Thermal optimization of composite PCM based large-format lithium-ion battery modules under extreme operating conditions

Thermal optimization of composite PCM based large-format lithium-ion battery modules under extreme operating conditions

Energy Conversion and Management 153 (2017) 22–33 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.e...

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Energy Conversion and Management 153 (2017) 22–33

Contents lists available at ScienceDirect

Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Thermal optimization of composite PCM based large-format lithium-ion battery modules under extreme operating conditions Weixiong Wu, Wei Wu, Shuangfeng Wang

MARK



Key Laboratory of Enhanced Heat Transfer and Energy Conservation of the Ministry of Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, Guangdong, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Battery thermal management Phase change material Thermal optimization High-rate discharge Dynamic cycle Thermal runaway

Thermal management is a crucial strategy that needs to be carefully considered for lithium-ion batteries under extreme operating conditions. One promising approach is the use of phase change material (PCM), which can bring benefits such as passively thermal buffering and extending lifespan. In this paper, a paraffin/expanded graphite (EG) composites based battery module (PCM module), as well as the two dimensional thermal model is proposed. The effects of different EG mass fractions associated with different phase change enthalpy and thermal conductivity are investigated firstly under high-rate discharge condition. The results show that an optimal mass fraction of EG is needed to achieve the best thermal performance and the EG mass fraction of 15–20% is recommended to be used for battery thermal management. In order to further improve the overall performance, we also design a novel pyrolytic graphite sheets (PGS)-enhanced paraffin/EG composites based battery module (PCM/PGS module). EG with porous structure can create the primary thermal conductive network for PCM to increase the heat absorption rate. PGS forms the secondary thermal conductive network for the module to improve the whole thermal homogeneity. As a result, the as-designed PCM/PGS module presents much better heat dissipation performance and temperature uniformity compared with the PCM module during discharge–charge cycles. Also, the introduction of PGS is beneficial for thermal fluctuations and energy saving, for instance, the thermal performance of PCM/PGS module with a convective heat transfer coefficient of 50 W m−2 K−1 is comparable to PCM module with 200 W m−2 K−1. In the case of failure mode, to achieve the prevention of thermal runaway propagation, the spacing between cells for PCM module is up to 14 mm. However, the use of PGS lowers the spacing (i.e. less composite PCM consumption) by a decreasing rate of ∼71.4% in comparison with PCM module.

1. Introduction Environmental concerns such as emissions legislation and the limitation of fossil fuels have induced the growing interests of research and development of rechargeable battery technology for energy storage and green power source for transportation. Secondary lithium-ion (Li-ion) batteries are a promising candidate amongst various battery types due to their good stability, no memory effect, low self-discharge rate and high energy density [1]. In order to meet the operational requirements of energy and power, large scales and capacities of Li-ion cells are always favored and integrated in large scale modules and packs [2,3]. However, the electrochemical performances of these cells are considered strongly sensitive to temperature [4]. In practical applications, large-format cells are prone to overheating from complex operating conditions such as rapid discharging and dynamic cycling, during which abundant heat is continuously generated



inside the cells [5]. If the heat transfer from cells to the ambient is not sufficient, the cells will subject to several adverse effects such as capacity and power fade [6]. Besides, the temperature difference between cells in a module could result in different charging/discharging behaviors and the reduction of lifespan. Moreover, failure modes of cells such as short circuits and internal defects could initiate the chemical reaction of active materials [7]. When this process is out of control, catastrophic destruction such as fire and explosion occurs, namely, thermal runaway [8]. In extreme cases, failure of a single cell will potentially induce the cell-to-cell propagation of thermal runaway if no appropriate means are adopted to prevent it in the battery module [9]. Therefore, a robust thermal management system is essential to operate the battery module in normal and extreme conditions. In general, the existed types of battery thermal management (BTM) systems can be categorized as air, liquid, and phase change material (PCM) system based on the heat transfer medium [10]. For air based

Corresponding author. E-mail address: [email protected] (S. Wang).

http://dx.doi.org/10.1016/j.enconman.2017.09.068 Received 24 July 2017; Received in revised form 25 September 2017; Accepted 27 September 2017 0196-8904/ © 2017 Elsevier Ltd. All rights reserved.

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Rake A 7#

8#

9#

10#

11#

12#

T8

T1

1#

T6

T3

2#

3#

4#

5#

6#

Battery module Fig. 1. Schematic diagram of battery module with composite PCM (PCM module).

the temperature rise could be slowed down by doping EG and the temperature uniformity could be improved by increasing the EG mass fraction. Furthermore, foam-stable and modular PCM/EG blocks associated with the thermos-mechanical properties such as bending strength, compression strength and tensile strength were also studied in Ref. [36]. In order to further improve the thermal conductivity and skeleton strength, a copper mesh-enhanced PCM/EG composite with a EG mass fraction of 20% was proposed for BTM system by Wu et al. [27]. By taking into account the literature survey, it is obvious that the thermal performance of PCM/EG composite based system is significantly influenced by the thermal-physical characteristics such as phase change enthalpy and thermal conductivity, which vary with density and EG mass fraction of the composites. Despite a large number of studies on PCM based BTM system, there have been relatively few investigations on the optimum conditions of thermal management of Liion cells using PCM/EG composites. Literature reports of experimental and numerical studies of PCM based BTM system under normal conditions such as low-rate discharge and one charge–discharge process are abundant [37–39]. In practical applications, it is increasingly important to understand the capabilities of PCM when the cells are under extreme conditions that are outside their normal operational range [39]. The battery module could generate a large amount of heat in a short time during high discharge rate and also suffer from heat accumulation during dynamic cycling, which may lead to the running out of the available latent heat of PCM [40]. Moreover, propagation of thermal runaway occurs if the volume of PCM in between cells is not enough to absorb the heat from failed cell [41]. Arguably, operating in extreme conditions stresses the PCM based BTM system thermally and introduces new challenges that are needed to be understood and overcome. In this paper, a two-dimensional model is developed for the thermal management of composite PCM based large-format battery module in extreme conditions. Paraffin/EG composites with various mass fractions of EG are considered and the effects of phase change enthalpy and thermal conductivity on the thermal performance of battery module under high-rate discharge are studied and optimized. To further improve the overall performance in dynamic cycling and failure conditions, a novel heat transfer enhancement method is proposed, in which pyrolytic graphite sheets are introduced to form a thermal conductive network for the composited PCM based battery module.

