historically, coal itself has been used directly for transportation purposes in vehicles and boats using steam engines. And compressed natural gas is being used in special circumstances fuel, for instance in busses for some mass transit agencies. SYNTHETIC HYDROCARBON FUEL Carbon dioxide in the atmosphere has been, experimentally, converted into hydrocarbon fuel with the help of energy from another source. To be useful industrially, the energy will probably have to come from sunlight using, perhaps, future artificial photosynthesis technology. Another alternative for the energy is electricity or heat from solar energy or nuclear power. Compared to hydrogen, many hydrocarbons fuels have the advantage of being immediately usable in existing engine technology and

Introduction

29

existing fuel distribution infrastructures. Manufacturing synthetic hydrocarbon fuel reduces the amount of carbon dioxide in the atmosphere until the fuel is burned, when the same amount of carbon dioxide returns to the atmosphere. If usable on a wide scale, this approach may help in the long term to avoid some of the deleterious effects of greenhouse gas emission.

BORON, SILICON, AND ZINC Boron, silicon, and zinc have been proposed as energy storage solutions. MECHANICAL STORAGE Energy can be stored in water pumped to a higher elevation, in compressed air, or in spinning flywheels, but mechanical methods of storing energy on a large scale are expensive and water pumping systems require considerable capital investment. Several companies have done preliminary design work for vehicles using compressed air power. INTERMITTENT POWER Many renewable energy systems produce intermittent power. Other generators on the grid can be throttled to match varying production from renewable sources, but most of the existing throttling capacity is already committed to handling load variations. Further development of intermittent renewable power will require some combination of grid energy storage, demand response, and spot pricing. Intermittent energy sources is limited to at most 20-30% of the electricity produced for the grid without such measures. If electricity distribution loss and costs are managed, then intermittent power production from many different sources could increase the overall reliability of the grid. Non-intermittent renewable energy sources include hydroelectric power, geothermal power, solar thermal, tidal power, Energy tower, ocean thermal energy conversion, high altitude airborne wind turbines, biofuel, and solar power satellites. Solar photovoltaics, although technically intermittent, produce electricity

30


Solar Variation

31

largely during peak periods (ie, daylight), and hence do reduce the need for peak power generation, though somewhat unreliably in most areas since weather conditions interfere with terrestrially mounted solar cells. On the demand side, demand response programs, which send market pricing signals to consumers (or their equipment), can be a very effective way of managing variations in electricity production. For example, electrically powered hydrogen production can be set to increase when electricity is being produced beyond current demand (and prices will be lowest), and conversely, hot water heaters can be automatically set to a lower temperature when demand is high and pricing is also high.

2 SOLAR VARIATION Solar variations are changes in the amount of solar radiation emitted by the Sun. There are periodic components to these variations, the principal one being the 11-year solar cycle (or sunspot cycle), as well as aperiodic fluctuations. Solar activity has been measured via satellites during recent decades and through 'proxy' variables in prior times. Climate scientists are interested in understanding what, if any, effect variations in solar activity have on the Earth. Effects on the earth caused by solar activity are called "solar forcing". The variations in total solar irradiance (TSI) remained at or below the threshold of detectability until the satellite era, although the small fraction in ultra-violet wavelengths varies by a few percent. Total solar output is now measured to vary (over the last three 11-year sunspot cycles) by approximately 0.1% or about 1.3 W/m² peak-to-trough during the 11 year sunspot cycle. The amount of solar radiation received at the outer surface of Earth's atmosphere varied little from an average value of 1,366 watts per square meter (W/m²). There are no direct measurements of the longer-term variation and interpretations of proxy measures of variations differ; recent results suggest about 0.1% variation over the last 2,000 years, although other sources suggest a 0.2% increase in solar irradiance since 1675. The combination of solar variation and volcanic effects has very likely been the cause of some climate change, for example during the Maunder Minimum. A 2006 study and review of existing literature, published in Nature, determined that there has been no net increase in solar brightness since the mid 1970s, and that changes in solar output

32


within the past 400 years are unlikely to have played a major part in global warming. However, the same report cautions that "Apart from solar brightness, more subtle influences on climate from cosmic rays or the Sun's ultraviolet radiation cannot be excluded, say the authors. They also add that these influences cannot be confirmed because physical models for such effects are still too poorly developed." HISTORY OF STUDY INTO SOLAR VARIATIONS The longest recorded aspect of solar variations are changes in sunspots. The first record of sunspots dates to around 800 BC in China and the oldest surviving drawing of a sunspot dates to 1128. In 1610, astronomers began using the telescope to make observations of sunspots and their motions. Initial study was focused on their nature and behavior. Although the physical aspects of sunspots were not identified until the 1900s, observations continued. Study was hampered during the 1600s and 1700s due to the low number of sunspots during what is now recognized as an extended period of low solar activity, known as the Maunder Minimum. By the 1800s, there was a long enough record of sunspot numbers to infer periodic cycles in sunspot activity. In 1845, Princeton University professors Joseph Henry and Stephen Alexander observed the Sun with a thermopile and determined that sunspots emitted less radiation than surrounding areas of the Sun. The emission of higher than average amounts of radiation later were observed from the solar faculae. Around 1900, researchers began to explore connections between solar variations and weather on Earth. Of particular note is the work of Charles Greeley Abbot. Abbot was assigned by the Smithsonian Astrophysical Observatory (SAO) to detect changes in the radiation of the Sun. His team had to begin by inventing instruments to measure solar radiation. Later, when Abbot was head of the SAO, it established a solar station at Calama, Chile to complement its data from Mount Wilson Observatory. He detected 27 harmonic periods within the 273-month Hale cycles, including 7, 13, and 39 month patterns. He looked for connections to weather by means such as matching opposing solar trends during a month to opposing temperature and precipitation trends

Solar Variation

33

in cities. With the advent of dendrochronology, scientists such as Waldo S. Glock attempted to connect variation in tree growth to periodic solar variations in the extant record and infer long-term secular variability in the solar constant from similar variations in millennial-scale chronologies. Statistical studies that correlate weather and climate with solar activity have been popular for centuries, dating back at least to 1801, when William Herschel noted an apparent connection between wheat prices and sunspot records. They now often involve high-density global datasets compiled from surface networks and weather satellite observations and/or the forcing of climate models with synthetic or observed solar variability to investigate the detailed processes by which the effects of solar variations propagate through the Earth's climate system. SOLAR ACTIVITY

SUNSPOTS Sunspots are relatively dark areas on the surface of the Sun where intense magnetic activity inhibits convection and so cools the surface. The number of sunspots correlates with the intensity of solar radiation. The variation is small (of the order of 1 W/m² or 0.1% of the total) and was only established once satellite measurements of solar variation became available in the 1980s. Based on work by Abbot, Foukal et al. (1977) realised that higher values of radiation are associated with more sunspots. Nimbus 7 (launched October 25, 1978) and the Solar Maximum Mission (launched February 14, 1980) detected that because the areas surrounding sunspots are brighter, the overall effect is that more sunspots means a brighter sun. There had been some suggestion that variations in the solar diameter might cause variations in output. But recent work, mostly from the Michelson Doppler Imager instrument on SOHO, shows these changes to be small, about 0.001% (Dziembowski et al., 2001). Various studies have been made using sunspot number (for which records extend over hundreds of years) as a proxy for solar output (for which good records only extend for a few decades). Also, ground instruments have been calibrated by comparison

34


with high-altitude and orbital instruments. Researchers have combined present readings and factors to adjust historical data. Other proxy data - such as the abundance of cosmogenic isotopes - have been used to infer solar magnetic activity and thus likely brightness. Sunspot activity has been measured using the Wolf number for about 300 years. This index (also known as the Zürich number) uses both the number of sunspots and the number of groups of sunspots to compensate for variations in measurement. A 2003 study by Ilya Usoskin of the University of Oulu, Finland found that sunspots had been more frequent since the 1940s than in the previous 1150 years. Sunspot numbers over the past 11,400 years have been reconstructed using dendrochronologically dated radiocarbon concentrations. The level of solar activity during the past 70 years is exceptional - the last period of similar magnitude occurred over 8,000 years ago. The Sun was at a similarly high level of magnetic activity for only ~10% of the past 11,400 years, and almost all of the earlier high-activity periods were shorter than the present episode. A list of historical Grand minima of solar activity includes also Grand minima ca. 690 AD, 360 BC, 770 BC, 1390 BC, 2860 BC, 3340 BC, 3500 BC, 3630 BC, 3940 BC, 4230 BC, 4330 BC, 5260 BC, 5460 BC, 5620 BC, 5710 BC, 5990 BC, 6220 BC, 6400 BC, 7040 BC, 7310 BC, 7520 BC, 8220 BC, 9170 BC.

Solar Variation

35

• 22 years: Hale cycle, named after George Ellery Hale. The magnetic field of the Sun reverses during each Schwabe cycle, so the magnetic poles return to the same state after two reversals. • 87 years (70-100 years): Gleissberg cycle, named after Wolfgang Gleißberg, is thought to be an amplitude modulation of the 11-year Schwabe Cycle (Sonnett and Finney, 1990).Braun, et al, (2005) • 210 years: Suess cycle (a.k.a. de Vries cycle). Braun, et al, (2005). • 2,300 years: Hallstatt cycle Other patterns have been detected: • In carbon-14: 105, 131, 232, 385, 504, 805, 2,241 years (Damon and Sonnett, 1991). • During the Upper Permian 240 million years ago, mineral layers created in the Castile Formation show cycles of 2,500 years. The sensitivity of climate to cyclical variations in solar forcing will be higher for longer cycles due to the thermal inertia of the ocean, which acts to damp high frequencies. Scafetta and West (2005) found that the climate was 1.5 times as sensitive to 22 year cyclical forcing relative to 11 year cyclical forcing, and that the thermal inertial induced a lag of approximately 2.2 years in cyclic climate response in the temperature data.

Predictions Based on Patterns SOLAR CYCLES Solar cycles are cyclic changes in behavior of the Sun. Many possible patterns have been suggested; only the 11 and 22 year cycles are clear in the observations. • 11 years: Most obvious is a gradual increase and decrease of the number of sunspots over a period of about 11 years, called the Schwabe cycle and named after Heinrich Schwabe. The Babcock Model explains this as being due to a shedding of entangled magnetic fields. The Sun's surface is also the most active when there are more sunspots, although the luminosity does not change much due to an increase in bright spots (faculae).

• A simple model based on emulating harmonics by multiplying the basic 11-year cycle by powers of 2 produced results similar to Holocene behavior. Extrapolation suggests a gradual cooling during the next few centuries with intermittent minor warmups and a return to near Little Ice Age conditions within the next 500 years. This cool period then may be followed approximately 1,500 years from now by a return to altithermal conditions similar to the previous Holocene Maximum. • There is weak evidence for a quasi-periodic variation in the sunspot cycle amplitudes with a period of about 90 years. These characteristics indicate that the next solar

36

Solar Energy and its Uses cycle should have a maximum smoothed sunspot number of about 145±30 in 2010 while the following cycle should have a maximum of about 70±30 in 2023. • Because carbon-14 cycles are quasi periodic, Damon and Sonett (1989) predict future climate:

Cycle length

232

Cycle name

--?--

Last positive carbon-14 anomaly AD 1922 (cool)

Next "warming"

AD 2038

208

Suess

AD 1898 (cool)

AD 2002

88

Gleisberg

AD 1986 (cool)

AD 2030

SOLAR IRRADIANCE OF EARTH AND ITS SURFACE Solar irradiance, or insolation, is the amount of sunlight which reaches the Earth. The equipment used might measure optical brightness, total radiation, or radiation in various frequencies. Historical estimates use various measurements and proxies. There are two common meanings: • the radiation reaching the upper atmosphere • the radiation reaching some point within the atmosphere, including the surface. Various gases within the atmosphere absorb some solar radiation at different wavelengths, and clouds and dust also affect it. Hence measurements above the atmosphere are needed to observe variations in solar output, within the confounding effects of changes to the atmosphere. Indeed, there is some evidence that sunshine at the Earth's surface has been decreasing in the last 50

Solar Variation

37

years (see global dimming) possibly caused by increased atmospheric pollution, whilst over roughly the same timespan solar output has been nearly constant.

MILANKOVITCH CYCLE VARIATIONS Some variations in insolation are not due to solar changes but rather due to the Earth moving closer or further from the Sun, or changes in the relative amount of radiation reaching regions of the Earth. These have caused variations of as much as 25% (locally; global average changes are much smaller) in solar insolation over long periods. The most recent significant event was an axial tilt of 24° during boreal summer at near the time of the Holocene climatic optimum. SOLAR INTERACTIONS WITH EARTH There are several hypotheses for how solar variations may affect Earth. Some variations, such as changes in the size of the Sun, are presently only of interest in the field of astronomy.

Changes in Total Irradiance • Overall brightness may change. • The variation during recent cycles has been about 0.1%. • Changes corresponding to solar changes with periods of 9-13, 18-25, and >100 years have been measured in seasurface temperatures. • Since the Maunder Minimum, over the past 300 years there probably has been an increase of 0.1 to 0.6%, with climate models often using a 0.25% increase. • One reconstruction from the ACRIM data show a 0.05% per decade trend of increased solar output between solar minima over the short span of the data set. These display a high degree of correlation with solar magnetic activity as measured by Greenwich Sunspot Number. Wilson, Mordvinov (2003)

Changes in Ultraviolet Irradiance • Ultraviolet irradiance (EUV) varies by approximately 1.5 percent from solar maxima to minima, for 200 to 300 nm UV.

38

Solar Energy and its Uses • Energy changes in the UV wavelengths involved in production and loss of ozone have atmospheric effects. o The 30 hPa atmospheric pressure level has changed height in phase with solar activity during the last 4 solar cycles. o UV irradiance increase causes higher ozone production, leading to stratospheric heating and to poleward displacements in the stratospheric and tropospheric wind systems. • A proxy study estimates that UV has increased by 3% since the Maunder Minimum.

Changes in the Solar Wind and the Sun's Magnetic Flux • A more active solar wind and stronger magnetic field reduces the cosmic rays striking the Earth's atmosphere. • Variations in the solar wind affect the size and intensity of the heliosphere, the volume larger than the Solar System filled with solar wind particles. • Cosmogenic production of 14C, 10Be and 36Cl show changes tied to solar activity. • Cosmic ray ionization in the upper atmosphere does change, but significant effects are not obvious. • As the solar coronal-source magnetic flux doubled during the past century, the cosmic-ray flux has decreased by about 15%. • The Sun's total magnetic flux rose by a factor of 1.41 from 1964-1996 and by a factor of 2.3 since 1901.

EFFECTS ON CLOUDS • Cosmic rays have been hypothesized to affect formation of clouds through possible effects on production of cloud condensation nuclei. Observational evidence for such a relationship is, at best, inconclusive. • 1983-1994 data from the International Satellite Cloud Climatology Project (ISCCP) showed that global low cloud formation was highly correlated with cosmic ray flux; subsequent to this the correlation breaks down.

Solar Variation

39

• The Earth's albedo decreased by about 2.5% over 5 years during the most recent solar cycle, as measured by lunar "Earthshine". Similar reduction was measured by satellites during the previous cycle. • Merranean core study of plankton detected a solar-related 11 year cycle, and an increase 3.7 times larger between 1760 and 1950. A considerable reduction in cloud cover is proposed. • A laboratory experiment conducted by Henrik Svensmark at the Danish National Space Center was able to produce particles as a result of cosmic ray-like irradiation, though these particles do not resemble actual cloud condensation nuclei found in nature.

Other Effects Due to Solar Variation Interaction of solar particles, the solar magnetic field, and the Earth's magnetic field, cause variations in the particle and electromagnetic fields at the surface of the planet. Extreme solar events can affect electrical devices. Weakening of the Sun's magnetic field is believed to increase the number of interstellar cosmic rays which reach Earth's atmosphere, altering the types of particles reaching the surface. It has been speculated that a change in cosmic rays could cause an increase in certain types of clouds, affecting Earth's albedo. GEOMAGNETIC EFFECTS

Solar Particles Interact with Earth's Magnetosphere The Earth's polar aurorae are visual displays created by interactions between the solar wind, the solar magnetosphere, the Earth's magnetic field, and the Earth's atmosphere. Variations in any of these affect aurora displays. Sudden changes can cause the intense disturbances in the Earth's magnetic fields which are called geomagnetic storms.

Solar Proton Events Energetic protons can reach Earth within 30 minutes of a major flare's peak. During such a solar proton event, Earth is showered in energetic solar particles (primarily protons) released

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from the flare site. Some of these particles spiral down Earth's magnetic field lines, penetrating the upper layers of our atmosphere where they produce additional ionization and may produce a significant increase in the radiation environment.

Galactic Cosmic Rays An increase in solar activity (more sunspots) is accompanied by an increase in the "solar wind," which is an outflow of ionized particles, mostly protons and electrons, from the sun. The Earth's geomagnetic field, the solar wind, and the solar magnetic field deflect galactic cosmic rays (GCR). A decrease in solar activity increases the GCR penetration of the troposphere and stratosphere. GCR particles are the primary source of ionization in the troposphere above 1 km (below 1 km, radon is a dominant source of ionization in many areas). Levels of GCRs have been indirectly recorded by their influence on the production of carbon-14 and beryllium-10. The Hallstatt solar cycle length of approximately 2300 years is reflected by climatic Dansgaard-Oeschger events. The 80-90 year solar Gleissberg cycles appear to vary in length depending upon the lengths of the concurrent 11 year solar cycles, and there also appear to be similar climate patterns occurring on this time scale.

Cloud Effects Changes in ionization affect the abundance of aerosols that serve as the nuclei of condensation for cloud formation. As a result, ionization levels potentially affect levels of condensation, low clouds, relative humidity, and albedo due to clouds. Clouds formed from greater amounts of condensation nuclei are brighter, longer lived, and likely to produce less precipitation. Changes of 3-4% in cloudiness and concurrent changes in cloud top temperatures have been correlated to the 11 and 22 year solar (sunspot) cycles, with increased GCR levels during "antiparallel" cycles. Global average cloud cover change has been found to be 1.52%. Several studies of GCR and cloud cover variations have found positive correlation at latitudes greater than 50° and negative correlation at lower latitudes. However, not all scientists accept

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this correlation as statistically significant, and some that do attribute it to other solar variability (e.g. UV or total irradiance variations) rather than directly to GCR changes. Difficulties in interpreting such correlations include the fact that many aspects of solar variability change at similar times, and some climate systems have delayed responses.

Carbon-14 Production The production of carbon-14 (radiocarbon: 14C) also is related to solar activity. Carbon-14 is produced in the upper atmosphere when cosmic ray bombardment of atmospheric nitrogen (14N) changes the Nitrogen into an unusual form of Carbon with an atomic weight of 14 rather than the more common 12. Paradoxically, increased solar activity results in a reduction of cosmic rays reaching the earth's atmosphere and reduces 14C production. This is because cosmic rays are partially excluded from the Solar System by the outward sweep of magnetic fields in the solar wind. Thus the cosmic ray intensity and carbon-14 production vary oppositely to the general level of solar activity. Therefore, the 14C concentration of the atmosphere is lower during sunspot maxima and higher during sunspot minima. By measuring the captured 14C in wood and counting tree rings, production of radiocarbon relative to recent wood can be measured and dated. A reconstruction of the past 10,000 years shows that the 14C production was much higher during the mid-Holocene 7,000 years ago and decreased until 1,000 years ago. In addition to variations in solar activity, the long term trends in carbon-14 production are influenced by changes in the Earth's geomagnetic field and by changes in carbon cycling within the biosphere (particularly those associated with changes in the extent of vegetation since the last ice age). GLOBAL WARMING Researchers have correlated solar variation with changes in the Earth's average temperature and climate - sometimes finding an effect, and sometimes not. Researchers who have found an

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effect include Willie Soon and Sallie Baliunas or Douglass and Clader, Geophysical Research Letters, 2002. The IPCC questions the magnitude of long-term (last hundred or more years) solar variation in section 6.11 of the TAR and show various results including Lean et al. (1995). However the Lean 1995 value may well be too high: more recently Lean et al (GRL 2002,) say: Our simulation suggests that secular changes in terrestrial proxies of solar activity (such as the 14C and 10Be cosmogenic isotopes and the aa geomagnetic index) can occur in the absence of long-term (i.e., secular) solar irradiance changes. ...this suggests that total solar irradiance may also lack significant secular trends. ...Solar radiative forcing of climate is reduced by a factor of 5 when the background component is omitted from historical reconstructions of total solar irradiance ...This suggest that general circulation model (GCM) simulations of twentieth century warming may overestimate the role of solar irradiance variability. ...There is, however, growing empirical evidence for the Sun's role in climate change on multiple time scales including the 11-year cycle ...Climate response to solar variability may involve amplification of climate modes which the GCMs do not typically include. ...In this way, long-term climate change may appear to track the amplitude of the solar activity cycles because the stochastic response increases with the cycle amplitude, not because there is an actual secular irradiance change. More recently, a study and review of existing literature published in Nature in September 2006 suggests that the evidence is solidly on the side of solar brightness having relatively little effect on global climate, and downplays the likelihood of significant shifts in solar output over long periods of time. Lockwood and Fröhlich, 2007, find that there "is considerable evidence for solar influence on the Earth's pre-industrial climate and the Sun may well have been a factor in post-industrial climate change in the first half of the last century. Here we show that over the past 20 years, all the trends in the Sun that could have had an influence on the Earth's climate have been in the opposite direction to that required to explain the observed rise in global mean temperatures." This is now however

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disputed by a recent reply by Svensmark and Friis-Christensen which concludes that tropospheric air temperature records, as opposed to the surface air temperature data used by Lockwood and Fröhlich, do show a significant negative correlation between cosmic-ray flux and air temperatures up to 2006. They also point out that Lockwood and Fröhlich present their data by using running means of around 10 years, which shows a constant temperature rise. This reply has so far not been published in a peer-reviewed journal.

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3 SOLAR VARIATION THEORY There have been proposals that variations in solar output explain past climate change and contribute to global warming. The most accepted influence of solar variation on the climate is through direct radiative forcing. Various hypotheses have been proposed to explain the apparent solar correlation with temperatures that some assert appear to be stronger than can be explained by direct irradiation and the first order positive feedbacks to increases in solar activity. The meteorological community has responded with skepticism, in part because theories of this nature have come and gone over the course of the 20th century. Sami Solanki, the director of the Max Planck Institute for Solar System Research in Katlenburg-Lindau, Germany said: The sun has been at its strongest over the past 60 years and may now be affecting global temperatures... the brighter sun and higher levels of so-called "greenhouse gases" both contributed to the change in the Earth's temperature, but it was impossible to say which had the greater impact. Nevertheless, Solanki agrees with the scientific consensus that the marked upswing in temperatures since about 1980 is attributable to human activity. • Just how large this role of solar variation is, must still be investigated, since, according to our latest knowledge on the variations of the solar magnetic field, the significant increase in the Earth's temperature since 1980 is indeed to be ascribed to the greenhouse effect caused by carbon dioxide."

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Willie Soon and Sallie Baliunas of the Harvard Observatory correlated historical sunspot counts with temperature proxies. They report that when there are fewer sunspots, the earth cooled (see Maunder Minimum, Little Ice Age) - and that when there are more sunspots the earth warmed. The theories have usually represented one of three types: • Solar irradiance changes directly affecting the climate. This is generally considered unlikely, as the amplitudes of the variations in solar irradiance are much too small to have the observed relation absent some amplification process. • Variations in the ultraviolet component having an effect. The UV component varies by more than the total. • Effects mediated by changes in cosmic rays (which are affected by the solar wind, which is affected by the solar output) such as changes in cloud cover. Although correlations often can be found, the mechanism behind these correlations is a matter of speculation. Many of these speculative accounts have fared badly over time, and in a paper "Solar activity and terrestrial climate: an analysis of some purported correlations" (J. Atmos. and Solar-Terr. Phy., 2003 p801-812) Peter Laut demonstrates problems with some of the most popular, notably those by Svensmark and by Lassen (below). Damon and Laut report in Eos that the apparent strong correlations displayed on these graphs have been obtained by incorrect handling of the physical data. The graphs are still widely referred to in the literature, and their misleading character has not yet been generally recognized. In 1991, Knud Lassen of the Danish Meteorological Institute in Copenhagen and his colleague Eigil Friis-Christensen found a strong correlation between the length of the solar cycle and temperature changes throughout the northern hemisphere. Initially, they used sunspot and temperature measurements from 1861 to 1989, but later found that climate records dating back four centuries supported their findings. This relationship appeared to account for nearly 80 per cent of the measured temperature changes over this period (see graph). Damon and Laut, however, show that when the graphs are corrected for filtering errors, the

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sensational agreement with the recent global warming, which drew worldwide attention, has totally disappeared. Nevertheless, the authors and other researchers keep presenting the old misleading graph. Note that the prior link to "graph" is one such example of this. Sallie Baliunas, an astronomer at the Harvard-Smithsonian Center for Astrophysics, has been among the supporters of the theory that changes in the sun "can account for major climate changes on Earth for the past 300 years, including part of the recent surge of global warming." On May 6, 2000, however, New Scientist magazine reported that Lassen and astrophysicist Peter Thejll had updated Lassen's 1991 research and found that while the solar cycle still accounts for about half the temperature rise since 1900, it fails to explain a rise of 0.4 °C since 1980. "The curves diverge after 1980," Thejll said, "and it's a startlingly large deviation. Something else is acting on the climate.... It has the fingerprints of the greenhouse effect." Later that same year, Peter Stott and other researchers at the Hadley Centre in the United Kingdom published a paper in which they reported on the most comprehensive model simulations to date of the climate of the 20th century. Their study looked at both "natural forcing agents" (solar variations and volcanic emissions) as well as "anthropogenic forcing" (greenhouse gases and sulphate aerosols). They found that "solar effects may have contributed significantly to the warming in the first half of the century although this result is dependent on the reconstruction of total solar irradiance that is used. In the latter half of the century, we find that anthropogenic increases in greenhouses gases are largely responsible for the observed warming, balanced by some cooling due to anthropogenic sulphate aerosols, with no evidence for significant solar effects." Stott's team found that combining all of these factors enabled them to closely simulate global temperature changes throughout the 20th century. They predicted that continued greenhouse gas emissions would cause additional future temperature increases "at a rate similar to that observed in recent decades". It should be noted that


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their solar forcing included "spectrally-resolved changes in solar irradiance" and not the indirect effects mediated through cosmic rays for which there is still no accepted mechanism - these ideas are still being fleshed out. In addition, the study notes "uncertainties in historical forcing" - in other words, past natural forcing may still be having a delayed warming effect, most likely due to the oceans. A graphical representation of the relationship between natural and anthropogenic factors contributing to climate change appears in "Climate Change 2001: The Scientific Basis", a report by the Intergovernmental Panel on Climate Change (IPCC). Stott's 2003 work mentioned in the model section above largely revised his assessment, and found a significant solar contribution to recent warming, although still smaller (between 16 and 36%) than that of the greenhouse gases. HISTORICAL PERSPECTIVE Physicist and historian Spencer R. Weart in The Discovery of Global Warming (2003) writes: The study of sun spot cycles was generally popular through the first half of the century. Governments had collected a lot of weather data to play with and inevitably people found correlations between sun spot cycles and select weather patterns. If rainfall in England didn't fit the cycle, maybe storminess in New England would. Respected scientists and enthusiastic amateurs insisted they had found patterns reliable enough to make predictions. Sooner or later though every prediction failed. An example was a highly credible forecast of a dry spell in Africa during the sunspot minimum of the early 1930s. When the period turned out to be wet, a meteorologist later recalled "the subject of sunspots and weather relationships fell into dispute, especially among British meteorologists who witnessed the discomfiture of some of their most respected superiors." Even in the 1960s he said, "For a young climate researcher to entertain any statement of sun-weather relationships was to brand oneself a crank.")

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SOLAR CYCLE The solar cycle, or the solar magnetic activity cycle, is the main source of periodic variation of all solar phenomena driving variations in space weather. Powered by a hydromagnetic dynamo process driven by the inductive action of internal solar flows, the solar cycle • structures the sun's atmosphere, corona and wind; • modulates the solar irradiance; • modulates the flux of short-wavelength solar radiation, from ultraviolet to X-Ray; • modulates the occurrence frequency of flares, coronal mass ejections, and other geoeffective solar eruptive phenomena; • indirectly modulates the flux of high-energy galactic cosmic rays entering the solar system. HISTORY The solar cycle was discovered in 1843 by Samuel Heinrich Schwabe, who after 17 years of observations noticed a periodic variation in the average number of sunspots seen from year to year on the solar disk. Rudolf Wolf compiled and studied these and other observations, reconstructing the cycle back to 1745, eventually pushing these reconstructions to the earliest observations of sunspots by Galileo and contemporaries in the early seventeenth century. Starting with Wolf, solar astronomers have found it useful to define a standard sunspot number index, which continues to be used today. The average duration of the sunspot cycle is about 11 years (about 28 cycles in the 309 years between 1699 and 2008), but cycles as short as 9 years and as long as 14 years have been observed. Significant variations in amplitude also occur. Solar maximum and solar minimum refer respectively to epochs of maximum and minimum sunspot counts. Individual sunspot cycles are partitioned from one minimum to the next. Following the numbering scheme established by Wolf, the 1755-1766 cycle is traditionally numbered "1". The period between 1645 and 1715, a time during which very few sunspots were observed, is a real feature, as opposed to an artifact due to missing


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data, and coincides with the Little Ice Age. This epoch is now known as the Maunder minimum, after Edward Walter Maunder, who extensively researched this peculiar event, first noted by Gustav Spörer. In the second half of the nineteenth century it was also noted (independently) by Richard Carrington and by Spörer that as the cycle progresses, sunspots appear first at mid-latitudes, and then closer and closer to the equator until solar minimum is reached. This pattern is best visualized in the form of the so-called butterfly diagram, first constructed by the husband-wife team of E. Walter and Annie Maunder in the early twentieth century. Images of the sun are divided into latitudinal strips, and the monthly-averaged fractional surface of sunspots calculated. This is plotted vertically as a color-coded bar, and the process is repeated month after month to produce this time-latitude diagram. The physical basis of the solar cycle was elucidated in the early twentieth century by George Ellery Hale and collaborators, who in 1908 showed that sunspots were strongly magnetized (this was the first detection of magnetic fields outside the Earth), and in 1919 went on to show that the magnetic polarity of sunspot pairs: • is always the same in a given solar hemisphere throughout a given sunspot cycle; • is opposite across hemispheres throughout a cycle; • reverses itself in both hemispheres from one sunspot cycle to the next. The solar cycle, as seen in variations of the sunspot number index. Three historical reconstruction are shown, namely the monthly sunspot number (orange), and yearly sunspot number (red), and, from 1610 to 1750, the Group sunspot number (blue), generally deemed a more reliable reconstruction over this time interval. Hale's observations revealed that the solar cycle is a magnetic cycle with an average duration of 22 years. However, because very nearly all manifestations of the solar cycle are insensitive to magnetic polarity, it remains common usage to speak of the "11-year solar cycle". Half a century later, the father-and-son team of Harold Babcock and Horace Babcock showed that the solar surface is magnetized even outside of sunspots; that this weaker magnetic field is to first

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order a dipole; and that this dipole also undergoes polarity reversals with the same period as the sunspot cycle (see Fig. 3 below). These various observations established that the solar cycle is a spatiotemporal magnetic process unfolding over the sun as a whole. The basic causes of the solar cycle are still under debate, with some researchers suggesting a link with the tidal forces due to the gas giants Jupiter and Saturn

IMPACTS OF THE SOLAR CYCLE The sun's magnetic field structures its atmosphere and outer layers all the way through the corona and into the solar wind. Its spatiotemporal variations lead to a host of phenomena collectively known as solar activity. All of solar activity is strongly modulated by the solar magnetic cycle, since the latter serves as the energy source and dynamical engine for the former. SURFACE MAGNETISM Sunspots may exist anywhere from a few days to a few months, but they eventually decay, and this releases magnetic flux in the solar photosphere. This magnetic field is dispersed and churned by turbulent convection, and solar large-scale flows. These transport mechanisms lead to the accumulation of the magnetized decay products at high solar latitudes, eventually reversing the polarity of the polar fields. The dipolar component of the solar magnetic field is observed to reverse polarity around the time of solar maximum, and reaches peak strength at the time of solar minimum. Sunspots, on the other hand, are produced from a strong toroidal (longitudinallydirected) magnetic field within the solar interior. Physically, the solar cycle can be thought of as a regenerative loop where the toroidal component produces a poloidal field, which later produces a new toroidal component of sign such as to reverse the polarity of the original toroidal field, which then produces a new poloidal component of reversed polarity, and so on. SOLAR IRRADIANCE The total solar irradiance (TSI) is the amount of solar radiative energy impinging on the Earth's upper atmosphere. It is observed to vary in phase with the solar cycle, with yearly averages going


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from 1365.5 Watt per square meter at solar minimum, up to 1366.6 at solar maximum, with fluctuations about the means of about +/ - 1 Watt per square meter on timescales of a few days (see Figure 4, yellow and red curves). The min-to-max variation, at the 0.1% level, is far too small to affect Earth's climate directly, but it is worth keeping in mind that continuous reliable measurements of the TSI are only available since 1978; the minimum and maximum levels of solar activity have remained roughly the same from then to now, spanning cycle 21 through 23. Interestingly, the Sun is slightly brighter at solar maximum, even though sunspots are darker than the rest of the solar photosphere. This is because at solar maximum, a great many magnetized structures other than sunspots appear on the solar surface and many of them, such as faculae and active elements of the network, are brighter than the photosphere. They collectively end up slightly overcompensating for the overall irradiance deficit associated with the larger but less numerous sunspots. Recent observations indicate that the primary driver of TSI changes is the varying photospheric coverage of these different types of solar magnetic structures, although contributions from long-timescale variations associated with a deep-seated physical process, such as cycle-mediated small changes in the efficiency of convective energy transport, cannot be ruled out entirely as yet.

SHORT-WAVELENGTH RADIATION With a temperature of 5870 kelvin, the unmagnetized regions of the Sun's atmosphere emit very little short-wave radiation, such as extreme ultraviolet (EUV) and X-Rays. However, magnetized regions emit more short-wave radiation. Since surface coverage of magnetic structures varies markedly in the course of the cycle, the level of diffuse, non-flaring solar UV, EUV and XRay flux varies accordingly. Figure 5 illustrates this variation for soft X-Ray, as observed by the Japanese satellite YOHKOH. Similar cycle-related variations are observed in the flux of solar UV or EUV radiation, as observed, for example, by the SOHO or TRACE satellites. Even though it only accounts for a minuscule fraction of total solar radiation, the impact of solar UV, EUV and X-Ray radiation

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on the Earth's upper atmosphere is profound. Solar UV flux is a major driver of stratospheric chemistry, and increases in ionizing radiation significantly affect ionosphere-influenced temperature and electrical conductivity.

SOLAR RADIO FLUX Emission from the Sun at centimetric (radio) wavelength is due primarily to coronal plasma trapped in the magnetic fields overlying active regions. The F10.7 index is a measure of the solar radio flux per unit frequency at a wavelength of 10.7cm, near the peak of the observed solar radio emission. It represents a measure of diffuse, nonradiative heating of the coronal plasma trapped by magnetic fields over active regions, and is an excellent indicator of overall solar activity levels. The solar F10.7 cm record extends back to 1947, and is the longest direct record of solar activity available, other than sunspot-related quantities. Sunspot activity has a major effect on long distance radio communications particularly on the shortwave bands although medium wave and low VHF frequencies are also affected. High levels of sunspot activity lead to improved signal propagation on higher frequency bands. Although they also increase the levels of solar noise and ionospheric disturbances. These effects are caused by impact of the increased level of solar radiation on the ionosphere. It has been proposed that 10.7 cm solar flux can interfere with point-to-point terrestrial communications. GEOEFFECTIVE ERUPTIVE PHENOMENA The solar magnetic field structures the corona, giving it its characteristic shape visible at times of solar eclipses. Complex coronal magnetic field structures evolve in response to fluid motions at the solar surface, and emergence of magnetic flux produced by dynamo action in the solar interior. For reasons not yet understood in detail, sometimes these structures lose stability, leading to coronal mass ejections into interplanetary space, or flares, caused by sudden localized release of magnetic energy driving copious emission of ultraviolet and X-ray radiation as well as energetic particles. These eruptive phenomena can have a significant impact on Earth's upper atmosphere and space environment, and are the primary drivers of what is now called


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space weather. The occurrence frequency of coronal mass ejections and flares is strongly modulated by the solar activity cycle. Flares of any given size are some 50 times more frequent at solar maximum than at minimum. Large coronal mass ejections occur on average a few times a day at solar maximum, down to one every few days at solar minimum The size of these events themselves does not depend sensitively on the phase of the solar cycle. A good recent case in point are the three large X-class flares having occurred in December 2006, very near solar minimum; one of these (an X9.0 flare on Dec 5) stands as one of the brightest on record.

COSMIC RAY FLUX The outward expansion of solar ejecta into interplanetary space provides overdensities of plasma that are efficient at scattering high-energy cosmic rays entering the solar system from elsewhere in the galaxy. Since the frequency of solar eruptive events is strongly modulated by the solar cycle, the degree of cosmic ray scattering in the outer solar system varies in step. As a consequence, the cosmic ray flux in the inner solar system is anticorrelated with the overall level of solar activity. This anticorrelation is clearly detected in cosmic ray flux measurements at the Earth's surface. Some high-energy cosmic rays entering Earth's atmosphere collide hard enough with molecular atmospheric constituents to cause occasionally nuclear spallation reactions. Some of the fission products include radionuclides such as 14C and 10Be, which settle down on Earth's surface. Their concentration can be measured in ice cores, allowing a reconstruction of solar activity levels into the distant past. Such reconstructions indicate that the overall level of solar activity since the middle of the twentieth century stands amongst the highest of the past 10,000 years, and that Maunder minimum-like epochs of suppressed activity, of varying durations have occurred repeatedly over that time span. IMPACT ON BIOSPHERE AND HUMAN CIRCADIAN CYCLE The impact of Solar cycle on living organisms is covered in part by interdisciplinary studies in the fields of science known as Chronobiology, Heliobiology, and Astrobiology.

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UVB VARIATION The amount of UVB light at 300 nm reaching the Earth varies by as much as 400% over the solar cycle due to variations in the protective ozone layer. In the stratosphere ozone is continuously regenerated by the splitting of O2 molecules by ultraviolet light. During a solar minimum, the decrease in ultraviolet light received from the sun leads to a decrease in the concentration of ozone, allowing increased UVB to penetrate to the Earth's surface. SOLAR AND HELIOSPHERIC OBSERVATORY The Solar and Heliospheric Observatory (SOHO) is a spacecraft that was launched on a Lockheed Martin Atlas IIAS launch vehicle on December 2, 1995 to study the Sun, and began normal operations in May 1996. It is a joint project of international cooperation between the European Space Agency (ESA) and NASA. Originally planned as a two-year mission, SOHO currently continues to operate after over ten years in space. In addition to its scientific mission, it is currently the main source of near-real time solar data for space weather prediction. Along with the GGS Wind and Advanced Composition Explorer (ACE), SOHO is one of three spacecraft currently in the vicinity of the Earth-Sun L1 point, a point of gravitational balance located approximately 0.99 astronomical unit (AU)s from the Sun and 0.01 AU from the Earth. In addition to its scientific contributions, SOHO is distinguished by being the first three-axis-stabilized spacecraft to use its reaction wheels as a kind of virtual gyroscope; the technique was adopted after an on-board emergency in 1998 that nearly resulted in the loss of the spacecraft. ORBIT The 610 kg SOHO spacecraft is in a halo orbit around the SunEarth L1 point, the point between the Earth and the Sun where the balance of the (larger) Sun's gravity and the (smaller) Earth's gravity is equal to the centripetal force needed for an object to have the same orbital period in its orbit around the Sun as the Earth, with the result that the object will stay in that relative position. It is about 1.5 million kilometers from the Earth. Gravity from the Sun is 2% (118 µm/s²) more than at the Earth (5.9 mm/s²), while the reduction of required centripetal force is


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half of this (59 µm/s²). The sum of both effects is balanced by the gravity of the Earth, which is here also 177 µm/s². Although sometimes described as being at L1, the SOHO satellite is not exactly at L1 as this would make communication difficult due to radio interference generated by the Sun, and because this would not be a stable orbit. Rather it lies in the (constantly moving) plane which passes through L1 and is perpendicular to the line connecting the sun and the Earth. It stays in this plane, tracing out an elliptical orbit centered about L1. It orbits L1 once every six months, while L1 itself orbits the sun every 12 months as it is coupled with the motion of the Earth. This keeps SOHO at a good position for communication with Earth at all times.

COMMUNICATION WITH EARTH In normal operation the spacecraft transmits a continuous 200 kbit/s data stream of photographs and other measurements via the NASA Deep Space Network of ground stations. SOHO's data about solar activity are used to predict solar flares, so electrical grids and satellites can be protected from their damaging effects (mainly, solar flares may produce geomagnetic storms, which in turn produce geomagnetically induced current creating blackouts, etc.). In 2003 ESA reported the failure of the antenna Y-axis stepper motor, necessary for pointing the high gain antenna and allowing the downlink of high rate data. At the time, it was thought that the antenna anomaly might cause two to three week data-blackouts every three months. However, ESA and NASA engineers managed to use SOHO's low gain antennas together with the larger 34 and 70 meter DSN ground stations and judicious use of SOHO's Solid State Recorder (SSR) to prevent total data loss, with only a slightly reduced data flow every three months. NEAR LOSS OF SOHO The SOHO Mission Interruption sequence of events began on 24 June 1998, while the SOHO Team was conducting a series of spacecraft gyroscope calibrations and maneuvers. Operations proceeded until 23:16 UTC when SOHO lost lock on the Sun, and entered an emergency attitude control mode called Emergency

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Sun Reacquisition (ESR). The SOHO Team attempted to recover the observatory, but SOHO entered the emergency mode again on June 25 02:35 UTC. Recovery efforts continued, but SOHO entered the emergency mode for the last time at 04:38 UTC. All contact with SOHO was lost, and the mission interruption had begun. SOHO was spinning, losing electrical power, and no longer pointing at the Sun. Expert ESA personnel were immediately dispatched from Europe to the United States to direct operations. Days passed without contact from SOHO. On July 23, the Arecibo Observatory and DSN antennas were used to locate SOHO with radar, and to determine its location and attitude. SOHO was close to its predicted position, oriented with its side versus the usual front Optical Surface Reflector panel pointing toward the Sun, and was rotating at one RPM. Once SOHO was located, plans for contacting SOHO were formed. On 3 August a carrier was detected from SOHO, the first signal since June 25. After days of charging the battery, a successful attempt was made to modulate the carrier and downlink telemetry on August 8. After instrument temperatures were downlinked on August 9, data analysis was performed, and planning for the SOHO recovery began in earnest. The SOHO Recovery Team began by allocating the limited electrical power. After this, SOHO's anomalous orientation in space was determined. Thawing the frozen hydrazine fuel tank using SOHO's thermal control heaters began on August 12. Thawing pipes and the thrusters was next, and SOHO was reoriented towards the Sun on September 16. After nearly a week of spacecraft bus recovery activities and an orbital correction maneuver, the SOHO spacecraft (bus) returned to normal mode on September 25 at 19:52 UTC. Recovery of the instruments began on October 5 with SUMER, and ended on October 24, 1998 with CELIAS. Only one gyro remained operational after this recovery, and on December 21 that gyro failed. Attitude control was accomplished with manual thruster firings that consumed 7kg of fuel weekly, while ESA developed a new gyroless operations mode that was successfully implemented on February 1, 1999.


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SCIENTIFIC OBJECTIVES The three main scientific objectives of SOHO are: • Investigation of the outer layer of the Sun, which consists of the chromosphere, transition region, and the corona. CDS, EIT, LASCO, SUMER, SWAN, and UVCS are used for this solar atmosphere remote sensing. • Making observations of solar wind and associated phenomena in the vicinity of L1. CELIAS and CEPAC are used for "in situ" solar wind observations. • Probing the interior structure of the Sun. GOLF, MDI, and VIRGO are used for helioseismology. INSTRUMENTS The SOHO Payload Module (PLM) consists of twelve instruments, each capable of independent or coordinated observation of the Sun or parts of the Sun, and some spacecraft components. The instruments are: • Coronal Diagnostic Spectrometer (CDS) which measures density, temperature and flows in the corona. • Charge ELement and Isotope Analysis System (CELIAS) which studies the ion composition of the solar wind. • Comprehensive SupraThermal and Energetic Particle analyser collaboration (COSTEP) which studies the ion and electron composition of the solar wind. (COSTEP and ENRE are sometimes referred to together as the COSTEPERNE Particle Analyzer Collaboration (CEPAC). • Extreme ultraviolet Imaging Telescope (EIT) which studies the low coronial structure and activity. • Energetic and Relativistic Nuclei and Electron experiment (ERNE) which studies the ion and electron composition of the solar wind. (See note above in COSTEP entry.) • Global Oscillations at Low Frequencies (GOLF) which measures velocity variations of the whole solar disk to explore the core of the sun. • Large Angle and Spectrometric COronagraph experiment (LASCO) which studies the structure and evolution of the corona by creating an artificial solar eclipse.

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Solar Energy and its Uses • Michelson Doppler Imager (MDI) which measures velocity and magnetic fields in the photosphere to learn about the convection zone which forms the outer layer of the interior of the sun and about the magnetic fields which control the structure of the corona. The MDI is the biggest producer of data by far on SOHO. In fact, two of SOHO's virtual channels are named after MDI, VC2 (MDI-M) carries MDI magnetogram data, and VC3 (MDI-H) carries MDI Helioseismology data. • Solar Ultraviolet Measurement of Emitted Radiation (SUMER) which measures plasma flows, temperature and density in the corona. • Solar Wind ANisotropies (SWAN) which uses telescopes sensitive to a characteristic wavelength of hydrogen to measure the solar wind mass flux, map the density of the heliosphere, and observe the large-scale structure of the solar wind streams. • UltraViolet Coronagraph Spectrometer (UVCS) which measures density and temperature in the corona. • Variability of solar IRradiance and Gravity Oscillations (VIRGO) which measures oscillations and solar constant both of the whole solar disk and at low resolution, again exploring the core of the sun.

Observations from some of the instruments can be formatted as images, most of which are also readily available on the internet for either public or research use (see the official website). Others such as spectra and measurements of particles in the solar wind do not lend themselves so readily to this. These images range in wavelength or frequency from optical (H?) to extreme ultraviolet (UV). Images taken partly or exclusively with non-visible wavelengths are shown on the SOHO page and elsewhere in false color. Unlike many space-based and ground telescopes, there is no time formally allocated by the SOHO program for observing proposals on individual instruments: interested parties can contact the instrument teams directly via e-mail and the SOHO web site to request time via that instrument team's internal processes (some of which are quite informal, provided that the ongoing reference observations are not disturbed). A formal process (the "JOP"


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program) does exist for using multiple SOHO instruments collaboratively on a single observation. JOP proposals are reviewed at the quarterly Science Working Team ("SWT") meetings, and JOP time is allocated at monthly meetings of the Science Planning Working Group. As a consequence of its observing the Sun, SOHO (specifically the LASCO instrument) has inadvertently discovered comets by blocking out the Sun's glare. Approximately one-half of all known comets have been discovered by SOHO. Recently, it discovered its 1500th comet.

INSTRUMENT CONTRIBUTORS The Max Planck Institute for Solar System Research contributed to SUMER, LASCO and CELIAS instruments. The Smithsonian Astrophysical Observatory built the UVCS instrument. The Lockheed Martin Solar and Astrophysics Laboratory (LMSAL) built the MDI instrument in collaboration with the solar group at Stanford University. PHOTOVOLTAIC MODULE In the field of photovoltaics, a photovoltaic module is a packaged interconnected assembly of photovoltaic cells, also known as solar cells. An installation of photovoltaic modules or panels is known as a photovoltaic array or a solar panel. Photovoltaic cells typically require protection from the environment. For cost and practicality reasons a number of cells are connected electrically and packaged in a photovoltaic module, while a collection of these modules that are mechanically fastened together, wired, and designed to be a field-installable unit, sometimes with a glass covering and a frame and backing made of metal, plastic or fiberglass, are known as a photovoltaic panel or simply solar panel. A photovoltaic installation typically includes an array of photovoltaic modules or panels, an inverter, batteries (for off grid) and interconnection wiring. THEORY AND CONSTRUCTION The majority of modules use wafer-based Crystalline silicon cells or a thin film cell based on cadmium telluride or silicon (see photovoltaic cells for details) crystalline silicon, which is commonly

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used in the wafer form in photovoltaic (PV) modules, is derived from silicon, a relatively multi-faceted element. In order to use the cells in practical applications, they must be: • connected electrically to one another and to the rest of the system • protected from mechanical damage during manufacture, transport and installation and use (in particular against hail impact, wind and snow loads). This is especially important for wafer-based silicon cells which are brittle. • protected from moisture, which corrodes metal contacts and interconnects, (and for thin film cells the transparent conductive oxide layer) thus decreasing performance and lifetime. • electrically insulated including under rainy conditions • mountable on a substructure Most modules are rigid, but there are some flexible modules available, based on thin film cells. Electrical connections are made in series to achieve a desired output voltage and/or in parallel to provide a desired amount of current source capability. Diodes are included to avoid overheating of cells in case of partial shading. Since cell heating reduces the operating efficiency it is desirable to minimize the heating. Very few modules incorporate any design features to decrease temperature, however installers try to provide good ventilation behind the module, New designs of module include concentrator modules in which the light is concentrated by an array of lenses or mirrors onto an array of small cells. This allows the use of cells with a very high cost per unit area (such as gallium arsenide) in a cost-competitive way. Depending on construction the photovoltaic can cover a range of frequencies of light and can produce electricity from them, but cannot cover the entire solar spectrum. Hence much of incident sunlight energy is wasted when used for solar panels, although they can give far higher efficiencies if illuminated with monochromatic light. Another design concept is to split the light into different wavelength ranges and direct the beams onto different cells tuned to the appropriate wavelength ranges. This


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is projected to raise efficiency to 50%. Sunlight conversion rates (module efficiencies) can vary from 5-18% in commercial production. A group of researchers at MIT has recently developed a process to improve the efficiency of luminescent solar concentrator (LSC) technology, which redirects light along a translucent material to PV-modules located along its edge. The researchers have suggested that efficiency may be improved by a factor of 10 over the old design in as little as three years. 3 of the researchers involved have now started their own company, called Covalent Solar, to manufacture and sell their innovation in PV-modules. Finally, a whole range of other companies (eg HoloSun, Gamma Solar, NanoHorizons, ...) are emerging which are also offering new innovations in the modules. These new innovations include power generation at the front and back side, increased outputs, ... However, most of these companies have not yet produced working systems from their design plans, and are mostly still actively improving the technology.

RIGID THIN-FILM MODULES In rigid thin film modules, the cell and the module are manufactured in the same production line. The cell is created directly on a glass substrate or superstrate, and the electrical connections are created in situ, a so called "monolithic integration". The substrate or superstrate is laminated with an encapsulant to a front or back sheet, usually another sheet of glass. The main cell technologies in this category are CdTe, amorphous silicon, micromorphous silicon (alone or tandem), or CIGS (or variant). Amorphous silicon has a sunlight conversion rate of 5-9%. FLEXIBLE THIN-FILM MODULES Flexible thin film cells and modules are created on the same production line by depositing the photoactive layer and other necessary layers on a flexible substrate. If the substrate is an insulator (e.g. polyester or polyimide film) then monolithic integration can be used. If it is a conductor then another technique

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for electrical connection must be used. The cells are assembled into modules by laminating them to a transparent colourless fluoropolymer on the front side (typically ETFE or FEP) and a polymer suitable for bonding to the final substrate on the other side. The only commercially available (in MW quantities) flexible module uses amorphous silicon triple junction (from Unisolar). So-called Inverted Metamorphic (IMM) multi-junction solar cells made on compound-semiconductor technology is just be comming commercialized in July 2008. The University of Michigan's solar car won the North American Solar challenge in July 2008 used IMM thin-flim flexible solar cells. Emcore won an R&D 100 award in July 2008 for commericalization of this technology developed at the National Renewable Energy Laboratories (USA). Disclaimer: I work for Emcore. SOLAR HOT CARBON Solar Hot Carbon (also known as Solar Hot CO2, Solar Methane, SolarCarbon, Carbon Panels, Smoky Solar, etc.) is an somewhat uncommon method for capturing solar energy. It is essentially the same as Solar hot water except that the heat carrying medium is carbon dioxide, methane or smog. Carbon Dioxide and other green house gases, although invisible in visible light, are very dark or reflective in infrared light, meaning that heat cannot escape as easily through infrared means. As a result, when insulated by a second means, such as vacuum insulation, Solar Hot Carbon Systems can become as much as 95% efficientclarify although such a system has never been built. Green house gases, when not allowed to convey heat through direct means can hold much more heat than water. Essentially, green house gases are thermal insulators and will prevent infrared heat from getting out as well as heat from getting in.

SOLAR COOKER A solar oven or solar cooker is a device which uses sunlight as its energy source. Because they use no fuel and they cost nothing to run, humanitarian organizations are promoting their use worldwide to help slow deforestation and desertification,


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caused by using wood as fuel for cooking. Solar cookers are also sometimes used in outdoor cooking, especially in situations where minimal fuel consumption or fire risk are considered highly important.

