have improved during the last 10 years. Tajima also estimated that production energy would decrease to 5.25kWh for solar cells with an output power on Earth improved to 2.8 kWh/year in the future. It follows that the production energy for solar cells will decrease to a value of about 7 MJ/W in future. The GaAs solar cell is not considered because of its high cost. The production energy of other elements of the SPS are also ignored in this study. The relation between output power and mass for the totally assembled SPS in orbit is the most important parameter for defining the quantitative SPS model. According to the DOE/NASA reference system, the solar panel weighs 25,000 tons and generates 7.5 GW of electrical power in space, and so achieves a specific power of 300 W/kg [3], This value would apply to the most advanced totally assembled SPS in the future. The ratio of output power to mass for a completely assembled SPS including the power transmitting system of the reference design in the DOE/NASA study is 100 W/kg [3]. This value also applies to the future prospect, but it is already reported that 100 W/kg has been achieved using silicon crystal cells [6], So this seems to be a realistic value for the SPS, and 100 W/kg is adopted as the present value of the ratio of output power to mass for a completely assembled SPS in the present. These results are summarized in Table 1. Rocket propulsion is the only practical way to put space systems in orbit. The energy which is necessary to put an SPS in orbit using rocket propulsion is much greater than the theoretical value of the energy given by orbital mechanics. So, in addition to the energy cost of the SPS itself, the energy cost of rocket propulsion is analyzed as an energy cost of the SPS transportation system. Three elements are considered as the cost of the transportation system, the chemical energy of the propellants, the production energy for the propellants, and the energy for vehicle manufacture. It is assumed that SPS will be established in geosynchronous orbit (GEO) and chemical propulsion will be used for transportation of the SPS.
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