Solar cars
SOLAR CARS 1. AN OVERVIEW A solar car is a vehicle, which is powered by sun’s energy. A solar car is a light weight, low power vehicle designed and built with a single purpose in mind – racing. They have limited seating (usually one, sometimes two people), they have very little cargo capacity, and they can only be driven during the day. It does, however, offer an excellent opportunity to develop future technologies that can be applied to practical applications.
Figure 1: Energy Flow Diagram of a Solar Car The main component of a solar car is its solar array, consisting of photovoltaic cells, which collect the energy from the sun and converts it into
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usable electrical energy. The energy is passed either to the battery for storage, or to the motor to run the car, though a device called power tracker, which convert it into the required voltage. The decision on whether to transfer the power to the motor or battery is made by a small onboard computer called the motor controller. It is responsible for sending the electricity smoothly to the motor when the accelerator is depressed, controlling the torque that goes to the motor such that the car maintains the desired speed. Some cars also use a process called regenerative braking, which allows some of the kinetic energy stored in the vehicle’s translating mass to be stored in the battery when the car is slowing down. A solar car is made up of many components that have been integrated together so that they work as a single system. For the ease of explanation it has been broken down into five primary systems: •
Driver Controls & Mechanical Systems
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Electrical System
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Drive Train
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Solar Array
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Body and Chassis
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2. DRIVER CONTROLS & MECHANICAL SYSTEMS Solar cars do have some of the standard features found in conventional cars, such as turn signals (front & rear), brake lights, accelerator, rear view mirrors, fresh air ventilation, and usually cruise control. The drivers and passengers are protected safety harnesses and helmets. Drivers and passengers can look forward to uncomfortable seats, cramped positioning, and high cockpit temperatures as these cars have very few amenities for the driver. 2.1 REAR VISION: Mirrors mounted to a car's exterior greatly increase aerodynamic drag; therefore, an out-of-the–box thinking is required to find a solution. SUNRUNNER, a solar car developed by the University of Michigan in 1995, utilized a fibre optic cable connecting an eyepiece in the driver's area to a lens located in an aerodynamic fin mounted on top of the canopy. MAIZE & BLUE, a later model developed by the University, on the other hand, chose an electronic system consisting of a miniature camera installed in the car's trailing edge and a pocket television in the driver's area. Some cars also have externally mounted mirrors of mirrors within a bubble canopy. 2.2 VENTILATION: High temperatures are obviously bad for the driver (and passenger), but they are also bad for electrical and electronic components as high temperatures will generally reduce the efficiency and shorten the life of solar cells, batteries, motors, motor controllers and other electronic equipment.
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Figure 2: The comfortable interior temperature and air flow rate as a function of outside temperature Something like 10 kilograms of air would typically have to be provided every minute to approach passenger car comfort levels. Obviously, that's seldom feasible in a solar car due to the drag that it might impose on the vehicle, if such cooling flows are not also required by electrical, electronic and mechanical components of the vehicle. Vehicle designers usually use the same airflow several times over as it passes through the vehicle; for example cooling driver, electronics, electrics and motor sequentially. Placing a sizeable air inlet at the forward stagnation point of the vehicle minimises drag due to the opening. 'NACA ducts’ are an alternative for getting air into the car if there's a reasonably-flat, external surface nearby that doesn't have significant divergent (or convergent) flow. The mechanical systems of a solar car are designed to minimize friction and weight while maintaining the strength needed to handle the various road conditions. Lightweight metals like titanium and composites are commonly used to maximize the strength-to-weight ratio. It includes:
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2.3 STEERING: The major design factors for steering are reliability and efficient performance. The steering system is designed with precise steering alignment because even small misalignments can cause significant losses and increase tire wear. Different cars use different steering mechanisms depending on their budget and other considerations. The SUNRUNNER utilized a rack and pinion system that was attached to the steering arms by means of tie rods. TESSERACT, a single-seat high performance solar racecar, uses a centre mounted handlebars, much like that on bicycles that connect to a rack-andpinion steering system. 2.4 BRAKES: To maximize efficiency, the brakes are designed to move freely, eliminating brake drag, which is caused by brake pads rubbing against the brake surface. Hydraulic disc brakes are commonplace in solar cars because of their adjustability and good braking power. As a supplemental system, some teams have regenerative braking which allows some of the kinetic energy stored in the vehicle’s translating mass to be stored in the battery when the car is slowing down. Here the car's motor becomes a generator as regenerative braking is applied and adds energy to the batteries during deceleration. Both MAIZE& BLUE and SUNRUNNER had hydraulic disc brakes while only SUNRUNNER used regenerative braking. 2.5 SUSPENSION: Of the available front suspension variants, MacPhearson struts or double A arms are most common in solar cars. A MacPhearson strut requires a large vertical clearance since it is positioned perpendicular to the ground. Double A arms require less vertical clearance, but consist of more components. Depending
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upon the design a suitable one is chosen. The most common rear suspension is a trailing arm, similar to that found in motorcycles. Due to a single degree of motion, the trailing arm suspension allows for convenient packaging of dampers and the drivetrain. 2.6 WHEELS: Wheels, however, are the least efficient part of a solar car due to rolling resistance. About one third of the energy used by a solar car is lost due to this factor. Due to this limitation, contact with the ground should be minimized. Solar cars typically have three or four wheels. The common three-wheel configuration is two front wheels and one rear wheel (usually the driven wheel). Four-wheel vehicles are sometimes configured like a conventional vehicle (with one of the rear wheels being driven). Other four-wheel vehicles have the two rear wheels close together near the centre (similar to the common three wheel configuration). Solar car wheel designs are similar to those of bicycle tires. Generally, the wheel's rims and hubs are aluminium while the spokes are made of steel. A Mylar film is placed over the spokes to increase aerodynamic efficiency. Pneumatic tires are preferred over solid rubber tires because they weigh less and provide a smoother ride. The best tires currently available are the Bridgestone Ecopia tires made for solar cars. They are very thin and operate at over one hundred pounds/inch pressure.
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3. ELECTRICAL SYSTEM The heart of a solar car is the electrical system, which is made up of batteries and power electronics. Power electronics include the peak power trackers, the motor controller, and the data acquisition system. The primary function of the power electronics is to monitor and control the electricity within the system. 3.1 BATTERIES: A solar car uses the battery pack to store energy, which will be at a later time. The battery pack is made up of several individual modules wired together to generate the required system voltage. The types of batteries used include: •
Lead-Acid
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Nickel-Metal Hydride (NiMH)
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Nickel-Cadmium (NiCad)
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Lithium Ion
The NiCad, NiMH, and Lithium batteries offer improved power to weight ratio over the more common Lead-Acid batteries, but are more costly to maintain. The battery pack is made up of several individual modules wired together to generate the required system voltage. Typically, teams use system voltages between 84 and 108 volts, depending on their electrical system. For example, Tesseract uses 512 li-ion batteries, broken down into twelve modules, which are each equivalent to a car battery, but only weigh 5 lbs each. Through an innovative pack design, the batteries are ventilated with even airflow to minimize temperature differences between the modules.
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3.2 PEAK POWER TRACKERS: The peak power trackers condition the electricity coming from the solar array to maximize the power and deliver it either to the batteries for storage or to the motor controller for propulsion. When the solar array is charging the batteries, the peak power trackers help to protect the batteries from being damaged by overcharging. Peak power trackers can be very lightweight and commonly reach efficiencies above 95%. A maximum power point tracker (MPPT) is a DC-DC converter that matches the output of a PV string to the battery voltage in a way that maximises the power generated by the PV string. The power generated by a PV string depends on the operating voltage. PV power increases steadily with operating voltage to a maximum, and then drops off rapidly as the voltage is increased further to the open-circuit voltage. A tracker allows the PV string to always operate at the most efficient point, independently of the battery voltage. For example, if your battery voltage is 100V and the ideal operating point for an array string is 2A x 120V = 240W, the tracker output will be 2.4A x 100V = 240W. In practice, there is always a small loss of 1-2% due to inefficiencies in the tracker electronics. MPPTs are of three types: • down (buck) converters, which convert the PV voltage to a lower battery voltage; • up (boost) converters, which convert the PV voltage to a higher battery voltage; and • dual (buck-boost) converters, which will convert either way, though usually with a penalty in efficiency.