TYPES OF SOLAR COOKERS There are many different types of Solar cookers. All solar cookers use the sun's heat and light to cook food. The basic principles of solar cookers are: • Concentrating sunlight: Some device, usually a mirror or some type of reflective metal, is used to concentrate light and heat from the sun into a small cooking area, making the energy more concentrated and therefore more potent. • Converting light to heat: Any black on the inside of a solar cooker, as well as certain materials for pots, will improve the effectiveness of turning light into heat. A black pan will absorb almost all of the sun's light and turn it into heat, substantially improving the effectiveness of the cooker. Also, the better a pan conducts heat, the faster the oven will work. • Trapping heat: Isolating the air inside the cooker from the air outside the cooker makes an important difference. Using a clear solid, like a plastic bag or a glass cover, will allow light to enter, but once the light is absorbed and converted to heat, a plastic bag or glass cover will trap the heat inside using the Greenhouse Effect. This makes it possible to reach similar temperatures on cold and windy days as on hot days. Alone, each of these strategies for heating something with the sun is fairly ineffective, but most solar cookers use two or all three of these strategies in combination to get temperatures sufficient for cooking. The top can usually be removed to allow dark pots containing food to be placed inside. The box usually has one or more reflectors with aluminum foil or other reflective material to bounce extra light into the interior of the box. Cooking containers and the inside bottom of the cooker should be dark-colored or black. The inside walls should be reflective to reduce radiative heat loss and bounce

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the light towards the pots and the dark bottom, which is in contact with the pots.

BOX COOKERS The inside insulator for the solar box cooker has to be able to withstand temperatures up to 150°C (300 °F) without melting or off-gassing. Crumpled newspapers, wool, rags, dry grass, sheets of cardboard, etc. can be used to insulate the walls of the cooker, but since most of the heat escapes through the top glass or plastic, very little insulation in the walls is necessary. The transparent top is either glass, which is durable but hard to work with, or an oven cooking bag, which is lighter, cheaper, and easier to work with, but less durable. If dark pots and/or bottom trays cannot be located, these can be darkened either with flat-black spray paint (one that is non-toxic when warmed), black tempera paint, or soot from a fire. The solar box cooker typically reaches a temperature of 150 °C (300 °F). This is not as hot as a standard oven, but still hot enough to cook food over a somewhat longer period of time. Food containing moisture cannot get much hotter than 100 °C (212 °F) in any case, so it is not necessary to cook at the high temperatures indicated in standard cookbooks. Because the food does not reach too high a temperature, it can be safely left in the cooker all day without burning. It is best to start cooking before noon, though. Depending on the latitude and weather, food can be cooked either early or later in the day. The cooker can be used to warm food and drinks and can also be used to pasteurize water or milk. Solar box cookers can be made of locally available materials or be manufactured in a factory for sale. They range from small cardboard devices, suitable for cooking a single meal when the sun is shining, to wood and glass boxes built into the sunny side of a house. Although invented by Horace de Saussure, a Swiss naturalist, as early as 1767, solar box cookers have only gained popularity since the 1970s. These surprisingly simple and useful appliances are seen in growing numbers in almost every country of the world. An index of detailed wiki pages for each country can be found here.


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PANEL COOKERS Panel solar cookers are very inexpensive solar cookers that use shiny panels to direct sunlight to a cooking pot that is enclosed in a clear plastic bag. A common model is the CooKit. Developed in 1994 by Solar Cookers International, it is often produced locally by pasting a reflective material, such as aluminum foil, onto a cut and folded backing, usually corrugated cardboard. It is lightweight and folds for storage. When completely unfolded, it measures about three feet by four feet (1 m by 1.3 m). Using materials purchased in bulk, the typical cost is about US$5. However, CooKits can also be made entirely from reclaimed materials, including used carboard boxes and foil from the inside of cigarette boxes. The CooKit is considered a low-to-moderate temperature solar cooker, easily reaching temperatures high enough to pasteurize water or cook grains such as rice. On a sunny day, one CooKit can collect enough solar energy to cook rice, meat or vegetables to feed a family with up to three or four children. Larger families use two or more cookers. To use a panel cooker, it is folded into a bowl shape. Food is placed in a dark-colored pot, covered with a tightly fitted lid. The pot is placed in a clear plastic bag and tied, clipped, or folded shut. The panel cooker is placed in direct sunlight until the food is cooked, which usually requires several hours for a full familysized meal. For faster cooking, the pot can be raised on sticks or wires to allow the heated air to circulate underneath it. High-temperature plastic bags (oven roasting bags) can be reused for more than a month, but any plastic bag will work, if measures (such as sticks or wires) are taken to keep the bag from touching the hot cooking pot and melting to it. The purpose of the plastic bag is to trap heated air next to the pot; it may not be needed on very bright, windless days. A recent development is the HotPot developed by US NGO Solar Household Energy, Inc. The cooking vessel in this cooker is a large clear pot with a clear lid into which a dark pot is suspended. This design has the advantage of very even heating since the sun is able to shine onto the sides and the bottom of the pot during cooking. An added advantage is that the clear lid allows the food to be observed while it is cooking without removing

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the lid. The HotPot provides an alternative to using plastic bags in a panel cooker.

SOLAR KETTLES Solar kettles are solar thermal devices that can heat water to boiling point through the reliance on solar energy alone. Typically they use evacuated (or vacuum) solar glass tube technology to capture, accumulate and store solar energy needed to power the kettle. Besides heating liquids, since the stagnating temperature of solar vacuum glass tubes is a high 220 degrees Celsius (425 °F), Solar kettles can also deliver dry heat and function as ovens and autoclaves. Moreover, since solar vacuum glass tubes work on accumulated rather than concentrated solar thermal energy, solar kettles only need diffused sunlight to work and needs no sun tracking at all. If solar kettles uses solar vacuum tubes technologies, the vacuum insulating properties will keep previously heated water hot throughout the night. PARABOLIC COOKERS Although these types of solar cookers can cook as well as a conventional oven, they are difficult to construct. Parabolic cookers reach high temperatures and cook quickly, but require frequent adjustment and supervision for safe operation. Several hundred thousand exist, mainly in China. They are especially useful for large-scale institutional cooking. The solar bowl is a unique concentrating technology used by the Solar Kitchen in Auroville, India. Unlike nearly all concentrating technologies that use tracking reflector systems, the solar bowl uses a stationary spherical reflector. This reflector focuses light along a line perpendicular to the sphere's surface and a computer control system moves the receiver to intersect this line. Steam is produced in the solar bowl's receiver at temperatures reaching 150 °C and then used for process heat in the kitchen where 2,000 meals are prepared daily. HYBRID COOKERS A hybrid solar oven is a type of solar oven that uses both the regular elements of a solar box cooker as well as a conventional electrical heating element for cloudy days or nighttime cooking.


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Hybrid solar ovens are therefore more independent. However, they lack the cost advantages of some other types of solar cookers, and so they have not caught on as much in third world countries. A hybrid solar grill consists of an adjustable parabolic reflector suspended in a tripod with a movable grill surface. These outperform solar box cookers in temperature range and cooking times. When solar energy is not available, the design uses any conventional fuel as a heat source, including gas, electricity, wood, etc. The tripod hybrid grill is revolutionary in that many, if not all, of the parts required to build them can be scavenged from commonly thrown away items.

USING A SOLAR COOKER The different kinds of solar cookers have somewhat different methods for use, but most follow the same basic principles. Food is prepared as it would be for an oven or stovetop. Because food cooks faster when it is in smaller pieces, solar cookers usually cut the food into smaller pieces than they might otherwise. Potatoes, for example, are usually cut into bite-sized pieces rather than being roasted whole. Bread is usually baked as individual rolls instead of large loaves. When a food, such as rice, needs to be cooked in water, then the minimum necessary amount of water is used. The prepared food is placed in an appropriately sized heatproof container. Efficient containers are not significantly larger than necessary to hold the food are usually shallow. The most efficient containers are made of thin, dull metal, dark in color, with a lid that covers the food and reduces moisture loss, but does not completely seal. Glass containers and even plain paper bags are also used. Pottery and thick metal (such as cast iron) cook food more slowly, but retain their heat better when the sunlight has declined. For very simple cooking, such as melting butter or cheese, a lid may not be needed and the food may be placed on an uncovered tray or in a bowl. If several foods are to be cooked separately, then they are placed in different containers. The container of food is placed inside the solar cooker, perhaps elevated on a brick, rocks, metal trivet, or other heat sink, and the solar cooker is placed in direct sunlight. If the solar cooker is

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entirely in direct sunlight, then the shadow of the solar cooker will not overlap with the shadow of any nearby object. Foods that cook quickly may be added to the solar cooker later. Rice for a midday meal might be started early in the morning, with vegetables, cheese, or meat added to the solar cooker in the middle of the morning. Depending on the size of the solar cooker and the number and quantity of cooked foods, a family may use one or more solar cookers. The solar cooker is turned towards the sun and left until the food is cooked. Unlike cooking on a stove or over a fire, which may require more than an hour of constant supervision, food in a solar cooker is generally not stirred or turned over, both because it is unnecessary and also because opening the solar cooker allows the trapped heat to escape and thereby slows the cooking process. If wanted, the solar cooker may be checked every one to two hours, to turn the cooker to face the sun more precisely and to ensure that shadows from nearby buildings or plants have not blocked the sunlight. If the food will be left untended for many hours during the day, then the solar cooker is often turned to face the sun the point where the sun will be when it is higher in the sky, instead of towards its current position. The cooking time depends primarily on the equipment beings used, the amount of sunlight at the time, and the quantity of food that needs to be cooked. Air temperature, wind, and latitude also affect performance. Food cooks faster in the two hours before and after the local solar noon than it does in either the early morning or the late afternoon. Larger quantities of food, as well as larger individual pieces of food, take longer to cook. As a result, only general figures can be given for cooking time. For a small solar panel cooker, it might be possible to melt butter in fifteen minutes, to bake cookies in two hours, and to cook rice for four people in four hours. However, depending on the local conditions and the solar cooker type, these projects could take half as long, or twice as long. It is difficult to burn food in a solar cooker. Food that has been cooked even an hour longer than necessary is usually indistinguishable from minimally cooked food. The exception to this rule is some green vegetables, which quickly change from a


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perfectly cooked bright green to olive drab, while still retaining the desirable texture. For most foods, such as rice, the typical person would be unable to tell how it was cooked from looking at the final product. There are some differences, however: Bread and cakes brown on their tops instead of on bottom. Compared to cooking over a fire, the food does not have a smoky flavor. Certain foods require different cooking techniques. For example, to fry an egg in a solar cooker, a common method preheats an empty, heavy, dark-colored metal skillet in the solar cooker. Then oil or butter is melted in the pan until sizzling hot. Finally, an egg is added to the pan, where it can cook almost as quickly as it might on a stove top, largely from the stored heat in the pan. Cakes are often baked in a pre-heated solar cooker, perhaps while sitting on a hot brick.

ADVANTAGES Solar ovens are just one part of the alternative energy picture, but one that is accessible to a great majority of people. A reliable solar oven can be built from everyday materials in just a few hours or purchased ready-made. Solar ovens can be used to prepare anything that can be made in a conventional oven or stove - from baked bread to steamed vegetables to roasted meat. Solar ovens allow you to do it all, without contributing to global warming or heating up the kitchen and placing additional demands on cooling systems. Nearly threequarters of US households prepare at least one hot meal per day; one-third prepare two or more. Many of those meals could be made in an environmentally responsible way, using a solar oven. The World Health Organization reports that cooking with fuel wood is the equivalent of smoking two packs of cigarettes a day. Inhalation of smoke from cooking fires causes respiratory diseases and death. One of the solutions advocated to address this problem is solar cooking which makes no smoke at all. In advanced countries, the environmental advantages, a desire for energy independence, and not heating up the house on a hot day are usually cited as advantages. In the developing world, other advantages include:

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Solar Energy and its Uses • lower cost compared to firewood or cooking oil; • greater safety for children and the cook compared to a fire or stove; • lower likelihood of starting a fire that could destroy a family's home; and • less time spent cooking compared to tending a fire or stove.

DISADVANTAGES Solar cookers provide hot food during or shortly after the hottest part of the day, when people are less inclined to eat a hot meal. However, a thick pan that conducts heat slowly (such as Cast Iron) will lose heat at a slower rate, and that, combined with the insulation of the oven or an insulated basket, can be used to keep food warm well into the evening. Solar cookers take longer to cook food compared to an oven. Using a solar oven therefore requires that food preparation be started several hours before the meal. However, it requires less hands-on time cooking, so this is often considered a reasonable trade-off. SOLAR COOKING PROJECTS

Bakeries in Lesotho Michael Hönes of Germany has established solar cooking in Lesotho, enabling small groups of women to build up community bakeries using solar ovens.

Use in Darfur Refugee Camps Cardboard, aluminum foil, and plastic bags for well over 10,000 solar cookers have been donated to the Iridimi refugee camp and Touloum refugee camps in Chad by the combined efforts of the Jewish World Watch, the Dutch foundation KoZon, and Solar Cookers International. The refugees construct the cookers themselves, using the donated supplies and locally purchased Arabic gum, and use them for midday and evening meals. The goal of this project was to reduce the Darfuri women's need to leave the relative safety of the camp to gather firewood, which exposed them to a high risk of being beaten, raped, kidnapped,


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or murdered. It has also significantly reduced the amount of time women spend tending open fires each day, with the results that they are healthier and they have more time to grow vegetables for their families and make handicrafts for export.

Indian Solar Cooker Village Bysanivaripalle, a silk-producing village that is 125 km (80 mi) northwest of Tirupati in the Indian state of in Andhra Pradesh, is the first of its kind: an entire village that uses only solar cooking. Intersol, an Austrian non-governmental organisation, sponsored the provision of powerful "Sk-14" parabolic solar cookers in 2004.

SOLAR FURNACE A solar furnace is a structure used to harness the rays of the sun in order to produce high temperatures. This is achieved using a curved mirror (or an array of mirrors) that acts as a parabolic reflector, concentrating light (Insolation) onto a focal point. The temperature at the focal point may reach 3,000 degrees Celsius, and this heat can be used to generate electricity, melt steel, or make hydrogen fuel. The solar furnace at Odeillo in the Pyrenees of France was opened in 1970 and is the largest in the world. It employs an array of plane mirrors to gather the rays of light from the sun, reflecting them on to a larger curved mirror. The rays are then focused onto an area the size of a cooking pot and can reach 3,000 degrees Celsius. The first modern solar furnace is believed to have been built in France in 1949 by Professor Félix Trombe. It is still in place at Mont Louis, near Odeillo. The Pyrenees were chosen as the site for these furnaces due to sunny weather for up to 300 days a year. It has been suggested that solar furnaces could be used in space to provide energy for manufacturing purposes. Their reliance on sunny weather may mean that they are unlikely to be used as a major source of renewable energy on Earth. The ancient Greek / Latin term "heliocaminus" literally means "solar furnace" - A glass-enclosed sunroom intentionally designed to become hotter than the outside air temperature. Today, the term "solar furnace" has evolved to refer to solar concentrator

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heating systems using parabolic mirrors or heliostats. 1,000 degrees Fahrenheit (538 degrees Celsius) is now commonly achieved. The theoretical maximum is the 5778 degree Kelvin surface temperature of the sun , although the practical limit (due to atmospheric absorption and rapid heat transfer at high temperature differentials) is much lower. During the Second Punic War (218 - 202 BCE), the Greek scientist Archimedes is said to have repelled the attacking Roman ships by setting them on fire with a "burning glass" that may have been an array of mirrors. An experiment to test this theory was carried out by a group at the Massachusetts Institute of Technology in 2005. It concluded that although the theory was sound for stationary objects, the mirrors would not likely have been able to concentrate sufficient solar energy to set a ship on fire under battle conditions. The solar furnace principle is being used to make inexpensive solar cookers and solar-powered barbecues, and for solar water pasteurization. A prototype Scheffler reflector is currently being constructed in India for use in a solar crematorium. This 50 m² reflector will generate temperatures of 700 °C and displace 200300 kg of firewood used per cremation.

SOLAR STILL A solar still is a very simple way for distilling water, powered by the heat of the sun, especially when distillation equipment is unavailable. A few basic types of solar stills are cone shaped, boxlike, and pit. For cone solar stills, impure water is inserted into the container, where it is evaporated by the sun through clear plastic. The pure water vapor condenses on top and drips down to the side, where it is collected and removed. The most sophisticated of these are the box-shaped types. The least sophisticated are the pit types. Solar stills are used in cases where piped or well water is impractical, such as in remote homes or during power outages. In Florida and other hurricane target areas that frequently lose power for a few days, solar distillation can provide an alternate source of clean water. Solar stills are occasionally used on a longer term basis in developing world settings. However, they produce a relatively


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small amount of clean water, and even smaller amounts where the source water is saline or brackish. If the amount of water is inadequate, a compromise method is to mix the distilled water with the brackish or saline water purified with other methods this gives a more adequate quantity, while still lowering the salinity, and improving the taste. A larger scale version of the concept of the solar still is the Water Pyramid, which uses an inflatable dome as the condensing surface and can be applied in tropical, rural areas. Knowing how to put together a solar still is often billed as a useful survival skill and could provide an important means of potable water in the event of a wilderness emergency. Nevertheless, under typical conditions makeshift solar stills rarely produce enough water for survival, and the sweat expended in building one can easily exceed its daily output. Solar stills can extract water from moisture in the ground but to increase the amount of moisture available to a solar still, water (fresh or saline) can be added inside or along the edges of the still. Where no water sources are readily available, urine or shredded vegetation can be used inside the pit. To prevent losing moisture by taking apart the still to retrieve collected water a length of plastic tubing can be used to sip water as it accumulates. A simpler to put together solar still was presented on the TV show Survivorman, in which the host simply wrapped a plastic bag around a leafy branch on a live tree. The transpiration from the tree leaves provided the water source. No cup was used; water dripped straight into the bag. In the same episode, Les Stroud created a solar still by using urine for the source of water, arranged in the sand beside a cup. SOLAR WATER DISINFECTION Solar water disinfection, also known as SODIS is a method of disinfecting water using only sunlight and plastic PET bottles. SODIS is a cheap and effective method for decentralized water treatment, usually applied at the household level and is recommended by the World Health Organization as a viable method for household water treatment and safe storage. SODIS is already applied in numerous developing countries.

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PRINCIPLE Exposure to sunlight has been shown to deactivate diarrheacausing organisms in polluted drinking water. Three effects of solar radiation are believed to contribute to the inactivation of pathogenic organisms: • UV-A interferes directly with the metabolism and destroys cell structures of bacteria. • UV-A (wavelength 320-400nm) reacts with oxygen dissolved in the water and produces highly reactive forms of oxygen (oxygen free radicals and hydrogen peroxides), that are believed to also damage pathogens. • Infrared radiation heats the water. If the water temperatures raises above 50°C, the disinfection process is three times faster. At a water temperature of about 30°C (86°F), a threshold solar radiation intensity of at least 500 W/m2 (all spectral light) is required for about 5 hours for SODIS to be efficient. This dose contains energy of 555 Wh/m2 in the range of UV-A and violet light, 350nm-450nm, corresponding to about 6 hours of midlatitude (European) midday summer sunshine. At water temperatures higher than 45°C (113°F), synergetic effects of UV radiation and temperature further enhance the disinfection efficiency. GUIDELINES FOR THE APPLICATION AT HOUSEHOLD LEVEL • Water from contaminated sources are filled into transparent water bottles. For oxygen saturation, bottles can be filled three quarters, then shaken for 20 seconds (with the cap on), then filled completely. Highly turbid water (turbidity higher than 30 NTU) must be filtered prior to exposure to the sunlight. • Filled bottles are then exposed to the sun. Better temperature effects can be achieved if bottles are placed on a corrugated roof as compared to thatched roofs. • The treated water can be consumed. The risk of recontamination can be minimized if water is stored in the bottles. The water should be consumed directly from the


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bottle or poured into clean drinking cups. Re-filling and storage in other containers increases the risk of contamination. Suggested T reatm ent S chedule W eather C onditions

M inim u m T reatm ent D uration

sunny

6 hours

50% cloud y

6 hours

50-100% cloud y

2 days

continuous rainfall

unsatisfactory perform ance, use rainw ater harvesting

APPLICATIONS OF SODIS SODIS is an effective method for treating water where fuel or cookers are unavailable or prohibitively expensive. Even where fuel is available, SODIS is a more economical and environmentally friendly option. The application of SODIS is limited if enough bottles are not available, or if the water is highly turbid. In theory, the method could be used in disaster relief or refugee camps. However, supplying bottles may be more difficult than providing equivalent disinfecting tablets containing chlorine, bromine, or iodine. Additionally, in some circumstances, it may be difficult to guarantee that the water will be left in the sun for the necessary time. Other methods for household water treatment and safe storage exist, e.g. chlorination, different filtration procedures or flocculation/disinfection. The selection of the adequate method should be based on the criteria of effectiveness, the co-occurrence of other types of pollution (turbidity, chemical pollutants), treatment costs, labor input and convenience, and the user's preference.

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CAUTIONS If the water bottles are not left in the sun for the proper length of time, the water may not be safe to drink and could cause illness. If the sunlight is less strong, due to overcast weather or a less sunny climate, a longer exposure time in the sun is necessary. The following issues should also be considered: • Bottle material: Some glass or PVC materials may prevent ultraviolet light from reaching the water. Commercially available bottles made of PET are recommended. The handling is much more convenient in the case of PET bottles. Polycarbonate blocks all UVA and UVB rays, and therefore should not be used. Glass also blocks UV rays and therefore would be ineffective. • Aging of plastic bottles: SODIS efficiency depends on the physical condition of the plastic bottles, with scratches and other signs of wear reducing the efficiency of SODIS. Heavily scratched or old, blind bottles should be replaced. • Shape of Containers: the intensity of the UV radiation decreases rapidly with increasing water depth. At a water depth of 10cm and moderate turbidity of 26 NTU, UV-A radiation is reduced to 50%. PET soft drink bottles are often easily available and thus most practical for the SODIS application. • Oxygen: Sunlight produces highly reactive forms of oxygen (oxygen free radicals and hydrogen peroxides) in the water. These reactive molecules contribute in the destruction process of the microorganisms. Under normal conditions (rivers, creeks, wells, ponds, tap) water contains sufficient oxygen (more than 3 mg Oxygen per litre) and does not have to be aerated before the application of SODIS. • Leaching of bottle material: There has been some concern over the question whether plastic drinking containers can release chemicals or toxic components into water, a process possibly accelerated by heat. The Swiss Federal Laboratories for Materials Testing and Research have examined the diffusion of adipates and phthalates (DEHA and DEHP) from new and reused PET-bottles in the water


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during solar exposure. The levels of concentrations found in the water after a solar exposure of 17 hours in 60°C water were far below WHO guidelines for drinking water and in the same magnitude as the concentrations of phthalate and adipate generally found in high quality tap water. Concerns about the general use of PET-bottles were also expressed after a report published by researchers from the University of Heidelberg on antimony being released from PETbottles for soft drinks and mineral water stored over several months in supermarkets. However, the antimony concentrations found in the bottles are orders of magnitude below WHO and national guidelines for antimony concentrations in drinking water. Furthermore, SODIS water is not stored over such extended periods in the bottles.

HEALTH IMPACT, DIARRHEA REDUCTION It has been shown that the SODIS method (and other methods of household water treatment) can very effectively remove pathogenic contamination from the water. However, infectious diseases are also transmitted through other pathways, i.e. due to a general lack of sanitation and hygiene. Studies on the reduction of diarrhea among SODIS users show reduction values of 30-80%. SODIS RESEARCH AND DEVELOPMENT The effectiveness of the SODIS was first discovered by Professor Aftim Acra at the American University of Beirut in the early 1980s . Substantial follow-up research was conducted by the research groups of Martin Wegelin at the Swiss Federal Institute of Aquatic Science and Technology (Eawag) and Dr Kevin McGuigan at the Royal College of Surgeons in Ireland. Clinical control trials were pioneered by Professor Ronan Conroy of the RCSI team in collaboration with Dr T Michael Elmore-Meegan. Currently, a joint research project on SODIS is implemented by the following institutions: • Royal College of Surgeons in Ireland (RCSI), Ireland (coordination) • University of Ulster (UU), United Kingdom

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Solar Energy and its Uses • CSIR Environmentek, South Africa, Eawag, Switzerland • The Institute of Water and Sanitation Development (IWSD), Zimbabwe • Plataforma Solar de Almería (CIEMAT-PSA), Spain • University of Leicester (UL), United Kingdom • The International Commission for the Relief of Suffering & Starvation (ICROSS), Kenya • University of Santiago de Compostela (USC), Spain • Swiss Federal Insitute of Aquatic Science and Technology (Eawag), Switzerland

The project has embarked on a multi-country study including study areas in Zimbabwe, South Africa and Kenya.

WORLDWIDE APPLICATION OF SODIS The Swiss Federal Institute of Aquatic Science and Technology (Eawag), through the Department of Water and Sanitation in Developing Countries (Sandec), coordinates SODIS promotion projects in 33 countries including Bhutan, Bolivia, Burkina Faso, Cambodia, Cameroon, DR Congo, Ecuador, El Salvador, Ethiopia, Ghana, Guatemala, Guinea, Honduras, India, Indonesia, Kenya, Laos, Malawi, Mozambique, Nepal, Nicaragua, Pakistan, Perú, Philippines, Senegal, Sierra Leone, Sri Lanka, Togo, Uganda, Uzbekistan, Vietnam, Zambia, and Zimbabwe. Contact addresses and case studies of the projects coordinated by the Swiss Federal Insitute of Aquatic Science and Technology (Eawag) are available at sodis.ch. SODIS projects are funded by, among others, the SOLAQUA Foundation (), several Lions Clubs, Rotary Clubs, Migros, and the Michel Comte Water Foundation. SODIS has also been applied in several communities in Brazil, one of them being Prainha do Canto Verde north of Fortaleza. There, the villagers have been purifying their water with the SODIS method. It is quite successful, especially since the temperature during the day can go beyond the 40°C (100°F) and there is a limited amount of shade. SOLAR CAR RACING Solar car racing refers to competitive races of electric vehicles which are powered by solar energy obtained from solar panels


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on the surface of the car (solar cars). The first solar car race was the Tour de Sol in 1985 which led to several similar races in Europe, USA and Australia. Solar car races are often sponsored by government agencies who are keen to promote the development of alternative energy technology (such as solar cells). Such challenges are often entered by universities to develop their students' engineering and technological skills, but many business corporations have entered competitions in the past. A small number of high school teams participate in solar car races designed exclusively for high school students.

NOTABLE DISTANCE RACES The two most notable solar car distance (overland) races are the World Solar Challenge and the North American Solar Challenge. They are contested by a variety of university and corporate teams. Corporate teams contest the race to give its design teams experience in working with both alternative energy sources and advanced materials (although some may view their participation as mere PR exercises). University teams enter the races because it gives their students experience in designing high technology cars and working with environmental and advanced materials technology. These races are often sponsored by agencies such as the US Department of Energy keen to promote renewable energy sources. The cars require intensive support teams similar in size to professional motor racing teams. This is especially the case with the World Solar Challenge where sections of the race run through very remote country. WORLD SOLAR CHALLENGE This race features a field of competitors from around the world who race to cross the Australian continent. In 2005, the Dutch Nuna 3 team won this challenge for a 3rd time in a record average speed of 102.75 km/h over a distance of 3000 km, followed by the Australian Aurora (92.03 km/h) and the University of Michigan (90.03 km/h). The increasingly high speeds of the 2005 race participants has led to the rules being changed for future

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solar cars starting in the 2007 race. The 20th Anniversary race of the World Solar Challenge ran in October of 2007. Major regulation changes were released in June 2006 for this race to increase safety, to build a new generation of solar car, which with little modification could be the basis for a practical proposition for sustainable transport and intended to slow down cars in the main event, which could easily exceed the speed limit (110 km/h) in previous years. The winner again was the Nuna 4 team averaging 90.87 km/h. The winner in the Adventure Class (driving under old rules) was the Ashiya University Solar Car Project team averaging 93.57 km/h.

NORTH AMERICAN SOLAR CHALLENGE The North American Solar Challenge, previously known as the 'American Solar Challenge' and 'Sunrayce USA', features mostly collegiate teams racing in timed intervals in the United States and Canada. The North American Solar Challenge was sponsored in part by the US Department of Energy. However, funding was cut near the end of 2005, and the NASC 2007 was cancelled. The North American solar racing community worked to find a solution, bringing in Toyota as a primary sponsor for a 2008 race. The next North American Solar Challenge will run from July 13-21, 2008, from Dallas, Texas to Calgary, Alberta. Other races: • Suzuka, a yearly track race in Japan. • Phaethon, part of the Cultural Olympiad in Greece prior to the 2004 Olympics. • World Solar Rally. • Dell-Winston School Solar Car Challenge is the best-known and longest-running high-school-level race. SOLAR DRAG RACES Solar drag races are another form of solar racing. Unlike long distance solar races, solar dragsters do not use any batteries or pre-charged energy storage devices. Racers go head-to-head over a straight quarter kilometer distance. Currently, a solar drag race is held each year on the Saturday closest to the summer solstice


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in Wenatchee, Washington, USA. The world record for this event is 29.5 seconds set by the South Whidbey High School team on June 23, 2007.

VEHICLE DESIGN Solar cars combine technology used in the aerospace, bicycle, alternative energy and automotive industries. Unlike most race cars, solar cars are designed with severe energy constraints imposed by the race regulations. These rules limit the energy used to only that collected from solar radiation, albeit starting with a full charged battery pack. As a result optimizing the design to account for aerodynamic drag, vehicle weight, rolling resistance and electrical efficiency are paramount. Conventional thinking has to be challenged, for example, rather than a conventional automobile seat which would weigh tens of pounds, one championship solar car employed a nylon mesh seat combined with a five-point harness that weighed less than 3 pounds. Solar race cars can be designed with a variety of basic configurations by varying the shape of the vehicle, the number and location of wheels, the location of solar cells, and other variables. These trade off the efficiency of the panel against aerodynamics, weight, controllability, and ease of manufacture. Since 1996 the leading WSC cars have tended to have a small canopy in the middle of a curved wing-like array, entirely covered in cells, with 3 wheels. Before then the cockroach style, as used in the GM Sunraycer with a smooth nose fairing into the panel were more successful. At lower speeds, with less powerful arrays, other configurations are viable and may be easier to construct. DRIVER'S COCKPIT Like many race cars, the driver's cockpit usually only contains room for one person, although a few cars do contain room for a second passenger. They contain some of the features available to drivers of traditional vehicles such as brakes, accelerator, turn signals, rear view mirrors (or camera), ventilation, and sometimes cruise control. A radio for communication with their support crews is almost always included. Solar cars are often fitted with gauges as seen in conventional cars. Aside from keeping the car on the road, the driver's main priority is to keep an eye on these

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gauges to spot possible problems. Cars without gauges available for the driver will almost always feature wireless telemetry. Wireless telemetry allows the driver's team to monitor the car's energy consumption, solar energy capture and other parameters and free the driver to concentrate on just driving. Drivers also have a safety harness, and optionally (depending on the race) a helmet similar to racing car drivers.

ELECTRICAL SYSTEM The electrical system is the most important part of the car's systems as it controls all of the power that comes into and leaves the system. The battery pack plays the same role in a solar car that a fuel tank plays in a normal car in storing power for future use. Solar cars use a range of batteries including lead-acid batteries, nickel-metal hydride batteries (NiMH), Nickel-Cadmium batteries (NiCd), Lithium ion batteries and Lithium polymer batteries. Leadacid batteries are less expensive and easier to work with but store less energy for a given mass. Typically, solar cars use voltages between 84 and 170 volts. Power electronics monitor and regulate the car's electricity. Components of the power electronics include the peak power trackers, the motor controller and the data acquisition system. The peak power trackers manage the power coming from the solar array to maximize the power and deliver it to be stored in the motor. They also protect the batteries from overcharging. The motor controller manages the electricity flowing to the motor according to signals flowing from the accelerator. Many solar cars have complex data acquisition systems that monitor the whole electrical system while even the most basic cars have systems that provide information on battery voltage and current to the driver. One such system utilizes Controller Area Network (CAN). A wide variety of motor types have been used. Usually there is a strong relationship between efficiency and cost. The most efficient motors exceed 98% efficiency. These are brushless three"phase" DC, electronically commutated, wheel motors, with a Halbach array configuration for the neodymium-iron-boron magnets, and Linz wire for the windings. Cheaper alternatives include motors from wind turbines, or brushed DC motors.


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MECHANICAL SYSTEMS The mechanical systems are designed to keep friction and weight to a minimum while maintaining strength and stiffness. Designers normally use aluminium, titanium and composites to provide a structure that meets strength and stiffness requirements whilst being fairly light. Steel is used for some suspension parts on many cars. Solar cars usually have three wheels, but some have four. Three wheelers usually have two front wheels and one rear wheel: the front wheels steer and the rear wheel follows. Four wheel vehicles are set up like normal cars or similarly to three wheeled vehicles with the two rear wheels close together. Solar cars have a wide range of suspensions because of varying bodies and chassis. The most common front suspension is the double wishbone suspension. The rear suspension is often a trailing-arm suspension as found in motor cycles. Solar cars are required to meet rigorous standards for brakes. Disc brakes are the most commonly used due to their good braking ability and ability to adjust. Mechanical and hydraulic brakes are both widely used. The brake pads or shoes are typically designed to retract to minimize brake drag, on leading cars. Steering systems for solar cars also vary. The major design factors for steering systems are efficiency, reliability and precision alignment to minimize tire wear and power loss. The popularity of solar car racing has led to some tire manufacturers designing tires for solar vehicles. This has increased overall safety and performance. All the top teams now use wheel motors, eliminating belt or chain drives. Testing is essential to demonstrating vehicle reliability prior to a race. It is easy to spend a hundred thousand dollars to gain a two hour advantage, and equally easy to lose two hours due to reliability issues.

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Solar Array

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consider wind energy to be solar energy, so their race regulations allow this practice. AERODYNAMICS

4 SOLAR ARRAY

Aerodynamic drag is the main source of losses on a solar race car. The aerodynamic drag of a vehicle is the product of the frontal area and its Cd. For most solar cars the frontal area is 0.75 to 1.3 m^2. While Cds as low as 0.10 have been reported, 0.13 is more typical. This needs a great deal of attention to detail.

The solar array consists of hundreds (or thousands) of photovoltaic solar cells converting sunlight into electricity. Cars can use a variety of solar cell technologies; most often polycrystalline silicon, monocrystalline silicon, or gallium arsenide.

MASS The vehicle's mass is also a significant factor. A light vehicle generates less rolling resistance and will need smaller lighter brakes and other suspension components. This is the virtuous circle when designing lightweight vehicles.

The cells are wired together into strings while strings are often wired together to form a panel. Panels normally have voltages close to the nominal battery voltage. The main aim is to get as much cell area in as small a space as possible. Designers encapsulate the cells to protect them from the weather and breakage.

ROLLING RESISTANCE Rolling resistance can be minimised by using the right tires, inflated to the right pressure, correctly aligned, and by minimising the weight of the vehicle.

Designing a solar array is more than just stringing a bunch of cells together. A solar array acts like many very small batteries all hooked together in series. The total voltage produced is the sum of all cell voltages.

PERFORMANCE EQUATION The design of a solar car is governed by the following work equation:

The problem is that if a single cell is in shadow it acts like a diode, blocking the current for the entire string of cells. To design against this, array designers use by-pass diodes in parallel with smaller segments of the string of cells, allowing current around the non-functioning cell(s). Another consideration is that the battery itself can force current backwards through the array unless there are blocking diodes put at the end of each panel. The power produced by the solar array depends on the weather conditions, the position of the sun and the capacity of the array. At noon on a bright day, a good array can produce over 2 kilowatts (2.6 hp). Some cars have employed free-standing or integrated sails to harness wind energy. Many races, including the WSC and NASC,

which can be usefully simplified to the performance equation

for long distance races, and values seen in practice. Briefly, the left hand side represents the energy input into the car (batteries and power from the sun) and the right hand side is the energy needed to drive the car along the race route (overcoming rolling resistance, aerodynamic drag, going uphill and accelerating). Everything in this equation can be estimated except u. Solving the long form of the equation for velocity results in a large equation (approximately 100 terms). Using the power

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equation as the arbiter, vehicle designers can compare various car designs and evaluate the comparative performance over a given route. Combined with CAE and systems modeling, the power equation can be a useful tool in solar car design.

RACE ROUTE CONSIDERATIONS The directional orientation of a solar car race route affects the apparent position of the sun in the sky during a race day, which in turn affects the energy input to the vehicle. • In a south-to-north race route alignment, for example, the sun would rise over the driver's right shoulder and finish over his left (due to the east-west apparent motion of the sun). • In an east-west race route alignment, the sun would rise behind the vehicle, and appear to move in the direction of the vehicle's movement, setting in the front of the car. • A hybrid route alignment includes significant sections of south-north and east-west routes together. This is significant to designers, who seek to maximize energy input to a panel of solar cells (often called an "array" of cells) by designing the array to point directly toward the sun for as long as possible during the race day. Thus, a south-north race car designer might increase the car's total energy input by using solar cells on the sides of the vehicle where the sun will strike them (or by creating a convex array coaxial with the vehicle's movement). In contrast, an east-west race alignment might reduce the benefit from having cells on the side of the vehicle, and thus might encourage design of a flat array. Because solar cars are often purpose-built, and because arrays do not usually move in relation to the rest of the vehicle (with notable exceptions), this race-route-driven, flat-panel versus convex design compromise is one of the most significant decisions that a solar car designer must make. For example, the 1990 and 1993 Sunrayce USA events were won by vehicles with significantly convex arrays, corresponding to the south-north race alignments; by 1997, however, most cars in that event had flat arrays to match the change to an east-west route.

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RACE STRATEGY

ENERGY CONSUMPTION Optimizing energy consumption is of prime importance in a solar car race. Therefore it is very important to be able to closely monitor the speed, energy consumption, energy intake from solar panel, among other things in real time. Some teams employ sophisticated telemetry that relays vehicle performance data to a computer in a following support vehicle. The strategy employed depends upon the race rules and conditions. Most solar car races have set starting and stopping points where the objective is to reach the final point in the least amount of total time. Since aerodynamic drag force rises quadratically with speed, the energy the car consumes per second rises cubically (per meter travelled it rises quadratically with speed). Given the varied conditions in all races and the limited (and continuously changing) supply of energy, most teams have race speed optimization programs that continuously update the team on how fast the vehicle should be traveling. RACE ROUTE The race route itself will affect strategy, because the apparent position of the sun in the sky will vary depending various factors which are specific to the vehicle's orientation. In addition, elevation changes over a race route can dramatically change the amount of power needed to travel the route. For example, the 2001 and 2003 North American Solar Challenge route crossed the Rocky Mountains. WEATHER FORECASTING A successful solar car racing team will need to have access to reliable weather forecasts in order to predict the power input to the vehicle from the sun during each race day.

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The Solar Cell

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THE SOLAR CELL

that the first solar cell was built, by Charles Fritts, who coated the semiconductor selenium with an extremely thin layer of gold to form the junctions. The device was only around 1% efficient. Russell Ohl patented the modern solar cell in 1946 (U.S. Patent 2,402,662 , "Light sensitive device"). Sven Ason Berglund had a prior patent concerning methods of increasing the capacity of photosensitive cells. The modern age of solar power technology arrived in 1954 when Bell Laboratories, experimenting with semiconductors, accidentally found that silicon doped with certain impurities was very sensitive to light.

A solar cell or photovoltaic cell is a device that converts solar energy into electricity by the photovoltaic effect. Photovoltaics is the field of technology and research related to the application of solar cells as solar energy. Sometimes the term solar cell is reserved for devices intended specifically to capture energy from sunlight, while the term photovoltaic cell is used when the source is unspecified. Assemblies of cells are used to make solar modules, which may in turn be linked in photovoltaic arrays.

This resulted in the production of the first practical solar cells with a sunlight energy conversion efficiency of around 6 percent. The first spacecraft to use solar panels was the US satellite Vanguard 1, launched in March 1958 with solar cells made by Hoffman Electronics. This milestone created interest in producing and launching a geostationary communications satellite, in which solar energy would provide a viable power supply. This was a crucial development which stimulated funding from several governments into research for improved solar cells.

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Solar cells have many applications. Individual cells are used for powering small devices such as electronic calculators. Photovoltaic arrays generate a form of renewable electricity, particularly useful in situations where electrical power from the grid is unavailable such as in remote area power systems, Earthorbiting satellites and space probes, remote radiotelephones and water pumping applications. Photovoltaic electricity is also increasingly deployed in grid-tied electrical systems. Similar devices intended to capture energy radiated from other sources include thermophotovoltaic cells, betavoltaics cells, and optoelectric nuclear battery. HISTORY The term "photovoltaic" comes from the Greek (phos) meaning "light", and "voltaic", meaning electrical, from the name of the Italian physicist Volta, after whom the measurement unit volt is named. The term "photo-voltaic" has been in use in English since 1849. The photovoltaic effect was first recognized in 1839 by French physicist A. E. Becquerel. However, it was not until 1883

In 1970 the first highly effective GaAs heterostructure solar cells were created by Zhores Alferov and his team in the USSR. Metal Organic Chemical Vapor Deposition (MOCVD, or OMCVD) production equipment was not developed until the early 1980s, limiting the ability of companies to manufacture the GaAs solar cell. In the United States, the first 17% efficient air mass zero (AM0) single-junction GaAs solar cells were manufactured in production quantities in 1988 by Applied Solar Energy Corporation (ASEC). The "dual junction" cell was accidentally produced in quantity by ASEC in 1989 as a result of the change from GaAs on GaAs substrates to GaAs on Germanium (Ge) substrates. The accidental doping of Ge with the GaAs buffer layer created higher open circuit voltages, demonstrating the potential of using the Ge substrate as another cell. As GaAs single-junction cells topped 19% AM0 production efficiency in 1993, ASEC developed the first dual junction cells for spacecraft use in the United States, with a starting efficiency of approximately 20%. These cells did not utilize the Ge as a second cell, but used another GaAs-based cell with different doping. Eventually GaAs dual junction cells reached

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production efficiencies of about 22%. Triple Junction solar cells began with AM0 efficiencies of approximately 24% in 2000, 26% in 2002, 28% in 2005, and in 2007 have evolved to a 30% AM0 production efficiency, currently in qualification. In 2007, two companies in the United States, Emcore Photovoltaics and Spectrolab, produce 95% of the world's Triple Junction solar cells which have a commercial efficiency of 38%. In 2006 Spectrolab's cells achieved 40.7% efficiency in lab testing. THREE GENERATIONS OF SOLAR CELLS Solar Cells are classified into three generations which indicates the order of which each became prominent. At present there is concurrent research into all three generations while the first generation technologies are most highly represented in commercial production, accounting for 89.6% of 2007 production.

FIRST GENERATION First generation cells consist of large-area, high quality and single junction devices. First Generation technologies involve high energy and labour inputs which prevent any significant progress in reducing production costs. Single junction silicon devices are approaching the theoretical limiting efficiency of 33% and combined with high production costs are unlikely to achieve cost parity with fossil fuel energy generation. SECOND GENERATION Second generation materials have been developed to address energy requirements and production costs of solar cells. Alternative manufacturing techniques such as vapour deposition and electroplating are advantageous as they reduce high temperature processing significantly. It is commonly accepted that as manufacturing techniques evolve production costs will be dominated by constituent material requirements, whether this be a silicon substrate, or glass cover. Second generation technologies are expected to gain market share in 2008. The most successful second generation materials have been cadmium telluride (CdTe), copper indium gallium selenide, amorphous silicon and micromorphous silicon. These materials

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are applied in a thin film to a supporting substrate such as glass or ceramics reducing material mass and therefore costs. These technologies do hold promise of higher conversion efficiencies, particularly CIGS-CIS, DSC and CdTe offers significantly cheaper production costs. Among major manufacturers there is certainly a trend toward second generation technologies however commercialisation of these technologies has proven difficult. In 2007 First Solar produced 200 MW of CdTe solar cells making it the fifth largest producer of solar cells in 2007 and the first ever to reach the top 10 from production of second generation technologies alone.. Wurth Solar commercialised its CIS technology in 2007 producing 15 MW. Nanosolar commercialised its CIGS technology in 2007 with a production capacity of 430 MW for 2008 in the USA and Germany. In 2007 CdTe production represented 4.7% of total market share, thin film silicon 5.2% and CIGS 0.5%.

THIRD GENERATION Third generation technologies aim to enhance poor electrical performance of second generation thin film technologies while maintaining very low production costs. Current research is targeting conversion efficiencies of 30-60% while retaining low cost materials and manufacturing techniques. There are a few approaches to achieving these high efficiencies: • Multijunction photovoltaic cell. • Modifying incident spectrum (concentration). • Use of excess thermal generation to enhance voltages or carrier collection. APPLICATIONS AND IMPLEMENTATIONS Solar cells are often electrically connected and encapsulated as a module. PV modules often have a sheet of glass on the front (sun up) side , allowing light to pass while protecting the semiconductor wafers from the elements (rain, hail, etc.). Solar cells are also usually connected in series in modules, creating an additive voltage. Connecting cells in parallel will yield a higher current. Modules are then interconnected, in series or parallel, or both, to create an array with the desired peak DC voltage and current.

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The power output of a solar array is measured in watts or kilowatts. In order to calculate the typical energy needs of the application, a measurement in watt-hours, kilowatt-hours or kilowatt-hours per day is often used. A common rule of thumb is that average power is equal to 20% of peak power, so that each peak kilowatt of solar array output power corresponds to energy production of 4.8 kWh per day. To make practical use of the solargenerated energy, the electricity is most often fed into the electricity grid using inverters (grid-connected PV systems); in stand alone systems, batteries are used to store the energy that is not needed immediately. THEORY

SIMPLE EXPLANATION 1. Photons in sunlight hit the solar panel and are absorbed by semiconducting materials, such as silicon. 2. Electrons (negatively charged) are knocked loose from their atoms, allowing them to flow through the material to produce electricity. The complementary positive charges that are also created (like bubbles) are called holes and flow in the direction opposite of the electrons in a silicon solar panel. 3. An array of solar panels converts solar energy into a usable amount of direct current (DC) electricity. PHOTOGENERATION OF CHARGE CARRIERS When a photon hits a piece of silicon, one of three things can happen: 1. the photon can pass straight through the silicon - this (generally) happens for lower energy photons, 2. the photon can reflect off the surface, 3. the photon can be absorbed by the silicon, if the photon energy is higher than the silicon band gap value. This generates an electron-hole pair and sometimes heat, depending on the band structure. When a photon is absorbed, its energy is given to an electron in the crystal lattice. Usually this electron is in the valence band, and is tightly bound in covalent bonds between neighboring atoms,

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and hence unable to move far. The energy given to it by the photon "excites" it into the conduction band, where it is free to move around within the semiconductor. The covalent bond that the electron was previously a part of now has one fewer electron - this is known as a hole. The presence of a missing covalent bond allows the bonded electrons of neighboring atoms to move into the "hole," leaving another hole behind, and in this way a hole can move through the lattice. Thus, it can be said that photons absorbed in the semiconductor create mobile electron-hole pairs. A photon need only have greater energy than that of the band gap in order to excite an electron from the valence band into the conduction band. However, the solar frequency spectrum approximates a black body spectrum at ~6000 K, and as such, much of the solar radiation reaching the Earth is composed of photons with energies greater than the band gap of silicon. These higher energy photons will be absorbed by the solar cell, but the difference in energy between these photons and the silicon band gap is converted into heat (via lattice vibrations - called phonons) rather than into usable electrical energy.

CHARGE CARRIER SEPARATION There are two main modes for charge carrier separation in a solar cell: 1. drift of carriers, driven by an electrostatic field established across the device 2. diffusion of carriers from zones of high carrier concentration to zones of low carrier concentration (following a gradient of electrochemical potential). In the widely used p-n junction solar cells, the dominant mode of charge carrier separation is by drift. However, in non-p-njunction solar cells (typical of the third generation of solar cell research such as dye and polymer thin-film solar cells), a general electrostatic field has been confirmed to be absent, and the dominant mode of separation is via charge carrier diffusion. THE P-N JUNCTION The most commonly known solar cell is configured as a largearea p-n junction made from silicon. As a simplification, one can

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imagine bringing a layer of n-type silicon into direct contact with a layer of p-type silicon. In practice, p-n junctions of silicon solar cells are not made in this way, but rather, by diffusing an n-type dopant into one side of a p-type wafer (or vice versa). If a piece of p-type silicon is placed in intimate contact with a piece of n-type silicon, then a diffusion of electrons occurs from the region of high electron concentration (the n-type side of the junction) into the region of low electron concentration (p-type side of the junction). When the electrons diffuse across the p-n junction, they recombine with holes on the p-type side. The diffusion of carriers does not happen indefinitely however, because of an electric field which is created by the imbalance of charge immediately on either side of the junction which this diffusion creates. The electric field established across the p-n junction creates a diode that promotes current to flow in only one direction across the junction. Electrons may pass from the n-type side into the ptype side, and holes may pass from the p-type side to the n-type side, but not the other way around. This region where electrons have diffused across the junction is called the depletion region because it no longer contains any mobile charge carriers. It is also known as the "space charge region".

CONNECTION TO AN EXTERNAL LOAD Ohmic metal-semiconductor contacts are made to both the ntype and p-type sides of the solar cell, and the electrodes connected to an external load. Electrons that are created on the n-type side, or have been "collected" by the junction and swept onto the n-type side, may travel through the wire, power the load, and continue through the wire until they reach the p-type semiconductor-metal contact. Here, they recombine with a hole that was either created as an electron-hole pair on the p-type side of the solar cell, or swept across the junction from the n-type side after being created there. EQUIVALENT CIRCUIT OF A SOLAR CELL To understand the electronic behavior of a solar cell, it is useful to create a model which is electrically equivalent, and is based on discrete electrical components whose behavior is well known. An ideal solar cell may be modelled by a current source

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in parallel with a diode; in practice no solar cell is ideal, so a shunt resistance and a series resistance component are added to the model. The resulting equivalent circuit of a solar cell is shown on the left. SOLAR CELL EFFICIENCY FACTORS

SUN UNIT "One sun" is a measurement equal to the solar power incident at noon on a clear summer day. I.e. in a 2300 sun system, approximately 230 watts per square centimeter are concentrated onto the cell system. ENERGY CONVERSION EFFICIENCY A solar cell's energy conversion efficiency (h, "eta"), is the percentage of power converted (from absorbed light to electrical energy) and collected, when a solar cell is connected to an electrical circuit. This term is calculated using the ratio of the maximum power point, Pm, divided by the input light irradiance (E, in W/ m²) under standard test conditions (STC) and the surface area of the solar cell (Ac in m²).

STC specifies a temperature of 25°C and an irradiance of 1000 W/m² with an air mass 1.5 (AM1.5) spectrum. These correspond to the irradiance and spectrum of sunlight incident on a clear day upon a sun-facing 37°-tilted surface with the sun at an angle of 41.81° above the horizon. This condition approximately represents solar noon near the spring and autumn equinoxes in the continental United States with surface of the cell aimed directly at the sun. Thus, under these conditions a solar cell of 12% efficiency with a 100 cm2 (0.01 m2) surface area can be expected to produce approximately 1.2 watts of power. The losses of a solar cell may be broken down into reflectance losses, thermodynamic efficiency, recombination losses and resistive electrical loss. The overall efficiency is the product of each of these individual losses. Due to the difficulty in measuring these parameters directly, other parameters are measured instead:

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Thermodynamic Efficiency, Quantum Efficiency, VOC ratio, and Fill Factor. Reflectance losses are a portion of the Quantum Efficiency under "External Quantum Efficiency". Recombination losses make up a portion of the Quantum Efficiency, VOC ratio, and Fill Factor. Resistive losses are predominantly categorized under Fill Factor, but also make up minor portions of the Quantum Efficiency, VOC ratio.

THERMODYNAMIC EFFICIENCY LIMIT Solar cells operate as quantum energy conversion devices, and are therefore subject to the "Thermodynamic Efficiency Limit". Photons with an energy below the band gap of the absorber material cannot generate a hole-electron pair, and so their energy is not converted to useful output and only generates heat if absorbed. For photons with an energy above the band gap energy, only a fraction of the energy above the band gap can be converted to useful output. When a photon of greater energy is absorbed, the excess energy above the band gap is converted to kinetic energy of the carrier combination. The excess kinetic energy is converted to heat through phonon interactions as the kinetic energy of the carriers slows to equilibrium velocity. Solar cells with multiple band gap absorber materials are able to more efficiently convert the solar spectrum. By using multiple band gaps, the solar spectrum may be broken down into smaller bins where the thermodynamic efficiency limit is higher for each bin. QUANTUM EFFICIENCY As described above, when a photon is absorbed by a solar cell it is converted to an electron-hole pair. This electron-hole pair may then travel to the surface of the solar cell and contribute to the current produced by the cell; such a carrier is said to be collected. Alternatively, the carrier may give up its energy and once again become bound to an atom within the solar cell without reaching the surface; this is called recombination, and carriers that recombine do not contribute to the production of electrical current. Quantum efficiency refers to the percentage of photons that are converted to electric current (i.e., collected carriers) when the

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cell is operated under short circuit conditions. External quantum efficiency is the fraction of incident photons that are converted to electrical current, while internal quantum efficiency is the fraction of absorbed photons that are converted to electrical current. Mathematically, internal quantum efficiency is related to external quantum efficiency by the reflectance of the solar cell; given a perfect anti-reflection coating, they are the same. Quantum efficiency should not be confused with energy conversion efficiency, as it does not convey information about the power collected from the solar cell. Furthermore, quantum efficiency is most usefully expressed as a spectral measurement (that is, as a function of photon wavelength or energy). Since some wavelengths are absorbed more effectively than others in most semiconductors, spectral measurements of quantum efficiency can yield information about which parts of a particular solar cell design are most in need of improvement.

