Our energy heritage Work and Energy -
Work done by a force = force x displacement = ∙ SI Unit : N·m = joule (J)
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Energy: a thermodynamic quantity equivalent to the capacity of a physical system to do work and produce change. SI Unit : N·m = joule (J) Unit of electrical energy is kilowatt-hour (kW.h) One kW.h = 3,600,000 J (3600 kJ or 3.6 MJ)
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Law of conservation of energy Energy can neither be created nor destroyed, it can only be transformed from one form to another. Examples -
Heat Engines, such as the internal combustion engine used in cars, or the Steam engine (Heat Mechanical energy) →
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Ocean Thermal energy (Heat
Electricity)
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Hydroelectric dams(Gravitational potential energy
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Electric Generator (Kinetic Energy or Mechanical Work
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Fuel Cells (Chemical energy
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electricity)
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Electricity)
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Electricity)
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Mechanical energy -
Kinetic energy: is the energy associated with the motion of a body o
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translational KE: = 2 2
Rotational KE: = 2 2
Potential energy o Gravitational: The energy possessed by a body by virtue of its height is known as gravitational potential energy, U. = o elastic potential energy
Power -
power: Rate of doing work or rate of consumption of energy
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Average power: =
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SI Unit of power: Watt, 1W = 1 J/s. Alternative unit of Power, 1 horsepower = 746 W
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Efficiency Energy Conversion Efficiency: It is defined as the effectiveness of converting from one form of input energy to a more useful form.
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Energy landscap l andscape e Present Energy utilization - Total world energy consumption: 2008: 505 quadrillion Btu o 2020: 619 quadrillion Btu o 2035: 770 quadrillion Btu o - Bottom Line: World energy consumption will increase by 53% from 2008-2035. - By 2035: The energy consumption share of OECD countries is expected to increase only by 18% - Non-OECD countries energy use is expected to grow by 85% from 2007 to 2035. - Energy use in Non OECD emerging countries is growing @ 2.3 % per annum. This is due to the growing economies of these countries. - China and India are the non-OECD fastest growing economies.
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There is strong correlation between standard of living per capita (per head of population) and the energy consumption per capita. Graph shows a large spread in energy consumption per capita between different highly developed countries. Less developed countries will increase their GDP and energy consumption per capita. Further the population also increases there by increasing energy consumption. In more developed countries, the population is roughly constant and they are increasing their energy efficiency-leading to decrease in energy consumption.
Energy supply - Share of coal, natural gas, nuclear, hydro and others have increased and oil has decreased - Overall TPES has increased by 122% in 40 years - Total primary energy supply by America and Europe has increased nominally - Total primary energy supply by Asia has increased by a good amount - Most of the energy currently used in the world comes from fossil fuels. - Fossil fuels: Coal, oil and gas: 81% of the total energy is supplied from non-renewable sources. - By 2030: Share of oil is expected to decline to 32% - : Share of coal is expected to grow to 28% Fossil fuel - Coal o o o
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Coal is a burnable carbonaceous rock that contains large amount of carbon Composition of Coal: Main ingredient is Carbon Additional ingredients are hydrogen, sulfur, nitrogen, water and ash forming mineral. Its greater carbon content and more impurities lead to more carbon di oxide and greater air pollution on burning. Primary use: Energy resource for electricity production In the absence of legislation, US, China and India may turn to coal in place of more expensive fuels. United States produces 18% of the world’s coal annually. Coal is more plentiful than oil and natural gas.
Energy landscap l andscape e Present Energy utilization - Total world energy consumption: 2008: 505 quadrillion Btu o 2020: 619 quadrillion Btu o 2035: 770 quadrillion Btu o - Bottom Line: World energy consumption will increase by 53% from 2008-2035. - By 2035: The energy consumption share of OECD countries is expected to increase only by 18% - Non-OECD countries energy use is expected to grow by 85% from 2007 to 2035. - Energy use in Non OECD emerging countries is growing @ 2.3 % per annum. This is due to the growing economies of these countries. - China and India are the non-OECD fastest growing economies.
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There is strong correlation between standard of living per capita (per head of population) and the energy consumption per capita. Graph shows a large spread in energy consumption per capita between different highly developed countries. Less developed countries will increase their GDP and energy consumption per capita. Further the population also increases there by increasing energy consumption. In more developed countries, the population is roughly constant and they are increasing their energy efficiency-leading to decrease in energy consumption.
Energy supply - Share of coal, natural gas, nuclear, hydro and others have increased and oil has decreased - Overall TPES has increased by 122% in 40 years - Total primary energy supply by America and Europe has increased nominally - Total primary energy supply by Asia has increased by a good amount - Most of the energy currently used in the world comes from fossil fuels. - Fossil fuels: Coal, oil and gas: 81% of the total energy is supplied from non-renewable sources. - By 2030: Share of oil is expected to decline to 32% - : Share of coal is expected to grow to 28% Fossil fuel - Coal o o o
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Coal is a burnable carbonaceous rock that contains large amount of carbon Composition of Coal: Main ingredient is Carbon Additional ingredients are hydrogen, sulfur, nitrogen, water and ash forming mineral. Its greater carbon content and more impurities lead to more carbon di oxide and greater air pollution on burning. Primary use: Energy resource for electricity production In the absence of legislation, US, China and India may turn to coal in place of more expensive fuels. United States produces 18% of the world’s coal annually. Coal is more plentiful than oil and natural gas.
Limitation: Its solid form causes difficulties in extraction, transportation and use. Crude oil: o Naturally occurring flammable liquid: They are found beneath the earth's surface. o They are composed of a complex mixture of hydrocarbons of various molecular weights, and other organic compounds, o Biggest reserve: Middle east: 56% of the World share o The price of oil may vary due to: Low Oil Price : Assumes greater competition and international cooperation in both consuming and producing nations. Reference case: Current practices, politics, levels of access and economics decides the trend. High Oil Price case: Assumption is a rebound in world oil prices due to economic growth and long-term restrictions on conventional liquid production. Natural gas: o mixture of gases formed from the fossil remains of ancient plants and animals buried deep in the earth. The main ingredient in natural gas is methane Used to heat buildings, cook food, dry clothes, heat water, power generation and o transportation. Compressed natural gas is a cleaner alternative to other automobile fuels such as o gasoline (petrol) and diesel o More efficient than oil and coal and less carbon intensive. o
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Transition to clean energy Reasons 1. Most of the world’s population is extremely energy poor. About 2.5 billion people, have no access to electricity. For example India, with over 1 billion people, it implies a twenty-fold increase in per capita energy use. 2. Depleting oil needs to be replaced by other energy resources. In the future, we’ll ha ve to create our new energy carriers, be they chemical batteries or oil-substitutes like methanol or hydrogen. 3. Growing human population, the impacts of climate change and other forms of environmental damage is escalating future demands for clean energy. 4. Fossil fuels cannot meet energy requirement forever a. Oil and gas reserves will become scarce in human life time. b. Oil reserves will last for next 40 years. c. Gas reserves will last for 60 years. d. Coal reserves will last for next 133 years. 5. Excessive usage of fossil fuels: CO2 and other pollutants emission Implication: enhanced greenhouse effect by earth’s atmosphere Global warming - Curves show a steep rise in temperature since 1970: This Rise is called GLOBAL WARMING. - Over 20th Century, average global temperature rose by 0.6+0.20C - IPCC(International Panel of climate change prediction): 1.4-5.80C rise in global temperature between 1990 and 2100. Effectsofglobalwarming
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Global sea level is rising at an average rate of 1.7 mm per year over past 100 years.
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This increase is due mainly to thermal expansion and contributions from melting alpine glaciers, and does not include any potential contributions from melting ice sheets in Greenland or Antarctica Increase in glacial melting, the size and number of glacial lakes, and ground instabilities in permafrost areas and change in Arctic/Antarctic ecosystem. Increased spring runoff and peak discharge in snow-fed rivers, warming of lakes and rivers. Earlier timing of spring events, such as leaf unfolding, bird migration, egg-laying. Poleward and upward shift in range of plant and animal species.
Immediate steps taken to reduce CO2 emission will take more than a century to see its elimination from the atmosphere due to its slow removal time from the atmosphere. Figure shows that the damage is already done. Even if CO2 emission peak declines, effects on CO2 concentration, temperature and sea level rise will continue.
To address daunting challenges, we need to switch from fossil fuels to clean energy
Clean energy Clean energy technologies refer to those technologies that will either replace existing supply of fossil fuels or use energy more efficiently and judiciously thereby minimizing environmental pollution. Include: Renewable
Non-renewable
1. Ocean energy (marine currents and 1. Nuclear energy 2. waves) 3. Solar energy 4. Hydro energy 5. Wind energy 6. Biomass 7. Geothermal energy 8. Hydropower has main contribution towards global electricity consumption
Renewable energy policies continue to be the main driver behind renewable energy growth. By early 2011, around 119 countries had some type of policy target or renewable support policy at national level, doubling from 55 countries in early 2005. Important clean energy systems are: Modern renewables and traditional biomass -
Renewable energy supplied an estimated 16% of global energy consumption. Solar PV received recognition, thanks to its declining cost. Hydropower has main contribution towards global electricity consumption Emerging and developing economies increase share of Policies, Investment, Supply and Use in renewables. Renewable energy policies continue to be the main driver behind renewable energy growth
Commented [JXP1]: Why is this non-renewable
Singapore’s energy industry - Singapore is One of the top oil refining centres in the world. - Singapore is the world’s busiest marine bunkering centre. - 80 per cent of Singapore’s electricity is produced from piped natural gas imported from Malaysia and Indonesia. - Singapore also imports all of its crude oil. - Singapore is a net importer of energy - Between 2009-2018, demand of electricity is expected to increase at an annual rate of between 2.5-3.0 % - Oil Industry contributes to 5% of gross domestic product (GDP). - Oil storage facilities are under operation on Jurong Island. - Singapore relies heavily on import of fuels to ensure a secure, reliable and diversified - supply of competitively-priced energy. Liquefiednaturalgas
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To meet the demand and diversify the sources, liquification of natural gas is an appropriate method. It decreases the volume of the fuel and makes it easy to transport and store. The liquefaction technique will expand the pool of natural gas suppliers for Singapore. Singapore has constructed an LNG terminal located on a 30-hectare site at Jurong Island.
Solarenergy
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Solar energy: Singapore is located on the tropical sunbelt and there is a good potential to harness the solar energy for power generation. Solar Photovoltaic systems have been incorporated in various pilot projects led by Housing and Development Board (HDB). Till June 2009, 31 commercial and 9 house hold solar PV installations have been connected to the grid in Singapore.
Biofuels
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Biofuels are a wide range of fuels which are in some way derived from biomass. These include solid biomass, liquid fuels and various biogases. Singapore has signed agreements for the development of biofuel technologies with foreign partners.
Thermal energy Heat and temperature -
If any substance of mass m absorbs heat (Q), it will: a. Increase in temperature Q=mc T b. Change state at constant temperature (solid to liquid at the melting point, liquid to gas at the boiling point) Q=mL
Modes of heat transfer Conduction - Conduction is the transfer of heat energy within a body due to random motion of molecules. - Conduction takes place in all forms of matter, viz. solids, liquids and gases. - It does not require any bulk motion of matter particles.
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In solids, it is due to the combination of vibrations of the molecules thereby transporting heat energy from one molecule to the other. In gases and liquids, conduction is due to the collisions and diffusion of the molecules during their random motion.
Convection - Heat transfer due to bulk motion of a fluid (liquid or gas) is known as convection. - It cannot take place in solids since the molecules in a solid are not free to move in the body of the solid. Radiation - Radiative heat transfer is the transport of heat energy by electromagnetic waves. - Unlike conduction and convection which needs a medium, heat can be transferred by radiation through vacuum. - Energy radiated per second per unit area( Power per unit area), Pe is given by Stefan’s Boltzmann’s Law: = 4 - T: absolute surface temperature of radiation emitting body - =emissivity of the surface, its value lies between 0 and 1 depending on the nature of the surface - =Stefan’s Boltzmann’s constant=5.67x 10-8 Wm-2K-4 - Energy absorbed per second per unit area (Power per unit area), Pa is : = 04 - Net rate of emission per unit area per second is : − = ( 4 − 04 ) - A surface that absorbs all radiations falling on it is known as black body.
Laws of thermodynamics First law of thermodynamics It is the law of conservation of energy applied to a thermodynamic system. The difference between the heat input Q and the work done by the system W is equal to the change in internal energy U of the thermodynamic system
− = ∆
Second law of thermodynamics Q2 > 0 η<1 Above Eqn is the one of the statements of second law of thermodynamics. Different statements of the second law are: 1. No system operating in a closed cycle can convert all the heat absorbed from a heat reservoir in to work. 2. Heat always flows spontaneously from a body at a higher temperature to a body at a lower temperature. 3. In any process, Entropy of the universe( system+surroundings) always increases .
Power plants Principle
1. Upon absorption of heat: Rise in temperature of the fluid and change of state. 2. Laws of thermodynamics 3. Heat exchange with the environment due to the temperature difference. Types of power plants 1. Steam Power Plants: Uses steam as the fuel. Operating temperature is low. 2. Gas Power Plant: Uses gas (natural gas) as the fuel. Operating temperature is high.
Steam power plant - The working fluid (water) undergoes a phase change at different stages in a closed cycle and is reused in subsequent cycles. Stages: 1. 2. 3. 4.
Compression: work done (Wcom) on the system to compress cold water to high pressure Boiling: Heat Q1 added to the system to convert cold water into steam. Turbine Rotation: Work Wt done by the system (steam) on the turbine blades. Condensation: Hear Q2 lost from the system to the environment in converting steam back to cold water.
After each complete cycle the working fluid has the same energy U as it had in the beginning of the cycle. Hence, U=0.
Efficiency A perfect system: 1. 2. -
No heat loss in the condenser and the heat input in the boiler. Entire heat supplied to the system will be used to do useful work (not possible). This heat increases the disorder (ENTROPY) of the steam. As a result, working substance rejects some heat to the environment to reduce the disorder of the fluid back to the original value. Amount of heat rejected depends on the temperature of the condenser. Since Q2 is always positive η<1 There is always an upper limit to the efficiency of a thermal power plant and the wasted thermal energy heats the external environment.
Thermal properties of water and steam - In a conventional thermal power plant: Working fluid is water - At various stages of cycle: Water changes its phase from water to a two phase mixture of water and steam to dry steam to water. - A convenient representation for describing operation of thermal power plant: T-S diagram
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Rankine cycle Salient features of Rankine Cycle without reheat 1. Compressor increases the pressure of the water adiabatically before entering the boiler(ef). (high pressure makes water harder to boil. Hence the water absorbs more heat before boiling improving the efficiency of the system) 2. Boiler has three sections a. Economizer (fa): Water is heated at high pressure until it starts boiling. b. Evaporator(ab): Two phase mixture of water and steam is heated at constant pressure until all the water is converted into dry steam. c. Superheater (bc): Dry steam is then heated at constant pressure in superheater. 3. Dry steam enters the turbine at high pressure and rotates the turbine, thereby doing work (cd). 4. On leaving the turbine, wet steam enters the condenser. Here, all the steam is converted into water (de) before entering the compressor. Limitations
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A pressure drop through the boiler due to frictional losses. Unable to completely eliminate the formation of water droplets in stage (cd). wet steam not only affects heat transfer efficiency. The droplets hit the turbine with high momentum and damage its blades.
Rankine cycle with reheat This steam power plant has two-three turbines: High pressure (HP), Intermediate pressure(IP) and low pressure (LP) turbines. -
Steam is reheated several times before entering the condenser. After the steam leaves the (HP), it is reheated and goes to (IP), followed by second reheating and turning (LP).
