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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. [ABSTRACT FROM AUTHOR]

We describe a CPV generator in which an off-axis paraboloidal dish reflector powers a small receiver near the focus, housing many individually illuminated multijunction cells. The receiver entrance window doubles as a field lens that forms a reduced scale image of the reflector, at concentration of ~30x. The image has a sharp boundary, and its position is stable against tracking errors. A fly's eye lens array divides the image into equal portions, and further concentrates it to ~500x onto the cells. This approach is in contrast to nearly all previous PV and CPV, where sunlight is equal apportioned (for simple series electrical connection) directly on entering the system. In our approach, small multijunction cells are packaged into a small receiver module that will be less expensive (per watt) to manufacture than large conventional PV or CPV modules, and can be economically upgraded for 40 year lifetime. Our concept differs from Rehnu's dish/receiver design [1] in its lack of obscuration and simpler cooling, using forced air convection rather than pumped liquid coolant, this made possible by the lower heat density at the cell array. In preliminary on-sun system data with a 2.4 m² prototype powering 5.2 mm cells at 500x concentration, we demonstrate good tolerance to mispointing (90% at 0.5° off-axis), good air cooling (cell mounting plate at 19°C above ambient) and uniform division of light between the cells (scatter of 3.3% rms). [ABSTRACT FROM AUTHOR]

Hybrid CPV-T with combined electrical and thermal output is well suited to solar generation from fixed limited areas, such as on the roof of an industrial or commercial facility with need for heat. This application will become especially attractive once overall electrical conversion efficiency of 40% is reached, as is projected for REhnu CPV systems using multijunction cells of 50% efficiency, anticipated in a few years. We outline here a configuration of dishbased CPV trackers optimized for close packing on a flat roof in a triangular grid, with a mirror area-to-ground area ratio of 50%. When the geometry of shadowing averaged over a year is taken into account, 80% of all the sunlight that would strike the rooftop is directed into the receivers. Such an array on a given area of flat roof will generate more electrical energy than would be possible with conventional PV panels, even if covering the entire rooftop, because of silicon's relative inefficiency. For example, in Tucson, the annual average global flux of 5.7 kWh/m2/day on a horizontal surface covered with 22% silicon modules will yield 1.25 kWh/m2/day. We show that a CPV system collecting 80% of all the direct sunlight of 7.0 kWh/m2 and converting it with 40% efficiency will yield 2.24 kWh/m2/day of rooftop area, nearly twice as much4. Thermal power will double again the total energy yield A dual a×is CPV-T tracker designed specifically very close spacing has been built to carry a single dish mirror of the standard type used in REhnu's M-8 generator, described by Stalcup et al in these proceedings1,2. Sunlight is collected and focused by a single square paraboloidal mirror, 1.65 × 1.65 m with focal length of 1.5 m. For closest possible packing without mechanical interference, and for broad distribution of load on a rooftop, the mirror and receiver are mounted to a C-ring structure, configured such that the elevation and azimuth a×es intersect at a virtual pivot, at the center of the sphere that just clears the receiver and the corners of the mirror. Initial tests of closed loop tracking show an accuracy of 0.03° rms under calm conditions, and 0.04° rms in 6 m/sec wind. [ABSTRACT FROM AUTHOR]