A thermal management system for rectangular LiFePO4 battery module using novel double copper mesh-enhanced phase change material plates

Accepted Manuscript A thermal management system for rectangular LiFePO4 battery module using novel double copper mesh-enhanced phase change material plates

Wenfu Situ, Guoqing Zhang, Xinxi Li, Xiaoqing Yang, Chao Wei, Mumin Rao, Ziyuan Wang, Cong Wang, Weixiong Wu PII:

S0360-5442(17)31604-3

DOI:

10.1016/j.energy.2017.09.083

Reference:

EGY 11577

To appear in:

Energy

Received Date:

18 November 2016

Revised Date:

31 August 2017

Accepted Date:

18 September 2017

Please cite this article as: Wenfu Situ, Guoqing Zhang, Xinxi Li, Xiaoqing Yang, Chao Wei, Mumin Rao, Ziyuan Wang, Cong Wang, Weixiong Wu, A thermal management system for rectangular LiFePO4 battery module using novel double copper mesh-enhanced phase change material plates,

Energy (2017), doi: 10.1016/j.energy.2017.09.083

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Highlights 1. The maximum thermal conductivity of the DCM-PCMP can be obviously improved. 2. The coupled system based on DCM-PCMP and air-cooling significantly enhanced the thermal management effect. 3. The outstretched copper mesh can effectively reduce the internal temperature of the battery. 4. The DCM-PCMP can effectively reduce the power consumption of battery module for secondary cooling.

ACCEPTED MANUSCRIPT

A thermal management system for rectangular LiFePO4 battery module using novel double copper mesh-enhanced phase change material plates Wenfu Situa, Guoqing Zhanga, Xinxi Lia,, Xiaoqing Yanga,, Chao Weia, Mumin Raob, Ziyuan Wanga, Cong Wanga, Weixiong Wua a

School of Materials and Energy, Guangdong University of Technology, Guangzhou, 510006, P. R. China

b

Shenzhen Optimum Nano Energy Co., Ltd., Shenzhen 518118, PR China

Abstract A coupled battery thermal management (BTM) system based on novel quaternary phase change material plate (PCMP) is developed to balance the temperature in rectangular LiFePO4 battery modules. Paraffin (PA), expanded graphite (EG), low-density polyethylene, and copper mesh were combined into a quaternary PCMP to strengthen the heat transfer. The thermal conductivity of the PCMP with double copper mesh (DCM-PCMP) was increased by 36.0% compared with that of PCMP composed of EG and PA. Accordingly, the DCM-PCMP reduced the maximum temperature and maximum temperature difference within the battery module to less than 52.8 and 3 °C, respectively, both the lowest among the four methods. The coupled system based on DCMPCMP and forced air convection showed excellent thermal performance, which contributed to a stable temperature during the cycling process. Thermal simulations showed that the double outstretched copper mesh through the DCM-PCMP disturbed the air flow tempestuously, giving rise to a decrease in thermal resistance. Thus, the temperature distribution inside the battery and temperature uniformity within the battery module were both better optimized. The analysis of the power consumption of the DCM-PCMP method revealed that the optimal heat dissipation performance for the battery module is achieved at an air velocity of 6 m/s.



Corresponding author Tel: +86-18310886308 (X. Li). E-mail addresses: [email protected] (X. Li).  Corresponding author Tel: +86-020-39322570 (X. Yang). E-mail addresses: [email protected] (X. Yang). -1-

ACCEPTED MANUSCRIPT Keywords: Battery thermal management; Quaternary phase change material composites; Copper mesh; optimal thermal performance.

Nomenclature ΔTmax Tmax ΔT Rb-PCMP RPCMP RP-air Tb TPCMP,1 TPCMP,2 Tair Q tcycle ρair Cp,air ΔTair Toutlet Tinlet ʋair Wdiss Pfans η

Maximum temperature difference (°C) Maximum temperature (°C) Temperature difference (°C) Thermal resistance between the battery and PCMP composite (°C/W) Thermal resistance of PCMP composite (°C/W) Surface thermal resistance from PCMP composite to air (°C/W) Thermal steady temperature of the battery (°C) Heat-end temperature of PCMP composite (°C) Cold-end temperature of PCMP composite (°C) Air temperature (°C) Power input (W) Duration of the cycle (s) Density of air (kg/m3) Specific heat capacity of air (J/kg·K) Temperature difference between inlet and outlet air (°C) Outlet air temperature (°C) Inlet air temperature (°C) Flow-rate of air (m3/s) Heat dissipated (J) Power consumed by the fans (W) The coefficient of heat dissipation performance of the battery module

Acronyms ANC AFC HEVs PEVs LIBs DOD BTM PCM PA EG LDPE PCMP WCM-PCMP SCM-PCMP DCM-PCMP EG/PA-PCMP

Air natural convection Air forced convection Hybrid electric vehicles Pure electric vehicles Lithium-ion batteries Depth of discharge Battery thermal management Phase change material Paraffin Expanded graphite Low-density polyethylene Phase change material plate Phase change material plate without copper mesh Phase change material plate with single copper mesh Phase change material plate with double copper mesh Phase change material plate composed of paraffin and expanded graphite -2-

