Research Progress of Electric Vehicle Thermal Management
System Integration Technology
1. Introduction to thermal management system
The thermal management system of electric vehicles evolved from the thermal management system of traditional fuel vehicles, and the system configuration has also gradually developed from the relative independence of each thermal management loop to the direction of integration. Since there is no engine waste heat to utilize, additional equipment is required to provide a heat source for passenger compartment heating. Currently, commonly used heating methods for electric vehicles include positive temperature coefficient (PTC) electric heater heating and heat pump air-conditioning heating.
1.1 Single cooling air conditioner + PTC
The resistance of the thermistor in the PTC electric heater will increase as the temperature increases, resulting in a decrease in heating power, so the PTC thermistor has constant temperature characteristics. Due to the simple composition and low price of the PTC heating system, most early electric vehicles used single-cooling air conditioning refrigeration and PTC electric heater heating to achieve the cooling and heating needs of the passenger compartment. Figure 1 is a system diagram of a single cooling air conditioner + PTC. During summer cooling, the evaporator arranged in the air duct is used to absorb heat to achieve the purpose of cooling. There are two options for heating. One is to arrange the PTC directly in the air duct of the air-conditioning box as shown in Figure 1(a). When there is a need for heating, the PTC is energized and the air in the heated air duct is passed into the passenger compartment; the other is As shown in Figure 1(b), the water PTC heats the refrigerant, and the refrigerant flows into the warm air core arranged in the air duct, indirectly heating the air in the air duct to meet the heating demand.
1.2 Heat pump air conditioner + PTC
Since PTC uses electric heating and the heating efficiency is less than 1, this heating method can reduce the cruising range of electric vehicles by 50%. The theoretical efficiency of heat pumps is greater than 1, and using heat pumps instead of PTC heating has become a development trend.
Heat pump air conditioning + PTC system using electromagnetic four-way reversing valve and three heat exchangers. The heat pump air conditioning system realizes the switching of system cooling, heating, dehumidification, defrost and other modes through the switch of the valve body. The four-way reversing valve has been widely used in the field of household air conditioning. Its application in the heat pump air conditioning system of electric vehicles can well solve the problem of refrigerant reversal during system cooling and heating, as shown in Figure 2(a) . This system solution has fewer parts, simple structure and low cost. However, the four-way reversing valve has defects such as the copper-aluminum welding process is difficult and is easily corroded. There is blow-by gas on the high and low pressure sides, which affects system performance. Passenger car heat pump air conditioning systems mostly use a three-heat exchanger solution with one outdoor heat exchanger and two indoor heat exchangers, and mode switching is performed through multiple solenoid valves, as shown in Figure 2(b).
1.3 Heat pump air conditioner + waste heat recovery + PTC
Our country has a vast territory and a large temperature zone across it, so it is necessary to expand the temperature range of heat pump air conditioners. The waste heat from motors and batteries is a valuable heat source in winter. Many manufacturers and scientific research institutions consider recovering this heat as an auxiliary heat source to expand the use scope of heat pump air conditioners.
Figure 3 is a diagram of an electric vehicle thermal management system using heat pump air conditioning + waste heat recovery + PTC. This system can realize functions such as passenger cabin refrigeration, heating, defrosting, defogging, and dehumidification, and can also heat or cool the battery and drive motor. , in addition, waste heat recovery from batteries and drive motors can be achieved. The working principle of this system is as follows: the functions of cooling, heating, dehumidification, defrosting and defogging of the passenger compartment are realized through the switch combination of the solenoid valve of the refrigerant circuit; a battery cooler (Chiller) connected in parallel with the evaporator is added to the refrigerant circuit. ), when the battery or drive motor has cooling requirements, the refrigerant flows through the Chiller to cool the secondary refrigerant in the secondary circuit; when the battery or drive motor does not have a large need for heat dissipation, the state of the three-way valve can be switched to control the secondary refrigerant. Flow direction, the heat is discharged to the outside of the vehicle through the low-temperature radiator to cool the battery or drive motor; when the battery needs heating, the PTC energizes the heating and realizes the heating function through the adjustment of the three-way valve on the refrigerant side; when the environment When the temperature is low, the heat pump air conditioner cannot be used and the battery or drive motor needs to dissipate heat, the secondary refrigerant absorbs the heat it needs to dissipate on the battery or drive motor side, flows through the three-way valve through the Chiller, and exchanges the heat to the refrigerant side. Heating the passenger compartment expands the system's operating temperature range and improves the system's energy efficiency.
