Solar car racing
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Solar car racing refers to competitive races of electric vehicles which are powered by solar energy obtained from solar panels on the surface of the car.
Solar car races are often sponsored by government agencies who are keen to promote the development of alternative energy technology (such as solar cells). Such challenges are often entered by universities to develop their students' engineering and technological skills, but many business corporations have entered competitions in the past. A small number of high school teams participate in solar car races designed exclusively for high school students.
[edit] Notable distance races
The two most notable solar car distance (overland) races are the World Solar Challenge and the North American Solar Challenge. They are contested by a variety of university and corporate teams. Corporate teams contest the race to give its design teams experience in working with both alternative energy sources and advanced materials (although some may view their participation as mere PR exercises). University teams enter the races because it gives their students experience in designing high technology cars and working with environmental and advanced materials technology. These races are often sponsored by agencies such as the US Department of Energy keen to promote renewable energy sources.
The cars require intensive support teams similar in size to professional motor racing teams. This is especially the case with the World Solar Challenge where sections of the race run through very remote country.
[edit] World Solar Challenge
This race features a field of competitors from around the world who race to cross the Australian continent. In 2005, the Dutch Nuna 3 team won this challenge for a 3rd time in a record average speed of 102.75 km/h over a distance of 3000 km, followed by the Australian Aurora (92.03 km/h) and the University of Michigan (90.03 km/h). The increasingly high speeds of the 2005 race participants has led to the rules being changed for future solar cars starting in the 2007 race.
The 20th Anniversary race of the World Solar Challenge ran in October of 2007. Major regulation changes were released in June 2006 for this race to increase safety, to build a new generation of solar car, which with little modification could be the basis for a practical proposition for sustainable transport and intended to slow down cars in the main event, which could easily exceed the speed limit (110 km/h) in previous years. The winner again was the Nuna 4 team averaging 90.87 km/h. The winner in the Adventure Class (driving under old rules) was the Ashiya University Solar Car Project team averaging 93.57 km/h.[1]
[edit] North American Solar Challenge
The North American Solar Challenge, previously known as the 'American Solar Challenge' and 'Sunrayce USA', features mostly collegiate teams racing in timed intervals in the United States and Canada.
The North American Solar Challenge was sponsored in part by the US Department of Energy. However, funding was cut near the end of 2005, and the NASC 2007 was cancelled. The North American solar racing community worked to find a solution, bringing in Toyota as a primary sponsor for a 2008 race.[2][3] The next North American Solar Challenge will run from July 13-21, 2008, from Dallas, Texas to Calgary, Alberta.
[edit] Other races
- Suzuka, a yearly track race in Japan.
- Phaethon[1], part of the Cultural Olympiad in Greece prior to the 2004 Olympics.
- World Solar Rally.
- Dell-Winston School Solar Car Challenge is the best-known and longest-running high-school-level race.
[edit] Solar drag races
Solar drag races are another form of solar racing. Unlike long distance solar races, solar dragsters do not use any batteries or pre-charged energy storage devices. Racers go head-to-head over a straight quarter kilometer distance. Currently, a solar drag race is held each year on the Saturday closest to the summer solstice in Wenatchee, Washington, USA. The world record for this event is 29.5 seconds set by the South Whidbey High School team on June 23, 2007.[4]
[edit] Vehicle Design
Solar cars combine technology used in the aerospace, bicycle, alternative energy and automotive industries. Unlike most race cars, solar cars are designed with severe energy constraints imposed by the race regulations. These rules limit the energy used to only that collected from solar radiation, albeit starting with a full charged battery pack. As a result optimizing the design to account for aerodynamic drag, vehicle weight, rolling resistance and electrical efficiency are paramount. Conventional thinking has to be challenged, for example, rather than a conventional automobile seat which would weigh tens of pounds, one championship solar car employed a nylon mesh seat combined with a five-point harness that weighed less than 3 pounds.
Solar race cars can be designed with a variety of basic configurations by varying the shape of the vehicle, the number and location of wheels, the location of solar cells, and other variables. These trade off the efficiency of the panel against aerodynamics, weight, controllability, and ease of manufacture. Since 1996 the leading WSC cars have tended to have a small canopy in the middle of a curved wing-like array, entirely covered in cells, with 3 wheels. Before then the cockroach style, as used in the GM Sunraycer with a smooth nose fairing into the panel were more successful. At lower speeds, with less powerful arrays, other configurations are viable and may be easier to construct.