system, since the specific heat capacity and heat transfer coefficient of air is much lower than many other media, it is very difficult to meet the demands of heat dissipation at high discharge rate and other extreme conditions. Liquid based thermal management strategy is more effective in heat rejection than air method due to its higher heat transfer coefficient [11], while it requires extra energy supply components and is possible to leak and cause the short circuit of cells. As an innovative solution for thermal management applications, PCM can absorb/release abundant latent heat during the melting/solidification process, keeping the PCM based system at a relatively constant temperature [12], which make this type of BTM system receive extensive attention and exploration in recent years. In 2000, Al Hallaj and Selman [13] first proposed a PCM based BTM system, which can absorb the heat generated inside the cells by means of large phase change enthalpy of PCM to keep the temperature within a proper range. Further works showed that PCM based system can bring many benefits such as passively buffering against high operating temperature [14], extending life cycle [15] and eliminating thermal runaway [16]. Currently, paraffin wax with the melting point range from 40 to 42 °C is the most widely used PCM for BTM for its low cost, hardto-decompose character and suitable phase-change temperature. Nevertheless, significant challenges still remain in pure paraffin: relatively low thermal conductivity and leakage problem in the molten form. To address these problems, composite PCM has been developed by introducing a second component made of high conductive materials such as metallic particle [17], metal foam/mesh [18–22], carbon fiber [23], graphene [24,25], carbon nanotubes [26] and expanded graphite (EG) [27] to enhance the overall thermal conductivity for BTM system. Among them, EG is the mostly employed material due to its high thermal conductivity and absorbability [28–32], which is favorable for the creation of thermal conductive networks within the shape-stabilized composited PCM. With EG impregnated in the PCM, the heat absorption rate of composite PCM could be amplified and thus improve the overall thermal performance of battery module [15]. Ling et al. [33] prepared a series of PCM/EG composites and then studied the influences of density and PCM mass fraction of the composites on a simulative BTM system. The thermal conductivity of the composites normally increased with higher bulk densities and the graphite-matrix bulk density was identified as an important parameter for such a thermal management strategy [34]. Fathabadi [35] proposed a hybrid active-passive BTM system, in which PCM/EG composite served as the passive part and it showed that 23

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2. Battery module

chemical reactions during charge/discharge operations. However, the battery may generate a large amount of heat in off-normal conditions. To evaluate the performance of designed module, three load profiles, i.e., high-rate discharge, dynamic cycles and failure mode were carried out to simulate the extreme conditions, as shown in Fig. 3. The first profile simulated a fairly high operational rate (Fig. 3a). The heat generation is generally described as the sum of reversible and irreversible heat due to entropic heating and joule heating, respectively. The joule heat can be calculated in terms of I2R, where the total internal resistance R is determined by the electrodes polarization and ohmic potential drop, which can be measured by a Hybrid Pulse Power Characterization (HPPC) test [42]. At a high constant discharge rate, the irreversible joule heat becomes dominant and accounts for a large proportion of the total generated heat and the heat generation rate can be calculated and averaged over discharge time. The heat load was modeled using an average volumetric heat generation rate of 12.70 × 10 4 W m−3, which was applied to each cell for 720 s, representing a high discharge rate of 5 C. In the dynamic cycle profile (Fig. 3b), it was assumed that the cells were cycled in the following steps with no rest period: (1) constantcurrent (CC) discharge with 5 C rate until the fully charged battery exceeds the lower voltage limit; (2) CC charge with 1 C rate until the battery exceeds the upper voltage limit; (3) go to the first step and continue with five cycles. Different currents lead to different heat generation rates. The average volumetric heat generation rates of discharge and charge process were 12.70 × 10 4 W m−3 and 5079 W m−3, respectively. In the failure profile (Fig. 3c), it simulated a fast and high energy exothermic reaction due to the occurrence of thermal runaway triggered by external heating, nail penetration, over charging/discharging, or external/internal short [43]. Cell #2 was selected as the failed cell that released energy over a fifty-second ramp profile while the other cells remained in rest state during the failure. The totally released heat of the failed cell was calculated based on the cell’s electrical energy at 100% state of charge (SOC) plus an additional 30% for exothermic chemical reactions as presented in Ref. [41]. This load profile aimed to investigate the thermal runaway propagation behavior from failed cell

2.1. Module design A generic battery module with 2 × 6 cells arrangement, as shown in Fig. 1, is considered because it can be applied in high power battery pack for electric vehicles and larger stationary energy storage system. The prismatic cell with multi-layer structure is wrapped around with shape-stabilized EG/PCM composites. The thickness of composite PCM surrounding a single cell is L. The nominal cell capacity is 12 Ah and the dimension is (height × width × depth) 90 mm × 70 mm × 27 mm. Each layer of the cell unit consists of two current collectors (copper for anode and aluminum for cathode), a separator and two electrodes (anode and cathode), which can be also seen in Fig. 1. This two-dimensional (2D) model is considered to reduce computation load and a 2D simulation is sufficient to investigate the thermal behavior of PCM/ EG based battery module [28]. To further improve the battery performance in extreme conditions, a new design for the battery module is proposed in this study. The cells with surrounding composite PCM are kept 1.5 mm apart from each other by placing pyrolytic graphite sheets (PGS) which also attached to the module sides as shown in Fig. 2. This new module can be classified into three areas according to their functions: (1) PCM, with large latent heat serves as the thermal buffer to absorb heat generated by batteries through solid-liquid phase change, (2) EG, with high thermal conductivity and porosity creates the primary thermal conductive network for PCM, and absorbs liquid phase PCM to address the leakage problem, and (3) PGS acts as a heat spreader to further increase the heat absorption rate of PCM, and also form the secondary thermal conductive network for module to enhance the thermal homogeneity across the module. 2.2. Load profile The heat generated from the battery module usually varies depending on the operating conditions. In normal use, the heat generation is relatively small, which is mainly caused by charge transport and

Fig. 2. New design of the battery module (PCM/PGS module).

