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An experimental study of heat pipe thermal management system with wet cooling method for lithium ion batteries
Journal of Power Sources 273 (2015) 1089e1097
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
An experimental study of heat pipe thermal management system with wet cooling method for lithium ion batteries Rui Zhao, Junjie Gu, Jie Liu* Department of Mechanical and Aerospace Engineering, Carleton University, Ottawa, ON K1S 5B6, Canada
h i g h l i g h t s A heat pipe e wet cooling combined battery thermal management system is developed. The performance of the proposed system is compared with other cooling systems. The combined system efficiently controls the temperature rise in discharge tests. The temperature gradient is minimized in the combined cooling system. An energy saving approach of using wet cooling system is proposed.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 August 2014 Accepted 2 October 2014 Available online 13 October 2014
An effective battery thermal management (BTM) system is required for lithium-ion batteries to ensure a desirable operating temperature range with minimal temperature gradient, and thus to guarantee their high efficiency, long lifetime and great safety. In this paper, a heat pipe and wet cooling combined BTM system is developed to handle the thermal surge of lithium-ion batteries during high rate operations. The proposed BTM system relies on ultra-thin heat pipes which can efficiently transfer the heat from the battery sides to the cooling ends where the water evaporation process can rapidly dissipate the heat. Two sized battery packs, 3 Ah and 8 Ah, with different lengths of cooling ends are used and tested through a series high-intensity discharges in this study to examine the cooling effects of the combined BTM system, and its performance is compared with other four types of heat pipe involved BTM systems and natural convection cooling method. A combination of natural convection, fan cooling and wet cooling methods is also introduced to the heat pipe BTM system, which is able to control the temperature of battery pack in an appropriate temperature range with the minimum cost of energy and water spray. © 2014 Elsevier B.V. All rights reserved.
Keywords: Lithium ion battery Battery thermal management Heat pipe Wet cooling Temperature
1. Introduction Lithium-ion (Li-ion) batteries have emerged as one of the most promising energy storage technologies due to their higher energy density, power density and no memory effect when compared with other secondary batteries [1]. Consequently, the advantages such as long driving range and fast acceleration capability of the Li-ion batteries make them highly recommended as power sources for electric vehicles (EV), hybrid electric vehicles (HEV) and plug-in hybrid electric vehicles (PHEV). Although offering many benefits and advantages, the poor performances of Li-ion batteries at either too high or too low temperatures, such as short supply of energy, fast degradation and safety issues [2e7], may constrain their real
* Corresponding author. Tel.: þ1 613 520 2600x8257; fax: þ1 613 520 5715. E-mail address: [email protected] (J. Liu). http://dx.doi.org/10.1016/j.jpowsour.2014.10.007 0378-7753/© 2014 Elsevier B.V. All rights reserved.
applications. Thus, the battery thermal management (BTM) system is essential for the Li-ion battery packs, especially for the ones commonly used under high rate discharges or in critical applications (e.g., aerospace and military). A proper thermal management system is able to control the temperatures of batteries in a proper range and to minimize the temperature gradient in battery packs. In general, a high operating temperature can accelerate the degradation rate of battery pack and lead to safety issues, e.g., overheating and thermal runaway; the uneven temperature distribution in battery pack may give rise to different capacity fading rates between batteries and finally change the charging and discharging behaviors of the entire battery pack. Motloch et al. [8] reported that the lifespan of Li-ion batteries can be reduced by about two months for every degree of temperature rise in an operating range of 30 Ce40 C. It is also found that the temperature gradient in both the battery level and pack level should be lower than 5 C [9].
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To achieve the goal of controlling battery temperature in a appropriate range, different mediums, such as air [10e12], liquid [13], phase change material (PCM) [14e16] and hydrogel [6,17], have been implemented in the previous thermal management systems. Among them, the air cooling method is generally performed in a forced convection way, in which the air is brought from the fan [11] or wind tunnel [18], and the researches on air cooling is mainly focusing on optimizing the layout of batteries and air flows in the pack, the wind speed and energy cost. For example, Park [10] designed and compared five air flow configurations for a specific battery pack layout, and it is demonstrated that the required cooling performance can be achieved by employing the tapered manifold and pressure relief ventilation on the cooling tunnel. In Ref. [12], an air-cooled Li-ion battery module for plug-in hybrid electric vehicles is studied, in which the influences of the gap spacing and air flow rate on the cooling effect of BTM system were fully explained. PCM cooling systems are also well investigated by Al-Hallaj’s research group [14e16]. Thanks to the cooling mechanism, use of latent heat, the PCM cooling system possesses high energy storage capacity and thus can effectively lower the temperature of battery pack. But due to the low conductivity, the PCM cooling system will suffer from the inherent limitations such as the moderate temperature gradient and long cooling time. Although many matrices were developed to improve the thermal conductivity of the PCM, the high cost and volume change of PCM cannot be well managed. Recently, heat pipe based BTM systems are fully investigated due to the advantages accompanied with heat pipe and the corresponding BTM system, such as high thermal conductivity, compact structure, flexible geometry, etc. So far, the researches related to heat pipe BTM system can be classified as air-cooled system [19,20] and water-cooled system [21] depends on different cooling strategies used in the cooling ends of heat pipes. Wu et al. [19] compared the cooling effectiveness of natural convection and forced convection with/without heat pipes in a Li-ion battery pack from both simulated and experimental perspectives, and it is found that the insertion of heat pipes into the cooling system can effectively lower the temperature and minimize the temperature difference inside the battery pack. A water-cooled heat pipe BTM system was also studied [21], in which a thermostat water bath was implemented to control the temperature rise. The result shows that the battery maximum temperature can be controlled below 50 C when the heat generation rate is under 50 W, and the maximum temperature gradient can be reduced to 5 C when the heat generation rate is decreased to 30 W. Although the relevant works revealed the excellence of heat pipe BTM system compared to other cooling systems, the cooling effectiveness can still be greatly improved by using well contacted pipes and wet cooling method. In this work, a mature cooling technology in industry, wet cooling, is combined with the existing heat pipe cooling system to investigate its cooling effect. Temperature histories and temperature gradients of the wet cooling pipe system are collected and compared with other BTM systems. The testing results demonstrate that the wet cooling combined system is the most effective BTM system in cooling and it can be coupled with the other heat pipe systems in real applications to control the temperature variation of Li-ion battery with the lowest energy consumption and to improve the performance of entire battery system. 2. Experimental Two battery packs, 3 Ah, four batteries in series (4S1P), and 8 Ah (4S1P), are used in the experiments to test the effectiveness of different cooling systems. The specifications of the batteries in two battery packs are listed in Table 1. Depends on whether heat pipes are involved in the BTM system, the cooling systems in the tests are
Table 1 Specifications of 3 Ah battery and 8 Ah battery.