Advantages
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Overall efficiency is increased. Higher the operating temperature of super heater, higher will be the efficiency. Problem of water droplets formation is decreased. Practically achieved efficiency: 40-45%
PracticalLimitation
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Highest temperature of the super heater is 6500C. Metal fatigue puts a limitation beyond this.
Gas turbines and Brayton cycle - Uses gas instead of water - Achievable temperature: 1300C - Turbine blades are covered with ceramic coating of low thermal conductivity to avoid metallurgical damage of blades. - The blade assembly is water cooled, to keep their temperature low. - Condenser is not needed: Direct impact is cost reduction. - Working substance is replenished with successive cycles. Process 1. COMPRESSION:(a) Air enters the compressor at atmospheric pressure and it is compressed to 10-20 bar(b). 2. COMBUSTION: Air is mixed with fuel and produces hot gases(c). 3. TURBINE: These hot gases rotate the turbine leading to electricity production. The exhaust gases are vented to atmosphere. Simple gas turbines, =40% Advantage: Low capital cost devices, can be set-up quickly.
Combined Cycle Gas Turbine (CCGT) - Net effect is a single cycle operating between the upper temperature of Brayton cycle and the lower temperature of Rankine cycle. - η=60% - η>60% is also achievable - Condenser in the steam power cycle is operated at a temperature higher than that of a conventional steam plant. - Waste heat from the condenser maybe used for district heating in local community. - η=80% - The cost involved is high: Finds application in industrial complexes or densely populated urban areas.
Wind energy Solar radiation: The ultimate source of all energy including wind energy. 1-2% of incident solar power is converted into wind.
WIND Pattern over a day: During day: Land is more warm than sea: Wind flows from sea to Land During Night: Sea is warmer than land: Wind flows from land to sea. Available wind power is ~109MW which is 100 times the total Global power usage. Limitation: It is Diffuse resource: Only a fraction of it could be harnessed.
Wind patterns Highest intensity of SOLAR RADIATION at equator causes warm air to rise up and cooler air to flow in from north and south. Wind varies both with time and location. Other effects: like varying effect of oceans, surface friction, large scale eddy motions and seasonal effects.
Coriolis Force -
Earth’s rotations determine the places of high and low wind. Wind moving north or south will have a component of velocity towards east to an observer in space. The eastward component of wind velocity increases with increase in distance from the equator, as the distance to t he earth’s axis decreases
At 30 latitude, wind flow becomes unstable and north-south motion of the wind dissipates. In northern hemisphere, the sinking air at 30 latitude gives rise to northeast trade winds and westerly wind belt. Westerly wind belt prevails over Europe.
Sites with strong wind - North America was found to have the greatest wind power potential. - Some of the strongest winds were observed in Northern Europe, along the North Sea. - Southern tip of South America and the Australian island of Tasmania also recorded significant and sustained strong winds at the turbine blade height. - In North America, the most consistent winds were found in the Great Lakes region and from ocean breezes along the eastern, western and southern coasts
Wind classification
Class 4, with wind speed ~7.0 m/s is considered as the threshold limit for economic viability of the wind turbines.
Kinetic energy and power of wind For a wind speed u and air density , the energy density (E, kinetic energy per unit volume) of wind
is given by: The volume of the wind flowing per second across a cross sectional area A = uA.
Wind power, P is the wind energy over an area A per second
∝ 3
Factors affecting wind turbine efficiency 1. Doubling the wind speed results in 8-fold increase in the power available in the wind 2. Doubling the blade diameter increases power by a factor of 4. 3. The wind speed increases with increase in height above the ground, hence, much more power is available at higher elevations. In day time, the variation follows the 1/7th power law, which predicts that wind speed rises proportionally to the seventh root of altitude. 4. Horizontal axis wind turbines are found better than the vertical axis turbines 5. Turbines should have yaw mechanism (direction of turbine facing the direction of incident wind) 6. Aerofoil shape: ensures good lift force. Pitch mechanism to tilt it in the direction of the wind 7. Longer blades: more power can be harnessed (PαAαR2) 8. Number of blades: 3
Types of wind turbines Horizontal axis wind turbines (HAWT) - The turbine consists of a tower. An enclosure called NACELLE is mounted atop the tower. - Nacelle houses bearings for turbine shaft, gear box and the generator. - Turbine blades generally 2 or 3 in number are mounted on the shaft. - The turbine blades are of the shape of aerofoil. - The aerofoil shape provides lift force to rotate the turbine. - YAW CONTROL is the drive mechanism which orients the nacelle in the direction of the incident wind. Upwind Design: Wind strikes the blades before reaching the tower. (more efficient) Downwind design: -
Wind strikes the blades after passing over the tower. Thus, wind always keeps the blades away from the tower. In this case presence of tower causes wind shadowing effect on the blades. It causes the blades flexing and therefore more fatigue in blades, leading to their early failure. Reduces output power and increases blade noise
Vertical axis wind turbines (VAWT)
Driving force: Lift force/drag force, depending on the blade design Torque is maximum when the blades are moving across the wind. Torque is minimum when the blades are moving along the direction of the wind. In this particular design, cables are required to support the top, this limits its height and the advantage of stronger winds at higher altitudes.
HAWT vs VAWT Advantages of VAWT over HAWT -
VAWT do not require any yaw mechanism. Gear box and generator are situated at ground level, maintenance is easy.
Limitations of VAWT over HAWT
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VAWT are proven to be less cost effective over HAWT.
Turbine blades Rotation of turbine blades is based on the principle of lift force and drag force -
For small angle of attack, the pressure distribution on the upper side of aerofoil is significantly lower than that of the lower side, resulting in a net lift and drag force on the aerofoil.
A good blade design is the one which maximizes the lift force and minimizes the drag force. As the wind flows through the turbine, part of the kinetic energy of the wind is transferred to the turbine. This causes the turbine to rotate. In this process the wind slows down Material -
Originally blades were made of wood, aluminium and steel. Nowadays, fibre glass and other composite materials are used due to their high strength, stiffness and low density. Blades are quite long. The study of their fatigue properties is needed since they have to rotate for years together (typically 30 years). Fatigue causes the blades to bend permanently and finally break.
Tip Speed Ratio (TSR)--This is defined as the ratio of the speed of rotation of the outer tip of the blade and the speed of the incident wind.
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The rotor efficiency is a function of the tip speed ratio as shown in figure. TSR is a measure of rotations per minute of the rotor.
Number of blades: -
For electricity generation: The tip speed of the blades is very high and the turbulence caused by one blade on another can significantly reduce the overall efficiency. The fewer the number of blades, the better it is. Most new turbines have three blades: They run smoother than 2 blade turbines because the impact of tower interference and the variation of wind speed with height are more evenly transferred from rotors to drive the shaft.
Maximum power extraction efficiency Using concepts of mechanics and fluid dynamics, Power extracted Pext, is given by
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Betz limit: Maximum theoretical efficiency of the rotor is called Betz Limit.
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According to Betz's law, no turbine can capture more than 16/27 (59.3%) of the kinetic energy in wind. Betz limit is achievable when the rotor slows down the wind speed by two- thirds. i.e. ratio of downstream to upstream wind: 1/3. Speed of incident wind decreases by 2/3
Capacity factor - The actual energy delivered and the deliverable energy at the rated power differ from each other. - The ratio of the annual energy yield to that which would be produced at the rated power is called capacity factor (CF). actual energy produced per unit time/ energy suggested by manufacturer per unit of time - It is typically~ 1/3 (30%) for modern wind turbines. - Wind plants installed in Class-4 and Class-5 sites, result in CFs of roughly 30% - 40%. - Comparison of capacity factor: Coal plants operate with CF of 80%-90%. CF is affected by wind speed -
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Vc: cut in wind speed, Vr: rated wind speed, Vf: furling or cut-out wind speed Below the cut in wind speed vc, the turbine is not turned on since the power generated is insufficient to offset generator losses. Above vc, the output power increases as cube of wind speed, till wind speed vR, when the output power is same as the rated power, PR. Above vR, the pitch of the turbine blades is reduced to shed some of the wind, to prevent the generator from overpowering. At vF, the cut off wind speed or furling, the winds are just high and too dangerous, so the turbine shuts down.
CF is very low if a lot of wind is below the cut in wind speed. At very high wind speed, the value CF attains is nearly constant. In the range of wind speeds between the above limits of speed, the variation of CF is almost linear. Thus, once the design specifications of actual turbines are known, the energy delivered by the turbine increase linearly with average wind speed.
Applications based on the output power 1. Large turbines are connected to the national electricity grid for power production (Capacity> 150 kW). 2. Intermediate size wind turbines find applications in hybrid energy systems: wind turbine generators could be connected to other energy sources such as photovoltaics/hydro/diesel used in small remote grids (size range, 10 kW-150 kW size range). 3. Small standalone turbines (<10 kW) are used for battery charging, water pumping, heating etc. For battery charging, the size range, 25-150 Watt is sufficient (i.e., blades with 0.5m-1.5 m diameter). Around 200,000 small battery charging wind turbines are now in use.
Wind farms -
A wind farm constitutes a group of wind turbines located close at a place.
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These are used to produce electric power. Individual turbines are interconnected with a medium voltage (usually 34.5 kV) power collection system and communications network. At a substation, this medium-voltage electrical current is increased in voltage with a transformer for connection to the high voltage transmission system. A large wind farm may consist of a few dozen to several hundred individual wind turbines, and cover an extended area of hundreds of square miles, but the land between the turbines may be used for agricultural or other purposes.
Wind farm classification based on location - Onshore, nearshore and offshore are the most widely used classification of wind farms by location. There are also aerial turbines. - Each class of turbines has unique design characteristics intended to suit their specific location. - The size of a turbine will influence its power generating capacity. The smaller turbines, which produce under 50 kilowatts, are most commonly used to power water pumps, telecommunication dishes, and homes. - A 5-15 kilowatt turbine should suffice the need of home that uses under 10,000 kilowatt hours of electricity per year. This type of system will cost between $6,000-$22,000 to install. - A hybrid wind system uses smaller turbines in combination with photovoltaic systems, rechargeable deep-cycle batteries, and diesel generators to provide storable, on-demand power in remote, off-the-grid locations. - Classification: o Onshore: more than 3km inland o Nearshore: less than 3km inland or less than 10km away from land o Offshore: more than 10km away from land Onshorewindfarms
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Onshore wind turbines are installed in hilly or mountainous regions on ridge lines generally three kilometres or more inland from the nearest shoreline. This is done to exploit the topographic acceleration of the wind due to its passage over the ridge. The additional wind speeds gained in this way contributes significantly towards enhancement in the amount of energy produced by the turbines. Hence, due care needs to be taken in deciding the location of the turbines since shifting of locations even by 30 metres can cause two folds increase in the power output.
NearshoreWindFarms
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Near shore turbine installations are on land within three kilometres of a shoreline or, on water within ten kilometres of land. These areas are good sites for turbine installation, because of high wind speeds produced by convection due to differential heating of land and sea each day. Wind speeds in these zones share the characteristics of both onshore and offshore wind. The province of Ontario in Canada is pursuing several proposed near shore locations in the Great Lakes fresh water -Hence, no problem of corrosion of towers.
Offshorewindfarms
Offshore wind power development zones are generally considered to be ten kilometres or more from land.
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The United Kingdom plans to use offshore wind turbines to generate enough power to light every home in the U.K. by 2020. Denmark has many offshore windfarms
Advantages: -
Offshore wind turbines are less obtrusive than turbines on land, as their apparent size and noise is mitigated by large distance from habitation. The average wind speed is usually considerably higher and stable over open water of the sea. Capacity factors (utilisation rates) are considerably higher than for onshore and nearshore locations. Wind turbines in offshore farms can also be bigger in size than those located on land because it is easier to transport very large turbine components by sea.
Limitations -
Compared to onshore wind towers, off shore winds tower is more complex and costly to install and maintain. Corroding of Offshore towers due to saltwater environment also enhances maintenance cost. Offshore foundations for towers are more expensive than the onshore foundations. Repair and maintenance of Offshore Turbines are usually costlier than those of the onshore turbines. Hence, for a desired power production, it is preferred to reduce the number of wind turbines by installing the largest available units.
Offshore windfarm technology
Existing offshore technology: Fixed-bottom, foundation-based tower technology In areas with extended shallow continental shelves, water not deeper than 40 m (130 feet), windy but without Category 4 or higher storms, fixed-bottom turbines are now available and in use as well. Offshore fixed-bottom towers are generally taller than onshore towers once their submerged height is included.
Future technology: -
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A floating wind turbine is an offshore wind turbine mounted on a floating structure that allows the turbine to generate electricity in water depths where bottom-mounted towers are not feasible. Floating wind parks are wind farms that site several floating wind turbines closely together to take advantage of common infrastructure such as power transmission facilities. The electricity generated is transmitted to onshore places through undersea cables. The initial capital cost of floating turbines is competitive with bottom-mounted, near-shore wind turbines.
Economics of wind power The cost of the new turbines is decreasing due to: -
Cost of a rotor is roughly proportional to its diameter but power delivered is proportional to the square of diameter.
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Taller towers reach in higher winds which increases energy faster than the tower cost. Planning, permitting, site preparation, and installation costs don’t increase much when size increases. Servicing large turbines is not much different from servicing small ones and newer turbines are designed to need less servicing in the first place. Wind power plants can be installed rapidly. Example: 50 MW power plant can be in operation in less than a year from signing the contract. All these factors have contributed towards reduction in the capital costs for US projects by 85% in the last two decades.
Environmental impact -
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Atmospheric emission: No direct atmospheric emissions are caused by the operation of wind turbines. Energy balance: Energy invested in production, installation, operation and maintenance of a typical wind turbine has a payback time of less than half a year of its operation. Land use: Wind farms have the advantage of dual land use. 99% of the area occupied by a wind farm can be used for agriculture. As a thumb rule wind farms require 0.08-0.13 km2/MW (8-13 MW/km2). Noise emission: Noise produced by wind turbines is composed of a mechanical component and an aerodynamic component. For rotor diameters up to 20 m, mechanical component dominates while for larger rotors, aero dynamic component dominates. Visual Impact: The modern wind turbines with hub height greater than 40m and blade length greater than 20m have a visual impact, which mainly is the effect of moving shadows of the rotor blades. Interference with electromagnetic communication systems: The wind turbines can reflect electromagnetic waves, which will be scattered and diffracted. As a result, the telecommunication links are disturbed. Safety of personnel: Accidents with wind turbines involving humans are extremely rare. Impact on birds: Birds mortality due to wind turbines is only a fraction of the overall birds mortality.
Wind technology in Singapore -
Limitations in land area, Singapore cannot replicate application of wind power in terms of large wind farms. other countries' Singapore does not have abundant winds except in the coastal areas and offshore islands (average wind speed is usually lower than 3.3 m/s) There is no grid-tied installation functioning on wind energy in the country at present. However, Singapore can look into micro-wind technology (which can generate electricity with wind speeds of less than 2m/s)
Solar power -
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The sun radiates energy at the rate of 3.9 x 1026W (watts) The fusion reactions in the solar core take place because of the very high temperatures (108K ) in this region of the Sun. Solar radiation received at the top of earth’s atmosphere comprises : Ultraviolet (UV) radiation: 9% o o Visible radiation: 40% o Infra-Red (IR) radiation: 51% Roughly half of it reaches the surface of earth. Much of the UV is absorbed by Oxygen, Nitrogen and Ozone in upper part of earth’s atmosphere. Some of the infrared rays are also absorbed by water vapours, carbon dioxide and methane in the lower atmosphere.