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-3

Heat generation rate (kW m )

W. Wu et al.

a

Fig. 3. Load profiles of: (a) high-rate discharge (5 C); (b) dynamic cycles; and (c) failure mode.

135

126

117 0

120

240

360

480

600

720

840

960

-3

Heat generation rate (kW m )

Time (s) 150

b 100

50

0 0

4320

8640

12960

17280

21600

-3

Heat generation rate (kW m )

Time (s) 60000

c 40000

20000

0 0

50

100

150

200

Time (s)

directions. For the operational temperature of battery cells, paraffin with phase change temperature range from 315.15 K to 317.15 K was used to construct the EG-based composite PCM. Phase change enthalpy of the composite was calculated using a simple mass-weighted average. The effective thermal conductivity was predicted by the mathematic model derived by Ling et al. [44]. This model is simple and has a wide applicability to PCM/EG composites by considering only two parameters: bulk density of the composites and mass fraction (φ ) of EG. In general, the PCM/EG composites could be put into a mold and then compressed to bulk material with different densities. Three different densities (i.e., 714 kg m−3, 850 kg m−3, 950 kg m−3) within the actual and acceptable ranges were adopted. Fig. 4 shows the calculated values of phase change enthalpy and thermal conductivity with different EG mass fractions and densities. It can be seen that, thermal conductivity increases with higher bulk density. For instance, when the density increases from 714 kg m−3 to 950 kg m−3 under the EG mass fraction of 30 wt%, the thermal conductivity will rise from 11.84 W m−1 K−1 to 15.16 W m−1 K−1, almost an increase by 28%. In addition, a higher mass fraction of EG in the composites can increase thermal conductivity, however, it simultaneously lowers the phase change enthalpy. The thermal conductive network material for battery module, PGS, is highly thermal conductive, flexible and heat-resistant [45,46]. The PGS is 1.5 mm thick and has a thermal conductivity of 800 W m−1 K−1 in the in-plane directions, which is twice than that of copper [47]. Thermal and physical properties of cell, paraffin, EG, composite PCM

to the other cells. The mechanism of thermal runaway propagation is induced by the failed cell when the contact surface temperature is above the onset temperature of thermal runaway. As indicated by Huang et al. [9], the self-accelerating decomposition temperature, namely, the onset temperature of thermal runaway was calculated as 126.1 °C and 139.2 °C. Thus, if the temperature exceeds 130 °C in the simulations, the cell is considered at a great risk to cause thermal runaway. 2.3. Material characteristics Based on the geometrical characteristics and thermos-physical properties of the multi-layer structure, the value of density, heat capacity and thermal conductivities of the prismatic cell can be calculated through:

ρb Cp,b =

kx =

ky =

∑i ρi Cp,i Vi ∑i Vi

(1)

∑i ki Li ∑i Li

(2)

∑i Li ∑i

Li ki

(3)

where Vi is the volume, Li is the thickness and i represents the part of the structures. k x and k y are the thermal conductivities in different 25

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190 -1

180

714 kg m

-3

850 kg m

-3

950 kg m

-3

16

12 -1

-1

170

k (W m K )

Phase change enthalpy (kJ kg )

Fig. 4. Thermal conductivity and phase change enthalpy of composites with different packing densities and EG mass fractions.

20

200

160

8

150 4

140 130

0

0

5

10

15

20

25

30

EG mass fraction (%)

Table 1 Thermal and physical properties of materials used in the study [44,47]. Cell

PGS

Paraffin

EG

Composite PCM

Density (kg m−3) Thermal conductivity (W m−1 K−1)

2335 0.9 (k x ) 2.6 (ky )

914 0.3

2333 129

714,850,950

Specific heat capacity (kJ kg−1 K−1) Phase change enthalpy (kJ kg−1) Phase change temperature (K)

0.950

2300 800 (in plane) 20 (vertical) 0.71





2.5





186.4



186.4 × (1−φ)





315.15 −317.15

_

315.15 −317.15

328

Module temperature (K)

Property

kEG ρPCM / EG φ ρEG

324

320

13195 21941 40516 59527

316 0

40

80

120

160

Rake A (mm)

and PGS used in the simulations are listed in Table 1.

Fig. 5. Independent test of grid number.

3. Simulation method

3.2. Governing equations and boundary conditions

3.1. Battery module model

In this study, different volumetric heat generations were considered to investigate the performance of the battery module under the extreme conditions. The UDF (user defined function) was used to define the heat generations. To deal with the mathematical modeling of composite PCM, enthalpy method was adopted and the composites were modeled by one-temperature energy equation using equivalent physical properties [28]. The thermal equilibrium between battery and PCM as well as PCM and PGS was achieved, thus the contact resistance was negligible and coupled boundary conditions satisfied the continuity of temperature profiles [48]. The initial temperature (T0) and environmental temperature (Tamb ) were all set as 298.15 K. The governing equations of each domain were summarized as: (a) Energy conservation equations for cell, composite PCM and PGS:

A simplified two dimensional geometry of the battery module was developed using the commercial software SolidWorks. The computational meshes were generated by ANSYS MESH and the numerical simulation in this study was developed by ANSYS FLUENT, a commercial CFD software package that could be used for the analysis of heat transfer and melting/solidification. To simplify the model, assumptions were made based on the module design. Firstly, the thermo-physical properties except the thermal conductivity were isotropic and uniform. Secondly, no liquid flow occurred during the phase change process and heat conduction was the only way of heat transfer in composite PCM. The variables (e.g. L and convection heat transfer coefficient) could be reasonably changed to improve the working performance. Independent test of grid number was initially carried out to ensure accuracy of the calculation. When the spacing between the cells was 4 mm (L = 2 mm), the temperature distribution in the created Rake A (Fig. 1) was shown in Fig. 5, the grid number of 40,516 was chosen for the domains of battery module with composite PCM.

