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J. Sarkar et al. / Energy Conversion and Management 46 (2005) 2053–2067 2065 5.2. Evaporator and gas cooler Irreversibilities in the evaporator and the gas cooler occur due to the temperature differences existing between the two heat exchanger fluids, pressure loss, flow imbalance and heat transfer with the ambient. The results show that almost 90% of the irreversibility occurs due to the fluid temper- ature difference and 10% due to the rest. At the mean conditions, the average fluid temperature dif- ferences in the evaporator and gas cooler are about 23 and 42 °C, respectively, whereas the pressure losses are 2.5 and 0.9 bar, respectively. The effective fluid temperature difference can be reduced by increasing the heat transfer area, either by increasing the heat exchanger length or incorporating fins, both of which will result in higher pressure drop. The system COP and exergetic efficiency of the system increase, first rapidly and then slowly, because of the increase in irreversibility due to pressure loss (Fig. 10). Heat transfer with the ambient is inconsequential for the system perfor- mance. If better insulation reduces the net outer wall conductivity from 20 to 1 W m1 K1, the exergetic efficiency will increase by a mere 0.2%. Irreversibility due to flow imbalance depends on the heat exchanger design. In this analysis, the heat exchanger was designed for nearly equal heat capacities of the two fluids, and hence, the flow imbalance is negligible for this system. 5.3. Expansion device Replacement of the expansion valve by a turbine is the only option available to improve the performance of the system and reduce the irreversibility of the expansion process. For such a tech- nique, it was reported [7] that an expansion work recovery turbine with isentropic efficiency of 60% would reduce the contribution of this process to total cycle irreversibility by 35% in the ther- modynamic cycle. In this system, at the mean condition stated above, by using an expansion work recovery turbine of 85% isentropic efficiency, both the system COP and the exergetic efficiency will improve by about 22%. Hence, improvement in system performance through this technique is quite significant. However, such extensive hardware addition may not be economically feasible in many practical applications, especially for small capacities. 4.8 28 27 System COP Exergetic efficiency 4.4 4 3.6 3.2 26 25 24 23 22 21 20 19 18 17 2.8 16 15 20 25 30 35 Total length (m) Fig. 10. System performance with varying total heat exchanger length. System COP Exergetic efficiency (%)PDF Image | Transcritical CO2 heat pump systems
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