Numerical Analyses on Transient Thermal Behavior of Micro Heat Pipe Array-Air Cooling Battery Thermal Management System
Authors: Dan Dan , Yao Chengning , Zhang Hu , Xu Xiaoming , Zhang Yangjun 
Published: February 2020

Power battery provides energy source for electric vehicles, and is an important part of electric vehicles. The performance of power battery directly restricts the output power, system safety and economy of electric vehicles: specific energy determines the driving mileage of electric vehicle; specific power determines the maximum climbing capacity and maximum speed and battery cycle life directly affects the automobile economy. Among many kinds of power batteries, lithium-ion battery has strong advantages in power output, mileage and service life. It is the mainstream technology of existing vehicle power battery, and is the focus of current research and development.

Lithium-ion power system is an electrochemical power source with complex flow and heat transfer process, temperature is one of the key factors affecting the system performance. Temperature and temperature distribution directly affect the electrical performance, charge/discharge efficiency, output power and cycle life. Overall, the effect of temperature on the battery is mainly reflected in three aspects: (1) When temperature is too high, the recession will aggravate the battery capacity and output power, high temperature will even cause thermal runaway. (2) When temperature is too low, the discharge capacity of the battery will be attenuated and the charge/discharge efficiency decreased; (3) inconsistence in temperature distribution will further exacerbate the inconsistency between the cells and the uneven rate of aging, and cause cycle life decay. The best working temperature range of lithium ion battery is about 25-40℃, and the temperature difference between cells should be controlled below 5℃. Therefore, it is necessary to design a reasonable thermal management system structure and to develop advanced control strategy, in order to ensure the power battery operate within the suitable temperature range, and effectively control the battery temperature difference, in order to improve the performance of power battery and ensure the efficient operation of electric vehicles.

Traditional battery thermal management method mainly adopts air cooling and liquid cooling structure. However, it is difficult for air cooling system to control the battery temperature and meet heat dissipation requirement under high discharge rate. The liquid cooling system has complicated structure, which may lead to an increase in weight and reduces the energy density of the battery system. Compared with the traditional air cooling and liquid cooling, heat pipe has advantages of high heat transfer coefficient, simple structure, light weight, and in the large current discharge conditions, which can achieve efficient heat dissipation enhancement and ensure smaller temperature difference. Therefore, battery thermal management system based on heat pipe technology is the choice of future HEV/EV.

A heat pipe-air cooling coupled heat dissipation system was designed in this paper. Micro heat pipe array (MHPA) was used as it has flat surface to ensure the contact surface and low contact resistance for better heat transfer efficiency. A fin structure was installed on the MHPA to enhance heat transfer. Simulation was conducted for both one battery cell and a battery pack using MHPA cooling system.

First, one cell-MHPA-fin structure was studied. A first order equivalent circuit model was used in the approach to predict the dynamics of the cell electrical performance. The electric model was then coupled with a lumped thermal model to describe the dynamics of heat generation of the battery cell. 3D CFD calculation was conducted to simulate the thermal response when the battery is operating under a variable operating condition with heat pipe cooling. Results show that MHPA-fin air cooling system has quick response for the sudden change of current. Comparison results showed that the cooling rate of battery with MHPA is twice than that with liquid cooling.

Secondly, a battery pack of 36 cells was studied. MHPA-fin system was installed between two adjacent cells. Same model with one cell analysis was adopted in this study. Results showed that the temperature of all cells within the pack responses nearly identical with a temperature difference below 3℃.

Finally, the cycle life of the battery cell with the above MHPA-air cooling system was analyzed. Under 1C discharge rate, the cell can work for more than 800 cycles with a capacity fading less than 20%.