PROJECT REPORT

ON

THERMOELECTRIC COOLER

A REPORT SUBMITTED IN PARTIAL FULLFILLMENT FOR THE AWARD OF DEGREE

INMECHANICAL ENGINEERING

AWARDED BY

PUNJAB TECHNICAL UNIVERSITY SUBMITTED TO SUBMITTED BY

Er. Rajesh chowdhary pardeep bansal (90501175422)(HOD of mech. Engineering department) Keshav shrma (80301114014) rahul (90501175423) akash kumar (90501175394) lakhwinder singh(90501175414)

DEPARTMENT OF MECHANICAL ENGINEERING

MALOT INSTITUTE OF MAMNAGEMENT AND INFORMATION TECHNOLOGY

MALOUT(2009-2012)

Acknowledgement

This project is made under the guidance of Er. Major singh sidhu. We thanks

them for their kind guidance and unique idea of the project name thermo electric cooler.

This was innovative and easy to make. And we thanks to all the mechanical department

for encouraging us. Last, we would like to extend our thanks to our families and friends

who have been there to help out and then, enjoy themselves along the way.

Contents

Chapter 1:- thermoelectricooling……………………………………………1.1 Historical background……………………………………………………1.2 Introduction………………………………………………………………1.3 Seedbeck effect……………………………………………………………1.4 Peltier effect………………………………………………………………1.5 Thomson effect……………………………………………………………1.6 Thermoelectric principle of operation……………………………………1.7 Uses………………………………………………………………………1.8 Thermoelectrical materials………………………………………………Chapter 2:- thermal analysis and design parameters………………………2.1 introduction……………………………………………………………….2.2 example……………………………………………………………………2.3 powering the thermoelectric………………………………………………2.4 other parameters to consider………………………………………………2.5 advantages of thermoelectric coolers………………………………………2.6 Reliability & Mean Time between Failures (MTBF)………………………2.7 Moisture and Vibration Effect…………………………………………….2.8 Comparison: Conventional Refrigeration…………………………………2.8 Thermoelectric Multistage (Cascaded) Device……………………………2.9 Summary…………………………………………………………………Chapter3:- the project………………………………………………………Chapter 4:- practical analysis of project……………………………………Chapter 4:- future aspects …………………………………………………

Chapter 1:-Thermoelectric Cooling

1.1 Historical Background

Although commercial thermoelectric modules were not available until almost

1960, the physical principles upon which modern thermoelectric coolers are based

actually date back to the early 1800s.The first important discovery relating to

thermoelectricity occurred in 1821 when German

Scientist Thomas Seebeck found that an electric current would flow continuously

in a closed circuit made up of two dissimilar metals, provided that the junctions of the

metals were maintained at two different temperatures. Seebeck did not actually

comprehend the scientific basis for his discovery, however, and falsely assumed that

flowing heat produced the same effect as flowing electric current.

In 1834, a French watchmaker and part-time physicist, Jean Peltier, while

investigating the Seebeck Effect, found that there was an opposite phenomenon where by

thermal energy could be absorbed at one dissimilar metal junction and discharged at the

other junction when an electric current flowed within the closed circuit. Twenty years

later, William Thomson (eventually known as Lord Kelvin) issued a comprehensive

explanation of the Seebeck andPeltier Effects and described their relationship. At the

time, however, these phenomena were still considered to be mere laboratory curiosities

and were without practical application.

In 1834, a French watchmaker and part-time physicist, Jean Peltier, while

investigating the Seebeck Effect, found that there was an opposite phenomenon where by

thermal energy could be absorbed at one dissimilar metal junction and discharged at the

other junction when an electric current flowed within the closed circuit. Twenty years

later, William Thomson (eventually known as Lord Kelvin) issued a comprehensive

explanation of the Seebeck andPeltier Effects and described their relationship. At the

time, however, these phenomena were still considered to be mere laboratory curiosities

and were without practical application.

1.2 Introduction

Thermoelectric are based on the Peltier Effect, The Peltier Effect is one of the

three thermoelectric effects; the other two are known as the Seebeck Effect and Thomson

Effect.Whereas the last two effects act on a single conductor, the Peltier Effect is a

typical junction phenomenon.

Thermoelectric coolers are solid state heat pumps used in applications where

temperature estabilization, temperature cycling, or cooling below ambient are required.

