CHAPTER 1:

INTRODUCTION

1.1Basic configuration

Heat sinks or liquid cooling plates are used as heat exchangers.

Figure -1.1

heat exchanger1.1.1 Types

Depending on the used heat exchangers the cooling units are

distinguished:

• air to air

• air to fluid

• fluid to fluid

• direct to air (cooling plate to air)

1

Figure -1.2 air to air cooler

1.2 Thermoelectric Cooling?

French watchmaker, Jean Charles Athanase Peltier,

discovered thermoelectric cooling effect, also known as Peltier cooling

effect, in 1834. Peltier discovered that the passage of a current through a

junction formed by two dissimilar conductors caused a temperature change.

However, Peltier failed to understand this physics phenomenon, and his

explanation was that the weak current doesn’t obey Ohm’s law. Peltier

effect was made clear in 1838 by Emil Lenz, a member of the St.

Petersburg Academy. Lenz demonstrated that water could be frozen when

placed on a bismuth-antimony junction by passage of an electric current

through the junction. He also observed that if the current was reversed the

ice could be melted. In 1909 and 1911 another scientist Altenkirch derived

2

the basic theory of thermoelectrics. His work pointed out that thermoelectric

cooling materials needed to have high Seebeck coefficients, good electrical

conductivity to minimize Joule heating, and low thermal conductivity to

reduce heat transfer from junctions to junctions. Shortly after the

development of practical semiconductors in 1950’s, bismuth telluride began

to be the primary material used in the thermoelectric cooling.The passage

of an electric current through junctions of dissimilar metals causes a fall in

temperature at one junction and a rise at the other, the Peltier effect.

Improvements in this method of cooling have been made possible in recent

years by the production of suitable semiconductors. Applications are limited

in size, owing to the high electric currents required, and practical uses are

small cooling systems for military, aerospace and laboratory use.

The Peltier effect to create a heat flux between the

junctions of two different types of materials. A Peltier cooler, heater, or

thermoelectric heat pump is a solid-state active heat pump which transfers

heat from one side of the device to the other side against the temperature

gradient (from cold to hot), with consumption of electrical energy. Such an

instrument is also called a Peltier device, Peltier heat pump, solid state

refrigerator, or thermoelectric cooler (TEC). The Peltier device is a heat

pump: when direct current runs through it, heat is moved from one side to

the other. Therefore it can be used either for heating or for cooling

(refrigeration), although in practice the main application is cooling. It can

also be used as a temperature controller that either heats or cools.

1.3 PELTIER HISTORY

Early 19th century scientists, Thomas Seebeck and Jean Peltier,

first discovered the phenomena that are the basis for today’s

thermoelectric industry. Seebeck found that if you placed a temperature

gradient across the junctions of two dissimilar conductors, electrical current

would flow. Peltier, on the other hand, learned that passing current

through two dissimilar electrical conductors ,caused heat to be either

emitted or absorbed at the junction of the materials. It was only after

mid-20th Century advancements in semiconductor technology,

3

however, that practical applications for thermoelectric devices became

feasible. With modern techniques, we can now produce

thermoelectric “modules” that deliver efficient solid state heat-pumping for

both cooling and heating; many of these units can also be

used to Generate DC power at reduced efficiency. New and often

elegant uses for thermoelectric continue tube developed each day.

1.4 PELTIER STRUCTURE

A typical thermoelectric module consists of an array of Bismuth

Telluride semiconductor pellets that have been “doped” so that one type of

charge carrier– either positive or negative– carries the majority of current.

The pairs of P/N pellets are configured so that they are connected

electrically in series, but thermally in parallel. Metallised ceramic

substrates provide the platform for the pellets and the small conductive tabs

that connect them.

4

Figure -1.2 constriction

1.5 PELTIER THEORY

When DC voltage is applied to the module, the positive and negative

charge carriers in the pellet array absorb heat energy from one substrate

surface and release it to the substrate at the opposite side. The surface

where heat energy is absorbed becomes cold; the opposite surface where

heat energy is released, becomes hot. Reversing the polarity will result in

reversed hot and cold sides.