[edit] Driver's cockpit
Like many race cars, the driver's cockpit usually only contains room for one person, although a few cars do contain room for a second passenger. They contain some of the features available to drivers of traditional vehicles such as brakes, accelerator, turn signals, rear view mirrors (or camera), ventilation, and sometimes cruise control. A radio for communication with their support crews is almost always included.
Solar cars are often fitted with gauges as seen in conventional cars. Aside from keeping the car on the road, the driver's main priority is to keep an eye on these gauges to spot possible problems. Cars without gauges available for the driver will almost always feature wireless telemetry. Wireless telemetry allows the driver's team to monitor the car's energy consumption, solar energy capture and other parameters and free the driver to concentrate on just driving. Drivers also have a safety harness, and optionally (depending on the race) a helmet similar to racing car drivers.
[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 store less energy for a given mass. Typically, solar cars use voltages between 84 and 170 volts.
Power electronics monitor and regulate the car's electricity. Components of the power electronics include the peak power trackers, the motor controller and the data acquisition system.
The peak power trackers manage the power coming from the solar array to maximize the power and deliver it to be stored in the motor. They also protect the batteries from overcharging. The motor controller manages the electricity flowing to the motor according to signals flowing from the accelerator.
Many solar cars have complex data acquisition systems that monitor the whole electrical system while even the most basic cars have systems that provide information on battery voltage and current to the driver. One such system utilizes Controller Area Network (CAN).
A wide variety of motor types have been used. Usually there is astrong relationship between efficiency and cost. The most efficient motors exceed 98% efficiency. These are brushless 3 'phase' DC, electronically commutated, wheel motors, with a Halbach array configuration for the neodymium-iron-boron magnets, and Linz wire for the windings.[5] Cheaper alternatives include motors from wind turbines, or brushed DC motors.
[edit] Mechanical systems
The mechanical systems are designed to keep friction and weight to a minimum while maintaining strength and stiffness. Designers normally use aluminium, titanium and composites to provide a structure that meets strength and stiffness requirements whilst being fairly light. Steel is used for some suspension parts on many cars.
Solar cars usually have three wheels, but some have four. Three wheelers usually have two front wheels and one rear wheel: the front wheels steer and the rear wheel follows. Four wheel vehicles are set up like normal cars or similarly to three wheeled vehicles with the two rear wheels close together.
Solar cars have a wide range of suspensions because of varying bodies and chassis. The most common front suspension is the double wishbone suspension. The rear suspension is often a trailing-arm suspension as found in motor cycles.
Solar cars are required to meet rigorous standards for brakes. Disc brakes are the most commonly used due to their good braking ability and ability to adjust. Mechanical and hydraulic brakes are both widely used. The brake pads or shoes are typically designed to retract to minimize brake drag, on leading cars.
Steering systems for solar cars also vary. The major design factors for steering systems are efficiency, reliability and precision alignment to minimize tire wear and power loss. The popularity of solar car racing has led to some tire manufacturers designing tires for solar vehicles. This has increased overall safety and performance.
All the top teams now use wheel motors, eliminating belt or chain drives.
Testing is essential to demonstrating vehicle reliability prior to a race. It is easy to spend a hundred thousand dollars to gain a two hour advantage, and equally easy to lose two hours due to reliability issues.
[edit] Solar array
The solar array consists of hundreds (or thousands) of photovoltaic solar cells converting sunlight into electricity. Cars can use a variety of solar cell technologies; most often polycrystalline silicon, monocrystalline silicon, or gallium arsenide. The cells are wired together into strings while strings are often wired together to form a panel. Panels normally have voltages close to the nominal battery voltage. The main aim is to get as much cell area in as small a space as possible. Designers encapsulate the cells to protect them from the weather and breakage.
Designing a solar array is more than just stringing a 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 design 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.[6] Many races, including the WSC and NASC, consider wind energy to be solar energy, so their race regulations allow this practice.
[edit] Aerodynamics
Aerodynamic drag is the main source of losses on a solar race car. The aerodynamic drag of a vehicle is the product of the frontal area and its Cd. For most solar cars the frontal area is 0.75 to 1.3 m^2. While Cds as low as 0.10 have been reported, 0.13 is more typical. This needs a great deal of attention to detail.[7]
[edit] Mass
The vehicle's mass is also a significant factor. A light vehicle generates less rolling resistance and will need smaller lighter brakes and other suspension components. This is the virtuous circle when designing lightweight vehicles.
[edit] Rolling resistance
Rolling resistance can be minimised by using the right tires, inflated to the right pressure, correctly aligned, and by minimising the weight of the vehicle.
