Solar car
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A solar car is an electric vehicle powered by solar energy obtained from solar panels on the surface of the car. Photovoltaic (PV) cells convert the sun's energy directly into electrical energy. Solar cars are not practical day-to-day transportation devices at present, but are primarily demonstration vehicles and engineering exercises. Solar cars compete in races (often called rayces) such as the World Solar Challenge and the American Solar Challenge. These events are often sponsored by government agencies, such as the United States Department of Energy, who are keen to promote the development of alternative energy technology (such as solar cells). Such challenges are often entered by universities to develop their students' engineering and technological skills, but many professional teams have entered competitions as well, including teams from GM and Honda.
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[edit] Design
Solar cars combine technology typically used in the aerospace, bicycle, alternative energy and automotive industries. Unlike typical race cars, solar cars are designed with severe energy constraints imposed by the race regulations. These rules limit the energy to only that collected from solar radiation and as a result optimizing the design to account for aerodynamic drag, vehicle weight, rolling resistance and electrical efficiency are paramount. Conventional thinking has to be challenged, for example, rather than a conventional automobile seat which would weigh tens of pounds, one solar car designed by Ford and General Motors for University of Michigan employed a nylon mesh seat combined with a five-point harness that weighed less than 3 pounds.
The design of a solar car is governed by the power equation:
Briefly, the left hand side represents the energy input into the car (batteries and power from the sun) and the right hand side is the energy needed to drive the car along the race route (overcoming rolling resistance, aerodynamic drag, going uphill and accelerating). Everything in this equation can be estimated except v. The parameters include:
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- η = Motor, controller and drive train efficiency (decimal)
- ηb = Watt-hour battery efficiency (decimal)
- E = Energy available in the batteries (joules)
- P = Estimated average power from the array (watts)
- x = Daily race route distance (meters)
- W = Weight of the vehicle (newtons)
= First coefficient of rolling resistance (non-dimensional)
= Second coefficient of rolling resistance (newton-seconds per meter)- N = Number of wheels on the vehicle (integer)
- ρ = Air density (kilograms per cubic meter)
- Cd = Coefficient of drag (non-dimensional)
- A = Frontal area (meters squared)
- h = Total height that the vehicle will climb (meters)
- Na = Number of times the vehicle will accelerate in a race day (integer)
- g = Gravity constant (meters per second squared)
- v = Average velocity over the route (meters per second)
- η = Motor, controller and drive train efficiency (decimal)
Solving the equation for velocity results in a large equation (approximately 100 terms). Using the power equation as the arbiter, vehicle designers can compare various car designs and evaluate the comparative performance over a given route. Combined with CAD and systems modeling, the power equation is a useful tool in solar car design.
[edit] Driver's cockpit
The driver's cockpit usually only contains a single seat, although a few cars do contain room for a second passenger. They contain some of the features available to drivers of traditional vehicles such as brakes, accelerator, turn signals, rear view mirrors (or camera), ventilation, and sometimes cruise control. A radio for communication with their support crews is almost always included.
Solar cars are fitted with some gauges seen in conventional cars. Aside from keeping the car on the road, the driver's main priority is to keep an eye on these gauges to spot possible problems. Drivers also have a safety harness, and optionally (depending on the race) a helmet similar to racing car drivers.
[edit] Electrical system
The electrical system is the most important part of the car's systems as it controls all of the power that comes into and leaves the system. The battery pack plays the same role in a solar car that a petrol tank plays in a normal car in storing power for future use. Solar cars use a range of batteries including lead-acid batteries, nickel-metal hydride batteries (NiMH), Nickel-Cadmium batteries (NiCd), Lithium ion batteries and Lithium polymer batteries. Lead-acid batteries are less expensive and easier to work with but have less power to weight ratio. Typically, solar cars use voltages between 84 and 170 volts.
Power electronics monitor and regulate the car's electricity. Components of the power electronics include the peak power trackers, the motor controller and the data acquisition system.
The peak power trackers manage the power coming from the solar array to maximize the power and deliver it to be stored in the motor. They also protect the batteries from overcharging. The motor controller manages the electricity flowing to the motor according to signals flowing from the accelerator.
Many solar cars have complex data acquisition systems that monitor the whole electrical system while even the most basic cars have systems that provide information on battery voltage and current to the driver. One such system utilizes Controller Area Network (CAN).
