R134a 냉매를 적용한 직접접촉 배터리 냉각시스템의 성능 고찰

Abstract

Fossil fuel consumption has increased significantly since the industrial revolution, which broke out in the 18th century, and fossil fuels are an important energy resource that contributed to the development of human civilization. However, fossil fuels emit pollutants such as carbon dioxide and nitrogen oxides, which are known to be the main causes of global warming and other environmental problems. Due to these problems, the rapid development of electric vehicles classified as eco-friendly transportation has been promoted. Lithium-ion batteries are being used as power sources for electric vehicles. However, while charging and discharging, lithium-ion batteries operating temperature changes significantly due to the electrochemical reaction, which directly affects the performance and life of the battery, thus it is essential to operate a battery thermal management system to control the heat generation. In this study a 21700 dummy batteries experiment device and a simulation model based on 21700 actual batteries were developed. And an effective cooling method for battery thermal management through indirect contact liquid cooling, direct contact cooling simulation, and direct contact refrigerant cooling experiments were investigated. First, using the 21700 dummy batteries, a performance study of battery thermal management where indirect contact liquid cooling is applied to the bottom of the batteries was carried out. Next, a simulation was performed under 2C-rate conditions and various working fluid changes in which direct contact cooling with 21700 batteries was applied. An optimal simulation model was achieved according to design factor changes of the simulation model, and cooling performance was analyzed according to ambient temperature and flow rate based on the the optimal model. Finally, a direct contact refrigerant cooling experiment system using R134a refrigerant was constructed with 21700 dummy batteries under 2 C-rate, and cooling performance was examined according to the pressure, mass flow rate, C-rate, and heat generation conditions of R134a refrigerant. As a result, the indirect contact liquid cooling with 21700 dummy batteries is not suitable for battery thermal management because of the non-uniformity of temperature distribution caused by thermal resistance between batteries and the cooling plate. As a result of applying direct contact cooling to the 21700 battery module, the R134a refrigerant, which has the most advantages in terms of pressure drop and price, was applied as a cooling fluid for a direct contact cooling. An optimal model was investigated by changing the port number and header width of the battery cooling block, which is a direct contact cooling model when R134a refrigerant is applied. Based on the optimal model, the optimal flow rate conditions were 3 l/min and 5 l/min at 25oC and 35oC, which are standard and summer condition, respectively. It was confirmed that the direct contact cooling method is an efficient battery thermal management method with sufficient cooling performance for the battery thermal management and uniformity of the battery temperature distribution even under summer conditions. The experiment of applying direct contact refrigerant cooling to the 21700 dummy batteries was conducted based on the experiment results of the 21700 actual batteries. By analyzing the cooling performance according to the pressure, mass flow rate and C-rate changes with R134a, the increase in mass flow positively affects battery thermal management. An increase in the pressure negatively affected battery thermal management because the enthalpy vaporization of R134a decreased and the latent cooling capacity decreased. Accordingly, the direct contact refrigerant cooling method using R134a has sufficient cooling performance for battery thermal management and battery temperature distribution uniformity even under high heat generation conditions and has good cooling potential for the battery cooling system even with more number of batteries.