Thickness Optimization for Passive Radiative Daytime Cooling with Polymeric Materials

Outer space is a hostile environment for living organisms, but special solutions are also necessary for non-living matter to endure harsh conditions and retain functionality. For example, thermal management in outer space requires a unique material approach due to the absence of a surrounding atmosphere. Terrestrial cooling applications can utilize conductive or convective heat dissipation pathways to keep the temperature within a specific operation window. In outer space, radiative heat dissipation becomes much more important. Materials are designed to reflect solar radiation to prevent heating. At the same time, the materials need to efficiently emit thermal infrared radiation at the operating temperature.^[1] This approach can also be utilized for terrestrial cooling applications. Materials and strategies to cool terrestrial objects with radiative heat dissipation to outer space are researched within the field of passive radiative cooling. For this cooling approach, materials are required that show strong emission in the mid-infrared wavelength region between 8 – 13 µm, where the earth's atmosphere is mostly transparent. At the same time, the materials should not absorb sunlight. Different strategies to realize those specific property combinations within a material have been reported in recent years. Important material parameters were discussed in detail, including tailored or broadband emissivity, angle selectivity, or the influence of non-radiative heat losses.^[2] The material thickness is typically chosen sufficiently thick to ensure high emission in the atmospheric transparency window but was far less researched. However, besides the material emittance, atmospheric and solar energy uptake also depend on the material thickness. This interplay of different contributions has been less addressed so far.<br/>In our current project, we show how the emitter thickness can be optimized when the optical properties of the material are known. We use complex refractive index data to estimate the cooling performance of Polydimethylsiloxane (PDMS) as example material. Our theoretical study discriminates between a day- and nighttime case and finds an optimum emitter thickness. We verify our results by preparing and characterizing a set of homogenous films with different thicknesses. UV-Vis and IR spectroscopy is used to measure the optical properties, and the cooling properties are characterized in a rooftop experiment.^[3] The presented approach is directly transferable to other materials and environmental conditions. Therefore, our work could help to improve existing and future materials.<br/><br/>References<br/>[1] H. Kim et al., ”VO₂-based switchable radiator for spacecraft thermal control”, <i>Scientific Reports</i>, <b>2019</b>, 9, 11329.<br/>[2] B. Zhao <i>et al., “Radiative Cooling: A Review of Fundamentals, Materials, Applications, and Prospects”,</i> <i>Applied Energy</i>, <b>2019</b>, 236, 489 – 513.<br/>[3] K. Herrmann <i>et al.</i>, “Homogeneous Polymer Films for Passive Daytime Cooling: Optimized Thickness for Maximized Cooling Performance”, Manuscript submitted.