4.6.5 Weight Analysis One of the most important characteristics of the SSPS that needs to be estimated at the earliest date is its total mass. This section of the analyses presents the masses of the various components of the design in Figure 4-21. The converter emitter consists of a circular 100 cm- molybdenum flat plate which is 5 mm thick, so that it can withstand the 1.2 mm loss of material (0.6 mm from each face) during the desired 30-year lifetime, and retain a reasonable electric conductivity. The complementary collector has similar dimensions. Cesium is permanently introduced into the interelectrode cavity at the beginning of operation. Any small loss of this element during the operating lifetime of the collector can be replaced with cesium-graphite pockets in the collector. The total weight of the electrodes is 950 gm per converter or for a 1033 W net power output, 0.92 kg per kWe. In Table 4-5, the sodium-filled heat pipe is estimated to have the following mass for a 1 mm-thick outer casing: Table 4-5. Mass of Heat Pipe Components or for the approximate 0.1 m- heat pipe radiating surface area, 26.7 kg per m-. The mass of ten layers of 1 mil-thick MULTI-FOIL insulation is a negligible 0.25 kg per m^. The masses for the rotary converters and for the solar concentrators plus frame and support arms has been calculated by Boeing to be 0.4 kg per kWe and 0.3 kg per kWy, respectively (Ref. 7). The mass of each of the 15-cm wide, 7-mm thick, 15.2-cm long copper leads connecting the converters, as shown in Figure 4-21, is approximately 1.42 kg, or .84 kg per converter, which means 2.75 kg per kWe. Table 4-6 summarizes these masses, and relates them all to electrical power output. The masses of the busbars connecting the fundamental 161 converter units to each other and the panels to the rotary converters have not been estimated. Table 4-6. Mass of Converter Assembly 4.6.6 Conclusions The major results of this study are: 1) Planar molybdenum-nickel thermionic energy converters with 5 mm-thick electrodes can be operated at emitter temperatures of 1800 K and be expected to provide electrical efficiencies of 21 and 24 percent, respectively, by the years 1985 and 1995. 2) The converter design judged best has electrodes 100 cm- in area and a gross output of 1200 W in 1955 and 1290 A and 0.93 V. Resistive power losses through the electrodes and interconverter leads will reduce the gross power 14 percent for a net output of 1032 W per converter. 3) The panel design judged best to minimize busbar resistive power loss consists of 3381 converters in twenty-one 161-series strings. Net power output from such a panel is 3.5 MW. Copper interconverter leads were found superior to sodium-filled stainless steel. The requirements of busbars connecting the modules to the rotary converters were not considered. Such leads should be tapered to carry a maximum of 27,090 A per panel. The mass of these busbars could be substantially reduced by connecting, say, two converter strings (i.e., 322 converters) in a panel in series, thereby doubling the output voltage to 258 V and reducing the maximum current by one-half to 13,545 A. The ability of electrical insulation to withstand this higher voltage at 1000 K must be investigated. For the 161-converter unit, a failure in operation of one converter causes a maximum loss in power of 5 percent of the panel output, or 0.004 percent from a 4 GW module. The reliability of operation over a 30-year period would be increased by connecting all converters in parallel as well as series. Such a configuration would, however, substantially increase the mass of the leads.
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