액침냉각을 적용한 21700 배터리 모듈의 냉각특성 분석

Abstract

Global energy consumption is increasing every year, and energy obtained through the combustion process of fossil fuels generates air pollutants such as oxides of nitrogen (NOx), sulfur (SOx), and carbon (COx), as well as greenhouse gases when used. It causes environmental pollution. Accordingly, interest in energy and environmental protection is increasing, and countries worldwide are declaring carbon neutrality and strengthening eco-friendly energy policies and regulations. In the transportation field, the use of electric vehicles using batteries as a power source is being considered as an alternative to internal combustion engine vehicles that use fossil fuels as a power source. Lithium-ion batteries are the most widely used in electric and hybrid electric vehicles due to their high energy density, low self-discharge characteristics, near-zero memory effect, high open circuit voltage, and long cycle life. The development of lithium-ion batteries with high capacity and high energy density to improve the performance of electric vehicles is actively underway, which results in high battery heat generation. Lithium-ion batteries are very sensitive to temperature, so a battery thermal management system that can maintain the battery in an appropriate temperature range is essential for better performance and stability. Immersion cooling is a method of cooling the battery by directly placing it in a dielectric fluid that does not conduct electricity and is attracting attention as a next-generation battery thermal management technique. There is no thermal contact resistance between the working fluid and the battery during the cooling process, thus it has high cooling efficiency. Additionally, the system design is simple, and many dielectric fluids can prevent thermal runaway and extinguish cascading battery fires. Therefore, immersion cooling was applied to the thermal management of the 21700 cylindrical battery in this study. An analysis was performed on the immersion cooling of a 16S2P battery module considering the electrochemical heat generation of the battery using the MSMD Empirical model. The cooling performance of the inlet/outlet location and cell spacing of the battery cooling device was analyzed, and the battery heat generation characteristics and immersion cooling performance according to C-rate, immersion fluid, and immersion cooling performance according to mass flow rate changes were analyzed. In addition, a multiple regression analysis was conducted to analyze the influence of cell spacing, C-rate, and mass flow rate on the battery maximum temperature and temperature difference between cells. Using the result of a multiple regression analysis, an equation was derived to predict the maximum temperature and temperature difference of the battery. A 16S2P battery module immersion cooling experimental device was constructed and the cooling performance was analyzed using Novec 649. An immersion cooling experiment was conducted according to C-rate and mass flow rate using Novec 649. In addition, the simulation and experimental results were comparatively investigated. As a result, the Z-type inlet/outlet location, Novec 649 working fluid, and 2 mm cell spacing were determined to be the most appropriate model. Based on this, simulation and experiments were conducted to analyze cooling performance. As a result, immersion cooling using Novec 649 showed excellent cooling performance, maintaining the maximum battery temperature below 45℃ even during the 3 C-rate discharge process. In addition, the temperature between cells was relatively uniform, but due to structural simplification, a large temperature difference between cells occurred at high discharge rates. However, it is believed that this unfavorable temperature difference can be improved by changing the cooling system. Therefore, it is confirmed that immersion cooling is an excellent cooling method with better cooling performance and can ensure the stability of high-capacity and energy-density batteries that generate high heat. Keyword : Lithium-ion battery(LIB), Battery thermal management system(BTMS), Immersion cooling, Dielectric fluid, Multi-Scale Multi-Dimensional(MSMD)