Accelerated Ageing of Solar Reflectors for Secondary Concentrators

Secondary concentrators are used in solar concentrating systems to redirect solar beams reflected by the primary concentrators to the focal point or line. Secondary concentrators allow smaller receiver apertures (and hence lower reradiation losses) and increase the concentrated solar flux density. Solar mirrors for secondary concentrators are permanently exposed to environmental conditions, a high radiation flux and elevated temperatures that potentially cause stress and degradations throughout the time. For most solar mirrors, exposures to sunlight during service, particulary ultraviolet wavelenghts, temperature, and moisture can lead to loss in reflectance. Insufficient cooling of mirror surfaces may lead to destruction of the mirror e.g. by melting. Therefore, analyzing solar reflectors of secondary concetrators by simulating the previouse conditions is crucial. The present work is focused on studying the degradation of the mirrors by simulating accelerated exposure, caused by several ambient paramenters and the effect of concetrated radiation. It has been done under the framwork of the SFERA project. The tests were performed at two installations. The salt spray tests and weathering tests (constant and cycle temperature, damp heat and humidity) were completed at the Solar Reflectors Durability Laboratory of PSA (Plataforma Solar de Almeria, Spain). The high flux exposure tests of mirrors with their cooling system were performed at the Solar Technology Laboratory (STL) of PSI (Paul Scherrer Institute, Switherland). A total of 9 selected mirror types were exposed to different levels of concentrated radiation and accelerated weathering parameters to simulate and analyze the effects caused by the ambient conditions. According to results obtained, aluminium reflectors and thin-glass silvered reflectors glued to an aluminium structure have demonstrated to be appropriate for cooled 3D secondary concentrators (tower systems). In addition, laminated silvered-glass mirrors have shown to be suitable for non-cooled 2D secondary concentrators (Fresnel and parabolic-trough collectors).