1. Overview of battery thermal management based on phase change materials
Phase change material (PCM) is a substance whose temperature remains constant or changes within a small range during the phase change process, but can absorb or release a large amount of latent heat. At present, PCM has been widely used in many fields, such as active or passive cooling system used in space field, electronic device and energy storage device as thermal protection system. The preparation and characterization of PCM, heat storage and heat transfer characteristics, and mathematical models and simulations have also received sufficient attention. PCM is used in electric vehicle battery thermal management system, which was first proposed by Al-Hallaj and others and patented by Al-Hallaj and Selman. When using PCM, the battery cells or modules can be directly immersed in the PCM, and the battery is thermally managed by the heat absorbed and released when the PCM melts or solidifies.
The increase in the size and discharge current of Li-ion battery packs will lead to more prominent thermal problems, especially heat dissipation. When the cost and system complexity of air-cooled battery cooling and liquid-cooled battery cooling become more prominent, the thermal management advantages of PCM in batteries become more obvious. For the Li-ion battery pack of electric scooters, Khateeb et al. established a two-dimensional unsteady model containing 18 18650 batteries, using PCM (melting point 40~44℃) for thermal management, in which the mass of the PCM is 28.6% of the battery, the module design is simple and the thermal management effect is ideal. Then, they compared the heat dissipation effect of natural convection cooling, foam aluminum, PCM, and PCM/foam aluminum on the basis of the module through experiments. The heat dissipation effect of PCM/foam aluminum is the most obvious, which is 17.5℃ lower than the maximum temperature under natural convection cooling. Numerical simulation analysis of a Li-ion battery pack consisting of 6 18650 cells by Mlls and A-Hallaj shows that the maximum temperature of the battery can be controlled within 55℃ as required. Sabbah et al. compared the thermal management effects of active air cooling and PCM cooling on high-power Li-ion batteries (18650, 1.5A). When the working temperature of the battery or the ambient temperature is high (40~45℃), the air cooling has failed, and the P℃M cooling can keep the battery temperature below 55℃ (6.67C discharge). For the discharge of high-energy Li-ion batteries (18650, 2.4A) at high current and high ambient temperature, Kizilel et al. conducted an experimental analysis. For different battery combinations in the module, PCM can enhance the thermal stability and electrochemical performance of the battery. Rao and Zhang designed a module consisting of 6 SC-type Ni-MH cells. In the experiment, the steel casing of the battery was directly contacted with the PCM, and the results showed that the temperature of the battery dropped by 14~18℃, and the heat dissipation effect was better than that of air cooling. Duan and Naterer used electric heating to simulate battery discharge characteristics, and conducted experimental analysis of battery PCM thermal management under constant heat generation rate, variable heat generation rate, variable ambient temperature and periodic ambient temperature, which further verifies the feasibility and efficiency of thermal management of PCM batteries.
Compared with the battery thermal management system using air and liquid as the medium, the battery thermal management system using PCM as the medium is simple, does not require additional moving parts, has low consumption or even zero consumption of battery energy, and is suitable for various practical working conditions such as high ambient temperature and non-steady battery discharge.
2. The basic principle of PCM battery thermal management system
As shown in Figure 1, when the PCM is used in the battery thermal management system, The battery pack is directly immersed in the PCM, or a jacket type structure can be used, and a layer of annular PCM is set outside the single cell to form a slightly larger single cell, and then a battery pack is formed. When the battery is discharged, the system stores the heat in the PCM in the form of latent heat of phase change, thereby absorbing the heat released by the battery and rapidly reducing the battery temperature. Batteries are usually arranged in a symmetrical form, with modules A, B, C, and D as the boundaries of the mathematical model, whose conditions are related to various heat transfer conditions such as module packaging materials and the location of the modules in the entire battery stack. The middle plane of symmetry of the battery is used as the adiabatic plane for heat transfer calculations.
Figure 1 - Structure of battery thermal management system using PCM
For a well-designed PCM battery thermal management system, the temperature of the PCM remains constant, which is comparable to the solid-liquid and solid-solid phase change temperature (PCT) of the PCM. When the battery is being charged, especially in cooler weather conditions (ie, the atmospheric temperature is much lower than the PCT), the PCM discharges heat into the environment. However, the total net heat flow of the whole process is towards the PCM, which normally has a large enough heat capacity to absorb the heat from the battery discharge process with only a small increase in its own temperature compared to the PCT. The excess heat in the PCM is derived from the net thermal effect of the discharge-charge process and will eventually be released to the environment as well. This means that the PCM must choose a material whose phase transition temperature is higher than the ambient temperature.
3. PCM performance requirements
The thermophysical properties of the PCM are the key factors that determine the thermal management effect of the battery thermal management system. Therefore, in the PCM-based battery thermal management system, the PCM should meet the following requirements:
(1) It has a suitable phase transition temperature (generally higher than the ambient temperature and lower than the highest target temperature of battery thermal management);
(2) The latent heat of phase transition is large, the specific heat capacity and thermal conductivity are large;
(3) The volume change during the phase transition process is small;
(4) The degree of supercooling is small or there is no supercooling phenomenon;
(5) Chemically stable, non-toxic, non-flammable and non-explosive;
(6) Large reserves and low prices.
According to the suitable working temperature range of the battery, the phase transition temperature of the PCM generally used in the battery thermal management system is mainly concentrated at 30~50℃. At present, the PCM used for the battery thermal management is mainly paraffin. Due to the shortcomings of low thermal conductivity of a single PCM, in order to better manage the thermal management of the battery, the method of adding other thermally conductive materials to the PCM is generally used to improve its heat transfer coefficient.
4. Basic types of power batteries
The inherent demand for improving the comprehensive performance of electric vehicles has promoted the development of power batteries in the direction of high power and high specific energy. For example, the miniaturization and lightweight characteristics of the whole vehicle determine that the mass and volume of the power battery pack should not be too large; the long driving distance and the low cost of the whole vehicle determine that the power battery must have the characteristics of long cycle life. Generally speaking, an electric vehicle battery pack is formed by stacking multiple battery modules or single cells in series and parallel. The commonly used single cells are cylindrical, square and oval in shape. Due to the battery composition and the compactness required for installation in electric vehicles, there are currently two main types of power batteries for electric vehicles: cylindrical and square. In recent years, some researches on electric vehicle power batteries, such as thermal analysis and thermal management of batteries, all take cylindrical and square batteries as research objects.
Different battery shapes will inevitably lead to differences in battery structure. The positive and negative electrodes of common cylindrical batteries are located at both ends of the battery, while the positive and negative electrodes of square batteries are generally located on the same side. During the charging and discharging process of the battery, the generation and transfer of heat will vary with the structure of the battery. Due to the difference in battery structure, it is particularly important to study the heat transfer law of the battery thermal management system for power batteries with specific shapes. In terms of battery heat dissipation, compared with the cooling method using air and liquid as the medium, the battery heat dissipation system based on phase change heat transfer medium (PCM) has more obvious energy saving effect because it does not require additional battery power consumption. At present, for cylindrical and square power batteries, both experiments and numerical simulations are mainly reflected in verifying the rationality and effectiveness of various heat dissipation methods. The following will take cylindrical and square Li-ion power batteries as examples to illustrate the influence of the thermal conductivity of PCM on the heat dissipation of the PCM battery module.