Solar car racing

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 cars). The first solar car race was the Tour de Sol in 1985 which led to several similar races in Europe, USA and Australia.

Solar car races are often sponsored by government agencies who are keen to promote the development of alternative energy technology (such as solar cells). Such challenges are often entered by universities to develop their students' engineering and technological skills, but many business corporations have entered competitions in the past. A small number of high school teams participate in solar car races designed exclusively for high school students.

Notable distance races

The two most notable solar car distance (overland) races are the World Solar Challenge and the North American Solar Challenge. They are contested by a variety of university and corporate teams. Corporate teams contest the race to give its design teams experience in working with both alternative energy sources and advanced materials (although some may view their participation as mere PR exercisesFact|date=August 2008). 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 government or educational agencies, and businesses such as Toyota [http://www.toyota.com/about/news/community/2008/07/30-2-solarchallenge.html] keen to promote renewable energy sources.

The cars require intensive support teams similar in size to professional motor racing teams. This is especially the case with the World Solar Challenge where sections of the race run through very remote country.

World Solar Challenge

This race features a field of competitors from around the world who race to cross the Australian continent. In 2005, the Dutch Nuna 3 team won this challenge for a 3rd time in a record average speed of 102.75 km/h over a distance of 3000 km, followed by the Australian Aurora (92.03 km/h) and the University of Michigan (90.03 km/h). The increasingly high speeds of the 2005 race participants has led to the rules being changed for future solar cars starting in the 2007 race. The 20th Anniversary race of the World Solar Challenge ran in October 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. [ [http://wsc.org.au/Media.Centre/Releases/2007103101.pdf WSC 2007 Final Results] ]

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. [ [http://tech.groups.yahoo.com/group/Solar-Vehicle-Challenge/message/4824 Official NASC2008 Announcement] ] [ [http://americansolarchallenge.org/ Official NASC Website] ] The last North American Solar Challenge was run from July 13-21, 2008, from Dallas, Texas to Calgary, Alberta. The race was won by the University of Michigan Solar Car Team.

Other races

* Suzuka, a yearly track race in Japan.
* Phaethon [http://www.phaethon2004.org] , 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.

olar 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. [ [http://users.applecapital.net/~jim/solardragrace.htm solar drag] ]

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. Some vehicle classes also allow human power input. As a result optimizing the design to account for aerodynamic drag, vehicle weight, rolling resistance and electrical efficiency are paramount.

A usual design for today's successful vehicles is a small canopy in the middle of a curved wing-like array, entirely covered in cells, with 3 wheels. Before, the cockroach style with a smooth nose fairing into the panel was more successful. At lower speeds, with less powerful arrays, other configurations are viable and easier to construct, e.g. covering available surfaces of existing electric vehicles with solar cells or fastening solar canopies above them.

Electrical system

The electrical system controls all of the power entering and leaving the system. The battery pack stores surplus solar energy produced when the vehicle is stationary or travelling slowly or downhill. 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.

Power electronics may be used to optimise the electrical system. The maximum power tracker adjusts the operating point of the solar array to that voltage which produces the most power for the given conditions, e.g. temperature. The battery manager protects the batteries from overcharging. The motor controller controls the desired motor power. Many controllers allow regenerative braking, i.e. power is fed back into the battery during decceleration.

Some solar cars have complex data acquisition systems that monitor the whole electrical system, while basic cars show battery voltage and motor current. In order to judge the range available with varying solar production and motive consumption, an Ah-meter multiplies battery current and rate, thus providing the remaining vehicle range at each moment in the given conditions.

A wide variety of motor types have been used. The most efficient motors exceed 98% efficiency. These are brushless three-"phase" DC, electronically commutated, wheel motors, with a Halbach array configuration for the neodymium-iron-boron magnets, and Linz wire for the windings. [ [http://www.csiro.au/resources/pf11g.html In-wheel motor for solar-powered electric vehicles: technical details (Publication - Technical) ] ] Cheaper alternatives are asynchronous AC or brushed DC motors.

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.

olar 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 many 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 current for the entire string of cells. To design against this, array designers use by-pass diodes in parallel with smaller segments of the string of cells, allowing current around the non-functioning cell(s). Another consideration is that the battery itself can force current backwards through the array unless there are blocking diodes put at the end of each panel.

The power produced by the solar array depends on the weather conditions, the position of the sun and the capacity of the array. At noon on a bright day, a good array can produce over 2 kilowatts (2.6 hp). A 6 m^2 array of 20% cells will produce roughly 6 kWh of energy during a typical day on the WSC.

Some cars have employed free-standing or integrated sails to harness wind energy.The Leading Edge, Tamai, Goro, Robert Bently, Inc., 1999, p. 137] Many races, including the WSC and NASC, consider wind energy to be solar energy, so their race regulations allow this practice.

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 C_d. For most solar cars the frontal area is 0.75 to 1.3 m^2. While C_ds as low as 0.10 have been reported, 0.13 is more typical. This needs a great deal of attention to detail. [Roche, Schinkel, Storey, Humphris & Guelden, "Speed of Light." ISBN 0 7334 1527 X]

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.

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.

Performance Equation

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

::$eta\; left\{eta\_bE\; +\; frac\{Px\}\{v\}\; ight\}\; =\; left\{W\; C\_\{rr1\}\; +\; N\; C\_\{rr2\}\; v\; +\; frac\{1\}\{2\}\; ho\; C\_d\; A\; v^2\; ight\}x\; +Wh\; +\; frac\{N\_a\; W\; v^2\}\{2g\}$Solar Vehicle Performance, Dr. Eric Slimko, December 1, 1991]

which can be usefully simplified to the performance equation

::$eta\; left\{eta\_bEv/x\; +\; P\; ight\}\; =\; left\{W\; C\_\{rr1\}\; v\; +\; frac\{1\}\{2\}\; ho\; C\_d\; A\; v^3\; ight\}$

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:

Note 1 For the WSC the average panel power can be approximated as 7/9*nominal power.

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.

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.

Race strategy

Energy consumption

Optimizing energy consumption is of prime importance in a solar car race. Therefore it is useful to be able to continually monitor and optimise the vehicle's energy parameters. Given the variable conditions, most teams have race speed optimization programs that continuously update the team on how fast the vehicle should be traveling. Some teams employ telemetry that relays vehicle performance data to a following support vehicle, which can provide the vehicle's driver with an optimum strategy.

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).

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.

ee also

* List of solar car teams
* Race the Sun
* South African Solar Challenge
* Tour de Sol

References

External links

* [http://videos.howstuffworks.com/2007-naias-solar-car-video.htm Howstuffworks.com: How solar cars work]
* [http://www.eere.energy.gov/solar_decathlon/ Solar Decathlon Web site] .
** [http://www.energy.gov/news/5026.htm DOE announced that the third Solar Decathlon competition will be held from 2007-October 12th through the 20th] in Washington, D.C.
* [http://inventors.about.com/od/sstartinventions/a/Solar_Cars.htm Solar cars in inventors.about.com]
* [http://www.formulasun.org/education/seles9.html/ American Solar Challenge on solar cars article]
* [http://www.wsc.org.au/history.htm/ World Solar Challenge website]
* [http://www.americansolarchallenge.org/event/asc2005/index.html/ North American Solar Challenge 2005]
* [http://www.speedace.info/solar_cars.htm International Solar Car A - Z]
* [http://www.winstonsolar.org/challenge/ The Dell-Winston Solar Challange]
* [http://www.solarchallenge.org.za South African Solar Challenge]
* http://web.ew.usna.edu/~bruninga/APRS-SPHEV.html