Teg Report Final

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.

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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.

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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.

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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

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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-

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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

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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.

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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

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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.

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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.

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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.

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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.

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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.

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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

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Fig. 8 P-N junction

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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.

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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.

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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.

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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.

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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

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3.2 GENERAL CALCULATIONS

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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

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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.

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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.

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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

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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.

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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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