Project 2 Thermal modeling of battery pack

                                                                                                                      Thermal Modelling of a Battery Pack

Aim –

1. To model a 10-cell series lithium-ion battery pack and simulate its thermal effects
2. To simulate and compare the performance of the cell at different temperature
3. To observe its charging and discharging rate

Model-

 To improve the performance of the EV vehicle, proper thermal management is necessary. During cruising the battery pack tends to get charge and discharge frequently

during this the battery tends to heat up, this could lead to a serious performance issue and can let to break down of vehicle or affect the life span of the entire battery pack.

Hence to eliminate such a problem, thermal management is necessary i.e. the battery pack should be cooled to maintain the efficiency of the vehicle.

There are different types of cooling method including Phase Change material, Fin Cooling, Air Cooling, and Liquid Cooling In this system we modeled a liquid cooling system using Matlab and Simulink, the liquid cooling system provides better cooling than other methods but also has some disadvantage of glycol leak in case of loose fixtures or vibration in the vehicles hence precise connections of tubing are necessary. The model consists of a battery pack, a cooling chamber, Reservoir, Radiator, Refrigeration, and a pump.

1.   Battery Pack –

A Battery Pack is a set of cells that are connected in series or parallel these are identical cells connected with each other forming a pack. In this model, there are 10 no cells connected in series altogether forming a battery pack.

Each cell is rated at 12V at 40Ahr, this pack is driven by a controlled current source whose output is the signal builder block. The battery used in the Matlab is a tubulated based type instrumented battery whose thermal port is enabled.

The instrumented type battery enables the SOC(state of charge ) port by which we can estimate the charge. The port H of the battery represents the thermal mass which connects to the heat flow rate sensor due to which we can monitor the heat flow rate of the battery.

A conductive heat transfer block is added to guide the heat flow to the cooling channel, where H1 and H1’ is the cooling channel. The heat is carried away from the battery through the channel a temperature sensor measures the difference across the channel giving output as pack temperature.

1. Cooling Chamber-

The cooling chamber consists of a TL (Thermal Liquid ) pipe and a constant volume chamber the initial liquid temperature is set to 40⁰C. The cooled liquid comes through the inlet to the constant volume chamber where A and B are the thermal conserving ports the heat enters through the conductive heat flow from H1 as a conserving port associated with the pipe wall.

The liquid then flows to the next chamber to the other pipe where H1’ is the next conserving port where the majority of the heat is transferred through the pipe wall, the hot liquid is then carried to the next chamber as an outlet.

1. Reservoir

The reservoir is basically a thermal liquid tank where the liquid coolant is stored the port B is where the hot liquid enters, the excess heat is released via a convective heat transfer, and then it is extracted through port A as an inlet to the cooling system. The tank here is 5 liters with atmospheric pressurization.

1. Cooling System

The cooling system is the most important part of this entire battery modeling system it is a process where the thermal liquid is cooled and provides cooling to the battery pack. The system consists of a refrigeration system a radiator and a temperature sensor block, the hot liquid flows through the port B into the TL pipe from the pipe it enters into the valve which is basically two sets of variable area orifices i.e. it opens and closes the valve depending upon the command given to it, the command is set by the physical port S. Hence depending upon the temperature of the liquid the command is given to the valve for positive and negative displacement and further the TL is cooled via a radiator and a refrigeration system.

1. Radiator

The radiator or heat exchanger works by passing the TL through the thin metal fins and allows the heat to flow, in this system, we have installed a fan to blow out the excess heat. The heat exchanger consists of a large metal frame with small metal fins that allows to vent heat to air surrounding where A2 is where the hot liquid is passed through the fins and A1, and B1 is where the air is bypassed through to cool the fins. The port B2 is where the cold liquid is passed.

1. Refrigeration System

This is another cooling system linked with the heat exchanger, if the coolant temperature increase beyond its limit then the refrigeration system activates by cooling the TL near the freezing point. The system maintains the constant cooling temperature sense by the mass and energy flow sensor where T is the output temperature from the refrigerant to the controlled temperature source which maintains the controlled temperature set by the refrigerant. The cold liquid flows to the outlet pipe, A heat flow rate sensor is added to measure the mass across and H is a thermal conserving port unit in Watts which is then converted to KW to measure the refrigeration power. Both the output from the radiator and refrigeration system are connected to a common constant volume chamber.

Port A from the CVC is the output port which is cooled liquid and is the inlet to the pump, a temperature sensor is connected within the port B which measure the temperature of the hot liquid entering the cooling system. The temperature is sensed by the TL pressure and temperature sensor which then reports to the controller i.e. is PID controller. The main principle of the controller is to sense the temperature and regulate the positive and negative displacement of the valve. For example, if the temperature reaches 30 or equal to 30 a positive error is generated which may lead to the positive displacement of the valve. For instance, the initial temperature is set to 40⁰C.

1. Pump

The thermal liquid pump is used to pump the liquid from the cooling system to the chamber, the port A is where the liquid is pushed to the TL pipes in the chamber. The port R is the shaft of the motor which is driven by the velocity source, this source is a physical signal which accepts input in rpm. The input is the speed of the vehicle obtains form the drive cycle, the drive cycle here is an FTP 75. The pump is set to 3500rpm and has a mass flow rate of 10kg/s.

1. Result and Conclusion

The model was run and simulated for T = 2474 sec

Result 1

The following scope shows the result of the pack temperature and refrigeration power we observe that the initial pack temperature was about 55⁰C and rises steadily around 63⁰C at 200sec but as refrigeration power increase from 24 to 38Kw  we can see a fall in the temperature. As the temperature rises the refrigeration power increase to lower the temperature and hence we can see an exponential decay in the temperature.

By zooming the X-axis and Y-axis we can clearly see the working of the cooling system

Result 2

The following scope shows the speed of the vehicle and current drawn from the battery pack, we observe that the speed of the vehicle reaches a max speed of 25km/hr and the current drawn was about 100Amps we also observe a negative spike current which indicates the charging current. This causes due to sudden braking of the vehicle the recharging of the battery takes place due to the regenerative braking of the vehicle.

Result 3

The following result shows the state of charge of each cell in the battery pack, the battery SOC decrease from 100 to 85 in 2474 sec

By zooming into the scope we see different soc at each cell we see that cell no 10 have slightly different soc rate compare to other cells by this we observe that each cell have their own characterization and may not be similar to other cells hence cell balancing along with battery management is necessary.

Result 4

Now the temperature of the cells 1-5 has been increased to check the thermal performance, the temperature of the cells is now set to 60⁰C by simulating the result we observe that there has been the effect of rising in temperature as shown in the purple graph. The cells with high-temperature discharges quickly compare to one with adequate thermal cooling the cells soc drop around 65% compare to other cells which are 85% hence there has been a significant drop in the performance of the first 5 cells.

1. Conclusion

From this experiment we learned thermal modeling and learned to model batter cooling system using Matlab and Simulink, we also learned how a liquid cooling system helps to control the heat flow effectively and the effect of rising in temperature in the battery pack Hence for obtaining efficient performance of the Electrical Vehicle proper thermal management of the battery pack is necessary.