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Project 1

RRohit Patil7 Oct 2020

Abstract

The operating temperature of Li-ion batteries used in modern electric vehicles should be maintained within an allowable range to avoid thermal runaway and degradation. One of the most challenging issues faced by the automobile industry is providing proper thermal management mechanisms to avert thermal runaways. In this work, the effect of operating parameters like volumetric heat generation (S̅q), conduction–convection parameter (ζcc), Reynolds number (Re), and Aspect ratio (Ar) on the thermal behavior of a prismatic battery cell is investigated numerically considering a realistic conjugate condition at the battery cell and coolant interface. Air is selected as the coolant that carries the heat generated uniformly in the modern battery cell during charging or discharging from its surface. For variations in S̅q from 0.1 to 1.0, ζcc from 0.06 to 0.1, Re from 250 to 2000, and Ar from 10 to 35, the temperature distribution, as well as maximum temperature variation in the battery cell, is determined. Further, the occurrence of the critical threshold of temperature and the necessary change in these operating parameters to avoid thermal runaway is proposed. Finally, the effect of flow Reynolds number and channel spacing on average Pressure and average Nusselt number is also discussed in this study. From the exhaustive analysis on the effect of considered parameters, it is observed that apart from heat generation parameter S̅q, other parameters like ζcc and Re play a prominent role in reducing the maximum temperature of the battery cell. However, Ar has a negligible impact on the thermal performance of battery cell irrespective of any value of other parameters considered in this study. It was interesting to find that for Re> 1000, the impact on temperature profiles of the battery cell is minimal, while the other parameters were either kept constant or varying, putting a limit to higher values of Re.

Abstract

The battery thermal management system (BTMS) using a unidirectional cooling strategy for a battery pack would cause the temperature gradient between the battery cells located at the inlet and outlet of the duct, which would reduce the temperature uniformity of the battery module. In order to solve the problem, the reciprocating cooling strategy was introduced in this paper to study the effect of switching time (τ) on the temperature uniformity of the battery module. Based on this, the flat heat pipe was combined with liquid cooling, in which the efficient heat transfer performance of the flat heat pipe could transfer the heat of the battery cells to the liquid. Using CFD to simulate the proposed BTMS, the result showed that the reciprocating cooling strategy could improve the temperature performance of the BTMS.

1. Introduction

At present, the development of EVs and HEVs is mainly constrained by the battery pack,(1,2) especially the lithium-ion battery cell.(3) However, the performance of the lithium-ion battery cell is limited by the working temperature. If the heat of the lithium-ion battery cannot be dissipated in time, it would lead to excessively high temperature and reduce the temperature uniformity of the battery pack.(4,5) In a word, an effective BTMS is essential(6) to make full use of each battery cell. The BTMS needs to meet the following requirements: (1) limit the temperature of the battery pack below 60 °C; and (2) minimize the temperature difference of the battery pack within 5 °C.(7,8)
Up to now, some cooling strategies have been implemented in the BTMS,(9) including the air cooling, liquid cooling, phase change material, heat pipe, and their combinations.(10−12) Based on its efficient heat transfer performance and light weight, the heat pipe was gradually applied into the BTMS.(13,14) Xu et al.(15) designed a multistage heat dissipation system based on the flat heat pipe combined with liquid cooling, which successfully controlled temperature of the battery module within the working temperature range. However, it led to a decrease in the temperature uniformity of the single battery cell in the battery module. On this basis, Yang et al.(16) placed the flat heat pipe between the cylindrical battery cells and filled them with a thermally conductive material. The result showed that the BTMS achieved the expected cooling effect, and it could alleviate the problem of excessive temperature difference of the single battery cell. Therefore, the combination of a flat heat pipe and a liquid cooling system could ensure that the temperature of the battery module is kept within the working temperature range.(17,18)
In addition, the traditional BTMS used a unidirectional flow strategy, which would lead to the inherent temperature gradient problem of conventional BTMS due to the heat exchange between the battery cells and the liquid. In this way, it would further reduce the temperature uniformity of the battery pack. Based on this, a strategy of reciprocating cooling was proposed. At present, there are few studies on the reciprocating cooling strategy, which mainly focused on the reciprocating flow used for super-adiabatic combustion and heat recovery applications.(19,20) Mahamud and Park(21) studied the use of reciprocating air flow cooling on BTMS to improve the temperature uniformity of the battery pack. They used flip door valves to achieve the reciprocating flow air, in which the BTMS with reciprocating flow could reduce the temperature difference by 4 °C. However, by using flip door valves and the unique duct to create reciprocating air would increase the volume and weight of the BTMS, and the flip door valves would greatly affect the service life of the entire system.
To alleviate the temperature gradient problem inherent in traditional battery systems, we used reciprocating cooling strategy in the paper. The reciprocating flow produced by using a reversible pump is shown in Figure 1. By using a reversible pump, a reciprocating cooling flow could be generated based on the original liquid duct. This paper focuses on analysis of the sensitivity of the switching time (τ) to the effectiveness of BTMS.

