Fig. 6-15. SPS Program Costs Net present value analysis is used to establish the present value of each system's costs and determine the value each KW-HR of ground available output must have to give the same present value. Summing the equal cost debits and generated power credits results in a zero net present value. The process of analysis uses the cost-time spreads and power output-time spread of each SPS program. A present value for each year's costs is calculated using the following formula: This results in 64 present values from 1978 through 2041. The present values are all summed to establish the system present value. All SPS options produce the same electric power. A 7.5% discount rate was used to determine the required busbar cost (at the ground rectenna output) to amortize the program costs given above. These required busbar costs are given in Figure 6-2. Power costs for the nuclear MSBR concept were estimated at 82 mills/kWh. The more advanced rotating particle bed reactor concept conceivably could provide costs in the 25 to 45 mills/kWh range. Present value for power output is established in the same manner, except that a cost rate must first be assumed. By trial and error the cost rate is adjusted until the power output has the same present value as the system costs. The final cost rate becomes the present value cost per KW-HR. The present value case represents a discount rate of 7.5%. A power output efficiency factor of 95% is used to allow for occultation and maintenance. The results are shown in Figure 6-16. The 7.5% case represents a possible cost of money for this project. It should be noted that 7.5% is calculated on constant 1976 dollars whereas a typical realtime rate in a 8% per year inflation would be 15.5%. The results are used for system comparison in Section 7.0. Fig. 6-16. Required Busbar Costs
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