OULU! 1 UWC! -JULCLLLICJ rrUUU LJy ILUIILLCJ kJ LIIL ULCLL LU IL where NRUR is the natural resource usage rate, and CIG is the capital investment generation. In this study, NRUR means the total energy resource usage rate per year on Earth including the energy from SPS. (3) Et(MJ/year): the total energy investment in SPS from the Earth per year, which is converted to capital investment in SPS from Earth per year, which is converted to total capital investment in SPS (CITS, capital unit/year) in the calculation by the following relation determined by the relation between NRUR and CIG. (4) SCIDN: the SPS capital investment discard rate, which is represented by the degradation rate of solar cells per year. In this study, we assumed that electrical energy will be used in the future in fields such as transportation, smelting etc., in which other forms of energy resources are used at present. Factors to Limit the Growth of SPS The growth of SPS will be limited by material resources, land on Earth and orbital constraints. If silicon is used as the material for solar cells (the most important part of SPS), the material resource will not restrict the growth of SPS because silicon is abundant on Earth. The maximum number of 5GW SPS which can be arranged on the equator of Earth is about 3000 if the size of the rectenna in the reference system of DOE/NASA is considered. The maximum number of SPS in GEO is 180 because of restriction on the angular separation in the orbit. But the maximum total capacity of SPS in GEO will be much higher than 3000 units of the 5GW type SPS because there is room for an SPS with much more than 5GW capacity at one location in GEO. In this study, we assumed that the growth of SPS is limited to 3000 units of 5GW capacity. Simulation Results Figures 3 and 4 show the representative simulation results for the cases where the values of Et are 0.003 x NRUR MJ/year and 0.01 x NRUR MJ/year respectively. Other parameters, IYEAR = 2000, RYEAR = 2100 and SCIDN = 0.01, are the same for both cases. The step size is 1 year. The limits to growth can not be avoided and CO, levels in the air will increase in the low investment case as shown in Fig. 3, because the growth of SPS cannot support the growth in energy consumption on Earth, and fossil fuels will be consumed on a large scale. On the other hand, the population and capital continue to increase and the CO2 levels stay low, and the quality of life begins to recover in the high investment case as shown in Fig. 4, because the rapid growth of SPS with high investment can support the growth of energy consumption on Earth, and the consumption of fossil fuels decreases with increasing energy from SPS.
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