Optical design for a Fly's eye CPV system with large, onaxis dish solar concentrator

Abstract

Here we describe an optical concentrator system for CPV which uses a large dish reflector to concentrate sunlight and a small receiver with fly's eye lens array to apportion the focused sunlight equally between many multi-junction cells. The small receiver is fan cooled. This architecture is aimed at a reduction in cost of complete CPV generating systems, which include dual axis tracking. In many CPV systems the light is evenly divided by identical Fresnel lenses facing the sun, and the cells are typically housed along with the lenses in modules, backed by large, passively cooled aluminum plates. The fly's eye system advantages are that the weight, complexity and cost of the inherently large scale optical collection is reduced to just that of the 4 mm thick glass mirrors, and the small receiver packages are easier and less expensive to manufacture than large modules. A specific fly's eye generator design using two large off-axis primary reflectors and nonobscuring receivers below the collection area has been described recently by Hyatt et al. [1], and the principles have been demonstrated in a prototype. Here we describe an alternative on-axis design optimized for greater uniformity of the light distributed to the cells, and excellent tolerance to mispointing. Ray tracing analysis shows deviations in uniformity of the full spectrum of sunlight distributed to the many cells of no more than 1% for sun mispointing angles of up to 0.5° and 80% of the maximum flux reaches the CPV cells when mispointing to +/- 0.9°. In this design, the central receiver blocks 4% of the entering sunlight, however, mechanical construction is simplified with a corresponding reduction in weight and cost. Also, not all the light blocked by the receiver is wasted, since it will be used to power the receiver's cooling fan via a built-in small PV panel, eliminating parasitic power loss.