ACCEPTED MANUSCRIPT 1. Introduction To satisfy the battery power requirements of pure electric vehicles (PEVs) and hybrid electric vehicles (HEVs), lithium-ion batteries (LIBs) must provide power continuously [1-3]. Under these circumstances, the dissipating heat of the inner battery can lead to the dangerous situations of fire and explosion [4], which is defined as thermal runaway [5-6]. Therefore, the heat must be removed to the ambient environment quickly enough to render the temperature of the battery within the safe range of 0 to 65 °C [7-8] and keep the maximum temperature difference (ΔTmax) below 5 °C [9]. Hence, the technology of the battery thermal management (BTM) system plays an important role in preventing thermal runaway for the battery module [10-11]. The traditional BTM methods, such as air-based thermal management systems (natural or air forced cooling) [12-13] and liquid-based thermal management systems (heat pipe or liquid cooling) [14-16] have been applied to the power battery modules of PEVs and HEVs. However, air-based thermal management systems hinder the dissipation of heat generated inside the batteries owing to their low heat transfer efficiency. Meanwhile, liquid-based thermal management systems can cause short circuit of the battery module if leakage of the liquid occurs in the system [17]. These undesirable disadvantages engender expectations for novel BTM methods, such as phase change material (PCM)-based thermal management systems. The application of PCM to BTM system can result in an efficient, reliable, and friendly solution for the thermal management of power batteries, and is especially suited to unsteady discharge systems [18-20]. A BTM system using pure PCM was previously proposed by Al-Hallaj [21]. Their results showed that the PCM-based BTM system had a better cooling performance than a conventional BTM system. To improve the thermal conductivity of pure PCM, paraffin was fabricated composites of nanofiber and nanoparticle, and the thermal performance of the resulting nano-PCM was significantly improved compared with that of the pure PCM [22]. Fereshteh Samimi et al. [23] simulated and experimentally measured the thermal performance of a 14500AA Li-ion battery in a cylindrical PCM composed of carbon fiber and paraffin (PA). They reported a thermal conductivity enhancement of 105% on average and revealed that the battery exhibited a more uniform temperature distribution as the percentage of carbon fiber in the PCM composite was increased. Abid Hussain et al. [24] designed and investigated a passive thermal management system for cylindrical batteries using nickel foam and PA that showed a temperature reduction of -3-

ACCEPTED MANUSCRIPT 31% and 24% compared with natural air convection and pure PCM, respectively, under a 2 C discharge rate. A novel PCM-based thermal management system containing a PA/expanded graphite (EG) composite was proposed by Alrashdan [25]. His results showed that as the mass fraction of paraffin wax in the composite material was increased, the thermal performance of the battery module was improved and the burst strength of the composite increased at room temperature. Lv et al. [26] proposed a ternary composite of PA/EG/low-density polyethylene (LDPE) coupled with low fins and applied it to the passive BTM system of a 18650 battery module. They reported that the PA/EG/LDPE composite PCM exhibited much better mechanical properties and cooling effect compared with those obtained with EG/PA composite and air cooling, and kept the maximum temperature (Tmax) and ΔTmax of the battery pack under 50 °C and 5 °C, respectively. The investigations on BTM systems described above were mostly carried out for cylindrical LIBs. There have been few studies on PCM-based BTM systems for rectangular LIBs. W.Q. Li et al. [27] designed a BTM system using paraffin and copper foam for a rectangular Li-ion battery module. Compared with the cases of natural air convection and pure PCM, their results showed that the use of copper foam with the PCM decreased the battery temperature at the end of discharge by approximately 4 °C. Lin et al. [28] conducted experiments and numerical simulations on a passive BTM system using a composite of EG, graphite sheets, and PCM for rectangular LiFePO4 battery module. They concluded that PCM cooling significantly reduced the increase in battery temperature and kept ΔTmax within the battery module below 5 °C at the end of 1 C and 2 C-rate discharge. Wu et al. [29] developed a copper mesh-enhanced PA/EG composite as a cooling method for a LiFePO4 battery module. They reported that the as-constructed copper meshenhanced PA/EG plate exhibited much better heat dissipation performance and temperature uniformity compared with a PA/EG plate without copper mesh. It is noticeable that most studies relating to PCM-based BTM system have been focused on cylindrical batteries and low rate discharge conditions. In practical applications, rectangular battery modules with large capacity usually discharge at high rate and subsequently generate large amounts of heat, which demands a superior heat dissipation and phase transition performance of the PCM. Meanwhile, the preparation process of the PCM should be simplified to avoid the leakage of PA in the heat pressing process. Moreover, the heat of the PCM should be removed to -4-

ACCEPTED MANUSCRIPT the ambient environment to keep the battery module working in the safe temperature range. Aircooling has been found to be an effective method of achieving this in previous studies [30]. Nevertheless, the optimization of the heat dissipation and phase transition performance of PCM in BTM systems for rectangular battery modules has not been adequately investigated either numerically or experimentally, and analysis of the power consumption of such a BTM system has not yet been conducted. In this work, to address the above-mentioned issues, a novel BTM system using a quaternary phase change material plate (PCMP) composed of PA, EG, LDPE, and copper mesh coupled with the air cooling is presented and its thermal performance studied by experimental measurements and simulation. Air natural convection (ANC) and the PCMP without copper mesh (WCMPCMP) were used as reference thermal dissipation methods. The PCMP with double copper mesh (DCM-PCMP) not only greatly simplified its preparation process according to its cooling forming characters whilst ensured its high thermal conductivity and latent heat during melting, but also showed superior heat dissipation performance for a rectangular LiFePO4 battery module. Furthermore, DCM-PCMP can effectively reduced the power consumption of the battery module for secondary cooling under the condition of air forced convection (AFC). This can provide basis for the application of coupling BTM system based on PCM and AFC from power consumption point of view.

2. Materials preparation 2.1. Material composites Table 1 Thermal and physical properties of PA, EG, LDPE and copper mesh.

