Thermoelectric Conversion of Waste Heat to Electricity in an IC Engine Powered Vehicle
Principal Investigator: Harold SchockPrepared by:
Harold Schock, Eldon Case, Jonathan D’Angelo, Andrew Hartsig, Tim Hogan, Mercouri Kanatzidis, James Novak, Fang Peng, Fei Ren,
Tom Shih, Jeff Sakamoto, Todd Sheridan, Ed Timm
08/06/2008
Supported By: Acknowledgement: ONR support US Department of Energy under MURI ProgramEnergy Efficiency Renewable Energy (EERE) Mihal Gross, Project MonitorJohn Fairbanks and Samuel Taylor, Contract Monitors
IOWA STATE
UNIVERSITY
3D CFD Analysis
Iowa State / MSU
• Couple and Module IssuesConvection and radiation between
legs with and without insulationCurrent distribution, Joule heating,
Heat fluxes
• Electrical energy production
• Unsteady heat transfer analysis to and from modules (3D, pulsatile, comp.)
• Complete engine system- f(x,t)
• Temperatures and heat flux
• EGR energy
• Energy in exhaust (T, P, m)
• Turbine work, inlet/outlet temperatures
TEG Design and Construction
MSU/JPL
• Generator design• TEG materials selection• Mechanical and TE material property characterization including Weibull analysis• FEA analysis• Leg and module fabrication methods
• Design of electrical energy conditioning and utilization system
• Control system design and construction
• Inverter, Belt Integrated Starter-Generator Selection
Systems for Utilizationof Electrical Power Recovered
MSU
Implementation of a Thermoelectric Generator with a Cummins ISX Over-the-Road Powerplant
Engine-TEG Simulation and Experimental Verification
MSU / Cummins
6 Cyl. Engine Test Data
Cummins
P2 - Single cylinder +TEG Demo
MSU
Goals and ObjectivesGoals and ObjectivesUsing a TEG, provide a 10% improvement in fuel economy by converting waste heat to electricity used by the OTR truckEvaluate currently available thermoelectric materials to determine optimum material selection and segmentation geometry for this applicationDevelop TEG fabrication protocol for module and system demonstrationDetermine heat exchanger requirements needed for building TEGs of reasonable lengthDetermine power electronic/control requirementsDetermine if Phase 2 results make an engine demo in Phase 3 reasonable
Important BarriersImportant Barriers• Design of heat exchanger is a major challenge with heat transfer
coefficients needed which are 5x higher than without enhanced heat transfer modes
• Reliable thermoelectric module fabrication methods need to be developed for the new high efficiency TE materials
• Status at MSU – Routine skutterudite production of hot pressed legs underway– Techniques for fabricating hot pressed LAST/LASTT still being developed– Module production methods are still under development
• Material strength and thermoelectric properties must meet life cycle performance criteria ….nanostructures in LAST have survived for six months at 600C
• Temperature dependant material properties are critical in order to conduct a detailed and accurate generator design
• Powder processing methods are being refined to provide increased strength while maintaining thermoelectric properties of ingot forms of the material
• ZT for the temperature ranges (700K) for best TE materials are about 1.4 and need to be closer to 3.0 to reach the efficiency goals requested by DOE
Accomplishment to dateAccomplishment to dateSystems for ingot synthesis and leg preparation demonstrated
70 hot pressed pucks of LAST, LASTT and skutterudite at MSU since 4/07Tube furnaces (~500 gms) and extensive powder processing facilities at MSUSegmented legs with an efficiency of 14.5% demonstrated at JPL
Module fabrication during past year at MSUFabrication of 2, 4 and 8 leg LAST/LASTT modulesFabrication of 2, 4 and 16 leg (40 watt) skutterudite modulesDemonstrated aerogel insulation technique for 16 leg module
Power electronic modules being designed and tested at MSUTemperature dependent elastic moduli and thermal expansion coefficients have been measured for LAST in collaboration with Oak Ridge National LabTransport measurements conducted by MSU have been verified by Northwestern, JPL, Iowa State and the general literatureNew material systems based on skutterudite composites and PbTe-PbSsystems have been examined and appear to offer promising improvementsAnalytical studies performed for various operation modes and conditions
Geometries for high efficiency heat transfer rates evaluatedFinite element analysis of pressing, contact metallization has been conducted and generator design is nextEfficiency improvements for various operational modes for the Cummins ISX engine evaluated for various geometriesHeat transfer studies excellent insulation and hot side convection required
Mechanical and thermal characterization of LAST and LASTT
•In service, the TE elements will be subjected to both thermal gradients and thermal cycling.
