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ABSTRACT
The main objective of this project is “To Study the Performance characteristics of a single
Thermo-Electric generator module to compare theoretical and experimental results and
also discuss its applications.”
Thermoelectric generators are all solid-state devices that convert heat into electricity.
Unlike traditional dynamic heat engines, thermoelectric generators contain no moving parts
and are completely silent. Such generators have been used reliably for over 30 years of
maintenance-free operation in deep space probes such as the Voyager missions of NASA.
Compared to large, traditional heat engines, thermoelectric generators have lower efficiency.
But for small applications, Thermoelectrics can become competitive because they are
compact, simple (inexpensive) and scalable. Thermoelectric systems can be easily designed
to operate with small heat sources and small temperature differences. Such small generators
could be mass produced for use in automotive waste heat recovery or home co-generation of
heat and electricity. Thermoelectrics have even been miniaturized to harvest body heat for
powering a wristwatch.
1
PREFACE
The following report consists in depth description and illustration of
Semiconductor Physics
The Seebeck effect
Materials required
Making of a Thermo-electric generator
Working of a Thermoelectric generator
Performance characteristics of a Thermoelectric generator
Application of Thermoelectric generators
Necessity of Thermoelectric power
Future of Thermoelectric power as an alternate source of energy.
2
1.1 INTRODUCTION TO SEMICONDUCTORS
A semiconductor is a material which has electrical conductivity between that of
a conductor such as copper and an insulator such as glass. The conductivity of a
semiconductor increases with increasing temperature, behaviour opposite to that of a metal.
Semiconductors can display a range of useful properties such as passing current more easily
in one direction than the other. Because the conductive properties of a semiconductor can be
modified by controlled addition of impurities or by the application of electrical fields or light,
semiconductors are very useful devices for amplification of signals, switching, and energy
conversion. Understanding the properties of semiconductors relies on quantum physics to
explain the motions of electrons through a lattice of atoms.
Current conduction in a semiconductor occurs via free electrons and "holes", collectively
known as charge carriers. Adding impurity atoms to a semiconducting material, known as
"doping", greatly increases the number of charge carriers within it. When a doped
semiconductor contains excess holes it is called "p-type", and when it contains excess free
electrons it is known as "n-type". The semiconductor material used in devices is doped under
highly controlled conditions to precisely control the location and concentration of p- and n-
type dopants. A single semiconductor crystal can have multiple p- and n-type regions; the p–
n junctions between these regions have many useful electronic properties and characteristics.
Semiconductors are the foundation of modern electronics, including radio, computers, and
telephones. Semiconductor-based electronic components include transistors, solar cells, many
kinds of diodes including the light-emitting diode (LED), the silicon controlled rectifier,
photo-diodes, and digital and analog integrated circuits. Increasing understanding of
semiconductor materials and fabrication processes has made possible continuing increases in
the complexity and speed of semiconductor devices, an effect known as Moore's law.
3
1.2 HISTORY OF SEMCONDUCTORS
The history of the understanding of semiconductors begins with experiments on the electrical
properties of materials. The properties of negative temperature coefficient of resistance,
rectification, and light-sensitivity were observed starting in the early 19th century.
In 1833, Michael Faraday reported that the resistance of specimens of silver sulfide decreases
when they are heated. This is contrary to the behavior of metallic substances such as copper.
In 1839, A. E. Becquerel reported observation of a voltage between a solid and a liquid
electrolyte when struck by light, the photovoltaic effect. In 1873 Willoughby Smith observed
that selenium resistors exhibit decreasing resistance when light falls on them. In 1874 Karl
Ferdinand Braun observed conduction and rectification in metallic sulphides, and Arthur
Schuster found that a copper oxide layer on wires has rectification properties that ceases
when the wires are cleaned. Adams and Day observed the photovoltaic effect in selenium in
1876.
A unified explanation of these phenomena required a theory of solid state physics which
developed greatly in the first half of the 20th Century. In 1878 Edwin Herbert
Hall demonstrated the deflection of flowing charge carriers by an applied magnetic field,
the Hall Effect. The discovery of the electron by J.J. Thomson in 1897 prompted theories of
electron-based conduction in solids. Karl Baedeker, by observing a Hall effect with the
reverse sign to that in metals, theorized that copper iodide had positive charge carriers. Johan
Koenigsberger classified solid materials as metals, insulators and "variable conductors" in
1914. Felix Bloch published a theory of the movement of electrons through atomic lattices in
1928. In 1930, B. Gudden stated that conductivity in semiconductors was due to minor
concentrations of impurities. By 1931, the band theory of conduction had been established
by Alan Herries Wilson and the concept of band gaps had been developed. Walter H.
Schottky and Nevill Francis Mott developed models of the potential barrier and of the
characteristics of a metal-semiconductor junction. By 1938, Boris Davydov had developed a
theory of the copper-oxide rectifer, identifying the effect of the p–n junction and the
importance of minority carriers and surface states.
Agreement between theoretical predictions (based on developing quantum mechanics) and
experimental results was sometimes poor. This was later explained by John Bardeen as due to
the extreme "structure sensitive" behavior of semiconductors, whose properties change
dramatically based on tiny amounts of impurities. Commercially pure materials of the 1920s
4
containing varying proportions of trace contaminants produced differing experimental results.
This spurred the development of improved material refining techniques, culminating in
modern semiconductor refineries producing materials with parts-per-trillion purity.
Devices using semiconductors at first were constructed based on empirical knowledge, but
semiconductor theory provided a guide to construction of more capable and reliable devices.
