micro thermoelectric generator

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A REPORT ON MICRO

THERMOELECTRIC GENERATOR

DEVICE

INSTRUCTOR IN-CHARGE: DR. N.N. SHARMA

BITS F415 INTRODUCTION TO MEMS

BIRLA INSTITUTE OF TECHNOLOGY AND SCIENCE, PILANI

By:

KSHITIZ GUPTA 2012A4PS141P

ABHISHEK BANERJEE 2012A4PS330P

SHUBHAM MAURYA 2012A4PS430P


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Acknowledgement

We extend our sincere Gratitude towards Dr. N.N.Sharma for giving us this

opportunity and guiding us through not only the project but also the course

Introduction to MEMS. He introduced us to the vast field of MEMS and

inspired us to work in the same.

Also, we thank Mr. Vijay Kumar and Mrs. Tamalika Bhakat for their continuoussupport in modelling and simulation in COMSOL Multiphysics.


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Contents

TOPIC Pg. No.

Introduction 3

Abstract 3

Working Principle 4

Application 6

Material Used 7

Governing Equations 8

Design/Geometry 11

Simulation 14

Literature Review 17

Innovation 18

References 19


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Introduction

We in our daily lives can see numerous instances where different appliances because of

their inefficiencies loose energy in form of heat for example a simple 60W incandescent

light bulb has an overall luminous efficiency of only 2.1% . Many other electrical devices

loose energy due to heating. In this background harnessing this waste energy via

Thermoelectric Generators which converts a temperature gradient into electrical potential,

is the key to this problem.

AbstractThis projects aims at reading the currently available literature on electricity generation from

waste heat using Thermoelectric Generators, understanding the principles working in the

background. It also includes re-creating the effect from a reference model and attempting

to increase the output potential by proposing changes in the model. COMSOL Multiphysics

Software is used for modelling and simulation of the device.

The device has a Thermopile i.e. series of thermocouples made of Bismuth Telluride. The

Thermoelectric module of COMSOL is used to solve the simulation.


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Working PrincipleThermoelectric effect: The term "Thermoelectric Effect" encompasses three separately

identified effects: the Seebeck effect, Peltier effect, and Thomson effect.

1. Seebeck Effect:The Seebeck effect is the conversion of temperature differences

directly into electricity and is named after the Baltic German physicist Thomas

Johann Seebeck.

Eemf= - S. T

where S is the Seebeck Co-efficient of the material.

2. Peltier Effect:The Peltier effect is the presence of heating or cooling at an

electrified junction of two different conductors and is named for French physicist

Jean Charles Athanase Peltier.

Q= ( a- b ). I

here is the Peltier Coefficient of the material.

Fig 1: Schematic diagram for a Thermo-electric device


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Fig 2: Schematic view of a thermoelectric generator that consists of n- and p-type thermoelectric

legs which are electrically connected in series and that are thermally arranged in parallel;Micromachined CMOS thermoelectric generators as on-chip power supply;

M strasser, Aigner et al. [2004]

3. Thomson Effect: This Thomson effectwas predicted and subsequently

observed byLord Kelvinin 1851. It describes the heating or cooling of a current-

carrying conductor with a temperature gradient.

If a current density is passed through a homogeneous conductor, the Thomsoneffect predicts a heat production rate per unit volume of:

Where, is the temperature gradient and is the Thomson coefficient.