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3.2.1 FINDING THE MAXIMUM POWER POINT: There are two methods to find the maximum power point. 1. Open-circuit voltage tracking: The tracker periodically measures
the open circuit voltage, VOC, of the PV string, then sets the operating voltage to Vmp = k VOC, where k is a constant. The method is simple, and reasonably effective. This method is used by AERL trackers. 2. Power tracking: The tracker measures changes in output power as it makes small changes to the operating point, and adjusts the operating point to maximise output power. 3.3 MOTOR CONTROLLERS: This component performs the complex task of deciding how much current actually reaches the motor at a given time. This determination of current by the motor controller allows the car to accelerate, decelerate, or stay at a constant speed. The better motor controllers are up to 90% efficient. 3.4 TELEMETRY: A team's telemetry system is used for data acquisition. A commercial or custom system monitors conditions such as speed, battery voltage, power collection and consumption, and motor temperature. The system then relays that information to the driver and team strategists. Most telemetry systems allow for two-way data transmissions and are based on microcontrollers and radio modems.
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4. DRIVE TRAIN The drive train will consist of the electric motor and the means by which the motor's power is transmitted to the wheel causing the vehicle to move. Due to the low amount of power generated (less than 5 hp) usually only one wheel in the rear of the car is driven by the electric motor. The motor types that have been used in solar cars include •
brushed DC motors
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DC brushless motors
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induction motors
DC brushless motors are commonplace in solar car racing. Rare-earth, permanent magnets mounted on the rotor, reacts to magnetic fields produced by the motor's windings. Three-phase windings allow the rotor remain at constant torque. A motor controller sends signals to the windings, regulating the magnetic field around the rotor. The most common type of motor used in solar cars is the dual-winding DC brushless. It is fairly lightweight and can reach efficiencies of 98% at their rated rpm. The dual-winding motor is sometimes used as an electronic transmission. Switching between the dual windings changes the speed rating of the motor. The low speed windings provide high torque for starting and passing, while the high speed windings have higher efficiencies and are best for cruising. There are several variations of two basic types of transmissions used in solar cars. 1.
single reduction direct drive
2.
variable ratio belt drive
3.
hub motor
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In the past, the most common type was the direct drive transmission where the motor is connected to the wheel through a chain or belt with a single gear reduction. This is a reliable and easily maintained transmission if special care is taken when aligning the components. Efficiencies above 75% can be achieved when designed properly. For a variable ratio belt drive, gear ratio changes as the speed of the motor increases. This gives the motor more starting torque at lower speeds, but still allows the car to run efficiently at higher speeds. Variable belt drives require precise alignment and careful setup to work efficiently. A hub motor eliminates the need for any external transmission because the motor shaft is connected directly to the wheel hub. This greatly increases the efficiency of the drive train and reduces the number of moving parts necessary to drive the wheel. A hub motor uses low rpm to account for the lack of gear reduction, which tends to drop their efficiency slightly, but they still can achieve efficiencies in excess of 95%.
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5. SOLAR INSOLATION The energy from the sun strikes the earth throughout the entire day. However, the amount of energy changes due to the time of day, weather conditions and geographic location. The amount of available solar energy is known as the solar insolation or irradiance and is most commonly measured in watts per meter squared or W / m 2.
Figure 3: Typical solar insolation for a sunny day. Solar irradiance is generally modelled as having three components: •
direct beam irradiance,
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diffuse irradiance, from the sky, and
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reflected irradiance, from the ground.