Figure 1

Figure 1. Schematic view of the BTMS with reciprocating cooling combined with a flat heat pipe.

2. Experimental Section (Battery Thermodynamic Parameters Acquisition)

In this paper, a 60 A h lithium-ion battery cell was selected. We adopted a simulation method to obtain the heating power of the battery cell and then conducted experiments to verify the result of simulations.

2.1. The Simulation of the Battery Cell

COMSOL was utilized to simulate the battery cell used in this paper (the electrochemical parameters provided by the manufacturer). The model consisting of a negative electrode material layer, negative electrode current collector, positive electrode material layer, positive current collector, and separator was established in COMSOL. We obtained the temperature and heating power of the battery cell at rates of 1/3, 2/3, 0.8, 1.0, 1.2, 1.5, and 2.0 C.
The temperature distribution of the lithium-ion battery cell at rates of 1 and 2 C is shown in Figure 2. It showed a temperature gradient that diffused from the middle to the surroundings of the battery cell. At a discharge rate of 2 C, the temperature trend was consistent with the discharge rate of 1 C.

Figure 2

Figure 2. Temperature distribution of the battery cell at the discharge rate of (a) 1 and (b) 2 C.

2.2. Verification of the Battery Cell Model

The experiment was designed and conducted to ensure the accuracy of simulation results. The test condition was consistent with the simulation condition. Comparing the differences between simulation and experimental results under the same operating conditions, we believed that the final temperature difference range was acceptable within 5% (E = |T1 – T2|/T2, where E is the temperature difference range, T1 is the temperature in simulation, and T2 is the temperature in the experiment).
The ambient temperature was 20 ± 0.5 °C. Eight temperature sensors were arranged in this experiment, including the positive and negative electrodes, the remaining five faces of the battery cell, and the ambient temperature shown in Figure 3. The experiment setting is shown in Figure 4.

Figure 3

Figure 3. Temperature measurement distribution.

Figure 4

Figure 4. Measurement distribution in a thermostat.

The experiment time was 7200 s. The battery cell was charged and discharged at a rate of 1 C. The temperature curves measured by eight temperature sensors are shown in Figures 5 and 6. The surface temperatures of the positive and negative electrodes of the battery cell were slightly higher than those of other positions, and the positive electrode surface temperature of the battery cell was higher than the negative electrode surface temperature.

Figure 5

Figure 5. Cell temperature at a charge rate of 1 C.

Figure 6

Figure 6. Cell temperature at a discharge rate of 1 C.

2.3. Comparison of Results and Thermal Characteristics of Battery Cells

Figures 7 and 8 show the temperature difference of the battery cell under simulation and experiment conditions. Under the discharge condition, the temperature difference range was 3.6%; under the charge condition, the temperature difference range was 2.8%, which both meet the demands.

