Infrared emissivity of copper-alloyed spinel black coatings for concentrated solar power systems

The directional spectral emissivities of four new copper-alloyed spinel coatings for concentrated solar power applications were measured up to 800 °C and compared Pyromark 2500®, deposited in the same conditions on Inconel 625. Reproducible results were found for all coatings at all temperatures, with similar spectral features at working temperatures. The temperature and angular dependences are related to the morphology and composition of the samples. The total hemispherical emissivity increases up to 400 °C for all coatings and then stabilizes, with similar values for most materials, except for the porous Cu0.5Cr1.1Mn1.4O4 coating. This coating offers a reduced total hemispherical emissivity due to increased semitransparency at high angles arising from its porosity. This porosity is linked to an increase in both the solar absorptance and the emissivity in the normal direction due to enhanced light trapping, which means that this coating shows signs of directional selectivity. These results, together with the data dispersion reported for Pyromark, suggest that structural properties are key for the high-temperature emissivity of the coatings and highlight the importance of direct emissivity characterization. Combined with absorptance measurements, these emissivity measurements allow for accurate calculations of the high-temperature efficiencies of the coatings, which reach values up to 0.929.