Air-stable selective surfaces for solar energy collectors

The design of highly selective air stable solar absorber coatings is elaborated. Systematic design has been done of absorber designs employing metals and dielectrics for operation at temperatures in excess of 150C. Results of heat stability tests and measurements of the optical properties of the films at temperatures up to 600C are presented. The program demonstrated optical and thermal behavior of interference selective surfaces of the general Ag-Al2O3-Cr-Al2O3 family with cutoff wavelengths ranging from 1.2 to 2.5 micrometers, absorbtances from 0.90 to 0.95, and emittances from 0.02 to 0.04 at operating temperatures. The Helio coatings appear to be highly stable in air at 150C.