•Analytical or numerical analysis of the stress-strain behavior requires knowledge of the elastic moduli and the coefficient of thermal expansion as functions of temperature
•For both LAST and LASTT, we have determined the elastic moduli by the Resonant Ultrasound Spectroscopy technique and the coefficient of thermal expansion by thermal-mechanical analysis and by high temperature x-ray diffraction.
Temperature dependent elastic moduli and thermal expansion coefficient
300 400 500 600 70035
40
45
50
55
/ Heating/Cooling, MSUHP6 (n-type, Ag0.43Pb18Sb1.2Te20)
/ Heating/Cooling, MSUHP4 (p-type, Ag0.9Pb9Sn9Sb0.6Te20)
You
ng's
mod
ulus
(GPa
)
Temperature (K)300 350 400 450 500 550 600 650
16
20
24
28
32 MSUHP18 (p-type, Ag0.9Pb9Sn9Sb0.6Te20) MSUHP6 (n-type, Ag0.43Pb18Sb1.2Te20)
CTE
(x10
-6/K
)
Temperature (K)
• The study on temperature dependent elastic moduli and thermal expansion has been conducted in collaboration with the High Temperature Materials Laboratory, Oak Ridge National Laboratory.
FEA Analysis of Metallization Process
1. Materials in this analysis: N,P type skutterudite, Cobalt and Titanium
2. Study’s goal was to determine stress concentrations in regions of metallization
3. Temperature dependent material properties including CTE and elastic modulus needed for FEA and later material fatigue properties
4. Heat transfer rates on the mold surface can influence cooling rates and stress concentrations
Bulk Thermoelectric Materials
0
0.5
1
1.5
2
200 400 600 800 1000 1200 1400
ZT
Temperature (K)
Bi2Te
3
Pb18
Ag0.86
SbTe20
PbTe
CoSb3
La2Te
3
SiGe
PbTe-PbS(8%)
n-type Materials
Ba0.30
Ni0.05
Co3.95
Sb12
Mg2Si
0.6Sn
0.4Mg2Si
0.4Sn
0.6
0
0.5
1
1.5
2
200 400 600 800 1000 1200 1400
ZTTemperature (K)
CsBi4Te
6
Bi2Te
3
Na0.95
Pb20
SbTe22
Zn4Sb
3
Ag0.5
Pb6Sn
2Sb
0.2Te
10
Tl9BiTe
6
CeFe4Sb
12
PbTeYb
14MnSb
11
(AgSbTe2)0.15
(GeTe)0.85
p-type Materials
Ce0.28
Fe1.5
Co2.5
Sb12
5nm5nm
Best Hot Pressed Samples
0
0.5
1
1.5
2
300 400 500 600 700 800 900
ZT
Small Ingot
Large Ingot
PbTe
Hot Pressed(Increasing #of Cycles)
Temperature (K)
Ag0.86Pb18SbTe20
0
0.5
1
1.5
2
300 400 500 600 700 800 900
ZT
Small Ingot
Large Ingot
PbTe
Hot Pressed(Increasing #of Cycles)
Temperature (K)
0
0.5
1
1.5
2
300 400 500 600 700 800 900
ZT
Small Ingot
Large Ingot
PbTe
Hot Pressed(Increasing #of Cycles)
Temperature (K)
Ag0.86Pb18SbTe20
• Hot pressed samples initially exhibit lower ZT than ingot, but improve with repeated temperature cycling (healing of grain boundaries? stress relaxation?)
• Nanostructures persist after powder processing and hot pressing
• ZT calculated above using thermal conductivity from measured cast samples.
