Enhancing Performance of Large-Scale Heat Pumps: The Impact of Refrigerant Choices and Off-Design Operations in District Heating Systems
Large-scale heat pumps (LSHPs) are a key technology for decarbonizing district heating systems. While simulation studies highlight their potential and optimization strategies, many simplify LSHP performance and neglect detailed refrigerant-cycle modeling, overlooking efficiency and heating capacity variations during offdesign operation—when source or sink temperatures differ from the design point. Off-design performance is strongly influenced by the refrigerant and cycle configuration. Considering these factors during system design can improve integration and efficiency in energy systems. Building on this, we analyze twelve refrigerant–flowsheet combinations in a district heating system to derive design recommendations for LSHP integration. We compare R717, R600a, R246fa, and R1224yd(Z) with standard, internal-heat-exchanger, and vaporinjection cycles. Off-design efficiency and heating capacity are calculated using detailed Python-based component models that account for refrigerant effects and advanced compressor and heat exchanger models. Annual simulations consider varying source and sink temperatures and interactions between system components, including LSHP, storage, heating rod, controller, and district heating network. The system’s maximum heat demand is 1.3 MW, with sink temperatures ranging from 70 °C to 90 °C and source temperatures from 9 °C to 25 °C. R717-LSHPs achieve, on average, 8% higher efficiency at the design point but also show greater reductions in heating capacity during off-design operation, increasing backup heater use by up to 6%. Depending on the backup system, this can influence the optimal refrigerant choice compared to neglecting off-design effects. The study demonstrates that both COP and available heating capacity across operating conditions significantly affect energy hub efficiency. It highlights the need for detailed off-design refrigerant-cycle analysis to inform reliable LSHP design and integration in district heating networks.