Switching on the lights at night is second nature to most people in the developed world. But electricity isn’t a given in many other parts of the globe. 

“Something like a billion people on our planet still lack reliable access to electricity. Think about folks in parts of the developing world that are living in off-grid locations. And for them, one of the central applications of electricity is lighting, and we need lighting the most at night.” 

U.C.L.A. materials scientist Aaswath Raman. Solar cells can provide remote areas with electricity during the day but require batteries to store that energy for use at night. Raman’s team has developed a potential solution: a simple thermoelectric device that generates power when it’s exposed to the cold night sky. It’s made possible by a phenomenon called radiative sky cooling.

All objects, Raman explains, radiate heat. “And so what that means from the perspective of a surface that’s looking up at the night sky is that it will, all by itself, send out more heat than the sky sends back to it. It escapes to the upper atmosphere and even out to outer space. And it’s something that anyone can observe at night. So if you go and measure the roof temperature on your house in the early morning hours, say, you should read a temperature that is much lower than the immediate ambient air temperature.”

Raman reasoned that this temperature difference could be exploited to generate electricity. His team built their device using an aluminum disk that acts as a radiative cooler. Its cool side faces the night sky, while its other side is warmed by the air around it. As heat escapes upwards, a thermoelectric generator converts the temperature difference into electricity that powers a small LED light.

For now, the energy output of the device is just a tiny fraction of what a solar cell can produce. But engineering improvements could eventually boost its performance. Raman sees the device as a complement to solar, providing inexpensive, 24-hour power generation to remote areas of the world—without the need for batteries.

“You can also think about, say, the polar regions, where for several months of the year, there is no sun at all. So in those kinds of regions, this might represent one of the few ways you can actually generate power naturally at night.” The research is described in the journal Joule. [Aaswath P. Raman et al., Generating light from darkness]

In addition to lighting, Raman says the device could be useful for specialized applications, such as powering wireless sensors and monitoring atmospheric conditions. Or it could be a really cool way to recharge your cell phone at night.

—Susanne Bard

[The above text is a transcript of this podcast.]