Research progress on thermal management technology of electric vehicles

Pure electric vehicles have high comprehensive energy efficiency and relatively low environmental pollution. They are a form of new energy vehicles that my country has given priority to developing. With the continuous development of pure electric vehicle related technologies, the scale of the industry has gradually expanded. Restricted by the energy density and material properties of power batteries, the range of pure electric vehicles has become a key issue restricting its development, and the demand and energy consumption of the vehicle thermal management system have gradually attracted widespread attention from the industry. The mobility of driving makes the environmental and climatic conditions faced by automobiles complex and changeable. For pure electric vehicles, there is no engine thermal system of traditional fuel vehicles. While meeting the requirements of cabin environment control, the vehicle thermal system also needs to meet the requirements of battery/motor/electronic control temperature control, heat exchanger defrosting, and window glass defogger. Thermal management technology is an important guarantee for vehicle driving safety and comfort, and has become a core key technology for the development of electric vehicles.

1 Electric vehicle thermal management requirements

The passenger compartment is the environment where the driver is located during the driving process of the car. In order to ensure a comfortable driving environment for the driver, the thermal management of the passenger compartment needs to control the temperature, humidity, air supply temperature, etc. of the interior environment of the vehicle. The thermal management requirements of the passenger compartment under different conditions are shown in Table 1.

Table 1 Passenger car cabin air conditioning requirements

Power battery temperature control is an important prerequisite for ensuring the efficient and safe operation of electric vehicles. When the temperature is too high, it will cause leakage, spontaneous combustion and other phenomena, affecting driving safety; when the temperature is too low, the battery charging and discharging capabilities will be attenuated to a certain extent. Due to its high energy density and lightweight, lithium batteries have become the most widely used power batteries for electric vehicles. The temperature control requirements of lithium batteries and the battery thermal load under different conditions estimated according to the literature are shown in Table 2. With the gradual increase in the energy density of power batteries, the expansion of the working environment temperature range and the increase in fast charging speed, the importance of power battery temperature control in the thermal management system has become more prominent. It is not only necessary to meet the temperature control load changes under different road conditions, different charging and discharging modes and other vehicle use conditions, the uniformity of the temperature field between battery packs and thermal runaway prevention and control, but also to meet all temperature control requirements under different environmental conditions such as severe cold, high heat and high humidity areas, hot summer and cold winter areas.

Table 2 Lithium battery temperature control requirements and heat load

The motor and electronic control are the key energy output links of electric vehicles. During the operation of the motor, a large amount of heat will be generated due to coil resistance heating, mechanical friction heating, etc. Excessive temperature will lead to internal short circuits in the motor and irreversible demagnetization of the magnet. According to the motor configuration of different models in the current electric vehicle market, the temperature control requirements of passenger car motors and electronic controls, as well as the motor heating power considering motor efficiency and motor power are shown in Table 3. With the popularization of electric vehicles and the increase in application scenarios, the demand for automotive power continues to increase. Electric vehicle motors require higher power, torque and speed, which also means higher heat generation. Therefore, the thermal management requirements of motor systems are gradually increasing.

Table 3 Passenger car motors, electronic temperature control requirements and motor heating power

2 Development History of Thermal Management Technology for Electric Vehicles

Vehicle thermal management is one of the core technologies for the development of electric vehicles, involving multi-target management such as passenger compartment temperature and humidity control, power system temperature control, and glass anti-fogging and defogger. According to the thermal management system architecture and integration level, the development of electric vehicle thermal management can be summarized into three stages, as shown in Figure 1. From single cooling with electric heating to heat pump with electric auxiliary heating to wide temperature range heat pump and vehicle thermal management, the thermal management technology of electric vehicles is gradually developing in the direction of high integration and intelligence, and its environmental adaptability in wide temperature range and extreme conditions is gradually improving.

Figure 1 Development trend of thermal management configuration of electric vehicles

2.1 The first stage of PTC heating

In the initial stage of electric vehicle industrialization, the core technology was basically developed based on the replacement of power systems such as batteries and motors. Auxiliary systems such as cabin air conditioning, window defogger, and power component temperature control were gradually improved on the basis of traditional fuel vehicle thermal management technology. Both pure electric vehicle air conditioning and fuel vehicle air conditioning achieve refrigeration function through vapor compression cycle. The difference between the two is that the fuel vehicle air conditioning compressor is indirectly driven by the engine through a belt, while pure electric vehicles directly use electric drive compressors to drive the refrigeration cycle. When heating in winter, fuel vehicles directly use the engine waste heat to heat the passenger compartment without the need for additional heat sources. However, the motor waste heat of pure electric vehicles cannot meet the heating needs in winter, so winter heating is a problem that pure electric vehicles need to solve. The positive temperature coefficient heater (PTC) consists of a PTC ceramic heating element and an aluminum tube. It has the advantages of low thermal resistance and high heat transfer efficiency, and has little modification on the body of a fuel vehicle. Therefore, early electric vehicles use vapor compression refrigeration cycle refrigeration plus PTC heating to achieve thermal management of the passenger compartment, such as the early Mitsubishi i-MIEV electric vehicle shown in Figure 2. Unlike fuel-powered vehicles, which are powered by fuel, electric vehicles are powered by power batteries. When electric vehicles are operating normally, the power batteries discharge and generate heat, which increases the temperature and requires cooling of the batteries. The main battery cooling methods include air cooling, liquid cooling, phase change material cooling, and heat pipe cooling. Air cooling has been widely used in early electric vehicles due to its simple structure, low cost, and easy maintenance. The main form of thermal management at this stage is that each independent subsystem meets the thermal management requirements.

Figure 2 Early electric vehicle thermal management system

2.2 Second stage heat pump technology application

In actual use, electric vehicles have a high demand for heating energy in winter. From a thermodynamic point of view, the COP of PTC heating is always less than 1, which makes PTC heating power consumption high and energy utilization rate low, which seriously restricts the mileage of electric vehicles. Heat pump technology uses vapor compression cycle to utilize low-grade heat in the environment. The theoretical COP during heating is greater than 1. Therefore, using a heat pump system instead of PTC can increase the mileage of electric vehicles under heating conditions. Figure 3 shows that the BMW i3 model uses a heat pump system to achieve winter heating. In addition, FAW Pentium, Hongqi, SAIC Roewe, etc. also use heat pump systems on some models. However, in low temperature environments, the heating capacity of traditional heat pump systems is severely attenuated and cannot meet the heating needs of electric vehicles in low temperature environments. Additional heaters are required for auxiliary heating. Therefore, the heating method of heat pump plus PTC auxiliary heating has become the main way to heat the passenger compartment of electric vehicles in low temperature environments in winter. With the further improvement of power battery capacity and power, the heat load of the power battery operation process is gradually increasing. The traditional air cooling structure cannot meet the temperature control requirements of the power battery, so liquid cooling has become the main method of current battery temperature control. In addition, since the comfortable temperature required by the human body is similar to the temperature at which the power battery works normally, the cooling needs of the passenger compartment and the power battery can be met separately by connecting a heat exchanger in parallel in the passenger compartment heat pump system. The heat of the power battery is indirectly taken away by the heat exchanger and secondary cooling, and the integration level of the thermal management system of the electric vehicle has been improved. Although the integration level has been improved, the thermal management system at this stage only simply integrates the battery cooling and the passenger compartment cooling, and the waste heat of the battery and motor is not effectively utilized.