Alexander Graham Bell used the light-sensitive property of selenium to Photophone transmit
sound over a beam of light in 1880. A working solar cell, of low efficiency, was constructed
by Charles Fritts in 1883 using a metal plate coated with selenium and a thin layer of gold;
the device became commercially useful in photographic light meters in the 1930s.[3] Point-
contact microwave detector rectifiers made of lead sulfide
were used by Jagadish Chandra Bose in 1904; the cat's-whisker detector using natural galena
or other materials became a common device in the development of radio. However, it was
somewhat unpredictable in operation and required manual adjustment for best performance.
In 1906 H.J. Round observed light emission when electric current passed through silicon
carbide crystals, the principle behind the light emitting diode. Oleg Losev observed similar
light emission in 1922 but at the time the effect had no practical use. Power rectifiers, using
copper oxide and selenium, were developed in the 1920s and became commercially important
as an alternative to vacuum tube rectifiers.
In the years preceding World War II, infra-red detection and communications devices
prompted research into lead-sulfide and lead-selenide materials. These devices were used for
detecting ships and aircraft, for infrared rangefinders, and for voice communication systems.
The point-contact crystal detector became vital for microwave radio systems, since available
vacuum tube devices could not serve as detectors above about 4000 MHz; advanced radar
systems relied on the fast response of crystal detectors. Considerable research and
development of silicon materials occurred during the war to develop detectors of consistent
quality.
Detector and power rectifiers could not amplify a signal. Many efforts were made to develop
a solid-state amplifier, but these were unsuccessful because of limited theoretical
understanding of semiconductor materials.[3] In 1922 Oleg Losev developed two-
terminal,negative resistance amplifiers for radio; however, he perished in the Siege of
Leningrad. In 1926 J.E. Lilenfeld patented a device resembling a modern field-effect
transistor, but it was not practical. R. Hilsch and R. W. Pohl in 1938 demonstrated a solid-
5
state amplifier using a structure resembling the control grid of a vacuum tube; although the
device displayed power gain, it had a cut-off frequency of one cycle per second, too low for
any practical applications, but an effective application of the available theory. [3] At Bell
Labs, William Shockley and A. Holden started investigating solid-state amplifiers in 1938.
The first p–n junction in silicon was observed by Russell Ohl about 1941, when a specimen
was found to be light-sensitive, with a sharp boundary between p-type impurity at one end
and n-type at the other. A slice cut from the specimen at the p–n boundary developed a
voltage when exposed to light.
In France, during the war, Herbert Mataré had observed amplification between adjacent point
contacts on a germanium base. After the war, Mataré's group announced their "Transistron"
amplifier only shortly after Bell Labs announced the "transistor".
Fig 1. Raw germanium Fig 2. Silicon
6
1.3 SEMICONDUCTOR MATERIALS
A large number of elements and compounds have semiconducting properties, including:
Certain pure elements found in Group IV of the periodic table; the most commercially
important of these elements are silicon and germanium.
Binary compounds, particularly between elements in Groups III and V, such as gallium
arsenide, Groups II and VI, groups IV and VI, and between different group IV elements,
e.g. silicon carbide.
Certain ternary compounds, oxides and alloys.
A number of organic compounds.
An intrinsic semiconductor is made up of one pure element or pure compound. At room
temperature, the conductivity of intrinsic semiconductors is relatively low because there are
very few charge carriers available. Conductivity is greatly enhanced by a process
called doping, in which very small amounts of other elements are added to the intrinsic
crystal to create what is called an extrinsic semiconductor.
Most common semiconducting materials are crystalline solids, but amorphous and liquid
semiconductors are also known. These include hydrogenated amorphous silicon and mixtures
of arsenic, selenium and tellurium in a variety of proportions. These compounds share with
better known semiconductors the properties of intermediate conductivity and a rapid variation
of conductivity with temperature, as well as occasional negative resistance. Such disordered
materials lack the rigid crystalline structure of conventional semiconductors such as silicon.
They are generally used in thin film structures, which do not require material of higher
electronic quality, being relatively insensitive to impurities and radiation damage.
7
Fig 3. Periodic table indicating semiconductors
1.4 ENERGY BANDS AND ELECTRICAL CONDUCTION
Semiconductors are defined by their unique electric conductive behaviour. Metals are
good conductors because at their Fermi level, there is a large density of energetically
available states that each electron can occupy. Electrons can move quite freely between
energy levels without a high energy cost. Metal conductivity decreases with temperature
increase because thermal vibrations of crystal lattice disrupt the free motion of
electrons. Insulators, by contrast, are very poor conductors of electricity because there is a
large difference in energies (called a band gap) between electron-occupied energy levels and
empty energy levels that allow for electron motion.
Insulator conductivity increases with temperature because heat provides energy to promote
electrons across the band gap to the higher electron conduction energy levels (called
the conduction band). Semiconductors, on the other hand, have an intermediate level of
8
electric conductivity when compared to metals and insulators. Their band gap is small enough
that small increase in temperature promotes sufficient number of electrons (to result in
measurable currents) from the lowest energy levels (in the valence band) to the conduction
band. This creates electron holes, or unoccupied levels, in the valence band, and very loosely
held electrons in the conduction band.
Fig 4. A simplified diagram illustrating the energy band levels of an insulator, a
semiconductor, and a conductor. Electrons can only exist in certain energy levels.
In the classic crystalline semiconductors, electrons can have energies only within certain
bands (ranges). The range of energy runs from the ground state, in which electrons are tightly
bound to the atom, up to a level where the electron can escape entirely from the material.
Each energy band corresponds to a large number of discrete quantum states of the electrons.
