A semi-cascade heat pump system for different temperature lifts

Conventional cascade heat pumps are widely applied to large temperature lifts between the heat sink and the heat source (Delta Tsource-sink). However, this system configuration struggles to maintain energy efficiency over a wide range of operating conditions, especially for small Delta Tsource-sink or large temperature lifts on the heat sink side (Delta Tsink). To address this challenge, a novel semi-cascade heat pump system is proposed for flexible operation across varying temperature lifts. The system can operate in three modes, including single-stage mode, conventional cascade mode, and semi-cascade mode with stepped heating. To verify the advantages of multi-mode operation, various conditions are considered for Delta Tsource-sink of 20-110 degrees C and Delta Tsink of 10-100 degrees C. A steadystate thermodynamic system model using GREATLAB solver is developed and validated. Based on the parameter optimization, the energy performance of each mode is analyzed and mode switching correlations are developed. The results show that the single-stage mode performs best for both small Delta Tsource-sink and Delta Tsink, while the conventional cascade mode is suitable for relatively large Delta Tsource-sink and small Delta Tsink. The semi cascade mode maintains high efficiency over a wider range of Delta Tsource-sink and Delta Tsink. The novel system achieves up to 22.3 % efficiency improvement versus the conventional cascade system, demonstrating promise for applications with a wide range of temperature lifts.