The Thomson coefficient is related to the Seebeck coefficient

as (seebelow). This equation however neglects Joule heating, and

ordinary thermal conductivity
http://en.wikipedia.org/wiki/William_Thomson,_1st_Baron_Kelvinhttp://en.wikipedia.org/wiki/William_Thomson,_1st_Baron_Kelvinhttp://en.wikipedia.org/wiki/William_Thomson,_1st_Baron_Kelvinhttp://en.wikipedia.org/wiki/Thermoelectric_effect#Thomson_relationshttp://en.wikipedia.org/wiki/Thermoelectric_effect#Thomson_relationshttp://en.wikipedia.org/wiki/Thermoelectric_effect#Thomson_relationshttp://en.wikipedia.org/wiki/Thermoelectric_effect#Thomson_relationshttp://en.wikipedia.org/wiki/William_Thomson,_1st_Baron_Kelvin


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ApplicationAll the electrical and electronic devices and mechanical equipment suffer from various

inefficiencies due to which they generate a lot of heat, which is pure loss of energy. In this

modern era where the deficiency of energy is of utmost importance, such devices which try

to harness the waste energy is the need of the hour. The device can be used in car radiators,

PC heat sinks, incandescent bulbs, household electrical appliances. As a future aspect, if this

device is further developed it can be used in coasters used for coffee/tea mugs and also can

be incorporated with garments as wearable technology, this way well be able to harness

the waste heat from various domains of our world which we tend to neglect.


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Material usedThermocouple: Bismuth Telluride

Sr.

No.

Property Value Unit

1 Seebeck Co-efficient p: 200 e-6

n: -200 e-6

V/K

2 Electrical Conductivity 1.1 e5 S/m

3 Thermal Conductivity 1.6 W/mK

4 Density 7740 Kg/m2

5 Heat Capacity 154.4 J/KgK

Bismuth Telluride (Bi2Te3) :

It is a grey powder that is a compound of bismuth and tellurium.

Used for efficient thermoelectric material for refrigeration or portable power

generation

Known to have one of the highest Seebeck Co-efficient

Alloyed with bismuth, antimony, tellurium, and selenium to get n-doped and p-

doped semi-conductors.

Electrical Contacts:

For the thermal electrodes at the junction a material which is good thermal as well as

electrical conductor was needed. Copper is one of the best suited materials for this

application.

Thermal Conductivity: 401 W/mK

Electrical Conductivity: 5.9 x 107

-1m

-1


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Governing EquationsThe principle of working of our device can be explained on the basis of the following two governing

equations:

1. Electric current balance:

(V) = 0 (1)

2. Heat energy balance:

Cp + q = Q

(2)

q = - kT + PJWhere,

a.

: electric conductivity [S/m]

b.

V : electric potential [V]

c. : density [kg/m2]

d. Cp : heat capacity [J/kg K]e.

T : temperature [K]

f.

q : heat flux [W/m2]

g. k : thermal conductivity [W/mK]h. P : Peltier coefficient [V]

i.

J : current density [A/m2]

j.

The combination of seebeck-peltier effect and the variation of these properties of the

material with temperature were first studied in detail by Lord Kelvin who called it the

Thomson effect. He gave 2 relationships which relate the Peltier, Seebeck and Thomson co-

efficients of a given material [1]. These are given below:

The first relation is-

where is the absolute temperature, is the Thomson coefficient, is the Peltier coefficient,

and is the Seebeck coefficient.


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The second relation is-

= -TS

which shows that Peltier and Seebeck coefficients of any thermoelectric material are linear

related.

3. Weak form Derivation:

(throughout this derivation, bold characters are used to represent vector quantities)

o To transfer energy balance to weak form. We multiply each side of energy balance by a

test function (Ttest) and integrate over the computational domain .

o The main steps to be followed are the manipulation done using Gauss Theorem and

substitution of flux value from the heat energy balance equations mentioned above

(equations (2))

(reference: Multiphysics Analysis of Thermoelectric Phenomenon)

We start with the heat energy balance governing equation:

() + () =

Now, using the vector identity (Ttestq) = q(Ttest ) + Ttest (q), we can write the aboveequation as:

() + [( ) ( ) =

To convert the above written volume integral to a surface (area) integral, we will use

Gauss theorem:

( )

= (.) ,

where nis the unit normal vector to the boundary of domain .

