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CHAPTER − 1

INTRODUCTION

1.1. Basics of Thermoelectricity

Thermoelectricity is a branch of science which introduces the experimental theme for

the conversion of heat into electricity with the advent of some special materials

called thermoelectric materials. This was introduced by Seebeck in 1817 by some

materials like Iron, Copper, Lead and Bismuth etc. He also explored a long series

of such materials called Seebeck series to select the required thermoelectric

materials on the basis of their electron density. The assembly of two different

materials (wires) having two junctions is called the thermocouple and there is a

generation of thermo emf due to contact potential at these two junctions for a

temperature gradient. In the recent years an increasing concern of environmental

issues especially the global warming and limitations of energy resources motivate

the researchers towards thermo power generation. Recently, owing to the

thermoelectric modules having efficient results in power generation and energy

recycling systems without any content of toxic or pollutants, this technology is

regarding as an alternative Green Technology (Ismail et al., 2009; Bulusu et al.,

2008). Thermoelectricity is considered as a key to overcome the energy crisis in

all the technical and scientific regions because of its some special characteristics

as:

This technology is portable and totally free from any type of pollution and

external age1ncies.

Its operation is easy and there is no use of moving parts.

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All the thermoelectric materials are non toxic and non radioactive which is one of

the chief characteristic of eco friendly system.

A very wide range of thermoelectric materials (all metals, non metals and

semiconductors) is available that means the materials can be selected in order of

the requirements of cost, dimensions, physical and chemical conditions etc.

The chip sized thermoelectric devices are also possible by nano and thin film

technologies.

Thermoelectric power sources are flexible and capable to operate at the elevated

temperatures.

Thermoelectric devices are generally used as the residential heating systems due

their safety nature and their reliability to install in any dimensions of homes. The self

powered heating equipments have comparatively better efficiency to provide the heating

facilities especially in the remote communities; where the connection to the grid is not

cost effective. A thermoelectric module with a power generation capacity of 550W

integrated into a fuel fired furnace (Qiu et al., 2008) is one of the latest achievements.

The excess power of the self-powered heating system can be used to charge the other

electrical units. The thermoelectric devices are also used to control the temperature of

vehicles (cars) i.e. to install the air-conditioned system. There is also a mathematical

model of the car seat proposed by Choi et al. (2007) to utilize the exhaust heat of

vehicles. The efficiency of hybrid solar systems are improved by several means on cost

of the heat utilization characteristics of thermoelectrics that provide a new direction to

the solar cell technology and solar energy regions (Vorobiev et al., 2006). In the thermal

photovoltaic (PV) solar hybrid system the two options are discussed by Vorobiev et al.

(2006) one with a special PV cell and the other coupled with the thermoelectric

materials. This research work explores the possibilities to increase the efficiency of

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solar to electric energy conversion, and these results can be used as guidelines for the

development of some new photovoltaic and thermoelectric devices. In other words, this

technology known as a key for the waste heat recovery systems (WHRS) in

Thermoelectric Generators that involves the heat of boilers using a variety of

semiconducting materials (Haidar et al., 2002; Eschenbach et al., 2006; Nnanna et al.,

2009; Munoz et al., 2008; Lineykin et al., 2005). Some of the switching methods are

also presented that make thermoelectric refrigerators more efficient and improve the

coefficient of performance (COP) during cooling operations (Ghoshal et al., 2009;

Nirmala et al., 2000).

This all explore the importance of thermoelectric based equipments and

thermoelectric materials that are playing an important role in the energy conversion

techniques in these days. In the presented thesis we investigate the generation of thermo

emf, an important parameter for the suitability of any thermoelectric device, and hence

the thermo power generation characteristics of a variety of thermoelectric materials.

This research work tends to improve the conversion efficiencies by the selection and

development of cheap and easily available materials (Kumar et al., 2009; Yamashita et

al., 2007).

Figure 1.1 describes the generation of thermo emf from an “assembly” of two

dissimilar metals (materials) called thermocouple. When one of the two junctions of the

thermocouple is kept hot and the other is cold then the temperature gradient is

established which causes the generation of thermo-emf.

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Figure 1.1 Generation of thermo emf with the temperature gradient at the two

junctions of a thermocouple

The generation of thermo-emf with temperature gradient is:

(1.1)

Where „T‟ is the temperature gradient in Kelvin (K) and α and β are the Seeback

coefficients in μV/K and μV/K2 respectively.

