Design and optimization of liquid-cooled plate structure for power battery of the pure electric excavator

With the increasing pressure of energy transformation and environmental protection, the trend of electrification of construction machinery is becoming more and more obvious. In this paper, based on a small pure electric excavator which is still in the stages of research and development, a liquid-cooled heat dissipation structure (liquid-cooled plate) is designed according to the power battery pack scheme. The overall shape of the liquid-cooled plate is designed as a symmetrical serpentine flow channel. Geometrically, the symmetrical serpentine flow channel is a combination of a straight flow channel and a runway-shaped flow channel. Then the conjugate heat transfer simulation model was established by using the CFD software Ansys Fluent. The results show that the initial scheme can not guarantee the temperature homogeneity of the battery pack. Therefore, the liquid-cooled heat dissipation scheme was optimized and compared. Firstly, a liquid-cooled plate was added on the original basis, and multiple sets of simulations were carried out with the length of the linear flow channel as a variable. The results show that appropriately increasing the length of the straight flow channel at the entrance of the liquid-cooled plate has a marked effect on improving the heat dissipation performance.


Introduction
Driven by environmental protection pressure and energy transformation, the construction machinery industry is developing in the direction of electrification.With the fast development of the new source of energy automotive industry, the three-electric technology (motor, battery, electronic control) technology has gradually matured, which provides support for the electrification of construction machinery.
In this paper, a set of liquid cooling structures is designed for a small pure electric excavator of a company.The power battery will produce a lot of heat during the discharge process, and its optimal operating temperature is generally 25 ~ 40°C, and the Difference in temperature is maintained within 5°C [1] .Although the air-cooled heat dissipation structure is simple and the cost is low, the cooling effect is relatively poor and there may be noise problems.Therefore, the liquid-cooled rejection of heat method is adopted.
The research on the liquid-cooled rejection of heat structure is generally focused on the design of the coolant flow channel.Scholars adopted a two-way linear flow channel as an improved heat dissipation scheme [1] .Chung and Kim [2] used a linear flow channel to perform thermal analysis and hierarchical design of the battery thermal management system; Sheng et al. [3] designed a serpentine flow channel with double inlets and outlets to availably control the temperature rise of the battery; Wei et al. [4] proposed a symmetrical serpentine channel.Compared with the traditional serpentine channel, the drop in pressure is greatly reduced and the temperature uniformity is better.Kong et al. [5] evaluated a divergent linear flow channel with two inlets and one outlet and found that it can reduce the drop in pressure and the temperature difference of the battery.Monika and Datta [6] compared the heat dissipation performance of six microchannel liquid cooling plates, including linear, serpentine, pumpkin-shaped, spiral,Ushaped and hexagonal grids.Their work shows that the serpentine and hexagonal grid flow channel design is the best choice for a liquid cooling plate.Asymmetrical serpentine flow channel will also be used in this paper.

Power battery scheme
Figure 1.Physical drawing of a small excavator.Figure 1 is a small fuel excavator, and the electric excavator is still in the stages of research and development.The structural engineer reserved 1200 × 600 × 500 mm 3 space for the pack of power batteries.The battery capacity of the whole machine is 70 KW • h, the indicative running time is 4-6 h, and the permanent magnet synchronous motor with a peak power of 60 KW is selected.In this paper, the Guoxuan high-tech IFP27175200A-105 Ah square lithium iron phosphate battery is selected.Its capacity is 3.2 × 105 × 10-3 = 0.336 KW•h, and the size is 21 × 175 × 200 mm 3 .The physical picture is shown in Figure 2.

