Transcript contents - UET Mechanical 09

THERMOELECTRIC COOLING
CONTENTS
1.
INTRODUCTION
2.
BASIC PRICIPLES OF THERMOELECTRIC
MODULES
3.
BASIC MECHANISM OF THERMOELECTRIC COOLING
4.
THERMOELECTRIC COOLING MODULES
5.
HEAT SINK CONSIDERATIONS
6.
PERFORMANCE GRAPH OF THERMOELECTRIC MODULE
7.
APPLICATIONS OF THERMOELECTRIC COOLERS
8.
ADVANTAGES OF THERMOELECTRIC COOLING
9.
THERMOELECTRIC COOLING VERSUS TRADITIONAL
REFRIGERATION
1. INTRODUCTION
 A thermoelectric (TE) cooler, sometimes called a
thermoelectric
module
semiconductor-based
or
Peltier
electronic
cooler,
component
is
a
that
functions as a small heat pump. By applying a low
voltage DC power source to a TE module, heat will be
moved through the module from one side to the other.
One module face, therefore, will be cooled while the
opposite face simultaneously is heated.
2. BASIC PRICIPLES OF THERMOELECTRIC
MODULES
THERMOELECTRICITY IS BSED UPON THREE
BSIC PRINCIPLES
1. SEEBECK EFFECT
2. PELTIER EFFECT
3. THOMSON EFFECT
SEEBECK EFFECT
VO = AXY * (TH - TC)
Where:
VO :- is the output voltage in volts.
AXY :- is the differential Seebeck coefficient between the
two materials, x and y, in volts/K .
TH and TC, are the hot and cold thermocouple temperatures, respectively
PELTIER EFFECT
QC or QH =PXY * I
Where:
PXY is the differential Peltier coefficient between the two
materials, x and y, in volts .I is the electric current flow in
amperes. QC, QH is the rate of cooling and heating,
respectively, in watts.
THOMSON EFFECT
 When an electric current is passed through a
conductor having a temperature gradient over its
length, heat will be either absorbed by or expelled
from the conductor. Whether heat is absorbed or
expelled depends upon the direction of both the
electric current and temperature gradient. This
phenomenon is known as the Thomson Effect
3. BASIC MECHANISM OF
THERMOELECTRIC
N-TYPE SINGLE SEMICONDUCTOR
PELLET
P-TYPE SINGLE SEMICONDUCTOR
PELLET
ELECTRICALLY AND THERMALLY
PARALLEL MULTIPLE PELLETS
THERMALLY PARALLEL AND ELECTRICALLT
IN SERIES MULTIPLE PELLETS
N AND P-TYPE PELLETS
N AND P-TYPE MULTIPLE PELLETS
THERMOELECTRIC MATERIALS
 The most often used in today's TE coolers is an alloy of Bismuth
Telluride (Bi2Te3).
 In addition to Bismuth Telluride (Bi2Te3), there are other
thermoelectric materials including Lead Telluride (Pb-Te), Silicon
Germanium (Si-Ge) and Bismuth-Antimony (Bi-Sb) alloys that
may be used in specific situations.
Thermoelectric Materials should posses: Large Seebeck Coefficients (to minimize Joule heating).
 High Electrical Conductivity.
 Low Thermal Conductivity (to retain heat at the junctions)
APPROXIMATE FIGURE-OF-MERIT(Z)FOR
VARIOUS TE MATERIALS
4. THERMOELECTRIC COOLING
MODULES
 thermoelectric modules ranging in size from approximately
2.5-50 mm (0.1 to 2.0 inches) square and 2.5-5mm (0.1 to
0.2 inches) in height.
5. Heat Sink Considerations
 A perfect heat sink would be capable of absorbing an unlimited quantity of
heat without exhibiting any increase in temperature.
 A heat sink temperature rise of 5 to 15°C above ambient (or cooling fluid)
is typical for many thermoelectric applications.
 Heat sink performance:- Qs= (Ts-Ta)/Q
Where
Qs:- Thermal Resistance in Degrees centigrade per Watt.
Ts:- Heat
Sink Temperature in Degrees Centigrade.
Ta:- Ambient or Coolant Temperature in Degrees Centigrade.
Q :- Heat Input to Heat Sink in Watts.
TYPES OF HEAT SINKS
 NATURAL CONVECTION HEAT SINKS
 FORCED CONVECTION HEAT SINKS
 LIQUID-COOLED HEAT SINKS
Forced Convection Heat Sink System
Showing Preferred Air Flow
6. PERFORMANCE GRAPH OF TE MODULE
7. APPLICATIONS OF
THERMOELECTRIC COOLERS
 Include equipment used by military, medical,
industrial, consumer, scientific/laboratory, and
telecommunications organizations.
 Uses range from simple food and beverage
coolers for an afternoon picnic to extremely
sophisticated temperature control systems in
missiles and space vehicles.
8. ADVANTAGES OF THERMOELECTRIC
COOLING
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No Moving Parts
Small Size and Weight
Ability to Cool Below Ambient
Ability to Heat and Cool With the Same module
Precise Temperature Control
High Reliability
Electrically "Quiet" Operation
Operation in any Orientation
Spot Cooling
Ability to Generate Electrical Power
Environmentally Friendly
Limitations of Thermoelectric Cooling
Devices
 Low C.O.P. and efficiencies make them
unsuitable in places where economy is
concerned.
 There is also a limitation of their use in larger
units.
9. THERMOELECTRIC COOLING VERSUS
TRADITIONAL REFRIGERATION
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Solid state design
 No moving parts
 Integrated chip design
 No hazardous gases
 Silent operation
Compact and lightweight
 Low profile
 Sizes to match your component footprint
 No bulky compressor units
Precise temperature stability
 Tolerances of better than +/- 0.1°C
 Accurate and reproducible ramp and dwell times
Cooling/heating mode options
 Fully reversible with switch in polarity
Localized Cooling
 Spot cooling for components or medical applications
 Perfect for temperature calibration in precision detection systems
Rapid response times
 Instantaneous temperature change
 Reduced power consumption
Dehumidification
 Efficient condensation of atmospheric water vapor
CONCLUSION
 In spite of the fact that it has some disadvantages like low coefficient of
performance and high cost, thermoelectric refrigerators are greatly needed,
particularly for developing countries where long life, low maintenance and
clean environment are needed. There is a lot of scope for developing
materials specifically suited for TE cooling purpose and these can greatly
improve the C.O.P. of these devices. Development of new methods to
improve efficiency catering to changes in the basic design of the
thermoelectric set up like better heat transfer, miniaturization etc. can give
very effective enhancement in the overall performance of thermoelectric
refrigerators. Finally, there is a general need for more studies that combine
several techniques, exploiting the best of each and using these practically.
REFRENCES
 http://www.thermoelectrics.com/introduction.h
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tm
http://www.educypedia.be/electronics/thermoe
lectric.htm
http://www.peltier-info.com/info.html
http://www.tellurex.com/12most.html
http://www.ferrotec.com/technology/thermoele
ctric/thermalRef01.php
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