Investigation and optimization of thermal management system for lithium-ion battery packs used in EVs and HEVs

Transportation electrification has been universally regarded as a critical method to tackle the more and more severe global warming and climate change issues nowadays. The greenhouse gas (GHG) emissions from the conventional internal combustion engine (ICE) were found to be one of the main causes of the average temperature rise and environment deterioration worldwide. As a result, electric vehicles (EVs) had been rampantly developed in the last two decades. The sales figures in recent years indicated the pivotal sign of the momentum change from conventional ICE vehicles to EVs.
The quintessential component of an EV is its onboard energy storage system, i.e., the battery pack, which is also the fundamental difference between conventional vehicles and new energy vehicles. There are many optional battery technologies available for EVs, such as lead-acid battery, nickel-metal hydride battery (Ni-MH), lithium-ion battery (LIB), and fuel cell, etc. In 2019, the Nobel Prize in Chemistry¹ was awarded to three scientists who developed LIB and revolutionized portable electronics by the Royal Swedish Academy of Sciences, manifesting its ubiquitous significance in both larger examples such as EVs and miniaturized versions including lifesaving medical devices and mobile phones. LIB was recognized as “laid the foundation of a wireless, fossil fuel-free society” and “transformed people’s lives”. Undoubtedly, LIB technology has dominated the current EV onboard power battery market. To follow this sensational technical mega trend, the author also selected the LIB as the only battery type for the battery pack in this research.
LIB is famous for its high energy density and long cycle life. However, its downsides are also obvious such as sensitive to temperature and degradation with age. Both high and low temperatures are detrimental to the normal operation of the LIB cells. Long-term exposure to abnormal temperatures could also shorten the useful remaining life and accelerate the capacity fading. Moreover, in addition to the uncontrollable ambient temperatures, heat generations during the battery's normal and abnormal operations are unavoidable. If the heat accumulation exceeds the desired range and the heat dissipation is not well practiced, overheat will happen and damage the LIB cells.
To solve this conundrum, an effective and reliable battery thermal management system (BTMS) needs to be carried out and integrated into the EV onboard power battery pack. Among different types of BTMS types, including air cooling, liquid cooling, and phase change material (PCM) cooling types, etc., we chose the air-cooling BTMS as the main objective in this research. Admittedly, there are pros and cons for each type including the air-cooling approach, but we want to magnify the advantages of the air-cooling method to the greatest extent by improving and optimizing its BTMS designs. The most desirable traits of the air-cooling BTMS are its high reliability, simple structure, and low cost. In this research, we focused on the design improvement and optimization of the air-cooling BTMS to further boost its heat transfer coefficient and enhance the overall cooling performance.
To increase the heat transfer coefficient, we conceived several novel design optimization mechanisms: multiple inlets/outlets structure, air flow channel modification, trapezoid battery pack layout, gradient vertical spacings, and vortex adjustment auxiliary structure, etc. To prove and validate the conceptual designs, the 21700 cylindrical LIB cell and its heat generation models are devised to simulate both its heat generation and dissipation processes during normal discharging operations. The major three-dimensional (3D) modelling and simulation software is ANSYS® Fluent ² . Many analytical methodologies in the discipline of thermodynamics have been adopted to observe and generalize the design optimization practices, such as the field synergy principle (FSP), the aerodynamic and turbulence theory, and the influences of several major thermodynamic properties on the heat transfer process, etc. The orthogonal design method is proposed to find the optimal parameters.
In the analysis and evaluation process, maximum temperature (Tmax), minimum temperature (Tmin), maximum temperature difference (ΔT), and maximum pressure difference (ΔP) are selected as the four major cooling performance indicators. The simulation results have proven the cooling performance enhancements and heat transfer coefficient increase for each improved design. Other advanced aerodynamic and thermodynamic properties, including turbulent eddy frequency (TEF), turbulent kinetic energy (TKE), Reynolds number (RE), and Nusselt number (NU), etc., are all thoroughly investigated as the design optimization indicators or basis. The relationships between these properties and the cooling performance are not linear but predictable and regular, which could be utilized as a reference for the design improvement and optimization.
All the optimal air-cooling BTMS designs have been proven by the numerical calculations to meet two critical objectives:
(1) the operating temperature range of the battery pack is within 25 – 40 °C during different normal discharging processes from 100% state-of-charge³ (SoC) to 0% SoC;
(2) the transient ΔT is always less than 5 °C at any time during different operating conditions. These optimal designs have just small modifications to the original battery pack structure, which only occurs a small cost increase to the whole EV battery system. In addition, the ΔP values of these optimal designs also just increased a relatively small mount compared with the original design, indicating a inegligible rise in the power consumption.
After the numerical simulations, experimental works have also been conducted to validate the optimal design obtained by the theoretical analysis. The research team has set up a modern battery pack testing rig for the proposed air-cooling BTMS prototypes and conducted comprehensive constant current discharging tests for all the derivative designs to explore the optimal design with the top cooling performance. At the 1C discharging test, the lowest temperature (22.0 °C) appeared in the optimal trapezoid design, which was lower than those of the other three designs by 1.1 °C, 0.3 °C, and 0.2 °C, respectively. At the 1.5C discharging test, the lowest temperature (24.8 °C) appeared in the optimal trapezoid design again, which was lower than those of the other three designs by 2.0 °C, 0.7 °C, and 0.1 °C, respectively. The experimental results conformed well to the numerical results and have proven the effective cooling performance of the optimal design among all different air-cooling BTMS designs.
In the era of the highly competitive EV market nowadays, low manufacturing cost and low energy consumption are favourable traits to the economic and commercial success of modern EV models, both of which demonstrate exceptional values, visionary insights, reliable solutions, and practical contributions of this research to the development of the next generation onboard energy storage systems for the future EV industry.

¹ The Nobel Prize in Chemistry is awarded annually by the Royal Swedish Academy of Sciences to scientists in the various fields of chemistry. The award is presented in Stockholm at an annual ceremony on 10 December, the anniversary of Nobel's death.

² ANSYS® Fluent is the industry-leading fluid simulation software known for its advanced physics modeling capabilities and industry leading accuracy.

³ The state-of-charge of a battery describes the difference between a fully charged battery and the same battery in use. It is associated with the remaining quantity of electricity available in the cell.