Cell operating temperatures as a function of concentration ratio also are shown in Figure 2.2-2. The use of bandpass filters which reject a part of the solar energy to which the cell is not responsive has the benefit of reducing the operational temperature of the arrays. A 1 x 1 cm GaAlAs cell developed by Rockwell is shown in Figure 2.2-3. Several 1 x 1 cm cells and smaller cells have been fabricated and tested at Rockwell. The performance curve for the 1 x 1 cm cell is shown in Figure 2.2-4. The cell has an air mass zero efficiency of 14.3 percent and an open circuit voltage of 0.97. A cell review of the literature was performed and discussions were held with investigators working on GaAlAs solar cells to determine current technology and to attempt to project the technology for a post-1985 SPS. The basic performance and projected technology for both Si and GaAlAs cells are shown in Figure 2.2-5. Experimental data points by Rockwell, Hughes, and IBM, and projected development efforts of the USAF are shown. Based on the present rate of development, it is projected that the GaAlAs solar cell has the potential of attaining a 20-percent efficiency (AMO, 30°C) for the post-1985 period. In the SPS study, a 20-percent efficiency AMO GaAlAs cell was assumed for sizing the solar array. The existing experimental data for cell efficiency as a function of temperature for the 14.3-percent efficient GaAlAs cell and the Si cell is shown in Figure 2.2-6. The efficiency temperature coefficient of the GaAlAs test cell is seen to be -0.0245%/°C. Utilizing the projection data of Figure 2.2-5 for GaAlAs and assuming that the efficiency curve for higher efficient GaAlAs cells will approximate the shape of the 14.3-percent cell Figure 2.2-3. Typical 1 x 1 cm GaAlAs/GaAs Heteroface Solar Cell
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