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

1

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

2

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

3

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

7

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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