thermoelectric ty

7/27/2019 Thermoelectric Ty

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ThermoelectricityPresented By-:

Mohammad Rameez


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ENERGY ROADMAP

100% energy

from power

source

25% effective

power

5% parasitic

losses

30% coolant

40% Heat Losses Can we convert this heat

into some useful energy?


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Introduction

The pioneer in thermoelectrics was a German scientistThomas Johann Seebeck (1770-1831)

Thermoelectricity refers to a class of phenomena in

which a temperature difference creates an electric potential

or an electric potential creates a temperaturedifference.

Thermoelectr ic power generator is a device that

converts the heat energy in to electr ical energy based on

the pr incip les of Seebeck effect

Later, In 1834, French scientist, Peltier and in 1851,

Thomson (later Lord Kelvin) described the thermal

effects on conductors


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Thermoelectricity - known in physics as the

"Seebeck Effect"

In 1821, Thomas Seebeck, a German physicist,

twisted two wires of different metals together

and heated one end.

Discovered a small current flow and so

demonstrated that heat could be converted to

electricity.

Seebeck Effect


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Seebeck Effect

Metal rod

Electron mobility Phonon motion

Photon

Phonon motion

Electron mobility

Electrons in the hot region are more

energetic and therefore have greater

velocities than those in the cold region

dT

dVS

Seebeck Coefficient

Heat transfer through electrons

and phonons (lattice vibrations)

Al Al


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Seebeck Effect


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PELTIER EFFECT

In 1834, a French watchmaker and part time

physicist, Jean Peltier found that an electricalcurrent would produce a temperature gradient atthe junction of two dissimilar metals.


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PELTIER COOLING

>0 ; Positive Peltier coefficient

High energy holes move from left toright.

Thermal current and electric currentflow in same direction.

q=*j, where q is thermal current density and j is electrical currentdensity.

= S*T (Volts) S ~ 2.5 kB/e for typical TE materials

T is the Absolute Temperature


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PELTIER EFFECT

Peltier Effect Thermoelectric

Cooler Diagram:


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PELTIER EFFECT

Peltier Effect Animation:

As current passes through the 1st plate, the negative electrons and positive holes(called carriers) transport the heat making the 1st plate to be warm (heat isabsorbed) and the 2nd plate to be cold (heat is released).


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THOMSON EFFECT

Discovered by William Thomson (Lord Kelvin)When an electric current flows through a conductor, the

ends of which are maintained at different temperatures,

heat is evolved at a rate approximately proportional to the

product of the current and the temperature gradient.

dx

dTI

dx

dQ

is the Thomson coefficient in Volts/Kelvin

Seebeck coeff. S is temperature dependent

dT

dSTRelation given by Kelvin:


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THE SEEBECK EFFECT:

EMF caused by temperaturegradient across two dissimilarconducting metals, whichform a closed loop.1

THE PELTIER EFFECT:

Temperature differentialcaused at the junctions ofdissimilar conductors, withthe passing of current.1

THE THOMSON EFFECT:

Electrical current causedby a temperature gradientin a single homogeneousconductor. 1

THERMOELECTRIC EFFECTS(SUMMARY)


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2S

z


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Efficiency of Thermoelectric Devices

Desirable > 0.2


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EFFICIENCY OF THERMOELECTRIC DEVICES

The Figu re of Meritof a thermoelectric material

S= Seebeck coefficient

= Electrical conductivity

ke= Electronic contribution to thermal conductivity

kp= Phonon contribution to thermal conductivity

Tkk

SZTpe

2

Conflicting Issues in Design:

Increasing (through increase in n) reduces S

Increase in accompanied by an increase in eIncreasing effective mass m*: increase in S, but decrease in

Attempts to change p interferes with changes in (mobility)

Metals: S ~ 10 V/K, Semiconductors: S ~ 100 V/K

Degenerate semiconductors, heavy atoms, soft spring constants ofbonds: Bi2Te3 and its alloys

*

2

mne

3

2

2

2

B

2

)3n

(m3eh

Tk8S

m*/(n1/3[k/])


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Thermoelectric materials

The good thermoelectric materials should possess

1. Large Seebeck coefficients

2. High electrical conductivity

3. Low thermal conductivity

The example for thermoelectric materials

BismuthTelluride (Bi2Te3), Lead Telluride (PbTe),

SiliconGermanium (SiGe),

Bismuth-Antimony (Bi-Sb)


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LT


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THERMOELECTRIC EFFECT IN SEMICONDUCTORS

Thermoelectric power generation is explained by a gradient in conduction band energy, across a

material. This gradient in conduction band energy is caused by an applied thermal gradient.

For homogeneous materials the conduction band energy is directly related to temperature.

Electrons on the hot side of a material have greater conduction band energy than those on the

cold side producing an EMF.3

e

e

ee

e

e

3. http://ecee.colorado.edu/~bart/book/


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h

h

hh

h

h

Ev

EC

e

e

ee

e

e

As for conduction band energy, valence band energy is also varied across a material with anapplied thermal gradient. In this case, valence band carriers are termed holes, and correspond to

an absent electron.3


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e

e

e

e

e

e


e

e

ee

h

h

h

h

h

h

h

h

hh

n-type Materials

For n-type materials, electrons are the primary charge carriers for which appliedthermal gradients produce an EMF in the direction shown above.3

p-type Materials

For p-type materials, holes are the primary charge carriers for which appliedthermal gradients produce an EMF in the direction shown above.3


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e

e

e

e

e

e


h

h

h

h

h

h

n-type Material

p-type Material

L

OAD

+

-

When connected in series, the two materials produce thermoelectric power capable of powering aload.


