Optical characterization of TiAlNx/TiAlNy/Al2O3 tandem solar selective absorber coatings

Solar selective absorber coatings, consisting in a TiAlNx / TiAlNy tandem absorber with low (metallic-like) and high (semiconductor-like) nitrogen content, and an AL(2)O(3) antireflective coating, were deposited on stainless steel. A full optical characterization was carried out on these coatings. Reflectance spectra were measured by spectrophotometry in the UV to mid-IR range (0.25-25 pm), allowing for the calculation of total solar absorptance and thermal emittance. An absorptance of 0.93 and an emittance of 0.22 at 550 degrees C were obtained for the best coating. The temperature dependence of reflectance was investigated in the infrared region and results demonstrated the validity of the classical room temperature measurement approximation to calculate thermal emittance at high temperature for these coatings. The total hemispherical emittance of the coatings was calculated from hemispherical directional reflectance spectra measured at different angles, showing that the near normal hemispherical emittance is a good approximation of the total hemispherical emittance. The total solar absorptance was also directly measured under natural solar irradiation using a unique solar facility.