How to Dissipate Heat to Lithium Battery?
Serious environmental pollution and the depletion of fossil fuels have made the development of new energy vehicles one of the important strategies. The energy and environmental benefits of new energy vehicles are becoming more and more significant, which are reflected in energy saving and low pollution. Most traditional fuel vehicles are powered by gasoline and diesel, and the emissions during driving have caused serious pollution to the environment; new energy vehicles are powered by the electrical energy of high-energy batteries, and there is basically no pollution during driving. Therefore, new energy vehicles will gradually replace traditional fuel vehicles. Lithium-ion batteries have the advantages of large capacity, long life, high energy density, and stable operation. They are one of the most advanced electrical energy storage and conversion technologies for new energy electric vehicles. However, the disadvantages of lithium-ion batteries are also more obvious. In a low-temperature environment, the capacity of lithium-ion batteries becomes smaller. In a high-temperature environment, the heat of lithium-ion batteries will gather, affecting their safe operation, and even cause the equipment to burn and explode in severe cases. Therefore, the battery thermal management system maintains the battery in the optimal temperature range (20~45℃) and controls the temperature difference below 5℃. It is necessary.At present, the mature cooling methods of the battery thermal management system include air cooling, liquid cooling, phase change cooling and heat pipe cooling. Whether it is a single cooling method or a variety of coupled cooling methods, they have their own advantages and disadvantages.
Air cooling is mainly based on air as the medium for heat exchange, and the heating lithium battery pack is cooled by the circulation of air. It is widely used in early battery thermal management design because of its low cost, simple structure, and easy maintenance. Air cooling can be divided into natural convection cooling and forced convection cooling. Natural convection cooling refers to the use of the natural flow of air to achieve the purpose of heat dissipation; forced convection cooling refers to the use of fans or specially designed air ducts to form a corresponding air flow in a specific space to achieve the purpose of heat dissipation.
The study found that compared with the horizontal battery pack, the heat dissipation performance of the longitudinal battery pack is improved due to the shortening of the air flow path. Due to the increase of the bottom air duct, the contact thermal conductivity area can be increased, and natural convection will occur in the top area of the battery pack, so the heat dissipation performance of the bottom air duct method is better.For battery packs in the bottom air duct mode, the heat dissipation performance of the double “U” air duct is improved than that of the double “I” air duct.
Air cooling is the simplest cooling method in the thermal management system of the battery. It has been extensively discussed in terms of flow channel, wind speed, import and export quantity and location, and flow type. However, due to the low specific heat capacity of air, it is difficult for air cooling to handle a large amount of heat, and its application has certain limitations.
LIQUID COOLING
Liquid cooling means that the battery module can be cooled with liquid cooling media such as water, mineral oil, ethylene glycol, dielectric, etc.Since liquid cooling has a high heat transfer coefficient, compared with air cooling systems or PCM cooling systems, liquid cooling systems can provide better heat exchange capabilities.
The same as air cooling, differences in structure will lead to different heat dissipation effects, and the number of liquid cooling channels will also have an impact on the heat dissipation effect.The study found that the number of channels has a significant impact on the maximum temperature and temperature difference of the lithium-ion battery pack, but this effect is limited.Increasing the aspect ratio of the rectangular channel can reduce the maximum temperature and temperature difference of the lithium-ion battery pack.Through the comparison of multiple sets of experimental studies, it is concluded that the C-type liquid cooling system and the alternating arrangement in the channel inlet arrangement can obtain the most balanced temperature distribution.
When conducting experiments and research on liquid cooling, the structural design is often considered, and the heat dissipation is improved by changing the number of flow channels, the direction of import and export, and the setting of baffles.Compared with air cooling, liquid cooling has excellent heat dissipation effect, but its structure is more complex and it must be sealed, otherwise it will cause leakage problems. When conducting experiments and research on liquid cooling, the structural design is often considered, and the heat dissipation is improved by changing the number of flow channels.
Phase change materials can be divided into inorganic phase change materials, organic phase change materials, composite phase change materials, etc. according to the material composition.Among them, inorganic phase change materials are mainly graphite, molten salt, crystalline water, etc., which have the advantages of high phase change enthalpy and high thermal conductivity, but the sub-cooling degree is also high, and the thermal stability is poor; organic phase change materials, such as paraffin wax, acetic acid, etc., have the characteristics of non-corrosive, low sub-cooling, and good chemical stability; composite phase change materials are used in combination with organic materials and inorganic materials, each taking its own strengths.In contrast, composite phase change materials have a better effect on the thermal management of lithium batteries.
The use of a single PCM in battery thermal management does not meet the heat dissipation requirements, and provides a safe and efficient environment for lithium battery work. PCM can be used in combination with porous and highly conductive materials to enhance the thermal diffusion of the battery.
Different cooling methods have their own advantages and disadvantages.Although the air cooling method has the advantages of simple structure and low cost, the unsatisfactory cooling effect limits the development of the air cooling system. The efficient cooling method of air cooling and heat pipe cooling can be combined, or the method of combining PCM and air cooling can be used. This coupled cooling method has a simple structure and the design is not too complicated.
Air cooling and liquid cooling are difficult to combine, because liquid cooling requires a relatively sealed working environment to prevent liquid leakage, while air cooling requires air circulation. The working conditions of the two make it difficult to combine them.Most of the liquid cooling design starts from the heat dissipation and cooling structure, changing its flow rate, number of flow channels, etc.
The combination of liquid cooling and PCM is more common, because PCM can effectively distribute the temperature evenly and store heat, while liquid cooling has a better heat transfer effect, which can take away the heat before the phase change material reaches the limit, thereby avoiding the occurrence of thermal runaway.PCM must first study the material composition. PCM made of different materials and ratios have different heat dissipation and cooling effects. At present, PCM cooling and cooling has been widely used in China.
