Highly Solar-Reflective Structures for Daytime Radiative Cooling under High Humidity.

Daytime radiative cooling is a passive strategy to cool down a terrestrial object under direct sunlight without the need of electricity input. It functions by simultaneously reflecting solar irradiance and sending heat as infrared (IR) thermal radiation through the atmospheric transparent window into the cold outer space. In spite of extensive studies on daytime radiative cooling, most of previous works were conducted in dry regions mainly in North America. Here, we explore the feasibility of achieving efficient radiative cooling in humid subtropical areas such as Hong Kong, where abundant atmosphere water vapor exists. In this case, the atmospheric transparent window is almost closed since water is highly absorptive of IR radiation. We report a simple approach to achieve efficient daytime radiative cooling in Hong Kong. Our design comprises an expanded polytetrafluoroethylene (ePTFE) film and a Ag layer deposited on a transparent glass substrate. We show that the combination of highly diffusely reflective ePTFE and all-band reflective Ag results in a reflectivity of 98% in the solar spectrum, allowing for a temperature drop up to 2.7 °C under a solar intensity of 1000 W/m on a humid day in Hong Kong.