VOC RATIO Due to recombination, the open circuit voltage (VOC) of the cell will be below the band gap voltage of the cell. Since the energy of the photons must be at or above the band gap to generate a carrier pair, cell voltage below the band gap voltage represents a loss. This loss is represented by the ratio of VOC divided by VG MAXIMUM-POWER POINT A solar cell may operate over a wide range of voltages (V) and currents (I). By increasing the resistive load on an irradiated cell continuously from zero (a short circuit) to a very high value (an open circuit) one can determine the maximum-power point, the point that maximizes V×I; that is, the load for which the cell can deliver maximum electrical power at that level of irradiation. (The output power is zero in both the short circuit and open circuit extremes). A high quality, monocrystalline silicon solar cell, at 25 °C cell temperature, may produce 0.60 volts open-circuit (Voc). The cell temperature in full sunlight, even with 25 °C air temperature, will probably be close to 45 °C, reducing the open-circuit voltage to 0.55 volts per cell. The voltage drops modestly, with this type of

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cell, until the short-circuit current is approached (Isc). Maximum power (with 45 °C cell temperature) is typically produced with 75% to 80% of the open-circuit voltage (0.43 volts in this case) and 90% of the short-circuit current. This output can be up to 70% of the Voc x Isc product. The short-circuit current (Isc) from a cell is nearly proportional to the illumination, while the open-circuit voltage (Voc) may drop only 10% with a 80% drop in illumination. Lower-quality cells have a more rapid drop in voltage with increasing current and could produce only 1/2 Voc at 1/2 Isc. The usable power output could thus drop from 70% of the Voc x Isc product to 50% or even as little as 25%. Vendors who rate their solar cell "power" only as Voc x Isc, without giving load curves, can be seriously distorting their actual performance. The maximum power point of a photovoltaic varies with incident illumination. For systems large enough to justify the extra expense, a maximum power point tracker tracks the instantaneous power by continually measuring the voltage and current (and hence, power transfer), and uses this information to dynamically adjust the load so the maximum power is always transferred, regardless of the variation in lighting.

FILL FACTOR Another defining term in the overall behavior of a solar cell is the fill factor (FF). This is the ratio of the maximum power point divided by the open circuit voltage (Voc) and the short circuit current (Isc):

COMPARISON OF ENERGY CONVERSION EFFICIENCIES At this point, discussion of the different ways to calculate efficiency for space cells and terrestrial cells is necessary to alleviate confusion. In space, where there is no atmosphere, the spectrum of the sun is relatively unfiltered. However on earth, with air filtering the incoming light, the solar spectrum changes. To account for the spectral differences, a system was devised to calculate this filtering effect. Simply, the filtering effect ranges from Air Mass 0 (AM0) in space, to approximately Air Mass 1.5 on earth.

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Multiplying the spectral differences by the quantum efficiency of the solar cell in question will yield the efficiency of the device. For example, a Silicon solar cell in space might have an efficiency of 14% at AM0, but have an efficiency of 16% on earth at AM 1.5. Terrestrial efficiencies typically are greater than space efficiencies. Solar cell efficiencies vary from 6% for amorphous siliconbased solar cells to 40.7% with multiple-junction research lab cells and 42.8% with multiple dies assembled into a hybrid package. Solar cell energy conversion efficiencies for commercially available multicrystalline Si solar cells are around 14-19%. The highest efficiency cells have not always been the most economical - for example a 30% efficient multijunction cell based on exotic materials such as gallium arsenide or indium selenide and produced in low volume might well cost one hundred times as much as an 8% efficient amorphous silicon cell in mass production, while only delivering about four times the electrical power. However, there is a way to "boost" solar power. By increasing the light intensity, typically photogenerated carriers are increased, resulting in increased efficiency by up to 15%. These so-called "concentrator systems" have only begun to become cost-competitive as a result of the development of high efficiency GaAs cells. The increase in intensity is typically accomplished by using concentrating optics. A typical concentrator system may use a light intensity 6-400 times the sun, and increase the efficiency of a one sun GaAs cell from 31% at AM 1.5 to 35%. A common method used to express economic costs of electricity-generating systems is to calculate a price per delivered kilowatt-hour (kWh). The solar cell efficiency in combination with the available irradiation has a major influence on the costs, but generally speaking the overall system efficiency is important. Using the commercially available solar cells (as of 2006) and system technology leads to system efficiencies between 5 and 19%. As of 2005, photovoltaic electricity generation costs ranged from ~0.60 US$/kWh (0.50 /kWh) (central Europe) down to ~0.30 US$/ kWh (0.25 /kWh) in regions of high solar irradiation. This electricity is generally fed into the electrical grid on the customer's side of the meter. The cost can be compared to prevailing retail electric pricing (as of 2005), which varied from between 0.04 and

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0.50 US$/kWh worldwide. (Note: in addition to solar irradiance profiles, these costs/kwh calculations will vary depending on assumptions for years of useful life of a system. Most c-Si panels are warrantied for 25 years and should see 35+ years of useful life.) The chart at the right illustrates the various commercial largearea module energy conversion efficiencies and the best laboratory efficiencies obtained for various materials and technologies.

WATTS PEAK Since solar cell output power depends on multiple factors, such as the sun's incidence angle, for comparison purposes between different cells and panels, the measure of watts peak (Wp) is used. It is the output power under these conditions known as STC: 1. insolation (solar irradiance) 1000 W/m² 2. solar reference spectrum AM (airmass) 1.5 3. cell temperature 25°C SOLAR CELLS AND ENERGY PAYBACK In the 1990s, when silicon cells were twice as thick, efficiencies were 30% lower than today and lifetimes were shorter, it may well have cost more energy to make a cell than it could generate in a lifetime. In the meantime, the technology has progressed significantly, and the energy payback time of a modern photovoltaic module is typically from 1 to 4 years depending on the type and where it is used (see net energy gain). With a typical lifetime of 20 to 30 years, this means that modern solar cells are net energy producers, i.e they generate much more energy over their lifetime than the energy expended in producing them. LIGHT-ABSORBING MATERIALS All solar cells require a light absorbing material contained within the cell structure to absorb photons and generate electrons via the photovoltaic effect. The materials used in solar cells tend to have the property of preferentially absorbing the wavelengths of solar light that reach the earth surface; however, some solar cells are optimized for light absorption beyond Earth's atmosphere as well. Light absorbing materials can often be used in multiple

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physical configurations to take advantage of different light absorption and charge separation mechanisms. Many currently available solar cells are configured as bulk materials that are subsequently cut into wafers and treated in a "top-down" method of synthesis (silicon being the most prevalent bulk material). Other materials are configured as thin-films (inorganic layers, organic dyes, and organic polymers) that are deposited on supporting substrates, while a third group are configured as nanocrystals and used as quantum dots (electron-confined nanoparticles) embedded in a supporting matrix in a "bottom-up" approach. Silicon remains the only material that is well-researched in both bulk and thinfilm configurations. There are many new alternatives to Silicon photocells. Proprietary nano-particle silicon printing processes promises many of the photovoltaic features that conventional silicon can never achieve. It can be printed reel to reel on stainless steel or other high temperature substrates. However, most of the work on the next generation of photovoltaics is directed at printing onto low cost flexible polymer film and ultimately on common packaging materials. The main contenders are currently CIGS, CdTe, DSSC and organic photovoltaics. The following is a current list of light absorbing materials, listed by configuration and substance-name:

BULK These bulk technologies are often referred to as wafer-based manufacturing. In other words, in each of these approaches, selfsupporting wafers between 180 to 240 micrometers thick are processed and then soldered together to form a solar cell module. A general description of silicon wafer processing is provided in Manufacture and Devices. SILICON By far, the most prevalent bulk material for solar cells is crystalline silicon (abbreviated as a group as c-Si), also known as "solar grade silicon". Bulk silicon is separated into multiple categories according to crystallinity and crystal size in the resulting ingot, ribbon, or wafer.

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Solar Energy and its Uses 1. Monocrystalline silicon (c-Si): often made using the Czochralski process. Single-crystal wafer cells tend to be expensive, and because they are cut from cylindrical ingots, do not completely cover a square solar cell module without a substantial waste of refined silicon. Hence most c-Si panels have uncovered gaps at the four corners of the cells. 2. Poly- or multicrystalline silicon (poly-Si or mc-Si): made from cast square ingots - large blocks of molten silicon carefully cooled and solidified. These cells are less expensive to produce than single crystal cells but are less efficient. 3. Ribbon silicon: formed by drawing flat thin films from molten silicon and having a multicrystalline structure. These cells have lower efficiencies than poly-Si, but save on production costs due to a great reduction in silicon waste, as this approach does not require sawing from ingots.

THIN FILMS The various thin-film technologies currently being developed reduce the amount (or mass) of light absorbing material required in creating a solar cell. This can lead to reduced processing costs from that of bulk materials (in the case of silicon thin films) but also tends to reduce energy conversion efficiency, although many multi-layer thin films have efficiencies above those of bulk silicon wafers.

CDTE Cadmium telluride is an efficient light-absorbing material for thin-film solar cells. Compared to other thin-film materials, CdTe is easier to deposit and more suitable for large-scale production. Despite much discussion of the toxicity of CdTe-based solar cells, this is the only technology (apart from amorphous silicon) that can be delivered on a large scale. The perception of the toxicity of CdTe is based on the toxicity of elemental cadmium, a heavy metal that is a cumulative poison. However it has been shown that the release of cadmium to the atmosphere is lower with CdTe-

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based solar cells than with silicon photovoltaics and other thinfilm solar cell technologies.

COPPER-INDIUM SELENIDE The materials based on CuInSe2 that are of interest for photovoltaic applications include several elements from groups I, III and VI in the periodic table. These semiconductors are especially attractive for thin film solar cell application because of their high optical absorption coefficients and versatile optical and electrical characteristics which can in principle be manipulated and tuned for a specific need in a given device. CIS is an abbreviation for general chalcopyrite films of copper indium selenide (CuInSe2), CIGS mentioned below is a variation of CIS. CIS films (no Ga) achieved greater than 14% efficiency. However, manufacturing costs of CIS solar cells at present are high when compared with amorphous silicon solar cells but continuing work is leading to more cost-effective production processes. The first large-scale production of CIS modules was started in 2006 in Germany by Wuerth Solar. When gallium is substituted for some of the indium in CIS, the material is sometimes called CIGS , or copper indium/gallium diselenide, a solid mixture of the semiconductors CuInSe2 and CuGaSe2, often abbreviated by the chemical formula CuInxGa(1x)Se2. Unlike the conventional silicon based solar cell, which can be modelled as a simple p-n junction (see under semiconductor), these cells are best described by a more complex heterojunction model. The best efficiency of a thin-film solar cell as of March 2008 was 19.9% with CIGS absorber layer. Higher efficiencies (around 30%) can be obtained by using optics to concentrate the incident light. The use of gallium increases the optical bandgap of the CIGS layer as compared to pure CIS, thus increasing the open-circuit voltage. In another point of view, gallium is added to replace as much indium as possible due to gallium's relative availability to indium. Approximately 70% of indium currently produced is used by the flat-screen monitor industry. Some investors in solar technology worry that production of CIGS cells will be limited by the availability of indium. Producing 2 GW of CIGS cells (roughly

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the amount of silicon cells produced in 2006) would use about 10% of the indium produced in 2004. For comparison, silicon solar cells used up 33% of the world's electronic grade silicon production in 2006. Nanosolar claims to waste only 5% of the indium it uses. As of 2006, the best conversion efficiency for flexible CIGS cells on polyimide is 14.1% by Tiwari et al, at the ETH, Switzerland.

germanium metal prices have risen substantially to $1000-$1200 per kg this year. Those materials include gallium (4N, 6N and 7N Ga), arsenic (4N, 6N and 7N) and germanium, pyrolitic boron nitride (pBN) crucibles for growing crystals, and boron oxide, these products are critical to the entire substrate manufacturing industry.

That being said, indium can easily be recycled from decommissioned PV modules. The recycling program in Germany is an example that highlights the regenerative industrial paradigm: "From cradle to cradle".

Triple-junction GaAs solar cells were also being used as the power source of the Dutch four-time World Solar Challenge winners Nuna in 2005 and 2007, and also by the Dutch solar cars Solutra (2005) and Twente One (2007).

Selenium allows for better uniformity across the layer and so the number of recombination sites in the film are reduced which benefits the quantum efficiency and thus the conversion efficiency.

GALLIUM ARSENIDE (GAAS) MULTIJUNCTION High-efficiency multijunction cells were originally developed for special applications such as satellites and space exploration, but at present, their use in terrestrial concentrators might be the lowest cost alternative in terms of $/kWh and $/W. These multijunction cells consist of multiple thin films produced using Metalorganic vapour phase epitaxy. A triple-junction cell, for example, may consist of the semiconductors: GaAs, Ge, and GaInP2. Each type of semiconductor will have a characteristic band gap energy which, loosely speaking, causes it to absorb light most efficiently at a certain color, or more precisely, to absorb electromagnetic radiation over a portion of the spectrum. The semiconductors are carefully chosen to absorb nearly all of the solar spectrum, thus generating electricity from as much of the solar energy as possible. GaAs based multijunction devices are the most efficient solar cells to date, reaching a record high of 40.7% efficiency under solar concentration and laboratory conditions. This technology is currently being utilized in the Mars rover missions. Tandem solar cells based on monolithic, series connected, gallium indium phosphide (GaInP), gallium arsenide GaAs, and germanium Ge pn junctions, are seeing demand rapidly rise. In just the past 12 months (12/2006 - 12/2007), the cost of 4N gallium metal has risen from about $350 per kg to $680 per kg. Additionally,

LIGHT-ABSORBING DYES (DSSC) Typically a ruthenium metalorganic dye (Ru-centered) is used as a monolayer of light-absorbing material. The dye-sensitized solar cell depends on a mesoporous layer of nanoparticulate titanium dioxide to greatly amplify the surface area (200-300 m²/ g TiO2, as compared to approximately 10 m²/g of flat single crystal). The photogenerated electrons from the light absorbing dye are passed on to the n-type TiO2, and the holes are passed to an electrolyte on the other side of the dye. The circuit is completed by a redox couple in the electrolyte, which can be liquid or solid. This type of cell allows a more flexible use of materials, and is typically manufactured by screen printing, with the potential for lower processing costs than those used for bulk solar cells. However, the dyes in these cells also suffer from degradation under heat and UV light, and the cell casing is difficult to seal due to the solvents used in assembly. In spite of the above, this is a popular emerging technology with some commercial impact forecast within this decade. ORGANIC/POLYMER SOLAR CELLS Organic solar cells and Polymer solar cells are built from thin films (typically 100 nm) of organic semiconductors such as polymers and small-molecule compounds like polyphenylene vinylene, copper phthalocyanine (a blue or green organic pigment) and carbon fullerenes. Energy conversion efficiencies achieved to date using conductive polymers are low compared to inorganic materials, with the highest reported efficiency of 6.5% for a tandem cell architecture. However, these cells could be beneficial

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for some applications where mechanical flexibility and disposability are important. These devices differ from inorganic semiconductor solar cells in that they do not rely on the large built in electric field of a PN junction to separate the electrons and holes created when photons are absorbed. The active region of an organic device consists of two materials, one which acts as an electron donor and the other as an acceptor. When a photon is converted into an electron hole pair, typically in the donor material, the charges tend to remain bound in the form of an exciton, and are separated when the exciton diffuses to the donor-acceptor interface. The short exciton diffusion lengths of most polymer systems tend to limit the efficiency of such devices. Nanostructured interfaces, sometimes in the form of bulk heterojunctions, can improve performance .

SILICON THIN FILMS Silicon thin-films are mainly deposited by chemical vapor deposition (typically plasma-enhanced (PE-CVD)) from silane gas and hydrogen gas. Depending on the deposition's parameters, this can yield: 1. Amorphous silicon (a-Si or a-Si:H) 2. Protocrystalline silicon or 3. Nanocrystalline silicon (nc-Si or nc-Si:H). These types of silicon present dangling and twisted bonds, which results in deep defects (energy levels in the bandgap) as well as deformation of the valence and conduction bands (band tails). The solar cells made from these materials tend to have lower energy conversion efficiency than bulk silicon, but are also less expensive to produce. The quantum efficiency of thin film solar cells is also lower due to reduced number of collected charge carriers per incident photon. Amorphous silicon has a higher bandgap (1.7 eV) than crystalline silicon (c-Si) (1.1 eV), which means it absorbs the visible part of the solar spectrum more strongly than the infrared portion of the spectrum. As nc-Si has about the same bandgap as c-Si, the two material can be combined in thin layers, creating a layered

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cell called a tandem cell. The top cell in a-Si absorbs the visible light and leaves the infrared part of the spectrum for the bottom cell in nanocrystalline Si. Recently, solutions to overcome the limitations of thin-film crystalline silicon have been developed. Light trapping schemes where the incoming light is obliquely coupled into the silicon and the light traverses the film several times enhance the absorption of sunlight in the films. Thermal processing techniques enhance the crystallinity of the silicon and pacify electronic defects. A silicon thin film technology is being developed for building integrated photovoltaics (BIPV) in the form of semi-transparent solar cells which can be applied as window glazing. These cells function as window tinting while generating electricity.

NANOCRYSTALLINE SOLAR CELLS These structures make use of some of the same thin-film light absorbing materials but are overlain as an extremely thin absorber on a supporting matrix of conductive polymer or mesoporous metal oxide having a very high surface area to increase internal reflections (and hence increase the probability of light absorption). Using nanocrystals allows one to design architectures on the length scale of nanometers, the typical exciton diffusion length. In particular, single-nanocrystal ('channel') devices, an array of single p-n junctions between the electrodes and separated by a period of about a diffusion length, represent a new architecture for solar cells and potentially high efficiency. CONCENTRATING PHOTOVOLTAICS (CPV) Concentrating photovoltaic systems use a large area of lenses or mirrors to focus sunlight on a small area of photovoltaic cells. If these systems use single or dual-axis tracking to improve performance, they may be referred to as Heliostat Concentrator Photovoltaics (HCPV). The primary attraction of CPV systems is their reduced usage of semiconducting material which is expensive and currently in short supply. Additionally, increasing the concentration ratio improves the performance of general photovoltaic materials. Despite the advantages of CPV technologies their application has been limited by the costs of focusing, tracking

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and cooling equipment. On October 25, 2006, the Australian federal government and the Victorian state government together with photovoltaic technology company Solar Systems announced a project using this technology, Solar power station in Victoria, planned to come online in 2008 and be completed by 2013. This plant, at 154 MW, would be ten times larger than the largest current photovoltaic plant in the world.

SILICON SOLAR CELL DEVICE MANUFACTURE Because solar cells are semiconductor devices, they share many of the same processing and manufacturing techniques as other semiconductor devices such as computer and memory chips. However, the stringent requirements for cleanliness and quality control of semiconductor fabrication are a little more relaxed for solar cells. Most large-scale commercial solar cell factories today make screen printed poly-crystalline silicon solar cells. Single crystalline wafers which are used in the semiconductor industry can be made into excellent high efficiency solar cells, but they are generally considered to be too expensive for large-scale mass production. Poly-crystalline silicon wafers are made by wiresawing block-cast silicon ingots into very thin (180 to 350 micrometer) slices or wafers. The wafers are usually lightly p-type doped. To make a solar cell from the wafer, a surface diffusion of n-type dopants is performed on the front side of the wafer. This forms a p-n junction a few hundred nanometers below the surface. Antireflection coatings, which increase the amount of light coupled into the solar cell, are typically next applied. Over the past decade, silicon nitride has gradually replaced titanium dioxide as the antireflection coating of choice because of its excellent surface passivation qualities (i.e., it prevents carrier recombination at the surface of the solar cell). It is typically applied in a layer several hundred nanometers thick using plasma-enhanced chemical vapor deposition (PECVD). Some solar cells have textured front surfaces that, like antireflection coatings, serve to increase the amount of light coupled into the cell. Such surfaces can usually only be formed on single-crystal silicon, though in recent years methods of forming them on multicrystalline silicon have been developed. The wafer then has a full area metal contact made on

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the back surface, and a grid-like metal contact made up of fine "fingers" and larger "busbars" are screen-printed onto the front surface using a silver paste. The rear contact is also formed by screen-printing a metal paste, typically aluminium. Usually this contact covers the entire rear side of the cell, though in some cell designs it is printed in a grid pattern. The paste is then fired at several hundred degrees Celsius to form metal electrodes in ohmic contact with the silicon. After the metal contacts are made, the solar cells are interconnected in series (and/or parallel) by flat wires or metal ribbons, and assembled into modules or "solar panels". Solar panels have a sheet of tempered glass on the front, and a polymer encapsulation on the back. Tempered glass cannot be used with amorphous silicon cells because of the high temperatures during the deposition process.

LIFESPAN A solar cell must be capable of producing electricity for at least twenty years, without a significant decrease in efficiency. LOW COST SOLAR CELLS This cell is extremely promising because it is made of lowcost materials and does not need elaborate apparatus to manufacture, so it can be made in a DIY way allowing more players to produce it than any other type of solar cell. In bulk it should be significantly less expensive than older solid-state cell designs. It can be engineered into flexible sheets. Although its conversion efficiency is less than the best thin film cells, its price/ performance ratio should be high enough to allow them to compete with fossil fuel electrical generation. CURRENT RESEARCH ON MATERIALS AND DEVICES There are currently many research groups active in the field of photovoltaics in universities and research institutions around the world. This research can be divided into three areas: making current technology solar cells cheaper and/or more efficient to effectively compete with other energy sources; developing new technologies based on new solar cell architectural designs; and developing new materials to serve as light absorbers and charge carriers.

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SILICON PROCESSING One way of reducing the cost is to develop cheaper methods of obtaining silicon that is sufficiently pure. Silicon is a very common element, but is normally bound in silica, or silica sand. Processing silica (SiO2) to produce silicon is a very high energy process - at current efficiencies, it takes over two years for a conventional solar cell to generate as much energy as was used to make the silicon it contains. More energy efficient methods of synthesis are not only beneficial to the solar industry, but also to industries surrounding silicon technology as a whole. The current industrial production of silicon is via the reaction between carbon (charcoal) and silica at a temperature around 1700 degrees Celsius. In this process, known as carbothermic reduction, each tonne of silicon (metallurgical grade, about 98% pure) is produced with the emission of about 1.5 tonnes of carbon dioxide. Solid silica can be directly converted (reduced) to pure silicon by electrolysis in a molten salt bath at a fairly mild temperature (800 to 900 degrees Celsius). While this new process is in principle the same as the FFC Cambridge Process which was first discovered in late 1996, the interesting laboratory finding is that such electrolytic silicon is in the form of porous silicon which turns readily into a fine powder, (with a particle size of a few micrometres), and may therefore offer new opportunities for development of solar cell technologies. Another approach is also to reduce the amount of silicon used and thus cost, is by micromachining wafers into very thin, virtually transparent layers that could be used as transparent architectural coverings. . The technique involves taking a silicon wafer, typically 1 to 2 mm thick, and making a multitude of parallel, transverse slices across the wafer, creating a large number of slivers that have a thickness of 50 micrometres and a width equal to the thickness of the original wafer. These slices are rotated 90 degrees, so that the surfaces corresponding to the faces of the original wafer become the edges of the slivers. The result is to convert, for example, a 150 mm diameter, 2 mm-thick wafer having an exposed silicon surface area of about 175 cm² per side into about 1000 slivers having dimensions of 100

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mm x 2 mm x 0.1 mm, yielding a total exposed silicon surface area of about 2000 cm² per side. As a result of this rotation, the electrical doping and contacts that were on the face of the wafer are located the edges of the sliver, rather than the front and rear as is the case with conventional wafer cells. This has the interesting effect of making the cell sensitive from both the front and rear of the cell (a property known as bifaciality). Using this technique, one silicon wafer is enough to build a 140 watt panel, compared to about 60 wafers needed for conventional modules of same power output.

THIN-FILM PROCESSING Thin-film solar cells use less than 1% of the raw material (silicon or other light absorbers) compared to wafer based solar cells, leading to a significant price drop per kWh. There are many research groups around the world actively researching different thin-film approaches and/or materials, however it remains to be seenvague if these solutions can generate the same space-efficiency as traditional silicon processing. One particularly promising technology is crystalline silicon thin films on glass substrates. This technology makes use of the advantages of crystalline silicon as a solar cell material, with the cost savings of using a thin-film approach. Another interesting aspect of thin-film solar cells is the possibility to deposit the cells on all kind of materials, including flexible substrates (PET for example), which opens a new dimension for new applications. One of the R&D Magazine's prestigious R&D 100 Awards also called the "Oscars of Invention"- for 2008, has gone to National Renewable Energy Laboratory Hybrid CGIS (or Thin-Film Photovoltaic Manufacturing Process) - hybrid CIGS cells that are manufactured in layers by using ink-jet and ultrasonic technology to precisely apply metal-organic inks in separate layers directly into common building materials such as metal and glass . METAMORPHIC MULTIJUNCTION SOLAR CELL The National Renewable Energy Laboratory won another R&D Magazine's R&D 100 Awards for its Metamorphic Multijunction Solar Cell, an ultra-light and flexible cell that converts solar energy

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with record efficiency . The ultra-light, highly efficient solar cell was developed at NREL and is being commercialized by Emcore Corp. of Albuquerque, N.M., in partnership with the Air Force Research Laboratories Space Vehicles Directorate at Kirtland Air Force Base in Albuquerque. It represents a new class of solar cells with clear advantages in performance, engineering design, operation and cost. For decades, conventional cells have featured wafers of semiconducting materials with similar crystalline structure. Their performance and cost effectiveness is constrained by growing the cells in an upright configuration. Meanwhile, the cells are rigid, heavy and thick with a bottom layer made of germanium. In the new method, the cell is grown upside down. These layers use high-energy materials with extremely high quality crystals, especially in the upper layers of the cell where most of the power is produced. Not all of the layers follow the lattice pattern of even atomic spacing. Instead, the cell includes a full range of atomic spacing, which allows for greater absorption and use of sunlight. The thick, rigid germanium layer is removed, reducing the cell's cost and 94% of its weight. By turning the conventional approach to cells on its head, the result is an ultralight and flexible cell that also converts solar energy with record efficiency (40.8 percent under 326 suns concentration).

POLYMER PROCESSING The invention of conductive polymers (for which Alan Heeger, Alan G. MacDiarmid and Hideki Shirakawa were awarded a Nobel prize) may lead to the development of much cheaper cells that are based on inexpensive plastics. However, all organic solar cells made to date suffer from degradation upon exposure to UV light, and hence have lifetimes which are far too short to be viable. The conjugated double bond systems in the polymers, which carry the charge, are always susceptible to breaking up when radiated with shorter wavelengths. Additionally, most conductive polymers, being highly unsaturated and reactive, are highly sensitive to atmospheric moisture and oxidation, making commercial applications difficult.

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NANOPARTICLE PROCESSING Experimental non-silicon solar panels can be made of quantum heterostructures, eg. carbon nanotubes or quantum dots, embedded in conductive polymers or mesoporous metal oxides. In addition, thin films of many of these materials on conventional silicon solar cells can increase the optical coupling efficiency into the silicon cell, thus boosting the overall efficiency. By varying the size of the quantum dots, the cells can be tuned to absorb different wavelengths. Although the research is still in its infancy, quantum dot-modified photovoltaics may be able to achieve up to 42 percent energy conversion efficiency due to multiple exciton generation(MEG). Researchers at the University of California, San Diego have come up with a way of making solar photovoltaic cells more efficient by making them fuzzy with indium phosphide nanowires. It sounds similar to a project announced by a consortium of German universities, working in concert with Harvard University Science department. TRANSPARENT CONDUCTORS Many new solar cells use transparent thin films that are also conductors of electrical charge. The dominant conductive thin films used in research now are transparent conductive oxides (abbreviated "TCO"), and include fluorine-doped tin oxide (SnO2:F, or "FTO"), doped zinc oxide (e.g.: ZnO:Al), and indium tin oxide (abbreviated "ITO"). These conductive films are also used in the LCD industry for flat panel displays. The dual function of a TCO allows light to pass through a substrate window to the active light absorbing material beneath, and also serves as an ohmic contact to transport photogenerated charge carriers away from that light absorbing material. The present TCO materials are effective for research, but perhaps are not yet optimized for large-scale photovoltaic production. They require very special deposition conditions at high vacuum, they can sometimes suffer from poor mechanical strength, and most have poor transmittance in the infrared portion of the spectrum (e.g.: ITO thin films can also be used as infrared filters in airplane windows). These factors make large-scale manufacturing more costly.

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A relatively new area has emerged using carbon nanotube networks as a transparent conductor for organic solar cells. Nanotube networks are flexible and can be deposited on surfaces a variety of ways. With some treatment, nanotube films can be highly transparent in the infrared, possibly enabling efficient low bandgap solar cells. Nanotube networks are p-type conductors, whereas traditional transparent conductors are exclusively n-type. The availability of a p-type transparent conductor could lead to new cell designs that simplify manufacturing and improve efficiency.

SILICON WAFER BASED SOLAR CELLS Despite the numerous attempts at making better solar cells by using new and exotic materials, the reality is that the photovoltaics market is still dominated by silicon wafer-based solar cells (firstgeneration solar cells). This means that most solar cell manufacturers are equipped to produce these type of solar cells. Therefore, a large body of research is currently being done all over the world to create silicon wafer-based solar cells that can achieve higher conversion efficiency without an exorbitant increase in production cost. The aim of the research is to achieve the lowest cost per watt solar cell design that is suitable for commercial production. IBM has a semiconductor wafer reclamation process that uses a specialized pattern removal technique to repurpose scrap semiconductor wafers to a form used to manufacture siliconbased solar panels. The new process was recently awarded the "2007 Most Valuable Pollution Prevention Award" from The National Pollution Prevention Roundtable (NPPR). MANUFACTURERS Solar cells are manufactured primarily in Japan, China, Germany, Taiwan and the USA , though numerous other nations have or are acquiring significant solar cell production capacity. While technologies are constantly evolving toward higher efficiencies, the most effective cells for low cost electrical production are not necessarily those with the highest efficiency, but those with a balance between low-cost production and efficiency high enough to minimize area-related balance of systems cost. Those

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companies with large scale manufacturing technology for coating inexpensive substrates may, in fact, ultimately be the lowest cost net electricity producers, even with cell efficiencies that are lower than those of single-crystal technologies.

ENERGY DEVELOPMENT Higher electricity use per capita correlates with a higher score on the Human Development Index (1997). Developing nations score much lower on these variables than developed nations. The continued rapid economic growth and increase in living standards in developing nations with large populations, like China and India, is dependent on a rapid and large expansion of energy production capacity. Energy development is the ongoing effort to provide sufficient primary energy sources and secondary energy forms to power the world economy. It involves both installation of established technologies and research and development to create new energyrelated technologies. Major considerations in energy planning include cost, impact on air pollution, and whether or not the source is renewable. SUSTAINABILITY The environmental movement emphasizes sustainability of energy use and development. Renewable energy is sustainable in its production; the available supply will not be diminished for the foreseeable future - millions or billions of years. "Sustainability" also refers to the ability of the environment to cope with waste products, especially air pollution. Sources which have no direct waste products (such as wind, solar, and hydropower) are seen as ideal in this regard. The status of nuclear power is controversial. The supply of usable uranium might last a very long time, with an almost unlimited supply of sea water uranium available once ground based mining is exhausted, but nuclear waste must be stored in a shielded location for hundreds or thousands of years without investment in new reactor designs. Fossil fuels such as petroleum, coal, and natural gas are not renewable. For example, the timing of worldwide peak oil

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production is being actively debated, but it has already happened in some countries. Fossil fuels also make up the bulk of the world's current primary energy sources. With global demand for energy growing, the need to adopt alternative energy sources is also growing. Fossil fuels are also a major source of greenhouse gas emissions, leading to concerns about global warming if consumption is not reduced. Energy conservation is an alternative or complementary process to energy development. It reduces the demand for energy by using it more efficiently.

DEPENDENCE ON EXTERNAL ENERGY SOURCES AND ENERGY RESILIENCE Technologically advanced societies have become increasingly dependent on external energy sources for transportation, the production of many manufactured goods, and the delivery of energy services. This energy allows people, in general, to live under otherwise unfavorable climatic conditions through the use of heating, ventilation, and/or air conditioning. Level of use of external energy sources differs across societies, as do the climate, convenience, traffic congestion, pollution, production, and greenhouse gas emissions of each society. Increased levels of human comfort generally induce increased dependence on external energy sources, although the application of energy efficiency and conservation approaches allows a certain degree of mitigation of the dependence. Wise energy use therefore embodies the idea of balancing human comfort with reasonable energy consumption levels by researching and implementing effective and sustainable energy harvesting and utilization measures. ENERGY RESILIENCE Andy Grove argues that energy independence is a flawed and infeasible objective, particularly in a network of integrated global exchange. He suggests instead that the objective should be energy resilience: resilience goes hand in hand with adaptability, and it also is reflected in important market ideas like substitutability. In fact, resilience is one of the best features of market processes; the information transmission function of prices means that individual

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buyers and sellers can adapt to changes in supply and demand conditions in a decentralized way. His suggestion for how to increase the resilience of the U.S. energy economy is to shift use from petroleum to electricity (electrification), that is sticky and can be produced using multiple sources of energy, including renewables.

PRIMARY ENERGY SOURCES Primary energy sources are substances or processes with concentrations of energy at a high enough potential to be feasibly encouraged to convert to lower energy forms under human control for human benefit. Except for nuclear fuels, tidal energy and geothermal energy, all terrestrial energy sources are from current solar insolation or from fossil remains of plant and animal life that relied directly and indirectly upon sunlight, respectively. And ultimately, solar energy itself is the result of the Sun's nuclear fusion. Geothermal power from hot, hardened rock above the magma of the earth's core is the result of the accumulation of radioactive materials during the formation of Earth which was the byproduct of a previous supernova event. FOSSIL FUELS Fossil fuels, in terms of energy, involve the burning of coal or hydrocarbon fuels, which are the remains of the decomposition of plants and animals. There are three main types of fossil fuels: coal, petroleum, and natural gas. Another fossil fuel, liquefied petroleum gas (LPG), is principally derived from the production of natural gas. Heat from burning fossil fuel is used either directly for space heating and process heating, or converted to mechanical energy for vehicles, industrial processes, or electrical power generation. PROS • The technology and infrastructure already exist for the use of fossil fuels. • Commonly-used fossil fuels in liquid form such as light crude oil, gasoline, and LPG are easy to distribute. • Petroleum energy density in terms of volume (cubic space) and mass (weight) is superior to some alternative energy

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sources (or energy storage devices, like a battery (electricity)). Energy density is important in land-and-air transportation fuel tanks.

CONS • Petroleum-powered vehicles are very inefficient. Only about 15% of the energy from the fuel they consume is converted into useful motion. The rest of the fuel-source energy is inefficiently expended as waste heat. The heat and gaseous pollution emissions harm our environment. • The inefficient atmospheric combustion (burning) of fossil fuels in vehicles, buildings, and power plants contributes to urban heat islands. • The combustion of fossil fuels leads to the release of pollution into the atmosphere. According to the Union of Concerned Scientists, a typical coal plant produces in one year: o 3,700,000 tons of carbon dioxide (CO2), could be the primary cause of global warming. o 10,000 tons of sulfur dioxide (SO2), the leading cause of acid rain. o 500 tons of small airborne particles, which result in chronic bronchitis, aggravated asthma, and premature death, in addition to haze-obstructed visibility. o 10,200 tons of nitrogen oxides (NOx), (from hightemperature atmospheric combustion), leading to formation of ozone (smog) which inflames the lungs, burning lung tissue making people more susceptible to respiratory illness. o 720 tons of carbon monoxide (CO), resulting in headaches and additional stress on people with heart disease. o 220 tons of hydrocarbons, toxic volatile organic compounds (VOC), which form ozone. o 170 pounds (77 kg) of mercury, where just 1/70th of a teaspoon deposited on a 25 acre lake can make the fish unsafe to eat.

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o 225 pounds (102 kg) of arsenic, which will cause cancer in one out of 100 people who drink water containing 50 parts per billion. o 114 pounds (52 kg) of lead, 4 pounds (1.8 kg) of cadmium, other toxic heavy metals, and trace amounts of uranium. Dependence on fossil fuels from volatile regions or countries creates energy security risks for dependent countries. Oil dependence in particular has led to war, major funding of radical terrorists, monopolization, and socio-political instability. Fossil fuels are non-renewable, un-sustainable resources, which will eventually decline in production and become exhausted, with dire consequences to societies that remain highly dependent on them. (Fossil fuels are actually slowly forming continuously, but we are using them up at a rate approximately 100,000 times faster than they are formed.) The Moss Landing Power Plant burns natural gas to produce electricity in California. Extracting fossil fuels is becoming more difficult as we consume the most accessible fuel deposits. Extraction of fossil fuels is becoming more expensive and more dangerous as mines get deeper and oil rigs must drill deeper, and go further out to sea. Extraction of fossil fuels results in extensive environmental degradation, such as the strip mining and mountaintop removal of coal.

Since these power plants are thermal engines, and are typically quite large, waste heat disposal becomes an issue at high ambient temperature. Thus, at a time of peak demand, a power plant may need to be shut down or operate at a reduced power level, as sometimes do nuclear power plants, for the same reasons. NUCLEAR ENERGY The status of nuclear power globally. Nations in dark green have reactors and are constructing new reactors, those in light green are constructing their first reactor, those in dark yellow are considering new reactors, those in light yellow are considering

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their first reactor, those in blue have reactors but are not constructing or decommissioning, those in light blue are considering decommissioning and those in red have decommissioned all their commercial reactors. Brown indicates that the country has declared itself free of nuclear power and weapons.

NUCLEAR FISSION Nuclear power stations use nuclear fission to generate energy by the reaction of uranium-235 inside a nuclear reactor. The reactor uses uranium rods, the atoms of which are split in the process of fission, releasing a large amount of energy. The process continues as a chain reaction with other nuclei. The heat released, heats water to create steam, which spins a turbine generator, producing electricity. Depending on the type of fission fuel considered, estimates for existing supply at known usage rates varies from several decades for the currently popular Uranium-235 to thousands of years for uranium-238. At the present use rate, there are (as of 2007) about 70 years left of known uranium-235 reserves economically recoverable at a uranium price of US$ 130/kg. The nuclear industry argue that the cost of fuel is a minor cost factor for fission power, more expensive, more difficult to extract sources of uranium could be used in the future, such as lower-grade ores, and if prices increased enough, from sources such as granite and seawater. Increasing the price of uranium would have little effect on the overall cost of nuclear power; a doubling in the cost of natural uranium would increase the total cost of nuclear power by 5 percent. On the other hand, if the price of natural gas was doubled, the cost of gas-fired power would increase by about 60 percent. Opponents on the other hand argue that the correlation between price and production is not linear, but as the ores' concentration becomes smaller, the difficulty (energy and resource consumption are increasing, while the yields are decreasing) of extraction rises very fast, and that the assertion that a higher price will yield more uranium is overly optimistic; for example a rough estimate predicts that the extraction of uranium from granite will

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consume at least 70 times more energy than what it will produce in a reactor. As many as eleven countries have depleted their uranium resources, and only Canada has mines left which produce better than 1% concentration ore. Seawater seems to be equally dubious as a source. As a consequence an eventual doubling in the price of uranium will give a marginal increase in the volumes that are being produced. Another alternative would be to use thorium as fission fuel. Thorium is three times more abundant in Earth's crust than uranium, and much more of the thorium can be used (or, more precisely, bred into Uranium-233, reprocessed and then used as fuel). India has around 32 percent of the world's reserves of thorium and intends on using it for itself because the country has run out of uranium. Current light water reactors burn the nuclear fuel poorly, leading to energy waste. Nuclear reprocessing or burning the fuel better using different reactor designs would reduce the amount of waste material generated and allow better use of the available resources. As opposed to current light water reactors which use uranium-235 (0.7 percent of all natural uranium), fast breeder reactors convert the more abundant uranium-238 (99.3 percent of all natural uranium) into plutonium for fuel. It has been estimated that there is anywhere from 10,000 to five billion years worth of Uranium-238 for use in these power plants. Fast breeder technology has been used in several reactors. However, the fast breeder reactors at Dounreay in Scotland, Monju in Japan and the Superphénix at Creys-Malville in France, in particular, have all had difficulties and were not economically competitive and most have been decommissioned. The People's Republic of China intends to build breeders. India has run out of uranium and is building thermal breeders that can convert Th-232 into U-233 and burn it. Some nuclear engineers think that pebble bed reactors, in which each nuclear fuel pellet is coated with a ceramic coating, are inherently safe and are the best solution for nuclear power. They can also be configured to produce hydrogen for hydrogen vehicles. China has plans to build pebble bed reactors configured to produce hydrogen. The possibility of nuclear meltdowns and other reactor accidents, such as the Three Mile Island accident and

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the Chernobyl disaster, have caused much public fear. Research is being done to lessen the known problems of current reactor technology by developing automated and passively-safe reactors. Historically, however, coal and hydropower power generation have both been the cause of more deaths per energy unit produced than nuclear power generation. Various kinds of energy infrastructure might be attacked by terrorists, including nuclear power plants, hydropower plants, and liquified natural gas tankers. Nuclear proliferation is the spread from nation to nation of nuclear technology, including nuclear power plants but especially nuclear weapons. New technology like SSTAR ("small, sealed, transportable, autonomous reactor") may lessen this risk. The long-term radioactive waste storage problems of nuclear power have not been fully solved. Several countries have considered using underground repositories. Nuclear waste takes up little space compared to wastes from the chemical industry which remain toxic indefinitely. Spent fuel rods are now stored in concrete casks close to the nuclear reactors. The amounts of waste could be reduced in several ways. Both nuclear reprocessing and fast breeder reactors could reduce the amounts of waste. Subcritical reactors or fusion reactors could greatly reduce the time the waste has to be stored. Subcritical reactors may also be able to do the same to already existing waste. The only way of dealing with waste today is by geological storage. The economics of nuclear power is not simple to evaluate, because of high capital costs for building and very low fuel costs. Comparison with other power generation methods is strongly dependent on assumptions about construction timescales and capital financing for nuclear plants. See Economics of new nuclear power plants. Depending on the source different energy return on energy investment (EROI) are claimed. Advocates (using life cycle analysis) argue that it takes 4-5 months of energy production from the nuclear plant to fully pay back the initial energy investment. Opponents claim that it depends on the grades of the ores the fuel came from, so a full payback can vary from 10 to 18 years, and that the advocates' claim was based on the assumption of high grade ores (the yields are getting worst, as the ores are leaner, for

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less than 0.02% ores, the yield is less then 50%). Advocates also claim that it is possible to relatively rapidly increase the number of plants. Typical new reactor designs have a construction time of three to four years. In 1983, 43 plants were being built, before an unexpected fall in fossil fuel prices stopped most new construction. Developing countries like India and China are rapidly increasing their nuclear energy use. However, a Council on Foreign Relations report on nuclear energy argues that a rapid expansion of nuclear power may create shortages in building materials such as reactor-quality concrete and steel, skilled workers and engineers, and safety controls by skilled inspectors. This would drive up current prices. On the other hand, in stark contrast to the claims of the nuclear industry and its talk of a renaissance, nuclear energy is in decline, according to a report 'World Nuclear Industry Status Report 2007' presented by the Greens/EFA group in the European Parliament. The report outlines that the proportion of nuclear energy in power production has decreased in 21 out of 31 countries, with five less functioning nuclear reactors than five years ago. There are currently 32 nuclear power plants under construction or in the pipeline, 20 fewer than at the end of the 1990s.

Pros • The energy content of a kilogram of uranium or thorium, if spent nuclear fuel is reprocessed and fully utilized, is equivalent to about 3.5 million kilograms of coal. • The cost of making nuclear power, with current legislation, is about the same as making coal power, which is considered very inexpensive (see Economics of new nuclear power plants). If a carbon tax is applied, nuclear does not have to pay anything because nuclear does not emit toxic gases such as CO2, NO, CO, SO2, arsenic, etc. that are emitted by coal power plants. • Nuclear power plants are guarded with the nuclear reactor inside a reinforced containment building, and thus are relatively impervious to terrorist attack or adverse weather conditions (see Nuclear safety in the U.S.).

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Solar Energy and its Uses • Because of the fear of a nuclear disaster, nuclear safety has become a major issue. • Nuclear power does not produce any primary air pollution or release carbon dioxide and sulfur dioxide into the atmosphere. Therefore, it contributes only a small amount to global warming or acid rain. • Coal mining is the second most dangerous occupation in the United States. Nuclear energy is much safer per capita than coal derived energy. • For the same amount of electricity, the life cycle emissions of nuclear is about 4% of coal power. Depending on the report, hydro, wind, and geothermal are sometimes ranked lower, while wind and hydro are sometimes ranked higher (by life cycle emissions). • According to a Stanford study, fast breeder reactors have the potential to power humans on earth for billions of years, making it sustainable.

Cons • The improper operation of a badly designed nuclear reactor with no containment vessel near human settlements can be catastrophic in the event of an uncontrolled power increase in the reactor, as shown by the Chernobyl disaster in the Ukraine (former USSR), where large areas of Europe were affected by moderate radioactive contamination and the parts of the Ukraine and one fifth of Belarus continue today to be affected by radioactive fallout as of 2008. • The human, environmental, and economic costs from a successful terrorist attack on a nuclear power reactor that results in the release of substantial quantities of radioactive material to the environment could be great. • Waste produced from nuclear fission of uranium is both poisonous and highly radioactive, requiring maintenance and monitoring at the storage sites. However, if nuclear fuel is reprocessed, the separated radioactive fission product waste will decay to such a level of radioactivity in 300-500 years.

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• There can be connections between nuclear power and nuclear weapon proliferation, since many reactor designs require large-scale uranium enrichment facilities. • The limited liability for the owner of a nuclear power plant in case of a nuclear accident differs per nation while nuclear installations are sometimes built close to national borders. • Since nuclear power plants are typically quite large power plants, and are, fundamentally, thermal engines, waste heat disposal becomes an issue at high ambient temperature. Thus, at a time of peak demand, a power reactor may need to be shut down or operate at a reduced power level, as do large coal-fired plants, for the same reasons.

NUCLEAR FUSION Fusion power could solve many of the problems of fission power (the technology mentioned above) but, despite research having started in the 1950s, no commercial fusion reactor is expected before 2050. Many technical problems remain unsolved. Proposed fusion reactors commonly use deuterium, an isotope of hydrogen, as fuel and in most current designs also lithium. Assuming a fusion energy output equal to the current global output and that this does not increase in the future, then the known current lithium reserves would last 3000 years, lithium from sea water would last 60 million years, and a more complicated fusion process using only deuterium from sea water would have fuel for 150 billion years. RENEWABLE SOURCES Renewable energy is the alternative to fossil fuels and nuclear power.

BIOMASS, BIOFUELS, AND VEGETABLE OIL Biomass production involves using garbage or other renewable resources such as corn or other vegetation to generate electricity. When garbage decomposes, the methane produced is captured in pipes and later burned to produce electricity. Vegetation and

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• Algaculture has the potential to produce far more vegetable oil per acre than current plants. • Infrastructure for biodiesel around the World is significant and growing.

wood can be burned directly to generate energy, like fossil fuels, or processed to form alcohols. Vegetable oil is generated from sunlight and CO2 by plants. It is safer to use and store than gasoline or diesel as it has a higher flash point. Straight vegetable oil works in diesel engines if it is heated first. Vegetable oil can also be transesterified to make biodiesel, which burns like normal diesel.

Pros • Biomass production can be used to burn organic waste products resulting from agriculture. This type of recycling encourages the philosophy that nothing on this Earth should be wasted. The result is less demand on the Earth's resources, and a higher carrying capacity for Earth because non-renewable fossil fuels are not consumed. • Biomass is abundant on Earth and is generally renewable. In theory, we will never run out of organic waste products as fuel, because we are continuously producing them. In addition, biomass is found throughout the world, a fact that should alleviate energy pressures in third world nations. • When methods of biomass production other than direct combustion of plant mass are used, such as fermentation and pyrolysis, there is little effect on the environment. Alcohols and other fuels produced by these alternative methods are clean burning and are feasible replacements to fossil fuels. • Since CO2 is first taken out of the atmosphere to make the vegetable oil and then put back after it is burned in the engine, there is no net increase in CO2. • Vegetable oil has a higher flash point and therefore is safer than most fossil fuels. • Transitioning to vegetable oil could be relatively easy as biodiesel works where diesel works, and straight vegetable oil takes relatively minor modifications. • The World already produces more than 100 billion gallons a year for food industry, so we have experience making it.

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Cons • Direct combustion of any carbon-based fuel leads to air pollution similar to that from fossil fuels. • Some researchers claim that when biomass crops are the product of intensive farming, ethanol fuel production results in a net loss of energy after one accounts for the fuel costs of petroleum and natural-gas fertilizer production, farm equipment, and the distillation process. • There is a long list of reasons why even non-food based cellulosic ethanol cannot solve our energy crisis or global warming problems. • Direct competition with land use for food production and water use. • Current production methods would require enormous amounts of land to replace all gasoline and diesel. With current technology, it is not feasible for biofuels to replace the demand for petroleum. • Even with the most-optimistic current energy return on investment claims, in order to use 100% solar energy to grow corn and produce ethanol (fueling machinery with ethanol, distilling with heat from burning crop residues, using NO fossil fuels at all), the consumption of ethanol to replace only the current U.S. petroleum use would require three quarters of all the cultivated land on the face of the Earth. GEOTHERMAL ENERGY Geothermal energy harnesses the heat energy present underneath the Earth. Two wells are drilled. One well injects water into the ground to provide water. The hot rocks heat the water to produce steam. The steam that shoots back up the other hole(s) is purified and is used to drive turbines, which power electric generators. When the water temperature is below the

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boiling point of water a binary system is used. A low boiling point liquid is used to drive a turbine and generator in a closed system similar to a refrigeration unit running in reverse.

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Geothermal energy is base load power. Economically feasible in high grade areas now. Low deployment costs. Geothermal power plants have a high capacity factor; they run continuously day and night with an uptime typically exceeding 95%. Once a geothermal power station is implemented, the energy produced from the station is practically free. A small amount of energy is required in order to run a pump, although this pump can be powered by excess energy generated at the plant. Geothermal power stations are relatively small, and have a lesser impact on the environment than tidal or hydroelectric plants. Because geothermal technology does not rely on large bodies of water, but rather, small, but powerful jets of water, like geysers, large generating stations can be avoided without losing functionality. Geothermal is now feasible in areas where the earth's crust is thicker. Using enhanced geothermal technology, it's possible to drill deeper and inject water to generate geothermal power. Geothermal energy does not produce air or water pollution if performed correctly.

Cons • Geothermal power extracts small amounts of minerals such as sulfur that are removed prior to feeding the turbine and re-injecting the water back into the injection well. • Geothermal power requires locations that have suitable subterranean temperatures within 5km of surface. • Some geothermal stations have created geological instability, even causing earthquakes strong enough to damage buildings.

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HYDROELECTRIC ENERGY In hydro energy, the gravitational descent of a river is compressed from a long run to a single location with a dam or a flume. This creates a location where concentrated pressure and flow can be used to turn turbines or water wheels, which drive a mechanical mill or an electric generator.

Pros • Hydroelectric power stations can promptly increase to full capacity, unlike other types of power stations. This is because water can be accumulated above the dam and released to coincide with peak demand. • Electricity can be generated constantly, so long as sufficient water is available. • Hydroelectric power produces no primary waste or pollution. • Hydropower is a renewable resource. • Hydroelectricity assists in securing a country's access to energy supplies. • Much hydroelectric capacity is still undeveloped, such as in Africa.

Cons • The construction of a dam can have a serious environmental impact on the surrounding areas. The amount and the quality of water downstream can be affected, which affects plant life both aquatic, and land-based. Because a river valley is being flooded, the local habitat of many species are destroyed, while people living nearby may have to relocate their homes. • Hydroelectricity can only be used in areas where there is a sufficient supply of water. • Flooding submerges large forests (if they have not been harvested). The resulting anaerobic decomposition of the carboniferous materials releases methane, a greenhouse gas.

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• Dams can contain huge amounts of water. As with every energy storage system, failure of containment can lead to catastrophic results, e.g. flooding. • Hydroelectric plants rarely can be erected near load centers, requiring long transmission lines. • Global warming is causing reduced rainfall in some regions, reducing the available water in dammed reservoirs (such as Lake Powell in the Southwestern United States).

• Solar power generation releases no water or air pollution, because there is no combustion of fuels. • In sunny countries, solar power can be used in remote locations, like a wind turbine. This way, isolated places can receive electricity, when there is no way to connect to the power lines from a plant. • Solar energy can be used very efficiently for heating (solar ovens, solar water and home heaters) and daylighting. • Coincidentally, solar energy is abundant in regions that have the largest number of people living off grid - in developing regions of Africa, Indian subcontinent and Latin America. Hence cheap solar, when available, opens the opportunity to enhance global electricity access considerably, and possibly in a relatively short time period. • Photovoltaic systems are subsidized, up to $5 USD per watt in some American states. • Passive solar building design and zero energy buildings are demonstrating significant energy bill reduction, and some are cost-effectively off the grid. • Photovoltaic equipment cost has been steadily falling, the production capacity is rapidly rising, and the U.S. Administration expects its Solar America Initiative to help make amortized PV electricity price competitive for the new generation of zero energy buildings. • Distributed point-of-use photovoltaic systems eliminate expensive long-distance electric power transmission losses. • Photovoltaics are much more efficient in their conversion of solar energy to usable energy than biofuel from plant materials.

