54068063 thermoelectric cooler

8/6/2019 54068063 Thermoelectric Cooler

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A Seminar Report

On

THERMOELECTRIC COOLER

Submitted for the partial fulfilment for the award of the degree of

Bachelor of Technology

In

Electrical and Electronics Engineering

by

Anandjee sah

B070470EE

Department of Electrical Engineering

NATIONAL INSTITUTE OF TECHNOLOGY CALICUT

NIT Campus P.O, Calicut 673601, India

2011


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CERTIFICATE

This is to certify that the seminar report entitled THERMOELECTRIC

COOLERS is a bona fide record of the seminar presented by ANANDJEE SAH (Roll No.

B070470EE), in partial fulfillment of the requirements for the award of Degree of Bachelor of

Technology in Electrical & Electronic Engineering from National Institute of Technology

Calicut for the year 2011.

Dr. Elizabeth P. Cherian Dr. Sreeram Kumar

(Seminar Coordinator) Professor & Head

Assistant. Professor Dept. of Electrical EngineeringDept. of Electrical Engineering

Place: NIT CalicutDate:


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Acknowledgement

At first, I thank the Lord almighty for the grace, strength and hope to make my endeavour a

success.

I wish to express my sincere gratitude to thank, Dr. Elizabeth P. Cheriyan Assistant Professor,

Department of Electrical Engineering, National Institute of Technology Calicut, for providing

me an opportunity to present this work.

I express my deep felt gratitude to Dr. Muraleedharan C Professor, Department of Mechanical

Engineering for his constant encouragement and instructions.

I express my gratitude to all the lecturers of Electrical &Electronics department, for the interestthey have shown in this presentation

I also like to thank my friends for being there with me always and for helping me to gain

valuable information. I wish to convey my sincere thanks to my parents for their

generosity and support shown during my work and further.

Finally I thank my institute NIT Calicut for giving me the opportunity to prepare and present a

seminar to all my classmates and for helping me improve my communication skills, presenting

skills and to share knowledge.

ANANDJEE SAH

(B070470EE)


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Contents

Title Page

Abstract

1 Introduction 2

2 Principle of operation 2

3 Construction 3

3.1 Thermoelectric material 3

3.2 Ceramic plates 3

3.3 Thermal interface material 3

3.4 Heat sink 4

3.5 Uses of two type of material (p and n type) 4

4 Working of Thermoelectric Module 5

5 Thermoelectric design 6

5.1 Figure of merit 6

5.2 Condensation 6

5.3 Thermoelectric performance 7

5.4 Design methodology 8

5.5 Improving performance 9

5.5.1 Thin film TE cooler 9

5.5.2 Multistage TE cooler 10

6A

dvantages and disadvantages 11

7 A pplication of TE cooling 11

8 Conclusions 11

9 References 13


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List of Figures

Title Page

1 Peltier effect 2

2 A typical Thermoelectric Module 3

3 N and P type pallet 4

4 Several N and P type pallets connected together 5

5 Working of Thermoelectric Modules 6

6 Performance curve 8

7 Thin film TE cooler 10

8 Multistage TE cooler 10

9 Cooling power densities for different design 10

ABSTRACT

Thermoelectric coolers also called thermoelectric modules or Peltier coolers. They are

semiconductor based electronic components that function as small heat pumps. On applying a

low DC voltage to the thermoelectric module, heat moves through the module from one side to

the other. One face of the module is therefore cooled, while the other face simultaneously heats

up. This phenomenon can also be reversed by changing the polarity of the applied DC

voltage causing the heat to flow in the opposite direction. Thereby, a thermoelectric module may

be used for both heating and cooling which makes it highly suitable for precise temperature

control applications.

Conventional coolers used in homes and industries depend on refrigerants such as hydro fluoro

carbon, which is a threat to the ozone layer. On the other hand, thermoelectric coolers are

environment friendly, compact and affordable. Thermoelectric coolers are semiconductor based

devices. They have got several advantages like solid construction, quiet & reliable operation, no


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CFCs, precise temperature control. A thermoelectric cooler permits lowering the temperature of

an object below ambient as well as maintaining the temperature of objects above ambient

temperatures. Thermoelectric coolers can be used for applications that require heat removal

ranging from milli-watts up to several thousand watts. Therefore they are used for the most

demanding industries such as medical, laboratory, aerospace, semiconductor, telecom, industrial,

and consumer. Uses range from simple food and beverage coolers for an afternoon picnic to

extremely sophisticated temperature control systems in missiles and space vehicles. A

thermoelectric cooler provide a solution that is smaller, weighs less, and is more reliable than a

comparatively small compressor system. It offers a convenient earth friendly alternative.

