A battery air-cooled heat dissipation

Lithium-ion batteries have become one of the main battery types for energy storage battery compartments due to their excellent characteristics. However, lithium-ion batteries continuously generate heat during charging and discharging. At the same time, due to the limitation of the battery compartment, the heat generated by the lithium-ion battery will continue to accumulate in the battery compartment and cannot be diffused to the external environment in time. As a result, the internal temperature of the battery compartment continues to rise, causing the risk of thermal runaway and bringing safety hazards to the battery compartment. The safety and economy of energy storage battery compartments have always been the main issues affecting its development. The key to the sustainable development of energy storage battery compartments lies in the solution of the heat dissipation problem of the battery compartment.

The traditional battery compartment heat dissipation system is simple and the temperature distribution in the battery compartment is uneven. Long-term operation will lead to poor consistency among battery modules, which will seriously affect the service life of battery modules.

We use SolidWorks and Ansys to solve the problem of poor air cooling and heat dissipation in the battery compartment.

By numerically simulating the temperature field of the battery compartment, the heating characteristics of the battery module are studied. Based on relevant theories and formulas, an optimal design scheme for the air-cooled heat dissipation system of the battery compartment is proposed. The selected battery heat generation calculation model is as follows.

ρ - the average density of the battery module; Cp - the specific heat capacity of the battery module; T - the Kelvin temperature of the battery; t - time; kx, ky, kz - the heat conduction of the battery along the x-axis, y-axis, and z-axis rate; q - heat generation rate per unit volume of the battery module

For the numerical calculation of the unit volume heat generation rate q of the battery module, the Bernardi model is mainly used, and its calculation formula is as follows.

Vb - the volume of the battery module; I - the rated current during the charging and discharging process of the battery; U , U0 - the rated voltage and the open circuit voltage of the battery module; dU0 /dT is a constant under a certain charge and discharge rate, indicating the temperature coefficient

There are mainly two methods for calculating the specific heat capacity Cp of the battery module. The experimental method is to use a calorimeter to directly measure the battery module. The theoretical method is to obtain the specific heat capacity of the battery module through numerical calculation. The calculation formula is as follows.

m - the mass of the battery module; mi - the mass of the material i contained in the battery module; Ci - the specific heat capacity of the material i contained in the battery module; n - the different types of materials in the battery module

According to the measurement, the specific heat capacity of the lithium iron phosphate single battery used is 1 329 J/(kg·K). Through experimental measurement and formula calculation, the heat generation rate of the lithium iron phosphate battery module used in this paper is 13 757.2 W/m3 when charging at 1 C.

There are three main modes of heat transfer between objects, heat conduction, heat convection and heat radiation. During the heat transfer process of the battery, heat conduction mainly occurs inside the battery and heat convection mainly occurs at the contact surface between the battery module and the air.

Heat conduction inside the battery is conducted through internal materials, including current collectors, electrode materials, separators and electrolytes. The heat conduction inside the battery is mainly through solid matter, so the heat conduction in the electrolyte can be ignored and the energy equation of the battery heat conduction can be expressed as follows.

∇2 T - Laplace operator of battery temperature transfer to space; λ - battery thermal conductivity; Q - heat generation rate of internal heat conduction of battery

The heat convection of the battery is mainly realized through the contact between the surface of the battery module and the air. The heat convection between the battery surface and the outside world can be expressed as follows using Newton's law of cooling.

h - convective heat transfer coefficient of different materials under natural conditions; Tab - ambient temperature

The heat exchange in the air-cooled heat dissipation of the battery compartment mainly occurs at the contact surface between the battery module and the air. To study the air-cooled heat dissipation of the battery compartment, it is necessary to establish a fluid-solid coupling model for the battery compartment to analyze the heat transfer relationship between the surface of the battery module and the air.

u - fluid field velocity; k - thermal conductivity of the battery

According to the fluid-solid coupling heat transfer expression, it is more convenient to choose the k-epsilon model for simulation calculation.

The model is established according to the proportion of the actual energy storage compartment and the basic model of the battery compartment is established by using SolidWorks, with a length of 12 m, a width of 2.4 m and a height of 2.8 m. A total of 12 battery clusters are placed in the battery compartment, with 6 groups placed on each side and each battery cluster consists of 15 battery modules. The actual appearance of the lithium-ion battery compartment and the internal structure of the model are shown in Figure 1.