Using this theorem, the last equation mentioned above can be transformed to-

() + (.) - [.] d =

Now, the net energy flux coming out from the domain considered is given by:

q= -k + PJ


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Substituting this relation, the last equation above equation can be written as:

(.) = [ + (). () + ]

4.

Peltier Weak Contribution:

The Petier weak contribution in this analysis is given by the 2nd

term of the RHS of last

equation above. It can be expanded into the following form:

5.

(PJ)Ttest= PJx Ttest + PJy Ttest

+ PJz Ttest

z

= P*ec.*Jx*test(Tx) + P*ec.Jy*test(Ty) + P*ec.Jz*test(Tz)

[In terms of COMSOL notations]


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Design/GeometryThermo-leg:The original model has a thermo-leg which is a rectangular column of cross

section 50x50 microns and height 200 microns made of n-type and p-type Si-Ge, howeverthe new proposed model has a cylindrical thermo leg with OD=50 microns and height 200

microns made of Bismuth Telluride.

Original Thermocouple Proposed model of thermocouple

Copper Contacts: The lower contact is a pad with cross section 50X50 microns and height 10

microns, the upper contact is a rectangular pad with cross section 50x200 microns and

height 10 microns.

Thermopile: The thermopile is the collection of 64 thermocouples connected in series


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The following tables sum up the properties of the materials used.

Material parameters

Name Value Unit

Electrical conductivity 5.998e7[S/m] S/m

Heat capacity at constant

pressure

385[J/(kg*K)] J/(kg*K)

Relative permittivity 1 1

Density 8700[kg/m^3] kg/m^3

Thermal conductivity 400[W/(m*K)] W/(m*K)

Seebeck coefficient 1.5e-6 V/K

Copper

Material parameters

Name Value Unit

Thermal conductivity k(T[1/K])[W/(m*K)] W/(m*K)


pressure

C(T[1/K])[J/(kg*K)] J/(kg*K)

Density rho(T[1/K])[kg/m^3] kg/m^3

Seebeck coefficient -190e-6 V/K

Electrical conductivity 16129 S/m


n-type silicon germanium

Material parameters

Name Value Unit

Thermal conductivity k(T[1/K])[W/(m*K)] W/(m*K)


pressure

C(T[1/K])[J/(kg*K)] J/(kg*K)

Density rho(T[1/K])[kg/m^3] kg/m^3

Seebeck coefficient 35e-6 V/K


Electrical conductivity 90909 S/m

p-type silicon germanium


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Material parameters

Name Value Unit


pressure

154[J/(kg*K)] J/(kg*K)


Seebeck coefficient -200e-6 V/K

Electrical conductivity sigma(T) S/m

Thermal conductivity k(T) W/(m*K)


n-bismuth telluride

Material parameters

Name Value Unit


pressure

154[J/(kg*K)] J/(kg*K)


Seebeck coefficient 200e-6 V/K

Electrical conductivity sigma(T) S/m

Thermal conductivity k(T) W/(m*K)


p-bismuth telluride


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SimulationThe Thermoelectric module of COMSOL Multiphysics was used to simulate the above

mentioned design. Each thermocouple was subject to a temperature gradient of 550 Kelvin(assuming the device is installed in a radiator of a vehicle) with the lower temperature being

323.15K and higher being 873.15K.

Simulation Result:

Each thermocouple generated an electrical potential of 0.22V and the net potential

generated by the thermopile is nearly 12.5V

The following images show the temperature and electric potential distribution along theheight of a single thermocouple:


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The following graph shows the data depicted in the above images in a graphical manner :

The following images show the temperature and electric potential distribution from the 1st

to 64th

thermocouple:


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The following graph shows the data depicted in the above images in a graphical manner


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Literature Review

Premise

The idea for a thermoelectric generator was first obtained from a paper presented by

Selemani Seif and Kenneth Cadien in a COMSOL conference in 2012 in Boston. This served as

a template for our design of the micro-thermoelectric generator as presented in this report.

