Measurement of high temperature emissivity and photothermal conversion efficiency of TiAlC/TiAlCN/TiAlSiCN/TiAlSiCO/TiAlSiO spectrally selective coating

A spectrally selective TiAlC/TiAlCN/TiAlSiCN/TiAlSiCO/TiAlSiO coating was deposited on stainless steel substrate by unbalanced magnetron sputtering system. Each individual layer of the tandem absorber was optimized by varying the reactive gas flow rates (C2H2, N2 and O2) and target power densities (Ti, Al and Si). The optimized tandem absorber shows a solar absorptance of 0.960 and an emittance of 0.15 at 82 °C, measured using solar spectrum reflectometer and emissometer, respectively. In order to study the optical properties of the deposited tandem absorber at high operating temperatures the reflectance spectra of the tandem absorber were measured at temperatures ranging from 80 °C to 500 °C by UV-Vis-NIR spectrophotometer and FTIR spectrometers. The reflectance spectra of the as-deposited sample and after high temperature reflectance measurements did not show any significant changes. The thermal emittance of the tandem absorber at high temperatures (80-500 °C) was studied in detail. At the temperature of 200 °C, 300 °C, 400 °C and 500 °C the tandem absorber shows the emittance of 0.152-0.157, 0.181-0.19, 0.214-0.246 and 0.251-0.275, respectively with an absorptance of ~0.930. These results show the good selectivity of the tandem absorber even at high operating temperatures (e.g., 500 °C) with a photothermal conversion efficiency of 88%, thus demonstrating that the tandem absorber is suitable for solar thermal power generation applications. Reflectance and roughness data of the absorber coating post annealing in air up to 600 °C for 2 h, carried out independently, corroborated the present results.