PHOTOVOLTAICS: Small and efficient wins the race

Bedecked from stem to stern with solar cells, the Aurora 101 won the 1999 World Solar Challenge, a race from Darwin to Adelaide, Australia, for cars powered by photovoltaics.

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Bedecked from stem to stern with solar cells, the Aurora 101 won the 1999 World Solar Challenge, a race from Darwin to Adelaide, Australia, for cars powered by photovoltaics. The October event had 42 entries from 11 countries.

The Aurora, which has entered the race since the first event in 1987, covered the 3010-km course in 41 hours and six minutes, averaging 72.96 km/h, though it sometimes reached 100 km/h. Second- and third-place finishers made the trek in 41 hours 33 minutes and 41 hours 50 minutes, respectively.

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The car was powered by passivated-emitter, rear locally diffused (PERL) solar cells designed by Martin Green of the Photovoltaics Special Research Centre at the University of New South Wales (UNSW; Sydney, NSW, Australia). The 3800 cells had a conversion efficiency of from 21% to 25%, a cell area of 23.4 cm2, and an illumination area of 21.6 cm2. Each PERL cell is constructed on a very pure wafer of silicon, with a thin layer of silicon oxide on the top and bottom surfaces to passivate the emitter, reducing efficiency losses at those layers (see photo inset). The electrodes are made from very thin strands of silver to minimize shadows cast on the surface of the cell and are connected to the silicon through holes in the oxide layer. Areas of silicon near the rear contact are heavily doped with boron to improve the electrical contact, which is why the cell is called locally diffused.

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The top surface of the cell is etched to form inverted pyramids, reducing reflection. A double-layer antireflection coating is applied, and a rear surface mirror of aluminum-oxide-silicon completes the light-trapping structure that captures weakly absorbed long-wavelength light.

The UNSW center says PERL cells are unlikely to ever become cost-effective for conventional photovoltaic applications. But for a specialized application, such as solar-car racing, where very high efficiency is important, they are valuable, and the work done on PERL cells has helped make other photovoltaic technologies more cost-effective, the center said.

Even with these highly efficient cells, the Aurora, which used less than 1.5 kW, had to be designed to minimize air resistance and use power sparingly. "We won because of our superior aerodynamics and race energy management," said David Fewchuk, Aurora team manager.

Neil Savage

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