CH04 TLOGY

Embed Size (px)

Citation preview

  • 7/31/2019 CH04 TLOGY

    1/23

    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

  • 7/31/2019 CH04 TLOGY

    2/23

    CHAPTER IVTHERMOELECTRIC

    TECHNOLOGY

    4

    Introduction.

    Principles of thermoelectric effect.

    What is the thermoelectric.

    History of thermoelectric.

    How do the thermoelectric work?

    Thermoelectric applications.

  • 7/31/2019 CH04 TLOGY

    3/23

    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

  • 7/31/2019 CH04 TLOGY

    4/23

    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.

  • 7/31/2019 CH04 TLOGY

    5/23

    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

  • 7/31/2019 CH04 TLOGY

    6/23

    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.

  • 7/31/2019 CH04 TLOGY

    7/23

    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

  • 7/31/2019 CH04 TLOGY

    8/23

    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.

  • 7/31/2019 CH04 TLOGY

    9/23

    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

  • 7/31/2019 CH04 TLOGY

    10/23

    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

  • 7/31/2019 CH04 TLOGY

    11/23

    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

  • 7/31/2019 CH04 TLOGY

    12/23

    2

    23

    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).

  • 7/31/2019 CH04 TLOGY

    13/23

    3

    25

    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.

  • 7/31/2019 CH04 TLOGY

    14/23

    4

    27

    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.

  • 7/31/2019 CH04 TLOGY

    15/23

    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

  • 7/31/2019 CH04 TLOGY

    16/23

    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

  • 7/31/2019 CH04 TLOGY

    17/23

    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.

  • 7/31/2019 CH04 TLOGY

    18/23

    8

    35

    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

  • 7/31/2019 CH04 TLOGY

    19/23

    9

    37

    38

  • 7/31/2019 CH04 TLOGY

    20/23

    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

  • 7/31/2019 CH04 TLOGY

    21/23

    1

    Ice Making

    Movie

    7:30 minutes

    41

    Heat to Work,

    The Peltier Effect42 seconds

    42

  • 7/31/2019 CH04 TLOGY

    22/23

    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?

  • 7/31/2019 CH04 TLOGY

    23/23

    45

    Thermoelectric Refrigeration

    THE ENDTHE ENDTHE ENDTHE END

    46