ground surface, will depend on the types of collectors used, and the spacing (or "packing factor") of the collectors along east-west and polar axes. Black and Veatch (1977) analyzed a hypothetical photothermal collecting facility, and determined that an array of heliostats covering between 42% and 56% of the ground surface would intercept 59% and 78% of the annual daily direct radiation, respectively. Their calculations were made at the equinoxes, and thus did not account for changes in azimuth of the sun with season. They assumed that interception of diffuse radiation would be equal to ground coverage of the heliostats (i.e., 42-56%), although acknowledging that this is probably an underestimate because tracking mirrors should intercept a greater proportion of the more intense diffuse radiation from the sunward part of the sky than would a horizontal surface. Assuming total incident radiation in the desert Southwest to be 80% direct and 20% diffuse (Liu and Jordan 1960), between 55% and 73% of the total incident radiation will be intercepted by heliostats and removed from the desert ecosystem (based on Black and Veatch 1977). A reduction of such proportion is highly significant. A more recent study of the "power tower" type of solar collection facilities indicates that only 25% of the ground area will be covered by heliostats (Hildebrandt and Vant-Hull 1977). The heliostat coverage will vary from 40% near the tower to approximately 10% near the periphery of the heliostat field, because with increasing distance from the tower greater amounts of blocking of reflected sunlight by adjacent heliostats occur. With reduced ground cover there will be less shading than the figures given by Black and Veatch (1977). The design suggested by Hildebrandt and Vant-Hull (1977) would probably result in removal of less than 50% of the total incident radiation from the system. Unfortunately, we were unable to obtain any values for ground coverage of projected photovoltaic arrays. Photovoltaic arrays may suffer greater penalties from shading loss due to adjacent collectors, and thus should be installed at reduced packing factors compared to photothermal systems (Arizona State University 1977). If so, solar radiation losses would be reduced in photovoltaic arrays but still should not be substantially below 50% of total incoming radiation.
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