Some of the more prominent features of the solar cells used in this study are: 18 percent efficiency cells, and very thin cell covers and substrates. More detailed features of the SEPS and SPS solar array characteristics are given in Table 7-2. The selected solar cells are similar to those in present use upgraded to higher performance. The characteristics assumed for the solar cells seem reasonable for advanced silicon solar cells. While the concept of Figure 7-4(a) represents a conservative adequate design, other concepts (Fig. 7-4, b through d) are postulated that have inherent advantages of higher concentration ratios and efficiency as a function of better temperature control characteristics. The performance features presented in Table 7-2 can be obtained by minor technology improvements — decreasing assembly losses, reducing the unused planform area, and improving thermal characteristics of the solar cells. Low radiation degradation in geosynchronous orbit and low thermal cycle degradation (because of the relatively small number of eclipses) help maintain the array performance. These performance figures take into account the effects of off-normal reflectance and absorption of sunlight, optical characteristics of the reflector and solar cells, variation in solar insolation due to Earth orbital eccentricity, and minimal solar radiation. Geosynchronous micrometeoroid damage is small. No allowance has been made for energy storage for eclipse operations. Table 7-2 compares the characteristics of a typical present lightweight array design with those projected for the SPS, based on SEPS technology. The improvements lie almost entirely in the area of solar cell performance and the handling of the large solar cell blanket/concentrators, including the automated manufacture of these assemblies. 7.1. 2.1 HIGH EFFICIENCY SOLAR CELLS Most of the SPS study activity to date has concentrated on silicon cells of moderate performance, but a key to the success of SPS is the selection and development of a high efficiency solar cell technology (silicon or other). Many new types of cells are postulated, and some of these are shown in Figure 7-5. The vertical multijunction (VMJ) cell in Figure 7-5(a) supplies power at a high voltage dependent upon the number of junctions used in its manufacture. The use of high voltage power transmission is desirable to reduce conductor losses, but many problems remain before large quantities of VMJ cells of high performance can be assured. Thin film photovoltaics, typified by the cadmium sulfide (CdS) system, are shown in Figure 7-5(b). Thin film photovoltaics are
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