The Tech Behind the Winning Solar Car

A mix of satellite-grade solar cells, good batteries, a new motor, and a little luck

3 min read

The race stats are impressive: 3000 kilometers over four days on zero gas, zero emissions, and an average speed of 100 kilometers per hour. That's how Japan's Tokai Challenger solar car came in first at the 2009 Global Green Challenge in Australia on 29 October, roughly three hours ahead of second place Nuon Solar Team from the Netherlands and the team from the University of Michigan, which came in third. There were 29 other also-rans as well. The race (previously called the World Solar Challenge) has taken place every two or three years since 1987 and runs from Darwin in northern Australia through the country's sun-baked center to Adelaide, the capital of South Australia.

The victorious Tokai Challenger broke a streak of four previous consecutive wins by Nunas,  a succession of cars designed and built by teams at the Netherland's Delft University of Technology. The only other Japanese team to triumph before was one from Honda Motor Co., in 1993 and 1996.

The team behind the winning vehicle was made up mostly of Tokai University engineering students plus a couple of advisers, including Hideki Kimura, a professor in the university's department of electrical and electronic engineering. He says there were several reasons behind Tokai's win, but essentially, "Our solar-car design was superior, and we were also lucky: Only one puncture, and one power-tracking circuit needed replacing."

To a great extent, the team made its own luck. In choosing the technologies to make the Tokai Challenger, it opted for the best available, granting sponsorship rights in return. For solar power, it turned to Sharp Corp., which supplied compound solar cells used to energize space satellites that boasted a conversion efficiency of 30 percent, considerably higher than the 20 percent efficiency level typical of even advanced crystalline-silicon solar cells. The compound materials enable these cells to absorb more of the available light spectrum than silicon but are more expensive to make.

Each compound cell is composed of three main layers: germanium at the bottom, indium gallium arsenide (InGaAs) or gallium arsenide (GaAs) in the middle, and indium gallium phosphide (InGaP) on top. Just before the race began, Sharp announced the development of a new compound solar cell with a 35.8 percent conversion efficiency, the world's highest, according to the company. Sharp achieved this by replacing the bottom germanium layer with one of InGaAs, enabling it to absorb more of the light. The cell is constructed using an inverted growth fabrication process, where the first layer to be grown is the InGaP, then GaAs, and then the InGaAs layer last. The layers are then flipped so that the original bottom layer becomes the top layer in the cell. This is done to achieve high crystal quality in the middle and top layers. The company said it aims to commercialize the new solar cells by 2012, targeting the satellite market.

More than 2000 of the current compound cells were mounted over the surface of the one-man three-wheeled vehicle; they produced a peak output of 1.8 kilowatts at noon. "By contrast, silicon cells have an output of 1.2 kW," says Kimura. And even when factoring in aerodynamic drag, which increases rapidly as the speed rises, Kimura says that a car using silicon solar cells would be able to achieve an average speed of only 86 km/h compared with the Tokai Challenger's 100 km/h.

The lithium-ion battery used to store the generated solar power came from Panasonic Corp. The battery employed more than 500 cells of the type used in notebook computers. "There was a maximum weight regulation of 25 kilograms, so we compared many companies' batteries," says Kimura. He adds that the Panasonic batteries had the best safety record, the largest storage  capacity (at 5.6 kWh), and the greatest energy density on a  watt-hour per kilogram basis.

The electric motor used to drive the rear wheel was a Mitsuba Corp. brushless DC direct-drive motor with an efficiency of 97 percent and an output of 2 kW. "The motor is one of a kind that I designed with Mitsuba," notes Kimura. The usual silicon-doped iron core was replaced with an iron-based amorphous core, a feature of which is low iron loss resulting in low hysteresis and eddy current losses, "which increased the motor's efficiency," he explains.

Developing the Tokai Challenger required the team to become "specialists in many technologies," says Kimura. "And it helped improve our human communications." As for the future, he believes a hybrid solar-electric car will become commercially available within 10 years, depending on oil prices.

About the Author

John Boyd writes about science and technology from Japan. In June 2009, he reported on a wireless 1000-year data-storage system.

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