Modelling and simulation of a multi-valve heat pump integrating a heat storage for flexible operating modes

Essadik, Miryam (2025) Modelling and simulation of a multi-valve heat pump integrating a heat storage for flexible operating modes. PhD thesis, University of Glasgow.

Abstract

To address the urgent need to reduce global warming, it is essential to transition to low emission technologies in the heating sector. In this context, heat pumps are expected to play a major role. The UK government has set ambitious targets for heat pump adoption, with air-source heat pumps seen as a promising alternative to gas boilers for individual households due to their affordability and flexibility. However, research shows two main challenges with air-source heat pumps that can significantly affect performance in a UK setting: lack of flexibility and high operating costs when supplying hot water at high temperatures; frosting issues in cold and humid conditions. These challenges could reduce the economic appeal of heat pumps compared to gas boilers. This raises the question: What modifications or improvements could be made to the heat pump cycle to address both the need for cost-efficient operation at a 60 °C supply temperature and effective defrosting in the UK climate, without making the system overly complex or costly, while still allowing for flexible operation?

This thesis addresses this question by establishing a theoretical foundation through a benchmark numerical study of a multi-valve heat pump concept with integrated heat storage for flexible operation. The study analyses this system’s potential to address these challenges and deliver efficient, cost-competitive performance in the UK. The flexible heat pump’s four operating modes and thermodynamic benefits are evaluated relative to a conventional heat pump cycle in different scenarios. Economic and environmental impacts are also assessed against an A-rated gas boiler and a conventional air-source heat pump water heater, forming a basis for future experimental validation, model improvements, and commercial applications.

The defrosting process is examined under ideal conditions, comparing the thermodynamic behaviour and performance of the flexible heat pump to that of a standard reverse cycle defrosting system. By using latent heat storage as the evaporating unit during defrosting, the flexible heat pump can effectively defrost the evaporator while maintaining continuous heating capacity, saving power, and achieving a COP increase from 8.9 % to 13.2 % at maximum improvement storage temperature, depending on refrigerant choice. Key parameters, including storage temperature and defrost duration, are examined in detail. An exergy analysis highlights the main sources of losses in the cycle, identifying the compressor and heat exchangers as major contributors due to the deviation from isentropic compression and temperature differentials. The latent heat storage also affects these losses during discharging/defrosting, especially if the temperature difference between the storage and the refrigerant is not minimised. System exergy efficiency is notably higher in discharging/defrosting mode than in heating/charging mode due to the lower compression ratio.

In addition to thermodynamic performance, the economic and environmental assessments highlight the flexible heat pump’s potential as a viable UK alternative to gas boilers and conventional air-source heat pumps. The system’s power-saving mode demonstrated a lower total annual cost rate than a conventional air-source heat pump water heater, with an average payback period of about four years. Additionally, it showed potential operating cost savings compared to an A-rated gas boiler. The Total Equivalent Warming Impact of the flexible heat pump was also lower than that of a conventional air-source heat pump, supporting its environmental benefits. With rising energy costs and the UK’s commitment to Net Zero Emissions, these findings suggest that the flexible heat pump system could be an economically attractive option for sustainable heating.

The thesis also explores a dedicated charging mode to store heat during periods when heating is not required, which could take advantage of off-peak electricity rates. Both latent and sensible heat storage configurations are evaluated, and results indicate that this mode still improves performance compared to a conventional heat pump cycle. Analyses of extended parameters, such as thermal storage losses and temperature variations in the water storage tank, provide further insights into how to increase cycle efficiency. A roadmap for selecting the best refrigerants across the flexible heat pump’s operating modes is also provided.

Overall, this thesis shows that the flexible heat pump with integrated heat storage has strong potential for residential applications in the UK and could play an important role in reducing carbon emissions in the heating sector. Further research is encouraged to support the development and deployment of practical flexible heat pump systems.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Subjects: T Technology > T Technology (General)
• Colleges/Schools: College of Science and Engineering > School of Engineering
• Supervisor's Name: You, Dr. Siming
• Date of Award: 2025
• Depositing User: Theses Team
• Unique ID: glathesis:2025-85324
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 10 Jul 2025 09:14
• Last Modified: 10 Jul 2025 09:21
• Thesis DOI: 10.5525/gla.thesis.85324
• URI: https://theses.gla.ac.uk/id/eprint/85324

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