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energies
Review
Hybrid Battery Thermal Management System
in Electrical Vehicles: A Review
Chunyu Zhao , Beile Zhang, Yuanming Zheng, Shunyuan Huang, Tongtong Yan
and Xiufang Liu *
School of Energy and Power, Xi’an JiaoTong University, Xi’an 710049, China;
zcy1041085222@stu.xjtu.edu.cn (C.Z.); zbeile@stu.xjtu.edu.cn (B.Z.); z18760661517@stu.xjtu.edu.cn (Y.Z.);
hsy50288@stu.xjtu.edu.cn (S.H.); ytt2174420426@stu.xjtu.edu.cn (T.Y.)
*Correspondence: liuxiufang@mail.xjtu.edu.cn
Received: 4 September 2020; Accepted: 9 November 2020; Published: 27 November 2020
Abstract: The Li-ion battery is of paramount importance to electric vehicles (EVs). Propelled by the
rapid growth of the EV industry, the performance of the battery is continuously improving. However,
Li-ion batteries are susceptible to the working temperature and only obtain the optimal performance
within an acceptable temperature range. Therefore, a battery thermal management system (BTMS) is
required to ensure EVs’ safe operation. There are various basic methods for BTMS, including forced-air
cooling, liquid cooling, phase change material (PCM), heat pipe (HP), thermoelectric cooling (TEC),
etc. Every method has its unique application condition and characteristic. Furthermore, based on
basic BTMS, more hybrid cooling methods adopting different basic methods are being designed to
meet EVs’ requirements. In this work, the hybrid BTMS, as a more reliable and environmentally
friendly method for the EVs, will be compared with basic BTMS to reveal its advantages and potential.
By analyzing its cost, efficiency and other aspects, the evaluation criterion and design suggestions are
put forward to guide the future development of BTMS.
Keywords: electric vehicles; Li-ion battery; hybrid battery thermal management system
1. Introduction
As a substitute for fossil-fueled vehicles, electric vehicles (EVs) have advantages such as low
pollution and high efficiency [
1
]. The Li-ion battery is a crucial component of EVs. The inappropriate
working temperature (high temperature, low temperature, and high differential temperature) will
affect batteries’ performance and lifespan [2], which seriously affects EVs’ capability.
At high temperature, capacity/power fade, self-discharging and other adverse effects will cause
a massive loss of batteries’ available energy [
3
,
4
]. In extreme situations like excessive ambient
temperature, high temperature will contribute to the thermal runaway and threaten EVs’ safety
when overheating from the short circuit is out of control. Low temperature is detrimental to charge
acceptance [
5
], capacity/power [
6
], lifespan and round-trip efficiency [
7
]. Data show that when the
temperature falls to
−
40
◦
C, the power that can be supplied is only 1.25% compared with the battery at
20
◦
C. Furthermore, due to the differences in electrochemical properties during charging or discharging
processes caused by the non-uniformity in temperature distribution in the cell, module or pack, the
cell performance and cycle life deteriorate [
8
]. Experiments show that when the temperature difference
(
∆
T) rises by 5
◦
C, power supply capacity suffers 1.5–2% more loss [
9
]. To hold batteries’ working
temperature within an appropriate range and improve temperature uniformity are the primary goals of
a battery thermal management system (BTMS) in EVs. Researchers and manufacturers have designed
and tested different kinds of BTMS to solve this problem.
Energies 2020,13, 6257; doi:10.3390/en13236257 www.mdpi.com/journal/energies
Energies 2020,13, 6257 2 of 18
The temperature on the battery surface needs to be evaluated combined with the temperature
profile inside the battery. To obtain a more accurate simulation of BTMS, heat-transfer models of
outer cooling structures should be built. Combined with the electrochemical and thermal model,
many battery models serve to deduce the inner situation of battery cells [
10
–
14
]. Because it is not easy to
obtain the temperature inside the battery cells, the internal temperature could mostly be calculated by
measuring the temperature profile on the surface of batteries [
15
]. Mahamud and Park [
16
] developed a
spatial-resolution, lumped-capacitance thermal model which could quickly predict the cell temperature
under different working cycles.
The principal evaluations for each BTMS are based on the range of operating temperature and
device temperature uniformity. The ideal BTMS could maintain the battery temperature within an
appropriate scope as well as obtain uniform temperature to ensure the long-time safe and efficient
operation of the battery. Since temperature adjustment has hysteresis, the BTMS with fast response
speed is required to respond to the change of battery temperature quickly and control the temperature
within a reasonable range within a short time. In order to achieve optimal BTMS, more factors such
as system weight, volume, response speed and stability, etc. [
17
] should be taken into consideration
for the comprehensive evaluation. Performance evaluation index (PEI) and standard test conditions
are needed.
The temperature uniformity of battery cells is also sensitive to the discharge rate and boundary
condition [
18
]. As a consequence, BTMS needs to be tested and compared in different working
environments, especially in a harsh external environment and a heavy workload, such as deserts and
polar regions. It is necessary to design BTMS to achieve stable temperature distribution of battery
modules in various settings. One thorny issue of EVs is hotspots on the surface of battery cells, which is
dangerous, especially under extreme driving situations [
19
–
21
]. In the overcharging test, thermal
runaway may happen so that the temperature is out of the appropriate scope. Reducing pressure
inside the cell could prevent an accident to some extent [22].
The basic types of BTMS use air, liquid, heat pipe (HP) and phase change material (PCM) as
heat conduction fluids or structures to deliver the waste heat from the battery to the outer space.
Previous literature [
23
–
27
] has given a very detailed description and discussion of each basic BTMS,
including designing, performance, development trends and applications, etc. Although many review
articles include a discussion of hybrid BTMS, there are a lack of systematic and targeted discussions.
BTMS could be classified by different criteria [
25
]. Generally, BTMS could be divided into the active or
passive system by the use of extra energy source. For active BTMS, extra energy is consumed to power
fans or pumps, which commonly exists in the air [
28
] and liquid cooling systems [
29
]. For passive
BTMS, particular structures will be attached on the battery surface to achieve a higher heat transfer
capacity between the battery and the outer space, such as PCM [
30
] and heat pipe [
31
]. Active BTMS
shows a significant difference from passive BTMS, such as stability and complexity. Active BTMS has a
more powerful heat dissipation capacity by consuming more energy and adopting complex devices.
Passive BTMS could achieve some particular targets like uniform temperature (PCM-based BTMS)
and quick response (Heat pipe-based BTMS). However, if there is a greater heat load for the system,
active BTMS always acts as a preferred method. In many situations, several basic BTMSs should be
combined together to reach different goals simultaneously. Research increasingly focuses on hybrid
BTMS, and the integration method has been widely applied in many situations.
In this article, a new classification criterion is provided to analyze the existed BTMS systematically
(as shown in Figure 1). Besides basic BTMS, hybrid BTMS are divided into five groups. A common
characteristic is the combination of active and passive methods. Unlike basic BTMS, hybrid BTMS
emphasizes combination and integration. With higher requirements of BTMS, more attention and
systematic evaluation methods are needed for hybrid BTMS. Furthermore, BTMS is not separate from
vehicle thermal management. For further researches, the investigation of BTMS should take VTM into
consideration due to the interaction between them.
Energies 2020,13, 6257 3 of 18
Energies 2020, 13, x FOR PEER REVIEW 3 of 18
Figure 1. Classification of battery thermal management systems (BTMS).
