Design and fabrication of highly thermally stable HfMoN/HfON/Al2O3 tandem absorber for solar thermal power generation applications

A new HfMoN(H)/HfMoN(L)/HfON/Al2O3 tandem absorber is designed and developed for high temperature solar thermal applications. The first absorber layer, HfMoN(H) is designed to have higher metallic content than the second HfMoN(L) layer. By varying the nitrogen flow rate, two different HfMoN layers with different refractive indices were deposited on SS substrates. The optical constants (n and k) measured using spectroscopic ellipsometry showed that HfMoN(H) and HfMoN(L) are the main absorber layers and HfON/Al2O3 acts as a double layer antireflection coating. The gradual decrease in the refractive indices from the substrate to the top resulted in increase in the absorptance, which was confirmed by the ellipsometric measurements. The optimized four layer tandem absorber exhibited high absorptance (α=0.94–0.95) and low thermal emittance (ε82 °C=0.13–0.14). The four layer tandem absorber was thermally stable up to 600 °C for 450 h and 650 °C for 100 h in vacuum. Whereas, coatings heat-treated in air were thermally stable up to 475 °C for 34 h.