Length (mm) Width (mm) Height (mm) Weight (g) Nominal voltage (V) Nominal capacity (mAh) Maximum charge rate (C) Maximum discharge rate (C) Working temperature range ( C)
Cell 1
Cell 2
125 40 5.5 70 3.7 3000 5 25 20e60
180 45 9 185 3.7 8000 5 25 20e60
classified as two main classes: pack without heat pipes (natural convection) and pack with heat pipes. In natural convection cooling system, all the batteries in pack are attached closely with the neighboring batteries. While in heat pipe cooling systems, heat pipes are inserted between batteries with leaving the cooling ends outside. Two types of heat pipes with different dimensions are selected to meet the sizes of the battery packs. Meanwhile, double side adhesive high thermal conductive pads are also used in between batteries and heat pipes to increase the contact areas. The specifications of the heat pipes and thermal pads are given in Table 2. The photographs of two battery packs assembled with heat pipes are given in Fig. 1. Different lengths of cooling ends and battery sizes are chose to test the capability of cooling of heat pipe BTM systems under various conditions. In the tests with 8 Ah battery pack, two more heat pipes are used on the top and bottom of the pack surface to improve the temperature uniformity of the battery pack and is not shown here. Fig. 2 shows the schematic illustrations of battery pack with heat pipe BTM system, in which the temperatures are collected in the assigned points in figures through K-type thermocouples. Through the comparison of the temperatures, the maximum/minimum temperatures and the temperature gradients in pack level and battery level can be obtained. As for heat pipe BTM systems, totally five cooling methods are used: 1) heat pipes in ambient; 2) heat pipes cooled by horizontal fan (hori-fan); 3) heat pipes cooled by vertical fan (verti-fan); 4) heat pipes cooled by thermostat bath and 5) heat pipes cooled through wet cooling. The schematic illustrations of the method 2) e 4) are given in Fig. 3, and the illustration for method 1) is same with other types except the cooling strategy and is not shown here. In cooling fan involved BTM system, the distance between the fan and heat pipes is kept at 5 cm, and the radius and rotate speed of the fan are 4 cm and 1500 rpm, respectively. The medium in thermostat bath in Fig. 3c is water and the temperature is kept at 25 C ± 0.2 C during operation. For wet cooling method, the water volume per spray is 0.7 ml, and the frequency of spray is 1 min1 or 0.5 min1 in the tests. To increase the water adsorption of heat pipes, thin layers of super absorbent fiber are attached to the surfaces of the cooling ends of heat pipes. Table 2 Specifications of heat pipes and thermal pads. Heat pipe 1 Heat pipe 2 Thermal pad 1 (2) Length (mm) Width (mm) Thickness (mm) Weight (g) Material Wick structure Working fluid Working temperature ( C) Thermal conductivity (W m1 K1)
250 40 2 25 Aluminum Groove Acetone 50e125 e
250 50 2.5 40 Aluminum Groove Acetone 50e125 e
125 (175) 40 (45) 0.5 4 (6.5) Silicone compound e e 40e200 3.2
R. Zhao et al. / Journal of Power Sources 273 (2015) 1089e1097
Fig. 1. 8 Ah (top) and 3 Ah (bottom) battery packs with heat pipes.
3. Results and discussion 3.1. Temperature control Before experimental tests, both battery packs are conditioned at room temperature (298 K) by cycling 5 times at 1C rate using ESI battery analyzer (PCBA 5010-4) with a cutoff voltage of 3 V during
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discharging, which allows the formation of solid electrolyte interface (SEI) and eliminates the impact from irreversible capacity fade of new Li-ion batteries [22]. In Fig. 4, the maximum temperature curves of 3 Ah battery pack equipped with six cooling methods under 3C discharge rate are compared. Among all the cooling systems, the battery pack cooled through natural convection experiences the highest temperature elevation, which is mainly attributed to the limited cooling surface area, the accumulation of heat in the center battery and the poor thermal properties of air. Generally, an operating temperature higher than 40 C is not recommended for Li-ion battery system [8], which may accelerate the aging rate and reduce the stability and safety of the energy system, although the capacities obtained at the initial cycles are higher than the battery systems discharged at lower temperature. With the implementation of heat pipes, the cooling area of entire battery pack is increased. As indicated from the red circles in Fig. 4, when the pipe system cooled in ambient, the maximum temperature is decreased from 49 C to 41 C comparing to the system without heat pipes. Interestingly, the heat pipe cooling system with thermostat bath also experiences a relatively high temperature rise, and the main reason is due to the bubble accumulation at the surfaces of the heat pipe cooling ends during the discharging process, which will dramatically decrease the thermal conductivity of the heat pipe and shutdown the heat transfer between the heat pipes and cooling water. Although the increase of the water flow rate and use of defoamer may reduce the amount of bubbles, the costs of energy and expense will limit its application. Then, comparison of temperature curve is made between heat pipe BTM system cooled by hori-fan and verti-fan. Although previous researches have tested that the heat pipe works differently in different inclination angles, the heat pipe used in this study is a micro-channeled ultra-thin pipe, which is able to work in different angles with the same performance. In the ultrathin heat pipe, the gas and liquid inside the micro-channel is discontinuously connected, and the liquid cannot accumulated together due to the gravity. As validated from the results, the temperature curves of the fan-cooling systems in different angles basically overlapped with each other. For wet cooling system, the battery pack discharges until the temperature is stable in the wet cooling condition, and the low initial temperature is attributed to the energy absorption during the evaporation of the water spray. Apparently, the heat pipe BTM system cooled through wet cooling is the most efficient method in suppressing the temperature rise of battery pack among all the methods, with the maximum temperature of 21 C at the end of discharge. Due to the similar results of hori-fan and verti-fan in cooling, and the low cooling efficient of the heavy thermostat bath, discharging processes under rates of 1C and 2C are only performed on four cooling systems (ambient, heat pipes in ambient, hori-pipe with fan and hori-pipe with wet cooling), and the maximum temperature elevations of the four systems are summarized in the bar chart as shown in Fig. 5. It can be seen that the natural convection cooling method is the worst among all the BTM systems, with all the temperature rises higher than 12 C. While for the wet cooling method, the temperature increases are all below 4 C at the assigned discharge rates, which again demonstrates its superiority over other BTM systems in controlling the temperature rise of battery pack, and this will benefits the wellbeing and performance of battery pack in both the short and long term run. 3.2. Temperature distribution
Fig. 2. Schematic illustrations of 8 Ah battery pack with heat pipes. Among the temperature collection points, T1 to T6 are used to measure the temperature gradient in pack level and Ta to Te are used to test the center cell temperature uniformity.
