138 kilowatt hours of d.c. energy while the microwave power transmission system as a whole consumes over 9000 kilowatt hours of energy, as shown in Fig. 5, for an efficiency of about 1.5%. By contrast, the microwave power transmission system in the solar power satellite system was confidently predicted to be better than 50%. If the 60-cycle electrical energy can be obtained at a cost of 5 cents per kilowatt- hour, the total cost of the 60-cycle energy consumed is $452 for the scenario selected, while for the 50% efficiency power transfer, the cost would be only $13.80. However, the cost of transferring the argon propellant from Earth to LEO, based upon an assumed transportation cost of $1000 per kilogram, is $144 (from Table 2). A lower cost Earth to LEO transportation system is therefore necessary before invoking a more efficient LEO to GEO system. The major cost of the microwave power transmission system will be the development and deployment of the ground based transmitter. Its radiating area will be two square kilometers and its maximum radiated microwave power is 400 MW for the scenario chosen. The estimated cost for the development and deployment of this system is in the range of $600,000,000 to $1,200,000,000. The estimate is based upon technology developments in Refs. 6,7,8 and upon ongoing refinements of the results of these studies. TECHNOLOGY BACKGROUND Microwave Power Transmission The technology of microwave power transmission is unique in that its development has not been forced and that with a relatively low investment of effort over a relatively long time period it has reached an advanced level, ready for deployment in several applications. An important feature of this advanced level of development is the modular configuration in which it can be deployed and the readily-available, low-cost components for such modules (8,9). For the needs of an interorbital transfer vehicle propelled by electric engines, the thin-film, printed-circuit rectenna represents a technology breakthrough that has the following features: • Specific d.c. power output of 1 kW/kg; • Overall efficiency greater than 80%; • Minimal use of GaAs (10 5 that of solar cells); • Can be easily shielded from radiation damage; • Physical flexibility allows easy transport and deployment; • Minimizes power conditioning; • Can be produced in the large quantities needed with existing facilities. The further development of the thin-film rectenna is currently receiving support from NASA and the Canadian Government (5). Turning now to the transmitter, which consists of the radiating antenna and the generation and distribution of microwave power, there are significant developments. The magnetron directional amplifier originally evaluated during the SPS program is a key component because of its high efficiency, high quality of its microwave output, and the low cost of its components. It is also currently undergoing additional development (9). The slotted waveguide array radiator makes use of a new method of fabrication
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