Approaching optimum COP by refrigerant charge management in transcritical CO2 heat pump water heater

•Transcritical CO2 HPWH can approach optimum COP without high pressure control.•Charge management method is developed to replace high pressure control.•Average COP loss of the redesigned system without high pressure control is −0.2%. Optimal high pressure is crucial to the performance of transcritic...

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Bibliographic Details
Published in:International journal of refrigeration 2020-10, Vol.118, p.161-172
Main Authors: He, Yu-Jia, Liang, Xing-Yu, Cheng, Jia-Hao, Shao, Liang-Liang, Zhang, Chun-Lu
Format: Article
Language:English
Subjects:
Ambient temperature
Carbon dioxide
Charge en frigorigène
Chauffe-eau à pompe à chaleur
Cycle au CO2 transcritique
Haute pression
Heat exchangers
Heat pump water heater
Heat pumps
High pressure
Inlet temperature
Mathematical analysis
Optimization
Pressure
Pumps
Refrigerant charge
Refrigerants
Studies
Systems design
Temperature effects
Transcritical CO2 cycle
Water heaters
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Summary:•Transcritical CO2 HPWH can approach optimum COP without high pressure control.•Charge management method is developed to replace high pressure control.•Average COP loss of the redesigned system without high pressure control is −0.2%. Optimal high pressure is crucial to the performance of transcritical CO2 heat pump water heater (HPWH). From the perspective of refrigerant charge management, this work aims to approach optimum COP in CO2 HPWH without high pressure control. By modeling a running CO2 HPWH, we in-depth investigated the relation between optimal high pressure and refrigerant charge. It was found that the essence of optimal high pressure control is to manage optimum refrigerant charge of the system. If the optimum charge of the system could remain constant under any working conditions, the system would operate automatically at optimum COP with fixed refrigerant charge. Then influences of water inlet temperature, water outlet temperature and ambient temperature on optimum refrigerant charge were numerically analyzed. Finally, system design methods to approach optimum COP by minimizing the variation of optimum charge under variable operating conditions were developed. As a result, the average COP loss of fixed-charge system after resizing heat exchangers is only −0.8%. By comparison, with a high-pressure reservoir at the outlet of gas cooler, the average COP loss can be as low as −0.2%.
ISSN:0140-7007
1879-2081
DOI:10.1016/j.ijrefrig.2020.06.011