Diffuse and direct solar radiation As solar radiation passes through the atmosphere, part of the radiation is absorbed, scattered and reflected by the following: -
Air molecules Water vapour Clouds Dust Pollutants
The above component is called diffuse solar radiation or diffuse insolation. The solar radiation that reaches the earth's surface without being diffused is called direct solar radiation or direct insolation. The sum of the diffuse and direct solar radiation forms global insolation.
Diffuse component depends on the clarity of the sky. Atmospheric conditions can reduce direct beam radiation by 10% on clear, dry days and by 100% during thick cloudy days.
Total Incident solar radiation is distributed as follows: -
30% is reflected back to space by atmosphere, clouds and earth’s surface. This component is called albedo. 19% is absorbed by atmosphere and clouds. Remaining 51% of the incident radiation is absorbed by earth’s surface.
The relatively constant temperature of earth is the energy balance between the incoming and outgoing radiations. Amount of sunlight collected also depends on changes in the radiant intensity of the sun. The sun’s irradiance will be higher on a dry still day compared to a windy humid day.
Intensity of sunlight increases with higher altitude.
Harnessing solar energy Advantages
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No deadly radioactive waste Free-Renewable Fuel Already heating Earth (No added Heat) Dependable-Sustained Clean Energy Natural Power
The range of solar technologies can be split into three categories: (i) Solar thermal devices for direct heat applications. (ii) Concentrating solar power (CSP) thermal devices which use heat for electricity production in a steam turbine. (iii) Photovoltaic devices that produce electricity directly from solar radiation (PV).
Solar heating systems - Solar cookers, solar furnaces, solar steam boilers 1. Active solar systems: the solar heated fluid is circulated by a fan or pump. a. Example: Heating of Swimming Pools, Domestic water heating Systems b. Most common use of solar heating systems today is to provide hot water for domestic purposes (DHW) or for swimming pools. c. DWH Collectors operate at temperatures varying from 60-820C. 2. Passive solar systems: Uses no external power but allows the fluid heated by the sun to circulate by natural means a. Example: Passive space heating in buildings Other applications: There are a variety of uses for this energy, such as hydrogen fuel production, foundry applications and high temperature materials testing.
Solar cookers The basic purpose of a solar box cooker is to heat things up -cook food, purify water, and sterilize instruments -
The interior of the box is heated by the energy of the sun. Sunlight, both direct and reflected, enters the solar box through the glass or plastic top. Single or multiple reflectors bounce additional sunlight through the glass and into the solar box. This additional input of solar energy results in higher cooker temperatures. The temperature inside the box rises until the heat loss of the cooker is equal to the solar heat gain Heating of the pots inside the box is done by direct absorption and by convection.
Solar water heating systems - Solar water heating systems use the sun's energy to heat water in liquid-based solar collectors. - These are usually used along with conventional water heaters. - Solar collectors for these systems are typically 3 –6 m2 in area. - A typical solar water heating system can meet approx. 50% of the water heating requirements in a home. There are two types of solar water heating systems: -
(i) Active, which have circulating pumps. (ii) Passive, which are based on natural convection.
Solar water heating systems include storage tanks and solar collectors. -
The storage tanks are well insulated. Types of solar collectors: o Flat plate collectors (active) o Batch collectors (passive) o Excavated tube solar collectors (active)
Flatplatecollector
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It has thin flat metal plate, painted black, to absorb sun’s radiation. Absorber plate is in contact with fluid tubes. Fluids absorb heat and circulated by a pump to take away heat. In an air based collector the circulating fluid is air, whereas in a liquid based collector it is usually water. This set up is covered with one or two sheets of glazing. Achievable temperature is 30-70°C (86-158°F)
PassiveBatchSolarWaterHeater
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Also known as a bread box system or integral collector storage system,
Structure -
This solar collector consisting of one or more storage tanks placed inside an insulated box that has a glazed side facing the sun. A batch collector Is mounted on the ground or on the roof of the building. Choice of materials for surfaces on the tank(s): The surfaces of the tank should have good absorbers of solar infrared radiation and inhibit radiative loss. On an area basis, batch collector systems are less costly than glazed flat-plate collectors but energy delivered per year by them is less .
Process -
Cold water enters a pipe and can either enter a solar storage/ backup water heater tank or the batch collector, depending on which bypass valve is open. Water, upon entering the tank, Is heated in it. Hot water from the batch collector is carried into the solar storage/backup water heater and thence to the house .
Evacuatedtubecollectors
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This type of solar collector can achieve high temperatures in the range 77°C to 177°C and under the clear sky conditions and proper orientation, work very efficiently. Evacuated-tube collectors are, however, quite expensive, with unit area costs about twice that of flat-plate collectors. They are well-suited to commercial and industrial heating applications and also for cooling applications. They can also be an effective alternative to flat-plate collectors for domestic space heating, especially in regions where it is often cloudy.
Structure:
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An evacuated-tube collector consists of parallel rows of evacuated glass tubes connected to a header pipe. This eliminate heat loss through convection and radiation. A highly selective absorption coating is applied to the inner tube. The heat gained is conducted by special aluminium lamellas into copper tubes. The water is circulated through the inner tubes and gets heated and sent to the header pipe. The collector header consists of two copper pipes. The lower pipe brings liquid into the collector, the upper pipe takes the warmed up liquid from the collector.
PassiveSolarSpaceHeatingSystem
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The house itself acts as the solar collector and the storage facility. Heat energy flow is by natural means. No need of pumps and fans. Sunlight enter through the sun facing window and is stored in. Principle of passive solar heating is that the amount of thermal energy transmitted in the house from the south facing window during a clear day is more than the thermal energy loss from house over 24 hours period. This is achieved by enhancing thermal mass i.e., using such materials which have high heat absorption capacity. Ex: Concrete, water and stone.
To improve efficiency: 1. Maximising solar heat gain: It depends on: (a) The solar radiation available at the location of the building. (b) Orientation of the building (South facing windows). ( the sun rises in the east and sets in the west, the side of the building that is utilized for solar gain needs to be facing the south to take maximum advantage of the sun’s potential energy.) (c) The characteristics of the collection areas (their solar transmittance/absorption and heat transfer). 2. Minimising heat losses Heat losses may be minimised by following methods: (a) (b) (c) (d)
Applying thermal insulation of high quality. avoiding thermal bridges. providing air tightness. installing multiple-glazed windows.
These methods help to reduce heat transmission and air in filtration which are the main avenues for heat transport through the building envelope (physical separator between the interior and exterior of a building)
3 types of passive systems 1. Direct gain 2. Indirect gain (Trombe wall) - System collects and stores heat in one part of the house and uses natural heat transfer (conduction and convection) to distribute the heat to the rest of the house. - A massive concrete wall is placed 10cm behind the glass area. - Solar radiations are absorbed by the wall, which reradiates heat in the space between glass and wall and heats up the air.
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Warm air rises and circulates in the room through vents and is replaced by cooler air from the bottom. 3. Attached solar greenhouse - Greenhouse is attached on the south side of the house. - It acts as extended thermal wall. - Serve dual purpose (food production and space heating)
Passive cooling 1. Minimizing solar heat gain by (a) (b) (e) (c) (d)
Increasing the building mass. Increasing thermal protection. Air tightness of the building. Reflective coating (white) on exposed surfaces. Curtailing solar radiations using shading devices.
2. Removing unwanted heat (a) Use of technologies for cooling of buildings. (b) Unwanted heat in hot and dry climates could be removed by: (i) (ii) (iii)
(iv)
evaporative cooling Nocturnal ventilation in hot humid climate, a thermo-active ceiling could be installed (see figure), which would however need a pump. BUT as no energy for cooling is required, such systems are usually classed as passive cooling. By providing adequate cross ventilation on buildings.
Solar Thermal Power Plants -
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Power plants use a curved, mirrored troughs (collectors) which reflects the direct solar radiation onto glass tubes containing a fluid running along the length of the trough and positioned at the focal point of the reflectors. The hot fluid is transported to a turbine where about a third of the heat is converted into electricity. The fluid (also called heat transfer fluid) becomes very hot. Common fluids are synthetic oil, molten salt and water.
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Eurotrough reflector: The focus pipe has flexible pipes on the ends to allow rotary motion to track the sun, while the absorber pipe is kept at the focus.
Solar power tower - SOLAR POWER TOWERS capture and focus the sun's thermal energy with thousands of tracking mirrors (called heliostats) in roughly a two square mile field. - A tower is located in the centre of the heliostat field. - The heliostats focus concentrated sunlight on a receiver which is mounted atop of the tower. - Within the receiver the concentrated sunlight heats molten salt to over 550C. - The heated molten salt then flows into a thermal storage tank where it is stored, maintaining 98% thermal efficiency, and eventually pumped to a steam generator. - The steam drives a standard turbine to generate electricity. This process, also known as the "Rankine cycle” is similar to a standard coal-fired power plant.
Solar Photovoltaics Photovoltaic (PV) systems (solar cells) convert solar energy directly into electricity.
Nature of light Light exhibits wave nature, i.e., it shows the property of rectilinear propagation, interference and diffraction. Like any wave, the velocity of light c, wavelength and frequency v are related as c=v c=3x108m/s in vacuum Light also exhibited particle nature, i.e., experiment of photoelectric effect suggested that light could be considered as composed of fast moving particles called photons. Each photon possesses and energy E given by = ℎ =
h(Planks constant)=6.63×10 -34 Js
Energy bands - When an electron breaks loose and becomes a conduction electron, a hole is also created - Energy states of Si atom expand into energy bands of Si crystal - The lower bands are filled and higher bands are empty in a semiconductor - The highest filled band is the valence band Ev - The lowest empty band is the conduction band Ec - Ev and Ec are separated by the band gap energy Eg Semiconductors,insulatorsandconductors
Conductors: conduction band is half-filled Semiconductors: small bandgap Insulators: large bandgap
Types of semiconductors Type of semiconductors: -
Intrinsic semiconductors (pure semiconductors) Extrinsic semiconductors (doped semiconductors)
Intrinsicsemiconductor
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Pure semiconductor (intrinsic): contains the right number of electrons to fill valence band, therefore, conduction band is empty. Because electrons in full valence cannot move at absolute zero, the pure semiconductor acts like an insulator.
Extrinsicsemiconductor Dopants
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As (arsenic), a Group V element, introduces conduction electrons and creates N-type silicon and is called a donor B (Boron), a Group III element, introduces holes and creates P-type silicon and is called an acceptor
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Donors and acceptors are known as dopants
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prevailing charge carrier in n-type: electrons prevailing charge carrier in p-type: holes
p-n junctions - When a p-n junction is formed, some of the free electrons in the n-region diffuse across the junction and combine with holes to form negative ions. In so doing they leave behind positive ions at the donor impurity sites. - A space charge builds up, creating a depletion region which inhibits any further electron transfer unless it is helped by putting a forward bias on the junction. Electriccurrentinp-njunctionunderexternalbias
Under forward biased conditions, the direction of positive charge flow (opposite to the direction of electrons flow) is from p to n side. If V is the forward bias voltage, the total forward current I is = (
− 1)
Where VT= kT/e= 0.026V at room temperature Generationofelectron-holepairwithlight
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Photon is absorbed by the electron in the valance band, and it is shifted to the CB. This process empties energy level in the previously filled valance band. Therefore a hole (with positive charge numerically equal to the charge on an electron) is left behind in VB. This creates an electron-hole pair. The energy of photon required for the transfer of electron from VB to CB must be at least equal to the width of the energy gap between the two bands (Eg). Thus, only a part of solar energy is utilized by the solar cells. The photon flux converted by a solar cell is about 2/3 of total flux.
Principle of solar cell - When light strikes a shallow p-n junction, electron and hole pairs are created in the junction by photoelectric effect. - These charges are separated by potential barrier at the junction. - When n and p sides of the solar cell are connected by an external circuit, the electrons will flow in the outer circuit from n-side, through a load into the p-side. The direction of current in the outer circuit, is opposite to the direction of flow of electrons. Thus, current flows from p side to n side.
IVgraphofasolarcell
ISC: max. current when there is no load Imp: current at max power VOC: max. voltage when there is no current Vmp: Voltage at max power
FF defines how close the I-V characteristics are to a rectangle. Good solar cells have FF>0.7. Typically, FF lies between 0.75-0.85.
Materials for solar cells Types of silicon 1. Polycrystalline 2. Amorphous 3. Crystalline
Advantages
Crystalline & Polycrystalline - High Efficiency (14-22%) - Established technology (The leader) - Stable
Amorphous - High absorption (doesn’t need a lot of material) - Established technology - Ease of integration into buildings - Excellent ecological balance.
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Disadvantages
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Expensive production Low absorption coefficient Large amount of highly purified feedstock
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Cheaper than the glass, metal, or plastic used for depositing amorphous silicon there on Can be rolled into sheets Only moderate stabilized efficiency 7-10% Instability-It degrades when light hits it
Design of commercial solar cells Thin top layer is made of n-type silicon about 1μm thick. On this layer a thin conducting grid is attached , arranged as fingers to avoid blocking out too much of light. These fingers are the connected to bus bar. Fingers are covered by antireflection coating (ARC) to minimize light reflection from top surface. ARC is made with thin layer of dielectric material. The bottom p layer of silicon is about 400μm thick.
Metal contact is attached at the bottom.
Power losses in solar cells - Out of 100% Incident solar radiation on a solar cell, photons with energy less that the band gap is 23%, so they cannot produce power. 30% goes in the form of heat and so only 47% is useful for interacting with solar cell. - Not all electron hole-pairs produced by incident photons are collected by field across the junction’ about 10% recombine. - Collection efficiency - Incomplete absorption - A potentially large loss (~40% ) from the surface of silicon can be reduced by using antireflective coatings. - Top contact shading - Series resistance losses - Fill factor
Band gap and Efficiency of Solar Cells -
Efficiency depends on band gap: Smaller band gap Increase photo current. But this decreases maximum output voltage as eVoc
Angle of incidence -
A solar collector receives maximum radiation, Smax, when the incoming sunlight has normal incidence to the collector’s surface as shown in Figure on left. When light is incident on a collector with a non-zero incident angle, the amount of energy collected is reduced by a factor equal to the cosine of the incident angle. A collector positioned horizontally flat with sunlight falling on the collector at an incident angle of A where the collected energy is equal to Smax´Cos(A).
Solar panels Solar panels are made by connecting single unit solar cells in series. -
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SOLAR PANELS WITH BATTERY: Solar panels are often used with battery storage. This allows operation of equipment at night also. Solar panels can provide power in remote locations: Example telecommunication equipment and lighting, small electronic devices. Solar home systems, supplying small amounts of energy in off grid household. These comprise one or several PV modules mounted onto a suitable support structure. SOLAR PANELS WITHOUT BATTERY: Solar panels without battery include applications for water pumping. In this case water reservoir itself provides storage. STAND ALONE HYBRID SYSTEMS: A hybrid system is a standalone system used in combination with another power source. The other power source could be used as back-up power generator. OTHER APPLICATIONS: parking meters, emergency telephones, temporary traffic signs, and remote guard posts & signals. GRID CONNECTED SYSTEMS: Here the PV power generator feeds the grid via an i nverter. Grid connected systems normally do not include batteries. Where an AC power is required, an inverter is used that converts the DC power to AC power. Applications of Solar Panels SOLAR POWER SATELLITES: Design studies of large solar power collection satellites have been conducted for decades. TRANSPORT: PV has traditionally been used for electric power in space. PV is rarely used to provide motive power in transport applications, but is being used increasingly to provide auxiliary power in boats and cars. A self-contained solar vehicle would have limited power and low utility, but a solar charged vehicle would allow use of solar power for transportation. Solar-powered cars and aeroplanes (SOLARIMPULSE) have been demonstrated.
