Thermodynamic Modeling of Refrigerant Cycle in an Air-Source Heat Pump Boiler and Performance Validation

Building electrification has driven growing interest in domestic hot water (DHW) systems based on air-source heat pump boilers (ASHPBs). However, heat pump performance regression models based on manufacturer-specific data have limited applicability to alternative refrigerants and off-design conditions. This study develops a physics-based dynamic ASHPB model that couples a thermodynamic analysis of the refrigerant cycle with the effectiveness-number of transfer units (ε-NTU) method and incorporates an optimization routine to determine the refrigerant evaporation and condensation temperatures that minimize total power input. The model performance is validated against manufacturer data over leaving water temperatures of 40-65℃ and outdoor air temperatures of -10 to 30℃. The results show satisfactory agreement, with a mean absolute percentage error (MAPE) of 10.1% and a mean absolute error (MAE) of 0.35 for the coefficient of performance (COP). The proposed framework thus supports the manufacturer-independent evaluation of heat pump systems without relying on proprietary empirical curve fits.