It was also mentioned that low development cost means that the capital cost for production has to be reasonably low as well. There was a discussion of what low capital cost means, applied to space solar arrays. George Vendura of TRW pointed out that amorphous silicon may have a high capital cost if a new production facility must be built for the space product, but that TRW was pursuing a low capital cost approach for a-Si arrays by leveraging the huge (many megawatt) a-Si production capability in place for terrestrial markets. On the other hand, it was pointed out that a typical amorphous silicon production facility has a capital cost of about $10M. If this were expensed over a year's production of 50 kW, the cost would be $200 per watt. This is only a small fraction of the current space-cell production cost, and if other costs (such as the cost of assembling the array) were reduced, it might be acceptable. The current industry trend is toward extremely fast cycle time: getting a product to market as swiftly as possible. Several of the participants suggested that for a new produce to fly, development time ought to be three years or less. Frank Ho said that getting MO-CVD GaAs cells to market took four years from the 1982 manufacturing technology (mantech) program. GaAs on Ge cells [2,3] took three years after the mantech. Lew Fraas said that his experience at Boeing was that they had their research breakthrough in GaSb in 1989, found a flight opportunity in 1992 for a flight in 1994, and the program was terminated by Boeing in 1992. The time scale of 5 years from technology to flight test was too long. It was debated whether a 3 year cycle time was possible. It was concluded that it may be possible for developments with low technical risk and the ability to use existing system heritage, as the GaAs/Ge cell did, where system components other than the cell can be transferred unchanged. It was concluded that space experience was the big stumbling block to short cycle times. It is important for NASA to use advanced cells on actual missions, in order to get the space heritage demanded by mission designers. A scientific satellite, for example, could be designed so that one of the panels of an array is made with advanced cells. Solar Cell Technologies Seven different advanced cell technologies were discussed in some detail. Amorphous silicon, copper indium diselenide, and cadmium telluride thin films were discussed as systems that could have lower cost at the cell and array levels, and have the potential for very low mass and good radiation tolerance [4,5]. However, it was expected that to take maximum advantage of these systems, new array technologies would be needed. The workshop was divided on this issue. Ultra-thin (5 micron) gallium arsenide was discussed. The costs were considered higher, but the reduced cell mass would improve the specific power of arrays. High- efficiency monolithic tandem cells, GaInP2 on GaAs/Ge [6], and GaAs on active germanium, were discussed as ways of improving efficiency. Since these cells could be used directly as replacement for existing GaAs/Ge cells in existing arrays, this was considered a very promising approach.
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