Enhancing the compactness of propane heat pumps: a sensitivity analysis via dynamic modelling
The transition from conventional refrigerants, such as hydrofluorocarbons, to low-global-warming-potential alternatives, like natural hydrocarbons, is essential. Propane has been demonstrated to be a good option among the alternatives for residential heat pump systems despite its high flammability. To ensure safety when using flammable refrigerants, one practical approach is to reduce the refrigerant charge. Compact heat exchangers can facilitate this by minimizing the required refrigerant volume and maintaining reliable operation. However, this problem involves many interdependencies between the heat pump design and its operational behavior. Furthermore, over two-thirds of the refrigerant charge resides in the heat exchangers, highlighting their significant role in the system dynamics. In this context, this study investigates different heat pump sizing using a dynamic simulation model based on a moving-boundary approach. The work aims to analyse the refrigerant charge in an air-source heat pump across various operating conditions in order to provide indications for a more effective design. The results show the possibility to reduce the charge by one-third compared to typical design approaches based on manufacturer’s specifications, while maintaining the outlet water temperature within ±1 K of the setpoint. These results demonstrate the need for more sophisticated tools to take into account the actual operating conditions in the design stage.