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Numerical analysis and experimental visualization of phase change material melting process for thermal management of cylindrical power battery
Accepted Manuscript Research Paper Numerical analysis and experimental visualization of phase change material melting process for thermal management of cylindrical power battery Huan Yang, Hengyun Zhang, Yang Sui, Chun Yang PII: DOI: Reference:
S1359-4311(17)31763-5 http://dx.doi.org/10.1016/j.applthermaleng.2017.09.022 ATE 11079
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
6 August 2017 5 September 2017 6 September 2017
Please cite this article as: H. Yang, H. Zhang, Y. Sui, C. Yang, Numerical analysis and experimental visualization of phase change material melting process for thermal management of cylindrical power battery, Applied Thermal Engineering (2017), doi: http://dx.doi.org/10.1016/j.applthermaleng.2017.09.022
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Numerical analysis and experimental visualization of phase change material melting process for thermal management of cylindrical power battery
Huan Yang1, Hengyun Zhang*1, Yang Sui1, Chun Yang2 1
College of Automotive Engineering, Shanghai University of Engineering Science 333 Longteng Road, Songjiang, Shanghai, China 201620
2
School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue Singapore 639798 *Correspondence Email: [email protected]
Abstract
Organic phase change materials (PCMs) have been of increasing interest in battery thermal management due to their high latent heat and excellent cycling stability. This paper reports both numerical analysis and visualization experiments for the melting process of paraffin PCM surrounding a cylindrical power battery. Specifically, two different housing materials are considered, one made of metal and the other made of acrylics. For the metal housing case, the PCM exhibits a melting front isolated both from the battery and housing wall with accelerated melting in the B~C stage, accounting for the isothermal temperature plateau observed in both the numerical modeling and experimental measurements. For the acrylic housing case, the unmelted PCM instead adheres to the inner wall of housing all the way, melting at the rate slower than the metal housing case. In both housing cases, the numerically predicted liquid fractions are found to agree with our experimentally visualized results. The battery top to bottom temperature variation is also examined with regard to the melting process. Moreover, the instantaneous Nusselt number 1
is obtained to identify the different heat transfer characteristics in the different melting stages.
Keywords: Phase change material (PCM); melting front; numerical model; visualization experiment; characteristic points
NOMENCLATURE
Ab surface area of battery (mm2) Db diameter of battery (mm) cp
specific heat(J/kg·K)
g
gravitational acceleration (m/s 2)
h
heat transfer coefficient (W/m2·K)
H b height of battery (mm)
H enthalpy (J)
k
thermal conductivity (W/m·K)
L
latent heat of paraffin (J/kg)
m
mass (kg)
P pressure (Pa) q
heating power (W)
r
radial coordinate (mm)
S
momentum source term
t
time (s)
T
temperature (°C) 2
T0 ambient temperature(°C） Tt
battery temperature at height Z=55mm
Tb battery temperature at height Z=10mm
V velocity vector of liquid paraffin V
volume (cm3)
W
gap between battery and housing
Z
axial coordinate (mm)
Greek symbols
thermal expansion coefficient (K-1)
liquid fraction
dynamic viscosity (kg/m·s)
density (kg/m3)
difference
Subscripts bot bottom f
liquid paraffin
ht
heater
ins insulation pcm phase change material ref reference top top w
housing 3
1 Introduction The energy shortage and environmental pollution issues have drawn increasing concern worldwide in recent decades. Specifically, transportation sector produces significant combustion-related emission from fossil fuels, such as carbon-hydrogen, oxynitride, carbon dioxide, etc. This calls for imperative replacement with substitute driving sources of high energy density and low emission [1]. Electric vehicles (EVs) and hybrid electric vehicles (HEVs), equipped with the lithium-ion batteries (LIBs), with high specific energy and high operating voltage, are the most feasible candidates to stand out from the traditional transportation tools [2, 3]. Notably, in 2015 China produced 331,092 EVs and HEVs, ranking first in the world [4]. As the core component of EVs, LIBs provide the power source central to the performance of electric vehicle directly. Nonetheless, the LIBs can get overheated with exothermic reactions and experience large temperature excursion during operation, leading to battery capacity fades and performance drops, or even possibly thermal runaway [5]. Thermal runaway tends to occur at elevated temperature, risking the loss of battery cell, pack or even whole vehicle [6]. High temperature affects the battery performance which is related to electrochemistry, round trip efficiency, charge acceptance, power and energy capability, reliability and cycle life [7]. It was reported that the add-on of indirect liquid cooling option can reduce the capacity fade from 33% to 27% [8]. As such, it is necessary to develop an effective thermal management system to maintain the battery within acceptable temperature range under various charge/discharge conditions. A phase change material (PCM) is referred to a material that changes from one aggregation 4
into another at a particular temperature accompanied by a large amount of heat storage or exotherm. Heat from the hot component is stored during melting process and released to the ambient during freezing process. Mainly due to their characteristics of high latent heat of fusion per weight, adjustable phase change temperature, as well as good thermal cycling and chemical stability, organic phase change materials (PCMs) such as paraffin waxes and fatty acids have been investigated recently as an alternative cooling method in the field of battery thermal management [9-15], along with the development in other applications such as heat storage [16-17], electronic cooling [18-20]. Al-Hallaj and Selman [10] proposed a battery module simply filled with a PCM in the gap between batteries with a module capacity of 100Ah, which could maintain the battery temperature 8 C below the one with no PCM. The passive cooling test with graphite and paraffin wax composites was also conducted in [13] for the 8S2P pack, and the center to corner battery temperature difference was measured to be 4 C, much smaller than the natural convection case. Duan et al [14] experimentally studied two types of PCMs with different melting temperatures for the thermal management of battery cell used in electric vehicles, one with a PCM cylinder surrounding the heater, and the other with PCM jackets wrapping the heater. Both configurations exhibited good effectiveness in maintaining the heater within a desired temperature range. Zhang et al [15] experimentally designed the battery cell and pack using composite phase change material, and found that the battery temperature at 1C discharge rate decreased by 14~18°C and 9~14°C, respectively, compared with the natural cooling and forced air cooling. Ling et al [11] numerically investigated the performance of passive battery thermal management systems using paraffin/EG composites. The influences of phase change temperature, composite density and paraffin mass fraction of the composite PCMs were discussed. Compared with natural air cooling 5
and liquid cooling, PCM was capable of keeping the components at a smaller range of temperature swing and minimizing the temperature difference among cells. Some visualization was reported in references such as [21] for the melting process in a rectangular slot bounded by an isothermal wall with horizontal fins and a plain wall. Nonetheless, most of the previous studies focus on the battery temperature profiles without in-depth investigation on the role of melting process in battery thermal control, especially in the presence of the thermally conductive housing. The connection between the melting process and the thermal performance improvement is not well understood and the isothermal control of the heat generating battery may not necessarily be achieved during the PCM melting stage. There is still a lack of correlation linking the phase change process with the heat transfer characteristics. In our recent work [22], we examined the thermal behavior with PCM-fin composite structure in battery thermal management and determined the characteristic temperature points through experimental measurements without visualizing the phase change process. Nonetheless, further investigation is required to explore the underlying heat transfer mechanism along with the help of experimental visualization. The present work aims to investigate the correlation between the melting process and the heat transfer characteristics for the use of paraffin PCM in temperature control of a cylindrical battery. A numerical analysis is conducted in combination with the visualization experiment to examine the melting process and the corresponding thermal characteristics. The configuration consists of a cylindrical battery cell allocated in the center of a concentric container made of either metal housing or acrylic housing. Detailed flow circulation, liquid fraction and heat transfer characteristics are obtained under different battery heating conditions. The characteristic 6
temperature-time points A, B, C and E indicated in the battery temperature evolution curve are found to correlate well with the different locations of the melting front. Moreover, the formation of the isothermal temperature plateau is revealed for the metal housing, which is rarely reported in literature. The corresponding experimental visualization prototype was also fabricated to visualize the melting process and to correlate with the heat transfer process.
2 Numerical Model The physical problem of the baseline case is depicted in Fig. 1(a) and the adiabatic base-plate case is shown in Fig. 1(b). An 18650 battery cell is allocated in the center of a container to simulate the battery heating configuration. The battery is 18 mm in diameter and 65 mm in height, with an electrical heater cylinder of 6.35mm (1/4 inch) in diameter and 40mm in length allocated at the battery center. The housing is made of aluminum and it has an inner diameter of 31 mm, a depth of 66 mm and a wall thickness of 4 mm. The thickness of the insulating spacer made of polycarbonate is 1 mm. Here we focus on a representative single battery cell configuration, in attempt to investigate the fundamental connection between the melting process and the heat transfer characteristics. The extension of the findings in the present work in the battery pack system is to be conducted in the next stage. Due to symmetry, a two-dimensional model is developed for numerical analysis. The battery generates a constant heat source q. The outer wall surface of the cylindrical housing is set with a constant convective heat transfer coefficient h and the base-plate of the housing is thermally insulated. The configuration in Fig. 1(b) is slightly different from that in Fig. 1(a) as the metal base of the housing is removed in purpose to examine the effect of the fully adiabatic base-plate case. The following assumptions are made in the modeling development: 7
The thermophysical properties of the materials including PCM are assumed constant.
