Battery Thermal Management System Design Modeling

Battery Thermal Management System

Design Modeling

Gi-Heon Kim, Ph.D

Ahmad Pesaran, Ph.D (

[email protected]

)

National Renewable Energy Laboratory, Golden, Colorado, U.S.A.

EVS 22

EVS 22

With support from

High Power Energy Storage Program (Tien Duong and Dave Howell) Office of FreedomCAR and Vehicle Technologies

Energy Efficiency and Renewable Energy Office U.S. Department of Energy

3

Motivations & Objectives

Motivations

ƒ Battery thermal management is critical in achieving performance and

extended life of batteries in electric and hybrid vehicles.

ƒ Appropriate models that can predict thermal behaviors of batteries

shorten the development process for improving battery system design.

Objectives of this Study

ƒ To investigate the impact of cooling strategies with different coolant

systems; air and direct/indirect liquid cooling

ƒ To evaluate system thermal responses and their sensitivities as a

function of controllable system parameters

ƒ To provide battery thermal management system design insight by

identifying analyses and approaches that engineers should consider when they design a battery thermal management system for electric and hybrid vehicles

4

Battery Thermal Responses

3D Component Analysis System Analysis Cell Characteristics

• Shape: Prismatic/Cylinder/Oval etc

• Materials/Chemistries

• Size/Dimensions/Capacity

• Thermal/Current Paths inside a Cell

Module Cooling Strategy

• Passive control with phase change

• Coolant type: Air/Liquid

• Direct Contact/Jacket Cooling

• Serial/Parallel Cooling

• Terminal/Side Cooling

• Module Shape/Dimensions

• Coolant Path inside a Module

• Coolant Flow Rate

• etc

Battery Thermal Management

Battery Thermal Management

Modeling at NREL

Modeling at NREL

• Temperature History Cells/Module/Pack

• Temperature Distribution in a Cell

• Cell-to-Cell Temperature Imbalance in a Module

• Battery Performance Prediction

• Pressure Prop and Parasitic Power

• Vehicle Driving Cycles

• Control Strategy

• Ambient Temperature

• etc

Operating Conditions

5

Approach

ƒ

NREL has developed a 3-D electro-thermal model to predict

thermal response of real cells – focus on cell internal

temperature (EVS-21)

ƒ

In this study, focus is mostly external to cell – fluid side

– Air cooling – Liquid cooling

•Direct •Indirect

ƒ

The battery management system response were evaluated

using

– Fully developed laminar channel flow analysis – Computational Fluid Dynamic (CFD) analysis

ƒ

The system responses oof interest were

– Coolant temperature change (outlet - inlet)

– Temperature difference between cell surface and bulk coolant – Pressure drop in coolant channel (fluid power requirements)

Typical Parallel Cooling Analysis

(all the cells are treated the same)

cell surface temperature coolant mean temperature pressure

Heat Transfer Fluid Properties ƒ Density ƒ Specific Heat ƒ Thermal Conductivity ƒ Viscosity Cell Specifics ƒ Dimensions ƒ Heat Generation Control Parameters

ƒ Coolant Mass-flow Rate (m_c) ƒ Coolant Channel Dimension (D_h)

ΔT₂

ΔT₁

ΔP : Channel Pressure Loss

: Coolant Temperature Change

: Coolant-Cell Temperature Difference

System Responses of Interest

P x T x ΔT₂ ΔT₁ ΔP D_cell L_cell Q 0.5D_h m

.

.

.

.

