project work on Thermoelectric refrigeraton

7/30/2019 project work on Thermoelectric refrigeraton

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CHAPTER1

INTRODUCTION

1.1 WHAT IS THERMOELECTRIC REFRIGERATION?

Refrigeration is the process of pumping heat energy out of an insulated chamber in order toreduce the temperature of the chamber below that of the surrounding air. Thermoelectric

refrigeration uses a principle called the "PELTIER" effect to pump heat electronically. The

Peltier effect is named after a French scientist who discovered it in 1834.

1.2 CONCEPT OF THERMOELECTRIC

REFRIGERATION :

There used a so called thermoelectric refrigerator based on Peltier effect. The given effect wascalled after a french watchmaker (1785-1845.), who discovered it in 1834.

If you put a drop of water in the hollow on the joint of 2 semiconductors Sb and Bi, and switch

on the current, the drop would freeze (with the reverse direction of the current the drop would

melt ). This is how Peltier effect works.

Jean Peltier noted that when an electrical current is applied across the junction of two dissimilar

metals, heat is removed from one of the metals and transferred to the other. This is the basis of

thermoelectric refrigeration. Thermoelectric modules are constructed from a series of tiny metal

cubes of dissimilar exotic metals which are physically bonded together and connectedelectrically. When electrical current passes through the cube junctions, heat is transferred from

one metal to the other. Solid-state thermoelectric modules are capable of transferring large

quantities of heat when connected to a heat absorbing device on one side and a heat dissipating

device on the other. The internal aluminium cold plate fins absorb heat from the contents, (food

and beverages), and the thermoelectric modules transfer it to heat dissipating fins under the

control panel. Here, a small fan helps to disperse the heat into the air. The system is totally


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environmentally friendly and contains no hazardous gases, nor pipes nor coils and no

compressor. The only moving part is the small 12-volt fan. Thermoelectric modules are too

expensive for normal domestic and commercial applications which run only on regular

household current. They are ideally suited to recreational applications because they are

lightweight, compact, insensitive to motion or tilting, have no moving parts, and can operate

directly from 12-volt batteries.

Fig.1


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WHY IS IT BETTER THAN AN ICE CHEST?Food and beverages are kept cold and dry. No space is wasted for ice (unless of course

you want ice, in which case we can help to preserve it 3 or 4 times longer than a plain

cooler).

1.3 THERMOELECTRIC MODULES:

A Peltier module consists of semiconductors mounted successively, which form p-n- and n-p-

junctions. Each junction has a thermal contact with radiators. When switching on the current of

the definite polarity, there forms a temperature difference between the radiators: one of them

warms up and works as a heatsink, the other works as a refrigerator.

Fig.2

A typical module provides a temperature difference of several tens degrees Celsius. With forcedcooling of the hot radiator, the second one can reach the temperatures below 0 Celsius. For more

temperature difference the cascade connection is used.

Fig.3

The cooling devices based on Peltier modules are often called active Peltier refrigerators or

Peltier coolers.


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Peltier module's power depends on its size. The modules of low power might not be efficient

enough. But the usage of the modules of too high power might cause moisture condensation,

what is dangerous for electronic circuits. The distance between conductors on the modern printed

circuit boards constitutes parts of a millimeter. Nevertheless, they were powerful Peltier modules

and additional cooling systems which helped us to overclock .We should notice here, that the

systems work was stable and reliable enough. Similar experiments were made with Intel Celeron,

Pentium II, Pentium III, which achieved tremendous performance growth.

We should point out that Peltier modules dissipates a lot of heat. That's why it's necessary to use

not only a powerful fan in the cooler, but also other different fans inside the case.

Fig.4

Unlike the Joule heat which is proportional to the current strength squared (Q=RIIt), thePeltier is proportional to the current strength and changes the sign (-/+) if the current changes the

direction. The Peltier heat equals:

Qp = P q

q=It, P is a Peltier factor that depends on contacting materials and temperature.


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Peltier heat is considered positive in case of dissipation, and negative in case of absorption.