2. Basic Battery Thermal Management System (BTMS)
A basic BTMS adopts a single type of BTMS individually so that the way to increase the single
BTMS performance becomes the primary issue. When adopting forced air or liquid as heat-transfer
fluid (HTF), flow channel design is an important part, including the channel shape, position of
channel inlet and outlet, channel parameter and flow direction. The work aims to obtain the
optimized parameters according to different operating environments and requirements. For
thermoelectric cooling (TEC), BTMS cannot be used individually without the assistance of other
basic BTMSs to cool down the hot side near the battery surface. Therefore, TEC is an auxiliary device
used in hybrid BTMS to improve the battery module surface’s local heat transfer capability. The
discussion about TEC is in the section on hybrid BTMS. Different kinds of basic BTMSs have
different characteristics, and they are applied to different situations and battery packs. More
importantly, the disadvantages of basic BTMS and corresponding solutions will be summarized.
2.1. Active BTMS
An active BTMS mainly includes forced-air cooling, liquid cooling and TEC. An active BTMS
needs to balance the benefit and cost of consuming extra energy. At relatively low temperature,
forced-air BTMS could satisfy heat dissipation requirements without complex devices and high
energy consumption compared with liquid-based BTMS. Under the condition of a high charge rate
or high heat generation, liquid cooling is necessary and energy-saving. The system needs to
minimize energy consumption while achieving necessary thermal management purposes.
2.1.1. Forced-Air Cooling
There are two kinds of air-based BTMS: one is based on natural convection of air, and the other
on air-forced convection. Due to the thermophysical properties of air (the low heat capacity and low
thermal conductivity), it is nearly impossible to use natural air to cool down the battery individually.
For the forced-air BTMS, a higher flow rate is needed to obtain a similar cooling performance of the
liquid-based BTMS. Due to low heat capacity, the forced-air cooling system’s temperature
distribution is more uneven, which is a vital problem that needs to be solved. Two factors contribute
to the uneven temperature distribution. On the one hand, the air temperature changes along the flow
channel. On the other hand, the gaps between cells have different distances to the inlet and outlet so
that the flow rate in different gaps varies. To solve this problem, symmetrical systems with uneven
cell-spacing distribution and tapered cooling ducts were used in air-based BTMS [32]. Figure 2a–f
shows “Z-type” and “U-type” flow channels, which have symmetrical modification and a tapered
ducts design. Figure 2c,f presents a more uniform temperature distribution and could achieve higher
performance with symmetrical designs. Except for the flow channel and cell arrangement, it is
practical to enhance the heat dissipation of battery cells where the heat dissipation condition is poor.
Figure 2g shows the design of adopting an extra duct to cool down the battery module center
directly. Figure 2h shows reciprocating air flow in two directions could make the temperature more
uniform. Both designs could be applied in poor local cooling environments.
Although air-based BTMS has many disadvantages compared with liquid-based BTMS,
forced-air BTMS is simple and has a low cost. Different research works focus on different aspects of
Figure 1. Classification of battery thermal management systems (BTMS).
2. Basic Battery Thermal Management System (BTMS)
A basic BTMS adopts a single type of BTMS individually so that the way to increase the single
BTMS performance becomes the primary issue. When adopting forced air or liquid as heat-transfer fluid
(HTF), flow channel design is an important part, including the channel shape, position of channel inlet
and outlet, channel parameter and flow direction. The work aims to obtain the optimized parameters
according to different operating environments and requirements. For thermoelectric cooling (TEC),
BTMS cannot be used individually without the assistance of other basic BTMSs to cool down the hot
side near the battery surface. Therefore, TEC is an auxiliary device used in hybrid BTMS to improve
the battery module surface’s local heat transfer capability. The discussion about TEC is in the section
on hybrid BTMS. Different kinds of basic BTMSs have different characteristics, and they are applied
to different situations and battery packs. More importantly, the disadvantages of basic BTMS and
corresponding solutions will be summarized.
2.1. Active BTMS
An active BTMS mainly includes forced-air cooling, liquid cooling and TEC. An active BTMS
needs to balance the benefit and cost of consuming extra energy. At relatively low temperature,
forced-air BTMS could satisfy heat dissipation requirements without complex devices and high energy
consumption compared with liquid-based BTMS. Under the condition of a high charge rate or high
heat generation, liquid cooling is necessary and energy-saving. The system needs to minimize energy
consumption while achieving necessary thermal management purposes.
2.1.1. Forced-Air Cooling
There are two kinds of air-based BTMS: one is based on natural convection of air, and the other
on air-forced convection. Due to the thermophysical properties of air (the low heat capacity and low
thermal conductivity), it is nearly impossible to use natural air to cool down the battery individually.
For the forced-air BTMS, a higher flow rate is needed to obtain a similar cooling performance of the
liquid-based BTMS. Due to low heat capacity, the forced-air cooling system’s temperature distribution
is more uneven, which is a vital problem that needs to be solved. Two factors contribute to the uneven
temperature distribution. On the one hand, the air temperature changes along the flow channel. On the
other hand, the gaps between cells have different distances to the inlet and outlet so that the flow rate in
different gaps varies. To solve this problem, symmetrical systems with uneven cell-spacing distribution
and tapered cooling ducts were used in air-based BTMS [
32
]. Figure 2a–f shows “Z-type” and “U-type”
flow channels, which have symmetrical modification and a tapered ducts design. Figure 2c,f presents
a more uniform temperature distribution and could achieve higher performance with symmetrical
designs. Except for the flow channel and cell arrangement, it is practical to enhance the heat dissipation
of battery cells where the heat dissipation condition is poor. Figure 2g shows the design of adopting an
extra duct to cool down the battery module center directly. Figure 2h shows reciprocating air flow in
two directions could make the temperature more uniform. Both designs could be applied in poor local
cooling environments.
Energies 2020,13, 6257 4 of 18
Although air-based BTMS has many disadvantages compared with liquid-based BTMS, forced-air
BTMS is simple and has a low cost. Different research works focus on different aspects of forced-air
BTMS. Generally, it can be concluded as the geometry of flow channels, cell arrangement and air flow
configuration [
33
]. These factors are interlinked so that researchers usually focus on two or more
factors together to investigate their influence on the performance of BTMS. By considering some factors
together with experimental or numerical methods, we can optimize the existing structures to enhance
cooling performance.
Energies 2020, 13, x FOR PEER REVIEW 4 of 18
forced-air BTMS. Generally, it can be concluded as the geometry of flow channels, cell arrangement
and air flow configuration [33]. These factors are interlinked so that researchers usually focus on two
or more factors together to investigate their influence on the performance of BTMS. By considering
some factors together with experimental or numerical methods, we can optimize the existing
structures to enhance cooling performance.
(a) (b) (c)
(d) (e) (f)
(g) (h)
Figure 2. Designs of the flow channel and flow pattern (a,b,d,e) “Z-type” flow channel,
Symmetrically modified “Z-type”, “U-type” flow channel, Symmetrically modified “U-type” [1], (c)
Tapered “Z-type” [2], (f) Tapered “U-type”, (g) Additional duct, Reprint with permission [34]; 2020,
Elsevier. (h) Reciprocating air flow in two directions. Reprint with permission [35]; 2020, Elsevier.
2.1.2. Liquid Cooling
More BTMSs are based on liquid due to its high heat-transfer efficiency compared with
forced-air cooling systems, which means liquid cooling systems consume much less energy than
forced-air cooling systems, especially under the high heat load of battery cells [36]. The liquid
cooling method also has some disadvantages, including complex devices, high cost and long startup
period. A00/A0 class electric vehicles usually adopt air-based BTMS due to high price sensitivity.
A-class EVs have higher requirements for endurance and adopt liquid-based BTMS. It is estimated
that the price of liquid-based BTMS is 40% more than that of air-based BTMS [37]. Active BTMS
based on the liquid can be categorized as the direct contact mode and the indirect contact mode [2].
