Analysis of the effects of outdoor and accelerated ageing on the optical properties of reflector materials for solar energy applications

Lifetime tests of solar reflector materials are eligible prior to application, to prove the stability of the optical properties. In this work, six reflector materials were aged outdoors and in a climatic test chamber. The surfaces of the samples were inspected using profilometry and optical microscopy. The total and specular reflectance spectra were measured using spectrophotometry and the weighted solar reflectance values were calculated before and after ageing. Reflectors of silvered glass, anodised aluminium, thin film-coated anodised aluminium, and lacquered rolled aluminium withstood accelerated testing well, while a laminated evaporated aluminium reflector, which was specular initially, became diffuse. Laminated and lacquered reflectors withstood outdoor ageing better than expected from accelerated ageing and better than unprotected thin film-coated and anodised aluminium, which degraded significantly outdoors. Thus, optical degradation depends on climatic conditions and on the protective layer, if applicable. The discrepancy between results from outdoor and accelerated ageing shows that a thorough understanding of corrosion processes is necessary for drawing conclusions about long-term performance from accelerated ageing tests.