Ground to Space Power Beaming using Microwaves The Net Present Value model described above is not specific to laser power beaming techniques. However development and ground station costs for microwave technology are likely to be lower, since the technology is more mature, and the implementation simpler. Development costs may be of order a few hundred million dollars, and ground station cost perhaps $100M (since large antenna areas are required). Maintenance and operating costs are unchanged, since the size of the microwave facility will in part balance the reduced complexity over the laser ground station. In the tables above, however, it was shown that the sensitivity of the project NPV to development and ground station costs was small. Indeed, the inaccuracies in other baseline assumptions will clearly swamp the small variations in development and ground station costs considered. However, space based receivers will be considerably more expensive, since even with large ground antennas microwave beam spreading is significant and the spacecraft rectenna must be some hundred meters in dimensions (see section 7.2 for an explanation of transmission fundamentals). This may limit the market size for microwave beaming to space users, but since in the mid to long term we expect power customers' spacecraft design to reflect the available beamed power source, we make the baseline assumption that the market would be the same irrespective of whether a laser or microwave ground to space power system is implemented. This assumption is optimistic in the case of microwave beaming. In the near term, the potential of microwave beaming is further limited since power delivery to existing spacecraft and solar arrays is not possible: a dedicated rectenna is required. There can be little doubt therefore, in view of the results of the near term laser analysis above, that near term microwave is also not commercially viable. For completeness, however, the mid term case with modified development and ground station costs for the microwave case is shown in Table 11.3 as follows: Table 11.3 NPV - Mid term Ground to Space Microwave Though this shows a slightly increased profit over the laser beaming model, this is of a sufficiently small value that the conclusions that may be drawn are the same as those in the discussion of the laser beaming venture. The lunar market, however, may not extend to microwave beaming simply because of the transmission distances and beam spreading: a 20km circular antenna on Earth would still require a 5km square rectenna on the lunar surface at 2.45 GHz transmission frequency. With the exception of the lunar market, we may consider the results of the analyses relating to laser transmission above to apply equally to microwave beaming in all time-scales considered. Conclusion We conclude that the near term market is not a viable one for laser or microwave Earth to space power beaming. Both transmission technologies look promising in the mid and long term, and may
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