Mode-dependent reconfiguration of exergy destruction and optimization of an air-source heat pump with a liquid-storage gas-liquid separator

Frost formation significantly degrades the performance of air-source heat pumps (ASHPs) in cold climates. A previous study on a novel ASHP incorporating a liquid-storage gas-liquid separator (Ls-Gls) demonstrated effective frost suppression based on first-law analysis. However, second-law aspects-particularly the reorganization of irreversibility across operational modes-remain insufficiently understood. To address this gap, this study proposes a coupled exergy-pinch analysis framework for both heating and defrosting modes. The results reveal a clear mode-dependent reconfiguration of dominant exergy destruction sources: despite increased compressor work, total system exergy destruction during defrosting is 7.9% lower than during heating,mainly due to a reduced pressure ratio that suppresses compressor-related irreversibility while heat-transfer losses intensify in the outdoor heat exchanger under the fixed 0 °C frost-layer constraint. Pinch analysis further quantifies the spatial shift of dominant irreversibility from the evaporator outlet during heating to the frost-layer interface during defrosting. Compressor isentropic efficiency is identified as the most influential parameter governing overall exergy performance. More importantly, a mode-specific optimization principle is established: an optimal internal heat-transfer temperature difference of 5 K is identified for the Ls-Gls in heating mode, while defrosting performance is primarily governed by the energy grade of the stored refrigerant. Collectively, these findings establish mode-specific principles to guide strategic optimization. This study shifts the optimization paradigm from component-based to mode-aware system design, providing a foundational guideline for next-generation adaptive ASHPs.