Recent progress in high-temperature solar selective coatings

This paper reviews two recent significant innovations relating to solar selective absorbing coatings. Through fundamental analysis and computer modelling, we have developed a double cermet layer film structure for solar selective surfaces with better solar performance than surfaces using a homogeneous cermet layer or conventional graded film structure. A second innovation reduces the cost of depositing high-temperature solar coatings. This innovation has two main features: (1) the ceramic and metallic components in the cermet are simultaneously deposited by direct current (DC) sputtering, and (2) the ceramic component is deposited by DC reactive sputtering and the metallic component by DC non-reactive sputtering. Metal-aluminium nitride (M-AlN) cermet solar coatings have been deposited by two-target DC magnetron sputtering technology. An Al metal target is used to deposit the AlN ceramic component in the cermet by DC reactive sputtering in a gas mixture of argon and nitrogen. Tungsten, molybdenum and stainless steel (SS), which have good nitriding resistance, are used to deposit the metallic component by DC non-reactive sputtering in the same gas mixture. M-AlN cermet solar coatings with the double cermet layer film structure were successfully deposited, achieving a solar absorptance of 0.92–0.96 and normal emittance of 0.04–0.05 at room temperature. A commercial-scale cylindrical DC magnetron sputter coater for depositing the SS-AlN cermet selective surfaces on batches of tubes has been constructed and successfully operated. SS-AlN cermet solar collector tubes have been produced. Minor modifications to this commercial-scale coater, such as exchanging the SS target for a tungsten or molybdenum target, would enable the production of W-AlN or Mo-AlN cermet solar collector tubes. Good thermal stability of SS-AlN, W-AlN and Mo-AlN cermet solar collector tubes at a high temperature of 350–500°C in vacuum is expected. The cost of these high-temperature solar collector tubes should be much lower than solar collector tubes produced using conventional sputtering technology, DC sputtered Mo metal component and RF-sputtered Al2O3 ceramic component, for solar thermal electricity applications.