While placing a rectenna 10 km by 20 km over the sea would add $3500 m to the total cost of the Reference System, the “reoptimized” design adds roughly half this figure. The addition to the total system cost is approximately 12%, and so the cost of electricity generated would be increased in the same proportion. A rectenna aperture of half that of the Reference Design, delivering 5 GWC, would entail an average microwave intensity across the beam of more than half the maximum permitted intensity of 23 mW/cm2 in the ionosphere. Thus, in order for the peak intensity to be less than the maximum, a rather flatter illumination pattern than the 10 dB Gaussian would be required — unless the limit of 23 mW/cm2 is found to be unnecessarily strict. Lund and Rathjen (25) consider the possibility of combining two or more functions in order to synthesize such a distribution and, interestingly, they quote the overall transmission efficiency as being higher than that for simple functions. As yet relatively little detailed work has been done on this subject, but since an active, automatic beam control system is required even for simple distributions, an increase in the complexity of the control circuitry is not likely in itself to pose serious difficulties. There are two further features of the marine location of rectennas which will affect the design. First, if the rectenna is some kilometres offshore, the limits for radiation around the rectenna will be less strict than near human habitations, and so sidelobe suppression should be less costly. Ships need be nearby for no more than an hour, for which current industrial limits would probably be sufficient. Second, since the marginal cost of a square metre of marine rectenna is rather more than twice that of a land rectenna, it ceases to be economical to collect the power in the edge of the beam at an intensity about twice that for a land rectenna, i.e. at 2-3 mW/cm2. The effect of this on the overall transmission efficiency would depend on the microwave illumination pattern. 6. RECOMMENDATIONS There are many areas of the SPS project which require more detailed study, but if the cost estimates made above are reasonable, the siting of rectennas in the North Sea looks promising enough to justify a detailed study. Such a study would need to address a large number of issues, some of which are quite new: (1) The identification of suitable offshore sites. Relevant factors to be considered include distance to centres of electricity demand; water depths; tides and currents; climatic conditions; shipping lanes, as well as all the environmental factors affecting rectenna sites on land (26). (2) Detailed design of the marine rectenna. The minimum cost design might or might not be that favoured above, but specific matters for investigation would be the design of horizontal rectenna arrays and/or of non-directional rectenna elements, and analysis of the dynamic behaviour and stresses in the rectenna structure under both normal and extreme environmental conditions. Data from oil and gas industry operations in the North Sea would be extremely valuable for estimating corrosion, fouling, environmental stresses and operation-and- maintenance costs. However, the study of the behaviour of such large structures would be a major new departure. (3) Detailed optimization of the microwave power transmission system (MPTS) for the case of a marine rectenna. This should include the optimum sizing of antenna and rectenna; the use of “flattened” microwave illumination functions; and the possibility of transmitting a non-circular micro wave beam so as
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