Neutral Temperature and Temperature of Inversion

It has been observed that the graph for the variation of thermo-emf and temperature

of hot junction is parabolic. The temperature of hot junction at which thermo-emf is

maximum, called the neutral temperature “θn” and the temperature of hot junction at

which thermo-emf reverses is called the temperature of inversion, “θi”. The neutral and

inversion temperatures are related by the equation:

(1.2)

where, tc is the temperature of the cold junction.

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Figure 1.2 Parabolic behavior of thermo emf with respect to the temperature gradient

Figure of Merit

One of the important aspects of thermoelectricity is the figure of merit

(dimensionless parameter) of a thermoelectric material; which is the ability of a

material to convert the heat into electricity, denoted by ZT and its expression (Rowe

D.M., 1995) is:

ZT =2

T (1.3)

where „α‟ is the Seeback coefficient in „μV/K‟, „σ‟ is the electrical conductivity of

thermoelectric material in Simen meter-1

(Sm-1

) and „λ‟ is the thermal conductivity of

the materials in WK-1

m-1

The figure of merit of a given thermocouple can be calculated (Trit, M. 2001) from

the parameters of selected thermoelectric materials:

ZT = (1.4)

Where, , are the Seebeck constants; , are

the thermal conductivities of two thermoelectric materials used to assemble the

thermocouple and T is The temperature gradient between the two junctions.

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Significance of Figure of Merit (ZT)

Greater the value of ZT more will be the conversion efficiency of a thermoelectric

material and vice versa. So it is clear that to improve the performance of a

thermocouple the electrical conductivity should be increased and thermal conductivity

should be reduced. Some researchers tend to improve ZT with different advanced

methods like combination of suitable materials, palleting techniques and nano

technology etc (Kantser et al., 2006; Bejenari et al., 2010; Kuei et al., 2004; Bilu et al.,

2001). Generally, the phonon waves are responsible for the thermal conductivity, so to

reduce it, the flow of phonons should face some interactions. The nano techniques; in

which the nano size particles able to distort the oscillations of phonons that reduce the

thermal conductivity and hence a significant improvement in the figure of merit which

has been employed in the silicon nano wires successfully (Zheng, 2008). The figure of

merit also studied for the oxygen deficient perovskites that determines their thermal and

electrical properties and concluded to the enhancement of seebeck coefficient

(Rodriguez et al., 2007; Brown et al., 2006; Jianlin et al., 2009; Mingo N., 2004;

Bhandari et al., 1980; Micheal et al., 2008) and hence the thermo power generation. In

this presented work we compare the experimental and theoretical values of the figure of

merit (ZT) for some of the common thermeoelctric materials like Cu, Fe, Constantan

and Nichrome.

Dependences of Figure of Merit (ZT)

It is clear that the figure of merit of a thermocouple is affected directly by the

electrical conductivity but inversely by the thermal conductivity of the thermoelectric

material.

(a) The thermal conductivity of a material is the ease with which the heat flows

through itself and its expression from literature is given by:

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1Q x

t A T (1.5)

Where, Q

t is the rate of heat flow,

T

x is the temperature gradient, t is the time

for which the heat flow, A is the area of cross section of the thermoelectric materials

with thickness x .

(b) The electrical conductivity of a thermoelectric material is the ease of the

material to allow the passage of electric current and is given in unit of (Sm-1

) by

1 l

RA (1.6)

Where „R‟ is the resistance of thermoelectric material in ohms, and „l

RA

‟,

Where „ ‟ is the resistivity (specific resistance) of the material in „Ωm‟, „l’ and „A‟ are

the length and area of cross-section of the material respectively.

Origin of Thermo EMF

There is a contact potential at the two junctions of a thermocouple due to the

difference of electron densities of the selected materials used to assemble a

thermocouple. But there is no excitement of electrons at the normal temperatures (from

a material of higher electron density to a material of lower electron density), which

becomes possible by the establishment of temperature gradient at the two junctions,

hence the generation of a current (also called thermoelectric current) across the

thermocouple and the corresponding emf is known as thermo emf.

Seebeck Series

The great scientist Thomson Johann Seebeck (German Physicist) introduced the

Seebeck series (in 1821) of a number of materials to indicate the direction and

magnitude of thermoelectric current through a thermocouple. According to this

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hypothesis “The thermoelectric current will flow through the cold junction from a

material placed earlier in the series to a material placed later in the series. Greater the

separation between two thermoelectric materials in the series; greater will be the

magnitude of thermo emf across that thermocouple.” This series is also known as

thermoelectric series:

Antimony (Sb), Iron (Fe), Cadmium (Cd), Zinc (Zn), Silver (Ag), Gold (Au),

Chromium (Cr), Strontium (Sn), Lead (Pb), Mercury (Hg), Manganese (Mn), Copper

(Cu), Platinum (Pt), Cobalt (Co), Nickel (Ni), Bismuth (Bi).