SOLAR POWER Solar power involves using solar cells to convert sunlight into electricity, using sunlight hitting solar thermal panels to convert sunlight to heat water or air, using sunlight hitting a parabolic mirror to heat water (producing steam), or using sunlight entering windows for passive solar heating of a building. It would be advantageous to place solar panels in the regions of highest solar radiation. In the Phoenix, Arizona area, for example, the average annual solar radiation is 5.7 kWh/m²/day, or 2080.5 kWh/m²/ year. Electricity demand in the continental U.S. is 3.7*1012 kW·h per year. Thus, at 100% efficiency, an area of 1.8x10^9 sq. m (around 700 square miles) would need to be covered with solar panels to replace all current electricity production in the US with solar power, and at 20% efficiency, an area of approximately 3500 square miles (3% of Arizona's land area). The average solar radiation in the United States is 4.8 kwh/m²/day, but reaches 89 kWh/m²/day in parts of Southwest. The cost, assuming $500/meter², would be about $5-10 trillion dollars. China is aggressively more-than-doubling worldwide silicon wafer capacity for photovoltaics to 2,000 metric tons by July 2008, and over 6,000 metric tons by the end of 2010. Significant international investment capital is flowing into China to support this opportunity. China is building large subsidized off-the-grid solar-powered cities in Huangbaiyu and Dongtan Eco City. Much of the design was done by Americans such as William McDonough.

Pros • Solar power imparts no fuel costs. • Solar power is a renewable resource. As long as the Sun exists, its energy will reach Earth.

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Cons • Solar electricity is currently more expensive than grid electricity. • Solar heat and electricity are not available at night and may be unavailable due to weather conditions; therefore, a storage or complementary power system is required for off-the-grid applications.

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Solar Energy and its Uses • Limited energy density: Average daily insolation in the contiguous U.S. is 3-7 kW·h/m². • Solar cells produce DC which must be converted to AC (using a grid tie inverter) when used in currently existing distribution grids. This incurs an energy loss of 4-12%. • A photovoltaic power station is expensive to build, and the energy payback time - the time necessary for producing the same amount of energy as needed for building the power device - for photovoltaic cells is about 1-5 years, depending primarily on location. • Solar panels collect dust and require cleaning. Dust on the panels significantly reduces the transfer of energy from solar radiation to electric current.

TIDAL POWER GENERATION Tidal power can be extracted from Moon-gravity-powered tides by locating a water turbine in a tidal current, or by building impoundment pond dams that admit-or-release water through a turbine. The turbine can turn an electrical generator, or a gas compressor, that can then store energy until needed. Coastal tides are a source of clean, free, renewable, and sustainable energy. WIND POWER This type of energy harnesses the power of the wind to propel the blades of wind turbines. These turbines cause the rotation of magnets, which creates electricity. Wind towers are usually built together on wind farms.

Pros • Wind power produces no water or air pollution that can contaminate the environment, because there are no chemical processes involved in wind power generation. Hence, there are no waste by-products, such as carbon dioxide. • Power from the wind does not contribute to global warming because it does not generate greenhouse gases. • Wind generation is a renewable source of energy, which means that we will never run out of it.

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• Wind towers can be beneficial for people living permanently, or temporarily, in remote areas. It may be difficult to transport electricity through wires from a power plant to a far-away location and thus, wind towers can be set up at the remote setting. • Farming and grazing can still take place on land occupied by wind turbines. • Those utilizing wind power in a grid-tie configuration will have backup power in the event of a power outage. • Due to the ability of wind turbines to coexist within agricultural fields, siting costs are frequently low.

Cons • Wind is unpredictable; therefore, wind power is not predictably available. When the wind speed decreases less electricity is generated. This makes wind power unsuitable for base load generation. • Wind farms may be challenged in communities that consider them an eyesore or view obstructor. • Wind farms, depending on the location and type of turbine, may negatively affect bird migration patterns and may pose a danger to the birds themselves. Newer, larger wind turbines have slower moving blades which are visible to birds.

INCREASED EFFICIENCY IN ENERGY USE Efficiency is increasing by about 2% a year, and absorbs most of the requirements for energy development. New technology makes better use of already available energy through improved efficiency, such as more efficient fluorescent lamps, engines, and insulation. Using heat exchangers, it is possible to recover some of the energy in waste warm water and air, for example to preheat incoming fresh water. Hydrocarbon fuel production from pyrolysis could also be in this category, allowing recovery of some of the energy in hydrocarbon waste. Meat production is energy inefficient compared to the production of protein sources like soybean or Quorn. Already existing power plants often can and usually are made more efficient with minor modifications due to new

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technology. New power plants may become more efficient with technology like cogeneration. New designs for buildings may incorporate techniques like passive solar. Light-emitting diodes are gradually replacing the remaining uses of light bulbs. Note that none of these methods allows perpetual motion, as some energy is always lost to heat. Mass transportation increases energy efficiency compared to widespread conventional automobile use while air travel is regarded as inefficient. Conventional combustion engine automobiles have continually improved their efficiency and may continue to do so in the future, for example by reducing weight with new materials. Hybrid vehicles can save energy by allowing the engine to run more efficiently, regaining energy from braking, turning off the motor when idling in traffic, etc. More efficient ceramic or diesel engines can improve mileage. Electric vehicles such as Maglev, trolleybuses, and PHEVs are more efficient during use (but maybe not if doing a life cycle analysis) than similar current combustion based vehicles, reducing their energy consumption during use by 1/2 to 1/4. Microcars or motorcycles may replace automobiles carrying only one or two people. Transportation efficiency may also be improved by in other ways, see automated highway system. Electricity distribution may change in the future. New small scale energy sources may be placed closer to the consumers so that less energy is lost during electricity distribution. New technology like superconductivity or improved power factor correction may also decrease the energy lost. Distributed generation permits electricity "consumers", who are generating

ENERGY TRANSPORTATION While new sources of energy are only rarely discovered or made possible by new technology, distribution technology continually evolves. The use of fuel cells in cars, for example, is an anticipated delivery technology. This section presents some of the more common delivery technologies that have been important to historic energy development. They all rely in some way on the energy sources listed in the previous section.

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Fuels Shipping is a flexible delivery technology that is used in the whole range of energy development regimes from primitive to highly advanced. Currently, coal, petroleum and their derivatives are delivered by shipping via boat, rail, or road. Petroleum and natural gas may also be delivered via pipeline and coal via a Slurry pipeline. Refined hydrocarbon fuels such as gasoline and LPG may also be delivered via aircraft. Natural gas pipelines must maintain a certain minimum pressure to function correctly. Ethanol's corrosive properties prevent it from being transported via pipeline. The higher costs of ethanol transportation and storage are often prohibitive.

Electric Grids Electricity grids are the networks used to transmit and distribute power from production source to end user, when the two may be hundreds of kilometres away. Sources include electrical generation plants such as a nuclear reactor, coal burning power plant, etc. A combination of sub-stations, transformers, towers, cables, and piping are used to maintain a constant flow of electricity. Grids may suffer from transient blackouts and brownouts, often due to weather damage. During certain extreme space weather events solar wind can interfere with transmissions. Grids also have a predefined carrying capacity or load that cannot safely be exceeded. When power requirements exceed what's available, failures are inevitable. To prevent problems, power is then rationed. Industrialised countries such as Canada, the US, and Australia are among the highest per capita consumers of electricity in the world, which is possible thanks to a widespread electrical distribution network. The US grid is one of the most advanced, although infrastructure maintenance is becoming a problem. CurrentEnergy provides a realtime overview of the electricity supply and demand for California, Texas, and the Northeast of the US. African countries with small scale electrical grids have a correspondingly low annual per capita usage of electricity. One of the most powerful power grids in the world supplies power to the state of Queensland, Australia.

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• Battery electric vehicles are quiet compared to internal combustion engines. • Multiple electric vehicles sold out including the General Motors EV1 and the Tesla Roadster proving the demand for battery electric vehicles. • Operation of a battery electric vehicle is approximately 2 to 4 cents per mile. About a sixth the price of operating a gasoline vehicle. • The use of battery electric vehicles may reduce the dependency on fossil fuels, depending on the source of the electricity.

ENERGY STORAGE Methods of energy storage have been developed, which transform electrical energy into forms of potential energy. A method of energy storage may be chosen based on stability, ease of transport, ease of energy release, or ease of converting free energy from the natural form to the stable form. COMPRESSED AIR VEHICLES The Indian company, Tata, is planning to release a compressed air powered car in 2008. BATTERY-POWERED VEHICLES Batteries are used to store energy in a chemical form. As an alternative energy, batteries can be used to store energy in battery electric vehicles. Battery electric vehicles can be charged from the grid when the vehicle is not in use. Because the energy is derived from electricity, battery electric vehicles make it possible to use other forms of alternative energy such as wind, solar, geothermal, nuclear, or hydroelectric.

Cons • Current battery technology is expensive. • Battery electric vehicles have a relative short range compared to internal combustion engine vehicles. • Batteries are highly toxic. Spent vehicle batteries present an environmental hazard. • Grid infrastructure and output would need to be improved significantly to accommodate a mass-adoption of gridcharged electric vehicles.

Pros • Produces zero emissions to help counteract the effects of global warming, as long as the electricity comes from a source which produces no greenhouse gases. • Batteries are a mature technology, no new expensive research and development is needed to implement technology. • Current lead acid battery technology offers 50+ miles range on one charge. • The Tesla Roadster has a 200-mile (320 km) range on one charge. • Batteries make it possible for stationary alternative energy generation such as solar, wind, hydroelectric, or nuclear • Electric motors are 90% efficient compared to about 20% efficiency of an internal combustion engine. • Battery electric vehicles have fewer moving parts than internal combustion engines, thus improving the reliability of the vehicle.

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HYDROGEN ECONOMY Hydrogen can be manufactured at roughly 77 percent thermal efficiency by the method of steam reforming of natural gas. When manufactured by this method it is a derivative fuel like gasoline; when produced by electrolysis of water, it is a form of chemical energy storage as are storage batteries, though hydrogen is the more versatile storage mode since there are two options for its conversion to useful work: (1) a fuel cell can convert the chemicals hydrogen and oxygen into water, and in the process, produce electricity, or (2) hydrogen can be burned (less efficiently than in a fuel cell) in an internal combustion engine.

Pros • Hydrogen is colorless, odorless and entirely non-polluting, yielding pure water vapor (with minimal NOx) as exhaust when combusted in air. This eliminates the direct

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Solar Energy and its Uses production of exhaust gases that lead to smog, and carbon dioxide emissions that enhance the effect of global warming. • Hydrogen is the lightest chemical element and has the best energy-to-weight ratio of any fuel (not counting tank mass). • Hydrogen can be produced anywhere; it can be produced domestically from the decomposition of water. Hydrogen can be produced from domestic sources and the price can be established within the country.

Cons • Other than some volcanic emanations, hydrogen does not exist in its pure form in the environment, because it reacts so strongly with oxygen and other elements. • It is impossible to obtain hydrogen gas without expending energy in the process. There are three ways to manufacture hydrogen; o By breaking down hydrocarbons - mainly methane. If oil or gases are used to provide this energy, fossil fuels are consumed, forming pollution and nullifying the value of using a fuel cell. It would be more efficient to use fossil fuel directly. o By electrolysis from water - The process of splitting water into oxygen and hydrogen using electrolysis consumes large amounts of energy. It has been calculated that it takes 1.4 joules of electricity to produce 1 joule of hydrogen (Pimentel, 2002). o By reacting water with a metal such as sodium, potassium, or boron. Chemical by-products would be sodium oxide, potassium oxide, and boron oxide. Processes exist which could recycle these elements back into their metal form for re-use with additional energy input, further eroding the energy return on energy invested. • There is currently modest fixed infastructure for distribution of hydrogen that is centrally produced, amounting to several hundred kilometers of pipeline. An

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alternative would be transmission of electricity over the existing electrical network to small-scale electrolyzers to support the widespread use of hydrogen as a fuel. • Hydrogen is difficult to handle, store, and transport. It requires heavy, cumbersome tanks when stored as a gas, and complex insulating bottles if stored as a cryogenic liquid. If it is needed at a moderate temperature and pressure, a metal hydride absorber may be needed. The transportation of hydrogen is also a problem because hydrogen leaks effortlessly from containers. • Some current fuel cell designs, such as proton exchange membrane fuel cells, use platinum as a catalyst. Widescale deployment of such fuel cells could place a strain on available platinum resources. Reducing the platinum loading, per fuel cell stack, is the focus of R&D. • Electricity transmission and battery electric vehicles are far more efficient for storage, transmission and use of energy for transportation, neglecting the energy conversion at the electric power plant. As with distributed production of hydrogen via electrolysis, battery electric vehicles could utilize the existing electricity grid until widespread use dictated an expansion of the grid.

ENERGY STORAGE TYPES

Chemical Some natural forms of energy are found in stable chemical compounds such as fossil fuels. Most systems of chemical energy storage result from biological activity, which store energy in chemical bonds. Man-made forms of chemical energy storage include hydrogen fuel, synthetic hydrocarbon fuel, batteries and explosives such as cordite and dynamite.

Gravitational Dams can be used to store energy, by using excess energy to pump water into the reservoir. When electrical energy is required, the process is reversed. The water then turns a turbine, generating electricity. Hydroelectric power is currently an important part of the world's energy supply, generating one-fifth of the world's electricity.

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Electrical Capacitance Electrical energy may be stored in capacitors. Capacitors are often used to produce high intensity releases of energy (such as a camera's flash).

Mechanical Pressure: Energy may also be stored pressurized gases or alternatively in a vacuum. Compressed air, for example, may be used to operate vehicles and power tools. Large scale compressed air energy storage facilities are used to smooth out demands on electricity generation by providing energy during peak hours and storing energy during off-peak hours. Such systems save on expensive generating capacity since it only needs to meet average consumption rather than peak consumption.

Flywheels and Springs Energy can also be stored in mechanical systems such as springs or flywheels. Flywheel energy storage is currently being used for uninterruptible power supplies.

FUTURE ENERGY DEVELOPMENT Extrapolations from current knowledge to the future offer a choice of energy futures. Some predictions parallel the Malthusian catastrophe hypothesis. Numerous are complex models based scenarios as pioneered by Limits to Growth. Modeling approaches offer ways to analyze diverse strategies, and hopefully find a road to rapid and sustainable development of humanity. Short term energy crises are also a concern of energy development. Some extrapolations lack plausibility, particularly when they predict a continual increase in oil consumption. Existing technologies for new energy sources, such as renewable energy technologies, particularly wind power and solar power, are promising. Nuclear fission is also promoted, and each need sustained research and development, including consideration of possible harmful side effects. Jacques Cousteau spoke of using the salinization of water at river estuaries as an energy source, which would not have any consequences for a million years, and then stopped to point out that since we are going to be on the

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planet for a billion years we had to be looking that far into the future. Nuclear fusion and artificial photosynthesis are other energy technologies being researched and developed. Energy production usually requires an energy investment. Drilling for oil or building a wind power plant requires energy. The fossil fuel resources (see above) that are left are often increasingly difficult to extract and convert. They may thus require increasingly higher energy investments. If the investment is greater than the energy produced, then the fossil resource is no longer an energy source. This means that a large part of the fossil fuel resources and especially the non-conventional ones cannot be used for energy production today. Such resources may still be exploited economically in order to produce raw materials for plastics, fertilizers or even transportation fuel but now more energy is consumed than produced. (They then become similar to ordinary mining reserves, economically recoverable but not net positive energy sources.) New technology may ameliorate this problem if it can lower the energy investment required to extract and convert the resources, although ultimately basic physics sets limits that cannot be exceeded. It should be noted that between 1950 and 1984, as the Green Revolution transformed agriculture around the globe, world grain production increased by 250%. The energy for the Green Revolution was provided by fossil fuels in the form of fertilizers (natural gas), pesticides (oil), and hydrocarbon fueled irrigation. The peaking of world hydrocarbon production (Peak oil) may test Malthus critics.neutrality disputed

HISTORY OF PREDICTIONS ABOUT FUTURE ENERGY DEVELOPMENT Ever since the beginning of the Industrial Revolution, the question of the future of energy supplies has occupied economists. • 1865 - William Stanley Jevons published The Coal Question in which he claimed that reserves of coal would soon be exhausted and that there was no prospect of oil being an effective replacement. • 1885 - U.S. Geological Survey: Little or no chance of oil in California. • 1891 - U.S. Geological Survey: Little or no chance of oil in Kansas or Texas.

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Solar Energy and its Uses • 1914 - U.S. Bureau of Mines: Total future production of 5.7 billion barrels (910,000,000 m³). • 1939 - U.S. Department of the Interior: Reserves to last only 13 years. • 1951 - U.S. Department of the Interior, Oil and Gas Division: Reserves to last 13 years. (Data from Kahn et al. (1976) pp.94-5 infra) • 1956 - Geophysicist M. King Hubbert predicts U.S. oil production will peak between 1965 and 1970 (peaked in 1971). Also predicts world oil production will peak "within half a century" based on 1956 data. This is Hubbert peak theory. • 1989 - Predicted peak by Colin Campbell ("Oil Price Leap in the Early Nineties," Noroil, December 1989, pages 3538.) • 2004 - OPEC estimates it will nearly double oil output by 2025 (Opec Oil Outlook to 2025 Table 4, Page 12)

The history of perpetual motion machines is a long list of failed and sometimes fraudulent inventions of machines which produce useful energy "from nowhere" - that is, without requiring additional energy input.

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6 SUPREMACY OVER OTHER ENERGY SOURCES Renewable energy is energy generated from natural resourcessuch as sunlight, wind, rain, tides and geothermal heat-which are renewable (naturally replenished). Renewable energy technologies range from solar power, wind power, hydroelectricity/micro hydro, biomass and biofuels for transportation. In 2006, about 18% of global final energy consumption came from renewables, with 13% coming from traditional biomass, such as wood-burning. Hydropower was the next largest renewable source, providing 3%, followed by hot water/heating, which contributed 1.3%. Modern technologies, such as geothermal, wind, solar, and ocean energy together provided some 0.8% of final energy consumption. The technical potential for their use is very large, exceeding all other readily available sources. Renewable energy technologies are sometimes criticised for being intermittent or unsightly, yet the market is growing for many forms of renewable energy. Wind power is growing at the rate of 30 percent annually, with a worldwide installed capacity of over 100 GW, and is widely used in several European countries and the United States. The manufacturing output of the photovoltaics industry reached more than 2,000 MW in 2006, and photovoltaic (PV) power stations are particularly popular in Germany. Solar thermal power stations operate in the USA and Spain, and the largest of these is the 354 MW SEGS power plant in the Mojave Desert. . The world's largest geothermal power installation is The Geysers in California, with a rated capacity of

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750 MW. Brazil has one of the largest renewable energy programs in the world, involving production of ethanol fuel from sugar cane, and ethanol now provides 18 percent of the country's automotive fuel. Ethanol fuel is also widely available in the USA. While there are many large-scale renewable energy projects and production, renewable technologies are also suited to small off-grid applications, sometimes in rural and remote areas, where energy is often crucial in human development. Kenya has the world's highest household solar ownership rate with roughly 30,000 small (20-100 watt) solar power systems sold per year. Climate change concerns coupled with high oil prices, peak oil and increasing government support are driving increasing renewable energy legislation, incentives and commercialization. European Union leaders reached an agreement in principle in March 2007 that 20 percent of their nations' energy should be produced from renewable fuels by 2020, as part of its drive to cut emissions of carbon dioxide, blamed in part for global warming. Investment capital flowing into renewable energy climbed from $80 billion in 2005 to a record $100 billion in 2006. This level of investment combined with continuing double digit percentage increases each year has moved what once was considered alternative energy to mainstream. Wind was the first to provide 1% of electricity, but solar is not far behind. Some very large corporations such as BP, General Electric, Sharp, and Royal Dutch Shell are investing in the renewable energy sector.

MAIN RENEWABLE ENERGY TECHNOLOGIES The majority of renewable energy technologies are directly or indirectly powered by the sun. The Earth-Atmosphere system is in equilibrium such that heat radiation into space is equal to incoming solar radiation, the resulting level of energy within the Earth-Atmosphere system can roughly be described as the Earth's "climate." The hydrosphere (water) absorbs a major fraction of the incoming radiation. Most radiation is absorbed at low latitudes around the equator, but this energy is dissipated around the globe in the form of winds and ocean currents. Wave motion may play a role in the process of transferring mechanical energy between the atmosphere and the ocean through wind stress. Solar energy


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is also responsible for the distribution of precipitation which is tapped by hydroelectric projects, and for the growth of plants used to create biofuels. Renewable energy flows involve natural phenomena such as sunlight, wind, tides and geothermal heat, as the International Energy Agency explains: "Renewable energy is derived from natural processes that are replenished constantly. In its various forms, it derives directly from the sun, or from heat generated deep within the earth. Included in the definition is electricity and heat generated from solar, wind, ocean, hydropower, biomass, geothermal resources, and biofuels and hydrogen derived from renewable resources." Each of these sources has unique characteristics which influence how and where they are used. WIND POWER Airflows can be used to run wind turbines. Modern wind turbines range from around 600 kW to 5 MW of rated power, although turbines with rated output of 1.5-3 MW have become the most common for commercial use; the power output of a turbine is a function of the cube of the wind speed, so as wind speed increases, power output increases dramatically. Areas where winds are stronger and more constant, such as offshore and high altitude sites, are preferred locations for wind farms. Since wind speed is not constant, a wind farm's annual energy production is never as much as the sum of the generator nameplate ratings multiplied by the total hours in a year. The ratio of actual productivity in a year to this theoretical maximum is called the capacity factor. Typical capacity factors are 20-40%, with values at the upper end of the range in particularly favourable sites. For example, a 1 megawatt turbine with a capacity factor of 35% will not produce 8,760 megawatt-hours in a year, but only 0.35x24x365 = 3,066 MWh, averaging to 0.35 MW. Online data is available for some locations and the capacity factor can be calculated from the yearly output. Globally, the long-term technical potential of wind energy is believed to be five times total current global energy production, or 40 times current electricity demand. This could require large

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amounts of land to be used for wind turbines, particularly in areas of higher wind resources. Offshore resources experience mean wind speeds of ~90% greater than that of land, so offshore resources could contribute substantially more energy. This number could also increase with higher altitude ground-based or airborne wind turbines. Wind power is renewable and produces no greenhouse gases during operation, such as carbon dioxide and methane. WATER POWER Energy in water (in the form of motive energy or temperature differences) can be harnessed and used. Since water is about 800 times denser than air, even a slow flowing stream of water, or moderate sea swell, can yield considerable amounts of energy. There are many forms of water energy: • Hydroelectric energy is a term usually reserved for largescale hydroelectric dams. Examples are the Grand Coulee Dam in Washington State and the Akosombo Dam in Ghana. • Micro hydro systems are hydroelectric power installations that typically produce up to 100 kW of power. They are often used in water rich areas as a Remote Area Power Supply (RAPS). There are many of these installations around the world, including several delivering around 50 kW in the Solomon Islands. • Damless hydro systems derive kinetic energy from rivers and oceans without using a dam. • Wave power uses the energy in waves. The waves will usually make large pontoons go up and down in the water, leaving an area with reduced wave height in the "shadow". Wave power has now reached commercialization. • Tidal power captures energy from the tides in a vertical direction. Tides come in, raise water levels in a basin, and tides roll out. Around low tide, the water in the basin is discharged through a turbine. • Tidal stream power captures energy from the flow of tides, usually using underwater plant resembling a small


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wind turbine. Tidal stream power demonstration projects exist, and the first commercial prototype will be installed in Strangford Lough in September 2007. • Ocean thermal energy conversion (OTEC) uses the temperature difference between the warmer surface of the ocean and the colder lower recesses. To this end, it employs a cyclic heat engine. OTEC has not been field-tested on a large scale. • Deep lake water cooling, although not technically an energy generation method, can save a lot of energy in summer. It uses submerged pipes as a heat sink for climate control systems. Lake-bottom water is a year-round local constant of about 4°C. • Blue energy is the reverse of desalination. This form of energy is in research.

SOLAR ENERGY USE In this context, "solar energy" refers to energy that is collected from sunlight. Solar energy can be applied in many ways, including to: • Generate electricity using photovoltaic solar cells. • Generate electricity using concentrated solar power. • Generate electricity by heating trapped air which rotates turbines in a Solar updraft tower. • Heat buildings, directly, through passive solar building design. • Heat foodstuffs, through solar ovens. • Heat water or air for domestic hot water and space heating needs using solar-thermal panels. • Heat and cool air through use of solar chimneys. • Generate electricity in geosynchronous orbit using solar power satellites. • Solar air conditioning BIOFUEL Plants use photosynthesis to grow and produce biomass. Also known as biomatter, biomass can be used directly as fuel or to

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produce liquid biofuel. Agriculturally produced biomass fuels, such as biodiesel, ethanol and bagasse (often a by-product of sugar cane cultivation) can be burned in internal combustion engines or boilers. Typically biofuel is burned to release its stored chemical energy. Research into more efficient methods of converting biofuels and other fuels into electricity utilizing fuel cells is an area of very active work.

LIQUID BIOFUEL Liquid biofuel is usually either a bioalcohol such as ethanol fuel or a bio-oil such as biodiesel and straight vegetable oil. Biodiesel can be used in modern diesel vehicles with little or no modification to the engine and can be made from waste and virgin vegetable and animal oil and fats (lipids). Virgin vegetable oils can be used in modified diesel engines. In fact the Diesel engine was originally designed to run on vegetable oil rather than fossil fuel. A major benefit of biodiesel is lower emissions. The use of biodiesel reduces emission of carbon monoxide and other hydrocarbons by 20 to 40%. In some areas corn, cornstalks, sugarbeets, sugar cane, and switchgrasses are grown specifically to produce ethanol (also known as grain alcohol) a liquid which can be used in internal combustion engines and fuel cells. Ethanol is being phased into the current energy infrastructure. E85 is a fuel composed of 85% ethanol and 15% gasoline that is sold to consumers. Biobutanol is being developed as an alternative to bioethanol. There is growing international criticism about biofuels from food crops with respect to issues such as food security, environmental impacts (deforestation) and energy balance. SOLID BIOMASS Solid biomass is mostly commonly usually used directly as a combustible fuel, producing 10-20 MJ/kg of heat. Its forms and sources include wood fuel, the biogenic portion of municipal solid waste, or the unused portion of field crops. Field crops may or may not be grown intentionally as an energy crop, and the remaining plant byproduct used as a fuel. Most types of biomass contain energy. Even cow manure, still contains


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two-thirds of the original energy consumed by the cow. Energy harvesting via a bioreactor is a cost-effective solution to the waste disposal issues faced by the dairy farmer, and can produce enough biogas to run a farm. With current technology, it is not ideally suited for use as a transportation fuel. Most transportation vehicles require power sources with high power density, such as that provided by internal combustion engines. These engines generally require clean burning fuels, which are generally in liquid form, and to a lesser extent, compressed gaseous phase. Liquids are more portable because they have high energy density, and they can be pumped, which makes handling easier. This is why most transportation fuels are liquids. Non-transportation applications can usually tolerate the low power-density of external combustion engines, that can run directly on less-expensive solid biomass fuel, for combined heat and power. One type of biomass is wood, which has been used for millennia in varying quantities, and more recently is finding increased use. Two billion people currently cook every day, and heat their homes in the winter by burning biomass, which is a major contributor to man-made climate change global warming. The black soot that is being carried from Asia to polar ice caps is causing them to melt faster in the summer. In the 19th century, wood-fired steam engines were common, contributing significantly to industrial revolution unhealthy air pollution. Coal is a form of biomass that has been compressed over millennia to produce a non-renewable, highlypolluting fossil fuel. Wood and its byproducts can now be converted through process such as gasification into biofuels such as woodgas, biogas, methanol or ethanol fuel; although further development may be required to make these methods affordable and practical. Sugar cane residue, wheat chaff, corn cobs and other plant matter can be, and are, burned quite successfully. The net carbon dioxide emissions that are added to the atmosphere by this process are only from the fossil fuel that was consumed to plant, fertilize, harvest and transport the biomass. Processes to harvest biomass from short-rotation poplars and willows, and perennial grasses such as switchgrass, phalaris, and

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miscanthus, require less frequent cultivation and less nitrogen than from typical annual crops. Pelletizing miscanthus and burning it to generate electricity is being studied and may be economically viable.

BIOGAS Biogas can easily be produced from current waste streams, such as: paper production, sugar production, sewage, animal waste and so forth. These various waste streams have to be slurried together and allowed to naturally ferment, producing methane gas. This can be done by converting current sewage plants into biogas plants. When a biogas plant has extracted all the methane it can, the remains are sometimes better suitable as fertilizer than the original biomass. Alternatively biogas can be produced via advanced waste processing systems such as mechanical biological treatment. These systems recover the recyclable elements of household waste and process the biodegradable fraction in anaerobic digesters. Renewable natural gas is a biogas which has been upgraded to a quality similar to natural gas. By upgrading the quality to that of natural gas, it becomes possible to distribute the gas to the mass market via gas grid. GEOTHERMAL ENERGY Geothermal energy is energy obtained by tapping the heat of the earth itself, usually from kilometers deep into the Earth's crust. It is expensive to build a power station but operating costs are low resulting in low energy costs for suitable sites. Ultimately, this energy derives from heat in the Earth's core. The government of Iceland states: "It should be stressed that the geothermal resource is not strictly renewable in the same sense as the hydro resource." It estimates that Iceland's geothermal energy could provide 1700 MW for over 100 years, compared to the current production of 140 MW. The International Energy Agency classifies geothermal power as renewable. Three types of power plants are used to generate power from geothermal energy: dry steam, flash, and binary. Dry steam plants take steam out of fractures in the ground and use it to directly


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drive a turbine that spins a generator. Flash plants take hot water, usually at temperatures over 200 °C, out of the ground, and allows it to boil as it rises to the surface then separates the steam phase in steam/water separators and then runs the steam through a turbine. In binary plants, the hot water flows through heat exchangers, boiling an organic fluid that spins the turbine. The condensed steam and remaining geothermal fluid from all three types of plants are injected back into the hot rock to pick up more heat. The geothermal energy from the core of the Earth is closer to the surface in some areas than in others. Where hot underground steam or water can be tapped and brought to the surface it may be used to generate electricity. Such geothermal power sources exist in certain geologically unstable parts of the world such as Chile, Iceland, New Zealand, United States, the Philippines and Italy. The two most prominent areas for this in the United States are in the Yellowstone basin and in northern California. Iceland produced 170 MW geothermal power and heated 86% of all houses in the year 2000 through geothermal energy. Some 8000 MW of capacity is operational in total. There is also the potential to generate geothermal energy from hot dry rocks. Holes at least 3 km deep are drilled into the earth. Some of these holes pump water into the earth, while other holes pump hot water out. The heat resource consists of hot underground radiogenic granite rocks, which heat up when there is enough sediment between the rock and the earths surface. Several companies in Australia are exploring this technology. RENEWABLE ENERGY COMMERCIALIZATION

COSTS Renewable energy systems encompass a broad, diverse array of technologies, and the current status of these can vary considerably. Some technologies are already mature and economically competitive (e.g. geothermal and hydropower), others need additional development to become competitive without subsidies. This can be helped by improvements to subcomponents, such as electric generators. The table shows an overview of costs of various renewable energy technologies. For comparison with the prices in the table,

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electricity production from a conventional coal-fired plant costs about 4¢/kWh. Though in some G8 nations the cost can be significantly higher at 7.88p (~15¢/kWh). Achieving further cost reductions as indicated in the table below requires further technology development, market deployment, an increase in production capacities to mass production levels, and of the establishment of an emissions trading scheme and/or carbon tax which would attribute a cost to each unit of carbon emitted; thus reflecting the true cost of energy production by fossil fuels which then could be used to lower the cost/kWh of these renewable energies. WIND POWER MARKET INCREASE As of April 2008, worldwide wind farm capacity was 100,000 megawatts (MW), and wind power produced some 1.3% of global electricity consumption, accounting for approximately 18% of electricity use in Denmark, 9% in Spain, and 7% in Germany. The United States is an important growth area and latest American Wind Energy Association figures show that installed U.S. wind power capacity has reached 18,302 MW, which is enough to serve 5 million average households. Horse Hollow Wind Energy Center, in Texas, is the world's largest wind farm at 735.5 MW capacity. It consists of 291 GE Energy 1.5 MW wind turbines and 130 Siemens 2.3 MW wind turbines. In the UK, a licence to build the world's largest offshore windfarm, in the Thames estuary, has been granted. The London Array windfarm, 20 km off Kent and Essex, should eventually consist of 341 turbines, occupying an area of 230 km². This is a £1.5 billion, 1,000 megawatt project, which will power one-third of London homes. The windfarm will produce an amount of energy that, if generated by conventional means, would result in 1.9 million tonnes of carbon dioxide emissions every year. It could also make up to 10% of the Government's 2010 renewables target. A proposed 4,000 MW facility, called the Pampa Wind Project, is to be located near Pampa, Texas.

NEW GENERATION OF SOLAR THERMAL PLANTS Since 2004 there has been renewed interest in solar thermal power stations and two plants were completed during 2006/2007:


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the 64 MW Nevada Solar One and the 11 MW PS10 solar power tower in Spain. Three 50 MW trough plants were under construction in Spain at the end of 2007 with 10 additional 50 MW plants planned. In the United States, utilities in California and Florida have announced plans (or contracted for) at least eight new projects totaling more than 2,000 MW. In developing countries, three World Bank projects for integrated CSP/combined-cycle gas-turbine power plants in Egypt, Mexico, and Morocco were approved during 2006/2007. There are several solar thermal power plants in the Mojave Desert which supply power to the electricity grid. Solar Energy Generating Systems (SEGS) is the name given to nine solar power plants in the Mojave Desert which were built in the 1980s. These plants have a combined capacity of 354 megawatts (MW) making them the largest solar power installation in the world.

WORLD'S LARGEST PHOTOVOLTAIC POWER PLANTS The Moura photovoltaic power station, located in the municipality of Moura, Portugal, is presently under construction and will have an installed capacity of 62 MWp. The first stage of construction should be finished in 2008 and the second and final stage is scheduled for 2010, making it one of the largest photovoltaic projects ever constructed. Construction of a 40 MW solar generation power plant is underway in the Saxon region of Germany. The Waldpolenz Solar Park will consist of some 550,000 thin-film solar modules. The direct current produced in the modules will be converted into alternating current and fed completely into the power grid. Completion of the project is expected in 2009. Three large photovoltaic power plants have recently been completed in Spain: the Parque Solar Hoya de Los Vincentes (23 MW), the Solarpark Calveron (21 MW), and the Planta Solar La Magascona (20 MW). Another photovoltaic power project has been completed in Portugal. The Serpa solar power plant is located at one of Europe's sunniest areas. The 11 megawatt plant covers 150 acres (0.61 km²) and comprises 52,000 PV panels. The panels are raised 2 metres off the ground and the area will remain productive grazing land.

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The project will provide enough energy for 8,000 homes and will save an estimated 30,000 tonnes of carbon dioxide emissions per year. A $420 million large-scale Solar power station in Victoria is to be the biggest and most efficient solar photovoltaic power station in the world. Australian company Solar Systems will demonstrate its unique design incorporating space technology in a 154MW solar power station connected to the national grid. The power station will have the capability to concentrate the sun by 500 times onto the solar cells for ultra high power output. The Victorian power station will generate clean electricity directly from the sun to meet the annual needs of over 45,000 homes with zero greenhouse gas emissions. However, when it comes to renewable energy systems and PV, it is not just large systems that matter. Building-integrated photovoltaics or "onsite" PV systems have the advantage of being matched to end use energy needs in terms of scale. So the energy is supplied close to where it is needed.

THE CALIFORNIA SOLAR INITIATIVE As part of Governor Arnold Schwarzenegger's Million Solar Roofs Program, California has set a goal to create 3,000 megawatts of new, solar-produced electricity by 2017 - moving the state toward a cleaner energy future and helping lower the cost of solar systems for consumers. This is a comprehensive $2.8 billion program. The California Solar Initiative offers cash incentives on solar PV systems of up to $2.50 a watt. These incentives, combined with federal tax incentives, can cover up to 50% of the total cost of a solar panel system. There are many financial incentives to support the use of renewable energy in other US states. USE

ETHANOL FOR TRANSPORTATION Brazil has one of the largest renewable energy programs in the world, involving production of ethanol fuel from sugar cane, and ethanol now provides 18 percent of the country's automotive fuel. As a partial result, Brazil, which years ago had to import a large share of the petroleum needed for domestic consumption, recently reached complete self-sufficiency in oil. OF


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Most cars on the road today in the U.S. can run on blends of up to 10% ethanol, and motor vehicle manufacturers already produce vehicles designed to run on much higher ethanol blends. Ford, DaimlerChrysler, and GM are among the automobile companies that sell "flexible-fuel" cars, trucks, and minivans that can use gasoline and ethanol blends ranging from pure gasoline up to 85% ethanol (E85). By mid-2006, there were approximately six million E85-compatible vehicles on U.S. roads. The challenge is to expand the market for biofuels beyond the farm states where they have been most popular to date. Flex-fuel vehicles are assisting in this transition because they allow drivers to choose different fuels based on price and availability. The Energy Policy Act of 2005, which calls for 7.5 billion gallons of biofuels to be used annually by 2012, will also help to expand the market.

WAVE FARMS EXPANSION Portugal now has the world's first commercial wave farm, the Aguçadora Wave Park, established in 2006. The farm will initially use three Pelamis P-750 machines generating 2.25 MW. Initial costs are put at 8.5 million. Subject to successful operation, a further 70 million is likely to be invested before 2009 on a further 28 machines to generate 525 MW. Funding for a wave farm in Scotland was announced in February, 2007 by the Scottish Government, at a cost of over 4 million pounds, as part of a £13 million funding packages for ocean power in Scotland. The farm will be the world's largest with a capacity of 3MW generated by four Pelamis machines. GEOTHERMAL ENERGY PROSPECTS The Geysers, is a geothermal power field located 72 miles (116 km) north of San Francisco, California. It is the largest geothermal development in the world outputting over 750 MW. By the end of 2005 worldwide use of geothermal energy for electricity had reached 9.3 GWs, with an additional 28 GW used directly for heating. If heat recovered by ground source heat pumps is included, the non-electric use of geothermal energy is estimated at more than 100 GWt (gigawatts of thermal power) and is used commercially in over 70 countries.( sec 1.2) During

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2005 contracts were placed for an additional 0.5 GW of capacity in the United States, while there were also plants under construction in 11 other countries.

DEVELOPING COUNTRY MARKETS Renewable energy can be particularly suitable for developing countries. In rural and remote areas, transmission and distribution of energy generated from fossil fuels can be difficult and expensive. Producing renewable energy locally can offer a viable alternative. Renewable energy projects in many developing countries have demonstrated that renewable energy can directly contribute to poverty alleviation by providing the energy needed for creating businesses and employment. Renewable energy technologies can also make indirect contributions to alleviating poverty by providing energy for cooking, space heating, and lighting. Renewable energy can also contribute to education, by providing electricity to schools. Kenya is the world leader in the number of solar power systems installed per capita (but not the number of watts added). More than 30,000 very small solar panels, each producing 12 to 30 watts, are sold in Kenya annually. For an investment of as little as $100 for the panel and wiring, the PV system can be used to charge a car battery, which can then provide power to run a fluorescent lamp or a small television for a few hours a day. More Kenyans adopt solar power every year than make connections to the country's electric grid. POTENTIAL FUTURE UTILIZATION Present renewable energy sources supply about 18% of current energy use and there is much potential that could be exploited in the future. As the table below illustrates, the technical potential of renewable energy sources is more than 18 times current global primary energy use and furthermore several times higher than projected energy use in 2100. Available renewable energy. The volume of the cubes represent the amount of available geothermal, wind and solar energy in TW, although only a small portion is recoverable. The small red cube shows the proportional global energy consumption.

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The Renewable Energy Resource Base (Exajoules per year)

Current use (2001)

]

Technical potential

Theoretical potential

Hydropower

9

50

147

Biomass energy

50

>276

2,900

640

6,000

3,900,000

Wind energy

0.12

Solar energy

0.1

>1,575

Geothermal energy

0.6

--

Ocean energy

not estimated

not estimated

Total

60

>1,800

--

7,400

>4,000,000

Current use is in primary energy equivalent. For comparison, the global primary energy use was 402 EJ per year in 2001. Source: World Energy Assessment 2001

There are many different ways to assess potentials. The theoretical potential indicates the amount of energy theoretically available for energy purposes, such as, in the case of solar energy, the amount of incoming radiation at the earth's surface. The technical potential is a more practical estimate of how much could be put to human use by considering conversion efficiencies of the

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available technology and available land area. To give an idea of the constraints, the estimate for solar energy assumes that 1% of the world's unused land surface is used for solar power. The technical potentials generally do not include economic or other environmental constraints, and the potentials that could be realized at an economically competitive level under current conditions and in a short time-frame is lower still. Sustainable development and global warming groups propose a 100% Renewable Energy Source Supply, without fossil fuels and nuclear power. Scientists from the University of Kassel have been busy proving that Germany can power itself entirely by renewable energy.

TRENDS FAVORING RENEWABLES The renewable market will boom when cost efficiency attains parity with other competing energy sources. The following trends are a few examples by which the renewables market is being helped to attain critical mass so that it becomes competitive enough vs fossil fuels: Other than market forces, renewable industry often needs government sponsorship to help generate enough momentum in the market. Many countries and states have implemented incentives - like government tax subsidies, partial copayment schemes and various rebates over purchase of renewables - to encourage consumers to shift to renewable energy sources. Government grants fund for research in renewable technology to make the production cheaper and generation more efficient. Development of loan programs that stimulate renewable favoring market forces with attractive return rates, buffer initial deployment costs and entice consumers to consider and purchase renewable technology. A famous example is the solar loan program sponsored by UNEP helping 100,000 people finance solar power systems in India. Success in India's solar program has led to similar projects in other parts of developing world like Tunisia, Morocco, Indonesia and Mexico. Imposition of fossil fuel consumption and carbon taxes, and channel the revenue earned towards renewable energy development. Also oil peak and world petroleum crisis and


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inflation are helping to promote renewables. Many think-tanks are warning that the world needs an urgency driven concerted effort to create a competitive renewable energy infrastructure and market. The developed world can make more research investments to find better cost efficient technologies, and manufacturing could be transferred to developing countries in order to use low labor costs. The renewable energy market could increase fast enough to replace and initiate the decline of fossil fuel dominance and the world could then avert the looming climate and peak oil crises. Most importantly, renewables is gaining credence among private investors as having the potential to grow into the next big industry. Many companies and venture capitalists are investing in photovoltaic development and manufacturing. This trend is particularly visible in Silicon valley, California, Europe, Japan.

CONSTRAINTS AND OPPORTUNITIES Critics suggest that some renewable energy applications may create pollution, be dangerous, take up large amounts of land, or be incapable of generating a large net amount of energy. Proponents advocate the use of "appropriate renewables", also known as soft energy technologies, as these have many advantages. AVAILABILITY AND RELIABILITY Available renewable energy. The volume of the cubes represent the amount of available geothermal, wind and solar energy in TW, although only a small portion is recoverable. The small red cube shows the proportional global energy consumption. There is no shortage of solar-derived energy on Earth. Indeed the storages and flows of energy on the planet are very large relative to human needs. • The amount of solar energy intercepted by the Earth every minute is greater than the amount of energy the world uses in fossil fuels each year. • Tropical oceans absorb 560 trillion gigajoules (GJ) of solar energy each year, equivalent to 1,600 times the world's annual energy use. • The energy in the winds that blow across the United States each year could produce more than 16 billion GJ of

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Solar Energy and its Uses electricity-more than one and one-half times the electricity consumed in the United States in 2000. • Annual photosynthesis by the vegetation in the United States is 50 billion GJ, equivalent to nearly 60% of the nation's annual fossil fuel use.

A criticism of some renewable sources is their variable nature. But renewable power sources can actually be integrated into the grid system quite well, as Amory Lovins explains: Variable but forecastable renewables (wind and solar cells) are very reliable when integrated with each other, existing supplies and demand. For example, three German states were more than 30 percent wind-powered in 2007-and more than 100 percent in some months. Mostly renewable power generally needs less backup than utilities already bought to combat big coal and nuclear plants' intermittence. The challenge of variable power supply may be readily alleviated by energy storage. Available storage options include pumped-storage hydro systems, batteries, hydrogen fuel cells, and thermal mass. Initial investments in such energy storage systems may be high, although the costs can be recovered over the life of the system. Lovins goes on to say that the unreliability of renewable energy is a myth, while the unreliability of nuclear energy is real. Of all U.S. nuclear plants built, 21 percent were abandoned and 27 percent have failed at least once. Successful reactors must close for refueling every 17 months for 39 days. And when shut in response to grid failure, they can't quickly restart. This is simply not the case for wind farms, for example. Wave energy and some other renewables are continuously available. A wave energy scheme installed in Australia generates electricity with an 80% availability factor.

AESTHETICS Both solar and wind generating stations have been criticized from an aesthetic point of view. However, methods and opportunities exist to deploy these renewable technologies


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efficiently and unobtrusively: fixed solar collectors can double as noise barriers along highways, and extensive roadway, parking lot, and roof-top area is currently available; amorphous photovoltaic cells can also be used to tint windows and produce energy. Advocates of renewable energy also argue that current infrastructure is less aethetically pleasing than alternatives, but sited further from the view of most critics.

ENVIRONMENTAL

AND

SOCIAL CONSIDERATIONS

While most renewable energy sources do not produce pollution directly, the materials, industrial processes, and construction equipment used to create them may generate waste and pollution. Some renewable energy systems actually create environmental problems. For instance, older wind turbines can be hazardous to flying birds.

LAND AREA REQUIRED Another environmental issue, particularly with biomass and biofuels, is the large amount of land required to harvest energy, which otherwise could be used for other purposes or left as undeveloped land. However, it should be pointed out that these fuels may reduce the need for harvesting non-renewable energy sources, such as vast strip-mined areas and slag mountains for coal, safety zones around nuclear plants, and hundreds of square miles being strip-mined for oil sands. These responses, however, do not account for the extremely high biodiversity and endemism of land used for ethanol crops, particularly sugar cane. In the U.S., crops grown for biofuels are the most land- and water-intensive of the renewable energy sources. In 2005, about 12% of the nation's corn crop (covering 11 million acres (45,000 km²) of farmland) was used to produce four billion gallons of ethanol-which equates to about 2% of annual U.S. gasoline consumption. For biofuels to make a much larger contribution to the energy economy, the industry will have to accelerate the development of new feedstocks, agricultural practices, and technologies that are more land and water efficient. Already, the efficiency of biofuels production has increased significantly and there are new methods to boost biofuel production.

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HYDROELECTRIC DAMS The major advantage of hydroelectric systems is the elimination of the cost of fuel. Other advantages include longer life than fuelfired generation, low operating costs, and the provision of facilities for water sports. Operation of pumped-storage plants improves the daily load factor of the generation system. Overall, hydroelectric power can be far less expensive than electricity generated from fossil fuels or nuclear energy, and areas with abundant hydroelectric power attract industry. However, there are several major disadvantages of hydroelectric systems. These include: dislocation of people living where the reservoirs are planned, release of significant amounts of carbon dioxide at construction and flooding of the reservoir, disruption of aquatic ecosystems and birdlife, adverse impacts on the river environment, potential risks of sabotage and terrorism, and in rare cases catastrophic failure of the dam wall. Hydroelectric power is now more difficult to site in developed nations because most major sites within these nations are either already being exploited or may be unavailable for other reasons such as environmental considerations. WIND FARMS A wind farm, when installed on agricultural land, has one of the lowest environmental impacts of all energy sources: • It occupies less land area per kilowatt-hour (kWh) of electricity generated than any other energy conversion system, apart from rooftop solar energy, and is compatible with grazing and crops. • It generates the energy used in its construction in just 3 months of operation, yet its operational lifetime is 20-25 years. • Greenhouse gas emissions and air pollution produced by its construction are tiny and declining. There are no emissions or pollution produced by its operation. • In substituting for base-load coal power, wind power produces a net decrease in greenhouse gas emissions and air pollution, and a net increase in biodiversity.


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• Modern wind turbines are almost silent and rotate so slowly (in terms of revolutions per minute) that they are rarely a hazard to birds. Studies of birds and offshore wind farms in Europe have found that there are very few bird collisions. Several offshore wind sites in Europe have been in areas heavily used by seabirds. Improvements in wind turbine design, including a much slower rate of rotation of the blades and a smooth tower base instead of perchable lattice towers, have helped reduce bird mortality at wind farms around the world. However older smaller wind turbines may be hazardous to flying birds. Birds are severely impacted by fossil fuel energy; examples include birds dying from exposure to oil spills, habitat loss from acid rain and mountaintop removal coal mining, and mercury poisoning.

LONGEVITY ISSUES Though a source of renewable energy may last for billions of years, renewable energy infrastructure, like hydroelectric dams, will not last forever, and must be removed and replaced at some point. Events like the shifting of riverbeds, or changing weather patterns could potentially alter or even halt the function of hydroelectric dams, lowering the amount of time they are available to generate electricity. Although geothermal sites are capable of providing heat for many decades, eventually specific locations may cool down. It is likely that in these locations, the system was designed too large for the site, since there is only so much energy that can be stored and replenished in a given volume of earth. Some interpret this as meaning a specific geothermal location can undergo depletion. BIOFUELS PRODUCTION All biomass needs to go through some of these steps: it needs to be grown, collected, dried, fermented and burned. All of these steps require resources and an infrastructure. Some studies contend that ethanol is "energy negative", meaning that it takes more energy to produce than is contained in the final product. However, a large number of recent studies, including a 2006 article in the journal Science offer the opinion

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that fuels like ethanol are energy positive. Furthermore, fossil fuels also require significant energy inputs which have seldom been accounted for in the past. Additionally, ethanol is not the only product created during production, and the energy content of the by-products must also be considered. Corn is typically 66% starch and the remaining 33% is not fermented. This unfermented component is called distillers grain, which is high in fats and proteins, and makes good animal feed. In Brazil, where sugar cane is used, the yield is higher, and conversion to ethanol is somewhat more energy efficient than corn. Recent developments with cellulosic ethanol production may improve yields even further. According to the International Energy Agency, new biofuels technologies being developed today, notably cellulosic ethanol, could allow biofuels to play a much bigger role in the future than previously thought. Cellulosic ethanol can be made from plant matter composed primarily of inedible cellulose fibers that form the stems and branches of most plants. Crop residues (such as corn stalks, wheat straw and rice straw), wood waste, and municipal solid waste are potential sources of cellulosic biomass. Dedicated energy crops, such as switchgrass, are also promising cellulose sources that can be sustainably produced in many regions of the United States. The ethanol and biodiesel production industries also create jobs in plant construction, operations, and maintenance, mostly in rural communities. According to the Renewable Fuels Association, the ethanol industry created almost 154,000 U.S. jobs in 2005 alone, boosting household income by $5.7 billion. It also contributed about $3.5 billion in tax revenues at the local, state, and federal levels.

DIVERSIFICATION The U.S. electric power industry now relies on large, central power stations, including coal, natural gas, nuclear, and hydropower plants that together generate more than 95% of the nation's electricity. Over the next few decades uses of renewable energy could help to diversify the nation's bulk power supply. Already, appropriate renewable resources (which excludes large


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hydropower) produce 12% of northern California's electricity. Although most of today's electricity comes from large, centralstation power plants, new technologies offer a range of options for generating electricity nearer to where it is needed, saving on the cost of transmitting and distributing power and improving the overall efficiency and reliability of the system. Improving energy efficiency represents the most immediate and often the most cost-effective way to reduce oil dependence, improve energy security, and reduce the health and environmental impact of the energy system. By reducing the total energy requirements of the economy, improved energy efficiency could make increased reliance on renewable energy sources more practical and affordable. OTHER ISSUES

SUSTAINABILITY Renewable energy sources are generally sustainable in the sense that they cannot "run out" as well as in the sense that their environmental and social impacts are generally more benign than those of fossil. However, both biomass and geothermal energy require wise management if they are to be used in a sustainable manner. For all of the other renewables, almost any realistic rate of use would be unlikely to approach their rate of replenishment by nature. TRANSMISSION If renewable and distributed generation were to become widespread, electric power transmission and electricity distribution systems might no longer be the main distributors of electrical energy but would operate to balance the electricity needs of local communities. Those with surplus energy would sell to areas needing "top ups". That is, network operation would require a shift from 'passive management' - where generators are hooked up and the system is operated to get electricity 'downstream' to the consumer - to 'active management', wherein generators are spread across a network and inputs and outputs need to be constantly monitored to ensure proper balancing occurs within the system. Some governments and regulators are moving to

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address this, though much remains to be done. One potential solution is the increased use of active management of electricity transmission and distribution networks. This will require significant changes in the way that such networks are operated. However, on a smaller scale, use of renewable energy produced on site reduces burdens on electricity distribution systems. Current systems, while rarely economically efficient, have shown that an average household with an appropriately-sized solar panel array and energy storage system needs electricity from outside sources for only a few hours per week. By matching electricity supply to end-use needs, advocates of renewable energy and the soft energy path believe electricity systems will become smaller and easier to manage, rather than the opposite. MARKET DEVELOPMENT OF RENEWABLE HEAT ENERGY Renewable heat is the generation of heat from renewable sources. Much current discussion on renewable energy focuses on the generation of electrical energy, despite the fact that many colder countries consume more energy for heating than as electricity. In 2005 the United Kingdom consumed 354 TWh of electric power, but had a heat requirement of 907 TWh, the majority of which (81%) was met using gas. The residential sector alone consumed a massive 550 TWh of energy for heating, mainly in the form of gas. Almost half of the final energy consumed in the UK (49%) was in the form of heat. Renewable electric power is becoming cheap and convenient enough to place it, in many cases, within reach of the average consumer. By contrast, the market for renewable heat is mostly inaccessible to domestic consumers due to inconvenience of supply, and high capital costs. Heating accounts for a large proportion of energy consumption, however a universally accessible market for renewable heat is yet to emerge. Solutions such as geothermal heat pumps may be more widely applicable, but may not be economical in all cases. Also see renewable energy development. CONTROVERSY OVER NUCLEAR RENEWABLE ENERGY SOURCE

POWER

AS

A

In 1983, physicist Bernard Cohen proposed that uranium is effectively inexhaustible, and could therefore be considered a


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renewable source of energy. He claims that fast breeder reactors, fueled by naturally-replenished uranium extracted from seawater, could supply energy at least as long as the sun's expected remaining lifespan of five billion years. Nuclear energy has also been referred to as "renewable" by politicians like George W. Bush, Charlie Crist, and David Sainsbury. In England and Wales there is a NonFossil Fuel Obligation, which provides support for renewable energy. Nuclear power production was also subsidised by this obligation from 1990 until 2002. Inclusion under the "renewable energy" classification could render nuclear power projects eligible for development aid under various jurisdictions. However, it is has not been established that nuclear energy is inexhaustible, and issues such as Peak uranium and Uranium depletion are ongoing debates. No legislative body has yet included nuclear energy under any legal definition of "renewable energy sources" for provision of development support (see: Renewable energy development). Similarly, statutory and scientific definitions of renewable energies usually exclude nuclear energy. Commonly sourced definitions of renewable energy sources often omit or explicitly exclude nuclear energy sources as examples. Nuclear fission is generally not regarded as renewable, as indicated by the U.S. DOE on the website "What is Energy?" There are also environmental concerns over nuclear power, including the dangerous environmental hazards of nuclear waste and concerns that development of new plants cannot happen quickly enough to reduce CO2 emissions, such that nuclear energy is neither efficient nor effective in cutting CO2 emissions.

