Stanford Engineers Invent a Solar Panel That Generates Electricity At Night

Research Sid Assawaworrarit and his colleagues have outfitted an ordinary solar panel with a thermoelectric generator, capable of generating a small amount of electricity from the slight difference in temperature between the ambient air and the surface of a solar panel pointed deep into space. Interesting Engineering reports: "During the day, there's a light coming in from the Sun and hitting the solar cell, but during the night, something of a reverse happens," Assawaworrarit says. That's because solar panels -- like everything warmer than absolute zero -- emit infrared radiation. "There's actually light going out [from the solar panel], and we use that to generate electricity at night. The photons going out into the night sky actually cool down the solar cell," he says. As those photons leave the skyward surface of the solar panel, they cary heat with them. That means that on a clear night -- when there are no clouds to reflect infrared light back toward the Earth -- the surface of a solar panel will be a few degrees cooler than the air around it. That temperature differential is what Assawaworrarit and his colleagues are taking advantage of. A device called a thermoelectric generator can capture some of the heat flowing from the warmer air to the cooler solar panel and convert it into electricity. On a clear night, the device Assawaworrarit tested on the Stanford rooftop generates roughly fifty milliwatts for every square meter of solar panel (50 mW/m2). "I think that's probably a record number," he says. But Assawaworrarit and his team aren't stopping there. He says that with a couple of improvements (and in a good location) such a device could generate twice that amount of electricity. "The theoretical limit is probably about one or two watts per square meter," he says. "That's not a huge number, but there are a lot of applications" where that kind of energy at night would come in handy.

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