New Way to Harness Solar Energy Mimics Greenhouse Effect, Multiplies Efficiency

A novel way to harness the sun's energy that mimics Earth's greenhouse effect has been discovered by researchers from MIT.

The MIT researchers, who pioneered the use of the sun's light concentrated by mirrors using solid-state devices called thermophotovoltaics, have found a simpler and less expensive system to concentrate the sunlight without mirrors.

According to researchers, the key is to prevent the escape of heat from the thermoelectric material by using a photonic crystal, which has precisely spaced microscopic holes in a top layer, mimicking the Earth's greenhouse effect.

Using this system, infrared radiation from the sun enters the chip through holes on the surface, but the reflected rays are blocked when they try to escape by a precisely designed geometry that limits the entry of rays to those that have a tiny range of angles to escape. The rest remains in the material and heats it up.

Peter Bermel of MIT's Research Laboratory of Electronics said that by concentrating sunlight with parabolic mirrors or a large array of flat mirrors, it iis possible to get much higher temperatures - but at the expense of a much larger and more complex system.

"What I'm looking at is an alternative to that paradigm," he said, by "concentrating the sunlight thermally" -- capturing it and reflecting it back into the material. According to Bermel, the result is that the device can absorb as much heat as a standard black object, but "in practice, we can get it extremely hot, and not reradiate much of that heat."

He said that at large scale, the new system is efficient enough to compete with more conventional forms of power and can be considered as an an alternative to concentrators.

The researcher said that the next step is testing different materials to find those that produce power most efficiently, since in existing solar thermophotovoltaic systems "the highest efficiency in converting solar energy to electricity is 10 percent, but with this angular-selective approach, maybe it could be 35 to 36 percent." This is higher than the theoretical maximum that can be achieved by traditional photovoltaic solar cells.

Bermel said the research has been "mainly theory" so far, but preliminary results validate it.

The research by Bermel and co-authors MIT's John Joannopoulos, the Francis Wright Davis Professor of Physics; professor of physics Marin Soljačić; and four students was published in October in the journal Nanoscale Research Letters.