The total number of units can be decreased by using a larger design, such as the 2.45 GHz bicycle wheel with mirror (Table 2, row 5), which supplies 533 MW to consumers. Thus, the annual deployment becomes: 230 x 103 MW 4- 533 MW/SPS - 430 SPS’s. For the NASA/US DOE reference design, (5 GW), this figure is: 230 GW 4- 5 GW/SPS = 46 SPS’s. The number of thin-film SPS’s needed may seem huge, but to nut them in perspective, 230 GW also corresponds to roughly 230 new nuclear power plants coming on line each year, or about one every day and a half. If constraints on orbital spaces necessitate fewer, larger SPS’s, and lower frequencies are not available, then large thin-film SPS’s can be constructed with only the central area used as the transmitting aperture. The large collector area will capture a great deal of solar energy, while the small transmitting aperture will cause the beam to spread out, keeping it from becoming too intense. Perhaps a more meaningful figure than the number of SPS’s that need to be deployed each year is the amount of material that will have to be launched each year. The 10 GHz bicycle wheel with mirror built of terrestrial materials has a specific mass of 0.68 kg/kW. Thus, the following amount of material will have to be launched annually: 230 x 106 kW x 0.68 kg/kW — 1.6 x 108 kg = 160,000 metric tonnes. For a conventional SPS, the amount of material needed annually would be: 230 x 106 kW x 10.2 kg/kW = 2.3 x IO9 kg = 2.3 x 106 metric tonnes. The reference system called for deploying two SPS’s each year3, for a total of up to 102,000 metric tonnes of material launched per year. Since this led to the suggestion that lunar materials be used, the need for lunar materials is suggested for the thin-film case as well. Although the thin-film SPS’s are much lighter than conventional designs, the higher estimates of global energy needs given here cause an increase in total mass requirements. The lunar bicycle wheel with mirror has a specific mass of 2.5 kg/kW, leading to an annual launching of: 230 x 106 kW x 2.5 kg/kW — 5.8 x 108 kg = 580,000 metric tonnes. Because of the difference in specific mass, the advantage of lunar materials is partly negated. However, while the lunar SPS is 2.5 kg/kW 4- 0.68 kg/kW — 3.7 times heavier per unit power delivered, launching from the Moon to geostationary orbit uses less than 8% of the energy needed from an Earth launch2. Due to the Moon’s lack of a significant atmosphere, as well as other factors, the savings in launch cost per unit mass (Moon versus Earth) may amount to a factor of about 50 [Ref. 2]. Thus, the cost of launching a given amount of power generating capacity is 50/3.7 — 14 times less from the Moon than from
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