There are many products using thermoelectric coolers, including CCD cameras (charge

coupled device), laserdiodes, microprocessors, blood analyzers and portable picnic

coolers. This article discussesthe theory behind the thermoelectric cooler, along with the

thermal and electrical parameters involved.

1.3 Seebeck Effect

The conductors are two dissimilar metals denoted as material A and material B.

The junction temperature at A is used as a reference and is maintained at a relatively cool

temperature (TC).

The junction temperature at B is used as temperature higher than temperature TC.

With heat applied to junction B, a voltage (Eout) will appear across terminals T1 and T2

and hence an electric current would flow continuously in this closed circuit. This voltage

as shown in Figure, known as the Seebeck EMF, can be expressed as.

Eout = α (TH – TC)

Where:

• α = dE / dT = α A – α B

• α is the differential Seebeck coefficient or (thermo electric power coefficient) between the two

materials, A and B, positive when the direction of electric current is same as the direction of

thermal current, in volts per oK.

• Eout is the output voltage in volts.

• TH and TC are the hot and cold thermocouple temperatures, respectively, in oK.

Image 1.1 Seebeck effect

1.4Peltier Effect

Peltier found there was an opposite phenomenon to the Seebeck Effect, whereby

thermal energy could be absorbed at one dissimilar metal junction and discharged at the

other junction when an electric current flowed within the closed circuit.

the thermocouple circuit is modified to obtain a different configuration that

illustratesthe Peltier Effect, a phenomenon opposite that of the Seebeck Effect. If a

voltage (Ein) is applied to terminals T1 and T2, an electrical current (I) will flow in the

circuit. As a result of the current flow, as light cooling effect (QC) will occur at

thermocouple junction A (where heat is absorbed), and a heating effect (QH) will occur

at junction B (where heat is expelled). Note that this effect may be reversed whereby a

change in the direction of electric current flow will reverse the direction of heat flow.

Joule heating, having a magnitude of I2 x R (where R is the electrical resistance), also

occurs in the conductors as a result of current flow. This Joule heating effect acts in

opposition to the Peltier Effect and causes net reduction of the available cooling. The

Peltier effect can be expressed mathematically as

QC or QH = β x I

= (α T) x I

Where:

• β is the differential Peltier coefficient between the two materials A and B in volts.

• I is the electric current flow in amperes.

• QC and QH are the rates of cooling and heating, respectively, in watts.

1.2 Peltier effectPeltier coefficient β has important effect on Thermoelectric cooling as following:

a) β <0 ; Negative Peltier coefficient

High energy electrons move from right to left.

Thermal current and electric current flow in opposite directions

b) β >0 ; Positive Peltier coefficient

High energy holes move from left to right.

Thermal current and electric current flow in same direction

Image 1.3 a) -ve Peltier coefficient b)+ve Peltier coefficient

Effect of Peltier coefficient on cooling Process

1.5 Thomson Effect

William Thomson, who described the relationship between the two phenomena,

later issued a more comprehensive explanation of the Seebeck and Peltier effects. 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 on the direction of both the electric current and temperature

gradient. This phenomenon is known as the Thomson Effect

1.6 Thermoelectric Principle of Operation

The typical thermoelectric module is manufactured using two thin ceramic wafers

with a series of P and N doped bismuth-telluride semiconductor material sandwiched

between them as shown in Figure. The ceramic material on both sides of the

thermoelectric adds rigidity and the necessary electrical insulation. The N type material

has an excess of electrons, while the P type material has a deficit of electrons. One P and

one N make up a couple, as shown in. The thermoelectric couples are electrically in

series and thermally in parallel. A thermoelectric module can contain one to several

hundred couples.

Image 1.4 TEC Principle of operation

Image 1.5 Cross section of a thermoelectric cooler

As the electrons move from the P type material to the N type material through an

electrical connector, the electrons jump to a higher energy state absorbing thermal energy

(cold side).Continuing through the lattice of material; the electrons flow from the N type

material to the P type material through an electrical connector dropping to a lower energy

state and releasing energy as heat to the heat sink (hot side).

Thermoelectric can be used to heat and to cool, depending on the direction of the

current. In an application requiring both heating and cooling, the design should focus on

the cooling mode. Using a thermoelectric in the heating mode is very efficient because all

the internal heating (Joulian heat) and the load from the cold side is pumped to the hot

side. This reduces the power needed to achieve the desired heating.

