micro thermoelectric generator
TRANSCRIPT
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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)
Heat capacity at constant
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
Relative permittivity 12 1
n-type silicon germanium
Material parameters
Name Value Unit
Thermal conductivity k(T[1/K])[W/(m*K)] W/(m*K)
Heat capacity at constant
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
Relative permittivity 12 1
Electrical conductivity 90909 S/m
p-type silicon germanium
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Material parameters
Name Value Unit
Heat capacity at constant
pressure
154[J/(kg*K)] J/(kg*K)
Density 7700[kg/m^3] kg/m^3
Seebeck coefficient -200e-6 V/K
Electrical conductivity sigma(T) S/m
Thermal conductivity k(T) W/(m*K)
Relative permittivity 1 1
n-bismuth telluride
Material parameters
Name Value Unit
Heat capacity at constant
pressure
154[J/(kg*K)] J/(kg*K)
Density 7700[kg/m^3] kg/m^3
Seebeck coefficient 200e-6 V/K
Electrical conductivity sigma(T) S/m
Thermal conductivity k(T) W/(m*K)
Relative permittivity 1 1
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