Solar Selective Absorber Coating with Multilayer Infrared Reflector for Receiver Tubes Operating up to 550 °C

This work presents a modular W/Ag/AlN infrared reflector for solar selective absorber coatings (SSACs) intended for CSP/CST receiver tubes operating under vacuum at 550 °C. Optical analyses showed that AlN deposition preserved Ag infrared reflectance. Two Ag/AlN bilayers reduced the W reflector’s hemispherical emittance from 7.5% to 2.9%, calculated from room-temperature reflectance weighted at 550 °C. The reflector was first integrated into an SSAC for durability assessment. Accelerated aging under vacuum at 620, 650, and 690 °C yielded an Arrhenius activation energy of 497 kJ/mol and predicted an emittance increase of 0.34 percentage points after 25 years at 550 °C (8 h/day). An optimized SSAC subsequently achieved 95.2% solar absorptance and 11.3% hemispherical emittance on stainless steel, based on room-temperature reflectance. Simulations incorporating temperature-dependent optical properties estimated an emittance of 13.4% at 550 °C, versus 14.7% for a W-based coating. The previous Ag-based coating had lower estimated emittance (11.0%) but required three times more Ag. A 96 h aging test at 650 °C showed that the optimized SSAC exhibited an emittance degradation rate comparable to those of the samples of the coating used in the durability study, supporting the long-term stability prediction for the optimized architecture.