ρc Cp,c

∂Tc = ∇ (kc ∇Tc ) + q ∂t

ρPCM Cp,PCM ρPGS Cp,PGS

26

∂TPCM = ∇ (kPCM ∇TPCM ) ∂t

∂TPGS = ∇ (kPGS ∇TPGS ) ∂t

(4)

(5)

(6)

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328.0

(b) Boundary conditions

-3

t = 0,T (x ,y ) = T0

327.5

∂T ∂T = −kPCM ∂n ∂n

−kPCM

(8)

∂T ∂T = −kPGS ∂n ∂n

Temperature (K)

−kc

714 kg m -3 850 kg m -3 950 kg m

(7)

(9)

For PCM module −kPCM

∂T = hPCM (T −Tamb) ∂n

For PCM/PGS module −kPGS

(10)

∂T = hPGS (T −Tamb) ∂n

327.0

326.5

326.0

(11)

325.5

4. Results and discussions

-5

0

5

25

30

35

Temperature (K)

324

321 1 wt.% EG 10 wt.% EG 20 wt.% EG 30 wt.% EG

318

5 wt.% EG 15 wt.% EG 25 wt.% EG

-3

714 kg m

315 0

20

40

60

80

100

120

140

160

180

Temperature (K)

327

324

321

318 -3

850 kg m

315 0

20

40

60

80

100

120

140

160

180

327

Temperature (K)

Temperature (K)

310

20

327

1 wt.% EG 5 wt.% EG 10 wt.% EG 15 wt.% EG 20 wt.% EG 25 wt.% EG 30 wt.% EG

315

15

Fig. 7. Effect of EG mass fraction on maximum temperature of PCM based battery module at the end of high-rate discharge.

In order to investigate the effect of EG mass fraction on thermal performance, three bulk densities with different EG mass fractions and corresponding thermal conductivity and phase change enthalpy, as shown in Fig. 4, were adopted in PCM module. The value of L is equal to 2 mm and the convective heat transfer coefficient is 10 W m−2 K−1. Fig. 6 shows the comparative temperature responses of cell 1 (T1) during the 5 C high-rate discharge process with different EG mass fractions under the density of 950 kg m−3. In the solid phase stage (< ∼400 s), the temperatures for all cases increase rapidly in an almost linear manner. Once T1 reaches the melting temperature’s lower limit (315.15 K), heat is absorbed as latent heat, thus the temperatures smoothly increase within the melting point range of 315.15–317.15 K. Because of the low heat absorption rate of PCM with 1 wt% EG, the temperature is slightly higher than the others during the melting process. However, for the composites with 30 wt% EG, in spite of highest thermal conductivity, the temperature again rises rapidly at ∼700 s due to the completion of melting. This illustrates that the introduction of EG can improve the heat transfer capability but shorten the thermal management time, due to the decrease of phase change enthalpy per unit mass in composite PCM. Fig. 7 shows the maximum temperature (Tmax) of PCM module under different EG mass fractions and densities at the end of discharge. With the increase of EG mass fraction, Tmax decreases firstly, and then increases gradually. Similar phenomenon can be seen elsewhere [49]. Higher EG mass fraction brings higher thermal conductivity, but lower phase change enthalpy in composites. As discussed above, the phase change enthalpy and thermal conductivity of the composite PCM govern the amount of heat that can be stored and the heat absorption 320

10

EG mass fraction (%)

4.1. High-rate discharge

324

321

318 -3

950 kg m

1%

305

315 0

318

20

40

60

80

100

120

140

160

180

Position along X (mm)

30%

300

Fig. 8. Temperature distribution along Rake A at the end of discharge. 316

295

400

290 0

300

500

Time (s)

600

rate at which this process occurs. Hence, there exists an optimal mass fraction of EG in composite PCM due to the competitive relation between the phase change enthalpy and thermal conductivity. Fig. 8 shows the temperature distribution along Rake A (Fig. 1) under different EG mass fractions and densities at the end of discharge. It can be clearly seen that higher EG mass fraction shows lower

700

600

Fig. 6. Temperature responses of cell 1 with different EG mass fraction with time.

27

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420

PCM PCM/PGS

PCM PCM/PGS

400 390

Tmax (K)

Tmax(K)

380 360

360 340

330 320 300

0

4320

8640

0

4320

8640

12960

17280

21600

12960

17280

21600

300 0

4320

Time (s)

8640

12960

17280

21600

17280

21600

Time (s)

-2

-2

-1

-1

(b) 50 W m K

(a) 10 W m K 16

16 PCM PCM/PGS

14

PCM PCM/PGS

14 12

10

10

T (K)

8

Δ

T (K)

12

6

6

4

4

2

2

0

0

Δ

8

0

4320

8640

12960

17280

0

21600

-2

4320

8640

12960

Time (s)

Time (s)

-2

-1

-1

(d) 50 W m K

(c) 10 W m K

Fig. 9. Maximum temperature variation and temperature difference in PCM and PCM/PGS module at different convective heat transfer. (a) and (c) 10 W m−2 K−1; (b) and (d) 50 W m−2 K−1.