5

Figure -1.4 Theory

Benefits of thermoelectric coolers:-

• Small size

• Light in weight

• No fluid

• Independent from the working position

• High reliability

• Exact temperature control

• Heating by changing the direction of the current

6

1.6 THERMOELECTRIC MODULE

In practical use, couples are combined in a module where they are

connected electrically in series, and thermally in parallel. Normally a module

is the smallest component commercially available. Modules are available in

a great variety of sizes, shapes, operating currents, operating voltages and

ranges of heat pumping capacity. The present trend, however, is toward a

larger number of couples operating at lower currents. The user can select

the quantity, size or capacity of the module to fit the exact requirement

without paying for excess power.

7

Figure -1.5

Figure -1.6

8

Figure -1.7

Figure -1.8

Interior structure of thermoelectric module

9

CHAPTER 2 :

LITERATURE REVIEW

Thermoelectric phenomenon was discovered nearly two hundred

years ago. The first breakthrough that would eventually be used to form the

thermoelectric effect was discovered in 1820 By William Thomson .

Thermoelectric property was also implemented in pick up refrigerated

trucks were conducted by Bulat and Nekhoroshev (2003).

Hyeung, (2007) have done a research on thermoelectric device to control

the temperature of car- seat surface.

Koetzsch and Madden (2009) examined on thermoelectric cooling

versus conventional cooling in industrial enclosures. It only requires TE

module, a fan and a power supply.

McStravick,et.al (2009) had invented a medical travel pack with

cooling system.

Bartlett and Sukuse (2007) have built an air-conditioned cooling helmet &

also discussed by Buist and Streitwieser(1988) with worked well to cool

the head of a race driver. The 225 grams helmet cooling system reduced 5

to 6 degree Celsius form ambient.

Lauwers and Angleo (2009) had conducted a study and

development of personal cooling vest which catered to maintain a core body

temperature even . So this was example, where they made use the

property of TEC to benefit for their country’s armed forces. One such

innovative is a thermoelectric air cooling device which is powered up by a

jack that is to inserted in to a cigarette lighter socket in a vehicle. It was

10

studied by Harrington (2009) and the device provided comfort cooling to a

persons head and face in a vehicle.

CHAPTER 3

WORKING PRINCIPLE

In a thermocouple, when a current is passed through the circuit, five

thermoelectric effects occur. Because of the Peltier Effect, the cold plate

will be cooled and the warm plate will be heated. Heat will flow from the

warm plate to the cold plate by Conduction. Heat will be generated in each

conductor and at each junction because of the Joule an Effect and part of

the Joule an heat will flow to each junction. It is usual to assume that one

half of the Joulean heat is transferred to each junction. Thomson Effect and

Seebeck Effect also occurs. The net Thomson coefficient (τp - τn) becomes

zero if (αp - αn) is considered constant. Therefore we neglect the Thomson

Effect and use mean thermoelectric power which gives results equivalent to

those obtained when the Thomson Effect is included. We also assume that

heat absorption and heat rejection occurs only at the junctions and that all

material property value are constants.

3.1 Peltier 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 conductor’s 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:

α=εx

dT /dX

11

where e is the electric field.The current is transported through charge

carriers (opposite the hole flow or with electron flow). Heat transfer occurs

in the direction of charge carrier movement.

Figure -3.1

Applying a current (e- carriers) transports heat from the warmer

junction to the cooler junction. Bismuth telluride (a semiconductor), is

sandwiched between two conductors, usually copper. A semiconductor

(called a pellet) is used because they can be optimized for pumping heat

and because the type of charge carriers within them can be chosen. The

semiconductor in this examples N type (doped with electrons) therefore, the

electrons move towards the positive end of the battery.

The semiconductor is soldered to two conductive materials, like

copper. When the voltage is applied heat is transported in the direction of

current flow.

12

Figure -3.2

When a p-type semiconductor (doped with holes) is used instead,

the holes move in a direction opposite the current flow. The heat is also

transported in a direction opposite the current flow and in the direction of

the holes. Essentially, the charge carriers dictate the direction of heat flow.