1.7 Uses

Peltier devices are commonly used in camping and portable coolers and for

cooling electronic components and small instruments. Some electronic equipment

intended for military use in the field is thermoelectrically cooled. The cooling effect of

Peltier heat pumps can also be used to extract water from the air in dehumidifiers.

Peltier elements are a common component in thermal cyclers, used for the

synthesis of DNA by polymerase chain reaction (PCR), a common molecular biological

technique which requires the rapid heating and cooling of the reaction mixture for

denaturation, primer annealing and enzymatic synthesis cycles.

The effect is used in satellites and spacecraft to counter the effect of direct

sunlight on one side of a craft by dissipating the heat over the cold shaded side,

whereupon the heat is dissipated by thermal radiation into space.

Photon detectors such as CCDs in astronomical telescopes or very high-end

digital cameras are often cooled down with Peltier elements. This reduces dark counts

due to thermal noise. A dark count occurs when a pixel generates an electron because of

a thermal fluctuation rather than because it has received a photon. On digital photos taken

at low light these occur as speckles (or "pixel noise").

Thermoelectric coolers can be used to cool computer components to keep

temperatures within design limits, or to maintain stable functioning when over clocking.

However, due to low efficiency, much more heat is generated than normally,

necessitating a very large and noisy fan or a liquid cooling system. In fiber optic

applications, where the wavelength of a laser or a component is highly dependent on

temperature, Peltier coolers are used along with a thermistor in a feedback loop to

maintain a constant temperature and thereby stabilize the wavelength of the device. A

Peltier cooler with a heat sink or water block can cool a chip to well below ambient

temperature.

Image 1.6 A USB-powered beverage cooler

Peltier devices are used in recent products that chill beverages. Some products can

also reverse the current to heat the beverage. Products such as the one pictured draw

power from the USB port found on computers. However, these products' ability to heat

and cool is limited, as the USB 2.0 standard guarantees only 500 mA of current (900 mA

in the USB 3.0 standard).

1.8 Thermoelecterical materials

Semiconductors are ideal thermoelectric devices because of their band structure

and electronic properties at high temperatures. Device efficiency is proportional to ZT, so

ideal materials have a large Z value at high temperatures. Since temperature is easily

adjustable, electrical conductivity is crucial. Specifically, maximizing electrical

conductivity at high temperatures and minimizing thermal conductivity optimizes ZT.

1.8.1Bismuth chalcogenides

Materials such as Bi2Te3 and Bi2Se3 comprise some of the best performing room

temperature thermoelectrics with a temperature-independent thermoelectric effect, ZT,

between 0.8 and 1.0.[15] Nanostructuring these materials to produce a layered super

lattice structure of alternating Bi2Te3 and Bi2Se3 layers produces a device within which

there is good electrical conductivity but perpendicular to which thermal conductivity is

poor. The result is an enhanced ZT (approximately 2.4 at room temperature for p-type).

Note that this high value has not entirely been independently confirmed.

Bismuth telluride and its solid solutions are good thermoelectric materials at room

temperature and therefore suitable for refrigeration applications around 300 K. The

Czochralski method has been used to grow single crystalline bismuth telluride

compounds. These compounds are usually obtained with directional solidification from

melt or powder metallurgy processes. Materials produced with these methods have lower

efficiency than single crystalline ones due to the random orientation of crystal grains, but

their mechanical properties are superior and the sensitivity to structural defects and

impurities is lower due to high optimal carrier concentration.

The required carrier concentration is obtained by choosing a nonstoichiometric

composition, which is achieved by introducing excess bismuth or tellurium atoms to

primary melt or by dopant impurities. Some possible dopants are halogens and group IV

and V atoms. Due to the small bandgap (0.16 eV) Bi2Te3 is partially degenerate and the

corresponding Fermi-level should be close to the conduction band minimum at room

temperature. The size of the band-gap means that Bi2Te3 has high intrinsic carrier

concentration. Therefore, minority carrier conduction cannot be neglected for small

stoichiometric deviations. Use of telluride com-pounds is limited by the toxicity and

rarity of tellurium.

