Thermal Management of Densely-packed EV Battery … Paper.pdf · EVS28 International Electric Vehicle Symposium and Exhibition 1 EVS28 KINTEX, Korea, May 3-6, 2015 Thermal Management

EVS28 International Electric Vehicle Symposium and Exhibition 1

EVS28

KINTEX, Korea, May 3-6, 2015

Thermal Management of Densely-packed EV Battery Set

Z. Lu1, X.Z. Meng1, W.Y. Hu2, L.C. Wei3, L.Y. Zhang1, L.W. Jin1*

1 Building Environment and Equipment Engineering, Xi’an Jiaotong University,

20 Xianning West Road, Shaanxi, 710049, China. Email: [email protected] 2 Chinese Association of Refrigeration, 67 Fucheng Road, Beijing, 100142, China.

3Shenzhen Envicool Technology Co., Ltd., 9 Building, Hongxin Industrial Park, Shenzhen, 518129, China.

*Corresponding author: [email protected]

Abstract

The modern development of electric vehicles requires higher power density to be packed into battery box.

It is always expected that the battery can be arranged as much as possible, which leads to the thermal

management issue due to the heat generation inside the battery packs. It is more serious when the battery

system is running at high power modes, such as large current charging/discharging and energy recovery

processes, etc. As an extreme temperature affects performance, reliability, safety and lifespan of batteries,

thermal management of battery system is critical to the success of all electric vehicles. The working

temperature range and temperature uniformity are major factors to maintain battery working at its ideal

conditions.

In this research, air cooling for a battery box is investigated numerically, of which 252 Li-ion batteries

(32650) are packed densely in a space with dimensions of 121(x) × 380(y) × 462(z) mm. The objective is to

explore the air cooling capability on the temperature uniformity and hotspots mitigation subject to various

flow paths, heat generation, and air inlet conditions. Different flow paths are designed and the results of the

thermal characteristics are compared and analyzed. It shows that a proper designed air cooling system is

able to maintain Li-ion batteries at optimal operating temperature and to minimize the hotspot for low and

moderate heat generation at 4.375 W∙m-2 and 8.75 W∙m-2. However, as for high heat generation at 16.5

W∙m-2, the temperature of battery packs inside battery box depends mainly on the inlet flow temperature

with reasonable flow paths. This implies that the battery thermal condition can be successfully controlled

using air cooling method if the inlet flow can be pre-cooled by EVs air-conditioning system or a dedicated

mini refrigeration system.

Keywords: battery packs, forced air cooling, flow paths, numerical simulation

1 Introduction

In response to energy crisis and environmental

problems, the pure electric vehicles, with the

advantages of low power consumption and zero

emissions, have developed rapidly in recent years.

As the only power of pure electric vehicles, the

battery working conditions directly affect the


performance of electric vehicle. Due to high

energy density, high voltage and low self

discharging and so on, Li-ion batteries are

becoming an attractive applications for pure

electric vehicles. As for Li-ion battery, the

optimal operating temperature is about 20-40oC

[1]. Nowadays, the battery can be arranged as

much as possible in battery box to meet required

higher power density of electric vehicle, which

could result in serious thermal management issue

because of heat generation inside the battery box.

For instance, high temperatures have the

devastating effects on the battery packs that the

life of battery packs can be severely shortened-

battery life cut in half for every 10oC increase.

Moreover, battery packs also need to be operated

at uniform temperatures because its fluctuations

results in the difference of charge/discharge

behavior, which lead to electrically unbalanced

modules and the reduction of battery packs’

performance [2]. Therefore, it is important to

maintain the battery packs optimum temperature

and temperature uniformity for ensuring battery

stability and extending battery lifespan through

proper thermal management system, which is

critical to safe and efficient operation of electric

vehicles. Several thermal management systems

have been carried out in the last years to keep the

battery packs at an optimum temperature with

small variations [3-12]. These thermal

management methods are mainly divided into air

cooling, liquid cooling and phase change cooling

manners. At present, compared to other cooling

methods, air cooling is a common method

because of reliable and simple battery cooling

system.

In this study, air cooling for a densely-packed

battery box is investigated numerically, where

252 Li-ion batteries (32650) are arranged in a

space with dimensions of 121(x) × 380(y) ×

462(z) mm. Numerical approach is performed

using CFD Fluent code. The objective is to

explore the air cooling capability on the

temperature uniformity and hotspots mitigation

under various flow paths, heat generation, air

inlet velocity and temperature in order to provide

some specific guidance for thermal characteristic

analysis of densely-packed battery packs.

