Effects of environmental and operational variability on the spectrally selective properties of W/WAlN/WAlON/Al2O3-based solar absorber coating

The stability of solar selective absorber coatings in hostile environments (e.g., humid condition, corrosive medium, longer exposure at elevated temperature) needs to be critically assessed to ensure durability of such coatings. In the present work, magnetron sputtered W/WAlN/WAlON/Al2O3-based absorber coating with a high absorptance (0.958) and low emittance (82 °C) has been tested in humid and corrosive environments. The selectivity (absorptance/emittance) of the coating did not change after keeping it in 95% humidity at 37 °C for 400 h. Corrosion study in 3.5% NaCl solution reveals that this novel multilayer coating has a better corrosion resistance than that of uncoated stainless steel (SS) substrate. The nanoindentation test on the coating indicates that it has a hardness of 9.6 ± 0.5 GPa. The performance of the coating did not degrade after heat treatment at 350 °C in air for 1000 h. Additionally, the activation energy for degradation has been determined to predict the stability of the coating at high temperature. Further, the normal spectral emissivity of the tandem solar absorber was measured in the full angular range from 200 to 500 °C. The results of emissivity measurements by varying observation angles are analysed using numerical integration to obtain the total hemispherical emissivity. In summary, W/WAlN/WAlON/Al2O3 stack is an attractive candidate absorber coating for photo-thermal conversion systems.