Durability of High-temperature Absorber Coatings

Abstract

Solar absorber coatings play a crucial role in concentrating solar power (CSP) plants by converting solar irradiance into thermal energy. The primary objective of the research presented in this doctoral thesis is to understand the optical and physical degradation mechanisms for such coatings while in operation. Such knowledge will assist future development of new coatings, with better durability and improved optical performance. This study focuses on three coatings: (1) Pyromark 2500, a coating in common use; (2) a so-called 'coral-structured coating' developed by industry partner Nano Frontier Technology (NFT); and (3) CoteRill750, a coating developed by John Cockerill. Aging mechanisms were studied for all three coatings using different accelerated aging tests. Isothermal aging tests at 800C and 900C showed that optical degradation of these coatings is primarily associated with changes in morphology and the formation of new phases. Analysis of coatings that failed mechanically during testing suggests key reasons for such failures include reduction in bonding strength owing to micro-defects and the formation of oxide layers, combined with stress caused by thermal expansion coefficient mismatch between layers. The coral-structured coating was found to be more resistant to thermal stress compared to Pyromark because of the macro- and micro-porous structures, which accommodate thermal stresses. For Pyromark, curing temperature was found to be an important factor in optical and mechanical stability. Isothermal aging testing cannot simulate conditions on a cloudy day, where coating temperatures may fluctuate significantly. Therefore, thermal cycling tests under a range of different conditions were conducted. The results showed that the combination of temperature ramping plus holding at high temperature accelerated aging mechanisms. Such mechanisms include diffusion, sintering and growth of oxide layers, which all contribute to optical degradation and reduction in the bonding strength between layers. Hence, this type of test is necessary for comparing the durability between different coatings. Further testing focussed on thermal cycling using on-sun and high-flux facilities, albeit for a shorter time. Another topic examined in this doctoral research is the measurement of coating emittance, which is a factor affecting emission loss from the receiver and efficiency at high temperatures. Show more