7

Fully Developed Laminar Flow Analysis

Annular Channel Cooling

24 Re = f c 5.385 Nu = ν h VD = Re where, ,

If coolant channel gap is small enough compared with cell diameter,

k hD_h = Nu D_cell=5cm L_cell=10cm Q=2W Cell Specifics 2.582e-6 5.6e-5 1.4607e-5 ν(m2/s) 0.3892 0.13 0.0242 k (W/m K) 3323 1900 1006.43 c_p(J/kg K) 1069 924.1 1.225 ? (kg/m3) Indirect Cooling Liquid Direct Cooling Liquid Air 2.582e-6 5.6e-5 1.4607e-5 ν(m2/s) 0.3892 0.13 0.0242 k (W/m K) 3323 1900 1006.43 c_p(J/kg K) 1069 924.1 1.225 ? (kg/m3) Indirect Cooling Liquid Direct Cooling Liquid Air

Heat Transfer Fluid Specifics

ΔT₂ = Q/(πD_cellLh)

ΔP = 4τ_oL/D_h ΔT₁ = Q/( c_{m& p})

hydraulic diameter

D_h= 4 A_c/ p where

A_c: area section of the channel p :perimeter of the channel

mean velocity

V = / (ρA_c) where

ρ : coolant density

: coolant mass flow rate

m& m& Control Parameters m& D_h System Responses ρ

Mineral Oil Water/Glycol

: Coolant Mass Flow Rate : Hydraulic Diameter of Coolant Channel

Channel Pressure Loss

ΔP

ƒ Large difference in kinematic viscosity Æ, ΔP varies in very different ranges ƒ ΔP inversely proportional to D_h3 _{at D}

cell >> Dh and laminar flow

ƒ The channel pressure loss changes are very sensitive to D_h when it is small.

ƒ Due to the much smaller fluid density and consequently larger volumetric flow rate at given mass flow rate, the air cooling system requires much higher flow power

1 2 3 4 5 6 7 8 10-1 100 101 102 103 104 Dh (mm) Air Mineral Oil Water/Glycol 1 2 3 4 5 6 7 8 10-1 100 101 102 103 104 105 106 Dh (mm) Air Mineral Oil Water/Glycol Pow er /m c 2 [W /( kg s -1 ) 2 ] . Pow er /m c 2 [W /( kg s -1 ) 2 ] . Δ P /m c [kP a/( kg s -1 )] . Δ P /m c [kP a/( kg s -1 )] . (b) (a) 3 ~ h c D m ΔP & ν ~ ₄ h c h D m D ΔP ₋ & ν ∂ ∂

Square-of-mass-rate specific power

Mass-rate specific pressure

3 2 ~ h c f D m W ρ ν &

9

Coolant Temperature Increase

ΔT

₁

0.5 1 1.5 2 2.5 3 3.5 4 0 0.5 1 1.5 2 2.5 3 3.5 4 Mass-Flow Rate (g/s) T em per at ur e ( o C) p Air Mineral Oil Water/Glycol ΔT₁ p c

c

m

ΔT

&

1

~

1

1

~

m

c

m

ΔT

&

&

−

∂

∂

ƒ To achieve the temperature uniformity over a cell, it is preferred to keep

coolant temperature change in the channel as small as possible.

ƒ ΔT₁ is inversely proportional to coolant heat capacity flow rate.

ƒ Therefore, increasing mass flow rate is not as effective for reducing coolant temperature change in large flow rate cooling as it is in a small flow rate cooling.

ƒ A little change of flow rate can greatly affect the coolant temperature change and consequently cell temperatures at small coolant flow rate (especially for air system having small c_p).

Temperature Difference

between Coolant Bulk and Cell Surface

1 2 3 4 5 6 7 8 0 100 200 300 400 500 600 700 800 900 1000 D_h (mm) h ( W /m 2 K)

Heat Transfer Coefficient

Air Mineral Oil Water/Glycol 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 D h (mm) T e m perat ure ( o C)

Temperature Difference Between Coolant & Cell Surface Air

Mineral Oil Water/Glycol

ΔT

₂

h

ƒ ΔT₂ varies linearly with D_h with slope being proportional to 1/k .

ƒ ΔT₂ rapidly increases with D_h in air cooling due to its small thermal conductivity.