Property

Value PA

EG

LDPE

Copper mesh

Melting point (°C)

50

—

—

—

Average particle size (μm) Expansion ratio (ml·g-1) Density (Kg/m3) Tensile strength (MPa) Breaking elongation (%) Melt flow-rate (g/10 min)

— — — — — —

150 220 — — — —

— — 920.5 8 50 50

— — — — — —

Aperture size (mm)

—

—

—

2.50

-5-

ACCEPTED MANUSCRIPT The PA, EG, LDPE and copper mesh were composed by physical method to prepare the quaternary PCMP. Their basic parameters were listed in Table 1. Firstly, Commercial PA (supplied by China Petroleum Chemical Co.) was melted at 100 °C in an oil bath for 20 min. Secondly, a predetermined amount of LDPE (supplied by China Petroleum Chemical Co.) was added to the melted PA with continuous stirring using a low-speed mixer (DC-1-100W, Changzhou Wanhe instrument manufacture Co., Ltd.) and the temperature of the oil bath was up to 155 °C for 1 h. After that, EG (provided by Qingdao Bai Xing Graphite Co., Ltd.) was added to the mixture and the temperature of the oil bath was maintained constant (155 °C) for 1 h. Finally, the mixture poured into the a specific mould containing the copper mesh. For comparison, another two PCMP composites were prepared by the same process, one was WCM-PCMP, while another one was composed of PA and EG (denoted as EG/PA-PCMP). 2.2. Designs of the PCMP composite The copper mesh (thickness: 0.5 mm, size: 79×89 mm) need to compact between PA/EG/LDPE composites as sandwiches structure. There were the design scheme of the PCMP with single copper mesh (SCM-PCMP) and DCM-PCMP, as shown in Fig. 1(a) and (b), respectively. Additionally, 5 mm copper mesh stretched out at each end of the PCMP composite. The side view and the magnification of the cross section of the SCM-PCMP (Fig. 1(c)) indicated that the copper mesh can be imbeded in the PCMP composite completely, it was the same to DCM-PCMP (Fig. 1(d)). For comparison, the WCM-PCMP was also prepared according the above mentioned size (69×89 mm). Their thermal conductivity was tested by a laser flash system (LFA447 NanoFlash™ system, range: 0.1–2000 W/m K, accuracy: ±5%, repeatability: ±3%).

Fig. 1. Design scheme of (a) SCM-PCMP and (b) DCM-PCMP. Photograph of (c) SCM-PCMP and (d) DCMPCMP.

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ACCEPTED MANUSCRIPT 3. Experimental 3.1. Battery A commercial prismatic LiFePO4 battery (supplied by Tianjin Lishen Joint-Stock Co.) with length of 69 mm, width of 26 mm and height of 89 mm was employed in this investigation. All the batteries used in the experiments were chosen from a same batch and their parameters were listed in Table 2. Table 2 Properties of the prismatic LiFePO4 battery a used in this study.

a

Parameter

Value

Nominal voltage (V) Nominal capacity (Ah) End voltage (V) Charging voltage (V) Max charging current (A) Max discharging current (A) Max operation temperature during charge (°C) Max operation temperature during discharge (°C) Optimum operation temperature during charge (°C) Optimum operation temperature during discharge (°C) Storage temperature within a month (°C) Storage temperature within six months (°C)

3.2 12 2.0 3.65 100 250 0-45 -20-60 15-35 15-35 -40-45 -20-35

In this paper, a single/individual battery refers to one that mainly contains an anode, cathode, separator and

electrolyte. While a battery module refers to one that contains several packaged single/individual batteries.

3.2. Design of experimental system WCM-PCMP, SCM-PCMP, and DCM-PCMP coupled with AFC were used in the passive BTM system of a battery module. For comparison, a battery module without the PCMP composite and only using AFC was also prepared. The test system is shown in Fig. 2. A CT-3001W battery testing instrument (50 V/120 A, Shenzhen Neware Electronics Co. Ltd., China) was used to measure the electrochemical reference parameters during the charge and discharge processes. An Agilent-34972A data acquisition system (Agilent Technologies Inc.) with a temperature acquisition time of 1 s was used to collect the temperature data of the battery modules. The air velocity through the battery modules was controlled with a DC power supply and measured with an anemograph. The battery module consisted of six PCMP composites and five prismatic LiFePO4 batteries

-7-

ACCEPTED MANUSCRIPT arranged in a compact sandwich structure. An image of the physical module and a sectional schematic view of the DCM-PCMP battery module are shown in Fig. 3(a) and (b), respectively. The heat flux, which was the same as those of the battery modules using SCM-PCMP and WCMPCMP, is illustrated in Fig. 3(c). The heat transfer within the battery module can be divided into three processes: First, the heat generated inside the batteries mainly transfers through the areas of direct contact between the batteries and the PCMP composites. Next, the heat is mainly transferred by the high thermal conductivity copper mesh through the PCMP composites to the section of outstretched copper mesh. Finally, the heat is removed by the disturbed air caused by the outstretched copper mesh. This phenomenon will be discussed further in Section 4.6.

