The design of the power battery thermal management system:
To adjust the battery temperature to keep it in the temperature range suitable for the battery to work; to reduce the difference between the highest temperature and the lowest temperature in the battery pack.
Composition of liquid cooling system
The liquid cooling heatsink system is a popular research direction for thermal management of power batteries at present. The optimal working temperature conditions of the battery pack can be achieved by utilizing the performance of the cooling liquid, which has a large thermal capacity and can take away the excess heat of the battery system through circulation.
The basic components of the liquid cooling heatsink system include: electric water pump, cell radiator (indirect cooling), temperature sensor, air conditioning system (compressor, condenser, evaporator), heater, and water to water heat exchanger.
Among them, the air-conditioning system is responsible for providing cooling in high temperature conditions; the heater, in low temperature conditions, is responsible for heating the coolant.
Principles of heat transfer
The intention of the design of the thermal management system is to transfer the excess heat of the new energy field power battery during the charging and discharging process, to keep the battery working within a suitable range, and the temperature difference of the cells in different positions should not be too large. In this way, the aging speed of the battery can be slowed down, and the degree of differentiation between different cells can be slowed down.
The reason why there are different cooling forms such as air cooling and liquid cooling is that the medium for heat transfer is different. In principle, it is necessary to start from the different transfer methods of heat. There are three main forms of heat transmission: heat radiation, heat conduction and convection.
Thermal radiation: Objects with a temperature higher than absolute zero are radiating thermal radiation. Thermal radiation does not require media and does not require contact, and transfers heat in the form of electromagnetic waves. The heat transferred from the sun to the earth is a typical process of thermal radiation.
Heat conduction: The process of transferring heat from a high temperature area to a low temperature area through a medium. Unlike thermal radiation, thermal conduction requires the existence of two conditions: temperature difference and medium.
Convection: Relative flow within a fluid, driven by temperature differences.
Heat, inside the power battery cell, is mainly transferred to the battery surface by heat conduction, and then spread to the surrounding space by radiation and convection. If a thermal management system is added to the system, the heat transfer process is partially altered. For example, in indirect heat dissipation, heat is transferred from the surface of the battery to the radiator shell mainly by heat conduction, and then the shell is transferred to the surface of the radiator flow channel by heat conduction; heat is transferred from the surface of the flow channel to the coolant by heat conduction. , the coolant transfers heat inside the coolant by convection, and follows the forced flow of the coolant to the outside of the battery pack.
Thermal management solutions for battery packs
The thermal management scheme of the battery pack involves three measures: cooling of the battery pack, low temperature preheating of the battery pack, and heat preservation of the battery pack.
Cooling of the battery pack
The cooling function of the liquid cooling system is mainly realized by circulating low temperature coolant. If the required heat dissipation power is relatively small, due to the relatively large heat capacity of the coolant itself, it is not necessary to start the cycle process, and the set temperature range requirements can already be met.
There are two main forms of battery pack cooling, direct cooling and indirect cooling. Direct cooling means that the cooling medium flows directly from the surface of the cell to take away excess heat; indirect cooling means that the cooling medium flows through the channels of the pipes and the radiator, and the radiator is in contact with the cell to transfer the heat of the cell to the cooling.
Low temperature warm-up of the battery pack
Originally, the compressor can have heating function, but its low temperature heating effect is not good, and the power consumption is relatively large, which has a great impact on the battery life; Too low or simply below the minimum discharge temperature to discharge. Therefore, the warm-up process before the car starts is designed into the thermal management strategy.
There are two basic forms of battery pack low temperature preheating: internal heating and external heating.
Internal heating, using the AC power outside the battery pack to heat the battery electrolyte until it reaches the applicable temperature range of the battery pack. The part that generates heat is the battery itself, so it is called internal heating.
External heating uses external power to heat the medium other than the battery, the medium transfers the heat to the battery, and gradually increases the battery temperature until it reaches the suitable temperature range of the battery. The external medium includes air medium and liquid medium, and the heat generating elements include PTC and heating film.
External heating is the more common method. The general implementation form is that the battery pack is equipped with a heater inside, which does not use the power of the power battery, but in the parking state, turns on the power supply outside the battery pack, and supplies power to the PTC or the heating film. The external power supply is generally the electric energy from the large power grid. The heater can work according to the applicable maximum power without worrying about the waste of electric energy, and the overall heating rate is relatively high.
Battery pack insulation
For new energy field power battery packs used in low temperature areas, the box body generally needs to be designed with thermal insulation measures to slow down the loss of preheating heat. Prevents the battery from falling below the operating temperature again when the vehicle is stopped for a short time while driving. Experiments have shown that the ambient temperature is minus 20°C. During the preheating process, the battery is heated to 25°C, and the vehicle is left standing for 8 hours, and the temperature drops to about 18°C.
Insulation measures are not provided on every vehicle with thermal management features. After the vehicle is preheated and the battery pack enters the working state, the battery itself will generate a lot of heat. If it is not an extremely cold environment and there is no need for long-term parking, the operating temperature of the battery pack can be maintained by self-heating.
The main factors that affect the cooling effect
Coolant temperature. During the cooling process, the lower the temperature of the coolant, the lower the maximum and minimum temperatures of the battery, but the gap between the two is large. During the heating process, the higher the temperature of the coolant, the greater the temperature difference of the battery. That is to say, the greater the temperature difference between the coolant and the battery, the greater the temperature difference between the cells at different positions inside the battery pack.
This phenomenon is mainly related to the different degrees of influence of the temperature regulation of the thermal management system on the cells at different positions. Some cells have a large contact area with the radiator, while others are relatively small; on the other hand, during the circulation of the coolant inside the battery pack, the temperature is constantly changing from the inlet to the outlet. At different locations, the temperature difference between the coolant and the cells with the same body temperature is different. Only accurate thermal design can solve this problem, not simply adjusting the coolant temperature.
Coolant flow. The greater the flow of coolant, the more heat it takes away in the same period of time. Some simulations have specifically observed the liquid cooling model, other parameters remain unchanged, and only the cooling liquid flow is adjusted, the effect of the cooling liquid flow on the cooling effect. As the coolant flow increases, the maximum temperature of the battery system decreases, but the temperature difference increases. After passing a maximum temperature difference, the flow continues to increase, and the temperature difference begins to decrease. In the process of continuing to increase the flow rate, the maximum temperature and the temperature difference have been decreasing in one direction.
In the first half of the process of flow increase, the maximum temperature decreases and the temperature difference increases. The reasons are consistent with the effect of the continuous decrease of the coolant temperature, which is related to the specific thermal structure design. Different cooling effects bring different temperature changes. In the second half of the flow rate increase test, with the increase of flow rate, the temperature difference began to decrease and continued to decrease, because the flow rate of the coolant increased to a certain extent, relative to the heat absorption capacity of the coolant, the battery transferred to the coolant heat is relatively small. In this way, on the one hand, the influence on the temperature of the cooling liquid becomes smaller, and the temperature difference between the cooling liquid at different positions near the system inlet becomes smaller and smaller; on the other hand, the difference in heat transfer capacity caused by the difference in the heat transfer area of different cells , is relatively smaller. As a result, the overall temperature difference of the system continues to decrease.
But the traffic cannot continue to increase. On the one hand, it is related to the amount of energy consumed, and it is inevitable to choose a flow with the best cost performance. On the other hand, maintaining a large flow rate for a long time is a test of the strength of the coolant circulation system, the life of the equipment may be reduced, and at the same time, the risk of accidents will increase.
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