Solar Power with a Side of Hot Running Water
Researchers at the Massachusetts Institute of Technology in Cambridge, Mass., and their collaborators have come up with an unusual, highly efficient and possibly less expensive way of turning the sun’s heat into electricity.
Their system, described in a paper published online in the journal Nature Materials, produces power with an efficiency roughly eight times higher than ever previously reported for a solar thermoelectric device such as one that produces electricity from solar heat. It does so by generating and harnessing a temperature difference of about 392°F (200°C) between the interior of the device and the ambient air.
The concept “is very radical,” says 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 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.
While solar thermal electricity systems are not 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.
Like the silicon photovoltaic cells that produce electricity when struck by sunlight, 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 also can 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 United States, 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 MIT lab 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.
Chen points out that the DOE has programs to develop thermoelectric systems, mostly geared toward harnessing waste heat from car and truck engines. He says that solar applications for such devices also can have an important role to play in reducing carbon emissions.
The system would not be a substitute for solar photovoltaics, Chen says, but offers another way of tapping into the enormous amount of solar energy that bathes the Earth every day. And because it can be piggybacked onto the existing solar hot-water industry, the thermoelectric device could be a relatively inexpensive addition, with no subsidies required, he says.