Sunless solar energy

May 6, 2009

sunless solar

Growing up in north central China, graduate student Jing Cheng recalls that “there was always black ash. The sky was always gray.” And, too, there was a constant smell of sulfur in the air.

Her home province, Shanxi, is known as the “kingdom of coal.” And in consuming this abundant fuel to meet the country’s exploding demand for electricity, Shanxi not only experienced breakneck economic growth but also a rapid rise in pollution. “I got a sense of the environment at an early age,” notes Cheng.

The need for cleaner energy sources spurred the materials science and engineering Ph.D. student to focus on solar power technologies. With her colleagues in Professor Lionel “Kim” Kimerling’s lab, she is developing a special type of solar cell that could generate electricity not from sunlight, but from the infrared radiation emitted by industrial waste heat.

Called a thermophotovoltaic, or TPV, cell, the technology is unlike a conventional photovoltaic cell in that it is tuned to the infrared end of the light spectrum, which is invisible to the naked eye. Any warm object radiates infrared light, but some are warmer than others. Coke ovens used in steelmaking, for example, can reach temperatures up to 2,500 degrees F. As a result, such ovens can function as small suns for TPV cells — but providing radiation in the form of heat rather than light.

Already, small TPV devices are used in conjunction with heaters and cook stoves to generate electricity in remote locations. A scaled-up version of the technology, though, would be a boon to energy efficiency. In the U.S. alone, industries waste enough heat to generate nearly 20 percent of the power the nation uses, according to recent studies. That’s enough to replace up to 400 coal-fired power plants.

However, TPV faces twin challenges familiar to anyone working in the photovoltaics industry: its materials and manufacturing processes are too costly — and its energy-conversion efficiency too low — for it to compete on a large scale with conventional electricity-generating technologies.

The novel TPV cell Cheng is helping to develop could change that equation. It would cost much less to manufacture and be capable of converting as much, if not more, infrared light to electricity than current TPV cells. The technology could one day be a desirable alternative to current methods of capturing industrial waste heat.

After she earns a Ph.D., Cheng will likely pursue a career in solar technology, either in the U.S. or China. She says she came to MIT to study engineering because she “wanted to do something useful. I wanted to solve some of the big problems we face in the future.” By advancing a solar technology that addresses the need for clean power, she’s on her way.

by Lauren Clark

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