In order to optimize the heat dissipation effect of the improved battery compartment, a deflector is added on the top of the battery compartment and the width of the deflector is 500 mm. There are two different optimization methods of adding a single deflector and adding a double deflector to the battery compartment during design. The layout of two different battery compartment models with deflectors added is shown in Figure 2.

The internal structure of the battery module is complex, and its material distribution in different directions and the thermal conductivity are different. Table 1 shows the thermal characteristic parameters of the lithium iron phosphate battery module casing and the x-axis, y-axis, and z-axis directions.

Through the calculation of the surface area of the battery module, the heat dissipation area of the battery can be obtained as 178.85 m2. Setting the solver inside the battery compartment as a fluid-solid coupling field can directly use the convective heat transfer conditions.

The setting of the boundary conditions of the simulation model mainly includes temperature, velocity and pressure. The inlet boundary condition is velocity inlet, the wind speed is 4 m/s, and the inlet air temperature is set to be consistent with the ambient temperature, which is 25 °C. Using the Fluent module to simulate and calculate, the heat dissipation surface temperature cloud diagram of the basic battery compartment model can be obtained, as shown in Figure 3.

In Figure 3, the high temperature area in the battery compartment is concentrated in the center of the battery compartment and the temperature distribution is very uneven. The average temperature in the battery compartment is 46.3 °C and the difference between high and low temperature in the battery cluster area is 26.5 °C. The temperature in the central area of the battery compartment is the highest, as high as 57.15 °C. The lowest temperature is 27.5 ℃ near the top and entrance of the battery compartment. The temperature change curve of the average temperature of the battery cluster area along the air inlet direction is shown in Figure 4.

The highest average temperature of the battery cluster area appears in the middle, which is 56.2 °C. The lowest value appears at the edge of the battery compartment, which is 39.91 °C. The flow field at the edge of the battery compartment is faster for better heat exchange with the battery module. The flow field effect in the central area of the battery compartment is weak and cannot form a good heat exchange with the battery module. This view can also be verified by combining the streamline diagram of the flow field, as shown in Figure 5.

In Figure 5, the air velocity in the upper part of the battery compartment is large, which is 3.2 m/s. Most of the air flows out directly from the outlet and only a small part of the air drives the air in the lower half of the battery compartment to form a circulation. The air velocity in the middle part of the battery compartment is 0.8 m/s. A lot of air does not conduct sufficient heat exchange with the battery module, resulting in waste of resources and the battery compartment cannot obtain a good heat dissipation effect.

In order to obtain better heat dissipation of the battery compartment, the heat exchange intensity between the battery module and the air can be enhanced. By rationally arranging the deflectors, the flow field in the battery compartment is changed to make it more evenly distributed so that the air and the battery module can be more fully contacted.

Usually, the air inlet and outlet of the battery compartment are placed near the top of the upper part. At this time, the airflow will only circulate above the battery compartment, which makes the cooling effect of the modules located at the lower part of the battery compartment unsatisfactory. The temperature of the cooling surface of the battery compartment model after adding a single deflector is shown in Figure 6.

In Figure 6, when the deflector is added near the air inlet of the battery compartment, the temperature distribution in the battery compartment changes greatly. The high temperature area is smaller and more evenly distributed. Both the maximum temperature and the average temperature in the battery compartment are dropping. The average temperature in the battery compartment is 43.4 °C, and the temperature difference between the battery cluster areas is 24.1 °C. The highest temperature only appeared in the rear half of the battery compartment, which was 52.65 °C. The lowest temperature occurs near the inlet and deflector, which is 26.85 ℃. The temperature change curve of the average temperature of the battery cluster area along the air inlet direction is shown in Figure 7.

In Figure 7, the overall temperature curve presents a hump shape. The highest temperature in the first peak area is 43.41 ℃. This part is the area in front of the deflector and the air has not been changed by the deflector, so the average temperature is slightly higher than other areas. The highest temperature in the second peak area is 50. 13°C. This part is the second half of the battery compartment. The lowest temperature in the battery cluster area occurs in the trough, which is 29.04 °C. This part is the area where the air is directly cooled after the deflector changes the wind direction. And the battery module can perform better heat exchange with the air with a lower temperature.

When the air enters the battery compartment from the inlet, the flow channel is blocked by the deflector. The flow direction of the air is artificially changed so that it has to flow to the lower half of the battery compartment for more sufficient heat exchange with the battery module. The line diagram can also verify this point of view. The flow field velocity is shown in Figure 8 .

