Beam-Down Mirror: Thermal and Stress Analyses

The “beam-down” optics or solar tower reflector, developed and demonstrated at the Weizmann Institute of Science during the past 9 years, could be a useful modification of the classic solar tower technology, especially for solar applications where reacting solids are involved or heavy equipment has to be placed on top of a conventional tower. The theory of this optics has been thoroughly studied and reported elsewhere. This paper details the development and experience gained with the mirror facets of the tower reflector. Thermal and stress analyses are presented here, validated by temperature measurements and calculated incident flux map. The projection for a large scale solar plant of about 100 MW at the aperture of the receiver is illustrated. The current basic design of the facet made of a sandwich of mirrors glued back-to-back seems to be a feasible solution for future applications. Aluminum-glass facets failed, and cracks in the glass were observed in the course of time. Years of experience proved that using only natural cooling to the surrounding, for the glass/glass facets, which can reach $130–140°C$ during operation under average incident solar flux of about $30kW/m2$⁠, is a viable design. Maximum working temperatures of $160°C$ were experienced without any degradation of the reflectivity and the performance of these facets after several hundreds of operation hours.