Thermoelectric Generators for Body Heat Energy Harvesting PIs: Mehmet C. Ozturk, Ki Wook Kim & Daryoosh Vashaee
Graduate Students: Namita Narendra, Amin Noziariasbmarz, Viswanath Ramesh, Francisco Suarez
Electrical and Computer Engineering Department
NC State University
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SAP Gen-1 Task Presentations
Cloud StorageSmart Phone
IOIO
User Interface
Radio
ASSIST Custom
Off the Shelf Components
Antenna
Power Management
SPI
RADIO
Energy Harvesting
ECG Electrodes
SOC
Armband w/ ECG Electrodes
Aggregator
AFE
On Node DSP
ULP Accel
Signal Processing
Signal ProcessingEnergy Storage
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Thermoelectric Energy Harvesting From the Body
Two reasons: Large temperature
drop across the skin –essentially a thermal insulator
Large temperature drop across the heatsink if there is no air flow (i.e. convection)
While the temperature difference between the body and the ambient is about 10 – 15 degrees, very little of this drops across the TEG
∆T
25 °C
37 °C
Rheatsink
RTEG
RSkin
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TEG Objectives - System Requirements
Gen 1: 50 μW power
Higher TEG voltage increases the efficiency of the DC/DC boost convertor:
Thermoelectric Generator (TEG)
DC/DC boost convertor
10-500 mV
1.4 V
Sensors and electronic circuits
Input Voltage Booster Efficiency
10mV 21%20mV 56%50mV 72%
100mV 79%4
TEG System Model
5
ZT=2ZT=1
Pow
er (μ
W)
Material Optimization for Body Heat Energy Harvesting ZT is NOT everything Low thermal
conductivity is more important than Seebeck due to the large parasitic resistances.
Assumptions:Fill factor = 25%TEG Area = 10 cm2
Heat Spreader = NoneHot Side Heat Transfer Coeff. = 55 W/m2KCold Side Heat Transfer Coeff. = 100 W/m2K 6
Theoretical Calculations
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Multi-Scale Modeling of Thermoelectric Properties
Tailoring the Seebeck coefficient Exploit non-local transport and/or DoS modification
for enhanced contribution of high energy carriers Tailoring electrical/thermal interfacial resistance Nano-crystals with randomly or preferentially
aligned grains Impact of heterojunctions
Thin-Film Materials:
Nano-Bulk Materials:First principles Band structure, Band gap, Effective mass, Non-
parabolicity, Dielectric constants, Grüneisenparameter, Deformation potentials, Lattice
thermal conductivity.
Multi-band BoltzmannElectrical cond., Seebeck coeff., Total thermal
conductivity
Coherent Potential AppRelaxation times, Corrections to E-k,
Mobility edges
Material Design Rules
Optimizing TE properties in a multilayer design Effect of film thickness and surface quality Effect of adjacent layer (strain, surface charge
defects, etc.)8
Electronic band properties obtained by DFT
Bi2Te3
Sb2Te3
Band gap (eV) Bi2Te3 Sb2Te3
Calculation 0.114 0.049
Experiment 0.15 ± 0.02 0.28
Effective mass tensors (m0):
Band structures of Bi2Te3 & Sb2Te3
Assume an ellipsoidal band expression:
α11k12 + α22 k2
2 + α33 k32 + 2α23 k2k3 = 2m0E/2
Bi2Te3 Sb2Te3Valence
bandConduction
bandValence
bandConduction
bandα11 32.3 36.02 11.62 8.73
α22 6.4 7.18 2.32 3.9
α33 10.3 12.01 7 6.76
α23 2.1 3.1 1.1 0.76
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Development of nanocomposite TE materials
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Bulk Nanocomposite Thermoelectric Materials
Phonon
ElectronPoudel at al, Science, 2008
Inexpensive to makeCompatible with the existing
from of the TE devices Less sensitive to electrical
contacts Less sensitive to thermal
contactsAppropriate for large scale
production
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Start with melting the mixed elemental of Bi, Sb and Te
Induction furnace melting/mixing
Description of Technology: Alloy
Nanopowder Synthesis and Mechanical Alloying
High Energy Ball Mills
1µm
5nm
Nanocrystals
Description of Technology: Powder
Plasma Pressure Compaction (P2C) of TE Nanocomposites
DC current (0-3000A)
Description of Technology: Ingot
sampletube
In-situ Decrystallization in MW cavity
Sample
CirculatorMicrowave source
WaveguideTuner
Sliding Short
Dummy Load
E
Description of Technology: Amorphization
Description of Technology: Amorphization
Crystalline
Amorphous
Bi-rich
Crystalline-Amorphous nanocomposite C
ryst
allin
e
Am
orph
ous
TEM Images of the Nano Bulk Ingots
X-ray diffraction
Alignmentc
a
