Thermodynamic modeling and performance analysis of freezing concentration for frost-free air source heat pumps solution regeneration in cold regions

Frost-free air source heat pumps use antifreeze solution as an intermediate heat-transfer medium and therefore require regeneration. Freezing concentration needs only the latent heat of fusion, which is about one-seventh that of thermal regeneration, showing strong energy-saving potential. However, key parameter effects and mechanism-based comparisons with thermal regeneration remain insufficiently studied. This study proposes a progressive freezing concentration model to simulate the regeneration dynamics of calcium chloride solution using a vapor-compression refrigeration cycle, and systematically reveals the thermodynamic mechanisms behind its performance advantages over conventional thermal regeneration. Simulation results reveal that the coefficient of performance declines steadily throughout the icing process—from approximately 3.4 initially to below 1—mainly due to the increasing thermal resistance induced by ice accumulation. The average coefficient of performance was found to be around 1.86. The evaporation temperature and outdoor air temperature have significant influences on regeneration performance. The energy consumption of the melting process accounts for only 2.3% under the baseline operating conditions of this study. The specific regeneration energy reached 97.75 kWh/t, and stepwise regeneration can further reduce this value by 21% when the prescribed thickness is 10 mm. Thermodynamic analysis reveals the underlying reason for the superior performance of freezing concentration compared with conventional regeneration, and indicates that this advantage becomes more pronounced at lower ambient temperatures. The results of this study provide valuable guidance for the selection and design of antifreeze solution regeneration strategies for frost-free air source heat pump systems in cold regions.