Economics of photovoltaics - The cost of solar panels has decreased over the years. - Besides the cost, location also matters. In addition to a place being sunny, it is important whether there is electricity grid or not. - For locations that are far from grid, the cost of solar power is 0.5 of any other technology. This has resulted in a good market of solar PV systems for remote areas. - Total Global market share of Solar Photovoltaics is: Remote Industrial: 22% o Remote Domestic: 17% o Grid Connected Applications: 59% o Small Items (calculators): 2% o
Environmental impacts of solar photovoltaic technology -
Solar PV power in operation produces no pollutants and in particular no greenhouse gases. It is visually unobtrusive and there are no moving parts, which reduces maintenance and noise pollution. This technology is ideal for distributed power generation not requiring a grid. In production some hazardous materials like Cd and As are used but the quantities are small. Solar energy falling on earth is used, no additional energy is needed for operation.
Hydro Power 70% of the earth’s surface is covered by water water is the biggest reserve on earth
the water cycle ensures that we never run out of water the energy is not very intermittent Power generation from water is possible via three different ways: 1. Hydro Power: An Established technology 2. Tidal Power 3. Wave Power 4. Ocean Thermal Energy
History of hydro power Water wheels - Hydro Power is harnessed into more useful form using water wheels. - A Water wheel is the simplest and oldest device which converts the kinetic energy of flowing water into useful forms of work. - Applications of waterwheels include grinding grains and pumping water. - The Romans were known to use waterwheels extensively in mining projects. Enormous Roman-era waterwheels are found in places like modern-day Spain.
Water mills Water mills use the flow of water to rotate a large wheel. A shaft connected to the wheel axle transmits the energy from the water through a system of gears and cogs to operate machinery. Applications: -
Grist Mills or corn mills, grind grains into flour.
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Saw Mills cut timber into lumber.
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Bark Mills strip bark from trees or ground it to powder for use in tanneries.
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Spoke Mills turn lumber into spokes for carriage wheels.
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Cotton Mills usually power a water wheel at the beginning of the industrial revolution.
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Bobbin Mills made wooden bobbins for the cotton and other textile industries.
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Carpet Mills for making rugs were sometimes water-powered.
Ancient applications: -
Textile Mills for weaving cloths were also water-powered .
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Powder Mills for making gunpowder - black powder or smokeless powder were usually water-powered. Iron Mills, also known as furnaces and forges, and tin plate works were water powered. Blade Mills were used for sharpening newly made blades.
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Slitting mills were used for slitting bars of iron into rods, which were then made into nails.
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Rolling mills shaped metal by passing it between rollers.
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Smelt Mills were used to smelt the Lead prior to the introduction of the cupola (a reverberatory furnace).
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Paper Mills used water not only for motive power, but also required it in large quantities in the manufacturing process.
Modern applications
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By the early 20th century, the water wheel was incorporated in the design and development of water turbine for the generation of electricity.
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This resulted in availability of cheap electrical energy. This made the watermills obsolete in developed countries. However, in some developing countries watermills are still in use for grinding grains. The number of such machines in operation in Nepal and India are 25,000 and 200,000 respectively.
Three gorges dam - Location: Yangtze river in China - Cost: $24-billion - Power capacity: Twenty-six turbines are meant to produce18,000 MW - Dimensions: 2.3 kilometers long and 185 meters tall - The dam's 660-kilometer-long) reservoir will flood about 632 square kilometers of land Advantages: -
Boost Yangtze river trade Block garbage from going into the sea Control flooding downstream Produces large amounts of power
Disadvantages: -
1.3 million people displaced drop in delta sediment
Types of water wheels Name Description Undershot A vertically-mounted water Water wheel rotated by striking Wheel water on paddles or blades at the bottom of the wheel
Advantages cheapest
Disadvantages 1. least efficient 2. can only be used where the water flow rate is high. 3. derive no advantage from the water head. They are therefore, most suited to shallow streams in the flat terrain.
Overshot Water Wheel
Backshot water wheels
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A vertically-mounted water The overshot design can wheel rotated by falling use all the water flow water striking paddles or for power production blades near the top of the and does not require wheel. rapid flow of water. - It has the water channelled (1) The force of the to the wheel slightly to one flowing water partially side at the top in the transferred to the direction of rotation. wheel. - The water collects in the (2) The weight of the buckets on that side of the water descending in the wheel, making it heavier wheel's buckets also than the other "empty" imparts additional side. energy. - The difference in weight on the two sides of the wheel turns the wheel. - After about one quarter rotation of wheel, the buckets get inverted and the water flows out into the tail-water. The water is introduced just 1. technique behind the apex of the wheel. particularly suitable Entire amount of the for streams that potential energy released by experience extreme the falling water is harnessed seasonal variations as the water descends the in flow, and reduces back of the wheel. the need for A backshot wheel continues complex sluice and to function until the water in tail race the wheel pit rises well above configurations the height of the axle, when 2. A backshot wheel any other overshot wheel will may also gain power be stopped or even from the water's destroyed. current past the bottom of the wheel, and not just the weight of the water falling in the wheel's buckets.
Types of hydro power technologies 1.
Hydroelectric Power from Dams
Run of The River Plants (diversion) 3. Pumped Storage Technology 4. Damless Hydro Power 2.
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Principle of hydroelectric power plant -
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Hydroelectric power is the production of electricity by using the gravitational force of falling water.
potential energy of water
kinetic energy of water
rotational kinetic energy of turbine
electrical energy
Hydroelectric power is the most widely used form of renewable and clean energy . Hydro power contributes about 20% to the world's electricity and accounts for over 63% of the electricity harnessed from renewable sources. A breakthrough in hydropower generation occurred with the advent of Fourneyron’s turbine.
Power output Usually hydro electricity comes from dammed water which is released to drive a water turbine and generator.
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Head (h)=Vertical distance between the turbine and water surface in the reservoir. =Efficiency of the turbine. Q=Volume of water flowing per second on the turbine. Potential energy per unit volume of water =gh
Potential energy per unit mass of water falling on turbine per (revise it) second= Q gh Power output, P=Efficiency×Energy/Time
P=ghQ -
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To obtain very high head, water is routed through a large pipe/ channel called a penstock the head can be controlled by controlling the amount of water passing through the plant The choice of design of Hydroelectric power plant depends on the site at which the plant is intended to be put up and desired values of Q and h.
Water turbine The water turbine was developed in the nineteenth century and was widely used for supply of industrial power prior to electrical grids. -
As water sources vary, water turbines have been designed to suit the different locations.
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Selection depends on head height and desired running speed of the generator.
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They are classified by the way of operating and can be either impulse or reaction turbines.
All turbines have a power-speed characteristic. They will tend to run most efficiently at a particular speed, head and flow combination.
Impulse turbines
Reaction turbines
High head Pelton Turgo
Medium head Cross-flow Multi-jet Pelton Turgo Francis
Low head Cross-flow
Propeller
Kaplan
Type Impulse turbine: pelton turbine
Reaction turbine
Description Advantages In an Impulse turbine, the - Impulse turbines blades are fixed to a rotating are usually cheaper wheel . than reaction Each blade rotates in air turbines. There is except the time when it is in no need for a line with the high speed jet specialist pressure of water. casing, nor for carefully engineered clearances. - Impulse turbines are generally more suitable for microhydro applications. - Greater tolerance of sand and other particles in the water. - Better access to working parts. - Easier to fabricate and maintain. - The rotating element - Reaction turbines (called `runner') of a rotate faster than reaction turbine is fully impulse turbines immersed in water and is given the same enclosed in a pressure head and flow casing. The runner conditions. blades are profiled so that pressure differences - A reaction turbine across them impose lift can often be forces, like those on coupled directly to aircraft wings, which an alternator cause the runner to without requiring a rotate. speed-increasing - The spiral casing drive system. (volute)is tapered to - Significant cost distribute water savings are made in uniformly around the eliminating the entire perimeter of the drive and the runner. maintenance of the
Disadvantages - unsuitable for low-head sites because of their low speeds.
These are more expensive than impulse turbines because of the need of specialist pressure casing and carefully engineered clearances.
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The guide vanes feed the water into the runner at the correct angle. The runner blades direct the water to exit axially from the centre of the runner. The water imparts most of its energy to the runner before leaving the turbine.
hydro unit is very much simpler. High energy conversion suitable for microhydro projects
Advantages of hydroelectric power plants - Hydro power is considered to be a clean, renewable source of energy. - It emits a very low level of greenhouse gases in comparison to fossil fuelled plants. - Hydro power has a low operating cost and can be highly automated. - Plant life is long, ~40 years before major refurbishment. - Power is generally available on demand as the flow of water can be controlled. Limitations of hydroelectric power plants - Capital cost is high and pay back time is very long. - Serious social issues are to be addressed while deciding the site; mainly the displacement of population. - Dams can block fish passage to spawning grounds. Many plants now have measures in place to help reduce this impact. - The diversion of water can impact stream flow, or even cause a river to dry out. This leads to degradation of both aquatic and streamside habitats. - Hydroelectric plants can also have an impact on water quality by lowering the amount of dissolved oxygen in the water. In the reservoir, sediments and nutrients can build up and the reduced water flow can create undesirable growth and spread of algae and aquatic weeds. -
Run of the river plants -
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Hydroelectric plants with no dam or reservoir capacity are called run-of-the-river plants, as they do not store water. Run of the river hydro usually involves a low level diversion weir or a stream bed intake and is usually located on a fast flowing, non seasonal stream or river. In run of river systems, running water is diverted from a flowing river and guided down a channel, or penstock, which leads to a generating house. There the force of the moving water spins a turbine and drives a generator. The water is fed back into the main river further downstream. The difference between run-of-river and large hydropower is that run of river systems do not dam the river to create a water reservoir. Because they can't store water they usually generate much less power than hydroelectric dams. They minimize the impact on the environment.
Pumped storage hydroelectricity -
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This type of hydroelectric power generation is used by some power plants for load balancing. The method stores energy in the form of water, pumped from a lower elevation reservoir to a higher elevation. Low-cost off-peak electric power is used to run the pumps. During periods of high electrical demand, the stored water is released through turbines. Although the losses of the pumping process makes the plant a net consumer of energy overall, the system increases revenue by selling more electricity during periods of peak demand, when electricity prices are highest. Pumped storage is the largest-capacity form of grid energy storage now available.
Damless hydro Damless hydro is a less common type of hydro scheme. It uses the kinetic energy in flowing water to create electricity. This method is being regarded as quite important as the future for hydroelectric power from river systems. Damless hydro is a relatively new technology based on capturing kinetic energy in rivers, spillways, and channels, without the need to build dams. A series of turbines can be used as the river progresses. These systems require little to no maintenance. The initial setup cost and environmental impact is minimal in comparison to the cost of building dams.
Advantages - There is no risk of flash flooding caused by a breached dam, and no risks of accidents caused during construction of a dam. The environmental benefits speak for themselves: - No flooding of large catchment areas, hence in no effect on the natural ecosystem in the river valley. - No silt accumulation in the dam basin. No need for fish ladders. - No additional greenhouse gases (Dams create greenhouse gases).
Status of hydropower -
Hydropower is currently being utilised in some 150 countries, utilising 11,000 stations with around 27,000 generating units. Global installed capacity estimates from different sources range from 860GW to 950GW. Europe has the highest installed capacity (~260GW). Eastern Asia, lead by China, is rapidly developing its hydro resources and is expected to become the region with the greatest level of deployment within the next two to three years. South America, lead by Brazil, is also developing rapidly. Africa remains the region with the poorest ratio of deployment to potential.
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China is driving the development of the resource.
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The North American region, for example, has more than 19GW of development under planning, of which some 11GW is identified in Canada.
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Europe and North America, despite their existing levels of hydropower deployment, are continuing to develop substantial new hydropower capacity.
Ocean Energy Systems -
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The oceans cover a little more than 70 percent of the Earth's surface. They form the world's largest solar energy collectors and energy storage systems. On an average day, 60 million square kilometres (23 million square miles) of tropical seas absorb an amount of solar radiation with heat content equal to that of about 250 billion barrels of oil. The significance of the magnitude of this energy can be easily understood by the fact that even less than 0.1% (one-tenth of one percent) of this stored solar energy when converted into electric power, would supply more than 20 times the total amount of electricity consumed in the United States on any given day.
Based on its source, the ocean energy can be divided into the following categories:
1. Tidal Energy: Gravitational fields of the sun and moon are the contributors to formation of tides and the energy contained therein.
2. Ocean Wave Energy: Wind blowing over the ocean surface drags water with it and produce ocean waves.
3. Ocean Thermal Energy: This is the component of energy received by sea directly from the sun.
Tidal energy What are tides? - Tides are periodic rise and fall of large bodies of sea water.
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Tides are caused by the gravitational interaction between the Earth and the Moon.
The
gravitational attraction of the moon causes the oceans to bulge out in the direction of the moon.
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Another bulge occurs on the opposite side, since the Earth is also being pulled toward the moon (and away from the water on the far side).
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During this process, earth is also rotating. Hence, the two tides are formed everyday.
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The low and high tides occur simultaneously at two places located at longitudes differing by about 90.
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At a given longitude the interval between two high tides is approximately 12 hrs and 25 minutes.
Sun’sinteractionwithtides
Spring tides
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Spring tides are especially strong tides (they have nothing to do with the season Spring). They occur when Earth, Sun, and Moon are in a straight line. In such a situation, the gravitational forces of the Moon and the Sun jointly contribute to the formation of tides. Spring tides occur during the full moon and the new moon.
Proxigean spring tides
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The Proxigean Spring Tide is a rare and unusually high tide.
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This type of tide occurs when the moon is both unusually close to the Earth (at its closest perigee, called the proxigee) and in the New Moon phase (when the Moon is between the Sun and the Earth).
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The proxigean spring tide occurs at the most once every 1.5 years.
Neap tides
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Neap tides are especially weak tides.
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Neap tides occur during quarter moons.
They occur when the gravitational forces of the Moon and the Sun are perpendicular to one another (with respect to the Earth).
Tidal range
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The difference between the height of a high tide and a low tide is called the tidal range. Mid Ocean tidal range is 0.5 m-1.0 m. In the restricted passages between the islands and straits, the tidal range is significantly enhanced. As an example it is ~12 m in Bristol Channel (UK).
Sites with large tidal ranges Country Argentina Canada India Russia Russia UK
Site Golfo Nuevo Cobequid Gulf of Khambat Mezen Penzhinsk Severn
Mean Tidal Range (m) 3.7 12.4 7.0 6.7 11.4 7.0
Tidal barrages - A huge dam (called a "barrage") is built across a river estuary. - During high tide, when the level of water in the sea is high, sea-water flows into the reservoir of the barrage and rotates the turbine blades and also the shaft which in turn generate electricity. - During low tide the above process is reversed. The sea-water stored in the barrage reservoir is allowed to flow out into the sea. This flowing water also turns the turbines and generates electricity.
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Thus, as the sea-water flows in and out of the tidal barrage during high and low tides, the turbine rotates continuously to generate electricity.
Total mass m of water in the tidal basin of area A above the water level h is given by m=Ah
Height of centre of mass(m) = h/2 Work done in raising the water from sea level to the top of the tidal basin mgh/2 gAh2/2 is the density of water = 1025 kg/m3 (seawater varies between 1021 and 1030 kg/m3) =
=
is the acceleration due to the Earth’s gravity= 9.81 m/s2 T=Tidal period = Time interval between two successive high tides (or low tides) The above work done is stored in the form of potential energy of water which is used to drive the turbine. g
factor of 2 since within T, there is high tide and a low tide which drives the turbine advantages of tidal power 1. Once tidal barrage is built, tidal power is free. 2. It needs no fuel. 3. It produces no greenhouse gases or other waste. 4. Tides are totally predictable. 5. It produces electricity reliably. Not expensive to maintain. 6. Offshore turbines are not ruinously expensive to build and do not cause a large environmental impact. Disadvantages of tidal power 1. Once tidal barrage is built, tidal power is free. 2.
