2*4.3.5 Recuperator/Cooler The recuperator faces relatively high temperatures from the working fluid coming out of the turbine. It helps save some of the energy of the hot gas by transferring the heat to the cold gas coming out of the compressor. Therefore it needs to be made of materials with good thermal conductivity and resistant to temperatures of up to 1200 K (for the high temperature cycle). Some parts of this device, such as the outer shell and some of the pipes, can be made of lunar alloys like croloy, stainless steel, or other steels. The high temperature cycle cooler operates at moderate temperatures. It uses NaK as the working fluid (which is assumed to be lunar) and it can be made mostly out of lunar metals such as aluminum or nickel. The low temperature cycle cooler operates at lower temperatures and is more massive. It has a smaller percentage of non-lunar materials but a higher non-lunar mass. It is estimated that 25Z of the high temperature recuperator/cooler system and 10% of the low temperature system will require non-lunar materials. The mass estimate for each cycle is shown in Table 2.4-7. TABLE 2.4-7 RECUPERATOR/COOLER MASS FOR 9 GW BRATTON CYCLE 2.4.3.6 Primary Structure The mass of the primary structure is not included in specific mass estimates for power conversion systems. This information on the Brayton SPS primary structure mass (also used in the TPV, Rankine, and Stirling) is provided for comparison with the silicon planar structure discussed in chapter 3. The primary structure holds the modules together. It receives direct sunlight. Aluminum can be used, though passive thermal control measures like those discussed in the thermal control section of chapter 3 may be needed. The estimated mass is shown in Table 2.4-8. TABLE 2.4-8 PRIMARY STRUCTURE MASS FOR 9 GW BRAYTON CYCLE
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