2. Gallium arsenide photovoltaic cells have certain advantages over the more common silicon cells, especially in orbital applications. Early in the program, gallium arsenide cells were not strongly considered because of limited supply of gallium. This limit arises from the fact that though widely distributed, gallium is generally found in very low concentrations averaging less than 15 parts per million. Gallium is found in higher concentrations in bauxite and can be recovered in association with aluminum production. At this time, gallium recovery from bauxite is the only economically available process. Gallium arsenide cells are being considered because of technical advantages over silicon cells. However to avoid exceeding the projected availability of gallium, the gallium aluminum arsenide solar cell design was driven toward very thin cells (5 micrometers) and synthetic sapphire replaced additional gallium arsenide as the substrate material. In addition, a further demand reduction is achieved by designing a solar concentration of two into the collection system. Table A-7 shows changes in the estimated materials requirements for the silicon systems between mid-1976 and January 1978 for both the satellite and rectenna in columns 2 and 3. In columns 4 and 5, the estimated materials requirements for the gallium arsenide system as projected in mid-1977 and January 1978 are shown.
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