As a final note, although it is not a direct cost item and perhaps may be viewed in the context of an “insurance cost,” the mature LPS system can be made far more secure in a military-terrorist-sabotage sense than SPS or any combination of Earthbased utility systems. IV. CRITIQUE OF “COST COMPARISONS OF SOLAR POWER SATELLITES VERSUS LUNAR BASED POWER SYSTEMS” By E. M. Kerwin and G. D. Arndt Before discussing specific topics in the subject study it may be well to note the following: 1. Overall cost figures for the reference SPS baseline system are probably unduly optimistic and may have relatively little value in power cost comparisons with terrestrial alternatives. 2. While uniform percentage errors in the costs of major SPS subsystems would not directly affect the cost comparison between SPS and LPS models, non-uniform errors can (and probably have) lead to substantial errors in overall costs biased in favor of SPS. (For example, the NRC (1981) study (24) concluded that orbital transportation costs would probably be 2 to 3 times study estimates and solar cell costs would be at least 10 times and more likely 50 times study estimates. Adoption of such estimates would multiply the cost of the reference SPS between 2.5 and 9 times original levels.) Methodology The procedure used in the subject study included: A. Selection of model, including sizing, duty cycles, etc. B. Selection of unit cost criteria based on collector or RF aperture area, mass, peak power or other presumed applicable factors. C. Adoption of (mostly subjective) cost comparison factors to account for manufacturing, assembly and transportation differences between LPS and SPS. D. Calculation of line item comparative costs for 39 categories, results of which appear in Table 1. We claim there are serious flaws in both logic and judgement applied to steps A through C resulting in costs in D heavily biased against LPS. Lunar Model There are serious deficiencies in the LPS model selected by Kerwin and Arndt. Equation 1 is virtually irrelevant to the modeling of an oversized lunar transmission system projecting a beam onto a rectenna 2 or more times the diffraction limited size. Past SPS work has emphasized minimizing the sizes and expenses of building the transmitter in space and very large rectenna on Earth (15). Assuming a 100 km diameter LPS aperture and 10 cm microwaves, Eq. 1 would be appropriate to a rectenna the order of 400 m across. Even a 1 km LPS rectenna would be much smaller than possible with SPS. Next, they compare the economics of a production line 5 GWe SPS against transmitting only 5 GWe of power from a pair of lunar facilities capable of transmitting far more power with only the addition of photocon-
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