The classification of liquid cooling and examples are shown in Figure 3.
Figure 2.
Designs of the flow channel and flow pattern (
a
,
b
,
d
,
e
) “Z-type” flow channel,
Symmetrically modified “Z-type”, “U-type” flow channel, Symmetrically modified “U-type” [
1
],
(
c
) Tapered “Z-type” [
2
], (
f
) Tapered “U-type”, (
g
) Additional duct, Reprint with permission [
34
];
2020, Elsevier. (
h
) Reciprocating air flow in two directions. Reprint with permission [
35
]; 2020, Elsevier.
2.1.2. Liquid Cooling
More BTMSs are based on liquid due to its high heat-transfer efficiency compared with forced-air
cooling systems, which means liquid cooling systems consume much less energy than forced-air
cooling systems, especially under the high heat load of battery cells [
36
]. The liquid cooling method
also has some disadvantages, including complex devices, high cost and long startup period. A00/A0
class electric vehicles usually adopt air-based BTMS due to high price sensitivity. A-class EVs have
higher requirements for endurance and adopt liquid-based BTMS. It is estimated that the price of
liquid-based BTMS is 40% more than that of air-based BTMS [
37
]. Active BTMS based on the liquid
can be categorized as the direct contact mode and the indirect contact mode [
2
]. The classification of
liquid cooling and examples are shown in Figure 3.
Energies 2020,13, 6257 5 of 18
•Direct contact mode
In this mode, the battery surface is always directly immersed in the liquid. It brings the significant
advantage of this mode—high heat-transfer efficiency. The convection always takes place on the surface
of batteries or most of it. Although the direct contact mode is not so practical, it can be used in extreme
situations like high charge rate and high-power Li-ion batteries. However, a significant disadvantage
of direct liquid cooling is that it is hard to integrate heating into the thermal management, which means
that if the ambient temperature is below 0 ◦C, other types of BTMS should be adopted [29].
The direct contact mode could be divided into the phase change and single phase. If the boiling
point of HTF is lower than the maximum temperature (
Tmax
) of batteries, the cooling process will be
accompanied by the phase change process. This is also called boiling cooling. If liquid cooling involves
the phase change process, the temperature rise near the boiling point would be significantly slowed
down due to the high latent heat. One practical way to utilize the phase change is to employ porous
materials like hydrogel [
38
] or film materials like thin sodium alginate film (SA-1 film) [
39
]. In this way,
a small amount of water can form a water film attached on the battery surface. If the direct contact
mode only involves a single phase, it is similar to forced-air cooling. The flow channel design like the
symmetrical design is key to achieve optimal heat transfer performance.
Energies 2020, 13, x FOR PEER REVIEW 5 of 18
• Direct contact mode
In this mode, the battery surface is always directly immersed in the liquid. It brings the
significant advantage of this mode—high heat-transfer efficiency. The convection always takes place
on the surface of batteries or most of it. Although the direct contact mode is not so practical, it can be
used in extreme situations like high charge rate and high-power Li-ion batteries. However, a
significant disadvantage of direct liquid cooling is that it is hard to integrate heating into the thermal
management, which means that if the ambient temperature is below 0 °C, other types of BTMS
should be adopted [29].
The direct contact mode could be divided into the phase change and single phase. If the boiling
point of HTF is lower than the maximum temperature (T) of batteries, the cooling process will be
accompanied by the phase change process. This is also called boiling cooling. If liquid cooling
involves the phase change process, the temperature rise near the boiling point would be significantly
slowed down due to the high latent heat. One practical way to utilize the phase change is to employ
porous materials like hydrogel [38] or film materials like thin sodium alginate film (SA-1 film) [39].
In this way, a small amount of water can form a water film attached on the battery surface. If the
direct contact mode only involves a single phase, it is similar to forced-air cooling. The flow channel
design like the symmetrical design is key to achieve optimal heat transfer performance.
• Indirect contact mode
Compared with the direct contact mode, the indirect contact mode is more practical and
commonly used in commercial EVs because of its safety and stability. The core concept is conducting
heat to the outer space by setting a plate exchanger or tube exchanger onto the battery cells’ surface.
Basically, a liquid cold plate (LCP) is suitable for prismatic cells or pouch cells due to its large contact
area and simple structure. The LCP or corrugated channel could be categorized as the surface
thermal contact mode, which means the flow channel is attached to the cells’ surface. In the other
mode, the flow channels go through a thermal conductive structure (TCS) that attaches to the battery
cells. Therefore, we can name this mode the channel thermal contact mode.
Figure 3. Classification of liquid cooling and corresponding classic examples [40–43].
Figure 3. Classification of liquid cooling and corresponding classic examples [40–43].
•Indirect contact mode
Compared with the direct contact mode, the indirect contact mode is more practical and commonly
used in commercial EVs because of its safety and stability. The core concept is conducting heat
to the outer space by setting a plate exchanger or tube exchanger onto the battery cells’ surface.
Basically, a liquid cold plate (LCP) is suitable for prismatic cells or pouch cells due to its large contact
area and simple structure. The LCP or corrugated channel could be categorized as the surface thermal
contact mode, which means the flow channel is attached to the cells’ surface. In the other mode,
the flow channels go through a thermal conductive structure (TCS) that attaches to the battery cells.
Therefore, we can name this mode the channel thermal contact mode.
Energies 2020,13, 6257 6 of 18
2.2. Passive BTMS
2.2.1. Phase Change Material (PCM)
PCM is a recurring composite of BTMS. This section only discusses the solid-liquid material,
which is used in BTMS. Sharma et al. [
44
] made a very detailed classification and investigation of PCMs.
There are many kinds of material that can be used in PCM-based BTMS, including organic materials
(paraffin wax, alkane and organic acid), inorganic materials (aqueous solution, salt hydrate and molten
salt) and eutectic [
30
]. It is not sufficient for pure PCM to transfer heat from the batteries to the outer
space due to its low heat conductivity. Many kinds of composite phase change materials (CPCM)
are designed for heat transfer enhancement to solve this problem. Usually, the thermal conductive
enhancement materials used in pure PCM are graphite [
45
], metal foam and carbon fiber [
46
]. Moreover,
the enhancement method like attaching fins on the surface of battery cells is also adopted in PCM-based
BTMS enhancing heat transfer due to the larger contact area.
Using PCM has many significant improvements for the overall performance of BTMS. One of the
improvements is to improve thermal uniformity due to its fluidity, which is similar to the direct liquid
cooling mode. Another distinctive advantage is the high efficiency of energy utilization because of
the latent heat of the phase change. PCM is also widely used in the pre-heating process for EVs to
save energy. BTMSs based on PCM are flexible because PCM’s melting point could be changed with
different components. By adjusting its melting point, the BTMS could work in different situations,
and its latent heat helps BTMS work in extreme cases longer.
PCM is considered a practical method to replace forced-air cooling and simplify the structure of
BTMS [
47
]. However, due to the low latent heat of phase change for many kinds of PCM, the heat
saturation always happens with long-time working under extreme situations of the BTMS. To solve
this problem, the PCM usually is coupled with active cooling strategies that can recover the PCM’s
thermal energy storage capacity. It is a common type of hybrid BTMS.
2.2.2. Heat Pipe (HP)
Compared with other BTMS, the heat pipe has many obvious advantages, including high
thermal conductivity, contact structures, flexible geometry [48], etc. The working principle of a HP is
simple—working medium evaporates at the heating side (heat source) and condenses at the cooling
side (heat sink). In the heat sink, the waste heat is transferred to the outer space. The shape of HP
has significant effects on the thermal performance of BTMS. In particular, a flat HP has good thermal
performance [
49
]. A microscale HP could be used for internal cooling, which is more efficient than
external cooling [50].