It needs no fuel.
It produces no greenhouse gases or other waste.
3.
Tides are totally predictable. It produces electricity reliably. Not expensive to maintain.
Offshore turbines are not ruinously expensive to build and do not cause a large environmental impact. 5. The barrages act as a major blockage to navigation and require the installation of locks to allow navigation to pass through. 6. The water quality in the basin is altered since the natural flushing of silt and pollution is impeded, affecting fish and bird life. 7. There is also an economic drawback associated with the tidal barrages. It involves the large capital cost, long construction period and intermittent operation. This issue could be addressed by building tidal lagoons. 4.
Wave power -
Wave power varies considerably in different parts of the world, and wave energy can’t be harnessed effectively everywhere. Wave-power rich areas of the world include the western coasts of Scotland, Northern Canada, Southern Africa, Australia, and the North-Western coasts of the United States.
Important questions:
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What Are Ocean Currents or Ocean waves?
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How are they produced? Properties of Ocean Waves? Methods of harnessing Ocean Wave Energy?
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Advantages and Concerns of Ocean Wave Energy?
Ocean currents An ocean current is defined as the horizontal movement of seawater in the ocean. Some of the factors affecting the movement of ocean currents are: -
intensity of solar radiation air temperature wind speed and direction the gravitational pull of sun and moon.
Formation -
Unequal heating of the earth and sea water due to the solar radiation falling thereon causes the atmospheric air to move from above the sea towards the land. This fast moving air over the sea surface imparts momentum to the top surface layer of the sea water. In this process the water at the surface is dragged along the wind. Heating of the ocean also alters the density of the ocean surface directly by changing its temperature and/or its salinity.
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Coriolis force acting due to the rotation of earth deviates the flow of ocean currents.
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It exists because the ocean water is affected by friction with the Earth only at the seafloor, and because the eastward linear velocity of the earth decreases from a maximum at the equator to zero at the poles (but the rotational velocity does not change).
This force makes the water move towards right in Northern Hemisphere and towards left in the Southern Hemisphere.
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The ocean currents carry immense amount of energy which is directly related to the density of water. Many Asian, European and North American countries have recently undertaken R&D studies on problems related to utilization of energy from ocean waves.
Characteristics of ocean waves Strong wind blowing for a long time over a large distance results in the formation of a large wave. -
A wave’s highest point is called its crest. The low point between two crests is called a trough. The vertical distance between crest and trough is called the wave height. The distance between two crests is called the wavelength and it is usually measured either from one crest to the next or from one trough to the next.
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Some waves are larger than average and others are smaller than average. On an average, a wave that is about double the size of the others is formed about once in every hour.
Over deep water, the energy in a wave moves forward, but the water does not. The water moves up and down in circles. (transverse waves) Motion of a particle in an ocean wave. A - At deep water. The orbital motion of fluid particles decreases rapidly with increasing depth below the surface. B - At shallow water (ocean floor is now at B). The elliptical movement of a fluid particle flattens with increasing depth. 1 - Direction of propagation of wave. 2 - Wave crest. 3 - Wave trough. -
Wave energy technology Oscillatingwatercolumndevices
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The base of the device is open to the sea. During the propagation of the wave, its crests and troughs pass alternately over the base of the device. As a result the water is pushed up and down alternately. Thus, the air in the column above the water surface moves up and down in phase with the incident crests and troughs . The movement of the air and rotate the blades of the turbine. Speed of the air is increased by reducing the cross- sectional area of the air column. Electricity is then brought ashore through an undersea cable. The technology is unusual among wave energy systems because all the moving parts are located above the water line.
Buoyantmooreddevice
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This type of device floats on the surface of the water or below it. It is moored to the seabed by either a taught or loose mooring system. An example of this type of device is the Edinburgh or Salter Duck. Ducks rotate independently about a long linkage. The front edge of the duck matches the wave particle motion. The device requires a water depth of at least 80 metres and uses a system of weights and floats to give almost constant tension in the mooring cables.
HingedContourDevice:Pelamis
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It is semi submerged serpentine construction consisting of series of cylindrical hinged segments that are pointed towards the incident wave. This type of device follows the motion of the waves; it creates power using the motion at the joints. It is commonly moored slackly to hold it in place.
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As the Pelamis moves with the waves, the segments rock back and forth and the relative motion between adjacent segments activates hydraulic pumps which drives electricity generators.
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A 750kW device would be 150m long and 3.5m in diameter and comprise five sections.
ArchimedesWaveSwing
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submerged device The wave action powers the floater which moves up and down, generating a reciprocating movement. When the wave crest approaches, pressure on the top of the floater increases, which pushes mechanism inside the cylinder downwards, compressing the gas within the cylinder to balance the pressure. When the wave trough passes over the floater, the reverse process takes place, moving the floater upwards and decompressing the gas inside the cylinder. This reciprocating motion generated by the floater is converted into electricity by means of a hydraulic system or a motor-generator set. Having only one moving part makes the system is more reliable with less need of maintenance.
Reliable - AWS is reliable due to its simple arrangement and less moving parts. -
It is submerged atleast 6 meter below the sea surface and is thus not affected by high storms. This reduces the mooring cost and risk of damage.
Advantages and limitations of wave power Advantages: -
Wave Power doesn’t generate harmful greenhouse gases. The global potential for wave power is very large: 1-10 TW
Limitations: -
Opposition to shore-based sites could be an issue in areas of scenic beauty. Noise generated by air turbines of oscillating water column is unacceptable. The visual impact is much less significant for off-shore devices but providing cables for electricity transmission to the shore adds to the cost. Main challenge is to reduce the capital cost of construction of the system to make electricity available at a competitive price.
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Another challenge is to withstand extreme weather conditions at sea.
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Niche market for wave power is limited to remote areas where electricity supply by other sources is either unavailable or expensive.
Moving to shore based and near shore devices reduces the risk but the power available is less than that available at offshore places.
Ocean Thermal Energy Conversion (OTEC) -
OTEC process uses the heat energy stored in the Oceans to generate electricity. OTEC systems works best when the temperature difference between the warmer, top layer of the ocean and the colder, deep ocean water is about 20°C. These conditions exist in tropical coastal areas, lying between the Tropic of Capricorn and the Tropic of Cancer. To bring the cold water to the surface, OTEC plants require an expensive, large diameter intake pipe, which is submerged a mile or more deep into the oceans. Earlier OTEC systems had an overall efficiency of only 1 to 3% (the theoretical maximum efficiency lies between 6 and 7%).
Ocean water temperature variation with depth Findings of some of temperature distribution of seawater measured in vertical direction in tropical and subtropical zones are shown in Fig. It is observed that sea water temperature in the surface layer is around 20~30°C, while in the layer at about 700 m depth the water temperature is 2~7°C.
Operating principle of OTEC Ocean thermal energy conversion systems (OTEC) make use of the different high and low temperatures that exists in deep and shallow water to the process of rotation of turbine. The heat cycle suitable for OTEC is the Rankine Cycle using a low-pressure turbine. The OTEC Systems are of three types: (i)closed-cycle (ii) open-cycle (iii) hybrid cycle. The main components of the systems are : a feed pump, an evaporator, a turbine and a condenser. It may be emphasized that in all of the three cycles, it is necessary to reach up to a sea depth where water is cool enough to condense the working fluid. The desired depth is 1,000 meters (3,200 feet), where the temperature of water is approximately 4C.
Closed cycle:
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In this system ammonia is used a working fluid. On its passage through the evaporator, ammonia absorbs heat from the hot sea water and gets evaporated. Then the vaporized ammonia passes over the turbine causes it to rotate and generate electricity.
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The fluid exhausted from the turbine is cooled down and reliquefied in the condenser by cold seawater present at a greater depth.
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By repeating this cycle, power can be generated without assistance of any extra fuel.
Open cycle:
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In the open-cycle system, warm surface water is introduced into a vacuum chamber where it is flash-vaporized.
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The water vapor so formed drives a turbine to generate electricity. The remaining water vapor (essentially distilled water) is condensed using cold water. The condensed water can either return back to the ocean or be collected for the production of potable water.
Hybrid cycle (HC) :
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A HC system possesses combination of the characteristics of the closed cycle and the open cycle.
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Such a system has great potential for applications requiring higher efficiencies for the coproduction of energy and potable water.
Multiple industrial complex with OTEC Apart from generation of electricity, the potential of deep ocean water (DOW) has been explored and has several other applications. Some of the utilizations already in practice are: -
Helps produce fuels such as hydrogen, ammonia, and methanol. Extracting lithium chloride dissolved in seawater is one of considerable method of industrial lithium production. Produces base load electrical energy.
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Produces desalinated water for industrial, agricultural, and residential uses.
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Provides air-conditioning for buildings and moderate-temperature refrigeration. The temperature of depth cold water after utilization for OTEC is still low; e.g. the temperature is around 10°C. It is cold enough to use as chilling source of air cool conditioning. Such air conditioning system requires much less energy than that is needed by ordinary electrical refrigeration method. It means that OTEC makes electricity demand decrease ecologically.
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DOW is rich in mineral. It is possible to produce ‘Mineral water’ as by-product of the OTEC. Providing with ion- exchanger and mineralizer, a part of desalinated water comes more valuable as an industrial product.
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The characteristics of DOW are cold, pure and nutrient. These characteristics can be used effectively to aquaculture by getting rapid growth and less disease.
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Is a resource for on-shore and near-shore mariculture operations.
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Hydrogen Production: "Offshore Hydrogen Production" by OTEC: Completely clean hydrogen is produced by using only natural renewable energy.
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Food, Cosmetics, Medical Science, etc. : Due t o surpassing characteristics DOW draws attention in the various fields of industry and science.
advantages and limitations of OTEC Advantages: 1. 2. 3. 4.
Clean and renewable (solar source energy). Stable throughout a moment, a day and a year. Huge amount but low-density energy. We can assess the value of an ocean thermal energy conversion (OTEC) plant and continued OTEC development by both its economic and noneconomic benefits. 5. Enhances energy independence and energy security. 6. Has potential to mitigate greenhouse gas emissions resulting from burning fossil fuels. Disadvantages: OTEC involve transportation large amounts of water. This brings up three important concerns: (1) Marine organisms entrainment and impingement through the water current; (2) The effect of chemicals used to reduce/control biofouling buildup inside the seawater pipes and heat exchangers; (3) The effect known as “upwelling”, or rise of the d eep cold water to the surface. All three problems can be controlled and mitigated during system design and/or through preventive measures during operation.
Biomass energy -
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Biomass refers to plant and animal derived materials such as straw, logs, dung and crop residue. These are used directly or indirectly as fuel which are called as BIO FUELS. The attraction of bio mass is that, it is carbon neutral. The amount of CO2 released in the combustion of bio fuels has been previously o removed from the atmosphere when CO2 was converted by photosynthesis into making the plant material. In photosynthesis, CO2 and water are converted into oxygen and carbohydrate. o o CO2 + H2O + h O2 + [CH2O] where h are Sunlight photons and [CH2O] is Carbohydrates o In photosynthesis a minimum number of 8 photons each with energy ~1.8eV( 14.4 eV in all) are needed to produce one O2 molecule and one C atom fixed in carbohydrate that stores ~4.8 eV of energy. o A rough estimate of the overall efficiency of the photosynthesis process is only 0.5 % Examples of bio energy crops: Hybrid Poplar - Corn Corn Stover - Soybeans - Sorghum Wood Chips Sawdust Municipal Solid Waste - Sugar Cane Bagasse - Switchgrass
Biofuels -
Biomass is the only renewable source of carbon based fuels and chemicals. A variety of fuels can be made from biomass resources including the liquid fuels such as, ethanol, methanol, biodiesel, and gaseous fuels such as hydrogen and methane. Biofuels are primarily used to fuel vehicles, but can also fuel engines or fuel cells for electricity generation.
Liquidbiofuelyields
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In Europe, the annual yield of bioethanol from sugar beet is ~4.5 ton ha-1 and from wheat 2.1 ton ha-1, while in US from switch grass it is ~2.8 ton ha-1. In Europe Biodiesel comes from rapeseed and the annual yield is ~1.3 ton ha-1. Higher yields (2.3-3 ton ha -1) are obtained from the Jatropha plant whose seeds have an oil content of 37%. The annual biodiesesl yield from Palm trees is ~4-6 ton ha-1. The production of Palm oil is currently concentrated in Malaysia and Indonesia.
Potential and use -
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The mass of plants and animals produced on land each year is about 4x1011 ton and contains 1.5 x 10 11 ton of carbon. An average of ~7.5 MJ kg-1 is stored in biomass making the annual amount of terrestrial bioenergy stored equal to ~3 x 1015 MJ. This energy is many times the global power usage. The current use of Biomass energy is mainly (70%) for residential cooking and heating in the developing countries in the form of burning wood, dung and plant residues. The production of temperate cereals such as wheat, maize, sugar, rice etc yield residues which are used as biofuels. Example: bagasse, the residue from sugarcane, is used in sugar factories as a fuel for producing electricity and hot water.
1. 2. -
Energy is produced from two main sources of Biofuels: Agricultural and Municipal Waste Energy Crops These have low energy content per kilogram compared with fossil fuel and relatively low density making them bulky and expensive to transport. Due to this reason bioenergy production is currently often combined with crop production or as useful way of disposing organic waste, both municipal and agricultural.
Energy generation from biomass
Thermochemical Conversion of Biomass: 1. Direct Combustion 2. Gasification 3. Pyrolysis Biochemical Conversion Processes 1. Anaerobic Digestion 2. Fermentation 3. Extraction (from seeds)
Biomass direct combustion - Biomass direct combustion involves burning the solid biomass and production of thermal energy. - This thermal energy can be used for power production in any given power plant.
Gasification - Biomass when heated in absence of oxygen or in a reduced supply of oxygen, gasifies to a mixture of carbon monoxide and hydrogen forming synthesis gas or syngas. - Gaseous fuels mix with oxygen more easily than liquid fuels. - Syngas, therefore, inherently burns more efficiently and cleanly than the solid biomass from which it is made. - Biomass gasification can thus improve the efficiency of large- scale biomass power facilities. - Like natural gas, syngas can also be burned in gas turbines, a more efficient electrical generation technology than steam - boilers whose operation is based on burning of solid biomass and fossil fuels.
Example: McNeil Generating Station biomass gasifier -
200 tons of wood chips daily
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Forest thinnings; wood pallets
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Converted to gas at ~1000 C
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Combined cycle gas turbine
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8MW power output
liquefied by pyrolysis - Solid biomass can be liquefied by pyrolysis, hydrothermal liquefaction, or other thermochemical technologies. Pyrolysis and gasification are related processes of heating with limited oxygen. - Bio-material is heated under the following conditions: o Temperature: 500-1300C Pressure 50-150 atmospheres o Carefully controlled air supply o - Pyrolysis oil or other thermochemically-derived biomass liquids can be used directly as fuel. Further, these oils also holds great promise as platform intermediates for production of high-value chemicals and materials. - Up to 75% of biomass are converted to liquid which are usable in engines, turbines and boilers.
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Anaerobic digestion - Anaerobic Digestion is the decomposition of organic matter in the absence of air by bacteria. - Bacteria break down the organic matter and produces a gas consisting of methane (65%) and carbon dioxide (35%) with traces of other gases. - Anaerobic digestion occurs naturally and it takes place in landfill sites over a period of years, with methane production occurring after 10 years. - In digesters where temperature is kept at 30-60 0C, the methane production occurs within a few weeks.