To increase heat transport, several p type or n type thermoelectric(TE)

components can be hooked up in parallel.

However, the device requires low voltage and therefore, a large current

which is too great to be commercially practical.

13

The TE components can be put in series but the heat transport

abilities are diminished because the interconnecting between the

semiconductor creates thermal shorting.

The most efficient configuration is where a P and N TE component is

put electrically in series but thermally in parallel . The device to the right is

called a couple. One side is attached to a heat source and the other a heat

sink that converts the heat away. The side facing the heat source is

considered the cold side and the side facing the heat sink the hot side.

Between the heat generating device and the conductor must be an

electrical insulator to prevent an electrical short circuit between the module

and the heat source. The electrical insulator must also 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.

14

The most common devices use 254 alternating p and n type TE

devices. The devices can operate at 12-16 V at 4-5 amps. These values are

much more practical for real life operations

3.2 SEMICONDUCTORS FOR USE IN TE REFRIGERATORS

The best thermoelectric materials currently available, compounds of

doped Bi2Te3, have ZT¿1 at room temperature and attain maximum

temperature differential of ¿82K. Some of the commonly used conventional

thermoelectric materials are as follows:

Bi2Te3, Bi2Se3 and Sb2Te3 ; ZnSb, PbTe and PbSe

The essence of a good thermoelectric is given by the determination of

the material’s dimensionless figure of merit, ZT = (α2σ/k)T, where α is the

Seebeck coefficient, σ the electrical conductivity, and k the total thermal

conductivity (k = kL + kE ; the lattice and electronic contributions,

respectively). High mobility carriers which have the highest electrical

conductivity for a given carrier concentration are most desirable, and

typically the most promising materials have carrier concentrations of

approximately 1019 carriers/cm3 . The most promising thermoelectric

materials should behave as a phonon-glass/electron crystal (PGEC). The

paradigm of the PGEC material is that it should behave thermally as a glass

(large phonon scattering and thus low lattice thermal conductivity) and as

an electronic crystal (low scattering for the electrons, thus high electrical

conductivity).

3.2.1 Semiconductor Doping: N Type :

N doped semiconductors have an abundant number of extra

electrons to use as charge carriers. Normally, a group IV material (like Si)

with 4 covalent bonds (4 valence electrons) is bonded with 4 other Si. To

produce an N type semiconductor, Si material is doped with a Group V

15

metal (P or As) having 5 valence electrons, so that an additional electron on

the Group V metal is free to move and are the charge carriers

3.2.2 Semiconductor Doping: P Type:

For P type semiconductors, the dopants are Group III (In, B) which have 3

valence electrons, these materials need an extra electron for bonding which

creates “holes”. P doped semiconductors are positive charge carriers.

There’s an appearance that a hole is moving when there is a current

applied because an electron moves to fill a hole, creating a new hole where

the electron was originally. Holes and electrons move in opposite directions.

Thermoelectric Materials:

16

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 investigated with

possibly better performance.

Alternative materials include:

Alternating thin film layers of Sb2Te3 and Bi2Te3.

Lead telluride and its alloys

SiGe

Materials based on nanotechnology

A plot of various p-type semiconductor figures of merit times temperature vs.

temperature are shown. Within the temperature ranges concerned in electronics

cooling (0-200°C) Bi2Te3 performs the bes

Similar results are shown for n-type semiconductors:

Figure of Merit:

The figure of merit represents the quality of performance of a

thermoelectric material, sometimes it is multiplied by temperature. It is

defined as:

Where ρ is the electrical resistivity, k is the thermal conductivity, and

a is the Seebeck Coefficient.

Note: low electrical resistivity and thermal conductivity are required for high

high figure of merit. These values are temperature dependent therefore, the

figure of merit is temperature dependent. P and N type material have

different figures of merit and are averaged to determine a materials overall

quality.

17

Z= αρk

Thermoelectric Performance:

TE performance depends on the following factors:

The temperature of the cold and hot sides.

Thermal and electrical conductivities of the device’s materials.