Compared with a traditional HP, pulsating (oscillating) heat pipe (PHP or OHP) has some excellent
characteristics, including the simple structure, low cost, small size, high heat flow density and flexibility.
After optimizing its structure and design parameters, its operating performance is basically not affected
by gravity. Another available method to improve performance is to improve heat transfer conditions
in the heat sink (condenser) by adopting forced-air cooling or liquid cooling. In this way, HP could
achieve accurate and rapid heat transfer between the area close to the battery surface and the outer
environment. It is also a common type of hybrid BTMS.
3. Hybrid BTMS
Hybrid BTMS means the combination of two or more basic BTMSs. Different basic BTMSs have
their advantages and disadvantages, respectively. The hybrid BTMS can combine these advantages
and reach higher thermal performance. However, hybrid BTMS may involve some problems with
volume, weight and energy consumption. The main types of hybrid BTMSs and their remarks are
listed in Table 1.
Energies 2020,13, 6257 7 of 18
Table 1. Main types of hybrid BTMS.
Type Hybrid BTMS Remark
HP coupled with air or liquid
active cooling
HP+Air 1. Active cooling methods enhance the heat transfer process in
passive cooling methods. Combining them is the basic idea of
hybrid BTMS.
2. The enhanced effect of liquid cooling is more robust than air, but
forced air cooling is enough if the heat load is low.
3. Adding PCM increases the thermal uniformity and overall
thermal performance due to the substantial latent heat. Active
cooling methods solve the heat saturation problem in
pure PCM-BTMS.
4. HP and TEC enhance the local heat transfer significantly quickly.
A combination of HP/TEC and PCM has better performance and
stability. HP can increase the performance by adjusting the shape
of its heat sink.
HP+Liquid
PCM coupled with HP
PCM+HP
PCM+HP+Air
PCM+HP+Liquid
PCM coupled with air or liquid
active cooling
PCM+Liquid
PCM+Air
TEC coupled with other BTMS TEC+Air+Liquid
PCM+TEC
Liquid coupled with air Liquid+Air
3.1. HP Coupled with Air- or Liquid-Cooling Method
Compared with HP-based BTMS in a passive cooling system, a hybrid BTMS obviously has better
thermal performance with extra power consumption and a complicated structure. In this system, HP
is always assisted by active cooling methods, including forced-air cooling (see Figure 4a) and liquid
cooling (see Figure 4b). The BTMS in Figure 4a adopts an ultra-thin micro heat pipe (UMHP) coupled
with a fan. Adding a UMHP can decrease
Tmax
by 7.1
◦
C from the beginning of discharging at a 2 C
rate compared with that without HP and
Tmax
can be kept below 40
◦
C with a fan speed of 4 m/s [
48
].
Liu et al.
[
51
] and Gan et al. [
52
] estimated and proved the necessity of employing cooling fluid in
specific working experiments, respectively (see as Figure 4c). Hybrid BTMSs in these works could
significantly reduce the temperature by about 14
◦
C in the 5C discharge rate compared with the natural
cooling method. Jouhara et al. [
53
] (see as Figure 4d) applied a flat heat pipe (heat mat) in BTMS
and used it to transfer the waste heat to an external liquid cooling medium. It was shown that the
Tmax
in the cell was kept below 28
◦
C. Wei et al. [
54
] (see as Figure 4e) developed a proof-of-concept
plug-in pulsating heat pipe (PHP) with a flat-plate evaporator and tube condenser and found that
PHP charged with ethanol-water mixtures had a quicker response and achieved superior thermal
performance. Under the condition that the power input is 56 W, the battery pack’s average temperature
can be kept below 46.5 ◦C.
Before using these systems, the enhancement of assisting active cooling methods should be
evaluated. Liang et al. [
31
] found that when the ambient temperature is under 35
◦
C, reducing coolant
temperature has a limited influence on the thermal performance, which means there is no need to use
hybrid cooling methods. Another point is that the low temperature of the coolant could be harmful
to the battery. Although decreasing the coolant temperature could lower
Tmax
, the inhomogeneity
of temperature distribution reaches the peak and will be higher with the lower coolant temperature.
The result is that the battery module’s available capacity and voltage decrease by nearly 1.17% and
0.88% when the coolant temperature is reduced by 10 ◦C at 5 C discharge [55].
Energies 2020,13, 6257 8 of 18
Energies 2020, 13, x FOR PEER REVIEW 8 of 18
(a) (b)
(c) (d) (e)
Figure 4. Examples for HP-based hybrid BTMS coupled with active cooling methods. (a) Hybrid
BTMS with an ultra-thin micro heat pipe, Reprint with permission [48]; 2020, Elsevier., (b) HP-based
BTMS associated with liquid cooling, Reprint with permission [56]; 2020, Elsevier., (c) A HP-based
BTMS with cylindrical battery cells, Reprint with permission [52]; 2020, Elsevier., (d) Battery module
is set above the heat mat [3], (e) Plug-in oscillating heat pipes (OHPs) are sandwiched by battery
packs. Reprint with permission [54]; 2020, Elsevier.
3.2. PCM Coupled with Air or Liquid Active Cooling Method
Pure passive BTMS based on PCM or CPCM is always not enough to maintain the temperature
of the battery pack in an appropriate range because of heat accumulation caused by the inefficient
cooling of natural air cooling. In this way, active cooling strategies play an essential role in
recovering the thermal energy storage capacity of PCMs. The BTMS in Figure 5a adopts CPCM
(copper mesh and enhanced paraffin/expanded graphite) and copper fins exposed from the CPCM
to enhance the heat transfer [57]. The BTMS in Figure 5b adopts cooling water pipes and PCM [58].
The structures of this kind of hybrid BTMSs are like the indirect contact mode in the liquid-based
BTMS.
(a) (b)
Figure 5. Examples for PCM-based hybrid BTMS (a) PCM+Air. Reprint with permission [57]; 2020,
Elsevier., (b) PCM+Liquid. Reprint with permission [58]; 2020, Elsevier.
Cooling liquid pipes and LCP are the most common types, while PCM is filled between the
pipe/LCP and battery cells. A common combination of PCM and liquid cooling adopts the
mini-channel through the PCM in which HTF transfers heat with PCM. Rao et al. [59] investigated
the effects of fluid flow rate, the number of channels, melting point and thermal conductivity of
PCM on mini-channel/PCM-based BTMS. Bai et al. [60] developed a BTMS (see as Figure 6a). They
found that PCM composed of 20% n-octadecane microcapsules and 80% water has superior
Figure 4.
Examples for HP-based hybrid BTMS coupled with active cooling methods. (
a
) Hybrid
BTMS with an ultra-thin micro heat pipe, Reprint with permission [
48
]; 2020, Elsevier., (
b
) HP-based
BTMS associated with liquid cooling, Reprint with permission [
56
]; 2020, Elsevier., (
c
) A HP-based
BTMS with cylindrical battery cells, Reprint with permission [
52
]; 2020, Elsevier., (
d
) Battery module is
set above the heat mat [
3
], (
e
) Plug-in oscillating heat pipes (OHPs) are sandwiched by battery packs.
Reprint with permission [54]; 2020, Elsevier.
3.2. PCM Coupled with Air or Liquid Active Cooling Method
Pure passive BTMS based on PCM or CPCM is always not enough to maintain the temperature
of the battery pack in an appropriate range because of heat accumulation caused by the inefficient
cooling of natural air cooling. In this way, active cooling strategies play an essential role in recovering
the thermal energy storage capacity of PCMs. The BTMS in Figure 5a adopts CPCM (copper mesh and
enhanced paraffin/expanded graphite) and copper fins exposed from the CPCM to enhance the heat
transfer [
57
]. The BTMS in Figure 5b adopts cooling water pipes and PCM [
58
]. The structures of this
kind of hybrid BTMSs are like the indirect contact mode in the liquid-based BTMS.
