cooling and preheating of batteries in hybrid electric ... · pdf filenational renewable...

National Renewable Energy Laboratory

66thth ASMEASME--JSME Thermal Engineering ConferenceJSME Thermal Engineering ConferenceHawaii Island, HawaiiHawaii Island, Hawaii

March 17March 17--20, 200320, 2003

Cooling and Preheating of Batteries in Hybrid Electric Vehicles

Ahmad A. PesaranNational Renewable Energy Laboratory

Golden, Colorado, USA

Andreas VlahinosAdvanced Engineering Solutions

Tom StuartUniversity of Toledo

Sponsored by US Department of Energy, Office of FreedomCAR and VSponsored by US Department of Energy, Office of FreedomCAR and Vehicle Technologiesehicle Technologies


Outline

BackgroundBackgroundBattery Thermal Management Battery Thermal Management Cooling Cooling Preheating Preheating •• Finite Element AnalysisFinite Element Analysis•• ExperimentsExperiments

Concluding RemarksConcluding Remarks


BackgroundHybrid electric vehicles (HEV) entering the marketHybrid electric vehicles (HEV) entering the market

•• EngineEngine•• BatteryBattery--powered motorpowered motor

HEV success depends on battery performance, life, and costHEV success depends on battery performance, life, and cost


Battery Temperature is Important

Battery temperature affects vehicle performance, reliability, safety, and life cycle cost

Temperature affects battery:Temperature affects battery:Operation of the electrochemical systemOperation of the electrochemical systemRound trip efficiency Round trip efficiency Charge acceptanceCharge acceptancePower and energy availabilityPower and energy availabilitySafety and reliabilitySafety and reliabilityLife and life cycle costLife and life cycle cost


Background – cont.Consumers expect satisfactory performance from hybrid vehicles at all climatesGenerally, as battery temperature increases

Power and energy capability of battery increaseCalendar and cycle life decrease

At very cold temperatures batteries could not deliver the needed power and energy.


Impact of Temperature on Battery Discharge Power

0

100

200

300

400

500

600

700

800

-40 -30 -20 -10 0 10 20 30 40 50Temperature (C)

PNG

V M

axim

um D

isch

arge

Pow

er(W

atts

)

Panasonic 6.5 Ah NiMH prismatic Module (7.2 Volts) and 55% SOC (based on 18 sec discharge pulses)

Based on experiments at NREL

General trend in life


Why Battery Thermal Management?Regulate battery to operate in the desired temperature range for optimum performance and lifeReduce uneven temperature distribution in a pack batteries to avoid unbalanced modules/pack and thus, avoid reduced performance

Cooling in hot climates, mostly for avoiding premature degradation and improving safety.

Battery internal heating is dominant due to resistive heatingPreheating in very cold climates, to overcome poor performance


Cooling using Air Ventilation

Battery PackOutside Air ExhaustFan

Battery PackCabin Air

ExhaustFan

Outside Air

ReturnVehicle heater and evaporator cores

Battery Pack Exhaust

Fan

Outside Air

ReturnAuxiliary or Vehicleheater and evaporator cores

Passive cooling- Outside Air Ventilation

Passive cooling/heating- Cabin Air Ventilation

Active cooling/heating- Outside or Cabin Air

Prius & Insight


Cooling using Liquid Circulation

Liquid

Pump

Liquid/air heat exchanger

Fan

Outside Air Exhaust

Battery PackLiquid

Pump

Liquid/air heat exchanger

Fan

Outside Air Exhaust

Battery Pack

Liquid direct contact or jacketed

Liquid

Pump

Liquid/liquid heat exchanger

Pump

Vehicle enginecoolant Return

Battery Pack

Liquid direc contact or jacketed

Air from Evaporatoror Refrigerant from Condenser

Liquid

Pump

Liquid/liquid heat exchanger

PumpVehicle enginecoolant Return

Battery Pack

AC Heat Exchanger

Liquid direct contact or jacketed

PassiveCooling

Active moderatecooling/heating

Activecooling/heating


Series vs. Parallel Air Distribution

Parallel flowIn this casemodules uprightairflow up

Balancing pressure dropswith proper manifold

Air

Series flowIn this case, modules

on side airflow across


Preheating StudySources of energy for onSources of energy for on--board preheatingboard preheating

