7, SPS Size and Power Level When the first nine efficiencies in Table 1 are multiplied together and the result substituted for T| in Equation 7c, the size of an SPS whose peak beam intensity (Inc) is 30 mW/cm2 at the Earth's surface can be computed. For a non-tracking bicycle wheel SPS beaming power to a latitude of 35’ at a frequency of 10 GHz, the diameter is 2021 meters and the power incident on the Earth's surface (Equation 6) is 153 megawatts. When the last three efficiencies in Table 1 are accounted for, it is seen that 109 megawatts is available to consumers. The rectenna energy collection efficiency is based on the percent of the power of an untapered beam that falls within the main lobe5-14. The mass of the solar cell/transmitter array is 51 metric tonnes. This does not include the mass of the support structure, which is quite low, and will be assessed in Section 8. Results were also obtained for a bicycle wheel design with a mirror, an inflatable sphere, as well as SPS's with other frequencies and latitudes. A peak beam intensity of 30 mW/cm2 at the Earth's surface was assumed in all cases. The results are summarized in Table 2. The types of SPS referred to in Table 2 are non-tracking bicycle wheel (b.w.), bicycle wheel with mirror (b.w.m.), and inflatable sphere (i.s.). The first three rows of the table can be thought of a reference designs, with the rest of the table included for comparison. Note that rows 9 through 11 show that the design is only mildly sensitive to latitude (at least, for latitudes in which an SPS is practical). Rows 4, 7, and 8 show that the design is highly sensitive to power beam frequency. The 10 GHz SPS’s shown in the first (b.w.) and third (i.s.) rows served as the basis for calculations of structural design and total mass seen in Section 8 (for the inflatable sphere, there is no support structure, except for a low pressure gas whose mass was assessed in Section 8.5). The masses in parentheses for the bicycle wheel with mirror are simply double the array masses (since the mirror is assumed to have the same design as the bicycle wheel SPS, except that the substrate is coated with reflective material such as aluminum, instead of amorphous silicon and solid state transmitters), and are included for comparison. The masses of the inflatable sphere design were obtained by considering a flat array with no tracking loss and multiplying the mass by 4 (since the surface area of a sphere is 4 times its cross-sectional area). The specific power levels (power per unit mass) were computed by dividing the power delivered to consumers by the masses of the arrays (including a mirror where appropriate) built from tetrestrial materials. Results indicate that frequencies of 10 GHz and below have the highest specific power, due to the higher microwave transmitter efficiencies and lesser atmospheric absorption than at higher frequencies. The non-tracking
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