This sequence of materials is very useful to frame a thermocouple combination (to

generate the required thermo power) but on these days a large number of materials

(alloys, semiconductors, thin films and three layer configurations) other than this series

are also used as the thermo generator elements.

Thermo Power

The magnitude of the thermoelectric voltage in response to temperature gradient

across the thermoelectric materials is called the thermo power. If the temperature

difference between the two ends of the materials is ΔT then the thermo power (Ismail et

al., 2009) of the materials given by

VS

T (1.6)

Where „ V ‟ is the thermoelectric voltage developed at the terminals. This can also

be written in relation to the electric field „E‟ and temperature gradient „ T ‟ by the

equation:

ES

T (1.7)

Characteristics of Thermo Power

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The sign of thermo power “S” relates to the charge at the cold junction. If the

majority of the negative charges are at cold junction then the sign of S is negative and

vice versa. Charge carrier tends to responds to a temperature gradient by moving from

the hot and to the cold end. They tend to respond to an electric field in different ways

depending on their charge: Positive charges tend to move in the same direction as the

field, while negative charges move in the opposite direction of field. For equilibrium to

be reached these two tendencies have to cancel each other. So for metals thermo power

is small. Thus, power for purely p-type materials which have only positive mobile

charges (holes) the electric field and temperature field gradient will point in the same

direction in equilibrium given S > 0. Likewise power for purely n-types material which

has only negative mobile charges (electrons), the electric field and temperature gradient

should point in opposite direction in equilibrium giving S < 0. In practice the real

materials often have both + ve and – ve charge carrier and the sign of “S” usually

depend on which of them predominates.

The efficiency with which the thermoelectric material can generate electrical power

depends on material‟s several properties out of which most important is the thermo

power. A larger induced thermoelectric voltage for a given temperature gradient will

lead to a higher efficiency. Ideally one would want very large thermo-power values

since only a small amount of heat is then necessary to create a large voltage. This

voltage can then be used to provide required power.

Theories of Thermoelectricity

There are two basic theories which enlighten the path for research on thermoelectrics:

(a) Phonon Drag Theory

In this theory the phonons are treated as the heat carrying particles. Phonons are not

always in the local thermal equilibrium; they move against the thermal gradient. They

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lose momentum by the interfacing with electrons (or other carriers) and imperfections

in the crystal. If the phonon-electron interaction is predominant, the phonons tend to

push the electrons to one end of the material, losing momentum in this process. This

contributes to the already present thermoelectric field. This contribution is most

important in the temperature region where phonon-electron scattering is predominant.

This happens for T≈ (1/5)θD , where θD is the Debye temperature. At lower

temperatures there are fewer phonons available for drag, and at higher temperatures

they tend to lose momentum in phonon-phonon scattering instead of phonon-electron

scattering.

(b) Diffusion Theory

This theory relates to the charge concentration of the thermoelectric material.

According to this theory, when two ends of a conductor are kept at different

temperatures; the hot carriers are diffused from the hot end to cold end and the cold

carriers are diffused from cold to hot end. This diffusion leads to the heat current but

also the electric current due to the flow of charge carriers. This diffusion creates the

higher density of charge carriers at one end than at the other end. So it leads to the

potential difference, also known as the electrostatic field. The diffusion of charge

carriers is affected by their motion in opposite directions, imperfections, impurities and

the structural changes also. So the thermo power is a collection of number of effects on

the material.

1.2. Energy Crisis

World from the last few years face the difficulties regarding energy management,

energy consumption and the sources of energy (renewable and non renewable) not be

sufficient in comparison of the future energy trends. This is not only due to world

population but a long range of electrical and electronics based demands of modern life

are also responsible. This all causes the world energy crisis which leads to the need of

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introduction of some techniques, modifications, nuclear power plants and nano cells etc.

to overcome the energy crisis (Bhandari et al., 1998; Mingo, 2004). Thermoelectric

modules playing an important role by the conversion of waste heat into electricity. The

implementation of thermoelectric devices with cheap, single step power generation and

without any pollution can be regarded as a key to green energy generations. The energy

management (in its form of electricity) in the rural regions or icy areas is also a big

deterrent in the daily life activities. In some of these areas the electric power is neither

feasible to supply nor be economic for production. In such regions the thermoelectric

generators can be used to produce a sufficient amount of power (Rowe, 2006) by the

heat of stoves, wood, daily wastage etc. Such crisis also appear in the engineering and

technical fields due to a large scale consumption of energy and low efficiency of

modern high facilitated devices.