Temperature differences across pack level and battery level are significant factors that may influence the long term performance of battery system. A large temperature gradient in both the battery
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Fig. 3. Schematic illustrations of four BTM systems, a) horizontal heat pipes with cooling fan; b) vertical heat pipes with cooling fan; c) heat pipes in thermostat water bath and d) horizontal heat pipes with wet cooling system.
level and the pack level can lead to different fading rate and unbalanced voltage inside the pack. And thus, a small temperature distribution is necessary for Li-ion battery pack and it is also an indicator of a satisfactory BTM system. In Fig. 6, the maximum temperature differences of 3 Ah battery pack with different BTM systems discharged at 3C rate are shown.
As can be seen, the temperature gradient of battery pack cooled through natural convection method experiences a dramatic rise with the proceeding of discharge, and at the end of discharge, the temperature difference is as high as 6.8 C, which already exceeds the maximum difference recommended by Pesaran [9]. The large gradient of battery pack cooled in ambient is mainly attributed to
Fig. 4. Maximum temperature curves of 3 Ah battery pack equipped with different BTM systems at the discharge rate of 3C.
Fig. 5. Maximum temperature elevations of 3 Ah battery pack implemented with different BTM systems at assigned C rates.
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is because different regions along the center cell have basically the same heat transfer surroundings in different cooling systems. Finally, the results of temperature change and temperature gradients in pack and cell level of selected BTM systems are listed in Table 3. 3.3. Various operating conditions
Fig. 6. Temperature gradients of 3 Ah battery pack with different BTM systems at 3C discharge rate.
the different cooling surroundings across the battery pack, where the exterior parts can easily transfer the heat to the ambient through convection while the interior regions can only transfer the heat through conduction between the batteries, which will limit the heat dissipation. However, for battery pack equipped with heat pipes, the maximum temperature gradients are all below 3 C, and for wet cooling system, the temperature difference at the end of discharge is even lower than 1.5 C. The improved temperature distribution inside pack is mainly due to the involvement of heat pipes, which can efficiently conduct the heat from interior part of battery pack to the ambient at the cooling ends. Meanwhile, during the heat transfer, the phase change inside the pipe can absorb the heat and can keep the battery with low temperature. Fig. 7 depicts the maximum temperature gradients (max (Ta to Te) e min (Ta to Te)) across the center cell with different BTM systems. As shown, although the trends of gradients are similar with the pack temperature gradients (largest gradient in natural convection cooling method and smallest gradient in wet cooling method), all the temperature differences are below 2.5 C, and this
The discharging current of Li-ion batteries may vary with time in real applications, and the capability of a BTM system in keeping the battery temperature stable and uniform under different working conditions is crucial and essential. To test the effectiveness of the wet cooling system, a testing schedule as shown in Fig. 8b is applied on three cooling systems, natural convection, hori-pipe with fan and the wet cooling system, for comparison. In Fig. 8a, the temperatures at points 1 (surface corner) and 4 (pack center) of three cooling systems are given, it can be seen that, as discharge proceeds, the temperatures and temperature difference of battery pack cooled through natural convection experience dramatic rises, and the temperature change is obvious when the current rate varies. However, for wet cooling system, the temperature curves of two assigned points are basically overlapped and the temperature even decreases when the C-rate switches from 3C to 2C and from 2C to 1C. With the cooling medium switches from water to air, the fan cooling system performs moderately. The cooling time of the previous three BTM systems are compared in Fig. 9. In the test, the 3 Ah battery pack is discharged at 10 A to reach the highest temperature, and the cooling systems are kept on working when the discharge stops. As shown, due to the limitation of temperature dissipation, the maximum temperature of battery cooled through natural convection takes more than 2500 s to decrease to 30 C. While for heat pipe cooling systems, the times to recovery to the initial temperature are much shorter, and the time is even decreased to less than 300 s in wet cooling system, which again prove its excellent cooling ability. 3.4. Cooling effects on large battery pack A compact battery pack is required in some mission-critical applications, therefore, the space for cooling ends may be limited. To validate the wet cooling system in controlling the temperature rise of large sized battery pack with limited cooling area, an 8 Ah battery pack is constructed, where the cooling ends of heat pipes is decreased from previous 12 cm in 3 Ah battery pack to 6.5 cm, as shown in Fig. 1. Meanwhile, the spray frequency is decreased from 1 spray per minute to 0.5 spray per minute. In all the tests on 8 Ah
Table 3 Summary of experimental results of different cooling systems on 3 Ah battery pack. Cooling methods Ambient
Fig. 7. Temperature gradients of center cell in 3 Ah battery pack with different BTM systems at 3C discharge rate.
C Rate Tmax ( C) Televation Tmax, pack Tmax, cell gradient ( C) gradient ( C) ( C)
1C 2C 3C Heat pipes in ambient 1C 2C 3C Hori-pipe with fan 1C 2C 3C Verti-pipe with fan 3C Heat pipes in 3C water bath Hori-pipe with 1C wet cooling 2C 3C
37.2 45 48.8 30.7 38 41.1 27.5 29.6 31.8 31.8 38.2
12.2 20.1 23.6 5.7 13 16.1 2.5 4.6 6.7 6.8 13.2
e e 6.8 e e 2.9 e e 2.2 2.2 6.2
e e 2.1 e e 0.8 e e 0.7 0.7 1.5
20.5 21 21.5
1.5 1.9 2.5
e e 1.2
e e 0.5
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Then, cycle tests are carried out on 8 Ah battery pack in ambient and with wet cooling method. During the test, both the maximum charging and the discharging current are set at 10 A, and the cut-off current for the potentiostatic charging process is C/50. In Fig. 12, the temperatures of points 1 and 4 are collected in the two systems with giving the pack temperature gradient on the bottom of each
Fig. 8. a) Temperature curves of 3 Ah battery pack equipped with different BTM systems during a current changing discharge process; b) Testing schedule of the current changing discharge process.
battery pack, two more heat pipes are added to the battery pack, which are attached on the top and bottom surfaces of the battery pack to decrease the temperature gradient in pack level. In Fig. 10, the temperature curves of points 1 and 4 are collected from natural convection cooling system and the wet cooling system. It can be seen that the wet cooling system outperforms the natural convection system in temperature control of battery pack. Although the cooling ability of wet cooling system is slightly degraded compared to that of 3 Ah battery pack, which has longer cooling ends and smaller battery size, the maximum temperature is much lower than 30 C, and it is still in the optimum temperature range for Li-ion battery operation. With the implementation of two additional heat pipes, the temperature gradients of wet cooling systems in pack level and in cell level are slightly decreased and stabilized, as depicted in Fig. 11a and b, respectively. At the end of discharge, the pack temperature gradient in natural convection cooling system reaches as high as 9.3 C, almost twice that of the highest recommended temperature gradient in Li-ion battery. While for wet cooling system, the temperature gradients, 1.1 C and 0.4 C in pack level and cell level, respectively, are lower than the recommended threshold, which will ensure a same capacity fading rate across the cell and battery pack and guarantee a high energy output and long life span.