Boundary conditions and Governing equations
The governing equations of the coolant are: The energy balance equations of the liquid-cooled plate and cells are [8] : In the above formula, the subscripts 'f', 'c', and 'b' represent coolant, liquid cooling plate, and battery respectively; represent dynamic viscosity, density, Acceleration of gravity, temperature, specific heat capacity, and thermal conductivity, respectively.p 、 v represent velocity vector and pressure respectively; q is the heat source of the cell.According to the theoretical hypothesis of Bernardi [9] , when the cell is regarded as a uniform heat source, the heat generated is chiefly made up of reaction heat and Joule heat.The computing formula is : In the above formula, q is the Heat-generating power of the cell, W / m 3 ; i is charge-discharge current, A; vb is the cell volume, m 3 ; e is the electromotive force of the cell, V; uL is the cell terminal voltage, V; is the reversible reaction heat of the cell, J.The whole machine is designed to work continuously for 4 hours, and the average discharge rate of the power battery is 0.25 C. Substituted into the numerical value, the volume heat source of the battery is 4555 W/m 3 .The material properties in the model are displayed in Table 1.1073.55 0.38 The default initial temperature of all materials is 293.15K (20°C), and the environmental temperature is 300 K (26.95°C).The inlet boundary condition is mass inlet, 10 g/s, and the temperature is 293.15K; the open boundary condition is the pressure outlet, and the uge pressure is 0; the outer surface of the cell and the liquid-cooled plate is set to natural convection with air, and the convection coefficient is 5 W/(K•m 2 ).

2.3Mesh model
The mesh model is established in Fluent meshing.In order to ensure the accuracy of the calculation results, the mesh independence is verified by monitoring the maximum temperature of the battery pack and the fluid pressure drop in the two-layer liquid-cooled plate [7] .According to the boundary conditions mentioned above, following the same setting process, five different numbers of grid models of 5593144, 7154830, 8608920, 10378808, and 12578996 were divided to compare the simulation results.When the calculation error caused by the change in the number of grids is less than 3% [10] , the grid accuracy can be considered to be up to standard.As shown in Figure 5, when the number of grids increases from 10.37 million to 12.57 million, Tmax decreases from 309.09 K to 308.97 K; P1 decreases from 189.83 Pa to 189.64 Pa.P2 decreases from 189.74 Pa to 189.69 Pa.The change range is less than 3 %, so the number of grids of 10.37 million has met the calculation requirements.

Calculation results of the original scheme
Figure 6 shows the pressure distribution in the upper coolant.Because the boundary conditions are the same, the results of the lower liquid-cooled plate are almost the same as those of the upper liquid-cooled plate.Along the flow direction the pressure gradually decreases, the total import and export pressure drop is 189.83Pa., the exit pressure is 0, and the negative pressure appears near the export.Figure 7 is the temperature distribution cloud diagram of the power battery.In general, the left-handed temperature (entrance) is low, and the right-handed temperature (exit) is high.It is well understood that the coolant temperature at the access point is low and the heat dissipation effect is the best.Along the flow direction, the coolant exchanges heat with the liquid-cooled plate, and its temperature is gradually increasing, and the heat dissipation effect becomes worse.The minimum temperature is 295.88K,the maximum temperature of the cell reaches 309.09K, and the temperature variation is 13.21 K, which is greater than 5 K and does not meet the requirements [1] .

Program improvement
As shown in Figure 7, the temperature of the superstratum cell is obviously higher than that of the substratum cell.This is because the lower cell contacts with two layers of liquid cooling plates and indirectly heats up with two layers of coolant.The upper cell only has the lower surface in contact with the liquid-cooled plate, and the upper surface and the side are natural convection heat dissipation.Therefore, the first improvement plan first plans to add a layer of liquid-cooled plate (the coolant flow rate is constant), although this may increase energy consumption.The calculation results are shown in Figs. 8  and 9.The cooling hydraulic drop is almost the same as the initial scheme.The maximum temperature of the cell is 302.74K, which is 6.35 K lower than the initial scheme.In addition, the temperature of the center point of each cell was monitored, as shown in Figure 10.The flow channel of the initial scheme and the improved scheme 1 is symmetrical in the flow direction, and the length of the straight flow channel at the entrance is R.As shown in Figure 9, the temperature of the cell at the inlet is significantly lower than that at the outlet.From improvement scheme 2 to improvement scheme 4, we try to continuously extend the linear flow channel at the entrance and actively delay the distribution of the runway-shaped flow channel.This is because the coolant temperature at the entrance is low and the heat dissipation performance is strong.In the linear flow channel can achieve very good heat dissipation effect, and delay the layout of the runway-shaped flow channel to improve the homogeneity of temperature distribution.