Researchers are working on improving the efficiency of thermoelectric devices, reducing their

cost and increasing their applications.

1 IntroductionAlthough the principle of thermoelectricity dates back to the discovery of the Peltier effect in

1834[1], there was little practical application of the phenomenon until the middle 1950s. Prior to

then, the poor thermoelectric properties of known materials made them unsuitable for use in a

practical refrigerating device. It is only from the mid-1950s at which the major thermoelectric

material design approach was introduced by A.V.Ioffe, leading to the inventory of

semiconducting compounds such as Bi2Te3, which is currently used in thermoelectric coolers.

These materials made possible the development of practical thermoelectric devices for attaining

temperatures below ambient without the use of vapour compression refrigeration [1].

Thermoelectric coolers offer the potential to enhance the cooling of electronic module packages,

to reduce chip operating temperatures or to allow higher module powers. Thermoelectric coolers

also offer the advantages of being compact, quiet, free of moving parts, and their amount of

cooling can be controlled by the current supplied. Unfortunately, compared to vapor-

compression refrigeration, they are limited in the heat flux that they can be accommodated and

exhibit a lower coefficient of performance (COP). These two limitations have generally limited

thermoelectric cooling to be used in broad expect [1].


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In recent years there has been increased interest in the application of thermoelectric to electronic

cooling, followed by efforts to improve their performance through the development of new bulk

materials and thin film micro coolers.

2 Principle of operationPeltier Effect- when a voltage or DC current is applied to two dissimilar conductors, a circuit

can be created that allows for continuous heat transport between the conductors junctions. The

Seebeck Effect- is the reverse of the Peltier Effect. By applying heat to two different conductors

a current can be generated. The Seebeck Coefficient is given by:

, Where Ix is the electric field

Figure 1 Peltier Effect

The current is transported through the charge carriers (opposite to the electron flow). Heat trans-

fer occurs in the direction of charge carrier movement. A pplying a current (e- carriers)transports heat from the warmer junction to the cooler junction.

3 ConstructionA typical Thermoelectric Module consists of the following components:

3.1 Thermoelectric Material: The materials used for this purpose are generally semiconductors.

They are the optimum choice to sandwich between two metal conductors because of the ability

to control the semiconductors charge carriers as well as increase the heat pumping ability. The

most commonly used semiconductor for electronics cooling applications is Bi2Te3 because of its

relatively high figure of merit. However, the performance of this material is still relatively low

and alternate materials are being studied with possibly better performance.Alternative materials include: Alternating thin film layers of Sb2Te3 and Bi2Te3, Lead telluride

and its alloys, SiGe (Silicon Germanium), Materials based on nanotechnology.

dxdT

x

/

IE !


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Figure 2 A Typical Thermoelectric Module

3.2 Ceramic Plates: An electrical insulator is provided between the heats generating device and

the conductor to prevent an electrical short circuit between the module and the heat source. Theelectrical insulator must have a high thermal conductivity so that the temperature gradient

between the source and the conductor is small. Ceramics like alumina are generally used for this

purpose.

3.3 TIM (Thermal Interface Material): This principle requires the heat to be transferred from

the object being cooled (or heated) to the Peltier module and heat must be transferred from the

Peltier module to the heat sink. Realistically, the interface between the Peltier module surfaces to

the object being cooled and to the heat sink will not be perfect. There will be peaks and valleys

in the surfaces resulting in tiny air pockets which conduct heat poorly. So it is required to place a

"thermal interface material" (TIM) between the Peltier module surfaces. These include mainlysilicone based greases, elastomeric pads, thermally conductive tapes, thermally conductive

adhesives, old fashioned zinc oxide silicone heat sink grease.

3.4 Heat Sink: A heat sink is required for either heating or cooling a thermal load. They are

employed either to collect heat (in heating mode) or dissipate collected heat into another medium

(e.g., air, water, etc.). Without such provisions, the TE device will be vulnerable to overheating.

Once TE device reaches the reflow temperature of the solder employed, the unit will be

destroyed. When the heat sink is exchanging heat with air a fan is usually required to minimize

the size of the sink required.

