Experimental investigation of a two-stage R290 heat pump with economizer and internal heat exchanger: Impact of subcooling and intermediate pressure

Two-stage vapor compression cycles with vapor injection are a promising approach to improve the performance of air-source heat pumps at high temperature lifts. This study experimentally investigates a fully separated two-stage R290 heat pump equipped with an economizer and an internal heat exchanger. The analysis focuses on the interaction between economizer subcooling, injection ratio, intermediate pressure, compressor efficiency distribution, and component-wise exergy destruction under controlled steady-state operating conditions. The results show that increasing the injection ratio improves the normalized Coefficient of Performance (COP) by up to 17 % compared to operation without vapor injection. However, the COP improvement saturates at high subcooling levels, indicating a practical thermodynamic limit. A component-resolved exergy analysis indicates that this behavior is not solely governed by reduced flash-gas formation at the main expansion valve, but is strongly affected by the internal heat exchanger and the redistribution of irreversibilities within the cycle. Overall, the investigated system is characterized by a favorable steady-state operating range rather than a sharply defined global optimum. The results provide experimentally based component-level insight into the loss mechanisms of a real two-stage R290 economizer/ internal-heat-exchanger system and show that intermediate-pressure selection cannot be described by classical idealized criteria alone.