What Is Thermal Management System?

What Is Thermal Management System?

Battery thermal management, based on the impact of temperature on battery performance, combined with the battery's electrochemical characteristics and heat generation mechanisms, and grounded in the optimal charge/discharge temperature range of a specific battery, is a technology that addresses heat dissipation or thermal runaway caused by excessively high or low temperatures during battery operation. This is achieved through rational design and is grounded in materials science, electrochemistry, heat transfer, molecular dynamics, and other disciplines. Maintaining a reasonable operating temperature range is essential for the battery pack to maintain good performance. Therefore, designing a reasonable thermal management scheme for lithium-ion battery packs is of great significance for improving the overall performance of the battery system.

The battery pack thermal management system has the following five main functions: ① accurate measurement and monitoring of battery temperature; ② effective heat dissipation and ventilation when the battery pack temperature is too high; ③ rapid heating under low-temperature conditions; ④ effective ventilation when harmful gases are generated; and ⑤ ensuring a uniform temperature distribution within the battery pack.

A high-performance battery pack thermal management system requires a systematic design approach. Currently, many design methodologies for thermal management systems exist. The most commonly used is a battery pack thermal management system designed by the National Renewable Energy Laboratory (NREL) in the United States, whose design process includes seven steps:

1) Determine the self-caliber and requirements of the thermal management system. Based on the battery's temperature characteristics and suitable operating temperature range, determine the control self-caliber of the thermal management system. For example, the suitable operating temperature for lithium-ion power batteries is 10~40℃, with a low-temperature limit of 0℃ and a high-temperature limit of 45℃. Therefore, the design of the thermal management system should, while meeting the battery's extreme operating temperatures, strive to meet the battery's suitable operating temperature requirements.

2) Measure or estimate module heat generation and heat capacity. Through battery charge-discharge tests and simulation calculations based on the battery's specific heat capacity, determine the heat dissipation or heating power.

3) Initial evaluation of the thermal management system, including selecting the heat transfer medium and designing the heat dissipation structure. Generally, battery cooling is achieved through air cooling or liquid cooling. Air cooling systems are relatively simple in structure but inefficient; liquid cooling systems are complex in structure but highly efficient. There are also different forms of heating methods, such as circulating hot air heating, liquid flow heating, and direct thermal radiation heating from the heat source.

4) Predict the thermal behavior of the module and battery pack. Based on the battery pack's operating conditions, predict and assess the heat dissipation and heating requirements during application.

5) Preliminary design of the thermal management system. Based on the determined heat medium and the thermal behavior assessment results, conduct the principle and engineering design of the thermal management system.

6) Design and test the thermal management system. Produce scaled-down or full-scale battery systems and the battery thermal management system, and verify the effectiveness of the thermal management system under simulated actual operating conditions on a test bench.

7) Optimize the thermal management system. Improve and optimize the thermal management system based on the experimental results.

Batteries are not good conductors of heat. Knowing only the surface temperature distribution is insufficient to fully understand the internal thermal state of the battery. Calculating the internal temperature field using mathematical models and predicting the battery's thermal behavior is an indispensable step in designing battery thermal management systems. Currently, the mainstream mathematical models include two-dimensional and three-dimensional models. Among these, the three-dimensional model, due to its excellent accuracy and adaptability, has been widely used in numerous battery thermal management systems. The model is as follows:

Battery Thermal Field Calculation and Temperature Prediction

Where T is the temperature;

ρ is the average density;

c_p is the specific heat capacity of the battery;

λ_x, λ_y, λ_z are the thermal conductivity of the battery in the x, y, and z directions, respectively;

q is the rate of heat generation per unit volume.