In this case the Joule heat in both calorimeters is the same (since R = R(Cu)+R(Bi)). But the

Peltier heat differs in the sign. So, this experiment allows to calculate the Peltier factor.

In the table below you can see some Peltier factors for different pairs of metals.

In theory, the Peltier effect is explained the following way: electrons speed up or slow down

under the influence of contact potential difference. In the first case the kinetic energy of the

electrons increases, and then, turns into heat. In the second case the kinetic energy decreases and

the joint temperature falls down.

In case of usage of semiconductors of p- and n- types the effect becomes more vivid. On the

scheme you can see how it works.

Fig.5


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CHAPTER

2

LITERATURE REVIEW

TYPES OF THERMOELECTRIC EFFECTS:

PELTIER EFFECT SEEBECK EFFECT THOMPSON EFFECT

2.1 WHAT IS PELTIER EFFECT?

In 1834,French scientist Jean Peltier discovered a reversed phenomenon to that of Seebeck

Effect.If two dissimilar metals are joined together so as to form a closed circuit, there will be two

junctions where they meet each other. If an electric current is made to flow across these two

junctions, one of the junctions become hot and the other junction becomes cold. This is actually

the reverse of Seebeck Effect.

The PeltierSeebeck effect, or thermoelectric effect, is the direct conversion of thermal

differentials to electric voltage and vice versa. Related effects are the Thomson effect and Joule

heating. The PeltierSeebeck and Thomson effects are reversible (in fact, the Peltier and Seebeck

effects are reversals of one another); Joule heating cannot be reversible under the laws of

thermodynamics.


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2.2 WHAT IS SEEBECK EFFECT?If two dissimilar metals are joined together so as to form a closed circuit, there will be two

junctions where they meet each other. If one of these junctions is heated then a current flows in

the circuit which can be detected by a galvanometer. The amount of the current produced

depends on the difference in temperature between the two junctions and on the characteristics of

the two metals. The Seebeck effect is the conversion of temperature differences directly into

electricity. This effect was first discovered, accidentally, by the German physicist Thomas

Johann Seebeck in 1821, who found that a voltage existed between two ends of a metal bar when

a temperature difference T existed in the bar.

He also discovered that a compass needle would be deflected when a closed loop was formed of

two metals with a temperature difference between the junctions. This is because the metals

respond differently to the temperature difference, which creates a current loop, which in turnproduces a magnetic field.

The effect is that a voltage, the thermoelectric EMF, is created in the presence of a temperature

difference between two different metals or semiconductors. This causes a continuous current to

flow in the conductors if they form a complete loop. The voltage created is of the order of

several microvolts per degree difference.

In the circuit:

Fig.6


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(which can be in several different configurations and be governed by the same equations), thevoltage developed can be derived from:

SA and SB are the Seebeck coefficients (also called thermoelectric powerorthermopower) of themetals A and B, and T1 and T2 are the temperatures of the two junctions. The Seebeckcoefficients are non-linear, and depend on the conductors' absolute temperature, material, and

molecular structure. If the Seebeck coefficients are effectively constant for the measured

temperature range, the above formula can be approximated as:

Thus, the working principle of thermocouple depends on the thermo electric effect. It can be

used to measure a temperature difference directly, or to measure an absolute temperature, by

setting one end to a known temperature. Several thermocouples in series are called a thermopile.

This is also the principle at work behind thermal diodes and thermoelectric generators (such as

radioisotope thermoelectric generators or RTGs) which are used for creating power from heat

differentials.

The Seebeck effect is due to two effects: charge carrier diffusion and phonon drag. If both

connections are held at the same temperature, but one connection is periodically opened and

closed, an AC voltage is measured, which is also temperature dependent. This application of the

Kelvin probe is sometimes used to argue that the underlying physics does only need one

junction. And this effect is still visible if the wires only come close, but do not touch, thus no

diffusion is needed.

