Determining The Optical Quality of Focusing Collectors without Laser Ray Tracing

Abstract

This paper describes a novel alternative to the laser ray trace technique for evaluating the optical Quality of focusing solar collectors. The new method does not require any equipment beyond that which is used for measuring collector efficiency; it could therefore become part of routine collector testing. The total optical errors resulting from imperfect specularity and from inaccuracies in reflector position or slope are characterized by an angular standard deviation σoptical, the rms deviation of the reflected rays from the design direction. The method is based on the fact that the off-axis performance of a concentrator depends on σoptical. An angular scan is performed; i.e., the collector output is measured as a function of misalignment angle over the entire range of angles for which there is measurable output (typically a few degrees). This test should be carried out on a very clear day, with the receiver close to ambient temperature (if the latter condition cannot be satisfied, appropriate corrections are necessary). The parameter σoptical is then determined by a least-squares fit between the measured and the calculated angular scan. We tested the method on a parabolic trough collector manufactured by Hexcel, and it is suitable for parabolic dishes as well.