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176 S. Quoilin et al. / Renewable and Sustainable Energy Reviews 22 (2013) 168–186 for each specific thermodynamic property independently. The most common approach consists in simulating the cycle with a thermodynamic model while benchmarking different candi- date working fluids. (2) Positive or isentropic saturation vapor curve: as previously detailed in the case of water, a negative saturation vapor curve (‘‘wet’’ fluid) leads to droplets in the later stages of the expansion. The vapor must therefore be superheated at the turbine inlet to avoid turbine damage. In the case of a positive saturation vapor curve (‘‘dry’’ fluid), a recuperator can be used in order to increase cycle efficiency. This is illustrated in Fig. 9 for isopentane, R11 and R12. (3) High vapor density: this parameter is of key importance, especially for fluids showing a very low condensing pressure (e.g. silicon oils). A low density leads to a higher volume flow rate: the sizes of the heat exchangers must be increased to limit the pressure drops. This has a non-negligible impact on the cost of the system. It should however be noted that larger volume flow rates can allow a simpler design in the case of turboexpanders, for which size is not a crucial parameter. (4) Low viscosity: low viscosity in both the liquid and vapor phases results in high heat transfer coefficients and low friction losses in the heat exchangers. (5) High conductivity is related to a high heat transfer coeffi- cient in the heat exchangers. (6) Acceptable evaporating pressure: as discussed for the case of water as working fluid, higher pressures usually lead to higher investment costs and increased complexity. (7) Positive condensing gauge pressure: the low pressure should be higher than the atmospheric pressure in order to avoid air infiltration into the cycle. (8) High temperature stability: unlike water, organic fluids usually suffer chemical deterioration and decomposition at high temperatures. The maximum heat source temperature is therefore limited by the chemical stability of the working fluid. (9) The melting point should be lower than the lowest ambient temperature through the year to avoid freezing of the working fluid. (10) High safety level: safety involves two main parameters— toxicity and flammability. The ASHRAE Standard 34 classifies refrigerants in safety groups and can be used for the evaluation of a particular working fluid. (11) Low Ozone Depleting Potential (ODP): the ozone depleting potential is 11, expressed in terms of the ODP of the R11, set to unity. The ODP of current refrigerants is either null or very close to zero, since non-null ODP fluids are progressively being phased out under the Montreal Protocol. (12) Low Greenhouse Warming Potential (GWP): GWP is mea- sured with respect to the GWP of CO2, chosen as unity. Although some refrigerants can reach a GWP value as high as 1000, there is currently no direct legislation restricting the use of high GWP fluids. (13) Good availability and low cost: fluids already used in refrigeration or in the chemical industry are easier to obtain and less expensive. While fluid selection studies in the scientific literature cover a broad range of working fluids, only a few fluids are actually used in commercial ORC power plants. These fluids are summarized in Table 4, classified in terms of critical temperature [32]. In general, the selected fluid exhibits a critical temperature slightly higher than the target evaporation temperature: if the evaporation temperature is much higher than the critical temperature—for example if toluene (Tc 1⁄4 319 1C) is evaporated at 100 1C—vapor densities become excessively low in both the high and low pressure lines. Table 5 summarizes the scientific literature in the field of working fluid selection for ORC systems. To compare the different papers, three characteristics are taken into account: the target application and the considered condensing/evaporating tempera- ture ranges. The papers comparing the working fluid performance as a function of the turbine inlet pressure (for example [57]) and not the temperature are excluded since the main limitation in the ORC technology is the heat source temperature and not the high pressure. From Table 5 it becomes apparent that, despite the multiplicity of working fluid studies, no single fluid has been identified as optimal for the ORC. This is due to the different hypotheses used to perform fluid comparisons: Some authors consider the environmental impact (ODP, GWP), the flammability, and the toxicity of the working fluid, while other authors do not. Table 4 Common working fluids in commercial ORC installations. Fig. 9. Isentropic, wet and dry working fluids. HFC-134a HFC-245fa n-pentane Solkatherm OMTS Toluene Used in geothermal power plants or in very low temperature waste heat recovery. Low temperature working fluid, mainly used in waste heat recovery. Used in the only commercial solar ORC power plant in Nevada. Other applications include waste heat recovery and medium temperature geothermy. Waste heat recovery Biomass-CHP power plants Waste heat recoveryPDF Image | Techno-economic survey of Organic Rankine Cycle (ORC) systems
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