Solar power that works at night
A specially designed photovoltaic cell could be capable of generating power at night, according to new research.
The article, authored by Jeremy Munday, professor in the Department of Electrical and Computer Engineering at UC Davis, and graduate student Tristan Deppe, said that the photovoltaic cell could generate up to 50 W of power per square metre under ideal conditions at night.
This is about one-quarter of what a conventional solar panel can generate in daytime.
The researchers are developing prototypes of these night-time solar cells that can generate small amounts of power, in the hopes of improving the power output and efficiency of the devices.
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 very cold, so if a warm object is pointed at the sky, it will radiate heat in that direction. People have been using this phenomenon for night-time 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 is another kind of device called a thermoradiative cell that generates power by radiating heat to its surroundings. Researchers have explored the idea of using these to capture waste heat from engines.
"We were thinking, what if we took one of these devices and put it in a warm area and pointed it at the sky," Munday said.
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 steps were taken either to block direct sunlight or have it pointing 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.
This research was published in ACS Photonics.
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