In solar thermal and thermodynamic applications a coating material is usually defined by its values of absorptance and thermal emittance; new solutions for increased absorptance and reduced thermal emittance are constantly sought, though the improvement of one factor typically leads to a performance reduction in the other. Moreover, absorptance and thermal emittance alone are not sufficient to fully compare different coating candidates for a specific application. A simplified lumped element analysis, based on energy balances, shows that parameters such as working temperature, concentration ratio, optical efficiency and direct normal irradiation should also be taken into account in the process of choosing the best candidate. Assuming a simplified correlation between absorptance and thermal emittance, it is then possible to identify the optimal combination which maximizes the system efficiency. Such analytical model is validated against a detailed 2D simulation on the parabolic trough cross section, in which a typical solar heat flux distribution is applied. Convenience in moving towards improved absorptance or thermal emittance compounds depends on the specific application and on the operating parameters of the overall process, such as concentration ratio and working temperature. For medium-high temperature applications, the tube itself is subject to different concentration ratios along its circumference. These observations have been summarized in an example application of a two-region tube, in which different coatings have been applied for the high and low concentration regions. A simplified lumped model is used to compare performances and optimize the proposed solution. The model is again validated by using 2D finite elements heat transfer simulations.

Coating Candidates Comparison and a Two-Zone Coating Strategy for Parabolic Trough

Alberti, Fabrizio;Cozzini, Marco;Crema, Luigi;Bartali, Ruben
2012

Abstract

In solar thermal and thermodynamic applications a coating material is usually defined by its values of absorptance and thermal emittance; new solutions for increased absorptance and reduced thermal emittance are constantly sought, though the improvement of one factor typically leads to a performance reduction in the other. Moreover, absorptance and thermal emittance alone are not sufficient to fully compare different coating candidates for a specific application. A simplified lumped element analysis, based on energy balances, shows that parameters such as working temperature, concentration ratio, optical efficiency and direct normal irradiation should also be taken into account in the process of choosing the best candidate. Assuming a simplified correlation between absorptance and thermal emittance, it is then possible to identify the optimal combination which maximizes the system efficiency. Such analytical model is validated against a detailed 2D simulation on the parabolic trough cross section, in which a typical solar heat flux distribution is applied. Convenience in moving towards improved absorptance or thermal emittance compounds depends on the specific application and on the operating parameters of the overall process, such as concentration ratio and working temperature. For medium-high temperature applications, the tube itself is subject to different concentration ratios along its circumference. These observations have been summarized in an example application of a two-region tube, in which different coatings have been applied for the high and low concentration regions. A simplified lumped model is used to compare performances and optimize the proposed solution. The model is again validated by using 2D finite elements heat transfer simulations.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11582/100201
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