Homogeneous Polymer Films for Passive Daytime Cooling: Optimized Thickness for Maximized Cooling Performance

Passive radiative cooling materials that spontaneously cool below ambient temperature can save tremendous amounts of energy used for cooling applications. A multitude of materials, structures, and fabrication strategies have been reported in recent years. Important material parameters like a tailored or broadband emissivity, angle selectivity, or the influence of nonradiative heat losses were discussed in detail. The material thickness has been far less researched and is typically chosen sufficiently thick to ensure high emission in the atmospheric transparency window between wavelengths of 8–13 μm. However, not only the material emittance but also atmospheric and solar energy uptake depend on the material thickness. This broadband interplay has been less addressed so far. Herein, it is shown how an optimum thickness of a passive cooling material can be predicted when the optical properties of the material are known. Using complex refractive index data, the thickness‐dependent cooling performance of polydimethylsiloxane (PDMS) in back‐reflector geometry as exemplary material is calculated. For both day‐ and nighttime operation, an optimum emitter thickness is reported. The findings are verified experimentally by measuring the equilibrium temperatures of PDMS films with different thicknesses in a rooftop experiment. The presented analytical approach is directly transferable to other materials. Materials that spontaneously cool below ambient temperature are of great interest to fight global warming. Increased film thicknesses typically improve thermal radiation, but simultaneously increase uptake of solar radiance. A general approach is presented to balance these two contributions and to determine the optimum thickness for passive daytime cooling films.