Contact resistance between the TE device and heat source/heat

sink.

Thermal resistance of the heat sink.

3.3 Coefficient of Performance:

In a thermocouple, when a current is passed through the circuit, five

thermoelectric effects occur. [2] Because of the Peltier Effect, the cold plate

will be cooled and the warm plate will be heated. Heat will flow from the

warm plate to the cold plate by Conduction. Heat will be generated in each

conductor and at each junction because of the Joulean Effect and part of

the Joulean heat will flow to each junction. It is usual to assume that one

half of the Joulean heat is transferred to each junction. Thomson Effect and

See back Effect also occurs. The net Thomson coefficient (τp - τn) becomes

zero if (αp - αn) is considered constant. Therefore we neglect the Thomson

Effect and use mean thermoelectric power which gives results equivalent to

those obtained when the Thomson Effect is included. We also assume that

heat absorption and heat rejection occurs only at the junctions and that all

material property value are constants.

18

The Coefficient of Performance of the system as a refrigerating device is

defined as-

C.O.P. = QO / W

Therefore For a completely reversible thermoelectric system (no Joulean

Effect and no Conduction Effects) above equation becomes,

C .O . P .= T 0

T 1 - T 0 which is the Carnot cycle value.

A typical AC unit has a COP of approximately 3. TE coolers usually

have COP’s below 1; 0.4 to 0.7 is a typical range.

19

CHAPTER-4

METHODLOGY

4.1 EQUIPMENTS REQUIRED:-

Thermoelectric cooler(2 MODULES OF 37watts)

Multi meter

Thermometer

Thermocouple

Water heat exchanger

Pump

Silicon grease

The experiment was started at room temperature (23°C) and zero

current. A direct current was led through the Peltier element and increased

stepwise from 0 to 2.5A in steps of 0.1A. The voltage, current and

temperature in the cavity were registered after each step, giving sufficient

time for stationary temperature to establish. The current was increased until

the temperature reached its minimum value and started increasing.

20

CHAPTER 5

HEAT TRANSFER FORMULAE

1) Heat gained or lost through walls

Q = (A x ΔT x K) / (ΔX)

Where:Q = Heat (Watts)

A = External surface area of container (m2)

ΔT = Temperature difference (inside vs. outside of container)

(Kelvin)

K = Thermal conductivity of insulation (Watt / meter Kelvin)

ΔX = Insulation thickness (m)

2) Time required to change the temperature of an object

t = (m x Cp x ΔT) / Q

Where:

t = Time interval (seconds)

m = Weight of the object (kg)

Cp = Specific heat of material (J / (kg K))

ΔT = Temperature change of object (Kelvin)

Q = Heat added or removed (Watts)

21

Note: It should be remembered that thermoelectric devices do not add or

remove heat

at a constant rate when ΔT is changing.

An approximation for average Q is: Qave = (Q (ΔT max) + Q (ΔT min)) / 2

3) Heat transferred to or from a surface by convection

Q = h x A x ΔT

Where:Q = Heat (Watts)

h = Heat transfer coefficient (W / (m2 K)), 1 ~ 30 = “Free”

convection –

gasses, 10 ~ 100 = “Forced” convection - gasses

A = Exposed surface area (m2)

ΔT = Surface temperature - Ambient (Kelvin)

22

ADVANTAGES:

No moving parts make them very reliable; approximately 105 hrs of

operation at 100 degrees Celsius, longer for lower temps .

Ideal when precise temperature control is required.

Ability to lower temperature below ambient.

Heat transport controlled by current input.

Able to operate in any orientation.

Compact sizes make them useful for applications where size or

weight is a constraint.

Ability to alternate between heating and cooling.

Excellent cooling alternative to vapor compression coolers for

systems that are sensitive to mechanical vibration.

DISADVANTAGES:

Able to dissipate limited amount of heat flux.

Lower coefficient of performance than vapor-compression

systems.

Relegated to low heat flux applications.

More total heat to remove than without a TEC.

