Passive radiative cooling below ambient air temperature under direct sunlight
Passive radiative cooling below ambient air temperature under direct sunlight
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DOI：
10.1038/nature13883
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发表时间：
2014-11-27
期刊：
影响因子：
64.8
通讯作者：
Fan, Shanhui
中科院分区：
文献类型：
--
作者：
Raman, Aaswath P.;Abou Anoma, Marc;Fan, Shanhui
冷却是全球能源的重要终端使用，也是高峰电力需求的主要驱动力。例如，空调占美国建筑物一次能源消耗的近15%(1)。因此，不需要任何电力投入就能冷却的被动冷却战略可能会对全球能源消耗产生重大影响。为了实现降温，人们需要能够达到并保持低于环境空气的温度。在夜间，使用一种名为辐射冷却的技术证明了低于环境空气温度的被动冷却，在该技术中，暴露在天空中的设备通过大气中8至13微米(2-11)的透明窗将热量辐射到外层空间。然而，降温需求的高峰期出现在白天。在阳光直射下，白天辐射冷却到低于环境的表面温度(3，4，12，13)，因为白天进入天空会导致太阳对辐射冷却器进行加热。在这里，我们实验演示了在阳光直射下，辐射冷却到低于环境空气温度近5摄氏度。利用热光子方法(14-25)，我们介绍了一种集成的光子太阳能反射器和热发射器，它由七层HfO2和SiO_2组成，反射97%的入射太阳光，同时在大气透明窗口强而选择性地发射。当在屋顶上暴露在超过每平方米850瓦的直射阳光下时，光子辐射冷却器可以冷却到低于环境空气温度4.9摄氏度，在环境空气温度下的冷却功率为每平方米40.1瓦。这些结果表明，量身定做的光子方法可以从根本上实现新的技术可能性，以提高能源效率。此外，即使在一天中最热的几个小时，宇宙中寒冷的黑暗也可以用作可再生的热力学资源。
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(2-11). 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(14-25), we introduce an integrated photonic solar reflector and thermal emitter consisting of seven layers of HfO2 and SiO2 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.