1.8.2 Lead telluride

Jeffrey Snyder and his colleagues have shown in 2008 that with thallium doped

lead telluride alloy (PbTe) it is possible to achieve zT of 1.5 at 773 K (Heremans et al.,

Science, 321(5888): 554-557). In an article published in January 2011, they showed that

replacing thal-lium with Sodium zT~1.4 at 750 K is possible (Y. Pei et al., Energy

Environ. Sci., 2011). In May 2011 they reported in Nature in collaboration with Chinese

research group that PbTe1-xSex alloy doped with sodium gives zT~1.8±0.1 at 850 K (Y.

Pei et al., Nature, 473 (5 May, 2011)). Snyder’s group has determined that both thallium

and sodium alter the electronic structure of the crystal increasing electric conductivity.

The Snyder group also claims that selenium increases further electric conductivity and

also reduces thermal conductivity. These works show that other bulk alloys have also

potential for improvement, which could open many new applications for thermoelectrics.

1.8.3 Inorganic clathrates

Inorganic clathrates have a general formula AxByC46-y (type I) and AxByC136-

y (type II), in these formulas B and C are group III and IV atoms, respectively, which

form the framework where “guest” atoms A (alkali or alkaline earth metal) are

encapsulated in two different polyhedra facing each other. The differences between types

I and II comes from number and size of voids present in their unit cells. Transport

properties depend lot on the properties of the framework, but tuning is possible through

the “guest” atoms.

The most direct approach to the synthesis and optimization of thermoelectric

properties of semiconducting type I clathrates is substitutional doping, where some

framework atoms are replaced with dopant atoms. In addition, powder metallurgical and

crystal growth techniques have been used in the synthesis of clathrates. The structural and

chemical properties of clathrates enable the optimization of their transport properties with

stoichiometry. Type II materials should be investigated in future because their structure

allows a partial filling of the polyhedron enabling a better tuning of the electrical

properties and therefore a better control of the doping level. Partially filled variant can

also be synthesized as semiconducting or even insulating.

Blake et al have predicted zT~0.5 at room temperature and zT~1.7 at 800 K for

optimized compositions. Kuznetsov et al measured electrical resistance and Seebeck

coefficient for three different type I clathrates above room temperature and by estimating

high temperature thermal conductivity from the published low temperature data they

obtained zT~0.7 at 700 K for Ba8Ga16Ge30 and zT~0.87 at 870 K for Ba8Ga16Si30.

1.8.4 Magnesium group IV compounds

Mg2BIV (BIV=Si, Ge, Sn) compounds and their solid solutions are good

thermoelectric mate-rials and their figure of merit values are at the level with established

materials. Due to the lack of the systematic studies about their thermoelectric properties

suitability of these materials, and in particular their quasi-ternary solutions, for

thermoelectric energy conversion remains in question. The appropriated production

methods are based on direct comelting but mechanical alloying has also been used.

During synthesis, magnesium losses due to evaporation and segregation of components

(especially for Mg2Sn) need special attention. Directed crystallization met-hods can

produce single crystalline material. Solid solutions and doped compounds have to be

annealed in order to get homogeneous samples. At 800 K Mg2Si1-xSnx may have a

figure of merit about 0.9)

1.8.5 Silicides

Higher silicides seem promising materials for thermoelectric energy conversion,

because their figure of merit is at the level with materials currently in use and they are

mechanically and chemically strong and therefore can often be used in harsh

environments without any protection. More detailed studies are needed to assess their

potential in thermoelectrics and possibly to find a way to increase their figure of merit.

Some of possible fabrication methods are Czochralski and floating zone for single

crystals and hot pressing and sintering for polycrystalline.

1.8.6 Skutterudite thermoelectrics

Recently, skutterudite materials have sparked the interest of researchers in search

of new thermoelectric These structures are of the form (Co,Ni,Fe)(P,Sb,As)3 and are

cubic with space group Im3. Unfilled, these materials contain voids into which low-

coordination ions (usually rare earth elements) can be inserted in order to alter thermal

conductivity by producing sources for lattice phonon scattering and decrease thermal

conductivity due to the lattice without reducing electrical conductivity. Such qualities

make these materials exhibit PGEC behavior.

The composition of skutterudites corresponds to the chemical formula

ReTm4M12, where Re is a rare-earth metal, Tm a transition metal and M a metalloid,

which are chemical elements whose properties are between metals and nonmetals such as

phosphor, antimony, or arsenic. These materials could be potential in multistage

thermoelectric devices as it has been shown that they have zT>1.0, but their properties

are not well known and optimization of their structures is under way.

