De Maio, Davide (2021) Design, Simulation and Optimization of Selective Solar Absorbers for High Efficiency Solar Collectors. [Tesi di dottorato]

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Vacuum technology applied to flat plate solar collectors allows to reduce convective and conductive losses due to the internal gas, moving the operating temperature from the 80 °C (classic domestic flat plate solar panel) to up 200 °C (Mid-Temperature application) without concentration. The realization of an evacuated flat plate solar collector has been a technological challenge for decades, and just recently reliable products have been commercialised. The industrial field of solar thermal flat panels working at mid temperature is new and therefore not deeply investigated, so there is extensive room for improvement in performances: the scope of this PhD project is to explore ways to improve the efficiency in solar to thermal energy conversion of these new generation of solar thermal collectors. For the evacuated flat plate collectors, the power emitted by the absorber represents the main source of thermal losses, affecting the panel efficiency at high temperature: particular attention will be paid to this component. To efficiently convert solar energy into heat, a selective solar absorber is required. A selective absorber should efficiently absorb the light in the solar spectrum, while minimizing emission in the infra-red. In this work Cr2O3/Cr based multilayer selective solar absorbers have been optimized for evacuated flat plate solar collectors. The optimized absorbers can be 10% more efficient than the commercial alternative at 250 °C operating temperatures, reaching 400 °C stagnation temperature without solar concentration. Analysis on the robustness of the optimized coatings confirmed efficiency losses well below 5 percentage points for layer thickness deviations up to ±20% from the optimal thickness value, ensuring the industrial mass production feasibility of the solution. Values of solar absorptance as high as 0.97 and thermal emittance as low as 0.017 at 100 °C and 0.025 at 300 °C, validated by measurements on the fabricated samples promise unparalleled performances for evacuated flat plate solar collectors, whereas the robustness of the performances of the coatings related to the errors on layer thickness ensures the industrial feasibility of the coating, confirming that this emerging technology can give important contribution to the energy transition from fossil fuels to renewable energy for efficient heat production.

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