The liquid paraffin is considered as an incompressible Newtonian fluid
Boussinesq’s approximation is used.
The effect of radiation heat transfer is negligible.
Then, the governing equations are: Continuity equation ( V ) 0 t
(1)
Momentum equation
V (V )V P 2V g (T Tref ) S t
(2)
Energy equation ( H ) ( VH ) (kT ) t
(3)
Here, is the liquid paraffin density, V is the velocity vector of the liquid paraffin, P is the
pressure, is the dynamic viscosity, is the thermal expansion coefficient of liquid paraffin, g
is the gravity vector, Tref is the reference temperature and S is the momentum source term [23]. The enthalpy-porosity approach is used to solve the phase change problem and the solid or liquid phase can be represented by the liquid fraction , and thus the enthalpy H is expressed as [23]: T
H href
c
pcm
dT L
Tref
(4)
where href is the enthalpy at the reference temperature, L is the latent heat, is the liquid fraction defined by
0, T Tm1 (T Tm1 ) /(Tm 2 Tm1 ), Tm1 T Tm 2 1, T Tm 2 8
(5)
Note that 0 represents solid phase, 1 represents liquid phase, and between 0 and 1 represents the mushy zone. Here Tm1 is the onset melting temperature and Tm2 is the peak melting temperature of the PCM according to the DSC test curve. The numerical model is solved using Fluent 15.0 software. Specifically, the PISO algorithm is used for dealing with pressure-velocity coupling, and quadrilateral surface mesh is utilized. The time step t is set to as small as 0.1s after examination of the time step effect. The convergence is achieved when the residual error is below 110-5 for velocity components and 110-7 for energy quantity. A grid-independence test shows that the deviation in battery temperature level is smaller than 0.6% when computational grids of 17260 and 35060 are used. The results from the former grid is presented throughout this study. The PCM used here is a commercially available paraffin wax, with its thermo-physical properties tabulated in Table 1. Table 2 shows the thermo-physical properties of the other materials as indicated in Fig. 1(a). For the numerical simulations, several representative temperatures are selected, including the housing outer wall temperatures and the battery temperature at two different heights, Z=10mm (Tbot) and Z=55mm (Ttop) above the battery bottom. At the same time, the velocity field and liquid fraction of PCM are also obtained.
3 Numerical Simulation Results 3.1 Characteristic temperature points with respect to the melting front location During normal operation of lithium ion batteries, heat is generated mainly due to two parts: ohmic heat and entropic heat [24]. It was reported that the heat generation of battery can reach a high level near to the nominal electric power at high discharge rates associated with discharge time and the state of charge [25, 26]. Especially, the generated heat reached 12W at a high 9
discharge current of 25A~ 9.5C for 26650 battery cell [25]. Our lab test of a commercial 18650 battery up to 10C (10 times of the nominal current) showed an approximate quadratic dependence on the discharge rate, with a heat generation of 9.2W obtained at 10C discharge. For analysis purpose, three constant battery heating powers of 6.6W, 8.8W and 13.2W are used to simulate the battery heating at large electrical discharge rates. In the simulation, the ambient temperature is set to 25°C, and a constant heat transfer coefficient of the housing outer wall is 10W/m2K. As will be discussed later, such a heat transfer coefficient is found to agree with the experimental measurement. Fig. 2 shows a comparison of the simulation results at different power levels Denoting the starting time point as Point “O”, t
10
the housing wall and enables direct convection of battery heat to the housing wall, which minimizes the thermal resistance, accelerates the heat dissipation and
Point C as the transition point when the mid-height temperature of the housing wall reaches the peak melting temperature, which actually
° 11
C. Beyond Point C, the melting front passes over the corner of the housing, and thus the heat transfer gets deteriorated, raising the battery temperature again. Another empirical way is to obtain Point C based on the intersecting point of the two tangent lines extrapolated from the temperature curves B~C and C~E, which deviates less than 40s from the present numerical evaluation. This extrapolation technique also applies to Point B which can be obtained as the intersection of the two tangent lines extrapolated from the A~B and B~C curves with a deviation within 40s. Not limited to the present metal housing with the isothermal plateau, the extrapolation determination of the characteristic point is also useful in evaluating the ending of thermal control in other thermal management system involving PCM, applicable for various enhancement techniques such as filling thermally conductive particles and adding fins or metal/graphite foams. The thermal control duration before Point C for different thermal management systems [11, 13, 22] are shown in Fig. 5, as a function of the heat density per PCM volume. The comparison includes the 8-fin experimental case from Wang et al [22], and the optimal case with PCM/expanded graphite foam from Ling et al [11] with 75% PCM in mass. Kizil et al’s result [13] based on the PCM/graphite composite for the 18650 pack is also plotted for comparison even though the exact percentage of PCM and graphite was not given. The duration time in the cited references has been obtained using the extrapolation technique. Due to the highest power density level considered in the present case, the duration time before the thermal control point is shorter than the cases with lower power. Once the melting front passes over battery temperature increases at an accelerated rate, which is considered the end of the desirable isothermal temperature control. Combining Fig. 4(b) and Fig. 3 with Fig. 2, the PCM 12
melts completely at Point E, corresponding to the liquid fraction of = 1.
°C
3.2 Effects of acrylic housing As shown in Section 3.1, temperature plateau for the PCM melting can be achieved in the metal housing. Such a temperature control, nonetheless, may not necessarily hold when the housing is made from a low thermal conductivity material. In this study, the acrylic housing is considered as against the metal housing made of aluminum, whereas the other parameters are kept the same as the metal housing case. Fig. 7 shows the battery temperature comparison between acrylic and metal housings at the heating power of 8.8W. It is seen that the isothermal temperature plateau almost disappears for the acrylic housing, and the battery temperature ramps up with similar characteristic points A, B, C and E. Due to the weak heat conduction along the acrylic wall, the melting is not accelerated by the housing and thus an isothermal temperature plateau is not well formed in the battery temperature curve. The resulting battery temperature is 6.3~16°C higher than that of the metal housing case after Point B, indicating the inefficiency of the acrylic 13
housing in view of thermal enhancement. The velocity field and liquid fraction distributions of the PCM for the acrylic housing case are presented in Fig. 8. Different from the metal housing case shown in Fig. 4, the melting front for the acrylic housing descends along the housing wall, moves faster than the metal housing case at around 1100-1600s but advances more slowly after 1800s. The morphological evolution of the unmelted PCM is also different. It shows a tapering peak adhering to the acrylic wall after Point B, as against the isolated peak for the metal housing case. As discussed earlier, the metal housing accelerates the heat spreading and promotes the melting process of PCM near the housing, forming an isolated peak of the unmelted PCM. The mushy zone for the metal housing case is more broadened at the bottom of the PCM domain after 1600s, and the complete melting is attained at 2200s. For the acrylic housing case, a small amount of PCM at the bottom corner of the housing keeps unmelted due to the weak heat conduction along the acrylic housing wall. Fig. 9 shows a comparison of the melted liquid fraction for the metal housing and the acrylic housing. In the early stage A~B, the PCM melting rate in the metal housing case is slower than that of the acrylic housing due to more heat loss to the ambient for the former. As heating evolves beyond Point B, the liquid fraction of the PCM increases above the trend line for the metal housing, and the melting rate is gradually accelerated due to the conjugated effects of direct convection and the subsequent heat conduction along the housing wall. Slower than the metal housing case after 1750s, the melting rate for the acrylic housing is decelerated in absence of heat conduction along the housing wall, falling below the trend line. In practice, such a difference in battery temperature level with different housings can be utilized to enhance heat transfer for better battery thermal management. The housing of high thermal conductivity acts not only as a 14
container, but also as an extended surface to enhance both the convection and melting, inclining to form the isothermal temperature plateau during battery heating process. The computed melting front lines (for the case of = 0.2) at different time steps are processed and vividly illustrated in Figs. 10 for both the metal housing and the acrylic housing at 8.8W. In the first period of time (tA t 400s), the melting front almost parallels to the battery wall, indicating a conduction-dominated melting process. At approximately 500s the melting front line bends toward to the right in the upper zone indicating the convection regime, while the lower part of the PCM remains conduction-dominated. With the elapse of time, the upper melting interface gradually approaches to the inner wall of the housing. As shown in Fig. 10(a), we confirm that the point B ~1100s in the computed battery temperature curve is consistent with the experimental visualization when the liquid paraffin contacts the inner wall of housing. After this time point, the gap between the solid-liquid interface lines becomes wider, indicating a faster melting rate and the melting interface descends faster along the inner wall of housing. At 1400~1500s, the melting interface is isolated away from the housing wall due to the stronger heat transfer from the metal housing wall. Fig. 10(b) shows the solid-fluid interface for the acrylic housing case. The interface evolution is similar to that of the metal housing case before Point B, though Point B comes slightly earlier than that for the metal housing case. After Point B, the interface evolution is obviously different. The melting front in the acrylic housing case descends along the housing inner wall gradually, adhering to the housing wall instead of being isolated in the metal housing case.