23

APPLICATIONS { Cooling purpose}:

Electronic enclosures

Laser diodes

Laboratory instruments

Temperature baths

Refrigerators

Telecommunications equipment

Temperature control in missiles and space systems

Heat transport ranges vary from a few milliwatts to several thousand

watts.

Demand for vaccine storage in developing Demand for vaccine

storage in developing communities communities

Way to store other perishable items in remote Way to store other

perishable items in remote locations locations

24

CHAPTER 6

CONCLUSION

The study shows how the manufacturer’s data for the thermo-electric

cooler can be used to extract the parameters of the pro-posed model. The

model could be helpful for analysing the drive requirements of the TEC.

Another important application of the

proposed model is to analyse the performance of the TEC under specific

conditions such as thermal leakage, non ideal thermal insulation, etc. Using

the model, one can analyse not only existing modules, but also specify an

optimal TEC for a specific problem.

The paper is based on data given by many different manufacturers

that were used to reproduce accurately the performance of commercial

TEMs. An important feature of the model is its ability to generate small-

signal transfer functions that can be used to design a feedback network in

temperature-control application. Obtained changes in resistance and Z

factor lay in the range of random error of measurements. temperature

cycles from 10 to 55°C do not affect TECs efficiency.

25

CHAPTER 7 REFERANCE

Goldsmid H…... Thermoelectric Refrigeration.

Lasance, C.J.M., and Simmons, R.E. Advances In High-

Performance Cooling For Electronics. Electronics Cooling.

Goldsmid H. J Electronic Refrigeration. Englewood Cli€s (NJ):

Prentice-Hall, Inc, 1986. (chapter 8).

Andersen JR. Thermoelectric air conditioner for sub-marines. Adv

Energy Conv 1962;2:241±8.

Gray PE. The dynamic behavior of thermoelectric devices. New

York and London: John Wiley and Sons, Inc, 1960.

Stoecker WF, Chaddock JB. Transient performance of a

thermoelectric refrigerator under step-current control.ASHARE

1963;5:61±7.

Bywaters RP, Blum HA. The transient behavior of cascade

thermoelectric heat pumps. Energy Conversion1970;10:193±200.

Hoyos GE, Rao KR, Jerger D. Numerical analysis of transient

behavior of thermoelectric cooler. Energy Con-version 1977;17:23±9.

Soo SL. Direct energy conversion. Englewood Cli€s (NJ):Prentice-

Hall Inc., 1968 (chapter 5).

Phelan RM. Feedback control system. Cornel University, Itheca

(NY): Sibley School of Mechanical and Aero-nautical Engineering,

(privately published notes),

26

Leu MC. PDF subvariable control and its application to robot motion

control. ASME Journal of Dynamic System,

Measurement, and Control. 1989; 452±61.

4.0 Advantages of Thermoelectric Cooling

4.1

 The use of thermoelectric modules often provides solutions,

and in some cases the ONLY solution, to many difficult thermal

management problems where a low to moderate amount of heat must

be handled. While no one cooling method is ideal in all respects and

the use of thermoelectric modules will not be suitable for every

application, TE coolers will often provide substantial advantages over

alternative technologies. Some of the more significant features of

thermoelectric modules include:

No Moving Parts: A TE module works electrically without any moving

parts so they are virtually maintenance free.

Small Size and Weight: The overall thermoelectric cooling system is

much smaller and lighter than a comparable mechanical system. In

addition, a variety of standard and special sizes and configurations

are available to meet strict application requirements.

Ability to Cool Below Ambient: Unlike a conventional heat sink

whose temperature necessarily must rise above ambient, a TE cooler

attached to that same heat sink has the ability to reduce the

temperature below the ambient value.

Ability to Heat and Cool With the Same module: Thermoelectric

coolers will either heat or cool depending upon the polarity of the

applied DC power. This feature eliminates the necessity of providing

separate heating and cooling functions within a given system.27

Precise Temperature Control: With an appropriate closed-loop

temperature control circuit, TE coolers can control temperatures to better

than +/- 0.1°C.

High Reliability: Thermoelectric modules exhibit very high reliability due to

their solid state construction. Although reliability is somewhat application

dependent, the life of typical TE coolers is greater than 200,000 hours.

