Wide-angle lenses and image collapsing subreflectors for nontracking solar collectors

This paper presents new optical methods for the design of nontracking solar energy concentrators with acceptance angles of 60 degrees in the elevation (altitude) plane and +/-50 degrees in azimuth sectors. We have designed and constructed a two-point corrected cylindrical stepped prism lens (SPL) with 30.48-cm aperture height and F/D congruent with 1, which focuses well over the acceptance interval. Image collapsing subreflector (ICS) surfaces are synthesized that reflect the incident illumination refracted by the lens onto a small fixed absorbing area or shelf ~7.6 cm wide resulting in near maximum theoretical concentration ratios for these broad acceptance angles. Nearly 100% of the incident optical rays intercept the absorber shelf. The wide-angle and image collapsing optical properties were confirmed by laser and solar experiments. Rooftop thermal tests on a 30.5 x 30.5-cm collector section using selectively absorbing tubes with water as circulant were conducted that indicate aperture efficiencies of ~60% could be exected on large area collectors based on this design.