Fig. 9. Cooling ability of a) natural convection, b) heat pipes with fan and c) wet cooling systems on 3 Ah battery pack discharged at 10 A rate.
R. Zhao et al. / Journal of Power Sources 273 (2015) 1089e1097
Fig. 10. Temperature variations of points 1 and 4 in 8 Ah battery pack discharged in ambient and with wet cooling system equipped at 10 A discharge rate.
Fig. 11. Temperature gradients of 8 Ah battery pack cooled by natural convection and wet cooling systems in a) pack level and b) cell level.
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figure. As shown, when battery pack is discharged with natural convection cooling method, the maximum temperature reaches as high as 51.5 C at the end of 2nd and 3rd cycles, with the temperature gradient of 12.4 C, and it takes 3 cycles for the battery to reach a stable stage (same temperature variation with previous cycle). The tests are carried out at room temperature, however, in summer period, the initial temperature may be higher than 35 C, and thus the battery operated in ambient will have the potential to exceed the suggested operating temperature range in the specification and may cause safety concerns. In comparison, battery pack equipped with wet cooling system is able to recoveries to its initial temperature after the first cycle, and the maximum temperature during test is as low as 26 C. Moreover, the gradient in pack level is same for both cycles with a maximum value of 1 C. To save the energy and reduce the water supply in operation, another approach to implement the wet cooling system is to use the water spray at assigned temperature points. To verify the feasibility of this approach, in the test, the fan is turned on and one spray of water is sprinkled to the cooling ends of heat pipes when the maximum temperature of battery pack reaches 30 C. As shown in Fig. 13, the battery pack equipped with heat pipes is initially discharged in ambient at the rate of 10 A, and at 900 s, the cooling system is turned on due to the temperature at point 4 reaches the threshold. Thereafter, the temperatures of both points experience
Fig. 12. Cycle tests on 8 Ah battery with a) natural convection cooling method and b) wet cooling method.
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Fig. 13. Wet cooling on 8 Ah battery pack with water spray at 30 C.
dramatic decrease and the cooling fan is kept on working until the end of discharge. During the whole test, totally four sprays of water are used, and the temperature of the battery pack is successfully controlled below the set temperature of 30 C with temperature difference lower than 1 C. As a final note, the water spray used in the wet cooling system can be replaced by micro water channels (tubes), which can accurately and automatically control the volume of water and can be programmed with different battery sizes and cooling areas, and can ease the assembly of cooling system and save the energy cost. This programmed cooling system is suitable for big sized battery pack used in industries and for battery pack used in electric vehicles, and will be discussed in future works. 4. Conclusions An ultra-thin heat pipe battery thermal management (BTM) system for pouch Li-ion battery pack is investigated in this study. The combination of wet cooling method with heat pipe system is introduced, which cools down the battery pack on the basis of the heat absorption during the evaporation of water and the phase change inside the heat pipe between the cooling ends and battery sides. The proposed BTM system has the advantages such as compactness, light weight and superiority in controlling battery temperature during discharges. Other types of BTM systems are also tested and compared with the wet cooling system on the 3 Ah battery pack. The natural convection cooling method is not recommended for battery pack discharged at high rate due to the high temperature and large temperature gradient inside the battery at the end of discharge. Battery pack assembled with heat pipes discharged in ambient has an improved temperature uniformity inside battery pack with a relatively alleviated temperature rise. Heat pipe BTM system cooled by water bath is not suggested due to the accumulation of bubbles during the discharge process, which dramatically reduces the thermal conductivity of system and hinders the heat exchange at the interface between pipes and water. The comparison between the vertical and horizontal heat pipe cooling system verifies that
the angle of the ultra-thin heat pipes has no impact on the performance of the cooling system, which is mainly attributed to the internal structure of the heat pipe. As for wet cooling integrated heat pipe BTM system, the effectiveness in cooling is validated through a series tests. In the tests with 3 Ah battery pack, the temperature elevations during all C-rate discharges are below 4 C, and the temperature difference across the pack and center cell are lower than 1.5 C and 0.5 C, respectively. Meanwhile, the proposed BTM system is able to work stable under unsteady discharging condition and can decrease the battery temperature to its initial value after high rate discharge in a very short time. In the discharging and cycling tests of 8 Ah battery pack, both the operating temperature and the temperature gradient are well controlled with the reduced length of cooling ends as well as the decreased spraying frequency. In addition, the wet cooling system is capable of keeping the temperature of 8 Ah battery pack below 30 C with only four sprays of water. Acknowledgments This project is supported in-part by Ontario-China Research and Innovation Fund (OCRIF). References [1] V. Etcheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Energy Environ. Sci. 4 (2011) 3243e3262. [2] S.S. Zhang, K. Xu, T.R. Jow, Electrochim. Acta 49 (2004) 1057e1061. [3] J. Fan, J. Power Sources 117 (2003) 170. [4] S.S. Zhang, K. Xu, T.R. Jow, J. Power Sources 115 (2003) 137e140. [5] D. Doughty, E.P. Roth, Electrochem. Soc. Interface 21 (2012) 37e44. [6] S. Zhang, R. Zhao, J. Liu, J. Gu, Energy 68 (2014) 854e861. [7] T.M. Bandhauer, S. Garimella, T.F. Fuller, J. Electrochem. Soc. 158 (2011) R1eR25. [8] C.G. Motloch, J.P. Christopheresen, J.P. Belt, R.B. Wright, G.L. Hunt, R.A. Sutula, T. Duong, T.J. Tartamella, H.J. Haskins, T.J. Miller, High-power Battery Testing Procedures and Analytical Methodologies for HEV's, SAE 2002-01-1950. [9] A.A. Pesaran, J. Power Sources 110 (2002) 377e382. [10] H. Park, J. Power Sources 239 (2013) 30e36. [11] R. Mahamud, C. Park, J. Power Sources 196 (2011) 5685e5696. [12] L. Fan, J.M. Khodadadi, A.A. Pesaran, J. Power Sources 238 (2013) 301e312. [13] K. Chen, X. Li, J. Power Sources 247 (2014) 961e966.
