Passive radiative cooling below ambient air temperature under direct sunlight

• Published in: Nature (London) Vol. 515; no. 7528; pp. 540 - 544
• Main Authors: Raman, Aaswath P., Anoma, Marc Abou, Zhu, Linxiao, Rephaeli, Eden, Fan, Shanhui
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 27.11.2014; Nature Publishing Group
• Subjects: 639/624/1111/1114; Air temperature; Ambient temperature; Atmosphere; Clouds; Cooling systems; Electricity; Emissions; Energy consumption; Energy efficiency; Heat; Humanities and Social Sciences; letter; multidisciplinary; Passive cooling; Radiative transfer; Science; Silicon wafers; Solar energy; Sunlight; Temperature; United States
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature13883

A multilayer photonic structure is described that strongly reflects incident sunlight while emitting heat selectively through an atmospheric transparency window to outer space; this leads to passive cooling under direct sunlight of 5 degrees Celsius below ambient air temperature, which has potential applications in air-conditioning and energy efficiency. Passive cooling in direct sunlight 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. Cooling is a significant end-use of energy globally and a major driver of peak electricity demand. Air conditioning, for example, accounts for nearly fifteen per cent of the primary energy used by buildings in the United States 1 . A passive cooling strategy that cools without any electricity input could therefore have a significant impact on global energy consumption. To achieve cooling one needs to be able to reach and maintain a temperature below that of the ambient air. At night, passive cooling below ambient air temperature has been demonstrated using a technique known as radiative cooling, in which a device exposed to the sky is used to radiate heat to outer space through a transparency window in the atmosphere between 8 and 13 micrometres 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 . Peak cooling demand, however, occurs during the daytime. Daytime radiative cooling to a temperature below ambient of a surface under direct sunlight has not been achieved 3 , 4 , 12 , 13 because sky access during the day results in heating of the radiative cooler by the Sun. Here, we experimentally demonstrate radiative cooling to nearly 5 degrees Celsius below the ambient air temperature under direct sunlight. Using a thermal photonic approach 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , we introduce an integrated photonic solar reflector and thermal emitter consisting of seven layers of HfO 2 and SiO 2 that reflects 97 per cent of incident sunlight while emitting strongly and selectively in the atmospheric transparency window. When exposed to direct sunlight exceeding 850 watts per square metre on a rooftop, the photonic radiative cooler cools to 4.9 degrees Celsius below ambient air temperature, and has a cooling power of 40.1 watts per square metre at ambient air temperature. These results demonstrate that a tailored, photonic approach can fundamentally enable new technological possibilities for energy efficiency. Further, the cold darkness of the Universe can be used as a renewable thermodynamic resource, even during the hottest hours of the day.