consequence, the batteries are cycled between being charged and discharged nearly 6.000 times per year. Thus, after several years, the batteries will have to be completely replaced because of limitations on cell cycle life. The total mass to be replaced every 5.5 years is about 5 tonnes in the 57 kW AC configuration, and about 3 tonnes in the 37.5 kW PMC configuration.[7] Two possible Powersat options are analysed in the following subsections. The scenario takes Freedom in its permanently manned 37.5 kW configuration as a starting point. However, instead of adding an additional pair of solar arrays to upgrade the station's power to the original goal of 75 kW, an alternative is proposed using Powersats. This is shown schematically in Figure 4.1-2. Using Freedom as the example is considered a reasonable approach to take because the specific quantities (i.e. logistics upload, power requirements etc.) are very well defined. In this sense, it is not proposed that Freedom itself should use a Powersat - although it could - but rather that its defined configuration provides '‘hard’’ numbers that can be analysed with a certain degree of precision. It is important to note that the user power referred to above does not include the power needed by Freedom for operational purposes. The operating power is a little more than half the user power level (i.e. about 60%). Hence, the Powersat concepts defined must be sized to deliver that much more power in addition to the 37.5 kW of user power. 4.1.2 Columbus Free-Flyer Before discussing Freedom, it is worth briefly discussing the Columbus Free-Flyer. It is very important to note that Freedom is designed from the very beginning to eventually support the 75kW user power configuration. Therefore,
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