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7/31/2019 CH04 TLOGY
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NGP 322, 4 cr
20111
Dr. Hamada Mohamed Gad
CHAPTER ICHAPTER ICHAPTER ICHAPTER I
CHAPTER IVCHAPTER IVCHAPTER IVCHAPTER IV
CHAPTER IIICHAPTER IIICHAPTER IIICHAPTER III
GENERAL INTRODUCTIONGENERAL INTRODUCTIONGENERAL INTRODUCTIONGENERAL INTRODUCTION
THERMOELECTRICTHERMOELECTRICTHERMOELECTRICTHERMOELECTRIC TECHNOLOGYTECHNOLOGYTECHNOLOGYTECHNOLOGY
CHAPTERCHAPTERCHAPTERCHAPTER IIIIIIII COMBUSTION OF FOSSIL FUELSCOMBUSTION OF FOSSIL FUELSCOMBUSTION OF FOSSIL FUELSCOMBUSTION OF FOSSIL FUELS
SOLAR ENERGY TECHNOLOGYSOLAR ENERGY TECHNOLOGYSOLAR ENERGY TECHNOLOGYSOLAR ENERGY TECHNOLOGYCHAPTER VCHAPTER VCHAPTER VCHAPTER V
FUEL CELL TECHNOLOGYFUEL CELL TECHNOLOGYFUEL CELL TECHNOLOGYFUEL CELL TECHNOLOGY
ENERGY CONSERVATIONENERGY CONSERVATIONENERGY CONSERVATIONENERGY CONSERVATIONCHAPTER VICHAPTER VICHAPTER VICHAPTER VI
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CHAPTER IVTHERMOELECTRIC
TECHNOLOGY
4
Introduction.
Principles of thermoelectric effect.
What is the thermoelectric.
History of thermoelectric.
How do the thermoelectric work?
Thermoelectric applications.
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In the early 1800s, Thomas Seebeck discovered that the
junction between two metals generates a voltage that is a
function of temperature.
Introduction
5
It is a temperature sensor that consists of two wires of
dissimilar metals joined at one end. These metals are shown as
"Alloy 1" and "Alloy 2" in the figure and form junctions J1
and J4.
A thermocouple is simply a practical application of the
"Seebeck Effect".
ColdJunction
HotJunction
6
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Historically, temperature measurement with thermocouples relied on a
second thermocouple element to sense a known temperature as a
reference.
V = (TUNKNOWN TREF)
where is the Seebeck Coefficient. 7
The magnitude of the voltage generated in this scenario now depends on
the temperature difference between J1 and J4, and the types of metals
used for Alloy 1 and Alloy 2. The result can be expressed by the
following equation:
The easiest and most precise way of producing a reference temperature
was to immerse this reference junction (J4) in an ice bath, which gave it
the name "cold junction".
Different types of thermocouples have different coefficients, which are
listed in most thermocouple references.
Note that the connection of the thermocouple to the voltmeter forms
additional, potentially unwanted junctions J2 and J3. These junctions will
have no effect on the measurement.
8
With this configuration, it was only necessary to read the voltage, then
look up the corresponding temperature in a table for the Alloy1 / Alloy 2
thermocouple type referenced to 0C.
We now have a basic model that can be used to develop a more
sophisticated instrumentation system for reading thermocouples.
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9
Seebeck
Effect
The thermoelectric effect is the direct conversion of temperature
differences to electric voltage and vice versa.
Temperature
Difference
Electric
Voltage
T EMF
10
Peltier
Effect
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The term thermoelectric effect or thermoelectricity
encompasses three separately identified effects:
11
The Seebeck effect
In many textbooks, thermoelectric effect may also be called the
PeltierSeebeck effect.
The Thomson effect.
The Peltier effect.
Seebeck Effect
The Seebeck effectis the conversion of temperature differences directly
into electricity.
12
This is because the metals respond differently to the temperature
difference, which creates a current loop, which produces a magnetic
field.
Seebeckdiscovered that a compass needle would be deflected when a
closed loop was formed of two metals joined in two places with a
temperature difference between thejunctions.
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Seebeck, however, at this time did not recognize there was an electric
current involved, so he called the phenomenon the thermomagnetic
effect, thinking that the two metals became magnetically polarized by the
temperaturegradient.
13
The effect is that a voltage, the thermoelectric EMF, is created in the
presence of a temperature difference between two different metals or
semiconductors.
One such combination, copper-constantan, has a Seebeck coefficient of
41 microvolts per Kelvin at room temperature.
This causes a continuous current in the conductors if they form a
complete loop.
The voltage created is of the order of several microvolts per Kelvin
difference.
In the circuit:
which can be in several different
configurations and be governed by the same
equations), the voltage developed can be
derived from:
SA andSB arethe Seebeck coefficients(alsocalledthermoelectric power
or thermopower) ofthe metals A and B as a function of temperature, and
T1 andT2 are the temperatures of the two junctions .
14
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The Seebeck coefficientsare non-linear as a function of temperature,and depend on the conductors' absolute temperature, material, and
molecular structure .
The Seebeck effect is commonly used in a device called a
thermocouple (because it is made from a coupling or junction ofmaterials, usuallymetals)to measure a temperature difference directly or
to measure an absolute temperature by setting one end to a known
temperature .
V = (SB SA) * (T2 T1)
15
If the Seebeck coefficientsare effectively constant for the measuredtemperature range, the above formula can be approximated as:
Several thermocoupleswhen connected in series are calleda thermopile,
which is sometimes constructed in order to increase the output voltage
since the voltage induced over each individual couple is small.
A thermopile is an electronic device that converts thermal energy into
electrical energy.
16
Thermopiles do not measure the absolute temperature, but generate an
output voltage proportional to a local temperature difference or
temperature gradient.
