Wavelength dependent scattering caused by dust accumulation on solar mirrors

Abstract

The reflectance properties of several solar mirror materials have been studied after the mirrors were subjected to ten months of outdoor exposure in Albuquerque, NM. The mirror materials include second-surface, silvered float glass; an aluminized acrylic film; an aluminized teflon film; and a roll polished aluminum sheet. The specular reflectance properties were measured as a function of angular aperture using a bidirectional reflectometer while the diffuse reflectance and hemispherical reflectance were measured using an integrating sphere reflectometer. The outdoor exposure resulted in a buildup on the mirror surface of particulates, which can both scatter and absorb incident solar radiation. For the materials studied, the reduction in specular reflectance was approximately five times greater than the reduction in hemispherical reflectance, indicating that the deposited particles are much more effective in scattering radiation than absorbing it. The specular reflectance loss caused by the accumulated dust particles was measured for each mirror during the 10 month exposure. When normalized by the hemispherical reflectance of each mirror, the wavelength dependence of the normalized scattering curves were identical for all mirrors after the same exposure time. The normalized scattering curve changes during the course of the outdoor exposure, suggesting that the particle size distribution of accumulated dust is changing with time. Over the 10 month period, the average decrease in the specular reflectance measured at 500 nm was slightly smaller for the silvered glass mirror as compared to the aluminized acrylic and aluminized teflon mirrors. The measurement statistics associated with determining the mean reflectance value after outdoor exposure from a number of discrete measurements are briefly discussed. In addition, data are presented which indicate that a decrease in the Fe 2+ absorption band after outdoor exposure for the silvered float glass mirror.