Coupled effect of operation conditions and refrigerant charge on the performance of a transcritical CO₂ automotive air conditioning system

Abstract

Refrigerant charge is one of the most important factors affecting the performance of the transcritical CO₂ automotive air conditioning system. In the presented study, a transcritical CO₂ automotive air conditioning system test bench was set up, and the effect of normalized charge on the temperature, pressure and system performance were studied. A novel standard for judging the appropriate normalized charge was proposed. Meanwhile, the pressure-enthalpy diagrams under different charge was shown. Moreover, the influence of the ambient temperature and the compressor frequency on the charge characteristic were also experimentally studied and analyzed. Results revealed that the discharge temperature, the suction temperature and the temperature before the EXV were the optimal parameters to judge the charge characteristic. As the ambient temperature and compressor speed increased, the required charge amount decreased. It was concluded that the optimal normalized charge range of the transcritical CO₂ air-conditioning system was recommended to be set between 0.111 and 0.321. The results provide good guides in charge of automobile air conditioning system.

Introduction

With the development of science and technology, energy shortages and environmental pollution have gradually intensified. Automotive have developed rapidly with the advantages of energy saving. However, the environmental pollution caused by the traditional refrigerants in the air-conditioning industry has aroused a huge amount of attention. Meanwhile, the adoption of the Paris Agreement has accelerated the pace of the withdrawal of traditional refrigerants from the historical stage. Due to the fact that the European Communities motor vehicle directive (EU 2006) refrigerants with a GWP higher than 150 are not allowed from January 1, 2017 in new passenger cars (Yu et al., 2019), R134a is to be eliminated. It has become a research hotspot to find new natural and environmentally friendly alternative refrigerants (Vaghela, 2017, Kumar, 2017, Yu et al., 2018, Yang et al., 2019, Meng et al., 2018, Joudi et al., 2003). Hafner et al (Hafner et al., 2004). declared that the LCCP of CO₂ was 18–49% lower than that of R134a at the different European countries. This meant that CO₂ still has its own advantages in consideration of LCCP. CO₂ refrigerant stands out among many alternative refrigerants due to its advantage of pollution-free and low price (Ma and Tian, 2013). Besides, the heating performance of CO₂ at low temperature is exceptionally good, which perfectly solves the problem of winter mileage anxiety of electric vehicle. In view of this, CO₂ has become one of the most promising alternative refrigerants (Tamura and Nishiwaki, 2005, Li and Norris, 2016, Hoffmann, 2017, Wang et al., 2018).

As an alternative refrigerant of automotive air-conditioning, CO₂ has been studied for several years. Many efforts have been done to improve the system performance of a CO₂ automotive air conditioner (Yiming et al., 2018, zhen et al., 2011),but refrigerant charge is one of the most important factors for the development of all types of the system. On the one hand, the refrigerant charge of CO₂ automotive air-conditioning has a great influence on the service life of the components. A serious insufficient charge will cause an excessive high discharge temperature, which might damage the compressor. On the other hand, charge is closely related to the parameters of the system, such as COP, mass flow rate, temperature and pressure parameters, the performance of the accumulator and so on. Improper charge will cause the COP of the air-conditioning system to be greatly reduced. Hazarika and Bhattacharyya (2018) concluded that COP decreased significantly if the charge varied more than ±18% from the optimum value. Lixin et al. (2012) Studied the pressure drop of CO₂ under different mass flow rates. Their conclusion had a clear guidance for the pressure drop analysis of CO₂ air conditioning systems. Cho et al. (2005)studied the influence of pressure and performance parameters with the increases of charge. From the results, they pointed out that the expansion losses and gas-cooler losses were the main reasons for the rapid decrease in system performance under insufficient and excessive charge. Liu et al. (2005) summarized the characteristics of the discharge pressure, the outlet temperature of the EXV, the inlet refrigerant dryness of the evaporator and the inlet temperature of the compressor under an appropriate charge operation condition. Furthermore, they proposed these four parameters as the standards for the judgement of the charge amount. Because of the complex and diverse operation condition of the automotive air-conditioning, the optimal charge often varied heavily. In view of this, an accumulator was used, and the behavior of the accumulators were widely investigated (Wang and Dickson, 2006). Choi and Mand (2020) and Wang et al. (2019) studied the working behavior of the accumulator under different refrigerant charge, and summarized the characteristic of the refrigerant migration during the process of the starting and stopping of the system. Moreover,Choi and Mand (2020) suggested that degree of superheat at evaporator outlet (DSH_e,out) and degree of superheat of compressor suction (DSH_suc) could be used to determine whether the charge was appropriate. Generally speaking, the current research was mainly focused on the impact of charge on the system performance under a single condition. It has a limited application scope of the appropriate charge range under a certain ambient temperature or compressor speed. When the operating conditions change, it is difficult to meet the system's charging demand again. In view of the limitations of the above researches, this paper explored the coupling influence characteristics of variable operating conditions and charge on system performance and studied the influence of changes in ambient temperature and compressor speed on the range of normalized charge, which satisfies the system charge requirements under variable working conditions. The automotive air conditioner needs to deal with the variable operation conditions, including the dynamic heat demand, ambient temperature, air flow rate, air flow temperature and so on. It was still doubt whether the charge was significant affected by the variable conditions. The deep understanding of thermodynamic performance on the variable operation condition and the optimal performance could provide the reference for the refrigerant charge during the practical engineering or even for the air conditioner service. Thus, the CO₂ automotive air conditioner had the best performance at the whole operation condition. Hence the content of these aspects is also worthy of further research to more clearly grasp the optimal charge judgement in the actual project.

A transcritical CO₂ automotive air conditioning system was built to study the following two aspects. First, a novel standard for judging whether the charging is appropriate was proposed by studying the influence of normalized charge on the temperature, pressure and performance parameters. Then, the effect of variable conditions on the appropriate normalized charge required by the system was researched. The driving conditions of the car are changeable and complicated, which has a significant impact on the performance of air conditioners. Therefore, performance research under variable operating conditions is particularly important. In this paper, the effects of variable ambient temperature and variable compressor speed on the normalized charge of the air conditioning system were experimentally studied. The former simulated the effect of ambient temperature changes caused by day-night replacement or weather changes on the automotive air-conditioning system during actual driving conditions. The latter simulated the different comfort requirements of passengers. Some passengers prefer a warmer space, while others are used to living in colder spaces. Therefore, the cooling capacity of the air conditioner is not static. It needs to be adjusted according to the personal needs of passengers. The adjustment of the cooling capacity is achieved by adjusting the speed of the compressor. In addition, the optimal normalized charge range of the air conditioning system was given under both operating conditions.