Steffen Ulmer

The solar field is the major cost component of a solar thermal power plant and the optical quality of the concentrators has a significant impact on the field efficiency and thus on the performance of the power plant. Measuring slope deviations in the parabolic shape of the mirror panels in the accuracy and resolution required for these applications is a challenge as it is not required with the same characteristics in other industries. Photogrammetry and deflectometry are the two optical measurement methods that are typically used to measure this shape accuracy of mirror panels used in CSP applications. They have been compared and validated by measuring a typical mirror panel under optimal conditions. Additionally, a flat water surface has been measured as an absolute reference object using deflectometry. The remaining deviations between the results of both methods and to the reference object are discussed and possible sources of errors during the measurement are identified. A detailed error analysis is conducted for both methods and compared to the experimental findings. The results show that both methods allow for surface slope measurement with the necessary accuracy for present CSP applications and that among the two, deflectometry exhibits advantages in speed, measurement accuracy, and spatial resolution. However, for obtaining correct results several sources of errors have to be addressed appropriately during measurement and postprocessing.

A new and fast method for optically measuring the reflector slope of parabolic troughs with high accuracy has been developed. It uses the reflection of the absorber tube in the concentrator, as seen from some distance, and is therefore called “absorber reflection method.” A digital camera is placed at a distant observation point perpendicular to the trough axis with the concentrator orientated toward it. Then, a set of pictures from the absorber tube reflection is taken with slightly different tilt angles of the concentrator. A specially developed image analysis algorithm detects the edges of the absorber tube in the reflected images. This information, along with the geometric relationship between the components, the relative collector tilt angles, and the known approximately parabolic shape of the concentrator, is used to calculate the slopes perpendicular to the trough axis. Measurement results of a EuroTrough segment of four facets are presented and verified with results from a reference measurement using high-resolution close-range photogrammetry. The results show good agreement in statistical values as well as in local values of the reflector slope. Compared to the existing photogrammetric method, the new technique reduces drastically the time measurement.