THERMOELECTRIC COOLING

CONTENTS INTRODUCTION BASIC PRICIPLES OF THERMOELECTRIC MODULES BASIC MECHANISM OF THERMOELECTRIC COOLING THERMOELECTRIC COOLING MODULES HEAT SINK CONSIDERATIONS PERFORMANCE GRAPH OF THERMOELECTRIC MODULE APPLICATIONS OF THERMOELECTRIC COOLERS ADVANTAGES OF THERMOELECTRIC COOLING THERMOELECTRIC COOLING VERSUS TRADITIONAL REFRIGERATION

1. INTRODUCTION A thermoelectric (TE) cooler, sometimes called a thermoelectric module or Peltier cooler, is a semiconductor-based electronic component 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 SEEBECK EFFECT PELTIER EFFECT 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 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 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.htm http://www.educypedia.be/electronics/thermoelectric.htm http://www.peltier-info.com/info.html http://www.tellurex.com/12most.html http://www.ferrotec.com/technology/thermoelectric/thermalRef01.php

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