Calculation results of improved schemes
Figs. 14 and 15 are the calculation results of the improved scheme 4. Compared with the initial scheme, the pressure drop increases slightly, and the temperature distribution uniformity is improved.Figs.16 and 17 show the data statistics of cell temperature monitoring under four improved schemes.From scheme 1 to scheme 4, as the length of the linear flow channel at the inlet of the coolant increases, the maximum cell temperature decreases slightly, and the minimum cell temperature continues to rise, so the temperature variation between the cells continues to decrease.The cell temperature variation of the improved scheme 4 is 3.55 K, which achieves the expected goal [1] .In addition, the temperature variance of the cell, which characterizes the homogeneity of temperature distribution, is also significantly reduced, which shows the rationality of the improved scheme.

Conclusions
In this paper, a liquid-cooled heat dissipation scheme based on a symmetrical serpentine flow channel is designed for the power battery pack of a small pure electric excavator.After grid independence verification, multiple sets of CFD simulations are carried out, and the following conclusions are obtained : (1) The arrangement of three layers of the liquid-cooled plate (i.e., the superstratum and substratum surfaces of all cells can be in direct contact with the liquid-cooled plate) can significantly improve the temperature distribution uniformity of the power battery pack and improve the heat dissipation effect.
(2) The symmetrical serpentine flow channel can be regarded as a combination of a linear flow channel and a runway-shaped flow channel.Under the premise of ensuring that the energy consumption (fluid pressure drop) is unchanged, appropriately increasing the length of the straight-line flow channel at the coolant inlet and actively delaying the distribution of the runway-shaped flow channel can significantly decrease the cell temperature variance and ameliorates the temperature distribution homogeneity of the power battery pack.

Figure 2 .
Figure 2. Guoxuan High-tech 105 ah square lithium iron phosphate battery physical map.The connection mode of the planned cell is 210S1P ( the total capacity is 70.56 KW•h ), and the spatial arrangement is 5 × 3 × 7 × 2. The schematic diagram is shown in Figure 3.The size of the liquidcooled plate in the figure is 900 × 560 × 15 mm 3 .Every five cells form a module, which is separated by a layer of silicone heat conduction pad ( size of 200 × 175 × 1 mm 3 ).The overall size of the power battery pack is 900 × 560 × 430 mm 3 , and the design requirements have been met.

Figure 3 .
Figure 3.The Spatial arrangement of cells.

Figure 4 .
Figure 4. Size parameters of symmetrical serpentine flow channel.

Figure 5 .
Figure 5. Grid independence verification.When the calculation error caused by the change in the number of grids is less than 3%[10] , the grid accuracy can be considered to be up to standard.As shown in Figure5, when the number of grids increases from 10.37 million to 12.57 million, Tmax decreases from 309.09 K to 308.97 K; P1 decreases from 189.83 Pa to 189.64 Pa.P2 decreases from 189.74 Pa to 189.69 Pa.The change range is less than 3 %, so the number of grids of 10.37 million has met the calculation requirements.

Figure 6 .
Figure 6.Upper coolant pressure distribution of the initial scheme.

Figure 7 .
Figure 7. Temperature distribution of power battery in the initial scheme.

Figure 9 .
Figure 9. Improvement scheme 1 Temperature distribution of power battery.

Figure 10 .
Figure 10.Frequency histogram of cell temperature distribution.Other improvement schemes will have three layers of liquid cooling plates and change the length of the linear channel in the symmetrical serpentine channel.The specific parameters are shown in Figs.11-13.

Figure 11
Figure 11 Improvement scheme 2 channel design.

Figure 12
Figure 12 Improvement scheme 3 channel design.

Figure 13 .
Figure 13.Improvement scheme 4 channel design.The flow channel of the initial scheme and the improved scheme 1 is symmetrical in the flow direction, and the length of the straight flow channel at the entrance is R.As shown in Figure9, the temperature of the cell at the inlet is significantly lower than that at the outlet.From improvement scheme 2 to improvement scheme 4, we try to continuously extend the linear flow channel at the entrance and actively delay the distribution of the runway-shaped flow channel.This is because the coolant temperature at the entrance is low and the heat dissipation performance is strong.In the linear flow channel can achieve very good heat dissipation effect, and delay the layout of the runway-shaped flow channel to improve the homogeneity of temperature distribution.

8 Figure 16 .
Figure 16.The temperature difference of Battery pack.

Figure 17 .
Figure 17.Judgment index of cell temperature uniformity.

Table 1 .
Physical properties of materials.