It should be noted at this point that by tuning the laser frequency to the quantum band gap of the solar array, significantly increased array efficiencies are achieved over normal solar radiation (approximately 50% as opposed to 10-15% [Brauch et al 91]). Since poor weather will interrupt ground based lasing, a continuous supply of power to the client could not be ensured. Ground laser stations could be based in areas with excellent weather conditions, such as the Sahara Desert or south western Arizona, or more than one ground station in different locations could further reduce the risk of the power supply being “weathered out”. However, uninterrupted power beaming cannot be guarantied, and this is an important feature of ground based laser power beaming. Power Beaming for Geostationary Based End Users The majority of geostationary satellites are for communications. These satellites have design lifetimes of typically 10 to 15 years. Meteorological satellites and some other proposed earth observation platforms are also geostationary orbit users. It has been proposed that geostationary satellites' lifetimes could be extended by power beaming to degraded satellite power systems. This can be analyzed as follows: Geostationary satellites become unusable for one of the following reasons: 1. Malfunction of payload or satellite bus 2. Exhaustion of on board propellant for station keeping and attitude control 3. Degradation of batteries to the degree that operation in eclipse is no longer possible or degradation of solar arrays to a degree that they are unable to power the payload and charge the batteries after eclipse. Cases 1 and 2 are clearly not power system issues. Case 3 might be alleviated by providing the spacecraft with beamed laser power which could prevent deep battery discharge during eclipse, since battery deep cycling is the main contribution to their degradation, or by boosting degraded solar array output in general. Received power levels should be of order 1 kW to be useful. This application scenario is one that is popular with researchers working on space beamed power, and is often vaunted as one of the viable space power missions. Geostationary Market Size and Value However, there is no significant market for power beaming to existing communications satellites for life extension purposes, and the optimism of some in the field is misplaced. Satellite design attempts to balance the life limiting factors so that they reach life expiration simultaneously. Solar arrays are sized to give operational power at end of life, and battery systems are massive enough that they, likewise, provide sufficient performance throughout the satellite lifetime. The end of the satellite's useful life is almost always brought about by propellant exhaustion. To our knowledge, there are no documented cases of battery or solar array degradation alone ending the productive service of a satellite, [conversation with Brandhorst, Lewis Research Center] In some cases, hardware failure of one power subsystem has increased the demands on the remaining subsystems (such as failure of one battery pack), and in these cases, beamed power could be of use. The incidence of such failures, however, numbers one or two cases out of 200 or so geostationary satellites launched. The market value of such a small number of potential clients can be established: information from the US organization Comsat indicates that revenue from regenerating a spent satellite would total US$10M per year, and the organization would be willing to spend 20 % of this total to keep the satellite in operation. [Muelenberg, Comsat, via Brandhorst] The market value is therefore $2M per year for each satellite. Alternatively, if a hardware failure occurs early in the satellite's life that can be aided by beamed power, the value of this can be judged by considering the savings derived from a delay in procuring a replacement satellite. If this cost (including launch) is $400M, and assuming a 10% discount rate, a year's life extension is worth a nominal $40M. However, one must factor the increased complexity and capacity that the new satellite will have: possibly as much as twice the capability (though more likely less than this). This effectively reduces the value of life extension to $20M per year. The cost of establishing ground based laser systems in the near term is, however, of the order of billions of US dollars (see section 11.1.1). Such revenue, which is possible on a very occasional basis (when these very uncommon hardware failures occur), does not render this application a viable market for space beamed power, despite the optimism of some in the field.
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