The 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-target management such as passenger cabin temperature and humidity environment control, power system temperature control, glass anti-fogging and defogging, etc. According to the thermal management system architecture and degree of integration, the development of electric vehicle thermal management is summarized into three stages, as shown in Figure 1. From single cooling combined with electric heating to heat pumps combined with electric auxiliary heating to the gradual coupling of wide-temperature zone heat pumps with vehicle thermal management, the vehicle thermal management technology of electric vehicles is gradually developing in a highly integrated and intelligent direction, and in a wide temperature range The environmental adaptability ability under regional and extreme conditions is gradually improved.
ⅠFirst Stage PTC Heating
In the initial stage of industrialization of electric vehicles, they were basically developed with the replacement of power systems such as batteries and motors as core technologies. Auxiliary systems such as cabin air conditioning, window defogging, and power component temperature control were based on traditional fuel vehicle thermal management technology. Based on the gradual improvement. Both pure electric vehicle air conditioners and fuel vehicle air conditioners achieve refrigeration functions through a vapor compression cycle. The difference between the two is that the air conditioning compressor of fuel vehicles is indirectly driven by the engine through a belt, while pure electric vehicles directly use an electric drive compressor to drive refrigeration. cycle. When heating a fuel vehicle in winter, the engine waste heat is directly used 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. Therefore, winter heating is a problem that pure electric vehicles need to solve.
When an electric vehicle is operating normally, the power battery discharges and generates heat, causing the temperature to rise, requiring the battery to be cooled down. Battery cooling methods mainly include air cooling, liquid cooling, phase change material cooling, and heat pipe cooling. Because air cooling has a simple structure, low cost, and easy maintenance, it was widely used in early electric vehicles. The main form of thermal management at this stage is that each independent subsystem meets the needs of thermal management.
ⅡThe Second Stage of Heat Pump Technology Application
In actual use, electric vehicles have higher energy demand for heating 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 electric vehicles. mileage. Heat pump technology uses a 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 cruising range of electric vehicles under heating conditions.
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 pumps plus PTC auxiliary heat has become an important heating method for electric vehicles in low-temperature environments in winter. The main method of cabin heating. As the capacity and power of power batteries further increase, the thermal load during operation of power batteries also gradually increases. The traditional air-cooling structure cannot meet the temperature control needs of power batteries, so 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 operates normally, the cooling needs of the passenger compartment and the power battery can be met respectively by connecting heat exchangers in parallel in the passenger cabin heat pump system. The heat of the power battery is indirectly taken away through the heat exchanger and secondary cooling, and the degree of integration of the entire vehicle thermal management system of electric vehicles has been improved. Although the degree of integration has improved, the thermal management system at this stage only briefly integrates battery refrigeration and passenger compartment refrigeration, and the waste heat of the battery and motor has not been effectively utilized.
ⅢDevelopment of integrated technology for wide temperature zone heat pump and vehicle thermal management
Traditional heat pump air conditioners have low heating efficiency and insufficient heating capacity in high-cold environments, 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 reasonably adding a secondary heat exchange circuit, while cooling the power battery and motor system, the remaining heat is recycled to increase the heating capacity of electric vehicles under low-temperature conditions. Experimental results show that the heating capacity of waste heat recovery heat pump air conditioners is significantly increased compared with traditional heat pump air conditioners.
However, when the ambient temperature is lower and the amount of waste heat recovery is less, waste heat recovery alone still cannot meet the heating capacity demand in low-temperature environments. PTC heaters still need to be used to make up for the lack of heating capacity in the above situations. However, with the gradual improvement of the integration of thermal management of trams, the amount of waste heat recovery can be increased by reasonably increasing the heat generated by the motor, thereby increasing the heating capacity and COP of the heat pump system and avoiding the use of PTC heaters. It further reduces the space occupancy of the thermal management system while meeting the heating needs of electric vehicles in low-temperature environments.
In addition to recycling waste heat from batteries and motor systems, return air utilization is also a way to reduce energy consumption of thermal management systems under low-temperature conditions. Research results show that in low-temperature environments, reasonable return air utilization measures can avoid fogging and frosting on car windows while reducing the heating energy required by electric vehicles by 46% to 62%, and can reduce heating energy consumption by about 40% at most. . Nippon Denso has also developed a corresponding double-layer return air/fresh air structure, which can prevent fogging and reduce heat loss caused by ventilation by 30%. 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, the direct cooling and direct heating battery temperature control method that directly sends the refrigerant into the battery pack for heat exchange is also a current technical solution. The thermal management configuration of direct heat exchange between the battery pack and the refrigerant is shown in Figure 5. Direct cooling technology can improve heat exchange efficiency and heat transfer, achieve a more uniform temperature distribution inside the battery, reduce secondary loops and increase system waste heat recovery, thereby improving battery temperature control performance. However, since the direct heat exchange technology between the battery and the refrigerant requires the heat pump system to increase the cooling heat, on the one hand, the battery temperature control is limited by the start and stop of the heat pump air conditioning system, which has a certain impact on the performance of the refrigerant loop. It also limits the use of natural cold sources in transition seasons, so this technology still needs further research improvement and application evaluation.