Most of the states with low energy (closer to the nucleus) are occupied, up to the valence
band.
9
Semiconductors and insulators are distinguished from metals by the population of electrons in
each band. The valence band in any given metal is nearly filled with electrons under usual
conditions, and metals have many free electrons with energies in the conduction band. In
semiconductors, only a few electrons exist in the conduction band just above the valence
band, and an insulator has almost no free electrons.
The ease with which electrons in the semiconductor can be excited from the valence band to
the conduction band depends on the band gap. The size of this energy gap (band gap)
determines whether a material is semiconductor or an insulator (nominally this dividing line
is roughly 4 eV).
With covalent bonds, an electron moves by hopping to a neighbouring bond. The Pauli
Exclusion Principle requires the electron to be lifted into the higher anti-bonding state of that
bond. For delocalized states, for example in one dimension – that is in a nanowire, for every
energy there is a state with electrons flowing in one direction and another state with the
electrons flowing in the other. For a net current to flow, more states for one direction than for
the other direction must be occupied. For this to occur, energy is required, as in the
semiconductor the next higher states lie above the band gap. Often this is stated as: full bands
do not contribute to the electrical conductivity. However, as the temperature of a
semiconductor rises above absolute zero, there is more energy in the semiconductor to spend
on lattice vibration and on exciting electrons into the conduction band.
Electrons excited to the conduction band also leave behind electron holes, i.e. unoccupied
states in the valence band. Both the conduction band electrons and the valence band holes
contribute to electrical conductivity. The holes themselves don't move, but a neighbouring
electron can move to fill the hole, leaving a hole at the place it has just come from, and in this
way the holes appear to move, and the holes behave as if they were actual positively charged
particles.
One covalent bond between neighbouring atoms in the solid is ten times stronger than the
binding of the single electron to the atom, so freeing the electron does not imply destruction
of the crystal structure.
10
1.5 DOPING OF SEMICONDUCTORS
The conductivity of semiconductors may easily be modified by introducing impurities into
their crystal lattice. The process of adding controlled impurities to a semiconductor is known
as doping. The amount of impurity, or dopant, added to an intrinsic (pure) semiconductor
varies its level of conductivity. Doped semiconductors are referred to as extrinsic. By adding
impurity to pure semiconductors, the electrical conductivity may be varied by factors of
thousands or millions.
A 1 cm3 specimen of a metal or semiconductor has of the order of 1022 atoms. In a metal,
every atom donates at least one free electron for conduction, thus 1 cm3 of metal contains on
the order of 1022 free electrons. Whereas a 1 cm3 of sample pure germanium at 20 °C,
contains about 4.2×1022 atoms but only 2.5×1013 free electrons and 2.5×1013 holes. The
addition of 0.001% of arsenic (an impurity) donates an extra 1017 free electrons in the same
volume and the electrical conductivity is increased by a factor of 10,000.
The materials chosen as suitable dopants depend on the atomic properties of both the dopant
and the material to be doped. In general, dopants that produce the desired controlled changes
are classified as either electron acceptors or donors. Semiconductors doped with
donor impurities are called n-type, while those doped with acceptor impurities are known
as p-type. The n and p type designations indicate which charge carrier acts as the
material's majority carrier. The opposite carrier is called the minority carrier, which exists
due to thermal excitation at a much lower concentration compared to the majority carrier.
For example, the pure semiconductor silicon has four valence electrons which bond each
silicon atom to its neighbors. In silicon, the most common dopants are group III and group
V elements. Group III elements all contain three valence electrons, causing them to function
as acceptors when used to dope silicon. When an acceptor atom replaces a silicon atom in the
crystal, a vacant state ( an electron "hole") is created, which can move around the lattice and
functions as a charge carrier. Group V elements have five valence electrons, which allows
them to act as a donor; substitution of these atoms for silicon creates an extra free electron.
11
Therefore, a silicon crystal doped with boron creates a p-type semiconductor whereas one
doped with phosphorus results in an n-type material.
Fig 5 Pentavalent & trivalent impurities
1.6 P – TYPE SEMICONDUCTORS
A P - type semiconductor is formed when a small amount of trivalent impurity is added to
pure Germanium or silicon atom crystal. The addition of trivalent impurity produces a large
no. of holes to the host crystals. To explain the formation of P - type semiconductor, let us
introduce a trivalent impurity into the lattice of a pure silicon crystal. The trivalent atom has
3 valance electrons and form covalent bonds with neighbouring atoms. The 4th bond is
incomplete. The trivalent atom then attracts an electron from an adjacent atom there by
completing the 4th bond and forming a hole in the adjacent atom. Since a trivalent impurity
atom provides 1 hole, an enormous increase occurs in the number of holes. The
impure crystals so obtained are called P - type semiconductors where P represents the
positive charge on hole. Thus the majority carrier in a P - type semiconductor is holes. Free
electrons are also present in the P - type semiconductor. These are thermally generated and
since they relatively few, they are called minority carriers. The trivalent impurity atoms are
called acceptors because each accepts an electron when the atom is introduced into the host
crystal.
12
Fig 6 Trivalent impurity with silicon (p-type)
1.7 N – TYPE SEMICONDUCTORS
An N - type semiconductor is formed when a small amount of pentavalent impurity is added
to a pure Germanium or Silicon crystal. The addition of pentavalent impurity produces a
large no. of free electrons in the host crystal.
