The present thesis examines the influence of measurement parameters and boundary conditions on shape accuracy of parabolic trough concentrator mirrors of RP3 geometry by means of optical measurements and finite element analyses. Deflectometric measurements of mirror panels are performed in common laboratory setups: in vertical and horizontal measurement position, both with and without tightening the mirrors to a support frame with screws. The results serve for validation of finite element models which are applied to calculate the difference in shape accuracy between, firstly, the individual setups and secondly, between mirrors horizontally fixed to differently rigid support frames. The effect of angular deviations of the mounting pads is assessed. Gravity-induced deformation of mirrors mounted to two exemplary collector support structures is determined for the relevant working positions. The transferability of shape accuracy data measured in laboratory to data applying for mirrors mounted to a collector is assessed. For RP3 inner mirror panels a difference of up to 0.8 mrad in root mean square slope deviation (SDx) and 3.3 mm in root mean square focus deviation (FDx) from vertical to horizontal position is measured. Gravity-induced deformation and resulting slope and focus deviation on three different types of horizontally oriented support frames is modeled: an ideally rigid support frame, a laboratory support frame, and an ideal support frame with elastic brackets. A variation of 0.5 mrad (SDx) and 1.8 mm (FDx) is 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. In case of an angular deviation of the mounting pads mirror panels fixed to an ideally rigid support frame show additional local slope deviation in the magnitude of the angular pad deviation. Due to gravity-induced deformation and slope deviation the shape accuracy of RP3 inner mirrors mounted to an ideally rigid collector support structure varies up to 1.0 mrad in SDx and up to 1.2 mrad in SDx when compared to the ideal shape and the shape in zenith collector angle, respectively. For RP3 inner mirrors mounted to a collector structure with elastic brackets the values are 1.6 mrad and 2.1 mrad. Whereas the models of mirrors mounted to a laboratory support frame allow for a conversion and thus a comparison of results obtained in different setups, the models for the collector support structure need to be refined in order to allow for a conversion from laboratory to collector measurement results. The results demonstrate that measurement position, mounting mode and employed support structure have to be documented in order to obtain reproducible measurement results. They should hence be included as measurement boundary conditions in guidelines for the measurement and assessment of mirror shape accuracy for concentrating solar collectors. The inspection of the angular accuracy of the mounting pads is part of a complete optical mirror assessment. The data of standardized shape accuracy measurement serve for subsequent calculation of intercept factors and annual energy yields as well as for the evaluation of optimization approaches.
展开▼
