Battery Thermal Management System for an e-Vehicle

Battery Thermal Management System

1.1. System Functions

The battery thermal management system should be equipped with four essential functions:

• Cooling: Due to inefficiency, battery cells will not only generate electricity but also heat. This heat should be rejected form the battery pack to ensure the safety of the battery pack, thus, a cooling function is required.
• Heating: As set up that the battery does not perform up to its full potential in very harsh cold weather, hence a heater such as a PTC heater, is needed to help the battery pack to maintain the optimum temperature conditions.
• Insulation: To protect the battery from the influence of the ambient conditions, a proper insulation of the battery from the external conditions is needed, else the battery will be affected by the extreme temperature conditions. To prevent this a good insulation is needed.
• Ventilation: Ventilation is required to remove the hazardous gases from the battery pack.

1.2 Methods of BTMS

1.2.1. Air Cooling and Heating

Air systems use air as the thermal medium. The intake air could be direct either from atmosphere or from the cabin and could also be conditioned air after a heater or evaporator of an air conditioner. The former is called passive air system and the latter is active air system. Active systems can offer additional cooling or heating power. A passive system can offer some hundreds of watts cooling or heating power and an active system power is limited to 1 kW. Because in both cases the air is supplied by a blower, they are also called forced air systems. The following figure shows a schematic description of systems.

Fig. 1. Forced air systems

Note that the air system offers full functions of heating, cooling, and ventilation. There is no need to build an additional ventilator, but it must be noted that the exhaust air cannot be returned to the cabin again. In some cases, a heat recovery unit (air-air heat exchanger) is mounted after the battery pack to recover the heat from the exhaust air. It can prevent mixture of exhaust air with intake air and at the same time provide an extra saving potential. The forced air system with heat recovery is presented below.

Fig. 2. forced air system with heat recovery

1.2.2. Liquid Cooling and Heating

Besides air, liquid is another heat transfer fluid to transfer heat. There are generally two groups of liquids applied for thermal management systems. One is dielectric liquid (direct-contact liquid) which can contact the battery cells directly, such as mineral oil. The other is conducting liquid (indirect-contact liquid) which can only contact the battery cells indirectly, such as a mixture of ethylene glycol and water. Depending on the different liquids, different layouts are designed. For direct-contact liquid, the normal layout is to submerge modules in mineral oil. For indirect-contact liquid, a possible layout can be either a jacket around the battery module, discrete tubing around each module, placing the battery modules on cooling/heating plate or combining the battery module with cooling/heating fins and plates. Between these two groups, indirect contact systems are preferred in order to achieve better isolation between battery module and surroundings and thus better safety performance. By different heat-sinks for cooling, liquid systems can also be categorized into either passive systems or active systems. In passive liquid system, the heat-sink for cooling is a radiator. This system has no ability to heat. Figure 3 presents the systematic scheme of a passive liquid system. Heat transfer fluid is circulated by the pump within a closed system. The circulating fluid absorbs heat from battery pack and releases heat via a radiator. The cooling power depends strongly on the temperature between ambient air and battery. Fans behind the radiator can improve the cooling performance, but if ambient air is higher than the battery temperature or the difference between them is too small, the passive liquid system becomes ineffective.

Fig. 3. Passive Liquid cooling system

Figure 4 shows the systematic scheme of an active liquid system. There are two loops. The upper is called the primary loop and the lower the secondary loop. The primary loop is similar to the loop in a passive liquid system, where the heat transfer fluid is circulated by pump. The secondary loop is actually an air conditioning loop (A/C loop). The upper heat exchanger instead of being a radiator works as an evaporator (EVAP) for cooling operation and connects both loops. During heating operation, the 4 way valve will be switched, and the upper heat exchanger works as a condenser (COND) and the lower heat exchanger works as an evaporator. The heating operation loop is also called heat pump loop.

Fig. 4. Active Liquid cooling system

1.2.3. Direct Refrigerant Cooling and Heating

Similar to active liquid systems, a direct refrigerant system (DRS) consists of an A/C loop, but DRS uses refrigerant directly as heat transfer fluid circulating through battery pack.

Fig. 5. Direct refrigerant cooling system

1.2.4. PCM

During melting, heat is absorbed by PCM and is stored as latent heat until the latent heat is up to the maximum. The temperature is kept at melting point for a period and the temperature increase is delayed. Therefore, PCM is used as conductor and buffer in battery thermal management systems. Figure 6 shows the working mechanism of PCM on battery cells. Also, a PCM is always combined with air cooling system or liquid cooling system to manage the battery temperature.

Fig. 6. The working mechanism of PCM on battery cells

1.2.5 Thermo-electric Module

To improve cooling/heating power of passive air systems, there are two possible upgrades. One is through thermo-electric modules, which will be introduced here. Thermo-electric module can convert electric voltage to temperature difference and viceversa. Here the former effect is adopted. That means it transfers heat through the module by consuming electricity directly. The schematic structure is presented in Figure 7. Two fans are installed to improve heat transfer by forced convection. To combine a passive air system with thermo-electric module, the combined system is able to cool down the battery even lower than the intake air temperature, but the power is still limited to around some hundreds of watts and less than one kW. It’s easy to switch between cooling and heating operation. To achieve that, the poles of electrodes need to be reversed.

Fig. 7. Thermoelectric cooling/heating system

1.2.6 Heat Pipe

Besides thermo-electric modules, a heat pipe is another way to upgrade passive air systems. The structure of a heat pipe is shown in Figure 8. The flat copper envelope of the heat pipe was under partial vacuum. The capillary structure is made of sintered copper powder. The heat pipe uses water as the working fluid. Water on the evaporator side will absorb heat and become vapour lower 100°C due to low pressure inside. Water on the condenser will dissipate heat to the surrounding and become liquid again. This cycle repeats again and again.

Fig. 8. Structure of the heat pipe

The following figure shows the schematic description of a heat pipe cooling system. A battery as heat source sits below the heat pipe (on the evaporating side). Cooling fins as heat sinks are on the heat pipe (on the condensing side). Heat pipe cooling system can reduce the thermal resistance by 30% under natural convection as compared to without heat pipe . A thermal resistance reduction of 20% under low air velocity convection is possible. Note that, in comparsion to thermo-electric, a heat pipe is more reliable, because there are no moving parts and no energy consumption. However, a heat pipe is unable to heat the battery due to its fixed structural layout.


Fig. 9. Scheme of heat pipe cooling system

1.2.7 PTC Heater

PTC thermistors have many self-heated applications by utilizing their own voltagecurrent or current-time characteristics. One of the applications is as a self-regulating heater known as a PTC heater. The temperature of a PTC heater can be kept at a fixed point through adjusting the resistance of PTC heater automatically, see Figure 10.

Fig.10. Resistance versus Temperature for a PTC