The concepts for utilizing energy from space for commercial purposes are directly applicable only to peaceful purposes. Beamed-energy devices conceived to disable satellites or missiles require a much higher flux on target than is required for power transmission. Furthermore, their power supplies are designed for peak power generation and transient operations rather than the continuous supply of baseload power which is inherent in the SPS concept. The SPS concept represents a generic technology which can be designed to meet a wide range of commercial power generation requirements. One of the reasons for confidence in the technical feasibility of the SPS is that alternative technologies have been identified for nearly all components of the system. Most studies have been concerned with the SPS reference system which was evolved to provide a common basis for a broad variety of studies which were subsequently carried out in the U.S. Department of Energy concept development and evaluation program. The SPS reference system is a conservative design which uses only known technologies which require limited development; it does not represent an optimized system. An operational SPS which could be developed during the next 20 years would use some of the 80 alternative technologies which already have been identified for advanced SPS designs and would be quite different from the SPS reference system. Just as one aircraft design does not meet all of the requirements of the air transportation industry, one SPS design will not suffice: a variety of SPS designs for different purposes will be developed. The SPS represents a fertile field for innovations. Few of the potentially interesting technological concepts have been analyzed in detail and it would be premature to attempt a choice among them, since the consequences of the new technologies cannot be evaluated without a vigorous system design study of the impact of technical changes at the system and subsystem levels. The SPS design objectives based on the use of several alternative technologies include the following: • The lowest feasible cost per unit power output; • Reduction of environmental and other external costs; • Cost-optimum power outputs over a range from 100 MW to 10 GW; and • Demonstration of performance of preferred system designs at costs low enough to reduce investments needed before returns will be available. The development of the most effective SPS designs for intended uses represents significant challenges, and these challenges must be met realistically. But it is as inappropriate now to discount the SPS as a major option for the 21 st century as it was for Simon Newcomb, American astronomer, to state in 1906 that “the demonstration that no possible combination of known substances, known forms of machinery and known forms of force can be united in a practical machine by which man shall fly long distances through the air seems to the writer as complete as it is possible for the demonstration of any physical fact to be." The forces of man's creativity will be a major ingredient in the successful development of the SPS concept. ECONOMIC FACTORS FAVORING DEVELOPMENT OF THE SPS The variability of solar insolation presents serious impediments to the exploitation of solar energy for generating baseload power with terrestrially based solar energy conversion systems. Because of interruptions by inclement weather and the diurnal
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