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Sustainability 2015, 7 15377 The isentropic efficiency is considered to be at a maximum if the pressure after the isochoric pressurization section is the same as the condenser pressure. If the pressures are higher or lower, throttling or additional work input are needed. These additional processes introduce irreversibilities since they do not contribute to useful work. High pressure ratios introduce additional leakage since the leakage mass flow rate is a function of input and output pressures. The system expander using R134a has the highest isentropic efficiency (52%) while that using n-pentane has the lowest (50.3%). These values are relatively low and suggest that this expander has significant losses due to friction and leakage. From the exergo-economic analysis, the unit cost of electricity is calculated with the optimized results. The results are useful in identifying which fluid has the lowest electrical output unit cost. From Table 5, toluene and R123 have unit electrical costs of 1.14 USD/kWh and 0.49 USD/kWh, respectively, which are the highest of all the working fluids considered. Systems using R134a, iso-butane and R600 exhibit the lowest unit costs of electricity, with values of 0.08, 0.10 and 0.14 USD/kWh, respectively. These costs reflect the lowest price that needs to be charged for the electricity and depend significantly on the amount of work output. The working fluids with the highest unit cost for electricity are seen to have the lowest electrical work outputs. Conversely, the working fluids with the highest electrical work outputs have the lowest electricity rates. For instance, systems using R134a, iso-butane and R600 generate 396.7, 301.7 and 226 W of electrical power, respectively, while those using toluene and R123 generate 20.8 and 61.3 W of electrical power, respectively. The difference in pressure from inlet to outlet for the expander plays a large role in determining its work output. Since the torque produced by the expander is directly related to this pressure difference, it is useful to analyze this effect to help explain the work outputs of systems using each working fluid. The expander using R134a has the highest pressure difference (1389 kPa) and that using toluene the lowest (58.3 kPa). Other working fluids like R123 and n-pentane exhibit a low pressure difference, which corresponds to low electrical power outputs. The work output rate also corresponds to the heat input rate needed by the system. Systems using working fluids that lead to high work output rates such as R134a and iso-butane have high heat input rates. The system using R134a requires a heat input rate of almost 7.7 kW to produce 396.7 W of electrical power. The lowest heat input rate is needed for the system using toluene, which requires a heat input of 560 W to generate 20.8 W of electricity. 4.1. Exergy Analysis Table 6 shows the exergy destruction breakdown for the system for each working fluid considered. The exergy destruction rate for the overall system using R134a is 1.3 kW. Most of the exergy destruction occurs is in the boiler (59.7%), and is due to the irreversible heat transfer processes in that component. The expander is responsible for almost 32% of the exergy destruction. These results suggest that there the improvement potentials are large for these components. This large share of exergy destruction in the expander is a consequence of the low expander isentropic efficiency of 52%. Large-scale systems typically have much higher expander isentropic efficiencies, approaching 80% to 90%. In order for the systems considered here to be competitive, the isentropic efficiency of its expander likely needs to improve. For the system using R227ea, the boiler is responsible for most of the system exergy destruction. Due to the larger amount of regeneration needed for this fluid (since it is a dry fluid), it exhibits relatively more exergy destruction in the regenerator. The condenser exergy destruction isPDF Image | Selection of Optimum Working Fluid for Organic Rankine Cycles
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