An overview of ultra-refractory ceramic absorbers for thermodynamic solar energy generation at high temperature

An efficiency improvement of concentrating solar power systems relies on a significant increase of the operating temperatures, exceeding 600 C. This goal can be achieved through the use of solar absorbers possessing high spectral selectivity and stability at such temperatures. Suitable alternatives to the largely used silicon carbide can be found in the ultra-high temperature ceramics class. This study focuses on the effect of processing, microstructure evolution and surface texture on the optical properties at room and high temperature. ZrB2-based ceramics are taken as case study to detect any correlation amongst composition, porosity, mean grain size, roughness and spectral selectivity. In addition, the effect of surface variation, induced by chemical etching or by exposure to oxidizing environment, thus simulating the actual operation conditions, are evaluated and compared to SiC optical properties. Absorbance and solar selectivity are discussed as a function of the microstructural and surface properties upon detailed roughness characterization. Advantages in the use of UHTCs as solar absorbers, strength and criticalities related to the use of these ceramics in comparison with SiC are discussed.