3.3 Effect of the adiabatic base-plate 15
In the physical model depicted in Fig. 1 (a), the metal base-plate is thermally conductive and thus heat conduction through the metal base might affect the phase change characteristics. Here, we examine the adiabatic base-plate case by removing the base-plate, as has been indicated in Fig. 1(b). The computed battery temperature versus heating time has been shown in Fig. 7. It can be seen that the characteristic time points A, B, C and E are still prominent, nearly agreeing with the characteristic time points for the baseline case with the metal housing and metal base-plate. Nonetheless, the average battery temperature is 5~10°C higher than the baseline case due to the elimination of heat conduction along the metal base-plate. Interestingly, the battery top to bottom temperature difference as shown in Fig. 11 is also related to the melting process. Such a temperature difference is positive for both the metal base plate and the adiabatic base-plate, indicating that the temperature of the upper battery is relatively higher than that of the lower. This is mainly due to the upper allocation of the heater and the upward flowing of the liquid PCM subjected to battery heating due to buoyancy effect. At Point A, the temperature difference drops and then increases with the heating time. This can be explained from the inset figure of Fig. 11 with enlarged view of the computed temperature curve at 8.8W heating power. Due to the adjacent heating of the upper part of the battery, the upper temperature of the battery is higher than its lower part temperature before PCM starts to melt. When the battery temperature at the top exceeds the melting temperature of 47C (Point 1 in the inset for the computed temperature difference for the metal housing), the PCM adjacent to the upper part begins to melt, keeping the upper temperature almost unchanged whereas the bottom battery temperature keeps increasing, which reduces the top to bottom temperature difference to form the turning point A. As the lower part temperature of the battery reaches above the melting 16
point, the PCM adjacent to the lower part of PCM starts melting, which reduces the bottom battery temperature, causing the second increase in the top to bottom temperature difference, as indicated in the A-2 stage in the inset of Fig. 11. Though similar in trend, the battery top to bottom temperature difference is smaller for the adiabatic base-plate.
3.4 Heat dissipation analysis The battery heat is not only transferred to the PCM, but also heats up the battery itself and the other components. A lumped energy balance equation at the time interval of t and t+t can be expressed as:
(6)
where the subscripts pcm stands for PCM, w the housing material, b the battery, h the overall heat exchange coefficient of the system boundary to the ambient, the average liquid fraction, and A the heat transfer area to the ambient. The seven terms in the right-hand side of the equation corresponds to the heat capacities of the PCM, battery, housing, insulation and heater, the latent heat absorption, and the heat loss to the ambient, respectively. The corresponding proportion of the heat dissipation versus time is calculated and illustrated in Fig. 12. It is seen that the PCM latent heat absorption amounts to 42-60% of the heat dissipation during the B~C stage, which is a significant contribution.
17
3.5 Heat transfer analysis In this section, the Nusselt number with time is used to identify different heat transfer rates associated with the melting process for the cylindrical battery cell. Here, the instantaneous Nusselt number is defined with the instantaneous temperature difference between the battery and the housing. Namely,
Nubw
h(t )W kf
(7)
where h(t) is the instantaneous surface-average heat transfer coefficient and determined by:
h(t )
q Ab(Tb Tw )
(8)
where Ab is the total heat transfer area calculated by:
Ab Db H b
(9)
Fig. 13 shows the obtained instantaneous Nu versus the normalized heating time t/tE for both metal and acrylic housing cases. Here tE is the time to complete melt at Point E. It can be seen in common that initially the Nusselt number decreases sharply until Point A for all the cases since the regime is conduction-dominated. After Point A, Nu does not vary significantly with the normalized time, which indicates a quasi-steady state and the liquid layer thickness keeps growing to allow occurrence of the convection. As the melting front moves beyond Point B, the inner wall of housing is exposed to liquid PCM due to the descending of melting front to form the direct convective heat dissipation, as already explained in Section 3.1. Thus the heat transfer is enhanced in the B~C stage, which results in the local maximum of Nu, and then decreases during the C~E stage. Due to the weaker conduction and consequent convection, the instantaneous Nu for the acrylic housing is always lower than that for the metal housing. It is also noted that the Nu 18
keeps almost unchanged in the B~C and C~E stages in spite of different heating power for the metal housing case.
4 Visualization Experiment 4.1 Experimental setup To validate the numerical analysis, we carried out the experiments with both full-circular housing and semi-circular test sections. The full-circular test section is shown in Fig. 14(a) to examine the battery temperature evolution. A comparison of the experimentally obtained battery temperature with the numerical results has been demonstrated in Fig. 6 with reasonable agreement for the full-circular test section. As it was difficult to find a heater with the exact dimension of a commercial 18650 cell, the mock-up battery made of aluminum cylinder of 18mm in diameter and 65mm in length was fabricated. A hole of 6.5mm was drilled at the center of the mock-up battery for inserting a heater with standard size of 6.35mm in diameter and 40mm in length as supplied from Watlow. To reduce the contact resistance, thin aluminum foil had been used to wrap around the heater for tight contact with the aluminum. The heater was connected with a DC power supply to simulate the 18650 battery heat generation. To facilitate visual observation of the melting process, the mock-up battery and housing were made into semicircular shape through wire cutting to form an observation section. The semicircular incision was sealed with a 3mm thick transparent plate made of acrylics. A highly elastic silicone adhesive, branded Dow Corning 3145, was applied to glue the acrylic plate with both the semicircular battery and the housing wall, as depicted in Fig. 14(a). Epoxy adhesive lacks of elongation at high temperature and cannot be used for the visualization test. Two different housing materials were tested in the visualization experiments. One was aluminum, and the other 19
was acrylics, both having an inner diameter of 31mm and an outer diameter of 39mm. An electrically insulating polycarbonate spacer of 1mm thick inserted beneath the battery, which also minimize the heat transfer to the base. Due to the symmetry in the test section, the thermocouples were allocated in the right-hand side of the test section as shown in Fig. 14. The battery temperature was measured by mounting two K-type thermocouples in the holes drilled at the top and bottom of the battery on the right side. Each hole has a diameter of 1.5mm and a depth of 10mm, with its center 7.25mm away from the battery center. Another two thermocouples were mounted on the outer housing wall for measuring the housing temperature with the same height as for the battery temperature measurement. All these thermocouples were connected to the Hioki data acquisition instrument (Hioki LR8410) to record the temperature data as shown in Fig. 14. The uncertainty in the thermocouple measurement was estimated to be 0.5°C based on factory calibration, whereas the deviations among thermocouples are around 0.1°C. The maximum deviation in the heat input was estimated to be 0.5%. Photos were taken around several characteristic times with aid of the thermocouples readings and the melting process observation. Point A corresponded to the time when the battery wall temperature reached the onset melting temperature of PCM, Point B the time where the liquid paraffin contacted the inner wall of housing, Point C the time when the housing temperature reached the peak melting temperature of PCM, and Point E the complete melting point.
4.2 The comparison between visualization experiment and numerical results Figs. 15 (a) and (b) shows comparison of melting front evolution between the visualization 20
experiments and the numerical results for the metal housing case and the acrylic housing case at characteristic Points A, B, C and E under the heating power 8.8W. The left hand side denotes the visualization experimental results and the right depicts the numerical results. It should be pointed out that the melting front contains a larger mushy zone in the metal housing case than in the acrylic housing case due to the heating of the PCM not only from the battery but also from the housing wall. To avoid interfacial blurring, only the liquid fraction distribution at = 0.2 is chosen in the comparison of visualized interfacial locations for both the metal and acrylic housing cases. It can be seen that the numerically predicted interfacial location agrees reasonably well with the experimentally visualized phase change interface. As the melting evolves, the melting front intersects with the right upper wall of the housing at Point B, which appears similar in both shape and size to the numerical predictions. As the melting further evolves, the melting front moves away from the housing wall to form an isolated peak due to the downward heat conduction along the housing wall. Shown in Fig. 15(b) is the comparison of the phase change process for the acrylic housing case. Again good agreement is found for the melting fronts between the experimental observations and the numerical predictions. Due to the weak heat conduction along the acrylic housing, the unmelted PCM adheres to the wall without being isolated, exhibiting a slower melting rate. At Point E all the PCM gets melted, which can be seen from both the experimental and numerical results.
4 Conclusions In this work, we have numerically analyzed and experimentally investigated the melting 21
process of the paraffin PCM and the thermal characteristics of the cylindrical power battery configured with both metal housing and acrylic housing cases. The conclusions are drawn as follows. In the numerical analysis, the computed battery temperature profiles can be divided into four stages for the metal housing case: O~A, A~B, B~C and C~E. Among them, O~A is the initial stage before the onset of melting and the heat dissipation is mainly through
conduction and
sensible heat.
The isothermal temperature plateau phenomena B~C occurs when the latent heat and natural convection are sufficiently strong for the metal housing.
W
The evolution of the melting front is clearly visualized in both numerical simulations and visualization experiments (Fig. 15).
22
The findings in the present work would also be useful in investigating the heat transfer characteristics in a battery pack system, which is to be addressed in the future work.
Acknowledgements Financial supports under the program of Eastern Scholar and the project grant 14520501100 from Science and Technology Commission of Shanghai Municipality for are acknowledged.
REFERENCES [1]
Wada Masayoshi, Research and development of electric vehicles for clean transportation. J. Environmental Sciences 21(2009): 745-749.
[2]
B. Scrosati, J. Garche. Lithium batteries: status, prospects and future. J Power Sources 195(2010) 2419-2430.
[3]
S.A Khateeb, M.M Fraid, J.R Selman, S.A Hallaj. Design and simulation of a lithium-ion battery with a phase change material thermal management system for an electric scooter. Journal of Power Sources 128 (2004) 292-307.
