one a day, and the gallium aluminum arsenide option requires 225 HLLV flights to transport construction material for 10 GW (two 5 GW units) capability. The foregoing paragraphs have presented an overview of the SPS reference system. A more detailed description is presented in Ref. 1. III. ALTERNATE SYSTEM APPROACHES Alternate approaches to the reference system have been identified. A number are illustrated in Fig. 7. Because such a number of approaches exist, the SPS concept is not dependent on the successful development of any one technology. For example, the reference system photovoltaic energy conversion may employ silicon or gallium aluminum arsenide solar cells. Silicon has the advantage of operational experience, while gallium aluminum arsenide has the potential for providing a significantly lighter system. In addition, there are a number of promising advanced solar cell concepts that have the potential for higher efficiency and/or lower cost than the reference system solar cells. As an alternative to photovoltaics, solar thermal energy conversion systems employing either Brayton or Rankine cycles appear competitive in terms of system mass and cost. Construction and maintenance operations in space, however, appear to be more complex for these systems than for photovoltaic systems.
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