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J. Sarkar et al. / Energy Conversion and Management 46 (2005) 2053–2067 2063 4.1 4 3.9 3.8 2000 Fig. 7. Effect of compressor speed on system performance. 25 24 23 22 21 20 System COP Exergetic efficiency 2500 Compressor speed (rpm) 3000 3500 perature due to the rapid change of refrigerant outlet temperature in the gas cooler. Hence, the exergetic efficiency of the system deteriorates with a rise in water inlet temperature as the exergy losses in the evaporator and in the internal heat exchanger increase rapidly due to the increase in heat exchanger temperature differences. Although the irreversibility in the gas cooler remains fairly constant with changes in water inlet temperature, the exergy loss in the expansion valve in- creases. The effect of the compressor speed on the system performance at a heat exchanger area ratio of 1.8 and a water inlet temperature of 30 °C is presented in Fig. 7. It is observed that the system COP at optimum discharge pressure decreases as both compressor work and cooling out- put increase with compressor speed. The maximum increment of irreversibility occurred in the gas cooler. This may be attributed to an increase in the temperature difference in the gas cooler as well as the increase in pressure loss due to the rapid increase in flow velocity. So, an increase in com- pressor speed yields higher capacity due to the higher mass flow rate and higher frictional pressure loss due to the higher velocity that causes the increase in irreversibility as well. The exergy loss in the different components (Grassmann diagram) is shown in Fig. 8. Similar to the behaviour reported for systems based on conventional refrigerants such as R22, R12, R502 Compressor, 33.5 % cooler, 17.0 % Internal HEX, 2.7 % Gas Output 23.1 % Expansion device, 10.6 % Evaporator, 13.1 % Fig. 8. Exergy flow (Grassmann) diagram at mean operating condition. Comp- ressor input, 100% System COP Exergetic efficiency (%)PDF Image | Transcritical CO2 heat pump systems
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