Since the solar cells they could afford were only 15 percent efficient in capturing sunlight and converting it into energy, the Drexel students considered how much of an advantage it would be if they could incorporate panels that actually moved with the changing angle of the sun as the car circled the course on Houston's downtown streets. But such moving parts weren't allowed under the Eco-marathon rules.
As it turned out, their solar cells produced 305 kilojoules of energy on their competition run. The car used just 240 kilojoules to traverse the six-mile course--10 laps around the Discovery Green park in downtown Houston. That meant they had a valid run. Shell Eco-marathon rules for solar vehicles require that the car not be operating on the reserve battery power, but actually be generating enough solar energy to propel the car, or more. As it turned out, Drexel's car was the only entry among the five solar prototype teams to achieve this feat. Purdue University reached the same milestone with a solar car it entered in another category, urban concept vehicles, and took home first prize for solar cars in that grouping. (In the Eco-marathon, urban concept cars must meet safety requirements to actually operate on city streets.)
Even though the efficiency of the Drexel car at its peak outstripped that of any of the internal combustion-powered vehicles at the Eco-marathon, it was not eligible for the grand prize, which is always an internal combustion car.
Shell's technical manager for the Americas race, Adrian Juergens, who works in the company's fuel research division, explained in an email: "The solar car works well as long as the sky is clear, i.e. the solar cars are dependent on the sun and the efficiency of the array panels." Those stumbling blocks didn't deter the solar competitors.
The winner of the race, Université Laval of Québec City, constructed a sleek, white, bullet-shaped pod that cruised the six-mile track at 2,565 mpg (1,090 km/l). It was the third consecutive win in the Eco-marathon Americas for the Canadian team.
(Related: "Breaking 2,500 mpg, Canadian Team Wins High-Efficiency Race")