SOLAR THERMAL ENERGY Solar thermal energy (or STE) is a technology for harnessing solar energy for heat. Solar thermal collectors are characterized by the US Energy Information Agency as low, medium, or high temperature collectors. Low temperature collectors are flat plates generally used to heat swimming pools. Medium-temperature collectors are also usually flat plates but are used for creating hot water for residential and commercial use. High temperature collectors concentrate sunlight using mirrors or lenses and are generally used for electric power production. This is different

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from solar photovoltaics, which convert solar energy directly into electricity.

LOW-TEMPERATURE COLLECTORS Of the 21,000,000 square feet (2,000,000 m²) of solar thermal collectors produced in the United States in 2006, 16,000,000 square feet (1,500,000 m²) were of the low-temperature variety. Lowtemperature collectors are generally installed to heat swimming pools, although they can also be used for space heating. Collectors can use air or water as the medium to transfer the heat to its destination. HEATING, COOLING, AND VENTILATION In the United States, heating, ventilation, and air conditioning (HVAC) systems account for over 25 percent (4.75 EJ) of the energy used in commercial buildings and nearly half (10.1 EJ) of the energy used in residential buildings. Solar heating, cooling, and ventilation technologies can be used to offset a portion of this energy. Thermal mass materials store solar energy during the day and release this energy during cooler periods. Common thermal mass materials include stone, cement, and water. The proportion and placement of thermal mass should consider several factors such as climate, daylighting, and shading conditions. When properly incorporated, thermal mass can passively maintain comfortable temperatures while reducing energy consumption. A solar chimney (or thermal chimney) is a passive solar ventilation system composed of a hollow thermal mass connecting the interior and exterior of a building. As the chimney warms, the air inside is heated causing an updraft that pulls air through the building. These systems have been in use since Roman times and remain common in the Middle East. A Trombe wall is a passive solar heating and ventilation system consisting of an air channel sandwiched between a window and a sun-facing thermal mass. During the ventilation cycle, sunlight stores heat in the thermal mass and warms the air channel causing circulation through vents at the top and bottom of the wall. During the heating cycle the Trombe wall radiates stored


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heat. Solar roof ponds are a unique solar heating and cooling technology developed by Harold Hay in the 1960s. A basic system consists of a roof mounted water bladder with a movable insulating cover. This system can control heat exchange between interior and exterior environments by covering and uncovering the bladder between night and day. When heating is a concern the bladder is uncovered during the day allowing sunlight to warm the water bladder and store heat for evening use. When cooling is a concern the covered bladder draws heat from the building's interior during the day and is uncovered at night to radiate heat to the cooler atmosphere. The Skytherm house in Atascadero, California uses a prototype roof pond for heating and cooling. Active solar cooling can be achieved via absorption refrigeration cycles, desiccant cycles, and solar mechanical processes. In 1878, Auguste Mouchout pioneered solar cooling by making ice using a solar steam engine attached to a refrigeration device. Thermal mass, smart windows and shading methods can also be used to provide cooling. The leaves of deciduous trees provide natural shade during the summer while the bare limbs allow light and warmth into a building during the winter. The water content of trees will also help moderate local temperatures.

PROCESS HEAT Evaporation ponds are shallow ponds that concentrate dissolved solids through evaporation. The use of evaporation ponds to obtain salt from sea water is one of the oldest applications of solar energy. Modern uses include concentrating brine solutions used in leach mining and removing dissolved solids from waste streams. Altogether, evaporation ponds represent one of the largest commercial applications of solar energy in use today. Unglazed transpired collectors (UTC) are perforated sun-facing walls used for preheating ventilation air. UTCs can raise the incoming air temperature up to 22°C and deliver outlet temperatures of 45-60°C. The short payback period of transpired collectors (3 to 12 years) make them a more cost-effective alternative to glazed collection systems. As of 2003, over 80 systems with a combined collector area of 35,000 m² had been installed worldwide. Representatives include an 860 m² collector in Costa Rica used for

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drying coffee beans and a 1300 m² collector in Coimbatore, India used for drying marigolds. A food processing facility in Modesto, California uses parabolic troughs to produce steam used in the manufacturing process. The 5,000 m² collector area is expected to provide 4.3 GJ per year.

MEDIUM-TEMPERATURE COLLECTORS These collectors could be used to produce approximately 50% of the hot water needed for residential and commercial use in the United States. In the United States, a typical system costs $5000$6000 and 50% of the system qualifies for a tax cr. With this incentive, the payback time for a typical household is nine years. A crew of one plumber and two assistants with minimal training can install two systems per week. The typical installation has negligible maintenance costs and reduces a households' operating costs by $6 per person per month. Solar water heating can reduce CO2 emissions by 1 ton/year (if replacing natural gas for hot water heating) or 3 ton/year (if replacing electric hot water heating). Medium-temperature installations can use any of several designs: common designs are pressurized glycol, drain back, and batch systems. COOKING Solar cookers use sunlight for cooking, drying and pasteurization. Solar cooking offsets fuel costs, reduces demand for fuel or firewood, and improves air quality by reducing or removing a source of smoke. The simplest type of solar cooker is the box cooker first built by Horace de Saussure in 1767. A basic box cooker consists of an insulated container with a transparent lid. These cookers can be used effectively with partially overcast skies and will typically reach temperatures of 50-100°C. Concentrating solar cookers use reflectors to concentrate light on a cooking container. The most common reflector geometries are flat plate, disc and parabolic trough type. These designs cook faster and at higher temperatures (up to 350°C) but require direct light to function properly. The Solar Kitchen in Auroville, India uses a unique concentrating technology known as the solar bowl. Contrary to


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conventional tracking reflector/fixed receiver systems, the solar bowl uses a fixed spherical reflector with a receiver which tracks the focus of light as the Sun moves across the sky. The solar bowl's receiver reaches temperature of 150°C that are used to produce steam that helps cook 2,000 daily meals. Many other solar kitchen in India use another unique concentrating technology known as the Scheffler reflector. This technology was first developed by Wolfgang Scheffler in 1986. A Scheffler reflector is a parabolic dish that uses single axis tracking to follow the Sun's daily course. These reflectors have a flexible reflective surface that is able to change its curvature to adjust to seasonal variations in the incident angle of sunlight. Scheffler reflectors have the advantage of having a fixed focal point which improves the ease of cooking and are able to reach temperatures of 450-650°C. Built in 1999, the world's largest Scheffler reflector system in Abu Road, Rajasthan India is capable of cooking up to 35,000 meals a day. By early 2008, over 2000 large cookers of the Scheffler design had been built worldwide. DISINFECTION AND DESALINATION Solar water disinfection, also known as SODIS, is a simple method of disinfecting water using only sunlight and plastic PET bottles. SODIS is a cheap and effective method for decentralized water treatment, usually applied at the household level and is recommended by the World Health Organization as a viable method for household water treatment and safe storage. SODIS has over two million users in developing countries such as Brazil, Cameroon and Uzbekistan. A solar still uses solar energy to distill water. The main types are cone shaped, boxlike, and pit. The box shaped types are most sophisticated of these and the pit types the least sophisticated. In cone solar stills, impure water is inserted into the container, where it is evaporated by sunlight coming through clear plastic. Free of solids in suspension or solution, the water vapor condenses on top and drips down to the side, where it is collected and removed. The application of solar thermal power for desalination at large scale, especially in sunny regions such as the Middle East and North Africa, is the subject of a definitive 2007 report by the German research institute DLR.

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HIGH-TEMPERATURE COLLECTORS Where temperatures below about 95°C are sufficient, as for space heating, flat-plate collectors of the nonconcentrating type are generally used. The fluid-filled pipes can reach temperatures of 150 to 220 degrees Celsius when the fluid is not circulating. This temperature is too low for efficient conversion to electricity. The efficiency of heat engines increases with the temperature of the heat source. To achieve this in solar thermal energy plants, solar radiation is concentrated by mirrors or lenses to obtain higher temperatures - a technique called Concentrated Solar Power (CSP). The practical effect of high efficiencies is to reduce the plant's collector size and total land use per unit power generated, reducing the environmental impacts of a power plant as well as its expense. As the temperature increases, different forms of conversion become practical. Up to 600°C, steam turbines, standard technology, have an efficiency up to 41%. Above this, gas turbines can be more efficient. Higher temperatures are problematic because different materials and techniques are needed. One proposal for very high temperatures is to use liquid fluoride salts operating above 1100°C, using multi-stage turbine systems to achieve 60% thermal efficiencies. The higher operating temperatures permit the plant to use higher-temperature dry heat exchangers for its thermal exhaust, reducing the plant's water use - critical in the deserts where large solar plants are practical. High temperatures also make heat storage more efficient, because more watt-hours are stored per kilo of fluid. Since the CSP plant generates heat first of all, it can store the heat before conversion to electricity. With current technology, storage of heat is much cheaper and more efficient than storage of electricity. In this way, the CSP plant can produce electricity day and night. If the CSP site has predictable solar radiation, then the CSP plant becomes a reliable power plant. Reliability can further be improved by installing a back-up system that uses fossil energy. The back-up system can reuse most of the CSP plant, which decreases the cost of the back-up system. With reliability, unused desert, no pollution and no fuel costs, the only obstacle for large deployment for CSP is cost. Although


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only a small percentage of the desert is necessary to meet global electricity demand, still a large area must be covered with mirrors or lenses to obtain a significant amount of energy. An important way to decrease cost is the use of a simple design.

SYSTEM DESIGNS During the day the sun has different positions. If the mirrors or lenses do not move, then the focus of the mirrors or lenses changes. Therefore it seems unavoidable that there needs to be a tracking system that follows the position of the sun (for solar photovoltaics a solar tracker is only optional). The tracking system increases the cost. With this in mind, different designs can be distinguished in how they concentrate the light and track the position of the sun. PARABOLIC TROUGH DESIGNS Parabolic trough power plants use a curved trough which reflects the direct solar radiation onto a receiver (also called absorber or collector) running along the trough, above the reflectors. The trough is parabolic in one direction and just straight in the other direction. For change of position of the sun orthogonal to the receiver, the whole trough tilts so that direct radiation remains focused on the receiver. However, a change of position of the sun parallel to the trough, does not require adjustment of the mirrors, since the light is just concentrated on another part of the receiver. So, the trough design avoids a second axis for tracking. A substance (also called heat transfer fluid) passes through the receiver and becomes hot. Used substances are synthetic oil, molten salt and pressurized steam. The receiver can be in a vacuum chamber of glass. The light will shine through the glass and vacuum, but the vacuum will significantly reduce convective loss of the collected heat. The substance with the heat is transported to a heat engine where about a third of the heat is converted to electricity. Andasol 1 in Gaudix, Spain uses the Parabolic Trough design which consists of long parallel rows of modular solar collectors. Tracking the sun from East to West by rotation on one axis, the high precision reflector panels concentrate the solar radiation

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coming directly from the sun onto an absorber pipe located along the focal line of the collector. A heat transfer medium, a synthetic oil like in car engines, is circulated through the absorber pipes at temperatures up to 400°C and generates live steam to drive the steam turbine generator of a conventional power block. Full-scale parabolic trough systems consist of many such troughs laid out in parallel over a large area of land. Since 1985 a solar thermal system using this principle has been in full operation in California in the United States. It is called the SEGS system. Other CSP designs lack this kind of long experience and therefore it can currently be said that the parabolic trough design is the only proven CSP technology. The Solar Energy Generating System (SEGS) is a collection of nine plants with a total capacity of 350MW. It is currently the largest operational solar system (both thermal and non-thermal). A newer plant is Nevada Solar One plant with a capacity of 64MW. Under construction are Andasol 1 and Andasol 2 in Spain with each site having a capacity of 50MW. Note however, that those plants have heat storage which requires a smaller (but better utilized) generator. With day and night operation Andasol 1 produces more energy than Nevada Solar One. 553MW new capacity is proposed in Mojava Solar Park, California. Furthermore, 59MW hybrid plant with heat storage is proposed near Barstow, California. Near Kuraymat in Egypt, some 40MW steam is used as input for a gas powered plant. Finally, 25MW steam input for a gas power plant in Hassi R'mel, Algeria.

POWER TOWER DESIGNS Power towers (also known as 'central tower' power plants or 'heliostat' power plants) use an array of flat, moveable mirrors (called heliostats) to focus the sun's rays upon a collector tower (the receiver). The advantage of this design above the parabolic trough design is the higher temperature. Thermal energy at higher temperatures can be converted to electricity more efficiently and can be more cheaply stored for later use. Furthermore, there is less need to flatten the ground area. In principle a power tower can be built on a hillside. Mirrors can be flat and plumbing is concentrated


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in the tower. The disadvantage is that each mirror must have its own dual-axis control, while in the parabolic trough design one axis can be shared for a large array of mirrors. BrightSource Energy entered into a series of power purchase agreements with Pacific Gas and Electric Company in March 2008 for up to 900MW of electricity, the largest solar power commitment ever made by a utility. BrightSource is currently developing a number of solar power plants in Southern California, with construction of the first plant planned to start in 2009. In June 2008, BrightSource Energy dedicated its Solar Energy Development Center (SEDC) in Israel's Negev Desert. The site, located in the Rotem Industrial Park, features more than 1,600 heliostats that track the sun and reflect light onto a 60 meter-high tower. The concentrated energy is then used to heat a boiler atop the tower to 550 degrees Celsius, generating steam that is piped into a turbine, where electricity can be produced. The 15MW Solar Tres plant with heat storage is under construction in Spain. In South Africa, a 100MW solar power plant is planned with 4000 to 5000 heliostat mirrors, each having an area of 140 m². A 10MW power plant in Cloncurry, Australia (with purified graphite as heat storage located on the tower directly by the receiver). A cost/performance comparison between power tower and parabolic trough concentrators was made by the NREL which estimated that by 2020 electricity could be produced from power towers for 5.47 ?/kWh and for 6.21 ?/kWh from parabolic troughs. The capacity factor for power towers was estimated to be 72.9% and 56.2% for parabolic troughs. There is some hope that the development of cheap, durable, mass produceable heliostat power plant components could bring this cost down.

DISH DESIGNS A dish system uses a large, reflective, parabolic dish (similar in shape to satellite television dish). It focuses all the sunlight that strikes the dish up onto to a single point above the dish, where a receiver captures the heat and transforms it into a useful form. Typically the dish is coupled with a Stirling engine in a DishStirling System, but also sometimes a steam engine is used. These

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create rotational kinetic energy that can be converted to electricity using an electric generator .

at different receivers at different times of day), this can allow a denser packing of mirrors on available land area.

The advantage of a dish system is that it can achieve much higher temperatures due to the higher concentration of light (as in tower designs). Higher temperatures leads to better conversion to electricity and the dish system is very efficient on this point. However, there are also some disadvantages. Heat to electricity conversion requires moving parts and that results in maintenance. In general, a centralized approach for this conversion is better than the dencentralized concept in the dish design. Second, the (heavy) engine is part of the moving structure, which requires a rigid frame and strong tracking system. Furthermore, parabolic mirrors are used instead of flat mirrors and tracking must be dualaxis.

Recent prototypes of these types of systems have been built in Australia (CLFR) and by Solarmundo in Belgium. The Solarmundo research and development project, with its pilot plant at Liège, was closed down after successful proof of concept of the Linear Fresnel technology. Subsequently, Solar Power Group GmbH (SPG), based in Munich, Germany, was founded by some Solarmundo team members. A Fresnel-based prototype with direct steam generation was built by SPG in conjunction with the German Aerospace Center (DLR).

In 2005 Southern California Edison announced an agreement to purchase solar powered Stirling engines from Stirling Energy Systems over a twenty year period and in quantities (20,000 units) sufficient to generate 500 megawatts of electricity. Stirling Energy Systems announced another agreement with San Diego Gas & Electric to provide between 300 and 900 megawatts of electricity. However, as of October 2007 it was unclear whether any progress had been made toward the construction of the 1 MW test plant, which was supposed to come online some time in 2007.

FRESNEL REFLECTORS A linear Fresnel reflector power plant uses a series of long, narrow, shallow-curvature (or even flat) mirrors to focus light onto one or more linear receivers positioned above the mirrors. On top of the receiver a small parabolic mirror can be attached for further focusing the light. These systems aim to offer lower overall costs by sharing a receiver between several mirrors (as compared with trough and dish concepts), while still using the simple line-focus geometry with one axis for tracking. This is similar to the trough design (and different from central towers and dishes with dual-axis). The receiver is stationary and so fluid couplings are not required (as in troughs and dishes). The mirrors also do not need to support the receiver, so they are structurally simpler. When suitable aiming strategies are used (mirrors aimed

Based on the Australian prototype, a 177MW plant is proposed near San Luis Obispo in California and will be built by Ausra. Plants with smaller capacities being an enormous economical challenge for plants with conventional parabolic trough and drive design, only few companies intend to build such small projects. Plans were revealed for former Ausra subsidiary SHP Europe building a 6.5 MW project in Portugal as a combined cycle plant. The German company SK Energy) has published its intention to build various small 1-3 MW plants in Southern Europe, esp. in Spain on the basis of their own Fresnel mirror and steam drive technology (Press Release). In May 2008, the German Solar Power Group GmbH and the Spanish Laer S.L. agreed the joint execution of a solar thermal power plant in central Spain. This will be the first commercial solar thermal power plant in Spain based on the Fresnel collector technology of the Solar Power Group. The planned size of the power plant will be 10 MW a solar thermal collector field with a fossil co-firing unit as backup system. The start of constructions is planned for 2009. The project is located in Gotarrendura, a small renewable energy pioneering village, about 100 km northwest of Madrid, Spain.

LINEAR FRESNEL REFLECTOR (LFR) AND COMPACT-LFR TECHNOLOGIES Rival single axis tracking technologies include the relatively new Linear Fresnel reflector (LFR) and compact-LFR (CLFR) technologies. The LFR differs from that of the parabolic trough in that the absorber is fixed in space above the mirror field. Also,

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the reflector is composed of many low row segments, which focus collectively on an elevated long tower receiver running parallel to the reflector rotational axis. This system offers a lower cost solution as the absorber row is shared among several rows of mirrors. However, one fundamental difficulty with the LFR technology is the avoidance of shading of incoming solar radiation and blocking of reflected solar radiation by adjacent reflectors. Blocking and shading can be reduced by using absorber towers elevated higher or by increasing the absorber size, which allows increased spacing between reflectors remote from the absorber. Both these solutions increase costs, as larger ground usage is required. The compact linear Fresnel reflector (CLFR) offers an alternate solution to the LFR problem.9 The classic LFR has only one linear absorber on a single linear tower. This prohibits any option of the direction of orientation of a given reflector. Since this technology would be introduced in a large field, one can assume that there will be many linear absorbers in the system. Therefore, if the linear absorbers are close enough, individual reflectors will have the option of directed reflected solar radiation to at least two absorbers. This additional factor gives potential for more densely packed arrays, since patters of alternative reflector inclination can be set up such that closely packed reflectors can be positioned without shading and blocking. CLFR power plants offer reduced costs in all elements of the solar array. These reduced costs encourage the advancement of this technology. Features that enhance the cost effectiveness of this system compared to that of the parabolic trough technology include minimized structural costs, minimized parasitic pumping losses, and low maintenance.7 Minimized structural costs are attributed to the use of flat or elastically curved glass reflectors instead of costly sagged glass reflectors are mounted close to the ground. Also, the heat transfer loop is separated from the reflector field, avoiding the cost of flexible high pressure lines required in trough systems. Minimized parasitic pumping losses are due to the use of water for the heat transfer fluid with passive direct boiling. The use of glass-evacuated tubes ensures low radiative losses and is inexpensive. Studies of existing CLFR plants have


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been shown to deliver tracked beam to electricity efficiency of 19% on an annual basis as a preheater.

FRESNEL LENSES Prototypes of Fresnel lens concentrators have been produced for the collection of thermal energy by International Automated Systems. No full-scale thermal systems using Fresnel lenses are known to be in operation, although products incorporating Fresnel lenses in conjunction with photovoltaic cells are already available. The advantage of this design is that lenses are cheaper than mirrors. Furthermore, if a material is chosen that has some flexibility, then a less rigid frame is required to withstand wind load. MICROCSP "MicroCSP" references Solar Thermal Technologies in which Concentrating Solar Power (CSP) collectors are based on the designs used in traditional Concentrating Solar Power systems found in the Mojave Desert but are smaller in collector size, lighter and operate at lower thermal temperatures usually below 600 degrees F. These systems are designed for modular field or rooftop installation where they are easy to protect from high winds, snow and humid deployments . Solar manufacturer Sopogy is currently constructing a 1MW plant at the Natural Energy Laboratory of Hawaii HEAT EXCHANGE Heat in a solar thermal system is guided by five basic principles: heat gain; heat transfer; heat storage; heat transport; and heat insulation. Here, heat is the measure of the amount of thermal energy an object contains and is the product of temperature and mass. Heat gain is the heat accumulated from the sun in the system. Solar thermal heat is trapped using the greenhouse effect; the greenhouse effect in this case is the ability of a reflective surface to transmit short wave radiation and reflect long wave radiation. Heat and infrared radiation (IR) are produced when short wave radiation light hits the absorber plate, which is then trapped

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inside the collector. Fluid, usually water, in the absorber tubes collect the trapped heat and transfer it to a heat storage vault.

concrete, a variety of phase change materials, and molten salts such as sodium and potassium nitrate.

Heat is transferred either by conduction or convection. When water is heated, kinetic energy is transferred by conduction to water molecules throughout the medium. These molecules spread their thermal energy by conduction and occupy more space than the cold slow moving molecules above them. The distribution of energy from the rising hot water to the sinking cold water contributes to the convection process. Heat is transferred from the absorber plates of the collector in the fluid by conduction. The collector fluid is circulated through the carrier pies to the heat transfer vault. Inside the vault, heat is transferred throughout the medium through convection.

The PS10 solar power tower stores heat in tanks as pressurized steam at 50 bar and 285C. The steam condenses and flashes back to steam, when pressure is lowered. Storage is for one hour. It is suggested that longer storage is possible, but that has not been proven yet in an existing power plant.

Heat storage enables solar thermal plants to produce electricity during hours without sunlight. Heat is transferred to a thermal storage medium in an insulated reservoir during hours with sunlight, and is withdrawn for power generation during hours lacking sunlight. Thermal storage mediums will be discussed in a heat storage section. Rate of heat transfer is related to the conductive and convection medium as well as the temperature differences. Bodies with large temperature differences transfer heat faster than bodies with lower temperature differences. Heat transport refers to the activity in which heat from a solar collector is transported to the heat storage vault. Heat insulation is vital in both heat transport tubing as well as the storage vault. It prevents heat loss, which in turn relates to energy loss, or decrease in the efficiency of the system.

HEAT STORAGE Heat storage allows a solar thermal plant to produce electricity at night and on overcast days. This allows the use of solar power for baseload generation as well as peak power generation, with the potential of displacing both coal and natural gas fired power plants. Additionally, the utilization of the generator is higher which reduces cost. Heat is transferred to a thermal storage medium in an insulated reservoir during the day, and withdrawn for power generation at night. Thermal storage media include pressurized steam,

The proposed power plant in Cloncurry Australia will store heat in purified graphite. The plant has a power tower design. The graphite is located on top of the tower. Heat from the heliostats goes directly to the storage. Heat for energy production is drawn from the graphite. This simplifies the design. The Solar Tres power plant in Spain is expected to be the first commercial solar thermal power plant to utilize molten salt for heat storage and nighttime generation.

MOLTEN SALT STORAGE A variety of fluids have been tested to transport the sun's heat, including water, air, oil, and sodium, but molten salt was selected as best. Molten salt is used in solar power tower systems because it is liquid at atmosphere pressure, it provides an efficient, lowcost medium in which to store thermal energy, its operating temperatures are compatible with today's high-pressure and hightemperature steam turbines, and it is non-flammable and nontoxic. In addition, molten salt is used in the chemical and metals industries as a heat-transport fluid, so experience with molten-salt systems exists for non-solar. The molten salt is a mixture of 60 percent sodium nitrate and 40 percent potassium-nitrate, commonly called saltpeter. The salt melts at 430 F and is kept liquid at 550 F in an insulated cold storage tank. The uniqueness of this solar system is in de-coupling the collection of solar energy from producing power, electricity can be generated in periods of inclement weather or even at night using the stored thermal energy in the hot salt tank. Normally tanks are well insulated and can store energy for up to a week. As an example of their size, tanks that provide enough thermal storage to power a 100-megawatt turbine for four hours would be about 30 feet tall and 80 feet in diameter. Studies show that

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the two-tank storage system could have an annual efficiency of about 99 percent.

GRAPHITE HEAT STORAGE Molten salts coolants are used to transfer heat from the reflectors to heat storage vaults. The heat from the salts are transferred to a secondary heat transfer fluid via a heat exchanger and then to the storage media, or alternatively, the salts can be used to directly heat graphite. Graphite is used as it has relatively low costs and compatibility with liquid fluoride salts. The high mass and volumetric heat capacity of graphite provide an efficient storage medium. PHASE-CHANGE MATERIALS FOR STORAGE Phase-change materials (PCMs) offer an alternate solution in energy storage. Using a similar heat transfer infrastructure, PCMs have the potential of providing a more efficient means of storage. PCMs can be either organic or inorganic materials. Advantages of organic PCMs include no corrosives, low or no undercooling, and chemical and thermal stability. Disadvantages include low phase-change enthalpy, low thermal conductivity, and inflammability. Inorganics are advantageous with greater phasechange enthalpy, but exhibit disadvantages with undercooling, corrosion, phase separation, and lack of thermal stability. The greater phase-change enthalpy in inorganic PCMs make hydrates salts a strong candidate in the solar energy storage field. CONVERSION RATES FROM SOLAR ENERGY TO ELECTRICAL ENERGY Of all of these technologies the solar dish/stirling engine has the highest energy efficiency. A single solar dish-Stirling engine installed at Sandia National Laboratories National Solar Thermal Test Facility produces as much as 25 kW of electricity, with a conversion efficiency of 30%. Solar parabolic trough plants have been built with efficiencies of about 20%. Fresnel reflectors have an efficiency that is slightly lower (but this is compensated by the denser packing). The gross conversion efficiencies (taking into account that the solar dishes or troughs occupy only a fraction of the total area of


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the power plant) are determined by net generating capacity over the solar energy that falls on the total area of the solar plant. The 500-megawatt (MW) SCE/SES plant would extract about 2.75% of the radiation (1 kW/m²; see Solar power for a discussion) that falls on its 4,500 acres (18.2 km²). For the 50 MW AndaSol Power Plant that is being built in Spain (total area of 1,300×1,500 m = 1.95 km²) gross conversion efficiency comes out at 2.6% Furthermore, efficiency does not directly relate to cost: on calculating total cost, both efficiency and the cost of construction and maintenance should be taken into account.

LEVELIZED COST Since a solar power plant does not use any fuel, the cost consists mostly of capital cost with minor operational and maintenance cost. If the lifetime of the plant and the interest rate is known, then the cost per kWh can be calculated. This is called the levelised energy cost. The first step in the calculation is to determine the investment for the production of 1 kWh in a year. Example, the fact sheet of the Andasol 1 project shows a total investment of 310 million euros for a production of 179 GWh a year. Since 179 GWh is 179 million kWh, the investment per kWh a year production is 310 / 179 = 1.73 euro. Another example is Cloncurry solar power station in Australia. It produces 30 million kWh a year for an investment of 31 million Australian dollars. So, this price is 1.03 Australian dollar for the production of 1 kWh in a year. This is significantly cheaper than Andasol 1, which can partially be explained by the higher radiation in Cloncurry over Spain. The investment per kwh cost for one year should not be confused with the cost per kwh over the complete lifetime of such a plant. In most cases the capacity is specified for a power plant (for instance Andasol 1 has a capacity of 50MW). This number is not suitable for comparison, because the capacity factor can differ. If a solar power plant has heat storage, then it can also produce output after sunset, but that will not change the capacity factor, it simply displaces the output. The average capacity factor for a solar power plant, which is a function of tracking, shading and location, is about 20%, meaning that a 50MW capacity power

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plant will typically provide a yearly output of 50 MW × 24 hrs × 365 days × 20% = 87,600 MWh/year, or 87.6 GWh/yr. Although the investment for one kWh year production is suitable for comparing the price of different solar power plants, it doesn't give the price per kWh yet. The way of financing has a great influence on the final price. If the technology is proven, an interest rate of 7% should be possible. However, for a new technology investors want a much higher rate to compensate for the higher risk. This has a significant negative effect on the price per kWh. Independent of the way of financing, there is always a linear relation between the investment per kWh production in a year and the price for 1 kWh (before adding operational and maintenance cost). In other words, if by enhancements of the technology the investments drop by 20%, then the price per kWh also drops by 20%. If a way of financing is assumed where the money is borrowed and repaid every year, in such way that the debt and interest decreases, then the following formula can be used to calculate the division factor: (1 - (1 + interest / 100) ^ -lifetime) / (interest / 100). For a lifetime of 25 years and an interest rate of 7%, the division number is 11.65. For example, the investment of Andasol 1 was 1.73 euro, divided by 11.65 results in a price of 0.15 euro per kWh. If one cent operation and maintenance cost is added, then the levelized cost is 0.16 euro. Other ways of financing, different way of debt repayment, different lifetime expectation, different interest rate, may lead to a significantly different number. If the cost per kWh may follow the inflation, then the inflation rate can be added to the interest rate. If an investor puts his money on the bank for 7%, then he is not compensated for inflation. However, if the cost per kWh is raised with inflation, then he is compensated and he can add 2% (a normal inflation rate) to his return. The Andasol 1 plant has a guaranteed feed-in tariff of 0.21 euro for 25 years. If this number is fixed, it should be realized that after 25 years with 2% inflation, 0.21 euro will have a value comparable with 0.13 euro now. Finally, there is some gap between the first investment and the first production of electricity. This increases the investment with the interest over the period that the plant is not active yet.


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The modular solar dish (but also solar photovoltaic and wind power) have the advantage that electricity production starts after first construction. Given the fact that solar thermal power is reliable, can deliver peak load and does not cause pollution, a price of US$0.10 per kWh starts to become competitive. Although a price of US$0.06 has been claimed With some operational cost a simple target is 1 dollar (or lower) investment for 1 kWh production in a year. A description of the history can be found on the site of the company Ausra. Modern use of solar technology started after the 1973 and 1979 oil crises, which led to SEGS in California and some smaller projects. Due to low energy prices after 1990, no new commercial plans were made, but some research continued. New commercial plans were made from 2005 and onwards.

SOLAR HEATING Solar heating is the usage of solar energy to provide process, space or water heating. See also Solar thermal energy. The heating of water is covered in solar hot water. Solar heating design is divided into two groups: • Active solar heating uses pumps which move air or a liquid from the solar collector into the building or storage area. • Passive solar heating does not require electrical or mechanical equipment, and may rely on the design and structure of the house to collect, store and distribute heat throughout the building (passive solar building design). The very first solar heating factory in the world was built by Jewish immigrants, from South Africa, in Ashqelon Israel in 1952. In 1980 a law was passed in Israel making solar heating mandatory.

How Solar Heating Works A household solar heating system consists of a solar panel (or solar collector) with a heat transfer fluid flowing through it to transport the heat energy collected to somewhere useful, usually a hot water tank or household radiators. The solar panel is located somewhere with good light levels throughout the day, often on

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the roof of the building. A pump pushes the heat transfer liquid (often just treated water) through the panel. The heat is thus taken from the panel and transferred to a storage cylinder.

OTHER USES Solar heating also refers to the heating of any objects, including buildings, cars, through solar radiation. Solar heating depends on the solar radiation, surface area, surface reflectance, surface emissivity, ambient temperature, and thermal convection from wind. With most all objects on Earth, solar heating reaches a state of temperature equilibrium as the heat imparted by the sun is offset by the heat given off through reflection, radiation, and convection. White objects stay dramatically cooler than other objects because the most important variables are characteristics of the surface, reflectance, emissivity, convection and surface area. Silvery objects get hot even though they are excellent reflectors because they are very poor in heat emission. Human skin, and many other living surfaces, like tree leaves, have near perfect emissivity (~1.0), and so stay pretty cool. A perfect sunscreen is a dye that perfectly absorbs, with high emissivity, or perfectly reflects, ultraviolet and infrared while being transparent in visible light. It is worth noting that it is impossible for any material to be a good absorber of a given frequency and at the same time a poor emitter of the same frequency ( or the other way around). The difference in absorption and emission arises because the radiation emitted by a relatively cold object like a human, has much lower frequency than the radiation emitted by a hot object like the sun. Materials which have high emissivity for low frequencies but high absorption at higher frequencies will therefore stay much cooler than materials which have high absorption of high frequencies and low emission of low frequencies. SOLAR POWER PLANTS IN THE MOJAVE DESERT There are several solar power plants in the Mojave Desert which supply power to the electricity grid. Solar Energy Generating Systems (SEGS) is the name given to nine solar power plants in the Mojave Desert which were built in the 1980s. These plants have a combined capacity of 354 megawatts (MW) making them the largest solar power installation in the world. Nevada Solar


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One is a new solar thermal plant with a 64-MW generating capacity, located near Boulder City, NV. There are also plans to build other large solar plants in the Mojave Desert. The Mojave Solar Park will deliver 553 MW of solar thermal power when fully operational in 2011. Insolation (solar radiation) in the Mojave Desert is among the best available in the United States, and some significant population centers are located in the area. This makes the Mojave Desert particularly suitable for solar power plants. These plants can generally be built in a few years because solar plants are built almost entirely with modular, readily available materials. The southwestern United States is one of the world's best areas for insolation, and the Mojave Desert receives up to twice the sunlight received in other regions of the country. This abundance of solar energy makes solar power plants an attractive alternative to traditional power plants, which burn polluting fossil fuels such as oil and coal. Unlike traditional power plants, solar power stations provide an environmentally benign source of energy, produce virtually no emissions, and consume no fuel other than sunlight. Currently, the cost of solar thermal produced energy can be close to 12 cents (US) per kilowatt-hour (kWh). However, many economists predict that this price will gradually drop over the next ten years to 6 cents per kWh, as a result of economies of scale and technological improvements. While many of the costs of fossil fuels are well known, others (pollution related health problems, environmental degradation, the impact on national security from relying on foreign energy sources) are indirect and difficult to calculate. These are traditionally external to the pricing system, and are thus often referred to as externalities. A corrective pricing mechanism, such as a carbon tax, could lead to renewable energy, such as solar thermal power, becoming cheaper to the consumer than fossil fuel based energy. Solar thermal power plants can generally be built in a few years because solar plants are built almost entirely with modular, readily available materials. In contrast, many types of conventional power projects, especially coal and nuclear plants, require long lead times.

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SOLAR ONE AND SOLAR TWO Solar power towers use thousands of individual sun-tracking mirrors (called heliostats) to reflect solar energy onto a central receiver located on top of a tall tower. The receiver collects the sun's heat in a heat-transfer fluid that flows through the receiver. The U.S. Department of Energy, and a consortium of U.S. utilities and industry, built the first two large-scale, demonstration solar power towers in the desert near Barstow, CA. Solar One operated successfully from 1982 to 1988, proving that power towers work efficiently to produce utility-scale power from sunlight. The Solar One plant used water/steam as the heattransfer fluid in the receiver; this presented several problems in terms of storage and continuous turbine operation. To address these problems, Solar One was upgraded to Solar Two, which operated from 1996 to 1999. Both systems had the capacity to produce 10 MW of power. The unique feature of Solar Two was its use of molten salt to capture and store the sun's heat. The very hot salt was stored and used when needed to produce steam to drive a turbine/ generator that produces electricity. The system operated smoothly through intermittent clouds and continued generating electricity long into the night. Solar Two was decommissioned in 1999, and was converted by the University of California, Davis, into an Air Cherenkov Telescope in 2001, measuring gamma rays hitting the atmosphere. SOLAR ELECTRICITY GENERATING SYSTEMS Trough systems predominate among today's commercial solar power plants. Nine trough power plants, called Solar Energy Generating Systems (SEGS), were built in the 1980s in the Mojave Desert near Barstow. These plants have a combined capacity of 354 MW making them the largest solar power installation in the world. Today they generate enough electricity to meet the power needs of approximately 500,000 people. Trough systems convert the heat from the sun into electricity. Because of their parabolical shape, trough collectors can focus the sun at 30-60 times its normal intensity on a receiver pipe located along the focal line of the trough. Synthetic oil circulates through


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the pipe and captures this heat, reaching temperatures of 390 °C (735 °F). The hot oil is pumped to a generating station and routed through a heat exchanger to produce steam. Finally, electricity is produced in a conventional steam turbine. The SEGS plants are configured as hybrids to operate on natural gas on cloudy days or after dark, and natural gas provides 25% of the total output.

NEVADA SOLAR ONE Nevada Solar One has a 64-MW generating capacity and is located in Boulder City, Nevada. It was built by the U.S. Department of Energy, National Renewable Energy Laboratory, and Acciona Solar Power, Inc. formerly known as Solargenix Energy, Inc. Nevada Solar One uses parabolic troughs as thermal solar concentrators, heating tubes of liquid which act as solar receivers. These solar receivers are specially coated tubes made of glass and steel, and about 19,300 of these four meter long tubes are used in the newly built power plant. Nevada Solar One also uses a technology that collects extra heat by putting it into phase-changing molten salts. This energy can then be drawn at night. Solar thermal power plants designed for solar-only generation are ideally matched to summer noon peak loads in prosperous areas with significant cooling demands, such as the south-western United States. Using thermal energy storage systems, solar thermal operating periods can even be extended to meet base-load needs. The cost of Nevada Solar One is in the range of $220-250 million. The power produced is slightly more expensive than wind power, but less than photovoltaic (PV) power. NELLIS SOLAR POWER PLANT In December 2007, the U.S. Air Force announced the completion of a solar photovoltaic (PV) system at Nellis Air Force Base in Clark County, NV. Occupying 140 acres (57 ha) of land leased from the Air Force at the western edge of the base, this ground-mounted photovoltaic system employs an advanced sun tracking system, designed and deployed by PowerLight subsidiary of SunPower. Tilted toward the south, each set of solar panels rotates around a central bar to track the sun from east to west.

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The 14-megawatt (MW) system will generate more than 30 million kilowatt-hours of electricity each year and supply approximately 25 percent of the total power used at the base. The Nellis Solar Power Plant is the largest solar photovoltaic system in North America.

convection of air heated by passive solar energy. A simple description of a solar chimney is that of a vertical shaft utilizing solar energy to enhance the natural stack ventilation through a building. The solar chimney has been in use for centuries, particularly in the Middle east, as well as by the Romans.

MOJAVE SOLAR PARK Solel has signed a contract with Pacific Gas and Electric (PG&E) to build the world's largest solar plant in the Mojave Desert. When fully operational in 2011, the Mojave Solar Park will deliver 553 megawatts of solar power, the equivalent of powering 400,000 homes, to PG&E's customers in northern and central California. The plant will cover up to 6,000 acres (24 km²) of land.

DESCRIPTION In its simplest form, the solar chimney consists of a blackpainted chimney. During the day solar energy heats the chimney and the air within it, creating an updraft of air in the chimney. The suction created at the chimney's base can be used to ventilate and cool the building below. In most parts of the world it is easier to harness wind power for such ventilation as is done with a Badgir, but on hot windless days a Solar chimney can provide ventilation where otherwise there would be none. There are however a number of solar chimney variations. The basic design elements of a solar chimney are: • The solar collector area: This can be located in the top part of the chimney or can include the entire shaft. The orientation, type of glazing, insulation and thermal properties of this element are crucial for harnessing, retaining and utilizing solar gains • The main ventilation shaft: The location, height, cross section and the thermal properties of this structure are also very important. • The inlet and outlet air apertures: The sizes, location as well as aerodynamic aspects of these elements are also significant. A principle has been proposed for solar power generation, using a large greenhouse at the base rather than relying solely on heating the chimney itself.

STIRLING SOLAR DISH Stirling Energy Systems under an agreement with utility company Southern California Edison is planning to erect a 500megawatt, 4,600 acre (19 km²), solar power plant to open some time after 2009. This will be the first commercial application of the Stirling Solar Dish which concentrates solar energy by the use of reflective surfaces and uses a Stirling heat engine to convert the heat into electricity. Stirling Energy Systems have announced another agreement with San Diego Gas & Electric to provide between 300 and 900 megawatts of electricity. LAND USE ISSUES Solar thermal power plants are large and seem to use a lot of land, but when looking at electricity output versus total size, they use less land than hydroelectric dams (including the size of the lake behind the dam) or coal plants (including the amount of land required for mining and excavation of the coal). While all power plants require land and have an environmental impact, the best locations for solar power plants are deserts or other land for which there might be few other human uses. SOLAR CHIMNEY A solar chimney - often referred to as a thermal chimney is a way of improving the natural ventilation of buildings by using

SOLAR CHIMNEY AND SUSTAINABLE ARCHITECTURE Air conditioning and mechanical ventilation have been for decades the standard method of environmental control in many building types especially offices. Global warming, pollution and dwindling energy supplies have led to a new environmental approach in building design. Innovative technologies along with

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bioclimatic principles and traditional design strategies are often combined to create new and potentially successful design solutions. The solar chimney is one of these concepts currently explored by scientists as well as designers, mostly through research and experimentation. A Solar chimney can serve many purposes. Direct gain warms air inside the chimney causing it to rise out the top and drawing air in from the bottom. This drawing of air can be used to ventilate a home or office, to draw air through a geothermal heat exchange, or to ventilate only a specific area such as a composting toilet. Natural ventilation can be created by providing vents in the upper level of a building to allow warm air to rise by convection and escape to the outside. At the same time cooler air can be drawn in through vents at the lower level. Trees may be planted on that side of the building to provide shade for cooler outside air. This natural ventilation process can be augmented by a solar chimney. The chimney has to be higher than the roof level, and has to be constructed on the wall facing the direction of the sun. Absorption of heat from the sun can be increased by using a glazed surface on the side facing the sun. Heat absorbing material can be used on the opposing side. The size of the heat-absorbing surface is more important than the diameter of the chimney. A large surface area allows for more effective heat exchange with the air necessary for heating by solar radiation. Heating of the air within the chimney will enhance convection, and hence airflow through the chimney. Openings of the vents in the chimney should face away from the direction of the prevailing wind. To further maximize the cooling effect, the incoming air may be led through underground ducts before it is allowed to enter the building. The solar chimney can be improved by integrating it with a trombe wall. The added advantage of this design is that the system may be reversed during the cold season, providing solar heating instead. A variation of the solar chimney concept is the solar attic. In a hot sunny climate the attic space is often blazingly hot in the summer. In a conventional building this presents a problem as it leads to the need for increased air conditioning. By integrating the


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attic space with a solar chimney, the hot air in the attic can be put to work. It can help the convection in the chimney, improving ventilation. The use of a solar chimney may benefit natural ventilation and passive cooling strategies of buildings thus help reduce energy use, CO2 emissions and pollution in general. Potential benefits regarding natural ventilation and use of solar chimneys are: • Improved ventilation rates on still, hot days • Reduced reliance on wind and wind driven ventilation • Improved control of air flow though a building • Greater choice of air intake (i.e. leeward side of building) • Improved air quality and reduced noise levels in urban areas • Increased night time ventilation rates • Allow ventilation of narrow, small spaces with minimal exposure to external elements Potential benefits regarding passive cooling may include: • Improved passive cooling during warm season (mostly on still, hot days) • Improved night cooling rates • Enhanced performance of thermal mass (cooling, cool storage) • Improved thermal comfort (improved air flow control, reduced draughts)

PRECEDENT STUDY: THE ENVIRONMENTAL BUILDING The British Research Establishment (BRE) office building in Garston, incorporates solar assisted passive ventilation stacks as part of its ventilation strategy. Designed by architects Feilden Clegg Bradley, the BRE offices aim to reduce energy consumption and CO2 emissions by 30% from current best practice guidelines and sustain comfortable environmental conditions without the use of air conditioning. The passive ventilation stacks, solar shading, and hollow concrete slabs with embedded under floor cooling are key features of this building. Ventilation and heating systems are controlled by the

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building management system (BMS) while a degree of user override is provided to adjust conditions to occupants' needs. The building utilizes five vertical shafts as an integral part of the ventilation and cooling strategy. The main components of theses stacks are a south facing glass-block wall, thermal mass walls and stainless steel round exhausts rising a few meters above roof level. The chimneys are connected to the curved hollow concrete floor slabs which are cooled via night ventilation. Pipes embedded in the floor can provide additional cooling utilizing groundwater. On warm windy days air is drawn in through passages in the curved hollow concrete floor slabs. Stack ventilation naturally rising out through the stainless steel chimneys enhances the air flow through the building. The movement of air across the chimney tops enhances the stack effect. During warm, still days, the building relies mostly on the stack effect while air is taken from the shady north side of the building. Low-energy fans in the tops of the stacks can also be used to improve airflow. Overnight, control systems enable ventilation paths through the hollow concrete slab removing the heat stored during the day and storing 'coolth' for the following day. The exposed curved ceiling gives more surface area than a flat ceiling would, acting as a cool 'radiator', again providing summer cooling. Research based on actual performance measurements of the passive stacks found that they enhanced the cooling ventilation of the space during warm and still days and may also have the potential to assist night-time cooling due to their thermally massive structure.

PASSIVE DOWN-DRAFT COOLTOWER A technology closely related to the solar chimney is the evaporative down-draft cooltower. In areas with a hot, arid climate this approach may contribute to a sustainable way to provide air conditioning for buildings. Evaporation of moisture from the pads on top of the Toguna buildings built by the Dogon people of Mali, Africa contribute to the coolness felt by the men who rest underneath. The women's buildings on the outskirts of town are functional as more conventional solar chimneys.


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The principle is to allow water to evaporate at the top of a tower, either by using evaporative cooling pads or by spraying water. Evaporation cools the incoming air, causing a downdraft of cool air that will bring down the temperature inside the building. Airflow can be increased by using a solar chimney on the opposite side of the building to help in venting hot air to the outside. This concept has been used for the Visitor Center of Zion National Park. The Visitor Center was designed by the High Performance Buildings Research of the National Renewable Energy Laboratory (NREL). The principle of the downdraft cooltower has been proposed for solar power generation as well.

SOLAR COLLECTOR A solar collector is a device for extracting the energy of the sun not indirectly into a more usable or storable form. The energy in sunlight is in the form of electromagnetic radiation from the infrared (long) to the ultraviolet (short) wavelengths. The solar energy striking the earth's surface at any one time depends on weather conditions, as well as location and orientation of the surface, but overall, it averages about 1000 watts per square meter on a clear day with the surface directly perpendicular to the sun's rays. The best designed solar collectors are the ones that collect the most solars. Glazing is a common process used to increase the absorption rate of solars.

Design The solar heating system consists of the collector described above; a heat transfer circuit that includes the fluid and the means to circulate it; and a storage system including a heat exchanger (if the fluid cirulating through the collector is not the same liquid being used to heat the object of the system). The system may or may not include secondary distribution of heat among different storage reservoirs or users of the heat. The system can be used in a variety of ways, including warming domestic hot water, heating swimming pools, heating water for a radiator or floor-coil heating circuit, heating an industrial dryer, or providing input energy for a cooling system, among others. In

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addition, glazing is a process in which a thin layer of 5hydroxymethylfurfural is applied to improve heat rejection at low light wavelengths. The heat is normally stored in insulated storage tanks full of water. Heat storage is usually intended to cover a day or two's requirements, but other concepts exist including seasonal storage (where summer solar energy is used for winter heating by just raising the temperature by a few degrees of several million liters of water (numerous pilot housing projects in Germany and elsewhere use this concept).

System Types For solar heating of domestic hot water, two common system types are thermosyphon and pumped. In the thermosyphon system, a storage tank is placed above the collector. As the water in the collector is heated, it will rise and naturally start to circulate around the tank. This draws in colder water from the bottom of the tank. This system is self-regulating and requires no moving parts or external energy, so is very attractive. Its main drawback is the need for the tank to be placed at a level higher than the collector, which may prove to be physically difficult. A pumped system uses a pump to circulate the water, so the tank can be positioned independently of the collector location. This system requires external energy to run the pump (though this can be solar, since the water should only be circulated when there is incident sunlight). It also requires control electronics to measure the temperature gradient across the collector and modulate the pump accordingly. Systems using solar electric pumping and controls are known as zero carbon solar while those using mains electricity are known as low carbon, since they typically have a 10-20% carbon drawback.


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sunshine is readily available, a 2 to 10 square metre array will provide all the hot water heating required for a typical family house. Such systems are a key feature of sustainable housing, since water and space heating is usually the largest single consumer of energy in households. SOLAR THERMAL COLLECTORS A solar thermal collector that stores heat energy is called a "batch" type system. Other types of solar thermal collectors do not store energy but instead use fluid circulation (usually water or an antifreeze solution) to transfer the heat for direct use or storage in an insulated reservoir. Water/glycol has a high thermal capacity and is therefore convenient to handle. The direct radiation is captured using a dark colored surface which absorbs the radiation as heat and conducts it to the transfer fluid. Metal makes a good thermal conductor, especially copper and aluminium. In high performance collectors, a "selective surface" is used in which the collector surface is coated with a material having properties of high-absorption and low-emissivity. The selective surface reduces heat-loss caused by infrared radiant emission from the collector to ambient. Another method of reducing radiant heat-loss employs a transparent window such as clear UV stabilized plastic or Low-emissivity glass plate. Again, Low-E materials are the most effective, particularly the type optimized for solar gain. Borosilicate glass or "Pyrex" (tm) has low-emissivity properties, which may be useful, particularly for solar cooking applications.

Placement

As it heats up, thermal losses from the collector itself will reduce its efficiency, resulting in increased radiation, primarily infrared. This is countered in two ways. First, a glass plate is placed above the collector plate which will trap the radiated heat within the airspace below it.

Solar collectors can be mounted on a roof but need to face the sun, so a north-facing roof in the southern hemisphere, and a south-facing roof in the northern hemisphere is ideal. Collectors are usually also angled to suit the latitude of the location. Where

This exploits the so-called greenhouse effect, which is in this case a property of the glass: it readily transmits solar radiation in the visible and ultraviolet spectrum, but does not transmit the lower frequency infrared re-radiation very well. The glass plate

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The Solar Combisystem

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also traps air in the space, thus reducing heat losses by convection. The collector housing is also insulated below and laterally to reduce its heat loss. The second way efficiency is improved is by cooling the absorber plate. This is done by ensuring that the coldest available heat transfer fluid is circulated through the absorber, and with a sufficient flow rate. The fluid carries away the absorbed heat, thus cooling the absorber. The warmed fluid leaving the collector is either directly stored, or else passes through a heat exchanger to warm another tank of water, or is used to heat a building directly. The temperature differential across an efficient solar collector is usually only 10 or 20°C. While a large differential may seem impressive, it is in fact an indication of a less efficient design.

7 THE SOLAR COMBISYSTEM A solar combisystem is a solar heating system that provides both space heating and hot water from a common array of solar thermal collectors, normally linked to an auxiliary non-solar heat source. Solar combisystems may range in size from those installed in individual properties to those serving several in a block heating scheme. Those serving larger groups of properties via district heating tend to be called central solar heating schemes. A large number of different types of solar combisystems are produced - over 20 were identified in the first international survey, conducted as part of IEA Task 14 in 1997. The systems on the market in a particular country may be more restricted, however, as different systems have tended to evolve in different countries. Prior to the 1990s such systems tended to be custom-built for each property. Since then commercialised packages have developed and are now generally used. Depending on the size of the combisystem installed, the annual space heating contribution can range from 10% to 60% or more in ultra-low energy Passivhaus type buildings; even up to up to 100% where a large seasonal thermal store is used. The remaining heat requirement is supplied by one or more auxiliary sources in order to maintain the heat supply once the solar heated water is exhausted. Such auxiliary heat sources may also use other renewable energy sources. During 2001, around 50% of all the domestic solar collectors installed in Austria, Switzerland, Denmark and Norway were to supply combisystems, while in Sweden it was greater. In Germany, where the total collector area installed (900,000 m2) was much

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larger than in the other countries, 25% was for combisystem installations. Combisystems have also been installed in Canada since the mid 1980s. It has been suggested that in future combisystems might be able to incorporate absorption solar cooling in summer .

