Introduction

Heat pumps are a key technology for decarbonizing space heating and domestic hot water production. The selection of an appropriate refrigerant strongly influences system efficiency, operating range, safety, and environmental impact. Carbon dioxide (CO₂) has attracted considerable attention as a natural refrigerant due to its negligible Global Warming Potential (8GWP); however, its transcritical operation introduces challenges at high heat sink temperatures. In parallel, low-GWP alternatives such as propane (R290) and hydrofluoroolefins (HFOs), including R1234ze, have emerged as promising candidates for subcritical heat pump applications. A consistent and physically sound comparison between these refrigerants remains essential, particularly under water-to-water operating conditions relevant to building heating systems. The objective of this study is to develop a thermodynamic modelling framework for a water-to-water heat pump to compare the performance of CO₂, R290, and R1234ze under identical boundary conditions.

Methodology

A steady-state lumped-parameter model of a water-to-water heat pump was developed in MATLAB. The evaporator and condenser (or gas cooler in the case of CO₂) were modelled using prescribed water inlet and outlet temperatures, allowing heat transfer to be calculated directly from the water side. Compressor performance was modelled using a constant isentropic efficiency. For subcritical refrigerants, realistic operating conditions were imposed, including a small degree of superheating at the compressor inlet and subcooling at the condenser outlet. For CO₂, transcritical operation was considered, with the gas cooler outlet temperature determined by the heat sink conditions. Expansion was assumed to be isenthalpic for all cases. The model was first validated against experimental data obtained from a CO₂ water-to-water heat pump test rig. After validation, the same modelling assumptions, heat exchanger boundary conditions, and efficiency parameters were applied to all refrigerants to ensure a consistent basis for comparison.

Results

The model successfully reproduced the main thermodynamic trends observed in the experimental CO₂ heat pump, confirming the reliability of the modelling approach. Under the investigated operating conditions, the simulated coefficient of performance (COP) for CO₂ was approximately 4.9, which is consistent with values reported for similar experimental systems. When the same boundary conditions were applied to alternative refrigerants, significant differences in cycle performance were observed. The simulations predicted COP values of approximately 10 for both R290 and R1234ze, indicating substantially lower compressor work compared with the CO₂ cycle under the same heat sink conditions. In addition to efficiency differences, the refrigerants exhibited distinct thermodynamic behaviour. Subcritical refrigerants operated with compressor inlet states close to the saturation line and moderate pressure levels, whereas CO₂ required significantly higher operating pressures due to its transcritical cycle. The results also showed that CO₂ performance is strongly influenced by the gas cooler outlet temperature, whereas the performance of subcritical refrigerants is more stable under the same operating range. These findings highlight the importance of operating conditions and refrigerant thermophysical properties in determining heat pump performance.

Conclusions

A unified thermodynamic modelling framework for water-to-water heat pumps has been developed and validated using experimental CO₂ data. The main novelty of the study lies in the consistent comparison of transcritical and subcritical refrigerants using identical boundary conditions, modelling assumptions, and compressor performance parameters, enabling a fair evaluation of refrigerant performance. The comparative analysis indicates that, under the investigated operating conditions, R290 and R1234ze achieve significantly higher COP values than CO₂, primarily due to lower compression work and favourable thermodynamic properties in subcritical operation. However, the results also demonstrate that CO₂ performance is highly sensitive to gas cooler outlet temperature and may become more competitive at higher heat sink temperatures. The proposed modelling framework provides a robust tool for evaluating alternative refrigerants and supports the assessment of environmentally friendly working fluids for future heat pump applications.