Fig. I. Cost curves predicted for terrestrial and SPS energy resources. Find the crossing point of the two cost curves as an ideal time for changing the energy resources from the terrestrial to SPS. 2. Assess the maturity of technologies required to SPS development period e, which corresponds to the relaxation time of finite-time-availability thermodynamics as explained later. 3. Define the start time of SPS development as the ideal time minus e. The details of each step are described in the following. The example of cost curves are shown in Fig. I. The costs of terrestrial resources such as the oil, coal and nuclear are shown to be soaring in the beginning of the next century. Although the coal looks less expensive than the oil, its utilization may be limited by the environmental impacts. Therefore the oil shortage is assumed to occur first in the future as the real crisis. The SPS is considered to take over the resource position of oil. The cost estimation of SPS power contains uncertainty according to the studies in the late 1970s. The error involved is shown as a bandwidth of cost curve in Fig. 1. The cost curve will be revised and its accuracy will be increased as the technology is advanced. The mean value is taken as typical in order to simplify
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