UCLA team generates light from darkness with $30 rooftop setup

Radiative sky cooling causes water droplets to form on car windows, so a UCLA team collected that heat from a rooftop and converted it to electricity. (UCLA)

Researchers at UCLA are leveraging the physical principle of radiative sky cooling to produce renewable energy at night when solar panels do not function.

The concept was explained in a recent Joule article, “Generating Light from Darkness,” that describes how the team used a low-cost, off-the-shelf thermoelectric generator to generate electricity on a rooftop.

“Our device couples the cold side of the thermoelectric module to a sky-facing surface that radiates heat to the cold of space and has its warm side heated by the surrounding air, enabling electricity generation at night,” the researchers said. 

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Radiative sky cooling occurs naturally, especially on clear nights, explained Aaswath Raman, an assistant professor of materials science and engineering at UCLA’s Samueli School of Engineering, who led the study with support from the U.S. Department of Energy.

Evidence of radiative sky cooling is seen when frost forms on the ground overnight when temperatures are above freezing or when water droplets show up on a car windshield even on clear nights. As a result of radiative sky cooling, an object like a car or the ground or a building that ejects heat will be slightly cooler than the ambient temperature. 

The UCLA team’s cheap technology catches some of the heat from the surrounding air that would normally rise into the sky and converts it into electricity. The parts that the team cobbled together were purchased at hardware and electric supply stores for less than $30.

They used an aluminum disk that was painted black on one side, which faced the sky. A thermoelectric generator then converted the heat collected from near the bottom of the disk to electricity. A thermoelectric generator produces electric voltage when responding to a difference in temperature. 

The device generated up to 25 milliwatts per square meter which is enough to power a single LED light bulb. That’s far below the energy from a solar cell but might be useful in places where there’s no available electricity. Better components could help it generate 0.5 watts per square meter, 20 times more, and enough to charge a cell phone overnight or light a room.

Raman described the technology as complementary to solar energy.  A study by Raman and others published in Nature in 2014 demonstrated how radiative sky cooling could be used to cool hot buildings on sunny, warm days.

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