1.8.7 Half Heusler alloys

Half Heusler alloys have potential for high temperature power generation

applications especially as n-type material. These alloys have three components that

originate from different element groups or might even be a combination of elements in

the group. Two of the groups are composed of transition metals and the third group

consists of metals and metalloids. Currently only n-type material is usable in

thermoelectrics but some sources claim that they have achieved zT~1.5 at 700 K, but

according to other source only zT~0.5 at 700 Khas been achieved. They state that

primary reason for this difference is the disagreement between thermal conductivities

measured by different groups. These alloys are relatively cheap and also have a high

power factor.

1.8.8 Electrically conducting organic materials

Some electrically conducting organic materials may have a higher figure of merit

than existing inorganic materials. Seebeck coefficient can be even mill volts per Kelvin

but electrical conductivity is usually very low resulting small figure of merit. Quasi one-

dimensional organic crystals are formed from linear chains or stacks of molecules that are

packed into a 3D crystal. It has theoretically been shown that under certain conditions

some Q1D organic crystals may have zT~20 (Figure 13) at room temperature for both p-

and n-type materials. In the Thermoelectrics Handbook chapter 36.4 this has been

accredited to an unspecified interference between two main electron-phonon interactions

leading to the formation of narrow strip of states in the conduction band with a

significantly reduced scattering rate as the mechanism compensate each other causing

high zT.

1.8.9 Others

Silicon-germanium alloys are currently the best thermoelectric materials around

1000 ℃ and are therefore used in radioisotope thermoelectric generators (RTG) and

some other high temperature applications, such as waste heat recovery. Usability of

silicon-germanium alloys is limited by their high price and in addition, zT is also only in

the mid-range (~0.7).

With functionally graded materials, it is possible to improve the conversion

efficiency of existing thermoelectric materials. These materials have a non-uniform

carrier concentration distribution and in some cases also solid solution composition. In

power generation applications the temperature difference can be several hundred degrees

and therefore devices made from homogeneous materials have some part that operates at

the temperature where zT is substantially lower than its maximum value. This problem

can be solved by using materials whose transport properties vary along their length thus

enabling substantial improvements to the operating efficiency over large temperature

differences. This is possible with functionally graded materials as they have a variable

carrier concentration along the length of the material, which is optimized for operations

over specific temperature range.

Chapter 2:- Thermal Analysis and design Parameters

2.1 introductions

The appropriate thermoelectric for an application, depends on at least three

parameters. These parameters are the hot surface temperature (Th), the cold surface

temperature (Tc), and the heat load to be absorbed at the cold surface (QC).

load to be absorbed at the cold surface (QC).

The hot side of the thermoelectric is the side where heat is released when DC

power is applied. This side is attached to the heat sink. When using an air cooled heat

sink (natural or forced convection) the hot side temperature and its heat transferred can be

found by using Equations.

Th = Tamb + θ Qh

Where:

• Th = the hot side temperature (°C).

• Tamb = the ambient temperature (°C).

• θ = Thermal resistance of heat exchanger (°C/watt).

And

Qh = QC + Pin

COP = QC / Pin

Where:

• Qh = the heat released to the hot side of the thermoelectric (watts).

• QC = the heat absorbed from the cold side (watts).

• Pin = the electrical input power to the thermoelectric (watts).

• COP = coefficient of performance of the thermoelectric device, typically is between 0.4

and 0.7 for single stage applications.

Estimating QC, the heat load in watts absorbed from the cold side is difficult,

because all thermal loads in the design must be considered. Among these thermal loads

are:

1. Active:

i. I2R heat load from the electronic devices

ii. Any load generated by a chemical reaction

2. Passive:

i. Radiation (heat loss between two close objects with different temperatures)

ii. Convection (heat loss through the air, where the air has a different temperature than

the object)

iii. Insulation losses

iv. Conduction losses (heat loss through leads, screws, etc.)

v. Transient load (time required to change the temperature of an object)

By energy balance across the hot and cold junction it produces

Qh = (α Th) x I – C (Th – Tc) + I2 R/2

QC = (α Tc) x I – C (Th – Tc) - I2 R/2

R = RA + RB

C = (kA+ kB) (A/L)

To get the max the heat absorbed from the cold side (QC); by differentiate the Qc to the

electric current I,

d Qc /d I = 0

Then it produces

Iopt. = α Tc /R

Substitute for Iopt. In Equation 17.7 to get the max the heat absorbed from the cold side

QC (max) = [(Z Tc

2)/2 – (Th – Tc)] C (17.8)

Where:

Z = Figure of merit for the material A and B

= α2 / R C

The cold side of the thermoelectric is the side that gets cold when DC power is

applied. This side may need to be colder than the desired temperature of the cooled

object. This is especially true when the cold side is not in direct contact with the object,

such as when cooling an enclosure.

