To Study the Thermal Management of Battery.

AIM : To Study the Thermal Management of Battery.

OBJECTIVES :

1. Prepare a report summarizing thermal management techniques for electric car battery pack (not more than 500 words).

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

   (Hint: Table based battery, Temperature source)

3. Compare the resultants graph for different areas of contact of heat.

STUDY - 1 :

Preparing a report summarizing thermal management techniques for electric car battery Packs:

Battery Thermal Management Systems (BTMS) :

Electric vehicles (EV) develop fast and have become popular due to their zero-emission and high tank-to-wheels efficiency. However, some factors limit the development of the electric vehicle, especially performance, cost, lifetime and safety of the battery. Therefore, the management of batteries is necessary in order to reach the maximum performance when operating at various conditions.

The performance of an EV is highly dependant on the battery pack, and one of the most important parameters affecting the performance of the battery (and hence the vehicle) is the operating temperature of the cells and the battery pack. All batteries will eventually age and experience degradation in performance, but without adequate thermal management, it won't be possible to maximize the life cycle and performance of the battery pack.

Operating outside the limits specified, be that charge and discharge rates, or allowable temperatures at which the pack can function adequately, will accelerate the degradation in performance and efficiency of the EV.

Systems Functions -

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 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.

The Importance of a Cooling System :

While advancements have been made in electric vehicles batteries that allow them to deliver more power and require less frequent changes, one of the biggest challenges that remain for battery safety is the ability to design an effective cooling system.

Batteries work based on the principle of voltage differential and at high temperature, the electron inside becomes excited which deserves the difference in voltage between the two sides of the battery. Because batteries are only manufactured to work between certain temperature extremes, they will stop working if there is no cooling system to keep it in working range.

If there is a larger internal temperature difference it can lead to different charge and discharge rates for each cell and deteriorate the battery pack performance. potential thermal stability issues such as capacity degradation, thermal runaway, and fire explosion could occur if the battery overheats or if there is nonuniform temperature distribution in the battery pack. In the face of life-threatening safety issues, innovation is continually happening in the electric vehicle industry to improve the battery cooling system.

Different methods are used for Cooling the Battery : -

Direct Method -

The battery is submerged directly into the coolant. Requires coolant to be low to no conductivity to maintain vehicle safety. Not yet commercialized for electric vehicle use due to continuous R&D.

Indirect Method -

Coolant circulates through a system of pipes similar to the cooling system currently found in the internal combustion engine. Required a high heat capacity coolant with corrosion inhibitors.

On the basis of which type of Material is used for Cooling there are the following Types : -

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.

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.

The figure below shows Forced-air System with Heat Recovery -

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.

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 below 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.

Figure below 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 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 a battery pack. The systematic layout is in the Figure below.

Phase Change Material (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 below 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 -

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 of Thermoelectric cooling/heating system is presented in Figure below. 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.

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 below. 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.

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 below shows Resistance versus Temperature for a PTC.

STUDY - 2 :

Creating a simple simscape model with a battery and showing the temperature dependence in the performance. Also, connecting a resistor load of 10 Ohm :

Increasing Temperature Afecrt Battery Performance -

When temperatures increases, 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. However, there is a side effect to this increased performance - the lifecycle of the battery is decreased over time.

So, 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.

Simscape Model of Battery (Table Based) and its Temperature-Dependent Performance :

• A battery is discharging a fixed amount of current whilst being heated by a temperature source.
• Max battery capacity is 4.2Ah and 2.1A (0.5C rate) of current is being discharged for 2 hours. Here we give a 4.2 PS Constant for all the models which is a 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 1e-4 m².

Battery (Table-Based) Parameters :

• Apparently, first, we need to set up the battery model with its features. It's a table-based model so it has a lot of different options. as you can see from the above Figure.
• The surface of the battery has a default setup m^2.

Output Scope :

• Here we see that the battery SOC was reduced to 50% of the total value and the temperature 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 operate, etc.

Simscape Model of Battery (Table Based) and its Temperature-Dependent Performance with Load Resistor :

• We added the load resistor of 10 ohms parallel with a battery in the setup.

Resistor Parameters :

• Here we see the internal values of the resistor block with a set resistance of 10ohm.

Output Scope :

• Identical results of the above model there is no change in battery temperature or SOC with the attachment of a 10ohm resistor.
• This is probably because 10ohm is too low of a resistance to extract enough power from the circuit to affect battery parameters or performance.
• For a 10ohm resistor to make an impact, the battery would have to be made less powerful - lower capacity and lower voltage.

STUDY - 3 :

Comparing the Resultants Graph for Different Area of Contact of Heat :

Simscape Model of Battery (Table Based) and its Temperature-Dependent Performance with Different Area of Contact of Heat :

• The surface area of the battery in contact with the heating fluid is changed to 1m².

Output Scope :

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

Simscape Model of Battery (Table Based) and its Temperature-Dependent Performance with Different Area of Contact of Heat :

• The surface area of the battery in contact with the heating fluid is changed to 0.1m².

Output Scope :

• Since the contact area is very low the temperature of the battery reduces up to 296k from their max value of 298k and even the SOC also reduces slowly.

Simscape Model of Battery (Table Based) and its Temperature-Dependent Performance with Different Area of Contact of Heat :

• The surface area of the battery in contact with the heating fluid is changed to 10m².

Output Scope :

• Since the contact area is low the temperature of the battery reduces up to 293.2k from their max value of 298k and even the SOC also reduces slowly.

CONCLUSION :

• Successfully Prepared a report summarizing thermal management techniques 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 resultants graph for different areas of contact of heat.

REFERENCES :

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