Passive Daytime Radiative Cooling for Space and Terrestrial Applications based on Polymer Emitter Coating

Kurzfassung

Passive daytime radiative cooling is considered a green technology where the heat of an object is efficiently radiated to the coldness of deep space, acting as a thermal sink. It has found application in both space and terrestrial application, where the excess heat from satellites can be ejected to space using radiator surfaces and where terrestrial sky facing objects can radiate its heat through the atmospheric transmittance window (8–13 µm) in the Mid Infrared range respectively. Polymer emitters are particularly interesting due to easy applicability and cost effectiveness but lack in optical transparency in solar wavelength region, structural flexibility and stability. The goal of this thesis is to determine if and how a simple, thin polyorganosilazane polymer coating can be used as a thermal emitter layer to enhance the passive radiative cooling properties of flexible Copper Indium Gallium di-selenide solar cells for space and as a sub ambient radiative cooler for terrestrial applications. Therefore, the optical, electrical and thermal properties of the solar cells with and without the polyorganosilazane emitter coating were compared and analyzed. These optical properties of the coating were studied using Fourier transform infrared spectroscopy and Ultraviolet Visual Near Infrared spectroscopy techniques and the electrical performance of the CIGS modules was verified using EQE and IV measurements. The results suggested that with the enhanced emissivity of 0.72 in between 3–20 µm range, a temperature drop of 30 °C of the solar cells can be expected for a lower earth orbit without any significant power losses. Studies were carried out to gain an understanding of the emissivity response of the PSZ coating and explore the sub-ambient terrestrial cooling properties. A spectrally selective terrestrial passive daytime radiative cooler structure is proposed comprising of a 5µm thin silicon oxycarbonitride emitter on a silver reflector achieving 97% solar reflection and an emissivity of 0.86 in the atmospheric transmittance window. A sub-ambient cooling of 6.8°C was observed under sunlight yielding a cooling power of 93.7 Wm-2. The behavior of a radiative sky cooler was also studied in a controlled indoor environment with liquid nitrogen heat sink and under varying air pressure to understand the interaction of the parasitic conducting, convective and radiative heat fluxes on the passive radiative cooler. A lumped heat transfer model was prepared in Python to analyze and understand the performance of the deep sub-ambient radiative cooler in different pressure regimes and with varying sub-component system parameters like shield reflectivity, number of shields and thermal conductivity of holder. The findings presented in this work demonstrate the wide applicability and utility of the single layer polyorganosilazane polymer derived emitter coating and demonstrates advancements made in the field of the passive radiative cooling technology. The results are also expected to open new scopes in materials research and designing of radiative cooling system.