Modeling, Thickness Optimization, and Application Prospects of Radiative Cooling Performance in Polydimethylsiloxane Films

Abstract

This study focuses on the radiative cooling performance of polydimethylsiloxane (PDMS) films, aiming to establish a mathematical model describing the variation of emissivity with wavelength and thickness, and to evaluate its performance. Emissivity is considered a key property in thermal radiation processes, determined by the material's complex refractive index and thickness. According to Kirchhoff's law, emissivity equals absorptivity. The emissivity model was developed using thin-film optics theory, specifically the transfer matrix method, to calculate the reflectance and transmittance of the gas-PDMS-substrate three-layer structure under normal incidence conditions. The refractive index of PDMS varies with wavelength and can be fitted using the Cauchy model. The core evaluation metric for radiative cooling performance is net cooling power. This model is constructed based on the energy balance equation, encompassing key energy terms such as external radiative heat dissipation, solar radiation absorption, atmospheric longwave radiation absorption, and convective heat transfer. Through numerical simulation, the study found that the emissivity of PDMS films within the “atmospheric transparent window” exhibits oscillatory-convergent behavior with thickness while maintaining extremely low absorption in the solar spectrum. Analysis indicates an optimal thickness range of 10–20 μm for PDMS films, achieving net cooling power exceeding 90 W/m² within this range. Furthermore, the study applies this technology to polar ice storage cooling systems and proposes a cost-effective, high-performance PDMS/XPS composite insulation layer design.