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PETE could improve solar tech

Stanford engineers have found a new method for converting sunlight into electricity, which could significantly improve existing photovoltaic and solar thermal technologies.

This new method, called photon enhanced thermionic emission (PETE), was discovered by a University research group headed by materials science and engineering Prof. Nick Melosh. Melosh went public with the research on Aug. 1, publishing a paper on the subject in Nature Materials.

“Despite some of the outrageous claims on the Internet,” Melosh said, “this is not a panacea, but is a unique method that can capture both heat and light, and may one day be a valuable part of the energy solution.”

Photovoltaic solar technology currently relies on semiconductors, which use photons from the sun to excite electrons, ultimately generating an electrical current. This mechanism, however, becomes less efficient with high temperatures, precluding the possibility of using the waste heat to fuel a secondary generator. As a result, photovoltaic and thermal energy conversion are mutually exclusive processes, and current scientific efforts have focused on optimizing one of the two.

Melosh’s new approach, if successful, would solve this dilemma by making high temperatures favorable to semiconductor-mediated energy conversion. By using the semiconductor gallium nitrate, coated with cesium, the researchers constructed a parallel plate thermionic emission device, in which higher temperatures will excite more electrons from the semiconductor cathode and generate current.

When photons strike the cathode, they increase the population of electrons that can participate in the thermionic emission process, which Melosh and his team dubbed “photon enhancement.”

Since higher temperatures increase the efficiency of this process, the researchers envision solar concentrators, which can multiply the sun’s intensity by 500 times, focusing light on a PETE device and siphoning unused heat to drive other thermal conversion systems.

While current monocrystalline solar panels boast an efficiency of around 26 percent, Melosh expects this new process to increase efficiency to 50-60 percent.

And since PETE’s optimal temperature point is achieved well after 200 degrees Celsius, it can function in areas such as the Mojave Desert, whereas today’s solar technology usually fails in temperatures above 100 degrees. However, this new technology is not limited to such high-temperature climates, if large parabolic dishes are used to concentrate the direct sunlight.

“It will actually work a little better in cold but sunny climes,” Melosh said, “but mostly as long as there is direct sun, it should be able to work.”

“This needs solar concentrations of at least a few hundred times, thus is most likely to be used for large scale utilities, though could also be used for remote areas as well,” he added.

While the cost of these dishes, in addition to the semiconductor material and cesium, is not inexpensive, the output of this process has the potential to rival that of fossil fuel combustion.

At the moment, tests have been run using gallium nitride, a common material for household electronics. While the research team has demonstrated the PETE process, stability and cost-effectiveness remain obstacles for the technology.

“We have currently only shown the proof of principle experimentally, and shown theoretically that it could be quite efficient,” Melosh said.

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