Figure 7

Figure 7. Charge process at a rate of 1 C.

Figure 8

Figure 8. Discharge process at a rate of 1 C.

Detailed charge and discharge data for 1/3, 2/3, 1.0, 1.5, and 2.0 C and the specific thermodynamic parameters of battery cells are shown in Table1.
Table 1. Thermodynamic Parameters of the 60 A h Battery Cell at Difference Rates
charge process discharge process
rate temperature rise (°C) heating power (W) temperature rise (°C) heating power (W)
1/3 C 9.75 2.06 10.01 2.56
2/3 C 10.73 2.95 11.15 3.12
1 C 12.57 5.42 15.28 7.60
1.5 C 18.94 10.06 23.27 15.59
2 C 22.62 14.44 26.99 19.89

3. Methods (Design of BTMS, Governing Equation, and Boundary and Simulation Conditions)

3.1. Conceptual Design of the BTMS

This paper used numerical calculations to study the sensitivity of switching time (τ) to the temperature distribution of the battery module. We used STAR-CCM+ to simulate the proposed BTMS. Generally, a battery module has 12 battery cells, as shown in Figure 9.

Figure 9

Figure 9. Schematic view of a conventional module.

Considering the convenience of calculation and geometric symmetry of the model, the six battery cells shown in Table2 were taken as one unit in the paper. The computational domain of the unit is shown in Figure 10.

Figure 10

Figure 10. Computational domain of the unit.

Table 2. Battery Cell Parameters: Lithium-Ion Battery, Ternary Material NCM Thermodynamic Parameters
parameter cell flat heat pipe duct thermal adhesive
ρ (kg · m–3) 2123 4000 8978 1008
K (W·m–1·k–1) X Y Z X Y Z 387.6 4
2 2 20 200 9216 9216
Cp (J·kg–1·k–1) 913 871 381 1978
The BTMS proposed in this paper consisted of six battery cells, 10 flat heat pipes, five pieces of thermal adhesive, and a unidirectional liquid duct. The relative position relationship between them and their dimension parameters are shown in Figure 11. The flat heat pipe (sintered copper liquid) was inserted into the unidirectional liquid duct. The flat heat pipe and duct were welded to ensure tightness of the entire system. The cooling liquid in the one-way liquid duct flowed through the evaporation end of the flat heat pipe and generated vortices to enhance heat exchange. The 2 mm thick thermal adhesive was placed between two flat heat pipes, which was used to ensure the temperature uniformity of the battery cell located between the two flat heat pipes. In addition, the thermal grease with a thickness of about 0.3 mm was used between the battery cell and flat heat pipe to ensure the insulation and make it as a buffer.

Figure 11

Figure 11. Relative position relationship and dimension parameters of each component of the model.

The thermal conductivity of the heat pipe selected in this paper was based on experimental data determined by Ye et al.(22) They experimentally obtained the thermal resistance of the flat heat pipe. Based on the heat conduction process, the thermal conductivity of the flat heat pipe was derived by eq 1. The thermal conductivity of the flat heat pipe (khp) was 9216 W·m–1·k–1 in the present study under the condition.(1)Here, Rhp is the thermal resistance of the flat heat pipe. lhp is the length of the flat heat pipe. d is the diameter of the flat heat pipe. khp is the thermal conductivity of the flat heat pipe.(23)
Additionally, the characteristics of related materials proposed in this paper are shown in Table3. Among them, the flat heat pipe acted as a heat conductor with effective thermal conductivity, as shown in Figure 12.

Figure 12

Figure 12. Schematic view of the flat heat pipe.

