In concentrating solar systems, it is essential to study the optical losses of the collectors. A fundamental parameter is the intercept factor, namely, the fraction of sunrays reflected by the concentrator that reaches the receiver. Optical profilometry studies the relationship between the collector profile and the intercept factor, which influences the collection efficiency. Profilometric analyses were performed on a micro-parabolic trough collector (m-PTC), with reduced sizes and greater mirror curvature than a usual PTC. The proposed technique projects a luminous pattern (structured light) both on the collector with an opaque covering and on a flat reference plane. Measurement set-up and calibration technique were developed for m-PTC. A program coded in Python analyzed the images and reconstructs the mirror profile. The tilted reference plane was reconstructed using an original geometric model and a calibration procedure. The focal length of each parabolic section was calculated, providing information on surface defects in the mirror. An important parameter obtained was the displacement of the focus of the parabola with respect to the ideal position. Using this value, the intercept factor was estimated to be 0.89. The proposed technique was validated by comparing the results with an independent profilometric study applied to the same m-PTC.
The development of a new prototype of micro-PTC (Parabolic Trough Collector) system requests to study the optical characteristics of the mirrors in use. Two techniques were developed for this purpose. The first one is based on the projection of a Moiré pattern on the mirror surface covered with an opaque material. It provides a reconstruction of the profile from which it is possible to calculate focus position, vertex position and hence the focal length. This method represents a very fast estimation of the macroscopic defects of parabolic collectors, which is useful to determine the quality of the product. The second method is based on the analysis of the reflection of a laser beam, scanning the surface of the parabolic mirror. Despite this latter method is more time consuming, it is a more accurate technique that allows measuring both position and slope of each point of the mirror. In addition, this second method permits to estimate the intercept factor of the concentrator.
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