A one-dimensional approach to predict off-design performance of brazed plate evaporators in transcritical CO2 heat pump applications
A one-dimensional iterative model is developed to efficiently and accurately predict the thermal performance of a brazed plate heat exchanger (BPHX) evaporator in a transcritical CO2 heat pump designed to meet the heating and cooling demands of a residential complex. The evaporator draws energy from a water-ethylene glycol (W-EG) solution, supplied via a stratified thermal energy storage tank at an inlet temperature of 15°C, while CO2 evaporates at -1°C. The study aims to establish relationships among the overall pressure drops in both the CO2 and W-EG channels, the W-EG outlet temperature, the CO2 outlet condition (pressure, temperature, superheat degree, or quality), and variations in evaporator load for ±45% changes in the mass flow rate of each stream, reflecting seasonal building load fluctuations. The model applies continuity and energy equations at equidistant segments of the BPHX flow paths and incorporates state-of-the-art correlations for two-phase and single-phase heat transfer coefficients and pressure drop. It evaluates the off-design performance for a fixed plate configuration and dimensions. Results are presented as pressure drop profile, WEG outlet temperatures, CO2 outlet superheat or quality, and evaporator loads. The proposed model enables precise generation of performance maps for BPHX evaporators, advancing their optimization and integration in transcritical CO2 heat pumps.