2.3 THOMSON EFFECT ?

In 1851, Thomson pointed out the third effect.He related the heat absorbed or evolved in a

single conductor to the temperature gradient along it and current flowing through it.
http://en.wikipedia.org/wiki/Seebeck_coefficienthttp://en.wikipedia.org/wiki/Thermoelectric_powerhttp://en.wikipedia.org/wiki/Thermopowerhttp://en.wikipedia.org/wiki/Thermopowerhttp://en.wikipedia.org/wiki/Thermoelectric_powerhttp://en.wikipedia.org/wiki/Seebeck_coefficient


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2.4 ADVANTAGES/DISADVANTAGES OF

THERMOELECTRIC REFRIGERATION:

COMPACT SIZE: Since the cooling module is comparable to the size of a matchbox, so the

space required by the cooling system is very less.

LIGHT WEIGHT: A 36 qt. capacity unit weighs only 17 lbs. Portability is one of the most

attractive feature of thermoelectric refrigeration.

LOWER PRICED: It is 20% to 40% less expensive than compressor or absorption units.

LOW BATTERY:Battery consumption is quite low, averages approximately 4.5 amps - less

than your cars headlights.

SOME OTHER ADVANTAGES:

No moving parts ; noiseless. Simple and fewer parts required. Can operate in any position. These units are much more flexible than conventional units. Easy control. No leakage problem. Compact in size. Suitable for low capacity. Very long life. It can take over load simply by increasing power input. An interchange of heating and cooling process can be exercised by reversing the polarity.

DISADVANTAGES:

Low C.O.P. More power is needed to run the system. Advantageous only for units of smaller capacity. Unavailablity of suitable materials of high figure of merit.

2.5 APPLICATIONS:

Peltier refrigerators are widely used in several western countries.


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Peltier cooling is also used in air conditioning in of rooms where large cooling capacitiesare required but the temperature difference need not to be so large.

Such large system has a great advantage that it can be used for heating the room in

winter merely by reversing the direction of current.

The thermoelectric cooling can be effectively used in the following: Constant low temperature bath and chambers. Coooled baffles fopr oil diffusion pump in vacuum system. Dew point hygrometer for determining absolute humidity. Photo multiplier cooler. Serum coolers for perseveration of blood plasma and serums etc.

2.6 COMPARISON OF THERMOELECTRICREFRIGERATION AND OTHER METHODS OF

REFRIGERATION :

THERMOELECTRIC: Cooling is achieved electronically using the concept of "Peltier" effect

- heat is pumped with electrical energy.

COMPRESSOR : Whereas in conventional refrigeratorscooling is achieved by vapourising a

refrigerant (such as freon) inside the refrigerator - heat is absorbed by the refrigerant through the

principle of the "latent heat of vaporisation" and released outside the refrigerator where thevapour is condensed and compressed into a liquid again. Thermoelectric modules have no

moving parts and are free with chlorofluorocarbons . Therefore they are environment friendly,

reliable, and virtually maintenance free. They can be operated in any orientation and are ideal for

cooling devices where mechanical vibrations cannot be tolerated. Their compact size also makes

them ideal for applications that are size or weight limited where even the smallest compressor

would have excess capacity. Their ability to heat and cool by a simple reversal in polarity of

current is useful in applications where both heating and cooling is necessary or where

temperature control is of utmost importance.

ABSORPTION: In absorption units cooling is achieved by vapourising a refrigerant (ammonia

gas) inside the refrigerator by "boiling" it out of a water ammonia solution with a heat source

(electric or propane) using the principle of "latent heat of vapourisation". The vapour is

condensed and re-absorbed by the ammonia solution outside the refrigerator.


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2.6.1 COMPARISON OF THE FEATURES OF ALL THREE

SYSTEMS:

COMPACTNESS: They are the most compact because of the small size of the cooling

components - cooling module / heat sink / cold sink.

WEIGHT: Thermoelectric units weigh 1/3 to 1/2 as much as the other units because of the

lightweight cooling system - no heavy compressor.

PORTABILITY: Thermoelectric units are the most portable because they are light enough to

carry with one hand and are not affected by motion or tilting. Compressor models are quite

heavy and the absorption models must be kept level within 2 - 3 degrees.

PRICE: They cost 20% - 40% less than the equivalent sized compressor or absorption units

available for recreational use.