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[18] X. Li, F. He, L. Ma, J. Power Sources 238 (2013) 395e402. [19] M.S. Wu, K.H. Liu, Y.Y. Wang, C.C. Wan, J. Power Sources 109 (2002) 160e166. [20] T.H. Tran, S. Harmand, B. Desmet, S. Filangi, Appl. Therm. Eng. 63 (2014) 551e558. [21] Z. Rao, S. Wang, M. Wu, Z. Lin, F. Li, Energy Convers. Manage 65 (2013) 92e97. [22] J. Hong, H. Maleki, S. Al-Hallaj, L. Redey, J.R. Selman, J. Electrochem. Soc. 145 (1998) 1489e1501.
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
An experimental study of heat pipe thermal management system with wet cooling method for lithium ion batteries Rui Zhao, Junjie Gu, Jie Liu* Department of Mechanical and Aerospace Engineering, Carleton University, Ottawa, ON K1S 5B6, Canada
h i g h l i g h t s A heat pipe e wet cooling combined battery thermal management system is developed. The performance of the proposed system is compared with other cooling systems. The combined system efficiently controls the temperature rise in discharge tests. The temperature gradient is minimized in the combined cooling system. An energy saving approach of using wet cooling system is proposed.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 August 2014 Accepted 2 October 2014 Available online 13 October 2014
An effective battery thermal management (BTM) system is required for lithium-ion batteries to ensure a desirable operating temperature range with minimal temperature gradient, and thus to guarantee their high efficiency, long lifetime and great safety. In this paper, a heat pipe and wet cooling combined BTM system is developed to handle the thermal surge of lithium-ion batteries during high rate operations. The proposed BTM system relies on ultra-thin heat pipes which can efficiently transfer the heat from the battery sides to the cooling ends where the water evaporation process can rapidly dissipate the heat. Two sized battery packs, 3 Ah and 8 Ah, with different lengths of cooling ends are used and tested through a series high-intensity discharges in this study to examine the cooling effects of the combined BTM system, and its performance is compared with other four types of heat pipe involved BTM systems and natural convection cooling method. A combination of natural convection, fan cooling and wet cooling methods is also introduced to the heat pipe BTM system, which is able to control the temperature of battery pack in an appropriate temperature range with the minimum cost of energy and water spray. © 2014 Elsevier B.V. All rights reserved.
Keywords: Lithium ion battery Battery thermal management Heat pipe Wet cooling Temperature
1. Introduction Lithium-ion (Li-ion) batteries have emerged as one of the most promising energy storage technologies due to their higher energy density, power density and no memory effect when compared with other secondary batteries [1]. Consequently, the advantages such as long driving range and fast acceleration capability of the Li-ion batteries make them highly recommended as power sources for electric vehicles (EV), hybrid electric vehicles (HEV) and plug-in hybrid electric vehicles (PHEV). Although offering many benefits and advantages, the poor performances of Li-ion batteries at either too high or too low temperatures, such as short supply of energy, fast degradation and safety issues [2e7], may constrain their real
* Corresponding author. Tel.: þ1 613 520 2600x8257; fax: þ1 613 520 5715. E-mail address: [email protected] (J. Liu). http://dx.doi.org/10.1016/j.jpowsour.2014.10.007 0378-7753/© 2014 Elsevier B.V. All rights reserved.
applications. Thus, the battery thermal management (BTM) system is essential for the Li-ion battery packs, especially for the ones commonly used under high rate discharges or in critical applications (e.g., aerospace and military). A proper thermal management system is able to control the temperatures of batteries in a proper range and to minimize the temperature gradient in battery packs. In general, a high operating temperature can accelerate the degradation rate of battery pack and lead to safety issues, e.g., overheating and thermal runaway; the uneven temperature distribution in battery pack may give rise to different capacity fading rates between batteries and finally change the charging and discharging behaviors of the entire battery pack. Motloch et al. [8] reported that the lifespan of Li-ion batteries can be reduced by about two months for every degree of temperature rise in an operating range of 30 Ce40 C. It is also found that the temperature gradient in both the battery level and pack level should be lower than 5 C [9].
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To achieve the goal of controlling battery temperature in a appropriate range, different mediums, such as air [10e12], liquid [13], phase change material (PCM) [14e16] and hydrogel [6,17], have been implemented in the previous thermal management systems. Among them, the air cooling method is generally performed in a forced convection way, in which the air is brought from the fan [11] or wind tunnel [18], and the researches on air cooling is mainly focusing on optimizing the layout of batteries and air flows in the pack, the wind speed and energy cost. For example, Park [10] designed and compared five air flow configurations for a specific battery pack layout, and it is demonstrated that the required cooling performance can be achieved by employing the tapered manifold and pressure relief ventilation on the cooling tunnel. In Ref. [12], an air-cooled Li-ion battery module for plug-in hybrid electric vehicles is studied, in which the influences of the gap spacing and air flow rate on the cooling effect of BTM system were fully explained. PCM cooling systems are also well investigated by Al-Hallaj’s research group [14e16]. Thanks to the cooling mechanism, use of latent heat, the PCM cooling system possesses high energy storage capacity and thus can effectively lower the temperature of battery pack. But due to the low conductivity, the PCM cooling system will suffer from the inherent limitations such as the moderate temperature gradient and long cooling time. Although many matrices were developed to improve the thermal conductivity of the PCM, the high cost and volume change of PCM cannot be well managed. Recently, heat pipe based BTM systems are fully investigated due to the advantages accompanied with heat pipe and the corresponding BTM system, such as high thermal conductivity, compact structure, flexible geometry, etc. So far, the researches related to heat pipe BTM system can be classified as air-cooled system [19,20] and water-cooled system [21] depends on different cooling strategies used in the cooling ends of heat pipes. Wu et al. [19] compared the cooling effectiveness of natural convection and forced convection with/without heat pipes in a Li-ion battery pack from both simulated and experimental perspectives, and it is found that the insertion of heat pipes into the cooling system can effectively lower the temperature and minimize the temperature difference inside the battery pack. A water-cooled heat pipe BTM system was also studied [21], in which a thermostat water bath was implemented to control the temperature rise. The result shows that the battery maximum temperature can be controlled below 50 C when the heat generation rate is under 50 W, and the maximum temperature gradient can be reduced to 5 C when the heat generation rate is decreased to 30 W. Although the relevant works revealed the excellence of heat pipe BTM system compared to other cooling systems, the cooling effectiveness can still be greatly improved by using well contacted pipes and wet cooling method. In this work, a mature cooling technology in industry, wet cooling, is combined with the existing heat pipe cooling system to investigate its cooling effect. Temperature histories and temperature gradients of the wet cooling pipe system are collected and compared with other BTM systems. The testing results demonstrate that the wet cooling combined system is the most effective BTM system in cooling and it can be coupled with the other heat pipe systems in real applications to control the temperature variation of Li-ion battery with the lowest energy consumption and to improve the performance of entire battery system. 2. Experimental Two battery packs, 3 Ah, four batteries in series (4S1P), and 8 Ah (4S1P), are used in the experiments to test the effectiveness of different cooling systems. The specifications of the batteries in two battery packs are listed in Table 1. Depends on whether heat pipes are involved in the BTM system, the cooling systems in the tests are
Table 1 Specifications of 3 Ah battery and 8 Ah battery.