(a)

DC Power Supply

Power Batter y Testing Equipment Batter y module

Fan

Fan

Data Logger

Computer System

(b) Computer System

DC Power Supply Batter y module Data Logger

Fan Power Batter y Testing Equipment

Fig. 2. (a) Schematic diagram and (b) photograph of the experimental system. (a)

(b)

Air flow

5

+ (c)

+ 4

-

-

+

3

2

+

LiFePO 4 batter y

Copper Mesh

1

-

+

DCM-PCMP

-

+

+

Heat flux

-

Air flow

Fig. 3. (a) Physical layout, (b) sectional schematic view, and (c) heat flux diagram of the battery module assembled with DCM-PCMP. -8-

ACCEPTED MANUSCRIPT 3.3. Testing of battery modules During the thermal response of the battery module, in order to simplify test procedure and screening representative data, the highest temperature on the surface of the battery can be assumed to be on the central position. Furtherly, thermal imaging device (Ti9, Fluke Corporation, USA) was used to verify this assumption under the condition of ANC (at the temperature of 25 °C). In turn, the battery was discharged at 1 C-rate and 5 C-rate. The thermal images of this two periods were snapped at the 20% and 80% depth of discharge (DOD) respectively, as shown in Fig. 4, and seven temperature test points were measured with T-type thermocouples (Omega type TT-T-30SLE-1M, accuracy of ±1 °C) in two periods. As the above assumption, the highest temperature was appeared on the center position of the battery in the two images. Therefore, T-type thermocouples (Omega type TT-T-30-SLE-1M) with an accuracy of ±1 °C were placed in the central position on both sides of each battery in the following works, and then the average temperature was considered the highest surface temperature of the battery. Under the condition of constant temperature atmosphere of 25 °C, the parameters of the battery modules were measured through the following experiment. Simultaneously, the galvanostatic charge and discharge cycles of battery modules were executed by the battery testing instrument. The test stage was as follows: •Discharge stage: The battery modules were discharged under constant current of 12 A (1 C) , 36 A (3 C) and 60 A (5 C), respectively until the voltage decayed to 10 V. •Hold stage: 10 min. •Charge stage: The battery modules were charged under constant current of 12 A (1 C) until the voltage reached 18.25 V, and then the voltage was retained constant at 4.2 V until the charge current descended to 0.3 A. •Hold stage: 30 min. •Cycle times: 3 times.

-9-

ACCEPTED MANUSCRIPT +

-

(a)

-

27.8

27.5

28.0

28.1 27.7

27.5

27.2

30.1

39.1

39.3

39.3

39.0

30.0

30.5

29.8

DOD=20% +

(b)

+

29.7

30.2 29.8

DOD=80% +

39.3

60.9

39.2

61.0

38.9

60.7

DOD=20%

60.9

61.1

61.1 60.8

DOD=80%

Fig. 4. Thermal images of the prismatic LiFePO4 battery surface in the process of (a) 1 C and (b) 5 C discharge rate.

4. Results and discussion 4.1. Thermal characteristics of the PCMP composites The differential scanning calorimetry (DSC) results of the PA and the PCMP composites are shown in Fig. 5. The fusion point of the PA ranged from 47.35 to 49.59 °C, and its latent heat was 255.8 J/g. After addition of the EG and the LDPE, the fusion point of the resulting PCMP composite ranged from 46.26 to 48.47 °C, and its total latent heat decreased to 147.7 J/g. The main peak on the right side of the curve of the PCMP composite represents the solid-liquid phase change, and correspondingly the minor peak on the left side of the curve represents the solid-solid phase transition [29]. 1 0 Heat Flow (W/g)

47.35 oC 255.8 J/g

46.26 oC 147.7 J/g

-1 48.47 oC

-2 -3

PCMP composite PA

-4

49.59 oC

-5 20

30

40

50

60

Temperature (oC)

70

Fig. 5. DSC curves of PA and the PCMP composite.

- 10 -

80

ACCEPTED MANUSCRIPT Furthermore, the phase transition of the PA leads to a variation of the thermal conductivity. The testing results for thermal conductivity of the PCMP composites at different temperatures are shown in Fig. 6. The maximum thermal conductivity of the DCM-PCMP reached 8.327 W/m·K at 50 °C, which was about 17.5% and 36.0% higher than that of the SCM-PCMP (7.087 W/m·K) and EG/PA-PCMP (6.128 W/m·K), respectively. However, although the thermal conductivity of the DCM-PCMP varied relatively gradually from 25 to 40 °C, it dramatically increased within the phase change temperature range (45–50 °C). This steep change in thermal conductivity is mainly attributed to the superior capability of the DCM-PCMP in maintaining a steady temperature during the phase change process [31-32]. The high thermal conductivity double copper mesh can be considered to be a heat conduction framework that enhances the heat dissipation performance of PCMP composite, as well as disturbs the air around the batteries to improve the temperature uniformity within the battery module and decrease Tmax inside the battery. This phenomenon will

Thermal Conductivity (W/mK)

be discussed further in section 4.6. 9.0 DCM-PCMP SCM-PCMP WCM-PCMP EG/PA-PCMP PA

7.5 6.0 4.5 3.0 1.5 20

30

40 50 60 Temperature (oC)

70

80

Fig. 6. Thermal conductivity of the PCMP composites (DSM-PCMP, SCM-PCMP, WCM-PCMP, and EG/PAPCMP) and PA at different temperatures.

4.2. Temperature response of the battery modules at different discharge rate As is known, the Tmax of a whole battery module mostly occurs in the middle battery, for example, cell No. 3 in the battery module shown in Fig. 3(b). The time-dependent temperature curves of the battery modules measured at different discharge rates are displayed in Fig. 7. The Tmax of the battery modules using ANC, WCM-PCMP, SCM-PCMP, and DCM-PCMP was 74.5, 60.2, 59.0, and 55 °C, respectively, under the condition of 5 C discharge rate. A similar tendency was observed at 1 C and 3 C discharge rates. Moreover, the ΔTmax of the battery module using the DCM-PCMP was lowest among the four cases, which indicates that a better thermal effect was - 11 -

ACCEPTED MANUSCRIPT obtained with the DCM-PCMP method. It is notable that the cooling effect of the BTM system with DCM-PCMP was most effective compared with that of the other three cases during 5 C discharge, in which the Tmax of the battery module approached the melting range, and was 4 °C lower than that of SCM-PCMP. These results confirmed that the BTM system using DCM-PCMP also effectively reduced the rise in battery temperature during high-rate discharge. The ΔT within the module is discussed in the next section. 5C

