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The state of the race in photovoltaics and composites.
By Marie O’Mahony
If you want to stay abreast of future trends in solar, engineered and composite materials, you may want to tune in to a car race—the World Solar Challenge® in Australia. The race, in which solar racecars speed 1,864 miles through the Australian Outback, attracts the most creative minds from engineering, automotive and aerospace colleges and industries.
Race with the sun
Not surprisingly, the race acts as a showcase for photovoltaics (PV), which the teams use to harness solar power and convert it into energy to run the vehicles. The tightly regulated rules of the race require that participants use only solar energy as a source of power. Regulations also include a stipulation that the surface area of the PVs cannot exceed 6m2 (64.6 square feet). The efficiency, weight and positioning of the PVs is crucial to the success of the design.
The Dutch Solar Team Twente, a group of 18 college students from the University of Twente and Saxion College who have participated in several World Solar Challenge races, uses a special Fresnel lens system to capture sunlight from an area 7.2m2 (77.5 square feet) and concentrate it into the maximum surface area of 6m2. The positioning of the PVs allows for as much sunlight as possible to be captured, which is accomplished by using an innovative pivoting body to connect the PV panel to the solar car. Movement is achieved through the use of a thermoplastic elastomer that has a similar flexibility to rubber, allowing the panel to tilt so that the wing follows the direction of the sun. The team estimates that the combination of the two results in a 25 percent increase in energy.
The Australian University of New South Wales (UNSW) Solar Racing Team (Sunswift) won the Challenge Class Silicon award for vehicles deriving power exclusively from silicon-based photovoltaics. The team uses SunPower silicon single-crystal solar cells that incorporate TopCells to run the telemetry system in the car. A method of encapsulation is used to texture the cells through an etching process. This serves to maximize the absorption of the sun’s rays, which are further enhanced by the use of maximum power point trackers and 99-percent-efficient motor controllers. These cells ensure that as much power as possible is delivered to the wheels, while any surplus energy is stored in the lithium ion batteries used by peripheral devices.
The Dutch team from the Delft University of Technology in the Netherlands (TU Delft) used more than two thousand gallium arsenide photovoltaic cells in its racecar design for the Nuna5. Microscopic triangular prisms cover the surface of the cells using a special laminating process, which increases efficiency of the PV cells by redirecting the sun’s rays to be perpendicular to the cells.
Winning composites
The aerospace industry has long recognized the need for lighter aircraft to save on fuel costs, and the automotive industry is now following their lead. Composites are increasingly being used as a lighter alternative to heavier metal parts throughout vehicles. In his seminal book titled Lightness, Adriaan Beukers of TU Delft emphasizes the importance of the weight-to-strength ratio in transportation. “Now lightness, or performance per energy unit, is quickly gaining significance again because … cheap energy is getting scarce.”
Solar Team Twente worked with Dutch textile manufacturer TenCate Advanced Composites on its vehicle, using composite materials that are largely derived from the space and aerospace industries. The structural parts of the car are constructed using Centex®, a thermoplastic glass-reinforced carbon laminate. The curved areas of the car design include carbon, glass and aramid fibers to further reduce the overall weight without any noticeable loss of strength. An additional benefit is that the materials allow for a smoother and more aerodynamic finish, which translates into 25 percent less air resistance than earlier designs.
The Nuon Solar Team from TU Delft designed a vehicle that measures slightly less than one meter in height and weighs 160kg (352lbs), with the driver adding an additional 80kg (176lbs). Textile composites form the basis for this vehicle. Working with Schaap Composites, the company behind the ABN Amro Volvo Ocean Racer, the team used carbon and aramid fibers to form the main part of the composite for the body structure, with aramid wheel caps for impact protection. The aerodynamics of the vehicle were tested in a special wind tunnel. In order to gather performance data, liquid paints were applied over the surface of the vehicle to record the flow of air.
Material advantages
Speculating on what we are likely to see in the next World Solar Challenge race in just two years, it seems clear that there will be further refinements in both solar power and lightweight materials. One possibility for new innovations in the vehicle designs is that PVs will become even more flexible, able to move and realign themselves for maximum solar gain. Jaap de Carpentier Wolf at TenCate expects that we may soon see PVs that are sprayed onto the composite in a process based on a forthcoming inkjet technology.
A second area to watch is new developments in composites. Carbon fibers may be supplemented with new yarns such as the Innegra™ S polypropylene in composite structures, and functionally gradient materials and new fiber laying processes are already under development. As the most innovative students and textile manufacturers gather to plan for the next race in 2011, the only certainty is that the race will be to the truly innovative.