power to heating input power (called Q) must be about 30 for an economical power reactor, and that the power levels of fusion experimentshave become inconveniently high as Q has increased. Figure 9 shows the output power of fusion experiments versus the input power, where Q is approximately 1 in recent experiments. A commercial reactor would have an output of perhaps a gigawatt, which Furth predicts will come on line around 2020 and will be the largest engineering endeavor ever undertaken. Nuclear fusion thus will have to be a large-scale operation, and may be many years away from being feasible. After almost 40 years of research worldwide, with huge research budgets (for example, the U.S alone spends about $400 million each year), scientific and technical breakthroughs will have to be made before fusion power becomes a reality. In light of the above discussion, it appears that the most promising alternative that could make up the energy shortfall in the decades to come is the solar power satellite (SPS). The SPS is a satellite in geostationary orbit (GEO), 35,800 kilometers above the equator, which collects solar energy on arrays of photovoltaic panels and beams the energy to Earth using either microwaves or lasers. The energy is received on the Earth and is routed to users by electric power lines. The advantages of the SPS are that it uses proven technologies, and does not produce greenhouse gases or nuclear waste. Solar Power Satellite Technology Options Three SPS technologies are compared, which differ in their means of generation and transmission of power. These designs include a 2.45 GHz system similar to the NASA-DOE SPS reference design, a 35 GHz system, and a laser system. The mass of the reference design is about 50,000 metric tons for a 5 GW power level (see for example reference 19). Recent studies conducted at NASA's Langley Research Center have compared mass estimates for advanced laser power generation and beaming systems.20 On the basis of their comparison, we conclude that a laser SPS would be several times the mass of an SPS that would use microwaves. Thus we do not examine the laser option, though it might be pointed out that recent successes in concentrators might be able to reduce the mass involved.21 The Space Studies Institute's study19 investigated the feasibility of construction of the SPS using lunar materials. The reference design uses transmission through the atmosphere's microwave window at a wavelength of approximately 12 cm (2.45 GHz). The diffraction pattern for a 2.45 GHz beam was calculated assuming a quadratic aperture, and is shown in Figure 10. A rectenna large enough to capture 88% of the energy would have dimensions of approximately 9.7 x 9.7 km. An exclusion boundary set at a distance where the microwave intensity tapers off to 0.1 mW/cm2 would be a distance of approximately 6.5 km from the central maximum (assuming that the SPS delivers 6 gigawatts of power to the surface of the Earth). A question might be raised as to the value of this calculation, as extremely detailed calculations, designs, and simulations were done during the NASA-DOE SPS program. However, this calculation was done as a benchmark to compare it with the 35 GHz case.
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