Week 10 Thermal Management

Prepare a report summarizing thermal management techniques for electric car battery pack.

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.

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.

Conclusion

Generally, the two models for the two proposed systems have been built up fulfilling almost all the requirements asked for in the beginning. Numerical results with figures have been generated under different weather, control and driving conditions. The electric energy consumption predicted makes sense and had the correct tendency.
Moreover, the modelling showed also its flexibility to fit new systems with a few modification.

CLS model results have indicated how the driving behaviour, battery initial temperature and ambient temperature influenced the BTMS and the corresponding electricity consumption. From the results it was seen that different driving cycles would change the BTMS feature significant in hot weather but less significantly in cold weather. The
ambient temperature was the most important variable for the BTMS. The battery initial temperature representing pre-conditioning treatment changed the energy consumption significantly, especially in extreme hot and cold weather.

The PCM model showed that the electric range of EV was sensitive to the vehicle mass. PCM could bring significant improvement of BTMS, but the mass of PCM was difficult to choose. For this specific case, 10 kg of PCM was an optimal point. However, further investigation is still required to figure out the influence in detail.

Future Prospects

Though a Simulink model has been built up, a lot more work could be done in the topic of battery thermal management system. Firstly, this investigation as the first study focused more on the essential issues, such as system requirements and modelling functions. Thus, only a simplified model has been completed. This model has low accuracy and lacks validation with laboratory data. Another limitation of this thesis is the connection and coupling with other models. Currently this model can only process static data from file. The interface of the model should be therefore further developed to interact with other models of battery pack or vehicle components. Last but not least, optimisation of the thermal management system can also be achieved by pre- and postconditions, different control strategies, customer behaviour and other factors.

Accuracy improvement

In the Simulink model only main heat sources and heat sinks are included for calculation. The following factor may also have an effect:

• Heat transfer through the insulation material of battery pack case
• Heat dissipation from electric inverter, electric control unit and water pump

Also, many components could be improved to achieve a better prediction of energy balancing and energy consumption:

• Use CFD Simulation tools to predict the heat transfer condition of radiator,
  chiller and battery pack. The heat transfer between thermal fluid and battery
  cells should be thoroughly investigated.
  
  
• The pressure drop of each component and pipe should be added together and then the calculations of water pump power will be more accurate.
  
  
• Dynamic performance of heater, chiller and radiator could be introduced instead of the current static performance in order to improve the prediction, especially during start-up period and sudden power changes.
  
  
• Laboratory tests of the same BTMS system could be carried out. A comparison between modelling result and experimental data could further correct the error of the modelling.

Functionality Development

There are several points which could be extended in the future development:

• One of the high-priorities is to connect this Simulink model with available thermodynamic simulation tools such as GT-SUITE or StateFlow in order to achieve more accurate heat-transfer calculation.
  
  
• Also, a direct connection with the battery electro-thermal model is also needed to output the power of BTMS and to obtain the dynamic data of battery heat generation rate instead of static data.
  
  
• Another important part of BTMS is to monitor the battery thermal health in order to prevent battery from thermal issues and damage. It is important to send a warning in risk situations and to send an emergency stop signal when the system is in danger.

Alternative work

Beside the two topics above, the following points are also very interesting:

• Pre- and post-conditioning
  The most interesting potential energy-saving that should be further investigated is the battery behaviour before and after driving and the corresponding thermal treatment i.e. pre- and post-condition, which should be implemented by BTMS. The different control strategy can be developed according to different conditions. Both heating and cooling energy consumption will change and could be saved.
  
  
• Control strategy of BTMS for charging cycle
  During charging, the battery pack will also generate a large amount of heat requiring even more strict thermal demands. To develop a proper control strategy of BTMS for charging is highly recommended to ensure both safety and high charging rate.
  
  
• Control strategy optimization
  This thesis has generally analysed the increasing energy consumption with decreasing the desired temperature of the battery, which is charged by control unit. But there are some other factors affecting on the energy consumption of BTMS, such as the switching differential of control relays for radiator and air conditioner.
  
  
• Measurement, analysis and improvement of battery temperature. Each point on battery simultaneously had the same temperature, but in reality battery cells have a different temperature distribution. To measure this is interesting. It is also interesting to reduce the unevenness by re-design of the BTMS.

• BTMS simulation with customised conditions
  Evaluation of BTMS energy consumption is related not only to one given condition but on a combination of different situations. For example, both cold winter and hot summer conditions should be included for a comprehensive evaluation. It is reasonable to develop such simulation conditions with varying parameters, such as the whole year ambient temperatures in a city, charging behaviour, and BTMS ageing. This overall evaluation would present a more practical performance of BTMS