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The sum of these components is called global irradiance. The irradiance that will fall on a surface depends on the many factors, including: •
the day of the year
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the position of the sun in the sky
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the inclination of the surface
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cloud cover.
These factors should be taken into account while designing the solar array.
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6. SOLAR ARRAY Solar cells or photovoltaics collect the energy from the sun and converts it into usable electrical energy. They are made from silicon by joining an n-type and a p-type silicon semiconductor, creating an electron rich and an electron poor layer. When sunlight strikes the cell, photons cause atoms of the semiconductor to free electrons, leaving behind positive charges. The flow of electrons thus created constitutes an electromotive force that drives the current to charge a battery or power a motor. The cell's positive contact is on the bottom while the negative contact, or bus bar, is located on the top of the cell. Each cell produces approximately .5 volts and 3 amps of current. Connecting the cells in series, i.e., positive to negative, increases voltage. Parallel connections, i.e., negative to negative and positive to positive, increase current. Therefore, connecting the cells in various series and parallel configurations produces modules of different voltages and currents.
Figure 4: Schematic Diagram of a Solar
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Figure 5: Solar Cell Diagram Cells can be grouped into space grade and terrestrial grade categories: • Space grade cells are up to 29% efficient, and are used mainly in satellite production due to their high cost. These high efficiency cells cost in excess of $500 per square inch. •
Terrestrial grade cells having a efficiency of 14%, are much cheaper causing them to be the cells of choice for solar cars. Each cell measures 10cm x 10cm, costs approximately $6.00, and produces 1.5 watts of power.
A large number of solar cells are wired together to form a solar array. The entire solar cells together form the solar array. Solar cells should also be divided into several zones. For example, if you have 750 solar cells, you might want to wire 3 sets of 250 cells, each zone producing about 125 volts. If one zone fails, two other zones are still producing power. SUNRUNNER'S array consisted of 14,057 razor blade sized, 16% efficient space grade cells. The cells are extremely fragile. So many engineers put them through a process called encapsulation. Doing so strengthens solar cell durability, but decreases the efficiency. Encapsulation is the process of coating the cells with a tougher material like resins or sandwiching it between two sheets of fibre glass,
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which prevents the cells from being damaged. For cells 14% efficient, encapsulation would reduce the overall efficiency to12.5%.
6.1 PRACTICALPROBLEMS WITH USING SOLAR CELLS 6.1. 1 I-V CURVES AND SERIES MISMATCH: All silicon solar cells put out a voltage of about 0.5V. This is because they're a kind of diode, and this is analogous to the forward break over voltage of the diode. Now, if you have several cells in series and they're all the same they'll all give the same current, and the voltage from all the cells will add up neatly. But they're not all the same. The silicon is doped very subtly differently from cell to cell, or the purity of the silicon varies, or different cells are at different temperatures. Ss some cells will give more current than others. In a series string, they can't because all the cells are constrained to give exactly the same current. This will cause many of our cells to run sub-optimally. So after the cells are tabbed, they are measured, and grouped like with like. 6.1. 2 CURVED ARRAYS: The next hazard is curves on the array. Many arrays are not perfectly flat, which means that not all cells are receiving the same amount of sunlight. They have to be arranged in such a way that all the cells in the strings receive approximately the same illumination. A common way to achieve this would be to run each string parallel to the long axis of the car, so that all the cells in a string are pointing in approximately the same direction. Several strings are often wired together to form a section or panel that has a voltage close to the nominal battery voltage.