CLASSIFICATION Following the work of IEA Task 26 (1998 to 2002), solar combisystems can be classified according to two main aspects; firstly by the heat storage category (the way in which water is added to and drawn from the storage tank and its effect on stratification); secondly by the auxiliary heat management category (the way in which non-solar auxiliary heaters are integrated into the system). Maintaining stratification (the variation in water temperature from cooler at the foot of a tank to warmer at the top) is important so that the combisystem can supply hot water and space heating water at different temperatures. HEAT STORAGE CATEGORY

No Controlled Storage Device for Space Heating B Heat management and stratification enhancement by means of multiple tanks and/or by multiple inlet/outlet pipes and/or by three- or four-way valves to control flow through the inlet/outlet pipes C Heat management using natural convection in storage tanks and/or between them to maintain stratification to a certain extent. D Heat management using natural convection in storage tanks and built-in stratification devices. Heat management by natural convection in storage tanks and built-in stratifiers as well as multiple tanks and/or multiple inlet/outlet pipes and/or three-or four-way valves to control flow through the inlet/outlet pipes.

Auxiliary Heat Management Categories M Mixed mode: The space heating loop is fed from a single store heated by both solar collectors and the auxiliary heater


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P Parallel mode: The space heating loop is fed alternatively by the solar collectors (or a solar water storage tank), or by the auxiliary heater; or there is no hydraulic connection between the solar heat distribution and the auxiliary heat emissions S Serial mode: The space heating loop may be fed by the auxiliary heater, or by both the solar collectors (or a solar water storage tank) and the auxiliary heater connected in series on the return line of the space heating loop. A solar combisystem may therefore be described as being of type B/DS, CS, etc. Within these types, systems may be configured in many different ways. For the individual house they may - or may not - have the storage tanks, controls and auxiliary heater integrated into a single prefabricated package. In contrast, there are also large centralised systems serving a number of properties. The simplest combisystems - the Type A - have no 'controlled storage device'. Instead they pump warm water from the solar collectors through underfloor central heating pipes embedded in the concrete floor slab. The floor slab is thickened to provide thermal mass and so that the heat from the pipes (at the bottom of the slab) is released during the evening.

COMBISYSTEM DESIGN The size and complexity of combisystems, and the number of options available, mean that comparing design alternatives is not straightforward. Useful approximations of performance can be produced relatively easily, however accurate predictions remain difficult. Tools for designing solar combisystems are available, varying from manufacturer's guidelines to nomograms (such as the one developed for IEA Task 26) to various computer simulation software of varying complexity and accuracy. Among the software and packages are CombiSun (released free by the Task 26 team , which can be used for basic system sizing) and the free SHWwin (Austria, in German ). Other commercial systems are available.

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TECHNOLOGIES Solar combisystems use similar technologies to those used for solar hot water and for regular central heating and underfloor heating, as well as those used in the auxiliary systems microgeneration technologies or otherwise. The element unique to combisystems is the way that these technologies are combined, and the control systems used to integrate them, plus any stratifier technology that might be employed.

RELATIONSHIP

TO

LOW ENERGY BUILDING

By the end of the 20th century solar hot water systems had been capable of meeting a significant portion of domestic hot water in many climate zones. However it was only with the development of reliable low-energy building techniques in the last decades of the century that extending such systems for space heating became realistic in temperate and colder climatic zones. As heat demand reduces, the overall size and cost of the system is reduced, and the lower water temperatures typical of solar heating may be more readily used - especially when coupled with underfloor heating, but radiators no longer need to be grossly oversized to compensate if not. The volume occupied by the equipment also reduces, which also increases the flexibility in its location, which can be of particular importance in individual houses. In common with other heating systems in low-energy buildings, system performance is more sensitive to the number of occupants, room temperature and ventilation rates, when compared to regular buildings where such effects are small in relation to the higher overall energy demand.

SOLAR RADIATION Solar radiation is radiant energy emitted by a sun as a result of its nuclear fusion reactions. The spectrum of the Sun's solar radiation is close to that of a black body with a temperature of about 5,800 K. About half that lies in the visible short-wave part of the electromagnetic spectrum


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and the other half mostly in the near-infrared part. Some also lies in the ultraviolet part of the spectrum. When ultraviolet radiation is not absorbed by the atmosphere or other protective coating, it can cause a change in human skin pigmentation. Solar radiation is commonly measured with a pyranometer or pyrheliometer.

SOLAR CONSTANT The solar constant is the amount of the Sun's incoming electromagnetic radiation (Solar radiation) per unit area, measured on the outer surface of Earth's atmosphere in a plane perpendicular to the rays. The solar constant includes all types of solar radiation, not just the visible light. It is measured by satellite to be roughly 1366 watts per square meter (W/m²), though this fluctuates by about 6.9% during a year (from 1412 W/m² in early January to 1321 W/m² in early July) due to the earth's varying distance from the Sun, and by a few parts per thousand from day to day. Thus, for the whole Earth (which has a cross section of 127,400,000 km²), the power is 1.740×1017 W, plus or minus 3.5%. The Solar constant does not remain constant over long periods of time (see Solar variation). 1366 W/m² is equivalent to 1.96 calories per minute per square centimeter, or 1.96 langleys (Ly) per minute. The Earth receives a total amount of radiation determined by its cross section (?·RE²), but as it rotates this energy is distributed across the entire surface area (4·?·RE²). Hence the average incoming solar radiation (sometimes called the solar irradiance), taking into account the angle at which the rays strike and that at any one moment half the planet does not receive any solar radiation, is one-fourth the solar constant (approximately 342 W/m²). At any given moment, the amount of Solar radiation received at a location on the Earth's surface depends on the state of the atmosphere and the location's latitude. The solar constant includes all wavelengths of solar electromagnetic radiation, not just the visible light (see Electromagnetic spectrum). It is linked to the apparent magnitude of the Sun, ?26.8, in that the solar constant and the magnitude of the Sun are two methods of describing the apparent brightness of the Sun, though the magnitude only measures the visual output of the Sun.

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In 1884, Samuel Pierpont Langley attempted to estimate the Solar constant from Mount Whitney in California. By taking readings at different times of day, he attempted to remove effects due to atmospheric absorption. However, the value he obtained, 2903 W/m², was still too great. Between 1902 and 1957, measurements by Charles Greeley Abbot and others at various high-altitude sites found values between 1322 and 1465 W/m². Abbott proved that one of Langley's corrections was erroneously applied. His results varied between 1.89 and 2.22 calories (1318 to 1548 W/m²), a variation that appeared to be due to the Sun and not the Earth's atmosphere. The angular diameter of the Earth as seen from the Sun is approximately 1/11,000 radians, meaning the solid angle of the Earth as seen from the sun is approximately 1/140,000,000 steradians. Thus the Sun emits about two billion times the amount of radiation that is caught by Earth, in other words about 3.86×1026 watts.

CLIMATE EFFECT OF SOLAR RADIATION On Earth, solar radiation is obvious as daylight when the sun is above the horizon. This is during daytime, and also in summer near the poles at night, but not at all in winter near the poles. When the direct radiation is not blocked by clouds, it is experienced as sunshine, combining the perception of bright white light (sunlight in the strict sense) and warming. The warming on the body and surfaces of other objects is distinguished from the increase in air temperature. The amount of radiation intercepted by a planetary body varies inversely with the square of the distance between the star and the planet. The Earth's orbit and obliquity change with time (over thousands of years), sometimes forming a nearly perfect circle, and at other times stretching out to an orbital eccentricity of 5% (currently 1.67%). The total insolation remains almost constant but the seasonal and latitudinal distribution and intensity of solar radiation received at the Earth's surface also varies. For example, at latitudes of 65 degrees the change in solar energy in summer & winter can vary by more than 25% as a result of the Earth's orbital variation. Because changes in winter and summer


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tend to offset, the change in the annual average insolation at any given location is near zero, but the redistribution of energy between summer and winter does strongly affect the intensity of seasonal cycles. Such changes associated with the redistribution of solar energy are considered a likely cause for the coming and going of recent ice ages. In advanced countries, the environmental advantages, a desire for energy independence, and not heating up the house on a hot day are usually cited as advantages. In the developing world, other advantages include: • lower cost compared to firewood or cooking oil; • greater safety for children and the cook compared to a fire or stove; • lower likelihood of starting a fire that could destroy a family's home; and • less time spent cooking compared to tending a fire or stove.

DISADVANTAGES Solar cookers provide hot food during or shortly after the hottest part of the day, when people are less inclined to eat a hot meal. However, a thick pan that conducts heat slowly (such as Cast Iron) will lose heat at a slower rate, and that, combined with the insulation of the oven or an insulated basket, can be used to keep food warm well into the evening. Solar cookers take longer to cook food compared to an oven. Using a solar oven therefore requires that food preparation be started several hours before the meal. However, it requires less hands-on time cooking, so this is often considered a reasonable trade-off. SOLAR COOKING PROJECTS Michael Hönes of Germany has established solar cooking in Lesotho, enabling small groups of women to build up community bakeries using solar ovens. USE

DARFUR REFUGEE CAMPS Cardboard, aluminum foil, and plastic bags for well over 10,000 solar cookers have been donated to the Iridimi refugee IN

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camp and Touloum refugee camps in Chad by the combined efforts of the Jewish World Watch, the Dutch foundation KoZon, and Solar Cookers International. The refugees construct the cookers themselves, using the donated supplies and locally purchased Arabic gum, and use them for midday and evening meals. The goal of this project was to reduce the Darfuri women's need to leave the relative safety of the camp to gather firewood, which exposed them to a high risk of being beaten, raped, kidnapped, or murdered. It has also significantly reduced the amount of time women spend tending open fires each day, with the results that they are healthier and they have more time to grow vegetables for their families and make handicrafts for export.

INDIAN SOLAR COOKER VILLAGE Bysanivaripalle, a silk-producing village that is 125 km (80 mi) northwest of Tirupati in the Indian state of in Andhra Pradesh, is the first of its kind: an entire village that uses only solar cooking. Intersol, an Austrian non-governmental organisation, sponsored the provision of powerful "Sk-14" parabolic solar cookers in 2004. COSMIC RAY Cosmic rays are energetic particles originating from space that impinge on Earth's atmosphere. Almost 90% of all the incoming cosmic ray particles are protons, about 9% are helium nuclei (alpha particles) and about 1% are electrons (beta minus particles). The term "ray" is a misnomer, as cosmic particles arrive individually, not in the form of a ray or beam of particles. The variety of particle energies reflects the wide variety of sources. The origins of these particles range from energetic processes on the Sun all the way to as yet unknown events in the farthest reaches of the visible universe. Cosmic rays can have energies of over 1020 eV, far higher than the 1012 to 1013 eV that man-made particle accelerators can produce. There has been interest in investigating cosmic rays of even greater energies. COSMIC RAY SOURCES Most cosmic rays originate from extrasolar sources within our own galaxy such as rotating neutron stars, supernovae, and black holes. However, the fact that some cosmic rays have extremely


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high energies provides evidence that at least some must be of extra-galactic origin (e.g. radio galaxies and quasars); the local galactic magnetic field would not be able to contain particles with such a high energy. The origin of cosmic rays with energies up to 1014 eV can be accounted for in terms of shock-wave acceleration in supernova shells. The origin of cosmic rays with energy greater than 1014 eV remained unknown until recently, when a large collaborative experiment at the Pierre Auger Observatory appears to have answered this question. In preliminary results announced in November 2007, they showed a strong correlation between their 27 most energetic events and active galactic nuclei AGN. These results demonstrated that there is only a small chance (less than 1/100) that the highest energy protons originated from outside the AGN. Observations have shown that cosmic rays with an energy above 10 GeV (10 x 109 eV) approach the Earth's surface isotropically (equally from all directions); it has been hypothesized that this is not due to an even distribution of cosmic ray sources, but instead is due to galactic magnetic fields causing cosmic rays to travel in spiral paths. This limits cosmic ray's usefulness in positional astronomy as they carry no information of their direction of origin. At energies below 10 GeV there is a directional dependence, due to the interaction of the charged component of the cosmic rays with the Earth's magnetic field.

SOLAR COSMIC RAYS Solar cosmic rays or solar energetic particles (SEP) are cosmic rays that originate from the Sun. The average composition is similar to that of the Sun itself. There exists no clear and sharp boundary between the phase spaces of the solar wind and SEP plasma particle populations. The name solar cosmic ray itself is a misnomer because the term cosmic implies that the rays are from the cosmos and not the solar system, but it has stuck. The misnomer arose because there is continuity in the energy spectra, i.e., the flux of particles as a function of their energy, because the low-energy solar cosmic rays fade more or less smoothly into the galactic ones as one looks at increasingly higher energies. Until the mid-1960s the energy

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distributions were generally averaged over long time intervals, which also obscured the difference. Later, it was found that the solar cosmic rays vary widely in their intensity and spectrum, increasing in strength after some solar events such as solar flares. Further, an increase in the intensity of solar cosmic rays is followed by a decrease in all other cosmic rays, called the Forbush decrease after their discoverer, the physicist Scott Forbush. These decreases are due to the solar wind with its entrained magnetic field sweeping some of the galactic cosmic rays outwards, away from the Sun and Earth. The overall or average rate of Forbush decreases tends to follow the 11-year sunspot cycle, but individual events are tied to events on the Sun, as explained above. There are further differences between cosmic rays of solar and galactic origin, mainly in that the galactic cosmic rays show an enhancement of heavy elements such as calcium, iron and gallium, as well as of cosmically rare light elements such as lithium and beryllium. The latter result from the cosmic ray spallation (fragmentation) of heavy nuclei due to collisions in transit from the distant sources to the solar system.

ANOMALOUS COSMIC RAYS Anomalous cosmic rays (ACRs) are cosmic rays with unexpectedly low energies. They are thought to be created near the edge of our solar system, in the heliosheath, the border region between the heliosphere and the interstellar medium. When electrically neutral atoms are able to enter the heliosheath (being unaffected by its magnetic fields) subsequently become ionized, they are thought to be accelerated into low-energy cosmic rays by the solar wind's termination shock which marks the inner edge of the heliosheath. It is also possible that high energy galactic cosmic rays which hit the shock front of the solar wind near the heliopause might be decelerated, resulting in their transformation into lower-energy anomalous cosmic rays. The Voyager 1 space probe crossed the termination shock on December 16, 2004, according to papers published in the journal Science. Readings showed particle acceleration, but not of the kind that generates ACRs. It is unclear at this stage (September 2005) if this is typical of the termination shock (requiring a major


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rethink of the origin of ACRs), or a localised feature of that part of the termination shock that Voyager 1 passed through. Voyager 2 is expected to cross the termination shock during or after 2008, which will provide more data.

COMPOSITION Cosmic rays may broadly be divided into two categories, primary and secondary. The cosmic rays that arise in extrasolar astrophysical sources are primary cosmic rays; these primary cosmic rays can interact with interstellar matter to create secondary cosmic rays. The sun also emits low energy cosmic rays associated with solar flares. The exact composition of primary cosmic rays, outside the Earth's atmosphere, is dependent on which part of the energy spectrum is observed. However, in general, almost 90% of all the incoming cosmic rays are protons, about 9% are helium nuclei (alpha particles) and about 1% are electrons. The remaining fraction is made up of the other heavier nuclei which are abundant end products of star's nuclear synthesis. Secondary cosmic rays consist of the other nuclei which are not abundant nuclear synthesis end products, or products of the Big Bang, primarily lithium, beryllium and boron. These light nuclei appear in cosmic rays in much greater abundance (about 1:100 particles) than in solar atmospheres, where their abundance is about 10-7 that of helium. This abundance difference is a result of the way secondary cosmic rays are formed. When the heavy nuclei components of primary cosmic rays, namely the carbon and oxygen nuclei, collide with interstellar matter, they break up into lighter nuclei (in a process termed cosmic ray spallation), into lithium, beryllium and boron. It is found that the energy spectra of Li, Be and B falls off somewhat steeper than that of carbon or oxygen, indicating that less cosmic ray spallation occurs for the higher energy nuclei presumably due to their escape from the galactic magnetic field. Spallation is also responsible for the abundances of Sc, Ti, V and Mn elements in cosmic rays, which are produced by collisions of Fe and Ni nuclei with interstellar matter. In the past, it was believed that the cosmic ray flux has remained fairly constant over time. Recent research has, however, produced evidence for 1.5 to 2-fold millennium-timescale changes in the cosmic ray flux in the past forty thousand years.

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MODULATION The flux (flow rate) of cosmic rays incident on the Earth's upper atmosphere is modulated (varied) by two processes; the sun's solar wind and the Earth's magnetic field. Solar wind is expanding magnetized plasma generated by the sun, which has the effect of decelerating the incoming particles as well as partially excluding some of the particles with energies below about 1 GeV. The amount of solar wind is not constant due to changes in solar activity over its regular eleven-year cycle. Hence the level of modulation varies in autocorrelation with solar activity. Also the Earth's magnetic field deflects some of the cosmic rays, which is confirmed by the fact that the intensity of cosmic radiation is dependent on latitude, longitude and azimuth. The cosmic flux varies from eastern and western directions due to the polarity of the Earth's geomagnetic field and the positive charge dominance in primary cosmic rays; this is termed the eastwest effect. The cosmic ray intensity at the equator is lower than at the poles as the geomagnetic cutoff value is greatest at the equator. This can be understood by the fact that charged particle tend to move in the direction of field lines and not across them. This is the reason the Aurorae occur at the poles, since the field lines curve down towards the Earth's surface there. Finally, the longitude dependence arises from the fact that the geomagnetic dipole axis is not parallel to the Earth's rotation axis. This modulation which describes the change in the interstellar intensities of cosmic rays as they propagate in the heliosphere is highly energy and spatial dependent, and it is described by the Parker's Transport Equation in the heliosphere. At large radial distances, far from the Sun ~ 94 AU, there exists the region where the solar wind undergoes a transition from supersonic to subsonic speeds called the solar wind termination shock. The region between the termination shock and the heliospause (the boundary marking the end of the heliosphere) is called the heliosheath. This region acts as a barrier to cosmic rays and it decreases their intensities at lower energies by about 90% indicating that it is not only the Earth's magnetic field that protect us from cosmic ray bombardment. For more on this topic and how the barrier effects


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occur the agile reader is referred to Mabedle Donald Ngobeni and Marius Potgieter (2007), and Mabedle Donald Ngobeni (2006). From modelling point of view, there is a challenge in determining the Local Interstellar spectra (LIS) due to large adiabatic energy changes these particles experience owing to the diverging solar wind in the heliosphere. However, significant progress has been made in the field of cosmic ray studies with the development of an improved state-of-the-art 2D numerical model that includes the simulation of the solar wind termination shock, drifts and the heliosheath coupled with fresh descriptions of the diffusion tensor, see Langner et al. (2004). But challenges also exist because the structure of the solar wind and the turbulent magnetic field in the heliosheath is not well understood indicating the heliosheath as the region unknown beyond. With lack of knowledge of the diffusion coefficient perpendicular to the magnetic field our knowledge of the heliosphere and from the modelling point of view is far from complete. There exist promising theories like ab initio approaches, but the drawback is that such theories produce poor compatibility with observations (Minnie, 2006) indicating their failure in describing the mechanisms influencing the cosmic rays in the heliosphere.

DETECTION The nuclei that make up cosmic rays are able to travel from their distant sources to the Earth because of the low density of matter in space. Nuclei interact strongly with other matter, so when the cosmic rays approach Earth they begin to collide with the nuclei of atmospheric gases. These collisions, in a process known as a shower, result in the production of many pions and kaons, unstable mesons which quickly decay into muons. Because muons do not interact strongly with the atmosphere and because of the relativistic effect of time dilation many of these muons are able to reach the surface of the Earth. Muons are ionizing radiation, and may easily be detected by many types of particle detectors such as bubble chambers or scintillation detectors. If several muons are observed by separated detectors at the same instant it is clear that they must have been produced in the same shower event.

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DETECTION BY PARTICLE TRACK-ETCH TECHNIQUE Cosmic rays can also be detected directly when they pass through particle detectors flown aboard satellites or in high altitude balloons. In a pioneering technique developed by P. Buford Price et al., sheets of clear plastic such as 1/4 mil Lexan polycarbonate can be stacked together and exposed directly to cosmic rays in space or high altitude. When returned to the laboratory, the plastic sheets are "etched" literally, slowly dissolved in warm caustic sodium hydroxide solution, which slowly removes the surface material at a slow, known rate. Wherever a bare cosmic ray nucleus passes through the detector, the nuclear charge causes chemical bond breaking in the plastic. The slower the particle, the more extensive is the bond-breaking along the path; and the higher the charge the higher the Z, the more extensive is the bond-breaking along the path. The caustic sodium hydroxide dissolves at a faster rate along the path of the damage, but thereafter dissolves at the slower base-rate along the surface of the minute hole that was drilled. The net result is a conical shaped pit in the plastic; typically with two pits per sheet one originating from each side of the plastic. The etch pits can be measured under a high power microscope typically 1600X oil-immersion, and the etch rate plotted as a function of the depth in the stack of plastic. At the top of the stack, the ionization damage is less due to the higher speed. As the speed decreases due to deceleration in the stack, the ionization damage increases along the path. This generates a unique curve for each atomic nucleus of Z from 1 to 92, allowing identification of both the charge and energy speed of the particle that traverses the stack. This technique has been used with great success for detecting not only cosmic rays, but fission product nuclei for neutron detectors. INTERACTION WITH THE EARTH'S ATMOSPHERE When cosmic ray particles enter the Earth's atmosphere they collide with molecules, mainly oxygen and nitrogen, to produce a cascade of lighter particles, a so-called air shower. The general idea is shown in the figure which shows a cosmic ray shower produced by a high energy proton of cosmic ray origin striking


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an atmospheric molecule. Cosmic rays kept the level of carbon14 in the atmosphere roughly constant (70 tons) for at least the past 100,000 years, until the beginning of aboveground nuclear weapons testing in the early 1950s. This is an important fact used in radiocarbon dating which is used in archaeology.

UNUSUAL COSMIC RAYS In 1975, a team of researchers headed by P. Buford Price at U.C. Berkeley announced the discovery of a cosmic ray track in a particle detector slung under a high-altitude balloon that was significantly different from all others ever measured. Using the particle track-etch method pioneered by Price, et al., they discovered the track of a particle that had passed through 32 sheets of 1/4 mil Lexan plastic without any measurable change in ionization. Yet, the Cerenkov detector admitted only of particles less than 2/3 c the speed of light in the clear plastic. The charge was measured as being 137, the same as predicted by Paul Dirac who first predicted the theoretical existence of magnetic monopoles. The particle track preliminarily identified as having been caused by a magnetic monopole had been spotted by technical assistant Walter L. Wagner. A possible alternative explanation was offered by Alvarez. In his paper it was demonstrated that the path of the cosmic ray event that was claimed to be due to a magnetic monopole could be reproduced by a path followed by a platinum nucleus fragmenting to osmium and then to tantalum. RESEARCH AND EXPERIMENTS There are a number of cosmic ray research initiatives. These include, but are not limited to: • CHICOS • PAMELA • Alpha Magnetic Spectrometer • MARIACHI • Pierre Auger Observatory • Spaceship Earth After the discovery of radioactivity by Henri Becquerel in 1896, it was generally believed that atmospheric electricity (ionization of the air) was caused only by radiation from radioactive

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elements in the ground or the radioactive gases (isotopes of radon) they produce. Measurements of ionization rates at increasing heights above the ground during the decade from 1900 to 1910 showed a decrease that could be explained as due to absorption of the ionizing radiation by the intervening air. In 1912 Domenico Pacini observed simultaneous variations of the rate of ionization over a lake, and over the sea. Pacini concluded that a certain part of the ionization must be due to sources other than the radioactivity of the Earth or the air. Then, in 1912, Victor Hess carried three Wulf electrometers (a device to measure the rate of ion production inside a hermetically sealed container) to an altitude of 5300 meters in a free balloon flight. He found the ionization rate increased approximately fourfold over the rate at ground level. He concluded "The results of my observation are best explained by the assumption that a radiation of very great penetrating power enters our atmosphere from above." In 1913-14, Werner Kolhörster confirmed Victor Hess' results by measuring the increased ionization rate at an altitude of 9 km. Hess received the Nobel Prize in Physics in 1936 for his discovery of what came to be called "cosmic rays". For many years it was generally believed that cosmic rays were high-energy photons (gamma rays) with some secondary electrons produced by Compton scattering of the gamma rays. Then, during the decade from 1927 to 1937 a wide variety of experimental investigations demonstrated that the primary cosmic rays are mostly positively charged particles, and the secondary radiation observed at ground level is composed primarily of a "soft component" of electrons and photons and a "hard component" of penetrating particles, muons. The muon was initially believed to be the unstable particle predicted by Hideki Yukawa in 1935 in his theory of the nuclear force. Experiments proved that the muon decays with a mean life of 2.2 microseconds into an electron and two neutrinos, but that it does not interact strongly with nuclei, so it could not be the Yukawa particle. The mystery was solved by the discovery in 1947 of the pion, which is produced directly in high-energy nuclear interactions. It decays into a muon and one neutrino with a mean life of 0.0026 microseconds. The pion?muon?electron decay sequence was observed directly in a microscopic examination of


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particle tracks in a special kind of photographic plate called a nuclear emulsion that had been exposed to cosmic rays at a highaltitude mountain station. In 1948, observations with nuclear emulsions carried by balloons to near the top of the atmosphere by Gottlieb and Van Allen showed that the primary cosmic particles are mostly protons with some helium nuclei (alpha particles) and a small fraction heavier nuclei. In 1934 Bruno Rossi reported an observation of nearsimultaneous discharges of two Geiger counters widely separated in a horizontal plane during a test of equipment he was using in a measurement of the so-called east-west effect. In his report on the experiment, Rossi wrote "...it seems that once in a while the recording equipment is struck by very extensive showers of particles, which causes coincidences between the counters, even placed at large distances from one another. Unfortunately, he did not have the time to study this phenomenon more closely." In 1937 Pierre Auger, unaware of Rossi's earlier report, detected the same phenomenon and investigated it in some detail. He concluded that extensive particle showers are generated by high-energy primary cosmic-ray particles that interact with air nuclei high in the atmosphere, initiating a cascade of secondary interactions that ultimately yield a shower of electrons, photons, and muons that reach ground level. Homi J. Bhabha derived an expression for the probability of scattering positrons by electrons, a process now known as Bhabha scattering. His classic paper, jointly with Warren Heitler, published in 1937 described how primary cosmic rays from space interact with the upper atmosphere to produce particles observed at the ground level. Bhabha and Heitler explained the cosmic ray shower formation by the cascade production of gamma rays and positive and negative electron pairs. In 1938 Bhabha concluded that observations of the properties of such particles would lead to the straightforward experimental verification of Albert Einstein's theory of relativity. Measurements of the energy and arrival directions of the ultra-high-energy primary cosmic rays by the techniques of "density sampling" and "fast timing" of extensive air showers were first carried out in 1954 by members of the Rossi Cosmic Ray

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Group at the Massachusetts Institute of Technology. The experiment employed eleven scintillation detectors arranged within a circle 460 meters in diameter on the grounds of the Agassiz Station of the Harvard College Observatory. From that work, and from many other experiments carried out all over the world, the energy spectrum of the primary cosmic rays is now known to extend beyond 1020 eV (past the GZK cutoff, beyond which very few cosmic rays should be observed). A huge air shower experiment called the Auger Project is currently operated at a site on the pampas of Argentina by an international consortium of physicists. Their aim is to explore the properties and arrival directions of the very highest energy primary cosmic rays. The results are expected to have important implications for particle physics and cosmology. In November, 2007 preliminary results were announced showing direction of origination of the 27 highest energy events were strongly correlated with the locations of active galactic nuclei AGN, where bare protons are believed accelerated by strong magnetic fields associated with the large black holes at the AGN centers to energies of 1E20 eV and higher. Three varieties of neutrino are produced when the unstable particles produced in cosmic ray showers decay. Since neutrinos interact only weakly with matter most of them simply pass through the Earth and exit the other side. They very occasionally interact, however, and these atmospheric neutrinos have been detected by several deep underground experiments. The Super-Kamiokande in Japan provided the first convincing evidence for neutrino oscillation in which one flavour of neutrino changes into another. The evidence was found in a difference in the ratio of electron neutrinos to muon neutrinos depending on the distance they have traveled through the air and earth.

EFFECTS

Role in Ambient Radiation Cosmic rays constitute a fraction of the annual radiation exposure of human beings on earth. For example, the average radiation exposure in Australia is 0.3 mSv due to cosmic rays, out of a total of 2.3 mSv.


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Effect on Electronics Cosmic rays have sufficient energy to alter the states of elements in electronic integrated circuits, causing transient errors to occur, such as corrupted data in memory, or incorrect behavior of a CPU. This has been a problem in high-altitude electronics, such as in satellites, but as transistors become smaller it is becoming an increasing concern in ground-level equipment as well. To alleviate this problem, Intel has proposed a cosmic ray detector which could be integrated into future high-density microprocessors, allowing the processor to repeat the last command following a cosmic ray event.

Significance to Space Travel Galactic cosmic rays are one of the most important barriers standing in the way of plans for interplanetary travel by crewed spacecraft. See Health threat from cosmic rays.

Role in Lightning Cosmic rays have been implicated in the triggering of electrical breakdown in lightning. It has been proposed that essentially all lightning is triggered through a relativistic process, "runaway breakdown", seeded by cosmic ray secondaries. Subsequent development of the lightning discharge then occurs through "conventional breakdown" mechanisms.

Role in Climate Change Whether cosmic rays have any role in climate change is disputed. Different groups have made different arguments regarding the role of cosmic ray forcing in climate change. Shaviv et al. have argued that galactic cosmic ray (GCR) climate signals on geological time scales are attributable to changing positions of the galactic spiral arms of the Milky Way, and that cosmic ray flux variability is the dominant climate driver over these time periods. They also argue that GCR flux variability plays an important role in climate variability over shorter time scales, though the relative contribution of anthropogenic factors in relation to GCR flux presently is a matter of continued debate. Because of uncertainty about which GCR energies are the most important

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drivers of cloud cover variation (if any), and because of the paucity of historical data on cosmic ray flux at various ranges of energies, controversies remain. Henrik Svensmark et al. have argued that solar variations modulate the cosmic ray signal seen at the earth and that this would affect cloud formation and hence climate. Cosmic rays have been experimentally determined to be able to produce ultrasmall aerosol particles, orders of magnitude smaller than cloud condensation nuclei (CCN). Whether this mechanism is relevant to the real atmosphere is unknown; in particular, the steps from this to modulation of cloud formation and thence climate have not been established. The analogy is with the Wilson cloud chamber, however acting on a global scale, where earth's atmosphere acts as the cloud chamber and the cosmic rays catalyze the production of CCN. But unlike a cloud chamber, where the air is carefully purified, the real atmosphere always has many CCN naturally. Various proposals have been made for the mechanism by which cosmic rays might affect clouds, including ion mediated nucleation, and indirect effects on current flow density in the global electric circuit (see Tinsley 2000, and F. Yu 1999). Claims have been made of identification of GCR climate signals in atmospheric parameters such as high latitude precipitation (Todd & Kniveton), and Svensmark's annual cloud cover variations, which were said to be correlated to GCR variation. That Svensmark's work can be extrapolated to suggest any meaningful connection with global warming is disputed: At the time we pointed out that while the experiments were potentially of interest, they are a long way from actually demonstrating an influence of cosmic rays on the real world climate, and in no way justify the hyperbole that Svensmark and colleagues put into their press releases and more 'popular' pieces. Even if the evidence for solar forcing were legitimate, any bizarre calculus that takes evidence for solar forcing of climate as evidence against greenhouse gases for current climate change is simply wrong. Whether cosmic rays are correlated with climate or not, they have been regularly measured by the neutron monitor at Climax Station (Colorado) since 1953 and show


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no long term trend. No trend = no explanation for current changes. More recently a Lancaster University study produced further compelling evidence showing that modern-day climate change is not caused by changes in the Sun's activity.

Cosmic Rays and Fiction Because of the metaphysical connotations of the word "cosmic", the very name of these particles enables their misinterpretation by the public, giving them an aura of mysterious powers. Were they merely referred to as "high-speed protons and atomic nuclei" this might not be so. In fiction, cosmic rays have been used as a catchall, mostly in comics (notably the Marvel Comics group the Fantastic Four), as a source for mutation and therefore the powers gained by being bombarded with them. Also, in the book Atlas Shrugged by author Ayn Rand, Dr. Robert Stadler's research of cosmic rays is said to have contributed to Project X: a weapon of mass destruction.

Sunspot A sunspot is a region on the Sun's surface (photosphere) that is marked by a lower temperature than its surroundings and has intense magnetic activity, which inhibits convection, forming areas of reduced surface temperature. They can be visible from Earth without the aid of a telescope. Although they are blindingly bright at temperatures of roughly 4000-4500 K, the contrast with the surrounding material at about 5800 K leaves them clearly visible as dark spots, as the intensity of a heated black body (closely approximated by the photosphere) is a function of T (temperature) to the fourth power. If a sunspot was isolated from the surrounding photosphere it would be brighter than an electric arc. A minimum in the eleven-year sunspot cycle may happen during 2008. While the reverse polarity sunspot observed on 4 January 2008 may represent the start of Cycle 24, no additional sunspots have yet been seen in this cycle. The definition of a new sunspot cycle is when the average number of sunspots of the new cycle's magnetic polarity outnumbers that of the old cycle's polarity. Forecasts in

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2006 predicted cycle 24 to start between late 2007 and early 2008, but new estimates suggest a delay until 2009. Sunspots, being the manifestation of intense magnetic activity, host secondary phenomena such as coronal loops and reconnection events. Most solar flares and coronal mass ejections originate in magnetically active regions around visible sunspot groupings. Similar phenomena indirectly observed on stars are commonly called starspots and both light and dark spots have been measured.

Sunspot Variation Sunspot populations quickly rise and more slowly fall on an irregular cycle about every 11 years. Significant variations of the 11 year period are known over longer spans of time. For example, from 1900 to the 1960s the solar maxima trend of sunspot count has been upward; from the 1960s to the present, it has diminished somewhat.. The Sun is presently at a markedly heightened level of sunspot activity and was last similarly active over 8,000 years ago. The number of sunspots correlates with the intensity of solar radiation over the period (since 1979) when satellite measurements of absolute radiative flux were available. Since sunspots are darker than the surrounding photosphere it might be expected that more sunspots would lead to less solar radiation and a decreased solar constant. However, the surrounding margins of sunspots are hotter than the average, and so are brighter& overall, more sunspots increase the sun's solar constant or brightness. The variation caused by the sunspot cycle to solar output is relatively small, on the order of 0.1% of the solar constant (a peak-to-trough range of 1.3 W m-2 compared to 1366 W m-2 for the average solar constant). This range is slightly smaller than the change in radiative forcing caused by the increase in atmospheric CO2 since the 18th century. During the Maunder Minimum in the 17th Century there were hardly any sunspots at all. This coincides with a period of cooling known as the Little Ice Age. It has been speculated that there may be a resonant gravitational link between a photospheric tidal force from the planets, the dominant component by summing gravitational tidal force (75% being Jupiter's) with an 11 year cycle. Apparent references to sunspots were made by Chinese astronomers in 28 BC (Hanshu, 27), who probably could see the


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largest spot groups when the sun's glare was filtered by windborne dust from the various central Asian deserts. A large sunspot was also seen at the time of Charlemagne's death in 813 A.D. and sunspot activity in 1129 was described by John of Worcester. However, these observations were misinterpreted until Galileo gave the correct explanation in 1612. They were first observed telescopically in late 1610 by the English astronomer Thomas Harriot and Frisian astronomers Johannes and David Fabricius, who published a description in June 1611. At the latter time Galileo had been showing sunspots to astronomers in Rome, and Christoph Scheiner had probably been observing the spots for two or three months. The ensuing priority dispute between Galileo and Scheiner, neither of whom knew of the Fabricius' work, was thus as pointless as it was bitter. Sunspots had some importance in the debate over the nature of the solar system. They showed that the Sun rotated, and their comings and goings showed that the Sun changed, contrary to the teaching of Aristotle. The details of their apparent motion could not be readily explained except in the heliocentric system of Copernicus. The cyclic variation of the number of sunspots was first observed by Heinrich Schwabe between 1826 and 1843 and led Rudolf Wolf to make systematic observations starting in 1848. The Wolf number is an expression of individual spots and spot groupings, which has demonstrated success in its correlation to a number of solar observables. Also in 1848, Joseph Henry projected an image of the Sun onto a screen and determined that sunspots were cooler than the surrounding surface. Wolf also studied the historical record in an attempt to establish a database on cyclic variations of the past. He established a cycle database to only 1700, although the technology and techniques for careful solar observations were first available in 1610. Gustav Spörer later suggested a 70-year period before 1716 in which sunspots were rarely observed as the reason for Wolf's inability to extend the cycles into the seventeenth century. The economist William Stanley Jevons suggested that there is a relationship between sunspots and crises in business cycles. He reasoned that sunspots affect earth's weather, which, in turn, influences crop

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yields and, therefore, the economy. Edward Maunder would later suggest a period over which the Sun had changed modality from a period in which sunspots all but disappeared from the solar surface, followed by the appearance of sunspot cycles starting in 1700. Careful studies revealed the problem not to be a lack of observational data but included references to negative observations. Adding to this understanding of the absence of solar activity cycles were observations of aurorae, which were also absent at the same time. Even the lack of a solar corona during solar eclipses was noted prior to 1715. Sunspot research was dormant for much of the 17th and early 18th centuries because of the Maunder Minimum, during which no sunspots were visible for some years; but after the resumption of sunspot activity, Heinrich Schwabe in 1843 reported a periodic change in the number of sunspots. Since 1981, the Royal Observatory of Belgium keeps track of sunspots as the World data center for the Sunspot Index.

RADIO COMMUNICATIONS INTERFERENCE Solar flares also create a wide spectrum of radio noise; at VHF (and under unusual conditions at HF) this noise may interfere directly with a wanted signal. The frequency with which a radio operator experiences solar flare effects will vary with the approximately 11-year sunspot cycle; more effects occur during solar maximum (when flare occurrence is high) than during solar minimum (when flare occurrence is very low). A radio operator can experience great difficulty in transmitting or receiving signals during solar flares due to more noise and different propagation patterns. SIGNIFICANT EVENTS An extremely powerful flare was emitted toward Earth on 1 September 1859. It interrupted electrical telegraph service and caused visible Aurora Borealis as far south as Havana, Hawaii, and Rome with similar activity in the southern hemisphere. The most powerful flare observed by satellite instrumentation began on 4 November 2003 at 19:29 UTC, and saturated instruments for 11 minutes. Region 486 has been estimated to have produced


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an X-ray flux of X28. Holographic and visual observations indicate significant activity continued on the far side of the Sun.

PHYSICS Although the details of sunspot generation are still somewhat a matter of research, it is quite clear that sunspots are the visible counterparts of magnetic flux tubes in the convective zone of the sun that get "wound up" by differential rotation. If the stress on the flux tubes reaches a certain limit, they curl up quite like a rubber band and puncture the sun's surface. At the puncture points convection is inhibited, the energy flux from the sun's interior decreases, and with it the surface temperature. The Wilson effect tells us that sunspots are actually depressions on the sun's surface. This model is supported by observations using the Zeeman effect that show that prototypical sunspots come in pairs with opposite magnetic polarity. From cycle to cycle, the polarities of leading and trailing (with respect to the solar rotation) sunspots change from north/south to south/north and back. Sunspots usually appear in groups. The sunspot itself can be divided into two parts: • The central umbra, which is the darkest part, where the magnetic field is approximately vertical • The surrounding penumbra, which is lighter, where the magnetic field lines are more inclined. Magnetic field lines would ordinarily repel each other, causing sunspots to disperse rapidly, but sunspot lifetime is about two weeks. Recent observations from the Solar and Heliospheric Observatory (SOHO) using sound waves traveling through the Sun's photosphere to develop a detailed image of the internal structure below sunspots show that there is a powerful downdraft underneath each sunspot, forming a rotating vortex that concentrates magnetic field lines. Sunspots are self-perpetuating storms, similar in some ways to terrestrial hurricanes. Sunspot activity cycles about every eleven years. The point of highest sunspot activity during this cycle is known as Solar Maximum, and the point of lowest activity is Solar Minimum. At the start of a cycle, sunspots tend to appear in the higher latitudes and then move towards the equator as the cycle approaches

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maximum: this is called Spörer's law. Today it is known that there are various periods in the Wolf number sunspot index, the most prominent of which is at about 11 years in the mean. This period is also observed in most other expressions of solar activity and is deeply linked to a variation in the solar magnetic field that changes polarity with this period, too. A modern understanding of sunspots starts with George Ellery Hale, in which magnetic fields and sunspots are linked. Hale suggested that the sunspot cycle period is 22 years, covering two polar reversals of the solar magnetic dipole field. Horace W. Babcock later proposed a qualitative model for the dynamics of the solar outer layers. The Babcock Model explains the behavior described by Spörer's law, as well as other effects, as being due to magnetic fields which are twisted by the Sun's rotation.

SUNSPOT OBSERVATION Sunspots are observed with land-based solar telescopes as well as ones on Earth-orbiting satellites. These telescopes use filtration and projection techniques for direct observation, in additional to filtered cameras of various types. Specialized tools such as spectroscopes and spectrohelioscopes are used to examine sunspots and areas of sunspots. Artificial eclipses allow viewing of the circumference of the sun as sunspots rotate through the horizon. Since looking directly at the Sun with the naked eye, through binoculars or a telescope is extremely dangerous, amateur observation of sunspots with the unaided eye is generally done by projection or via using proper filtration. Small sections of very dark filter glass, such as a 14 welder's glass is sometimes employed. The eyepiece of a telescope is also used in the role of a "projector" to project the image, without filtration, on to a white screen where it can be viewed indirectly, and even traced, so sunspot evolution can be followed. Special purpose hydrogen-alpha narrow bandpass filters as well as aluminum coated glass attenuation filters (which have the appearance of mirrors due to their extremely high optical density) are also used on the front of a telescope to provide safe direct observation through the eyepiece.


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APPLICATION A large group of sunspots in year 2004. The grey area around the spots can be seen very clearly, as well as the granulation of the sun's surface. Due to their link to other kinds of solar activity, sunspots can be used to predict the space weather and with it the state of the ionosphere. Thus, sunspots can help predict conditions of short-wave radio propagation or satellite communications. Don Easterbrook, a Professor Emeritus of geology at Western Washington University, has claimed that there is a cause-andeffect relationship between sunspot activity and global temperatures on Earth. It is also believed that sunspot activity has a direct effect on weather and climate change on the planet, the controversy over global warming has put scientists in opposition to each other, until more is known about the effects of sunspots and solar radiation levels and their exact relationship with Earth and its weather patterns (including temperature change) we must accept that there may be a correlation between global warming and sunspot activity. STARSPOTS ON OTHER STARS Periodic changes in brightness had been first seen on red dwarfs and in 1947 G. E. Kron proposed that spots were the cause. Since the mid 1990s observations of starspots have been made using increasingly powerful techniques yielding more and more detail: photometry determined starspot regions grew and decayed and showed cyclic behaviour similar to the Sun's; spectroscopy examined the structure of starspot regions; Doppler imaging showed differential rotation of spots for several stars and distributions different from the Sun's; spectral line analysis measured the temperature range of spots and the stellar surfaces. For example, in 1999, Strassmeier reported the largest cool starspot ever seen rotating the giant K0 star XX Triangulum (HD 12545) with a temperature of 3500 kelvin, together with a warm spot of 4800 kelvin. SOLAR ECLIPSE A solar eclipse occurs when the Moon passes between the Sun and the Earth so that the Sun is wholly or partially obscured. This

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can only happen during a new moon, when the Sun and Moon are in conjunction as seen from the Earth. At least two and up to five solar eclipses occur each year on Earth, with between zero and two of them being total eclipses. Total solar eclipses are nevertheless rare at any location because during each eclipse totality exists only along a narrow corridor in the relatively tiny area of the Moon's umbra.

• A partial eclipse occurs when the Sun and Moon are not exactly in line, and the Moon only partially obscures the Sun. This phenomenon can usually be seen from a large part of the Earth outside of the track of an annular or total eclipse. However, some eclipses can only be seen as a partial eclipse, because the umbra never intersects the Earth's surface.

A total solar eclipse is a spectacular natural phenomenon and many people travel to remote locations to observe one. The 1999 total eclipse in Europe helped to increase public awareness of the phenomenon, as illustrated by the number of journeys made specifically to witness the 2005 annular eclipse and the 2006 total eclipse. The next solar eclipse will occur on August 1, 2008, and will be a total eclipse.

The match between the apparent sizes of the Sun and Moon during a total eclipse is a coincidence. The Sun's distance from the Earth is about 400 times the Moon's distance, and the Sun's diameter is about 400 times the Moon's diameter. Because these ratios are approximately the same, the sizes of the Sun and the Moon as seen from Earth appear to be approximately the same: about 0.5 degree of arc in angular measure.

In ancient times, and in some cultures today, solar eclipses have been attributed to supernatural causes. Total solar eclipses can be frightening for people who are unaware of their astronomical explanation, as the Sun seems to disappear in the middle of the day and the sky darkens in a matter of minutes.

Because the Moon's orbit around the Earth is an ellipse, as is the Earth's orbit around the Sun, the apparent sizes of the Sun and Moon vary. The magnitude of an eclipse is the ratio of the apparent size of the Moon to the apparent size of the Sun during an eclipse. An eclipse when the Moon is near its closest distance from the Earth (i.e., near its perigee) can be a total eclipse because the Moon will appear to be large enough to cover completely the Sun's bright disk, or photosphere; a total eclipse has a magnitude greater than 1.

ANNULAR ECLIPSE There are four types of solar eclipses: • A total eclipse occurs when the Sun is completely obscured by the Moon. The intensely bright disk of the Sun is replaced by the dark silhouette of the Moon, and the much fainter corona is visible. During any one eclipse, totality is visible only from at most a narrow track on the surface of the Earth. • An annular eclipse occurs when the Sun and Moon are exactly in line, but the apparent size of the Moon is smaller than that of the Sun. Hence the Sun appears as a very bright ring, or annulus, surrounding the outline of the Moon. • A hybrid eclipse is intermediate between a total and annular eclipse. At some points on the surface of the Earth it is visible as a total eclipse, whereas at others it is annular. Hybrid eclipses are rather rare.

Conversely, an eclipse when the Moon is near its farthest distance from the Earth (i.e., near its apogee) can only be an annular eclipse because the Moon will appear to be slightly smaller than the Sun; the magnitude of an annular eclipse is less than 1. Slightly more solar eclipses are annular than total because, on average, the Moon lies too far from Earth to cover the Sun completely. A hybrid eclipse occurs when the magnitude of an eclipse is very close to 1: the eclipse will appear to be total at some locations on Earth and annular at other locations. The Earth's orbit around the Sun is also elliptical, so the Earth's distance from the Sun varies throughout the year. This also affects the apparent sizes of the Sun and Moon, but not so much as the Moon's varying distance from the Earth. When the Earth approaches its farthest distance from the Sun (the aphelion) in July, this tends to favor a total eclipse. As the Earth approaches

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its closest distance from the Sun (the perihelion) in January, this tends to favor an annular eclipse.

TERMINOLOGY The Moon transiting in front of the Sun as seen from STEREOB on February, 25 2007 at 4.4 times the distance between the Earth and the Moon. Central eclipse is often used as a generic term for a total, annular, or hybrid eclipse. This is, however, not completely correct: the definition of a central eclipse is an eclipse during which the central line of the umbra touches the Earth's surface. It is possible, though extremely rare, that part of the umbra intersects with Earth (thus creating an annular or total eclipse), but not its central line. This is then called a non-central total or annular eclipse. The term solar eclipse itself is strictly speaking a misnomer. The phenomenon of the Moon passing in front of the Sun is not an eclipse, but an occultation. Properly speaking, an eclipse occurs when one object passes into the shadow cast by another object. For example, when the Moon disappears at full moon by passing into Earth's shadow, the event is properly called a lunar eclipse. Therefore, technically, a solar eclipse actually amounts to an eclipse of the Earth. PREDICTIONS

Geometry The Moon's orbit around the Earth is inclined at an angle of just over 5 degrees to the plane of the Earth's orbit around the Sun (the ecliptic). Because of this, at the time of a new moon, the Moon will usually pass above or below the Sun. A solar eclipse can occur only when the new moon occurs close to one of the points (known as nodes) where the Moon's orbit crosses the ecliptic. As noted above, the Moon's orbit is also elliptical. The Moon's distance from the Earth can vary by about 6% from its average value. Therefore, the Moon's apparent size varies with its distance from the Earth, and it is this effect that leads to the difference between total and annular eclipses. The distance of the Earth from the Sun also varies during the year, but this is a smaller effect.


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On average, the Moon appears to be slightly smaller than the Sun, so the majority (about 60%) of central eclipses are annular. It is only when the Moon is closer to the Earth than average (near its perigee) that a total eclipse occurs. The Moon orbits the Earth in approximately 27.3 days, relative to a fixed frame of reference. This is known as the sidereal month. However, during one sidereal month, the Earth has revolved part way around the Sun, making the average time between one new moon and the next longer than the sidereal month: it is approximately 29.5 days. This is known as the synodic month, and corresponds to what is commonly called the lunar month. • A Total eclipse in the umbra. • B Annular eclipse in the antumbra. • C Partial eclipse in the penumbra The Moon crosses from south to north of the ecliptic at its ascending node, and vice versa at its descending node. However, the nodes of the Moon's orbit are gradually moving in a retrograde motion, due to the action of the Sun's gravity on the Moon's motion, and they make a complete circuit every 18.6 years. This means that the time between each passage of the Moon through the ascending node is slightly shorter than the sidereal month. This period is called the draconic month. Finally, the Moon's perigee is moving forwards in its orbit, and makes a complete circuit in about 9 years. The time between one perigee and the next is known as the anomalistic month. The Moon's orbit intersects with the ecliptic at the two nodes that are 180 degrees apart. Therefore, the new moon occurs close to the nodes at two periods of the year approximately six months apart, and there will always be at least one solar eclipse during these periods. Sometimes the new moon occurs close enough to a node during two consecutive months. This means that in any given year, there will always be at least two solar eclipses, and there can be as many as five. However, some are visible only as partial eclipses, because the umbra passes above Earth's north or south pole, and others are central only in remote regions of the Arctic or Antarctic.

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Path During a central eclipse, the Moon's umbra (or antumbra, in the case of an annular eclipse) moves rapidly from west to east across the Earth. The Earth is also rotating from west to east, but the umbra always moves faster than any given point on the Earth's surface, so it almost always appears to move in a roughly west-east direction across a map of the Earth (there are some rare exceptions to this which can occur during an eclipse of the midnight sun in Arctic or Antarctic regions). The width of the track of a central eclipse varies according to the relative apparent diameters of the Sun and Moon. In the most favourable circumstances, when a total eclipse occurs very close to perigee, the track can be over 250 km wide and the duration of totality may be over 7 minutes. Outside of the central track, a partial eclipse can usually be seen over a much larger area of the Earth.

OCCURRENCE AND CYCLES Total solar eclipses are rare events. Although they occur somewhere on Earth every 18 months on average, it has been estimated that they recur at any given place only once every 370 years, on average. The total eclipse only lasts for a few minutes at that location, as the Moon's umbra moves eastward at over 1700 km/h. Totality can never last more than 7 min 31 s, and is usually much shorter: during each millennium there are typically fewer than 10 total solar eclipses exceeding 7 minutes. The last time this happened was June 30, 1973 (7 min 3 sec). Observers aboard a Concorde aircraft were able to stretch totality to about 74 minutes by flying along the path of the Moon's umbra. The next eclipse exceeding seven minutes in duration will not occur until June 25, 2150. The longest total solar eclipse during the 8,000-year period from 3000 BC to 5000 AD will occur on July 16, 2186, when totality will last 7 min 29 s. For comparison, the longest eclipse of the 21st century will occur on July 22, 2009 and last 6 min 39 sec. If the date and time of any solar eclipse are known, it is possible to predict other eclipses using eclipse cycles. Two such cycles are the Saros and the Inex. The Saros cycle is probably the best known, and one of the most accurate, eclipse cycles. The Inex


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cycle is itself a poor cycle, but it is very convenient in the classification of eclipse cycles. After a Saros cycle finishes, a new Saros cycle begins one Inex later, hence its name: in-ex. A Saros cycle lasts 6,585.3 days (a little over 18 years), which means that after this period a practically identical eclipse will occur. The most notable difference will be a shift of 120° in longitude (due to the 0.3 days) and a little in latitude. A Saros series always starts with a partial eclipse near one of Earth's polar regions, then shifts over the globe through a series of annular or total eclipses, and ends at the opposite polar region. A Saros lasts 1226 to 1550 years and 69 to 87 eclipses, with about 40 to 60 central.

FINAL TOTALITY Spectacular solar eclipses are an extreme rarity within the universe at large. They are seen on Earth because of a fortuitous combination of circumstances that are statistically very improbable. Even on Earth, spectacular eclipses of the type familiar to people today are a temporary (on a geological time scale) phenomenon. Many millions of years in the past, the Moon was too close to the Earth to precisely occult the Sun as it does during eclipses today; and many millions of years in the future, it will be too far away to do so. Due to tidal acceleration, the orbit of the Moon around the Earth becomes approximately 3.8 cm more distant each year. It is estimated that in 600 million years, the distance from the Earth to the Moon will have increased by 23,500 km, meaning that it will no longer be able to completely cover the Sun's disk. This will be true even when the Moon is at perigee, and the Earth at aphelion. A complicating factor is that the Sun will increase in size over this timescale. This makes it even more unlikely that the Moon will be able to cause a total eclipse. We can therefore say that the last total solar eclipse on Earth will occur in slightly less than 600 million years. ACTIVITIES DURING SOLAR ECLIPSE A marked drop of the intensity of the solar radiation occurs during solar eclipse. It influences the actions in the atmosphere.

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The variations of the atmospheric actions display in changes of standard meteorological and physical quantities. We can notice it by a measurement of the air temperature and other meteorological quantities (e.g.: air humidity, soil temperature, colour of the solar radiation). The progressions of the quantities are usually detected by special weather stations because of a short duration of a total (annular) solar eclipse. The properties of the devices usually are: high speed of measurement, high resolution and sensitivity. Acquired results show very interesting variations in progressions of meteorological and physical quantities (e.g.: colour of the light).

