Thermal photovoltaics use solar cells to convert the light that radiates from a hot surface into electricity. The first applications will be generating electricity from waste heat, eventually the technology could be used to generate electricity from sunlight far more efficiently than solar panels do. In such a system, sunlight is concentrated on a material to heat it up, and the light it emits is then converted into electricity by a solar cell.
In a thermal photovoltaic system, light is concentrated onto a material to heat it up. The material is selected so that when it gets hot, it emits light at wavelengths that a solar cell can convert efficiently. As a result, the theoretical maximum efficiency of a thermal photovoltaic system is 85 percent.
In practice, engineering challenges will make this hard to attain, but DiMatteo says that the company’s computer models suggest that efficiencies over 50 percent should be possible. The prototypes aren’t this efficient: they convert about 10 to 15 percent of the heat that they absorb from the glass-factory exhaust into electricity, which DiMatteo says is enough to make the devices economical.
Thermal Photovoltaics could be superior to both regular photovoltaics and to thermal electrics in terms of efficiency of converting heat or sunlight to electricity.
In a conventional TPV system, most of the photons generated in the heated material are reflected back into the material when they reach its surface; it’s the same phenomenon that traps light in fiber-optic cables. When the solar cell and the heated material are brought close together, so that the gap between the two is shorter than the wavelength of the light being emitted, the surface no longer reflects light back. The photons travel from one material to the other as if there were no gap between them. The close spacing also allows electrons on one side of the gap to transfer energy to electrons on the other side. (A vacuum between the heated material and the solar cell maintains a temperature difference between the two that is required to achieve high efficiencies.) Since the heated material emits more photons, the solar cell can generate 10 times as much electricity for a given area, compared with a solar cell in a conventional TPV.
That makes it possible to use one-tenth as much solar-cell material, which cuts costs significantly. Alternatively, it makes it possible to generate more power at lower temperatures, which Peter Peumans, a professor of electrical engineering at Stanford University, says is one of the key advantages of the approach. Conventional thermal photovoltaics can require temperatures of 1,500 °C, he says. The first prototypes from MTPV work well at less than 1,000 °C, and DiMatteo says that, in theory, the technology could economically generate electricity at temperatures as low as 100 °C. This large temperature range could make the technology attractive for generating electricity from heat from a variety of sources, including automobile exhaust, that would otherwise be wasted.
But Peumans says that the technology has a trade-off: because the heated material and solar cell are placed so close together, it’s not possible to put a filter between them to help tune the wavelengths of light that reach the solar cell. This could limit the ultimate efficiencies that the system can reach.