The effect of soiling on solar mirrors and techniques used to maintain high reflectivity

Solar mirrors are used to concentrate low-level solar radiation to power levels which are practical and efficient for consumption. Any interference with the collection of that energy not only decreases the power level but also increases the cost of the energy available from a solar power system. One of the most immediate and drastic effects of outdoor exposure is the reflectance loss due to the accumulation of foreign particles on the mirror surface. Specular reflectance losses as great as 25% have been observed for mirrors exposed for only a few weeks. The effect of the deposited particles is to reduce the reflected energy by both absorbing and scattering light. The degree to which the particles reduce the collection of reflected energy depends on their composition, number and size distribution. An additional factor is the optics of the collection system. The angular acceptance aperture of the system, defined as the angle subtended by the receiver at the concentrator surface, determines the relative importance of the scattering due to dust accumulation. For flat plate thermal and photovoltaic collectors which have essentially a 180/sup 0/ angular acceptance aperture, scattering of the incident light is not critical but absorption can be an important factormore » in the loss of energy. For concentrating collection systems, such as line focus collectors and central receivers, angular acceptance apertures of a few degrees make scattering at the concentrator surface much more important and can result in severe energy losses. Results of a study of each of these areas are presented and discussed. (WHK)« less