Energies 2020, 13, x FOR PEER REVIEW 8 of 18
(a) (b)
(c) (d) (e)
Figure 4. Examples for HP-based hybrid BTMS coupled with active cooling methods. (a) Hybrid
BTMS with an ultra-thin micro heat pipe, Reprint with permission [48]; 2020, Elsevier., (b) HP-based
BTMS associated with liquid cooling, Reprint with permission [56]; 2020, Elsevier., (c) A HP-based
BTMS with cylindrical battery cells, Reprint with permission [52]; 2020, Elsevier., (d) Battery module
is set above the heat mat [3], (e) Plug-in oscillating heat pipes (OHPs) are sandwiched by battery
packs. Reprint with permission [54]; 2020, Elsevier.
3.2. PCM Coupled with Air or Liquid Active Cooling Method
Pure passive BTMS based on PCM or CPCM is always not enough to maintain the temperature
of the battery pack in an appropriate range because of heat accumulation caused by the inefficient
cooling of natural air cooling. In this way, active cooling strategies play an essential role in
recovering the thermal energy storage capacity of PCMs. The BTMS in Figure 5a adopts CPCM
(copper mesh and enhanced paraffin/expanded graphite) and copper fins exposed from the CPCM
to enhance the heat transfer [57]. The BTMS in Figure 5b adopts cooling water pipes and PCM [58].
The structures of this kind of hybrid BTMSs are like the indirect contact mode in the liquid-based
BTMS.
(a) (b)
Figure 5. Examples for PCM-based hybrid BTMS (a) PCM+Air. Reprint with permission [57]; 2020,
Elsevier., (b) PCM+Liquid. Reprint with permission [58]; 2020, Elsevier.
Cooling liquid pipes and LCP are the most common types, while PCM is filled between the
pipe/LCP and battery cells. A common combination of PCM and liquid cooling adopts the
mini-channel through the PCM in which HTF transfers heat with PCM. Rao et al. [59] investigated
the effects of fluid flow rate, the number of channels, melting point and thermal conductivity of
PCM on mini-channel/PCM-based BTMS. Bai et al. [60] developed a BTMS (see as Figure 6a). They
found that PCM composed of 20% n-octadecane microcapsules and 80% water has superior
Figure 5.
Examples for PCM-based hybrid BTMS (
a
) PCM+Air. Reprint with permission [
57
]; 2020,
Elsevier., (b) PCM+Liquid. Reprint with permission [58]; 2020, Elsevier.
Cooling liquid pipes and LCP are the most common types, while PCM is filled between the
pipe/LCP and battery cells. A common combination of PCM and liquid cooling adopts the mini-channel
through the PCM in which HTF transfers heat with PCM. Rao et al. [
59
] investigated the effects
of fluid flow rate, the number of channels, melting point and thermal conductivity of PCM on
mini-channel/PCM-based BTMS. Bai et al. [
60
] developed a BTMS (see as Figure 6a). They found
that PCM composed of 20% n-octadecane microcapsules and 80% water has superior performance,
Energies 2020,13, 6257 9 of 18
especially in the high target temperature when the mass flow rate does not exceed the threshold level.
On the other hand, like the surface thermal contact mode in liquid cooling, the flow channels can also
be attached to the PCM plate [
61
]. The cells, PCM and cooling plates are set in alignment [
62
] under
compression. Bai et al. [
63
] designed a BTMS (see as Figure 6b), and the effects of different parameters
were investigated. The LCP near the near-electrode area of the battery dissipates the majority of the
heat generated by battery cells. At the same time, PCM increased thermal uniformity considerably.
Energies 2020, 13, x FOR PEER REVIEW 9 of 18
performance, especially in the high target temperature when the mass flow rate does not exceed the
threshold level. On the other hand, like the surface thermal contact mode in liquid cooling, the flow
channels can also be attached to the PCM plate [61]. The cells, PCM and cooling plates are set in
alignment [62] under compression. Bai et al. [63] designed a BTMS (see as Figure 6b), and the effects
of different parameters were investigated. The LCP near the near-electrode area of the battery
dissipates the majority of the heat generated by battery cells. At the same time, PCM increased
thermal uniformity considerably.
There are some novel designs to increase the performance of hybrid BTMS adopting forced-air
cooling coupled with PCM. Shi et al. [64] (see as Figure 6c) developed a BTMS and built an unsteady
mathematical model. This BTMS is proved to have the ability to hold the battery temperature in an
appropriate scope before the PCM completely melts. Jiang et al. [65] (see as Figure 6d) designed a
BTMS adopting baffles to change the airflow direction in order to enhance its heat transfer. Situ et al.
[66] (see as Figure 6e) developed a novel quaternary PCM plate consisting of paraffin, expanded
graphite, low-density polyethylene and double copper mesh (DCM). Lazrak et al. [67] (see as Figure
6f) developed a novel BTMS based on PCM, and adopted a new way to enhance heat dissipation in
the PCM by using copper grids. The two AI plates are on the two sides of the PCM and the battery
cells. A fan was used to ventilate.
The geometry of heating has a remarkable influence on thermal performance. Typical shapes of
battery cells are cylindrical, prismatic and pouch. If PCM is adopted in BTM, there is a need to
design the shape of PCM to cover the battery cells, which achieves better thermal performance and
less energy consumption. Safdari et al. [68] studied the effects of different shapes of container
cross-sections, including circular, rectangular and hexagonal, on the thermal performance (see as
Figure 7a). The conclusion is that a circular PCM container achieves superior performance with high
latent heat, and rectangular PCM configuration is the most efficient due to its uniform air channel.
Qin et al. [69] put forward a novel hybrid BTMS using forced air and PCM (see as Figure 7b).
Compared with the passive BTMS, the maximum ∆T reduces by 1.2 °C and T drops by 16 °C in
the hybrid BTMS under a 3 C rate, respectively.
(a) (b)
(c) (d)
Energies 2020, 13, x FOR PEER REVIEW 10 of 18
(e) (f)
Figure 6. Classic structures of hybrid BTMS combining PCM with air or liquid active cooling
method. (a) Combination of phase change slurry and mini channel liquid cold plate (LCP). Reprint
with permission [60]; 2020, Elsevier. (b) Combination of PCM with LCP. Reprint with permission
[63]; 2020, Elsevier. (c) PCM-based BTMS assisted with air cooling. The phase change storage energy
unit (PCSEU) is made up of a copper foam and n-Eicosane. Reprint with permission [64]; 2020,
Elsevier. (d) Integration of CPCM consisted of expanded graphite and paraffin and forced air cooling
Reprint with permission [65]; 2020, Elsevier. (e) BTMS with a novel quaternary PCM and DCM
Reprint with permission [66]; 2020, Elsevier.. (f) PCM-based BTMS with copper grids. Reprint with
permission [67]; 2020, Elsevier.
(a) (b)
Figure 7. The geometry in PCM-based hybrid BTMS (a) Three different shapes of the cross section for
PCM-based BTMS Reprint with permission [68]; 2020, Elsevier.; (b) Schematic of a PCM-based
BTMS. Reprint with permission [69]; 2020, Elsevier.
The system complexity is a problem for hybrid BTMS, but hybrid BTMS can cut down the
weight and increase the efficiency of pure passive BTMS. Compared with basic active BTMS, PCM
adds extra weight to the whole structure. Considering the heavier weight and larger volume of
hybrid BTMS, Ling et al. [70] analyzed the influence of the composition of PCM, the set of the battery
module, and the active cooling configuration on the heat-transfer capacity to minimize the mass of
PCM used in BTMS. They found that the optimized design of this hybrid BTMS helps to save up to
94.1% in mass and 55.6% in volume of PCM.
