Since the land for rectenna sites is so severely restricted and European industry has considerable experience in the construction of offshore structures (from the North Sea oil exploration), the problem of siting rectennas offshore has been further investigated by P. Collins (16). Neglecting the specific problem of interfacing power satellites with a utility grid, the 5 GW per unit of the United States1 reference system does not seem to be a problem in itself. At present, there are already blocks of three to five nuclear power plants with capacities of 1 GW each in the planning stage in several European countries. The national grids in Europe are also widely interconnected. In addition to the member states of the UCPTE (15) (Union for the Co-ordination of the Production and Transport of Electrical Energy) Austria, Belgium, Federal Republic of Germany, France, Italy, Luxemburg, Netherlands and Switzerland, Spain, Portugal, Yugoslavia, Denmark and Greece are connected to a European network. The United Kingdom and the Scandinavian countries are also connected to the UCPTE grid, via direct current lines. The exchange of electrical energy between national grids is constantly increasing. In 1977, 51,000 GWh, corresponding to 5.9% of the total electricity generated, were exchanged between the UCPTE member states alone. The maximum power exchange at any given time during 1977 between UCPTE countries was 8.3 GW. The capacity of the lines crossing the borders of the Federal Republic of Germany, for example, was 21,600 MW in 1977. In addition to restrictions imposed by the size of the sites needed for rectennas, the potential contribution of an SPS to the European energy supply could also be limited by spacecraft orbital location constraints. West to East Europe extends from 10°W at the West coast of Ireland, to 25°E, at the West coast of Turkey. It is not yet clear what the minimum separation for power satellites in geostationary orbit would be. Preliminary estimates assume a separation of 0.5° corresponding to a maximum of 70 satellites in geostationary orbit within the longitudinal boundaries of Europe. In practice, the number available to Western European countries would be lower than this because of the needs of East European or African countries falling within the same longitudinal band. The availability of orbital space may become an important factor in the assessment of Europe's interest in the SPS. It has been estimated (14) that the minimum energy production from the SPS necessary to justify the development of such a technology would be approximately 103 TWh/year, which corresponds to the EEC's present electrical power consumption. This lower limit would call for approximately 25 satellites, each with an output of 5 GW. 4. Prospects for European Activities Any future European activity in the SPS field will be strongly influenced by the fate of the SPS project in the United States. A significant effort in Europe can only be expected if the United States' Government decides to continue and increase its SPS activities beyond the present three-year assessment phase. Until this decision is taken, the European effort is expected to continue at its
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