1. Research background of lithium-ion power battery thermal management system
The performance, life and safety of lithium-ion power batteries are closely related to the temperature of the battery. If the temperature is too high, the side reactions will be accelerated, the decay will be accelerated (every time the temperature increases by 15°C, the life span will be reduced by half), and even safety accidents will occur. If the temperature is too low, the power and capacity of the battery will be significantly reduced. If the power is not limited, it may lead to the precipitation of lithium ions, causing irreversible attenuation and burying potential safety hazards. Generally, the suitable working temperature of lithium-ion power battery is between 10°C and 30°C. The operating ambient temperature of the lithium-ion battery for electronic products is not much different from this suitable temperature range, and no or only simple heat dissipation components are required. Vehicle power batteries are used in a wide range of ambient temperatures (-20°C to 50°C), and the thermal environment around the battery in the vehicle is often very uneven, which poses a serious challenge to the thermal management of the battery pack. The large-scale and grouped use of power batteries has led to the fact that the heat dissipation capacity of the battery (group) is much lower than the heat generation capacity. Especially for HEVs and PHEVs characterized by high-rate discharge, a complex heat dissipation system needs to be designed. When the single cells are used in parallel (the internal pole pieces of the single cells are also connected in parallel), the uneven temperature of the individual cells will cause thermoelectric coupling, that is, the battery (or part) with a high temperature has a smaller internal resistance and will share more current, resulting in The state of charge is not uniform, thereby accelerating the deterioration of the battery pack. Therefore, the thermal management technology of the power battery system is one of the key technologies to ensure its performance, life and safety.
The thermal management system of the power battery mainly realizes the following functions: first, heat dissipation when the temperature of the battery pack is high to prevent safety accidents caused by overheating of the battery; second, heating the battery pack when the temperature of the battery pack is low to ensure that the battery is in a low temperature environment The safety and use efficiency of lower charging and discharging; third, make the temperature difference between different positions of the battery and different parts of the battery as small as possible, suppress the formation of local hot spots or hot spots, and make the thermally induced decay rates of the batteries at different positions close to Consistent. Generally, the internal temperature difference of the battery pack is less than 5℃. GM’s Volt adopts a water-cooling design of thermoelectric integration, which can control the maximum temperature difference within 2℃, which strongly supports the 8-year life guarantee period (GM’s guarantee period for the internal combustion engine power system is 5 years). Table 4-1 shows typical automobile thermal management methods in the United States and Japan.
2. Research content of thermal management system of lithium-ion power battery
1) The main components of the thermal management system for lithium-ion power batteries
(1) Heat transfer medium: a medium in contact with the heat exchange surface of the battery pack, through which the heat generated in the battery pack is dissipated to the external environment through the flow of the medium.
(2) Flow field environment: the path through which the heat transfer medium flows and the distribution of velocity and pressure along the way.
(3) Temperature measuring element and control circuit: The temperature measuring element is used to measure the real-time temperature of different positions of the battery pack; the control circuit makes the action decision of the cooling actuator according to the real-time temperature.
(4) Heat dissipation actuator: The device that drives the heat transfer medium to circulate, with fans and pumps being the most common. Thermal management systems with natural ventilation do not contain thermal actuators.
2) The main heat transfer medium of the thermal management system of the lithium-ion power battery pack
(1) Air is used as the heat transfer medium. In a thermal management system that uses air as the heat transfer medium, the air from the outside environment or the passenger compartment enters the flow channel of the thermal management system, directly contacts the heat exchange surface of the battery pack, and takes away heat through the air flow. According to the spontaneous degree of air flow, it is divided into two categories: natural ventilation and forced ventilation. Natural ventilation includes natural convection and air movement that occurs with the vehicle. Forced ventilation is primarily driven by fans whose instantaneous power is determined by the control circuit of the thermal management system.
(2) Use liquid as heat transfer medium. Thermal management systems using liquid as heat transfer medium are mainly divided into contact and non-contact thermal management systems. The contact type uses highly insulating liquids such as silicon-based oil, mineral oil, etc., and the battery pack can be directly immersed in the heat transfer liquid. The non-contact type uses conductive liquids such as water, ethylene glycol or coolant, and the battery pack cannot be in direct contact with the heat transfer liquid. At this time, distributed closed pipes must be arranged inside the battery pack, and the heat transfer liquid flows through the pipes to take away the heat. The material of the pipe and its tightness ensure the electrical insulation between the conductive liquid and the battery body. The liquid flow in the contact or non-contact liquid cooling system is mainly driven by oil pumps/water pumps.
Since the specific heat capacity and thermal conductivity of liquid are much higher than that of air, the heat dissipation effect of liquid-cooled thermal management system is theoretically better than that of air-cooled system. However, the following two characteristics of the liquid cooling system reduce its heat dissipation efficiency in practical use:
①The heat transfer medium insulating oil of the contact liquid cooling system has a high viscosity, which requires a high oil pump power to maintain the required flow rate.
②The non-contact liquid cooling system needs to design distributed closed flow channels inside the battery pack, which increases the overall mass of the battery pack and reduces the heat transfer efficiency between the battery surface and the heat transfer medium.
(3) The phase change material is used as the heat transfer medium. Certain substances undergo a phase change at a specific temperature and absorb or release energy, and these substances are called phase change materials (PCM). The phase change temperature can be adjusted near the upper limit of the suitable working range of the battery by adjusting the types and composition ratios of phase change materials and additives. Using this type of phase change material to wrap the battery pack, when the battery temperature rises to the phase change temperature, the phase change material will absorb a large amount of latent heat, so that the battery temperature is maintained within the suitable working range of the battery, and the battery pack is effectively prevented from overheating.
The thermal management system using phase change material as heat transfer medium has the advantages of simple overall structure, high system reliability and safety. At 40℃~45℃ and high rate discharge, the effect of using composite PCM material to dissipate heat from the battery pack is better than using a fan within the general power range for air cooling. At present, paraffin wax (and its additives) has received more attention as the mainstream battery thermal management phase change material, because the phase change temperature of paraffin wax is close to the upper limit of the optimal operating temperature of the battery, and the cost is low and the latent heat is high. But the main problem is its low thermal conductivity. Therefore, other substances with high thermal conductivity are often added to paraffin to make composite PCM materials. The results show that the mechanical properties are gradually improved with the increase of the paraffin mass fraction at low temperature, while the mechanical properties are gradually deteriorated with the increase of the paraffin mass fraction at high temperature. In addition, adding heat pipes, foamed aluminum and aluminum heat sinks inside the battery pack phase change material can further improve the heat dissipation capacity of the PCM.