Length (mm) Width (mm) Height (mm) Weight (g) Nominal voltage (V) Nominal capacity (mAh) Maximum charge rate (C) Maximum discharge rate (C) Working temperature range ( C)
Cell 1
Cell 2
125 40 5.5 70 3.7 3000 5 25 20e60
180 45 9 185 3.7 8000 5 25 20e60
classified as two main classes: pack without heat pipes (natural convection) and pack with heat pipes. In natural convection cooling system, all the batteries in pack are attached closely with the neighboring batteries. While in heat pipe cooling systems, heat pipes are inserted between batteries with leaving the cooling ends outside. Two types of heat pipes with different dimensions are selected to meet the sizes of the battery packs. Meanwhile, double side adhesive high thermal conductive pads are also used in between batteries and heat pipes to increase the contact areas. The specifications of the heat pipes and thermal pads are given in Table 2. The photographs of two battery packs assembled with heat pipes are given in Fig. 1. Different lengths of cooling ends and battery sizes are chose to test the capability of cooling of heat pipe BTM systems under various conditions. In the tests with 8 Ah battery pack, two more heat pipes are used on the top and bottom of the pack surface to improve the temperature uniformity of the battery pack and is not shown here. Fig. 2 shows the schematic illustrations of battery pack with heat pipe BTM system, in which the temperatures are collected in the assigned points in figures through K-type thermocouples. Through the comparison of the temperatures, the maximum/minimum temperatures and the temperature gradients in pack level and battery level can be obtained. As for heat pipe BTM systems, totally five cooling methods are used: 1) heat pipes in ambient; 2) heat pipes cooled by horizontal fan (hori-fan); 3) heat pipes cooled by vertical fan (verti-fan); 4) heat pipes cooled by thermostat bath and 5) heat pipes cooled through wet cooling. The schematic illustrations of the method 2) e 4) are given in Fig. 3, and the illustration for method 1) is same with other types except the cooling strategy and is not shown here. In cooling fan involved BTM system, the distance between the fan and heat pipes is kept at 5 cm, and the radius and rotate speed of the fan are 4 cm and 1500 rpm, respectively. The medium in thermostat bath in Fig. 3c is water and the temperature is kept at 25 C ± 0.2 C during operation. For wet cooling method, the water volume per spray is 0.7 ml, and the frequency of spray is 1 min1 or 0.5 min1 in the tests. To increase the water adsorption of heat pipes, thin layers of super absorbent fiber are attached to the surfaces of the cooling ends of heat pipes. Table 2 Specifications of heat pipes and thermal pads. Heat pipe 1 Heat pipe 2 Thermal pad 1 (2) Length (mm) Width (mm) Thickness (mm) Weight (g) Material Wick structure Working fluid Working temperature ( C) Thermal conductivity (W m1 K1)
250 40 2 25 Aluminum Groove Acetone 50e125 e
250 50 2.5 40 Aluminum Groove Acetone 50e125 e
125 (175) 40 (45) 0.5 4 (6.5) Silicone compound e e 40e200 3.2
R. Zhao et al. / Journal of Power Sources 273 (2015) 1089e1097
Fig. 1. 8 Ah (top) and 3 Ah (bottom) battery packs with heat pipes.
3. Results and discussion 3.1. Temperature control Before experimental tests, both battery packs are conditioned at room temperature (298 K) by cycling 5 times at 1C rate using ESI battery analyzer (PCBA 5010-4) with a cutoff voltage of 3 V during
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discharging, which allows the formation of solid electrolyte interface (SEI) and eliminates the impact from irreversible capacity fade of new Li-ion batteries [22]. In Fig. 4, the maximum temperature curves of 3 Ah battery pack equipped with six cooling methods under 3C discharge rate are compared. Among all the cooling systems, the battery pack cooled through natural convection experiences the highest temperature elevation, which is mainly attributed to the limited cooling surface area, the accumulation of heat in the center battery and the poor thermal properties of air. Generally, an operating temperature higher than 40 C is not recommended for Li-ion battery system [8], which may accelerate the aging rate and reduce the stability and safety of the energy system, although the capacities obtained at the initial cycles are higher than the battery systems discharged at lower temperature. With the implementation of heat pipes, the cooling area of entire battery pack is increased. As indicated from the red circles in Fig. 4, when the pipe system cooled in ambient, the maximum temperature is decreased from 49 C to 41 C comparing to the system without heat pipes. Interestingly, the heat pipe cooling system with thermostat bath also experiences a relatively high temperature rise, and the main reason is due to the bubble accumulation at the surfaces of the heat pipe cooling ends during the discharging process, which will dramatically decrease the thermal conductivity of the heat pipe and shutdown the heat transfer between the heat pipes and cooling water. Although the increase of the water flow rate and use of defoamer may reduce the amount of bubbles, the costs of energy and expense will limit its application. Then, comparison of temperature curve is made between heat pipe BTM system cooled by hori-fan and verti-fan. Although previous researches have tested that the heat pipe works differently in different inclination angles, the heat pipe used in this study is a micro-channeled ultra-thin pipe, which is able to work in different angles with the same performance. In the ultrathin heat pipe, the gas and liquid inside the micro-channel is discontinuously connected, and the liquid cannot accumulated together due to the gravity. As validated from the results, the temperature curves of the fan-cooling systems in different angles basically overlapped with each other. For wet cooling system, the battery pack discharges until the temperature is stable in the wet cooling condition, and the low initial temperature is attributed to the energy absorption during the evaporation of the water spray. Apparently, the heat pipe BTM system cooled through wet cooling is the most efficient method in suppressing the temperature rise of battery pack among all the methods, with the maximum temperature of 21 C at the end of discharge. Due to the similar results of hori-fan and verti-fan in cooling, and the low cooling efficient of the heavy thermostat bath, discharging processes under rates of 1C and 2C are only performed on four cooling systems (ambient, heat pipes in ambient, hori-pipe with fan and hori-pipe with wet cooling), and the maximum temperature elevations of the four systems are summarized in the bar chart as shown in Fig. 5. It can be seen that the natural convection cooling method is the worst among all the BTM systems, with all the temperature rises higher than 12 C. While for the wet cooling method, the temperature increases are all below 4 C at the assigned discharge rates, which again demonstrates its superiority over other BTM systems in controlling the temperature rise of battery pack, and this will benefits the wellbeing and performance of battery pack in both the short and long term run. 3.2. Temperature distribution
Fig. 2. Schematic illustrations of 8 Ah battery pack with heat pipes. Among the temperature collection points, T1 to T6 are used to measure the temperature gradient in pack level and Ta to Te are used to test the center cell temperature uniformity.