ƒ The heat transfer coefficient (h) evaluated at cell surface for water/glycol jacket cooling is greatly reduced and not sensitive to channel height due to added thermal resistances between coolant and cell surface.

ƒ High h system reduces ΔT , and removes heat fast from small temperature difference.

hat direct contact surface

for water/glycol system

11 1 .6 1 .8 2 2 2.2 2 .2 2.4 2.4 2.6 2.6 2.8 2 .8 3 3 3 3.2 3.2 3 .2 3.4 3.4 3 .4 3.6 3.6 3.6 3.8 3.8 3.8 4 4 4 4.2 4.2 4 .2 4.4 4.4 4.4 4.6 4.6 4 .6 4.8 4.8 4.8 5 5 5 5 5.2 5.2 5.2 5.4 5.4 5.4 5.6 5.6 5 .6 5.8 5.8 5.8 6 6 6 6 6.2 6.2 6.2 6.4 6.4 6.4 6.6 6.6 6 .6 6.8 6.8 6.8 7 7 7 7 7.2 7.2 7 .2 7.4 7.4 7.4 7.6 7.6 7.6 7.8 7.8 7 .8 8 8 8 8 8.2 8.2 8 .2 8.4 8.4 8.4 8.6 8.6 8.8 8.8 9 9 9.2 9.2 9.4 9.69. 8 1010.210.10.610.411 8 D h(mm) m a s s fl o w r a te ( g /s ) DT_max (oC) 1 2 3 4 5 6 7 8 0.5 1 1.5 2 2.5 3 3.5 4 0.6 0.8 0.8 1 1 1 1.2 1.2 1.2 1.4 1.4 1.4 1.4 1.6 1.6 1.6 1.6 1.8 1.8 1.8 1.8 2 2 2 2.2 2.2 2.2 2.4 2.4 2.4 2.6 2.6 2.8 2.8 3 3 3.2 3.4 D h(mm) DT max ( o_C) 1 2 3 4 5 6 7 8 0.5 1 1.5 2 2.5 3 3.5 4 1 1 1.2 1.2 1.2 1.4 1.4 1.4 1.6 1.6 1.6 1.8 1.8 1.8 2 2 2 2 2 2.2 D h(mm) DT max ( o_C) 1 2 3 4 5 6 7 8 0.5 1 1.5 2 2.5 3 3.5 4 1 .6 1 .8 2 2 2.2 2 .2 2.4 2.4 2.6 2.6 2.8 2 .8 3 3 3 3.2 3.2 3 .2 3.4 3.4 3 .4 3.6 3.6 3.6 3.8 3.8 3.8 4 4 4 4.2 4.2 4 .2 4.4 4.4 4.4 4.6 4.6 4 .6 4.8 4.8 4.8 5 5 5 5 5.2 5.2 5.2 5.4 5.4 5.4 5.6 5.6 5 .6 5.8 5.8 5.8 6 6 6 6 6.2 6.2 6.2 6.4 6.4 6.4 6.6 6.6 6 .6 6.8 6.8 6.8 7 7 7 7 7.2 7.2 7 .2 7.4 7.4 7.4 7.6 7.6 7.6 7.8 7.8 7 .8 8 8 8 8 8.2 8.2 8 .2 8.4 8.4 8.4 8.6 8.6 8.8 8.8 9 9 9.2 9.2 9.4 9.69. 8 1010.210.410.61110.8 D h(mm) m a s s fl o w r a te ( g /s ) DT_max (oC) 1 2 3 4 5 6 7 8 0.5 1 1.5 2 2.5 3 3.5 4 0.6 0.8 0.8 1 1 1 1.2 1.2 1.2 1.4 1.4 1.4 1.4 1.6 1.6 1.6 1.6 1.8 1.8 1.8 1.8 2 2 2 2.2 2.2 2.2 2.4 2.4 2.4 2.6 2.6 2.8 2.8 3 3 3.2 3.4 D h(mm) DT max ( o_C) 1 2 3 4 5 6 7 8 0.5 1 1.5 2 2.5 3 3.5 4 1 1 1.2 1.2 1.2 1.4 1.4 1.4 1.6 1.6 1.6 1.8 1.8 1.8 2 2 2 2 2 2.2 D h(mm) DT max ( o_C) 1 2 3 4 5 6 7 8 0.5 1 1.5 2 2.5 3 3.5 4 Air m as s fl o w ra te ( g /s )

