GALLIUM ARSENIDE (GaAs) POWER CONVERSION CONCEPT A.A. Nussberger, Rockwell International 12214 Lakewood Boulevard, Downey, CA 90241 The Rockwell Satellite Power System concept utilizes gallium arsenide (GaAs) solar cells and flat plate concentrators (CR=2) to generate 9.52 GW of power at the array sufficient for the satellite microwave antenna system to deliver 5 GW at the utility interface. The solar array bay configuration and design factors are shown in Figure 1. This concept shows a 3-bay by 10-bay matrix 3,900 m wide by 16,000 m long exclusive of the antenna. Each bay contains two panels 600 m by 750 m, providing a voltage string of 45.7 kV. The 600-m width consists of 24 rolls each 25-m wide. The solar array is sized for worst conditions using summer solstice values (1311.5 W/m^), end of life reflector values (CR effective = 1.83), solar cell degradation allowance (4% non-annealable loss), operating temperature of 113°C at summer soltice (solar cell temperature coefficient of Atj/AT = 0.0282%/°C), north-south seasonal inclination (altitude tipping of the SPS configuration accounts for 9.05° of the nominal misorientation of 23.5°, resulting in a seasonal, factor of 0.968), packaging and array voltage mismatch factor of 0.89, and switch gear factor of 0.997. Array power output is calculated to be 352.6 W/m^. The solar cell array area of 27 x 10^ provides a 1.7% margin. Key functional requirements include: delivery of 5 GW at constant power (except during solar eclipse) to the utility network; operation in geosynchronous orbit for 30 years (size for end of life); and cost-competitiveness with ground-based power generation. The last requirement (cost competitive with ground-based power generation) has driven the Rockwell design toward use of higher technology hardware. The solar cell used in the satellite system design is a GaAs cell having a nominal efficiency of 20% (AMO, 28°C). Based on today's technology, 20% cell efficiency is expected by the year 1990. The best laboratory GaAs cells are presently around 18% (Hughes, Rockwell International). The basic SPS cell concept is an inverted GaAs/sapphire design having a specific mass of 0.252 kg/m^ (Figure 2). This cell design has a 20 gm sapphire (AI2O3) substrate upon which is grown a 5 ^im single crystal GaAs junction. The Electronic Research Center (ERC) of Rockwell has supported this effort with investigations of the development and mass producibility requirements of the baseline GaAs/A^Og cells using a metallic oxide-chemical vapor deposition (MO-CVD) process. Figure 3 shows a production model of inverted structure GaAs/Al2O3 continuous ribbon solar cell. Trade studies by Rockwell on the system level have shown GaAs to be the preferred cell material compared to silicon. This is based on its higher efficiency (20% versus 17.3%); potential for cell efficiency improvements (the multi-bandgap concept is essentially a gallium arsenide cell with potential of 25-30% or greater); lower space radiation degradation damage (125°C threshold temperature for 1 annealing versus >500°C); lower specific mass (0.252 kg/m^ versus 0.427 kg/m^); better compatibility with concentrators (improved temperature coefficient, A77/AT = 0.028%/°C versus 0.043%/°C); and lower overall SPS cost. A comparison of GaAs solar cell annealing effects after proton irradiation is presented in Figure 4. Over 400 small-area (0.4-cm-square) solar cells were tested by Rockwell J Both typical and best cell annealing results are shown in Table 1. The SPS design assumes that nearly all radiation damage can be self-annealed out or annealed out with sufficient time and oroper temperature. A cost comparison was made of single-crystal GaAs, single-crystal silicon (Si), and amorphous silicon (A-Si). The baseline GaAs configuration was utilized at a mass of 0.252 kg/m^, the single-crystal silicon cell stack configuration was taken from the DOE/NASA reference system report^ 0.427 kg/m^, and an amorphous silicon configuration was modeled from an RCA paper.3 It was assumed that the A-Si cell stack weight was equivalent to 1-mil glass (0.143 kg/m^) and that tne blanket configuration was the same as in the baseline GaAs. Figure 5 summarizes results and shows that A-Si must achieve near theoretical efficiency (~15%) and low cell cost (~$20/m^) to provide a cost-competitive SPS system. Single-crystal silicon (even at the high efficiency of 17.3%) appears to result in a significantly higher SPS cost (A cost ^2.13 B) compared to the GaAs CR=2 baseline. ^Gallium Arsenide Solar Concentrator Hardness Study, Rockwell International, Technical Report AFAPL-TR-78-30 (May 1978). Concept Development and Evaluation Program, U.S. Department of Energy and NASA Reference System Report (October 1978). ^Twelfth IEEE Photovoltaic Specialist Conference, p 893.
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