above 220 Whr/Kg (100 Whr/lb) for missions lasting up to 15 years in earth orbit. This type of battery will make large solar power systems in the 50 Kw range feasible, and indeed attractive in the future. Summary From the previous discussion it can be seen that improvements in solar cell efficiency, solar array specific power, and battery specific energy have been significant and are expected to continue for a long time. Figure 3 summarizes some of these expectations for several solar power system parameters. The present state of the art is represented by technologies that have been utilized in flight. Specific power values of solar cell arrays today, as represented by the FRUSA and SAFE arrays, are at 66 W/Kg. This can be increased to 120 W/Kg using the SEP array design with 30% multi band-gap solar cells, or even to 350 W/Kg using thin film solar cell technology. Specific energy values for storage subsystems are at 30 Whr/Kg for NiH2 cells today, and this can be expected to go to 150 Whr/Kg with NaS technology and 400 Whr/Kg with regenerative fuel cells. Consequently, the complete photovoltaic power system specific power of about 7 W/Kg today for rigid silicon solar cell panels and NiCd batteries, can now be increased to 22 W/Kg using silicon cells on flexible panels along with NiH2 batteries, and in the near future this can be increased to 50 W/Kg using 30% efficiency multi band-gap solar cells on a flexible array design with NaS batteries. These advances will make it possible for photovoltaic system power levels to increase from the present day 1 to 10 KWe levels to 25 KWe level anticipated for
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