Shanhui Fan and colleagues demonstrate a practical radiative cooling device that is effective in direct sunlight, requires only sky access and needs no electricity input. The device operates by — whilst avoiding sunlight absorption — radiating heat into the cold darkness of space via what is known as the atmospheric infrared transparency window, wavelengths of 8 and 13 micrometres. The device differs from previous designs in that it can function in full daylight. The authors have designed and fabricated a multilayered photonic structure that reflects 97% of incoming sunlight while emitting strongly in the atmospheric transparency window. When exposed to direct sun, the device cools to a temperature 5 °C below ambient with a cooling power of 40 watts per square metre. The authors calculate potential annual energy savings for a typical roof covered with this passive cooling system to be equivalent to 120,000 kilowatt hours.
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