JX Crystals

Demonstration of a 33% efficient Cassegrainian Solar Module

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Abstract
The Cassegrainian solar concentrator module concept we shall describe here uses a primary and a dichroic secondary mirror to split the solar spectrum into two parts and direct the infrared and near visible portions of the spectrum to two separate cell locations. An efficiency of 32.9% (STC) is reported measured outdoors for a solar concentrator PV module using InGaP/GaAs dual junction (DJ) cells located at the near-visible focus at the center of the primary and GaSb infrared solar cells located behind the secondary.

Cassegrain Module Concept
High efficiency monolithic InGaP/GaAs/Ge multijunction (MJ) solar cells have been used in space for powering satellites. These monolithic MJ cells are grown on thin germanium wafers in order to be lightweight because launch costs dominate the hardware costs for space. However, lightweight is not the criterion for terrestrial applications. These monolithic MJ cells are very difficult to fabricate and the lattice match constraint for growing all junction materials in succession on germanium is quite restrictive. Using two separate cells relaxes this constraint and potentially allows even higher conversion efficiency. It has been previously noted that the theoretical limit efficiency for the InGaP/GalnAs cell in combination with a separate GaSb IR cell is as high as 59.5% [1].

The Cassegrainian solar concentrator module concept shown in figures 1 and 2 uses a 25 cm x 25 cm primary and a dichroic secondary mirror to split the solar spectrum into two parts and direct the infrared and near visible portions of the spectrum to two separate cell locations. Herein, we report the fabrication and outdoor testing of prototype solar concentrator PV module using InGaP/GaAs dual junction (DJ) cells located at the near-visible focus at the center of the primary and GaSb infrared solar cells located behind the secondary.

Component Cell Fabrication & Test Results
Under an NREL contract, we have previously described the fabrication of our first prototype Cassegrainian module and opportunities for improvement [2, 3]. In our earlier unit, we used InGaP/GaAs cells from Spectrolab with a 1 cm2 aperture area operating at a geometric concentration ratio of 600 suns. Since these cells operated outdoors at only 3.5 C above ambient, we decided to decrease the InGaP/GaAs cell aperture area to 0.5 cm2. We ordered and obtained 50 DJ cells from Spectrolab and we were very pleased with the results. Nineteen of the cells had measured efficiencies over 31.5%. Table I shows the reported performances at the chip level for a representative group of these cells.

A group of GaSb IR cells was also designed and fabricated in house at JX Crystals Inc. The aperture area for the IR cells is 1 cm2. Both types of cells were then mounted onto alumina substrates as shown in figure 3. A bypass diode is mounted near the DJ cell to protect it against back bias if shaded. A single cell is used at each focus with the plan being to eventually current match and series connect both types of cells in a Cassegrainian panel.

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