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6.1. 3 SHADOWS AND BROKEN CELLS: Sometimes there will be shadows on the array. This could be caused by the driver bubble, or by trees or other obstructions near the road, or by passing traffic. When a cell in a string is shaded, its output goes down. Since the other cells continue to force current through it, this cell actually dissipates power instead of generating, and it gets dissipated as heat. Now that this cell is warmer, it's less efficient than the others, and so even when the light comes back, it'll want to generate less current, which means it'll wind up dissipating some power as heat. This is called Thermal runaway. This is prevented in the following way: Every cell (or, more often, every small group of cells) has a diode across it. When a cell in that group is shaded, current flows through the diode. If you have 60 cells in your string, and they're in groups of 6, then when a single cell is shaded, your output voltage will drop by 10%, as the bypass diode for that group comes into play, and your current output will drop not at all. This is better than having your voltage drop 0.6V for the dark cell, and having your current output drop by some large amount, as current is forced through the dark cell. The other time that the bypass diodes come in handy is when a cell gets damaged. This may be due to a stone being flicked up from the road, a camera falling out of someone's pocket or a small child running up the array. The damaged cell may go open-circuit, meaning that without the bypass diode, output from the string would drop to zero. With the bypass, output drops only proportionately to the percentage of cells bypassed.
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6.2 LIMITATIONS: To put the limitations of a solar car in perspective, a simple calculation will suffice. Only 1000 W/m2 of energy reaches the earth’s surface in an hour of “peak sun”. This term can be thought of as the amount of sunlight that reaches a sunny area on cloudless, summer day around noon. An average solar array configuration span 8m, meaning the total amount of energy hitting the solar car during peak sun is 8KWh/m2. Of this energy, average solar cells are only able to convert 12.5% to electricity. As a result, the total amount of converted energy available to a car consists of 1 KW/h, approximately the same amount of energy used to run a hairdryer. With cars running on 700-1500 Watts, efficiency is hypercritical. Therefore, advances in all aspects of engineering, from mechanical to electrical to materials and computer science are the key. The three primary areas of energy loss consist of aerodynamic drag, braking, and rolling resistance. To minimize aerodynamic drag, engineers make solar cells as sleek as possible. Rolling resistance is proportional to weight. Hence solar cars should be engineered to be very light.
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7. BODY & CHASSIS The most distinctive part of solar cars is their bodies. The sleek and exotic shapes are eye catching. The main goals when designing the body are to minimize the aerodynamic drag, maximize the exposure to solar insolation, minimize weight, and maximize safety.
Figure 6: Body and Chassis of a Solar Car
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7.1 BODY SHAPES Although cars differ in design, their shapes can be grouped into four categories. A unified aero body and panel allows for a small frontal area, low weight, and a wide range of visibility around the canopy. Fixed or tilting, flat panels with a separate driver cab are simple, lightweight, and inexpensive to construct; however, aerodynamic efficiency is compromised due to exposed suspension components and vulnerability to cross winds. Catamaran shapes offer reduced frontal area and low aerodynamic drag. For north/south race routes, the curved array becomes very powerful in the early morning and late afternoon as the sun travels across the horizon. Finally, are uniquely designed vehicles whose aerodynamic efficiency and power collection capabilities differ from design to design. MAIZE & BLUE and SUNRUNNER were catamaran shaped.
Figure 8: Body shapes of common solar cars
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7.2 CHASSIS Generally, there are three types of chassis used in solar cars: 1.
space frame
2.
semi-monocoque or carbon beam
3.
monocoque
A space frame uses a welded tube structure to support the loads and the body. The body is a lightweight, non-load bearing, composite shell that is attached to the chassis separately. The semi-monocoque or carbon beam chassis uses composite beams and bulkheads to support the loads and is integrated into a non-load bearing composite belly pan. The top sections of the car are often separate body pieces that are attached to the belly pan. A monocoque chassis uses the body structure to support the loads. Many solar cars use a combination of the chassis categories mentioned above. The image above is an example of a semi-monocoque chassis with an integrated space frame used to protect the driver.
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8. MATERIALS USED A composite material is the combination of a filler material sandwiched between layers of a structural material. Carbon fibre, Kevlar and fibreglass are common composite structural materials. Honeycomb and foam are common composite filler materials. These materials are bonded together using epoxy resins and in the cases of Kevlar and carbon fibre, can obtain impressive strengths (equal to steel) but remain very lightweight. SUNRUNNER used Kevlar as the fabric with a Nomex honeycomb spacer while MAIZE& BLUE used carbon fibre fabric.
