Experimental analysis of a liquid-to-liquid two-stage R290 heat pump system for residential cold-climate applications

Abstract

The phasedown of hydrofluorocarbon (HFC) refrigerants with high global warming potential (GWP), alongside growing restrictions on PFAS compounds, has intensified the search for sustainable and efficient alternatives in residential space conditioning. Propane (R290) has emerged as an appealing environmentally friendly refrigerant, but its flammability requires the utilization of indirect system architectures. In parallel, electrification efforts demand highly efficient heat pumps to replace traditional fossil fuel-based heating systems. This study experimentally evaluates a novel two-stage, variable-speed, liquid-to-liquid heat pump with R290 refrigerant for cold climates. Unlike prior studies, it introduces independent speed control of the low- and high-stage compressors, providing the first experimental dataset for R290 in two-stage systems. The system achieved an 18.3% increase in heating capacity and a 5.8% improvement in the coefficient of performance (COP) at an ambient temperature of −8.3 °C in two-stage mode compared to single-stage operation, although with an 11% increase in refrigerant charge level. The study also highlights the critical role of the secondary fluid in system design, overall performance, and thermal comfort. Namely, a design mismatch was identified between hydrocarbon refrigerants and aqueous secondary fluids in the evaporators of indirect systems operating at low evaporation temperatures. These findings provide key insights for designing high-performance regulation-compliant heat pumps suitable for safe residential use in cold climates.