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Anaerobic digestion is widely used in Asian villages where the biogas is used for heating and cooking.
Anaerobicdigesters
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Anaerobic digesters are animal waste tanks or storage ponds sealed with covers that trap the biogas produced in a digester, creating a sort of “biogas plant”.
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The biogas is then pulled out from the digester by providing a slight vacuum on a pipe with a gas pump or blower.
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Biogas, which contains 60% – 80% methane, is used to produce energy. Methane can power a generator to produce electricity. The methane gas can also be used for direct heat application, such as operating a boiler or space heater, as well as chilling and refrigeration equipment.
Fermentation - Carbohydrate portion of biomass is converted into sugar and subsequently into ethanol in a fermentation process is known as bioethanol . - Fermentation is carried out by yeast or bacteria. Thus the carbohydrate is converted into to ethanol and CO2.
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As the heat released is small, nearly all the energy contained in the sugar is stored in the alcohol. Ethanol, derived from starch crops such as corn, is the most widely used biofuel today with current capacity of 1.8 billion gallons per year. Ethanol produced from cellulosic biomass is currently the subject of extensive research, development and demonstration efforts.
Diesel oil can be derived from any fossil fuel or any vegetable oil. When it is derived from vegetable oil, it is known as biodiesel. Biodiesel synthesis involves the reaction of oil with either methanol or ethanol using sodium hydroxide or potassium hydroxide as a catalyst and formation of ethyl or methyl esters. This process is known as trans esterification. The efficiency of the process is high (>97%) requirement of the alcohol is about 10% of the weight of vegetable oil.
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Environmental impact of biomass Advantages -
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Biomass is a carbon-neutral source of energy provided the biocrop is replanted. Biomass is also a sustainable source of energy as long as the land quality is maintained fertile. Irrigation, fertilizers, harvesting and processing of the biomass consume energy which is derived from fossil fuels, The carbon emissions associated with theses processes are a small fraction of those given off by the fossil fuels producing the same amount of energy. Biomass combustion generally produces low emissions. The combustion of wood gives much less SO2 less than emitted by coal. Hence there will be less acid rain.
Disadvantages -
Large areas of energy crops may reduce biodiversity.
Economics and potential of biomass - Sustainable Development: Biomass technologies discourage the use of fossil fuels and help us to adopt sustainable energy production.
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Energy Security: As a domestic energy source, biomass can substantially reduce dependence on imported crude oil. Biomass is more evenly distributed over the earth's surface than
Commented [Office3]: rephrase
other finite energy sources and therefore provides opportunities for local, regional, and national energy self-sufficiency.
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Rural Economic Growth: Producing biomass and using agricultural residues for biomass technologies will stimulate rural development efforts in farming, forestry, and associated service industries by creating new products, markets, and jobs.
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Land Use: The lands must be used in balanced ways, supporting agricultural and forestry production, environmental preservation, human and wildlife habitats, as well as biomass production.
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Combustion and gasification of biomass is the most economical competitive use of biomass, with gasification having greater potential with its use in high temperature gas turbines.
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In Europe biomass combustion and gasification could provide up to ~15% of the projected demand for electricity by 2020.
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Europe has promoted biofuels as a way of reducing its greenhouse gas emission. Biodiesel is Europe’s main biofuel and tax exemptions and national targets are increasing its demand.
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Since the energy density of biocrops is low and the cost of transportation is high, hence small bioplants near the biofarms are more economical.
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As the cost of non-biomass energy rises, different biomass supplies become increasingly attractive.
Future of biomass technology - Biomass has the potential to provide 10-20% of the primary energy needs of developed countries and a large percentage in developing countries where there is lot of scope of improvement in Biomass Technology.
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Unlike other renewable technologies, biomass can be stored.
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Its development aids rural economics and its use locally avoids high transport costs.
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Genetic engineering of Bioenergy crops may increase yield and allow more harvests per year. However, with present technology the potential contribution from biomass to projected global energy needs would appear to be limited to 20-30%.
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Biomass can provide energy security and, if used for liquid biofuel production, can reduce dependence on foreign imports. The introduction of tax incentive and in particular a percentage share of energy production has helped the development of Biomass Technology.
Geothermal energy -
geothermal energy: energy extracted from heat stored in the earth. The geothermal energy originates from the original formation of the planet, from radioactive decay of minerals, from volcanic activity and from solar energy absorbed by the surface.
Source of geothermal energy -
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Heat Flows outward from the Earth’s Interior The Curst insulates us from Earth’s interior heat The Mantle is semi-molten, the outer core is liquid and the inner core is solid Earth's core maintains temperatures in excess of 5000 K. It is due to the gradual radioactive decay of elements present in the earth’s core. Heat energy continuously flows from hot core by -Conduction & Convection.
Mean heat flux reaching at earth's surface Dissipates to the atmosphere and space.
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Tends to be strongest along tectonic plate boundaries Volcanic activity transports hot material to near the surface -Only a small fraction of molten rock actually reaches surface.
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is left at depths of 5-20 km beneath the surface,
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Hydrological convection forms high temperature geothermal systems at shallow depths of 500-3000m.
Geothermal sites Name Hot springs
Phases gushes of hot water that are found on the land surface.
Fumaroles
vents from which volcanic gas escapes into the atmosphere.
Description As molten materials deep in the earth cool down, they give off water vapor and carbon dioxide. This hot vapor then find its way upward through the cracks in the rocks, cooling as it goes, until it condenses to become water. This water may be pure and clear, but it is rich in mineral salts dissolved from the rocks it has passed through on its way to the surface. These hot springs can be found in Japan, New Zealand, Kenya and Iceland. Fumaroles may occur along tiny cracks or long fissures, in chaotic clusters or fields, and on the surfaces of lava flows and thick deposits of pyroclastic flows. They may persist for decades or centuries if they are above a persistent heat source (active Magma chamber) or disappear within weeks to months if they occur atop a fresh volcanic deposit that quickly cools.
Geysers
A geyser is a type of hot spring that erupts periodically, ejecting a column of hot water and steam into the air.
The temperatures of volcanic gases escaping from it is 70 70C - 100 100C or more. In some cases they are hidden in the ground and can be broken into. The gases are dangerous and a gas- mask is often needed. They are always a sign of active volcanism. The formation of geysers requires a favourable hydrogeology which exists in only a few places on Earth, and so they are fairly rare phenomena. About 1000 exist worldwide, with about half of these in Yellowstone National Park, USA. Geyser eruptive activity may change or cease due to ongoing mineral deposition within the geyser plumbing, exchange of functions with nearby hot springs, earthquake influences, and human intervention
Extracting geothermal energy -
Three types of geothermal power plant technologies are in use to convert hydrothermal fluid energy into electricity. The type of conversion depends on the state of the fluid (whether steam or water) and its temperature.
The conversion technologies are: 1. Dry Steam System: Dry steam power plants use the steam from the geothermal reservoir as it comes from wells, and route it directly through turbine/generator units to produce electricity. 2. Flash System: Flash steam plants are the most common type of geothermal power generation plants in operation today. They use water at temperatures greater than 182°C that is pumped under high pressure to the generation equipment at the surface. 3. Binary Cycle System: Binary cycle geothermal power generation plants differ from Dry Steam and Flash Steam systems in the way that the water or steam from the geothermal reservoir never comes in contact with the turbine/generator units.
Dry steam power plants - “Dry” steam extracted from nat ural reservoir: o
180-225 180-225C
o
4-8 MPa
o
200 km/hr (100+ mph)
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Steam is used to drive a turbo-generator
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Steam is condensed and pumped back into the
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Energy generation = 1 kWh per 6.5 kg of steam
ground
Process
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The dry steam power plants are suitable where the geothermal steam is not mixed with water.
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Production wells are drilled down to the aquifer and the superheated, pressurised steam (180°-350°C) is brought to the surface at high speeds, and passed through a steam turbine to generate electricity.
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The steam is passed through a condenser to convert it to water. This improves the efficiency of the turbine and avoids the environmental problems associated with the direct release of steam into the atmosphere.
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The waste water is then reinjected into the field via reinjection wells.
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The waste heat is vented through cooling towers. low, around 30% .
Economics
The energy conversion efficiencies are
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The efficiency and economics of dry steam plants are affected by the presence of noncondensable gases such as carbon dioxide and hydrogen sulphide. The pressure of these gases reduces the efficiency of the turbines. In addition to this, the removal of the gases for environmental considerations also adds to the cost of operation. Dry steam power plants are the simplest and most economical technology, and therefore are widespread. The United States and Italy have the largest dry steam geothermal resources, but these resources are also found in Indonesia, Japan and Mexico.
Single flash steam power plants - Single flash steam technology is used where the hydrothermal resource is in a liquid form. - The fluid is sprayed into a flash tank, causing it to vaporise (or flash) rapidly to steam. - The steam is then passed through a turbine coupled to a generator as for dry steam plants. - To prevent the geothermal fluid flashing inside the well, the well is kept under high pressure. - The major part of the geothermal fluid does not flash. This fluid is reinjected into the reservoir or used in a local direct heat application.
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1. 2. 3. 4. 5. 6. 7.
Alternatively, if the fluid left in the tank has a sufficiently high temperature, it can be passed into a second tank, where a pressure drop induces further flashing to steam. This steam, together with the exhaust from the principal turbine, is used to drive a second turbine or the second stage of the principal turbine to generate additional electricity.
Steam with water extracted from ground Pressure of mixture drops at surface a nd more water “flashes” to steam Steam separated from water Steam drives a turbine Turbine drives an electric generator Generate electricity between 5 and 100MW Use 6 to 9 tonnes of steam per hour
Doubleflashpowerplants
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Similar to single flash operation Unflashed liquid flows to low-pressure tank –flashes to steam Steam drives a second-stage turbine Also uses exhaust from first turbine Increases output 20-25% for 5% increase in plant costs
Binary cycle power plant - Geothermal resource with low temperatures (100 and 150C) can be used. - Working liquids with boiling points less than that of water such as iso-butane, iso-pentane are used to form vapour to operate the turbine. -
Vapour used to drive turbine is condensed and recycled continuously.
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Typically 7 to 12 % efficient
Process:
Commented [Office4]: is it still a single flash steam power plant?
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Binary cycle power plants are used where the geothermal resource is not hot enough to efficiently produce steam, or where the resource contains too many chemical impurities to allow flashing. Further, the fluid remaining in the tank of flash steam plants can also be utilised in binary cycle plants (eg Kawerau, New Zealand). In the binary cycle process, the geothermal fluid is passed through a heat exchanger. The secondary fluid , which has a lower boiling point than water (eg isobutane or pentane), is vaporised, and expanded through a turbine to generate electricity. The secondary fluid also known as working fluid is condensed and recycled for another cycle. All of the geothermal fluid is reinjected into the ground in a closed- cycle system.
Advantages:
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Binary cycle power plants can achieve higher efficiencies than flash steam plants, and they allow the utilisation of lower temperature resources.
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Corrosion problems are avoided.
Disadvantages: -
Binary cycle plants are more expensive, and large pumps are required which consume a significant percentage of the power output of the plants.
Geothermal heat pump (GHP) The GHP can transfer heat in two ways: -
During the winter, heat is withdrawn from the earth and fed into the building; In the summer, heat is removed from the building and stored under-ground.
In some GHP systems heat is removed from shallow ground by means of an antifreeze/water solution circulating in plastic pipe loops (either inserted in vertical wells less than 200 m deep which are then backfilled or buried horizontally in the ground). In other GHP systems flow of water produced from a shallow borehole through the heat pump, discharges the water either in another well or at surface. The heat pump unit is located inside the building and is coupled either with a low-temperature floor or wall heating net or with a fan delivering hot and cold air.
Advantages -
Environmentally very attractive Low CO2 emissions Attractive energy source in right locations Unlike solar and wind energy, geothermal energy is always available, 365 days a year Likely to remain an adjunct to other larger energy sources Part of a portfolio of energy technologies Exploration risks and up-front capital costs remain a barrier
Drawbacks Technological issues: -
Geothermal fluids can be corrosive – Contain gases such as hydrogen sulphide – Corrosion,
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scaling Requires careful selection of materials and diligent operating procedures
Commented [Office5]: ??????
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Typical capacity factors of 85-95%
Environmental issues: Land - Vegetation loss - Soil erosion - Landslides Air - Slight air heating - Local fogging Ground - Reservoir cooling - Seismicity (tremors) Non-renewable
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Water - Hydrothermal eruptions - Lower water table - Subsidence: Sinking of ground to lower level- affects builiding foundations. Noise Benign overall -
geothermal sites are capable of providing heat for many decades, eventually specific locations may cool down. Heat depleted as ground cools Not steady-state Earth’s core does not replenish heat to crust quickly enough
Economics 1. Temperature and depth of resource: - A shallow resource means minimum drilling costs. High temperatures (high enthalpies) mean higher energy capacity. 2. Type of resource (steam, liquid, mix): - A dry steam resource is generally less expensive to develop as reinjection pipelines, separators and reinjection wells are not required 3. Available volume of resource 4. Chemistry of resource : - A resource with high salinity fluids, high silica concentrations, high gas content, or acidic fluids can pose technical problems which may be costly to overcome. 5. Permeability of rock formations: - A highly permeable resource means higher well productivity, and therefore fewer wells required to provide the steam for the power plant. - Size and technology of plant: - As with most types of power plant, large power plants are generally cheaper in terms of $/MW. - Infrastructure (roads, transmission lines) development considerations. - Costs of geothermal energy is highly variable from site to site. - The cost of drilling boreholes to depths of several kilometers is very high and nature of rock formation and rock temperature are unknown in advance. The initial capital cost is high but the operating cost is low because the fuel is free.
Present implementation of geothermal power generation 1. World production of 8 GW – 2.7 GW in US 2. The Geyers (US) is world’s largest site – Produces 2 GW 3. Other attractive sites
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Rift region of Kenya, Iceland, Italy, France, New Zealand, Mexico, Nicaragua, Russia, Phillippines, Indonesia, Japan
Nuclear energy -
Nuclear energy is the energy in the nucleus, or core, of an atom. 1. Fission is a process in which a nucleus with a large mass number splits into two nuclei, which have smaller mass numbers. - neutrons are usually released when fission takes place - fission of a nucleus may be spontaneous, that is, it may happen at random due to internal processes within the nucleus - fission can also be induced by bombarding a nucleus with a neutron. Induced fission is used to generate nuclear power and for weapons - the products formed during fission gain kinetic energy. It is this energy that is harnessed in nuclear power stations 2. Fusion is a process in which two nuclei combine to form a nucleus of larger mass number. - Fusion is the main nuclear process that occurs in the Sun and other stars. - The products of fusion reactions also gain kinetic energy that can be harnessed.
Binding energy and stability of nucleus -
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Commented [Office6]: refer to pc1144 notes and add on
Nucleus of an atom consists of positively charged protons and neutrons (called nucleons). Nucleons are bound together by short range attractive forces called nuclear forces. The mass of a nucleus is less than the sum of masses of its nucleons. This mass difference known as mass defect provides the binding energy to hold the nucleons together in the nucleus of the atom. The mass energy relationship is given by E=∆Mc2 . Where, E= Binding Energy.
∆M=Mass of the nucleons (Protons+Neutrons) - Mass of the
nucleus c=Speed of light in vacuum. -
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Thus, Binding Energy needed to break the nucleus into its constituent nucleons determines whether a nucleus is stable or unstable. The nuclear force between the nucleons is short range and attractive, unless the separation between the nucleons is very small. Nucleons are therefore on average the same distance apart and interact primarily with their nearest neighbours. The total binding energy per nucleon in a nucleus, is roughly constant for nucleus with number of nucleons more than 12. This is maximum near Fe with nucleons ~56 The binding energy curve is obtained by dividing the total nuclear binding energy by the number of nucleons. = ∑ − ∑
Commented [Office7]: ???
