Research Progress of Electric Vehicle Thermal Management Technology

Pure electric vehicles have high comprehensive energy efficiency and relatively little environmental pollution. With the continuous development of pure electric vehicle related technologies, the scale of the industry is gradually expanding. Constrained by the energy density and material properties of power batteries, the cruising range of pure electric vehicles has become a key issue restricting its development. The demand and energy consumption of the vehicle thermal management system has gradually attracted widespread attention in the industry. The mobility of driving makes the environmental and climatic conditions faced by automobiles complex and changeable. For pure electric vehicles, without the engine thermal system of traditional fuel vehicles, the automotive thermal system needs to meet the requirements of battery/motor/electronic temperature control, heat exchanger defrosting, window glass Fog removal and other needs. Thermal management technology is an important guarantee for the safety and comfort of automobile driving, and has become the core key technology for the development of electric vehicles.

1. Electric Vehicle Thermal Management Requirements

The passenger compartment is the environmental space where the driver is in 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, and air supply temperature of the cabin environment. The thermal management requirements of the passenger compartment under different conditions are shown in Table 1.

Power battery temperature control is an important prerequisite to ensure the efficient and safe operation of electric vehicles. When the temperature is too high, it will cause liquid leakage, spontaneous combustion and other phenomena, which will affect driving safety. When the temperature is too low, the battery charge and discharge capacity will be attenuated to a certain extent. Due to its high energy density and light weight, lithium batteries have become the most widely used power batteries for electric vehicles. The temperature control requirements of lithium batteries and the battery heat 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 temperature range of the working environment, and the increase in fast charging speeds, the importance of power battery temperature control in the thermal management system has also become more prominent. This not only needs to meet the temperature control load changes under different road conditions, different charging and discharging modes and other vehicle operating conditions, the uniformity of the temperature field between battery packs and the prevention and control of thermal runaway, but also needs to meet the temperature control requirements in different environments in high heat and high humidity areas etc.

The motor and electronic control are the key energy output links of electric vehicles. During the working process of the motor, a large amount of heat will be generated due to coil resistance heating, mechanical friction heat generation and other reasons. Excessive temperature causes problems such as internal short circuit of the motor and irreversible demagnetization of the magnet. According to the motor configuration of different models in the current electric vehicle market, the motor and electric temperature control requirements of passenger cars, and the motor heating power considering the motor efficiency and motor power are shown in Table 3. With the popularity of electric vehicles and the increase of application scenarios, the demand for vehicle power continues to increase. Electric vehicle motors require higher power, torque and speed, which also means higher heat generation. As a result, the thermal management requirements of electric motor systems are gradually increasing.

2. Development history of electric vehicle thermal management technology

Vehicle thermal management is one of the core technologies for the development of electric vehicles, involving multi-objective management such as temperature and humidity environment control of the passenger compartment, power system temperature control, and glass anti-fog and demist. According to the thermal management system architecture and integration degree, the development of electric vehicle thermal management is summarized into three stages, as shown in Figure 1. From single cooling with electric heating to heat pump with electric auxiliary heating, and then to the gradual coupling of wide temperature zone heat pump and vehicle thermal management, the thermal management technology of electric vehicles is gradually developing towards a highly integrated and intelligent direction. And the environmental adaptability in wide temperature range and extreme conditions is gradually improved.

In the initial stage of the industrialization of electric vehicles, it is basically developed with the replacement of power systems such as batteries and motors as the core technology. Auxiliary systems such as cabin air conditioning, window defogging, and power component temperature control are gradually improved on the basis of traditional fuel vehicle thermal management technologies. Both pure electric vehicle air conditioners and fuel vehicle air conditioners realize the cooling function through the vapor compression cycle. The difference between the two is that the air-conditioning compressor of a fuel vehicle is indirectly driven by the engine through a belt, while a pure electric vehicle directly uses an electric drive compressor to drive the refrigeration cycle. When fuel vehicles are heated in winter, the waste heat of the engine is directly used to heat the passenger compartment without an additional heat source. However, the waste heat of the motor of pure electric vehicles cannot meet the needs of winter heating. Therefore, winter heating is a problem that pure electric vehicles need to solve. . Positive temperature coefficient heater (positive temperature coefficient, PTC) is composed of PTC ceramic heating element and aluminum tube, which has the advantages of small thermal resistance and high heat transfer efficiency. And there are minor changes based on the body of the fuel car. Therefore, early electric vehicles used vapor compression refrigeration cycle cooling plus PTC heating to achieve thermal management of the passenger compartment, as shown in Figure 2. Unlike gasoline vehicles, which are powered by fuel, electric vehicles are powered by power batteries. When the electric vehicle is running normally, the power battery discharges heat and the temperature rises, which requires battery cooling. The battery cooling methods mainly include air cooling, liquid cooling, phase change material cooling, and heat pipe cooling. Due to its simple structure, low cost and easy maintenance, air cooling was widely used in early electric vehicles. The main form of thermal management at this stage is that each independent subsystem meets the requirements of thermal management respectively.