[4]
X.Zhang, X.Bai, Incentive policies from 2006 to 2016 and new energy vehicle adoption in
23
2010-2020 in China. Renewable and Sustainable Energy Reviews, 2017, 70: 24-43. [5]
Yuksel T, Michalek J. Development of a simulation model to analyze the effect of thermal management on battery life. SAE Technical Paper, 2012 [01-0671].
[6]
Ramadass P, Haran B, White R, Popov B.N, Capacity fade of Sony 18650 cells cycled at elevated temperatures: Part I. Cycling performance. J. Power Sources 2002, 112, 606-613.
[7]
A.A .Pesaran, Battery thermal models for hybrid vehicle simulation, J. Power Sources 110 (2) (2002) 377-382.
[8]
Jeremy Neubauer, Eric Wood. Thru-life impacts of driver aggression, climate, cabin thermal management, and battery thermal management on battery electric vehicle utility, Journal of Power Sources 259 (2014) 262-275.
[9]
A. Mills, S.Al-Hallaj, Simulation of passive thermal management system for lithium-ion battery packs. Journal of Power Sources 141 (2005) 307-315.
[10] S. Al-Hallaj, J.R. Selman, Thermal modeling of secondary lithium batteries for electric vehicle/hybrid electric vehicle applications. J Power Sources 110(2002) 341-348. [11] Ziye Ling, Jiajie Chen, Xiaoming Fang, Zhengguo Zhang, Tao Xu, Xuenong Gao, Shuangfeng Wang, Experimental and numerical investigation of the application of phase change materials in a simulative power batteries thermal management system. Applied Energy 727 (2014), 704-773. [12] Ziye Ling, Z.G. Zhang, G.Q. Shi et al, Review on thermal management systems using phase change materials for electronic components, Li-ion batteries and photovoltaic modules. Renewable and Sustainable Energy Reviews 31 (2014) 427-438. 24
[13] Kizilel R, Lateef A, Sabbah R, Farid MM, Selman JR, Al-Hallaj S, Passive control
of
temperature excursion and uniformity in high-energy Li-ion battery packs at high current and ambient temperature. Journal of Power Sources183 (2008) 370–375. [14] X. Duan, G.F. Naterer, Heat transfer in phase change materials for thermal management of electric vehicle battery modules, Int. J. Heat Mass Transfer 53 (2010) 5176-5182. [15] G.Q. Zhang, C.Y. Zhao, Z.J. Wu, et al, Experimental investigation on the heat dissipation effect of power battery pack cooled with phase change materials, Chemical industry and engineering progress 28(2009)23-27. [16] Y. Shiina, T. Inagaki, Study on the effective thermal conductivities on melting characteristics of latent heat storage capsules. International Journal of Heat and Mass Transfer 48 (2005) 373-383. [17] Sharma A., Tyagi V.V, Chen C.R, et al, Review on thermal energy storage with phase change materials and applications. Renewable and Sustainable Energy Reviews 13 (2009) 318-345. [18] W. Zhou, F. Zhang, X.Q. Wang, Prospect of thermal control phase change in electronic devices. Chinese J. Electron Devices 30 (2007) 344-348. [19] R. Kandasamy, X.Q Wang, A.S Mujumdar, Application of phase change materials in thermal management of electronics. Applied Thermal Engineering 27 (2007) 2822-2832. [20] W.X. Wu, G.Q. Zhang, X.F. Ke et al, Preparation and thermal conductivity enhancement of composite phase change materials for electronic thermal management. Energy Conversion and Management 101 (2015) 278-284. 25
[21] B. Kamkari, H. Shokouhmand, Experimental investigation of phase change material melting in rectangular enclosures with horizontal partial fins, Int. J. Heat Mass Transfer 78 (2014) 839–851. [22] Z.W. Wang, H.Y. Zhang, X. Xia, Experimental investigation on the thermal behavior of cylindrical battery with composite paraffin and fin structure. Int. J. Heat Mass Transfer 109 (2017) 958-970. [23] A. Sciacovelli, F. Colella, V. Verda, Melting of PCM in a thermal energy storage unite: Melting of PCM in a thermal energy storage unit: Numerical investigation and effect of nanoparticle enhancement. Int. J. Energy Res. 37 (2013) 1610-1623. [24] W.B. Gu and C.Y. Wang, Thermal-electrochemical modeling of battery systems. J. Electrochem. Soc., 147 (2000) 2910-2922. [25] Drake S.J., Martin M., Wetz D.A., Ostanek J.K., Miller S.P., Heinzel J.M., Jain A., Heat generation rate measurement in a Li-ion cell at large C-rates through temperature and heat flux measurements. J. Power Sources 285 (2015) 266-273. [26] Y.H. Ye, Y.X. Shi, L.H. Saw, Andrew A.O. Tay, An electro-thermal model and its application on a spiral-wound lithium ion battery with porous current collectors. ElectrochimicaActa 121 (2014) 143-153. [27] P. Jany, A. Bejan, Scaling theory of melting with natural convection in an enclosure. Int. J. Heat Mass Transfer 31(1988), 1221-1235
26
List of Tables
Table 1 Thermo-physical properties of paraffin wax. Table 2 Thermophysical properties of other materials.
1
Table 1 Thermo-physical properties of paraffin wax.
C
Table 2 Thermophysical properties of other materials.
2
List of Figures Fig. 1 The geometrical model used in the present study: (a) the baseline case; (b) the case with fully adiabatic base-plate Fig. 2 The computed battery temperature results at three different heating power levels Fig. 3 Comparison of the computed liquid fraction of PCM at different time steps for three heating power levels Fig.4 (a) Velocity field and (b) liquid fraction distribution for the baseline case at different time steps for a heating power of 8.8W Fig. 5 Comparison of the duration before the thermal control point C for different thermal management systems. Fig. 6 The computed battery temperature evolutions for different test cases: the baseline ( in comparison with experimental measurement), no convection, and no latent heat. Fig. 7 Comparison of battery temperature evolution: metal housing with base-plate, metal housing without base-plate (adiabatic base-plate), acrylic housing with base-plate. Fig.8 (a) Velocity field and (b) liquid fraction distribution for the acrylic housing case at different time points Fig. 9 Comparison of the liquid fraction of PCM for the metal housing and acrylic housing Fig. 10 The temporal interfacial locations during the melting of PCM: (a) the metal housing case and (b) the acrylic housing case Fig. 11 The battery temperature difference Ttop- Tbot versus time Fig. 12 The proportion of heat dissipation in the thermal management system versus time Fig. 13 The instaneous Nusselt number vs normalized time for both metal and acrylic housing cases Fig. 14 (a) Schematic of test section with full circular metal housing (half of housing wall is removed for better view), (b) paraffin PCM used in this study and (c) schematic of visualization experimental system with semi-circular housing Fig. 15 Comparison of solid-liquid interfacial locations between the visualization experiments (left) and the numerical results (right) at four characteristic points for (a) the metal housing and (b) the acrylic housing 3
Fig. 1 The geometrical model used in the present study: (a) the baseline case; (b) the case with full adiabatic base-plate
Fig. 2 The computed battery temperature results at three different heating power levels 4
Fig. 3 Comparison of the computed liquid fraction of PCM at different time steps for three heating power levels
5
(a)
（b） Fig.4 (a) Velocity field and (b) liquid fraction distribution for the baseline case at different time steps for a heating power of 8.8W
6
Fig. 5 Comparison of the duration before the thermal control point C for different thermal management systems.
Fig. 6 The computed battery temperature evolutions: the baseline ( in comparison with experimental measurement), no convection, and no latent heat. 7
Fig. 7 Comparison of battery temperature evolution: metal housing with base-plate, metal housing without base-plate (adiabatic base-plate), acrylic housing with base-plate.
8
(a)
(b) Fig.8 (a) Velocity field and (b) liquid fraction distribution for the acrylic housing case at different time points
9
Fig. 9 Comparison of the liquid fraction of PCM for the metal housing and acrylic housing
(a)
(b) 10
Fig. 10 The temporal interfacial locations during the melting of PCM: (a) the metal housing case and (b) the acrylic housing case
Fig. 11 The battery temperature difference Ttop- Tbot versus time
11
Fig. 12 The proportion of heat dissipation in the thermal management system versus time
Fig. 15 The instantaneous Nusselt number for different cases.
Fig. 13 The instaneous Nusselt number vs normalized time for both metal and acrylic housing cases
12
housing
heater H=70mm
Fig. 14 (a) Schematic of test section with full circular metal housing (half of
simulated battery
housing wall is removed for better view), (b) paraffin PCM used in this study and (c) schematic of visualization experimental system with semi-circular housing
insulation
K2
(a)
K4
(b)
DC power supply PCM Housing
Tt
Acrylic T w
Heater HIOKI Simulated battery Tb camera Insulation
computer
(a)
(b) Fig. 15 Comparison of solid-liquid interfacial locations between the visualization experiments (left) and the numerical results (right) at four characteristic points for (a) the metal housing and (b) the acrylic housing 14
Highlights Numerical analysis and experimental visualization are presented for PCM surrounding battery Characteristic battery temperature and heat transfer are related to the melting stages Isothermal temperature plateau is obtained for the metal housing, not the acrylic housing Numerical analysis agrees with the experimental visualization
41
S1359-4311(17)31763-5 http://dx.doi.org/10.1016/j.applthermaleng.2017.09.022 ATE 11079
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
6 August 2017 5 September 2017 6 September 2017
Please cite this article as: H. Yang, H. Zhang, Y. Sui, C. Yang, Numerical analysis and experimental visualization of phase change material melting process for thermal management of cylindrical power battery, Applied Thermal Engineering (2017), doi: http://dx.doi.org/10.1016/j.applthermaleng.2017.09.022
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.