3.5 Uses of two type of Material (p and n type): A simple thermoelectric device can be made

with a single semiconductor pellet, but it can't pump an appreciable amount of heat through it. In

order to give a TE device greater heat pumping capacity multiple pellets are used together. The

initial attempt would be to simply connect them in parallel both electrically and thermally. While

this is possible, it is not suitable for a practical device. The drawback is that the typical TE

semiconductor pellet is rated for only a very small voltage as small as tens of mill volts so it can

draw a substantial amount of current. For example, a single pellet in an ordinary TE device might


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draw five amps or more with only 60 mV applied, if connected in parallel in a typical 254 pellet

configuration, the device would draw over 1270 amps with the application of that 60 mV

(assuming that the power supply could deliver that much current).

The only realistic solution is to connect the semiconductors in series and doing so in a way that

keeps them thermally in parallel (i.e., pumping together in the same direction). Here we might

think to simply zigzag the electrical connections from pellet to pellet to achieve a series circuit.

This is theoretically workable however; the interconnections between pellets will create thermal

shorting that significantly compromises the performance of the device. Fortunately, there is

another option which gives us the desired electrical and thermal configuration while better

optimizing the thermoelectric effect. By arranging N and P-type pellets in a 'couple' (see Figure

3) and forming a junction between them with a plated copper tab, it is possible to configure a

series circuit which can keep all of the heat moving in the same direction.

Figure 3 N and P type pellet

As shown in the illustration, with the free (bottom) end of the P-type pellet connected to the

positive voltage potential and the free (bottom) end of the N-type pellet connected to thenegative side of the voltage, an interesting phenomenon takes place. The positive charge carriers

(holes) in the P material are repelled by the positive voltage potential and attracted by the

negative pole and the negative charge carriers (electrons) in the N material are similarly repelled

by the negative voltage potential and attracted by the positive pole of the voltage supply. In the

copper tabs and wiring, electrons are the charge carriers. When these electrons reach the P

material, they simply flow through the holes within the crystalline structure of the P-type pellet.

Thus the electrons flow continuously from the negative pole of the voltage supply, through the N

pellet, through the copper tab junction, through the P pellet, and back to the positive pole of the

supply. This happens because two different types of semiconductor material are being used. The

charge carriers and heat are flowing in the same direction through the pellets (bottom to top inthe figure 3). Using these special properties of the TE 'couple', it is possible to combine many

pellets together in rectangular arrays to create practical thermoelectric modules (see Figure 4).

These devices can not only pump appreciable amounts of heat but with their series electrical

connection are suitable for commonly available DC power supplies. Thus the most common TE

devices now in use connecting 254 alternating P and N-type pellets and can run from a 12 to 16

VDC supply and draw only 4 to 5 amps (rather than 1270 amps at 60 mV, for example).


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Figure 4 Several P and N type pellets connected together

In fabricating devices with multi pellet arrays a means to mechanically hold everything togethermust be provided. A solution to this is to mount the copper tabs to thin ceramic substrates; the

outer faces of the ceramics are then used as the thermal interface between the Peltier device andthe outside world.

4 WorkingFigure in the next page explains the working of the thermoelectric device. Electrons can travel

freely in the copper conductors but not so freely in the semiconductor. As the electrons leave the

copper tabs and enter the hot side of the p-type, they must fill a hole in order to move through the

p-type. When the electrons fill a hole, they drop down to a lower energy level and release heat in

the process. Then, as the electrons move from the p-type into the copper conductor on the cold

side, the electrons are again taken back to a higher energy level and in the process they absorbs

heat. Next, the electrons move freely through the copper tabs until they reach the cold side of the

n-type semiconductor. Now when the electrons move into the n-type material they must boost up

Figure 5 Working of thermoelectric device

to a higher energy level in order to move through the n-type semiconductor. Heat is absorbed

when this occurs. Finally, when the electrons leave the hot side of the n-type, they can move


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freely in the copper conductors. They drop down to a lower energy level and release heat in the

process. Thus the charge carriers, negative electrons and positive holes transfer heat.

5 Thermoelectric DesignThere are various factors that have to be considered in designing thermoelectric module. These

are discussed below:

5.1 Figure of merit: The figure of merit represents the quality of performance of thermoelectric

material, sometimes it is multiplied by temperature. It is defined as:

Where is the electrical resistivity, k is the thermal conductivity, and E is the Seebeck

Coefficient. So lower electrical resistivity and thermal conductivity higher the figure of merit.

These values are temperature dependent therefore the figure of merit is also temperature

dependent. P and N type material have different figures of merit and are averaged to determinematerials overall quality.

