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
50th AIAA Aerospace Science Meeting, Nashville, TN - January 9-12, 2012
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.
50th AIAA Aerospace Science Meeting, Nashville, TN - January 9-12, 2012
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
50th AIAA Aerospace Science Meeting, Nashville, TN - January 9-12, 2012
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)
50th AIAA Aerospace Science Meeting, Nashville, TN - January 9-12, 2012
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
50th AIAA Aerospace Science Meeting, Nashville, TN - January 9-12, 2012
• System efficiency
• Heat input, no heat recirculation
• Heat input with heat recirculation
mRh
e
in
h
in
es Q
QW
QW
..
System Efficiency
][ 300,, Kinhnrin HHQ
][ ,,, outcinhwrin HHQ
Qc
Qh
Qin
Qc
Qh
Qin
50th AIAA Aerospace Science Meeting, Nashville, TN - January 9-12, 2012
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
50th AIAA Aerospace Science Meeting, Nashville, TN - January 9-12, 2012
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
50th AIAA Aerospace Science Meeting, Nashville, TN - January 9-12, 2012
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
50th AIAA Aerospace Science Meeting, Nashville, TN - January 9-12, 2012
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
50th AIAA Aerospace Science Meeting, Nashville, TN - January 9-12, 2012
• 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
50th AIAA Aerospace Science Meeting, Nashville, TN - January 9-12, 2012
Absolute Axial Velocity Profiles
No fins Finned Water-cooled
50th AIAA Aerospace Science Meeting, Nashville, TN - January 9-12, 2012
Temperature Profiles
No fins Finned Water-cooled
50th AIAA Aerospace Science Meeting, Nashville, TN - January 9-12, 2012
Heat Flux Vector Plot
No fins Finned Water-cooled
• Significant axial conduction in the metal conductor
50th AIAA Aerospace Science Meeting, Nashville, TN - January 9-12, 2012
Axial Mean Temperature Profile
Hot Fluid Cold Fluid
50th AIAA Aerospace Science Meeting, Nashville, TN - January 9-12, 2012
Axial Mean Temperature Profile
Mean Junction Temperature Hot Junction - Cold Junction
50th AIAA Aerospace Science Meeting, Nashville, TN - January 9-12, 2012
TE Module Performance
Heat Input Rate, Qh TE Power Generation RateThermoelectric Efficiency, ηTE
.ch
eTE Q
W hTEe QW .
50th AIAA Aerospace Science Meeting, Nashville, TN - January 9-12, 2012
Carnot Efficiency TE material parameter, γ
.ch
eTE Q
W
TE Module Performance
50th AIAA Aerospace Science Meeting, Nashville, TN - January 9-12, 2012
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 %
50th AIAA Aerospace Science Meeting, Nashville, TN - January 9-12, 2012
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
50th AIAA Aerospace Science Meeting, Nashville, TN - January 9-12, 2012
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.
50th AIAA Aerospace Science Meeting, Nashville, TN - January 9-12, 2012
Acknowledgments
Troy Dent is supported byGraduate Assistance in Areas of National Need
(GAANN) Fellowship program of theUS Department of Education