Durability of solar reflector materials for secondary concentrators used in CSP systems

Abstract

a b s t r a c tSecondary 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.

Investigation of temperature distribution on a new linear Fresnel receiver assembly under high solar flux

“…A comparatively smaller peak in absorbed solar flux exists at the center point of the secondary reflector in Figure ; however, the surface temperature at this point is comparatively higher than that at ±0.12 m. This is because the secondary reflector traps the hot air rising from the glass envelope, and thus, the middle portion of the secondary reflector experiences the highest temperature under the no‐wind condition. That a large portion of the secondary reflector experiences a temperature above 80°C suggests that a regular glass mirror is no longer suitable and a high‐temperature glass mirror is required for the secondary reflector . The glass envelope experiences a temperature ranging from 130°C to 170°C.…”

“…At this solar flux concentration, elevated operating temperatures are expected to impose additional material requirements for components such as a secondary reflector and glass envelope. 36,37 An understanding of expected material temperatures is a prerequisite to both material selection and a final viable detailed engineering design.…”

Thermal modeling of a secondary concentrator integrated with an open direct-absorption molten-salt volumetric receiver in a beam-down tower system

“…Typical reflecting materials used in secondary concentrators are thin silvered glass or enhanced aluminum reflectors 5 . For the project's first prototype, polished aluminum was chosen as it was the cheapest and easiest option despite its relatively low specular reflectance.…”

“…Reflectance loss from exposure to high temperatures 5 would lead to even higher absorbed flux. Three dimensional secondary concentrators are typically water-cooled and kept at a temperature around 85 °C 5 to keep cooling water in liquid state and avoid possible degradation of the glue if applicable. However, for our open tank concept, water-cooling of FOE is not desirable in order to avoid any possible water intrusion in the molten salt tank, as contact of water and hot molten nitrate salts presents risks of explosive generation of steam 6 .…”

Energetic and exergetic analysis of Rankine cycles for solar power plants with parabolic trough and thermal storage

“…The Concentrated Solar Power (CSP) technology gets to high temperatures, depending of capture system: heliostats and towers [1][2][3], or parabolic troughs [4][5][6][7][8].…”