System size effects must be considered. For example, if heat pipes are to be used without pumped manifolds a maximum practical heat pipe length of = 10m (33 ft) limits the radiator area to 628m^ (2060 ft^). At a cell and radiator temperature of 333K (140°F), the heat rejection from the radiator is only about 0.63 kW/m^ (0.06 kW/ft^), limiting the total heat rejection to approximately 400 kW. This would limit the module which this radiator cools to an output of under 200 kWe. The associated solar collector would be only approximately 500m^ (5380 ft^), and at least 160,000 such modules would be required for a 20 GW ground output SPS. The solar collectors are too small to use the steerable reflector facet concept, since the facets would be under 0.5m (20 inches) on a side. The mass of the facet pointing servomechanism would contribute to an overall specific mass in the range of 2 to 3 Kg/m^ (0.4 to 0.6 Ibm/ft^). This is in contrast to the 0.24 Kg/m^ (0.05 Ibm/ft^) of the large steerable facet concentrators used with the Brayton concept. Thus a heat pipe radiator without pumped manifolds appears to be limited to use with low to moderate (up to 200) solar concentration ratios. Of course linear (two dimensional) solar concentrators of any length may be envisioned, so that large power levels may be handled with heat pipe radiators without manifolds. These considerations are included in the modeling effort to select a solar concentration ratio and cooling system (if any) for use with photovoltaics. REFERENCES 1)NASA TMX-64627 "Space and Planetary Environment Guidelines for Use in Space Vehicle Development,''Nov. 15, 1971. 2) McGraw Hill "Space Exploration." 3) NASA CR-54201, "Meteoroid Protection For Spacecraft Structures." 4.11 POWER DISTRIBUTION 4 11.1 The Problem The baseline microwave power transmitter requires 16.17 GW at the interface to achieve 10 GW on the ground, as shown in Figure 4-89. With 20,000 volts direct current to the amplitrons, the required current is 808,500 amperes. In photoFig. 4-89. Microwave Power Transmission Efficiency Chain voltaic systems, power is generated over the entire SPS; in thermal engine systems power generation is more localized. In both cases power distribution over distances exceeding 15 km (9.3 mi.) is required (in nuclear SPS types the generators can be located relatively near the transmitters). Also, in solar SPS types the transmitter must constantly face the receiver on Earth while the power generation system faces the sun; thus either slip rings or a rotary transformer are required to pass the power. 4.11.2 Conductors In a 5 GWe ground output powersat approximately 8 GWe at 20,000 vdc must be delivered to the antenna; twice this is required for 10 GWe ground output. If 8 GWe is moved over two conductors the current is 400,000 amperes. In Figure 4-90 minimum system mass is determined for three material temperatures. This analysis technique works for any "unit length"; one centimeter (0.4 inch) was used. The three conductor sections necessary to dissipate the I^R losses for the three temperatures are shown. 500K (440°F) is probably the highest practical temperature for aluminum structure. The principle shown is to select that conductor diameter which causes the total SPS mass to be a minimum. At the bottom of Figure 4-90 is shown the baseline structure (size based on mechanical loads alone). This structural element can carry the 382,000 volts baselined for a.c. distribution without overheating.
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