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Nuclear fission -
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low-energy (slow, or thermal) neutrons are able to cause fission only in those isotopes of uranium and plutonium whose nuclei contain odd numbers of neutrons (e.g. U-233, U-235, and Pu-239). Bombardment of Uranium by slow moving neutrons results in splitting of the nucleus into two smaller nuclei along with emission of neutrons and huge amount of energy. This process is known as Nuclear Fission. The smaller nuclei are of Ba and Kr and each reaction release of three more neutrons.
Chain reaction - More neutrons (2 – 3) are released during each fission process. - These neutrons could cause further fission of other nuclei if absorbed. - It could cause a chain reaction – either uncontrolled (atomic bomb) or in a controlled manner where it proceeds slowly.
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Whether a chain reaction occurs or not, depends on the relative probability of neutroninduced fission compared to neutron loss.
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The main reactions of neutron with Uranium are scattering, capture and induced fission. It depends on the energy of incident neutron. The dominant causes of neutron loss are neutron capture followed by gamma rays emission. Natural Uranium has 99.28% of 238 U and 0.72% of 235 U. The average number of neutrons emitted per fission is 2.4 and their energies range between ~0-10 MeV. In natural uranium, these neutrons are most likely scattered off 238 U and it is only when their energy is less than 5 MeV that neutron-induced fission of 235 U is more likely to occur. Thus the chain reaction dies off. To sustain a chain reaction, either, the 235 U proportion should be increased: Enrichment or the capture by 238 U should be decreased. Addition of nuclei with a low atomic number, called moderators, changes the energy of neutrons which suffer elastic/inelastic collision with moderators and helps in sustaining chain reaction with neutrons with energy as low as 0.05 eV. Neutrons with theses energies are called thermal neutrons and they are at the same temperature as Uranium fuel.
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Nuclear fuels -
Most reactors use 235U as the fuel for fission as it readily undergoes fission after absorbing a neutron. However, natural uranium contains only 0.7% 235U and 99.3% 238U. 238U can absorb the neutrons and fission but cannot sustain the chain reaction.
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Thus, to sustain the chain reaction it is necessary to increase the concentration of (or enrich) 235U. This can not be done chemically (same element) and instead makes use of the small mass difference between 235U and 238U, e.g., in a high-speed centrifuge,
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Alternatively, a different fuel such as plutonium-239 could be used. This does not occur naturally and need to be produced in a breeder reactor through the following reaction scheme:
Thermal nuclear reactor design Out of a total of 439 Nuclear Reactors in 2003, 263 were pressurized light water reactors (PWR). 1. Fuel Rods: Uranium is the basic fuel 2. Chemically Inert Fluid 3. Moderator: Material in the core which slows down the neutrons released from fission so that they cause more fission. It is usually water, but may be heavy water or graphite 4. Control Rods: these are made with neutron-absorbing material such as cadmium, hafnium or boron, and are inserted or withdrawn from the core to control the rate of reaction, or to halt it. 5. Heat Exchangers: A fluid circulating through the core so as to transfer the heat from it
Process
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The fuel is in the form of rods which allows for easy refuelling. The rods are immersed in a chemically inert fluid such as water, CO2 or He which is heated. The control rods are located above the core, their depth of insertion controls the reaction. Energy released from the nuclear fission reaction heats up the fluid. The hot fluid is pumped through the primary loop and releases its heat to the heat exchanger, and the steam produced drives the turbine. The fuel rods are surrounded by the moderator which helps in inducing chain reaction. Neutrons from fissions have energy ~ 1 MeV, while the 235U are more likely to absorb thermal neutrons (< 1 eV). They are slowed down by moderators (made of carbon, helium or “heavy water”). Hydrogen nuclei (protons) in normal water would be ideal in terms of taking away the excess KE through collision, but will also absorb neutrons to form deuterium. Primary Loop: The energy from the fission is used to heat the water surrounding the reactor core. The pressure in the reactor core is high, 15 Mpa and it keeps water in liquid phase at a temperature of 315C. This in turn heats the water in the steam generator through a heat exchanger. The neutrons in the reactor core makes the water in primary loop as radioactive water. But this is not in direct contact with the turbine and rest of the electricity-generating system. Secondary Loop: The water in the secondary loop is at a relatively low pressure of 5 Mpa. Steam is formed due to the heating from the water in the primary loop. It drives the turbine to generate electricity (similar to a conventional generator). Unused steam is cooled at the steam condenser and pumped back to the steam generator. Over a long period, the high neutron flux causes embrittlement of the reactor vessel as the metal becomes less ductile and this affects the lifetime of the reactor. Corrosion in the steam generating tubes must also be monitored.
Reactor control -
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As the fuel is burnt, the number of neutrons start to decrease. Thus to maintain the nuclear reaction, the control rods are inserted or removed from the core to maintain the reaction. Control can also be maintained by altering the absorption of coolant by adding a chemical containing a nucleus with a large neutron absorption property. Example boric acid in water can absorb neutrons. Another method to control the reaction is to use burnable poisons. Examples are Gd2O3 or Er2O3 and are included in fuel rods.
Applications: - The PWR was initially developed for submarines since, unlike internal combustion engines, nuclear-powered submarines do not need oxygen and can remain underwater for much longer time. - The heat from the reactor produces steam to drive a turbine and the relative compact core proved a cost effective design that could be scaled up to 1GW. Radiation Fission products:
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During the operation of a reactor, there is a build up of fission products within fuel rods. Some are very long lived actinides arising through successive neutron capture reaction on uranium.
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When the amount of fissile material in fuel rod is insufficient to maintain the controlled chain reaction, the rod is removed and the remaining fissile material is extracted chemically.
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It is reutilized in new fuel and the waste products are separated for storage.
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The presence of these actinides means that the waste must be stored safely for many thousand years. The spent fuel is first stored on site for several years to allow the intense short-lived activity to decay. There it is kept in storage pools to remove the heat and prevent the radiation. The spent fuel is reprocessed to recover the Uranium and plutonium. The residue is immobilized by incorporating in borosilicate glass. This spent fuel can then be placed in a corrosion-resistant can and stored in an underground respository.
Radiation effects
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Radiation affects tissues as it causes ionization, which breaks molecules apart and gives rise to free radicals, which can damage cells. The scale of effect depends on the energy deposited per unit mass of tissue, the dose of the radiation, and on the type of radiation.
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Charged particles, such as -particles cause relatively more damage than -rays or electrons depositing the same energy since their energy loss per unit length is higher.
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Safety of nuclear power There is considerable public concern over the use of nuclear power due to three serious accidents. 1. In 1952 there was a fire at a gas-cooled graphite-moderated reactor at Windscale in the UK. 2. In 1979 there was an accident in a PWR at Three Miles Island in Pennsylvania in the USA. This was caused by both mechanical and human failures, resulting in a 20% core meltdown but only a small release of radioactivity. 3. In 1986, there was an uncontrolled reactor power increase in a water cooled graphite moderated reactor at Chernobyl in the Ukraine, causing a steam explosion and huge release of radioactivity. 4. The Fukushima, Japan nuclear disaster has 5 nuclear reactors burning, 2 in partial meltdown and 3 in full meltdown- and they've ALL been uncontrollably burning since March 11th . Its been over 3 months and this nuclear disaster remains completely out of control. In the USA there have been almost 3000 reactor years of operation during which time there has been only one serious accident.
Economics of nuclear power -
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New reactor designs with both passive and active safety features will reduce this accident probability. Improvements are done in training and instilling the importance safety at work. Nuclear reactor plants require large capital costs and those operating today were originally state-owned or run by regulated utility monopolies. Most of the financial burden is absorbed by the state-frame work. Although capital cost is high, but operational cost is low.
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There are other factors such as reduction in CO2 content as a result of usage of nuclear power which give an upper hand to Nuclear Power Plants in comparison to fossil fuel based power plants.
Environmental impact of nuclear power -
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The principal environmental advantage of nuclear power is the very small amount of CO2 emission. This makes it very good energy resource in light of global warming. Main environmental considerations are related to location of nuclear reactors. These include, the seismology, risk of flooding, meteorology, geology and population distribution in the area of reactor. Other matters of consideration are the effects of thermal discharge to the environment and in particular the storage and disposal of nuclear waste.
Outlook of nuclear power -
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The production of electricity from nuclear power has been increasing steadily but its share of the total world production has been decreasing slightly since 1990. New nuclear projects are promoted in countries – China, India, Japan, Korea and Russia. Nuclear energy is a highly compact fuel resource with a high energy density that can be easily stockpiled. As there are diversified sources of supply, it is unlikely that the supply of fuel would be interrupted, making this a very secure energy resource. In 2005, 28% of Germany’s electricity was provided by nuclear power. But, they have decided to phase out all their nuclear reactors by 2025 and replace them by Renewable energy resources.
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The desire of both a secure energy supply and a reduction in global carbon emission is causing nations to re-evaluate nuclear power as a source of carbon free electricity.
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Currently there are plans to expand nuclear power in India, China, Korea, Japan and Russia but the economic benefits are unattractive in the short term, except when the oil and natural gas prices are high.
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More standardization and simplicity of design will improve the competitiveness of nuclear power and the inclusion of both passive and active safety systems will reduce the likelihood of accidents.
Fusion -
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Fusion power is the power generated by nuclear fusion reactions. In this type of reaction, two light nuclei fuse together to form a heavier nucleus accompanied by the release of large amount of energy. Most designs for fusion power plants are based on the use of fusion reactions to produce heat which is used to operate a steam turbine for generation of electricity. The basic concept behind any fusion reaction is to bring two or more atoms close enough together so that the nuclear force in their nuclei will pull them together into one larger atom. If two light nuclei fuse, they will generally form a single nucleus with a slightly smaller mass than the sum of their original masses.
Conditions for fusion power generation 1. Very high temperatures (~106K) must be attained so that kinetic energies of the nuclei will be enough to overcome their electrostatic repulsion. At high temperatures gas molecules
decomposes into positive nuclei and free electrons. This ionised gas with equal number of positive and negative charges is called plasma. Examples: Sun and fluorescent tubes. 2. Confinement of plasma is difficult. As the temperature of the gas increases, its volume or its pressure both must increase. So the gas must be confined to a fixed volume to allow the nuclei to come close enough to fuse. 3. The fusion energy released must be converted into a useful form, such as electricity.
Outlook of fusion Despite decades of research, we're still a long way from replicating the proton- proton fusion reaction that powers the sun. Fusion power has always seemed thirty or forty years away. Producing commercial power from fusion is highly challenging, because it requires solving fundamental problems of science and engineering. The potential technological problems include: -
Achieving temperatures several time that at the core of the sun, Creation of uniform magnetic fields around 100,000 times stronger than that of the earth. Achieving densities of laser fusion pellets a thousand times that of a normal solid, Focussing of laser beam to within millionths of an inch. The radiation and heat damage to the chamber walls. The fast neutrons from the fusion reactor can lead to metal fatigue and embrittlement.
Advantages The advantages of fusion power include: 1. Infinite fuel supply 2. Higher thermal efficiency 3. Few radioactive waste problems 4. No global warming. Although the technological issues are met individually in the lab framework, but not all at once. The outlook continues to be optimistic and the vision is to have first commercial fusion reactor in 3040 years time.
Energy storage devices Need for energy storage devices -
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Solar power & wind power is an intermittent energy source, meaning that solar power and wind is not available at all times, and is normally supplemented by storage or another energy source Under circumstances of peak power consumption, load shedding is done to meet the grid power supply. Under these condition one may like to switch to storage power supply. From very small devices such as wrist watches to sophisticated devices such as powering of space shuttles rely on power storage devices.
Energy storage systems 1. Fuel Cells 2. Hydrogen production and Storage 3. Li-ion batteries
Fuel cells -
A fuel cell is an electrochemical device that converts the fuel into an electric current. Electricity is generated through chemical reactions between the fuel and an oxidant. The reactants flow into the cell, and the reaction products flow out of the cell while the electrolyte remains within it. Fuel cells consume reactant from an external source, which must be replenished . Many combinations of fuels and oxidants are possible. A hydrogen fuel cell uses hydrogen as its fuel and oxygen (usually from air) as its oxidant. Other fuels include hydrocarbons and alcohols. Other oxidants include chlorine and chlorine dioxide.
Design - Fuel cells are available in a number of varieties. However the way of their functioning is almost identical. - Each type of Fuel cell consists of 3 parts : (1) An anode (2) An electrolyte, and (3) A cathode. - The most common fuel used in the cells is hydrogen. - The anode catalyst is usually made up of very fine platinum powder. The anode catalyst, accelerates up the dissociation of the fuel into electrons and ions. - The cathode catalyst is often made up of nickel which converts the ions into the waste chemicals like water or carbon dioxide - A typical fuel cell produces a voltage from 0.6 V to 0.7 V at full rated load Performance and efficiency of fuel cell - Energy conversion of a fuel cell may be represented as : Chemical energy of fuel = Electrical energy + Heat energy - The input energy is the energy produced during reactions at the electrodes. - The above energy balance could be explained using the first and second laws of thermodynamics. Applications 1. Stationary power includes any application in which the fuel cells are operated at a fixed location, either for primary or for backup power, or for combined heat and power generation(CHP). 2. Transportation applications include motive power for cars, buses and other fuel cell vehicles (FCV) and auxiliary power units (APUs) for highway and off-road vehicles, as well as specialty vehicles.
Types of fuel cells 1. Polymer Electrolyte Membrane Fuel Cell (PEMFC) 2. Alkaline Fuel Cell (AFC) 3. Phosphoric Acid Fuel Cells (PAFC) 4. Molten Carbonate Fuel Cells (MCFC) 5. Solid Oxide Fuel Cells (SOFC) PolymerElectrolyteMembraneFuelCell(PEMFC)
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Polymer electrolyte membrane (PEM) fuel cells are also called proton exchange membrane fuel cells. These cells use a solid polymer as an electrolyte and porous carbon electrodes containing a platinum catalyst. H2: Fed on Anode Air (O2): Fed on Cathode Polymer electrolyte membrane fuel cells operate at relatively low temperatures, around 80°C.
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It deliver high-power density and offer the advantages of low weight and volume, compared with other fuel cells.
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PEM fuel cells are used primarily for transportation applications such as cars and buses and some stationary applications due to their fast startup time,
Commented [Office8]: show reaction at anode and cathode
low sensitivity to orientation, and favourable power-to-weight ratio.
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It requires a noble-metal catalyst (typically platinum) to separate the hydrogen's electrons and protons, adding to system cost.
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A limitation is hydrogen storage. Most fuel cell vehicles (FCVs) powered by pure hydrogen must store the hydrogen on-board as a compressed gas in pressurized tanks. Due to the lowenergy density of hydrogen, it is difficult to store enough hydrogen on-board to allow vehicles to travel the same distance as gasoline-powered vehicles before refuelling, 300 –400 miles.
AlkalineFuelCell
These fuel cells use solution of potassium hydroxide in water as the electrolyte and can use a variety of nonprecious metals as a catalyst at the anode and cathode. At anode, hydrogen is oxidized according to the reaction: producing water and releasing two electrons. The electrons flow through an external circuit and return to the cathode, reducing oxygen in the reaction:
producing hydroxide ions. Combined reaction products are water molecule electricity and heat. -
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Alkaline fuel cells (AFCs) were one of the first fuel cell technologies developed with efficiency of 60% and they were the first type widely used in the U.S. space program to produce electrical energy and water on-board spacecrafts. New AFC designs operate at lower temperatures varying from ~23°C to 70°C AFC stacks have been shown to maintain sufficiently stable operation for more than 8,000 operating hours. The disadvantage of this fuel cell is its poor resistance to poisoning by carbon dioxide (CO2) and affecting cell’s life. In fact, even a small amount of CO2 in air can affect its operation, making it necessary to purify both the hydrogen and oxygen used in the cell. This purification process is costly.