1.3. Utilization of Waste Heat to Overcome the Energy Crisis

Thermoelectric devices like cooling devices, refrigerators, picnic bottles, energy

recycled devices, thermal-photovoltaic solar hybrid system and thermoelectric

generators are becoming much familiar with time. In many countries the waste heat

from exhaust pipes is also utilized for the working of sound system and other audio-

video systems. In medical treatment this conversion is also employed to awake some

body organs and to refine their functioning. Among these, the thermoelectric generators

have been receiving renewed interest in recent years in a wide range of applications like

waste heat recovery from different sources like transformers, body-heat, computers etc.

which is of a great importance in the era of growing energy crisis (Ismail et al., 2009;

Bulusu et al., 2008; Hyeung et al., 2007; Gravier et al., 2004; Qiu K et al., 2008). Hence

there is a wide scope to utilize the waste heat by its conversion into electricity (Michael

Freunek et al., Leonov et al., T. Goto et al., 1997) with the advent of efficient

thermoelectric generators. The other remarkable aspect is that the input does not require

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any production but already available heat and the electrical energy (output) can be

recycled to improve the efficiency of the same system.

Theory of Thermoelectric Generators

The thermoelectric generation is based on the Seebeck Phenomenon (Riffat S.B. et

al., 2003) and is carried out when a temperature gradient is established at the two

junctions of two dissimilar materials. The Figure 1.3 shows the schematic diagram

where the electrical power output (We) is obtained due to the temperature gradient of

two junctions QH (higher temperature) and QL (Lower temperature) corresponding to

the heat source and heat sink respectively. This is also in accordance with the first law

of thermodynamics (energy conservation principle) that the difference between QH and

QL is the output electrical power We (Cengel Y.A. et al., 2008).

Figure 1.3 Schematic diagrams showing the basic concept of a simple thermoelectric

power generator operating based on Seebeck effect (Basel Ismail I. et al., 2009)

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Composition and Specifications of a Thermoelectric Power Generator

It consists of the two ceramic plates called the substrates for n-type and p-type

semiconductor thermo elements. These substrates provide the mechanical integrity and

electrical insulation to these semiconducting elements. These ceramic plates are made

from alumina (Al2O3), but when large lateral heat transfer is required, materials with

higher thermal conductivity (beryllia and aluminum nitride) are used. The

semiconductor thermo elements like SiGe, PbTe and their alloys are sandwiched

between the ceramic plates; are thermally in parallel and electrical in series to form a

thermoelectric module (Basel Ismail I. et al., 2009) (Figure 1.4).

Figure 1.4 Schematic diagram showing components and arrangement of a typical

single-stage thermoelectric power generator (Basel Ismail I. et al., 2009)

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Size of the Conventional Thermoelectric Modules

The conventional thermoelectric modules lie in the range of 3 mm2 of 4 mm thick to

75 mm2 of 5 mm thickness. To face the mechanical aspects, the length of thermoelectric

modules is not more than 50 mm generally. The height of the single stage

thermoelectric modules ranges from 1 mm to 5 mm. The multi stage thermoelectric

modules are also designed for the higher temperature gradient regions. The height of

such multistage thermoelectric modules can be up to 20 mm depending upon the

number of stages (Basel Ismail I. et al., 2009) (Figures 1.5 and 1.6).

Figure 1.5 Single stage thermoelectric modules (Basel Ismail I. et al., 2009)

Figure 1.6 Multi stage thermoelectric modules (Basel Ismail et al., 2009)

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Performance of Thermoelectric Power Generators

The performance of thermoelectric power generators (thermoelectric materials) can

be explained (Rowe DM, 2006) in terms of the figure of merit as:

Z = (1.8)

Where Z is the figure of merit of the thermoelectric material, α is the Seebeck

Coefficient in µV/K, given by

α= (1.9)

R and k are the electrical resistivity and thermal conductivity respectively.

If T = where TH and TL are the temperatures of hot and cold junctions

respectively.

Then, ZT = becomes a dimensionless parameter also known as the

thermoelectric material figure of merit. The term is referred as the electrical power

factor. The figure of merit parameter is very significant to compare the efficiency of

thermoelectric materials and modules. The efficiency of a thermoelectric module can be

defined as the ratio of the output thermoelectric power generated by the module to the

input heat energy. This is also concluded by Rowe D.M. (2006) that the efficiency of

conventional thermoelectric devices is comparatively low due to lower values of figure

of merit i.e. ZT ≤ 1 of currently available materials. This leads to the thermo emf

generations for novel semiconducting thermoelectric materials like Bi2Te3 and Pb2Te3

which having ZT values around unity in the high temperature range of 500 to 700 K

with their compatibility in the power generation systems.

