array is determined by balancing the average load (Kw hr) requirements against the array generation capacity while considering the efficiency factors of the battery, power electronics and distribution system. Battery sizing (Kw hr) is determined by the peak power (Kw) requirements and the maximum depth of discharge desired. Although high performance and reliability are key factors desired in a solar power system, cost is still the deciding factor in most cases. There are exceptions for some missions where cost may not be a major consideration but this is usually related to cases where there is some other restraining design factor such as a limited array area or a weight restriction. In these cases, the higher cost of an advanced technology, such as a higher efficiency GaAs solar cell, can be justified if it allows power growth in existing spacecraft designs. In these cases the cost of the increased performance is offset by the cost saving of using an existing design versus redesign and requalification. Another way that one can justify the higher cost of an advanced technology is to provide weight savings or reduced drag area which can be balanced against launch costs or fuel expenditures. These types of trade-offs however, are often difficult to quantify and require multidisciplinary analysis, so a solar power system design will usually be selected based on performance and cost. Solar Array Technology Silicon solar cells in the range of 14 to 16% AMO efficiency are typically used in most missions today. However, there are many opportunities for improved solar cell performance or weight reduction using new materials or cell designs (see Table III). Conventional single junction GaAs solar cells of 19% AMO efficiency are now available in production quantities, but they are about two to three times the cost and about twice the weight of silicon solar cells. This limits their use to only a few applications where the improved output performance justifies their use.
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