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Pyromark 2500 is a widely used coating for concentrated solar power centralreceiver systems due to its high absorptivity, ease in application, andrelatively low cost. Pyromark's performance is quantified by its figure ofmerit (FOM), which relates the coating's heat losses to its solar-to-thermalconversion efficiency. After long-term exposure to high temperatures(>750{\deg}C) and irradiance levels, Pyromark's absorptivity and FOM decrease.The aim of this research is to evaluate changes in Pyromark's absorptivity,heat losses, and FOM as a function of thermal aging. This work also comparesthe most common FOM expression, which neglects convection losses, to an FOMthat includes all heat losses experienced by a central receiver. Isothermalaging experiments are conducted on Pyromark-coated Inconel 600 substrates at750{\deg}C. The spectral, hemispherical absorptivity of the samples is measuredat room temperature with a spectrophotometer and input into a finite elementanalysis model that includes radiation and convection boundary conditions. Theheat flux and temperature output by the model are used to determine the heatlosses and FOM of the Pyromark samples. After 151 h of thermal aging, thesample with the thinnest Pyromark coat maintains the most stable total,hemispherical absorptivity. Conversely, the total, hemispherical absorptivityof the sample with the thickest Pyromark coat drops by a maximum of 1.73%, andthe corresponding maximum drop in FOM is 1.90% when windy conditions (which areexpected around central receivers) are assumed. In windy conditions, convectionlosses constitute between 21% and 24% of the samples' total heat loss; thus,the most common FOM expression in the literature overestimates the samples' FOMby ~4.40%. An analysis of the samples' heat losses indicates that reflectionlosses exceed emission losses when the absorptivity declines significantly.

Impacts of thermal aging and associated heat losses on the performance of a Pyromark 2500-coated concentrated solar power central receiver