70

56

65

o

Temperature ( C)

60

48

55 50

Melting Range

44

45 200

80

Melting Range

400

600

Time (s)

800

40

1000

400

600

800

1000

Time (s)

1200

1400

TMAX-ANC TMIN-ANC

70

TMAX-WCM-PCMP

37

TMIN-WCM-PCMP

60

TMAX-SCM-PCMP TMIN-SCM-PCMP

50

TMAX-DCM-PCMP TMIN-DCM-PCMP

40

36 o

o

Temperature ( C)

3C

52

1C

Temperature ( C)

o

Temperature ( C)

75

35 34 33 32

Melting Range

31

30

30

0

500 1000 1500 2000 2500 3000 3500 Time (s)

2600

2800

3000

Time (s)

3200

3400

Fig. 7. Experimentally measured temperature response of the battery modules with different heat dissipating methods at different discharge rate.

4.3. Temperature uniformity of the battery modules at different discharge rate The ΔT of the four battery systems was measured at different discharge rates, as shown in Fig. 8, to evaluate their temperature uniformity. The temperature uniformity of all modules deteriorated over time. The ΔT of the battery module with DCM-PCMP heat dissipation was the minimum among the four cases, which indicates that DCM-PCMP balanced the temperature in the battery more efficiently. Except for 1 C discharge rate, the ΔT of the battery modules containing PCMP composites exhibited a gradual small decrease, while after discharge, ΔT increased slowly during the rest period. This can be ascribed to the following two reasons: (1) The PCMP composites absorbed a large amount of the heat generated inside the battery and underwent liquidsolid phase transition, which can give rise to a better temperature balance in the battery module [29, 33]. (2) The thermal conductivity of the PCMP composite greatly decreased when it reached

- 12 -

ACCEPTED MANUSCRIPT or exceeded the thermal saturation state (as shown in Fig. 3(b)), which impeded heat dissipation from battery module and so caused ΔT to increase slowly during the rest period. Consequently, the heat generated inside the battery would build up continuously in the PCMP composite with cycling, which could result in a poor heat dissipation performance in practical applications. Temperature Difference ( C)

3.0

o

(a)

2.5

ANC WCM-PCMP SCM-PCMP DCM-PCMP

2.0 1.5 1.0 0.5

Rest

0.0

Discharge

o

Temperature Difference ( C)

0 5

600 (b)

ANC WCM-PCMP SCM-PCMP DCM-PCMP

4 3 2 1

Discharge

0 0

10

o

Temperature Difference ( C)

1200 1800 2400 3000 3600 Time (s)

300

600 900 Time (s)

Rest

1200

1500

(c)

8

ANC WCM-PCMP SCM-PCMP DCM-PCMP

6 4 2 0

Discharge

0

200

400 600 Time (s)

Rest

800

1000

Fig. 8. ΔT of the battery modules with different heat dissipating methods at (a) 1 C, (b) 3 C, and (c) 5 C discharge rate.

4.4. Cycle performance of the battery modules at different discharge rate To further investigate the heat dissipation of the PCMP composites for rectangular battery modules during cycling in practical application, the No. 3 cell (see Fig. 3(b)) in each battery module was chosen for further study. The relationship observed between the temperature variation and time at different discharge rate is displayed in Fig. 9. The No. 3 cell in the battery module with DCM-PCMP heat dissipation exhibited the lowest temperature among the four heat dissipation methods. Taking the results for 5 C discharge rate as an example, at the beginning of the first, second, and third cycles, the temperature of the DCM-PCMP battery module was 25.0, - 13 -

ACCEPTED MANUSCRIPT 44.7, and 48.9 °C, respectively, which caused Tmax to gradually increase with the number of cycles (57.5, 72.5, and 77.0 °C, respectively). The same tendency was observed for the other three systems. These results confirmed that the DCM-PCMP endowed the rectangular battery module with excellent thermal performance. However, more and more heat will accumulate in the PCMP composites with cycling if it cannot be transferred to the ambient environment quickly and effectively, which will easily lead to thermal runaway, especially at high rate discharge. 44

(a)

o

Temperature ( C)

40 36 ANC WCM-PCMP SCM-PCMP DCM-PCMP D-Discharge C-Charge/Rest D D C

28 24 0

o

o

DCM-PCMP

o

25.0 C

D

C

C

ANC WCM-PCMP SCM-PCMP DCM-PCMP

50

o

o

o

45.7 C

30

75

51.0 C

o

D

0 90

48.4 C

o

D-Discharge C-Charge/Rest

20

o

40.0 C

34.4 C

o

33.4 C

(b)

40

Temperature ( C)

38.2 C

6000 12000 18000 24000 30000 Time (s)

Temperature ( C)

60

o

o

39.7 C

o

32

C

35.3 C

DCM-PCMP

o

25.0 C

C

D

37.1 C

C

D

5000 10000 15000 20000 25000 Time (s)

(c)

60 45

o

ANC WCM-PCMP SCM-PCMP DCM-PCMP D-Discharge C-Charge/Rest D C D

30 15 0

0

o 72.5 C 77.0 C o

57.5 C o

o

44.7 C 25.0 C C

48.9 C DCM-PCMP

o

D

C

3000 6000 9000 12000 15000 18000 Time (s)

Fig. 9. Temperature variation in the battery modules with different heat dissipating methods at (a) 1 C, (b) 3 C and (c) 5 C discharge rate in the process of cycles.

