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4.3 Criteria and methodology for fluid selection 4.3.1 Overview of selection criteria A selection process of an item in a precise group of similar items requires criteria and selection procedure. This applies also to the selection of working fluids for Rankine cycles. For this particular case of organic and inorganic fluids, several works have been reported in literature and a non-exhaustive list of papers dealing with the working fluids selection for organic Rankine cycle applications is given in Table 4.2. However, the study is one of the first works dealing with selection of fluids for solar applications. Majority of works available in literature focus the waste heat recovery application. Nevertheless there exist general criteria that apply to almost all applications: Critical parameters The critical point is the point on a thermodynamic diagram for which the thermodynamic properties of liquid and vapor meet and become indistinguishable. The temperature, density and composition of the substance are the same for a liquid or gas at this point. The properties at the critical point are referred to as the critical density, critical temperature, critical volume and critical pressure. Depending on the type of the cycle (supercritical or subcritical), the critical parameters should be compared to the set operating conditions of the cycle. In case the cycle is subcritical as in most cases, the critical point should be appropriately above the system evaporating point in order to optimize the heat transfer (Badr et al., 1985; Tchanche et al., 2009b). The cycle pressures The pressure in the evaporator should not be excessive. Moderate pressure is recommended in order to avoid mechanical stress problems. Very high pressure will cause safety problems and increase the cost of the system by requiring additional safety equipments. The condenser saturation pressure should be kept above the atmospheric pressure. A low condenser pressure could lead to problems of sealing against infiltration of the atmospheric air into the system. Therefore, for safety and economic reasons, the pressure in the heat exchange units should be kept above 1 bar for condensers and below 25 bar for evaporators (Maizza & Maizza, 1996; Badr et al., 1985). The slope in a T-s diagram Usually, three groups of fluids are distinguished: dry (positive slope), isentropic (infinite slope) and wet (negative slope) fluids (Badr et al., 1985). For low-temperature systems and particularly with low power output levels, wet fluids like water, methanol or ethanol are not suitable (Hung et al., 2001). This category of fluids requires a superheat in order to avoid moisture after the expansion process. Fortunately, there are dry and isentropic fluids that do not exhibit excessive moisture during the expansion process. Isentropic expansion of isentropic fluids ends in saturated or superheated vapor regions and the erosion of the blades is avoided. Dry fluids with high positive slopes yield low thermal efficiencies and this can be overcome by integrating a regenerative heat exchanger in the cycle as mentioned in chapter 2. Page | 85PDF Image | Low-Grade Heat Conversion into Power Using Small Scale Organic Rankine Cycles
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