Several configuration approaches were considered and two typical examples were studied in some detail for the purpose of defining ranges of weight, cost, and construction approaches. These two examples are referred to as the "column/cable" and "truss" configurations. A. System Analysis 1. Efficiencies The energy collection, conversion, and transmission process involves a number of steps, each having an associated efficiency. An initial task of the study was to estimate these efficiencies. Three estimates were made of the efficiency of each step, including a minimum efficiency that could be achieved with virtual certainty, a probably achievable (nominal) efficiency, and the best, or maximum, efficiency that might be achieved. These estimates are presented in figure IV-2. The estimated overall efficiencies from incident sunlight to de output were 4.2, 5.4, and 8 percent for the "minimum," "probable," and "maximum" cases, respectively. The estimated efficiencies of the system excluding photovoltaic conversion of sunlight to electrical energy were 41, 52, and 69 percent, respectively. These estimated efficiencies were used for collector sizing and weight estimates. Revised efficiency estimates indicated that the "probable" achievable (nominal) efficiency was more appropriately 58 percent than 52 percent. The efficiencies of the various steps resulting in this revised "probable" estimate are also presented in figure IV-2. 2. MPTS/MRCS Analysis An analysis was conducted to determine the appropriate size of the power station, defined in terms of the de output power at the rectenna and the overall microwave system(s) parameters. Two specific constraints were identified that would limit the maximum power output. These constraints were maximum allowable power densities of 21 kW/m at the transmitting antenna and 23 mW/cm at the ionosphere. The former is the result of the thermal limitations of the aluminum waveguides. The latter is the result of a theoretical analysis (ref. Meltz) which indicates that nonlinear interactions between the beam and the ionosphere will not exist below this level. Given a system frequency (2.45 GHz) and the estimated efficiencies of steps in the transmission process, the two aforementioned constraints can be related to de output power and transmitting antenna diameter. These relationships are illustrated in figure IV-3. It can be seen from the figure that the maximum power output that does not exceed the constraints is 5 GW, achieved with a transmitting antenna diameter of 1 km. Accordingly, a 5-GW de output power at the rectenna and a 1-km antenna diameter have been used as nominal, or reference, values throughout the study.
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