Solar Researchers Turn Up Heat To Boost Efficiency

Researchers are claiming a possible advance in turning the sun’s heat – not just the easily accessible infrared light that falls directly on solar cells – into electricity. The payoff, if the new approach pans out, would be to stretch the bounds of efficiency for single-junction photovoltaic cells, where the upper limit is now around 34 percent and real-world returns tend to be in the teens.

This is a realm of solar power known as thermophotovoltaics. We touched on it a few years ago, reporting on an MIT-built small power generator that burned butane from a replaceable cartridge to create heat and that was able to run a device three times longer than a lithium-ion battery of the same weight.

stanford thermophotovoltaics

A cross-section micrograph of the thermal emitter shows it holding up well to 2,500 F. (image via Stanford/Kevin Arpin)

The focus in the new research, out of Stanford and also involving scientists from Illinois and North Carolina State, was on improving the heat resistance of the thermal emitter, the piece in a thermophotovoltaic system that turns absorbed heat into infrared light.

Infrared light is the part of the light spectrum that silicon semiconductors can use to make electricity while higher-energy light waves are lost as heat and lower-energy waves simply aren’t absorbed. The more heat these thermal emitters can stand, the more infrared light that can be sent to the solar cell, and the more electricity that can be produced.

According to Stanford, the material of choice for thermal emitters had been tungsten, which could work up to about 1,800 (limiting the efficiency of the system to 8 percent). In the new research, the tungsten was coated with a nanolayer of ceramic material called hafnium dioxide.

“The results were dramatic,” Stanford said. “When subjected to temperatures of 1,800 F (1000 C), the ceramic-coated emitters retained their structural integrity for more than 12 hours. When heated to 2,500 F (1,400 C), the samples remained thermally stable for at least an hour.”

So what now? “Hopefully these results will motivate the thermophotovoltaics community to take another look at ceramics and other classes of materials that haven’t been considered,” said Illinois’ Paul Braun.

The research discussed here was published in the Oct. 16 edition of the journal Nature Communications.

Pete Danko is a writer and editor based in Portland, Oregon. His work has appeared in Breaking Energy, National Geographic's Energy Blog, The New York Times, San Francisco Chronicle and elsewhere.