A specially designed photovoltaic cell was developed that 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 the daytime.

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 toward it.

Another kind of device, called a thermoradiative cell, generates power by radiating heat to its surroundings. Researchers have explored using them to capture waste heat from engines. 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 power is still generated. The device would work during the day as well if steps were taken to either block direct sunlight or point it away from the Sun.

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