convective heat transfer coefficient of 50 W m−2 K−1 which represents the active method, is performed for PCM and PCM/PGS modules and the results are shown in Fig. 9(b). It can be clearly seen that during the cycles, the temperature of each cycle for PCM module increases gradually, while PCM/PGS module reaches a stable stage (same temperature profile with previous cycle) after the first cycle. Furthermore, the Tmax at the end of discharge/charge with different convective heat transfer coefficients are tabulated in Table 2. The thermal performance of PCM/PGS module with 50 W m−2 K−1 is comparable to PCM module with 200 W m−2 K−1, indicating that the use of PGS is beneficial for thermal fluctuations and energy saving. Fig. 9(c) and (d) show the comparison of temperature uniformity between PCM and PCM/PGS modules under different convective heat transfer coefficients. The uniformity index of ΔT is equal to Tmax minus T1. The PCM/PGS module shows a much lower and stable ΔT than that of PCM. For instance, the average ΔT of PCM/PGS module has a decrease rate of ∼68% (i.e. 2.67 K vs. 8.37 K) compared to PCM module during the whole cycles when the convective heat transfer coefficient is 50 W m−2 K−1. It is noted that the ΔT of PCM/PGS does not exceed 5 K for the most, except during the discharging process in which a peak ΔT appears. As the cells start to discharge, the generated heat could not be transferred timely from cell core to surface because of its relatively low internal thermal conductivity, thus the Tmax increases rapidly. While the T1 is assumed to be a constant value because of the small temperature variation during melting process, as a result, a sharp rise in the ΔT. Further insight into the temperature distribution of PCM and PCM/ PGS modules during the dynamic cycles is shown in Fig. 10. For the

amplitude of temperature variation. This is ascribed to the high thermal conductivity of increased EG mass fraction. The high thermal conductivity of composite allows high rate of heat removal and minimizes non uniform temperature distribution in the battery module. By taking into account liquid PCM leakage problem during the phase change process, the composite PCM with 15–20 wt% EG is recommended for BTM. Therefore, EG mass fraction of 20% with the density of 950 kg m−3 was adopted in the following study. 4.2. Dynamic cycles In order to reflect the actual circumstances better, the effect of heat accumulation on the thermal performance of battery module during dynamic cycles was simulated. During the dynamic cycling process, most heat generated by cells is stored as latent heat in PCM. If the absorbed heat cannot be transferred to the ambient effectively, accumulated consumption of latent heat will result in a higher initial temperature for next cycle. As shown in Fig. 9(a), the Tmax at the end of discharge for PCM module is 325.9 K, 353.4 K, 375.8 K, 390.9 K and 401.2 K during the first, second, third, fourth and fifth cycles, respectively, giving rise to an increasing Tmax at the end of charge (320.5 K, 344.9 K, 361.2 K, 372.3 K and 379.9 K). In the terms of PCM/PGS module, a lower increasing rate of Tmax is obtained and the Tmax at the end of fifth discharge is reduced by almost 21 K compared to PCM module. Previous studies showed that passive BTM using PCM combined with active cooling such as forced air flow can avoid potential failures and improve its long term operational stability [40]. A high 28

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Table 2 Temperatures recorded at the end of each discharge/charge cycle. Convective heat transfer coefficient (W m−2 K−1)

Module

Temperature (K) End of discharge

PCM

10 50 100 150 200 10 50

PCM/PGS

End of charge

1

2

3

4

5

1

2

3

4

5

325.9 325.7 325.3 325.1 324.9 325.3 325.0

353.4 341.7 333.4 329.5 327.9 343.9 326.5

375.8 348.0 337.8 332.0 329.1 361.7 327.0

390.9 349.8 338.8 332.6 329.4 372.9 327.1

401.2 350.3 339.0 332.7 329.4 379.8 327.2

320.5 317.2 316.0 310.6 308.4 317.2 305.9

344.9 320.5 317.0 315.9 310.8 332.7 307.9

361.2 322.0 317.1 316.1 311.4 344.8 308.5

372.3 322.5 317.1 316.2 311.5 352.2 308.7

379.9 322.6 317.1 316.2 311.5 356.8 308.8

900

PCM module, it does not favor uniformity of the temperature distribution from cell to cell after the first cycle and the maximum temperature region appears in the middle of module. Such uneven temperature distribution may cause unbalancing between the cells, which will affect the working performance drastically. However, the temperature uniformity is excellent in the PCM/PGS module. The in-cell temperature contours are almost identical in adjacent cells. It can be concluded that the secondary thermal conductive network formed by PGS plays an important role in ensuring high rate of heat transfer from cell to cell and the removal of heat across the module.

Temperature (K)

Tmax 800

T1

700

T6

T3 T8

600

500

4.3. Failure mode

400

In this section, cell 2 was used to represent the trigger cell on which the thermal runaway took place along with the load profile shown in Fig. 3c. In addition to monitoring the maximum temperature of cell 2, the temperatures of adjacent cells (T1, T3 and T8) and a far cell (T6) in the corner were also recorded to investigate the thermal runaway propagation behavior of different modules. Fig. 11 shows the temperature response of PCM module under failure condition when the L and hPCM are 2 mm and 10 W m−2 K−1, respectively. Thermal runaway of the trigger cell occurs instantaneously. In this situation, Tmax rises rapidly because of the

300 0

200

400

600

800

Fig. 11. Temperature profiles of PCM module under failure condition.

thermal abuse and peaks around 850 K. The adjacent cell temperatures T1, T3 and T8 peak in the range of 450–500 K before declining, and T6 shows a very small increase because of its distance from cell 2 and

390

Temperature (K)

360

330

300

270

240 0

4320

1000

Time (s)

8640

12960

17280

21600

Time (s) Fig. 10. Temperature distribution in PCM and PCM/PGS modules under dynamic cycles at a convective heat transfer coefficient of 50 W m−2 K−1.