3.3. PCM Coupled with HP and Active Cooling Methods
PCM is easy to integrate into hybrid BTMS, and the adoption of PCM increases the thermal
uniformity. Similar to the mode “PCM plus active cooling”, HP is adopted to solve the heat
saturation in PCM due to its high efficiency and quick response. PCM could be filled between HP
and HTF [71] (see as Figure 8a) or battery cells. Lei et al. [72] combined PCM, HP and spray cooling
to manage the battery pack temperature (see as Figure 8b). This BTMS controls the temperature rise
of the battery surface by less than 8 °C, even in 24 A discharge current and a high ambient
temperature (40 °C). Amin et al. [73] design a BTMS (see as Figure 8c) that can maintain the battery
temperature below 50 °C at the maximum heat load of 50 W. Huang et al. [74] designed a BTMS (see
as Figure 8d), in which HP makes a huge contribution to heat transfer and thermal uniformity. Wu et
Figure 6.
Classic structures of hybrid BTMS combining PCM with air or liquid active cooling method.
(
a
) Combination of phase change slurry and mini channel liquid cold plate (LCP). Reprint with
permission [
60
]; 2020, Elsevier. (
b
) Combination of PCM with LCP. Reprint with permission [
63
];
2020, Elsevier. (
c
) PCM-based BTMS assisted with air cooling. The phase change storage energy unit
(PCSEU) is made up of a copper foam and n-Eicosane. Reprint with permission [
64
]; 2020, Elsevier.
(
d
) Integration of CPCM consisted of expanded graphite and paraffin and forced air cooling Reprint
with permission [
65
]; 2020, Elsevier. (
e
) BTMS with a novel quaternary PCM and DCM Reprint with
permission [
66
]; 2020, Elsevier.. (
f
) PCM-based BTMS with copper grids. Reprint with permission [
67
];
2020, Elsevier.
There are some novel designs to increase the performance of hybrid BTMS adopting forced-air
cooling coupled with PCM. Shi et al. [
64
] (see as Figure 6c) developed a BTMS and built an unsteady
mathematical model. This BTMS is proved to have the ability to hold the battery temperature in an
appropriate scope before the PCM completely melts. Jiang et al. [
65
] (see as Figure 6d) designed a BTMS
Energies 2020,13, 6257 10 of 18
adopting baffles to change the airflow direction in order to enhance its heat transfer.
Situ et al.
[
66
]
(see as Figure 6e) developed a novel quaternary PCM plate consisting of paraffin, expanded graphite,
low-density polyethylene and double copper mesh (DCM). Lazrak et al. [
67
] (see as Figure 6f) developed
a novel BTMS based on PCM, and adopted a new way to enhance heat dissipation in the PCM by using
copper grids. The two AI plates are on the two sides of the PCM and the battery cells. A fan was used
to ventilate.
The geometry of heating has a remarkable influence on thermal performance. Typical shapes
of battery cells are cylindrical, prismatic and pouch. If PCM is adopted in BTM, there is a need
to design the shape of PCM to cover the battery cells, which achieves better thermal performance
and less energy consumption. Safdari et al. [
68
] studied the effects of different shapes of container
cross-sections, including circular, rectangular and hexagonal, on the thermal performance (see as
Figure 7a). The conclusion is that a circular PCM container achieves superior performance with
high latent heat, and rectangular PCM configuration is the most efficient due to its uniform air
channel.
Qin et al.
[
69
] put forward a novel hybrid BTMS using forced air and PCM (see as Figure 7b).
Compared with the passive BTMS, the maximum
∆
T reduces by 1.2
◦
C and
Tmax
drops by 16
◦
C in the
hybrid BTMS under a 3 C rate, respectively.
Energies 2020, 13, x FOR PEER REVIEW 10 of 18
(e) (f)
Figure 6. Classic structures of hybrid BTMS combining PCM with air or liquid active cooling
method. (a) Combination of phase change slurry and mini channel liquid cold plate (LCP). Reprint
with permission [60]; 2020, Elsevier. (b) Combination of PCM with LCP. Reprint with permission
[63]; 2020, Elsevier. (c) PCM-based BTMS assisted with air cooling. The phase change storage energy
unit (PCSEU) is made up of a copper foam and n-Eicosane. Reprint with permission [64]; 2020,
Elsevier. (d) Integration of CPCM consisted of expanded graphite and paraffin and forced air cooling
Reprint with permission [65]; 2020, Elsevier. (e) BTMS with a novel quaternary PCM and DCM
Reprint with permission [66]; 2020, Elsevier.. (f) PCM-based BTMS with copper grids. Reprint with
permission [67]; 2020, Elsevier.
(a) (b)
Figure 7. The geometry in PCM-based hybrid BTMS (a) Three different shapes of the cross section for
PCM-based BTMS Reprint with permission [68]; 2020, Elsevier.; (b) Schematic of a PCM-based
BTMS. Reprint with permission [69]; 2020, Elsevier.
The system complexity is a problem for hybrid BTMS, but hybrid BTMS can cut down the
weight and increase the efficiency of pure passive BTMS. Compared with basic active BTMS, PCM
adds extra weight to the whole structure. Considering the heavier weight and larger volume of
hybrid BTMS, Ling et al. [70] analyzed the influence of the composition of PCM, the set of the battery
module, and the active cooling configuration on the heat-transfer capacity to minimize the mass of
PCM used in BTMS. They found that the optimized design of this hybrid BTMS helps to save up to
94.1% in mass and 55.6% in volume of PCM.
3.3. PCM Coupled with HP and Active Cooling Methods
PCM is easy to integrate into hybrid BTMS, and the adoption of PCM increases the thermal
uniformity. Similar to the mode “PCM plus active cooling”, HP is adopted to solve the heat
saturation in PCM due to its high efficiency and quick response. PCM could be filled between HP
and HTF [71] (see as Figure 8a) or battery cells. Lei et al. [72] combined PCM, HP and spray cooling
to manage the battery pack temperature (see as Figure 8b). This BTMS controls the temperature rise
of the battery surface by less than 8 °C, even in 24 A discharge current and a high ambient
temperature (40 °C). Amin et al. [73] design a BTMS (see as Figure 8c) that can maintain the battery
temperature below 50 °C at the maximum heat load of 50 W. Huang et al. [74] designed a BTMS (see
as Figure 8d), in which HP makes a huge contribution to heat transfer and thermal uniformity. Wu et
Figure 7.
The geometry in PCM-based hybrid BTMS (
a
) Three different shapes of the cross section for
PCM-based BTMS Reprint with permission [68]; 2020, Elsevier.; (b) Schematic of a PCM-based BTMS.
Reprint with permission [69]; 2020, Elsevier.
The system complexity is a problem for hybrid BTMS, but hybrid BTMS can cut down the weight
and increase the efficiency of pure passive BTMS. Compared with basic active BTMS, PCM adds extra
weight to the whole structure. Considering the heavier weight and larger volume of hybrid BTMS,
Ling et al. [
70
] analyzed the influence of the composition of PCM, the set of the battery module, and the
active cooling configuration on the heat-transfer capacity to minimize the mass of PCM used in BTMS.
They found that the optimized design of this hybrid BTMS helps to save up to 94.1% in mass and 55.6%
in volume of PCM.
