engine efficiency. About 3% of the overall system efficiency is lost because of the loss of 80°C of the superheat due to transport heat leak. Of the total energy needed to raise the fluid to superheated steam, 15% goes into preheating from 250°C to 313°C, 56% is required to boil the water at 1500 psi, and 29% is used to superheat to 510°C. Thus, 71% of the energy is used to generate saturated vapor. It is possible to reduce the heat leak by devoting the outer 71% of the solar field to vaporizing the water. This fluid would be transported at 313°C to the 29% inner ring of the field where superheating would take place. By using condensate separators, only vapor would be introduced to the inner ring of collectors. Thus, the higher temperature (510°C) superheated steam would move a shorter distance to the central plant. This approach would save approximately 1/3 of the total heat leak and would reduce the superheat temperature loss by nearly 70%. Thus, the loss of superheat could only reduce the overall system efficiency by 1% instead of 3%. Although the superheating collectors would require larger cavity receivers that operated at lower efficiencies, the average collector efficiency of the field would remain unchanged. This use of two types of receivers in the collector field should also reduce any control difficulty in producing superheated steam in a once-through boiler with varying solar input. 4.2.1.3 NaK and Helium Transport. As indicated in Section 3.2.1.2, both of these fluids operate between 370 and 650°C. Superheated steam at 540°C (1800 psi) is generated with 210°C superheat. The Rankine heat engine is 41.5% efficient based on a FTR of 0.6. The collector efficiency is 75% at 650°C. The cavity heat exchanger can have a compact design because of the excellent heat transfer characteristics of the liquid metal, but will be expensive because of the need for
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