A semi-cascade heat pump system for different temperature lifts

Abstract

Conventional cascade heat pumps are widely applied to large temperature lifts between the heat sink and the heat source (ΔTsource-sink). However, this system configuration struggles to maintain energy efficiency over a wide range of operating conditions, especially for small ΔTsource-sink or large temperature lifts on the heat sink side (ΔTsink). 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 ΔTsource-sink of 20–110 °C and ΔTsink of 10–100 °C. A steady-state 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 ΔTsource-sink and ΔTsink, while the conventional cascade mode is suitable for relatively large ΔTsource-sink and small ΔTsink. The semi-cascade mode maintains high efficiency over a wider range of ΔTsource-sink and ΔTsink. 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.