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2066 J. Sarkar et al. / Energy Conversion and Management 46 (2005) 2053–2067 6. Conclusions Exergetic analyses of a transcritical carbon dioxide based heat pump to provide heating and cooling simultaneously have been presented here. Unlike previous studies reported in the litera- ture, realistic heat transfer and fluid flow effects have been included. A computer model has been developed first to simulate the system at steady state for different operating conditions and then to evaluate the system performance based on COP as well as exergetic efficiency. Component level irreversibility analyses have been performed. The highly variable thermophysical and transport properties of the refrigerant near the critical point have been taken into account to obtain better accuracy. Spatial discretization of all the heat exchangers has been used as well to yield better pre- cision. Results are obtained by varying the important operating and design parameters such as heat exchanger area ratio, compressor speed, water inlet temperature and ambient temperature over a given range. The results show that: 1. The optimum heat exchanger area ratio ranges between 1.8 and 1.9 for maximum system COP as well as maximum exergetic efficiency at optimum discharge pressure. 2. The favourable heat transfer properties of carbon dioxide in both the two phase and super- critical region and an efficient compression process contribute significantly toward high sys- tem COPs and exergetic efficiency values. The temperature differences in the heat exchangers contribute more that 90% of their irreversibilities, whereas the rest occurs due to pressure drop and the system imbalance in the heat exchanger. 3. It is more effective to maintain the secondary fluid inlet temperature as low as possible to get higher COP and exergetic efficiency within the given range. 4. The compressor, evaporator, gas cooler and expansion device contribute to system irrevers- ibility to a larger extent, while the internal heat exchanger has a negligible effect. The expan- sion valve contributes a significant amount of exergy loss here, whereas it is negligible for a conventional system. 5. Itiseffective,intermsofimprovementinCOPandexergeticefficiency,toemployalargeheat exchanger area by increasing the length or by using fins, which will also attract additional investment and higher pressure drop. Hence, there is an optimal trade-off between the two. 6. Replacement of the expansion valve with a turbine will increase the COP as well as the exer- getic efficiency significantly, but it will also raise issues related to cost, design and dynamic balancing of the system. It is advisable to employ a turbine for large systems, such as a large dairy plant or other large system where simultaneous cooling and heating is useful. References [1] Bridges BD, Harshbarger DS, Bullard CW. Second law analysis of refrigerator and air conditioners. ASHRAE Trans 2001:644–51. [2] Yumrutas R, Kundus M, Kanoglu M. Exergy analysis of vapor compression refrigeration systems. Exergy, An Int J 2002;2(4):266–72. [3] Bilgen E, Takahashi H. Exergy analysis and experimental study of heat pump systems. Exergy, An Int J 2002;2(4):259–65.PDF Image | Transcritical CO2 heat pump systems
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