A new study is looking at the possibility of using thermophotovoltaic devices without mirrors to concentrate sunlight, potentially making a much simpler and less expensive system. The goal is to prevent the heat from escaping the thermoelectric material by using a photonic crystal — essentially an array of precisely spaced microscopic holes in a top layer of the material. Diagram of angle-selective solar thermophotovoltaic system. (Image: Bermel et al. Nanoscale Research Letters 2011 6:549 doi:10.1186/1556-276X-6-549) Infrared radiation from the sun can enter the device through the holes on the surface, but the reflected rays are blocked when they try to escape (similar to the Earth’s greenhouse effect). This blockage is achieved with a precisely designed geometry that allows only rays that fall within a very tiny range of angles to escape, while the rest stay in the material and heat it up. In direct sunlight, an ordinary dark-colored, light- and heat-absorbing material can’t get much hotter than boiling water because the object reradiates heat almost as fast as it absorbs it; power generation requires 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. Peter Bermel, a scientist at MIT’s Research Laboratory of Electronics, suggests that by concentrating the sunlight thermally — capturing it and reflecting it back into the material — the device could absorb as much heat as a standard black object, yet not reradiate much of the heat, becoming in theory extremely hot. Such a system “at large scale, is efficient enough to compete with more conventional forms of power,” Bermel said. “This is an alternative to concentrators.” In addition, he said the system is simple to manufacture using standard chip-fabrication technology. By contrast, the mirrors used for traditional concentrating systems require extremely good optics, which are expensive. While efficiency of ordinary solar energy harnessing systems is around 10 percent, MIT’s theoretical material could achieve 32 to 36 percent, which could be really useful given that an increase in efficiency of even 1 percent is considered important. Jason Fleischer, an associate professor of electrical engineering at Princeton University who was not involved in this work, said the advance made by Bermel and his colleagues is envisioning the use of existing light-absorbing material to create a photonic structure that preferentially emits light in a direction and wavelength range that are optimal for photovoltaic conversion. By doing so, he said, “This increases the efficiency significantly beyond classical predictions based on unconcentrated sunlight, enabling a small device to generate as much electricity as a conventional one that is much larger.” The new device was described in the October issue of Nanoscale Research Letters. For more information, visit: www.mit.edu
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