Energy Analysis of the Development of SPS To make a model of SPS to apply the world dynamics model to, the energy cost and the relation between output power and mass of an SPS are analyzed. As an energy cost of the SPS itself, the energy to produce solar cells, which dominates the total SPS energy cost, is estimated per unit output power based on Tajima’s study [4]. According to this data, a square solar cell of Si with 10cm sides which has output energy of 2.1 kWh/year requires an energy of 8.77 kWh for production with current technology. This output energy is the value on the ground where a solar energy availability of 20% is assumed, allowing for the diurnal cycle and inclement weather. On the other hand, in space, the availability is almost 100% because the solar cell can generate electric power continuously during the greater part of the year, and the intensity of the sunlight increases to a value about 1.75 times larger than that on the ground. So, the output power in space is about 2.1 W and the energy to produce the solar cell per output power is approximately 15 MJ/W. This value is about two orders of magnitude lower than assumed in Herendeen’s study in 1979 [5], and shows how much the energy cost of solar cells
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