New cermet coatings for mid-temperature applications for solar concentrated combine heat and power system

New cermet (ceramic-metal) composite coatings have been developed for solar absorbers in a solar concentrating system for combined heat and power operating in a mid-temperature range between 250 to 350 °C. The coatings were applied on stainless steel substrates. Two types of cermet with expected good duration properties were chosen: Nb-TiO2 and W-SiO2. The basic layer-structure concept consisted of four sub-layers, counted from the substrate: molybdenum infrared reflector, high metal concentration cermet of either Nb-TiO2 or W-SiO2, low metal concentration cermet of either Nb-TiO2 or W-SiO2 and SiO2 antireflection layer. The results from optimised coating fabrication gave solar absorptance and thermal emittance of 0.93 and 0.09 respectively for the Nb-TiO2 cermet and 0.91 and 0.08 for the W-SiO2 cermet based absorbers. Annealing at 350 °C did not change the absorptance but decreased the thermal emittance with 0.01 units. Adhesion to substrate and between sub-layers was good and even improved after annealing. In a next step up-scaling to deposition on tubes has been made for the Nb-TiO2 cermet type coating and such absorbers are now operating in the solar concentrating combined heat and power demonstration plant in Malta.