Week 10 Thermal Management
Prepare a report summarizing thermal management techniques for electric car battery pack.
ANS:
Battery Thermal Management Systems (BTMS):
The battery pack for safety, performance (both power and capacity) and lifespan reasons should be stored in a controlled surrounding where the temperature is controlled and there is no risk of thermal runaway. According to a foundational research (Pesaran, 2001), 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.
In the following sections, different technologies of heating and cooling will be introduced in detail and their possibility and ease of ventilation will be evaluated. Insulation affects mainly stationary conditions rather than driving conditions, so it will not be further discussed in this thesis.
Technologies of BTMS:
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. (Valeo, 2010) 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.
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 in order to recovery 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,
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. 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 3.3 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.
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. The systematic layout is in Figure .
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 3.6 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.
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 will be discussed in Section 3.2.6.
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 3.7. 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. (Valeo, 2010) It’s easy to switch between cooling and heating operation. To achieve that, the poles of
electrodes need to be reversed.
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 3.8. 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.
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). According to the conclusion of an experiment (Tran, 2014), 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.
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 3.10.
Liquid System:
There are three types of cooling systems, namely passive cooling system, active cooling system, and refrigerant cooling system. The passive cooling system is affected by the ambient temperature, because the heat dissipation is dependent on the radiator and the radiator dissipates heat through the temperature difference between liquid and the ambient temperature. Under normal circumstances, it works well, but under high ambient temperature it is insufficient. Active cooling systems have good thermal performance which can keep the battery pack within the operating temperature and keep temperature distribution between cells even because of high heat co-efficient of the coolant. Due to many auxiliaries and moving parts, the structure is complicated and difficult to maintain. It also has the tendency of leaking out. Compared to active cooling systems, direct refrigerant cooling systems are more
efficient because they use refrigerant directly to cool the system instead of using refrigerant to cool coolant first and then using coolant to cool the system. The weaknesses of refrigerant cooling system are complicated structure and difficult maintenance, as well as potential to leak out and so on.
PCM System:
PCM cooling systems perform well on thermal management. Even under high ambient temperatures from 45°C to 50°C, the temperature inside the cell pack is always below 55°C because of high thermal conductivity and latent heat. (Sabbah, et al., 2008) Table 3.2 shows the characteristics of PCMs. In the case of stressful situations e.g. at current discharge rate 6.67C-rate and ambient temperatures of 45°C, the cell maximum temperature difference doesn’t exceed 0.5K. And at normal conditions the difference of temperature between cells is negligible. (Sabbah, et al., 2008) If one cell fails, thermal runaway will not propagate because the PCM-graphite matrix absorbs and
spreads the heat quickly. (Kizilel, et al., 2009) The weakness of PCM is that it is more suitable for cold conditions or in space. When the temperature of the battery pack is higher than the temperature of the PCM melting CHALMERS, Applied Mechanics, Master’s Thesis 2014:42 21 point, heat is stored as latent heat and will be released to the module of battery when the ambient temperature is lower than the temperature of the PCM melting point. The structure of PCM without new components is simple, light and space-saving. But it still has the potential of flammability and electrical conductivity as well as volume change due to the characteristic of the PCM-graphite.
Table 3.2 The thermal characteristics of PCM.
TESLA:
The Tesla models S battery cooling system consists of a patented serpentine cooling pipe that winds through the battery pack and carries a flow of water-glycol coolant, thermal contact with the cells is through their sides by thermal transfer material. Again this will remove heat from the side of the cells rather than from the tabs, and overheating a Tesla battery pack under hard driving is easy to do.
AUDI:
The Audi battery module design probably comes the closest to directional tab cooling due to the orientation of the battery cells. For example, on the A3 e-tron, there are 4 heat exchangers which are interleaved within the battery pack, but the heat exchangers appear to be deliberately positioned at the base end of the cells away from the tabs.
