This device harnesses the cold night sky to generate electricity in the dark

A new device is an anti-solar panel, harvesting energy from the cold night sky.

By harnessing the temperature difference between Earth and outer space, a prototype of the device produced enough electricity at night to power a small LED light. A bigger version of this nighttime generator could someday light rooms, charge phones or power other electronics in remote or low-resource areas that lack electricity at night when solar panels don’t work, researchers report online September 12 in Joule. 

The core of the new night-light is a thermoelectric generator, which produces electricity when one side of the generator is cooler than the other (SN: 6/1/18). The sky-facing side of the generator is attached to an aluminum plate sealed beneath a transparent cover and surrounded with insulation to keep heat out. This plate stays cooler than the ambient air by shedding any heat it absorbs as infrared radiation (SN: 9/28/18). That radiation can zip up through the transparent cover and the atmosphere toward the cold sink of outer space. 

Meanwhile, the bottom of the generator is attached to an exposed aluminum plate that is continually warmed by ambient air. At night, when not baking under the sun, the top plate can get a couple of degrees Celsius cooler than the bottom of the generator. 

Engineer Wei Li of Stanford University and colleagues tested a 20-centimeter prototype of the device on a clear December night in Stanford, Calif. The generator produced up to about 25 milliwatts of power per square meter of device — enough to light a small light-emitting diode, or LED, bulb. The team estimates that further design improvements, like better insulation around the cool top plate, could boost production up to at least 0.5 watts per square meter.

“It’s a very clever idea,” says Yuan Yang, a materials scientist at Columbia University not involved in the work. “The power generation is much less than solar panels,” which generally produce at least 100 watts per square meter. But this nighttime generator may be useful for emergency backup power, or energy for people living off the grid, Yang says. 

A typical lamp bulb might consume a few watts of electricity, says Shanhui Fan, an electrical engineer at Stanford University who worked on the device. So a device that took up a few square meters of roof space could light up a room with energy from the night sky.

Aaswath Raman, a materials scientist and engineer at UCLA, also envisions using their team’s generator to help power remote weather stations or other environmental sensors. This may be especially useful in polar regions that don’t see sunlight for months at a time, Raman says. “If you have some low-power load and you need to power it through three months of darkness, this might be a way.”