Nomenclature

U Velocity vector

Ø General variables

ρ Density of fluid [kg∙m3]

Γϕ generalized diffusion coefficient

Ѕϕ Source term

2 Configuration of Densely-

Packed Battery Box

The thermal management issue of a densely-

packed battery box is studied in this paper, whose

dimensions are of 121(x) × 380(y) × 462(z) mm. In

this study, the densely-packed battery box is

designed to house 252 Li-ion batteries (32650)

arranged into six rows and has five air baffles to

fix batteries. Three kinds of flow paths, namely, 15

vents, 17 vents and 59 vents are investigated under

various air inlet conditions and heat generation by

batteries. The details of the densely-packed battery

box are shown in Fig. 1.

Figure 1: The schematic diagram of densely-packed battery box configuration with three flow paths


3 Numerical Simulation

3.1 Airflow modeling

In this study, the air flow and temperature

distributions in the densely-packed battery box

are numerically simulated using Fluent 6.3.26.

The flow is assumed to be steady, three-

dimensional, incompressible and turbulent. The

Boussinesq approximation is used to model the

buoyancy effects. Turbulence is resolved using

the standard k-ε turbulence model.

All the variables (velocities, temperature,

turbulent energy and dissipation energy) to be

solved are denoted by ϕ. The general transport

equation for ϕ can be written as [13]:

div U div grad S

t

(1)

where ρ is the density of the fluid, U = (u, v, w) is

the velocity vector, Γϕ is the generalized

diffusion coefficient and Ѕϕ is the source term.

With properly prescribed Γϕ, Ѕϕ and ϕ, Equation

(1) can be taken as the continuity, momentum,

energy or other scalar equations.

3.2 Boundary conditions

In this study, the boundary conditions include

velocity-inlet, outflow-outlet and no-slip

condition at all walls of battery box and battery

surfaces. In addition, all walls of battery box are

taken as adiabatic wall and the heat fluxes of

battery surfaces are set based on different heat

generation corresponding to different

charge/discharge rates.

3.3 Numerical scheme

The grid is generated using the Gambit 2.4.6 pre-

processor and the discretization of the

computational domain is achieved using an

unstructured mesh. The solution method is based

on the following main hypothesis: the diffusion

terms are second-order central-differenced and

the second-order upwind scheme for convective

terms is adopted to reduce the numerical

diffusion. The coupled velocity-pressure terms

are resolved using the SIMPLE algorithm.

4 Results and Discussion

4.1 Grid independency analysis

The grid independency analysis is carried out to

ensure that the numerical results are not

influenced by the cell numbers. We take same

grid numbers for the battery box with different

flow paths due to similar geometry. Therefore,

three kinds of grid numbers of battery box with 15

vents, namely, 4118732 (coarse), 4671297 (regular) and 5203321 (fine) are chosen to investigate grid

independency analysis for numerical simulation.

Figure 2 presents the trends of temperature

variation along z direction at location of x = 74.9

mm and y = 133.6 mm. It is obvious that the

temperature differences between the regular mesh

and the fine mesh are rather small. Therefore, the

regular meshes are used for densely-packed battery

box under different flow paths in this article.

Figure 2: The trends of temperature variation along z direction at location of x = 74.9 mm and y = 133.6 mm.

4.2 Flow fields and temperature fields

analysis for 15 air inlets

Figures 3(a) and 3(b) show temperature fields and

flow fields of the densely-packed battery box with

15 air inlets under the heat flux of battery surfaces

and airflow rate set at 8.75 W∙m-2 and 22.4 m3∙h-1

respectively.

It can be seen that the high temperature area of

battery packs is around the center and at the

bottom near air outlet; the temperature of battery

packs at the top of the box is relatively low due to

the effect of air inlet. The maximum temperature

of battery packs is 316 K and the maximum

temperature differences is 22 K.

From a closer view of Fig. 3(b), it is observed that

the air velocities are rather small at the bottom of

battery box and there are downdrafts at the last two

rows of battery packs. These observations are in

agreement with the temperature fields shown in

Fig. 3(a) that the temperature of battery packs at

the bottom of battery box increases along the

airflow direction and reaches the maximum;

battery surface temperature decreases with the

growth of height. Therefore, when the heat flux of

battery surfaces is 8.75 W∙m-2, the air cooling

performance of a densely-packed battery box with


15 air inlets could not meet the requirements of

operating temperature for Li-ion batteries.

(a) temperature field

(b) velocity field

Figure 3: The temperature field and velocity field for

15 air inlets at 293 K



According to the results of 15 air inlets,

additional two air inlets are designed to improve

air cooling performance at the bottom of battery

box.