4.5. Effect of air velocity on heat dissipation performance Next, the coupling of the above four heat dissipation methods with AFC was investigated. The dependence of the temperature of the No. 3 cell in different battery modules on the AFC air velocity during cycling at 5 C discharge rate is shown in Fig. 10. For the DCM-PCMP battery - 14 -

ACCEPTED MANUSCRIPT module, an air velocity of 2 m/s kept the Tmax of the battery module below 52.8 °C during the cycling process, compared with 68.0, 59.7, and 54.4 °C for the modules with ANC, WCM-PCMP, and SCM-PCMP. At the first, second, and third cycle, the Tmax of the battery module using DCMPCMP heat dissipation was 52.5, 52.6 and 52.8 °C, respectively, and increased by 0.3 °C at the end of each cycle. Thus, with forced air cooling, the battery modules containing PCMP composite exhibited little temperature fluctuation with cycling. It is notable that, at the same inlet air velocity, the temperatures measured in the DCM-PCMP case were lower than those in the other three cases. As shown in Fig. 3(b) and (c), the airflow mainly carried out heat exchange with the copper mesh and the PCMP composites, and thus removed heat generated inside the batteries. As a result, DCM-PCMP heat dissipation with forced air coupling showed good performance under driving conditions. To further verify the cooling effect of DCM-PCMP heat dissipation with forced air coupling, the Tmax distributions of the different battery modules measured at various air velocities during 5 C discharge are shown in Fig. 11. In each case, Tmax was lowest for the battery with DCM-PCMP heat dissipation, and moreover, ΔTmax remained below 3 °C. Notably, at all air velocities, the hottest part of the battery module was the middle (No. 3 cell). The main reason for this phenomenon is a decrease in air cooling capacity in the middle of the battery module, which is attributed to space constraints and airflow variation [29-30].

80 70 60

80 (b)

65 60 55 50 45 40

14400 15200

50

60

60

0 m/s 2 m/s 4 m/s 5 m/s 6 m/s 7 m/s 8 m/s

55 50 45 40

50

14000

15000

40

40

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Fig. 10. Temperature variation of the battery modules with (a) ANC, (b) WCM-PCMP, (c) SCM-PCMP, and (d) DCM-PCMP heat dissipation at different air velocities during cycling (5 C discharge rate).

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ACCEPTED MANUSCRIPT 90 5 4 3 2 1

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Fig. 11. Tmax distributions of the battery modules with (a) ANC, (b) WCM-PCMP, (c) SCM-PCMP, and (d) DCM-PCMP heat dissipation at different air velocities during cycling (5 C discharge rate).

4.6. Effect of outstretched copper mesh on heat dissipation performance Through the above experimental analysis, it can be inferred that the DCM-PCMP method endowed excellent surface thermal response and temperature uniformity to the rectangular battery module. The copper mesh used in this method has the advantage of high thermal conductivity, and the outstretched copper mesh in the module plays an important role in disturbing the air flowing through the module to strengthen its heat transfer capability. To further research the resulting effect on the internal temperature distribution of the battery, battery modules using SCM-PCMP and DCM-PCMP composites with and without outstretched copper mesh were simulated [30]. The velocity vectors and temperature distributions of these four battery modules at the end of 3C discharge under an air velocity of 8 m/s are shown in Fig. 12. In the case of DCM-PCMP with outstretched copper mesh, the Tmax inside the battery is 43 °C, which is lowest among the four different conditions. This result is attributed to the intensive air disturbance caused by the double outstretched copper mesh, which aided the optimization of the temperature distribution inside the battery and improved the temperature uniformity within the battery module. The result also indicates that the double outstretched copper mesh disturbs the air to the most extent among the four different cases, which gives rise to a decrease in the surface thermal resistance of the copper mesh and accelerates heat transfer. This will be discussed further in the next section.

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ACCEPTED MANUSCRIPT

Fig. 12. Velocity vectors and temperature distributions in battery modules with (a) SCM-PCMP, (b) SCM-PCMP without outstretched copper mesh, (c) DCM-PCMP and (d) DCM-PCMP without outstretched copper mesh (discharge rate = 3 C, air velocity = 8 m/s).

To further improve the BTM system, the thermal resistances of the four above-mentioned PCMP composites were analyzed based on the following formula [34-36]: Rall  Rb  PCMP  RPCMP  RP  air 

Tb  TPCMP ,1 Q



TPCMP ,1  TPCMP , 2 Q



TPCMP , 2  Tair Q

(1)

As shown in Fig. 13, a PTC heating element was used as the stable heat source between two PCMPs. One of the thermocouples was mounted on the central position of the PCMP surface, while one of them was mounted near the edge of PCMP. The thermal resistance was tested under different air velocities (0 m/s, 2 m/s, 4 m/s, 5 m/s, 6 m/s, 7 m/s and 8 m/s) and was achieved by the average value in the temperature range of 45-55 °C.

(a)

+ (b)

PCMP

-

Heat source

PCMP

Copper mesh

Thermocouples

Thermal insulation material Fan

Fig. 13. The (a) actual diagrams and (b) schematic of the thermal resistance test device. - 17 -

ACCEPTED MANUSCRIPT The variation in the thermal resistances calculated with Eq. (1) with air velocity is shown in Fig. 14. The thermal resistance of DCM-PCMP with outstretched copper mesh is lowest among the four different conditions. The main reason for this is that the outstretched copper mesh severely disturbs the surrounding air, thereby greatly enhancing the heat dissipation. To supplement this theory, the Tmax distributions of the four different battery modules were investigated. The temperature distribution in the battery modules at the end of cycling at different air velocities are shown in Fig. 15. At different discharge rates and air velocities, the temperature of the battery modules with outstretched copper mesh are usually lower than those of the modules without it. Furthermore, the smoother camber of the plots for the battery modules with outstretched copper mesh indicates a more uniform temperature distribution in these cases. Finally, DCM-PCMP with outstretched copper mesh exhibits excellent heat dissipation performance and temperature

o

Thermal resistance ( C/W)

equilibrium capability.