N-type skutterudite material development
Skutterudite crystal structure
• Background– High ZT reported in the 300-800K temperature range for
Bax Yby Co4 Sb12 skutterudite compositions1
– High ZT values mainly attributed to low lattice thermal conductivity due to the broad range of resonant phonon scattering provided by the Ba and Yb fillers
– Samples used for this study were prepared by a multi-step synthesis process, potentially difficult to scale-up
• Goal– Develop a scalable synthesis process for Bax Yby Co4 Sb12 skutterudite
compositions and evaluate TE properties in a first step– Evaluate applicability for integration into advanced TE couples for
waste heat recovery applications• Approach
– Ball milling • High-energy ball
mills: ≤ 15 g loads• Planetary ball mill:
≥ 50 g loads– Hot-pressing
• Graphite dies and plungers
1X. Shi et al. APL 92, 182101 (2008)
Planetary ball millHigh-energy ball mill
Hot-pressed pucks and disks of Bax Yby Co4 Sb12
Bax Yby Co4 Sb12 : initial transport properties results
• Ball milled Bax Yby Co4 Sb12 - initial transport properties– ZT ~ 1.3 at 873K (consistent with previous report)– ~ 40% improvement over n-type PbTe in the 873K-373K
temperature range– ZT improvement over doped-CoSb3 appears to be due to:
• Lower thermal conductivity (double rattler)• But also higher carrier mobility
0.00.10.20.30.40.50.60.70.80.91.01.11.21.31.41.5
300 400 500 600 700 800 900 1000
Temperature (K)
BaxYbyCo4Sb12
Pd & Te doped CoSb3
n-PbTe
1
10
100
1000
1E+18 1E+19 1E+20 1E+21 1E+22
Hall carrier concentration (cm-3 )
Pd & Te doped n-type CoS3
BaxYbyCo4Sb12
p-Ce
1 Fe3 R
u1 Sb
12
n-Ba
x Yby C
o4 Sb
12
p- Bi0.4Sb1.6Te3 n- Bi2Te2.9Se0.1
Cold-shoe Cold-shoe
Hot-shoe
≤ 873K
373K
~ 473KIllustration of skutterudite-
Bi2 Te3 couple
TH = 873K - TC =373K
TH = 773K - TC =373K
TH = 773K - TC =373K
With Bi2 Te3 segments 11.8 10.0 7.9
Without Bi2 Te3 segments
10.7 8.8 6.75
Couple efficiency (%)
At equivalent carrier concentration, the Hall mobility for Bax Yby Co4 Sb12 is higher than that
for doped CoSb3
PbTe – PbS system
1Androulakis, J. et al., JACS 2007, 129, (31), 9780-9788.
Background
•The material PbTe – PbS 8% has been shown to exhibit at enhanced ZT ~1.41 700 K.
•Reason for high ZT:
•high power factor at 700 K (17-19 uW/cmK2))
•Very low total thermal conductivity (0.8 W/mK)
•Low lattice thermal conductivity is the result of nanostructures formed by the spinodal decomposition and nucleation and growth phenomena
PbTe – PbS 8% nucleation and growth creates inhomogeneities
on the micro and naoscale
PbTe matrix PbS
S
TePb
EDS linescans show presence of PbS particles within the PbTe matrix.
SEM micrographs show presence of spinodal decomposition, nucleation and growth
EDS analysis
Kanatzidis effort
22 mm
Large-scale Synthesis of (Pb0.95 Sn0.05 Te) – PbS 8%
300 400 500 600 700 800
0.4
0.6
0.8
1.0
1.2
1.4
Pb0.95Sn0.05Te - PbS 8% : small scale large scale
ZT
Temperature, K
300 400 500 600 700 8000.40.60.81.01.21.41.61.82.0
Ther
mal
Con
duct
ivity
, W/m
K
Temperature, K
Pb0.95Sn0.05Te - PbS 8% κtot
κlat
Nanostructures in PbTe – PbS reduce lattice thermal conductivity, increasing ZT.
