Workshop co-leader Ed Gaddy of NASA Goddard Spaceflight Center suggested that there will be only a small number of NASA missions devoted to solar system exploration, and a large number of future missions to LEO and GEO orbits, where LEO missions will include many polar- and near-polar (sun synchronous) orbits. The consensus of the workshop was that commercial missions will also be primarily to LEO and GEO orbits. Thus, the overall consensus of the group was that the most important applications for future solar cells will be for satellites in the orbits which are important today: LEO and GEO. Two strong trends were identified, with important implications for the future: a trend toward small satellites (less than a few kilowatts, as opposed to the 10 kW+ projects envisioned just a few years ago), and an industry-wide trend toward fast cycle times for development of new technology. Goals The next question is: in which solar cell parameters are advances most needed by the user community? The answer is: cost. Array volume and reliability of interconnects were mentioned, but the overwhelming answer of the participants was that cost is critical. However, in the ensuing discussion, it became clear that the important cost is not cell cost, but life-cycle system cost. The typical purchase cost to users of space solar arrays ranges from $1000 to $2000 per watt today. Some of this is non-recurring cost, since power requirements are different for each satellite. One vendor said that they are providing arrays for $800 per watt to a military customer, where the non-recurring cost is amortized over many satellites. Frank Ho provided the following "typical" numbers. For a GaAs array of a few kilowatts, roughly 25% of the cost of the power system is the cost of the solar panels. Of the panel cost, roughly half of the cost is cell cost (for GaAs/Ge cells). Thus, about 12% of the cost of a power system is attributable to the cost of the cells for (relatively expensive) GaAs cells, and a few percent for (relatively cheap) silicon cells. From this we conclude that cutting the cell cost can have at most a 12% impact on the power system cost, and, considering launch costs and other system costs, is likely to have a much lower impact. In itself, cell cost is not a major issue. To achieve low system cost, the workshop participants suggested that the single most important factor is conversion efficiency, since an increased efficiency reduces the entire array cost. In addition, in order to get a new technology into the marketplace, investors require a low development cost, and a fast cycle time. Lew Fraas emphasized that low development cost is critical. He said that Boeing estimated that the development cost to bring their 30% efficient tandem GaAs/GaSb concentrator system to market would be $100 M, and that this high cost made it impossible to attract investors. Representatives of the space-cell industry said that development of the GaAs on Ge cell [2,3] required "lots of millions of dollars" and took over three years, but that it had a strong selling point in that the cells already had a customer, since they were direct replacements for existing GaAs on GaAs cells developed for an unnamed (presumably military) customer.
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