In the presented research work, we investigate the thermo emf generation

characteristics of semiconducting thermoelectric pallets (Bi2Te3, Bi2Pb3 and Pb2Te3) not

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only in the normal mode but also under the effect of applied electric and magnetic fields

of various magnitudes. These studies can provide some useful ideas to improve the

figure of merit (by the utilization of heat along with electric and magnetic fields) and

hence to improve the efficiency of thermoelectric modules.

Output of the Conventional Thermoelectric Power Generators

The researchers oriented to improve the thermo power generations with the advent

of advanced materials, operation in different orientations, their thermal conductivities,

electrical properties, their ability to withstand at higher temperature gradients etc. This

has been observed that the output power of the commercially available thermoelectric

power generators ranges from microwatts to multi-kilowatts (Riffat S.B. et al., 2003;

Rowe D.M., 1999). A standard thermoelectric module consists of 71 thermocouples

with the size of 75 mm2 that can deliver the output power about 19 W (Riffat S.B. et al.,

2003). The maximum thermo power generation depends upon the temperature

difference between the hot and cold plates of the module specifications and their length,

area of cross section, area of contact, resistivity and thermal conductivity etc. This has

been observed that the selection of materials, their geometry and the corresponding

temperature gradients affect the thermo emf generations (Rowe D.M. et al., 1998).

To elaborate such dependencies, we selected three types of thermoelectric materials

(classical thermocouples, RTD thermoelectric materials and semiconducting

thermoelectric pallets) in this research work for the investigations on the generation of

thermo emf and hence, the thermoelectric power. Due to the dependence of thermo

power on the physical parameters i.e. length, area of cross section, resistance, resistivity

and their electrical conductivities, are measured. In addition of such parameters the

effect of electric field and magnetic field of various magnitudes are also considered,

along with the different orientations (parallel and perpendicular) of all the

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thermocouples so that the electric field and magnetic field energy as applied from

outside or available there itself, can also be utilized along with the waste heat energy

during the operation of thermoelectric modules.

1.4. Availability of Low Grade Waste Heat and its Utilization by the

Thermoelectric Devices

The major attraction of thermoelectricity is the utilization of waste heat that is

available approximately in all fields of science, engineering and technology. Waste heat

is the byproduct of machines and technical processes for which no useful application

has been found so far. In the present times of industrial revolution, factories, data

centers, kitchen, stoves, gas burners, back of refrigerator, computers, laptops, cameras,

screen instruments and even our clothes dryer throw off waste heat that could be a

useful source of small but free energy.

Domestic Waste Heat

This has been investigated that the thermoelectric power generator can be used in

the domestic dimensions located properly (Rowe, D.M. 2006) between the heat and

water sources. This work reports that the two thermoelectric modules based on PbTe

technology when operated at the hot and cold side temperatures of 5500C and 50

0C

respectively then it generates the 50W required to power the circulating pump.

In the rural regions where the electric power is feasible, the waste heat energy

utilized from the wood or diesel-heated stoves (Nuwayhid et al., 2003) can be an

additional supplement proportional to 20-50 kW electric power. This work results that a

thermoelectric power generator to produce electricity from stove-top surface

temperatures of 100-3000C is designed and operated. On the surface of a stove about

500 K temperature is available that generates the electric power of about 100W with the

advent of FeSi2 (used in open flames due its excellent stability at high temperatures),

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PbTe (advantageous in power factor and Z), Bi2Te3 (Peltier modules in power

generation mode). A similar application is reported by Rowe D.M. that when a

thermoelectric generator is used to generate small amounts of electrical power in remote

regions of Northern Sweden. This generator uses heat from a wood burning stove with

cold side cooled with a 12 volt, 2.2 W fan and produces around 10 watts thermo power.

Waste Heat from the Exhaust Gases Generated from Automobiles

The utilization of waste heat energy from the exhaust gases, the combustion of fuel

in the automobiles is also a novel application of electricity generation using

thermoelectric devices. However, a lot of heat energy gets wasted in various other

energy producing and utilizing devices, especially in cars with gasoline engines. This

has been observed that in a gasoline powered engine, about 30% fuel energy gets

wasted as heat and discharging in the gases (Riffat S.B. et al., 2009). This is reported by

Rowe D.M. that a thermoelectric generator powered by exhaust heat could meet the

electrical requirements of a medium sized automobile (Rowe D.M., 1999). This is also

observed by Rowe D.M. that the thermoelectric modules developed by using PbTe

materials are more suitable for the energy requirements of automobiles. These ideas of

utilizing thermoelectric power generation can lead to some reduction in the fuel

consumption and thus the environmental global warming.