Figure 4(a) shows the temperature contours of

the densely-packed battery box with 17 air inlets

under same conditions with 15 air inlets. It is

observed that the high temperature area of

battery packs is near the center and the maximum

temperature of battery packs at the bottom of

battery box decreases due to the effect of airflow.

However, the maximum temperature of battery

packs is 318 K which is about 2 K higher than

that of 15 air inlets.

Combination with air velocity fields shown in

Fig. 4(b), it can be seen that the air velocity in

the central of battery box is slightly lower than

that of 15 air inlets, which may affect heat

dissipation of battery packs far away from all air

vents.

Figures 5(a) and 5(b) present the horizontal

temperature profiles along z direction (airflow

direction) at locations of x = 74.9 mm, y = 133.6

mm and x = 46.2 mm, y = 183.4 mm for these two

kinds of flow paths. The positions of selected

temperature points are shown in Fig. 6. As

expected, the temperatures of selected points (T1,

T2, T3, T4, T5, T6) far away from air vents for 17

air inlets with increasing more than 2 K, in

comparison with that of 15 air inlets. While, the

temperatures of selected points (T7, T8, 9, T10, T11,

T12) near air vents have similar values for these

two kinds of flow paths.

Based on the above analysis, it is found that

additional two air inlets could reduce the

maximum temperature of battery packs at the

bottom of battery box. However, the smaller air

inlet velocity significantly affects heat dissipation

of battery packs from all vents.


(b) velocity field

Figure 4: The temperature field and velocity field for 17 air inlets at 293 K


(a) 17 air inlets

(b) 15 air inlets

Figure 5: The horizontal temperature profiles along z direction at location of x = 74.9 mm, y = 133.6 mm

and x = 46.2 mm, y = 183.4 mm

(a) 17 air inlets

(b) 15 air inlets

Figure 6: The schematic diagram of different selected temperature points



In order to avoid the smaller air inlet velocity

affecting the heat dissipation of battery packs from

air vents, 59 air inlets are designed to keep all

battery packs close to air vents.

Figures 7(a) and 7(b) show temperature fields and

flow fields of a densely-packed battery box with

59 air inlets under same conditions with above two

flow paths. As expected, the maximum

temperature of the battery packs is 310 K, which

occurs at the central of cells. It indicates that this

flow path can significantly reduce the maximum

temperature of the battery packs higher than 6 K

and 8 K, in comparison with the maximum

temperature of 316 K for 15 air inlets and 318 K

for 17 air inlets. Therefore, when the heat flux of

battery surfaces is 8.75 W∙m-2, this flow path can

improve the heat dissipation performance of forced

air cooling system.

From Fig. 7(b), it can be seen that although the air

velocity is smaller than that of above two flow

paths, the airflow around battery packs is relatively

uniform that can effectively avoid the heat

dissipation problem of battery packs from air

vents.

From Fig. 8, it can be observed that the

temperature profile of selected points (T1, T2, T3,

T4, T5, T6) is similar to those of T7, T8, T9, T10, T11,

T12, which is different from above two flow paths,

i.e., 15 and 17 air inlets. This observation is in

agreement with uniform flow fields around battery

packs. The positions of selected temperature points

are shown in Fig. 9.



(b) velocity field



Figure 8: The horizontal temperature profiles along z

direction at location of x = 74.9 mm, y = 133.6 mm and x = 46.2 mm, y = 183.4 mm with 59 air inlets

Figure 9: The schematic diagram of different selected temperature points with 59 air inlets


analysis for 59 air inlets at different heat

generation by battery packs

Figure 10 shows the temperature fields and flow

fields of a densely-packed battery box with 59 air

inlets under air inlet temperature and airflow rate

set at 293 K and 22.4 m3∙h-1 respectively. It can be

seen that the maximum temperature and maximum

temperature difference of battery packs are 301.5 K

and 8 K respectively.

Table 1 compares the thermal characteristics of

battery packs for 59 air vents at different heat fluxes

of battery surfaces. Obviously, the maximum

temperature and maximum temperature difference

increase along with increasing the heat flux of battery

surfaces. Compared with heat flux of battery surfaces

set at 16.5 W∙m-2, the heat dissipation performance of

battery packs could meet the operating temperature

requirement of Li-ion battery when heat flux of

battery surfaces is 4.375 W∙m-2 or 8.75 W∙m-2, and

the air inlet temperature is 293 K.

Therefore, as for low and moderate heat generation,

a proper designed air cooling system is able to

maintain Li-ion batteries at operating temperature

and to minimize the hotspot. However, as for high

heat generation, the lower air inlet temperature or

the higher air inlet velocity is needed to improve

forced air cooling performance of a densely-

packed battery box with reasonable flow paths.