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550

900 800

500

T1 (K)

Tmax (K)

700

Cell #1

Cell #2

4mm 8mm 12mm 14mm 16mm

600

450

400

500 350

400

300

300 0

200

400

600

800

1000

0

200

Time (s)

400

600

1000

550

550

Cell #3

Cell #8

500

500

450

450

T8 (K)

T3(K)

800

Time (s)

400

400

350

350

300

300 0

200

400

600

800

1000

0

200

Time (s)

400

600

800

1000

Time (s)

Fig. 12. Temperature profiles of PCM module at different cell-to-cell spacing under failure condition.

relatively low heat transfer. Once the temperature of battery reaches the threshold point, the thermal runaway will be triggered locally, which will result in a chain reaction in the module. It can be concluded that the thermal runaway on adjacent cells (#1, #3 and #8) is triggered and then propagation occurs in the PCM module when the thickness of composite PCM is 2 mm. Since the thermal buffer characteristic of PCM, T1, T3 and T8 show a slow heating rate at the beginning, it is expected that thicker composite PCM between cells can mitigate propagation. The overall results for the PCM module with 5 different cell-to-cell spacing (4 mm, 8 mm, 12 mm, 14 mm, 16 mm) are presented in Fig. 12. It can be observed that spacing has a little effect on the Tmax due to the relatively low internal thermal conductivity of battery cell. However, the effect of spacing has a significant influence on the temperatures of adjacent cells associated with the thermal runaway propagation behavior during the failure condition. For instance, the duration of PCM melting is much longer for thicker L, indicating that the amount of PCM available for absorbing heat during melting process is increased when increasing spacing. The peak temperatures and times at which they are reached are summarized in Table 3. It is clearly seen that the propagation of thermal runaway can be prevented by leaving spacing up to 14 mm in which the peak temperature is below the threshold point (130 °C). Except the thermal performance of cells, space occupation rate (ξ; the ratio of all cells volume over module volume) is another important criterion for battery module [50]. A higher ξ means better space utilization. The ξ of PCM module is 0.8239, 0.6923, 0.5910, 0.54878 and 0.5111 for the cell-to-cell spacing 4 mm, 8 mm, 12 mm, 14 mm and 16 mm, respectively. The thicker the composite PCM between cells is, the heavier the structure is. Also, there is a geometrical restriction in putting thicker composite PCM around the cell. To investigate the effect of as-constructed PCM/PGS module on the

Table 3 Effect of cell-to-cell spacing on peak temperatures during failure condition.

Cell #2 Cell #1 Cell #3 Cell #8

Tpeak t (s) Tpeak t (s) Tpeak t (s) Tpeak t (s)

(K) (K) (K) (K)

4 mm

8 mm

12 mm

14 mm

16 mm

849.0 50 495.9 144 495.9 144 457.7 65

849.0 50 444.5 177 444.5 177 397.0 80

848.9 50 409.6 207 409.5 206 363.5 106

848.9 50 396.1 221 396.1 221 352.4 123

848.9 50 384.6 236 384.6 235 343.7 145

450 Tmax

Temperature (K)

T1 T3

400

T6 T8

350

300 0

200

400

600

800

1000

Time (s) Fig. 13. Temperature profiles of PCM/PGS module under failure condition.

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50 s

100 s

150 s

200 s

250 s

300 s

Fig. 14. Contour plots of PCM module under failure condition.

50 s

100 s

150 s

200 s

250 s

300 s

Fig. 15. Contour plots of PCM/PGS module under failure condition.

consumption up to 28.6% to achieve the prevention of thermal runaway propagation as compared with PCM module. Figs. 14 and 15 show the evolution of temperature distribution with time for PCM and PCM/PGS modules (L = 2 mm) during the failure condition. It is seen that the PCM/PGS module displays a more uniform distribution of the temperature compared to the PCM module. The trigger cell generated heat can be absorbed efficiently by surrounding PCM for the primary thermal conductive network of EG. For PCM module, the large-format geometry of prismatic cell induces a high thermal resistance along the composite PCM path within the battery module, causing the fast temperature rise of adjacent cells and ultimately thermal runaway. However, for PCM/PGS module, the

prevention of thermal runaway propagation, the failure condition was also simulated for PCM/PGS module. Fig. 13 shows the temperature response of PCM/PGS module under failure condition when the L and hPGS are 2 mm and 10 W m−2 K−1, respectively. Although the trigger cell also far exceeds 450 K, the neighbor cell temperatures T1, T3 and T6 peak below the threshold point. For instance, the peak temperature of T1 is much lower than that of the PCM module (400.6 K v.s. 495.9 K) and the time to reach the peak temperature is much shorter (68 s v.s. 144 s) when the L is 2 mm. This result indicating that thermal runaway occurs in one failed cell could be prevented from propagating by using PCM/PGS. Moreover, the ξ of PCM/PGS module is 0.7569 when the L is 2 mm, implying that PCM/PGS module can reduce composite PCM 31

