Review of daytime radiative cooling technologies and control methods

Abstract

Cooling demand in buildings accounts for a significant portion of global energy consumption and is estimated to increase tenfold by 2050. Daytime radiative cooling shows great potential for reducing surface temperatures and cooling energy consumption of buildings by dissipating heat into cosmic space without consuming any energy. However, further commercial application of radiative cooling technologies requires improvements in their cooling efficiency, reduction of manufacturing costs, and optimisation of their seasonal suitability. Meanwhile, typical daytime radiative coolers with constant solar spectral reflective and mid-infrared emissivity cannot automatically switch off in cold winter and would lead to an energy penalty for heating. To overcome the above limitations, this paper reviews the types of daytime radiative coolers in terms of their structure and related temperature-adaptive control methods. The literature is also analysed to compare and evaluate the performance indicators, design methods, numerical simulation methods and manufacturing processes of daytime radiative coolers. Furthermore, the insulation and convection shielding methods during daytime radiative cooling experiments are also critically reviewed. The study also summarises recently developed temperature-adaptive daytime radiative coolers that utilize a daytime radiative cooler with a switching accessory or temperature-switching material. This paper critically reviews the switching control technologies, switching accessories and materials for the temperature-adaptive daytime radiative cooler to analyze their characteristics and their effect on the heat transfer of daytime radiative coolers. Finally, the potential temperature-adaptive switching materials that can be used for temperature-adaptive daytime radiative coolers are presented. The review demonstrated that the higher the solar reflectance and mid-infrared emissivity of the daytime radiative cooler, the larger the cooling temperature drop during daylight. Mie theory, Maxwell's equations and finite difference time domain are commonly used for the design and simulation of daytime radiative coolers with commercial software. Daytime radiative cooler structures of nanoparticles in polymers, porous and random nanofibers structures are technically mature, less costly and suitable for mass production. The mechanical switching control system for daytime radiative cooling is sensitive, and the switching temperature can be set flexibly, but its limitations are complexity, durability and high cost. Temperature-adaptive switching materials (i.e. phase change materials and thermo-chromic materials) have the advantages of adaptive passive control and easy integration into daytime radiative coolers. Overall, this review contributes to guiding the development of radiative cooling technologies through the comparison of the cooling effect, manufacturing process, manufacturing cost, advantages and disadvantages of current control methods and materials for DRCers.