Radiative cooling structure optimization based on evolutionary computation and its cooling performance evaluation

Abstract

Radiative cooling is a spontaneous cooling technology at daytime outdoor condition by controlling a spectral emissivity of a cooling surface. Radiative cooling technology can be utilized as an eco-friendly, sustainable, and passive cooling-assistant technology, because a sub-ambient temperature of surface can be achieved with radiative heat transfer between a cold space through an atmospheric transparent window. To improve a performance of radiative cooling system, it is important to design a proper radiative cooling surface for application conditions and to develop a precise examination system of radiative cooling performance. Since the emissivity is physically same as an absorptivity, tailoring a selective emissivity spectrum can enhance a cooling power of the radiative cooling surface by maximizing thermal emissive power and minimizing absorption of solar irradiance and atmospheric emission. The most typical way to tailor selective emission is to utilize electromagnetic resonance effect of an optical structure. The selective emissivity spectrum (i.e., maximization and/or minimization of emissivity at a spectral range in demand) can be implemented via interaction between micro/nanostructure and electromagnetic wave as well as intrinsic emission of materials. Various design objectives can be regarded from simple performance estimation methods, such as an emissive power and an instantaneous cooling power, to more rigorous estimation method for realistic application, such as an accumulated cold energy production and contrast of switchable emissivity. Photonic structures, for example, multilayered and periodic grating structure, are optimized via evolutionary computation method to maximize radiative cooling performance under those objectives. In addition, to experimentally evaluate a radiative cooling performance, spectrometer-based measurement set-up is established for emissivity spectrum measurement as a basis of cooling performance estimation. Furthermore, radiative cooling power indoor measurement facility is developed for a simple and stable evaluation of a radiative cooling performance. The proposed optimization method using evolutionary computation can successfully design highly performing radiative cooling structures for a more applicable radiative cooling. A cooling performance of fabricated radiative cooling structures can also be quantified using a developed measurement scheme. Consequently, results of this study can contribute to developing realistic radiative cooling as well as energy technology.