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Subcooling effect on the optimal performance for a transcritical CO2 heat pump with cold thermal energy storage

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Abstract

This paper studies the combined heating and cooling thermal performance of a CO2 heat pump system considering the subcooling effect. For such a system without cold thermal energy storage (CTES), the gas cooler outlet temperature normally needs to be controlled to match the cooling load required. However, the integration of CTES would enable the system to be operated under its optimal conditions depending on the ambient temperatures, i.e. a considerable amount of cooling capacity can be generated and stored for later use. A configuration of a CO2 heat pump integrated with CTES is described in this paper. A thermodynamic cycle and a simulation model considering the subcooling effect have been developed. The mathematical model for the pinch point analysis has been newly validated against published experimental data with acceptable agreements. In the case study, the impacts of the subcooling temperature on the optimal combined performance under four ambient temperatures (5 °C, 15 °C, 25 °C, and 32 °C) have been studied. The highest optimal combined COP of 5.38 can be achieved when the ambient temperature is 5 °C. The detailed profiles of CO2 temperatures, heating and cooling loads, and the COPs when the CTES is in operation have been revealed for the first time. It is found when the ambient temperature is higher than the water inlet temperature (plus the pinch point temperature), the optimal cooling COPs can even have a surge without the subcooling effect, due to a lower sCO2 temperature leaving the heat exchanger compared to the ambient temperature. Additionally, performance analysis for the CO2 heat pump system with or without CTES is compared, and it is concluded that all optimal heating, cooling, and combined COPs integrated with CTES surpass those without CTES.

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Abbreviations

Cp:

Specific heat capacity, kJ/kg.K

h :

Enthalpy, kJ/kg

k :

A slope of a curve

\(\dot{m}\) :

Mass flow rate, kg/s

P :

Pressure, bar

ΔP :

Pressure difference, bar

Q :

Heating power, kW

r :

Compression ratio

T :

Temperature, °C

ΔT :

Temperature difference, K or °C

\(\dot{V}\) :

Volumetric flow rate, m3/ s

\({V}_{s}\) :

Displacement rate of the compressor, m3/ s

\({W}_{comp}\) :

Compression work, kW

x :

Liquid mass fraction

\(\eta\) :

Efficiency

η v :

Volumetric efficiency of a CO2 compressor

ρ:

Density, kg/\({\mathrm{m}}^{3}\)

ν :

Specific volume, \({\mathrm{m}}^{3}/\mathrm{kg}\)

COP:

Coefficient of performance

GWP:

Global warming potential

HX:

Heat exchanger

ODP:

Ozone depletion potential

PCM:

Phase change material

sCO2 :

Supercritical CO2

CTES:

Cold thermal energy storage

amb:

Ambient

cool:

Cooling

comb:

Combined

comp:

Compressor

dis:

Discharge

dif:

Difference

dyn:

Dynalene

ev:

Evaporation

gc:

Gas cooler

heat:

Heating

hx:

Heat exchanger

in:

Inlet

inter:

Interstage

isentr:

Isentropic

liq:

Liquid

out:

Outlet

opt:

Optimal

suc:

Suction

vap:

Vapour

w:

Water

1 to 12:

Refrigerants states

5’ to 11’:

Refrigerants states during subcooling

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Acknowledgements

The authors would like to acknowledge the Australian Government for the Research Training Program (RTP) scholarship. The author: Ji Wang, would also like to thank Dr Ming Liu from the University of South Australia for giving the explanations on selecting intermittent heat transfer fluid and phase change material (PCM) for cold thermal energy storage (CTES).

Funding

The author would like to acknowledge the Australian Government for the Research Training Program (RTP) scholarship. The author declares that no other funds, grants, or other support were received during the preparation of this manuscript.

Author information

Authors and Affiliations

  1. Future Industries Institute, University of South Australia, Mawson Lakes Campus, Mawson Lakes, SA, 5095, Australia

    Ji Wang, Ming Liu & Frank Bruno

  2. UniSA STEM, University of South Australia, Mawson Lakes Campus, Mawson Lakes, SA, 5095, Australia

    Michael Evans

  3. Mondial Advisory, Malvern, South Australia, 5061, Australia

    Martin Belusko

  4. School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, Darlington, NSW, 2006, Australia

    Chunrong Zhao

Authors
  1. Ji Wang
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  2. Michael Evans
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  3. Martin Belusko
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  4. Chunrong Zhao
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  5. Ming Liu
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  6. Frank Bruno
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Contributions

Ji Wang: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Writing – Original Draft, Writing – Final review & Editing. Michael Evans: Supervision, Investigation, Writing – Review & Editing. Martin Belusko: Investigation, Writing – Review. Chunrong Zhao: Investigation, Writing – Review. Ming Liu: Investigation, Writing – Review. Frank Bruno: Supervision, Investigation, Writing –Review & Editing, Writing – Final review & Editing.

Corresponding author

Correspondence to Ji Wang.

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The authors have no relevant financial or non-financial interests to disclose.

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Highlights

• A configuration of a CO2 heat pump integrated with CTES is described.

• A validated numerical model with a pinch point effect is developed.

• Detailed temperature and efficiency profiles with the subcooling are revealed.

• The current configuration with an air sub-cooler is not suitable for hot climate.

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Wang, J., Evans, M., Belusko, M. et al. Subcooling effect on the optimal performance for a transcritical CO2 heat pump with cold thermal energy storage. Heat Mass Transfer 59, 1257–1275 (2023). https://doi.org/10.1007/s00231-022-03333-9

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  • DOI: https://doi.org/10.1007/s00231-022-03333-9

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