5.2 Condensation: This is the most serious fact that has to be taken in to account while

designing a TE device. A common problem with TE cooling is that condensation may occur

causing corrosion and eroding the TEs inherent reliability. Condensation occurs when the dew

point is reached. The dew point is the temperature to which air must be cooled at constant

pressure for the water vapor to start to condense. Condensation occurs because the air loses the

ability to carry the water vapor that condenses. As the airs temperature decreases its water vapor

carrying capacity decreases. Since TE coolers can cool to low and even below ambient

temperatures condensation is a problem. The most common sealant employed is silicon rubber.Research has been performed to determine the most effective sealing agent used to protect the

chip from water. Four sealants were used to seal a TE cooling device and the weight gain due to

water entering the device measured. The best sealants should have the lowest weight gain.

5.3 Thermoelectric Performance: TE performance depends on the following factors:

1. The temperature of the cold and hot sides.2. Thermal and electrical conductivities of the devices materials.3. Contact resistance between the TE device and heat source or heat sink.4. Thermal resistance of the heat sink.

The current yielding the maximum COP is given by:

The maximum COP is given by:

kZ

V

E 2!

]1)1[(

))((2/1

!

m

chnp

ZTR

TTI

EEJ


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Where Tm= (TH+TC)/2

The COP corresponding to the maximum heat pumping capacity is:

The current corresponding to the maximum heat pumping capacity is:

A simplified way of determining the voltage and the heat load are given by:

Where V is the voltage and Qc is the heat load, N is the number of couples, L is the element

height. R thermal resistance, K thermal conductivity, seeback coefficient, I current required, A

area of the module.

5.4 Design methodology: There are various design methodology that is being adopted in out of

which Mollar design is mostly used.

Mollar design method: This method involves various steps and can be better explained by the

help of example. In this design a TE performance curve is used. The various steps involved are

discussed below

1. Calculations of total heat load, all the heat loads are to be taken in account. Let the heat loads

be as describe below. The heat loads are expressed in watts.

Type of heat load Example (watts)

Active load 8.0

Radiation load 0.2

Convective load 0.8

Conductive load 0.0

]1)1)[((

]/)1[(2/1

12

12

2/1

1

max

!!

m

m

in

c

ZTTT

TTZTT

W

QJ

cH

cHc

qTZT

TTZT )(2/12

!J

R

TI

cnp

q

)( EE !

RITTKITQc cHcnp22/1)()( ! EE

!

A

I

L

V ch )(2 E


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Total heat load 9.0

2. Defining the temperature of hot and cold side.

Temperature Example (

0

C)Hot side (Th) 27

Cold side (Tc) -8

DT (Th-Tc) 35

Figure 6 Performance curve

3. Determining the number of stages required. The minimum number of stages required is

selected from the table below. This should meet the required DT.

Stage DTmax (Dry

N2 @ 1 atm)

DT max

(Vaccum)

1 64 67

2 84 91

3 95 109

4 - 115

5 - 121

4. After doing this an appropriate TEC is selected or designed.

4(a). Determining the ratio of DT/DTmax

DT (from step 2) 350

C

DTmax (from step 3) 640 C

DT/DTmax 350/64

0

4(b). A horizontal line is drawn on the performance graph corresponding to DT/DTmax .

4(c). The optimum value of Q/Qmax is obtained at the intersection of the horizontal line just

drawn and the diagonal optimum Q/Qmax line. Interpolation between curves may be necessary.

Optimum value of Q/Qmax is obtained to be 0.25 in this case.

4(d). The maximum value of Q/Qmax is obtained at the intersection of the horizontal line (drawn

in step 4b) and the right vertical axis.

Maximum value of Q/Qmax is obtained to be 0.45 in this case.

Note: maximum Q refers to the maximum heat the thermoelectric device can pump which will

occur when DT = 0.


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4(e). Calculation of the optimum and maximum Qmax. The total heat load obtained in step 1 is

divided by optimum and maximum value of Q/Qmax.

Optimum value of Q/Qmax = 9/0.25 = 36 watts

Maximum value of Q/Qmax

= 9/0.45 = 20 watts

For this example, let us assume maximum efficiency is desired. Thus, the 5.6 amp, 8.2 volt

cooler is selected, because between these two potential TECs, its Qmax (30 watts) is closest to the

optimum Qmax (36 watts).

4(f). Select or design a TEC from standard product list with Qmax greater than the maximum Qmax

(20 watts in this example), but less than the optimum Qmax (36 watts in this example). Keep in

mind that within this range a TEC with a Qmax close to the optimum Qmax will provide maximum

efficiency and will yield smaller and possibly less expensive TECs.

