Discussion on thermal management technology of new energy vehicle batteries
Power batteries are the main power source for new energy vehicles. Batteries generate a lot of heat during vehicle operation, and as time goes by, the heat accumulates in a relatively small space. Due to the dense stacking of cells in the battery pack, it is also more difficult to dissipate heat in the middle area to a certain extent, which aggravates the temperature inconsistency between cells. As a result, the battery's charging and discharging efficiency will be reduced, affecting the battery's power. In severe cases, it will also lead to thermal runaway, affecting the safety and life of the system. Especially in terms of temperature management, thermal runaway of the battery may cause fire and performance degradation. Therefore, research on thermal management technology of new energy vehicle batteries is of great significance to the development of new energy vehicles.
1. Basic components of vehicle thermal management system
The thermal management system of new energy vehicles includes four parts: battery system, motor system, air conditioning system, and other components. Compared with the thermal management of traditional fuel vehicles, the thermal management system of new energy vehicles is more complex. The battery system is a vital component of new energy vehicles. Engineers need to start with the thermal management of the battery system to design a good thermal management system for the whole vehicle.
Since the birth of new energy vehicles, experts and scholars in related industries have conducted a lot of research on the heat dissipation of their batteries, and have achieved a lot of results. The mainstream cooling method of battery thermal management has changed from air cooling to liquid cooling, phase change material cooling and heat pipe cooling. The following analyzes the cooling technologies such as air cooling, liquid cooling, phase change material cooling and heat pipe cooling.
2.1 Air Cooling
Air cooling is a heat dissipation method that uses air as a medium and utilizes heat convection in the air to allow the battery to directly exchange heat with the air, thereby reducing the battery temperature. Air cooling can be divided into natural cooling and forced cooling according to whether a fan is used. Natural air cooling is used without a fan; forced air cooling is used with a fan. A large number of studies have shown that the heat dissipation effect of forced air cooling is much higher than that of natural air cooling.
Air cooling can also be divided into serial cooling and parallel cooling according to different heat dissipation structures. In the serial cooling method, the airflow flows in from one side and flows out from the other side. This method will lead to poor cooling of batteries far away from the air inlet flow channel, uneven heat dissipation of the batteries, and large temperature differences in the batteries; in the parallel cooling method, the airflow generally enters from the bottom and flows out from the top. As can be seen from the figure, the cooling airflow can basically flow through the surface of each battery, so the temperature difference between each battery will be smaller than that of serial cooling, but it also brings the problem of uneven heat distribution.
2.2 Liquid Cooling
Power battery liquid cooling technology is one of the thermal management technologies. This technology usually uses a coolant with a high heat transfer coefficient to allow the battery to exchange heat with the coolant, thereby reducing the battery temperature.
The heat dissipation performance of air-cooled and liquid-cooled battery packs was compared and analyzed. The results show that the maximum temperature of the battery pack is lower and the temperature consistency is better in the liquid cooling system. Liquid cooling is an extremely effective heat dissipation method, and its heat transfer coefficient is higher than that of air cooling. The liquid cooling system of electric vehicles can be divided into direct contact and indirect contact according to the contact form between the insulating liquid and the battery. The form of immersing the battery cell or module in the liquid for heat exchange is the direct contact form; in addition, a cooling channel can be set between the battery modules, or a cooling plate can be used at the bottom of the battery. The heat of the power battery is transferred to the coolant through the cooling plate. This liquid cooling form is indirect contact. These two forms have high requirements for the air tightness of the liquid cooling system. In addition, the mechanical strength requirements are high, and the vibration resistance and the life requirements of the cooling system need to be guaranteed.
The electric vehicle liquid cooling system is mainly composed of coolant, cooling plate, electronic water pump, temperature sensor, radiator, etc. The compressor, as the power source of refrigeration, determines the heat exchange capacity of the entire system. The chiller (cooling device) plays the role of exchanging refrigerant and coolant, and the amount of heat exchange directly determines the temperature of the coolant. The water pump determines the flow rate of the coolant in the pipeline. The faster the flow rate, the better the heat exchange performance, and vice versa.
2.3 Phase Change Material Cooling
Phase change material (PCM) heat dissipation technology uses the principle that phase change materials absorb heat when they undergo phase change. The phase change material is placed around the battery pack and reaches the phase change temperature under certain conditions. The phase change material undergoes phase change and absorbs the heat generated when the battery is working, thereby effectively avoiding the process of overheating the battery module. Because the phase change process is a constant temperature process, the battery temperature can be well maintained near the phase change temperature of the phase change material, thereby suppressing the battery temperature from continuing to rise. However, the use of phase change material cooling requires attention to sealing issues, and will increase the volume of the battery pack and reduce energy density. In addition, the heat preservation function can only be maintained within a limited parking time. Long-term battery preheating still depends on the built-in heat source, and heat preservation generally requires a lower thermal conductivity, which may cause problems with uneven temperature distribution.
2.4 Heat pipe cooling
Heat pipe cooling is to divide the heat pipe into evaporation section, heat transfer section and condensation section. Its main heat dissipation principle is to remove the heat in the battery pack through the heat pipe evaporation section absorbing heat. The heat in the evaporation section is transferred to the condensation section through the middle section of the heat pipe due to the pressure difference formed, thereby achieving effective thermal management of the battery pack. Regarding the research on heat pipe cooling, the cooling effects of three cooling methods of power batteries, namely air, liquid and heat pipe cooling, were compared. The results show that heat pipe cooling has the best cooling effect on battery packs.
