It is then recommended to include those developments in a more general plan to share the costs with other projects. The field of telecommunications could be used to get a first return on investment from those technologies. A point of great interest, even if not directly linked with the spacecraft itself, is the energy density at rectenna level. This density has a direct impact on the efficiency of the system and depends highly on the performances of diodes at GHz frequencies. Within the present technology frame and for high orbits, it is necessary to consider either extremely large spacecraft (kilometers wide) to allow a large amount of energy to be beamed to relatively small areas on the ground, or to use concentrators. Neither of those solutions are perfectly satisfactory. It is therefore recommended to push the development of better diodes or rectifying devices and to investigate the possibility of ‘burst' transmission. This later point could be a joint development with radar users. Finally it should be pointed out that this study does not cover the reliability and safety aspects, which should be carefully detailed for an operational usage. Energy Aspects It is of some interest to compare the amount of energy collected in this design with what could be collected at ground level with solar cells. The average solar power at ground level over the US is pessimistically one quarter (Whitehouse, 1989) of the value in space. Considering that the rectenna complex is roughly 10 km x 10 km wide, and even considering a very inefficient conversion ratio of the ground system, that is, 5%, the available energy would be 10000 m x 10000 m x 0.25 kW/m2 x 0.05 = 1.25 GW. This figure would improve still further using a thermodynamic system. It seems then, that even not mentioning the tremendous difference in cost between space and earth operations, covering the area of the rectenna with solar cells could warrant further investigation! 10.4.3 Concept Summary The following concepts are discussed in the previous text, they both fill potentially the requirements expressed in item 0 and allow consideration of alternatives. The baseline is concept number 2. ________________Table 10.4.4 Concept 0 SPS-2000_______________
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