Texturing Nano Bulk Bi0.5Sb1.5Te
S021 Sb2Te3
Bi2Te3
(Bi2Te3)x(Sb2Te3)1-x Nanocomposites
ZT=2
ZT=1
Pow
er (μ
W)
Comparison of Nanocomposite Materials
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ZT=2
ZT=1
Pow
er (μ
W)
Comparison of Nanocomposite Materials
S103
S104
S120a40
S120S021
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Hard Substrate TEG devices
Ingots were diced to 0.6x0.6x2mm legs
Used nanocomposite p-type and commercial n-type legs
TEG devices were bonded using Bi0.57Sn0.42Ag0.01 solder
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Metronome:Set to Beats per minute (BPM)
Swing distance:fixed distance in between two markers
Velocity = Arclength x BPM / 60 s
Data Collection
(Collaboration with Myers/Jur)
0
100
200
300
400
500
0 20 40 60
Mea
sure
d Po
wer
(uW
)
Time (s)
00.250.50.821.131.4
TEG wristband
Air Velocity (m/s):
No heatsink
Measured and Projected TEG Power
Using COTS TEGs without a spreader or a heatsink, the wristband generates 40 – 400 μW
Projected power with optimized leg dimensions and nanostructured legs is 100 – 1000 μW
0
200
400
600
800
1000
0 0.5 1 1.5
Pow
er (μW
)
Air Velocity (m/s)
Measured COTS TEG
Nano p – type / STD n – type2 mm legs
Nano n & p – type / 2 mm legs
Nano p-type /STD n-type /1.3mm legs
TEG Area = 16.5 cm2
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Comparison with Commercial TE Devices
Voc
(mV/cm2)Isc
(mA/cm2)Pout
(μW/cm2)COTS 18.4 1.5 5.7 StationaryCOTS 52.9 3.2 35.5 Airflow
ASSIST 49.7 3.9 44.2 StationaryASSIST 97.4 7.1 156.5 Air flow
Used 14.3 cm2 spreader on both sides.
ASSIST COTS
Air flow
Air flow
Stat.
Stat.
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Compare with other body heat TE energy harvestersVoc (mV/cm2) Power per cm2 Condition Location Ref
Jo et al 0.2 0.004 nW No heat sink Body Electronics Letters, 48, 16, 2012
Wang et al 37.5 0.08 nW No heat sink Wrist Sensors and Actuators A: Physical, 156, 1, 2009
Kim at al 0.1 0.42 nW No heat sink Wrist Transducers, Barcelona, SPAIN, 16-20, 2013
Jo et al 0.4 4 nW No heat sink Body16th Int. Conf. on Miniaturized Systems for Chemistry and Life Sciences, Oct. 28 - Nov. 1, 2012, Okinawa, Japan
Kim et al - 8.1 nW No heat sink Chest Smart Mater. Struct. 23,105002, 2014
Im et al - 46 nW No heat sink Chest Nano Research, 7, 4, 2014
Strasser et al - <100 nW No heat sink Exp. setup Sensors and Actuators A: Physical, 114, 2–3, 2004
Wahbah et al 1.6 2.22 μW Large heat sink WristIEEE J Emerging and Selected Topics in Circuits and Systems, 4, 3, 2014
Leonov et al - 20 μW Very large heat sink Wrist IEEE Sensors J, 13, 6, 2013
Settaluri et al 10.8 21.6 μW 1.1mm grooved heat sink and spreader, 2mm TE legs, no air Wrist Journal of Electronic Materials, 41,
6, 2012
ASSIST 49.7 44.2 μW 0.1mm flexible spreader, 2mm TE legs, no air Wrist 25
Flexible TEG
Large area non-burdening flexible lightweight conformal to the body We do not want large
clunky heat-sinks
Compatible with industrial TE legs and soldering process
Body Comfort Compatibility
Commercial TE legs
Conformal to the body –small contact resistance
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Flexible TEG Development
Collaboration with Zhu/Jur
Compatible with:Bulk Thermoelectrics including ASSIST NanocompositesPick-and-place tooling & Thermal Compression Bonding
(Low Cost-of-Ownership – Easy Adaptation)
Patent PendingFlexible Thermoelectric Modules and Methods of Fabrication
PCT/US2015/026376 – filed on April 17, 2015 27
Stretchable Flexible Thermoelectrics
Continue exploring materials to reduce the thermal conductivity of the stretchable medium
Continue exploring methods to achieve low-resistance stretchable metal interconnects
PDMS embedded with hollow glass microspheres
Stretchable Materials with Low Thermal Conductivity Stretchable Metals as TEG
interconnects
Collaboration with Dr. Zhu
Stretchable Ag NWs on PDMS
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Summary of TEG research projects
A novel process for fabrication of flexible TE devices were developed, which is compatible with bulk TE legs
Theoretical codes for material and device optimization Dense, crack free, p type nanocomposites with small
thermal conductivity and high ZT ASSIST TEGs generate 44-156 μW/cm2, i.e. 4-7 times more
than COTS TEGs
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Thank you.
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