On further exploration, we came across Micro Electronic and Mechanical Systems by Chris

Gould and Noel Shammas,which provided a very extensive review of the existing scenario

with respect to Micro thermoelectric devices. We studied the methods presented by various

other researchers to thoroughly understand the principles of thermoelectric and peltier

effects. These included research by N. Wojtas, L. Rthemann, W. Glatz, C. Hierold as well as

by Xiaodong Jia, Yuanwen Gao in Elsevier.

Material Selection

A number of semiconductors were surveyed for the list of possible materials which

could be used for our project. This was because we had studied that the

semiconducting materials generally have high Seebeck coefficient, which is due to the

fact that they can generate a large number of charge carriers even for a small

temperature gradient. A report by Lobat Tayebi, Zahra Zamanipour, Daryoosh Vashaee

and another by Zhao-kun Cai

gave a lot of insight into the use of Bismuth Tellurium as

a viable and highly effective alternative to Silicon Germaniumwhich was used in the

design by Selemani Seif and Kenneth Cadien. A number of other polymer based

materials were also reviewed. However, these couldnt be implemented as these

didnt exist in the COMSOL library.

Application

A number of applications ranging from solar power generation, cooling of microscale

devices as well as uses in the automotive and aerospace industry were explored. Y.Y.

Hsiao, W.C. Chang, S.L. Chenpresented an idea of using a thermoelectric device in the

exhaust system to generate power. This greatly influenced our vision and we decided

to use the thermocouple to develop a thermopile for this very same use. Similar

research was also done by Yuchao Wang, Chuanshan Dai, Shixue Wang and C.Q. Su,

W.S. Wang, X. Liu, Y.D. Dengand proper insight was gained from these.


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Innovation We incorporated circular thermoelectric legs in the design instead of the common

rectangular legs. This helped us in obtaining marginal increase in the potential output,

as mentioned by Ahmet Z. Sahin, Bekir S. Yilbas

The material used in the original paper was Silicon Germanium. On proper literature

review, we decided to go ahead with Bismuth Tellurium as this would give a higher

potential output. The simulation results were positive in this aspect as there was a

more than 100% increase in the potential output than was presented in the paper.

Realizing that this device could have far extensive implications, we decided to make a

thermopile consisting of 64 thermocouples which would be utilized to generate a

potential difference utilizing the waste heat of an automobile exhaust. Our simulation

results showed that a potential output of nearly 12 V could be obtained from the

designed thermopile. The design for the thermopile was done independently by our

team, with a brief idea from the aforementioned reports.


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References A Review of Thermoelectric MEMS Device for Micro-power Generation, Heating and

Cooling Applications

By Chris Gould and Noel Shammas

Thickness Designs for Micro-Thermoelectric Generators using Three Dimensional PDE

Coefficient-Comsol Multiphysics 4.2a Analysis

By Selemani Seif and Kenneth Cadien

Optimized thermal coupling of micro thermoelectric generators for improved output

performance

By N. Wojtas, L. Rthemann, W. Glatz, C. Hierold

Characteristics analysis and parametric study of a thermoelectric generator by

considering variable material properties and heat losses

By Jing-Hui Meng, Xin-Xin Zhang, Xiao-Dong Wang

Design optimization of micro-fabricated thermoelectric devices for solar power

generation

By Lobat Tayebi, Zahra Zamanipour, Daryoosh Vashaee The thermoelement as thermoelectric power generator: Effect of leg geometry on the

efficiency and power generation

By Ahmet Z. Sahin, Bekir S. Yilbas

Theoretical analysis of a thermoelectric generator using exhaust gas of vehicles as

heat source

By Yuchao Wang, Chuanshan Dai, Shixue Wang

Simulation and experimental study on thermal optimization of the heat exchanger for

automotive exhaust-based thermoelectric generators

By C.Q. Su, W.S. Wang, X. Liu, Y.D. Deng

micro thermoelectric generator

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