May/June 2011

Stationary solar panels heating up

MIT researchers and their collaborators have come up with an unusual, high performance and possibly less expensive way of turning the sun's heat into electricity. Their system, described in the journal Nature Materials, produces power with an efficiency roughly eight times higher than ever previously reported for a solar thermoelectric device-one that produces electricity from solar heat. It does so by generating and harnessing a temperature difference of about 200°C between the interior of the device and the ambient air.

The concept "is very radical," said Gang Chen, MIT's Carl Richard Soderberg Professor in Power Engineering and director of the Pappalardo Micro and Nano Engineering Laboratories, who co-authored the new paper with MIT doctoral student Daniel Kraemer and collaborators from Boston College and GMZ Energy. The work is funded by the Solid-State Solar-Thermal Energy Conversion Center, an Energy Frontier Research Center at the U.S. Department of Energy that is supported by MIT's Materials Processing Center.

While solar thermal electricity systems aren't a new idea, they typically involve vast arrays of movable mirrors that track the sun and focus its rays on a small area. The new approach uses flat, stationary panels similar to traditional solar panels, eliminating the need for tracking systems.

Chen's system is a solid-state device with no moving parts. A thermoelectric generator, placed inside a vacuum chamber made of glass, is covered with a black plate of copper that absorbs sunlight but does not re-radiate it as heat. The other side of the generator is in contact with ambient temperatures. Placed in the sun, the entire unit heats up quickly, even without facing the sun directly.

The device requires much less material than conventional photovoltaic panels, and could therefore be much less expensive to produce. It can also be integrated into solar hot water systems, allowing the expenses of the structure and installation to serve two functions at once. Such solar water heaters are rarely seen in the U.S., but are already a highly successful mass-market product in China and Europe, where they provide households with hot water and in some cases space heating as well.

The materials used to build such solar thermoelectric generators, made through a nanostructured process, were developed jointly a few years ago in Chen's lab at MIT and in co-author Zhifeng Ren's lab at Boston College. Their teams have continued to work on improving these materials and integrating them into complete systems.

Li Shi, associate professor of mechanical engineering at the University of Texas at Austin, said this approach to solar power is "very novel, simple, and easy for low-cost implementation." The efficiency level they have demonstrated so far, at 4.6%, is "already quite impressive," he said.