Intrinsic spectral selectivity in ultra-high temperature ceramics for solar applications

Different concepts for solar receiver systems have been developed, and each collector architecture (linear parabolic systems, solar dishes, solar tower plants, etc) demands peculiar system solutions. It is a general rule that the efficiency of solar thermal systems rapidly increases with increasing working temperature. In solar tower plants, a critical parameter for temperature increasing is the receiver, where the whole heliostat field concentrates the collected sunlight. The family of Ultra High Temperature Ceramics (UHTCs) can be interesting for this application because of its unique combination of properties. In fact UHTCs are characterized by some of the highest melting points of any known material (>3900°C for the monocarbides of Ta and Hf), high hardness, good wear resistance, good chemical stability and mechanical strength at high temperatures and high thermal conductivities. Up to now, UHTCs are employed mainly in the aerospace industry for hypersonic vehicles, rocket motor nozzles or atmospheric entry probes capable of the most extreme entry conditions. The ultra-high melting point of UHTCs, together with the unique combination of good thermal conductivity and chemical stability appear intriguing for employing them in high temperature solar furnaces. To assess the potentialities of UHTCs as selective sunlight absorber in solar cavities