Transcritical CO₂ thermal systems have emerged as leading solutions to addressing challenges such as "range anxiety in winter" and pronounced greenhouse effects associated with the working fluid in electric vehicle thermal systems. Nevertheless, the intricate interplay of transcritical cycles in the varied scenarios of electric vehicles introduces complexity, with performance and operational stability intricately linked to the refrigerant charge. This study conducts simulations to investigate the variability in refrigerant charging requirements for transcritical CO₂ thermal management systems under diverse operating conditions. We specifically examined the impact of three critical factors, namely ambient temperature, indoor airflow rate, and outdoor air velocity, on refrigerant requirements in different modes and their underlying mechanisms. In the heat pump mode, the demand for refrigerant charge increases with ambient temperatures and wind speed and decreases with cabinet air flow rate, with changes of 18.6%, 18.9%, and 6.16%, respectively. In the cooling mode, the refrigerant charge requirement decreases with ambient temperatures and cabinet air flow rate and increases with outdoor wind speeds, with changes of 7.03%, 7.85%, and 2.27%, respectively. In situations of nonoptimal charging, potential alterations in the interaction between system control variables and target variables contribute to system instability. This necessitates adjustments to refrigerant distribution to mitigate instability under specific operating conditions. The research outcomes hold substantial reference value for the optimization of electric vehicle air-conditioning accumulator designs, enhancement of energy efficiency, and improvement of overall thermal comfort.

Optimal discharge pressure is the most important factor affecting the performance of a transcritical CO₂ heat pump. In this study, the reasons for the optimal discharge pressure were theoretically analyzed based on the heat transfer pinch point. The factors influencing the optimal discharge pressure and their influence mechanism were determined by constructing a transcritical CO₂ heat pump simulation model using the AMESim software. The model took into consideration the internal heat transfer pinch point of the gas cooler. Under inlet water temperature of 10 ℃-40 ℃, outlet water temperature of 60 ℃-90 ℃, and ambient temperature-30 ℃-25 ℃, the influence of inlet temperature, outlet temperature, ambient temperature, and heat recovery rate on the optimal discharge pressure was studied quantitatively. The results showed that the lower the inlet, outlet, and ambient temperatures, the lower the optimal discharge pressure of the system. Moreover, the optimal discharge pressure could be reduced by using an internal heat exchanger. The influence mechanism of each factor on the optimal discharge pressure was found to be closely related to the heat transfer pinch point and the physical properties of CO₂ near the critical point and supercritical zone. The research results can provide a reference for the design and performance optimization of CO₂ heat pump systems.

With the adoption and implementation of the Kigali Amendment in China, the R134a refrigerant has gradually reduced and could be eliminated in China′s mobile air conditioning (MAC). Currently, the most important alternative refrigerants for MAC are R1234yf, CO₂, and R290; however, China has no criteria for the selection of these three refrigerants. Recently, Germany and five other countries have proposed regulations to restrict PFASs, and R1234yf has also been included in the restricted list; therefore, the prospect of R1234yf is not optimistic. In this context, this study compares R1234yf, CO₂, and R290 in terms of environmental protection, safety, economy, and system performance. Relevant research results and conclusions are summarized, and the latest PFAS substance restriction proposal involving MAC was organized and analyzed. These efforts provide direction for the research of alternative refrigerants in the field of MAC in China, which may be used as a basis for the selection of alternative refrigerants for MAC in the future.

Lubricating oil is a key factor affecting the performance and reliability of transcritical CO₂ heat pump air conditioners dealing with complicated application environments and the requirements of new energy vehicles. The type of oil products and the amount of charged oil play an important role in maintaining high performance and ensuring the safe and stable operation of the system. This study discusses the function and influence of lubricating oil in transcritical CO₂ automotive heat-pump air conditioners. Several common lubricating oils used in CO₂ automobile heat-pump air conditioners are summarized, and the performance of each lubricating oil is compared. The effects of adding lubricating oil on the performance of each component and the overall performance of the transcritical CO₂ heat-pump air-conditioning system are analyzed. Lubricating oil reduces the abrasion and working noise of moving parts and prevents refrigerant leakage and excessive discharge temperature in the compressor. However, excessive oil accumulation in the compression chamber leads to reduced volumetric efficiency and increases power consumption. In the heat exchanger and pipeline, the oil slick that forms on the heat exchanger wall increases the flow resistance and thermal resistance. Simultaneously, oil has an effect of restraining nuclear boiling and bubble disturbance in the evaporator, which deteriorates two-phase heat transfer. Finally, the existing research results, conclusions, and key research directions in related fields are discussed.

The thermal-management system of a new energy vehicle is a key to ensuring thermal safety and comfort. Battery thermal management and cabin cooling are strongly coupled, resulting in poor dynamic characteristics. Thus, this study examined a transcritical CO₂ vehicle thermal-management system, built an Amesim model of a thermal-management system and control system based on the co-control of thermal comfort and battery temperature, and analyzed the thermal-management system under different scenarios and conditions. It was demonstrated that the thermal-management system was able to quickly achieve a comfortable thermal environment of the cabin and ensure a suitable battery temperature under various conditions, with less fluctuation, prompt response, and excellent anti-disturbance performance. Compared to the traditional control method based on cabin temperature, this method could provide excellent cabin thermal comfort at all times, which verified the superiority of a thermal-management system based on this control method.