Week 10 Thermal Management
Aim : To study about Thermal Management Techniques for Electric Car Battery Pack
Explanation :
The battery thermal management system (BTMS) plays a vital role in the control of the battery thermal behaviour. These systems are analysed through a trade-off between performance, weight, size, cost, reliability, safety and energy consumption. According to the analysis two prime battery thermal management systems are recommended: combined liquid system (CLS) and a variant system with PCM.
Unlike conventional fuel, battery cells as an energy source have stricter requirement on working environment. They are especially sensitive to temperature. To ensure a proper thermal working environment, a Battery Thermal Management System (BTMS) will normally be integrated with battery cells. Thus, knowledge about the proper working requirements of battery is vital, and what kind of management systems can sufficiently and efficiently meet these requirements. With this cornerstone, the performance and durability of battery pack can be maximized in an electric vehicle.
The main purpose of this master thesis is to develop a BTMS model for balancing the different cooling and heating circuits within the battery pack to fulfil the performance requirements.
BTMS should be equipped with four essential functions to ensure the right operation conditions of the battery pack:
1) Cooling
Due to inefficiency, battery cells will not only generate electricity but also heat. This heat should be moved from the battery pack when battery temperature reaches the optimum temperature or even in advance. Thus, a cooling function is required in BTMS.
2. Heating
In cold climates, battery pack temperature probably falls below the lower temperature limit. Hence, a heating function, such as PTC heater, is required to assist the battery pack to reach the proper temperature range in a shorter time.
3. Insulation
In extreme cold or hot weather, the temperature difference between the inside and outside of the battery pack is much larger than that in mild weather. Battery temperature will thus fall (cold) or rise (hot) sooner out of the proper temperature range. To prevent this, good insulation can slow down the falling or rising of battery temperature, especially when the vehicle is parked outdoors.
4. Ventilation
Ventilation is required to exhaust the hazardous gases within battery pack. In some systems, such as air systems, this function is combined with cooling and heating functions.
Different technologies of heating and cooling :
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.
Fig.1.1.
Because in both cases the air is supplied by a blower, they are also called forced air systems.
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.
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 1.2. 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.1.2.
Figure 1.3. 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 rimary 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.1.3.
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.1.4.
4) PCM ( Phase change material )
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 1.5. 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.1.5.
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. The other is a heat pipe.
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 1.6. a 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.1.6.
6) Heat Pipe
The flat copper envelope of the heat pipe was under partial vacuum. The capillary structure is made of sinteredbcopper 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.1.7.
7) PTC Heater
PTC thermistors have many self-heated applications by utilizing their own voltage current 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.
Fig.1.8.