To explain the formation of N - type semiconductor, let us introduce a pentavalent impurity
atom into the lattice of pure silicon crystal. The pentavalent atom has 5 valance electrons, but
only 4 form covalent bonds with the neighbouring atoms. The 5th electron finds no place in
the covalent bonding so becomes free. Since an impurity atom provides one free electron, an
enormous increase occurs in the no. of free electrons. The impure semiconductor so obtained
is then called as N - type semiconductor where N represents negative charge on an electron.
Thus the majority carrier in N - type semiconductor is free electrons. Holes are also present in
the N - type semiconductor. These are thermally generated and since they are relatively few,
they are called minority carrier.
The pentavalent impurity atom are called donor because each donate a free electron to the
host crystal.
13
Fig 7 Pentavalent impurity with silicon (n-type)
1.8 P – N JUNCTION
A p–n junction is a boundary or interface between two types of semiconductor material, p-
type and n-type, inside a single crystal of semiconductor. It is created by doping, for example
by ion implantation, diffusion of dopants, or by epitaxy(growing a layer of crystal doped with
one type of dopant on top of a layer of crystal doped with another type of dopant). If two
separate pieces of material were used, this would introduce a grain boundary between the
semiconductors that severely inhibits its utility by scattering the electrons and holes
14
Fig. 8 P-N junction
15
2.1 THE THERMOELECTRIC EFFECT
The thermoelectric effect is the direct conversion of temperature differences to electric
voltage and vice-versa. A thermoelectric device creates voltage when there is a different
temperature on each side. Conversely, when a voltage is applied to it, it creates a temperature
difference. At the atomic scale, an applied temperature gradient causes charge carriers in the
material to diffuse from the hot side to the cold side.
This effect can be used to generate electricity, measure temperature or change the
temperature of objects. Because the direction of heating and cooling is determined by the
polarity of the applied voltage, thermoelectric devices can be used as temperature controllers.
The term "thermoelectric effect" encompasses three separately identified effects: the Seebeck
effect, Peltier effect and Thomson effect. Textbooks may refer to it as the Peltier–Seebeck
effect. This separation derives from the independent discoveries of French physicist Jean
Charles Athanase Peltier and Baltic German physicist Thomas Johann Seebeck. Joule
heating, the heat that is generated whenever a voltage is applied across a resistive material, is
related though it is not generally termed a thermoelectric effect. The Peltier–Seebeck and
Thomson effects are thermodynamically reversible,[1] whereas Joule heating is not.
2.2 SEEBECK EFFECT
Fig 9 seebeck effect
A thermoelectric circuit composed of materials of different Seebeck coefficient (p-doped and
n-doped semiconductors), configured as a thermoelectric generator. If the load is removed
then the current stops, and the circuit functions as a temperature-sensing thermocouple.
16
The Seebeck effect is the conversion of temperature differences directly into electricity and is
named after the Baltic German physicist Thomas Johann Seebeck, who, in 1821 discovered
that a compass needle would be deflected by a closed loop formed by two metals joined in
two places, with a temperature difference between the junctions. This was because the metals
responded differently to the temperature difference, creating a current loop and a magnetic
field. Seebeck did not recognize there was an electric current involved, so he called the
phenomenon the thermomagnetic effect. Danish physicist Hans Christian Ørsted rectified the
mistake and coined the term "thermoelectricity".
where is the Seebeck coefficient (also known as thermopower), a property of the
local material, and is the gradient in temperature .
The Seebeck coefficients generally vary as function of temperature, and depend
strongly on the composition of the conductor. For ordinary materials at room
temperature, the Seebeck coefficient may range in value from -100 μV/K to +1000
μV/K (see Thermoelectric materials)
If the system reaches a steady state where , then the voltage gradient is given
simply by the emf: . This simple relationship, which does not
depend on conductivity, is used in the thermocouple to measure a temperature
difference; an absolute temperature may be found by performing the voltage
measurement at a known reference temperature. Conversely, a metal of unknown
composition can be classified by its thermoelectric effect if a metallic probe of known
composition, kept at a constant temperature, is held in contact with it (the unknown
material is locally heated to the probe temperature). Industrial quality control
instruments use this as thermoelectric alloy sorting to identify metal alloys.
Thermocouples in series form a thermopile, sometimes constructed in order to
increase the output voltage, since the voltage induced over each individual couple is
small. Thermoelectric generators are used for creating power from heat differentials
and exploit this effect.
17
2.3 THE PELTIER EFFECT
Fig 10 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, who
discovered it in 1834. When a current is made to flow through a junction between two
conductors A and B, heat may be generated (or removed) at the junction. The Peltier heat
generated at the junction per unit time, , is equal to
where ( ) is the Peltier coefficient of conductor A (B), and is the electric
current (from A to B). Note that the total heat generated at the junction is not determined
by the Peltier effect alone, as it may also be influenced by Joule heating and thermal
gradient effects (see below).
The Peltier coefficients represent how much heat is carried per unit charge. Since charge
current must be continuous across a junction, the associated heat flow will develop a
discontinuity if and are different. The Peltier effect can be considered as the
back-action counterpart to the Seebeck effect (analogous to the back-emf in magnetic
induction): if a simple thermoelectric circuit is closed then the Seebeck effect will drive a
current, which in turn (via the Peltier effect) will always transfer heat from the hot to the
cold junction. A typical Peltier heat pump device involves multiple junctions in series,
through which a current is driven. Some of the junctions lose heat due to the Peltier
effect, while others gain heat. Thermoelectric heat pumps exploit this phenomenon, as
do thermoelectric cooling devices found in refrigerators.
18
3.1 THERMOELECTIC GENERATOR
Fig 11 thermoelectric generator
Thermoelectric generators (also called Seebeck generators) are devices which convert heat
(temperature differences) directly into electrical energy, using a phenomenon called the
"Seebeck effect" (or "thermoelectric effect").
