Better Thermal Photovoltaics

A new way to convert heat into electricity could lead to more efficient solar power.

A new approach to converting heat into electricity using solar cells could make a technology called thermal photovoltaics (TPVs) more practical. MTPV, a startup based in Boston that has raised $10 million, says that it has developed prototypes that are large enough for practical applications. The company recently announced agreements to install the devices in glass factories to generate electricity from hot exhaust.

Hot spot: A microscopic spacer used to support solar cells less than a micrometer above another material in a thermal photovoltaic device. Credit: Robert DiMatteo, MTPV

In general, thermal photovoltaics use solar cells to convert the light that radiates from a hot surface into electricity. While 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.

So far, the technology has been impractical for commercial applications, in part because of the high temperatures required and in part because of competition from existing technologies, such as steam turbines, for converting heat into electricity. MTPV's innovation is a method to increase the flow of photons from the heated material to the solar panel by 10 times compared with typical thermal photovoltaic systems, which could make its systems smaller, less expensive, and practical at lower temperatures, says Robert DiMatteo, MTPV's CEO.

A conventional solar panel absorbs light from the entire spectrum, but it only converts certain colors efficiently. Much of the energy in the other wavelengths of light goes to waste. As a result, the maximum theoretical efficiency of a conventional solar cell is 30 percent, or 41 percent if the sunlight is first concentrated using a mirror or lens. 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.

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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. (The expected efficiency of TPV devices is also much higher than efficiencies anticipated for thermoelectric devices, which directly convert heat into electricity.)

The key difference between MTPV's technology and other thermal photovoltaics is the positioning of solar cell and the heated material (MTPV stands for "micron-gap TPVs"). In his work first as a student at MIT and later as a researcher at Draper Laboratories, in Cambridge, MA, DiMatteo found that putting the heated material extremely close to the solar cell allowed far more photons to escape a given area of the material and be absorbed by the solar cell.