BATTERY DRAIN: but may average slightly less depending on thermostatic control settings.

Absorption portables draw 6.5 to 7.5 amps when running and may average about 5 amps They

have a maximum current drain on 12 volts of 4.5 amps. Compressor portables draw slightly more

current when running draw.

COOLING PERFORMANCE: Compressor systems are the most efficient in hot weather.

Some models will perform as a portable freezer and will refrigerate in ambient temperatures of

up to 110 degrees F. Such units will refrigerate in sustained ambient temperatures of up to 95degrees F. If they are kept full, they will refrigerate satisfactorily even if peak daytime

temperatures reach 110 degrees F because the contents temperature will lag behind the ambient.

The food will be just starting to warm up when the air cools off in the evening which will bring

the food temperature back down to normal. Absorption type refrigerators provide almost the

same cooling performance as thermoelectric portables but are less efficient at high ambient

temperatures.

FREEZING ICE CUBES: Compressor systems will usually make a quantity of small ice cubes

except in very hot weather. Gas absorption systems can do the same except in hot weather.

Thermoelectric units do not make ice cubes but can preserve them in a plastic container for 2 - 3days which is often adequate for most applications.

SAFETY: Such systems are completely safe because they use no gases or open flames and run

on just 12 volts. Compressor systems can leak freon which can be extremely dangerous

especially if heated. Absorption systems may use propane which can be extremely dangerous in

the event of a leak.


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RELIABILITY: Thermoelectric modules do not wear out or deteriorate with use. They have

been used for military and aerospace applications for years because of their reliability and other

unique features. Compressors and their motors are both subject to wear and freon-filled coils are

subject to leakage and costly repairs. Absorption units are somewhat temperamental and may

require expert servicing from time to time, especially if jarred when travelling.

EASE OF SERVICING AND MAINTENANCE: Thermoelectric units have only one moving

part, a small fan (and 12 volt motor) which can easily be replaced with only a screw driver. Most

parts are easily replaced by the end-user. Compressor and absorption units both require trained

(expensive) mechanics and special service equipment to service them.

2.6.2 COMPARISON BETWEEN THERMOELECTRIC

REFRIGERATION AND VAPOUR COMPRESSION SYSTEM:

In the thermoelectric refrigeration system the electrons are pumped by the wheres in caseof vapour compression system the vapour from the evaporator (to a condenser) is

deliveredby the compressor at a high pressure.

When the electrons reach the junction of the dissimilar reconductors, their energydecreases due to heat transfer to the surroundings.This corresponds to the cooling or

condensation of compressed vapour.

After this the electrons are reduced to lower potential which resembles the process ofthrottling as in vapour compression system.

The heat transfer to the cold junction imparts energy to electrons which again move tobattery similar to transfe of vapour from evaporator to the compressor.

2.7 MATERIALS USED IN THERMOELECTRIC

REFRIGERATION SYSTEM:

There are various combinations of thermoelectric materials which are used in thermoelectric

refrigeration system. In the table below you can see Peltier factors for different pairs of metalsand their calculated efficiency.


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Fe-constantan Cu-Ni Pb-constantan

P,mv C.O.P P,mv C.O.P P,mv C.O.P

13.0 0.17 8.0 - 8.7 -

15.0 0.27 9.0 - 11.8 0.12

19.0 0.48 10.3 - 16.0 0.34

Table-1

In recent times, the most commonly used material for thermoelectric convertors is lead

telluride.The efficiency of such a thermoelectric convertor is,however,only about 5 to 7 percent.

Taking into consideration mechanical characteristics, stability under operating conditions and

ease of fabrication,Bismuth telluride appears to be amply suitable material.It can be alloyed with

such materials as Bismuth selenide, antimony telluride,lead selenide and tin telluride to give

improved properties.

Research is being made to find more efficient thermocouple materials.To achieve higher

efficiency, thermoelectric material should have a high value of Z and be able to operate upto

very high temperature .The following points are worthnoting in this regard:

The component thermal conductivity of semiconductor should be as low as possible. The mobility of current carriers should be as high as is compatible with condition 1. One of the arms should consists of a purely hole type and the other of a purely electronic

type semiconductor.

