Since solar arrays are designed to operate year around, packing factors should be designed to enable the solar facilities to produce sufficient quantities of energy in the winter, when shading losses from neighboring collectors will be highest. During the peak growing season in late spring to early fall, the amount of intercepted radiation, and thus shading of the ground, would be expected to be less than values estimated at the equinoxes due to the higher azimuth of the sun. On the other hand, shading of the ground surface in the winter months, also a very productive season in the Southwest desert ecosystems, could be nearly complete, especially during the morning and afternoon. The types of solar collectors used will play an important role in how much of the ground surface is shaded, and how that varies on a temporal basis. For instance, heliostats (designed by Martin Marietta) tested at the Sandia Laboratories, Albuquerque solar thermal test site had spacing between the mirrors while others to be tested may be solid. This difference in reflectors may not cause significant differences in absolute incident radiation reaching the ground surface, but could be important with regard to duration of shading of a given area of ground, as sun spots will move across the desert floor as the system tracks the sun. Different types of proposed photovoltaic collectors will produce different seasonal patterns of shading of the ground surface (Arizona State University 1977). Collectors which rotate on a single horizontal axis will produce almost complete shading of the ground surface within the collector array in the winter (with collectors rotating on a polar axis) or on a diurnal basis (early morning and late afternoon for east-west axis rotation of collectors). Conversely, a collector tilted at 45° from the zenith and rotating around a vertical axis to track the azimuth of the sun (similar to the simulators constructed in Phase I) will not exhibit significantly different seasonal shading (Arizona State University 1977). Collectors rotating on vertical and horizontal axes simultaneously exhibit the smallest shading losses, and thus can be placed at the highest packing factors. It is evident that knowledge of specifics involved with collector design is a necessary pre-requisite in order to predict interception of incident radiation, and thus energy loss from the ecological system.
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