Evolution of subcritical and transcritical cycles and the quantitative influence of heat exchangers configuration on CO₂ automotive heat pumps

Traditional CO₂ heat pumps are designed to operate using transcritical cycles to achieve optimal heating performance. However, in scenarios where the inlet air temperature is low, these systems may inadvertently transition to subcritical cycles. This transition is particularly influenced by the sensitivity of the working medium state before the expansion valve (EXV) to the rejection pressure, and system performance can drop if it operates improperly. To well address this problem, a mathematical model was established to reveal the underlying reason for optimal operation between subcritical and transcritical CO₂ cycles. The heat transfer in the gas cooler was first evaluated and analyzed. The effect of parameters, including air flow rate, inlet and supply air temperature, on heat transfer performance were investigated. Moreover, the evolution boundary between subcritical and transcritical CO₂ cycles were summarized. The quantitative influence of heat exchanger configuration on the evolution characterize was further analyzed, and the impact weights of various parameters under different heat exchanger designs on the anticipated operating pressure were studied. Based on simulation results, the inlet air temperature had the highest influence on the sub-transcritical cycle evolution, accounting for > 81 % across various systems. This study explained why different system designs correspond to different optimal pressure correlation equations for both transcritical and subcritical CO₂ cycles, as these were affected by the unique characteristics of sub- to trans-critical evolution for electric CO₂ heat pumps in winter. It provides new insights into achieving universal optimal operation of CO₂ heat pumps for low-temperature heating applications.