5.5 Improving TE performance: Various methods have been used to improve the performanceof TE coolers which are its major drawback. Examples: thin film coolers or multistage (bulk)

coolers.

5.5.1 Thin film coolers: Thin films are material layers of about one micrometer thickness.

Alternating layers of Sb2Te3 (antimony telluride) and Bi2Te3 (bismuth telluride) are used to

produce thin film TE coolers. An example is shown below where the highest power components

are mounted on a diamond substrate which would be the top or cold side substrate of a thin film

TE cooler. Power densities were found to be above 100W/cm2. Thin film coolers considerably

Figure 7 Thin film thermoelectric cooler

reduce the size of TE devices because the cooling density of a Peltier cooler is inversely

proportional to its length. Smaller size is desirable

5.5.2 Multistage Modules: When the desired temperature difference between the cold and hot

side cannot be obtained with a single stage module, or when the cold side temperature must be

lower than a one stage cooler will provide a multistage module may need to be applied.

Multistage modules are essentially single stage modules stacked up in a vertical pyramid shaped

array. As the number of stages increases, the minimum cold side temperature will decrease. Also,


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increasing the number of stages increases the coefficient of performance for a given cold side

temperature.

Figure 8 Multistage Modules

The Figure below compares the three types of coolers bulk (multistage), thin film, and current.

Figure 9 cooling power densities for different power design

6 Advantages and disadvantagesAdvantages

1. No moving parts make them very reliable; approximately 105 hrs of operation at 100degrees Celsius, longer for lower temperature.

2. Ideal when precise temperature control is required.3. Ability to lower temperature below ambient.4. Heat transport controlled by current input.5. Able to operate in any orientation.6. Compact size makes them useful for applications where size or weight is a constraint.7.

Ability to alternate between heating and cooling.

8. Excellent cooling alternative to vapor compression coolers for systems that are sensitiveto mechanical vibration.

Disadvantages

1. Able to dissipate limited amount of heat flux.2. Lower coefficient of performance than vapor-compression systems.


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3. Relegated to low heat flux applications.7 Application

The industries which use TE cooling are Electronics, Medical, Aerospace and

telecommunication and their applications are1. Electronic enclosures2. Laser diodes3. Laboratory instruments, DNA synthesis, Blood analyzers, Tissue preparation4. Temperature baths5. Refrigerators6. Telecommunications equipment7. Temperature control in missiles and space systems, CCD/LED/Infrared detector cooling,

Night vision equipment

8. Heat transport ranges vary from a few mill watts to several thousand watts, however,since the efficiency of TE devices are low, smaller heat transfer applications are morepractical.

8 ConclusionA new dimension has been added to the cooling challenge by the requirement to reduce

operating temperatures to achieve enhanced speed. With the continued demand for improved

cooling technology to enhance the performance and reliability of CMOS applications,

thermoelectric cooling may be considered a potential candidate for cooling enhancement.

To use the equations, detailed information in terms of the parameters , , K, and R pertaining tothe thermoelectric module under consideration is required. Unfortunately, today only onethermoelectric vendor provides such information in their data sheets. It is to be hoped that in the

future other thermoelectric vendors will also make such information available for users.

As shown in the example, the application of thermoelectric coolers could provide cooling

enhancement for a limited range of powers. Unfortunately, in many cases MCM powers may besimply too high for current thermoelectric modules to handle effectively. The current figure of

merit, Z, of the available candidate materials, and the coefficient of performance (COP)attainable with existing thermoelectric coolers, need to be increased.

Until and unless improvements can be made to enhance heat pumping capability and COP,

thermoelectric will not be a serious candidate for higher power electronic cooling applications. Itis hoped that the work being done on development of improved thermoelectric materials and thin

film thermoelectric will eventually alter this situation.


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9 References[1]. Nolas, G.S., Slack, G.A., Cohn, J.L., and Schujman, S.B., The Next Generation of

Thermoelectric Materials, Proceedings of the 17th IEEE International Conference on

Thermoelectrics, PP. 294-297, 1998.

[2].Tellurex (2002) The 12 Most Frequently Asked Questions A bout ThermoelectricCooling. Retrieved May 2006, http://www.tellurex.com/12most.html

[3].Wikipedia the Free Encyclopedia (May 2006). Semiconductor Retrieved May 2006,http://en.wikipedia.org/wiki/Semiconductor.

54068063 thermoelectric cooler

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