Electrically "Quiet" Operation: Unlike a mechanical refrigeration system,

TE modules generate virtually no electrical noise and can be used in

conjunction with sensitive electronic sensors. They are also acoustically

silent.

Operation in any Orientation: TEs can be used in any orientation and in

zero gravity environments. Thus they are popular in many aerospace

applications.

Convenient Power Supply: TE modules operate directly from a DC power

source. Modules having a wide range of input voltages and currents are

available. Pulse Width Modulation (PWM) may be used in many

applications

Spot Cooling: With a TE cooler it is possible to cool one specific

component or area only, thereby often making it unnecessary to cool an

entire package or enclosure.

Ability to Generate Electrical Power: When used "in reverse" by applying

a temperature differential across the faces of a TE cooler, it is possible to

generate a small amount of DC power.

Environmentally Friendly: Conventional refrigeration systems can not be

fabricated without using chlorofluorocarbons or other chemicals that may be

harmful to the environment. Thermoelectric devices do not use or generate

gases of any kind.

28

Application MarketInfrared Detectors Military guidance systems

Environmental analyzers

Night vision systems

Black Body References Military

Air to Air Exchangers Portable cool boxes

Small refrigerators

Electronic cabinets

Charge Coupled Device (CCD) Commercial and military cameras

Space telescopes

Satellites

Liquid Exchangers Semiconductor processing equipment

Constant temperature baths

Blood analyzers

Laser Diodes Telecommunications switching

Medical diagnostics equipment

Commercial electronic scanners

Guidance systems

High Speed Integrated Circuits Commercial, Military

29

5conversion efficiency also increases with increase in the strength of the

applied electric field strength. Hence the research work provides the way to

overcome the energy crisis in future without any special production of heat but

only by the utilization of the available waste heat by the proper selection of

materials and operating parameters; which is free from any type of the pollution

or complexities. The reliable thermoelectric equipments/modules can be

framed of these materials in the selective orientations of electric or magnetic

fields. (b) Precise Measurements by RTD Materials There is a

variety of temperature sensing systems in each of the engineering and

industrial areas which can be categorized according to the requirements and

the range of temperatures to be measured. In high temperature situations

(molten state of materials, separation of ores etc.) the Resistive Temperature

Detective (RTD) materials are generally used which are basically

thermocouples, works on Seebeck Effect. This research work puts a protest for

the alteration of these temperature accuracies due to the effect of electric and

magnetic fields, which are generally present in such operating conditions and

affect the thermoelectric properties. Hence, this research work inspires for the

consideration of effect of such external parameters for the precise

measurements; to seek the reliability of a system. (c) Refrigeration

Thermoelectric devices achieved an importance in recent years as viable

solutions for applications such as spot cooling of electronic components,

remote power generation in space stations and satellites etc. These solid state

thermoelectric devices are free from moving parts, having good reliability

however their efficiency depends on the selection of materials. Such devices

with higher efficiency can be implemented for refrigeration also. Actually the

combination of Seebeck Effect and Peltier Effect is the 122

absolute advent for such refrigeration. If heat from solar energy is provide as

the input to this implementation the cooling will be the output. Surely this

research work will be an idea for better refrigeration and becomes an effort to

overcome the energy crisis by the means of refrigeration from waste heat. In

the instruments like computers, laptops, dynamos and vehicles the low grade

waste heat can be utilized for cooling and can also be recycled to improve their

performances. To reduce the thermal conductivity the heat resistant

membranes can also be used. In a large number of devices metallic blocks are

30

used, which causes eddy currents due to non uniform magnetic fields. If these

magnetic fields are synchronized to the thermo coupled devices then it is

observed that the thermo emf will enhanced. This results to more cooling

without any extra input. Such thermoelectric cooling devices can be applied to

industries, buildings of hot regions and to the houses in summer. However,

they require some modifications related to their size and the selection of

materials but their cheapness, eco friendly nature, no cause to global warming

are enough inputs to motivate the engineers for their implementations in almost

all the suitable applications of daily life in near future.

31