9. FALLING SHORT There are several characteristics that a commercially viable car must have. Commercial cars typical can hold at least 4 passengers. It must be extremely reliable, comfortable, and be able to function on its own. It must also be able to maintain the required speed. In addition, commercial cars typically have amenities such as air conditioning, radio, and power locks and windows. Solar vehicles when driven on highways, experienced many flat tires and often were incapable of maintaining highway speeds of fifty-five miles per hour. With the energy available to solar cars, the type of amenities described above is impossible. The car is also a very cramped one. Hence it failed to break into the commercial car market as of now
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10. THE FUTURE 10.1 IN THE SHORT TERM One plausible market for solar vehicles is a terrestrial application of the rovers that NASA uses in space for data collection in a hot, sun rich area where manual labor is difficult. The vehicles would recharge autonomously, and the driver’s discomfort would not be an issue because there would not be a driver. In addition, these vehicles could be kept lightweight and simple without a need for too many amenities. 10.1.1 CARRY-OVER OF EXPERTISE: The solar car rush brought substantial advances to the design of electric vehicles, starting with the use of solar power. It led to better motors, better use of batteries, and better motor controller design which have been adopted by some electrical vehicle manufacturers. 10.1.2 COMPOSITE HULL CAR: Another key area that has been charging ahead is the composite hull car. Composites were not only lighter, but they also made cars safer than their steel counterparts. 10.1.3 HARNESSING SOLAR POWER Solar cars also helped spread the word about the use of harnessing solar power. By spreading the word about solar energy in this exciting way, the cars contributed to the rise in the use of solar cells for other, more practical uses. Lastly, it created a sport that has an educational as well as a social benefit.
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10.2 IN THE LONG RUN Whatever be its limitations, the future is definitely full of promise for the solar cars. It took us about a hundred years after electricity was invented, to develop a commercially viable electric vehicle. Similarly solar cars too need a suitable incubation period, to successfully foray into the commercial vehicle segment. With the crude prices hitting upwards of $50 a barrel and still looking bullish, it is certain that the current preference for petroleum based automobiles will change in the not too distant future. The solar car with no fuel expenses will certainly be preferred for short distance commutation in the future. Though it may offer only a significantly reduced performance compared to the conventional vehicle it will then be looked upon as a cost effective option. Also, there are many areas of the solar car, which can be improved upon, starting with the solar array. At present the solar array is only 12.5% efficient. What would be the case if it were made atleast 50% efficient? I’m sure that much of the current problems in solar cars can be overcome.
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CONCLUSION The solar cars are used exclusively for racing in tournaments, at present. Though they have been around for about twenty five years now, the technology is still in the developmental stages. Hence they can not be used as a practical means of transport. The challenge lies in making it a viable means of transport. Further research is needed in this regard to improve solar panels, reduce weight, to improve reliability and to reduce the cost. Research is being carried out on many semi-conductors and their alloys to develop more efficient solar cells. It can be safely assumed that with the advent of mass production there would be greatly reduced. Thus this technology will definitely live up to its potential some time in the future.
Louis Palmer standing in the Solartaxi
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REFERENCES 1. http://www.americansolarcarchallenge.org 2. http://www.solarcar.mcmaster.ca 3. http://www.formulasun.org 4. http://scg.levels.unisa.edu.au/src/pmwiki.php 5. http://www.raccoon.com/~cpraven/thesis/ 6. http://www.umr.edu/~dougc/solar/sun.html 7. http://web.umr.edu/~wif/experimental/Beijing.Kevlar.html 8. http://sunsite.anu.edu.au/questacon/aimscc_main.html 9. http://www.wikipedia.com
Ned, constructed in 1999 by the South Australian Solar Car Consortium, can speed up to 120km/h.
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