Battery thermal management system is a complete set of system used to ensure that the battery works within the appropriate temperature range from the perspective of users, including battery box, fan, heat transfer medium, monitoring equipment and other components. The thermal management system of battery pack has five main functions: accurate measurement and monitoring of battery temperature; Effective heat dissipation and ventilation when the battery pack temperature is too high; Rapid heating under low temperature conditions enables the battery pack to work normally; Effective ventilation when harmful gas is generated; Ensure the uniform distribution of the temperature field of the battery pack.
Systematic design method should be adopted to design the thermal management system of battery pack with good performance. Many researchers have published literature to introduce their own methods of designing thermal management system. Ahmad a. Pesaran of National Renewable Energy Laboratory (NREL) and others introduced the general process of battery thermal management system design in document , which is specific and systematic. The design process includes seven steps. Due to the differences of actual conditions such as budget and time requirements, the actual design process can simplify the process or use this process for multiple rounds of design. The specific steps are: determine the objectives and requirements of the thermal management system; Measure or estimate the heat generation and heat capacity of the module; First round evaluation of thermal management system; Predict the thermal behavior of the module and battery pack; Preliminary design of thermal management system, including selection of heat transfer medium, design of heat dissipation structure, etc; Design and experiment the thermal management system; Optimization of thermal management system.
- Determine the optimal working temperature range of the battery pack
One of the main functions of the battery pack thermal management system is to ensure that the battery pack operates within a safe temperature range under different climatic conditions and different vehicle operating conditions, and try to keep the working temperature of the battery pack within the optimal working temperature range. Therefore, the premise of designing battery thermal management system is to know the optimal working temperature range of battery pack.
A large number of literatures have studied the effect of temperature on battery life. Generally speaking, the service life of lead-acid battery decreases linearly with the increase of temperature. The charging acceptance capacity of lead-acid battery decreases with the decrease of battery temperature. The temperature gradient between battery modules will damage the capacity of the whole battery pack. The working temperature of lead-acid battery should be controlled between 35 ~ 40 ℃. The performance of Ni MH battery is also related to temperature. When the temperature exceeds 50 ℃, the battery charging efficiency and battery life will be greatly attenuated. At low temperature, the discharge capacity of the battery is much smaller than the normal temperature. Figure 4-22 shows the discharge efficiency experiment of an 80A · h Ni MH battery at different temperatures in the author’s laboratory. It can be seen that the discharge efficiency of the battery decreases significantly when the temperature is higher than 40 ℃ and lower than 0 ℃. If only according to this limit, the operating range of the battery should be between 0 ~ 40 ℃. More attention has been paid to the safety and poor low-temperature performance of lithium-ion batteries. Lithium ion batteries have higher volume and power than nickel hydrogen batteries and valve regulated lead-acid batteries, resulting in more heat generation, so the heat dissipation also needs to be more effective.
- Battery thermal field calculation and temperature prediction
Battery is not a good conductor of heat. Only mastering the surface temperature distribution of battery can not fully explain the internal thermal state of battery. Predicting the thermal behavior of battery is an indispensable link for the design of battery thermal management system.
(1) Battery heat generation rate
In engineering application, it is difficult to accurately obtain the expression of heat generation rate per unit volume of battery, so it is the difficulty to solve the battery temperature field. At present, it is mainly obtained by theoretical calculation and experiment.
(2) Acquisition of thermophysical parameters of battery
To apply the battery thermal model, the thermophysical parameters of the battery must be measured, that is, the specific heat capacity CP and thermal conductivity KX, KY and KZ of the battery. In the calculation process, accurate boundary conditions must be set. It is not easy to obtain the above parameters accurately in engineering application. The specific heat capacity of the battery can be measured directly by the calorimeter defined in physics or by theoretical calculation. According to the specific heat capacity of each material in the battery cell, the specific heat capacity of the battery cell can be calculated by mass weighted average method.
The cell is anisotropic, and the average thermal conductivity is generally not equal in all directions. The battery is composed of many parts and electrolyte. It is difficult to measure the thermal conductivity directly by experimental method. The theoretical estimation method and finite element (FEA) method are usually used.
According to the temperature difference between the battery wall and the environment and the surface heat transfer coefficient, the boundary conditions of the thermal model can be determined. The heat transfer coefficient between the wall of the battery module and the environment can be obtained by CFD (Computational Fluid Dynamics) method or experiment.
