Week 10 - BATTERY THERMAL MANAGEMENT SYSTEM(BTMS) ANALYSIS

AIM:

Perform the heat transfer analysis using ANSYS-Fluent software for the cylindrical 4 by 2 Lithium-ion Battery with heat generation boundary conditions and study the battery thermal management system(BTMS).

OBJECTIVES:

• Perform Mesh independent study for any one case(For velocity) and use the more suitable model for further possibilities.
• Simulation is performed by varying the velocity of air to 3, 5, and 10m/sec and discussing the results.
• Determining the maximum temperature and heat transfer coefficient obtained by the
  battery.
• Based on the results and your judgment, select one case (3, 5, or 10 m/s) and vary the space between the batteries as 2, 4, and 6 mm, then discuss the results.
• Create the animation to observe the temperature distribution and velocity vector.
• Summarize the results in tabulation/ graphical form. Identify potential hotspots on the model. State the inferences based on the results

THEORY OF BATTERY THERMAL MANAGEMENT SYSTEM(BTMS):

Battery thermal management system(BTMS) is a design approach of cooling technologies. Before studying BTMS we should know the application and main usage of battery management systems.

As we know conventional vehicles use petrol and diesel which indeed cause environmental pollution and energy scarcity which are the two major problems faced in the world. Considering the above situation we have come up with electric vehicles where the vehicle's main power source depends on the battery. The market of electric vehicles consists of different types of batteries such as lead-acid, nickel-cadmium, Lithium-ion, Plastic lithium, etc and their energy source are shown below.

By comparing to all other batteries Lithium-Ion battery has a good energy density of around 350Whr/L(in terms of size) or 200Whr/kg(in terms of weight).

PRINCIPLE OF LITHIUM-ION BATTERY

The construction of Lithium-ion batteries consists of the positive(cathode) and negative(anode) electrodes. Here the cathode consists of lithium-ion-based (LiCoO2) and the anode consists of graphite (carbon). A thin plastic membrane called a separator separates both the cathode and anode. The above entire setup is put into the electrolyte called lithium salt-based electrolyte. The working principle is that ions are exchanged between the cathode and anode during the charging or discharging electrons move in the external circuit. During charging elections move from cathode to anode similarly during discharging electrons move from anode to cathode through the external circuits. The battery comes in different shapes such as a cylinder, circle, etc.

Difference between the cell, module, and pack

CELL: A single-cell unit device that converts chemical energy to electrical energy.

MODULE: A module is a collection of cells connected in series or parallel. The application of modules is in two-wheeler vehicles.

PACK: A pack is a series of individual modules and protection systems organized in a shape that will be installed in the vehicles. For heavy vehicle applications, we use pack systems.

Types of connections in battery pack/module system

SERIES: Each battery has milliamp hours and volts as specifications, Here each battery has 4.2Volts and 3200mAh so most applications have 12V. Hence 4.2V is not sufficient in order to increase the voltage we connect the batteries in series(we can multiply the voltage hence we get 12V but I can discharge 3400mampH for an hour) which is a positive side to another end and a negative side to another end of the battery as shown in the above figure. Similarly in the PARALLEL connection if we want to increase the amp rating that is 3400mAh is a very small current rating in order to increase the current flow then we have to connect the batteries in parallel which is 3400mAh multiplied by 6 is 20400mAH with a max voltage of 4.2V.

So here I am considering a 4 by 2 Lithium-Ion battery where 2 batteries are connected in series and 2 batteries are connected in parallel.

C-rating is a unit to declares a current value that is used for estimating or designating the expected sufficient time of the battery under variable charge or discharge conditions. As the C-rating increases, the heat generation of the battery increases due to the flow of electrons from the charging or discharging of the battery.

THERMAL RUNAWAY

Thermal runaway is the condition in which the battery catches the fire due to rapid propagation from one damaged cell to another cell. The process of thermal is shown below.

Battery temperature varies due to internal heat generation during the charging and discharging process. The uneven temperature distribution causes temperature non-uniformity which leads to a reduction in the life cycle and performance. It is recommended that the maximum temperature difference inside the battery pack does not exceed 5 degrees. BTMS is necessary to maintain the battery temperature in the desired range and it plays a crucial role in the battery pack life and its overall performance. It is difficult to maintain 15-35 degrees unless we use water/air as the cooling medium we can maintain at least 40 degrees. It all depends on the design of the battery. Hence convection(newton's law of cooling) plays an important role in cooling the system. Here we are using forced convection for cooling the battery system and calculating the heat transfer coefficient.