In actual use, the heating energy consumption demand of electric vehicles in winter is relatively high. From a thermodynamic point of view, the COP of PTC heating is always less than 1, which makes the power consumption of PTC heating high and the energy utilization rate low, which seriously restricts the mileage of electric vehicles. The heat pump technology uses the vapor compression cycle to utilize the low-grade heat in the environment, and the theoretical COP for heating is greater than 1. Therefore, using a heat pump system instead of PTC can increase the cruising range of electric vehicles under heating conditions.

With the further improvement of the capacity and power of the power battery, the heat load during the operation of the power battery is also gradually increasing. The traditional air cooling structure cannot meet the temperature control requirements of power batteries. Therefore, liquid cooling has become the main method of battery temperature control. Moreover, since the comfortable temperature required by the human body is similar to the temperature at which the power battery works normally, the cooling requirements of the passenger compartment and the power battery can be met by connecting heat exchangers in parallel in the passenger compartment heat pump system. The heat from the power battery is indirectly taken away through the heat exchanger and secondary cooling. The degree of integration of the thermal management system of the electric vehicle has been improved. Although the degree of integration has increased, the thermal management system at this stage only simply integrates battery cooling and passenger compartment cooling. Waste heat from batteries and motors has not been effectively utilized.

Traditional heat pump air conditioners have low heating efficiency and insufficient heating capacity in the cold environment, which restricts the application scenarios of electric vehicles. Therefore, a series of methods to improve the performance of heat pump air conditioners under low temperature conditions have been developed and applied. By rationally increasing the secondary heat exchange circuit, while cooling the power battery and the motor system, the remaining heat is recycled to improve the heating capacity of electric vehicles under low temperature conditions. The experimental results show that the heating capacity of the waste heat recovery heat pump air conditioner is significantly improved compared with the traditional heat pump air conditioner.

In addition to the recovery and utilization of waste heat from batteries and motor systems, the utilization of return air is also a way to reduce the energy consumption of the thermal management system under low temperature conditions. The research results show that in a low temperature environment, reasonable return air utilization measures can reduce the heating capacity required by electric vehicles by 46% to 62% while avoiding fogging and frosting of the windows. It can reduce heating energy consumption by up to 40%. At this stage, the environmental adaptability of electric vehicle thermal management under extreme conditions is gradually improving, and it is developing in the direction of integration and greening.

In order to further improve the thermal management efficiency of the battery under high power conditions and reduce the complexity of thermal management, a technical solution is proposed, which is a direct cooling and direct heating battery temperature control method in which the refrigerant is directly sent into the battery pack for heat exchange. A thermal management configuration of direct heat exchange between the battery pack and the refrigerant is shown in Figure 3. The direct cooling technology can improve the heat exchange efficiency and heat exchange capacity, obtain a more uniform temperature distribution inside the battery, reduce the secondary loop and increase the waste heat recovery of the system. This in turn improves battery temperature control performance. However, due to the direct heat exchange technology between the battery and the refrigerant, it is necessary to increase the cooling and heat through the work of the heat pump system. On the one hand, the temperature control of the battery is limited by the start and stop of the heat pump air conditioning system, and has a certain impact on the performance of the refrigerant loop. On the other hand, it also limits the use of natural cold sources in transitional seasons, so this technology still needs further research, improvement and application evaluation.

3. Development trend of electric vehicle thermal management

Although compared with the early stage, the current thermal management system of electric vehicles has made great progress in terms of integration, energy saving and high efficiency, but there are still big challenges in the replacement of refrigerants, the development of heat pump systems in all climates and wide temperature zones, and intelligent control.

