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efficiency increase at a continuously lower rate (as seen in Fig. 2). The optimal evaporation pressure corresponds to the pressure beyond which the benefits of the increasing thermal efficiency (effect 2) cannot compensate for the decreasing heat input (effect 1) that is supplied to the ORC. At this point the value of ηthηHS,u is maximized before starting to decline. The efficiency curve of Propane is an exception to the above pattern. Its exergetic efficiency curve increases monotonously with increasing the maximum operating pressure. The reason for this difference is the fluids’ significantly lower critical temperature, which results in smaller evaporation temperatures. Given the constant pinch point assumption, it is thus possible to better make use of the heat content of the heat source over a larger pressure range by cooling it to even lower temperatures. The decrease rate of ηHS,u is hence lower as the pressure increases, compared to the other pure fluids. The described effect can be seen in more detail in Fig. 6, where the heat source utilization efficiency of Propane (Tcrit=96.74 oC), Butane (Tcrit=151.98 oC) and Cyclopentane (Tcrit=238.54 oC) for heat source temperatures of 150, 225 and 300oC are plotted as a function of the maximum operation cycle pressure. It can be seen that for the lower heat source temperatures (150 and 225 oC) the high critical temperature fluids have a significantly lower ηHS,u, which rapidly decreases as the evaporation pressure (and temperature) increases. When the heat source temperature is sufficiently high (300 oC), the heat source utilization efficiency is much higher, and its rate of its decrease as the evaporation temperature of the working fluids becomes higher is less significant. 100 90 80 70 60 50 40 30 20 10 0 Propane Butane Cyclopentane 150 oC 225 oC 300oC 0 5 10 15 20 25 30 35 40 Maximum operation pressure (bar) Fig. 6. Heat source utilization efficiency as a function of the maximum operation pressure for heat source temperatures of 150, 225 and 300 oC for Propane, Butane and Cyclopentane The dependence of the first law and heat source utilization efficiency on the critical temperature of the fluids can also help explain why for the heat source temperature of 150 oC, high critical temperature working fluids tend to perform better in the lower pressure range (before their heat source utilization efficiency dramatically diminishes). The importance of the heat source utilization efficiency can be stressed by the fact that despite propane exhibiting the lowest energetic efficiency, its exergy efficiency, which is the primary evaluation parameter of the WHR-ORC system, is significantly higher than in the case of the other pure fluids. 3.1.2. Binary zeotropic mixtures -THS,in=150oC When using zeotropic mixtures, the exergetic efficiency is mainly affected by two effects. The first concerns the temperature glide of the working fluid during its evaporation (in subcritical cycles) and its condensation. Depending on the concentration of the mixture components, the temperature glide can help improve the matching of the temperatures between the heat source (heater), cooling medium (condenser) and the working fluid. This can in turn lead to reduced irreversibility and exergy losses during the heat exchange processes of the WHR-ORC system. 14 Heat source utilization efficiency (%)PDF Image | Thermodynamic investigation of waste heat recovery
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