In the low temperature zone the impurity concentration should be lower than in the highertemperature zone.

It should be resist chemical influences such as oxidation. It should have a good mechanical strength. It should be amply elastic.


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2.7.1 DESIRABLE PROPERTIES OF THERMOELECTRIC

MATERIALS:

A thermoelectric material should possess the following properties :

It must be excellent conductor of electricity so that ohmic losses are minimum. It must be a very poor conductor of heat because the heat must be absorbed at one end,

and rejected at the other.

It must have high thermoelectric power. This means it must have a high rate of change involtage with temperature.

A good thermoelectric material should have a high electrical conductivity ,low thermalthermal conductivity and a high Seebeck coefficient. Semiconductor is the proper

material for thermoelectric refrigeration.

2.8 WORKING OF A COOLING MODULE:

Thermoelectric modules are solid-state heat pumps which operate on the Peltier effect (seedefinitions). A thermoelectric module consists of an array of p- and n-type semiconductor

elements that are heavily doped with electrical carriers. The elements are arranged in a way that

they connected in series electricallybut thermally connected in parallel. This array is then fixedto two ceramic substrates, one on each side of the elements (see figure below). We should

examine how the heat transfers as electrons flow through one pair of p- and n-type elements

within the thermoelectric module:


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Fig.7

The p-type semiconductor is doped with certain atoms that less electrons than necessary to

complete the atomic bonds within the crystal lattice. By applying some voltage, there is a

tendency for conduction electrons to complete the atomic bonds. When this happens, they leaveholes which essentially are atoms within the crystal lattice that now have local positive

charges. Electrons are then continually dropping in and being bumped out of the holes andmoving on to the next available hole. In effect, it is the holes that are acts as an electrical carriers.Now, electrons move easily in the copper conductors than the semiconductors. When electrons

leave the p-type and enter into the copper on the cold-side, holes are created in the p-type as theelectrons jump out to a higher energy level to match the energy level of the electrons already

moving in the copper. The extra energy to create these holes comes due to absorption of heat.

Meanwhile, the newly created holes travel downwards to the copper on the hot side. Electronsfrom the hot-side of the copper move into the p-type and drop into the holes, releasing the excess

energy in the form of heat.The n-type semiconductor is doped with atoms that provide more electrons than necessary tocomplete the atomic bonds within the crystal lattice. On application of voltage, these extra

electrons easily move into the conduction band. However, some additional energy is required to

get the n-type electrons to match the energy level of the incoming electrons from the cold-side ofcopper. The extra energy comes by absorption of heat. Finally, when the electrons leave the hot-

side of the n-type, they once again can move freely inside the copper. They drop down to a lower

energy level, and release heat in the process.


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The above explanation does not cover all the details, but it serves to explain in words what are

otherwise very complex physical interactions. The main point is that heat is always absorbed at

the cold side of the n- and p- type elements, and heat is always released at the hot side of

thermoelectric element. The heat pumping capacity of a module is proportional to the current and

is depends on the element geometry, number of couples, and material property.

2.9 MATHEMATICAL MODEL OF COOLING

MODULE:

Fig.8

The figure above represents a thermoelectric couple. It shows some terms used in themathematical equation:

L = element height A = cross-sectional area Qc = heat load

Tc = cold-side temperature Th = hot-side temperature I = applied current

Additionally, there is the following:

S = Seebeck coefficient R = electrical resistivity K = thermal conductivity

V = voltage N = number of couples

Here are the basic equations:

Qc = 2 N[S I Tc -

R

K

(ThTc)]REF 8

V = 2 N [S (ThTc) + I R

]REF- 8


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The first Qc term, SITc, is the peltier cooling effect. The second term,

R

, represents

the Joule heating effect associated with passing an electrical current through a resistance. TheJoule heat is distributed throughout the element, so 1/2 the heat goes towards the cold side, and

1/2 the heat goes towards the hot side. The last term, K

(Th-Tc), represents the Fourier effect

in which heat is conducted from a higher temperature to a lower temperature. So, the peltiercooling is reduced by the losses associated with electrical resistance and thermal conductance.For a voltage, the first term, S(Th-Tc) represents the Seebeck voltage. The second term,

IRL/A represents the voltage related by Ohms law.These are simplified equations which show the basic idea in calculations. The actual differential

equations do not have a closed-form solution because S, R, and K are temperature dependent.