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MATERIAL OF CHOICE FOR THERMOELECTRICITY

TE Parameters

Materials

Metals

Insulators

Semiconductors

Semiconductors most suitable TE material.

Allow separate control of G (electrons) and (phonons).

Electrical

Conductivity(G)

Seebeck

Coefficient(S)

Thermal

Conductivity()

High~102 W/m-K

High

Moderate10-3S/m

High~120 V/K

Very High~107 S/m

Low~ 10V/K

Low~10-2-10-4 W/m-K

Low~10 W/m-K

Extremely

low (~10-10S/m)


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Conventional Thermoelectric Materials

Optimized Bi2Te3 though Sb/Se substitution


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POTENTIAL ZENHANCEMENT IN LOW-DIMENSIONAL MATERIALS

Increased Density of States near the Fermi Level:

high S2(power factor)

Increased phonon-boundary scattering: low

high Z = S2/:


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THIN FILM SUPERLATTICE THERMOELECTRIC MATERIALS

Thin film superlattice

Approaches to improve ZS2/:

--Frequent phonon-boundary scattering: low

--High density of states nearEF: high S2in QWs

Quantum well

(smallerEg)Barrier(largerEg)

Incidentphonons Reflection

Transmission

Phonon (lattice vibration wave)

transmission at an interface

Interface

LOW DIMENSIONAL THERMOELECTRIC MATERIALS


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LOW-DIMENSIONAL THERMOELECTRIC MATERIALSThin Film Superlattices of

Bi2Te3,Si/Ge, GaAs/AlAs

Ec

Ev

x

E

Quantum wellBarrier

Top View

Nanowire

Al2O3 template

Nanowires of

Bi, BiSb,Bi2Te3,SiGe


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THERMOELECTRIC PROPERTIES OF THIN-FILM MATERIALS

USED FOR THERMOELECTRIC MICROSENSORS


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THERMOELECTRIC DEVICES


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SEMICONDUCTOR PELTIER COOLERS

Bismuth-Telluride n and p

blocks

An electric current forces

electrons in n type and holes in ptype away from each other on the

cold side and towards each other

on the hot side.

The holes and electrons pull

thermal energy from where theyare heading away from each other

and deliver it to where they meet.


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Electrical Power Generation and Cooling

Wasted heat to electricity

Environment-friendly

No moving parts: easy maintenance

Long life

Precision control of T with spatial resolution


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THERMOELECTRIC MEMS DEVICES

The miniaturisation and development of MEMS based

thermoelectric devices has the potential to improve the

performance of thermoelectric devices, and create new

applications for the technology. Thermoelectric MEMS based

devices, based on thin-film technology, that are compatiblewith modern semiconductor processing techniques have

now started to enter the market place.


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THERMOELECTRIC MICRO POWER GENERATORS

Miniaturized, fully-integrated, monolithically

and batch-fabricated thermoelectric power

generator capable of powering MEMS devices

Sufficient power (individually or in small

arrays) to replace batteries in macroscale

systems such as weapons,man-portablecomputers, radios, and GPS receivers

Thermocouple probe

Micromachined scanning thermocouple

probe for high resolution temperaturemapping, topographical mapping, surface

imaging

e.g. for ULSI diagnostics


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APPLICATIONS

Deep space probes

Microprocessor cooling

Laser diode temperature stabilization

Temperature regulated flight suits

Air conditioning in submarines

Portable DC refrigerators

Automotive seat cooling/heating

Radioisotopic Thermoelectric

Generator (RTG)


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Some Applications of Thermoelectrics


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APPLICATIONSWater/Beer Cooler

Cooled

Car Seat

Electronic Cooling

Cryogenic IR Night Vision

Laser/OE Cooling

TE

Si bench

AUTOMOBILE


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In ATEGs, thermoelectric materials arepacked between the hot-side and the cold-

side heat exchangers. The temperature

difference between the two surfaces of

thethermoelectric module(s) generates

electricity.

Thermoelectric generator in a Volkswagen

Golf Plus lowers fuel use by 5% .

AUTOMOBILE
http://en.wikipedia.org/wiki/Heat_exchangershttp://en.wikipedia.org/wiki/Thermoelectricityhttp://en.wikipedia.org/wiki/Thermoelectricityhttp://en.wikipedia.org/wiki/Heat_exchangers


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Thermionic Watch absorbs heat fromthe wrist and dissipates it through thefront side of the watch. The internalthermoelectric generator converts thetemperature into electricity and drives thewatch. The heat powered Thermicwatches utilizes heat energy

continuously while wearing on the wrist.The memory chip inside the watch willkeeps tracks of time even when not incontact with the body.

The Seiko watch Introduced in 1988 .

under normal operation the watchproduces 22W of electrical power. With

only a 1.5K temperature drop across theintricately machined thermoelectricmodules, the open circuit voltage is 300mV, and thermal to electric efficiency isabout 0.1%.

BIO WATCHES (THERMIC WATCHES)


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