The device, say the researchers, generates energy by harnessing the cold darkness of space using a passive cooling mechanism – known as radiative sky cooling – to maintain the cold side of a thermoelectric generator several degrees below ambient. The surrounding air heats the warm side of the thermoelectric generator, with the ensuing temperature difference converted into usable electricity.
The approach, say the researchers, could be adapted into a low-cost technology that could eventually be used by the more than one billion people around the world who, according to the International Energy Agency, lack reliable access to electricity. The concept could be used as a standalone technology or work in combination with solar energy to produce electricity throughout the day and night.
Radiative sky cooling is a natural phenomenon in which a surface that faces the sky ejects its heat into the air as thermal radiation. Some of that heat eventually rises to the upper atmosphere and then into colder reaches of space.
“This effect occurs naturally all the time, especially on clear nights,” says Aaswath Raman, an assistant professor of materials science and engineering at the UCLA Samueli School of Engineering who led the study. “The result is that the object ejecting the heat, whether it’s a car, the ground or a building, will be slightly cooler than the ambient temperature.”
To demonstrate the technology, the researchers developed a simple and inexpensive device that they say was built from parts purchased at hardware and electronic supply stores for a total cost of less than $30. Located on the roof of a building, their device included an aluminum disk that was painted black on one side, which faced the sky. The disk was used to radiate the heat being given off by the surrounding air while a thermoelectric generator then converted that heat into electricity.
The device generated up to 25 milliwatts per square meter – enough to power a single LED light bulb. Although the device generates substantially less energy than a similarly sized solar cell, say the researchers, it could be used to generate power at night in locations that are off of the electrical grid or for users who don’t have easy access to batteries.
The researchers say the technology could be improved with better components, and that it could potentially generate as much as 0.5 watts per square meter — about 20 times more than the device the researchers demonstrated — especially in hot, dry climates where the radiative cooling effect is the strongest. With that output, says Raman, a small setup on the roof of a home could provide enough power overnight to charge a cell phone or to light a room with LED bulbs.
“We think this is an intriguing demonstration of how the cold of space can be accessed as a renewable energy resource and result in modest yet usable amounts of electricity,” says Raman. “We think it also could form the basis of a complementary technology to solar energy. While the power output will always be substantially lower than that of solar devices, this new technology can operate at hours when solar cells cannot.”
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