Numerical analysis and experimental visualization of phase change material melting process for thermal management of cylindrical power battery
Huan Yang1, Hengyun Zhang*1, Yang Sui1, Chun Yang2 1
College of Automotive Engineering, Shanghai University of Engineering Science 333 Longteng Road, Songjiang, Shanghai, China 201620
2
School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue Singapore 639798 *Correspondence Email: [email protected]
Abstract
Organic phase change materials (PCMs) have been of increasing interest in battery thermal management due to their high latent heat and excellent cycling stability. This paper reports both numerical analysis and visualization experiments for the melting process of paraffin PCM surrounding a cylindrical power battery. Specifically, two different housing materials are considered, one made of metal and the other made of acrylics. For the metal housing case, the PCM exhibits a melting front isolated both from the battery and housing wall with accelerated melting in the B~C stage, accounting for the isothermal temperature plateau observed in both the numerical modeling and experimental measurements. For the acrylic housing case, the unmelted PCM instead adheres to the inner wall of housing all the way, melting at the rate slower than the metal housing case. In both housing cases, the numerically predicted liquid fractions are found to agree with our experimentally visualized results. The battery top to bottom temperature variation is also examined with regard to the melting process. Moreover, the instantaneous Nusselt number 1
is obtained to identify the different heat transfer characteristics in the different melting stages.
Keywords: Phase change material (PCM); melting front; numerical model; visualization experiment; characteristic points
NOMENCLATURE
Ab surface area of battery (mm2) Db diameter of battery (mm) cp
specific heat(J/kg·K)
g
gravitational acceleration (m/s 2)
h
heat transfer coefficient (W/m2·K)
H b height of battery (mm)
H enthalpy (J)
k
thermal conductivity (W/m·K)
L
latent heat of paraffin (J/kg)
m
mass (kg)
P pressure (Pa) q
heating power (W)
r
radial coordinate (mm)
S
momentum source term
t
time (s)
T
temperature (°C) 2
T0 ambient temperature(°C） Tt
battery temperature at height Z=55mm
Tb battery temperature at height Z=10mm
V velocity vector of liquid paraffin V
volume (cm3)
W
gap between battery and housing
Z
axial coordinate (mm)
Greek symbols
thermal expansion coefficient (K-1)
liquid fraction
dynamic viscosity (kg/m·s)
density (kg/m3)
difference
Subscripts bot bottom f
liquid paraffin
ht
heater
ins insulation pcm phase change material ref reference top top w
housing 3
1 Introduction The energy shortage and environmental pollution issues have drawn increasing concern worldwide in recent decades. Specifically, transportation sector produces significant combustion-related emission from fossil fuels, such as carbon-hydrogen, oxynitride, carbon dioxide, etc. This calls for imperative replacement with substitute driving sources of high energy density and low emission [1]. Electric vehicles (EVs) and hybrid electric vehicles (HEVs), equipped with the lithium-ion batteries (LIBs), with high specific energy and high operating voltage, are the most feasible candidates to stand out from the traditional transportation tools [2, 3]. Notably, in 2015 China produced 331,092 EVs and HEVs, ranking first in the world [4]. As the core component of EVs, LIBs provide the power source central to the performance of electric vehicle directly. Nonetheless, the LIBs can get overheated with exothermic reactions and experience large temperature excursion during operation, leading to battery capacity fades and performance drops, or even possibly thermal runaway [5]. Thermal runaway tends to occur at elevated temperature, risking the loss of battery cell, pack or even whole vehicle [6]. High temperature affects the battery performance which is related to electrochemistry, round trip efficiency, charge acceptance, power and energy capability, reliability and cycle life [7]. It was reported that the add-on of indirect liquid cooling option can reduce the capacity fade from 33% to 27% [8]. As such, it is necessary to develop an effective thermal management system to maintain the battery within acceptable temperature range under various charge/discharge conditions. A phase change material (PCM) is referred to a material that changes from one aggregation 4
into another at a particular temperature accompanied by a large amount of heat storage or exotherm. Heat from the hot component is stored during melting process and released to the ambient during freezing process. Mainly due to their characteristics of high latent heat of fusion per weight, adjustable phase change temperature, as well as good thermal cycling and chemical stability, organic phase change materials (PCMs) such as paraffin waxes and fatty acids have been investigated recently as an alternative cooling method in the field of battery thermal management [9-15], along with the development in other applications such as heat storage [16-17], electronic cooling [18-20]. Al-Hallaj and Selman [10] proposed a battery module simply filled with a PCM in the gap between batteries with a module capacity of 100Ah, which could maintain the battery temperature 8 C below the one with no PCM. The passive cooling test with graphite and paraffin wax composites was also conducted in [13] for the 8S2P pack, and the center to corner battery temperature difference was measured to be 4 C, much smaller than the natural convection case. Duan et al [14] experimentally studied two types of PCMs with different melting temperatures for the thermal management of battery cell used in electric vehicles, one with a PCM cylinder surrounding the heater, and the other with PCM jackets wrapping the heater. Both configurations exhibited good effectiveness in maintaining the heater within a desired temperature range. Zhang et al [15] experimentally designed the battery cell and pack using composite phase change material, and found that the battery temperature at 1C discharge rate decreased by 14~18°C and 9~14°C, respectively, compared with the natural cooling and forced air cooling. Ling et al [11] numerically investigated the performance of passive battery thermal management systems using paraffin/EG composites. The influences of phase change temperature, composite density and paraffin mass fraction of the composite PCMs were discussed. Compared with natural air cooling 5
and liquid cooling, PCM was capable of keeping the components at a smaller range of temperature swing and minimizing the temperature difference among cells. Some visualization was reported in references such as [21] for the melting process in a rectangular slot bounded by an isothermal wall with horizontal fins and a plain wall. Nonetheless, most of the previous studies focus on the battery temperature profiles without in-depth investigation on the role of melting process in battery thermal control, especially in the presence of the thermally conductive housing. The connection between the melting process and the thermal performance improvement is not well understood and the isothermal control of the heat generating battery may not necessarily be achieved during the PCM melting stage. There is still a lack of correlation linking the phase change process with the heat transfer characteristics. In our recent work [22], we examined the thermal behavior with PCM-fin composite structure in battery thermal management and determined the characteristic temperature points through experimental measurements without visualizing the phase change process. Nonetheless, further investigation is required to explore the underlying heat transfer mechanism along with the help of experimental visualization. The present work aims to investigate the correlation between the melting process and the heat transfer characteristics for the use of paraffin PCM in temperature control of a cylindrical battery. A numerical analysis is conducted in combination with the visualization experiment to examine the melting process and the corresponding thermal characteristics. The configuration consists of a cylindrical battery cell allocated in the center of a concentric container made of either metal housing or acrylic housing. Detailed flow circulation, liquid fraction and heat transfer characteristics are obtained under different battery heating conditions. The characteristic 6
temperature-time points A, B, C and E indicated in the battery temperature evolution curve are found to correlate well with the different locations of the melting front. Moreover, the formation of the isothermal temperature plateau is revealed for the metal housing, which is rarely reported in literature. The corresponding experimental visualization prototype was also fabricated to visualize the melting process and to correlate with the heat transfer process.
2 Numerical Model The physical problem of the baseline case is depicted in Fig. 1(a) and the adiabatic base-plate case is shown in Fig. 1(b). An 18650 battery cell is allocated in the center of a container to simulate the battery heating configuration. The battery is 18 mm in diameter and 65 mm in height, with an electrical heater cylinder of 6.35mm (1/4 inch) in diameter and 40mm in length allocated at the battery center. The housing is made of aluminum and it has an inner diameter of 31 mm, a depth of 66 mm and a wall thickness of 4 mm. The thickness of the insulating spacer made of polycarbonate is 1 mm. Here we focus on a representative single battery cell configuration, in attempt to investigate the fundamental connection between the melting process and the heat transfer characteristics. The extension of the findings in the present work in the battery pack system is to be conducted in the next stage. Due to symmetry, a two-dimensional model is developed for numerical analysis. The battery generates a constant heat source q. The outer wall surface of the cylindrical housing is set with a constant convective heat transfer coefficient h and the base-plate of the housing is thermally insulated. The configuration in Fig. 1(b) is slightly different from that in Fig. 1(a) as the metal base of the housing is removed in purpose to examine the effect of the fully adiabatic base-plate case. The following assumptions are made in the modeling development: 7
The thermophysical properties of the materials including PCM are assumed constant.
The liquid paraffin is considered as an incompressible Newtonian fluid
Boussinesq’s approximation is used.
The effect of radiation heat transfer is negligible.
