deep wells all along the dikes and an open drainage system with two pump stations maintain a groundwater table in the polder permanently below the ground surface level. The outer slopes of the dikes are protected against wave attack by a hard sea defence construction with a primary armour layer consisting of concrete cubes. A work island is attached to the polder to serve as a production and supply base. The studies have shown that it is technically feasible to construct polders with offshore rectenna structures in the North Sea. The estimated cost is about US $5 billion (1980) and its impact on SPS electricity generation cost is in the order of 2 cent/kWh on the basis of a 5 GW reference design and a capital cost spread over 30 years (design lifetime of the reference SPS). In order to confirm that the use of offshore in Europe is feasible further SPS system aspects should be studied with priority for • increasing the intensity of microwave beams; • determining precise limits of safety zones; • determining the optimum of rectenna sites. Other uses of the rectenna site and its periphery need also to be considered, such as industrial activities, agriculture and biomass production, fish nursery and other energy conversion systems (wind, wave, tidal energy) in order to make the energy polder economically more attractive. FRONT-END COST OF POWER SATELLITES Another aspect worrying the Europeans is the immense cost and the time required to develop an SPS. It is well known that the cost estimates obtained by NASA are in the order of 100 billion dollars for development and construction of the first SPS satellite. We note that these figures are preliminary. A commercial SPS fleet for Europe consisting of twenty 5-GW satellites with an electrical output to the utility grid of about 100 GW (electric) could possibly cost more than 500 billion dollars (according to NASA nominal figures). Cost estimates performed by European authors confirm the order of magnitude. Presently there is no government in Western Europe or elsewhere prepared to take the risk of investing billions of dollars in such a development programme which has a payback time of at least 20 to 30 years. In order to bring these cost figures into perspective the following facts should be considered. Firstly, the assumed 20 power satellites could eventually replace approximately 200 million tons of oil per year, valued at about 60 billion dollars per year at present prices. This could greatly reduce the dependence of Western Europe on energy imports which at present is at least 50% (for comparison: USA about 10%). These figures do not include uranium fuel for nuclear reactors. Secondly, we can assume that only fractions of the development cost have to be spent on a specific SPS research and development programme. Much of the development effort in the world for other (civilian and military) space programmes in the next two decades will be vigorously pursued in areas such as space transportation, manned space activities, large space structures, and power supply systems for spacecraft in the range of hundreds of kilowatts. The results will be vital to future consideration of power satellites. A rough calculation will illustrate the indirect benefits in the SPS exploration induced by this effort: supposing only 10% of the results of such generic space
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