arrays and the space transportation. The cost estimates for solar cells generally represent extrapolations from current programs which are directed towards the development of solar cells for Earth use. Goals of the Department of Energy photovoltaic program are for costs of $2000/kW by 1982, $500/kW by 1986, and $100/kW to $300/kW by the 1990's. Confidence is building that these goals will be met, which in turn provides a degree of confidence that estimates used in the Solar Power Satellite assessment can be achieved by the turn of the century. Space transportation costs will represent approximately one-third of the total cost of implementing a Solar Power Satellite. The major element within this cost is the transportation of material to low Earth orbit using the large launch system previously described. Based on very high frequency of use and full reusability, launch costs for this vehicle are predicted to be similar to those for the Shuttle. The costs of fuel used in this estimate are those for hydrogen and methane obtained from the gasification of coal. The Solar Power Satellite electricity cost estimate of 1.5 cents/MJ (5.5 cents/kWh) assumed a transportation cost of $33/kg ($ 15/lb) to low Earth orbit. Obviously, this represents a significant reduction from projected Shuttle costs. Analysis indicates, however, that complete reusability and large-scale operations offer this potential. One of the negative concerns often expressed is the “front end” money required to implement a full-scale Solar Power Satellite program. There is no question that a considerable amount of funding would be required. Estimates have ranged from $50 to $100 billion over a 15- to 20-year period. This subject certainly deserves careful consideration by individuals skilled in the appropriate fields of economics and national investment. It is suggested, however, that we should view this $100 billion in the context of a total national energy development and supply program which certainly will involve trillions of dollars between now and the year 2000. As previously stated, the projection of costs for energy systems involving advanced technology is difficult and generally controversial. It is expected, however, that these estimates will improve as more definitive data are obtained. In the meantime, they are useful in identifying the more important cost parameters within the system and the sensitivity of total costs to changes in the cost of various system elements. 4. FUTURE ACTIVITIES The development of Solar Power Satellites on a commercial scale would obviously be a very large undertaking. It is equally obvious that inadequate information exists at the present time to commit to a full-scale development program. For these reasons, many, if not all, of the individuals who have studied the concept would propose a phased and evolutionary activity, where each phase was based on the success of the previous phase. The succeeding phases would tend to be greater in scope and cost but would be based on increased confidence in the technical, environmental, and economic viability of the concept. Figure 26 presents typical phases which might be included in such an evolutionary plan. The phases include concept identification and preliminary studies; a concept evaluation program now underway; an exploratory research phase to answer critical questions through laboratory development and testing; and a series of space technology projects to develop operational techniques and to demonstrate key elements of the system. The combined results of these four phases of activity would provide the necessary information on which to base a
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