Solar Device That Likes It Hot Hikes Efficiency

There’s still a ways to go, but PETE is moving along.

“Photon enhanced thermionic emission,” the conceptual breakthrough that Stanford researchers introduced three years ago as a way to capture and make use of both the light and heat in a solar device, has undergone improvements that boost its efficiency by 100 times.

Part of a 2-inch-diameter gallium-arsenide wafer used as a base for photon-enhanced thermionic emission chips. (image via Brad Plummer / SLAC)

Part of a 2-inch-diameter gallium-arsenide wafer used as a base for photon-enhanced thermionic emission chips. (image via Brad Plummer / SLAC)

That’s pretty remarkable, but we need to point out that it’s a jump “from a few hundredths of a percent to nearly 2 percent” efficiency. The researchers said they “expect to achieve at least another 10-fold gain in the future.”

Still, in a statement, Nicholas Melosh, of the Stanford Institute for Materials and Energy Sciences, called the advance “a major step toward making practical devices based on our technique for harnessing both the light and heat energy provided by the sun.”

This attempt to pull together thermal and photovoltaic solar conversion technologies is attractive to scientists because a lot of the wide spectrum of solar energy that hits a standard crystalline silicon photovoltaic cell isn’t captured. Much is lost as heat.

PETE is built to make use of that heat.

image via Nick Melosh/Stanford University

image via Nick Melosh/Stanford University

Here’s how the improved system — described by Stanford as “a sandwich of two semiconductor layers” — works: In the diagram above, concentrated sunlight (that’s the red arrows at the top) heats up the device’s semiconductor cathode (the beige and gray upper plate) to more than 400 degrees Celsius – 750 degrees Fahrenheit. Photoexcited hot electrons (blue dots) then stream out of the cathode’s nanotextured underside down to the anode (white/gray surface), where they are collected as direct electrical current.

Now the question: How do the researchers imagine this device being used? Well, because PETE does so well at really hot temperatures, their thought is that it could be added “to utility-scale concentrating solar power plants, such as multi-megawatt power tower and parabolic trough projects in California’s Mojave Desert.” There, they say, PETE could increase their electrical output by 50 percent.

“When placed where the sunlight is focused, our PETE chips produce electricity directly; and the hotter it is, the more electricity it will make,” Melosh said.

Stanford said the PETE research “has received support from Stanford’s Global Climate and Energy Project, the Gordon and Betty Moore Foundation, the Department of Energy’s SunShot Initiative and the Defense Advanced Research Projects Agency.”

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.