3.3. PCM Coupled with HP and Active Cooling Methods
PCM is easy to integrate into hybrid BTMS, and the adoption of PCM increases the thermal
uniformity. Similar to the mode “PCM plus active cooling”, HP is adopted to solve the heat saturation
in PCM due to its high efficiency and quick response. PCM could be filled between HP and HTF [
71
]
(see as Figure 8a) or battery cells. Lei et al. [
72
] combined PCM, HP and spray cooling to manage the
battery pack temperature (see as Figure 8b). This BTMS controls the temperature rise of the battery
surface by less than 8
◦
C, even in 24 A discharge current and a high ambient temperature (40
◦
C).
Amin et al.
[
73
] design a BTMS (see as Figure 8c) that can maintain the battery temperature below 50
◦
C
at the maximum heat load of 50 W. Huang et al. [
74
] designed a BTMS (see as Figure 8d), in which HP
makes a huge contribution to heat transfer and thermal uniformity. Wu et al. [
75
] designed a HP-based
Energies 2020,13, 6257 11 of 18
BTMS assisted by PCM and forced-air cooling (see as Figure 8e). Even under the highest discharge rate
(5 C), Tmax of the battery pack could be controlled below 50 ◦C. For this combination, the necessity of
adopting HP in this kind of BTMS should be discussed. The result shows that when the discharge rate
is low, air velocity doesn’t have a strong connection to the thermal performance due to the PCM.
Energies 2020, 13, x FOR PEER REVIEW 11 of 18
al. [75] designed a HP-based BTMS assisted by PCM and forced-air cooling (see as Figure 8e). Even
under the highest discharge rate (5 C),
of the battery pack could be controlled below 50 °C. For
this combination, the necessity of adopting HP in this kind of BTMS should be discussed. The result
shows that when the discharge rate is low, air velocity doesn’t have a strong connection to the
thermal performance due to the PCM.
(a) (b)
(c) (d)
(e)
Figure 8. Classic examples of hybrid BTMS associated with PCM and HP (a) HP-assisted PCM based
BTMS.
Reprint with permission [71]; 2020, Elsevier.
(b) HP-assisted PCM associated with spray
cooling. Reprint with permission [72]; 2020, Elsevier. (c) Adopting ‘L’ type of HP and beeswax PCM
[73]. (d) Combination of PCM and HP assisted by air/liquid cooling.
Reprint with permission [74];
2020, Elsevier.
(e) Each HP was set between two PCM plates and embedded in the surface of PCM
plates.
Reprint with permission [75]; 2020, Elsevier.
3.4. Thermoelectric Cooling Coupled with Other Basic BTMS
Thermoelectric cooling (TEC) does not have a wide application on BTM of EV due to its low
efficiency. However, it has been widely used in cooling electronics for its compact structure.
Researchers add TEC in hybrid BTMS to enhance the heat transfer or achieve some special purposes.
Figure 9 shows a typical structure of TEC in hybrid BTMS. Lyu et al. [76] combined TEC with active
cooling methods. TEC transfers the heat from the condenser side and forced air assists TEC to move
the heat to the outer space. The result shows that the temperature of the battery surface battery
drops to 12 °C from 55 °C. Song et al. [77] designed a BTMS for standby batteries combining
semiconductor thermoelectric device and PCMs (see as Figure 10) and tested the cooling time (14 h)
and heat preservation (4.15 days) time in a circular way under the ambient temperature (323 K).
Figure 8. Classic examples of hybrid BTMS associated with PCM and HP (a) HP-assisted PCM based
BTMS. Reprint with permission [
71
]; 2020, Elsevier. (
b
) HP-assisted PCM associated with spray
cooling. Reprint with permission [
72
]; 2020, Elsevier. (
c
) Adopting ‘L’ type of HP and beeswax
PCM [
73
]. (
d
) Combination of PCM and HP assisted by air/liquid cooling. Reprint with permission [
74
];
2020, Elsevier. (
e
) Each HP was set between two PCM plates and embedded in the surface of PCM
plates. Reprint with permission [75]; 2020, Elsevier.
3.4. Thermoelectric Cooling Coupled with Other Basic BTMS
Thermoelectric cooling (TEC) does not have a wide application on BTM of EV due to its low
efficiency. However, it has been widely used in cooling electronics for its compact structure. Researchers
add TEC in hybrid BTMS to enhance the heat transfer or achieve some special purposes. Figure 9shows
a typical structure of TEC in hybrid BTMS. Lyu et al. [
76
] combined TEC with active cooling methods.
TEC transfers the heat from the condenser side and forced air assists TEC to move the heat to the outer
space. The result shows that the temperature of the battery surface battery drops to 12
◦
C from 55
◦
C.
Song et al. [
77
] designed a BTMS for standby batteries combining semiconductor thermoelectric device
Energies 2020,13, 6257 12 of 18
and PCMs (see as Figure 10) and tested the cooling time (14 h) and heat preservation (4.15 days) time
in a circular way under the ambient temperature (323 K).
Energies 2020, 13, x FOR PEER REVIEW 12 of 18
Figure 9. The schematic diagram of TEC in hybrid BTMS [78].
Figure 10. A classic example of hybrid BTMS of PCM coupled with TEC. Reprint with permission
[77]; 2020, Elsevier.
3.5. Liquid Coupled with Air
It is important to note that for nearly all liquid cooling methods, the hot liquid needs to be
cooled down in a heat exchanger assisted by forced air. Cooling methods in this paper belong to
hybrid BTMS because the forced air cooling is not applied to the battery pack directly. It is not
necessary to employ both air path and HTF path for their different heat dissipation capacity and
devices for most cases. There is no doubt that adding forced air into liquid-based BTMS can enhance
the heat transfer in areas distant from LCP or other HTF channels.
Few researchers use forced-air cooling and liquid cooling simultaneously. A practical way is to
combine LCP with forced-air cooling. Wang et al. [79] designed a BTMS under the space
environment combining the gas circle and a LCP (see as Figure 11). They investigated the effects of
different assembly structures, the intensity of the gas and liquid cycles on the thermal performance
of BTMS. It was found that the structure with a fan under the LCP could make the flow field fully
developed. Compared with pure LCP cooling in the vacuum packaged battery, the general ∆T and
T are reduced by 3.45 K and 3.88 K under the condition that the total heat generation is 576 W.
The direct contact mode based on liquid cooling is hard to integrate with forced-air cooling.
Figure 9. The schematic diagram of TEC in hybrid BTMS [78].
Energies 2020, 13, x FOR PEER REVIEW 12 of 18
Figure 9. The schematic diagram of TEC in hybrid BTMS [78].
Figure 10. A classic example of hybrid BTMS of PCM coupled with TEC. Reprint with permission
[77]; 2020, Elsevier.
3.5. Liquid Coupled with Air
It is important to note that for nearly all liquid cooling methods, the hot liquid needs to be
cooled down in a heat exchanger assisted by forced air. Cooling methods in this paper belong to
hybrid BTMS because the forced air cooling is not applied to the battery pack directly. It is not
necessary to employ both air path and HTF path for their different heat dissipation capacity and
devices for most cases. There is no doubt that adding forced air into liquid-based BTMS can enhance
the heat transfer in areas distant from LCP or other HTF channels.
Few researchers use forced-air cooling and liquid cooling simultaneously. A practical way is to
combine LCP with forced-air cooling. Wang et al. [79] designed a BTMS under the space
environment combining the gas circle and a LCP (see as Figure 11). They investigated the effects of
different assembly structures, the intensity of the gas and liquid cycles on the thermal performance
of BTMS. It was found that the structure with a fan under the LCP could make the flow field fully
developed. Compared with pure LCP cooling in the vacuum packaged battery, the general ∆T and
T are reduced by 3.45 K and 3.88 K under the condition that the total heat generation is 576 W.
The direct contact mode based on liquid cooling is hard to integrate with forced-air cooling.
