Infrared radiation from the sun can enter the chip through the holes on the surface, but the reflected rays are blocked when they try to escape (e.g., similar to the greenhouse effect). 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.

Diagram of angle-selective solar thermophotovoltaic system. Bermel et al. Click to enlarge.

The new angle-selective solar thermophotovoltaic system was described in an open access paper by Research Laboratory of Electronics research scientist Peter Bermel and other MIT researchers, published in October in the journal Nanoscale Research Letters.

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.

By concentrating the sunlight thermally—capturing it and reflecting it back into the material—the device can absorb as much heat as a standard black object, Bermel says, 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.”

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

The next step in the research is to test different materials in this configuration to find those that produce power most efficiently. With existing solar thermophotovoltaic systems the highest efficiency is 10%; Bermel expects that with this angular-selective approach, it could be 35–36%. That, in turn, exceeds the Shockley-Quiesser efficiency limit of 31% for photovoltaic conversion under unconcentrated sunlight.

The paper was co-authored by MIT’s John Joannopoulos, the Francis Wright Davis Professor of Physics; professor of physics Marin Soljačić; 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.

Resources

• Peter Bermel, Michael Ghebrebrhan, Michael Harradon, Yi X Yeng, Ivan Celanovic, John D Joannopoulos and Marin Soljacic (2011) Tailoring photonic metamaterial resonances for thermal radiation. Nanoscale Research Letters 6:549 doi: 10.1186/1556-276X-6-549