might be compensated for by the greater flexibility of delivery and the potential for somewhat higher satellite load factors due to the possibility of shutting down one antenna for maintenance work while maintaining more than a 60% power output from the other antenna. Seasonal SPS Capacity Sharing A major opportunity for sharing the output of a satellite between two rectennas results from the fact that the periods of maximum demand on different electricity grids arise during different seasons of the year. (For instance, the maximum demand on some American electricity grids arises during the summer when the demand for cooling is highest, while in Northern Europe the maximum demand arises during the winter). Consequently a single satellite could be operated in such a way as to supply baselaod power to one rectenna during the winter months and to supply the same to another rectenna during the summer. In this way each rectenna would achieve a load factor of approximately 0.45 (ie 50% of 0.9). If each rectenna also received six hours of daytime fuel-saving supplies for approximately half the year, they would achieve overall load factors of 0.57. Such a winter/summer peak delivery pattern would make a convenient match with the fact that for three weeks either side of the equinoxes (March 21s' and September 23rd) geostationary satellites are eclipsed for a few minutes around midnight. Thus, by concentrating some of the planned outages for necessary maintenance into these periods, an SPS would be able to achieve higher load factors during the months of maximum demand. This potential for sharing capacity on a seasonal basis between two widely separated utility grids is a capability that is unique to the SPS. Coordinated Operation of a Global SPS System As the number of satellites and rectennas in a global system increased, the average longitude offset between them would decrease, and the scope for economising on satellite capacity through coordinatedoperation of satellites would increase. Such arrangements would be facilitated by some formation of long-term supply agreements between several different utilities. A computer model has been used to estimate the scale of savings that might be achievable in practice through the coordinated switching of power between several satellites and rectennas. Optimal Operation of SPS Systems In order to study the potential benefits of switching power supplied from satellites between several rectennas according to the changing pattern of demand during the day, a system of sixteen rectennas was selected, sited at major population centres around the world. For this exercise, assumptions were made about demands to be served at peak and off peak-times (see Table 4). A critical parameter in determining the relative profitability of supplying peak or base load power from an SPS is the ratio of the marginal costs of power at peak and
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