2.10 EFFICIENCY OF COOLING MODULES:

Efficiency relates to the ratio of the amount of work output to the amount of power input. In heat

pumping applications, this term is rarely used because it is possible to remove more heat than the

amount of power input it takes to move that heat. For thermoelectric modules, it is standard touse the term "coefficient of performance" rather than "efficiency." The coefficient of

performance (COP) is the amount of heat pumped divided by the amount of supplied electrical

power.The COP depends on the factors like heat load, input power, and the required temperature

differential. Typically, the COP is between 0.3 and 0.7 for single-stage applications. However,

COPs greater than 1.0 can be achieved especially when the module is pumping against a positive

temperature difference (that is, when the module is removing heat from an object that is warmer

than the ambient).

2.11 RELIABILITY OF THERMOELECTRIC SYSTEM:

Thermoelectric systems are very reliable provided they are used in an appropriate manner. The

specific reliability of thermoelectric coolers tends to be difficult to define though because failure

rates are highly dependent upon the manner of application. Thermoelectric modules that are atsteady state (constant power, heat load, temperature, etc.) can have mean time between failures(MTBFs) more than 200,000 hours. However, applications involving thermal cycling show

significantly worse MTBFs, especially when TE coolers are cycled up to a high temperature.

With thermal cycling, a more appropriate measure of reliability is not time but rather number of

cycles.


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All materials expand and contract as they are heated and cooled. Different materials will expand

at differently. The rate of expansion depends upon the material property called the coefficient of

thermal expansion (CTE). Generally, as the cold side of a module gets colder, it will contract,and as the hot side becomes hotter, hence it will expand. This flexes the thermoelectric elements

and their solder junctions. Furthermore, because the module is constructed of several different

materials, there is added stress simply because the materials themselves areexpanding/contracting at different rates. After repeated thermal cycling, the solder junctionswithin the module fatigue, and the electrical resistance increases. Cooling performance is

reduced, and eventually the module is damaged. The "failure point" is thus a function of

operating temperature, the amount of temperature cycling, and how much degradation theparticular system can tolerate before performance becomes unacceptable. All thermoelectric

modules experience the same stresses of operation, but how they tolerate these stresses depends

on build qualityselecting a manufacturer with good, strong solder junctions is a must.

2.12 THERMOELECTIC SYSTEM AS HEATER:

Thermoelectric coolers can indeed be used very effectively and efficiently for heating. Sincethermoelectric coolers are solid-state heat pumps, they can pump heat from the ambient in

addition to the heating effect that comes from the electrical resistance of the cooler itself. So, the

thermoelectric cooler can be more efficient than a resistive heater (within limits).If you are interested in building your own assembly, you can use the cooling performance graphs

of the thermoelectric module to estimate how much heating can be done. The total heating load

is calculated by first estimating a temperature difference across the module and assuming aninput current for any particular module. This defines the active amount of heat that the module

can pump from the ambient. Combining this with the total power input determines how much

total heating the module can do. You would then iterate the temperature difference guess basedon the thermal resistances to and from the module and the corresponding heat loads being

transferred.

2.13 COMPARISON WITH OTHER PAPERS:

The papers we have gone through has been compared with our project. Most of the papers have

used multiple cooling modules with high power supply. And most of the papers have

transformers for power supply. But we have used Switched Mode Power Supply as the power

supply. It makes the sensitive cooling module safe. The SMPS restricts the fluctuating load and

makes it stable. And in addition to it, our project has the uniqueness of both heating and

cooling. The COP of our project is also compared to other research papers.


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Documents

project work on Thermoelectric refrigeraton