Table 3. Dimensions of the Battery Cell, Module, and the Working Condition Used for the Simulation and Thermophysical Parameters
parameters of cells
L (mm) 148 liquid parameters (at 20 °C)
W (mm) 26 ρf(kg · m–3) 805.3
H (mm) 97 Kf (W·m–1·k–1) 0.1543
parameters of battery module Cf (kJ·kg–1·k–1) 2.2071
number of cells 12 μf (kg·m–1·s–1) 0.00759
L2 (mm) 148 boundary condition (at 295 K)
D3 (mm) 324 charging/discharging rate (C) 0.5/1/2
parameters of flat heat pipe T (°C) 20
L1 (mm) 680 t (s) 7200
D1 (mm) 500 flow rate (L·h–1) 400

3.2. CFD Simulation Governing Equation

According to above assumptions, as well as the obtained thermal characteristics of the battery cell, and the thermodynamic parameters of flat heat pipe, the energy conservation equation(24) of the BTMS proposed in this paper is as follows:(2)Here, ρb is the density of the battery module, Cpb is the specific heat capacity of the battery module, T is the temperature of the batter module, kb is the thermal conductivity of the battery module, and Q• is the source term that can be calculated as follows:
Here, Qgen is the heat generation power of the battery cell. Vb is the volume of the battery cell.
The governing equations for liquid include the continuous equation,(25) momentum conservation equation,(26) and energy conservation,(27) which are shown as follows:
Continuous equation:(4)
Momentum conservation equation:(5)
Energy conservation:(6)
Here, v means the velocity vector, P means the pressure, ρw means the water density, and kw means the thermal conductivity of water.
The heat generation power Qgen of the battery cell was obtained by simulation, as shown in Table1.

3.3. Boundary and Simulation Conditions

The ambient temperature was set to 20 °C. The inlet of the BTMS duct was set as the speed inlet, the inlet flow was set to 400 L h–1, and the turbulence was 2%. The water inlet temperature was the same as ambient temperature. The outlet of the BTMS duct was set as a pressure outlet. The heat transfer surfaces were set as a coupling surface. In addition, other surfaces were set as heat insulation walls. Additionally, in order to facilitate the numerical simulation, we make the following assumptions: The flat heat pipe was considered as a solid conductor with high thermal conductivity.
During the running of EVs, the charge and discharge rates of the battery pack changed with the driving conditions. Therefore, the charge and discharge strategy shown in Table 4 were formulated to simulate the real operating condition in this paper, verifying the thermal efficiency of proposed BTMS. Among them, working condition A performed charge and discharge with a smaller current, while working condition B was based on working condition A. The charge and discharge currents were doubled, and the simulation time was unchanged.
Table 4. Simulation Working Condition Data
stage serial number operation state time rate (A condition) rate (B condition)
conditions 1 1 discharging 430 s 1/3 C (20 A) 2/3 C (40 A)
2 shelve 20 s 0 0
3 charging 150 s 1 C (60 A) 2 C (120 A)
4 shelve 20 s 0 0
conditions 2 1 discharging 430 s 1/3 C (20 A) 2/3 C (40 A)
2 shelve 20 s 0 0
3 charging 150 s 1 C (60 A) 2C (120 A)
4 shelve 20 s 0 0
conditions 3 1 discharging 430 s 1/3 C (20 A) 2/3 C (40 A)
2 shelve 20 s 0 0
3 charging 150 s 1 C (60 A) 2 C (120 A)
4 shelve 20 s 0 0
conditions 4 1 discharging 800 s 1/3 C (20 A) 2/3 C (40 A)
2 shelve 20 s 0 0
3 charging 280 s 1 C (60 A) 2 C (120 A)
4 shelve 20 s 0 0

4. Results and Discussion

Based on previous research, this paper mainly studied the maximum temperature TMax, the minimum temperature TMin, the maximum temperature rise TMaxRise, and the maximum temperature difference TMaxDiff of the battery module as indicators to evaluate the thermal efficiency of BTMS.

