Analyzing releasable refrigerant charge in a residential R290 heat pump through 1D dynamic distribution modeling
Despite the ongoing energy transition, gas boilers remain widely used in European cities, especially in large multi-family houses. Heat pumps offer a sustainable alternative, but the tightening of the F-Gas Regulation and the shift to natural refrigerants, such as propane (R290), pose challenges due to flammability and the need for advanced safety concepts. The Austrian national project WISE is developing a compact indoor heat pump concept with a refrigerant charge above 150 g, while limiting the releasable charge to below 150g in accordance with IEC 60335-2-40. To support this design, a physics-based one-dimensional (1D) model was developed in Dymola/Modelica to predict the component-wise refrigerant distribution and to improve charge estimation accuracy beyond conventional homogeneous equilibrium models (HEM). The model replaces the HEM assumption with separated-flow formulations based on established slip-ratio correlations (Zivi, Premoli, Ali–Sadatomi–Kawaji) and accounts for refrigerant solubility in compressor oil using experimental data for RENISO PAG 100 from Fuchs Lubricants. Separated-flow modeling and oil solubility together increase the predicted refrigerant charge significantly compared to HEM, emphasizing the importance of these effects for safety-critical design. The resulting component-wise charge breakdown provides a foundation for assessing releasable charge and optimizing component volumes. Simulations across representative operating points, B0/W35 @ 120 Hz (nominal heating), B-15/W70 @ 80 Hz (domestic hot water generation), and B5/W24 @ 20 Hz (low-load heating), were conducted to determine the maximum admissible system charge while maintaining less than 150 g in each component. The results show that the condenser consistently stores the largest portion of the total charge, making it the most critical component for compliance with the releasable-charge limit. Using the conservative Premoli correlation, the maximum permissible total system charge was found to be approximately 269 g for the analyzed heat pump, limited by the domestic-hot-water operating point (B-15/W70 @ 80 Hz).