Heat from engine Heat from engine Electricity from batteryElectricity from batteryElectricity from generator/inverterElectricity from generator/inverter

Identify the most effective preheating technique Identify the most effective preheating technique External heating techniques:External heating techniques:

•• Electrically heated thermal jacketsElectrically heated thermal jackets•• A sealed enclosure with an internal heating elementA sealed enclosure with an internal heating element•• Circulating a fluid heated from the engineCirculating a fluid heated from the engine

Internal heating techniques:Internal heating techniques:•• Resistive heating elements embedded within the batteriesResistive heating elements embedded within the batteries•• Apply current to the battery terminal Apply current to the battery terminal

Perform FEA thermal analysis for a rectangular modulePerform FEA thermal analysis for a rectangular modulePerform feasibility tests.Perform feasibility tests.


Internal Core Heating using Battery ResistanceCase 1

Geometry: rectangular modules consisting of six cellsHeat transfer by conduction from core to exteriorHalf FEA model

Battery Core

Plastic case

Contact resistance

Small convection losses to surroundings from case

h = 1-3 W/m2 /°C


0 100 200 300 400 500 6000

0.5

1

1.5

2

2.5

3

3.5x 10

6

Time (sec )

Vol

umet

ric H

eat

Gen

erat

ion

(W

atts

/m3 )

Volumetric Heat Generation Vers us Time for Various Values of Input P owe r

Input Energy 1.09 WhrInput Energy 2.90 WhrInput Energy 4.71 WhrInput Energy 6.53 Whr

Transient Volumetric Heat Generation AppliedHeat generated in the core from battery resistive heating Heat generated in the core from battery resistive heating (decreases as temperature increases) (decreases as temperature increases)

Case 1

6.53 Whr

1.09 Whr

2.90 Whr

4.71 WhrTotal energy input in ¼ of a module over 10 minutes


0 100 200 300 400 500 600-40

-35

-30

-25

-20

-15

-10

-5

Time (sec )

Tem

pera

ture

(C)

Maximum Ce ll Te mpe rature Versus Time for Various Value s of Input P owe r

Input Energy 1.09 WhrInput Energy 2.90 WhrInput Energy 4.71 WhrInput Energy 6.53 Whr

Maximum Temperature versus time for Internal Core Heating

Temperature increases with time and amount of Temperature increases with time and amount of internal heating energyinternal heating energy

Case 1

After 2 minutes After 2 minutes the slope of the slope of temperature rise temperature rise decreases because decreases because heating rate heating rate decreases.decreases.

6.53 Whr

1.09 Whr

2.90 Whr

4.71 Whr

Total energy input in ¼ of module over 10 minutes


Case 2

External Electric Jacket Heating around ModuleGeometry: rectangular modules consisting of six cells

Jacket heater: 0.0625 inch thick with additional 0.0625 insulation

Half FEA Model

Battery Core

Plastic case

Contactresistance Heating jacket Insulation padSmall convection losses to surroundings

from the heater insulation

Heat transfer is by conduction from heater to core of module


Case 3

Internal Electric Jacket Heating around Cells Geometry: Geometry: rectangular modules consisting of six cellsmodules consisting of six cells

Jacket heater: 0.0625Jacket heater: 0.0625”” thick with additional 0.0625thick with additional 0.0625”” insulationinsulation

Battery Core

Plastic case

Heating pad Insulation jacket


Case 4

Internal Heating Using Fluid between each CellGeometry: rectangular modules consisting of six cells

