This thermoelectric generator harnesses temperature differences to generate renewable electricity. Here the device is powering a small LED light. (Image credit: Aaswath Raman)

No sun? No problem.

A new thermoelectric device can generate electricity for an LED light bulb even during the blackest night, according to a report by researchers.

The secret is using a phenomenon known as radiative cooling, which happens when surfaces on the ground radiate heat into the atmosphere. This process can make a surface cooler than the air surrounding it, which explains why frost forms on grass even if the air temperature is above freezing.

Researchers say their device is a useful form of renewable energy , especially because lighting demand peaks at night. "Beyond lighting, we believe this could be a broadly enabling approach to power generation suitable for remote locations, and anywhere where power generation at night is needed," lead author Aaswath Raman, an assistant professor of materials science and engineering at the University of California, Los Angeles, said in a statement .

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The prototype device was tested on a table 3 feet (1 meter) above the ground on a rooftop in Stanford, California, in late December. It was placed in a polystyrene enclosure covered in aluminized mylar (which minimizes thermal radiation) and protected by a wind cover. Inside the protective layers, researchers made the device draw heat from the air and send it back into the atmosphere, using a black emitter.

The researchers managed to power an LED using a voltage boost converter, and measured that over 6 hours the device can generate as much as 25 milliwatts of energy per square meter. That's much lower than typical solar cells , but the advantage is that the device works at night, while solar cells have no sunlight to convert into energy.

The researchers say that with some modifications, the device could be used over a wider scale since the radiative cooler is simple (an aluminum disk covered in paint) and the other components are commercially available. For example, the researchers suggest increasing heat-exchange efficiency by reducing heat gain in the radiative cooling part of the device. The device may also work best in hotter and drier climates, the team noted.

A schematic of the thermoelectric generator. (Image credit: Aaswath Raman)

"Our work highlights the many remaining opportunities for energy by taking advantage of the cold of outer space as a renewable energy resource," Raman said. "We think this forms the basis of a complementary technology to solar. While the power output will always be substantially lower, it can operate at hours when solar cells cannot."

This research was published Sept. 12 in the journal Joule.