Slope Error Measurements of Parabolic Troughs Using the Reflected Image of the Absorber Tube

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.