(the direct, or beam component of the insolation). As discussed in previous work [1-3], the direct component of the insolation is about 50% of the total sunlight during the (northern hemisphere) spring and summer, but can drop to 10% or less of the total available light during periods of highly dust-laden air, particularly during the period around perihelion (northern hemisphere winter), when global dust storms can occur. An application requiring a concentrator to produce process heat must thus be designed for operation during relatively clear periods only. This implies that the mission be scheduled to avoid perihelion, and that the processing system must be tolerant of the possibility of intermittent shut-down if dusty atmospheric conditions arise. A concentrator system will also have to be tolerant of dust loading of the mirror or lens surfaces. Environment In addition to the solar insolation and the fraction of direct and diffuse sunlight, other environmental factors of importance to photovoltaic system operation on the Mars surface are the temperature and the wind. The Martian surface temperature varies from a minimum of 130K to as high as 300K, with a mean of 215K. Air temperatures were measured by Viking at a height of 1.6 meters above the surface over a (Martian) year of measurement, including both local and global dust storms. Peak daytime temperatures varied from about 170K at the VL2 site during a global storm, to almost 250K near the summer solstice [2]. Photovoltaic cell performance increases with decreasing temperatures, with peak efficiency occurring at 150-200K; at lower temperatures the efficiency decreases. The temperature coefficient of efficiency depends on the material, and in general increases as the material bandgap decreases [8]. Thus, low bandgap materials such as silicon and CuInSe2, which have high coefficients, increase in performance rapidly at the low temperatures to be found at Mars. Wind was measured by the Viking landers. Average wind speed at site VL2 was about 2 m/sec [4], with winds of over 17 m/sec observed less than 1% of the time. The atmospheric density varies considerably with season and temperature; at typical atmospheric conditions, dynamic pressure at the maximum likely wind velocity of 30 m/sec will be 10 nt/m2. This is a small pressure loading by terrestrial standards, however, Martian solar arrays will have to be somewhat more robust than arrays for the Moon or deep space. A final aspect of the Mars environment not well characterized is the soil. One interpretation of the Viking lander life science experiments is that the Martian soil contains large amounts of peroxides and superoxides [4]. This hypothesis needs to be confirmed by direct chemical analysis of the soil. If it is confirmed, it will be important that solar arrays be built using materials not subject to attack by oxidants.
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