4.1. Validation of the CFD Model Using the Unidirectional Flow System

In this paper, the working condition A is shown in Figure 13. The calculation data was derived and post-processed after the calculation was completed. Figure 14 shows the temperature distribution of the battery module under working condition A. Figures 15 and 16 show TMax and TMin of the battery module under working condition A. Among them, TMax of the battery module was 23.7 °C, and TMin of the battery module was 20.28 °C. So, the temperature difference of the battery module was 3.42 °C; excluding the #1 and #6 cells, TMax of the system was 22.4 °C, TMin was 20.28 °C, and the temperature difference was 2.12 °C. The temperature distribution on the cell was relatively uniform, which was due to the flat heat pipe thermal conductivity and thermally conductive adhesive designed between battery cells.

Figure 13

Figure 13. Working condition A.

Figure 14

Figure 14. Temperature distribution of the battery module.

Figure 15

Figure 15. TMax of the battery module under condition A.

Figure 16

Figure 16. TMin of the battery module under condition A.

Figure 14 also shows that temperature at the inlet was always lower than that at the outlet due to unidirectional flow. This caused temperature of battery cells located at the outlet to be higher than those located at the inlet. With the temperature of liquid increasing, the temperature uniformity would be further weakened.
It is worth noting that the temperature of the liquid continued to increase during the flow of the liquid in the duct. It could also be seen in Figure 16 that the temperatures of battery cells (#2 and #3) were slightly higher than those of the battery cells (#4 and #5). Based on this, we considered using the reciprocating liquid strategy to optimize temperature distribution inside the battery module.
Figures 17 and 18 show the curve of TMax, TMin, TMaxRise, and TMaxDiff under working condition A and condition B. Among them, TMax of the battery module under working condition A was 23.7 °C; TMin was 20.28 °C; TMaxDiff was 3.42 °C; TMaxRise was 3.7 °C. TMax of the battery module under working condition B was 25.31 °C, TMin was 21.23 °C, TMaxDiff was 4.08 °C; TMaxRise was 5.31 °C.

Figure 17

Figure 17. Temperature characteristic curve of the battery module under working condition A.

Figure 18

Figure 18. Temperature characteristic curve of the battery module under working condition B.

4.2. Analysis of the Effect of Switching Time (τ) on the Thermal Performance of BTMS

Figure 19a shows that the average temperature of cell #6 was the highest under working condition B, reaching 22.39 °C, which was 0.13 °C higher than the average temperature of cell #1., and there were obvious temperature gradients between these three groups of cells ((#6, #1), (#5, #2), and (#4, #3)). Compared with working condition A, the temperature uniformity of the battery module was further weakened due to the balance between heat generation of the battery module and heat dissipation of the BMTS. At 5400 s, the simulation showed a steady-state convergence trend, and the temperature difference between the battery modules gradually decreased.

Figure 19

Figure 19. Average temperature curve of the battery module under different working conditions.

Further simulation was performed based on working condition B, and a reciprocating liquid cooling strategy was adopted. This paper identified switching times (τ) of 3600, 1800, and 900 s. It could be seen from Figure 19 that, as the switching time (τ) decreased from 3600 to 900 s, TMaxDiff between the battery cells decreased, and the temperature uniformity tended to be consistent. In particular, the temperature curves of four battery cells (#5, #2, #4, and #3) in the middle of the battery module were closer, which means that the temperature of four battery cells was basically consistent. Due to the reduction of TMaxDiff between the liquid inlet and the liquid outlet by the using reciprocating liquid strategy. TMaxDiff and TMaxRise would reduce. Taking cell #4 as an example, as the switching time (τ) decreased, the temperature of cell #4 decreased from 21.875 to 20.875 °C; the performance was obvious.

4.3. Analysis of Single Battery Cell Temperature

According to the above study, switching the flow of liquid could improve effectiveness of the BTMS. This chapter focuses on the temperature characteristics of each battery cell under the unidirectional cooling strategy and the reciprocating cooling strategy, as shown in Figure 20. This paper analyzed TMax, TMin, TMaxRise, and TMaxDiff of the battery cell. Among these four temperature characteristics, TMin of the battery cell was most affected by switching times (τ), and its sensitivity was the highest, as shown in Figure 20b.