It is composed of thermocouples connected usually in series, or less
commonly in parallel.
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Thermocouple Rangesand Limits ofError.
17
Thetable below shows temperature ranges and accuracy for J, K, E and
T thermocouples.
Common forms of thermocouple construction
18
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0
Methods for insulating thermocouple
19
Thermopiles
Multiple-Junction Thermocouple Circuits
Thermopile is a term used to describe a multiple-junction
thermocouple circuit that designed to amplify the output of the
circuit.
Thermocouple wires
1
N
2
B
A
A
Cu
B
B
Copper connecting wires
CuA
PotentiometerMeasuring junction
Reference junction
20
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1
Thermocouples in parallel
Multiple-Junction Thermocouple Circuits
When a spatially averaged temperature is desired, multiple
thermocouple junctions can be arranged in parallelThermocouple wires
1
B
A
ACu
B
B
Copper connecting wires
CuA
PotentiometerMeasuring junction
N
Reference junctions
2
=
=
N
i
iemfN
emf1
)(1
=
=
N
i
iTN
T1
1
21
Thermocouple Types
5
5
4
4
3
3
2
21 VAVAVAVAVAAT o +++++=
Type E
(Chromel/Constantan)
Type T
(Copper/Constantan)
Type S
(Platinum10% Rhodium/Platinum)
Types R
(Platinum 13% Rhodium/Platinum)
Type B
(Platinum-13% Rhodium/Platinum 6% Rhodium)
Type J
(Iron / Constantan)
Type K
(Chromel / Alumel)
22
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2
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Peltier Effect
This effect bears the name of Jean-Charles Peltier (a french physicist)
who discovered in 1834, the calorific effect of an electrical current at the
junction of two different metals.
24
Whereis the Peltier coefficient AB of the entire thermocouple, and Aand B are the coefficients of each material.
The Peltier heat absorbed ( )by the lower junction per unit time, is
equal to:
When a current I is made to flow through the circuit, heat is evolved atthe upper junction (at T2), and absorbed at the lower junction (at T1).
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Peltier effect
Schematic presentation of Peltier effect
This effect was discovered in 1834 by Jean Peltier in the course of the
following experiment:
Definition: cooling and heating at the junctions of two dissimilar metalswhen an electrical current is passed through those metals.
The electrical current was passed through the loop made of Cu and Bi
one junction heated up; another cooled down.
Peltier used a Cu-Bi junction to freeze water.
26
1. Seebeck effect
It is the reverse of the Peltier effect. When applying a temperaturedifference between two junctions, it produces electricity.
The Seebeck effect is transduction of temperature difference to
electricity.
This effect can be used for electric generation, but there are fewer
applications than Peltier effect as the efficiency is still not enough.
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4
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2-Peltier effect
The Peltier effect is generally used in cooling, refrigeration, heat
pump,
If the current direction is reversed, hot and cold junction is reversed.
When direct current is passed across the different materials (metal or
semiconductor) that are connected at two junctions, one junction absorbs
heat and the other diffuses it.
The Peltier effect is a creation of temperature difference from electriccurrent.
Thomson Effect
The Thomson effect was predicted and subsequently experimentally
observed by William Thomson (Lord Kelvin) in 1851.
28
Any current-carrying conductor with a temperature difference between
two points, will either absorb or emit heat, depending on the material.
It describes the heating or cooling of a current-carrying conductor with a
temperature gradient.
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5
In metals such as zinc and copper, which have a hotter end at a higher
potential and a cooler end at a lower potential, when current moves from
the hotter end to the colder end, it is moving from a high to a low
potential, so there is an evolution of heat.
29
This is called the positive Thomson effect.
Hotter
End
Cooler
End
Higher
Potential
Lower
Potential
Current
Copper
Positive Thomson Effect
Heat
Rejected
30
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6
In metals such as cobalt, nickel, and iron, which have a cooler end at a
higher potential and a hotter end at a lower potential, when current
moves from the cooler end to the hotter end, it is moving from a high to a
low potential, there is an absorption of heat.
31
This is called the negative Thomson effect.
Hotter
End
Cooler
End
Higher
Potentiallower
Potential
Current
Iron
Negative Thomson Effect
Heat
Absorbed
32
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7
The Seebeck effect is actually a combination of the Peltier and Thomson
effects. In fact, in 1854 Thomson found the following relationship, now
called the Thomson or Kelvin relationship, between the corresponding
coefficients.
The Thomson relationships
33
Which predicted the Thomson effect before it was actually formalized.
The absolute temperature T, the Peltier coefficient and Seebeck
coefficient S are related by the first Thomson relation
34
Peltier Effect
In 1834, Jean Peltier found that an electrical current would produce a
temperature gradient at the junction of two dissimilar metals.
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Peltier Cooling
>0 ; Positive Peltier coefficient
High energy holes move from left
to right.
Thermal current and electric
current flow in same direction.
P-TypeSemiconductor
q=*j, where q is thermal current density and j is electrical current density.
Thermoelectric Cooler Diagram
36
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38
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0
39
A p-n couple can be used as a thermoelectric couple based on the Peltier
effect (left) or a thermoelectric generator based on the Seebeck effect
(right).
40
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1
Ice Making
Movie
7:30 minutes
41
Heat to Work,
The Peltier Effect42 seconds
42
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2
Heat to Volts
20 Seconds
43
44
Important Questions
Define Seebeck, Peltier and Thomson effects.
What is the thermoelectric effect?
What is the positive and negative Thomson effect?
What is negative and positive Peltier effect?
What is the thermoelectric cooler diagram?
What is the thermocouple and the thermopile?
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Thermoelectric Refrigeration
THE ENDTHE ENDTHE ENDTHE END
46