for rectennas in densely populated areas. This criteria is the first chosen in an American study conducted for the United States mainland under DOE sponsorship. The authors of this study have excluded, as unsuitable, regions with population density higher than 20 p/km2, and this choice had led to the exclusion of nearly V3 of the United States, which is in fact a very sparsely populated country. We have chosen for our own assessment a higher density limit: 100 p/km2. The rational behind this choice is the consideration of the European case. It woudl be very difficult in France, a medium European country from the population density point of view, to find suitable 500 km2 smooth areas for implementing rectennas. The population density in France, about 100 p/km2, is thus clearly a maximal limit. Other European authors (9) have reached the same conclusion, stating in their study that the only solution for Western European countries would be to locate rectennas offshore, a possibility we consider very remote. Taking into account a density limit of 100 p/km2, our computations show that 70% of world population will live in 2025 in geographical zones with density higher than this limit, and that 50% of electricity demand will occur in these densely populated areas (Figs. 3 and 4). This problem seems very serious for the future of SSPS outside United States. Limitations Due to Electricity Distribution Cost In our previous studies, we have shown that the density of electrical demand was an important criteria for the trade-off between centralized and uncentralized electricity generation. The rational behind this assumption is that, for the same power, the total investment cost of an electrical energy system increases as density of demand decreases, due to higher costs of the distribution grid (this cost goes up like
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