News and analysis about renewable energy in California.

Breakthrough in Solar Thermal PV

Photovoltaic Thermal Hybrid system | Photo: Julian/Flickr/Creative Commons License

It may be hard to imagine in heat-wave-stricken California, but people in many places around the world rely on solar energy for heating, using rooftop sunlight to heat water or air. This poses an obstacle to full acceptance of rooftop photovoltaic panels: if you're counting on rooftop solar to make your house warm and your water hot, you may be reluctant to sacrifice valuable rooftop real estate to generate electricity.

Research announced today at Queens University in less-than-tropical Kingston, Ontario, Canada may offer one way to have the best of both worlds: a efficient rooftop system that generates both electricity and heat.

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The study by Stephen Harrison and Joshua Pearce builds on an existing technology called "photovoltaic thermal hybrid" (PVT). It's long been known that PV cells lose efficiency when they heat past a certain point; PVT systems use heat exchangers to cool the PV cells, at the same time heating either water or air that can then be used in the building. Such hybrid systems, however, are less efficient at heating than a standard solar thermal collector.

PV cells made of amorphous silicon can operate at higher temperatures than their still more-common crystalline silicon counterparts, but amorphous silicon cells lose their ability to turn light into electricity the longer they're exposed to bright light, a process called the Staebler-Wronski effect, not a quality you generally want in PV cells. It's been known for some time that the Staebler-Wronski effect can be reversed by heating the amorphous silicon. Harrison and Pearce determined that temperatures easily reached in solar thermal collectors are sufficient to reverse the Staebler-Wronski effect, suggesting that a new generation of PVT collectors designed to operate at higher temperatures could be an efficient source of both power and heat. Amorphous silicon is also less expensive than crystalline silicon, offering better cost-per-unit.

"These studies open up an entirely new application of amorphous silicon and make a highly-economic PVT possible," Pearce said in a Queens University press release. "We need both solar electricity and solar heating in Canada but we are running into 'roof real estate' issues. Now people can have both their solar electricity and solar heating combined in a nice tidy package."

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About the Author

Chris Clarke is a natural history writer and environmental journalist currently at work on a book about the Joshua tree. He lives in Joshua Tree.
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