D' Alessandro, Carmine (2021) Production of Selective Solar Absorbers for Evacuated Thermal Collectors and Measurements of their Radiative Properties. [Tesi di dottorato]

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Abstract

Decarbonization of energy is one of the most important challenges of today. In Europe the amount of energy demanded for only heating and cooling is half of the total. Within the industrial sector, process heat is the most relevant and for mid-temperatures applications, 100 - 200 °C, the amount reaches 21% of the whole. To renewably respond to such a demand, solar thermal receivers are the most feasible solution. Usually, architectures adopt vacuum insulation to minimize thermal losses of the absorber encapsulating it within evacuated glass tubes, eventually equipped with external reflectors for solar light centration. A new frontier is the solar thermal evacuated flat panel equipped with an unconcentrated plate absorber. The reduced encumbrance, larger fill factor, wider acceptance angle of the solar light and no need for any tracking system makes this solution more appealing than traditional evacuated tube receivers. Even if the original idea is almost 50 years old, just recently appeared commercial devices with unrivalled conversion efficiency at mid-temperatures, as 50% at 160 °C of working temperature. Such a performance can be further improved by the adoption of a properly designed absorber since the only commercially available ones are optimized for unconcentrated low-temperature applications. The scope of this research activity was to produce and test Selective Solar Thermal Absorbers designed to operate at mid-temperatures under high vacuum insulation without concentration. The selectivity is referred to high values of light absorptance in the solar spectrum and, simultaneously, very low emissivity at longer wavelengths where the spectral emission of the Black Body occurs. Absorber samples were produced by Physical Vapor Deposition of Cr, Cr2O3, SiO2 nanometric films over commercial copper substrates, and their characterization was conducted by optical analysis of reflectivity with traditional measurement devices. To overcome all the limits of the traditional approach, a novel test facility has been designed to carry out outdoor and indoor calorimetric experiments to measure the radiative properties under realistic operating conditions. During outdoor tests the produced samples were exposed to the actual solar irradiation, whereas an accurately calibrated custom-made LED solar simulator was adopted for the indoor procedure. Experimental results reveal solar absorptance and total thermal emittance can be tuned by changing the thicknesses of each layer of the coating and the proper combination is proposed for mid-temperature applications. A slight reduction of the solar absorptance guarantees sensibly lower values of total thermal emittance leading to an increase up to +25% on the annual panel efficiency with respect to commercial flat absorbers at 200 °C of operating temperature. Finally, deposition tests of the proposed layer materials were conducted also with an industrial machine to assess the feasibility of large scale production and the desired level of stability and reproducibility of the deposition process was achieved.

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