Robert DiMatteo, the CEO of a startup based in Boston named MTPV has a new approach to converting heat into electricity using solar cells could make a technology called thermal photovoltaics more practical. Thermal photovoltaics are solar cells that convert the light that radiates from a hot surface into electricity. The first applications will be generating electricity from waste heat, and then the technology could be developed to generate electricity from sunlight far more efficiently than photovoltaic solar cells. In the thermal photovoltaics system, sunlight is concentrated on a device to heat it up, and the light it emits is then converted into electricity by the photovoltaic portion of the cell. That’s a very interesting combination of engineering steps.
Till now the technology has been impractical for commercial applications, in part because of the high temperatures required the devices need and in part because of production cost competition from the existing technologies. MTPV’s innovation is a method that increases the flow of photons from the heated material of the solar panel to the photovoltaic section by 10 times compared with existing thermal photovoltaic technologies. The company’s innovations should make its systems smaller, less expensive, and practical at lower operating temperatures.
Regular readers and solar cell industry observers are aware that today’s photovoltaic cells only convert certain frequencies (colors of the light) efficiently. The rest of the light is just lost. The maximum efficiency is thought to be 30% with concentration using lenses or mirrors pushing that to perhaps 41%. The process change offered by MTPV concentrates the light onto a material to heat it, then when hot the material emits wavelengths that the devices photovoltaic section is optimized for resulting in a new theoretical maximum efficiency of 85%. Combining engineering steps can have a large payoff.
Theory aside, the practical engineering challenges are much harder to achieve. DiMatteo says that the company’s computer models suggest that efficiencies over 50 percent should be possible. That would be a great result.
The company’s prototypes aren’t this efficient: they’re converting about 10 to 15 percent of the heat that they absorb into electricity, which DiMatteo says is enough to make the devices economical. Keep in mind, these are early prototype rigs. Also a comparison with thermoelectric devices could be in order, as the thermal photovoltaics don’t have to source from the visible light spectrum, infra red will do. Early applications may be secondary recovery of heat to electricity.
The innovation is the positioning of solar cell and the heated material. As a student at MIT and later as a researcher at Draper Laboratories, DiMatteo found when putting the heated material extremely close to the solar cell far more photons are absorbed by the solar cell. In older technology most of the photons generated in the heated material are reflected back into the material when they reach its surface, the same phenomenon that traps light in fiber-optic cables. Bringing the solar cell and the heated material closer 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. Operation in a vacuum between the heated material and the solar cell maintains a temperature difference between the two that’s required to achieve the high efficiencies. Because the heated material emits more photons, the solar cell can generate 10 times as much electricity for a given area.
These close dimensions make it possible to use one-tenth as much solar-cell material, which cuts costs significantly, while making it possible to generate more power at lower temperatures. Peter Peumans, a professor of electrical engineering at Stanford University concurs, conventional thermal photovoltaics can require temperatures of 1,500 °C.
MTPV’s first prototypes 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. Such a 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.
Peumans says in the other hand 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.
Di Matteo began work and publishing on the concept in the late 1990s with engineering finally skilled enough for large prototypes to be practical. A primary effort is to create a heating material to photovoltaic 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. With at least 2/3rds and more of the fuel energy lost to the atmosphere now, this company’s efforts seek a valuable reward from an existing resource.