Industrial Waste Heat

The thermoelectric modules are also feasible for the conversion of industrial waste

heat into electricity in the effective means (Riffat S. B. et al., 2003). The large content

of heat is being rejected from the industries, power transfer modes, manufacturing

plants, in the exhaust gases and liquids but the corresponding temperatures are not

suitable in the conventional generating units. At the same time the green technology of

thermoelectric modules can be implemented not only to overcome the worldwide

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industrial energy crisis but also to diminish the pollution image of industries. Figure 1.7

shows the simple thermoelectric generator making use of the temperature difference

between hot and cold legs of a glycol natural gas dehydrator cycle (Weiling L et al.,

2004) with good results of thermo power generation.

Figure 1.7 Photograph of a thermoelectric power generator produced power for cathodic

protection of the well and gas line, which used the temperature difference

between hot and cold legs of glycol natural gas dehydrator cycle (Weiling L. et

al., 2004)

Thermoelectric power generators have also been successfully applied in recovering

waste heat from steel manufacturing plants. In this application, large amounts of

cooling water are typically discharged at the temperature of 900C when operated for the

cooling purpose in steel plants. This is reported by Rowe D.M. (2006) that total electric

power of around 8 MW can be produced by employing the thermoelectric modules

fabricated using Bi2Te3 thermoelectric materials. The application of thermo power

generation by using the waste heat energy has a potential use in the industrial

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cogeneration systems (Yodovard et al., 2001; Min et al., 2002). According to this work,

the thermopower generation is carried out for diesel cycle and gas turbine cogeneration

in the manufacturing industrial sector of Thialand. The data is collected from 27,000

factories from different sectors likely chemical product, food processing, oil refining,

paper mills and textiles etc. This is observed that this system produced about 100MW.

This can be applied to any type of industrial waste heat and even the energy generation

centers can also be established along with the industrial products. This is invented by

Dell et al., (2008) based on the thermoelectric power generation system designed to be

coupled onto the outer wall of a steam pipe. This work includes a number of assemblies

mounted on the sides of a pipe. Each assembly can include a hot block, an array of

thermoelectric modules and a cold block system. This is a unique and efficient

thermoelectric module to utilize the industrial waste heat.

Waste Heat from the Burning of Municipal Solid Wastage

The possibility of utilizing the heat from incinerated municipal solid waste has also

been considered. This is carried out by Rowe D.M. (2006) when the incinerator waste

gas temperature varied between 823 and 973 K and with an air flow on the cold side,

the estimated conversion efficiency of about 4.5% is achieved. This is also discussed

here that around 426 kW electric powers can be delivered by the burning of 100 ton

solid waste during a 16 hour day.

In this waste heat from incineration applications, the thermoelectric modules are

typically placed on the walls of the furnace‟s funnels. This construction can be

eliminating the by-heat furnace, gas turbine and other appending parts of the steam

recycle (Weiling L. et al., 2004) (Figure 1.8).

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Figure 1.8 Photograph of a thermoelectric power generator produced by the Japanese

Energy Conservation Centre, which used waste heat as energy source to generate

an electric power density of 100 kW/m3 (Weiling et al., 2004)

Micro Scale Waste Heat Utilization

The micro scale waste heat (i.e., low grade waste heat) can also be utilized with the

use of miniature thermoelectric power generators. This is one of the most important

aspects of the growing autonomous micro-systems and the wearable electronic devices.

The micro thermoelectric power generators can be fabricated using the integrated

circuit technology (Rowe D.M., 1999). This is also suggested by Rowe D.M. that the

alternate n- and p- type thermo elements are ion implanted into an undoped silicon

substrate. A miniature thermoelectric generator designed by Rowe D.M. (1999) is

shown in the Figure 1.9 in which the metallization of thermo elements connecting strips

and the output contacts enable to connect few hundred thermocouples electrically in

series and occupy an area about 25 mm2. This is very useful to provide the electrical

power to the chip sized electronic devices. This has been observed that about 1.5 Volts

can be produced when a temperature difference of few 10 degrees established across the

junctions of a thermoelectric module.

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Figure 1.9 Miniature of thermoelectric generator (Basel Ismail et al., 2009)

Suggested by Saiki et al. (1985), the body heat can be utilized to power a

thermoelectric watch battery in which the thermocouples are prepared by depositing

germanium and indium antimonide on either side of a 1 mm thick insulator. It is

observed that about 2875 thermo elements connected in series are required to get 2 V to

operate the watch.