[edit] Drive train
The setup of the motor and transmission is unique in solar cars. The electric motor normally drives only one wheel (usually at the back of the car) due to the low amount of power it generates. Solar car motors are normally rated at between 2 and 10 hp (1 and 7.5 kW); the most common type of motor is a dual-winding DC brushless. The dual-winding motor is sometimes also used as a transmission because multi-geared transmissions are rarely used.
There are three basic types of transmissions used in solar cars:
- a single reduction direct drive
- a variable ratio drive belt
- a direct drive transmission (hub motor)
There are several varieties of each type. The most common is the direct drive transmission.
[edit] Mechanical systems
The mechanical systems are designed to keep friction and weight to a minimum while maintaining strength. Designers normally use titanium and composites to ensure a good strength-to-weight ratio.
Solar cars usually have three wheels, but some have four. Three wheelers usually have two front wheels and one rear wheel: the front wheels steer and the rear wheel follows. Four wheel vehicles are set up like normal cars or similarly to three wheeled vehicles with the two rear wheels close together.
Solar cars have a wide range of suspensions because of varying bodies and chassis. The most common front suspension is the double-A-arm suspension found in traditional cars. The rear suspension is often a trailer-arm suspension found in motor cycles.
Solar cars are required to meet rigorous standards for brakes. Disc brakes are the most commonly used due to their good braking ability and ability to adjust. Mechanical and hydraulic brakes are both widely used with the brakes designed to move freely by minimise brake drag.
Steering systems for solar cars also vary. The major design factors for steering systems are efficiency, reliability and precision alignment to minimise tire wear and power loss. The popularity of solar car racing has led to some tire manufacturers designing tires for solar vehicles. This has increased overall safety and performance.
[edit] Solar array
The solar array consists of hundreds of photovoltaic solar cells converting sunlight into electricity. Cars can use a variety of solar cell technologies; most often polycrystalline silicon, monocrystalline silicon, or gallium arsenide. The cells are wired together into strings while strings are often wired together to form a panel. Panels normally have voltages close to the nominal battery voltage. The main aim is to get as many cells in as small a space as possible. Designers encapsulate the cells to protect them from the weather and breakage.
Designing a solar array is more than just stringing bunch of cells together. A solar array acts like a lot of very small batteries all hooked together in series. The total voltage produced is the sum of all cell voltages. The problem is that if a single cell is in shadow it acts like a diode, blocking the flow of current for the entire string of cells. To correct against this, array designers use by-pass diodes in parallel with smaller segments of the string of cells, allowing current to flow around the non-functioning cell(s). Another consideration is that the battery itself can force current backwards through the array unless there are blocking diodes put at the end of each panel.
The power produced by the solar array depends on the weather conditions, the position of the sun and the capacity of the array. At noon on a bright day, a good array can produce over 2 kilowatts (2.6 hp).
Some cars have employed free standing or integrated sails to harness wind energy[2], which is allowed by the race regulation.
[edit] Bodies and chassis
Solar cars have very distinctive shapes as there are no established standards for design. Designers aim to minimize drag, maximize exposure to the sun, minimize weight and make vehicles as safe as possible.
In chassis design the aim is to maximize strength and safety while keeping the weight as low as possible. There are three main types of chassis:
- space frame
- semi-monocoque or carbon stream
- monocoque
The space frame uses a welded tubed structure to support the body which is a lightweight composite shell attached to the body. The semi-monocoque chassis uses composite beams and bulkheads to support the weight and is integrated into the belly with the top sections often being attached to the body. A monocoque structure uses the body of the car as an integrated load bearing structure.
Composite materials are widely used in solar cars. Carbon fiber, Kevlar and fiberglass are common composite structural materials while foam and honeycomb are commonly used filler materials. Epoxy resins are used to bond these materials together. Carbon fiber and Kevlar structures can be as strong as steel but with a much lighter weight.
[edit] Race Strategy
Optimizing energy consumption is of prime importance in a solar car race. Therefore it is very important to be able to closely monitor the speed, energy consumption, energy intake from solar panel, among other things in real time. Some teams employ sophisticated telemetry that relays vehicle performance data to a computer in a following support vehicle.
The strategy employed depends upon the race rules and conditions. Most solar car races have set starting and stopping points where the objective is to reach the final point in the least amount of total time. Since aerodynamic drag rises exponentially with speed, the energy the car consumes also rises exponentially. This simple fact means that the optimum strategy is to travel at a single steady speed during all phases of the race. Given the varied conditions in all races and the limited (and continuously changing) supply of energy, most teams have race speed optimization programs that continuously update the team on how fast the vehicle should be traveling.