50

DCM-PCMP with outstretched CM DCM-PCMP without outstretched CM SCM-PCMP with outstretched CM SCM-PCMP without outstretched CM

45 40 35 30 25 0

2 4 6 Air velocity (m/s)

8

Fig. 14. Thermal resistance distributions of battery modules containing PCMPs with and without outstretched copper mesh at different air velocities.

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ACCEPTED MANUSCRIPT

Fig. 15. Tmax distributions of battery modules containing PCMPs with and without outstretched copper mesh during cycling at different air velocities and discharge rates.

4.7. Air velocity analysis for optimum DCM-PCMP heat dissipation performance The present DCM-PCMP heat dissipation method not only achieved a better cooling effect, but also decreased the power consumption of the fans. However, the cooling effect was not obvious until the air velocity was increased to a certain amount [37]. According to the energy balance, the air velocity at which optimum heat dissipation performance is obtained was analyzed. The following equation was used in the calculation [31]:

Tair  Toutlet - Tinlet

(2)

Wdiss   Tair C p ,air air  air dt cycle

(3)



Wdiss Pfans t cycle

(4)

where tcycle is the duration of the cycle, ρair is the density of air (1.29 kg/m3), Cp,air is the specific heat capacity of air (1010 J/kg·K), ΔTair is the temperature difference between the inlet and outlet air, Toutlet is the outlet air temperature, Tinlet is the inlet air temperature, ʋair is the air flow rate, Wdiss is the heat dissipated, Pfans is the power consumed by the fans, and η is the coefficient of heat - 19 -

ACCEPTED MANUSCRIPT dissipation performance of the battery module. As shown in Fig. 16, during 5 C discharge, the peak value (2.0936) of the coefficient of heat dissipation performance of the battery module with DCM-PCMP was obtained at an air velocity of 6 m/s. A similar tendency was observed at discharge rates of 1 C and 3 C. The main reason for this result that Cp,air decreases with increasing air velocity to a certain degree, leading to a reduction in air cooling capacity [35]. 2.0 1.6



1.2 0.8 1C 3C 5C

0.4 0.0 0

2 4 6 Air velocity (m/s)

8

Fig. 16. The coefficient of heat dissipation performance of the DCM-PCMP method at different air velocities.

5. Conclusion A quaternary PCMP composite was applied to the BTM system of a LiFePO4 rectangular battery module and its properties were investigated. The heat dissipation performance and temperature uniformity of the battery modules with the quaternary PCMP composites were evaluated during galvanostatic discharge. With the presence of outstretched copper mesh, the thermal conductivity of DCM-PCMP was increased by 17.5% and 36.0% compared with that of SCM-PCMP and EG/PA-PCMP, respectively. As a result, the Tmax of the battery module with DCM-PCMP heat dissipation was about 55 °C during galvanostatic discharge testing, 4.0, 5.2, and 19.5 °C lower than that observed in the cases of SCM-PCMP, WCM-PCMP and ANC heat dissipation. Moreover, ΔTmax was also lowest for DCM-PCMP. Simulated and experimental cycle testing results showed that the battery module with DCM-PCMP exhibited much better heat dissipation performance and temperature uniformity than the other three methods, on both the inside and outside surfaces of the battery. Furthermore, coupling DCM-PCMP method with AFC maintained the temperature of the battery module in the relatively stable range. Consequently, DCM-PCMP heat dissipation with forced air coupling exhibited excellent performance under - 20 -

ACCEPTED MANUSCRIPT driving conditions. The heat dissipation performance was found to be optimal at an air velocity of 6 m/s. Thus, the battery module with DCM-PCMP heat dissipation not only exhibited excellent cycle performance but also the highest coefficient of heat dissipation performance, which efficiently decreased extra power consumption by as much as possible. The DCM-PCMP heat dissipation method presented in this work provides an efficient solution for the thermal management of rectangular battery modules. At high temperature and discharge rate conditions, HEV and PEV batteries may not work normally unless they are matched with an effective BTM system. Battery modules containing DCM-PCMP coupled with AFC exhibit excellent temperature balance and heat dissipation performance, and can be expected to achieve the aims of high coefficient of heat dissipation performance and prominent cooling effect.

Acknowledgment This work is supported by Science and Technology Planning Project of Guangdong Province, China (2014B010128001), South Wisdom Valley Innovative Research Team Program (2015CXTD07), Scientific and technological project of Administration of Quality and Technology Supervision of Guangdong Province (2015PJ03), Science and technology application research and development projects of Guangdong Province, China (2015B010135010), Science and Technology Plan Projects of Guangdong Province. China (2016B090918015). References [1] Yang YL, Hu XS, Pei HX, Peng ZY. Comparison of power-split and parallel hybrid powertrain architectures with a single electric machine: Dynamic programming approach. Appl Energy 2016;168:683-90. [2] Hu XS, Martinez CM, Yang YL. Charging, power management, and battery degradation mitigation in plug-in hybrid electric vehicles: A unified cost-optimal approach. Mech Syst Signal Pr 2017;87:4-16. [3] Hu XS, Jiang JC, Cao DP. Battery Health Prognosis for Electric Vehicles Using Sample Entropy and Sparse Bayesian Predictive Modeling. IEEE T Ind Electron 2016;63(4):2645-56. [4] Zhang SL, Qin J, Bao W, Feng Y,Xie KL. Thermal management of fuel in advanced aeroengine in view of chemical recuperation. Energy 2014;77:201-11.