The temperature difference across the thermoelectric (ΔT) relates to Th and Tc according

to Equation

ΔT = Th – Tc

The thermoelectric performance curves in Figures show the relationship between ΔT and

the other parameters.

ΔT (OC)Image 2.1 Performance curve (ΔT vs. Voltage)

ΔT (OC)Image 2.2 Performance curve (ΔT vs. QC)

2.2 Example:

A thermoelectric cooling system is to be designed to cool a PCB through cooling

a conductive plate mounted on the back surface of the PCB. The thermoelectric cooler is

aimed to maintain the external surface of the plate at 40 oC, when the environment is 48

oC. Each thermoelectric element will be cylindrical with a length of 0.125 cm and a

diameter of 0.1 cm. The thermoelectric properties are.

p nα (V/K) 170 x 10-6 -190 x 10-6ρ (Ω.cm) 0.001 0.0008k (W/cm K) 0.02

Assume the cold junction at 38 oC and the warm junction at 52 oC, and the

electrical resistance of theleads and junctions = 10 % of the element resistance and design

for maximum refrigeration capacity. If10 W are being dissipated through the plate and

steady-state conditions then

Determine:

1- Number of couples required.

2- Rate of heat rejection to the ambient.

3- The COP.

4- The voltage drop across the d.c. power source

Solution:

Th = 52 oC = 325 K

Tc = 38 oC = 311 K

d = 0.1 cm

L = 0.125 cm

A = (π/4) (0.1)2 =7.854 x 10-3 cm2

Overall electric resistance (R) = Relement + Rjunction

= 1.1 Relement

= 1.1(ρp + ρn) (L/A)

= 1.1 (0.001 + 0.0008) (0. 125 / 7.854 x 10-3)

= 0.0315 Ω

Conduction coefficient (C) = (kp + kn) (A/L)

= (0.02 + 0.02) (7.854 x 10-3 /0.125)

= 2.513 x 10-3 W/K

Figure of merit (Z) = (αp - αn) 2/ RC

= (360 x 10-6)2/ (0.0315 x 2.513 x 10-3)

= 1.636 x 10-3 K-

1- Number of couples required.

QC = QC (max) = N C [(Z Tc2)/2 – (Th – Tc)]

10 = N (2.513 x 10-3) [0.5 (1.636 x 10-3 x (311)2) – (14)]

N ≈ 62 couples

2- Rate of heat rejection to the ambient (Qh).

Iopt. = (αp - αn) Tc /R

= (360 x 10-6) x 311/ 0.0315

= 3.55 A

Then

Qh = N [(αp - αn) Th x Iopt – C (Th – Tc) + I2opt R/2]

= 62 [(360 x 10-6) 325 x 3.55 - 2.513 x 10-3 (14) + (3.55)2 0.0315/2]

= 35.8 W

3- The COP.

COP = QC / Pin

Pin (Power input by power source to the thermoelectric) = Qh – QC

= 35.8 – 10 = 25.8 W

COP = 10 / 25.8

= 0.386

4- The voltage drop across the d.c. power source.

The voltage drop (ΔV) = Pin / I

= 25.8 / 3.55

= 7.27 volt

2.3 Powering the Thermoelectric

All thermoelectric are rated for Imax, Vmax, Qmax, and ΔTmax, at a specific

value of Th. Operating at or near the maximum power is relatively inefficient due to

internal heating (Joulian heat) at high power. Therefore, thermoelectric generally operate

within 25% to 80% of the maximum current. The input power to the thermoelectric

determines the hot side temperature and cooling capability at a given load

As the thermoelectric operates, the current flowing through it has two effects:

i. the Peltier Effect (cooling)

ii. The Joulian Effect (heating). The Joulian Effect is proportional to the square

of the current. Therefore, as the current increases, the Joule heating dominates

the Peltier cooling and causes a loss in net cooling. This cut-off defines Imax

for the thermoelectric.