Air gap between each cell: 3.1 mm

Battery Core

Plastic case

Air gap Air Flow Air Flow

Air Flow

h side = 25 W/m2 /°C

h horizontal= 2-5 W/m2 /°C


Comparison of Four Heating CasesThe most uniform heating was with internal core heating The most uniform heating was with internal core heating (Case 1)(Case 1)

Case 1 Internal core heating

Case 2 External jacket heating

Case 3 Internal jacket heating

The same temperature scaleCase 4 Internal air heating


Average Core Temperature at 2 minutes for various Heating Methods

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3-45

-40

-35

-30

-25

-20

-15

-10

Input Energy (Whr)

Ave

rage

Cor

e Te

mpe

ratu

re a

t 2 m

in (C

)

Average Core Temperature at 2 min vers us Input P ower for various heating methods

Internal heating us ing battery energy External heating us ing electric heaters Internal heating us ing electric heaters Internal Airflow Heating 100% EfficiencyInternal Airflow Heating 20% Efficiency

Uniform Input Heat over 2 minutes (Whr)

Ave

rage

Tem

pera

ture

at 2

min

utes

(°C

)

Internal core heatingCase 1

Other heating methods


Effect of aspect ratio studied to see if it has any impact on heating effectiveness

L/W = 17.2

L/W = 2.0

Small Large

6-cell Mass=2.12 kg

6-cell Mass=1.56 kg

Heat input = 6.62 Wh/kg core

Heat input = 6.62 Wh/kg core


Comparison of Heating Effectiveness for the same Whr heat/kg

0 20 40 60 80 100 120-40

-38

-36

-34

-32

-30

-28

-26

-24

-22

-20

Time (s e c)

Tem

pera

ture

(C)

Ave rage Core Te mpe ra ture Ve rs us Time

La rge As pe ct Ra tio - Core He a tingLa rge As pe ct Ra tio - J a cke t He a tingS ma ll As pe ct Ra tio - Core He a tingS ma ll As pe ct Ra tio - J a cke t He a ting Core Heating

Case 1

Jacket Heating


Thermal Analysis Observations Electric heating raises the battery Electric heating raises the battery temperature faster than heating with fluids.temperature faster than heating with fluids.The most uniform temperature distribution The most uniform temperature distribution was with internal core heating was with internal core heating With the same heat input, average core With the same heat input, average core temperature was raised faster with core temperature was raised faster with core heating. heating. These observation not changed for batteries These observation not changed for batteries with different aspect ratios. with different aspect ratios.


How much Energy and Power for Preheating a Battery?

To raise the temperature of a 40 kg battery pack from To raise the temperature of a 40 kg battery pack from --3030°°C to 0C to 0°°C in 2 C in 2 min, 9.76 kW of power (or about 325min, 9.76 kW of power (or about 325 WhWh of heat energy) is of heat energy) is required for a 100% efficient process. required for a 100% efficient process.


How to apply the more effective core heating method?

At low temperatures battery resistance is highAt low temperatures battery resistance is highCharging/discharging heats the battery (ICharging/discharging heats the battery (I22R)R)DC currents could damage batteriesDC currents could damage batteriesApplying high frequency alternating currents (AC) Applying high frequency alternating currents (AC) may heat up the battery without too much energy may heat up the battery without too much energy loss and battery damageloss and battery damageInitial feasibility work done in collaboration with Initial feasibility work done in collaboration with University of Toledo University of Toledo

60 Hz AC heating on lead acid and NiMH batteries 60 Hz AC heating on lead acid and NiMH batteries High frequency (10High frequency (10--20 kHz) AC heating to a NiMH pack20 kHz) AC heating to a NiMH pack


Applying 60 Hz AC Power is Effective in Warming Batteries

Measured pulse DC resistance (pulse power capability) Measured pulse DC resistance (pulse power capability) of a lead acid module at very cold temperatures before of a lead acid module at very cold temperatures before and after applying 60 Hz AC heatingand after applying 60 Hz AC heating

