2.5 Comparison of Conversion System Concepts 2.51 Working Fluid Cycles 2.511 The Rankine Cycle Temperature-entropy diagrams for two different types of Rankine cycle working fluids are shown in Figure rV-B-l-c-2. The difference between the two fluid types is basically in the shape of the saturated vapor line. Both of these cycles have saturated vapor at the turbine inlet; however, the Type-A fluid expands in the turbine through the "wet" region, whereas the Type-B fluid expands in the superheat region. Examples of a Type-A fluid are water, ammonia, and the liquid metals (sodium, rubidium, mercury, etc.). Examples of a Type-B fluid are many of the freons, Diphenyl, Dowtherm A, etc. A comparison of the Carnot cycle with that of a typical Rankine cycle with a Type-A fluid is shown in Figure IV-B-l-c-2b. Since any space (as opposed to terrestrial) working fluid cycle must operate with as high a radiator temperature as possible to minimize system weight, it is necessary to elevate peak cycle temperature to as high a temperature as can be tolerated within material limitations in order to maximize cycle efficiency. Thus while ground-based steam power systems operate typically below 1200F, space power systems may operate as high as 2000F. While the organic or Type-B fluids have been considered for certain space applications, e.g., the ORACLE (Organic Rankine Cycle) program, two problems exist which make their use difficult for SPS. First, at the high temperatures required, peak cycle pressures for these fluids are much greater than those for the liquid-metal systems, resulting in a larger system weight. Secondly, organic fluids can only be used at temperatures below approximately 700F for extended-duration missions due to decomposition of the working fluid. This reduced working fluid temperature would result in even further additional system weight. Thus Rankine cycle working fluids are essentially limited to Type-A fluids, meaning for the most part water, ammonia, or the liquid metals. Fluids such as water and ammonia have high vapor pressures at peak temperatures and would increase system weight substantially. For example, whereas peak pressures in terrestrial steam systems are on the order of 103 psia, they are generally an order of magnitude lower for space systems. On the other hand, while system operating pressures are low for liquid metal systems, corrosion is enhanced at elevated temperatures, and in view of the 30-year life requirement of the SPS, this is the biggest single disadvantage of the Rankine cycle. In addition to the various working fluid types, the cycles differ in their heat rejection methods. Figure IV-B-l-c-3 illustrates the direct and indirect cycles. In the direct cycle a condensing radiator is employed
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