Figure 20

Figure 20. Average temperature curve of the battery module under different working conditions.

The sensitivity of other three parameters was lower, and only minor fluctuations occurred. However, once the liquid cooling system changed from a unidirectional flow strategy to a reciprocating flow strategy, the three parameters TMax, TMaxRise, and TMaxDiff of the battery cells would change greatly. Figure 20a shows that TMax of the battery cell was reduced by 4.6%; Figure 20c shows that TMaxRise of the battery cell was reduced by 47.11%; Figure 20d shows that TMaxDiff of the battery cell was reduced by 75.6%. This result showed that the reciprocating cooling strategy could greatly improve the performance of BTMS.
We further analyzed the heat dissipation efficiency of the reciprocating cooling strategy taking the battery cells #3 and #4 as examples under initial conditions (unidirectional flow strategy). TMin of battery cell#3 near the liquid inlet was 21.95 °C, which was higher than the temperature of battery cell #4. With the decrease of switching times (τ), the temperature difference between battery cells #3 and #4 would further narrow. As shown in Figure 20c, when the switching times (τ) = 900 s, TMaxRise of battery cells #3 and #4 was approximately 2.34 °C. Therefore, as the switching times (τ) decreased, the temperature consistency of the battery module would improve.

5. Conclusions

This paper proposed a reciprocating cooling strategy to improve the temperature uniformity of the battery module for BTMS of the flat heat pipe and liquid cooling establishing the thermal model of the 60 A h lithium-ion battery cell and obtaining the thermal characteristics of the battery cell. Through simulations, the influence of switching time (τ) on temperature uniformity of the battery module was studied. Research showed that the reciprocating cooling strategy greatly reduced the temperature difference and the temperature rise of the battery module.
The main work and conclusions of this paper are as follows:
(1)

Under working condition A, the BTMS of the flat heat pipe combined with liquid cooling using a unidirectional cooling strategy could meet thermal management needs of the battery module. TMaxDiff of the battery module was 2.04 °C, and TMaxRise was 3.46 °C. The temperature gradient between the four cells (#3, #4, #2, and #5) did not change significantly.
(2)

Under working condition B, TMaxDiff of the battery module was 2.13 °C, and TMaxRise was 4.98 °C. TMaxDiff was basically unchanged, but TMaxRise increased by 43.9%. This was due to the temperature unevenness caused by unidirectional flow.
(3)

Adopting the switching time (τ) (τ = 3600, 1800, and 900 s), the results showed that, when the switching time was shorter, TMaxRise of the battery module was lower. Compared with the case of the unidirectional flowing strategy TMaxRise of the battery module was reduced by 8.7%, and TMaxDiff was reduced by 13.5%.
(4)

By analyzing the thermal characteristics of a single battery cell, it was found that the temperatures of four cells (#3, #4, #2, and #5) tended to be consistent with the decrease of switching time. Changing the switching time of liquid flow could greatly reduce TMin and TMaxDiff of the battery module.
For existing BTMS, whether it is an air cooling or liquid cooling system. The reciprocating cooling strategy proposed in this paper can be applied to the above BTMS, which can reduce the temperature difference of the battery module by replacing the unidirectional water pump with a reversible pump or replacing the fan with a reversible fan. The BTMS structure in which the scheme of placing a flat heat pipe on the side of the battery proposed in this paper needs further research to reduce the volume of the system in practical applications. This BTMS can be applied to other types of batteries, such as cylindrical batteries. Related research has also studied the unidirectional one-way flow strategy.

The authors declare no competing financial interest.

Acknowledgments

The authors appreciate the support of the National Natural Science Foundation of China (51875259), National Key Research and Development Program (2018YFC0810504), and Foundation of State Key Laboratory of Automotive Simulation and Control.

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