Temperature differences between different battery modules within the battery box exacerbate inconsistencies in battery internal resistance and capacity. Over time, this can lead to overcharging or over-discharging of some batteries, affecting their lifespan and performance, and creating safety hazards. The temperature differences between battery modules within the battery box are closely related to the battery pack arrangement. Generally, batteries in the middle tend to accumulate heat, while those at the edges have better heat dissipation. Therefore, when designing the battery pack structure and heat dissipation, it is crucial to ensure uniform heat dissipation. Taking air cooling as an example, there are generally two ventilation methods: series and parallel, to ensure uniform heat dissipation. Airflow design must adhere to the basic principles of fluid mechanics and aerodynamics.

When designing a battery thermal management system, the type and power of the fan, the number of temperature sensors, and the location of the measurement points must be carefully chosen.

Taking air cooling as an example, when designing the cooling system, while ensuring a certain cooling effect, flow resistance should be minimized to reduce fan noise and power consumption, thereby improving the overall system efficiency. The power consumption of the fan can be estimated by estimating pressure drop and flow rate using experimental, theoretical calculation, and fluid dynamics (CFD) methods. When the flow resistance is low, axial flow fans can be considered; when the flow resistance is high, centrifugal fans are more suitable. Of course, the space occupied by the fan and its cost must also be considered. Finding the optimal fan control strategy is also one of the functions of a thermal management system.

Schematic diagram of temperature measurement points in the battery box

The temperature distribution of the battery pack within the battery box is generally uneven, therefore, it is necessary to know the thermal field distribution of the battery pack under different conditions to determine the critical temperature points. More temperature sensors provide more comprehensive temperature measurement, but increase system cost and complexity. Depending on the specific engineering context, theoretically, finite element analysis, infrared thermal imaging in experiments, or real-time multi-point temperature monitoring can be used to analyze and measure the thermal field distribution of the battery pack, battery modules, and individual cells, determining the number of temperature measurement points and finding suitable points in different areas. A general design should ensure that the temperature sensors are not exposed to cooling airflow to improve the accuracy and stability of temperature measurements. When designing the battery, space should be reserved for temperature sensors; for example, suitable openings can be designed in appropriate locations. The battery pack of Toyota's Prius hybrid electric vehicle has 228 individual cells, and temperature monitoring is performed by 5 temperature sensors. The electric bus power battery system designed by Beijing Institute of Technology uses 6 temperature measurement points per box (see the circled area in Figure 8-16a), arranged at the positive and negative terminals and the power line output points of the battery box, as shown in Figure 8-16.

Based on the heat transfer medium, the cooling of battery pack thermal management systems can be divided into three types: air cooling, liquid cooling, and phase change material cooling. Considering material research and development and manufacturing costs, the most effective and commonly used heat dissipation system currently uses air as the heat dissipation medium.

Based on the heat dissipation airflow structure, air cooling systems can be further divided into two types: series ventilation and parallel ventilation, as shown in Figures 8-17 and 8-18, respectively.

In a series configuration, air typically flows from one side of the battery pack to the other to remove heat. However, this airflow carries heat from areas it passes through earlier to areas it passes through later, resulting in inconsistent temperatures and significant temperature differences. In a parallel configuration, the airflow between modules rises vertically, distributing air more evenly and ensuring consistent heat dissipation throughout the battery pack.

Thermal management systems can be categorized into passive and active systems based on whether they have internal heating or cooling devices. Passive systems are less expensive and require simpler infrastructure; active systems are more complex and require greater additional power, but offer better performance.

Figures 8-19, 8-20, and 8-21 show schematic diagrams of active and passive air heating and heat dissipation structures, respectively.

Thermal Management System Design and Implementation

In Figures 8-19 and 8-20, although the air has been cooled and heated by the car's air conditioning or heating system, it is still considered a passive system. With this passive system, due to the inconsistency in the temperature of the introduced ambient air, the ambient air must operate within a certain temperature range (10~35°C) for proper thermal management. Operating under extremely cold or hot conditions may result in greater unevenness in the battery pack.

In heating systems, besides introducing hot air into the battery pack, other methods can be used, as shown in Figures 8-22~8-25 (for prismatic batteries).

Other heating methods