Accelerated Durability Testing and Evaluation of Potential Improvements of Coatings for Solar Absorbers. Final Report

摘要

About one-hundred commercially available solar absorptive coatings, both selective and non-selective, were tested for durability by simultaneously exposing them in a collector box with a cover plate to seventy-eight temperature cycles between 450+35 exp 0 F (232+20 exp 0 C) and 200+25 exp 0 F and to solar radiation of about 22,000 langleys. Changes in the absorptance and emittance values were measured. Some of the coatings were also exposed to additional solar radiation of 156,000 langleys obtained at DSET Laboratories, Inc. of Phoenix, Arizona in EMMA (Equatorial Mount with Mirrors for Acceleration) under 8:1 concentration. Under the relatively high temperatures (180 exp 0 to 220 exp 0 C) and conditions of the combined thermal cycling and exposures described, most of the selective coatings, except cobalt oxide, zinc oxide, AMA sputtered on aluminum and stainless steel, nickel electroplated on anodized aluminum and copper oxide, did not show significant deterioration in optical properties. Majority of the non-selective coatings did not show significant changes in optical properties, but they exhibited physical, visual and mechanical damage. It is the conclusion of this work that the preceding test procedures provided valuable information on the long term durability of the coatings. Commercial black chrome coatings exhibit black, blue and reddish hues, which are due to different agglomerate sizes consisting of differing chromium metal to oxide ratios. Absorptance values vary slightly with hue. The presence of the hues did not affect the durability of the coatings under the above test sequence. Smut on black chrome contains a higher amount of oxide and exhibits somewhat higher Absorptivity and Emissivity values. (ERA citation 10:006637)