[edit] Performance Equation
The design of a solar car is governed by the following work equation:
which can be usefully simplified to the performance equation
for long distance races, and values seen in practice.
Briefly, the left hand side represents the energy input into the car (batteries and power from the sun) and the right hand side is the energy needed to drive the car along the race route (overcoming rolling resistance, aerodynamic drag, going uphill and accelerating). Everything in this equation can be estimated except v. The parameters include:
-
- 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 = Race route distance (meters)
- W = Weight of the vehicle (newtons)
- Crr1 = First coefficient of rolling resistance (non-dimensional)
- Crr2 = 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 = acceleration due to gravity constant (meters per second squared)
- v = Average velocity over the route (meters per second)
Solving the long form of 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 CAE and systems modeling, the power equation can be a useful tool in solar car design.
[edit] Race route considerations
The directional orientation of a solar car race route affects the apparent position of the sun in the sky during a race day, which in turn affects the energy input to the vehicle.
- In a south-to-north race route alignment, for example, the sun would rise over the driver's right shoulder and finish over his left (due to the east-west apparent motion of the sun).
- In an east-west race route alignment, the sun would rise behind the vehicle, and appear to move in the direction of the vehicle's movement, setting in the front of the car.
- A hybrid route alignment includes significant sections of south-north and east-west routes together.
This is significant to designers, who seek to maximize energy input to a panel of solar cells (often called an "array" of cells) by designing the array to point directly toward the sun for as long as possible during the race day. Thus, a south-north race car designer might increase the car's total energy input by using solar cells on the sides of the vehicle where the sun will strike them (or by creating a convex array coaxial with the vehicle's movement). In contrast, an east-west race alignment might reduce the benefit from having cells on the side of the vehicle, and thus might encourage design of a flat array.
Because solar cars are often purpose-built, and because arrays do not usually move in relation to the rest of the vehicle (with notable exceptions), this race-route-driven, flat-panel versus convex design compromise is one of the most significant decisions that a solar car designer must make.
For example, the 1990 and 1993 Sunrayce USA events were won by vehicles with significantly convex arrays, corresponding to the south-north race alignments; by 1997, however, most cars in that event had flat arrays to match the change to an east-west route.
[edit] Race strategy
[edit] Energy consumption
Optimizing energy consumption is of prime importance in a solar car race. Therefore it is very important to be able to closely monitor the speed, energy consumption, energy intake from solar panel, among other things in real time[citation needed]. Some teams employ sophisticated telemetry that relays vehicle performance data to a computer in a following support vehicle.
The strategy employed depends upon the race rules and conditions. Most solar car races have set starting and stopping points where the objective is to reach the final point in the least amount of total time. Since aerodynamic drag force rises quadratically with speed, the energy the car consumes per second rises cubically[citation needed] (per meter travelled it rises quadratically with speed). Given the varied conditions in all races and the limited (and continuously changing) supply of energy, most teams have race speed optimization programs that continuously update the team on how fast the vehicle should be traveling.
[edit] Race route
The race route itself will affect strategy, because the apparent position of the sun in the sky will vary depending various factors which are specific to the vehicle's orientation (see "Race Route Considerations," above).
In addition, elevation changes over a race route can dramatically change the amount of power needed to travel the route. For example, the 2001 and 2003 North American Solar Challenge route crossed the Rocky Mountains (see graph at right).
[edit] Weather forecasting
A successful solar car racing team will need to have access to reliable weather forecasts in order to predict the power input to the vehicle from the sun during each race day.
[edit] See also
[edit] References
- ^ WSC 2007 Final Results
- ^ Official NASC2008 Announcement
- ^ Official NASC Website
- ^ solar drag
- ^ In-wheel motor for solar-powered electric vehicles: technical details (Publication - Technical)
- ^ The Leading Edge, Tamai, Goro, Robert Bently, Inc., 1999, p. 137
- ^ Roche, Schinkel, Storey, Humphris & Guelden, "Speed of Light." ISBN 0 7334 1527 X
- ^ Solar Vehicle Performance, Dr. Eric Slimko, December 1, 1991
[edit] External links
- Howstuffworks.com: How solar cars work
- Solar Decathlon Web site.
- Solar cars in inventors.about.com
- American Solar Challenge on solar cars article
- World Solar Challenge website
- North American Solar Challenge 2005
- International Solar Car A - Z
- The Dell-Winston Solar Challange
- South African Solar Challenge
- http://web.ew.usna.edu/~bruninga/APRS-SPHEV.html
- How solar cars work
