Fig. 12. Advantages of the laser SPS relative to the microwave SPS. While closed-cycle chemical lasers are possible in principle, little effort has been devoted to their development (19). 4. ADVANTAGES OF LASERS FOR AN SPS The laser SPS has four significant advantages over the microwave alternative. In order to put these advantages into context, it is necessary to review four of the major concerns with the microwave SPS: [1] large land areas are required for the receiving antenna (rectenna) sites; [2] the effect on biosystems of long-term, low-level microwave radiation outside the rectenna site is at present unknown; [3] beam sidelobes might interfere with electromagnetic systems, including communication systems, although measures could be taken to mitigate such interference; and [4] any microwave demonstration system must be at nearly full-scale, because even a demonstration system would require both a large transmitting antenna and a large rectenna. With respect to each of these four concerns, the laser system offers significant advantages over the microwave SPS (see Fig. 12). First, land requirements for the laser SPS are much less than for the microwave system for the same power delivered to the utility busbar. In the SPS Reference Concept, each rectenna site produces 5000 MW(e). If each laser beam receiving site contributes 500 MW(e), then ten sites will produce the same power as one microwave rectenna site. At 0.6 km2 of land area per laser site (18), a total of 6 km2 is required for the laser SPS per 5000 MW(e) output. This compares very favorably with 129 km2 per 5000 MW(e) output for the microwave system. In other words, land requirements for a microwave SPS are greater by a factor of 21. The 129-km2 microwave site includes both the elliptical rectenna [having dimen-
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