Measurement Techniques for the Optical Quality Assessment of Parabolic Trough Collector Fields in Commercial Solar Power Plants

The optical quality of the collector field of concentrating solar power plants is a fundamental factor for their profitability. High optical quality can be achieved and guaranteed when the manufacturing process is continuously monitored and adjusted in its essential steps. Therefore, a reliable and automatic method is needed to check the geometric accuracy of the concentrator support structures during their manufacturing in the assembly workshop. To verify the overall optical quality of the solar field after completion, an additional method is needed that allows measuring the slope errors of large mirror surfaces in the field with reasonable effort. This paper presents two measurement techniques that fulfill the required measurement demands: The stationary, automatic photogrammetry system “Q-Foto” has been developed for the shape accuracy control of concentrator structures. The measurement system consists of a digital camera, moved around the structure automatically while shooting photos of the concentrator structure from various positions. The photos are evaluated with photogrammetry software to check the assembly quality. The whole measurement and evaluation procedure is computer-controlled, robust and fast enough to be integrated in a solar collector production line. This paper describes how to reach, maintain and control this accuracy in the rough environment of an on-site production line. The new optical method “TARMES” has been developed for on-site measurements of mirror slope errors in parabolic trough collector fields. It uses the reflection of the absorber tube in the concentrator and is therefore called “Trough Absorber Reflection Measurement System (TARMES)”. A set of pictures of the absorber tube reflection is taken with the concentrator in slightly different and known tilt angles. The developed image analysis algorithm detects the edges of the absorber tube in the reflected images and corrects for distortions from perspective. This information is then used to calculate the slope errors of the mirror surface with high spatial resolution and accuracy. The consequences of the measured reflector slope deviations on the collector performance are calculated with raytracing. The results give detailed information about the optical quality of the concentrator, systematic errors in the manufacturing process and their optical performance penalty.