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[10] Rao ZH, Wang SF. A review of power battery thermal energy management. Renew Sust Energ Rev 2011;15:4554–71. [11] Zhao J, Rao Z, Li Y. Thermal performance of mini-channel liquid cooled cylinder based battery thermal management for cylindrical lithium-ion power battery. Energ Convers Manage 2015;103:157–65. [12] Wu W, Zhang G, Ke X, Yang X, Wang Z, Liu C. Preparation and thermal conductivity enhancement of composite phase change materials for electronic thermal management. Energy Convers Manage 2015;101:278–84. [13] Al Hallaj S, Selman JR. A novel thermal management system for electric vehicle batteries using phase-change material. J Electrochem Soc 2000;147:3231–6. [14] Kizilel R, Lateef A, Sabbah R, Farid MM, Selman JR, Al-Hallaj S. Passive control of temperature excursion and uniformity in high-energy Li-ion battery packs at high current and ambient temperature. J Power Sources 2008;183:370–5. [15] Mills A, Farid M, Selman JR, Al-Hallaj S. Thermal conductivity enhancement of phase change materials using a graphite matrix. Appl Therm Eng 2006;26:1652–61. [16] Wilke S, Schweitzer B, Khateeb S, Al-Hallaj S. Preventing thermal runaway propagation in lithium ion battery packs using a phase change composite material: an experimental study. J Power Sources 2017;340:51–9. [17] Karimi G, Azizi M, Babapoor A. Experimental study of a cylindrical lithium ion battery thermal management using phase change material composites. J Energy Storage (2016). [18] Alipanah M, Li XL. Numerical studies of lithium-ion battery thermal management systems using phase change materials and metal foams. Int J Heat Mass Trans 2016;102:1159–68. [19] Rao ZH, Huo YT, Liu XJ, Zhang GQ. Experimental investigation of battery thermal management system for electric vehicle based on paraffin/copper foam. J Energy Inst 2015;88:241–6. [20] Azizi Y, Sadrameli SM. Thermal management of a LiFePO4 battery pack at high temperature environment using a composite of phase change materials and aluminum wire mesh plates. Energ Convers Manage 2016;128:294–302. [21] Qu ZG, Li WQ, Tao WQ. Numerical model of the passive thermal management system for high-power lithium ion battery by using porous metal foam saturated with phase change material. Int J Hydrogen Energ 2014;39:3904–13. [22] Li W, Qu Z, He Y, Tao W. Experimental and numerical studies on melting phase change heat transfer in open-cell metallic foams filled with paraffin. Appl Therm Eng 2012;37:1–9. [23] Azizi M, Samimi F, Babapoor A, Karimi G. Thermal management analysis of a Li-ion battery cell using phase change material loaded with carbon fibers. Energy 2016;96:355–71. [24] Goli P, Legedza S, Dhar A, Salgado R, Renteria J, Balandin AA. Graphene-enhanced hybrid phase change materials for thermal management of Li-ion batteries. J Power Sources 2014;248:37–43. [25] Mortazavi B, Yang H, Mohebbi F, Cuniberti G, Rabczuk T. Graphene or h-BN paraffin composite structures for the thermal management of Li-ion batteries: a multiscale investigation. Appl Energ 2017;202:323–34. [26] Shirazi AHN, Mohebbi F, Kakavand MRA, He B, Rabczuk T. Paraffin nanocomposites for heat management of lithium-ion batteries: a computational investigation. J Nanomater 2016;2016:1–10. [27] Wu W, Yang X, Zhang G, Ke X, Wang Z, Situ W, et al. An experimental study of thermal management system using copper mesh-enhanced composite phase change materials for power battery pack. Energy 2016;113:909–16. [28] Greco A, Jiang X. A coupled thermal and electrochemical study of lithium-ion battery cooled by paraffin/porous-graphite-matrix composite. J Power Sources 2016;315:127–39. [29] Lv Y, Yang X, Li X, Zhang G, Wang Z, Yang C. Experimental study on a novel battery thermal management technology based on low density polyethylene-enhanced composite phase change materials coupled with low fins. Appl Energ 2016;178:376–82. [30] Parsons KK, Mackin TJ. Design and simulation of passive thermal management system for lithium-ion battery packs on an unmanned ground vehicle. J Therm Sci Eng Appl 2017;9. [31] Yan J, Li K, Chen H, Wang Q, Sun J. Experimental study on the application of phase change material in the dynamic cycling of battery pack system. Energ Convers Manage 2016;128:12–9. [32] Zhao J, Lv P, Rao Z. Experimental study on the thermal management performance of phase change material coupled with heat pipe for cylindrical power battery pack. Exp Therm Fluid Sci 2017;82:182–8. [33] Ling ZY, Chen JJ, Fang XM, Zhang ZG, Xu T, Gao XN, et al. Experimental and numerical investigation of the application of phase change materials in a simulative power batteries thermal management system. Appl Energ 2014;121:104–13. [34] Greco A, Jiang X, Cao DP. An investigation of lithium-ion battery thermal management using paraffin/porous-graphite-matrix composite. J Power Sources 2015;278:50–68. [35] Fathabadi H. High thermal performance lithium-ion battery pack including hybrid active passive thermal management system for using in hybrid/electric vehicles. Energy 2014;70:529–38. [36] Alrashdan A, Mayyas AT, Al-Hallaj S. Thermo-mechanical behaviors of the expanded graphite-phase change material matrix used for thermal management of Liion battery packs. J Mater Process Tech 2010;210:174–9. [37] Li WQ, Qu ZG, He YL, Tao YB. Experimental study of a passive thermal management system for high-powered lithium ion batteries using porous metal foam saturated with phase change materials. J Power Sources 2014;255:9–15. [38] Schweitzer B, Wilke S, Khateeb S, Al-Hallaj S. Experimental validation of a 0-D numerical model for phase change thermal management systems in lithium-ion batteries. J Power Sources 2015;287:211–9. [39] Wang ZC, Zhang ZQ, Jia L, Yang LX. Paraffin and paraffin/aluminum foam

