Optimal heat rejection pressure of CO2 heat pump water heaters based on pinch point analysis

•The pinch effect considerably affects the optimal heat rejection pressure.•The optimal heat rejection pressure was underestimated by the commonly used model.•A new correlation of optimal pressure is proposed for CO2 heat pump water heaters. The control of heat rejection pressure is of great importa...

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Bibliographic Details
Published in:International journal of refrigeration Vol. 106; pp. 592 - 603
Main Author: Chen, Yun-Guang
Format: Journal Article
Language:English
Published: Paris Elsevier Ltd 01.10.2019
Elsevier Science Ltd
Subjects:
Carbon dioxide
Chauffe-eau à pompe à chaleur
Computer simulation
Cycle au CO2 transcritique
Gas cooling
Heat exchangers
Heat pump water heater
Heat pumps
Heat transfer
Impact analysis
Optimal heat rejection pressure
Parameter sensitivity
Pinch effect
Pinch point
Point de pincement
Pression optimale de rejet de chaleur
Pressure effects
Pumps
Rejection
Sensitivity analysis
Temperature gradients
Transcritical CO2 cycle
Water heaters
Water temperature
Heat pump water heater
Point de pincement
Pression optimale de rejet de chaleur
Chauffe-eau à pompe à chaleur
Optimal heat rejection pressure
Pinch point
Transcritical CO2 cycle
Cycle au CO2 transcritique
ISSN:0140-7007, 1879-2081
Online Access:Get full text
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Summary:•The pinch effect considerably affects the optimal heat rejection pressure.•The optimal heat rejection pressure was underestimated by the commonly used model.•A new correlation of optimal pressure is proposed for CO2 heat pump water heaters. The control of heat rejection pressure is of great importance for transcritical CO2 heat pump water heaters. However, almost all the existing correlations of optimal heat rejection pressure are based on an assumption of constant CO2 exit temperature from gas cooler when changing the high-side pressure. Actually, pinch effect occurring in CO2 gas cooler would have great impacts on CO2 exit temperature as the high-side pressure varies, which in turn affects overall system performance. A cycle simulation model was developed in the present study incorporating pinch point analysis for gas cooling process. The model is independent on heat exchanger geometry parameters, which guarantees the generality of the model. Sensitivity analyses of main impact factors were conducted in detail including inlet and outlet water temperature, refrigerant evaporation temperature and superheat degree, pinch temperature difference and isentropic efficiency of compressor. Furthermore, a new correlation of optimal heat rejection pressure was proposed.
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ISSN:0140-7007
1879-2081
DOI:10.1016/j.ijrefrig.2019.04.003