A patent research work carried out by Fleurial et al. (2002) explores the designing

of a micro thermoelectric device to operate the electronic components. This device

consists of a high thermal conductivity substrate like diamond that is deposited in

thermal contact with the high temperature region. In this module a Bi2Te3 alloy based

thin film is placed in contact of higher and lower temperature regions, the established

temperature gradient is sufficient to generate the thermo electric power as shown in

Figure 1.10.

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There is also another micro thermo electric generator suggested by Glatz et al.

(2006) for the non planar surfaces. Such power generators are fabricated by subsequent

electrochemical deposition of Cu and Ni in a 190-µm thick flexible polymer mold

formed by photolithographic patterning of SU-8. This is tested in this research work

that for the temperature difference of 0.12 K at the interface of thermoelectric generator

then the thermo power about 12±1.1 nW/cm2 is generated. The schematic diagram of

this micro thermoelectric generator is shown in Figure 1.11.

Figure 1.10 Schematic diagram of micro thermoelectric power generator that can be

used to convert waste heat into electrical power to drive an electronic chip

(Fleurial J.P. et al., 2002)

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Figure 1.11 Schematic diagram of the micro thermoelectric power generator that can be

used to convert waste heat into electrical power to drive an electronic chip (Glatz

W. et al., 2006)

1.5. Electric and Magnetic Field Dynamics

This is observed in the literature that the thermoelectric properties are affected by

the electric filed influences during the operation of thermoelectric modules. The

electrical properties of some metallic alloys are studied under the effect of applied

electric filed that results to improve the thermoelectric conversion efficiencies

(Smontara et al., 2007; S. Uda et al., 2004; Gitsu et al. 2002). A theoretical

thermoelectric cooler is proposed (Chung et al., 2003) and analyzed which uses an

electric filed modulated current to transport heat energy from a cold source to the hot

source via n- and p-type carriers. The cooling device here is shown to have the heat

energy transport per electron of about 500 meV depending on the concentration and

electric field values whereas in the good conventional thermoelectric coolers it is about

50 to 60 meV at the room temperatures. In the same way it is very interesting to know

about the investigations carried out by Gadzhialiev et al. (2004) about the effect of the

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thermoelectric field emerging under the effect of a high temperature gradient on the

current-voltage characteristics of hetrostuctures.

In the presented research work we carried out the investigations of thermo emf

generations for three types of thermoelectric materials (classical thermocouples, RTD

thermocouples and semiconducting thermoelectric pallets) under the effect of applied

electric field of various magnitudes in different orientations. Here we investigate each

of the thermocouple as a thermo generator element in the temperature range of 3150C

for classical and RTD thermocouples, 1550C for thermoelectric pallets and their

comparisons are also carried out to extract some ideas for a better precision

measurements and useful utilization of waste heat.

The effect of applied electric field on the thermo power generation is also explored

by Sandomirsky V. et al. According to which the applied electric field not only affect

the Fermi energy levels of thermoelectric materials but also the concentration of charge

carriers (electrons and holes). This effect is regarded as Electric Field Effect-EFE and

described that how an increase of electric field strength results in a further increase of

the introduced carriers and the conductivity, which finally affect the thermoelectric

properties. This can be viewed from the following formula of Seebeck coefficient ( )

that shows the dependence of net Seebeck coefficient of the material ( ) on the

corresponding Seebeck coefficients of holes ( ) and electrons ( ):

(1.10)

(1.11)

(1.12)

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The equations (1.11) and (1.12) describe the dependence of hole Seebeck

coefficient ( ) and electron Seebeck coefficient ( ), on their corresponding electrical

conductivities ( , ) where and are the electronic charge and Boltzmann

constant respectively. In order to take into account this electric field effect (EFE) we

apply the external electric field on all the selected thermoelectric materials of three

different magnitudes in the different orientations and then the generation of thermo emf

is analyzed.

Similar to that of the electric field, this is explained in the CRC Handbook of

Thermoelectrics (CRC Handbook of Thermolectrics by D.M. Rowe, Newyork, 1995,

Section 4.7) that the magnetic field also has a profound effect on the transport

coefficients in addition to a whole range of thermomagnetic phenomenon. The Lorentz

force acting on an electron in a magnetic field of a few kilogauss is usually greater than

the force exerted by attainable electric fields within the solid. The Boltzmann equation

in the presence of magnetic field includes an extra term

= (1.13)

Here is the Lorentz force in the applied magnetic field, is the conduction

electron wave vector, is the magnetic field strength, is the conduction electron

velocity, is the Planck‟s constant and is the electronic charge.