Thermoelectric properties are repeatable
•Successful preparation of ~100 g material per batch
300 400 500 600 700 8000
500
1000
1500
2000
2500
Elec
trica
l Con
duct
ivity
, S/c
m
Temperature, K
Pb0.95Sn0.05Te - PbS 8% small scale Pb
0.95Sn
0.05Te - PbS 8% large scale
Parasitic Contact Resistance and Efficiency
h
o
QP
=η
pn
chchccccc
RRRRRR
RR
++++
== 2121δ
M. H. Cobble, “Calculations of Generator Performance,” CRC Handbook of Thermoelectrics, CRC Press, 1995.
RRc=δ
0
0.05
0.1
0.15
0.2
0.25
0 0.5 1 1.5 2 2.5 3
η
ZTavg
δ = 0.0δ = 0.1
δ = 0.5
Th = 800KTc = 300K
2chR1chR
2ccR1ccRn p
oR
+-
I
cT
hT
hQ
Four Leg Modules – Soldered Contacts
0
0 .0 5
0 .1
0 .1 5
0 .2
0 .2 5
0 .3
0 .3 5
0 .4
0
0 .1
0 .2
0 .3
0 .4
0 .5
0 1 2 3 4 5 6
Out
put V
olta
ge (V
) Output Pow
er (W)
O u tp u t C u rre n t (A )
0
0 .0 5
0 .1
0 .1 5
0 .2
0 .2 5
0 .3
0 .3 5
0 .4
0
0 .1
0 .2
0 .3
0 .4
0 .5
0 1 2 3 4 5 6
Out
put V
olta
ge (V
) Output Pow
er (W)
O u tp u t C u r r e n t (A )
Voc
0.43W
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0 1 2 3 4 5
y = 0.39537 - 0.09177x R= 0.99995
Vol
tage
(V)
Current (A)
Thot = 885 KTcold = 300 K
Isc
Thot = 876 KTcold = 300 K
Voc
Isc
0.45 W
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0 1 2 3 4 5 6
y = 0.3171 - 0.057988x R= 0.99998
Vol
tage
(V)
Current (A)
8-Leg ModulesModule # Module RT
Resistancen-type leg
p-type leg
Module 1 0.026 Ω ETN204 ETP44
Module 2 0.026 Ω ETN204 ETP44
Module 3 0.027 Ω ETN204 ETP44
Module 4 0.027 Ω ETN204 ETP44
Module 5 0.026 Ω ETN204 ETP52
Module 6 0.027 Ω ETN204 ETP52
Module 7 0.025 Ω ETN204 ETP52
Expected module resistance = 24mΩ (~3mΩ per leg)
δ = 0.13
0
50
100
150
200
250
300
0 1 2 3 4 5 6
Vol
tage
(mV
)
Current (A)
Thot
= 387K
Thot
= 466K
Thot
= 637K
Thot
= 753K
Thot
= 814K 8-leg module #5
Tcold
= 300K
0
50
100
150
200
250
300
0 1 2 3 4 5 6
Volta
ge (m
V)
Current (A)
Thot
= 387K
Thot
= 466K
Thot
= 637K
Thot
= 753K
Thot
= 814K8-leg module #4
Tcold
= 300K
Module Resistance Values After Temperature Cycle
p n p n
10.19mΩ
3mΩ
3.3mΩ 6.73mΩ
8mΩ
7.76mΩ
6.13mΩ6.54mΩ
p n p
+ ‐
n
Total module resistance = 51.65 mΩ
δ = 0.91
Heater 700C
Cooling 100Cunicouple
p-leg n-leg
After testingTest set up
Skutterudite Unicouple made and tested at MSU
Belljar system (1atm argon)
•Unicouple power output ~ 2.5 Watts per 0.67cc of material
•Peak power at ~30 Amps, test limited by power supply current limitations
Skutterudite Unicouple 1
0
0.05
0.1
0.15
0.2
0 10 20 30 40 50 60
current (Amps)
pote
ntia
l (Vo
lts)
0
0.5
1
1.5
2
2.5
3
pow
er (W
atts
)
Skutterudite 16-leg module•Low temperature test confirmed Open Circuit Potential is in good agreement with what is expected
•Materials used in this module have ZT necessary for 40W power output: 700C- 100C assuming no parasitic contact resistance
16-leg Skutterudite module Fabrication and Testing
700W Heater
Water cooled
Cummins ISX 6 cylinder diesel engine
Thermal Power Split Hybrid – Options Using the electric power recovered from waste heat
I E
WheelsFD
Trans.
C
T
Σ
Induction Intercooler
InductionAir
EGR
EGRMixer
ESS, Batt+ Ultra-Cap
Th, qh
Pe @62% = Y kWPe @ 100% = Z kW
Pow
er E
lect
30 -
50 k
VA EGR Cooler
WP
Engine Coolant
Air
Exhaust
CoolantPump
TEG-1
Rad
iato
r
T/C or clutch
B-IM
G
ExcessElectricalPower
PoweredAncillaries
Cool Exhaust OutTEG-2
ηINV = 0.96ηBIMG =0.93ηmi = 0.89
X % of exhaust to EGR, (100- X) % of exhaust to turbine.