PHOTOGRAPHY OF THE PHENOMENON Photographing an eclipse is possible with fairly common film camera equipment. In order for the disk of the sun/moon to be easily visible, a fairly high magnification telephoto lens is needed (70-200mm for a 35mm camera), and for the disk to fill most of the frame, a longer lens is needed (over 500mm). As with viewing the sun directly, looking at it through the viewfinder of a camera can produce damage to the retina, so care is advised. MEASURING OF THE LIGHT EINSTEIN'S INFLEXION Historical eclipses are a valuable resource for historians, in that they allow a few historical events to be precisely dated, from which other dates and a society's calendar can be deduced. A solar eclipse of June 15, 763 BC mentioned in an Assyrian text is important for the Chronology of the Ancient Orient. Also known as the eclipse of Bur Sagale, it is the earliest solar eclipse mentioned in historical sources that has been successfully identified. Perhaps the earliest still-unproven claim is that of archaeologist Bruce Masse; on the basis of several ancient flood myths that mention a total solar eclipse, he links an eclipse that occurred May 10, 2807 BC with a possible meteor impact in the Indian Ocean. There have been other claims to date earlier eclipses, notably that of Mursili II (likely 1312 BC), in Babylonia, and also in China, during the 5th year (2084 BC) of the regime of king Zhong Kang of Xia dynasty, but these are highly disputed and rely on much supposition.


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Herodotus wrote that Thales of Miletus predicted an eclipse which occurred during a war between the Medians and the Lydians. Soldiers on both sides put down their weapons and declared peace as a result of the eclipse. Exactly which eclipse was involved has remained uncertain, although the issue has been studied by hundreds of ancient and modern authorities. One likely candidate took place on May 28, 585 BC, probably near the Halys river in the middle of modern Turkey. An annular eclipse of the Sun occurred at Sardis on February 17, 478 BC, while Xerxes was departing for his expion against Greece, as Herodotus recorded. Hind and Chambers considered this absolute date more than a century ago. Herodotus also reports that another solar eclipse was observed in Sparta during the next year, on August 1, 477 BC. The sky suddenly darkened in the middle of the day, well after the battles of Thermopylae and Salamis, after the departure of Mardonius to Thessaly at the beginning of the spring of (477 BC) and his second attack on Athens, after the return of Cleombrotus to Sparta. Note that the modern conventional dates are different by a year or two, and that these two eclipse records have been ignored so far. The Chronicle of Ireland recorded a solar eclipse on June 29, 512 AD, and a solar eclipse was reported to have taken place during the Battle of Stiklestad in the summer of 1030. It has also been attempted to establish the exact date of Good Friday by means of solar eclipses, but this research has not yielded conclusive results. Research has manifested the inability of total solar eclipses to serve as explanations for the recorded Good Friday features of the crucifixion eclipse. (Good Friday is recorded as being at Passover, which is also recorded as being at or near the time of a full moon.) The ancient Chinese astronomer Shi Shen (fl. 4th century BC) was aware of the relation of the moon in a solar eclipse, as he provided instructions in his writing to predict them by using the relative positions of the moon and sun. The 'radiating influence' theory for a solar eclipse (i.e., the moon's light was merely light reflected from the sun) was existent in Chinese thought from about the 6th century BC (in the Zhi Ran of Zhi Ni Zi), and opposed by the Chinese philosopher Wang Chong (27-97 AD),

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who made clear in his writing that this theory was nothing new. This can be said of Jing Fang's writing in the 1st century BC, which stated: The moon and the planets are Yin; they have shape but no light. This they receive only when the sun illuminates them. The former masters regarded the sun as round like a crossbow bullet, and they thought the moon had the nature of a mirror. Some of them recognized the moon as a ball too. Those parts of the moon which the sun illuminates look bright, those parts which it does not, remain dark. The ancient Greeks had known this as well, since it was Parmenides of Elea around 475 BC who supported the theory of the moon shining because of reflected light, and was accepted in the time of Aristotle as well. The Chinese astronomer and inventor Zhang Heng (78-139 AD) wrote of both solar and lunar eclipses in the publication of Ling Xian in 120 AD, supporting the radiating influence theory that Wang Chong had opposed (Wade-Giles): The sun is like fire and the moon like water. The fire gives out light and the water reflects it. Thus the moon's brightness is produced from the radiance of the sun, and the moon's darkness (pho) is due to (the light of) the sun being obstructed (pi). The side which faces the sun is fully lit, and the side which is away from it is dark. The planets (as well as the moon) have the nature of water and reflect light. The light pouring forth from the sun (tang jih chih chhung kuang) does not always reach the moon owing to the obstruction (pi) of the earth itself-this is called 'an-hsü', a lunar eclipse. When (a similar effect) happens with a planet (we call it) an occulation (hsing wei); when the moon passes across (kuo)(the sun's path) then there is a solar eclipse (shih). The later Chinese scientist and statesman Shen Kuo (10311035 AD) also wrote of eclipses, and his reasoning for why the celestial bodies were round and spherical instead of flat (WadeGiles spelling): The Director of the Astronomical Observatory asked me about the shapes of the sun and moon; whether they were like balls or (flat) fans. If they were like balls they would surely obstruct (ai) each other when they met. I replied that these celestial bodies were certainly like balls. How do we know this? By the waxing


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and waning (ying khuei) of the moon. The moon itself gives forth no light, but is like a ball of silver; the light is the light of the sun (reflected). When the brightness is first seen, the sun(-light passes almost) alongside, so the side only is illuminated and looks like a crescent. When the sun gradually gets further away, the light shines slanting, and the moon is full, round like a bullet. If half of a sphere is covered with (white) powder and looked at from the side, the covered part will look like a crescent; if looked at from the front, it will appear round. Thus we know that the celestial bodies are spherical...Since the sun and moon are in conjunction (ho) and in opposition (tui) once a day, why then do they have eclipses only occasionally?' I answered that the ecliptic and the moon's path are like two rings, lying one over the other (hsiang tieh), but distant by a small amount. (If this obliquity did not exist), the sun would be eclipsed whenever the two bodies were in conjunction, and the moon would be eclipsed whenever they were exactly in position. But (in fact) though they may occupy the same degree, the two paths are not (always) near (each other), and so naturally the bodies do not (intrude) upon one another.

VIEWING Looking directly at the photosphere of the Sun (the bright disk of the Sun itself), even for just a few seconds, can cause permanent damage to the retina of the eye, because of the intense visible and invisible radiation that the photosphere emits. This damage can result in permanent impairment of vision, up to and including blindness. The retina has no sensitivity to pain, and the effects of retinal damage may not appear for hours, so there is no warning that injury is occurring. Under normal conditions, the Sun is so bright that it is difficult to stare at it directly, so there is no tendency to look at it in a way that might damage the eye. However, during an eclipse, with so much of the Sun covered, it is easier and more tempting to stare at it. Unfortunately, looking at the Sun during an eclipse is just as dangerous as looking at it outside an eclipse, except during the brief period of totality, when the Sun's disk is completely covered (totality occurs only during a total eclipse and only very briefly; it does not occur during a partial or annular eclipse). Viewing the Sun's disk through any kind of optical aid (binoculars, a telescope,

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or even an optical camera viewfinder) is even more hazardous. Glancing at the Sun with all or most of its disk visible is unlikely to result in permanent harm, as the pupil will close down and reduce the brightness of the whole scene. If the eclipse is near total, the low average amount of light causes the pupil to open. Unfortunately the remaining parts of the Sun are still just as bright, so they are now brighter on the retina than when looking at a full Sun. As the eye has a small fovea, for detailed viewing, the tendency will be to track the image on to this best part of the retina, causing damage.

Partial and Annular Eclipses Viewing the Sun during partial and annular eclipses (and during total eclipses outside the brief period of totality) requires special eye protection, or indirect viewing methods. The Sun's disk can be viewed using appropriate filtration to block the harmful part of the Sun's radiation. Sunglasses are not safe, since they do not block the harmful and invisible infrared radiation which causes retinal damage. Only properly designed and certified solar filters should ever be used for direct viewing of the Sun's disk. The safest way to view the Sun's disk is by indirect projection. This can be done by projecting an image of the disk onto a white piece of paper or card using a pair of binoculars (with one of the lenses covered), a telescope, or another piece of cardboard with a small hole in it (about 1 mm diameter), often called a pinhole camera. The projected image of the Sun can then be safely viewed; this technique can be used to observe sunspots, as well as eclipses. However, care must be taken to ensure that no one looks through the projector (telescope, pinhole, etc.) directly. Viewing the Sun's disk on a video display screen (provided by a video camera or digital camera) is safe, although the camera itself may be damaged by direct exposure to the Sun. The optical viewfinders provided with some video and digital cameras are not safe. In the partial eclipse path one will not be able to see the spectacular corona or nearly complete darkening of the sky, yet, depending on how much of the sun's disk is obscured, some darkening may be noticeable. If two-thirds or more of the sun is obscured, then an effect can be observed by which the daylight


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appears to be dim, as if the sky were overcast, yet objects still cast sharp shadows.

TOTALITY It is safe to observe the total phase of a solar eclipse directly with the unaided eye, binoculars or a telescope, when the Sun's photosphere is completely covered by the Moon; indeed, it is too dim to be seen through filters. The Sun's faint corona will be visible, and the chromosphere, solar prominences, and possibly even a solar flare may be seen. However, viewing the Sun after totality can be dangerous.

Baily's Beads When the shrinking visible part of the photosphere becomes very small, Baily's beads will occur. These are caused by the sunlight still being able to reach Earth through lunar valleys, but no longer where mountains are present. Totality then begins with the diamond ring effect, the last bright flash of sunlight.

OTHER OBSERVATIONS For astronomers, a total solar eclipse forms a rare opportunity to observe the corona (the outer layer of the Sun's atmosphere). Normally this is not visible because the photosphere is much brighter than the corona. According to the point reached in the solar cycle, the corona can appear rather small and symmetric, or large and fuzzy. It is very hard to predict this prior to totality. During a solar eclipse, special (indirect) observations can also be done with the unaided eye only. Normally the spots of light which fall through the small openings between the leaves of a tree, have a circular shape. These are images of the Sun. During a partial eclipse, the light spots will show the partial shape of the Sun, as seen on the picture. Another famous phenomenon is shadow bands (also known as flying shadows), which are similar to shadows on the bottom of a swimming pool. They only occur just prior to and after totality, and are very difficult to observe. Many professional eclipse chasers have never seen them. During a partial eclipse, a related effect that can be seen is anisotropy in the shadows of objects. Particularly if the partial

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eclipse is nearly total, the unobscured part of the sun acts as an approximate line source of light. This means that objects cast shadows which have a very narrow penumbra in one direction, but a broad penumbra in the perpendicular direction.

1919 OBSERVATIONS The original photograph of the 1919 eclipse which was claimed to confirm Einstein's theory of general relativity. In 1919, the observation of a total solar eclipse helped to confirm Einstein's theory of general relativity. By comparing the apparent distance between two stars, with and without the Sun between them, Arthur Eddington stated that the theoretical predictions about gravitational lenses were confirmed, though it now appears the data were ambiguous at the time. The observation with the Sun between the stars was only possible during totality, since the stars are then visible. BEFORE SUNRISE, AFTER SUNSET The phenomenon of atmospheric refraction makes it possible to observe the Sun (and hence a solar eclipse) even when it is slightly below the horizon. It is however possible for a solar eclipse to attain totality (or in the event of a partial eclipse, neartotality) before (visual and actual) sunrise or after sunset from a particular location. When this occurs shortly before the former or after the latter, the sky will appear much darker than it would otherwise be immediately before sunrise or after sunset. On these occasions, an object (especially a planet, often Mercury) may be visible near the sunrise or sunset point of the horizon when it could not have been seen without the eclipse. ECLIPSES AND TRANSITS In principle, the simultaneous occurrence of a Solar eclipse and a transit of a planet is possible. But these events are extremely rare because of their short durations. The next anticipated simultaneous occurrence of a Solar eclipse and a transit of Mercury will be on July 5, 6757, and a Solar eclipse and a transit of Venus is expected on April 5, 15232. Only 5 hours after the transit of Venus on June 4, 1769, there was a total solar eclipse, which was visible in Northern America, Europe and Northern Asia as partial


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solar eclipse. This was the lowest time difference between a transit of a planet and a solar eclipse in the historical past. More common, but still quite rare, is a conjunction of any planet (not confined exclusively to Mercury or Venus) at the time of a total solar eclipse, in which event the planet will be visible very near the eclipsed Sun, when without the eclipse it would have been lost in the Sun's glare. At one time, some scientists hypothesized that there may be a planet (often given the name Vulcan) even closer to the Sun than Mercury; the only way to confirm its existence would have been to observe it during a total solar eclipse. However, it is now known that no such planet exists. While there does remain some possibility for small Vulcanoid asteroids to exist, none have ever been found.

ARTIFICIAL SATELLITES Artificial satellites can also pass in front of, or transit, the Sun as seen from Earth, but none are large enough to cause an eclipse. At the altitude of the International Space Station, for example, an object would need to be about 3.35 km across to blot the Sun out entirely. These transits are difficult to watch, because the zone of visibility is very small. The satellite passes over the face of the Sun in about a second, typically. As with a transit of a planet, it will not get dark.Artificial satellites do play an important role in documenting solar eclipses. Images of the umbra on the Earth's surface taken from Mir and the International Space Station are among the most spectacular eclipse images in history. Observations of eclipses from satellites orbiting above the Earth's atmosphere are of course not subject to weather conditions. The direct observation of a total solar eclipse from space is rather rare. The only documented case is Gemini 12 in 1966. The partial phase of the 2006 total eclipse was visible from the International Space Station. At first, it looked as though an orbit correction in the middle of March would bring the ISS in the path of totality, but this correction was postponed. ECONOMICS, ENERGY AND SYSTEM COSTS In sunny, warm locations, where freeze protection is not necessary, a batch type solar hot water heater can be extremely

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cost effective. In higher latitudes, there are often additional design requirements for cold weather, which add to system complexity. This has the effect of increasing the initial cost (but not the lifecycle cost) of a solar hot water system, to a level much higher than a comparable hot water heater of the conventional type. When calculating the total cost to own and operate, a proper analysis will take into consideration that solar energy is free, thus greatly reducing the operating costs, whereas other energy sources, such as gas and electricity, can be quite expensive over time. Thus, when the initial costs of a solar system are properly financed and compared with energy costs, then, in many cases the total monthly cost of solar heat can be less than other more conventional types of hot water heaters (and also in conjunction with an existing hot water heater). In addition, federal and local incentives can be significant. As an example, a 56ft2 solar water heater can cost US $7,500, but that initial cost is reduced to just $3,300 in the US State of Oregon due to federal and state incentives. The system will save approximately US $230 per year, with a payback of 14 years. Lower payback periods are possible based on maximizing sun exposure. In more northerly locations, solar heating used to be less efficient. Usable amounts of domestic hot water were only available in the summer months, on cloudless days, between April and October. During the winter and on cloudy days, the output was poor. Independent surveys have shown that modern systems do not suffer these limitations. There are cases of households in northern climates getting all of their domestic hot water year round from solar alone. Systems have been show to efficiently work as far north as Whitehorse, Yukon (latitude of 60 B 43' N ). The installation costs in the UK used to be prohibitive, on average about £9,000. This is reduced in more recent years to £3,000, with payback period reduced, with the rise in the gas price, to 12 years. As energy prices rise, payback periods shorten accordingly. According to ANRE (a Flemish energy agency, subsidised by the Flemish or Belgian government, a complete, commercial (active) solar hot water system composed of a solar collector (3-4 m²; this


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is large enough for 4 people), pipes and tank (again large enough for 4 people) costs around 4000 euro. The installation by a recognised worker costs another 800 euro. Electrabel's home magazine Eandismagazine stated in 2008 that a complete system (including 4m2 of solar collectors and a supply barrel of 200-240 liters) to cost 4500 euro. The system would then pay back itself in 11 years , when the returns are weighed off against a regular electric boiler. Calculation was as follows: a saving of 1875 kWh (which is 50% of the energy requirements in domestic hot water production) x 0.10 euro/kWh = 187, 5 euro's. This multiplied by 11,6 years made 2175 euro's (or the cost of the system with deducted regional tax benefits). In Australia, the cost for an average solar hot water system fully installed is between $1,800 and $2,800. This is after rebates (there is a federal rebate, some state rebates and Renewable Energy Certificates). According to the Department of Environment and Water Resources , the yearly electricity savings are between $300 and $700. This brings the payback period to under 2 years in the best case and under 10 years in the worst case.

SOLAR HOT WATER SYSTEMS Solar hot water systems can be classified in different ways: • The type of collector used • The location of the collector - roof mount, ground mount, wall mount • The location of the storage tank in relation to the collector • The requirement for a pump - active vs passive • The method of heat transfer - open-loop or closed-loop (via heat exchanger) COMPACT SYSTEMS (PASSIVE SYSTEMS) A passive system also known as a monobloc (thermosiphon) system, a compact system consists of a tank for the heated water, a solar collector, and connecting pipes all pre-mounted in a frame. The water flows upward when heated in the panel. When this water enters the tank (positioned higher than the solar panel), it expels some cold water from inside so that the heat transfer takes place without the need for a pump. A typical system for a four-

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person home in a sunny region consists of a tank of 150 to 300 litres (36.9 to 79.2 gallons) and three to four square metres of solar collector panels. A special type of compact system is the Integrated Collector Storage (ICS, Batch Heater) where the tank acts as both storage and solar collector. They are simple and efficient but only suitable in moderate climates with good sunshine. Direct ('open loop') compact systems, if made of metals are not suitable for cold climates. At night the remaining water can freeze and damage the panels, and the storage tank is exposed to the outdoor temperatures that will cause excessive heat losses on cold days. Some compact systems have a primary circuit. The primary circuit includes the collectors and the external part of the tank. Instead of water, a non-toxic antifreeze is used. When this liquid is heated up, it flows to the external part of the tank and transfers the heat to the water placed inside. ('closed loop'). However, direct ('open loop') systems are slightly cheaper and more efficient. A compact system can save up to 4.5 tonnes annually of greenhouse gas emissions. In order to achieve the aims of the Kyoto Protocol, several countries are offering subsidies to the end user. Some systems can work for up to 25 years with minimum maintenance. These kinds of systems can be redeemed in six years, and achieve a positive balance of energy (energy used to build them minus energy they save) of 1.5 years. Most part of the year, when the electric heating element is not working, these systems do not use any external source for power (as water flows due to thermosyphon principle). Flat solar thermal collectors are usually used, but compact systems using vacuum tube collectors are available on the market. These generally give a higher heat yield per square meter in colder climates but cost more than flat plate collector systems.

PUMPED SYSTEMS (ACTIVE SYSTEMS) How the solar water heating system is pumped and controlled determines whether it is a zero carbon or a low carbon system. Low carbon systems principally use electricity to circulate the fluid through the collector. The use of electricity typically reduces


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the carbon savings of a system by 10% to 20%. Conventional low carbon system designs use a mains powered circulation pump whenever the hot water tank is positioned below the solar panels. Most systems in northern Europe are of this type. The storage tank is placed inside the building, and thus requires a controller that measures when the water is hotter in the panels than in the tank. The system also requires a pump for transferring the fluid between the parts. The electronic controllers used by some systems permit a wide range of functionality such as measurement of the energy produced; more sophisticated safety functions; thermostatic and time-clock control of auxiliary heat, hot water circulation loops, or others; display or transfer of error messages or alarms; remote display panels; and remote or local datalogging. Newer zero carbon solar water heating systems are powered by solar electric (photovoltaic or PV) pumps. These typically use a 5-20W PV panel which faces in the same direction as the main solar heating panel and a small, low flow diaphragm pump to pump the water. The most commonly used solar collector is the insulated glazed flat panel. Less expensive panels, like polypropylene panels (for swimming pools) or higher-performing ones like evacuated tube collectors, are sometimes used.

SOLAR HEATING THERMAL COLLECTORS There are three main kinds of solar thermal collectors in common use. In order of increasing cost they are: Formed Plastic Collectors, Flat Collectors, and Evacuated Tube Collectors. The efficiency of the system is directly related to heat losses from the collector surface (efficiency being defined as the proportion of heating energy that can be usefully obtained from insulation). Heat losses are predominantly governed by the thermal gradient between the temperature of the collector surface and the ambient temperature. Efficiency decreases when either the ambient temperature falls or as the collector temperature increases. This decrease in efficiency can be mitigated by increasing the insulation of the unit by sealing the unit in glass e.g. flat collectors or providing a vacuum seal e.g. evacuated tube collector. The choice of collector

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is determined by the heating requirements and environmental conditions in which it is employed.

FORMED PLASTIC COLLECTOR Formed plastic collectors (such as polypropylene, EPDM or PET plastics) consist of tubes or formed panels through which water is circulated and heated by the sun's radiation. These are often used for extending the swimming season in swimming pools. In some countries, heating an open-air swimming pool with non-renewable energy sources is not allowed, and then these inexpensive systems offer a good solution. This panel is not suitable for year-round uses like providing hot water for home use, primarily due to its lack of insulation which reduces its effectiveness greatly when the ambient air temperature is lower than the temperature of the fluid being heated. FLAT PLATE COLLECTOR A flat plate collector consists of a thin absorber sheet (usually copper, to which a black or selective coating is applied) backed by a grid or coil of fluid tubing and placed in an insulated casing with a glass cover. Fluid is circulated through the tubing to remove the heat from the absorber and transport it to an insulated water tank, to a heat exchanger or to some other device for using the heated fluid. As an alternative to metal collectors, some new polymer flat plate collectors are now being produced in Europe. These may be wholly polymer, or they may be metal plates behind which are freeze-tolerant water channels made of silicone rubber instead of metal. Polymers, being flexible and therefore freeze-tolerant, are able to contain plain water instead of antifreeze, so that in some cases they are able to plumb directly into existing water tanks instead of needing the tank to be replaced with one using heat exchangers. EVACUATED TUBE COLLECTOR Evacuated tube collectors are made of a series of modular tubes, mounted in parallel, whose number can be added to or reduced as hot water delivery needs change. This type of collector consists of rows of parallel transparent glass tubes, each of which


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contains an absorber tube (in place of the absorber plate to which metal tubes are attached in a flat-plate collector). The tubes are covered with a special light-modulating coating. In an evacuated tube collector, sunlight passing through an outer glass tube heats the absorber tube contained within it. The absorber can either consist of copper (glass-metal) or specially-coated glass tubing (glass-glass). The glass-metal evacuated tubes are typically sealed at the manifold end, and the absorber is actually sealed in the vacuum, thus the fact that the absorber and heat pipe are dissimilar metals creates no corrosion problems. The better quality systems use foam insulation in the manifold. low iron glass is used in the higher quality evacuated tubes manufacture. Lower quality evacuated tube systems use the glass coated absorber. Due to the extreme temperature difference of the glass under stagnation temperatures, the glass sometimes shatters. The glass is a lower quality boron silicate material and the aluminum absorber and copper heat pipe are slid down inside the open top end of the tube. Moisture entering the manifold around the sheet metal casing is eventually absorbed by the glass fibre insulation and then finds its way down into the tubes. This leads to corrosion at the absorber/heat pipe interface area, also freeze ruptures of the tube itself if the tube fills sufficiently with water. Two types of tube collectors are distinguished by their heat transfer method: the simplest pumps a heat transfer fluid (water or antifreeze) through a U-shaped copper tube placed in each of the glass collector tubes. The second type uses a sealed heat pipe that contains a liquid that vapourises as it is heated. The vapour rises to a heat-transfer bulb that is positioned outside the collector tube in a pipe through which a second heat transfer liquid (the water or antifreeze) is pumped. For both types, the heated liquid then circulates through a heat exchanger and gives off its heat to water that is stored in a storage tank (which itself may be kept warm partially by sunlight). Evacuated tube collectors heat to higher temperatures, with some models providing considerably more solar yield per square metre than flat panels. However, they are more expensive and fragile than flat panels. Evacuated heat tubes perform better than flat plate collectors in cold climates because they only rely on the

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light they receive and not the outside temperature. The high stagnation temperatures can cause antifreeze to break down, so careful consideration must be used if selecting this type of system in temperate climates.Tubes come in different levels of quality so the different kinds have to be examined as well. High quality units can efficiently absorb diffuse solar radiation present in cloudy conditions and are unaffected by wind. They also have the same performance in similar light conditions summer and winter. For a given absorber area, evacuated tubes can maintain their efficiency over a wide range of ambient temperatures and heating requirements. The absorber area only occupied about 50% of the collector panel on early designs, however this has changed as the technology has advanced to maximize the absorption area. In extremely hot climates, flat-plate collectors will generally be a more cost-effective solution than evacuated tubes. When employed in arrays of 20 to 30 or more, the efficient but costly evacuated tube collectors have net benefit in winter and also give real advantage in the summer months. They are well suited to extremely cold ambient temperatures and work well in situations of consistently lowlight. They are also used in industrial applications, where high water temperatures or steam need to be generated. Properly designed evacuated tubes have a life expectancy of over 25 years which greatly adds to their value.

SOLAR THERMAL COOLING Solar thermal cooling can be achieved via absorption refrigeration cycles, desiccant cycles and solar-mechanical processes. The absorption cycle solar cooling system works like a refrigerator in that it uses hot water to compress a gas that, once expanded, will produce an endothermic reaction which cools the air. The main problem currently is that the absorber machine works with liquid at 90 °C, a fairly high temperature to be reached with pumped solar panels with no auxiliary power supply. The same pumped solar thermal installation can be used for producing hot water for the whole year. It can also be used for cooling in the summer and partially heating the building in winter.


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DIY SOLAR HOT WATER SYSTEMS With an ever rising diy-community and their increasing environmental awareness, people have begun building their own (small-scale) solar hot water systems from scratch. Through the internet, the community is able to attain plans to solar hot water systems. and people have sprung up building them for their own domestic requirements. DIY solar hot water systems are usually much cheaper than their commercial counterparts and installation costs can sometimes be avoided as well. The DIY-solar hot water systems are being used both in the developed world, as in the developing world to generate hot water. SOLAR MIRROR A Solar mirror is a reflective surface used for gathering and reflecting solar energy in a system being powered by solar energy. It comprises a glass substrate, a reflective layer for reflecting the solar energy, and an interference layer. The purpose of the solar mirror is to achieve a substantially concentrated reflection factor for solar energy systems. COMPONENTS

Glass Substrate The glass substrate is the top layer of the mirror in which solar energy is transmitted. Its purpose is to protect the other layers from abrasion and corrosion. Although glass is brittle, it is a good material for this purpose, because it is highly transparent (low optical losses), relatively inexpensive, resistant to UV, fairly hard (abrasion resistant), chemically inert, and fairly easy to clean. It is composed of a float glass with high optical transmission characteristics in the visible and infrared ranges, and is configured to transmit visible light and infrared radiation. The top surface, known as the "first surface", will reflect some of the incident solar energy, due to the reflection coefficient caused by its index of refraction being higher than air. Most of the solar energy is transmitted through the glass substrate to the lower layers of the mirror, possibly with some refraction, depending on the angle of incidence.

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Reflective Layer

Solar Thermal Applications

The reflective layer is designed to reflect the maximum amount of solar energy incident upon it, back through the glass substrate. The reflective layer comprises a highly reflective layer of thin silver plating. The use of silver as the reflective layer leads to higher efficiency levels, because it is the most reflective metal. Despite being relatively sensitive to abrasion and corrosion, the silver layer is protected by the (glass) substrate on top, and the bottom is covered with a protective coating which usually comprises a copper layer and varnish.

The intensity of solar thermal energy from solar radiation at the surface of the earth is is about 1 kilowatt of energy per square meter of area, normal to the direction of the sun, under clear-sky conditions. When solar energy is unconcentrated, the maximum collector temperature is about 80-100 deg C. This is useful for space heating and heating water. For higher temperature applications, such as cooking, or supplying a heat engine or turbineelectrical generator, this energy must be concentrated, a task normally assigned to flat or parabolic arrays of solar mirrors.

Despite the use of aluminum in generic mirrors, aluminum is often not used as the reflective layer for a solar mirror. This is because of aluminum's reflection factor in the UV region of the spectrum, wherein an aluminum layer would have to be placed on the top surface of the glass substrate, and not on the bottom surface.

Terrestrial Applications

Locating the aluminum layer on the first surface, exposes it to weathering, which reduces the mirror's resistance to corrosion and makes it more susceptible to abrasion (i.e. scratching). Adding a protective layer to the aluminum would reduce its reflectivity. For this reason silver is a higher performance reflector material, and (presumably) its higher cost is justified due to higher efficiency and longevity. The reflective layer has a high refractive index (see dielectric). In order to enhance reflection in the near-UV region of the spectrum, the thickness of this layer may be optimized for its interference effects.

Interference Layer An interference layer is located on the first surface of the glass substrate. It is designed for diffuse?-reflectance of near-ultraviolet radiation, in order to prevent it from passing through the glass substrate. Were this interference layer not present, it would allow near-ultraviolet radiation to pass into the glass substrate and through to the reflective layer. This would substantially enhance the overall reflection of near-ultraviolet radiation from the mirror. The interference layer is composed predominantly of titanium dioxide.

Solar thermal systems have been constructed to produce "concentrated solar power" (CSP), for generating electricity. The large Sandia Lab solar power tower uses a Stirling engine heated by a solar mirror concentrator. Another configuration is the trough system.

Space Power Application "Solar dynamic" energy systems have been proposed for various spacecraft applications, including solar power satellites, where a reflector focuses sunlight on to a heat engine such as the Brayton cycle type.

Photovoltaic Augmentation Photovoltaic cells (PV) which can convert solar radiation directly into electricity are quite expensive per unit area. Some types of PV cell, e.g. gallium arsenide, if cooled, are capable of converting efficiently up to 250 times as much radiation as is normally provided by simple exposure to direct sunlight. In tests done by Sewang Yoon and Vahan Garboushian, for Amonix Corp. photocell percent conversion efficiency actually increased at higher levels of concentration, often by significant amounts, provided external cooling is available to the photocells.

Terrestrial Application To date no large scale testing has been performed on this concept. Presumably this is because the increased cost of the reflectors and cooling generally is not economically justified.

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Solar Power Satellite Application Theoretically, for space-based solar power satellite designs, solar mirrors could reduce PV cell costs and launch costs since they are expected to be both lighter and cheaper than equivalent large areas of PV cells. Several options were studied by Boeing corporation. In their Fig. 4. captioned "Architecture 4. GEO Harris Wheel", the authors describe a system of solar mirrors used to augment the power of some nearby solar collectors, from which the power is then transmitted to receiver stations on earth.

Space Reflectors for Night Illumination Another advanced space concept proposal is the notion of Space Reflectors which reflect sunlight on to small spots on the night side of the Earth to provide night time illumination. An early proponent of this concept was Dr. Krafft Arnold Ehricke, who wrote about systems called "Lunetta", "Soletta", "Biosoletta", "Powersoletta". A preliminary series of experiments called Znamya was performed by Russia. The first, designated Znamya-2, was launched aboard Progress-TM-15 on 27 October 1992. After visiting the EO-12 crew aboard the Mir space station the Progress-TM-15 then undocked and deployed the reflector this mission was successful. The next flight Znamya-2.5 failed. Znamya-3 never flew. One interesting theoretical method to construct such an orbiting solar mirror is the "tension stabilized steerable orbiting mirror".

Formation and Evolution of the Solar System The formation and evolution of the Solar System is estimated to have begun 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass collected in the centre, forming the Sun, while the rest flattened into a protoplanetary disc out of which the planets, moons, asteroids, and other small Solar System bodies formed. This widely accepted model, known as the nebular hypothesis, was first developed in the 18th century by Emanuel Swedenborg, Immanuel Kant, and Pierre-Simon Laplace. Its subsequent development has interwoven a variety of scientific disciplines


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including astronomy, physics, geology, and planetary science. Since the dawn of the space age in the 1950s and the discovery of extrasolar planets in the 1990s, the models have been both challenged and refined to account for new observations. The Solar System has evolved considerably since its initial formation. Many moons have formed from circling discs of gas and dust around their parent planets, while other moons are believed to have been bodies captured by their planets or, as in the case of the Earth's Moon, to have resulted from giant collisions. Collisions between bodies have occurred continually up to the present day and have been central to the evolution of the solar system. The positions of the planets often shifted, and planets have switched places. This planetary migration now is believed to have been responsible for much of the Solar System's early evolution. In roughly 5 billion years, the Sun will cool and expand outward to many times its current diameter (becoming a red giant), before casting off its outer layers as a planetary nebula, and leaving behind a stellar corpse known as a white dwarf. In the far distant future, the gravity of passing stars gradually will whittle away at the Sun's retinue of planets. Some planets will be destroyed, others ejected into interstellar space. Ultimately, over the course of trillions of years, it is likely that the Sun will be left alone with no bodies in orbit around it. Ideas concerning the origin and fate of the world date from the earliest known writings; however, for almost all of that time, there was no attempt to link such theories to the existence of a "Solar System", simply because it was not generally known that the Solar System, in the sense we now understand it, existed. The first step toward a theory of Solar System formation and evolution was the general acceptance of heliocentrism, the model which placed the Sun at the centre of the system and the Earth in orbit around it. This conception had been gestating for millennia, but was widely accepted only by the end of the 17th century. The first recorded use of the term "Solar System" dates from 1704. The current standard theory for Solar System formation, the nebular hypothesis, has fallen into and out of favour since its formulation by Emanuel Swedenborg, Immanuel Kant, and Pierre-

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Simon Laplace in the 18th century. The most significant criticism of the hypothesis was its apparent inability to explain the Sun's relative lack of angular momentum when compared to the planets. However, since the early 1980s studies of young stars have shown them to be surrounded by cool discs of dust and gas, exactly as the nebular hypothesis predicts, which has led to its re-acceptance. Understanding of how the Sun will continue to evolve required an understanding of the source of its power. Arthur Stanley Eddington's confirmation of Albert Einstein's theory of relativity led to his realisation that the Sun's energy comes from nuclear fusion reactions in its core. In 1935, Eddington went further and suggested that other elements also might form within stars. Fred Hoyle elaborated on this premise by arguing that evolved stars called red giants created many elements heavier than hydrogen and helium in their cores. When a red giant finally casts off its outer layers, these elements would then be recycled to form other star systems.

Solar Power Satellite

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8 SOLAR POWER SATELLITE A solar power satellite, or SPS or Powersat, as originally proposed would be a satellite built in high Earth orbit that uses microwave power transmission to beam solar power to a very large antenna on Earth. Advantages of placing the solar collectors in space include the unobstructed view of the Sun, unaffected by the day/night cycle, weather, or seasons. It is a renewable energy source, zero emission after putting the solar cells in orbit, and only generates waste as a product of manufacture and maintenance. However, the costs of construction are very high, and SPS will not be able to compete with conventional sources (at current energy prices) unless at least one of the following conditions is met: • Sufficiently low launch costs can be achieved • A determination (by governments, industry, ...) is made that the disadvantages of fossil fuel use are so large they must be substantially replaced. • Conventional energy costs increase sufficiently to provoke serious search for alternative energy In common with other types of renewable energy such a system could have advantages to the world in terms of energy security via reduction in levels of conflict, military spending, loss of life, and avoiding future conflict over dwindling energy sources. The SPS concept was first described in November 1968 . At first it was regarded as impractical due to the lack of a workable method of sending power collected down to the Earth's surface. This changed in 1973 when Peter Glaser was granted U.S. patent number 3,781,647 for his method of transmitting power over long distances (eg, from an SPS to the Earth's surface) using microwaves

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from a very large (up to one square kilometer) antenna on the satellite to a much larger one on the ground, which came to be known as a rectenna. Glaser then worked at Arthur D. Little, Inc., as a vice-president. NASA became interested and signed a contract with ADL to lead four other companies in a broader study in 1974. They found that, while the concept had several major problems -- chiefly the expense of putting the required materials in orbit and the lack of experience on projects of this scale in space, it showed enough promise to merit further investigation and research. During the period from 1978 - 1981 the US Congress authorized DOE and NASA to jointly investigate. They organized the Satellite Power System Concept Development and Evaluation Program . The study remains the most extensive performed to date. Several reports were published addressing various issues, together investigating most of the possible problems with such an engineering project. They include: • Resource Requirements (Critical Materials, Energy, and Land) • Financial/Management Scenarios • Public Acceptance • State and Local Regulations as Applied to Satellite Power System Microwave Receiving Antenna Facilities • Student Participation • Potential of Laser for SPS Power Transmission • International Agreements • Centralization/Decentralization • Mapping of Exclusion Areas For Rectenna Sites • Economic and Demographic Issues Related to Deployment • Some Questions and Answers • Meteorological Effects on Laser Beam Propagation and Direct Solar Pumped Lasers • Public Outreach Experiment • Power Transmission and Reception Technical Summary and Assessment • Space Transportation • Office of Technology Assessment


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After these studies were published, there was no follow up work as the political climate had shifted against such projects. The DOE study conclusions were critical of the project's possibilities. Confused press reports claimed, incorrectly, that the concept had been found infeasible. More recently, the SPS concept has again become interesting, due to increased energy demand, increased energy costs, and emission implications, starting in 1997 with the NASA "Fresh Look" however funding is still minimal. In 2007, the US Department of Defense expressed interest in the concept. At some cost point, the high initial costs of an SPS project will become favourable due to the low-cost delivery of power. By some estimates, this has already happened in some locations, as a result of the widely varying costs of electricity which sometimes approach (or even exceed) this point. In addition, continued advances in material science and space transport continue to whittle away at the startup cost of an SPS. The SPS essentially consists of three parts: 1. a solar collector, typically made up of solar cells 2. a microwave antenna on the satellite, aimed at Earth 3. one or more paired, and much larger, antennas (rectennas) on the Earth's surface

SPACECRAFT DESIGN In many ways, the SPS is a simpler conceptual design than most power generation systems previously proposed. The simple aspects include the physical structure required to hold the SPS together and to align it orthogonally to the Sun. This will be considerably lighter than any similar structure on Earth since it will be in a zero-g, vacuum environment and will not need to support itself against a gravity field and needs no protection from terrestrial wind or weather. Solar photons will be converted to electricity aboard the SPS spacecraft, and that electricity will be fed to an array of Klystron tubes which will generate the microwave beam. SOLAR ENERGY CONVERSION (SOLAR PHOTONS TO DC CURRENT) Two basic methods of converting photons to electricity have been studied, solar dynamic (SD) and photovoltaic (PV).

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SD uses a heat engine to drive a piston or a turbine which connects to a generator or dynamo. Two heat cycles for solar dynamic are thought to be reasonable for this: the Brayton cycle or the Stirling cycle. Terrestrial solar dynamic systems typically use a large reflector to focus sunlight to a high concentration to achieve a high temperature so the heat engine can operate at high thermodynamic efficiencies; an SPS implementation will be similar. A major advantage of space solar is the efficiency with which huge mirrors can be supported and pointed in zero gravity and vacuum conditions of space. They can be constructed with very thin aluminum or other metal sheets and very light frames, easily constructed from materials available in space (eg, on the Moon's surface). PV uses semiconductor cells (e.g., silicon or gallium arsenide) to directly convert sunlight photons into voltage via a quantum mechanical mechanism. These are commonly known as "solar cells", and will likely be rather different from the glass panel protected solar cell panels familiar to many and in current terrestrial use. They will, for reasons of weight, probably be built in a membrane form not suitable to terrestrial use which is subject to considerable gravitational loading. It is also possible to use Concentrating Photovoltaic (CPV) systems, which like SD are a form of existing terrestrial Concentrating Solar Energy approaches which convert concentrated light into electricity by PV, thus avoiding thermodynamic constraints which apply to heat engines. On Earth, they also use tracking systems, mirrors, and lenses to achieve high concentration ratios and are able to reach efficiencies above 40% Concentrating Photovoltaic Technology. Because their PV area is rather smaller than for conventional PV, the majority of the deployed collecting area in CPV systems is mirrors, as with SD systems; so they share the advantages of building and pointing large (simple) mirror arrays in space as opposed to (complex) PV panels.

COMPARISON OF PV, CPV, AND SD The main problems with non-concentrating PV are that PV cells continue to be relatively expensive, and require a relatively


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large area to be acceptable for a significantly sized power station. In addition, semiconductor PV panels will require a relatively large amount of energy to produce; amorphous-silicon designs require much less energy to produce but are less efficient. CPV designs with a small area of 40%+ efficient cells and large reflector area are less expensive to produce. As well, the materials used in some PV cells (eg, gallium and arsenic) seem to be less common in lunar materials than is silicon; this may be significant if lunar manufacturing is planned. SD is a more mature technology, having been in widespread use on Earth in many contexts for centuries. Both CPV and SD systems have more severe pointing requirements than PV, because most proposed designs require accurate and stable optical focus. If a PV array orientation drifts a few degrees, the power being produced will drop a few percent. But, if an SD or CPV array orientation drifts a few degrees, the power produced will drop very quickly, perhaps to near zero. Aiming reflector arrays requires much less energy in space than on Earth, being without terrestrial wind and gravitation loads, but it has its own problems of gyroscopic action, vibration, limits on usable reaction mass (though electrically powered gyros would avoid that problem), solar wind, and meteorite strikes on control mechanisms. Currently, PV cells weigh between 0.5kg/kW and 10kg/kW depending on design. SD designs also vary but most seem to be heavier per kW produced than PV cells and thus have higher launch costs, all other things being equal. CPV should be lighter; since it replaces the thermal power plant (except for a radiator for waste heat) with a much lighter PV array.

WORKING LIFETIME The lifetime of a PV based SPS is limited mainly by the ionizing radiation from the radiation belts and the Sun. Without some method of protection, this is likely to cause the cells to continuously degrade by about a percent or two per year. Deterioration is likely to be more rapid during periods of high exposure to energetic protons from solar particle events. If some practical protection can be designed this also might be reducible (eg, for a CPV station, radiation and particle shields for the PV cells -- out of the energy

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path from the mirrors, of course). Lifetimes for SD based SPS designs will be limited by structural and mechanical considerations, such as micrometeorite impact, metal fatigue of turbine blades, wear of sliding surfaces (although this might be avoidable by hydrostatic bearings or magnetic bearings), degradation or loss of lubricants and working fluids in vacuum, from loss of structural integrity leading to impaired optical focus amongst components, and from temperature extremes effects. As well, most mirror surfaces will degrade from both radiation and particle impact, but such mirrors can be designed simply (and so light and cheap), so replacement may be practical.

about 6 weeks.For current silicon PV panels the energy needs are relatively high, and typically three-four years of deployment in a terrestrial environment is needed to recover this energy.

In either case, another advantage of the SPS design is that waste heat developed at collection points is re-radiated back into space, instead of warming the adjacent local biosphere as with conventional sources; thus thermal efficiency will not be in itself an important design parameter except insofar as it affects the power/weight ratio via operational efficiency and hence pushes up launch costs. (For example SD may require larger radiators when operating at a lower efficiency). Earth based power handling systems must always be carefully designed, for both economic and purely engineering reasons, with operational thermal efficiency in mind.

Clearly for a system (including manufacture, launch and deployment) to provide net power it must repay the energy needed to construct it. For current silicon PV panels the energy needs are relatively high, and typically several years of deployment in a terrestrial environment is needed to recover this energy. With SPS net energy received on the ground is higher (more or less necessarily so, for the system to be worth deploying), so this energy payback period would be somewhat reduced; however SD, being made of conventional materials, are more similar to conventional powerstations and are likely to be less energy intensive and would be expected to give quicker energy break even, depending on construction technology.

One useful 'trick' SPS has up its sleeve is that at the end of life, the material does not need to be launched a second time. In theory, it would be possible to recycle the satellite 'on-site', potentially at a significantly lower cost than launching an SPS from new. This might allow a very expensive launch cost to be paid for over multiple satellite lifetimes.

ENERGY PAYBACK Clearly for a system (including manufacture, launch and deployment) to provide net power it must repay the energy needed to construct it. Solar satellites pay back the lift energy in a remarkably short time. It takes 14.75 kWh/kg for a 100% efficiency system to lift a kg from the surface of the earth to GEO. If the satellite generated a kW with 2kg of mass, the payback time would be 29.5 hours. Even with 3% efficient rockets, the energy payback time is only

With SPS net energy received on the ground is higher (more or less necessarily so, for the system to be worth deploying), so this energy payback period would be reduced to about a year. Thermal systems being made of conventional materials, are more similar to conventional power stations and are likely to be less energy intensive. They would be expected to give quicker energy break even, depending on construction technology. The relative merits of PV vs SD is still an open question.

WIRELESS POWER TRANSMISSION TO THE EARTH Wireless power transmission was early proposed to transfer energy from collection to the Earth's surface. The power could be transmitted as either microwave or laser radiation at a variety of frequencies depending on system design. Whatever choice is made, the transmitting radiation would have to be non-ionizing to avoid potential disturbances either ecologically or biologically if it is to reach the Earth's surface. This established an upper bound for the frequency used, as energy per photon, and so the ability to cause ionization, increases with frequency. Ionization of biological materials doesn't begin until ultraviolet or higher frequencies so most radio frequencies will be acceptable for this. William C. Brown demonstrated in 1964 on CBS news with Walter Cronkite, a microwave-powered model helicopter that

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received all the power needed for flight from a microwave beam. Between 1969 and 1975 Bill Brown was technical director of a JPL Raytheon program that beamed 30 kW over a distance of 1 mile at 84% efficiency. To minimize the sizes of the antennas used, the wavelength should be small (and frequency correspondingly high) since antenna efficiency increases as antenna size increases. More precisely, both for the transmitting and receiving antennas, the angular beam width is inversely proportional to the aperture of the antenna, measured in units of the transmission wavelength. The highest frequencies that can be used are limited by water vapor and CO2 absorption of air at higher microwave frequencies. For these reasons, 2.45 GHz has been proposed as being a reasonable compromise. However, that frequency results in large antenna sizes at the GEO distance. A loitering stratospheric airship has been proposed to receive higher frequencies (or even laser beams), converting them to something like 2.45 GHz for retransmission to the ground. The proposal has not been as carefully evaluated for engineering plausibility as other aspects of SPS design.

SPACECRAFT SIZING The sizing will be dominated by the distance from Earth to geostationary orbit (22,300 miles, 35,700 km), the chosen wavelength of the microwaves, and the laws of physics, specifically the Rayleigh Criterion or Diffraction limit, used in standard RF (Radio Frequency) antenna design. For best efficiency, the satellite antenna should be circular and about 1 kilometers in diameter or larger; the ground antenna (rectenna) should be elliptical, 10km wide, and a length that makes the rectenna appear circular from GSO. (Typically 14km at some North American latitudes.) Smaller antennas would result in increased losses to diffraction/sidelobes. For the desired (23mW/ cm²) microwave intensity these antennas could transfer between 5 and 10 gigawatts of power. To be most cost effective, the system needs to operate at maximum capacity. And, to collect and convert that much power, the satellite would need between 50 and 100 square kilometers of collector area (if readily available ~14%


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efficient monocrystalline silicon solar cells were deployed). State of the art (currently, quite expensive, triple junction gallium arsenide) solar cells with a maximum efficiency of 40.7% could reduce the necessary collector area by two thirds, but would not necessarily give overall lower costs. In either cases, the SPS's structure would be kilometers wide, making it larger than most man-made structures here on Earth. While almost certainly not beyond current engineering capabilities, building structures of this size in orbit has not yet been attempted.

LEO/MEO INSTEAD OF GEO A LEO system of space power stations has been proposed as a precursor to GEO space power beaming system(s). There would be advantages, (much shorter path length allowing smaller antenna sizes, lower cost to orbit) and disadvantages (constantly changing antenna geometries, increased debris collision difficulties, etc). It might be possible to deploy LEO systems sooner than GEO because the antenna development would take less time. Ultimately, because full engineering feasibility studies have not been conducted, it is not known whether this would be an improvement over a GEO installation. EARTH BASED INFRASTRUCTURE The Earth-based receiver antenna (or rectenna) is a critical part of the original SPS concept. It would probably consist of many short dipole antennas, connected via diodes. Microwaves broadcast from the SPS will be received in the dipoles with about 85% efficiency. With a conventional microwave antenna, the reception efficiency is still better, but the cost and complexity is also considerably greater, almost certainly prohibitively so. Rectennas would be multiple kilometers across. Crops and farm animals may be raised underneath a rectenna, as the thin wires used for support and for the dipoles will only slightly reduce sunlight, so such a rectenna would not be as expensive in terms of land use as might be supposed. ADVANTAGES OF AN SPS The SPS concept is attractive because space has several major advantages over the Earth's surface for the collection of solar

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power. There is no air in space, so the collecting surfaces would receive much more intense sunlight, unaffected by weather. In geostationary orbit, an SPS would be illuminated over 99% of the time. The SPS would be in Earth's shadow on only a few days at the spring and fall equinoxes; and even then for a maximum of 75 minutes late at night when power demands are at their lowest. This allows the power generation system to avoid the expensive storage facilities (e.g., lakes behind dams, oil storage tanks, etc) necessary in many Earth-based power generation systems. Additionally, an SPS will avoid entirely the polluting consequences of fossil fuel systems, the ecological problems resulting from many renewable or low impact power generation systems (eg, dams).

However, economies of scale for expendable vehicles could give rather large reductions in launch cost for this kind of launched mass. Thousands of rocket launches could very well reduce the costs by ten to twenty times, using standard costing models. This puts the economics of an SPS design into the practicable range. Reusable vehicles could quite conceivably attack the launch problem as well, but are not a well-developed technology.

Politically, SPS would create new jobs and opportunities for companies. For nations on the equator, SPS provides an incentive to stabilise and a sustained opportunity to lease land for launch sites. SPS is also applicable on a global scale. Nuclear power especially is something many governments would be reluctant to sell to developing nations. Whether bio-fuels can support the western world, let alone the developed world, is currently a matter of debate. SPS poses no such problems.

Power beaming from geostationary orbit by microwaves has the difficulty that the required 'optical aperture' sizes are very large. For example, the 1978 NASA SPS study required a 1-km diameter transmitting antenna, and a 10 km diameter receiving rectenna, for a microwave beam at 2.45 GHz. These sizes can be somewhat decreased by using shorter wavelengths, although they have increased atmospheric absorption and even potential beam blockage by rain or water droplets. Because of the thinned array curse, it is not possible to make a narrower beam by combining the beams of several smaller satellites. The large size of the transmitting and receiving antennas means that the minimum practical power level for an SPS will necessarily be high; small SPS systems will be possible, but uneconomic.

The industrial capacity needed to construct and maintain such constructions would significantly reduce the cost of other space endeavours. A manned trip to mars (for example) might only cost hundreds of millions, instead of tens of billions. More long-term, the potential amount of power production is enormous. If power stations can be placed outside Earth orbit, the upper limit is vastly higher still. In the extreme, such arrangements are called Dyson spheres.

PROBLEMS

Launch Costs Without doubt, the most obvious problem for the SPS concept is the current cost of space launches. Current rates on the Space Shuttle run between $3,000 and $5,000 per pound ($6,600/kg and $11,000/kg) to low Earth orbit, depending on whose numbers are used. Calculations show that launch costs of less than about $180225 per pound ($400-500/kg) to LEO (Low Earth orbit) seem to be necessary.

Much of the material launched need not be delivered to its eventual orbit immediately, which raises the possibility that high efficiency (but slower) engines could move SPS material from LEO to GEO at acceptable cost. Examples include ion thrusters or nuclear propulsion. They might even be designed to be reusable.

To give an idea of the scale of the problem, assuming an (arbitrary, as no space-ready design has been adequately tested) solar panel mass of 20 kg per kilowatt (without considering the mass of the supporting structure, antenna, or any significant mass reduction of any focusing mirrors) a 4 GW power station would weigh about 80,000 metric tons, all of which would, in current circumstances, be launched from the Earth. Very lightweight designs could likely achieve 1 kg/kW,, meaning 4,000 metric tons for the solar panels for the same 4 GW capacity station. This would be the equivalent of between 40 and 80 heavy-lift launch vehicle (HLLV) launches to send the material to low earth orbit, where it would likely be converted into subassembly solar arrays, which then could use high-efficiency ion-engine style rockets to

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(slowly) reach GEO (Geostationary orbit). With an estimated serial launch cost for shuttle-based HLLVs of $500 million to $800 million, total launch costs would range between $20 billion (low cost HLLV, low weight panels) and $320 billion ('expensive' HLLV, heavier panels). Economies of scale on such a large launch program could be as high as 90% (if a learning factor of 30% could be achieved for each doubling of production) over the cost of a single launch today. In addition, there would be the cost of an assembly area in LEO (which could be spread over several power satellites), and probably one or more smaller one(s) in GEO. The costs of these supporting efforts would also contribute to total costs.


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In 1980, when it became obvious NASA's launch cost estimates for the space shuttle were grossly optimistic, O'Neill et al published another route to manufacturing using lunar materials with much lower startup costs This 1980s SPS concept relied less on human presence in space and more on partially self-replicating systems on the lunar surface under telepresence control of workers stationed on Earth. Again, this proposal suffers from the current lack of such automated systems on Earth, much less on the Moon.

So how much money could an SPS be expected to make? For every one gigawatt rating, current SPS designs will generate 8.75 terawatt-hours of electricity per year, or 175 TWoh over a twentyyear lifetime. With current market prices of $0.22 per kWoh (UK, January 2006) and an SPS's ability to send its energy to places of greatest demand (depending on rectenna siting issues), this would equate to $1.93 billion per year or $38.6 billion over its lifetime. The example 4 GW 'economy' SPS above could therefore generate in excess of $154 billion over its lifetime. Assuming facilities are available, it may turn out to be substantially cheaper to recast onsite steel in GEO, than to launch it from Earth. If true, then the initial launch cost could be spread over multiple SPS lifespans.

