Heat Transfer Simulation of a Hybrid Vehicle Battery Pack

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Introduction

The battery pack is the heart of every hybrid-electric vehicle on the road today. It supplies

the power to turn the wheels and stores the recovered energy to help these vehicles obtain

as many miles out of a single tank of fuel as possible. From this constant cycle of charging

and discharging, an immense amount of heat is generated by the battery pack which must be

mitigated, not only for the longevity of the pack but also for the safety of the passengers.

The design of such a cooling system requires powerful software throughout the design

phase, which CFdesign® upfront CFD was able to provide.

Problem Statement

The Penn State Advanced Vehicle Technology (AVT) Team has received a custom 330 Volt

Lithium-ion (Li-ion) battery pack. The pack is designed to power a Saturn Vue over 25 miles

before requiring the combustion engine to run a generator and begin to recharge the cells.

This extreme power demand means that the battery pack will be generating over 1.7 kW of

heat during operation which must be transferred safely out of the battery container and

radiated to the outside air. Due to the battery pack being a custom design, no cooling

system currently exists that has been optimized for the team’s application.

To tackle this heat transfer problem, the Penn State AVT Team has teamed up with Blue

Ridge Numerics, Inc. and utilized their CFdesign upfront CFD software to design and simulate

an effective cooling network for the Li-ion battery pack. By using this software, the Penn

State AVT Team hopes to simulate the heat generation cycles of the battery pack and design

an efficient cooling network. This will ensure peak battery performance throughout the life

of the vehicle.

Solution

Once the initial design of the battery pack container and coolant network had been

completed, CFdesign was loaded from the solid modeling program. This helped make

certain the design would stay as it was originally developed, eliminating any sources of

error associated with converting part files to another format.

After a couple simple simulations were completed to become acquainted with upfront CFD,

Penn State was then able to begin the cooling system analysis. Preliminary research had

defined that an air to water system was the most capable system that could effectively

transfer the heat and be manufactured in house using available aftermarket components.

This research then lead to the development of a coiled copper tubing design run within the

battery container and plumbed to an outside radiator. The initial procedure for analyzing

the design included defining points where heat generation sources were located (battery

module locations), defining the material compositions of the various components and

fluids, and highlighting the important factors of the design. With regards to important

factors, the team identified the temperature of the fluid running through the coils as the

most important factor to consider within the design. After identifying the critical factors

the program was then run through the cooling system analysis. The analysis ran quickly

considering the amount of information being processed and was attributed to CFdesign’s

advanced simulation ability.

Original Coil Design Run through CFdesign

Two heat generation cycles were used in the analysis of the battery pack. These heat

generation cycles were based off of the power needed to run the vehicle during steady

state and extended peak power output cases. The use of the two cycles would ensure a

robust design and the ability to handle worst case scenarios in terms of required cooling.

The output of the first iteration showed that the initial design was inadequate when

attempting to transfer the generated heat produced by the peak power cycle. As a result,

the Penn State AVT team began to progressively change the model to better account for

the coolant system requirements.

The team started with minor changes to the materials and eventually progressed to

dimensionally changing the components of the coolant system. Through this process,

design changes in coolant, tubing diameter, coil radius, and tubing length were all made

until a design was reached which was capable of transferring heat effectively through both

heat generation cycles.

Temperature Distribution Gradient of Coolant throughout the Coil

Reasons for Choosing CFdesign Upfront CFD

The Penn State AVT Team sought out the CFdesign upfront CFD software from Blue

Ridge Numerics, Inc. because of the software’s ability to provide an in-depth heat

transfer analysis, which was crucial in designing the team’s custom cooling system. Penn

State currently uses two different modeling programs in the design of its hybrid vehicle,

so the ability to have one analysis program that is compatible with both modeling

packages was a huge advantage. Using the same program minimizes the learning curve

of using the software and allowed the team to complete simultaneous analyses, even

when the designs were at different development stages. Many CFD analysis programs

require user input to completely define all objects such as boundary conditions and

meshes, but CFdesign is able to automatically perform these laborious functions with

minimal input required from the user. As changes were made to the model throughout

the design process, CFdesign would automatically adjust for the changes and output new

results. CFdesign also includes an extensive library of standard fluids, which made

defining a coolant for the cooling system much easier.

Penn State’s team member Shawn Getty gave the Help Tutorial PDF high marks, stating

that,

“It was easy to search for topics and the information provided was meaningful to the

solution of the problem.”

It was due to these advantages that the Penn State AVT Team was glad to have CFdesign

available throughout the battery pack design and looks forward to using the technology

as the design process for the rest of the vehicle continues.

Final Design

From the output results of CFdesign®, the final coolant system design will utilize a

copper tubing coil using a water and ethylene glycol mix coolant. An aluminum

support plate will also assist in removing heat from the battery pack.

Final Heat Transfer Plate and Coil Design Following CFdesign Upfront CFD Analysis

Design Conclusions

Hybrid-electric vehicles rely on the ability of a battery pack to undergo an extensive series

of discharge and recharge power cycles throughout a driving sequence. These intense

power cycles cause the battery pack to generate a great deal of waste heat which must

be effectively transferred out of the battery pack container to preserve both vehicle

performance and passenger safety. Thanks to the aid of CFdesign, the team was able to

analyze and improve the design of the battery pack cooling system. Throughout each

coolant system simulation, the team was able to see where design improvements were

required and what options were available to increase the performance in both transient

and steady state analyses. Overall, the custom cooling network was designed quickly and

efficiently, and the Penn State AVT Team is confident that the final design that will

integrate smoothly into the vehicle chassis and provide consistent cooling performance.

By: Mark Hull, Penn State AVT Team

Mark Hull is a senior in Mechanical Engineering at The Pennsylvania State

University and is a member of the Penn State AVT Team’s Auxiliary Power Unit

Group. He has experience with both solid modeling and computational fluid

dynamics software.

Publication Opportunities

- CFdesign promotional material or website as a product testimonial

- Penn State AVT Team website

- Mechanical engineering periodicals such as Machine Design

- Automotive engineering periodicals such as Automotive DesignLine and

Automotive Engineering International