1'ABLE 1 REQUIREMENTS AND CHARACTERISTICS OF A SPS WITH SILICON SOLAR CELL ARRAY and economically attractive. The associated requirements for the solar array, together with some essential programmatic assumptions for the SPS, are summarized in Table 1 (1,3). It is assumed that the first operational SPS can be launched in the year 2000. The number of SPS to be set in operation after this initial operation date shall be 2 satellites per year so that after a time period of 30 years a fleet of 60 SPS will be in operation in the USA. Because of economic and energy pay back time reasons, the lifetime of the satellites shall be 30 years. Consequently, the launch sequence, 2 SPS/yr, will have to be continued after the year 2030 unless it will be possible to fully refurbish a satellite in orbit after its actual operation lifetime is over. In order to meet these schedule requirements it is necessary that the technology for the SPS be available in 1990, i.e., 10 years from now! The primary technical requirements are the following: At the terrestrial interface between the rectenna and the electrical power grid networks the system shall generate a power output of 5 GW. Taking into account all conversion losses from the solar array to the ground interface the solar array must be designed to convert sun energy into 10.8 GW electrical energy. This output power requires array dimensions of 5.2 x 10.4 km, depending primarily on the attainable solar cell efficiency for end of life conditions. Because of power conditioning and distribution reasons, the solar array power shall be generated at an operation voltage of 40-45 kV in order to minimize mass and power loss for the distribution system. The solar cell efficiency significantly affects the SPS characteristics in terms of mass and economy. Therefore detailed requirements have been established in the system studies as a goal for further development. For economic reasons, large area solar cells (6.5 x 7.5 cm2) will be used with a thickness of 50 gm. The cells shall have wrap-around contacts to enable cell-to-cell interconnection on the rear side. For environmental (>3000 eclipse cycles) and thermal reasons, cell interconnection shall be performed by welding. The solar cell efficiency shall be 17.3% at 28°C. It is important to note that this efficiency is expected to be maintained for the total lifetime of the SPS. Since today's solar cells degrade in the geostationary particle environment, it has been proposed to
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