[edit] Solar car races
The two most notable solar car races are the World Solar Challenge and the North American Solar Challenge. They are contested by a variety of university and corporate teams. Corporate teams contest the race to give its design teams experience in working with both alternative energy sources and advanced materials (although some may view their participation as mere PR exercises). GM and Honda are among the companies who have sponsored solar teams. University teams enter the races because it gives their students experience in designing high technology cars and working with environmental and advanced materials technology. These races are often sponsored by agencies such as the US Department of Energy keen to promote renewable energy sources.
The cars require intensive support teams similar in size to professional motor racing teams. This is especially the case with the World Solar Challenge where sections of the race run through very remote country.
There are other races, such as Suzuka, Phaethon, and the World Solar Rally. Suzuka is a yearly track race in Japan and Phaethon was part of the Cultural Olympiad in Greece right before the 2004 Olympics.
The 2005 North American Solar Challenge, which ran from Austin, Texas, to Calgary, Canada, was the successor of the American Solar Challenge. The ASC ran in 2001 and 2003 from Chicago, Illinois, to Claremont, California along old Route 66. The ASC was in turn the successor to the old GM Sunrayce, which was run across the country in 1990, 1993, and then every two years through 1999.
The 2005 North American Solar Challenge had two classes:
- Open: where teams are allowed to use space-grade solar cells - won by the University of Michigan.
- Stock: limits the type of cells that can be used on solar arrays - won by Stanford University.
The North American Solar Challenge was sponsored in part by the US Department of Energy. However, funding was cut near the end of 2005, and the 2007 NASC will not happen. Recently, however, prospects of a NASC-like race in 2008 are looking up. 18 teams from around North America attended a conference in Topeka, KS on October 20-21 to decide upon rules for a 2008 race. Assuming that funding is found, this race is fairly sure to occur.
The 20th Anniversary race of the World Solar Challenge will be run in October of 2007, and is already shaping up to be a race to remember. Major regulation changes were released in June 2006 for this race, intended to slow down cars in the main event, which had been approaching the speed limit in previous years.
[edit] Solar bicycles and motorcycles
The first solar "cars" were actually tricycles or quadricycles built with bicycle technology. These were called solarmobiles at the first solar race, the Tour de Sol in Switzerland in 1985 with about 60 participants, 30 using exclusively solar power and 30 solar-human-powered hybrids. A few true solar bicycles were built, either with a large solar roof, a small rear panel, or a trailer with a solar panel. Later more practical solar bicycles were built with foldable panels to be set up only during parking. Even later the panels were left at home, feeding into the electric mains, and the bicycles charged from the mains. Today highly developed electric bicycles are available and these use so little power that it costs little to buy the equivalent amount of solar electricity. The "solar" has evolved from from actual hardware to an indirect accounting system. The same system also works for electric motorcycles, which were also first developed for the Tour de Sol. a solar car doesn't produce pollution.
[edit] Practical applications
Solar cars achieve their performance by extreme lightness of weight, and very efficient aerodynamics that force compromises that would not be acceptable in a day-to-day transportation device. Any vehicle built for passenger comfort and meeting contemporary safety standards would be much less aerodynamic and much heavier, thus requiring much more power to achieve highway speeds. Therefore, with current and foreseeable technologies it is unlikely a pure solar car will become commercially available. However, solar cars are essentially electric cars with an inbuilt recharging capability, so some of the engineering knowledge and technology developed in competition solar cars may help the development of battery electric vehicles and even hybrid vehicles. The Venturi AstroLab in 2006 was hailed as the world's first commercial electro-solar hybrid car due to be released in January 2008.[3]
The question arises as to whether, if battery electric vehicles become popular, it will be worthwhile fitting them with solar cells to extend their range and allowing recharge while parked anywhere in the sun. However, with present and near-term engineering considerations, it seems that the best place for solar cells will generally be on the roofs of buildings, where they are always exposed to the sky and weight is largely irrelevant, rather than on vehicle roofs.
One practical application for solar powered vehicles is possibly golf buggies, some of which are used relatively little but spend most of their time parked in the sun.
[edit] See also
[edit] Notes
- ^ Solar Vehicle Performance, Dr. Eric Slimko, December 1, 1991
- ^ The Leading Edge, Tamai, Goro, Robert Bently, Inc., 1999, p. 137
- ^ The first commercial solar-electric hybrid car