secondary thermal conductive network formed by PGS can transfer the heat absorbed in PCM near the trigger cell throughout the module. 5. Conclusion In order to optimize the thermal performance of PCM based battery thermal management system, paraffin/EG composites with various EG mass fraction was considered in a 2-D battery module model. A higher mass fraction of EG in the composites can increase the thermal conductivity and simultaneously lower the phase change enthalpy. The numerical results show that there exists an optimal mass fraction of EG in composite PCM and the EG mass fraction of 15–20% is recommended to be used for battery thermal management. To further improve the overall performance, a novel pyrolytic graphite sheets (PGS)-enhanced composite PCM based battery module was designed and simulated with the conditions of dynamic cycles and failure mode. The results show that the PCM/PGS module presents much better heat dissipation performance and temperature uniformity compared to PCM module during five discharge-charge cycles. For the case of failure mode, spacing between cells is significant in PCM module and thermal runaway occurs in one failed cell could be prevented from propagating when the spacing up to 14 mm. On the other hand, the PCM/PGS module could achieve the prevention of thermal runaway propagation with lower spacing (i.e. less composite PCM volume, a decreasing rate of 71.4%) as compared with PCM module. These excellent thermal performances can be attributed to the following factors: (1) PCM absorbs heat generated by cells through solid-liquid phase change; (2) EG adsorbs the liquid phase PCM without leakage problems and creates a primary thermal conductive network for PCM to increase the heat absorption rate; (3) PGS forms a secondary thermal conductive network for module to improve the thermal homogeneity. As the processes of the decomposition and reaction in the cells are very complex under extreme conditions, numerical simulation could overcome the limitations in the experiment and provide the information of inner temperature distribution, melting process and heat transfer process of battery module. However, simulation inevitably has some deviations with the practical applications. The conceptual battery module detailed in this paper is to provide an idea and guide the optimal design of battery thermal management. Acknowledgement The work was supported by National Natural Science Foundation of China (Grant No. 51536003) and Program of International Science and Technology Cooperation of China (Grant No. 2016YFE0118100). References [1] Etacheri V, Marom R, Ran E, Salitra G, Aurbach D. Challenges in the development of advanced li-ion batteries: a review. Energy Environ Sci 2011;4:3243–62. [2] Esmaeili J, Jannesari H. Developing heat source term including heat generation at rest condition for Lithium-ion battery pack by up scaling information from cell scale. Energy Convers Manage 2017;139:194–205. [3] Cicconi P, Landi D, Germani M. Thermal analysis and simulation of a Li-ion battery pack for a lightweight commercial EV. Appl Energ 2017;192:159–77. [4] Huo YT, Rao ZH. Investigation of phase change material based battery thermal management at cold temperature using lattice Boltzmann method. Energ Convers Manage 2017;133:204–15. [5] Wang CH, Lin T, Huang JT, Rao ZH. Temperature response of a high power lithiumion battery subjected to high current discharge. Mater Res Innov 2015;19. [6] Zhao JT, Rao ZH, Huo YT, Liu XJ, Li YM. Thermal management of cylindrical power battery module for extending the life of new energy electric vehicles. Appl Therm Eng 2015;85:33–43. [7] Zhao R, Liu J, Gu JJ. Simulation and experimental study on lithium ion battery short circuit. Appl Energ 2016;173:29–39. [8] Feng XN, Fang M, He XM, Ouyang MG, Lu LG, Wang H, et al. Thermal runaway features of large format prismatic lithium ion battery using extended volume accelerating rate calorimetry. J Power Sources 2014;255:294–301. [9] Huang PF, Ping P, Li K, Chen HD, Wang QS, Wen J, et al. Experimental and modeling analysis of thermal runaway propagation over the large format energy storage battery module with Li4Ti5O12 anode. Appl Energ 2016;183:659–73.

32

Energy Conversion and Management 153 (2017) 22–33

W. Wu et al.

[40]

[41]

[42]

[43] [44]

[45]

composite phase change material heat storage experimental study based on thermal management of Li-ion battery. Appl Therm Eng 2015;78:428–36. Wu W, Yang X, Zhang G, Chen K, Wang S. Experimental investigation on the thermal performance of heat pipe-assisted phase change material based battery thermal management system. Energ Convers Manage 2017;138:486–92. Coleman B, Ostanek J, Heinzel J. Reducing cell-to-cell spacing for large-format lithium ion battery modules with aluminum or PCM heat sinks under failure conditions. Appl Energ 2016;180:14–26. Liu F, Lan F, Chen J. Dynamic thermal characteristics of heat pipe via segmented thermal resistance model for electric vehicle battery cooling. J Power Sources 2016;321:57–70. Feng X, Lu L, Ouyang M, Li J, He X. A 3D thermal runaway propagation model for a large format lithium ion battery module. Energy 2016;115:194–208. Ling ZY, Chen JJ, Xu T, Fang XM, Gao XN, Zhang ZG. Thermal conductivity of an organic phase change material/expanded graphite composite across the phase change temperature range and a novel thermal conductivity model. Energ Convers Manage 2015;102:202–8. Wen CY, Lin YS, Lu CH. Performance of a proton exchange membrane fuel cell stack

[46]

[47]

[48]

[49]

[50]

33

with thermally conductive pyrolytic graphite sheets for thermal management. J Power Sources 2009;189:1100–5. Lin CJ, Xu SC, Chang GF, Liu JL. Experiment and simulation of a LiFePO4 battery pack with a passive thermal management system using composite phase change material and graphite sheets. J Power Sources 2015;275:742–9. Masaki K, Miyo Y, Sakurai S, Ezato K, Suzuki S, Sakasai A. Heat transfer characteristics of the first wall with graphite sheet interlayer. Fusion Eng Des 2010;85:1732–5. Javani N, Dincer I, Naterer GF, Yilbas BS. Heat transfer and thermal management with PCMs in a Li-ion battery cell for electric vehicles. Int J Heat Mass Trans 2014;72:690–703. Jiang GW, Huang JH, Fu YS, Cao M, Liu MC. Thermal optimization of composite phase change material/expanded graphite for Li-ion battery thermal management. Appl Therm Eng 2016;108:1119–25. Wang T, Tseng KJ, Zhao JY, Wei ZB. Thermal investigation of lithium-ion battery module with different cell arrangement structures and forced air-cooling strategies. Appl Energ 2014;134:229–38.