Temperature differences across pack level and battery level are significant factors that may influence the long term performance of battery system. A large temperature gradient in both the battery
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Fig. 3. Schematic illustrations of four BTM systems, a) horizontal heat pipes with cooling fan; b) vertical heat pipes with cooling fan; c) heat pipes in thermostat water bath and d) horizontal heat pipes with wet cooling system.
level and the pack level can lead to different fading rate and unbalanced voltage inside the pack. And thus, a small temperature distribution is necessary for Li-ion battery pack and it is also an indicator of a satisfactory BTM system. In Fig. 6, the maximum temperature differences of 3 Ah battery pack with different BTM systems discharged at 3C rate are shown.
As can be seen, the temperature gradient of battery pack cooled through natural convection method experiences a dramatic rise with the proceeding of discharge, and at the end of discharge, the temperature difference is as high as 6.8 C, which already exceeds the maximum difference recommended by Pesaran [9]. The large gradient of battery pack cooled in ambient is mainly attributed to
Fig. 4. Maximum temperature curves of 3 Ah battery pack equipped with different BTM systems at the discharge rate of 3C.
Fig. 5. Maximum temperature elevations of 3 Ah battery pack implemented with different BTM systems at assigned C rates.
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is because different regions along the center cell have basically the same heat transfer surroundings in different cooling systems. Finally, the results of temperature change and temperature gradients in pack and cell level of selected BTM systems are listed in Table 3. 3.3. Various operating conditions
Fig. 6. Temperature gradients of 3 Ah battery pack with different BTM systems at 3C discharge rate.
the different cooling surroundings across the battery pack, where the exterior parts can easily transfer the heat to the ambient through convection while the interior regions can only transfer the heat through conduction between the batteries, which will limit the heat dissipation. However, for battery pack equipped with heat pipes, the maximum temperature gradients are all below 3 C, and for wet cooling system, the temperature difference at the end of discharge is even lower than 1.5 C. The improved temperature distribution inside pack is mainly due to the involvement of heat pipes, which can efficiently conduct the heat from interior part of battery pack to the ambient at the cooling ends. Meanwhile, during the heat transfer, the phase change inside the pipe can absorb the heat and can keep the battery with low temperature. Fig. 7 depicts the maximum temperature gradients (max (Ta to Te) e min (Ta to Te)) across the center cell with different BTM systems. As shown, although the trends of gradients are similar with the pack temperature gradients (largest gradient in natural convection cooling method and smallest gradient in wet cooling method), all the temperature differences are below 2.5 C, and this
The discharging current of Li-ion batteries may vary with time in real applications, and the capability of a BTM system in keeping the battery temperature stable and uniform under different working conditions is crucial and essential. To test the effectiveness of the wet cooling system, a testing schedule as shown in Fig. 8b is applied on three cooling systems, natural convection, hori-pipe with fan and the wet cooling system, for comparison. In Fig. 8a, the temperatures at points 1 (surface corner) and 4 (pack center) of three cooling systems are given, it can be seen that, as discharge proceeds, the temperatures and temperature difference of battery pack cooled through natural convection experience dramatic rises, and the temperature change is obvious when the current rate varies. However, for wet cooling system, the temperature curves of two assigned points are basically overlapped and the temperature even decreases when the C-rate switches from 3C to 2C and from 2C to 1C. With the cooling medium switches from water to air, the fan cooling system performs moderately. The cooling time of the previous three BTM systems are compared in Fig. 9. In the test, the 3 Ah battery pack is discharged at 10 A to reach the highest temperature, and the cooling systems are kept on working when the discharge stops. As shown, due to the limitation of temperature dissipation, the maximum temperature of battery cooled through natural convection takes more than 2500 s to decrease to 30 C. While for heat pipe cooling systems, the times to recovery to the initial temperature are much shorter, and the time is even decreased to less than 300 s in wet cooling system, which again prove its excellent cooling ability. 3.4. Cooling effects on large battery pack A compact battery pack is required in some mission-critical applications, therefore, the space for cooling ends may be limited. To validate the wet cooling system in controlling the temperature rise of large sized battery pack with limited cooling area, an 8 Ah battery pack is constructed, where the cooling ends of heat pipes is decreased from previous 12 cm in 3 Ah battery pack to 6.5 cm, as shown in Fig. 1. Meanwhile, the spray frequency is decreased from 1 spray per minute to 0.5 spray per minute. In all the tests on 8 Ah
Table 3 Summary of experimental results of different cooling systems on 3 Ah battery pack. Cooling methods Ambient
Fig. 7. Temperature gradients of center cell in 3 Ah battery pack with different BTM systems at 3C discharge rate.
C Rate Tmax ( C) Televation Tmax, pack Tmax, cell gradient ( C) gradient ( C) ( C)
1C 2C 3C Heat pipes in ambient 1C 2C 3C Hori-pipe with fan 1C 2C 3C Verti-pipe with fan 3C Heat pipes in 3C water bath Hori-pipe with 1C wet cooling 2C 3C
37.2 45 48.8 30.7 38 41.1 27.5 29.6 31.8 31.8 38.2
12.2 20.1 23.6 5.7 13 16.1 2.5 4.6 6.7 6.8 13.2
e e 6.8 e e 2.9 e e 2.2 2.2 6.2
e e 2.1 e e 0.8 e e 0.7 0.7 1.5
20.5 21 21.5
1.5 1.9 2.5
e e 1.2
e e 0.5
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Then, cycle tests are carried out on 8 Ah battery pack in ambient and with wet cooling method. During the test, both the maximum charging and the discharging current are set at 10 A, and the cut-off current for the potentiostatic charging process is C/50. In Fig. 12, the temperatures of points 1 and 4 are collected in the two systems with giving the pack temperature gradient on the bottom of each
Fig. 8. a) Temperature curves of 3 Ah battery pack equipped with different BTM systems during a current changing discharge process; b) Testing schedule of the current changing discharge process.
battery pack, two more heat pipes are added to the battery pack, which are attached on the top and bottom surfaces of the battery pack to decrease the temperature gradient in pack level. In Fig. 10, the temperature curves of points 1 and 4 are collected from natural convection cooling system and the wet cooling system. It can be seen that the wet cooling system outperforms the natural convection system in temperature control of battery pack. Although the cooling ability of wet cooling system is slightly degraded compared to that of 3 Ah battery pack, which has longer cooling ends and smaller battery size, the maximum temperature is much lower than 30 C, and it is still in the optimum temperature range for Li-ion battery operation. With the implementation of two additional heat pipes, the temperature gradients of wet cooling systems in pack level and in cell level are slightly decreased and stabilized, as depicted in Fig. 11a and b, respectively. At the end of discharge, the pack temperature gradient in natural convection cooling system reaches as high as 9.3 C, almost twice that of the highest recommended temperature gradient in Li-ion battery. While for wet cooling system, the temperature gradients, 1.1 C and 0.4 C in pack level and cell level, respectively, are lower than the recommended threshold, which will ensure a same capacity fading rate across the cell and battery pack and guarantee a high energy output and long life span.
