Radiative Cooling: Harvesting the Coldness of the Universe

Wei Li and Shanhui Fan

The combination of the atmosphere’s transparency in the 8-to-13-μm wavelength band and outer space’s potential as a heat sink has opened up a new frontier in renewable-energy research.

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Under the second law of thermodynamics, efficient conversion of heat to work requires both a high-temperature heat source and a low-temperature heat sink. For humans, the sun, with a temperature of approximately 6000 K at the surface, represents the most important thermodynamic heat source, and harvesting solar energy has played a central role in the history of human civilization. Indeed, a significant portion of renewable-energy research today focuses on directly pulling energy from the sun—through such devices as solar thermal panels, which convert solar radiation to thermal energy, and photovoltaics, which convert it to electricity.

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