Influences on Mirror Shape Accuracy and Performance Prediction for Parabolic Trough Concentrating Solar Power Systems

Abstract

Solar thermal power plants employ sunlight directly to generate electricity. Although the solar energy is free of charge, the plant needs to operate cost-efficiently in order to compete with conventional coal-, gas-, and nuclear energy. One possibility is the optimization of the components used. Therefore, minimizing thermal and optical losses is essential. Different loads and boundary conditions have a significant influence on shape accuracy of parabolic solar mirrors. In general, it depends substantially on the support structure which usually differs from those used in standardized laboratory measurements. The aim of this work is to characterize influences on the shape accuracy in collector, the preparation of appropriate models and programs, as well as the evaluation of how well mirror shape in collector can be predicted based on laboratory results. The optical performance of the mirrors as well as the annual yield are quantified. As a result of this work, interfaces to the DLR internally developed tools SPRAY and Greenius are created and comprehensive workflows employing Matlab Tools provided. In laboratory and collector test benches optical measurements of the mirror shape, the support structure and occurring strains at the glass brackets holding the mirrors are carried out and reproduced with FE-models. An optimization workflow is implemented to support the evaluations. The investigations were carried out with the EuroTrough-geometry by using mirrors of RP 3 geometry. In the following, important results of this work are summarized: As an important finding, the consideration of further parts of the support structure than the glass brackets does not significantly influence dead load results. However, and as presented in exemplary simulations, geometrical deviations of the mounting elements of the support structure can have a significant effect on the shape accuracy. The resulting optical performance as well as the influence on the annual yield for the investigated load cases is evaluated. Mounting inaccuracies of the mirror mounting elements can induce significant shape deformations even in case of only a few mrad or mm deviation - and also affect the optical performance and the annual yield of the plant. Deriving technical specifications, but also the optimization of the geometries is made possible, since optical, thermal, and economical losses can be quantified due to the progress within this work. The predictability of mirror shape in collector is limited, and will be discussed, since the aforementioned geometrical deviations of the support structure are not universally known or predictable.