For each device, Qmax is the maximum heat load that can be absorbed by the cold

side of the thermoelectric. This maximum occurs at Imax, Vmax, and with ΔT = 0°C.

The ΔTmax value is the maximum temperature difference across the thermoelectric. This

maximum occurs at Imax, Vmaxand with no load (Qc = 0 watts). These values of Qmax

and ΔTmax are shown on the performance curve (Figures 17.7) as the end points of the

Imax line.

2.4 Other Parameters to Consider

The material used for the assembly components deserves careful thought. The

heat sink and cold side mounting surface should be made out of materials that have a high

thermal conductivity (i.e., copper or aluminum) to promote heat transfer. However,

insulation and assembly hardware should be made of materials that have low thermal

conductivity (i.e., polyurethane foam and stainless steel) to reduce heat loss.

Environmental concerns such as humidity and condensation on the cold side can

be alleviated by using proper sealing methods. A perimeter seal protects the couples

from contact with water or gases, eliminating corrosion and thermal and electrical shorts

that can damage the thermoelectric module.

Figure 2.3 Typical thermoelectric with a perimeter seal

The importance of other factors, such as the Thermoelectric's footprint, its height,

its cost, the available power supply and type of heat sink, vary according to the

application.

2.5 Advantages of Thermoelectric Coolers

Thermoelectric modules offer many advantages including:

• No moving parts

• Small and lightweight

• Maintenance-free

• Acoustically silent and electrically “quiet”

• Heat or cool by changing direction of current flow

• Wide operating temperature range

• Highly precise temperature control (to within 0.1°C)

• Operation in any orientation, zero gravity and high G- levels

• Environmentally friendly

• Sub-ambient cooling

• Cooling to very low temperatures (-80 °C)

2.6 Reliability & Mean Time between Failures (MTBF)

Thermoelectric devices are highly reliable due to their solid state construction.

MTBFcalculated as a result of tests performed by various customers are on the order of

200,000 to 300,000 hours at room temperature. Elevated temperature (80 °C) MTBF is

conservatively reported to be on the order of 100,000 hours.

2.7 Moisture and Vibration Effect

Moisture:

Moisture must not penetrate into the thermoelectric module area. The presence of

moisture will cause an electro-corrosion that will degrade the thermoelectric material,

conductors and solders. Moisture canals provide an electrical path to ground causing an

electrical short or hot side to cold side thermal short. A proper sealing method or dry

atmosphere can eliminate these problems.

Shock and Vibration:

Thermoelectric modules in various types of assemblies have for years been used

in different Military/Aerospace applications. Thermoelectric devices have been

successfully subjected to shock and vibration requirements for aircraft, ordinance, space

vehicles, shipboard use and most other such systems. While a thermoelectric device is

quite strong in both tension and compression, it tends to be relatively weak in shear.

When in a sever shock or vibration environment, care should be taken in the design of the

assembly to insure "compressive loading" of thermoelectric devices.

2.8 Comparison: Conventional Refrigeration

Because thermoelectric cooling is a form of solid-state refrigeration, it has the

advantage of being compact and durable. A thermoelectric cooler uses no moving parts

(except for some fans), and employs no fluids, eliminating the need for bulky piping and

mechanical compressors used in vapor-cycle cooling systems.

Compressors used in vapor-cycle cooling systems.

Such sturdiness allows thermoelectric cooling to be used where conventional

refrigeration would fail. In a current application, a thermoelectric cold plate cools radio

equipment mounted in a fighter jet wingtip. The exacting size and weight requirements,

as well as the extreme g forces in this unusual environment, rule out the use of

conventional refrigeration.

Thermoelectric devices also have the advantage of being able to maintain a much

narrower temperature range than conventional refrigeration. They can maintain a target

temperature to within ±1° or better, while conventional refrigeration varies over several

degrees. Unfortunately, modules tend to be expensive, limiting their use in applications

that call for more than 1 kW/h of cooling power. Owing to their small size, if nothing

else, there are also limits to the maximum temperature differential that can be achieved

between one side of thermoelectric module and the other.

However, in applications requiring a higher ΔT, modules can be cascaded by

stacking one module on top of another. When one module's cold side is another's hot side,

some unusually cold temperatures can be achieved.

