thermal management of densely-packed ev battery … paper.pdf · evs28 international electric...
TRANSCRIPT
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
EVS28 International Electric Vehicle Symposium and Exhibition 2
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
EVS28 International Electric Vehicle Symposium and Exhibition 3
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
EVS28 International Electric Vehicle Symposium and Exhibition 4
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
4.3 Flow fields and temperature fields
analysis for 17 air inlets
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.
(a) temperature field
(b) velocity field
Figure 4: The temperature field and velocity field for 17 air inlets at 293 K
EVS28 International Electric Vehicle Symposium and Exhibition 5
(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
4.4 Flow fields and temperature fields
analysis for 59 air inlets
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.
EVS28 International Electric Vehicle Symposium and Exhibition 6
(a) temperature field
(b) velocity field
Figure 7: The temperature field and velocity field for
59 air inlets at 293 K
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
4.5 Flow fields and temperature fields
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.
EVS28 International Electric Vehicle Symposium and Exhibition 7
(a) temperature field
(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
4.6 Flow fields and temperature fields
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
(a) temperature field
(b) velocity field
Figure 11: The temperature field and velocity field for
59 air inlets at 303 K
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
EVS28 International Electric Vehicle Symposium and Exhibition 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.
References [1] H. Teng, et al., An analysis of a lithium-ion battery
system with indirect air cooling and warm-up, SAE Tech. Pap. 4(3)(2011), 15.
[2] A. A. Pesaran, et al., Thermal performance of EV and HEV battery modules and packs, National
Renewable Energy Laboratory, 1997.
[3] R. Mahamud, et al., Reciprocating air flow for Li-
ion battery thermal management to improve
temperature uniformity, Journal of Power Sources,
196(13)(2011), 5685-5696.
[4] A. Jarrett., et al., Design optimization of electric
vehicle battery cooling plates for thermal performance, Journal of Power Sources,
196(23)(2011), 10359-10368.
[5] D. Linden, Handbook of batteries and fuel cells,
New York. 1984.
[6] A. S. Keller., et al., Thermal characteristics of electric vehicle batteries, Self, 2013, 08-27.
[7] C. Alaoui, et al., A novel thermal management for electric and hybrid vehicles, Vehicular Technology,
IEEE Transactions on, 54(2)(2005), 468-476 [8] A. A. Pesaran., Battery thermal management in EV
and HEVs: issues and solutions, Bttery Man,43(5)(2001), 34-49.
[9] S. Al-Hallaj., et al., Thermal modeling of secondary lithium batteries for electric vehicle/hybrid electric
vehicle applications, Journal of Power Sources, 110(2), 341-348.
[10] S. A. Khateeb., et al., Thermal management of Li-ion battery with phase change material for electric
scooters: experimental validation, Journal of Power Sources, 142(1)(2005), 345-353.
[11] R. Sabbah., et al., Active (air-cooled) vs passive (phase change material) thermal management of
high power lithium-ion packs: limitation of temperature rise and uniformity of temperature
distribution, Journal of Power Sources, 182(2008), 630-638.
[12] R. Kiziel., et al., Passive control of temperature excursion and uniformity in high-energy Li-ion
battery packs at high current and ambient temperature, Journal of Power Sources, 183(2008),
370-375.
[13] W. Q. Tao., Numerical heat transfer, Xi’an
Jiaotong University Press, Xi'an, 430-447, 2001.
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.
EVS28 International Electric Vehicle Symposium and Exhibition 9
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.