A thermoelectric generator is a device made up of p –n type semi conductors
A thermoelectric module is a array of thermocouples connected electrically in series
but thermally in parallel
Many couples are used becuause the voltage drop across one couple is only on the
order of millivolts.
Connecting many in series brings the voltage closer to that found in typical DC
power sources.
A thermoelectric device creates a voltage when there is a different temperature on
each side.
temperature difference provides the voltage but it is the heat flow which enables the
current.
19
Fig 12 .TEG effect
A thermoelectric produces electrical power from heat flow across a temperature gradient. As
the heat flows from hot to cold, free charge carriers (electrons or holes) in the material are
also driven to the cold end (Fig. 1). The resulting voltage (V) is proportional to the
temperature difference (∆T) via the Seebeck coefficient, α, (V = α∆T). By connecting an
electron conducting (n-type) and hole conducting (p-type) material in series, a net voltage is
produced that can be driven through a load. A good thermoelectric material has a Seebeck
coefficient between 100 µV/K and 300 µV/K; thus, in order to achieve a few volts at the
load, many thermoelectric couples need to be connected in series to make the thermoelectric
device
20
3.2 GENERAL CALCULATIONS
21
Many couples are used (in both power generation and cooling) becuause the voltage drop
across one couple is only on the order of millivolts. Connecting many in series brings the
voltage closer to that found in typical DC power souces. The Seebeck voltage (not including
the Ohmic, IR voltage drop) of the couple, S is derived from the Seebeck coefficient of the n-
type and p-type elements and the number of couples, n.
The electrical resistance of the device depends not only on the electrical resistance of the
thermoelectric materials but also the electrical resistnace of the metal interconnects and the
contact resistance between the interconnects and the thermoelectric materials. All of these
contributions are temperature dependent making the exact computation of the resistance
complex. The device resistance, R, can be approximated
assuming temperature independent properties. Here Rl is the interconnect and contact
resistance (loss) per couple, l is the length (height) and A is the cross-sectional area of the
thermoelectric elements.
Similar to the electrical resistance, the total thermal conductance of the device can be
approximated by
22
3.3 POWER OF A TEG
Just as the power in a resistor is V2/R the power produced in a thermoelectric
generator depends on the square of the voltage (Seebeck coefficient and temperature
difference) divided by the resistivity. Notice also that the power per area can be
arbitrarily adjusted with l (length).
3.4 ZT of a thermoelectric (figure of merit)
The efficiency of a thermoelectric material depends on the thermoelectric properties,
Seebeck coefficient, electrical resistivity and thermal conductivity
These material properties all appear together and thus form a new material property
which we call zT, the Thermoelectric Figure of Merit.
23
3.5 Efficiency of Thermoelectric generator
A thermoelectric generator converts heat (Q) into electrical power (P) with efficiency η.The
amount of heat, Q, that can be directed though the thermoelectric materials frequently
depends on the size of the heat exchangers used to harvest the heat on the hot side and reject
it on the cold side. As the heat exchangers are typically much larger than the thermoelectric
generators themselves, when size is a constraint (or high P/V is desired) the design for
maximum power P = ηQ. Small Thermoelectric Generators may take precedence over
maximum efficiency. In this case the temperature difference (and therefore thermoelectric
efficiency as described below) may be only half that between the heat source and sink.The
efficiency of a thermoelectric converter depends heavily on the temperature difference
∆T = Thot – Tcold
across the device. This is because the thermoelectric generator, like all heat engines, cannot
have an efficiency greater than that of a ( Carnot ). The efficiency of a thermoelectric
generator is typically defined as
Where the first term is the Carnot efficiency and ZT is the figure of merit for the device.
While the calculation ofefficiency Schematic of a thermoelectric generator. Many
thermoelectric couples (top) of n-type and p-type thermoelectric semiconductors are
connected electrically in series and thermally in parallel to make a thermoelectric generator.
The flow of heat drives the free electrons (e-) and holes (h+) producing electrical power from
heat. a thermoelectric generator efficiency can be complex, use of the average material figure
of merit, zT, can provide an approximation for ZT.
Here, Seebeck coefficient (α), electrical resistivity (ρ), and thermal conductivity (κ) are
temperature (T) dependent materials properties.
24
4.1 Materials used
The thermoelectric power factor maximizes somewhere between a metal and
semiconductors. Good thermoelectric materials are typically heavily doped
semiconductors with carrier concentration of 1019 to 1021 carriers/cm3.
To ensure that the net Seebeck effect is large, there should only be a single type of
carrier. Mixed n-type and p-type conduction will lead to opposing Seebeck effect and
low thermopower (defined here as absolute value of Seebeck coefficient).
By having a band gap large enough, n-type and p-type carriers can be separated, and
doping will produce only a single carrier type. Thus good thermoelectric materials
have band gaps large enough to have only a single carrier type but small enough to
sufficiently high doping and high mobility (which leads to high electrical
conductivity).
A material with a large thermoelectric power factor and therefore zT, needs to have a
large Seebeck coefficient (found in low carrier concentration semiconductors or
insulators) and a large electrical conductivity (found in high carrier concentration
metals)
Graph 1. Material selection
25
Using these principles, a variety of high zT materials have been developed. Many materials
have an upper temperature limit of operation, above which the material is unstable. Thus no
single material is best for all temperature ranges, so different materials should be selected for
different applications based on the temperature of operation.
