Table 8. Energy Ratio: SPS vs Conventional Electrical Technologies 8.2.10. External energy subsidies. In addition to other economic comparisons the external energy subsidies which are required to place an SPS in operation have to be considered, inasmuch as they determine how long a power plant would have to operate before all of the energy required — i.e. to process the materials, fabricate and assemble components and place the power plant in operation, including space transportation and the required propellants — would be paid back during normal operations. Figure 7 shows the external energy subsidies for a variety of present and future power plants (35). The shaded area of the bar represents the subsidy required for initial construction (capital subsidy) and the unshaded portion of the bar indicates the subsidy required for annual operation and maintenance (operating subsidy). On an energy subsidy basis, achievement of the system design goals would make the SPS very competitive with other systems. The results of the analyses of external energy subsidies based on a 30-year lifetime of the SPS can be translated into a 3.25-year energy payback time for the SPS, including the operating subsidy. Recent impact assessments indicate that energy payback will be achieved in less than 1.6 years (28). Because the GEO environment is benign compared to the terrestrial environment, with properly scheduled maintenance the projected lifetime of the SPS will be of the order of hundreds of years rather than an assumed 30-year life. Net energy analysis has been used to estimate the energy ratio for the SPS, which can be defined as the electrical energy delivered over the lifetime of the facility divided by the primary non-renewable energy required to construct the facility and operate it over its lifetime (36). The results of this analysis indicate that if the purpose is to produce electricity with a minimum of fossil fuel input, the SPS does a better job than coal and nuclear power plants (see Table 8). The conventional economic view, however, considers resources as free except for the development costs, and thus the energy ratio without fuel corresponds best to the present decision criteria. Furthermore, SPS has been found to be a net energy producer, particularly if the electrical output is multiplied by a factor of 3 to 4 to convert the electricity to fossil fuel equivalents.
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