Radiative Cooling under a Humid Subtropical Climate and Its Future Potential Development

As a passive and environmentally friendly type of cooling technology, a radiative cooler reflects almost all incident sunlight, while at the same time, emitting thermal radiation strongly and selectively through the atmospheric transparent window, having a wavelength range between 8 μm and 13 μm. As a result, thermal energy of an object can be transferred to the extremely cold outer space (i.e. ~2.7 K), providing a cooling effect to the object. In this study, photonic passive radiative coolers, consisting of multilayers of HfO₂ and SiO₂ on top of Ag film, were fabricated. Two distinct thermal designs, non-vacuumed and vacuumed configurations, were tested under 24-hour day and night time operations. The vacuumed design is confined in a vacuum chamber covered by a KCl infrared pass window on the top. The temperature difference between the radiative cooler and the ambient is the major figure of merit to be identified. Experimental results show that the radiative coolers provide a satisfactory cooling effect at night, but the cooling effect needs improvement during daytime operation under a humid subtropical climate. The underlying reason for this is explored, and could be attributed to reduced atmospheric transparency due to water droplet absorption. One possible direction, which could lead to successful daytime radiative cooling under a humid subtropical climate, is outlined. Plasmonic and Mie resonant radiators, which can emit thermal radiation strongly and selectively at specific wavelengths, could potentially replace their current photonic counterparts. This strategy is regarded as crucial to successful daytime radiative cooling under a humid subtropical climate.