SolidOxideFuelCells
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The cell is consists of two porous electrodes which sandwich an electrolyte. Air flows along the cathode. When an oxygen molecule from air comes in contact with cathode-electrolyte interface, it catalytically acquires four electrons from the cathode and splits into two oxygen ions. 2O + 4e 2O 2 The oxygen ions diffuse into the electrolyte material and migrate to the other side of the cell where they encounter the anode. The oxygen ions encounter the fuel at the anode/electrolyte interface and react catalytically, giving off water, carbon dioxide, heat, and -- most importantly -- electrons. The electrons transport through the anode to the external circuit and back to the cathode, providing a source of useful electrical energy in an external circuit.
Components
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Electrolyte: Ceramic material,
Yttrium-doped zirconium oxide (YSZ) is the potential electrolyte in SOFCs because of its sufficient ionic conductivity, chemical stability, and mechanical strength. The only drawback of stabilized YSZ is the low ionic conductivity in the lower cell operation temperature regime, below about o 750 C. -
Cathode: The oxidant gas is air or
oxygen at the SOFC cathode. Calcium or Strontium substituted Lanthanum manganite has good electronic conduction, porosity, thermal stability and thermal expansion match to YSZ. -
Anode: Nickel-YSZ composites are the most commonly used anode materials for SOFCs.
Nickel is an excellent catalyst for fuel oxidation. YSZ in the anode constrains nickel aggregation, decreases the effective thermal expansion coefficient bringing it closer to that of the electrolyte, and provides better adhesion of the anode with the electrolyte. Design configurations
Two design configurations for SOFCs have emerged: A planar design: In the planar design, the components are assembled in flat stacks, with air and fuel flowing through channels built into the cathode and anode. A tubular design: In the tubular design, components are assembled in the form of a hollow tube, with the cell constructed in layers around a tubular cathode. Air flows through the inside of the tube and fuel flows around the exterior.
Hydrogen production and storage -
Hydrogen can be produced from a variety of feedstocks. These include fossil resources such as natural gas and coal and also renewable resources such as biomass and water with input from renewable energy sources (e.g. sunlight, wind, wave or hydro-power).
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The technologies for hydrogen production include reforming of natural gas; gasification of coal and biomass; and the splitting of water by water-electrolysis, photo-electrolysis, photobiological production and high temperature decomposition. Each technology is in a different stage of development and offers unique opportunities and benefits. Water electrolysis and natural gas reforming are the technologies of choice in the current and near term. These are proven technologies usable in the early phases of building a hydrogen infrastructure for the transport sector.
Sources of hydrogen
Productionfromnaturalgas:
Hydrogen can currently be produced from natural gas by means of three different chemical processes: 1. Steam reforming (steam methane reforming – SMR). 2.
Partial oxidation (POX).
3. Autothermal reforming (ATR). 1. Steam Methane Reforming: Steam reforming involves the endothermic conversion of methane and water vapour into hydrogen and carbon monoxide (Eqn. 1). The heat is often supplied from the combustion of some of the methane feed-gas. The process typically occurs at temperatures of 700 to 850 °C and pressures of 3 to 25 bar (mm of Hg). The product gas contains approximately 12 % CO, which can be further converted toCO2 andH2throughthewater-gasshiftreaction(Eqn.2).
2. Partial oxidation of natural gas: -
Partial oxidation of natural gas is the process whereby partial combustion of methane with oxygen gas yields carbon monoxide and hydrogen ( Eqn 3).
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CH + 1/2O CO + 2H + heat (3) 4 2 2
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The above reaction is exothermic and heat is generated. Hence no external heating is required and comparatively more compact design of reactor is feasible. The CO produced is further converted to H as described in equation (2). 2
3. Autothermal reforming -
Autothermal reforming is a combination of both steam reforming (Eqn. 1) and partial oxidation (Eqn. 3).
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The total reaction is exothermic, and so it releases heat.
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The outlet temperature from the reactor is in the range of 950 to 1100°C, and the gas pressure can be as high as 100 bar. Again, the CO produced is converted to H2 through the water-gas shift reaction (Eqn. 2).
Productionfromcoal
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Hydrogen is produced from coal through gasification process. A typical reaction for the process is represented by equation (4), wherein carbon is converted to carbon monoxide and hydrogen.
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C(s) + H O + heat CO + H (4) 2 2
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This reaction is endothermic hence additional heat is required for carrying out the hydrogen production process. The CO is further converted to CO and H through the reaction, 2 2 described in equation (2).
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Hydrogen production from coal is commercially mature, but it is more complex than the production of hydrogen from natural gas. The cost of the hydrogen produced is also higher.
Productionfromsplittingofwater
Hydrogen can be produced from the splitting of water through various processes. These are: 1. Water electrolysis 2.
Photo-electrolysis
3.
Photo-biological production
4. High-temperature water decomposition. Water electrolysis
Water electrolysis is the process whereby water is split into hydrogen and oxygen through the application of electrical energy, as in equation (5). H2O + electricity H2 + 1/2O2 (5) Future potential costs for electrolytic hydrogen are presented in Figure, where the possibilities to considerably reduce the production cost are evident.
Hydrogen Storage Systems -
Hydrogen storage can be considered for onboard vehicular, portable, stationary, bulk, and transport applications, i.e. for fuel cell or ICE/electric hybrid vehicles.
Commented [Office9]: more details on the other proceseses
Gaseous hydrogen - The most common method to store hydrogen in gaseous form is in steel tanks, although lightweight composite tanks designed to endure higher pressures are also becoming more and more common. - Cryogas, gaseous hydrogen cooled to near cryogenic temperatures, is another alternative that can be used to increase the volumetric energy density of gaseous hydrogen. - A more novel method to store hydrogen gas at high pressures is to use glass microspheres. - Two of the most promising methods to store hydrogen gas under high pressure: 1. Composite tanks 2. Glass microspheres. Compositetanks
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Design of composite hydrogen Storage Tank.
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Composite tanks require no internal heat exchange and may be usable for cryogas.
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There are also some safety issues that still have not been resolved, such as the problem of rapid loss of H2 in an accident.
There are several advantages with such composite tanks. Their low weight meets key targets, and the tanks are already commercially available, wellengineered and safety- tested. Their main disadvantages are the large physical volume required (which does not meet targets).
Commented [Office10]: what are the features of the tank
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GlassMicrospheresforHydrogenStorage
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The basic concept for hydrogen gas storage in glass microspheres can be described in three steps:
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Charging, filling and discharging.
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Next, the microspheres are cooled down to room temperature and transferred to the lowpressure vehicle tank.
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Finally, the microspheres are heated to 200- 300 °C for controlled release of H2 to run the vehicle.
First, hollow glass spheres are filled with H2 at high pressure (350-700 bar) and high temperature (300 °C) by permeation in a high- pressure vessel.
Limitations
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The main problem with glass microspheres is the inherently low achievable volumetric density and the high pressure requirement for filling. The glass microspheres slowly leak hydrogen at ambient temperatures. Another challenge is the need to supply heat at temperatures higher than those available from the PEM fuel cell (70-80 °C). The required high temperature (300 °C) also makes rapid response-control difficult.
Advantages -
Glass microspheres have the potential to be inherently safe as they store H2 at a relatively low pressure onboard and are suitable for conformable tanks. This allows for low container costs.
Liquid hydrogen The three most promising methods are: 1. Cryogenic Hydrogen 2. Hydrogen as a constituent in NaBH4 solutions. 3. Hydrogen as a constituent in Rechargeable organic liquids.
Commented [Office11]: more details for these
The most common way to store hydrogen in a liquid form is to cool it down to cryogenic temperatures ( – 253 °C). Cryogenicliquidhydrogen(LH2 )
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Cryogenic hydrogen, usually simply referred to as liquid hydrogen (LH2), has a density of 70.8 kg/m3 at normal boiling point ( –253 °C).
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The main advantage with LH2 is the high storage density that can be reached at relatively low pressures.
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The theoretical gravimetric density of LH2 is 100%, but only 20 wt. % H2 of this can be achieved in practical hydrogen storage systems today. This means that liquid hydrogen has a much better energy density than the pressurised gas solutions.
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About 30-40% of the energy is lost when LH2 is produced.
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Liquid hydrogen has been demonstrated in commercial vehicles (particularly by BMW), and in the future it could also be co-utilized as aircraft fuel, since it provides the best weight advantage of any H2 storage.
Main disadvantage with LH2 is the boil-off loss during dormancy, and requirement of superinsulated cryogenic containers for storage.
Solid hydrogen Storage of hydrogen in solid materials has the potential to become a safe and efficient way to store energy, both for stationary and mobile applications. There are four main groups of solid materials suitable for this purpose: 1. Carbon and other high surface area materials H2O -reactive chemical hydrides 3. Thermal chemical hydrides 2.
Commented [Office12]: ?????
4.
Rechargeable hydrides
Carbonandotherhighsurfaceareamaterials
Carbon-based materials (nanotubes and graphite nanofibers) -
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Carbon-based materials, such as nanotubes and graphite nanofibers, have received a lot of attention in the research community and in the public press over the last decade. Pure H2 molecular physisorption has been clearly demonstrated, but is useful only at cryogenic temperatures (up to 6 wt.% H2), and extremely high surface area carbons are required. Pure atomic H-chemisorption has been demonstrated to 8 wt.% H2, but the covalent-bound H is liberated only at impractically high temperatures (above 400 °C). Room temperature adsorption up to a few wt.% H2 is occasionally reported, but has not been reproducible.
Rechargeablehydrides
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The two main reversible hydriding reactions in rechargeable metal hydride batteries are shown in Figure.
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From this it becomes clear that the complex hydrides provide the hope for the future, particularly the non-transition metal types such as borohydrides, alanates and amides
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Commented [Office13]: reactions in gas phase and electrochemical reaction
Comparison between various storage techniques - Comparisons between the three basic storage options shows that the potential advantages of solid H2-storage compared to gaseous and liquid hydrogen storage are: o o o
Lower volume. Lower pressure (greater energy efficiency). Higher purity H2 output.
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Compressed gas and liquid storage are the most commercially viable options today, but completely cost-effective storage systems have yet to be developed.
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The safety aspects with all storage options, particularly the novel hydride storage options, must not be underestimated.
Battery -
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A battery is a device that converts chemical energy to electrical energy. A battery is composed of two electrodes (cathode and anode) and an ionically conductive material called electrolyte. When these electrodes are connected by means of an external load or device, electrons spontaneously flow from negative to more positive potential and ions migrate through the electrolyte maintaining the charge balance, and electrical energy can be trapped by the external circuit. Two or more electrochemical cells can be connected in series or parallel combination to form a battery depending up on the required energy and voltage.
Commented [Office14]: ?????
Battery characteristics - A battery’s characteristics depend upon the internal chemistry, current drain and temperature. - The amount of energy per unit mass or volume (Watt. hours/kg or Watt. hours/litre) that a battery can deliver depends significantly on the cell’s voltage and capacity, which are dependent on the chemistry of the system. - Another important parameter is power which depends partly on t he battery’s engineering but crucially on the used chemicals in that battery. - Based on the usage and principle of operation, batteries are categorized mainly in two groups, namely primary (disposable) and secondary (rechargeable) and manufactured in various shapes and sizes according to usage. Primarybatteries(disposable)
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Primary batteries are manufactured to be used once only because its active material (chemicals) is consumed in a single discharge via an irreversible electrochemical reaction. These batteries are: o usually of low price o easy to carry because of light-weight o exhibit high energy density at low to moderate discharge o require minimum maintenance and are easy to use. The most common primary cells that are being used commercially, Zinc-MnO2 (1.5 V) and LiMnO2 (3.0 V). Depending upon their usage, different shape and sizes are available in market like, button and coin cells are widely used in watches, calculators, CD players and other portable appliances.
Primary batteries have created many environmental concerns, mainly toxic metal pollution.
Secondarybatteries(Rechargeable)
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These can be re-charged by applying the electrical current, which reverses the chemical reactions that occur during its use and regenerate its active material for further use. These batteries are also known as storage batteries or accumulators. Therefore, the rechargeable batteries are considered an eco- friendly alternative to the primary batteries as far as metal pollution is concerned. Many reclamation companies recycle batteries to reduce the number of batteries going in to landfills.
Characteristics of secondary batteries
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Energy densities o
Gravimetric (Wh/kg) lighter weight
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Volumetric (Wh/l) smaller size
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Optimised “Speck” battery
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Power (W) o
Potential (V)
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Current (I)
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Effects available capacity
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Recharge conditions and limits – cell protection
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Fabrication of micro-batteries
Types of secondary batteries
Nickel Cadmium (Ni-Cd) -
1.2V, 400 Cycles Inexpensive − Simple charging low energy density − Memory effect high self discharge (20% month) Toxic
Nickel Metal Hydride (Ni-MH) -
1.2V, 600 Cycles Simple to charge, High self discharge (30% month)
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reduced memory effect − Less -toxic
Silver Zinc (AgZn) -
1.5V, 300 Cycles
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Low energy density − Very difficult to recharge
Lithium Ion (Li-ion) based
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3.5V, 2000+ Cycles Higher energy density − No memory effect − Low self discharge Lower toxicity − More expensive − More complex charging Continuous current limited to 1.5C
Lithiumionbatteries
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Rechargeable LIBs involve a reversible insertion/extraction of Li ions (guest species) in to/from a host electrode material during discharge/charge. The Li intake/uptake process happening with a flow of ions through the electrolyte is accompanied by a redox (reduction/oxidation) reaction of the host matrix assisted with a flow of electrons through the external circuit. In the commercial LIBs, Li-containing metal oxides (LiCoO2, LiNiO2, LiMn2O4, and LiFePO4) are employed as cathodes (positive electrode). Graphitic carbons (MCMB: mesocarbon microbeads) or amorphous Sn-Co-C composite are used as anodes (negative electrode). The electrolyte allows the flow of Li-ions between the electrodes but prevents the electron flow. Due to the reversible motion of Li-ions between cathode and anode through electrolyte, the LIBs are also known as rocking chair, swing and shuttle-cock batteries.
Electrode redox reactions on charge: -
+ Cathode oxidation : LiCoO Li CoO + xLi + xe 2 1-x 2 + Anode reduction : xLi + xe + C LiC 6 6
Reverse reaction during discharging
Advantages
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Wide variety of shapes and sizes efficiently fitting the devices they power. Much lighter than other energy-equivalent secondary batteries.
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High open circuit voltage in comparison to aqueous batteries (such as lead acid, nickel-metal hydride and nickel-cadmium).
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This is beneficial because it increases the amount of power that can be transferred at a lower current.
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No memory effect. Self-discharge rate of approximately 5-10% per month, compared to over 30% per month in common nickel metal hydride batteries
Disadvantages
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Shelf life Charging forms deposits inside the electrolyte that inhibit ion transport. Over o time, the cell's capacity diminishes. The increase in internal resistance reduces the cell's ability to deliver current. This problem is more pronounced in high- current applications. The decrease means t hat older batteries do not charge as much as new ones (charging time required decreases proportionally). Internal resistance The internal resistance of lithium-ion batteries is high compared to other o rechargeable chemistries such as nickel-metal hydride and nickel-cadmium. Internal resistance increases with both cycling and age. Rising internal resistance causes the voltage at the terminals to drop under load, which reduces the maximum current draw. Eventually increasing resistance means that the battery can no longer operate for an adequate period. Safety Issues:
