Performance of low cost solar reflectors for transferring sunlight to a distant collector

Abstract

Efficiency of reflection and optical transmission to a distant collector is a critical parameter, along with cost per unit area, in the selection of a heliostat design for the Central Collector Solar Electric Plant. Efficient optical transmission is not easily accomplished because of the large distance to be spanned in a multi-MW facility. Depending on heliostat location, the transmission distance may vary from a few hundred to thousands of feet. Design conditions which influence optical transmission over these long distances are: heliostat pointing accuracy; spreading of the reflected solar beam due to the finite size of the Sun's image; beam spreading due to reflector misalignment or waviness; aberration present if curved heliostat reflectors are used and beam spreading due to microscopic irregularities (characteristic length less than 0.1 mm) in the reflective surface. These factors increase in importance as the transmission distance from heliostat to collector increases. Even the most preliminary heliostat design activity requires a detailed evaluation of beam spreading before the most cost effective heliostat concept, or family or concepts depending on transmission distance, can be defined. Data are presented here which will be of value in assessing one of the factors causing beam spreading. An experimental method has been utilized to determine beam spreading due to microscopic surface irregularities prevalent with “mill finished” materials. The test method provides a nearly independent measure of the effect of surface imperfections. Data are presented for five candidate materials and, as reference, an optical quality first surface mirror.