Project 2 Thermal modeling of battery pack

Aim:

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

Objective:

Create a simple simscape model with a battery and show the temperature dependence in the performance. Also connect a resistor load of 10 Ohm.

Compare the resultants graph for different area of contact of heat.

Battery Thermal Management Systems (BTMS):

The BTMS´s (Battery Thermal Management System) objective is to prevent accelerated battery deterioration by managing the heat generated by its components so that it operates continuously under optimum temperature conditions.

Although existing commercially available cells can operate safely between -40 and 60 ºC, the operating range preferred by manufacturers to maximize their performance is indeed between 15 and 35 ºC. In this sense, it is also recommended that within the battery-pack there should not be a difference of more than 5 ºC between the cells.

It should be noted that exposing the battery to extreme conditions can have fatal consequences. For example, its operation at very high temperatures (> 80 ºC) can cause the well-known thermal runaway, resulting in fire and, in the worst scenario, the explosion of the battery with the consequent personal safety implications.

The BTMS is the battery-pack component responsible for ensuring that the cells operate under the optimum temperature conditions specified by the manufacturer.

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. 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.

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.

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.

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.

2) Create a simple simscape model with a battery and show the temparature dependance in the performance.Also connect a resistor load of 10 Ohm.

When temperatures increase this affects the chemical reactions that occur inside a battery. As the temperature of the battery increases the chemical reactions inside the battery also quicken. At higher temperatures one of the effects on lithium-ion batteries’ is greater performance and increased storage capacity of the battery. A study by Scientific Reports found that an increase in temperature from 77 degrees Fahrenheit to 113 degrees Fahrenheit led to a 20% increase in maximum storage capacity. However there is a side effect to this increased performance, the lifecycle of the battery is decreased over time. In that same study, it was found that when the battery is charged at 113 degrees versus 77 the lifecycle degradation was much more significant at the higher temperature. For the first 200 cycles the battery performance only degraded 3.3% at 77 degrees; at 113 degrees the performance decreased by 6.7%.

That’s more than double the amount of degradation! Based on the greater degradation at higher temperatures, the battery lifecycle can be severely diminished due to consistent exposure to extreme heat. While heat exposure does temporarily increase battery capacity the damage that it does to the lifecycle can cause long term problems and prolonged heat exposure should be avoided.

• Battery is discharging a fixed amount of current whilst being heated by a temperature source.
• Max battery capacity is 4.2Ah and 2.1 of current is being discharged for 2hrs. Here we give a 4.2 PS constant for the model which is current Rate of 4.2Ah current Rate.
• The discharge current remains the same whilst there is a 10-degree increment in temperature every 10 minutes.
• The surface area of the battery in contact with the heating fluid is le-4m^2.

Battery Parameters:

• Here in variable we have to check the box of SOC and Temperature and check weather the temperature is in Kelvin and can set the temperature 298.15K. The priority should be set to high.

Output:

• Here we see the battery SOC was reduced ot 50% of the total value and thetemperature of the battery pack reached 298.163k.
• Beyond a certain point, however, an increase in temperature can lead to certain irreversible reactions such as the disintegration of the electrolyte and even thermal runaway if there is excess heat inside the cell.
• This is a very basic battery model of a general cell and is not fit to analyze the temperature sensitivity of a lithium-ion cell, beyond what temperature would the cell be unsafe to perate, etc.

Phase 2:

Comparing the Resultant Graph for Different Area of contact of Heat:

• The surface area of the battery in contact with the heating fluid is changed to 1m^2.

Output:

Since the contact area is low the temperature of the battery reduce up to 293.8K from their max value 298k and the even the Soc also reduce slowly.

Phase 3:

Simscape Model of battery and it’s temperature performance with Different Area of contact of Heat:

• The surface area of the battery in contact with the heating fluid is changed to 10m^2.

Output:

• Since the contact area is low the temperature of the battery reduce up to 293.8K from their max value 298k and the even the Soc also reduce slowly.

Conclusion:

• Successfully Prepared the report summarizing the thermal management techinques for the electric car battery pack.
• Also , created a simple simscape model with a battery and show the temperature dependence in the performance.
• Finally, compared the resultant graph for different areas of contact of heat.

References:

https://publications.lib.chalmers.se/records/fulltext/200046/200046.pdf