Toward Low-Charge R290 Water-to-Water Heat Pumps: Experimentally Validated Metric and Design Strategies

Decarbonizing residential heating requires replacing fossil-fuel boilers with electrically driven heat pumps, where renewable electricity enables near–zero-carbon operation. Propane (R290) is a promising low-GWP refrigerant, but its flammability necessitates minimizing charge while maintaining heating performance. This study develops and evaluates charge-reduction strategies for a residential water-to-water R290 heat pump using a combined experimental and simulation-based approach. A fixed-speed prototype was tested with symmetric (SPHE) and asymmetric (APHE1, APHE2) plate heat exchangers under representative winter conditions (source inlet: 5–10 °C; sink outlet: 45–50 °C; superheat: 5–15 K; refrigerant charge: 250–550 g) using closed-loop water circuits to limit internal refrigerant inventory. Degree of superheat (DSH), controlled via electronic expansion valve, strongly influenced performance, with heating capacity increasing up to 10 K and declining beyond 15 K, defining an optimal operating range. Experimentally, the lowest refrigerant charge capacity (RCC) was 47.58 g/kW at 330 g charge and 10 K DSH with APHE2, a 38% reduction compared with 76.75 g/kW for SPHE. APHE2 also delivered the highest COP of 2.75 (25% higher than SPHE) and a maximum heating capacity of 9 kW (40% higher). Building on these experimental results, a validated Python–CoolProp numerical model was employed to explore further charge reductions beyond the tested range, projecting ultra-low charges of 100–170 g, corresponding to RCCs near 20 g/kW. By clearly distinguishing experimentally demonstrated performance from simulation-based projections, this study provides a realistic and practical pathway toward compact, efficient, and low-charge R290 heat pumps, supporting the decarbonization of residential heating systems while ensuring safety and efficiency.