chapter − 1 - inflibnetshodhganga.inflibnet.ac.in/bitstream/10603/9242/10/10_chapter 1.pdf · a...
TRANSCRIPT
1
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.
2
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
3
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.
4
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.
5
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.
6
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:
7
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
8
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
9
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
10
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
11
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
12
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)
13
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)
14
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)
15
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
16
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
17
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),
18
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
19
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
20
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).
21
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.
22
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.
23
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)
24
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
25
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)
26
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)
27
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.
28
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.
29
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.