4.2 Bismuth Telluride Bi2Te3 ( ZT 0.8 - 1.0 @ room temp)
Bismuth telluride (Bi2Te3) is a gray powder that is a compound of bismuth and tellurium also
known as bismuth(III) telluride. It is a semiconductor which, when alloyed with
antimony or selenium is an efficient thermoelectric material for refrigeration or portable
power generation. Topologically protected surface states have been observed in Bismuth
telluride.
Fig 13.structure Bismuth telluride
26
Fig 14 .Bismuth telluride
band structure can be described as a many-ellipsoidal model with 6 constant-energy ellipsoids
that are centred on the reflection planes.[2] Bi2Te3 cleaves easily along the trigonal axis due
to Van der Waals bonding between neighbouring tellurium atoms. Due to this, bismuth
telluride based material that are used for power generation or cooling applications must be
polycrystalline. Furthermore, the Seebeck coefficient of bulk Bi2Te3 becomes compensated
around room temperature, forcing the materials used in power generation devices to be an
alloy of bismuth, antimony, tellurium, and selenium.[1]
Recently, researchers have attempted to improve the efficiency of Bi2Te3 based materials by
creating structures where one or more dimensions are reduced, such as nanowires or thin
films. In one such instance n-type bismuth telluride was shown to have an improved Seebeck
coefficient (voltage per unit temperature difference) of −287 μV/K at 54 Celsius, [3] However,
one must realize that Seebeck Coefficient and electrical conductivity have a trade-off; a
higher Seebeck coefficient results in decreased carrier concentration and decreased electrical
conductivity.[4] Bismuth telluride is a narrow gap layered semiconductor with a trigonal unit
cell. The valence and conduction
In another case, researchers report that bismuth telluride has high electrical conductivity of
1.1×105 S·m/m2 with its very low lattice thermal conductivity of 1.20 W/(m·K), similar to
ordinary glass.
27
4.3 OCCURENCE
The mineral form of Bi2Te3 is tellurobismuthite which is moderately rare. There are many
natural bismuth tellurides of different stoichiometry, as well as compounds of the Bi-Te-S-
(Se) system, like Bi2Te2S (tetradymite).
Fig 15 .bismuth telluride powder
Bismuth Telluride is prepared by sealing a sample of bismuth and tellurium metal in a quartz
tube under vacuum (critical, as an unsealed or leaking sample may explode in a furnace) and
heating it to 800°C in a muffle furnace.
28
4.4 Other materials
SKUTTERITE
Fig 16
Crystal Structure of Yb14MnSb11
Fig 17
29
5.1 THEORITICAL CALCULATION OF EFFICIENCY
According to theory , the efficiency of a thermo electric generator is given by the formula
For the given specimen ,i.e. commercial thermoelectric generator ,which is made of Bismuth
telluride , the value of its figure of merit ZT must be found out by using
Here ,
α = Seebeck coefficient of bismuth telluride
T = Operating Temperature
ρ = Resistivity
К = thermal conductivity
All these properties considered for bismuth telluride,, the average value of ZT of Bismuth
telluride over a vast temperature range lies between 0.8 – 1.0.
Let us assume ZT=0.9 for theoretical purpose
30
For comparison with the practical model yet to come, the temperatures have been assumed
similar to what have been done in the experimental model.
Thot
°C
Tcold
°C
ΔT
°C
ZT Carnot
efficiency
ΔT/ Thot
Efficiency
η
44.1 23.5 20.6 0.9 46.7 7.005
51.2 23.6 26.6 0.9 51.9 7.78
58.9 23.6 35.3 0.9 59.9 8.95
68.7 23.8 44.9 0.9 65.3 9.75
79.9 24 55.9 0.9 69.9 10.4
93.2 24.1 69.1 0.9 72.3 10.85
Table 1
The theoretical efficiency of the thermoelectric generator ranges between 7-11% within the
given temperature range.
31
6.1 PERFORMANCE OF THERMOELECTRIC GENERATOR
There are several ways to calculate the power generation performance of a thermoelectric
generator, either by averaging the schemes or by using finite element analysis. The advantage
of averaging schemes is that an immediate answer is obtained from simplified analytical
equations.
6.2 EXPERIMENTAL SETUP
A commercial thermoelectric device was used for the experimental testing and is a model
TEC1-
12706 Bismuth Telluride device with a physical size of 40mm x 40mm x 3.5 mm. The device
has 127 couples and a photo of the device is shown in Figure 1 below.
Figure 18: Photo of thermoelectric device used for testing (model TEC1-12706).
32
A testing assembly was constructed such that a known heat could be added to “hot” side of
the device. By measuring the power output of the thermoelectric device through a load, the
efficiency of the thermoelectric device can be calculated as follows:
η = P out / Q in
Where, η = thermal efficiency
Pout = measured power output of the device (watts)
Qin = measured input heat to the device (watts)
The testing assembly consisted of
Sno. Object
1 Thermoelectric generator (model TEC1-12706).
2 D.C. Power Supply
3 Heating coil
4 Hot water beaker
5 Cold water beaker
6 Thermometers (2)
7 Digital voltmeters
8 External load resistance (variable)
9 Aluminium plates assembly for TEG.
Table 2
33
6.3 SET UP
For experimentation, the thermoelectric generator was sandwiched between two
aluminium plates.
One plate was immersed into a hot water beaker.
The other was immersed into a cold water beaker.
Both were separated from physical contact.
A heating coil was immersed into the water of the hot water beaker.
The heating coil was connected to a D.C. power supply.
The two wires of the thermoelectric generator were connected to an external variable load
resistance.
