• Energy Research

Abstract Secondary concentrators are used in solar concentrating systems to redirect solar beams reflected by the primary concentrators to the focal point or line. These components allow to increase the concentrated solar flux density and hence to lower thermal radiation losses. Solar reflectors for secondary concentrators are permanently exposed to environmental conditions, high radiation fluxes and elevated temperatures that potentially cause stress and degradation throughout the time. Therefore, analyzing solar reflectors of secondary concentrators by simulating these conditions is crucial. No previous research works about the durability of solar reflector materials for secondary concentrators have been reported. The present work is focused on studying the degradation of the reflector materials by simulating accelerated aging, caused by several ambient parameters and the effect of concentrated radiation. Both cooled and uncooled systems for secondary concentrators are included in this study. According to results obtained, aluminum reflectors and thin silvered-glass reflectors glued to an aluminum structure showed minimum reflectance losses and structural degradation under the operation conditions of cooled 3D secondary concentrators (tower systems). Following critical aspects to avoid reflector degradation were identified: to select a suitable adhesive material to glue the thin silvered-glass reflector to the support aluminum structure, to properly protect reflectors edges, to design a suitable cooling system and to avoid the combination of high radiation fluxes with mechanical stress. In addition, laminated silvered-glass reflectors have shown to be suitable for uncooled 2D secondary concentrators (Fresnel collectors). Furthermore, a comparison with naturally aged secondary concentrators using silvered-glass reflectors glued to an aluminum structure revealed that the simulated degradation under accelerated conditions performed in this work did reproduce the most frequent degradation patterns suffered in real operating conditions.

Concentrating solar power (CSP) technologies are foreseen to be a crucial actor in the future renewable energy mix. Soil accumulation on the optical surfaces of CSP plants involves significant expenses of the operation and maintenance activities because a high cleanliness level is required to achieve proper plant revenues. Normally, only the front side of the solar reflectors is cleaned to reflect the maximum possible amount of direct solar radiation towards the receiver. However, soil deposited on the backside of the reflector could provoke degradation and might need to be considered in the cleaning strategy. As this possible degradation has never been studied, this work is dedicated to assess if the backside of reflectors should be regularly cleaned. The influence of the sand in the possible paint degradation depends on its chemical composition and the weather conditions. Therefore, several climatic conditions of artificially soiled reflector samples with different types of sand were simulated in accelerated aging tests. Concerning the results obtained, the ambient conditions simulated by the damp heat and thermal cycling tests were the only ones that produced a significant degradation of the backside paints. Also, the sand from Ouarzazate was responsible for higher deterioration.

The application of antireflective (AR) coatings on glass components for solar thermal collectors and PV modules increases the solar efficiency by increasing the sun energy that reaches the active layer. The low refractive index of this material required to satisfy the condition of producing destructive interference in AR/glass interfaces, makes necessary to deposit porous silica layers which are mechanically weaker than dense silica layers. Soiling of AR coated glass glazing not only can reduce the solar transmittance and increase the scattering but also can deteriorate the surface of the AR coating leading to irreversible damage by abrasion and reducing the long-term performance. The use of an anti-soiling (AS) coating on the top of the AR surface could avoid this yield loss due to soiling and increases the durability of the solar device. This paper describes the effect on the soiling and cleaning of an AS treatment on AR coated glass samples. The effect of the sample soiling, the type of sand applied and the brush used has been studied. The AS coating not only affects the sample soiling but also diminishes the surface damage