- 21 -

ACCEPTED MANUSCRIPT [5] Wang QS, Ping P, Zhao XJ, Chu GQ, Sun JH, Chen CH. Thermal runaway caused fire and explosion of lithium ion battery. J Power Sources 2012;208:210-24. [6] Feng XN, Lu LG, Ouyang MG, Li JQ, He XM. A 3D thermal runaway propagation model for a large format lithium ion battery module. Energy 2016;115(1):194-208. [7] Wright RB, Christophersen JP, Motloch CG, Belt JR, Ho CD, Battaglia VS, et al. Power fade and capacity fade resulting from cycle-life testing of Advanced Technology Development Program lithium-ion batteries. J Power Sources 2003;119:865-9. [8] Ramadass P, Haran B, White R, Popov BN. Capacity fade of Sony 18650 cells cycled at elevated temperatures Part II. Capacity fade analysis. J Power Sources 2002;112(2):614-20. [9] Pesaran AA. Battery thermal models for hybrid vehicle simulations. J Power Sources 2002;110(2):377-82. [10] Zhao R, Gu JJ, Liu J. Optimization of a phase change material based internal cooling system for cylindrical Li-ion battery pack and a hybrid cooling design. Energy 2017;135:811-22. [11] Zhang SJ, Zhao R, Liu J, Gu JJ. Investigation on a hydrogel based passive thermal management system for lithium ion batteries. Energy 2014;68:854-61. [12] Yang XH, Tan SC, Liu J. Thermal management of Li-ion battery with liquid metal. Energ Convers Manage 2016;117:577-85. [13] 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 Energy 2014;134:229-38. [14] Shah K, McKee C, Chalise D, Jain A. Experimental and numerical investigation of core cooling of Li-ion cells using heat pipes. Energy 2016;113:852-60. [15] Jin LW, Lee PS, Kong XX, Fan Y, Chou SK. Ultra-thin minichannel LCP for EV battery thermal management. Appl Energy 2014;113:1786-94. [16] Tran TH, Harmand S, Sahut B. Experimental investigation on heat pipe cooling for Hybrid Electric Vehicle and Electric Vehicle lithium-ion battery. J Power Sources 2014;265:262-72. [17] Wang Q, Jiang B, Li B, Yan YY. A critical review of thermal management models and solutions of lithium-ion batteries for the development of pure electric vehicles. Renew Sust Energy Rev 2016;64:106-28.

- 22 -

ACCEPTED MANUSCRIPT [18] Hassan F. High thermal performance lithium-ion battery pack including hybrid active– passive thermal management system for using in hybrid/electric vehicles. Heat Mass Transfer 2014;70:529-38. [19] Rao ZH, Wang SF. A review of power battery thermal energy management. Renew Sust Energy Rev 2011;15(9):4554-71. [20] Javani N, Dincer I, Naterer GF, Rohrauer GL. Modeling of passive thermal management for electric vehicle battery packs with PCM between cells. Appl Therm Eng 2014;73(1):307-16. [21] Al Hallaj S, Selman JR. A novel thermal management system for electric vehicle batteries using phase-change material. J Electrochem Soc 2000;147(9):3231-6. [22] Babapoor A, Karimi G, Khorram M. Fabrication and characterization of nanofibernanoparticle-composites with phase change materials by electrospinning. Appl Therm Eng 2016;99:1225-35. [23] Samimi F, Babapoor A, Azizi M, 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] Hussain A, Tso CY, Chao CYH. Experimental investigation of a passive thermal management system for high-powered lithium ion batteries using nickel foam-paraffin composite. Energy 2016;115(1):209-18. [25] Alrashdan A, Mayyas AT, Al-Hallaj S. Thermo-mechanical behaviors of the expanded graphite-phase change material matrix used for thermal management of Li-ion battery packs. J Mater Process Technol 2010;210(1):174-9. [26] Lv YF, Yang XQ, Li XX, Zhang GQ, Wang ZY, Yang CZ. Experimental study on a novel battery thermal management technology based on low density polyethylene-enhanced composite phase change materials coupled with low fins. Appl Energy 2016;178:376-82. [27] 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. [28] 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.

- 23 -

ACCEPTED MANUSCRIPT [29] Wu WX, Yang XQ, Zhang GQ, Ke XF, Wang ZY, Situ WF, Li XX, Zhang JY. An experimental study of thermal management system using copper mesh-enhanced composite phase change materials for power battery pack. Energy 2016;113:909-16. [30] Hemery CV, Pra F, Robin JF, Marty P. Experimental performances of a battery thermal management system using a phase change material. J Power Sources 2014;270:349-58. [31] 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. [32] Tian BQ, Yang WB, Luo LJ, Wang J, Zhang K, Fan JH, et al. Synergistic enhancement of thermal conductivity for expanded graphite and carbon fiber in paraffin/EVA form-stable phase change materials. Sol Energy 2016;127:48-55. [33] Rao ZH, Wang QC, Huang CL. Investigation of the thermal performance of phase change material/mini-channel coupled battery thermal management system. Appl Energy 2016;164:65969. [34] Harmel J, Ohms D, Guth U, Wiesener K. Investigation of the heat balance of bipolar NiMHbatteries. J Power Sources 2006;155(1):88-93. [35] Tiari S, Qiu SG, Mahdavi M. Numerical study of finned heat pipe-assisted thermal energy storage system with high temperature phase change material. Energ Convers Manage 2015;89:833-42. [36] Wu WX, Zhang GQ, Ke XF, Yang XQ, Wang ZY, Liu CZ. Preparation and thermal conductivity enhancement of composite phase change materials for electronic thermal management. Energ Convers Manage 2015;101:278-84. [37] Holman JP. Heat Transfer. Machinery Industry Press 2011.

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