Two thermometers were placed each in the hot water and the cold water beakers.
A digital voltmeter was connected to the resistance to measure the output voltage of the
TEG.
Fig 19 : Experimental setup
6.4 OPERATION The required parameters were
34
Temperature of the hot side of the thermoelectric device (thermometer 1)
Temperature of the cold side of the thermoelectric device (Thermometer 2)
Internal resistance of D.C. power source
Heater voltage, (D.C. input voltage )
Load voltage, measured across variable resistor
Load resistance
6.5 PROCEDURE
The D.C. power source was adjusted such that it gives a constant output of 12 volts,
Sufficient time was given for the heater coil to heat according to this voltage
The heater coil in turn heats the water in the hot water beaker and thus the aluminium
plate and the hot side of the thermoelectric generator
A thermometer was kept in the hot water beaker and after a homogeneous temperature
was obtained , it was noted down as Thot
Similarly ,another thermometer was kept in the cold water and temperature was
measured as Tcold
The temperature difference was calculated as ΔT.
The variable load resistance at the output was adjusted such that maximum power was
obtained for the given input and this resistance was noted down
The output voltage of the TEG was recorded on a digital voltmeter as Vout
The same procedure was repeated for 5 input D.C. voltages 12, 14, 16, 18 ,20 V.
All the results and calculations were tabulated.
7 RESULTS
35
In order to calculate efficiency using Equation (1), the input heat, Qin, and output power Pout, must be found. The input heat is found using the heater voltage and heater resistance as shownin Equation
Qin = Vin * Vin / Rin
Here Rin stands for the internal resistance of the D.C. power source (heating coil) which was
calculated as 12 Ω.
The Output is calculated using
Pout = Vout * Iout
Here Iout is given by Vout/Rout
Vin Thot
°C
Tcold
°C
ΔT
°C
12 44.1 23.5 20.6
14 51.2 23.6 26.6
16 58.9 23.6 35.3
18 68.7 23.8 44.9
20 79.9 24 55.9
22 93.2 24.1 69.1
Table 3
All the readings are seen and noted down for each value of input voltage Vin
36
The readings are tablualted as shown
Table 4
37
Vin ΔT
°C
Input(W) load
resistance Ω
Vout Output(W) η
12 20.6 12 3.01 0.54 0.10 0.83
14 26.6 16.33 2.81 0.65 0.15 0.91
16 35.3 21.33 3.02 0.87 0.25 1.17
18 44.9 27 2.76 1.22 0.54 1.99
20 55.9 33.33 3.38 2.48 1.83 5.41
22 69.1 40.33 2.81 2.62 2.46 6.12
Graph 2:Input voltage vs Temp difference ΔT
10 12 14 16 18 20 22 240
10
20
30
40
50
60
70
80
input voltage vs temp differenceLinear (input voltage vs temp dif-ference)
voltage
Tem
p. d
iffer
ence
38
Graph 3: Temperature difference vs. efficiency η
10 20 30 40 50 60 70 800
1
2
3
4
5
6
7
Series1Linear (Series1)Series2Linear (Series2)Temp difference vs efficiencyLinear (Temp difference vs ef-ficiency)
Temp difference
efficie
ncy
39
Graph 4: Input voltage (Vin) vs efficiency η
10 12 14 16 18 20 22 240
1
2
3
4
5
6
7
Series2Linear (Series2)
Input Voltage
efficie
ncy
7.2Comparison between theoretical model and experimental model:Table 5
40
Thot
°C
Tcold
°C
ΔT
°CEfficiency η
Theoretical %
Efficiency η
Experimental %
44.1 23.5 20.6 7.005 0.83
51.2 23.6 26.6 7.78 0.91
58.9 23.6 35.3 8.95 1.17
68.7 23.8 44.9 9.75 1.99
79.9 24 55.9 10.4 5.41
93.2 24.1 69.1 10.85 6.12
Graph 5 : Comparison graph (Temp difference ΔT vs Efficiency η)
10 20 30 40 50 60 70 800
2
4
6
8
10
12
TheoreticalLinear (Theoretical)ExperimentalLinear (Experimental)
Temp difference
Efficie
ncy
7.3 CONCLUSION
41
Results are presented for a particular Bismuth Telluride thermoelectric device (TEC1-
12706).
The load resistance is variable in the experimental setup and the power generation and
efficiency are both plotted versus the voltage produced.
The maximum temperature difference tested was 69.1C and this produced an
efficiency of 6.12% and an output power of 2.46 watts.
While this efficiency might seem low, thermoelectric generators are noted for their
relatively low conversion efficiency.
Also, the maximum temperature difference tested (69.1°C) is fairly modest, higher
temperature differences would result in higher efficiency.
If the graph plot of input temperature difference vs. Efficiency is extended further, it
is observed that the greater the temperature gradient ,the more drastic the increase in
efficiency
Typical thermoelectric devices require a temperature difference of approximately
500°C to achieve an efficiency of 15-20 % 9,10.
The Other factor that influences the efficiency is the internal material property ZT of
the material which determines how much of the heat is converted to electricity .
The current specimen (bismuth telluride ZT= 0.8-1.0) shows maximum
experimental efficiency of
6.12%
Compounds with a ZT value around 5 have been discovered,ZT values of 10 -15 in
the future will promise a gigantic increase in the efficieny in the order of 20-30%.
Note:
Due to economic considerations, one single TEG module was used for experimentation, A
group of modules packed together in series would result in higher efficiencies .
