  Pyromark 2500 is a widely used coating for concentrated solar power central
receiver systems due to its high absorptivity, ease in application, and
relatively low cost. Pyromark's performance is quantified by its figure of
merit (FOM), which relates the coating's heat losses to its solar-to-thermal
conversion 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 compares
the most common FOM expression, which neglects convection losses, to an FOM
that includes all heat losses experienced by a central receiver. Isothermal
aging experiments are conducted on Pyromark-coated Inconel 600 substrates at
750{\deg}C. The spectral, hemispherical absorptivity of the samples is measured
at room temperature with a spectrophotometer and input into a finite element
analysis model that includes radiation and convection boundary conditions. The
heat flux and temperature output by the model are used to determine the heat
losses and FOM of the Pyromark samples. After 151 h of thermal aging, the
sample with the thinnest Pyromark coat maintains the most stable total,
hemispherical absorptivity. Conversely, the total, hemispherical absorptivity
of the sample with the thickest Pyromark coat drops by a maximum of 1.73%, and
the corresponding maximum drop in FOM is 1.90% when windy conditions (which are
expected around central receivers) are assumed. In windy conditions, convection
losses constitute between 21% and 24% of the samples' total heat loss; thus,
the most common FOM expression in the literature overestimates the samples' FOM
by ~4.40%. An analysis of the samples' heat losses indicates that reflection
losses exceed emission losses when the absorptivity declines significantly.
