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investigated include the natural refrigerants butane, cyclopentane, hexane, pentane and propane, along with R245fa, which is used as benchmark fluid. Heptane, Octane etc. are not examined due to their significantly higher critical temperature, which results in their low efficiency for the heat source temperatures considered (as will be shown later). Methane was excluded from the study because of its extremely low critical temperature (-83.59 oC) which results in an entirely different cycle configuration (the condensation is replaced by supercritical cooling). Methanol, on the other hand, has a critical pressure which is too high (81 bar), so for a condensation temperature of around 40 oC (at 0.36 bar) the system would operate in technically unrealistic pressure ratios. The reasons for the selection of R245fa regard its reported favourable environmental properties and good thermodynamic performance in low/mid temperature ORC applications [3, 5, 13]. Table 3. Properties of the pure organic fluids that are examined Butane Cyclopentane Hexane Pentane Propane R245fa pcrit (bar) 37.96 45.15 30.34 33.7 42.51 36.51 Tcrit (oC) 151.98 238.54 234.67 196.55 96.74 154.01 ODP GWP 0 4 0 0 0 0 0 4±2 <0 3.3 0 950 ASHRAE Safety Group A3 - - A3 A3 A1 Molecular weight (kg/kmol) 58.1 70.1 86.2 72.1 44.1 134.1 The energetic efficiency for the pure fluids as a function of the maximum operation pressure of the ORC is plotted in Fig. 2. The second part of each curve corresponds to the supercritical pressure range. It can be seen that for all fluids the thermal efficiency increases for higher pressures while the supercritical ORC results in a higher thermal efficiency for all the refrigerants. It should be noted, though, that the thermal efficiency increase rate is much higher in the lower pressure range. For higher supercritical pressures, the energetic efficiency tends to gradually converge to a relatively steady value. 25 20 15 10 5 0 0 10 20 30 40 50 60 Maximum operation pressure (bar) cyclopentane hexane pentane butane R245fa Fig. 2. Energetic efficiency of the natural refrigerants examined A strong positive correlation between the critical temperature of each fluid and its thermal efficiency can also be observed. As a matter of fact, Propane, with a critical temperature of 96.74 oC, has the lowest thermal efficiency and Cyclopentane, with a much higher critical temperature (238.54 oC), has the highest. The link between the working fluid’s critical temperature and its overall thermal efficiency can be explained by the fact that, for fluids of higher critical temperatures, the turbine inlet temperature at a given pressure is generally at a higher level than for fluids of lower critical temperatures. This is due to the assumptions followed in the present work 10 Energeticc efficiency (%)PDF Image | Thermodynamic investigation of waste heat recovery
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