The effect of a magnetic field is quite unlike that of an electric field, it gives rise to

a drift which is balanced by the scattering processes. While considering the latter for

, giving rise to a drift which is balanced by the scattering processes. It follows that

(1.14)

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This becomes zero. Here is the Lorentz force due to internal magnetic field,

is the Energy wave.

The magnetic field has the effect of changing the direction of motion of the

electrons; it therefore, acts as the sort of scattering agent. Even the magnetoresistance

explore the importance of orientation of the applied magnetic field which in fact affect

the electrical and thermal properties, given by

(1.15)

Hence, according to Rowe D.M., this (magnetic field) is a powerful tool to probe

into an electronic structure. The experimental research addressing the influence of

magnetic field dynamics on the copper-constantan thermocouple performance is carried

out by Shir et al. (2005). In this research the various operational parameters of the

thermocouple are measured in an alternating magnetic field. Similarly the magnetic

field effects are carried out for the magneto-thermo-electromotive force (Gadzhialiev et

al., 2006), longitudinal-magneto-resistance (Gadzhialiev et al., 2005) and effect of the

mutual dragging of electrons and phonons on the thermo magnetic effects are carried

out (Bikkin et al., 1999; Herzer 1984; Ataev et al., 2002; Conover et al., 1991; Gravier

et al., 2004; Hamabe et al., 2008; MacDonald et al., 1957). Sometimes these external

parameters can be already available in the operating conditions of thermoelectric

systems but these can also be applied externally if their effect is favorable towards the

efficiency of the system. In this present research work we investigate the effect of

applied magnetic field of various magnitudes in different orientations for the selected

thermoelectric materials to awake some chances of better utilization of waste heat along

with the magnetic field.

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1.6. Role of Mechanical Stress

This has been observed that the applied stress produce some changes in the

thermoelectric properties of metals. The strains produced in the material due to stress

give rise to some phonon interactions that finally affect the thermoelectric properties

(Sawkey, 1998; Morgan, 1968; Inoue et al., 1965). The stress causes the plastic and

elastic deformations (Mortlock, 1953) in the material that actually affect the

transportation of electrons and phonons within the material. Hence, the effect of stress

on the thermoelectric properties is an important aspect due to which thermo-emf

generation gets affected for all the selected thermocouples and is investigated in a

temperature range of about 3300C.

The above discussions extract an idea that effect of operating parameters like

electric field, magnetic field and stress affect the thermoelectric properties hence the

temperature-emf measurements by RTD materials also get altered to an extent. The

alternation of such temperature-emf relations puts an objection on the accuracy and

reliability of these temperature sensing systems. So in the present research work, the

thermo emf generation characteristics of RTD thermocouples are studied which can be

put to ascertain the reliability and precise temperature measurements under the effect

of different operating parameters.

Objectives

1. Study of the cheap and easily available (in the market) thermoelectric materials like

Cu, Al, Nichrome, Constantan and Fe etc. in normal conditions i.e., without the

effect of any electric or magnetic fields as reliable thermocouples for waste heat

recovery (for temperature range of 300C to 330

0C).

2. Study of mechanical stress on the generation of thermo- emf for some selected

thermocouples of materials like Al, Fe, Constantan, Nichrome and Cu.

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3. Investigation of the effect of magnetic field and its different magnitudes on the

generation of thermo-emf in different orientations i.e., parallel and perpendicular in

all aspects of Fe, Cu, Constantan, Nichrome and Al combinations and their

comparisons.

4. Investigation of the effect of electric field and its different magnitudes on the

generation of thermo-emf in different orientations i.e., parallel and perpendicular in

all aspects of Fe, Cu, Constantan, Nichrome and Al combinations and their

comparison.

5. Standardization/Characterization of thermoelectric materials available in the market

and used in present studies is carried out for proper comparison of present

experimental results.

6. Synthesis (making pallets) of some advanced materials like Bi, Te and Pb and their

study from the point of view of thermoelectric properties like thermal conductivity,

electrical conductivity and the figure of merit under:

a) Norrmal conditions (without any electric or magnetic fields).

b) Applying electric and magnetic field dynamics

7. Finally, the comparison of all materials and their combinations for the better

performance in normal conditions as well as with the effect of electric and magnetic

field dynamics.

8. Simulation studies of thermo-emf generation to compile the theoretical equations for

the figure of merit, variation of figure of merit with the temperature gradient and

their comparison with the experimental results.

9. The selection of the best combinations on the basis that in which conditions the

thermo emf generation is optimum including the availability of electric or magnetic

fields, if available; so as to utilize the waste heat as efficiently as possible.