(c) a flexible structure supported on floats, anchored to the sea floor and protected by a perimeter sea-wall. The prospect of building a marine structure with a horizontal area in the region of 100 km2 is a daunting one, for which there are no real precedents. However, a variety of studies of very large marine structures have been performed elsewhere which provide enough information to estimate the feasibility and likely cost of the different design approaches with sufficient accuracy to produce useful conclusions. Some of these studies are: (1) A Dutch Consortium, the “North Sea Island Group” (NSIG) has completed a feasibility study of a 20 km2 industrial island to be built in the North Sea (14,15). The member companies have extensive experience in dredging, building and maintaining sea defences, and detailed cost information is available. (2) The Japanese Ministry of Transport is currently planning a new offshore airport to be built at Kansai during the 1980's. Two proposed designs for the 11 km2 structure, one based on 75,000 80 m piles, the other on 24,000 steel flotation chambers, each require some 5 million tons of steel. The landfill method used successfully in constructing Nagasaki airport is the officially favoured method (16). Though approximately an order of magnitude larger than this in area, a rectenna would have much lower gravitational loading and would require far less structural strength. (3) The possibility of building a cluster of large windmills for generating electricity off the East coast of Britain has been under investigation for some years. One recent study (17) proposes a 100 km2 site comprising 400 individual units of 2.5 MWe output, supported on concrete towers rising some 50 m above sea level. Much of the work on siting and structural requirements for these is relevant to marine rectennas. (4) The Universities of Yokohama and Miami are collaborating on the design of a system of floating solar-powered generators, each of 1 km2, for the production of hydrogen and oxygen from seawater (18). Interestingly the figures quoted for the energy output from this system suggest that a rectenna collecting power from an SPS would be likely to be a more economical source of electric power for hydrogen production. We now examine each of the proposed marine rectenna designs in turn: (a) For a rigid, piled support structure to carry the rectenna array, conventional design would require single concrete or steel spans between piles to be not more than about 100 metres long. A square lattice under a 160 km2 ellipse would therefore require at least 16,000 and possibly more than 60,000 piles. The rectenna arrays would have to stay above the maximum wave height. With sea depths of 20 m or more, tide and wave heights together reaching about 20 m, and a sea floor of deep mud, the piles would have to be more than 70 m long and correspondingly massive. It is not necessary to calculate the optimum pile separation, as a very rough cost estimate is adequate for the present purpose. If we assume a cost of only $1 m for each pile-plus-superstructure at a separation of 100 m (which broadly corresponds with cost estimates for offshore windmill towers (17)) we obtain a round figure of $100/m2 of support structure — i.e. $200m/km2 of beam aperture — which is almost certainly optimistic. Thus for a complete marine rectenna of this type we can write: (b) For constructing a complete artificial island of the scale of 10s of square
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