Then, the governing equations are: Continuity equation ( V ) 0 t
(1)
Momentum equation
V (V )V P 2V g (T Tref ) S t
(2)
Energy equation ( H ) ( VH ) (kT ) t
(3)
Here, is the liquid paraffin density, V is the velocity vector of the liquid paraffin, P is the
pressure, is the dynamic viscosity, is the thermal expansion coefficient of liquid paraffin, g
is the gravity vector, Tref is the reference temperature and S is the momentum source term [23]. The enthalpy-porosity approach is used to solve the phase change problem and the solid or liquid phase can be represented by the liquid fraction , and thus the enthalpy H is expressed as [23]: T
H href
c
pcm
dT L
Tref
(4)
where href is the enthalpy at the reference temperature, L is the latent heat, is the liquid fraction defined by
0, T Tm1 (T Tm1 ) /(Tm 2 Tm1 ), Tm1 T Tm 2 1, T Tm 2 8
(5)
Note that 0 represents solid phase, 1 represents liquid phase, and between 0 and 1 represents the mushy zone. Here Tm1 is the onset melting temperature and Tm2 is the peak melting temperature of the PCM according to the DSC test curve. The numerical model is solved using Fluent 15.0 software. Specifically, the PISO algorithm is used for dealing with pressure-velocity coupling, and quadrilateral surface mesh is utilized. The time step t is set to as small as 0.1s after examination of the time step effect. The convergence is achieved when the residual error is below 110-5 for velocity components and 110-7 for energy quantity. A grid-independence test shows that the deviation in battery temperature level is smaller than 0.6% when computational grids of 17260 and 35060 are used. The results from the former grid is presented throughout this study. The PCM used here is a commercially available paraffin wax, with its thermo-physical properties tabulated in Table 1. Table 2 shows the thermo-physical properties of the other materials as indicated in Fig. 1(a). For the numerical simulations, several representative temperatures are selected, including the housing outer wall temperatures and the battery temperature at two different heights, Z=10mm (Tbot) and Z=55mm (Ttop) above the battery bottom. At the same time, the velocity field and liquid fraction of PCM are also obtained.
3 Numerical Simulation Results 3.1 Characteristic temperature points with respect to the melting front location During normal operation of lithium ion batteries, heat is generated mainly due to two parts: ohmic heat and entropic heat [24]. It was reported that the heat generation of battery can reach a high level near to the nominal electric power at high discharge rates associated with discharge time and the state of charge [25, 26]. Especially, the generated heat reached 12W at a high 9
discharge current of 25A~ 9.5C for 26650 battery cell [25]. Our lab test of a commercial 18650 battery up to 10C (10 times of the nominal current) showed an approximate quadratic dependence on the discharge rate, with a heat generation of 9.2W obtained at 10C discharge. For analysis purpose, three constant battery heating powers of 6.6W, 8.8W and 13.2W are used to simulate the battery heating at large electrical discharge rates. In the simulation, the ambient temperature is set to 25°C, and a constant heat transfer coefficient of the housing outer wall is 10W/m2K. As will be discussed later, such a heat transfer coefficient is found to agree with the experimental measurement. Fig. 2 shows a comparison of the simulation results at different power levels Denoting the starting time point as Point “O”, t
10
the housing wall and enables direct convection of battery heat to the housing wall, which minimizes the thermal resistance, accelerates the heat dissipation and
Point C as the transition point when the mid-height temperature of the housing wall reaches the peak melting temperature, which actually
° 11
C. Beyond Point C, the melting front passes over the corner of the housing, and thus the heat transfer gets deteriorated, raising the battery temperature again. Another empirical way is to obtain Point C based on the intersecting point of the two tangent lines extrapolated from the temperature curves B~C and C~E, which deviates less than 40s from the present numerical evaluation. This extrapolation technique also applies to Point B which can be obtained as the intersection of the two tangent lines extrapolated from the A~B and B~C curves with a deviation within 40s. Not limited to the present metal housing with the isothermal plateau, the extrapolation determination of the characteristic point is also useful in evaluating the ending of thermal control in other thermal management system involving PCM, applicable for various enhancement techniques such as filling thermally conductive particles and adding fins or metal/graphite foams. The thermal control duration before Point C for different thermal management systems [11, 13, 22] are shown in Fig. 5, as a function of the heat density per PCM volume. The comparison includes the 8-fin experimental case from Wang et al [22], and the optimal case with PCM/expanded graphite foam from Ling et al [11] with 75% PCM in mass. Kizil et al’s result [13] based on the PCM/graphite composite for the 18650 pack is also plotted for comparison even though the exact percentage of PCM and graphite was not given. The duration time in the cited references has been obtained using the extrapolation technique. Due to the highest power density level considered in the present case, the duration time before the thermal control point is shorter than the cases with lower power. Once the melting front passes over battery temperature increases at an accelerated rate, which is considered the end of the desirable isothermal temperature control. Combining Fig. 4(b) and Fig. 3 with Fig. 2, the PCM 12
melts completely at Point E, corresponding to the liquid fraction of = 1.
°C
3.2 Effects of acrylic housing As shown in Section 3.1, temperature plateau for the PCM melting can be achieved in the metal housing. Such a temperature control, nonetheless, may not necessarily hold when the housing is made from a low thermal conductivity material. In this study, the acrylic housing is considered as against the metal housing made of aluminum, whereas the other parameters are kept the same as the metal housing case. Fig. 7 shows the battery temperature comparison between acrylic and metal housings at the heating power of 8.8W. It is seen that the isothermal temperature plateau almost disappears for the acrylic housing, and the battery temperature ramps up with similar characteristic points A, B, C and E. Due to the weak heat conduction along the acrylic wall, the melting is not accelerated by the housing and thus an isothermal temperature plateau is not well formed in the battery temperature curve. The resulting battery temperature is 6.3~16°C higher than that of the metal housing case after Point B, indicating the inefficiency of the acrylic 13
housing in view of thermal enhancement. The velocity field and liquid fraction distributions of the PCM for the acrylic housing case are presented in Fig. 8. Different from the metal housing case shown in Fig. 4, the melting front for the acrylic housing descends along the housing wall, moves faster than the metal housing case at around 1100-1600s but advances more slowly after 1800s. The morphological evolution of the unmelted PCM is also different. It shows a tapering peak adhering to the acrylic wall after Point B, as against the isolated peak for the metal housing case. As discussed earlier, the metal housing accelerates the heat spreading and promotes the melting process of PCM near the housing, forming an isolated peak of the unmelted PCM. The mushy zone for the metal housing case is more broadened at the bottom of the PCM domain after 1600s, and the complete melting is attained at 2200s. For the acrylic housing case, a small amount of PCM at the bottom corner of the housing keeps unmelted due to the weak heat conduction along the acrylic housing wall. Fig. 9 shows a comparison of the melted liquid fraction for the metal housing and the acrylic housing. In the early stage A~B, the PCM melting rate in the metal housing case is slower than that of the acrylic housing due to more heat loss to the ambient for the former. As heating evolves beyond Point B, the liquid fraction of the PCM increases above the trend line for the metal housing, and the melting rate is gradually accelerated due to the conjugated effects of direct convection and the subsequent heat conduction along the housing wall. Slower than the metal housing case after 1750s, the melting rate for the acrylic housing is decelerated in absence of heat conduction along the housing wall, falling below the trend line. In practice, such a difference in battery temperature level with different housings can be utilized to enhance heat transfer for better battery thermal management. The housing of high thermal conductivity acts not only as a 14
container, but also as an extended surface to enhance both the convection and melting, inclining to form the isothermal temperature plateau during battery heating process. The computed melting front lines (for the case of = 0.2) at different time steps are processed and vividly illustrated in Figs. 10 for both the metal housing and the acrylic housing at 8.8W. In the first period of time (tA t 400s), the melting front almost parallels to the battery wall, indicating a conduction-dominated melting process. At approximately 500s the melting front line bends toward to the right in the upper zone indicating the convection regime, while the lower part of the PCM remains conduction-dominated. With the elapse of time, the upper melting interface gradually approaches to the inner wall of the housing. As shown in Fig. 10(a), we confirm that the point B ~1100s in the computed battery temperature curve is consistent with the experimental visualization when the liquid paraffin contacts the inner wall of housing. After this time point, the gap between the solid-liquid interface lines becomes wider, indicating a faster melting rate and the melting interface descends faster along the inner wall of housing. At 1400~1500s, the melting interface is isolated away from the housing wall due to the stronger heat transfer from the metal housing wall. Fig. 10(b) shows the solid-fluid interface for the acrylic housing case. The interface evolution is similar to that of the metal housing case before Point B, though Point B comes slightly earlier than that for the metal housing case. After Point B, the interface evolution is obviously different. The melting front in the acrylic housing case descends along the housing inner wall gradually, adhering to the housing wall instead of being isolated in the metal housing case.
