Employing "thermoradiative" photovoltaics and concepts from the advancing field of radiative cooling, such "nighttime photovoltaic cells" could, say the researchers, 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. The devices operate similarly to the way normal solar cells work, but in reverse by generating power by radiating heat to their surroundings.
An object that is hot compared to its surroundings will radiate heat as infrared light, while a conventional solar cell is cool compared to the sun, so it absorbs light. A thermoradiative cell pointed at the night sky (i.e., toward deep space), say the researchers, 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," says Jeremy Munday, professor in the Department of Electrical and Computer Engineering at UC Davis. "In these new devices, light is instead emitted and the current and voltage go in the opposite direction, but you still generate power. You have to use different materials, but the physics is the same."
Such devices would work during the day as well if blocked from direct sunlight or pointed away from the sun. Because this new type of solar cell could potentially operate around the clock, say the researchers, it is an intriguing option to balance the power grid over the day-night cycle.
The researchers say they are developing prototypes of these nighttime solar cells that can generate small amounts of power, and hope to improve the power output and efficiency of the devices. For more, see " Nighttime Photovoltaic Cells: Electrical Power Generation by Optically Coupling with Deep Space ."