Caldarelli, Antonio (2023) Characterization and Thermal Aging Tests of Selective Solar Absorbers for Evacuated Flat Plate Collectors. [Tesi di dottorato]

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Abstract

Solar energy plays a key role in the energy transition from fossil fuels to renewable energy. The transition from these fuels to renewable energies could extensively and positively impact global warming. The goal is to reduce the use of fossil fuels and, consequently, greenhouse gas emissions, thereby mitigating climate change issues. A non-negligible portion of the energy resources employed by developed countries is currently used for heating and cooling. The industrial sector deserves particular attention because it comprises the largest portion of global energy consumption among the major energy-consuming sectors: industrial, transportation, residential, and commercial. Globally, the industrial sector is responsible for over one-third (37%) of the total energy consumption and a quarter (24%) of the global CO2 emissions. Solar collectors and thermo-photovoltaic (TPV) may be a gamechanger in the efforts related to industrial transitioning to renewable energy sources. This Ph.D. aims to improve the energy conversion efficiency in High Vacuum Flat Plate Collectors (HVFPCs) and TPV technologies. The HVFPCs are solar collectors capable of supplying mid-temperature output (up to 200 °C), retain all the advantages of a flat plate solar collector (net area is approximately 96% of the gross area), thanks to the vacuum insulation. However, owing to the present state of technology, they remain unsuitable for providing high-temperature heat on a large scale, and therefore, are unable to replace a significant portion of industrial process heat. An optimized solar selective coating and the reduction of the absorber’s substrate radiative losses has been proposed to improve the HVFPCs efficiency. A multi-layered structure (Cr-Cr2O3 based) appeared to be an interesting solution to achieve high working temperature. The results obtained show an absorber efficiency up to 75% at 300 °C working temperature. Moreover, copper and silver low emissive coating (LEC) seems to best suit the purpose of the absorber’s substrate radiative losses reduction. The use of these LEC led to almost doubled the HVFPCs performances. On the other hand, the optimization, development, and realization of selective emitters (SE) has been proposed for the TPV systems. Selective emitters could highly impact on the efficiency of these devices and in reducing heat losses. The proposed SE, is a SiNx-SiO2-TiO2 multilayer structure on tungsten substrate, demonstrating to have a good thermal stability (up to 1000 °C) and the best literature emitter efficiency in 0.63-0.72 eV range. Finally, a novel procedure for the prediction of the service lifetime of solar absorbers for HVFPCs depending on the operating temperatures is proposed to overcome the problems occurred using the current standard (ISO 22975-3:2014). A novel Performance Criterion (PC), that represents the degree of aging of a coating, an appropriate temperature frequency function f(T), that represents how many hours the absorber is at temperature T during one-year operation, were presented. The results obtained following the proposed procedure (decreasing service lifetime with increasing operating temperatures) appear to be more reasonable than those obtained following the standard procedure (increasing service lifetime with increasing operating temperatures), suggesting that the proposed procedure offers a more realistic prediction of service lifetime for absorbers used in HVFPCs.

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