Apparently, this very high load factor, which permits the amortization of both operating and capital costs over a much greater number of kilowatt hours per year, contributes to the low operating energy subsidy required for the SSPS. In this instance only 56 Btu per 1,000 (5.6%) external energy is required for the operations. This is substantially below any of the other technologies analyzed. 3. Estimates of Nominal Energy "Payback" The energy payback period (the time required to recover the energies expended in plant construction) is often used as an index of energy efficiency or balance for different technologies. The external energy subsidy concept of DSI is intended as part of the substitute for this approach which largely relates to process energies rather than total energies required. Nevertheless, it is possible to develop a nominal energy payback estimate using the external energy subsidy concept. The mechanics of this computation are summarized in Exhibit 43 . The basic approach is to take the capital energies required (per 1,000 Btu of energy output) and divide them by the annual output after subtracting the operating subsidy. This represents somewhat of a departure from present analyses which usually do not consider an external operating subsidy. For example, in the case of the oil system shown at the top of Exhibit 43 , a simplistic energy subsidy would be approximately 1.87 Btu (1,869 Btu of capital energies divided by 1,000 Btu annual output per year). By subtracting the operating energy subsidy of 335 Btu, the payback period becomes 2.81 years (1,869/665). Not surprisingly on this scale the SSPS has the highest or longest energy payback of 3.28 years. This compares to 2.81 years for oil and 2.71 for solar terrestrial. The various other coal, oil, and gas technologies proposed generally range from 1.2 to 2.0 years. Nuclear and geothermal are both considerably lower at 0.65 and 1.04 years, respectively. It should be noted that the payback period as calculated here is independent of any considerations of the use of a 20 or 30-year amortization period for the calculation of annual total subsidy. The energy payback period derived by this method is considerably higher than the two other estimates given for the SSPS. In the ECON study (Ref. All) a 1.6 year energy payback was calculated based almost
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