Fig. 9. Cooling ability of a) natural convection, b) heat pipes with fan and c) wet cooling systems on 3 Ah battery pack discharged at 10 A rate.
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Fig. 10. Temperature variations of points 1 and 4 in 8 Ah battery pack discharged in ambient and with wet cooling system equipped at 10 A discharge rate.
Fig. 11. Temperature gradients of 8 Ah battery pack cooled by natural convection and wet cooling systems in a) pack level and b) cell level.
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figure. As shown, when battery pack is discharged with natural convection cooling method, the maximum temperature reaches as high as 51.5 C at the end of 2nd and 3rd cycles, with the temperature gradient of 12.4 C, and it takes 3 cycles for the battery to reach a stable stage (same temperature variation with previous cycle). The tests are carried out at room temperature, however, in summer period, the initial temperature may be higher than 35 C, and thus the battery operated in ambient will have the potential to exceed the suggested operating temperature range in the specification and may cause safety concerns. In comparison, battery pack equipped with wet cooling system is able to recoveries to its initial temperature after the first cycle, and the maximum temperature during test is as low as 26 C. Moreover, the gradient in pack level is same for both cycles with a maximum value of 1 C. To save the energy and reduce the water supply in operation, another approach to implement the wet cooling system is to use the water spray at assigned temperature points. To verify the feasibility of this approach, in the test, the fan is turned on and one spray of water is sprinkled to the cooling ends of heat pipes when the maximum temperature of battery pack reaches 30 C. As shown in Fig. 13, the battery pack equipped with heat pipes is initially discharged in ambient at the rate of 10 A, and at 900 s, the cooling system is turned on due to the temperature at point 4 reaches the threshold. Thereafter, the temperatures of both points experience
Fig. 12. Cycle tests on 8 Ah battery with a) natural convection cooling method and b) wet cooling method.
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Fig. 13. Wet cooling on 8 Ah battery pack with water spray at 30 C.
dramatic decrease and the cooling fan is kept on working until the end of discharge. During the whole test, totally four sprays of water are used, and the temperature of the battery pack is successfully controlled below the set temperature of 30 C with temperature difference lower than 1 C. As a final note, the water spray used in the wet cooling system can be replaced by micro water channels (tubes), which can accurately and automatically control the volume of water and can be programmed with different battery sizes and cooling areas, and can ease the assembly of cooling system and save the energy cost. This programmed cooling system is suitable for big sized battery pack used in industries and for battery pack used in electric vehicles, and will be discussed in future works. 4. Conclusions An ultra-thin heat pipe battery thermal management (BTM) system for pouch Li-ion battery pack is investigated in this study. The combination of wet cooling method with heat pipe system is introduced, which cools down the battery pack on the basis of the heat absorption during the evaporation of water and the phase change inside the heat pipe between the cooling ends and battery sides. The proposed BTM system has the advantages such as compactness, light weight and superiority in controlling battery temperature during discharges. Other types of BTM systems are also tested and compared with the wet cooling system on the 3 Ah battery pack. The natural convection cooling method is not recommended for battery pack discharged at high rate due to the high temperature and large temperature gradient inside the battery at the end of discharge. Battery pack assembled with heat pipes discharged in ambient has an improved temperature uniformity inside battery pack with a relatively alleviated temperature rise. Heat pipe BTM system cooled by water bath is not suggested due to the accumulation of bubbles during the discharge process, which dramatically reduces the thermal conductivity of system and hinders the heat exchange at the interface between pipes and water. The comparison between the vertical and horizontal heat pipe cooling system verifies that
the angle of the ultra-thin heat pipes has no impact on the performance of the cooling system, which is mainly attributed to the internal structure of the heat pipe. As for wet cooling integrated heat pipe BTM system, the effectiveness in cooling is validated through a series tests. In the tests with 3 Ah battery pack, the temperature elevations during all C-rate discharges are below 4 C, and the temperature difference across the pack and center cell are lower than 1.5 C and 0.5 C, respectively. Meanwhile, the proposed BTM system is able to work stable under unsteady discharging condition and can decrease the battery temperature to its initial value after high rate discharge in a very short time. In the discharging and cycling tests of 8 Ah battery pack, both the operating temperature and the temperature gradient are well controlled with the reduced length of cooling ends as well as the decreased spraying frequency. In addition, the wet cooling system is capable of keeping the temperature of 8 Ah battery pack below 30 C with only four sprays of water. Acknowledgments This project is supported in-part by Ontario-China Research and Innovation Fund (OCRIF). References [1] V. Etcheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Energy Environ. Sci. 4 (2011) 3243e3262. [2] S.S. Zhang, K. Xu, T.R. Jow, Electrochim. Acta 49 (2004) 1057e1061. [3] J. Fan, J. Power Sources 117 (2003) 170. [4] S.S. Zhang, K. Xu, T.R. Jow, J. Power Sources 115 (2003) 137e140. [5] D. Doughty, E.P. Roth, Electrochem. Soc. Interface 21 (2012) 37e44. [6] S. Zhang, R. Zhao, J. Liu, J. Gu, Energy 68 (2014) 854e861. [7] T.M. Bandhauer, S. Garimella, T.F. Fuller, J. Electrochem. Soc. 158 (2011) R1eR25. [8] C.G. Motloch, J.P. Christopheresen, J.P. Belt, R.B. Wright, G.L. Hunt, R.A. Sutula, T. Duong, T.J. Tartamella, H.J. Haskins, T.J. Miller, High-power Battery Testing Procedures and Analytical Methodologies for HEV's, SAE 2002-01-1950. [9] A.A. Pesaran, J. Power Sources 110 (2002) 377e382. [10] H. Park, J. Power Sources 239 (2013) 30e36. [11] R. Mahamud, C. Park, J. Power Sources 196 (2011) 5685e5696. [12] L. Fan, J.M. Khodadadi, A.A. Pesaran, J. Power Sources 238 (2013) 301e312. [13] K. Chen, X. Li, J. Power Sources 247 (2014) 961e966.
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