Recent progress in high-temperature solar selective coatings

This paper reviews two recent significant innovations relating to solar sel ective absorbing coatings. Through fundamental analysis and computer modell ing, we have developed a double cermet layer him structure for solar select ive surfaces with better solar performance than surfaces using a homogeneou s cermet layer or conventional graded him structure. A second innovation re duces the cost of depositing high-temperature solar coatings. This innovati on has two main features: (1) the ceramic and metallic components in the ce rmet are simultaneously deposited by direct current (DC) sputtering, and (2 ) the ceramic component is deposited by DC reactive sputtering and the meta llic component by DC non-reactive sputtering. Metal-aluminium nitride (M-AI N) cermet solar coatings have been deposited by two-target DC magnetron spu ttering technology. An Al metal target is used to deposit the AlN ceramic c omponent in the cermet by DC reactive sputtering in a gas mixture of argon and nitrogen. Tungsten, molybdenum and stainless steel (SS), which have goo d nitriding resistance, are used to deposit the metallic component by DC no n-reactive sputtering in the same gas mixture. M-AlN cermet solar coatings with the double cermet layer film structure were successfully deposited, ac hieving 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 co ater for depositing the SS-AIN cermet selective surfaces on batches of tube s has been constructed and successfully operated. SS-AIN cermet solar colle ctor tubes have been produced. Minor modifications to this commercial-scale coater, such as exchanging the SS target for a tungsten or molybdenum targ et, would enable the production of W-AIN or Mo-AlN cermet solar collector t ubes. Good thermal stability of SS-AIN, W-AIN and Mo-AIN cermet solar colle ctor tubes at a high temperature of 350-500 degrees C in vacuum is expected . The cost of these high-temperature solar collector tubes should be much l ower than solar collector tubes produced using conventional sputtering tech nology, DC sputtered Mo metal component and RF-sputtered Al2O3 ceramic comp onent, for solar thermal electricity applications. (C) 2000 Elsevier Scienc e B.V. All rights reserved.