(b) velocity field

Figure 10: The temperature field and velocity field for 59 air inlets at 4.375 W∙m-2

Table 1: The comparisons of thermal characteristics of

59 air vents at different heat generation

Heat flux of battery surfaces (W∙m-2)

4.375 8.75 16.5

Airflow rate (m3∙h-1) 22.4 22.4 22.4

The maximum temperature (K) 302 310 324

The maximum temperature Difference (K)

9 16 30


analysis for 59 air inlets at different air

inlet temperatures

Figures 11(a) and 11(b) show temperature fields

and flow fields of a densely-packed battery box

with 59 air inlets under the heat flux of battery

surfaces and airflow rate set at 4.375 W∙m-2 and

22.4 m3∙h-1 respectively. It can be seen that the

maximum temperature and maximum temperature difference of battery packs are 311.5 K and 8 K

respectively.

Therefore, as for low heat generation, the higher air

inlet temperature could satisfy the requirements of

Li-ion batteries operating temperature.

Table 2 shows the comparison of thermal

characteristics of battery packs for 59 air vents at

different air inlet temperatures.

As expected, the maximum temperature rises

gradually with the increase of air inlet temperatures.

While, the maximum temperature difference does not

change. It indicates that air inlet temperatures have

little effect on temperature uniformity


(b) velocity field



Table 2: The comparison of thermal characteristics of 59

air vents at different air inlet temperatures

Air inlet temperature(K) 293 298 303

Airflow rate(m3∙h-1) 22.4 22.4 22.4

The maximum temperature(K) 301.5 306.5 311.5

The maximum temperature difference(K)

8 8 8


Conclusions

The forced air cooling of a densely-packed

battery box is investigated by numerical

simulation to explore the air cooling capability

on the temperature uniformity and hotspots

mitigation under various flow paths, heat

generation and air inlet conditions. Based on the

above research, the following conclusions may

be drawn:

(i) As for low and moderate heat generation, a

densely-packed battery box with a proper

designed air cooling system is able to maintain

Li-ion batteries at optimal operating temperature

within the range of 293 K - 313 K; while, as for

high heat generation, the lower air inlet

temperature or the higher air inlet velocity is

needed to improve forced air cooling

performance of a densely-packed battery box

with reasonable flow paths;

(ii) Despite of the lower air inlet velocity, the

forced air cooling performance of a densely-

packed battery box with 59 air inlets is stronger

than the other two kinds of flow paths discussed

in this article, which could provide uniform

temperature fields and mitigate hotspots. It

indicates that effective heat transfer area may

have a more significant effect on forced air

cooling performance than air velocity;

(iii) The proper flow path that makes air as

cooling medium contacted effectively with

battery surfaces is critical to forced air cooling

performance for a densely-packed battery box

(iv) The air inlet temperature is not critical for

maintaining the temperature uniformity; as for

low heat generation, the moderate air inlet

temperature should be chosen to cool densely-

packed battery box.

Acknowledgments

You may list acknowledgments here if

appropriate.

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Authors

Mr. Z. Lu is currently a Master Student studying in the Department of

Building Environment and Equipment Engineering of Xi’an Jiaotong

University. His research is related to the battery thermal management.

Dr. X.Z. Meng is a Senior Engineer

working at the School of Human Settles and Civil Engineering of Xi’an

Jiaotong University. His research includes both numerical and

experimental heat transfer.


Mr. W.Y. Hu obtained his Master

Degree from the School of Power and Energy Engineering of Xi’an Jiaotong

University. He is the Deputy General Secretary of Chinese Association of

Refrigeration. He is

Mr. L.C. Wei obtained his Master

Degree from the School of Power and Energy Engineering of Xi’an Jiaotong

University. He is the Chief Engineer of Shenzhen Envicool Technology

Co. Ltd. in charging of the development of cooling system of EV

and HEV.

Dr. L.Y. Zhang is an Associate

Professor working the Department of Building Environment and Equipment

Engineering of Xi’an Jiaotong University. Her research interests

include refrigeration system and building environment.

Dr. L.W. Jin obtained his Ph.D.

Degree from Nanyang Technological University, Singapore. He is now a

Professor at the Department of Building Environment and Equipment

Engineering of Xi’an Jiaotong University. His research focuses on

the thermal management of energy equipment.

Documents

Thermal Management of Densely-packed EV Battery … Paper.pdf · EVS28 International Electric Vehicle Symposium and Exhibition 1 EVS28 KINTEX, Korea, May 3-6, 2015 Thermal Management