With the development of electric vehicles and the continuous improvement of power system power, the density of battery packs has also increased more than before. The demand for battery fast charging has led to more heat generated by batteries during high-current charging and discharging. Under this trend, designing a reasonable battery cooling system has become the focus, which requires rapid heat dissipation at high temperatures and heating or heat preservation at low temperatures to improve the overall performance of electric vehicles.
1. Introduction to the battery cooling system
At present, electric vehicles have gradually become one of the mainstream trends in transportation. In order to meet the needs of high car battery voltage and large capacity of electric vehicles, power battery cells are used in the most common combination of series and parallel. During the operation or charging process of electric vehicles, the battery temperature will change greatly. If the heat is not dissipated in time, the heat will continue to accumulate in the battery module, resulting in continuous rise and uneven diffusion of battery module heat.
The internal chemical composition of lithium-ion batteries has poor heat resistance, so the reaction will be accelerated at high temperatures, resulting in qualitative changes in the internal structure of lithium-ion batteries, which will eventually cause serious safety consequences. Therefore, an efficient battery thermal management system (Battery thermal management system, BTMS) is needed to maintain an appropriate battery operating temperature range to prevent adverse effects caused by temperature changes.
The battery thermal management system can be divided into a battery cooling system and a battery heating system. Among them, the current mature battery cooling system can be divided into four parts according to the heat transfer medium, namely air cooling, liquid cooling, phase-change material cooling ( PCM) and heat pipe cooling.
2. Common types of lithium battery cooling system
① Air cooling system
Air cooling is currently the most widely used battery cooling system method, which can be combined with the driving characteristics design of the vehicle. The heat can be taken away by the natural wind formed by the speed of the vehicle, or the forced air flow can be generated by the operation of the fan. The natural convection of this battery cooling system has the advantages of simplicity, low cost, and natural convection as the main heat dissipation process. The disadvantage is that the wind force is uncontrollable.
Compared with natural convection, forced convection is more reliable and easier to maintain, so it has become a common battery cooling system. However, the disadvantage of forced convection is that the temperature distribution in the battery is uneven. Due to the characteristics of the air itself, the cooling effect has certain limitations. The advantages of air-cooled thermal management include: safety and reliability during operation, simple materials required and easy to implement, and timely and effective ventilation when harmful gases are generated.
Compared with the battery cooling system of liquid and phase change materials, the cooling ability of air as a cooling medium is obviously insufficient, and it is only suitable for low-density batteries. The heat generated by the bulky battery pack makes the active air cooling system increase in size, which affects the performance of electric vehicles and the comfort of passengers. In order to solve the problems faced by the air-cooled battery cooling system and improve its performance, many scholars have begun to study adding other cooling media to the air-cooled battery cooling system.
② Liquid cooling system
Compared with the air-cooled battery cooling system, the liquid-based battery cooling system has a higher heat transfer coefficient and specific heat capacity, which has a more significant effect on improving the energy density and thermal management capabilities of the battery pack.
According to the way the battery is in contact with the cooling liquid, the liquid-cooled battery cooling system can be divided into two types: direct contact type and indirect contact type. According to the structure of battery liquid cooling and heat dissipation, it can be divided into active and passive methods. In the passive system, the coolant exchanges heat with the outside air to send out the battery heat; in the active system, the battery heat is sent out through liquid-liquid exchange.
Direct contact cooling: The cooling liquid directly contacts the surface of the battery or battery module, which is direct contact liquid cooling, which can better dissipate heat from the battery compared to the air-cooled battery cooling system. The cooling liquid is characterized by high thermal conductivity and insulation, but because the cooling liquid used is not fluid, the heat dissipation effect will also be affected to a certain extent.
Indirect contact cooling: The indirect contact battery cooling system achieves the purpose of cooling the battery by contacting the battery with fins or heat sinks filled with coolant to remove heat. For the cylindrical battery, it can be set as an annular jacket structure, and the flow rate of the liquid is not restricted, so the liquid material with high thermal conductivity can be used.
The liquid-cooled battery cooling system has a good effect and can effectively reduce the working temperature and local temperature difference of the battery. At the same time, there are also adverse effects such as complex system structure, relatively large mass, liquid leakage, and frequent maintenance. However, in the electric vehicle thermal management system that requires relatively strict battery working conditions and prioritizes thermal management, the liquid-cooled battery cooling system has more obvious advantages than air-cooled.
In recent years, the more common research on liquid battery cooling system is to use new types of cooling fluids, such as liquid metals, nanofluids, etc., to optimize liquid cooling and heat dissipation. However, the current design of the liquid battery cooling system channel is still the research status of the liquid cooling system.
③ Heating pipe cooling system
Heating pipe(HP) is a high-efficiency heat exchange element that uses the phase change of the medium in the pipe to absorb and release heat. It is widely used in many fields such as industry. When the heating end of the heat pipe is heated, the working medium is heated and evaporated and flows to the condensing end under the force of the fluid in the pipe. Then the steam dissipates heat at the condensing end and becomes liquid again, and the liquid at the condensing end flows back to the evaporating end under the action of gravity or the capillary force of the porous material to achieve the purpose of heat dissipation.
Such a cycle transfers the heat generated by the battery to the outside air, thereby realizing the transmission of small temperature difference and large heat flow, and reducing the temperature of the battery. Heat pipes have become one of the important battery cooling system for electronic equipment due to their good heat flux variability, thermal conductivity, density variability, heat flow direction reversibility, excellent constant temperature thermal performance, and environmental adaptability. Compared with other battery cooling system, the heat pipe has a stronger heat transfer capacity, but it does not mean that its heat load can be increased infinitely.
The thermal efficiency of the heat pipe is restricted by many factors. When the heat pipe reaches the limit, the heat transfer will no longer continue to increase, and the heat transfer limit depends on the shape of the heat pipe, the structure of the internal wicking fluid, the working medium and the surrounding environment. Although heat pipes are widely used in the field of heat dissipation of electronic equipment, their disadvantages are the small capacity and small contact area of heat pipes. For large battery packs, more heat pipes are needed for heat dissipation, and the battery pack cannot be heated.
④ Phase change material cooling system
The physical state of phase change materials (PCM) changes with temperature. During the phase change process, the temperature range is small, but the latent heat absorbed or released is large. Phase change materials have the advantages of small volume change, large latent heat, and good stability. PCM relies on its own high latent heat capacity, however when the temperature exceeds its own melting point, the cooling performance of PCM will decrease significantly.
Therefore, some improvement schemes couple PCM with common cooling methods to form a hybrid battery cooling system to ensure long-term use. In the future, the demand for batteries with high power, high energy density and high charging efficiency will continue to grow, followed by the demand for more efficient, more stable, more economical and more compact battery thermal management systems. From the perspective of low energy consumption and structure, the heat dissipation system of PCM has more potential, and further research is needed to improve commercial applicability.