Analysis of Parabolic Trough Concentrator Mirror Shape Accuracy in different Measurement Setups

Abstract

Mirror shape accuracy as a key optical performance parameter for parabolic trough collectors can be assessed accurately by common measurement systems proving the high quality of state-of-the-art mirror panels. However, measurement results cannot always be compared directly because critical boundary conditions are not yet standardized. This paper quantifies the differences in shape accuracy results between the most common measurement setups for parabolic trough mirror panels and identifies measurement position, mounting mode and support frame employed for the measurement as relevant boundary conditions.Deflectometric measurements of mirror panels of RP3 geometry were performed at DLR's QUARZ Center Cologne in vertical (mounting points vertically and curved direction horizontally aligned) and horizontal measurement position (mirrors facing upward with mounting points horizontally aligned), both with and without tightening the mirrors to a support frame with screws. Finite element models were applied to calculate gravity-induced deformation and resulting slope and focus deviation on three different types of support frame: an ideally rigid support frame, a laboratory support frame, and an ideal support frame with elastic brackets.The measurement results demonstrate that the difference in position and mounting mode can lead to relevant deviations of the shape accuracy results higher than the uncertainty of the employed deflectometric measurement system. For RP3 inner mirror panels a difference of up to 0.7 mrad in root mean square slope deviation (SDx) and 3.3mm in root mean square focus deviation (FDx) from vertical to horizontal position was measured. Mirror shape specifications may thus not be applicable in all positions. Concerning the mounting onto different types of modeled support frame (in horizontal position) a variation of 0.5 mrad (SDx) and 1.8mm (FDx) was calculated for perfectly shaped RP3 inner mirrors mounted onto an ideally rigid support frame compared to the case when mounted to a support frame with elastic brackets.