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MIT solar-aided car shines in Tour de Sol

A vehicle built by members of MIT's Solar Electric Vehicle Club took first place in its category last month in the 1994 American Tour de Sol, the national solar and electric vehicle championship.

The Aztec, a three-wheeled, two-passenger vehicle that runs on batteries assisted by a solar array, finished first in the division of the commuter-car category. It also won an efficiency award by virtue of its energy use of approximately 45 watt-hours per mile during the event, in which cars were raced and showcased over the course of a week between New York's Battery Park City and the Franklin Institute in Philadelphia. The event is organized by the Northeast Sustainable Energy Association of Greenfield, MA.

Also taking honors were vehicles produced by Solectria Corp., an Arlington-based company headed by James Worden, who graduated from MIT in 1989 with an SB in mechanical engineering. The Solectria RS, a modified racing version of the Solectria Force (a Geo Metro altered to run on electricity rather than gas) also won in its commuter-car division (Aztec was entered in a division that set a limit on battery power). In the production vehicle category, the Solectria Long-Range E-10 electric pickup truck took fourth place and was named the event's best vehicle running on lead-acid batteries.

Aztec also runs on lead-acid batteries and has a range of about 140 miles, depending on terrain. Its 45 watt-hours per mile is the equivalent of approximately 735 miles per gallon for a gasoline-powered car. Running on experimental nickel metal hydride batteries, the Solectria Force RS broke the Tour de Sol distance record by going 214 miles on one charge. Although most of the participants were students and entrepreneurs, Ford also had an entry.

During the week-long race, vehicles recharged at night and drove about 60 miles a day toward Philadelphia, then had the opportunity to drive extra laps. Demonstrating additional daily range resulted in overall time reductions under rules of the event. However, organizers mandated minimum times for the on-road segments for reasons of safety. "It's not a speed race, but more of an energy management race," said Goro Tamai, a graduate student in mechanical engineering who has worked on Aztec as a hobby.

The Aztec was first built about three years ago but underwent several improvements in preparation for this year's Tour de Sol. A new motor, transmission and controller gave the vehicle more power, allowing it to accelerate and climb hills faster and run at greater speeds overall, Mr. Tamai said. The braking system has also been improved, with better regenerative braking (when braking, some of the car's kinetic energy is returned to the batteries). The brakes also have a better hydraulic assist capability, which improves safety, he added.

Safety and performance are important on the Tour de Sol, in which solar-electric vehicles drive on ordinary roads (and occasionally highways) alongside gas-burning cars, rather than on closed tracks. Those roads on the race route "are more demanding than typical everyday roads," because there were numerous hills and hairpin turns, Mr. Tamai noted.

To qualify as a commuter-class car, the 780-pound Aztec seats two people side by side, rather than the more aerodynamic arrangement of one behind the other. It also has headlights, seat belts, a horn and other equipment required by its vehicle registration as an experimental motorcycle.

But that's about where Aztec's resemblance to an ordinary car ends. There are no high-beams on the headlights and no windshield wipers. Instead of a gas gauge, radio and analog speedometer usually found on dashboards, the vehicle has digital readouts of volts and amps along with switches for getting readouts of other information such as motor temperature. Its body is plastered with decals from corporate sponsors who contributed components and services, such as Ciba (composite body materials), Van Dusen (composite oven-curing), True Temper (aircraft steel tubing), Alcan (aluminum), 3M (tape and plastics), Intel (computer technology), and Nissan (funding). A Cambridge business, Walnut Hill Auto Body, provided the paint job.

Aztec's appearance is a definite departure from Detroit standards as well. Its futuristic aerodynamic shape, including the large solar array attached to the sweeping back, attracted quite a few looks on the road during the Tour de Sol. In fact, the vehicle's approach was heralded more by its appearance than by its sound. "Sometimes people wouldn't even hear us coming. From the outside, it was whisper-silent, but inside it was one big echo chamber" because of the lack of an engine firewall, padding and other normal car amenities, Mr. Tamai said.

The car gets only about a tenth of its power from the solar cells, which harness about 12 percent of the solar energy hitting them. Solar cells are very expensive; to achieve 17 percent efficiency would cost more than 10 times as much, designers estimate. They are also fragile, being susceptible to damage from common hazards such as rocks tossed up by other cars.

The fragility of solar technology was demonstrated in another race last fall, the 1,860-mile Diado-Hoxan World Solar Challenge through the Australian outback. Team New England (a collaboration between the solar car clubs of MIT and the University of Lowell) entered with high hopes for its unorthodox strategy of driving without collecting solar power and periodically stopping to unfold a large solar array. However, Australia's "dust devils," or small but intense spinning winds, repeatedly damaged the solar array by twisting it, and the car had to withdraw.

"It was a fast and efficient car, but the technology needs refining," said Kathleen Allen, the MIT club's technical instructor and a part-time physics student. "For a car built in 18 weeks with prototype technology, it did very well."

The next project for the Solar Electric Vehicle Club is "Manta," a single-seat, three-wheeled car being built this summer in preparation for next summer's Sunrayce '95 from Indianapolis to Colorado. The vehicle will be much lighter than Aztec and will use proportionally five times as much solar power.

A version of this article appeared in the June 29, 1994 issue of MIT Tech Talk (Volume 38, Number 37).

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