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Wednesday, January 21, 2009

Better Thermo Photovoltaics

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

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.

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.

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.

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.

DiMatteo first published work on the MTPV concept in the late 1990s, but it has taken until now to engineer prototypes large enough to be practical. One main challenge has been finding ways to create a gap that's just one-tenth of a micrometer across and yet can be maintained over the relatively large areas needed for a practical device. DiMatteo says that the company will improve the performance of the devices by making the gap steadily smaller, which computer models suggest will improve efficiency.