8.1 APPLICATIONS OF THERMOELECTRIC GENERATORS
42
WASTE HEAT RECOVERY from
Cement plants Petrochemical plants Coal fired power plants Refineries Furnaces& most importantly
Automotive exhausts and surfaces
Fig 20. Waste heat
Fig 21. Automotive heat
43
Heat plays a major role in global energy consumption. Heat itself may be the final use
of energy (e.g. residential heating). Heat is also a waste product in the transformation
of energy, for example in electric power generation or transportation.
Thermal energy (heat) is a common link between many forms of energy. This means
that improving the net heat to electricity efficiency, or bypassing the thermal energy
step altogether (as in fuel cells), will improve energy utilization.
Fig 22. Power plant heat
As shown in the above illustration , out of 100 input units only a mere 33% is coming
out as useful electrical energy . Majority of the remaining 67 units dissipates as waste
heat contributing to global warming.
By use of thermoelectric generators in plants such as these the waste heat can be
harvested back in the form of useful electrical energy to further improve the overall
efficiency of the plant and reduce global warming effect.
44
8.2 AUTOMOTIVE HEAT RECOVERY
Fig 23
As shown above ,for every 100 units of fuel burnt only 12- 15 % is actually utilized in
propelling the vehicle forward.
62% goes dissipates as waste heat.
If this waste heat were recycled in the form of useful electrical energy via
thermoelectric generators, the overall efficiency of cars would increase drastically.
By utilizing a portion of the lost thermal energy to charge the battery instead of using
an alternator (adds drag on the engine) the overall fuel economy can be increased by
about 10%.
The present Zt figure of bismuth telluride offers an increase in efficiency by 5-6%.
45
The ZT 1.5 efficiency would translate into a 10 percent increase in the fuel economy
of cars if the devices are used to replace alternators in automobiles by generating
electricity from the heat in exhaust. The ZT 3.0 materials would be a 15% increase in
fuel economy of cars and trucks. The devices could begin selling in 3 to 4 years.
If you get up to ZT 5 or so with a cheap enough system then you can replace most of
the moving parts of an engine with thermoelectrics. You would generate heat and then
use thermoelectrics with no moving parts to convert the heat directly to electricity
with higher efficiency.
Fig 24,25. BMW TEG heat recovery system
46
As shown in the above illustrations , thermoelectric generators can be attached at the
exhaust of the car where waste heat is dissipated
Fig 26,27 TEG assembly at exhaust
By attaching so, the residual heat coming towards the catalytic convertor
from the engine exhaust can be trapped into these thermoelectric generators
via heat exchangers.
This would effectively trap almost all the waste heat and convert it into useful
electrical energy ,which can be used to charge the battery without a dynamo
It can also be used to power sub-systems relieving their dependency on battery,
therefore increasing the overall efficiency of the vehicle.
Fig 28. Heat to power flow
47
8.3 Cogeneration of Heat and Electricity
Because most electricity is produced by a heat engine, which is limited by Carnot efficiency,
much of the energy is lost in the heat rejected. A typical steam power plant is only 40%
efficient. The remaining heat is wasted, unless this rejected heat can be used for heating. The
use of this heat then can add to the energy utility.
Conversely, any time a fuel is burned to make low temperature heat (such as in a home) the
ability to produce useful work or lectricity from that heat is wasted. A small cogeneration
plant in the home would produce lectricity whenever the heat is needed. The added fuel
consumed to produce the electricity has essentially the same energy content as the electricity
produced. Thus in terms of energy untilization the efficiency of electricity generation
approaches 90% compared to the 40% in a typical power plant.
Fig 29.cogeneration
Thermoeletric systems are ideal for small (e.g. single family home) cogeneration because
they could be small and silent. Even with their lower thermal to electric efficiency compared
to dynamic heat engines, the electricity would be produced with high efficiency (electric
power/extra fuel consumed) because the heat rejected will not be wasted.
48
9. REPORT CONCLUSION:
-Detailed description and analysis of the physics behind the making of a thermoelectric
generator.
-Discussion of material properties suitable for thermoelectric generation
-Calculation of theoretical efficiency of TEG using given formulae.
-Performance test on single thermoelectric generator (model TEC1-12706)
-Comparison of both theoretical and practical models/
-Calculation of efficiency of the given thermoelectric generator (bismuth telluride)
-Necessity and applications of thermoelectric generator.
EFFICIENCY OF TEG(Theoretical mean)
8-10%
EFFICIENCY OF TEG( experimental mean)
3-5%
Note:
-Thermoelectric generators are small portable devices with almost negligible weight and no
noise of operation
-Since there are no internal moving parts, no mechanical losses due to friction occur and
device runs smoothly
-In applications such as home co-generation, the desire for silent, vibration, and maintenance
free operation will favour thermoelectrics. Residential co-generation and automotive waste
heat recovery are two examples where “small” systems could have an impact on the global
energy consumption if implemented on a large-scale.
49
BIBLIOGRAPHY
1.) Zorbas, K.T., E. Hatzikraniotis, and K.M. Paraskevopoulos. “Power and Efficiency
Calculation in Commercial TEG and Application in Wasted Heat Recovery in Automobile,”
Proceedings of the 5th European Conference on Thermoelectrics, September 10-12, 2007,
Odessa, Ukraine,
2.) Snyder, G. Jeffrey. “Small Thermoelectric Generators,” The Electrochemical Society
Interface, Fall 2008,
3.) Cengel, Y.A. and Boles, M.A., Thermodynamics: An Engineering Approach, 6th Edition,
McGraw-Hill, 2008,
4.) The Essential Guide to Semiconductors. Prentice Hall PTR,
5.) History of semiconductors John Wiley and Sons (WIE).
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