2.8 Thermoelectric Multistage (Cascaded) Device

A multistage thermoelectric device should be used only where a single stage

device does not fill the need

Given the hot side temperature, the cold side temperature and the heat load, a

suitable thermoelectric can be chosen. If ΔT across the thermoelectric is less than 55 °C,

then a single stage thermoelectric is sufficient. The theoretical maximum temperature

difference for a single stage thermoelectric is between 65 °C and 70 °C.

If ΔT is greater than 55 °C, then a multistage thermoelectric should be considered.

A multistage thermoelectric achieves a high ΔT by stacking as many as six or seven

single stage thermoelectric on top of each other.

The two important factors are ΔT and C.O.P. should affect on selection of the

number of stages. The following Figure 17.9 depicts ΔT, vs. C.O.P.max, vs. Number of

stages at Th = 35 oC.

Figure 17.4 ΔT vs. C.O.P. Max as a function of stages

There is another very significant factor that must always be considered and that is

the cost. Usually, as the number of stages increase, so does the cost. Certain applications

require a trade-off between C.O.P. and cost.

2.9 Summary

Although there are a variety of applications that use thermoelectric devices, all of

them are based on the same principle. When designing a thermoelectric application, it is

important that all of the relevant electrical and thermal parameters be incorporated into

the design process. Once these factors are considered, a suitable thermoelectric device

can be selected based on the guidelines presented in this article.

Chapter 3:- the Project

Our project is based on peltier effect and goal of this project is demonstration of

thermoelectric cooling.

3.1Parts used

3.1.1 Insulated container box

3.1.2 Hest sink and fan assembly

3.1.3 Peltier plate

3.1.4 Power supply

3.1.5 Switch and plug assembly

Image 3.1 Image of project closed

3.1.1 Insulated container box

This box is made for storage purpose . this is made from iron sheet metal.

Dimensions of this box is 28cm*20cm*15cm.

3.1.2 Heatsink and fan assembly

Heatsink and fan assembly is used for removing heat from the hot side of peltier

plate. Without it peltier plate will burn.in this a standard heat sink is used which is used

in computer for cooling the microprocessor.

3.1.3 Peltier plate

This plate is made from bismuth tulleride material. Model no.

Tec112709.dimesions 40*40mm

property specification

Model no. Tec112709

Dimensions 40*40mm

Imax(ampere) 9 amps

resistance 1.5 ohms

Maximum temp

difference

67 degrees

Q max 108 watt

Image 3.2Conceptual Drawing of Air-to-Air cooling system

3.1.4 Power supply

In power supply a 12 volt 3 amp centre tap transformer and two diodes are used

for converting 220 volt supply into 12 volt.

3.1.5 Switch and plug assembly

Switch and plug assembly is used for connecting it to power source.

Chapter 4:- Practical analysis of project

This practical experiment is done in rac lab of college.

Available data

Maximum power consumption of fan = 13 watt/hour

Maximum power consumption of peltier plate = 108 watt/hour

Total power consumption =121 watt

Specific heat of water =4.187 kj/kg/Kelvin

Data obtained from experiment:-

Weight of water = .2 kg

Temperature reading of water before experiment =29.9 degree Celsius

Temperature reading after experiment = 20.4 degree Celsius

Temperature difference:-9.5 degree

Time of experiment = 30 minutes

Power consumption =60.5 watt

Calculations:-

Cop of system =refrigeration effect/ work done

Refrigeration effect = mc∆ t = 4.187*.2*9.5 = 7.95

Cop =7.95/60.5 =.1314

Cop of system = .1314

Chapter 4:- Future aspects

Today’s thermoelectric modules have a cop of .25 maximum and 4 to 10%

efficiency and are widely used for wine coolers, picnic coolers and small beverage

coolers etc

With the arrival of new technology and better materials efficiency of these

devices are increasing day by day. in future it is possible that it can replace the

conventional vapour compression systems which is both good for us and for environment

New types of devices known as thin film thermoelectric modules are in under

research. They can be used to make temperature control wearable’s.

Figure 4.1: A thin film thermoelectric module available from Nextreme. This device is 3.5 mm x 3.0 mm

x 0.1 mm in size. Devices as small as 0.3 mm x 0.3 mm x 0.1 mm are feasible.