In Figure 8, it is blocked by the deflector and the flow direction changes downward. At this time, the flow velocity of the air flowing through the battery module is faster at 2.8 m/s, which can exchange heat more fully with the battery module. However, the second half of the battery compartment will still form a vortex, and the temperature in some areas will be too high.

In order to solve the situation that the temperature in some areas in the second half of the battery compartment is still too high, an additional single deflector is added near the air outlet of the battery compartment to achieve a uniform flow field. The temperature cloud diagram of the cooling surface of the battery compartment model after adding double deflectors is shown in Figure 9.

In Figure 9, when a deflector is added near the air outlet of the battery compartment, the maximum temperature and the average temperature of the battery compartment change significantly. The average temperature in the battery compartment is 40.8 °C, and the temperature difference between the battery cluster areas is 21.7 °C. The highest temperature still appeared in the rear half of the battery compartment, but the temperature dropped significantly to 47.55 °C. The lowest temperature occurs near the inlet and deflector, which is 26.85 ℃. The temperature change curve of the average temperature of the battery cluster area along the air inlet direction is shown in Figure 10.

In Figure 10, the overall temperature curve is similar to the case of a single deflector, showing two peak areas and one trough area. The lowest temperature in the battery cluster area occurs in the trough, which is 29.05 ℃. The highest temperature in the first peak area is 40.65 ℃, which is lower than that in the case of only a single deflector. The highest temperature in the second peak area is 46.05 ℃ and the temperature has dropped significantly. This shows that increasing the deflector near the air outlet helps to reduce the temperature in the battery compartment and the temperature difference.

Since the addition of double deflectors will make the flow field in the battery compartment more complicated, the heat exchange between the air and the battery module will be enhanced and the average temperature in the battery compartment will be further reduced compared with the case of a single board. The velocity of the flow field is shown in Figure 11.

In Figure 11, when the air circulates behind the battery compartment, it is blocked by the deflector. The flow field in the battery compartment is more complicated and all battery modules can fully contact with the air to obtain better heat dissipation. At the same time, the temperature distribution in the battery compartment is more uniform and the consistency of the battery is better, which can reduce the probability of fire and explosion accidents in the battery compartment. According to the analysis and optimization of its air-cooled heat dissipation based on SolidWorks and Ansys , the following conclusions are drawn.

(1) Adding an air-cooling system to the battery compartment can cool the battery module in the battery compartment, but limited by the structure of the battery compartment. The air can only form a simple circulation in the battery compartment, and the battery module cannot be cooled. The battery module in the central area of the battery compartment will generate a higher temperature rise, and the highest temperature in the central area is 57.15 °C. After running for a long time, the service life of the battery module in the central area will be shortened and the consistency between the battery modules will be worse, which will affect the normal operation of the battery compartment.

(2) After adding a single deflector in the battery compartment, the flow field of the air in the battery compartment becomes complicated. The air forms 2 main circulations in the battery compartment, gaining more contact with the battery modules for more sufficient heat exchange. The average temperature in the battery compartment decreased by 2.9 °C, the maximum temperature decreased by 4.5 °C, and the area where the maximum temperature occurred was smaller. The high and low temperature difference in the battery cluster area is reduced by 2.4 °C and the air-cooled heat dissipation effect of the battery compartment is improved.

(3) After adding double deflectors in the battery compartment, the flow field of the air in the battery compartment becomes more complicated. The air forms multiple cycles in the battery compartment and the heat exchange with the battery module is more sufficient. The average temperature in the battery compartment decreased by 5.5°C, the maximum temperature decreased by 8.6°C and the area where the maximum temperature occurred was smaller. The high and low temperature difference in the battery cluster area is reduced by 4.8 °C and the air-cooled heat dissipation effect of the battery compartment is improved.

Reasonable installation of deflectors in the battery compartment can effectively change the flow field in the battery compartment. The air can conduct more sufficient heat exchange with the battery module, thereby changing the temperature distribution of the battery module. The average temperature in the battery compartment is reduced and the consistency is better, which improves the service life of the battery module to a certain extent.

The optimized air-cooled heat dissipation system of the battery compartment can suppress the thermal runaway phenomenon of the lithium-ion battery module. Improve the safety of the energy storage battery compartment operation, prolong the service life of the battery, and improve the economy of the air-cooled heat dissipation of the battery compartment.