- Selection of heat transfer medium
The selection of heat transfer medium has a great impact on the performance of the heat management system. The heat transfer medium should be determined before designing the heat management system. According to the classification of heat transfer medium, the heat management system can be divided into air cooling, liquid cooling and phase change material cooling. Air cooling is the simplest way, just let the air flow through the battery surface. Liquid cooling can be divided into direct contact and indirect contact. The typical heat transfer medium can be oil or non direct contact mineral heat transfer medium. Liquid cooling must pass through heat exchange facilities such as water jacket to cool the battery, which reduces the heat exchange efficiency to a certain extent. The heat exchange rate between the battery wall and the fluid medium is related to factors such as fluid flow shape, flow rate, fluid density and fluid heat conductivity.
The main advantages of air cooling are: simple structure and relatively small weight; There is no possibility of liquid leakage; Effective ventilation when harmful gas is generated; Low cost. The disadvantage is that the heat transfer coefficient between it and the battery wall is low, and the cooling and heating speed is slow.
The main advantages of liquid cooling are: high heat transfer coefficient with the battery wall, fast cooling and heating speed; Small size. The main disadvantages are: there is the possibility of liquid leakage; Relatively large weight; Complex repair and maintenance; Water jacket, heat exchanger and other components are required, and the structure is relatively complex.
As an auxiliary power component, the battery pack of parallel hybrid electric vehicle has not very bad operating conditions, and the use of air cooling may meet the requirements. For pure electric vehicles and series hybrid vehicles, the battery pack, as the main power component, generates a lot of heat. In order to obtain better heat management effect, liquid cooling can be considered. Toyota’s hybrid electric vehicle Prius and Honda’s insight adopt air cooling. The fuel cell city bus jointly developed by Tsinghua University and many units also adopts air cooling.
- Heat dissipation structure design of thermal management system
The temperature difference between different battery modules in the battery box will aggravate the inconsistency of battery internal resistance and capacity. If accumulated for a long time, it will cause overcharge and overdischarge of some batteries, which will affect the service life and performance of batteries and cause potential safety hazards. The temperature difference of the battery module in the battery box has a great relationship with the layout of the battery pack. The battery in the middle is easy to accumulate heat, and the heat dissipation condition of the battery at the edge is better. Therefore, in the structural layout and heat dissipation design of the battery pack, we should try to ensure the uniformity of heat dissipation of the battery pack. Taking air cooling heat dissipation as an example, this paper introduces that in the serial ventilation mode, cold air is blown in from the left and out from the right. The air is constantly heated during the flow, so the cooling effect on the right is worse than that on the left. The temperature of the battery pack in the battery box increases from left to right. The first generation Toyota Prius and Honda Insight adopted Serial ventilation.
The parallel ventilation mode makes the air flow more evenly distributed among the battery modules. The parallel ventilation mode requires a good design of inlet and exhaust channels and battery layout positions. Toyota’s new Prius adopts a parallel ventilation structure. Its wedge-shaped inlet and exhaust channels basically keep the pressure difference between the gap between different modules consistent, ensuring the consistency of the air flow blowing through different battery modules, so as to ensure the consistency of the temperature field distribution of the battery pack.
- Selection of fan and temperature measuring point
When designing the battery thermal management system, the type and power of fans, the number of temperature sensors and the location of temperature measurement points are all appropriate. Taking the air-cooled heat dissipation mode as an example, when designing the heat dissipation system, under the condition of ensuring a certain heat dissipation effect, we should try to reduce the flow resistance, reduce the fan noise and power consumption, and improve the efficiency of the whole system. The power consumption of the fan can be estimated by estimating the pressure drop and flow with the methods of experiment, theoretical calculation and computational fluid dynamics (CFD). When the flow resistance is small, the axial flow fan can be considered; When the flow resistance is large, centrifugal fan is more suitable. Of course, the size of the space occupied by the fan and the cost should also be considered. Finding the optimal fan control strategy is also one of the functions of thermal management system.
The temperature distribution of the battery pack in the battery box is generally uneven, so it is necessary to know the thermal field distribution of the battery pack under different conditions to determine the dangerous temperature point. There are many temperature measurement sensors, which has the advantage of comprehensive temperature measurement, but it will increase the system cost. Considering that the temperature sensor may fail, the number of temperature sensors in the whole system should not be too small. Using the methods of finite element analysis and infrared thermal imaging, the thermal field distribution of battery pack, battery module and battery cell can be analyzed and measured, and the appropriate temperature measurement points in different areas can be found. The design of the temperature sensor should be improved to ensure the temperature measurement accuracy and instability of the cooling air. When designing the battery, we should consider reserving the space for the temperature sensor, for example, we can design appropriate holes at appropriate positions. The battery pack of Toyota’s hybrid electric vehicle new Prius has 228 battery cells, and the temperature monitoring is completed by five temperature sensors.
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