Additional energy recovery opportunity
Pm =249+TEG@ 100% , TEG@ 62%
Single TEG with exploded view of a module
ISX Engine Operating Conditions for ESC Duty Cycle Modes
HX: Heat Transfer Enhancement
Goal:
How to get high heat-transfer rate?
high heat transfer ratelow pressure drop
ribsDimplesvortex generatorshybrid
(combinations of ribs, dimples, …)
no rib
with rib
New Design Concepts w/ Heat Exchanger Cont.
• Dual TEGs Concept also modified
• 50cm Heat Exchanger added in EGR circuit after TEG
• Previous concept EGR temp was ~100K higher than Cummins test data
• Goal was to reduce EGR temperature with hope of increasing BHP
0.0
1.0
2.0
3.0
4.0
5.0
6.0
A-25 A-100 B-62 B-100 C-100
Operating Point
BSF
C Im
prov
emen
t (%
TE material efficiency 9.1% TE material efficiency scaled with packing factor and local temperature
Calculated BSFC Improvement LAST,LASTT-BiTe Materials
Collaborations/InteractionsCollaborations/Interactions
• MSU, JPL, Tellurex, Northwestern, Iowa State and Cummins Team continue to partner in this effort
• Office of Naval Research sponsored effort has provided the basis for new material exploration and assisted in module fabrication developments
• Oak Ridge DOE (High Temperature Materials Laboratory) has provided significant assistance in material property characterization
Publications/PatentPublications/Patent• Pilchak, A.L., Ren, F., Case, E.D., Timm, E.J., and Schock, H.J., (2007). “The
Effect of Milling Time and Grinding Media Upon the Particle Size Distribution for LAST (Lead, Antimony, Silver, Tellurium) Powders,” Philos. Mag. A.
• Ren, F., Case, E.D., Hall, B.D., Timm, E.J., and Schock, H.J., (2007). “Young’s Modulus of N-Type Last Thermoelectric Material Determined from Knoop Indention,” Chemistry of Physics and Materials.
• A. L. Pilchak, F. Ren, E. D. Case, E. J. Timm, H. J. Schock, C. -I. Wu, T. P. Hogan, (October 2007). “Characterization of Dry Milled Powders of LAST (lead-antimony-silver-tellurium) Thermoelectric Material,” Philosophical Magazine, v.87, Issue 29, pp. 4567-4591.
• Ren, F., Case, E.D., Timm, E.J., and Schock, H.J., (2007). “Young's Modulus as a Function of Composition for an N-Type Lead-Antimony-Silver-Telluride (LAST) Thermoelectric Material,” Philosophical Magazine, v.87, Issue 31, pp. 4907-4934.
• Ren, F., Case, E.D., Timm, E.J., and Schock, H.J., (2007). “Hardness as a Function of Composition for N-Type Last Thermoelectric Material,” Applied Physics Letters.
• Ren, F., Case, E.D., Timm, E.J., Jacobs, M.D., Schock, H.J., (2007). “Weibull Analysis of the Biaxial Fracture Strength of a Cast P-Type Last-T Thermoelectric Material,” Philosophical Magazine Letters.
• Case, E.J., Ren, F., Schock, H.J., and Timm, E.J., (2006). “Weibull Analysis of Biaxial Fracture Strength of Cast P-Type LAST-T Thermoelectric Material,” Philos. Mag. Lett. 86 [10]: pp. 673-682.
Major Plans for Remainder of Phase 2 EffortMajor Plans for Remainder of Phase 2 Effort
July ~ Aug, 2008: 2,4,8 and 16 leg modules being fabricated, segmented concepts evaluated and powder processing method development ongoingAug ~ Nov, 2008: Module construction and performance testing Sept ~ Nov, 2008: Advanced heat exchanger design and numerical simulation of expected system performanceDec, 2008: Choose a thermoelectric system to demonstrate a 500 watt generatorJanuary 2009: Complete 500 watt generator designFeb-June 2009: Construct the 500 watt generatorJuly-Nov. 2009: Generator testingDecember 2009: Complete report on generator performance
SummarySummary• Systems for material synthesis, powder processing, hot
pressing, leg and module fabrication are operational at MSU
• Performance testing of legs and modules at MSU is in agreement with others doing similar measurements
• Can produce materials required for a 500 watt module in one month
• Power conditioning electronics for being designed and tested
• Improved head exchanger designs are critical to success of TE effort for waste heat recovery
• Using TEG technology, a 5% improvement in bsfc for and OTR truck is a reasonable 5 year goal …10% improvement possible with new TE materials