Week 10 Thermal Management
Problem Statement - Prepare a report summarizing thermal management techniques for electric car battery pack.
Solution -
Lithium-ion cells performance depends on the temperature. Lithium-Ion cells work well when cells operate within limited voltage and temperature. Otherwise, damage will occur to the cells and will be irreversible. The battery temperature should be controlled carefully. Both excess heat and lack of heat will bring about problems.
Chemical reaction rates have a linear relation to temperature. The decrease of the operating temperature will reduce reaction rate and the capacity of carrying current during charging or discharging. In other words, the battery power capacity is decreased. Moreover, the reduction of reaction rate makes it harder to insert lithium ions into intercalation spaces. The result is the reduction of power and lithium plating causing the capacity loss. High temperature increases the reaction rate with higher power output, however, it also increases the heat dissipation and generates even higher temperatures. Unless heat is dissipated quicker than heat is generated, the temperature will be higher and finally a thermal runaway will result. The 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.
Technologies of BTMS -
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. 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. 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. (Pesaran, 2001) 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.
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.
4. Phase Changing material PCM - 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. Also, a PCM is always combined with air cooling system or liquid cooling system to manage the battery temperature.
5. Thermo-electric Module - 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. 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. It’s easy to switch between cooling and heating operation. To achieve that, the poles of electrodes need to be reversed.
6. Heat Pipe - 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.