The figure illustrates aspects of a lightweight, relatively inexpensive solar photovoltaic module suitable for use on Earth or in outer space in conjunction with at least a single-axis Sun-tracking apparatus. The module contains line-focus primary Fresnel lenses that act in conjunction with spot-focus, compound parabolic secondary solid optics to concentrate incident sunlight onto tandem, paired GaAs and GaSb photovoltaic cells arrayed along the focal lines. The tandem-photovoltaic-cell design and the optical and electrical configurations are combined to obtain high energy-conversion efficiency. In addition, the incorporation of the secondary solid optics increases resistance to ionizing radiation.

Fresnel Lenses and Secondary Optics are used together to concentrate light more effectively than could be done using either type of optic alone. The overall design affords enhanced energy-conversion efficiency.

Each line-focus (cylindrical) Fresnel lens comprises a thin, arched glass superstrate sheet that protects and supports a silicone rubber sheet, into which Fresnel-lens grooves have been molded. A spot-focus secondary solid optic is adhesively bonded to the input face of each tandem GaAs/GaSb cell pair. Each secondary optic, molded in silicone rubber, features rectangular entrance and exit apertures with parabolic side walls, on which incident light is totally internally reflected. The parameters of the parabolas are chosen in conjunction with the length and width of the tandem cell pair to maximize the concentration of light onto the cells over suitable acceptance and Sun-pointing-error angles. Taking advantage of the rectangularity of the entrance aperture, each secondary optic can be butted up against the adjacent secondary optic(s) to maximize utilization of the light focused by the Fresnel lens.

The tandem pairs of cells are designed to enhance energy-conversion efficiency by exploiting differing spectral sensitivities. The top (GaAs) cell in each pair is sensitive to the visible portion of sunlight and allows the infrared portion to pass through to the bottom (GaSb) cell, which is sensitive to infrared. The energy-conversion efficiency of the GaAs cell is 24 percent, while that of the GaSb cell is 6 percent; thus, the energy-conversion efficiency of the tandem cell pair is 30 percent.

The terminals of the tandem cells in each pair are arranged perpendicularly to each other. Tandem-cell pairs are grouped together in threes along each focal line, by use of a voltage-matching combination of series and parallel electrical connections: Each GaSb cell generates one-third the voltage of a GaAs cell. The GaSb cells in each triplet are connected in series along the focal line, while the GaAs cells in each triplet are connected in parallel. Thus, the voltage of the triplet series of GaSb cells matches the voltage of the GaAs cells, making it possible to connect both triplets in parallel. All of the series/parallel connected triplets along the focal line can then be strung together in series to obtain a higher output voltage.

The cells are mounted on ceramic substrates (omitted from the figure for clarity). The series and parallel electrical connections are formed in metal patterns on the substrates. Because the series connections are made over short distances along the focal lines, the overall series electrical resistance is relatively low.

A prototype module containing 24 tandem-cell pairs was constructed and tested under simulated sunlight in the absence of the atmosphere. The overall output of the module amounted to an average power of 2.61 W per cell — corresponding to an overall energy-conversion efficiency of 23.3 percent. In contrast, the energy-conversion efficiencies of relatively expensive, fragile, large-area arrays of flat Si and GaAs cells have generally been less than 20 percent.

This work was done by Lewis M. Fraas and James E. Auery of JX Crystals, Inc., and Mark J. O'Neill of Entech, Inc., for Lewis Research Center.For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Electronic Components and Circuits category, or circle no. 106 on the TSPOrder Card in this issue to receive a copy by mail ($5 charge).Inquiries concerning rights for the commercial use of this invention should be addressed to

NASA Lewis Research Center, Commercial Technology Office, Attn: Tech Brief Patent Status, Mail Stop 7-3, 21000 Brookpark Road, Cleveland, Ohio 44135.

Refer to LEW-16385.

Photonics Tech Briefs Magazine

This article first appeared in the April, 1998 issue of Photonics Tech Briefs Magazine.

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