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2064 J. Sarkar et al. / Energy Conversion and Management 46 (2005) 2053–2067 [4,17], the exergy loss is maximum in the compressor followed by that in the gas cooler, evapora- tor and expansion device, while the exergy loss in the internal heat exchanger is negligible. Because of the high pressure drop occurring in the system being studied, the expansion device contributes a much larger fraction of the irreversibility compared to conventional systems. 5. Measures to improve exergetic efficiency The exergy loss is relatively high in the compressor, gas cooler, evaporator and expansion valve, while that in the internal heat exchanger is insignificant. Hence, the contribution of the internal heat exchanger towards exergy destruction and its influence on the system performance is not pre- dominant, although by increasing the effective heat transfer area, we can modestly increase the effectiveness as well as the system COP and exergetic efficiency of the system. The primary chal- lenge is to improve the system performance and exergetic efficiency by improving the performance (by controlling exergy loss) of the above stated four influential components. Some of the impor- tant improvement measures are presented with their associated betterment in system performance. 5.1. Compressor Process irreversibility (due to mixing, throttling, internal convection, etc.), pressure loss due to friction in inlet and outlet valves and heat loss to the environment are the basic reasons for the exergy loss in the compressor, although the last two have been neglected since the heat loss is not so severe from having a modest temperature difference over the ambient. The system COP and the exergetic efficiency increase linearly with the isentropic efficiency as shown in Fig. 9. Isen- tropic efficiency primarily depends on the compressor design and the working pressure, so a supe- rior compressor design will lead to higher isentropic efficiency, resulting in a reduction in the process irreversibility, within certain limits. With a 10% increase in isentropic efficiency, the exer- getic efficiency improves by almost 3%. 5.5 40 38 System COP Exergetic efficiency 5 36 34 32 4.5 30 28 4 26 24 22 3.5 20 50 55 60 65 70 75 80 Compressor isentropic efficiency (%) Fig. 9. System performance with varying compressor isentropic efficiency. System COP Exergetic efficiency (%)PDF Image | Transcritical CO2 heat pump systems
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