Passive Radiative Cooling under a Humid Subtropical Climate – Development to Date and Future Potential

Passive Radiative Cooling under a Humid Subtropical Climate – Development to Date and Future Potential C.Y. TSO1,2, Christopher Y.H. CHAO1 1Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 2HKUST Jockey Club Institute for Advanced Study, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong Abstract In modern life, people spend a large portion of time in indoor environments. A huge amount of energy is consumed in order to provide a comfortable indoor environment. For example, 89% of the total electricity in Hong Kong is consumed by buildings at end-use level as stated by the government. Among different electricity end-uses, thermal systems contribute to a significant consumption of electricity in buildings, with 36% of electricity used to power space conditioning and refrigeration systems. Traditional active cooling systems relying on the vapor compression cycle require a huge electricity input. On the contrary, passive radiative cooling, which does not require an energy input in any form, offers a great potential to conserve energy use in buildings. A radiative cooler reflects almost all incident sunlight, while at the same time, emits thermal radiation strongly and selectively through the atmospheric transparency window, of which its wavelength lies in-between 8 and 13 ?m. As a result, thermal energy of an object can then be transferred to the extremely cold outer space (i.e. ~2.7 K), providing cooling effect to the object. In our knowledge, there is no field investigation of phontonic passive radiation cooling in Hong Kong. This work is intended to make an initial feasibility study on how effective this technique would perform under a tropical sky. Experimental results show that the radiative coolers provide a satisfactory cooling effect at night (i.e. the ambient air temperature is reduced by about 6 – 7 oC). The cooling capacity is achieved at 38 W/m2 at a clear sky night. However, the cooling effect needs improvement during the 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. Possible directions, which could lead to successful daytime radiative cooling under a humid subtropical climate, are outlined. Meanwhile, two possible solutions are proposed. First, plasmonic and Mie resonant radiators, which can emit thermal radiation strongly and selectively at specific wavelengths, could potentially replace their current photonic counterparts. Second, an asymmetric transmission window, which permits outgoing infrared propagation and obstructs incoming radiation at the same regime, could boost the net cooling power. These strategies are regarded as crucial to successful daytime radiative cooling under a humid subtropical climate.