3.3 Effect of the adiabatic base-plate 15
In the physical model depicted in Fig. 1 (a), the metal base-plate is thermally conductive and thus heat conduction through the metal base might affect the phase change characteristics. Here, we examine the adiabatic base-plate case by removing the base-plate, as has been indicated in Fig. 1(b). The computed battery temperature versus heating time has been shown in Fig. 7. It can be seen that the characteristic time points A, B, C and E are still prominent, nearly agreeing with the characteristic time points for the baseline case with the metal housing and metal base-plate. Nonetheless, the average battery temperature is 5~10°C higher than the baseline case due to the elimination of heat conduction along the metal base-plate. Interestingly, the battery top to bottom temperature difference as shown in Fig. 11 is also related to the melting process. Such a temperature difference is positive for both the metal base plate and the adiabatic base-plate, indicating that the temperature of the upper battery is relatively higher than that of the lower. This is mainly due to the upper allocation of the heater and the upward flowing of the liquid PCM subjected to battery heating due to buoyancy effect. At Point A, the temperature difference drops and then increases with the heating time. This can be explained from the inset figure of Fig. 11 with enlarged view of the computed temperature curve at 8.8W heating power. Due to the adjacent heating of the upper part of the battery, the upper temperature of the battery is higher than its lower part temperature before PCM starts to melt. When the battery temperature at the top exceeds the melting temperature of 47C (Point 1 in the inset for the computed temperature difference for the metal housing), the PCM adjacent to the upper part begins to melt, keeping the upper temperature almost unchanged whereas the bottom battery temperature keeps increasing, which reduces the top to bottom temperature difference to form the turning point A. As the lower part temperature of the battery reaches above the melting 16
point, the PCM adjacent to the lower part of PCM starts melting, which reduces the bottom battery temperature, causing the second increase in the top to bottom temperature difference, as indicated in the A-2 stage in the inset of Fig. 11. Though similar in trend, the battery top to bottom temperature difference is smaller for the adiabatic base-plate.
3.4 Heat dissipation analysis The battery heat is not only transferred to the PCM, but also heats up the battery itself and the other components. A lumped energy balance equation at the time interval of t and t+t can be expressed as:
(6)
where the subscripts pcm stands for PCM, w the housing material, b the battery, h the overall heat exchange coefficient of the system boundary to the ambient, the average liquid fraction, and A the heat transfer area to the ambient. The seven terms in the right-hand side of the equation corresponds to the heat capacities of the PCM, battery, housing, insulation and heater, the latent heat absorption, and the heat loss to the ambient, respectively. The corresponding proportion of the heat dissipation versus time is calculated and illustrated in Fig. 12. It is seen that the PCM latent heat absorption amounts to 42-60% of the heat dissipation during the B~C stage, which is a significant contribution.
17
3.5 Heat transfer analysis In this section, the Nusselt number with time is used to identify different heat transfer rates associated with the melting process for the cylindrical battery cell. Here, the instantaneous Nusselt number is defined with the instantaneous temperature difference between the battery and the housing. Namely,
Nubw
h(t )W kf
(7)
where h(t) is the instantaneous surface-average heat transfer coefficient and determined by:
h(t )
q Ab(Tb Tw )
(8)
where Ab is the total heat transfer area calculated by:
Ab Db H b
(9)
Fig. 13 shows the obtained instantaneous Nu versus the normalized heating time t/tE for both metal and acrylic housing cases. Here tE is the time to complete melt at Point E. It can be seen in common that initially the Nusselt number decreases sharply until Point A for all the cases since the regime is conduction-dominated. After Point A, Nu does not vary significantly with the normalized time, which indicates a quasi-steady state and the liquid layer thickness keeps growing to allow occurrence of the convection. As the melting front moves beyond Point B, the inner wall of housing is exposed to liquid PCM due to the descending of melting front to form the direct convective heat dissipation, as already explained in Section 3.1. Thus the heat transfer is enhanced in the B~C stage, which results in the local maximum of Nu, and then decreases during the C~E stage. Due to the weaker conduction and consequent convection, the instantaneous Nu for the acrylic housing is always lower than that for the metal housing. It is also noted that the Nu 18
keeps almost unchanged in the B~C and C~E stages in spite of different heating power for the metal housing case.
4 Visualization Experiment 4.1 Experimental setup To validate the numerical analysis, we carried out the experiments with both full-circular housing and semi-circular test sections. The full-circular test section is shown in Fig. 14(a) to examine the battery temperature evolution. A comparison of the experimentally obtained battery temperature with the numerical results has been demonstrated in Fig. 6 with reasonable agreement for the full-circular test section. As it was difficult to find a heater with the exact dimension of a commercial 18650 cell, the mock-up battery made of aluminum cylinder of 18mm in diameter and 65mm in length was fabricated. A hole of 6.5mm was drilled at the center of the mock-up battery for inserting a heater with standard size of 6.35mm in diameter and 40mm in length as supplied from Watlow. To reduce the contact resistance, thin aluminum foil had been used to wrap around the heater for tight contact with the aluminum. The heater was connected with a DC power supply to simulate the 18650 battery heat generation. To facilitate visual observation of the melting process, the mock-up battery and housing were made into semicircular shape through wire cutting to form an observation section. The semicircular incision was sealed with a 3mm thick transparent plate made of acrylics. A highly elastic silicone adhesive, branded Dow Corning 3145, was applied to glue the acrylic plate with both the semicircular battery and the housing wall, as depicted in Fig. 14(a). Epoxy adhesive lacks of elongation at high temperature and cannot be used for the visualization test. Two different housing materials were tested in the visualization experiments. One was aluminum, and the other 19
was acrylics, both having an inner diameter of 31mm and an outer diameter of 39mm. An electrically insulating polycarbonate spacer of 1mm thick inserted beneath the battery, which also minimize the heat transfer to the base. Due to the symmetry in the test section, the thermocouples were allocated in the right-hand side of the test section as shown in Fig. 14. The battery temperature was measured by mounting two K-type thermocouples in the holes drilled at the top and bottom of the battery on the right side. Each hole has a diameter of 1.5mm and a depth of 10mm, with its center 7.25mm away from the battery center. Another two thermocouples were mounted on the outer housing wall for measuring the housing temperature with the same height as for the battery temperature measurement. All these thermocouples were connected to the Hioki data acquisition instrument (Hioki LR8410) to record the temperature data as shown in Fig. 14. The uncertainty in the thermocouple measurement was estimated to be 0.5°C based on factory calibration, whereas the deviations among thermocouples are around 0.1°C. The maximum deviation in the heat input was estimated to be 0.5%. Photos were taken around several characteristic times with aid of the thermocouples readings and the melting process observation. Point A corresponded to the time when the battery wall temperature reached the onset melting temperature of PCM, Point B the time where the liquid paraffin contacted the inner wall of housing, Point C the time when the housing temperature reached the peak melting temperature of PCM, and Point E the complete melting point.
4.2 The comparison between visualization experiment and numerical results Figs. 15 (a) and (b) shows comparison of melting front evolution between the visualization 20
experiments and the numerical results for the metal housing case and the acrylic housing case at characteristic Points A, B, C and E under the heating power 8.8W. The left hand side denotes the visualization experimental results and the right depicts the numerical results. It should be pointed out that the melting front contains a larger mushy zone in the metal housing case than in the acrylic housing case due to the heating of the PCM not only from the battery but also from the housing wall. To avoid interfacial blurring, only the liquid fraction distribution at = 0.2 is chosen in the comparison of visualized interfacial locations for both the metal and acrylic housing cases. It can be seen that the numerically predicted interfacial location agrees reasonably well with the experimentally visualized phase change interface. As the melting evolves, the melting front intersects with the right upper wall of the housing at Point B, which appears similar in both shape and size to the numerical predictions. As the melting further evolves, the melting front moves away from the housing wall to form an isolated peak due to the downward heat conduction along the housing wall. Shown in Fig. 15(b) is the comparison of the phase change process for the acrylic housing case. Again good agreement is found for the melting fronts between the experimental observations and the numerical predictions. Due to the weak heat conduction along the acrylic housing, the unmelted PCM adheres to the wall without being isolated, exhibiting a slower melting rate. At Point E all the PCM gets melted, which can be seen from both the experimental and numerical results.
4 Conclusions In this work, we have numerically analyzed and experimentally investigated the melting 21
process of the paraffin PCM and the thermal characteristics of the cylindrical power battery configured with both metal housing and acrylic housing cases. The conclusions are drawn as follows. In the numerical analysis, the computed battery temperature profiles can be divided into four stages for the metal housing case: O~A, A~B, B~C and C~E. Among them, O~A is the initial stage before the onset of melting and the heat dissipation is mainly through
conduction and
sensible heat.
The isothermal temperature plateau phenomena B~C occurs when the latent heat and natural convection are sufficiently strong for the metal housing.
W
The evolution of the melting front is clearly visualized in both numerical simulations and visualization experiments (Fig. 15).
22
The findings in the present work would also be useful in investigating the heat transfer characteristics in a battery pack system, which is to be addressed in the future work.
Acknowledgements Financial supports under the program of Eastern Scholar and the project grant 14520501100 from Science and Technology Commission of Shanghai Municipality for are acknowledged.
REFERENCES [1]
Wada Masayoshi, Research and development of electric vehicles for clean transportation. J. Environmental Sciences 21(2009): 745-749.
[2]
B. Scrosati, J. Garche. Lithium batteries: status, prospects and future. J Power Sources 195(2010) 2419-2430.
[3]
S.A Khateeb, M.M Fraid, J.R Selman, S.A Hallaj. Design and simulation of a lithium-ion battery with a phase change material thermal management system for an electric scooter. Journal of Power Sources 128 (2004) 292-307.
