shows a diagram which compares the cost of photovoltaic and solar dynamic systems for providing additional power to Space Station Freedom. The trade looks at the whole life cycle cost of the two systems and takes into account also the quantity of propellant that is required to compensate for the atmospheric drag. It is predicted that there will be a cross-over point in favor of solar dynamic power in 2003 [Fordyce, 1992], Figure 4.3 Cost of Using Photovoltaic versus Solar Dynamic Power [Fordyce, 1992]. However, another trade that was made for Space Station Freedom indicated that by taking into consideration state-of-the-art solar cell technology the result would be in favor of photovoltaics [Bailey & Landis, 1990]. The trade assumes that the Si cells, which have an efficiency of 14.2%, are replaced by GaAs/Ge cells with an efficiency of 22%. It is also assumed in the trade that the NiH batteries are replaced by NaS batteries. The result is shown in Table 4.2, where Space Station 21 refers to the GaAs/Ge-cell design. Table 4.2 Trade between using photovoltaic and solar dynamic power for Snare <>tatinn Frapdnm FRailav A Lanriic 10001 Microwave and laser beaming are the two options for power transmission in space. The trade between microwave and laser transmission is determined by the characteristics of each particular application. Microwave transmission generally has higher efficiency than laser transmission, but in many cases, such as when beaming power to spacecraft, the large-size rectenna, which is required for receiving the microwave beam, may make it more favorable to use laser transmission despite its lower efficiency. Microwave transmission is a mature technology and has high efficiency, up to 60%, but the long wavelength makes it necessary to go to large transmitter and receiver diameters in order to achieve efficient power transmission. Going up in frequency has the drawback of increasing attenuation of the beam in the atmosphere. Environmental and safety aspects with regard to microwave radiation put an upper limit to the power density. The availability of suitable rectenna sites, which will need to have a
RkJQdWJsaXNoZXIy MTU5NjU0Mg==