Similarly battery developments have decreased the weights and increased capacity thus making solar power systems more competitive (see Table II). In the early 1960s NiCd cells had only 5 Ahr capacity and produced about 4.5 Whr/Kg (2 Whr/lb) while today NiCds are 50 Ahr and about 22 Whr/Kg (10 Whr/lb). Similarly the number of discharge cycles has been increased substantially so that missions could be increased from 3 years to 15 years today. Solar cell array structure development efforts have also progressed dramatically over the years. Early solar arrays were made from aluminum honeycomb panels and had specific power outputs of about 11 W/Kg (5 W/lb). More recently, lighter weight honeycomb panels covered with graphite/Kapton or Kevlar facesheets have been used to make large area cylindrical or flat plate arrays of about 22 W/Kg (10 W/lb). In 1971, the Hughes FRUSA flexible roll up array was placed into orbit demonstrating that lightweight solar cell arrays of about 66 W/Kg (30 W/lb) could be stored in a small volume. More recently, in 1984 Lockheed demonstrated the SAFE lightweight flexible array on a shuttle flight. This array utilized a foldable Kapton film substrate that was extended and held in position using a extendable truss beam. This design also demonstrated 66 W/Kg (30 W/lb) technology and the feasibility of stowing large area solar arrays in a small volume. This paved the way for missions with very high power levels, to be provided by solar array modules of this type. The present US space station power system design relies heavily on flexible array technology. Design Criteria A block diagram of a typical solar power system along with the key design factors influencing performance is shown in Fig. 2. Battery storage is typically needed to provide power during eclipse and when peak loads are encountered. Sizing of the solar
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