Mineral Oil Water/Glycol

D_h(mm) D_h(mm) D_h(mm)

Maximum Cell Surface Temperature

relative to Coolant Inlet

ΔT

(=

ΔT

+

ΔT

)

ƒ

Δ

T

in the air system are much higher compared with other fluid

systems due to its small heat capacity and thermal conductivity

ƒ

The air system:

Δ

T

is dominated by and sensitive to D

ƒ

The water/glycol jacket cooling system:

Δ

T

is not very sensitive to

D

, and

Δ

T

is not a strict limiting thermal design factor in a

water/glycol system.

12

Optimizing Operation Parameters

: maximum heat transfer coefficient (h) operating point

: lowest max temperature (ΔT₁+ ΔT₂ ) operating point : minimum pressure loss (ΔP) operating point

D_cell=5cm L_cell=10cm Q=2W D_cell=5cm L_cell=10cm Q=2W Cell Specifics

Re=2300

Δ

P=37 Pa

Δ

P=110 Pa

Δ

T

=1.5

_C

Δ

T

=4.5

_C

Δ

T

=3.43

_C

-

Air Cooling System

Colored Zone satisfies

Re < 2300

ΔP < 110 Pa

ΔT₁+ ΔT₂ < 4.5 o_C

ΔT₁ < 1.5 o_C

13 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 Mass Flow Rate (kg/s)

Coo lan t T em pera ture Rise ( oC) Co ol ant Temperat ure Ri se ( o C)

Mass Flow Rate (kg/s)

Trend Validation: Module Liquid Cooling

Experiment

C oolant Tem perature Inc reas e

Tested Module Cooling System

ƒ 12 li-ion cylindrical cells ƒ Indirect jacket cooling

ƒ Coolant channels not completely fully developed

ƒ Discharged at constant C rate

ƒ Coolant (water) temperature

increase is inversely proportional to the coolant mass flow rate.

ƒ Coolant temperature change at

high mass flow rates is not as sensitive to mass flow rate as it is at low mass flow rates.

ƒ Note that the magnitude of coolant temperature change at given heat removal rate is

relatively small due to large heat capacity of water/glycol.

prediction

Computational Fluid Dynamic (CFD) Evaluation

Cell Core Coolant Channel

Terminal Can

Terminal Current Collector

Temperature Distribution

Geometry & Mesh

NOTE: Radial direction length is exaggerated.

Direct Air Cooling

9 Channel Inlet Air Temperature: 35o_C

9 Air Mass Flow Rate: 1.33 g/s per Cell

15

CFD Predictions:

Temperatures, h and Surface Heat Flux Profiles

0 1 2 3 4 5 6 7 8 9 10 35 35.5 36 36.5 37 37.5 38 38.5 39 Axial Distance (cm) T em per at ur e ( oC)

Air Mean Temperature Cell Surface Temperature Cell Temperature at Axis

0 1 2 3 4 5 6 7 8 9 10 0 50 100 150 200 250 Axial Distance (cm) H e a t F lu x ( W /m 2) 0 1 2 3 4 5 6 7 8 9 10 0 20 40 60 80 100 120 A i l Di ( ) H e a t T rans fe r C oef fi c ient ( W /m 2K) ΔT₁ ΔT_max 60.96 3.41 2.89 1.50 114.2 CFD 59.24 N/A 3.64 1.49 109.1 Fully Developed Flow Relations

