Solar

MIT concentrates photovoltaics without mirrors

MIT researchers found a way to use thermophotovoltaic devices without mirrors to concentrate the sunlight, potentially making the system much simpler and less expensive. To prevent the heat from escaping the thermoelectric material, the MIT team used a photonic crystal: an array of precisely spaced microscopic holes in a top layer of the material.

December 5, 2011 — Researchers at MIT have found a way to use thermophotovoltaic devices without mirrors to concentrate the sunlight, potentially making the system much simpler and less expensive. To prevent the heat from escaping the thermoelectric material, the MIT team used a photonic crystal: an array of precisely spaced microscopic holes in a top layer of the material.

The approach mimics Earth?s greenhouse effect: Infrared radiation from the sun can enter the chip through the holes on the surface, but the reflected rays are blocked when they reflect back against the material. This blockage is achieved by a precisely designed geometry that only allows rays that fall within a very tiny range of angles to escape, while the rest stay in the material and heat it up.

The new device was described in a paper by Research Laboratory of Electronics research scientist Peter Bermel and other MIT researchers, published in October in the journal Nanoscale Research Letters. Access it here: http://www.nanoscalereslett.com/content/6/1/549

Bermel explains that if you put an ordinary, dark-colored, light- and heat-absorbing material in direct sunlight, ?it can?t get much hotter than boiling water,? because the object will reradiate heat almost as fast as it absorbs it. But to generate power efficiently, you need much higher temperatures than that. By concentrating sunlight with parabolic mirrors or a large array of flat mirrors, it?s 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,? Bermel says, by ?concentrating the sunlight thermally?: capturing it and reflecting it back into the material. The result, he says, 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.?

Such a system, he says, ?at large scale, is efficient enough to compete with more conventional forms of power. This is an alternative to concentrators.?

In addition, the system is simple to manufacture using standard chip-fabrication technology. By contrast, the mirrors used for traditional concentrating systems, he says, require ?extremely good optics, which are expensive.?

The next step in the research, Bermel says, is to test different materials in this configuration to find those that produce power most efficiently. With existing solar thermophotovoltaic systems, he says, ?the highest efficiency [in converting solar energy to electricity] is 10%, but with this angular-selective approach, maybe it could be 35 to 36 percent.? That, in turn, is higher than the theoretical maximum that could ever be achieved by traditional photovoltaic solar cells.

In the solar-cell business, Bermel points out, ?even small differences of 1% or so are considered important.? At this point, however, his research has been ?mainly theory,? so the next step is building and testing more actual devices. So far, he says, ?we have some preliminary results? that validate the theory.

The paper was co-authored by MIT?s John Joannopoulos, the Francis Wright Davis Professor of Physics; professor of physics Marin Solja?i?; and four students. It was funded by the National Science Foundation, the MIT S3TEC Energy Research Frontier Center of the Department of Energy, and the Institute for Soldier Nanotechnologies.

Written by David Chandler, MIT News Office