Figure 10.
A classic example of hybrid BTMS of PCM coupled with TEC. Reprint with permission [
77
];
2020, Elsevier.
3.5. Liquid Coupled with Air
It is important to note that for nearly all liquid cooling methods, the hot liquid needs to be cooled
down in a heat exchanger assisted by forced air. Cooling methods in this paper belong to hybrid BTMS
because the forced air cooling is not applied to the battery pack directly. It is not necessary to employ
both air path and HTF path for their different heat dissipation capacity and devices for most cases.
There is no doubt that adding forced air into liquid-based BTMS can enhance the heat transfer in areas
distant from LCP or other HTF channels.
Few researchers use forced-air cooling and liquid cooling simultaneously. A practical way is to
combine LCP with forced-air cooling. Wang et al. [
79
] designed a BTMS under the space environment
combining the gas circle and a LCP (see as Figure 11). They investigated the effects of different assembly
structures, the intensity of the gas and liquid cycles on the thermal performance of BTMS. It was found
that the structure with a fan under the LCP could make the flow field fully developed. Compared with
pure LCP cooling in the vacuum packaged battery, the general
∆T
and
Tmax
are reduced by 3.45 K and
Energies 2020,13, 6257 13 of 18
3.88 K under the condition that the total heat generation is 576 W. The direct contact mode based on
liquid cooling is hard to integrate with forced-air cooling.
Energies 2020, 13, x FOR PEER REVIEW 13 of 18
Figure 11. A classic example of BTMS based on liquid coupled with air. Reprint with permission [79];
2020, Elsevier.
4. Discussion
This paper puts forward a detailed classification rule to cover all existing BTMS and makes a
clear division of active cooling methods based on liquids (direct or indirect, single phase or phase
change, surface contact or channel contact). With regard to hybrid BTMS, these hybrid methods are
divided into five groups and nine child items. The application and characteristics of every group are
stated. There are some meaningful discussions about hybrid BTMS including necessity, design
method and evaluation criterion.
Hybrid BTMS is the main trend in the development of BTMS and will apply to more
applications, especially for extreme working situations. Hybrid BTMS also has some basic couples of
single BTMS. And basically, we combine passive BTMS with active BTMS, which is shown to have
great potential and practical use. However, not every situation is suitable for hybrid BTMS. Firstly,
we need to balance cost and performance. According to the requirements of BTM, passive BTMS
corresponds to low requirement (low heat load or short run time), while active BTMS satisfies higher
requirements. According to the cost sensitivity, forced-air BTMS has a low cost while liquid cooling
has a high consumption. For the combination of PCM and HP, if the ambient temperature is not very
high, natural convection is enough, and forced air cooling is unnecessary. We need to consider
natural air, forced air and liquid as a consequence and figure out the application condition of every
method. Hybrid BTMS has more flexibility. If the battery output is required for high stability, PCM
should be integrated to increase the thermal uniformity. TEC and HP could improve the local area’s
heat transfer pertinently and have a fast response velocity. They can operate as an auxiliary device
during a fast startup step.
Hybrid BTMS can overcome some disadvantages of basic BTMS, such as the heat saturation of
PCM-based BTMS. Simultaneously, they are flexible and efficient so that the mass of HTF or PCM
can be reduced in basic BTMS. However, there are some new problems like complexity and large
energy consumption. We must estimate the cost of hybrid BTMS and then decide whether to adopt
more complicated structures. It is suggested that we evaluate BTMSs with different indexes and give
comprehensive evaluation scores. The BTMS is tested in different ways (discharging rates, test time
and charging or cycling), and the evaluation indexes (T or ∆T) are often different, which is not
conducive to obtaining valid results through comparison. A dimensionless number is proposed to
evaluate the performance of different BTMSs.
Figure 11.
A classic example of BTMS based on liquid coupled with air. Reprint with permission [
79
];
2020, Elsevier.
4. Discussion
This paper puts forward a detailed classification rule to cover all existing BTMS and makes a clear
division of active cooling methods based on liquids (direct or indirect, single phase or phase change,
surface contact or channel contact). With regard to hybrid BTMS, these hybrid methods are divided
into five groups and nine child items. The application and characteristics of every group are stated.
There are some meaningful discussions about hybrid BTMS including necessity, design method and
evaluation criterion.
Hybrid BTMS is the main trend in the development of BTMS and will apply to more applications,
especially for extreme working situations. Hybrid BTMS also has some basic couples of single BTMS.
And basically, we combine passive BTMS with active BTMS, which is shown to have great potential
and practical use. However, not every situation is suitable for hybrid BTMS. Firstly, we need to
balance cost and performance. According to the requirements of BTM, passive BTMS corresponds to
low requirement (low heat load or short run time), while active BTMS satisfies higher requirements.
According to the cost sensitivity, forced-air BTMS has a low cost while liquid cooling has a high
consumption. For the combination of PCM and HP, if the ambient temperature is not very high,
natural convection is enough, and forced air cooling is unnecessary. We need to consider natural
air, forced air and liquid as a consequence and figure out the application condition of every method.
Hybrid BTMS has more flexibility. If the battery output is required for high stability, PCM should be
integrated to increase the thermal uniformity. TEC and HP could improve the local area’s heat transfer
pertinently and have a fast response velocity. They can operate as an auxiliary device during a fast
startup step.
Hybrid BTMS can overcome some disadvantages of basic BTMS, such as the heat saturation of
PCM-based BTMS. Simultaneously, they are flexible and efficient so that the mass of HTF or PCM
can be reduced in basic BTMS. However, there are some new problems like complexity and large
energy consumption. We must estimate the cost of hybrid BTMS and then decide whether to adopt
more complicated structures. It is suggested that we evaluate BTMSs with different indexes and give
comprehensive evaluation scores. The BTMS is tested in different ways (discharging rates, test time
Energies 2020,13, 6257 14 of 18
and charging or cycling), and the evaluation indexes (
Tmax
or
∆T
) are often different, which is not
conducive to obtaining valid results through comparison. A dimensionless number is proposed to
evaluate the performance of different BTMSs.
η=Actual heat accumulation
Theoretical heat output =
∆Tb·Cb·Mb+∆To·Co·Mo
I2α−∆P·Qvt(1)
here,
η
,
∆T
,
C
and M are PEI, temperature change, heat capacity and mass. The subscripts “b” and “o”
correspond to the battery and its external structure, including PCM.
I
is the charge/discharge current,
proportional to the charge/discharge rate.
α
represents the heating coefficient, and heat generation is
assumed to be proportional to the second power of the current.
∆P
is HTF-side pressure drop and
Qv
is the volume flow of HTF. t is testing time.
This PEI avoids a complex electrochemical model and takes power consumption and outer
structures into consideration. For the accuracy of the evaluation, standard test conditions are needed,
including standard charge/discharge rate and testing time. Under the same condition, we can compare
BTMS from different research. Hybrid BTMS will occupy a larger proportion in the future BTMS,
and has a broader range of applications other than EVs.
Author Contributions:
C.Z. and B.Z. finished the main content of this paper; S.H. and Y.Z. drawed the figures;
T.Y. designed the chart and table; X.L. guided the paper writing and proofreaded it. All authors contributed
equally in the writing and revision of this paper. All authors have read and agreed to the published version of
the manuscript.
Funding:
This research was funded by the Youth Innovation Team of Shaanxi Universities and the Innovation
and entrepreneurship project of Xi’an Jiaotong University (SJ202010698127).
Acknowledgments:
This work was supported by the Faculty of School of Energy and Power in Xi’an Jiaotong
University, Xi’an, China.
Conflicts of Interest: The authors declare no conflict of interest.
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