Asteroid mining has also been seriously considered. A NASA design studyevaluated a 10,000 ton mining vehicle (to be assembled in orbit) that would return a 500,000 ton asteroid 'fragment' to geostationary orbit. Only about 3000 tons of the mining ship would be traditional aerospace-grade payload. The rest would be reaction mass for the mass-driver engine; which could be arranged to be the spent rocket stages used to launch the payload. Assuming, likely unrealistically, that 100% of the returned asteroid was useful, and that the asteroid miner itself couldn't be reused, that represents nearly a 95% reduction in launch costs. However, the true merits of such a method would depend on a thorough mineral survey of the candidate asteroids; thus far, we have only estimates of their composition. There has been no such survey. Once built, NASA's CEV should be capable of beginning such a survey, Congressional money and imagination permitting.

Extraterrestrial Materials

Lofstrom Launch Loop

Gerard O'Neill, noting the problem of high launch costs in the early 1970s, proposed building the SPS's in orbit with materials from the Moon. Launch costs from the Moon are about 100 times lower than from Earth, due to the lower gravity. This 1970s proposal assumed the then-advertised future launch costing of NASA's space shuttle. This approach would require substantial up front capital investment to establish mass drivers on the Moon.

A Lofstrom loop could conceivably provide the launch capacity needed to make a solar power satellite practical. This is a high capacity launch system capable of reaching a geosynchronous transfer orbit at low cost (Lofstrom estimates a large system could go as low as $3/kg to LEO for example). The Lofstrom loop is expected to cost less than a conventional space elevator to develop and construct, and to provide lower launch costs. Unlike the conventional space elevator, it is believed that a launch loop could be built with today's materials.

Nevertheless, on 30 April 1979, the Final Report ("Lunar Resources Utilization for Space Construction") by General Dynamics' Convair Division, under NASA contract NAS9-15560, concluded that use of lunar resources would be cheaper than terrestrial materials for a system of as few as thirty Solar Power Satellites of 10GW capacity each.

Space Elevators More recently the SPS concept has been suggested as a use for a space elevator. The elevator would make construction of an SPS considerably less expensive, possibly making them competitive

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with conventional sources. However it appears unlikely that even recent advances in materials science, namely carbon nanotubes, can make possible such an elevator, nor to reduce the short term cost of construction of the elevator enough, if an Earth-GSO space elevator is ever practical. A variant to the Earth-GSO elevator concept is the Lunar space elevator, first described by Jerome Pearson in 1979. Because of the ~20 times shallower (than Earth's) gravitational well for the lunar elevator, this concept would not rely on materials technology beyond the current state of the art, but it would require establishing an Si mining and solar cell manufacturing facilities on the Moon, similar to O/Neill's lunar material proposal, discussed above.

Safety The use of microwave transmission of power has been the most controversial issue in considering any SPS design, but any thought that anything which strays into the beam's path will be incinerated is an extreme misconception. Consider that quite similar microwave relay beams have long been in use by telecommunications companies world wide without such problems. At the earth's surface, a suggested microwave beam would have a maximum intensity, at its center, of 23 mW/cm2 (less than 1/4 the solar irradiation constant), and an intensity of less than 1 mW/cm2 outside of the rectenna fenceline (10 mW/cm2 is the current United States maximum microwave exposure standard). At present, per OSHA, , the workplace exposure limit (10 mW/ cm2) is expressed in voluntary language and has been ruled unenforceable for Federal OSHA enforcement. The beam's most intense section (more or less, at its center) is far below dangerous levels even for an exposure which is prolonged indefinitely. Furthermore, exposure to the center of the beam can easily be controlled on the ground (eg, via fencing), and typical aircraft flying through the beam provide passengers with a protective shell metal (ie, a Faraday Cage), which will intercept the microwaves. Other aircraft (balloons, ultra-light, etc) can avoid exposure by observing airflight control spaces, as is currently done for military and other controlled airspace. Over 95% of the


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beam energy will fall on the rectenna. The remaining microwave energy will be absorbed and dispersed well within standards currently imposed upon microwave emissions around the world. The microwave beam intensity at ground level in the center of the beam would be designed and physically built into the system; simply, the transmitter would be too far away and too small to be able to increase the intensity to unsafe death ray levels, even in principle. In addition, a design constraint is that the microwave beam must not be so intense as to injure wildlife, particularly birds. Experiments with deliberate microwave irradiation at reasonable levels have failed to show negative effects even over multiple generations. Some have suggested locating rectennas offshore, but this presents serious problems, including corrosion, mechanical stresses, and biological contamination. A commonly proposed approach to ensuring fail-safe beam targeting is to use a retrodirective phased array antenna/rectenna. A "pilot" microwave beam emitted from the center of the rectenna on the ground establishes a phase front at the transmitting antenna. There, circuits in each of the antenna's subarrays compare the pilot beam's phase front with an internal clock phase to control the phase of the outgoing signal. This forces the transmitted beam to be centered precisely on the rectenna and to have a high degree of phase uniformity; if the pilot beam is lost for any reason (if the transmitting antenna is turned away from the rectenna, for example) the phase control value fails and the microwave power beam is automatically defocused. Such a system would be physically incapable of focusing its power beam anywhere that did not have a pilot beam transmitter. It is important for system efficiency that as much of the microwave radiation as possible be focused on the rectenna. Outside of the rectenna, microwave intensities would rapidly decrease, so nearby towns or other human activity should be completely unaffected. The long-term effects of beaming power through the ionosphere in the form of microwaves has yet to be studied, but nothing has been suggested which might lead to any significant effect.

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DEFENDING SOLAR POWER SATELLITES Solar power satellites would normally be at a high orbit that is difficult to reach, and hence attack. However, it has been suggested that a large enough quantity of granular material placed in a retrograde orbit at the geostationary altitude could theoretically completely destroy these kinds of system and render that orbit useless for generations. Whether this is a realistic attack scenario is arguable, and in any case at the present time there is only a small list of countries with the necessary launch capability to do this, such an attack would probably be considered an act of war by every single nation (except the attacker, which would lose its satellites, too) with satellites in geostationary orbit, and an attack with more conventional anti-satellite weapons would probably be considered an act of war by the nation whose satellite was attacked. In any case, the receiving stations on the ground, and conventional power generators (which are unlikely to be completely replaced by solar power satellites), are more easily attacked. Computer security may be a bigger issue than physical defense, since launch capabilities aren't necessary to hack a satellite for purposes of malicious orbital "corrections", extortion (by threatening to destabilize its orbit) or outright "grand theft satellite". SPS'S ECONOMIC FEASIBILITY

Current Energy Price Landscape In order to be competitive on a purely economic level, an SPS must cost no more than existing supplies. (Such costs must include the costs of cleaning waste from construction, operation and dismantling of the generating systems--including lifestyle and health costs.. Currently(2007) most Earth-based power generation does not include these costs. The cost figures below are undated, but are obsolete as of 2007. This greatly reduces the prices paid for power currently reducing the apparent benefits of SPS'.) This may be difficult, especially if it is deployed for North America, where energy costs have been relatively low. It must cost less to deploy, or operate for a very long period of time, or offer other advantages. Many proponentswho? have suggested that the lifetime is effectively infinite, but normal maintenance and


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replacement of less durable components makes this unlikely. Satellites do not, in our now-extensive experience, last forever. (But with regular maintenance there is no reason that a high orbit satellite has to 'die.' Currently (2007) the majority of such satellites-weather and communications, fail due to correctable maintenance issues which we do not correct because we have no repair people on site. Common failures are: running out of station keeping fuel or dead batteries-no longer holding a charge. Neither of these failure modes is much of a problem if service is available. With available refueling and battery replacement, the life of a satellite can be greatly increased. Structural components, which make up the largest percentage of mass, seldom fail. Nearly all of the other components can be modularized for easy replacement/upgrade.) Current prices for electricity on the public grid fluctuate depending on time of day, but typical household delivery costs about 5 cents per kilowatt hour in North America. If the lifetime of an SPS is 20 years and it delivers 5 gigawatts to the grid, the commercial value of that power is (5,000,000,000 watts)/(1000 watts/kilowatt) = 5,000,000 kilowatts, which multiplied by $.05 per kWoh gives $250,000 revenue per hour. $250,000 × 24 hours × 365 days × 20 years = $43,800,000,000. By contrast, in the United Kingdom (October 2005) electricity can cost 9-22 cents per kilowatt hour. This would translate to a lifetime output of $77-$193 billion for power delivered to the UK.

COMPARISON WITH FOSSIL FUELS The relatively low price of energy today is entirely dominated by the historically low cost of carbon based fossil fuels (e.g., petroleum, coal and natural gas). There are several problems with existing energy delivery systems. They are subject to (among other problems) • political instability for various reasons in various locations -- so that there are large hidden costs in maintaining military or other presence so as to continue supplies • depletion (some well regarded estimates suggest that oil and gas reserves have been in net decline for some time and that price increases and supply decreases are inevitable)

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Solar Energy and its Uses o oil prices rose from around $20/bbl in the early 2000s to over $130/bbl in early 2008, despite no major disruptions in supply, suggesting to some industry observers (e.g., Matthew Simmons) that the days of cheap oil are over • greenhouse pollution -- fossil fuel combustion emits enormous quantities of carbon dioxide (CO2), a greenhouse gas, contributing to global warming and climate change.

Following the Kyoto Treaty, 141 countries introduced the first system of mandatory emissions control via carbon crs. The ultimate direction of such policies is to increase efficiency of fossil fuel use, perhaps to the point of elimination in some countries or even globally. But, the energy requirements of Third World or developing countries (e.g., China and India) are increasing steadily. Because of the net increase in demand, energy prices will continue to increase, though how fast and how high are less easily predicted.

COMPARISON WITH NUCLEAR POWER (FISSION) Detailed analyses of the problems with nuclear power specifically (nuclear fission) are published elsewhere. Some are given below, with some comparative comments: • nuclear proliferation -- not a problem with SPS • disposal and storage of radioactive waste -- not a problem with SPS • preventing fissile material from being obtained by terrorists or their sponsors -- not a problem with SPS • public perception of danger -- problem with both SPS and nuclear power • consequences of major accident, e.g., Chernobyl -effectively zero with SPS, save on launch (during construction or for maintenance) • military and police cost of protecting the public and loss of democratic freedoms -- control of SPS would be a power/ influence center, perhaps sufficient to translate into political power. However, this has not yet happened in the developed world with nuclear power.


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• installation delays. These have been notoriously long with nuclear power plants (at least in the US), and may be reduced with SPS. With sufficient commitment from SPS backers, the difference may be substantial. On balance, SPS avoids nearly all of the problems with current nuclear power schemes, and does not have larger problems in any respect, although public perception of microwave power transfer (ie, in the beams produced by an SPS and received on Earth) dangers could become an issue.

COMPARISON WITH NUCLEAR FUSION Nuclear fusion is a process used in thermonuclear bombs (e.g., the H-bomb). Projected nuclear fusion power plants would not be explosive, and will likely be inherently failsafe. However, sustained nuclear fusion generators have only just been demonstrated experimentally, despite well funded research over a period of several decades (since approximately 1952). There is still no credible estimate of how long it will be before a nuclear fusion reactor could become commercially possible; fusion research continues to receive substantial funding by many nations. For example, the ITER facility currently under construction will cost 10 billion. There has been much criticism of the value of continued funding of fusion research. Proponents have successfully argued in favor of ITER funding. By contrast, SPS does not require any fundamental engineering breakthroughs, has already been extensively reviewed from an engineering feasibility perspective over some decades, and needs only incremental improvements of existing technology to be deployed. Despite these advantages, SPS has received minimal research funding to date. COMPARISON WITH TERRESTRIAL SOLAR POWER In the case of the United Kingdom, the country as a whole is further north than even most inhabited parts of Canada, and hence receives little insolation over much of the year, so conventional solar power is not competitive at 2006 per-kilowatthour delivered costs. However, per-kilowatt-hour photovoltaic costs have been in exponential decline for decades, with a 20-fold

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decrease from 1975 to 2001, so this situation may change. Let us consider a ground-based solar power system versus an SPS generating an equivalent amount of power. • Such a system would require a very large solar array built in a well-sunlit area, the Sahara Desert for instance. An SPS requires much less ground area per kilowatt (approx 1/5th). There is no such area in the UK. • The rectenna on the ground is much larger than the area of the orbiting solar panels. A ground-only solar array would have the advantage, compared to a GEO (Geosynchronous orbit) solar array, of costing considerably less to construct and requiring no significant technological advances. A small version of such a ground based array has recently been completed by General Electric in Portugal. • The receiving SPS rectenna will be quite simple, cheap, and even transparent, with fewer land use issues than a conventional terrestrial solar array. Crops could be grown beneath the rectenna, so the land needed could be dualuse. By comparison, ground-based solar panels would completely block sunlight thus destroying vegetation and having a considerable effect on local ecology, which in turn would result in increased soil erosion, drainage and runoff problems (increased flood risk) and loss of habitats, though this would be reduced somewhat for desert installations. • A terrestrial solar station intercepts an absolute maximum of only one third of the solar energy an array of equal size could intercept in space, since no power is generated at night and less light strikes the panels when the Sun is low in the sky or weather interferes. A solar panel in the contiguous United States on average delivers 19 to 56 W/ m² . By comparison an SPS rectenna would deliver about 23mW/cm² (230 W/m²) continuously, hence the size of rectenna required per collected watt would be about 8.2% to 24% that of a terrestrial solar panel array with equivalent power output, neglecting weather and night/day cycles. Assuming, of course, current levels of solar cell efficiency.


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• Further, if it is assumed that a ground-based solar array must supply baseload power (not true for every projected configuration), some form of energy storage would be required to provide power at night, such as hydrogen generation/storage, compressed air, or pumped storage hydroelectricity. With present technology, energy storage on this scale is prohibitively expensive, and will incur energy losses as well. • Weather conditions would also interfere with power collection, and will cause wear and tear on solar collectors which will be avoided in Earth orbit; for instance, sandstorms cause devastating damage to human structures via, for example, abrasion of surfaces as well as mechanically large wind forces causing direct physical damage. Terrestrial systems are also more vulnerable to terrorism than an SPS's rectenna since they are more expensive, complex, intolerant of partial damage, and harder to repair/replace. Wear and tear on orbital installations will be of very different character, for quite different reasons, and can be reduced by care in design and fabrication. Long experience with terrestrial installations shows that there is substantial, inescapable maintenance for any economically feasible electrical installation. • Terrestrial solar panel locations are inherently fixed, but beamed microwave power allows one to adaptively reroute delivered power near to places it is needed (within limits -- rectennas near the SPS's horizon (e.g., at high latitudes) will not be as efficient). A station in the Sahara could provide practical power only to the surrounding area; current demand is relatively low there. That is, at least until long distance superconducting distribution becomes possible, which will make remotely sited Earth surface collection systems more practical, and distribution of generated power equally so, including that from an SPS. • Remote tropical location of an extensive photovoltaic generator is a somewhat artificial scenario, as photovoltaic

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Solar Energy and its Uses costs continue to decline. Deployment of ground-based photovoltaics can be distributed (say to rooftops), but nevertheless, the required acreage (at any credible solar cell efficiency) will remain very large, and maintenance cost and effort will increase substantially compared to a large centralized design. In any case, dispersed installation is not possible for some terrestrial solar collectors. • Energy payback time for the capital costs of terrestrial PV cells has been typically in the 5-15 year range, depending largely on existing local cost structures. Payback for an orbital installations is likely to be quicker due to the higher total insolation rate, which will, of course be essentially continuous, without interruptions during nighttimes or bad weather. While it is true some of the potential energy available would not be collected (cell inefficienies will assure this in any case), that some would be lost internally at the SPS (no equipment is loss free), and that still more would be lost in transmission back to the Earth, the engineering feasibility studies have established that none of these losses will be large enough to make an SPS project infeasible on those grounds. Losses due to conventional fossil fuel generation are of larger magnitude than in an SPS design, and are more than merely lost efficiency as such losses all contribute to pollution (eg, exhaust gases).

Both SPS and ground-based solar power could be used to produce chemical fuels for transportation and storage, as in the proposed hydrogen economy. Or they could both be used to run an energy storage scheme (such as pumping water uphill at a hydropower generation station). Many advances in solar cell efficiency (eg, improved construction techniques) that make an SPS more economically feasible might make a ground-based system more economic as well. Also, many SPS designs assume the framework will be built with automated machinery supplied with raw materials, typically aluminium. Such a system could be (more or less easily) adapted for operation on Earth, no launching required. However, Earthbased construction already has access to inexpensive human labor that would not be available in space, so such construction


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techniques would have to be extremely competitive to be significant on Earth. SOLAR PANEL MASS PRODUCTION Currently the costs of solar panels are too high to use them to produce bulk domestic electricity in most situations. However, mass production of the solar panels necessary to build an SPS system would be likely to reduce those costs sufficiently to change this -- perhaps substantially -- especially as fossil fuel costs have been increasing rapidly. But, any panel design suited to SPS use is likely to be quite different than earth suitable panels, so not all such improvements will have this effect. This may benefit earth based array designs as costs may be lower (see the cost analysis above), but will not be able to take advantage of maximum economies of scale, and so piggyback on production of Earth based panels. It should be noted, however, that there are also frequent developments in the production of solar panels. Thin film solar panels and so-called "nanosolar" might increase collection efficiency, reduce production costs as well as weight, and therefore reduce the total cost of an SPS installation. In addition, private space corporations could become interested in transporting goods (such as satellites, supplies and parts of commercial space hotels) to LEO (Low Earth orbit), since they already are developing spacecraft to transport space tourists. If they can reduce costs, this will also increase the economic feasibility of an SPS.

COMPARISON WITH OTHER RENEWABLES (WIND, TIDAL, HYDRO, GEOTHERMAL) Most renewable energy sources (for example, tidal energy, hydro-electric, geothermal, ethanol), have the capacity to supply only a tiny fraction of the global energy requirement, now or in the foreseeable future. For most, the limitation is geography as there simply are very few sites in the world where generating systems can be built, and for hydro-electric projects in particular, there are few sites still open. For 2005, in the US, hydro-electric power accounted for 6.5% of electricity generation, and other renewables 2.3%. The U.S. Govt. Energy Information

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Administration projects that in 2030 hydro-power will decline to 3.4% and other renewables will increase to 2.9%.

COMPARISON WITH BIOFUELS Ethanol power production depends on farming in the case of corn or sugar cane, currently the two leading sources of ethanol fuel. There is insufficient farming capacity for both significant energy production and food production. Corn prices have risen substantially in 2006 and 2007, partly as a result of nascent ethanol production demand. Due to the high energy cost of industrial agriculture as well as the azeotropic distillation necessary to refine ethanol, serious questions remain about the EROEI of ethanol from corn. Ethanol from cellulose (eg, agricultural waste or purpose collected non-cultivated plants, eg, switchgrass) is not practicable as of 2007, though pilot plants are in development. Processing improvements (eg, a breakthrough in enzyme processing) may change this relative disadvantage. COMPARISON WITH WIND POWER Wind power is somewhat unique among the renewables as having emerged as competitive with fossil fuels on cost (similar to hydro), but unlike hydro has significant potential for expansion. Wind power has been the fastest-growing form of renewable energy throughout the 2000s, growing at an annual rate of approximately 30%. As of 2008, wind power's share of global energy output remained small, but wind power accounted for a large share of new power generation capacity in several countries including the United States and the United Kingdom. Improvements in technology, especially the trend toward larger wind turbines mounted on taller towers, has reduced the cost of wind power to be competitive with fossil fuel. The potential for wind power appears to be very large. For example, just the four windiest states in the United States (North Dakota, Texas, Kansas, and South Dakota), have wind resources that could equal the current electricity consumption of the entire country. Offshore wind resources appear to be even larger than on-shore wind resources. One advantage of wind farms is their ability to expand incrementally; individual wind turbines can be assembled on site at a typical rate of approximately one per week, and begin


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generating electricity (and thus revenue) as soon as they connect to the transmission grid. This gives wind power a lower capital risk compared to large-scale power generation schemes that require heavy investment for years before they become operational (e.g., hydroelectric power, nuclear power). Ocean-based windpower offers access to very large wind resources (there being large areas for potential installations, and winds tend to blow stronger and steadier over water than over land due to reduced surface friction), but it is strongly affected by two factors: the difficulty of long distance power transmission as many regions of high demand are not near the sea, and by the very large difficulty of coping with corrosion, contamination, and survivability problems faced by all seaborne installations. Some potential locations for offshore wind turbines suffer less from these problems, such as the Great Lakes of the United States and Canada, which are surrounded by well-developed power grids and large populations of electricity consumers. The lakes, being fresh water, would pose fewer corrosion problems, and construction in these environments is well-understood.

CURRENT WORK For the past several years there has been no line item for SPS in either the NASA nor DOE budgets, a minimal level of research has been sustained through small NASA discretionary budget accounts. NASDA (Japan's national space agency) has been researching in this area steadily for the last few years. In 2001 plans were announced to perform additional research and prototyping by launching an experimental satellite of capacity between 10 kilowatts and 1 megawatt of power. The National Space Society (a non-profit NGO) maintains a web page where the latest SPS related references are posted and kept current. In May 2007 a workshop was held at MIT in the U.S.A. to review the current state of the market and technology In 2007 the U.S. Department of Defense expressed interest in studying the concept. On 10/10/2007 The National Security Space

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Office of the US Department of Defense, published an assessment report . The report was released at a press conference which simultaneously announced the formation of the Space Solar Alliance for Future Energy which intends to pursue the recommendations of the NSSO-Led Study.

IN FICTION Space stations transmitting solar power have appeared in science-fiction works like Isaac Asimov's Reason (1941), that centers around the troubles caused by the robots operating the station. Solar Power Satellites have also been seen in the work of author Ben Bova's novels "Powersat" and "Colony". The anime series Gundam 00 explores the effects and politics of space based solar power. In both SimCity 2000 and 3000, plants that improvised solar satellite technology called microwave powerplants were available in the future. The plant was discontinued in SimCity 4 but several fan-made microwave powerplants were available on various SimCity 4 fan-sites. Solar Sats are used in the online browser-based game ogame. They are a means to supply power to planet production.

SOLAR POWER TOWER


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EXAMPLES OF HELIOSTAT POWER PLANTS The 10 MWe Solar One and Solar Two heliostat demonstration projects in the Mojave Desert have now been decommissioned. The 15 MW Solar Tres Power Tower in Spain builds on these projects. In Spain the 11 MW PS10 solar power tower was recently completed. In South Africa, a solar power plant is planned with 4000 to 5000 heliostat mirrors, each having an area of 140 m². A site near Upington has been selected. BrightSource Energy entered into a series of power purchase agreements with Pacific Gas and Electric Company in March 2008 for up to 900MW of electricity, the largest solar power commitment ever made by a utility. BrightSource is currently developing a number of solar power plants in Southern California, with construction of the first plant planned to start in 2009. In June 2008, BrightSource Energy dedicated its Solar Energy Development Center (SEDC) in Israel's Negev Desert. The site, located in the Rotem Industrial Park, features more than 1,600 heliostats that track the sun and reflect light onto a 60 meter-high tower. The concentrated energy is then used to heat a boiler atop the tower to 550 degrees Celsius, generating steam that is piped into a turbine, where electricity can be produced.

The solar power tower (also known as 'Central Tower' power plants or 'Heliostat' power plants or power towers) is a type of solar furnace using a tower to receive the focused sunlight. It uses an array of flat, movable mirrors (called heliostats) to focus the sun's rays upon a collector tower (the target). The high energy at this point of concentrated sunlight is transferred to a substance that can store the heat for later use.

COST The US National Renewable Energy Laboratory NREL has estimated that by 2020 electricity could be produced from power towers for 5.47 ?/kWh. Google.org hopes to develop cheap, low maintenance, mass produceable heliostat components to reduce this cost in the near future.

The most recent heat transfer material that has been successfully demonstrated is liquid sodium. Sodium is a metal with a high heat capacity, allowing that energy to be stored and drawn off throughout the evening. That energy can, in turn, be used to boil water for use in steam turbines. Water had originally been used as a heat transfer medium in earlier power tower versions (where the resultant steam was used to power a turbine). This system did not allow for power generation when the sun is not shining.

SOLAR PROTON EVENT A Solar proton event occurs when protons emitted by the Sun become accelerated to very high energies either close to the Sun during a solar flare or in interplanetary space by the shocks associated with coronal mass ejections. These high energy protons cause several effects. They can penetrate the Earth's magnetic field and cause ionization in the ionosphere. The effect is similar to auroral events, the difference being that electrons and not protons are involved. Energetic solar protons are also a significant radiation

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hazard to spacecraft and astronauts. Solar protons normally have insufficient energy to penetrate through the Earth's magnetic field. However, during unusually strong solar flare events, protons can be produced with sufficient energies to penetrate deeper into the Earth's magnetosphere and ionosphere. Regions where deeper penetration can occur includes the north pole, south pole, and South Atlantic magnetic anomaly. Protons are charged particles and are therefore influenced by magnetic fields. When the energetic protons leave the Sun, they preferentially follow (or are guided by) the Sun's powerful magnetic field. When solar protons enter the domain of the Earth's magnetosphere where the magnetic fields become stronger than the solar magnetic fields, they are guided by the Earth's magnetic field into the polar regions where the majority of the Earth's magnetic field lines enter and exit. Energetic protons that are guided into the polar regions collide with atmospheric constituents and release their energy through the process of ionization. The majority of the energy is extinguished in the extreme lower region of the ionosphere (around 50-80 km in altitude). This area is particularly important to ionospheric radio communications because this is the area where most of the absorption of radio signal energy occurs. The enhanced ionization produced by incoming energetic protons increases the absorption levels in the lower ionosphere and can have the effect of completely blocking all ionospheric radio communications through the polar regions. Such events are known as Polar Cap Absorption events (or PCAs). These events commence and last as long as the energy of incoming protons at approximately greater than 10 MeV (million electron volts) exceeds roughly 10 pfu (particle flux units) at geosynchronous satellite altitudes. The more severe proton events can be associated with geomagnetic storms that can cause widespread disruption to electrical grids. However, proton events themselves are not responsible for producing anomalies in power grids, nor are they responsible for producing geomagnetic storms. Power grids are only sensitive to fluctuations in the Earth's magnetic field. Extremely intense solar proton flares capable of producing energetic protons with energies in excess of 100 MeV can increase


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neutron count rates at ground levels through secondary radiation effects. These rare events are known as Ground Level Events (or GLE's). There is no substantive scientific evidence to suggest that energetic proton events are harmful to human health at ground levels, particularly at latitudes where most of the Earth's population resides. The Earth's magnetic field is exceptionally good at preventing the radiative effects of energetic particles from reaching ground levels. High altitude commercial transpolar aircraft flights have measured increases in radiation during energetic proton events, but a warning system is in place that limits these effects by alerting pilots to lower their cruising altitudes. Aircraft flights away from the polar regions are far less likely to see an impact from solar proton events. Significant proton radiation exposure can be experienced by astronauts who are outside of the protective shield of the Earth's magnetosphere, such as an astronaut in-transit to, or located on the Moon. However, the effects can be minimized if astronauts are in a low-Earth orbit and remain confined to the most heavily shielded regions of their spacecraft. Proton radiation levels in low earth orbit increase with orbital inclination. Therefore, the closer a spacecraft approaches the polar regions, the greater the exposure to energetic proton radiation will be. Astronauts have reported seeing flashes or streaks of light as energetic protons interact with their optic tissues. Similar flashes and streaks of light occur when energetic protons strike the sensitive optical electronics in spacecraft (such as star trackers and other cameras). The effect can be so pronounced that during extreme events, it is not possible to obtain quality images of the Sun or stars. This can cause spacecraft to lose their orientation, which is critical if ground controllers are to maintain control. Energetic proton storms can also electrically charge spacecraft to levels that can damage electronic components. They can also cause electronic components to behave erratically. For example, solid state memory on spacecraft can be altered, which may cause data or software contamination and result in unexpected (phantom) spacecraft commands being executed. Energetic proton storms also destroy the efficiency of the solar panels that are designed

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to collect and convert sunlight to electricity. During years of exposure to energetic proton activity from the Sun, spacecraft can lose a substantial amount of electrical power that may require important instruments to be turned off.

SOLAR RADIATION Solar irradiance spectrum at top of atmosphere, on a linear scale and plotted against wavenumber. Solar radiation is radiant energy emitted by a sun as a result of its nuclear fusion reactions. The spectrum of the Sun's solar radiation is close to that of a black body with a temperature of about 5,800 K. About half that lies in the visible short-wave part of the electromagnetic spectrum and the other half mostly in the near-infrared part. Some also lies in the ultraviolet part of the spectrum. When ultraviolet radiation is not absorbed by the atmosphere or other protective coating, it can cause a change in human skin pigmentation. Solar radiation is commonly measured with a pyranometer or pyrheliometer. SOLAR CONSTANT The solar constant is the amount of the Sun's incoming electromagnetic radiation (Solar radiation) per unit area, measured on the outer surface of Earth's atmosphere in a plane perpendicular to the rays. The solar constant includes all types of solar radiation, not just the visible light. It is measured by satellite to be roughly 1366 watts per square meter (W/m²), though this fluctuates by about 6.9% during a year (from 1412 W/m² in early January to 1321 W/m² in early July) due to the earth's varying distance from the Sun, and by a few parts per thousand from day to day. Thus, for the whole Earth (which has a cross section of 127,400,000 km²), the power is 1.740×1017 W, plus or minus 3.5%. The Solar constant does not remain constant over long periods of time (see Solar variation). 1366 W/m² is equivalent to 1.96 calories per minute per square centimeter, or 1.96 langleys (Ly) per minute. The Earth receives a total amount of radiation determined by its cross section (?·RE²), but as it rotates this energy is distributed across the entire surface area (4·?·RE²). Hence the average incoming solar radiation (sometimes called the solar irradiance), taking into account the angle at which the rays strike and that at any one


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moment half the planet does not receive any solar radiation, is one-fourth the solar constant (approximately 342 W/m²). At any given moment, the amount of Solar radiation received at a location on the Earth's surface depends on the state of the atmosphere and the location's latitude. The solar constant includes all wavelengths of solar electromagnetic radiation, not just the visible light (see Electromagnetic spectrum). It is linked to the apparent magnitude of the Sun, ?26.8, in that the solar constant and the magnitude of the Sun are two methods of describing the apparent brightness of the Sun, though the magnitude only measures the visual output of the Sun. In 1884, Samuel Pierpont Langley attempted to estimate the Solar constant from Mount Whitney in California. By taking readings at different times of day, he attempted to remove effects due to atmospheric absorption. However, the value he obtained, 2903 W/m², was still too great. Between 1902 and 1957, measurements by Charles Greeley Abbot and others at various high-altitude sites found values between 1322 and 1465 W/m². Abbott proved that one of Langley's corrections was erroneously applied. His results varied between 1.89 and 2.22 calories (1318 to 1548 W/m²), a variation that appeared to be due to the Sun and not the Earth's atmosphere. The angular diameter of the Earth as seen from the Sun is approximately 1/11,000 radians, meaning the solid angle of the Earth as seen from the sun is approximately 1/140,000,000 steradians. Thus the Sun emits about two billion times the amount of radiation that is caught by Earth, in other words about 3.86×1026 watts. CLIMATE EFFECT OF SOLAR RADIATION On Earth, solar radiation is obvious as daylight when the sun is above the horizon. This is during daytime, and also in summer near the poles at night, but not at all in winter near the poles. When the direct radiation is not blocked by clouds, it is experienced as sunshine, combining the perception of bright white light (sunlight in the strict sense) and warming. The warming on the body and surfaces of other objects is distinguished from the increase

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in air temperature. The amount of radiation intercepted by a planetary body varies inversely with the square of the distance between the star and the planet. The Earth's orbit and obliquity change with time (over thousands of years), sometimes forming a nearly perfect circle, and at other times stretching out to an orbital eccentricity of 5% (currently 1.67%). The total insolation remains almost constant but the seasonal and latitudinal distribution and intensity of solar radiation received at the Earth's surface also varies. For example, at latitudes of 65 degrees the change in solar energy in summer & winter can vary by more than 25% as a result of the Earth's orbital variation. Because changes in winter and summer tend to offset, the change in the annual average insolation at any given location is near zero, but the redistribution of energy between summer and winter does strongly affect the intensity of seasonal cycles. Such changes associated with the redistribution of solar energy are considered a likely cause for the coming and going of recent ice ages.

SOLAR SAIL Solar sails (also called light sails or photon sails, especially when they use light sources other than the Sun) are a proposed form of spacecraft propulsion using large membrane mirrors. Radiation pressure is about 10-5 Pa at Earth radii and decreases by the square of the distance from the light source (e.g. sun), but unlike rockets, solar sails require no reaction mass. Although the thrust is small, it continues as long as the light source shines and the sail is deployed. In theory a lightsail (actually a system of lightsails) powered by an Earth-based laser could even be used to decelerate the spacecraft as it approaches its destination. Solar collectors, temperature-control panels and sun shades are occasionally used as expedient solar sails, to help ordinary spacecraft and satellites make minor attitude control corrections and orbit modifications without using fuel. This conserves fuel that would otherwise be used for maneuvering and altitude control. A few have even had small purpose-built solar sails for this use. For example, EADS Astrium built Eurostar E3000 geostationary communications satellites use solar sail panels attached to their solar cell arrays to off-load transverse angular momentum, thereby saving fuel (angular momentum is accumulated over time as the


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gyroscopic momentum wheels control the spacecraft's attitude this excess momentum must be offloaded to protect the wheels from overspin). Some unmanned spacecraft (such as Mariner 10) have substantially extended their service lives with this practice. The science of solar sails is well-proven, but the technology to manage large solar sails is still undeveloped. Mission planners are not yet willing to risk multimillion dollar missions on unproven solar sail unfolding and steering mechanisms. This neglect has inspired some enthusiasts to attempt private development of the technology, such as the Cosmos 1. The concept was first proposed by German astronomer Johannes Kepler in the seventeenth century. It was again proposed by Friedrich Zander in the late 1920s and gradually refined over the decades. Recent serious interest in lightsails began with an article by engineer and science fiction author Robert L. Forward in 1984.

HOW THEY WORK The spacecraft deploys a large membrane mirror which reflects light from the Sun or some other source. The radiation pressure on the mirror provides a minuscule amount of thrust by reflecting photons. Tilting the reflective sail at an angle from the Sun produces thrust at an angle normal to the sail. In most designs, steering would be done with auxiliary vanes, acting as small solar sails to change the attitude of the large solar sail. The vanes would be adjusted by electric motors. In theory a lightsail driven by a laser or other beam from Earth can be used to decelerate a spacecraft approaching a distant star or planet, by detaching part of the sail and using it to focus the beam on the forward-facing surface of the rest of the sail. In practice, however, most of the deceleration would happen while the two parts are at a great distance from each other, and that means that, to do that focusing, it would be necessary to give the detached part an accurate optical shape and orientation. Sails orbit, and therefore do not need to hover or move directly toward or away from the sun. Almost all missions would use the sail to change orbit, rather than thrusting directly away from a planet or the sun. The sail is rotated slowly as the sail orbits

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around a planet so the thrust is in the direction of the orbital movement to move to a higher orbit or against it to move to a lower orbit. When an orbit is far enough away from a planet, the sail then begins similar maneuvers in orbit around the sun. The best sort of missions for a solar sail involves a dive near the sun, where the light is intense, and sail efficiencies are high. Going close to the Sun may be done for different mission aims: for exploring the solar poles from a short distance, for observing the Sun and its near environment from a nonKeplerian circular orbit the plane of which may be shifted some solar radii, for flying by the Sun such that the sail gets a very high speed. An unsuspected feature, until the first half of the 1990's, of the solar sail propulsion is to allow a sailcraft to escape the solar system with a cruise speed higher or even much higher than a spacecraft powered by a nuclear electric rocket system. The spacecraft mass-to-sail area ratio does not need to achieve ultralow values, even though the sail should be an advanced all-metal sail. This flight mode is also known as fast solar sailing. Proven mathematically (like many other astronautical items well in advance of their actual launches), such sailing mode has been considered by NASA/Marshall as one of the options for a future precursor interstellar probe exploring the near interstellar space beyond the heliosphere. Most theoretical studies of interstellar missions with a solar sail plan to push the sail with a very large laser Beam-powered propulsion Direct Impulse beam. The thrust vector (spatial)vector would therefore be away from the Sun and toward the target.

LIMITATIONS OF SOLAR SAILS Solar sails don't work well, if at all, in low Earth orbit below about 800 km altitude due to erosion or air drag. Above that altitude they give very small accelerations that take months to build up to useful speeds. Solar sails have to be physically large, and payload size is often small. Deploying solar sails is also highly challenging to date. Solar sails must face the sun to decelerate. Therefore, on trips away from the sun, they must arrange to loop behind the outer planet, and decelerate into the sunlight.


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There is a common misunderstanding that solar sails cannot go towards their light source. This is false. In particular, sails can go toward the sun by thrusting against their orbital motion. This reduces the energy of their orbit, spiraling the sail toward the sun, see Tack (sailing).

INVESTIGATED SAIL DESIGNS "Parachutes" would have very low mass, but theoretical studies show that they will collapse from the forces placed by shrouds. Radiation pressure does not behave like aerodynamic pressure. The highest thrust-to-mass designs known (2007) were theoretical designs developed by Eric Drexler. He designed a sail using reflective panels of thin aluminum film (30 to 100 nanometres thick) supported by a purely tensile structure. It rotated and would have to be continually under slight thrust. He made and handled samples of the film in the laboratory, but the material is too delicate to survive folding, launch, and deployment, hence the design relied on space-based production of the film panels, joining them to a deployable tension structure. Sails in this class would offer accelerations an order of magnitude higher than designs based on deployable plastic films. The highest-thrust to mass designs for ground-assembled deployable structures are square sails with the masts and guy lines on the dark side of the sail. Usually there are four masts that spread the corners of the sail, and a mast in the center to hold guide wires. One of the largest advantages is that there are no hot spots in the rigging from wrinkling or bagging, and the sail protects the structure from the sun. This form can therefore go quite close to the sun, where the maximum thrust is present. Control would probably use small sails on the ends of the spars. In the 1970s JPL did extensive studies of rotating blade and rotating ring sails for a mission to rendezvous with Halley's Comet. The intention was that such structures would be stiffened by their angular momentum, eliminating the need for struts, and saving mass. In all cases, surprisingly large amounts of tensile strength were needed to cope with dynamic loads. Weaker sails would ripple or oscillate when the sail's attitude changed, and the

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oscillations would add and cause structural failure. So the difference in the thrust-to-mass ratio was almost nil, and the static designs were much easier to control. JPL's reference design was called the "heliogyro" and had plastic-film blades deployed from rollers and held out by centripetal forces as it rotated. The spacecraft's altitude and direction were to be completely controlled by changing the angle of the blades in various ways, similar to the cycle and collective pitch of a helicopter. Although the design had no mass advantage over a square sail, it remained attractive because the method of deploying the sail was simpler than a strut-based design. JPL also investigated "ring sails" (Spinning Disk Sail in the above diagram), panels attached to the edge of a rotating spacecraft. The panels would have slight gaps, about one to five percent of the total area. Lines would connect the edge of one sail to the other. Weights in the middles of these lines would pull the sails taut against the coning caused by the radiation pressure. JPL researchers said that this might be an attractive sail design for large manned structures. The inner ring, in particular, might be made to have artificial gravity roughly equal to the gravity on the surface of Mars. A solar sail can serve a dual function as a high-gain antenna. Designs differ, but most modify the metallization pattern to create a holographic monochromatic lens or mirror in the radio frequencies of interest, including visible light. Pekka Janhunen from FMI has invented a type of solar wind sail called the electric solar wind sail. It has little in common with the solar wind sail design externally, bacause the sails are substituted with straigthened conducting tethers (wires) which are placed radially around the host ship. The wires are electrically charged and thus an electric field is created around the wires. The electric field of the wires extends a few tens of metres into the surrounding solar wind plasma. Because the solar wind electrons react on the electric field similarly as on a concrete solar wind sail, the function radius of the wires is based on the electric field that is generated around the wire rather than the actual wire itself. This fact also makes it possible to maneuver a ship with electric solar wind sail by regulating the electric charge of the wires. A


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full-sized functioning electric solar wind sail would have 50-100 straightened wires with a length of about 20 km each. NASA has successfully tested deployment technologies on small scale sails in vacuum chambers. No solar sails have been successfully deployed as primary propulsion systems, but research in the area is continuing. On August 9, 2004 Japanese ISAS successfully deployed two prototype solar sails from a sounding rocket. A clover type sail was deployed at 122 km altitude and a fan type sail was deployed at 169 km altitude. Both sails used 7.5 micrometer thick film. The experiment was purely a test of the deployment mechanisms, not of propulsion. A joint private project between Planetary Society, Cosmos Studios and Russian Academy of Science launched Cosmos 1 on June 21, 2005, from a submarine in the Barents Sea, but the Volna rocket failed, and the spacecraft failed to reach orbit. A solar sail would have been used to gradually raise the spacecraft to a higher earth orbit. The mission would have lasted for one month. A suborbital prototype test by the group failed in 2001 as well, also because of rocket failure. A 15-meter-diameter solar sail (SSP, solar sail sub payload, soraseiru sabupeiro-do) was launched together with ASTRO-F on a M-V rocket on February 21, 2006, and made it to orbit. It deployed from the stage, but opened incompletely.

NANOSAIL-D A team from the NASA Marshall Space Flight Center, along with a team from the NASA Ames Research Center, have developed a solar sail mission called NanoSail-D which is scheduled for launch aboard a Falcon 1 rocket in 2008. The structure is made of aluminum and plastic, with the spacecraft weighing less than 10 pounds (4.5 kg). The sail has about 100 square feet (9.3 m²) of light-catching surface. A NanoSail-D mission dashboard was recently released. SAIL MATERIALS The best known material is thought to be a thin mesh of aluminium with holes less than ½ the wavelength of most light. Nanometre-sized "antennas" would emit heat energy as infrared.

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Although samples have been created, it is too fragile to unfold or unroll with known technology. The most common material in current designs is aluminized 2 µm Kapton film. It resists the heat of a pass close to the Sun and still remains reasonably strong. The aluminium reflecting film is on the Sun side. The sails of Cosmos 1 were made of aluminized PET film. Research by Dr. Geoffrey Landis in 1998-9, funded by the NASA Institute for Advanced Concepts, showed that various materials such as Alumina for laser lightsails and Carbon fiber for microwave pushed lightsails were superior sail materials to the previously standard aluminum or Kapton films. In 2000, Energy Science Laboratories developed a new carbon fiber material which might be useful for solar sails. The material is over 200 times thicker than conventional solar sail designs, but it is so porous that it has the same weight. The rigidity and durability of this material could make solar sails that are significantly sturdier than plastic films. The material could selfdeploy and should withstand higher temperatures. There has been some theoretical speculation about using molecular manufacturing techniques to create advanced, strong, hyper-light sail material, based on nanotube mesh weaves, where the weave "spaces" are less than ½ the wavelength of light impinging on the sail. While such materials have as-of-yet only been produced in laboratory conditions, and the means for manufacturing such material on an industrial scale are not yet available, such materials could weigh less than 0.1 g/m² making them lighter than any current sail material by a factor of at least 30. For comparison, 5 micrometre thick Mylar sail material weighs 7 g/m², aluminized Kapton films weighs up to 12 g/m², and Energy Science Laboratories' new carbon fiber material weighs in at 3g/m².

APPLICATIONS Robert Forward proposed the use of lasers to push solar sails, providing beam-powered propulsion. Given a sufficiently powerful laser and a large enough mirror to keep the laser focused on the sail for long enough, a solar sail could be accelerated to a significant fraction of the speed of light. To do so, however, would require


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the engineering of massive, precisely-shaped optical mirrors or lenses (wider than the Earth for interstellar transport), incredibly powerful lasers, and more power for the lasers than humanity currently generates. A potentially easier approach would be to use a maser to drive a "solar sail" composed of a mesh of wires with the same spacing as the wavelength of the microwaves, since the manipulation of microwave radiation is somewhat easier than the manipulation of visible light. The hypothetical "Starwisp" interstellar probe design would use a maser to drive it. Masers spread out more rapidly than optical lasers thanks to their longer wavelength, and so would not have as long an effective range. Masers could also be used to power a painted solar sail, a conventional sail coated with a layer of chemicals designed to evaporate when struck by microwave radiation. The momentum generated by this evaporation could significantly increase the thrust generated by solar sails, as a form of lightweight ablative laser propulsion. To further focus the energy on a distant solar sail, designs have considered the use of a large zone plate. This would be placed at a location between the laser or maser and the spacecraft. The plate could then be propelled outward using the same energy source, thus maintaining its position so as to focus the energy on the solar sail. Spacecraft fitted with solar sails can also be placed in close orbits about the Sun that are stationary with respect to either the Sun or the Earth, a type of satellite called a statite. This is possible because the propulsion provided by the sail offsets the gravitational potential of the Sun. Such an orbit could be useful for studying the properties of the Sun over long durations. Such a spacecraft could conceivably be placed directly over a pole of the Sun, and remain at that station for lengthy durations. Likewise a solar sail-equipped spacecraft could also remain on station nearly above the polar terminator of a planet such as the Earth by tilting the sail at the appropriate angle needed to just counteract the planet's gravity. Additionally, it has been theorized by da Vinci Project contributor T. Pesando that solar sail-utilizing spacecraft successful in interstellar travel could be used to carry

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their own zone plates or perhaps even masers to be deployed during flybys at nearby stars. Such an endeavour could allow future solar-sailed craft to effectively utilize focused energy from other stars rather than from the Earth or Sun, thus propelling them more swiftly through space and perhaps even to more distant stars. However, the potential of such a theory remains uncertain if not dubious due to the high-speed precision involved and possible payloads required.

FUTURE VISIONS Despite the loss of Cosmos 1 (which was due to a failure of the launcher), scientists and engineers around the world remain encouraged and continue to work on solar sails. While most direct applications created so far intend to use the sails as inexpensive modes of cargo transport, some scientists are investigating the possibility of using solar sails as a means of transporting humans. This goal is strongly related to the management of very large (i.e. well above 1 km²) surfaces in space and the sail making advancements. Thus, in the near/medium term, solar sail propulsion is aimed chiefly at accomplishing a very high number of non-crewed missions in any part of the solar system and beyond. MISUNDERSTANDINGS Critics of the solar sail argue that solar sails are impractical for orbital and interplanetary missions because they move on an indirect course. However, when in Earth orbit, the majority of mass on most interplanetary missions is taken up by fuel. A robotic solar sail could therefore multiply an interplanetary payload by several times by reducing this significant fuel mass, and create a reusable, multimission spacecraft. Most near-term planetary missions involve robotic exploration craft, in which the directness of the course is unimportant compared to the fuel mass savings and fast transit times of a solar sail. For example, most existing missions use multiple gravitational slingshots to reduce necessary fuel mass, in order to save transit time at the cost of directness of the route. There is also a misunderstanding that solar sails capture energy primarily from the solar wind high speed charged particles emitted from the sun. These particles would impart a small amount of


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momentum upon striking the sail, but this effect would be small compared to the force due to radiation pressure from light reflected from the sail. The force due to light pressure is about 5000 times as strong as that due to solar wind. A much larger type of sail called a magsail would employ the solar wind.

TECHNICAL CRITICISM OF SOLAR SAIL CONCEPT It has been proposed that momentum exchange from reflection of photons is an unproven effect that may violate the thermodynamical Carnot rule. This criticism was raised by Thomas Gold of Cornell, leading to a public debate in the spring of 2003. This criticism has been refuted by Benjamin Diedrich pointing out that the Carnot Rule does not apply to an open system. Further explanation of lab results demonstrating is provided. James Oberg has also refuted Dr. Gold's analysis: "But 'solar sailing' isn't theoretical at all, and photon pressure has been successfully calculated for all large spacecraft. Interplanetary missions would arrive thousands of kilometers off course if correct equations had not been used. The effect for a genuine 'solar sail' will be even more spectacular." Another alleged solution is that when reflected by a solar sail, a photon undergoes a Doppler shift; its wavelength increases (and energy decreases) by a factor dependent on the velocity of the sail, transferring energy from the sun-photon system to the sail.

BIBLIOGRAPHY Adelberg, Edward A.: Papers on Bacterial Genetics, Boston, Brown and Company, 1960. Bailey, Conner and Charles E. Faupel: Environmentalism and Civil Rights in Sumter County, Colorado, Westview Press, 1992. Bailey, Ronald: Liberation Biology: The Scientific and Moral Case for the Biotech Revolution, Amherst, Prometheus Books, 2005. Campbell, R. R., and Wade, J. L.: Society and Environment: The Coming Collision, Boston, Allyn and Bacon, 1972. Chrispeels, Maarten : Plants, Genes and Crop Biotechnology, Sudbury MA, Jones and Barlett Publishers, 2003. Churchill, Ward: Struggle for the Land: Indigenous Resistance to Genocide, Ecocide, and Expropriation in Contemporary North America. Monroe, Common Courage Press, 1993. David Sadava: Plants, Genes and Crop Biotechnology, Sudbury MA, Jones and Barlett Publishers, 2003. Dodds, John H.: Plant Genetic Engineering, New York, Cambridge University Press, 1985. Erickson, Kai: A New Species of Trouble: Explorations in Disaster, Trauma, and Community, New York, W.W. Norton, 1994. Hill, David: The Quality of Life in America; Pollution, Poverty, Power, and Fear, New York, Holt, Rinehart and Winston, 1973. Murray, David: Seeds of Concern: The Genetic Manipulation of Plants, Sydney, University of New South Wales, 2003. Primrose, S.B.: Principles of Gene Manipulation, London, Blackwell Scientific Publications, 1989. Shetty, Kalidas: Food Biotechnology, New York, Dekker/CRC Press, 2005. Walden, Richard: Genetic Transformation in Plants, England, Open University Press, 1988.

INDEX A Administration, 131, 276. Agency, 54, 145, 150, 164, 240, 277. Agreement, 17, 46, 144, 190. Agriculture, 1, 4, 126, 141, Architecture, 3, 105, 107, 250.

Emergence, 52. Evolution, 57, 224, 250, 251. 167, 176, 276. 191,

B Business, 79, 221.

C Communication, 55, 81. Considerations, 1, 86, 115, 161, 162, 258. Contribution, 47, 161, 199, 217. Cooperation, 54. Corporate, 79. Corporation, 89, 250. Crops, 4, 24, 127, 148, 150, 161, 162, 164, 261, 272. Customers, 190.

D Deployment, 6, 16, 128, 139, 152, 158, 172, 254, 258, 259, 274, 287, 289.

E Ecology, 272. Economy, 16, 115, 117, 137, 161, 165, 222, 274.

F Finance, 158.

G Global Warming, 17, 32, 41, 44, 46, 47, 69, 116, 118, 124, 127, 130, 132, 136, 138, 144, 149, 158, 191, 218, 225, 270. Government, 79, 108, 144, 150, 155, 158, 240.

H Horticulture, 4. Hydrogen, 13, 25, 26, 27, 28, 30, 58, 71, 74, 76, 106, 121, 125, 137, 138, 139, 145, 160, 224, 252, 273, 274.

I Implications, 216, 255. Information, 82, 97, 98, 116, 167, 207, 275. Investment, 27, 29, 115, 122, 127, 130, 141, 144, 156, 183, 184, 185, 264, 277.

L Language, 266. Leaders, 144. Legislation, 9, 123, 144.

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M

S

Management, 165, 166, 194, 200, 254, 292. Manifestations, 49. Manipulation, 291. Measurement, 34, 58, 88, 92, 95, 97, 215, 232, 243. Mechanism, 4, 16, 45, 47, 187, 218, 256. Metabolism, 74. Morphology, 19.

Society, 116, 277, 289. Sodium, 138, 181, 212, 278. Solar Array, 12, 82, 84, 92, 178, 272, 273. Solar Electricity, 9, 131, 188. Solar Lighting, 3, 5. Solar Power, 1, 9, 11, 12, 15, 17, 29, 89, 95, 99, 108, 130, 131, 140, 143, 144, 147, 153, 154, 156, 158, 172, 175, 177, 179, 180, 181, 183, 184, 186, 187, 188, 190, 191, 195, 249, 250, 253, 261, 265, 268, 271, 272, 274, 278, 279. Solar Thermal, 1, 5, 11, 13, 29, 66, 130, 143, 152, 153, 167, 168, 171, 172, 174, 177, 179, 180, 181, 182, 185, 187, 189, 190, 197, 199, 242, 243, 246, 249. Solar Variation, 31, 33, 39, 41, 42, 44, 203, 282. Solar Vehicles, 1, 14, 83.

N Nitrogen, 26, 41, 118, 150, 212. Nuclear Energy, 119, 123, 124, 160, 162, 167.

O Operation, 10, 23, 55, 66, 124, 137, 146, 155, 162, 174, 179, 184, 188, 268, Organization, 8, 22, 69, 73,

112, 165, 274. 171.

P Parliament, 123. Partnership, 112. Photosynthesis, 2, 28, 141, 147, 160. Policy, 155. Propagation, 52, 222, 225, 254. Protection, 59, 236, 239, 255, 257.

R Reflections, 107. Relationship, 38, 45, 47, 82, 202, 221, 225. Renewable Heat Energy, 166. Revolution, 16, 141, 149.

T Tanks, 118, 139, 159, 181, 196, 200, 201, 244, 262. Terrorism, 162, 273.

W Water Heating, 5, 6, 17, 170, 185, 197, 242, 243. Water Power, 146. Weapon, 125, 219. Wind Power, 132, 133, 140, 141, 143, 145, 146, 152, 162, 185, 189, 191, 276, 277. World War, 28.


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CONTENTS 1. 2. 3. 4. 5. 6. 7. 8.

Preface Introduction Solar Variation Solar Variation Theory Solar Array The Solar Cell Supremacy Over other Energy Sources The Solar Combisystem Solar Power Satellite Bibliography Index

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