Three temperature control technologies of battery pack thermal management system

Air cooling

Air cooling is a heat dissipation method that uses low-temperature air as the medium to reduce the temperature of the battery by using heat convection. It is divided into natural cooling and forced cooling (using fans, etc.). This technology uses natural wind or a fan to cool the battery with the evaporator built in the car. The system has a simple structure and is easy to maintain. It is widely used in early electric passenger cars, such as Nissan Leaf, Kia SoulEV, etc. It is also widely adopted in the current electric buses and electric logistics vehicles.

The basic principle of air cooling is shown in Figure 1. The system is an air-cooled heat pump module unit. Its working principle is based on the compression refrigeration cycle, with refrigerant as the carrier, and forced heat exchange through the evaporator to extract or discharge heat from the atmosphere to meet the needs of cooling or heating. .

liquid cooling

Liquid cooling technology uses liquid convection heat exchange to remove the heat generated by the battery and reduce the battery temperature. The liquid medium has high heat transfer coefficient, large heat capacity and fast cooling speed, which has a significant effect on reducing the maximum temperature and improving the consistency of the temperature field of the battery pack. At the same time, the volume of the thermal management system is relatively small. The liquid cooling system is more flexible in form. It can immerse the battery cells or modules in liquid, or set up cooling channels between the battery modules, or use a cooling plate at the bottom of the battery. When the battery is in direct contact with the liquid, the liquid must be insulated (such as using mineral oil) to avoid short circuits. At the same time, the air tightness requirements for the liquid cooling system are also higher. In addition, there are mechanical strength, vibration resistance and life requirements. Liquid cooling is currently the preferred solution for many electric passenger vehicles. Typical domestic and foreign products such as BMW i3, Tesla, GM Volt, BMW Brilliance, and Geely Emgrand EV all use liquid cooling.

The cooling system of the car battery pack is a forced circulation water cooling system, that is, the water pump is used to increase the pressure of the coolant, and the coolant is forced to circulate through the energy exchanger in the battery pack. The liquid cooling system of the battery pack is mainly composed of water pump, battery, energy exchanger, water tank, condenser, etc.

In the cooling system, there are actually two thermal cycles, one is a cycle for cooling the battery pack, and the other is a heating cycle for the battery pack, which requires a special heater. Both cycles are centered around the battery pack and use the same coolant.

direct cold

Direct cooling (refrigerant direct cooling) utilizes the principle of evaporation and heat absorption of refrigerants (R1 34a, etc.) to establish an air conditioning system in the vehicle or battery system, install the evaporator of the air conditioning system in the battery system, and install the refrigerant in the evaporator. It evaporates and quickly and efficiently takes away the heat of the battery system, thereby completing the cooling operation of the battery system, as shown in Figure 3.

At present, the cooling method of direct cooling is basically applied in electric passenger cars, the most typical example is the BMW i3 (i3 has two cooling schemes of liquid cooling and direct cooling).

The advantage of direct cooling is that the cooling efficiency is 3 to 4 times higher than that of liquid cooling, which can better meet the needs of fast charging, has a compact structure, potentially reduces costs, and avoids the flow of ethylene glycol solution inside the battery box.

Direct cooling is performed by an evaporator, which is in direct contact with the inner wall of the battery pack for heat exchange in the battery pack. These evaporators are composed of S-type refrigeration coils. The cold air of the compression mechanism flows through the pipes, and the temperature of the pipes is very low. Through these pipes, the temperature of the inner wall of the battery pack is indirectly reduced, and heat exchange is realized, which gradually reduces the temperature of the battery pack. When the air in the battery pack encounters the inner wall of the low temperature battery pack, frost will form on its surface (frost is a white crystal of water vapor, when the water vapor suddenly encounters a low temperature object, its surface will condense and frost), which is directly The root cause of frost on the cooling battery pack. The battery pack can be automatically heated and removed by the heating system after frosting.