60 Hz AC (off60 Hz AC (off--board) more suitable for electric vehiclesboard) more suitable for electric vehicles

Initial Battery Temp = –40°CAC

Heating Internal Rmil Ohm

Peak current (A)

EstimatedT (°C)

None 108 100 -40

3 min 19.6 210 -4

6 min 14.7 250 +6

9 min 13.5 270 +9


High Frequency AC Heating EvaluationHigher frequencies reduce size of power electronics for Higher frequencies reduce size of power electronics for potential onpotential on--board hardware.board hardware.Applied 10Applied 10--20 kHz current to a Panasonic NiMH battery 20 kHz current to a Panasonic NiMH battery pack (16pack (16--module, 115.2 V, 6.5 Ah) using a special heater module, 115.2 V, 6.5 Ah) using a special heater circuit.circuit.Measured resistance and peak power before and after Measured resistance and peak power before and after applying high frequency currentapplying high frequency current

DC

I

ACB I

>>

B1C

B

2

ControlSystemI Q1,Q2

Driver

VVo/2

B2,

B

B

1

B1

R

B

= DC current.

AC

+

B

1IB2

I1

1

DC

2

Q

VB

I

V

V

DC

D

Q +I /2

L

2

L

or I

B

I

I

V

I

B2

= RMS value of 10-20 kHz current.

X

I

1

1

1

B

I

or V

= Battery temperature

R

DC

1Vo/2

B

=

2

AC

I

2

AC,

2

/2

T +

DC

I

L

B1,

T

V

DCharger

L

-

B

-

X

1

I

I


Obtained Battery Internal Resistance for various Temp and State of Charge

RB vs Tbat

0.1

0.4

0.7

1

1.3

-30 -20 -10 0 10 20 30 40

Tbat (deg.C)

R B (o

hms) SOC 55%

SOC 25%SOC 75%


High Frequency AC Heating on the NiMH Pack

T R(C) Ohm-20 1.024-10 0.6140 0.410

10 0.33325 0.20535 0.179

Rbat of (16) module Panasonic 6.5 Ah NiMH pack vs. time with High Frequency (10 kHz)

0

0.2

0.4

0.6

0.8

1

0 2 4 6 8 10 12

Time (min)

Rba

t (oh

m)

Resistance calculated over 25 A discharge pulse for 2 seconds.

AC heating current : 65 Arms 1. Pack SOC = 55 %.2. Temp = -20oC.3. IAC = 60 A.rms / 10 kHz.4. Rbat = 0.21 Ω @ 25oC.5. Soak time > 7 hrs.6. Pack Voltage = 130 Vdc

0°C

-20°C


Impact of Higher Current AmplitudesFrequency = 10 kHzFrequency = 10 kHz


Battery Capacity Improves with AC Heating

Discharge Tests at -30 deg.C

1.25

3.28

4.38

0.3 0.5 0.750.95

2.59

3.58

0

1

2

3

4

5

Am

per

e H

ou

rs (

AH

)

RTRT RTNo AC No AC No AC w/ACw/ACw/AC

SOC=25% SOC=55% SOC=75%

RT=25oC

Frequency = 10 kHzFrequency = 10 kHz


Concluding RemarksBattery thermal management necessary in HEVsBattery thermal management necessary in HEVsAnalysis showed that core heating is the most Analysis showed that core heating is the most effective method to preheat batterieseffective method to preheat batteries

Uses least amount of energy for the same Temp riseUses least amount of energy for the same Temp riseMore uniform temperature distributionMore uniform temperature distribution

Testing showed that core heating is feasible Testing showed that core heating is feasible through applying AC power through battery through applying AC power through battery terminals terminals Further analysis, testing, hardware evaluation, Further analysis, testing, hardware evaluation, and trade off analysis under way for onand trade off analysis under way for on--board board vehicle applications. vehicle applications.

cooling and preheating of batteries in hybrid electric ... · pdf filenational renewable...

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