Effects of refrigerant charge amount on the optimal discharge pressure of a transcritical CO₂ heat pump for electric vehicles

The operation of transcritical CO₂ heat pump systems at the optimal discharge pressure is crucial for achieving maximum efficiency. Existing correlations for optimal discharge pressure often overlook the influence of refrigerant charge, rendering them unsuitable for scenarios with varying charge amounts. This study investigates the relationship between optimal discharge pressure and refrigerant charge amount with a high accuracy system model, proposing new correlations to correct existing correlations. The optimal discharge pressure exhibits a linear increase with charge amount until reaching the charge plateau period, whether under constant rotary speed or fixed capacity conditions. As the refrigerant charge amount increases, the evaporator temperature steadily rises until it reaches the refrigerant charge plateau period. With decreasing refrigerant charge, the actual optimal pressure consistently decreases compared to predictions by existing correlations. The revised optimal pressure correlation proves adaptable to a diverse range of refrigerant charge variations, substantially reducing the maximum error from 13.67% to 2.51% in heating mode and 9.66% to 2.49% in cooling mode when compared to existing correlations. This refined correlation holds the potential to significantly enhance the precision of model-based methods and effectively evaluate refrigerant leakage levels in model-free methods.