Research progress of electric vehicle thermal management
system integration technology
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 when heating the passenger compartment. Currently, commonly used heating methods in electric vehicles include positive temperature coefficient (positive temperature coefficient, PTC) electric heater heating and heat pump air conditioning heating.
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. Therefore, the PTC thermistor has constant temperature characteristics. Due to the simple composition and low price of the PTC heating system, early PTC heating systems Most electric vehicles use single-cooling air conditioning refrigeration and PTC electric heater heating to achieve the cooling and heating needs of the passenger compartment.
During cooling in summer, 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 air PTC directly in the air duct of the air conditioning box. When there is a need for heating, the PTC is energized to heat the air. The air in the duct is introduced into the passenger compartment; another type uses water PTC to heat the secondary refrigerant, and the secondary refrigerant flows into the warm air core arranged in the air duct to indirectly heat the air in the air duct to meet the heating demand.
Heat pump air conditioner+PTC
Since PTC uses electric heating for heating, the heating efficiency is less than 1, so this heating method can reduce the cruising range of electric vehicles by 50%. The theoretical efficiency of heat pumps is greater than 1, so using heat pumps to replace PTC heating has become a development trend. ｡The research results of ZHANG et al. [18] show that the heat pump system can save 41.3% of energy compared with the PTC heater system. Since most electric vehicles currently use R134a as a working fluid, when the ambient temperature is lower than −10°C, the heat pump system The efficiency and reliability are reduced, so a PTC electric heater is still needed for auxiliary heating.
The heat pump air conditioning system realizes the switching of system refrigeration, 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, and its application in the heat pump air conditioning system of electric vehicles can It is a good solution to the problem of refrigerant reversal during system cooling and heating. This system solution has fewer parts, simple structure, and lower cost. However, the four-way reversing valve has a difficult copper-aluminum welding process and is easily corroded. High and low pressure There are defects such as air blow-by on the side, which affects system performance. Passenger car heat pump air conditioning systems mostly use a three-heat exchanger solution of one outdoor heat exchanger and two indoor heat exchangers, and mode switching is performed through multiple solenoid valves.
A heat pump air conditioning system using R134a refrigerant can quickly heat the passenger compartment when the ambient temperature is above 0°C. However, when operating at a lower temperature, due to the decrease in refrigerant mass flow, the evaporation heat absorption capacity is weakened; at the same time, the suction pressure decreases. , the compressor pressure ratio increases, resulting in reduced efficiency and attenuated system performance. Studies have shown that at an ambient temperature of −10°C, the heating capacity of the heat pump air conditioning system using R134a refrigerant has severely attenuated to the point that it cannot meet the cabin heating demand. At this time, The PTC electric heater needs to be used to heat the refrigerant to meet the heating needs of the system. Wang Ying et al. used experimental methods to compare the operating characteristics of the four-way valve and three heat exchanger systems. The research results show that the three heat exchanger system is better at defrosting. ､It has more advantages in dehumidification, and the energy efficiency of the four-way valve system is 7%~15% higher than that of the three heat exchanger system.
Heat pump air conditioner + waste heat recovery + PTC
Our country has a vast territory and spans a large temperature zone, so it is necessary to expand the temperature range used by heat pump air conditioners. The waste heat of motors and batteries is a valuable heat source in winter. Many manufacturers and scientific research institutions consider recycling this part of heat as an auxiliary heat source to expand Application range of heat pump air conditioners.
The system can realize functions such as passenger compartment cooling, heating, defrosting, defogging, and dehumidification. It can also heat or cool the battery and drive motor. In addition, it can also recover waste heat from the battery and drive motor. The working principle of the 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 driver When the motor has cooling needs, the refrigerant flows through the secondary refrigerant of the Chiller cooling 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 flow of the secondary refrigerant to transfer heat through the low temperature The radiator is discharged to the outside of the vehicle to cool down 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 ambient temperature is low, the heat pump When the air conditioner cannot be used and the battery or drive motor needs heat dissipation, the secondary refrigerant absorbs the heat that needs to be dissipated on the battery or drive motor side. It flows through the Chiller through the three-way valve and exchanges the heat to the refrigerant side to heat the passenger compartment. The operating temperature range of the system is increased and the energy efficiency of the system is improved.
