Published: Jan 1983
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This paper describes a matrix approach to testing mirrors (MATM) for solar concentrator applications. Specimens are exposed to elevated temperatures, high humidity, ultraviolet light, thermal cycling, atmospheric pollutants, and mechanical stresses at levels greater than those encountered in natural weathering but low enough that the degradation modes are expected to be similar to those observed in naturally weathered mirrors. The degradation of mirror performance is characterized by its reflectance. Microphotography and light-scattering measurements reveal defects in the silver layer.
In the initial phase of the research, specimens of commercially available wet chemical processed silver/glass mirrors were exposed for up to eight weeks at 80°C and 80 percent relative humidity (RH) and at 80 percent RH with a temperature cycle of 20 to 80 °C. A degradation in hemispherical reflectance of 15 percent and a tenfold increase in light scattering were observed for the least durable mirror types tested. The growth of defects causing light scattering is evident in the dark-field microphotography.
Removing the specimens, characterizing them optically, and returning them to the chambers did not alter their degradation rates in comparison with those of the specimens left undisturbed for equivalent exposure. Specimens subjected to temperature cycling between 20 and 80 °C showed less drop in reflectance than specimens left at high temperatures for equivalent exposure periods. However, temperature cycling caused severe edge corrosion in some mirror types. The differences in degradation rates between the specimen sets enabled a rank ordering of performance to be made. The precision of all the optical measurements was much greater than the specimen-to-specimen variation in the degradation rates. Hemispherical and diffuse reflectance measurements, especially at wavelengths less than 500 nm, provide a good means for assessing the degradation of silver/glass mirrors.
reflectance, durability, testing, mirrors, weathering, matrix, microstructure
Senior scientist, Solar Energy Research Institute, Golden, Colo
Manager, Section on Electro-Optics Systems, Pacific Northwest Laboratories, Richland, Wash.