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

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Herrmann, Kai ; Lauster, Tobias ; Song, Qimeng ; Retsch, Markus:
Homogeneous Polymer Films for Passive Daytime Cooling : Optimized Thickness for Maximized Cooling Performance.
In: Advanced Energy & Sustainability Research. Bd. 3 (2022) Heft 2 . - No. 2100166.
ISSN 2699-9412
DOI der Verlagsversion: https://doi.org/10.1002/aesr.202100166

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Abstract

Passive radiative cooling materials that spontaneously cool below ambient temperature could 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 non-radiative 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. Here we show, 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, we calculate the thickness dependent cooling performance of Polydimethylsiloxane (PDMS) in back-reflector geometry as exemplary material. For both day- and night-time operation, we report an optimum emitter thickness. We verify our findings experimentally by measuring the equilibrium temperatures of PDMS films with different thicknesses in a rooftop experiment. Our presented analytical approach is directly transferable to other materials.