The research and application of potential alternative refrigerants mainly focus on R1234yf, CO2 and R290. The main physical properties of the above refrigerants are shown in Table 4. R1234yf has similar thermodynamic properties to the traditional refrigerant R134a, and it is easy to replace the refrigerant, but the price is relatively high. R290 and CO2, as natural and environmentally friendly refrigerants, have the advantage of relatively low prices. CO2 is non-toxic, non-flammable, and has excellent thermal stability. And it has a large temperature glide when it releases heat in a supercritical state, so it has excellent heating performance. The R290 heat pump system has excellent cooling and heating performance. However, since R290 is a flammable refrigerant, solving the safety hazards caused by the flammability of R290 is the key issue to realize the application of R290 heat pump system in electric vehicles.

The thermal properties of R1234yf and R134a refrigerant are very close, and R1234yf can be directly used to replace R134a thermal management system, but the system performance will be slightly reduced. R1234yf has weak flammability, and the risk of combustion can be reduced by adding a secondary circuit. Due to reasons such as patents and synthesis technology, the high price of R1234yf has become an obstacle restricting its popularization and application.

As an inexpensive, environmentally friendly natural refrigerant. At present, the CO2 heat pump system has begun to be applied in real vehicles, but there are still problems such as insufficient cooling capacity in summer and low heating efficiency in extremely cold conditions. The main goal of the work in the research field is to further improve the performance of CO2 heat pump systems, especially the improvement of cooling performance in high temperature environments.

R290, as another potential alternative environmentally friendly natural refrigerant, has excellent cooling and heating performance.

On the other hand, mixed refrigerants can overcome the limitations of the physical properties of pure natural refrigerants, and it is also one of the development directions of new refrigerant heat pump systems in the future.

The efficient and intelligent thermal management system of electric vehicles and the thermal comfort of the passenger cabin have become the key guarantees for improving travel quality. According to the different driving conditions of the vehicle itself, the thermal load of each system of the electric vehicle will fluctuate dynamically. Moreover, the coupling degree of the thermal system of electric vehicles continues to deepen, which puts forward higher requirements for the control of the thermal management system. Therefore, an intelligent, integrated, and fine-grained control method will be the control method to reduce vehicle energy consumption and improve comfort.

The traditional control method of the heat pump system is to control each independent thermal management object and thermal management actuator through switch control, PID control and other methods. According to the deviation between the set value and the actual value, each control parameter is maintained within the set range by adjusting parameters such as compression speed, expansion valve opening, electric heater power, circulation pump power, and electronic fan air volume. However, with the deepening of thermal management integration, PID control is prone to problems such as overshoot or oscillation in the complex dynamic control process. While causing energy consumption to increase, driving comfort is reduced. The control method of the multi-branch coupled complex heat pump system is the current research focus of the control technology of the electric vehicle thermal management system.

In order to ensure the thermal comfort of the driver, it is necessary to control the temperature and humidity of the passenger compartment within a reasonable fluctuation range. For the control of the hot and humid environment in the car, the conventional control method is to control the temperature and humidity inside the car by adjusting the air supply volume and temperature on the premise of anti-fogging the front windshield and ensuring the safe operation of the vehicle. The environment is controlled.

In terms of vehicle thermal management, thermal management of the passenger compartment includes not only the traditional method of air conditioning, but also new methods such as seat heating, which have also been researched and promoted. In addition to the active adjustment method of thermal management, reasonable body insulation structure design and material selection can also reduce the volatility of the interior environment and improve thermal comfort. In addition, a comfortable driving environment for a long time is likely to cause fatigue to the driver and affect driving safety. Relevant research on intelligent control system improving driver's mental concentration through blowing or other stimulating means is also in progress.

4. Summary and Outlook

The electric vehicle thermal management system is improved from the traditional fuel vehicle air conditioning system, and gradually transitions to the heat pump system suitable for electric vehicles. Different from fuel vehicles, the thermal management objects of electric vehicles also include battery systems and motor systems. Through the three-electric coupling, the coupling degree of the thermal management system of the electric vehicle and the integration degree of components are continuously improved.

In order to improve the applicability of electric vehicles in multiple environments and further increase the cruising range of electric vehicles, it is necessary to develop heat pump systems that can adapt to wide temperature ranges and extreme conditions.

With the increasing demand for travel quality, it is necessary to increase the attention of thermal management to the thermal comfort of the human body, and implement people-oriented and intelligent automotive thermal management technology and control strategies.

In the face of more stringent environmental protection requirements, we should focus on the alternative research of environmentally friendly refrigerants. And through the development of technologies such as waste heat recovery and jet air supplementation, the construction of a green, energy-saving and efficient vehicle thermal management system is completed.