costs. Materials cost is reduced due to the small amount of materials required; the cost of labor and assembly is reduced by the fact that large-area, integrated assemblies are produced directly on the substrate sheet. A final photovoltaic alternative is to use a concentrator system to focus light onto small, extremely high efficiency solar cells. This approach has been tested in space only in small-scale experiments. While conversion efficiencies of over 30% have been demonstrated using such concentrator systems and high-efficiency tandem solar cells, concentrator systems will not be practical for Mars surface power. The difficulty is that concentrator systems can focus only the direct (beam) component of the light, and not the diffuse. Since a baseline power system must be sized to operate under worst-case conditions, where most of the incident sunlight is diffuse, concentrator systems can be ruled out. Some resource-utilization applications require high-temperature processing. For these applications it may be desirable to concentrate sun to high intensities using mirrors or lens concentrators. This will allow a system to use the sunlight directly as heat, rather than converting the sunlight first into electricity using solar cells. A concentration system can make use of only the light coming directly from the sun
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