Temperature- and Angle-Dependent Emissivity and Thermal Shock Resistance of the W/WAlN/WAlON/Al₂O₃-Based Spectrally Selective Absorber

Spectral emissivity is considered as one of the most critical thermophysical properties influencing photothermal conversion efficiency of solar selective absorbers. In addition, long-term stability at high temperature and thermal shock resistance are the performance-limiting properties of spectrally selective absorbers. In this context, this study reports the variation of emissivity with a change in emergence angles and operational temperatures for the newly developed W/WAlN/WAlON/Al2O3 absorber. An analysis of the experimental results demonstrates that hemispherical emissivity values at elevated temperature are comparable while calculated using both room temperature and high temperature reflectance data. Hence, the applicability of the room temperature measurement method is validated to evaluate high temperature emissivity. The analysis of angular measurements indicates an insignificant difference between hemispherical and near-normal emissivity values for W/WAlN/WAlON/Al2O3. The study suggests that hemispherical emissivity can be well approximated from near-normal emissivity values by avoiding complex angular measurement procedure. Importantly, one can achieve a combination of high solar absorptance (α = 0.90), low thermal emittance (ε = 0.15), and appreciable heliothermal efficiency (η = 87% at a concentration factor of 100) at 500 °C for the W/WAlN/WAlON/Al2O3 absorber. Thermal stability of this absorber was established by observing an insignificant change in the reflectance spectra while annealed at 80, 200, 300, and 400 °C. In addition, thermal cycling test for 30 times between room temperature and 450 °C in a high flux (40–60 kW/m2) solar simulator confirmed the efficacy of W/WAlN/WAlON/Al2O3 as a promising multilayer solar absorber for high temperature applications.