with Constant Heat Flux

h [W/m2_K] mean heat transfer coefficient ΔT_cell[o_C] maximum cell internal temperature to inlet air ΔT_max[o_C] maximum cell surface temperature to inlet air ΔT₁ [o_C] coolant temperature change ΔP [Pa] channel pressure loss 60.96 3.41 2.89 1.50 114.2 CFD 59.24 N/A 3.64 1.49 109.1 Fully Developed Flow Relations

with Constant Heat Flux

h [W/m2_K] mean heat transfer coefficient ΔT_cell[o_C] maximum cell internal temperature to inlet air ΔT_max[o_C] maximum cell surface temperature to inlet air ΔT₁ [o_C] coolant temperature change ΔP [Pa] channel pressure loss

Heat Flux

h

ƒ CFD captures entrance effects. ƒ CFD model addresses battery

internal heat flow and captures axially decreasing heat flux from cell to air.

ƒ Internal heat flow through high conductivity material distributed inside a cell (such as container can) makes the axial gradient of cell surface temperature smaller than that of air temperature.

ƒ This result strongly implies that capturing the internal heat flow paths and thermal resistances inside a cell are important for the improved prediction of

Air vs

Water/Glycol

CFD

cell core cell core aluminum can

air water/glycol

Performance comparison analyses were made between Air Cooling System and

Water/Glycol Jacket Cooling System in order to contrast the characteristics of each system.

9 Heat Generation: 4 W per Cell

9 Channel Inlet Air Temperature: 35o_C

9 Coolant Mass Flow Rate: 1.33 g/s 9 Channel Gap Height: 1.1 mm 9 5 cm diameter, 20 cm height cell

18

Concluding Remarks

ƒ The heat transfer coefficient (h) of an air cooling system is lower than that of liquid cooling systems.

ƒ Due to the small heat capacity of air, it is difficult to accomplish temperature uniformity inside a cell or between the cells in a module.

ƒ The temperature difference between coolant air and cell surface is sensitive to variations of channel height due to the small heat conductivity of air.

ƒ Heat transfer coefficient is inversely proportional to D_h, while friction pressure loss in channel is inversely proportional to D_h3_.

ƒ Increasing heat transfer coefficient by reducing channel thickness is limited by the required blower power.

ƒ The simplicity of an air cooling system is an advantage over a liquid coolant

system.

ƒ Air cooling could have less mass, has no potential for leaks, needs fewer

components, and could cost less.

Concluding Remarks

ƒ A water/glycol solution for a jacket cooling has much lower viscosity than dielectric mineral oil for direct cooling. Increasing the coolant flow rate in water/glycol system is not as severely restricted by the pump power as in a mineral oil system.

ƒ Cell/module temperature uniformity can be effectively achieved in liquid cooling system due to the large heat capacity of liquid coolant.

ƒ Water/glycol solutions generally have a higher thermal conductivity than oil. However, the effective heat transfer coefficient at the cell surface is greatly reduced due to the added jacket wall and air gap layer.

ƒ Because of the added thermal resistances, h is not as sensitive to the variation of channel height in indirect liquid cooling systems.

ƒ Liquid cooling systems are more effective in heat transfer and take up less volume.

ƒ However, the added complexity and cost may outweigh the merits.

ƒ Maintenance and repair of a liquid cooled pack is more involved and costlier.

Indirect liquid cooling, with jackets, is easer to handle than direct liquid cooling.

20

Concluding Remarks

ƒ

Selection between air or liquid cooling depends on applications.

ƒ

Trade-off between performance, application, and cost must be

considered

ƒ

It is recommended in liquid cooling system to use a small gap coolant

channel to reduce system weight and volume by minimizing the

amount of liquid coolant in a system operated at a given coolant flow

rate.

ƒ

Capturing the internal heat flow paths and thermal resistances inside

a cell using a sophisticated three-dimensional cell model is important

for the improved prediction of cell/battery thermal behaviors.

ƒ

With the model we can look at turbulent flows, mixed

conduction/convection, and phase change materials.