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Enhancing solar cells with heat

Thermophotovoltaic system converts high heat to narrow spectrum tailored to solar cell profile

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Cross-sectional view of an operating solar thermophotovoltaic (STPV) device shows the glowing side of the absorber/emitter substrate. By using photonic crystals and controlling absorption and emission wavelengths, Andrej Lenert and colleagues at MIT were able to maximize heat to electric conversion and minimize energy loss.
Image courtesy of the researchers

Mechanical engineering graduate student Andrej Lenert and colleagues at MIT recently demonstrated a photonic-crystal based thermophotovoltaic system, which converts heat to electricity.  Specifically, they paired a one-dimensional silicon/silicon dioxide photonic crystal emitter with an Indium Gallium Arsenide Antimonide (InGaAsSb) photovoltaic cell to reach thermophotovoltaic efficiency above 5 percent.

The work also predicted through theoretical modeling that solar thermophotovoltaic (STPV) system  efficiencies exceeding 10 percent could be achieved using two-dimensional tantalum photonic crystals for the emitter/absorber and a tandem filter with a simple planar layout. Data from the one-dimensional photonic crystal experiments matched predicted results within experimental uncertainty, thus validating the models.

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ANDREJ LENERT

Lenert, 27, presented the research in Washington, D.C., July 18-19, 2013, at a U.S. Department of Energy Frontier Research Centers Principal Investigators meeting.

His former post-doctoral associate Youngsuk Nam presented their paper, a joint effort under MIT Professors Evelyn Wang and Marin Soljacic, at the Transducers Conference in Barcelona, Spain in June.

Lenert, who conducted the experimental research in the Rohsenow Kendall Heat Transfer Lab at MIT, said the work represents the first time photonic crystals have been integrated in a solar thermophotovoltaic device.

"Our efforts show promise that you can get high efficiencies with the solar thermophotovoltaic process, which in theory can do a lot better than a single junction photovoltaic cell –¬ solar cell," Lenert said. "There hasn't been a lot of experimental work in this area so these are pioneering steps in that direction."

The efficiencies demonstrated by the nanostructured photonic crystal silicon emitter are more than two times the efficiency of a black body emitter, such as graphite.

"Since then, we've actually been able to validate these models with the device that was simulated," Lenert said. A paper on that work, experimentally demonstrating a two-dimensional tantalum photonic crystal emitter, is in preparation. The tantalum emitter replaces the silicon emitter in the earlier work and improves upon the results in the earlier experimental work. The team worked closely with Veronika Rinnerbauer, a former post-doc in Soljacic's group, who designed and fabricated the tantalum photonic crystals, and Walker Chan, an MIT graduate student, who designed and fabricated the silicon photonic crystal. Peter Bermel, a former research scientist at MIT and now assistant professor at Purdue University, and MIT post-doctoral associate Yi Xiang (Adrian) Yeng, contributed to modeling and design of photonic crystals.

Lenert's experiment focused on the power conversion aspects of the thermophotovoltaic system, relying on a Xe-arc light source to simulate sunlight. The light is further concentrated to 100 times the strength of ordinary sunlight before reaching the absorber.

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The power of the photonic crystal absorber-emitter structure comes from its ability to convert the full spectrum of sunlight into heat on the absorber side and to maximize emission of photons at or above the energy bandgap of the photovoltaic cell on the emitter side. The two-dimensional photonic crystal structures have a periodic array of cavities. The absorber and the emitter can be made from the same material but are nano engineered with different sized cavities to control their absorbance and emission. "Where you want that cutoff energy or that cutoff wavelength is slightly different between the absorber and the emitter," Lenert said.

The TPV system doesn't entirely eliminate energy loss but aims to maximize power from the photovoltaic cell while minimizing waste heat. The TPV typically operates at about 1,000 degrees Celsius for it to be an efficient process.

Lenert received his undergraduate degree in mechanical engineering from University of Iowa and his master's at MIT in 2010. He anticipates getting his MIT doctorate in spring 2014.

Written by Denis Paiste, Materials Processing Centerback

Massachusetts Institute of Technology
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