Role of Thermal Strategies in Thermoelectric Power Generation

50th AIAA Aerospace Science Meeting, Nashville, TN - January 9-12, 2012

Role of Thermal Strategies in Thermoelectric Power

GenerationTroy J. Dent Jr. and Ajay K. Agrawal

Department of Mechanical EngineeringThe University of Alabama, Tuscaloosa


Motivation

• Portable power generation– Thermoelectric power generation

• No moving parts or noise• Poor performance due to low heat transfer rate

between working fluids and TE module• TE research focus primarily been on

improving TE materials.


Thermoelectric (TE) Effects

n-type

Th

Tc

Th

Th

Tc

Tc

Tc

p-type

n-type

n-type

I

R0

Thomson Effect & Joule Heating

p-type

p-type

Qh

Th,∞

Qc

Tc,∞

y

x z

Peltier Effect (junctions)

• TE power generation from a temperature differential across the TE elements

• TE module formed by a series of TE elements

• TE effects– Joule heating– Peltier effect– Thomson effect


Thermoelectric (TE) Effects

• Joule Heating

• Thomson Effect– dV due to temperature difference

• Peltier Effect– dV due to material difference

dxdTJe dT

dT

dmdJe

T

2Je eJH

n-type

Th

Tc

Th

Th

Tc

Tc

Tc

p-type

n-type

n-type

I

R0

Thomson Effect & Joule Heating

p-type

p-type

Qh

Th,∞

Qc

Tc,∞

y

x z

Peltier Effect (junctions)


Thermoelectric Efficiency• Thermoelectric efficiency based on

• Thermoelectric figure-of-merit– Seebeck coefficient - (V/K)– Electrical resistivity - e (W·m)– Thermal conductivity - k (W/m·K)

• TE module efficiency

kZ

e

2

TZ

2ch TTT

.1

11c

hch

ch

h

eTE TTTZ

TZTTT

QW

h

em Q

W

Hot fluid flow

Cold fluid flow

Qh

Qc

Th,∞

Tc,∞

Tc

Th

TE module

x

z

y


• System efficiency

• Heat input, no heat recirculation

• Heat input with heat recirculation

mRh

e

in

h

in

es Q

QW

QQ

QW

..

System Efficiency

][ 300,, Kinhnrin HHQ

][ ,,, outcinhwrin HHQ

Qc

Qh

Qin

Qc

Qh

Qin


Objectives• Comparison of thermal strategies

− No fins, Finned, Water-cooled • Effect of thermal strategies on:

− Fluid and TE module temperature− Heat transfer rate between TE module / fluids− Heat input ratio, QR

− Thermoelectric module efficiency, ηm

− System efficiency, ηs


Model Layout

p-type leg

Cold Fluid Flow

(-z direction)

Hot Fluid Flow

(+z direction)

n-type leg

Col

d Ju

nctio

n

y

x z

Hot

Jun

ctio

n

6.00

Periodic Boundary

Periodic Boundary

Adiabatic B

oundary

Adiabatic B

oundary

3.25 1.00 5.00 3.25 1.00

n-type leg

p-type leg

3.25 1.00 5.00 3.25

1

1

1

1

1

0.5

0.5

y

x z

1.00

Col

d Ju

nctio

n

Hot

Jun

ctio

n

Periodic Boundary

Periodic Boundary

Adiabatic B

oundary

Adiabatic B

oundary

Cold water flow

n-type leg

p-type leg y

x z

Col

d Ju

nctio

n

Hot

Jun

ctio

n

3.25 1.00 5.00 3.25

Periodic Boundary

1

1

1

1

1

0.5

0.5

Adiabatic B

oundary

Periodic Boundary

Adiabatic B

oundary

1.00

No fins Finned

Water-cooled


CFD Parameters• Laminar Flow

− Hot Fluid - ReD = 211− Cold Fluid - ReD = 643

• Fluid flow inlets− Uniform temperature

Tc = 300 K, Th = 1500 K

− Uniform mass flux Air - 1.8 kg/m²∙s Water - 84.53 kg/m²∙s

− Temperature-dependent material properties• Silicon-Germanium TE material properties

− TE elements insulated− No axial conduction heat transfer

• DO Radiation Model


CFD Governing Equations• Conservation of Mass

• Conservation of Momentum

• Conservation of Energy

• Source Terms– Mass - Sm; Momentum - Sx, Sy & Sz; Energy - SE

mr

rz Srv

vr

vz

Eyxpypx SyTk

yxTk

xTcv

yTcv

x

xzxyxx

x Sxv

zv

zx

v

yv

yv

xv

xxPvv

322

yzyyxy

y Syv

z

v

zv

y

v

yyv

x

v

xyPvv

322

zzyzxz

z Svzv

zz

v

yv

yzv

xv

xzPvv

322


• Thermoelectric junctions− Hot junction

− Cold junction

• Thermoelectric legs− p-type

− n-type

• Jp = I/Ap; Anp = An/Ap

• Jp and Anp optimized for ηs

Thermoelectric Module

xT

JJS pppppepE

2

,,

xT

AJ

A

JS n

nnp

p

np

pnenE

2

2

,,

h

nppp

np

nepe

nphE t

TJJ

AAS

2,

,, 11

h

pnpp

np

nepe

npcE t

TJJ

AAS

2,

,, 11


Absolute Axial Velocity Profiles

No fins Finned Water-cooled


Temperature Profiles



Heat Flux Vector Plot


• Significant axial conduction in the metal conductor


Axial Mean Temperature Profile

Hot Fluid Cold Fluid


Axial Mean Temperature Profile

Mean Junction Temperature Hot Junction - Cold Junction


TE Module Performance

Heat Input Rate, Qh TE Power Generation RateThermoelectric Efficiency, ηTE

.ch

eTE Q

W hTEe QW .


Carnot Efficiency TE material parameter, γ

.ch

eTE Q

W

TE Module Performance


No Fins Finned Water-cooledThermoelectric efficiency, ηm 0.62 % 0.86 % 1.08 %Heat Input Ratio, QR,nr 0.17 0.46 0.81System efficiency, ηs,nr 0.10 % 0.40 % 0.87 %

Heat RecirculationHeat Input Ratio, QR,wr 0.20 0.84 4.05System efficiency, ηs,wr 0.13 % 0.72 % 4.36 %


Individual Thermoelectric Effects

Joule Heating Peltier EffectThomson Effect

• Heat source rate of individual TE effects− Joule heating & Thomson effect generate heat - Power loss− Peltier effect absorbs heat - TE power generation


Conclusions• Reduction of thermal resistance between TE module

and fluid can significantly improve system efficiency.• Good thermal strategies will result in improved system

efficiency, even with poor thermoelectric performance as in the water-cooled case.

• Improved system efficiency is possible through better understanding of the interaction of heat transfer, fluid flow, and thermoelectric power generation.

• Research of thermal strategies in combination with thermoelectric material research can yield better thermoelectric power generation.


Acknowledgments

Troy Dent is supported byGraduate Assistance in Areas of National Need

(GAANN) Fellowship program of theUS Department of Education

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Role of Thermal Strategies in Thermoelectric Power Generation

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