University of California Davis researchers are asking, “What if solar cells worked at night?” They are not joking. In fact, their specially designed photovoltaic cell could generate up to 50 watts of power per square meter under ideal conditions at night, about a quarter of what a conventional solar panel can generate in daytime, according to a recent concept article.

A conventional photovoltaic or solar cell (left) absorbs photons of light from the sun and generates an electrical current. A thermoradiative cell (right) generates electrical current as it radiates infrared light (heat) toward the extreme cold of deep space. UC Davis engineers propose that such cells could generate a significant amount of energy and help balance the power grid over the day-night cycle. Image Credit: Tristan Deppe/Jeremy Munday, UC Davis. Click image for the largest view.

Jeremy Munday, professor in the Department of Electrical and Computer Engineering at UC Davis and graduate student Tristan Deppe published the article in, and it became featured on the cover of the January 2020 issue of ACS Photonics.

Professor Munday, who recently joined UC Davis from the University of Maryland, is developing prototypes of these nighttime solar cells that can generate small amounts of power. The researchers hope to improve the power output and efficiency of the devices.

Munday said that the process is similar to the way a normal solar cell works, but in reverse. An object that is hot compared to its surroundings will radiate heat as infrared light. A conventional solar cell is cool compared to the sun, so it absorbs light.

Space is really, really cold, so if you have a warm object and point it at the sky, it will radiate heat toward it. People have been using this phenomenon for nighttime cooling for hundreds of years. In the last five years, Munday said, there has been a lot of interest in devices that can do this during the daytime (by filtering out sunlight or pointing away from the sun).

There’s another kind of device called a thermoradiative cell that generates power by radiating heat to its surroundings. Researchers have explored using them to capture waste heat from engines.

Munday said, “We were thinking, what if we took one of these devices and put it in a warm area and pointed it at the sky.”

This thermoradiative cell pointed at the night sky would emit infrared light because it is warmer than outer space.

“A regular solar cell generates power by absorbing sunlight, which causes a voltage to appear across the device and for current to flow. In these new devices, light is instead emitted and the current and voltage go in the opposite direction, but you still generate power,” Munday said. “You have to use different materials, but the physics is the same.”

The device would work during the day as well, if you took steps to either block direct sunlight or pointed it away from the sun. Because this new type of solar cell could potentially operate around the clock, it is an intriguing option to balance the power grid over the day-night cycle.

It looks that so far the idea has yet to be tried with prototyping and running some tests. It would likely work well under an open sky with a heat source warming the device during the darkness. Simple too, as the device needs only tilted out of the way during the heating period and back when dark.

Still, economics are going to matter. Perhaps the technology that works best will be the lowest cost. 50 watts is actually quite a bit of LED output. This just might have some good legs for more research.


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