Development of W-SiO2 and Nb-TiO2 solar absorber coatings for combined heat and power systems at intermediate operation temperatures

Two new absorber coatings for mid-temperature operation (300-350 °C) in collectors for solar thermal electricity plants are presented in this study. The absorbers consist of two cermet layers of either W-SiO2 or Nb-TiO2, deposited on a molybdenum infrared reflector and coated with an antireflection layer of silicon oxide. The optimization of the optical performance was made in two steps. First, the layer structure was optimized in model calculations. The optical constants used in this modelling were derived directly from sputtered films of the cermet constituents using reflectance and transmittance measurements. The absorber coatings were then sputter-deposited using parameters from the modelling. The results show good agreement between modelled and sputtered optical performance evaluated as solar absorptance and thermal emittance at 350 °C. The optimal values reached for W-SiO2 was 0.91 in combination with 0.08 and 0.93 in combination with 0.09 for Nb-TiO2. The materials characterization from XRD, AES and TEM shows that the composite coatings contain nano-metal inclusions, meaning that they are cermet coatings. Scratch tests show that the coatings adhere well to the substrate of stainless steel. Temperature testing at 350 °C in vacuum for up to 1500 h shows that both coatings are stable under such conditions. Only a slight change occurs during the first 72 h that decreases the emittance but does not change the solar absorptance. ERDA confirms that there is no detectable level of ion migration between layers, only a small decrease in hydrogen content was observed, which indicates outgassing.