TYPES OF COOLING LITHIUM_ION BATTERY

DETAILED PROCEDURE FOR PERFORMING THE ANALYSIS 4 BY 2 LITHIUM-ION BATTERY MODULE

GEOMETRY USING SPACECLAIM

• Initially, we draw the circles this is equally placed based on the spacing(2,6,8) provided but here we are considering 2mm and the diameter of the circle to be 18mm.

• I select all the circles once the sketching is done and use the pull option to extrude them into the cylinder with a length of 65mm as shown below.

• Create the enclosure around the battery module for performing the analysis.

• Finally use the share topology option for nodal information exchange and disable the preserve instance.

MESH

• We will sizing mesh option=> insert=>sizing methods(give respective mesh size) similarly for enclosure volume around the battery, we will sizing mesh option=> insert=>sizing methods(give respective mesh size).

Creating INFLATION LAYERS

• Before creating the inflation layers we have to do name selection which is for the inlet, outlet, convection wall, and battery wall.
• We are creating the inflation under the mesh options=> inflations=>select all the faces in chosen name selection option=>under named selection=>battery wall=> inflation option=>first layer thickness(0.3m)=>under number of layers(6)=>select update the mesh.

SETUP AND SOLUTION

• In this, we provide fundamental physics, types of solvers, boundary conditions, and the kind of turbulent model used.
• Initially, we go for the SOLVER option here we are the type of solver used is PRESSURE BASED SOLVER (M<0.3), and since the variable is not varying with respect to time select the STEADY STATE option.

• In the following step for the PHYSICS, the window selects the VISCOUS option we get the window opened up. Here we are selecting the type of viscous model, a TURBULENT model (k-omega standard). Since we have to calculate the temperature energy is turned on.

• Here for solids materials aluminum, Lithium this assigned under the solids sections there are properties are stated below.

ALUMINUM thermal conductivity(k=202.4 W/mK) specific heat capacity(cp=871 J/kg K) density(rho=2719 kg/m3)

LITHIUM thermal conductivity(k=7.14 W/mK) specific heat capacity(cp=900 J/kg K) density(rho=2700 kg/m3)

The heat generated per battery=10W

the volume of a cylinder is 1.65×10^-5 m3

the heat generated for battery =10/ 1.65×10^-5==>604594.9 W/m^3

• For the battery, I am assigning lithium in cell zone condition.
• For the convection wall for an enclosure, we are assigning the convection parameter with a heat transfer coefficient of 5W/mK
• In the cell, zone conditions check whether all materials are assigned appropriately. For heat generation under the battery, I give the heat source term 604594.9.
• We are giving the BOUNDARY conditions for the inlet velocity as 3m/s, 5m/s, 10m/s, and the default temperature as 300K.
• The hybrid method is used for initialization.
• Run this simulation based on solution convergence hence here this simulation converges.
• Finally, we are obtaining the graph heat transfer coefficient for the processor, temperature of the processor, and contour plots.

RESULTS

STUDY 1:

MESH INDEPENDENCE STUDY(STEADY STATE ANALYSIS)

MESH SIZE: 8mm external enclosure for a velocity of 3m/s

CONVERGENCE

HEAT TRANSFER COEFFICIENT PLOT

TEMPERATURE PLOT

The average temperature of the battery is 356K

The average heat transfer coefficient is -37.05W/m^2K

TEMPERATURE CONTOURS

MESH SIZE: 3mm external enclosure for a velocity of 3m/s

CONVERGENCE

TEMPERATURE PLOT

HEAT TRANSFER COEFFICIENT PLOT

The average temperature of the battery is 353K

The average heat transfer coefficient is -33.45W/m^2K

TEMPERATURE CONTOURS

STUDY 2:

SIMULATION OF LITHIUM-ION BATTERY WITHOUT COOLING

CONVERGENCE

TEMPERATURE PLOT

HEAT TRANSFER COEFFICIENT PLOT

The average temperature of the battery is 3048K

The average heat transfer coefficient is -0.8547W/m^2K

TEMPERATURE CONTOURS

STUDY 3:

SIMULATION FOR FORCED CONVECTION WITH VELOCITY OF 3, 5, and 10m/s(SPACE BETWEEN BATTERIES IS 2mm-STEADY STATE ANALYSIS)

CASE 1: For 3m/s

CONVERGENCE

TEMPERATURE PLOT

HEAT TRANSFER COEFFICIENT PLOT

The average temperature of the battery is 353K

The average heat transfer coefficient is -33.45W/m^2K

TEMPERATURE CONTOURS

CASE 2: For 5m/s

CONVERGENCE

HEAT TRANSFER COEFFICIENT PLOT

TEMPERATURE PLOT

The average temperature of the battery is 336.23K

The average heat transfer coefficient is -51.25W/m^2K

TEMPERATURE CONTOURS

CASE 3: For 10m/s

CONVERGENCE

HEAT TRANSFER COEFFICIENT PLOT

TEMPERATURE PLOT

The average temperature of the battery is 324K

The average heat transfer coefficient is -68.37W/m^2K

TEMPERATURE CONTOURS

From the above case studies with space between batteries for 2mm, we have observed the forced convection for different velocities and effective cooling is observed at 10m/s.

Now let's study for varying space between the batteries that is 4mm, and 6mm by considering the velocity of 10m/s. (Since I have already analyzed for 2mm spacing let's analyze for 4mm and 6mm)

STUDY 4:

VARYING THE SPACE BETWEEN THE LITHIUM-ION BATTERIES(STEADY STATE ANALYSIS)

CASE 1: The Space between the battery is 4mm with a forced convection velocity of 10m/s

CONVERGENCE

TEMPERATURE PLOT

HEAT TRANSFER COEFFICIENT PLOT

The average temperature of the battery is 320.765K

The average heat transfer coefficient is -74.618W/m^2K

TEMPERATURE CONTOURS

CASE 2: The space between the battery is 6mm with a forced convection velocity of 10m/s

CONVERGENCE

TEMPERATURE PLOT

HEAT TRANSFER COEFFICIENT PLOT

The average temperature of the battery is 320K

The average heat transfer coefficient is -74.343W/m^2K

TEMPERATURE CONTOURS

SIMULATION FOR FORCED CONVECTION WITH VELOCITY OF 3, 5, and 10m/s(SPACE BETWEEN BATTERIES IS 2mm-STEADY STATE ANALYSIS)

SERIAL NO	VELOCITY(m/s)	HEAT TRANSFER COEFFICIENT(W/m^2K)	TEMPERATURE(K)
1	3	33.45	353
2	5	51.25	336
3	10	68.37	324

VARYING THE SPACE BETWEEN THE LITHIUM-ION BATTERIES(STEADY STATE ANALYSIS) WITH VELOCITY=10m/s

SERIAL NO	SPACING(mm)	HEAT TRANSFER COEFFICIENT(W/m^2K)	TEMPERATURE(K)
1	2	68.37	324
2	4	74.618	320
3	6	74.343	320

CONCLUSIONS:

• In mesh independent study we can infer that in order to get accurate results we have to reduce the mesh element size as we see from the above study case from 8mm to 3mm. But here there is not much difference in the results hence we can go with the optimum mesh size(3mm- more the mesh elements computational time is more).
• When the Lithium-ion battery with 10W without cooling was performed we can see the temperature rising up to 381K. Hence effective cooling is required for preventing thermal runaway.
• For Lithium-ion battery 2mm space between the batteries, and effective force convection cooling was performed with velocities of 3, 5, and 10m/s. For 10m/s cooling is more effective compared to other velocities with a temperature of 324K.
• From the above inference, I have considered 10m/s to be the best cooling hence we going to study by varying the space between the batteries 2, 4, and 6mm. Hence effective cooling was observed in all the cases but there are red hotspots at the last end of two batteries in 2 midsections as shown in the figure(high temperature) for the 4mm and 6mm cases. Hence 2mm spacing is optimum for lithium-ion batteries.
• Lastly, I would like to conclude for Lithium-ion batteries 2mm space between the batteries with 10m/s forced convection cooling is an effective way of cooling the 4*2 battery module.