[4]
X.Zhang, X.Bai, Incentive policies from 2006 to 2016 and new energy vehicle adoption in
23
2010-2020 in China. Renewable and Sustainable Energy Reviews, 2017, 70: 24-43. [5]
Yuksel T, Michalek J. Development of a simulation model to analyze the effect of thermal management on battery life. SAE Technical Paper, 2012 [01-0671].
[6]
Ramadass P, Haran B, White R, Popov B.N, Capacity fade of Sony 18650 cells cycled at elevated temperatures: Part I. Cycling performance. J. Power Sources 2002, 112, 606-613.
[7]
A.A .Pesaran, Battery thermal models for hybrid vehicle simulation, J. Power Sources 110 (2) (2002) 377-382.
[8]
Jeremy Neubauer, Eric Wood. Thru-life impacts of driver aggression, climate, cabin thermal management, and battery thermal management on battery electric vehicle utility, Journal of Power Sources 259 (2014) 262-275.
[9]
A. Mills, S.Al-Hallaj, Simulation of passive thermal management system for lithium-ion battery packs. Journal of Power Sources 141 (2005) 307-315.
[10] S. Al-Hallaj, J.R. Selman, Thermal modeling of secondary lithium batteries for electric vehicle/hybrid electric vehicle applications. J Power Sources 110(2002) 341-348. [11] Ziye Ling, Jiajie Chen, Xiaoming Fang, Zhengguo Zhang, Tao Xu, Xuenong Gao, Shuangfeng Wang, Experimental and numerical investigation of the application of phase change materials in a simulative power batteries thermal management system. Applied Energy 727 (2014), 704-773. [12] Ziye Ling, Z.G. Zhang, G.Q. Shi et al, Review on thermal management systems using phase change materials for electronic components, Li-ion batteries and photovoltaic modules. Renewable and Sustainable Energy Reviews 31 (2014) 427-438. 24
[13] Kizilel R, Lateef A, Sabbah R, Farid MM, Selman JR, Al-Hallaj S, Passive control
of
temperature excursion and uniformity in high-energy Li-ion battery packs at high current and ambient temperature. Journal of Power Sources183 (2008) 370–375. [14] X. Duan, G.F. Naterer, Heat transfer in phase change materials for thermal management of electric vehicle battery modules, Int. J. Heat Mass Transfer 53 (2010) 5176-5182. [15] G.Q. Zhang, C.Y. Zhao, Z.J. Wu, et al, Experimental investigation on the heat dissipation effect of power battery pack cooled with phase change materials, Chemical industry and engineering progress 28(2009)23-27. [16] Y. Shiina, T. Inagaki, Study on the effective thermal conductivities on melting characteristics of latent heat storage capsules. International Journal of Heat and Mass Transfer 48 (2005) 373-383. [17] Sharma A., Tyagi V.V, Chen C.R, et al, Review on thermal energy storage with phase change materials and applications. Renewable and Sustainable Energy Reviews 13 (2009) 318-345. [18] W. Zhou, F. Zhang, X.Q. Wang, Prospect of thermal control phase change in electronic devices. Chinese J. Electron Devices 30 (2007) 344-348. [19] R. Kandasamy, X.Q Wang, A.S Mujumdar, Application of phase change materials in thermal management of electronics. Applied Thermal Engineering 27 (2007) 2822-2832. [20] W.X. Wu, G.Q. Zhang, X.F. Ke et al, Preparation and thermal conductivity enhancement of composite phase change materials for electronic thermal management. Energy Conversion and Management 101 (2015) 278-284. 25
[21] B. Kamkari, H. Shokouhmand, Experimental investigation of phase change material melting in rectangular enclosures with horizontal partial fins, Int. J. Heat Mass Transfer 78 (2014) 839–851. [22] Z.W. Wang, H.Y. Zhang, X. Xia, Experimental investigation on the thermal behavior of cylindrical battery with composite paraffin and fin structure. Int. J. Heat Mass Transfer 109 (2017) 958-970. [23] A. Sciacovelli, F. Colella, V. Verda, Melting of PCM in a thermal energy storage unite: Melting of PCM in a thermal energy storage unit: Numerical investigation and effect of nanoparticle enhancement. Int. J. Energy Res. 37 (2013) 1610-1623. [24] W.B. Gu and C.Y. Wang, Thermal-electrochemical modeling of battery systems. J. Electrochem. Soc., 147 (2000) 2910-2922. [25] Drake S.J., Martin M., Wetz D.A., Ostanek J.K., Miller S.P., Heinzel J.M., Jain A., Heat generation rate measurement in a Li-ion cell at large C-rates through temperature and heat flux measurements. J. Power Sources 285 (2015) 266-273. [26] Y.H. Ye, Y.X. Shi, L.H. Saw, Andrew A.O. Tay, An electro-thermal model and its application on a spiral-wound lithium ion battery with porous current collectors. ElectrochimicaActa 121 (2014) 143-153. [27] P. Jany, A. Bejan, Scaling theory of melting with natural convection in an enclosure. Int. J. Heat Mass Transfer 31(1988), 1221-1235
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List of Tables
Table 1 Thermo-physical properties of paraffin wax. Table 2 Thermophysical properties of other materials.
1
Table 1 Thermo-physical properties of paraffin wax.
C
Table 2 Thermophysical properties of other materials.
2
List of Figures Fig. 1 The geometrical model used in the present study: (a) the baseline case; (b) the case with fully adiabatic base-plate Fig. 2 The computed battery temperature results at three different heating power levels Fig. 3 Comparison of the computed liquid fraction of PCM at different time steps for three heating power levels Fig.4 (a) Velocity field and (b) liquid fraction distribution for the baseline case at different time steps for a heating power of 8.8W Fig. 5 Comparison of the duration before the thermal control point C for different thermal management systems. Fig. 6 The computed battery temperature evolutions for different test cases: the baseline ( in comparison with experimental measurement), no convection, and no latent heat. Fig. 7 Comparison of battery temperature evolution: metal housing with base-plate, metal housing without base-plate (adiabatic base-plate), acrylic housing with base-plate. Fig.8 (a) Velocity field and (b) liquid fraction distribution for the acrylic housing case at different time points Fig. 9 Comparison of the liquid fraction of PCM for the metal housing and acrylic housing Fig. 10 The temporal interfacial locations during the melting of PCM: (a) the metal housing case and (b) the acrylic housing case Fig. 11 The battery temperature difference Ttop- Tbot versus time Fig. 12 The proportion of heat dissipation in the thermal management system versus time Fig. 13 The instaneous Nusselt number vs normalized time for both metal and acrylic housing cases Fig. 14 (a) Schematic of test section with full circular metal housing (half of housing wall is removed for better view), (b) paraffin PCM used in this study and (c) schematic of visualization experimental system with semi-circular housing Fig. 15 Comparison of solid-liquid interfacial locations between the visualization experiments (left) and the numerical results (right) at four characteristic points for (a) the metal housing and (b) the acrylic housing 3
Fig. 1 The geometrical model used in the present study: (a) the baseline case; (b) the case with full adiabatic base-plate
Fig. 2 The computed battery temperature results at three different heating power levels 4
Fig. 3 Comparison of the computed liquid fraction of PCM at different time steps for three heating power levels
5
(a)
（b） Fig.4 (a) Velocity field and (b) liquid fraction distribution for the baseline case at different time steps for a heating power of 8.8W
6
Fig. 5 Comparison of the duration before the thermal control point C for different thermal management systems.
Fig. 6 The computed battery temperature evolutions: the baseline ( in comparison with experimental measurement), no convection, and no latent heat. 7
Fig. 7 Comparison of battery temperature evolution: metal housing with base-plate, metal housing without base-plate (adiabatic base-plate), acrylic housing with base-plate.
8
(a)
(b) Fig.8 (a) Velocity field and (b) liquid fraction distribution for the acrylic housing case at different time points
9
Fig. 9 Comparison of the liquid fraction of PCM for the metal housing and acrylic housing
(a)
(b) 10
Fig. 10 The temporal interfacial locations during the melting of PCM: (a) the metal housing case and (b) the acrylic housing case
Fig. 11 The battery temperature difference Ttop- Tbot versus time
11
Fig. 12 The proportion of heat dissipation in the thermal management system versus time
Fig. 15 The instantaneous Nusselt number for different cases.
Fig. 13 The instaneous Nusselt number vs normalized time for both metal and acrylic housing cases
12
housing
heater H=70mm
Fig. 14 (a) Schematic of test section with full circular metal housing (half of
simulated battery
housing wall is removed for better view), (b) paraffin PCM used in this study and (c) schematic of visualization experimental system with semi-circular housing
insulation
K2
(a)
K4
(b)
DC power supply PCM Housing
Tt
Acrylic T w
Heater HIOKI Simulated battery Tb camera Insulation
computer
(a)
(b) Fig. 15 Comparison of solid-liquid interfacial locations between the visualization experiments (left) and the numerical results (right) at four characteristic points for (a) the metal housing and (b) the acrylic housing 14
Highlights Numerical analysis and experimental visualization are presented for PCM surrounding battery Characteristic battery temperature and heat transfer are related to the melting stages Isothermal temperature plateau is obtained for the metal housing, not the acrylic housing Numerical analysis agrees with the experimental visualization
41