Evaluation and assessment of gravity load on mirror shape and focusing quality of parabolic trough solar mirrors using finite-element analysis

Kurzfassung

In order to achieve high optical efficiency of solar parabolic trough collectors and high performance of the solar field, the concentrator mirrors in concentrating solar power plants are expected to maintain accurate parabolic shape over the daily operation cycles. In addition to shape imperfections introduced by the manufacturing process, deformation due to gravity load and mounting forces is an inevitable factor affecting shape accuracy in all types of parabolic trough collectors. In this paper the effect of gravity load on mirror shape and resulting slope and focus deviation values is characterized and quantified in finite element analyses referenced to specific lab tests. Inner and outer ideally shaped parabolic mirror of RP3 geometry are evaluated for various discrete collector angles relevant for operation on different collector support structures. Three finite-element-models are included in the study: two with idealized support structures (ideal and elastic case) and one including the cantilever arms as relevant parts of the EuroTrough type collector support structure (cantilever case). Constructional design and stiffness of the support structure significantly determine characteristic and magnitude of deformation. Resulting rms values of the sagged mirror panels are as high as SDx = 1.7 mrad and FDx = 6.3 mm (inner mirror, elastic case) and SDx = 1.1 mrad and FDx = 5.6 mm (outer mirror, cantilever case). Depending on the type of support structure, minimum and maximum values occur at different collector angles. Rms slope and focus deviation values are closer to the 0° (zenith) collector angle case than to the non-deformed (ideal) mirror shape. This leads to optimizing the mirror shape for 0° (zenith) collector angle. Different support structures in design and stiffness for shape accuracy assessment in laboratory and those used in the collector make it difficult to find one optimum shape for all types of mirror and collector.