target is inversely proportional to antenna size. Positioning of a SPS in low orbit is much less desirable, due to the high required antenna slew rates and the longer power- out time spent in shadow. To develop a large program such as SPS, it is necessary to find a path that involves step by step progress, with immediate pay-offs at each step, and with experience gained at each step to refine and improve the technology in evolutionary steps. This paper is an attempt to define such a path. Ground-based Photovoltaic Power The first step is to demonstrate power production with ground-based solar arrays. A significant risk element for any satellite power system is the photovoltaic array. This was identified in the NRC review of SPS [4] as one of the most critical areas where extrapolations from current technology in terms of cost and performance were made. Proponents of SPS often disparage the potential use of ground-based solar energy, possibly considering ground-based systems as a competitor. Nothing could be further from the truth: ground-based and satellite-based solar power are complementary technologies, and satellite-based solar power will only be economically viable if terrestrial power is. Experience with ground-based solar power is a necessary step to shake-out the technology, define and trouble-shoot the manufacturing technologies, and move photovoltaics down the learning curve to low-cost production. Many ground sites exist in the USA with over 300 clear days per year. Flat-plate photovoltaic systems will also provide significant power during overcast days. The difficulty with terrestrial solar power is that it provides power only during the daytime. Ground-based solar power is viable due to a fortuitous match between daytime peak requirements and production [5]. Figure 1 shows a comparison of the generation profile for a photovoltaic system compared with the load profile of a utility [6]. The curve shown is for mid-summer in southern California, when power-requirements are typically highest, and the peak loads are in the daytime due to loads imposed by air conditioning. Utility-generated power can be usefully considered as divided into two categories, base load and peak load. The cost of generating peak power is higher than that of base power; and as long as the generation profile is primarily providing peak power the marginal benefit is high. Several analyses have shown that for generation fractions of up to about 20% of the current US production, photovoltaic power generation can provide primarily peak power without cutting into baseline power. Above about 20% penetration, however, photovoltaic generation begins to displace base capacity. This is a double liability, since the power displaced is produced at low cost, while high-cost peaking power must be generated to provide a level base during the times when the solar power is unavailable. However, 20% of the US power generation capacity is a huge amount, and a large amount of growth in the solar power industry is possible. An additional advantageous feature of terrestrial photovoltaics is the short construction lead-time required and the ability to add capacity in small, modular increments. As shown in the experience with the Carissa Plain field [7], it is possible to build a photovoltaic field in a year or two. Even the production factories to build arrays can be built relatively rapidly. This allows photovoltaic installations to avoid the uncertainty of forecasting power requirements far in advance, and also allows rapid progress down the learning curve.
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