191419188 Thermoelectric Refrigeration System

7/22/2019 191419188 Thermoelectric Refrigeration System

1/39

Objective of Project

The objective of the project is to develop Thermoelectric cooling System .Thermoelectriccoolinguses the Peltier effectto create a heat fluxbetween the junction of two different types ofmaterials. This effect is commonly used in camping and portable coolers.

7/22/2019 191419188 Thermoelectric Refrigeration System

2/39

INTRODUCTIONWhat is refrigeration?

Refrigeration is a process in which work is done to move heat from one location to another.Refrigeration has many applications including but not limited to; household refrigeratorsindustrial free!ers cryogenics air conditioning and heat pumps.

"old is the absence of heat hence in order to decrease a temperature one #removes heat# ratherthan #adding cold.# $n order to satisfy theSecond %aw of Thermodynamics some form of workmust be performed to accomplish this. The work is traditionally done by mechanical workbutcan also be done by magnetismlaseror other means.

Historical applications

Ice harvestingThe use of iceto refrigerate and thus preserve food goes back to prehistoric times. Through theages the seasonal harvesting of snow and ice was a regular practice of most of the ancientcultures& "hinese 'ebrews (reeks Romans Persians. $ce and snow were stored in caves ordugoutslined with strawor other insulating materials. The Persians stored ice in pits calledyakhchals. Rationing of the ice allowed the preservation of foods over the warm periods. Thispractice worked well down through the centuries with icehousesremaining in use into thetwentieth century.

$n the )*th century the discovery of chemical refrigeration was one of the first steps towardartificial means of refrigeration. Sodium nitrate orpotassium nitrate when added to waterlowered the water temperature and created a sort of refrigeration bath for cooling substances. $n

$taly such a solution was used to chill wine and cakes.+uring the first half of the ),th century ice harvesting became big business in -merica.ew/nglander 0rederic Tudor who became known as the #$ce 1ing# worked on developing betterinsulationproducts for the long distance shipment of ice especially to the tropics.

First refrigeration sste!s

The first known method of artificial refrigeration was demonstrated by2illiam "ullenat the3niversity of (lasgow in Scotlandin )45*. "ullen used a pump to create a partialvacuumover acontainer of diethyl ether which thenboiled absorbing heat from the surrounding air. Theexperiment even created a small amount of ice but had no practical application at that time.

$n )456 7enjamin 0ranklin and 8ohn 'adley professor of chemistry at "ambridge 3niversity

conducted an experiment to explore the principle of evaporation as a means to rapidly cool anobject. 0ranklin and 'adley confirmed that evaporation of highly volatile li9uids such as alcoholand ether could be used to drive down the temperature of an object past the free!ing point ofwater. They conducted their experiment with the bulb of a mercury thermometer as their objectand with a bellows used to #9uicken# the evaporation; they lowered the temperature of thethermometer bulb down to 4 :0 .0ranklin noted that soon after they passed the free!ing point of water ?@ :0> a thin film of iceformed on the surface of the thermometerAs bulb and that the ice mass was about a 9uarter inch

7/22/2019 191419188 Thermoelectric Refrigeration System

3/39

thick when they stopped the experiment upon reaching 4 :0

7/22/2019 191419188 Thermoelectric Refrigeration System

4/39

The first gas absorptionrefrigeration system using gaseous ammonia dissolved in water referredto as #a9ua ammonia#> was developed by0erdinand "arrIof 0rance in )65, and patented in)6*B. +ue to the toxicity of ammonia such systems were not developed for use in homes butwere used to manufacture ice for sale. $n the 3nited States the consumer public at that time stillused the ice boxwith ice brought in from commercial suppliers many of whom were stillharvesting ice and storing it in an icehouse.

Thaddeus %owe an -merican balloonist from the "ivil 2ar had experimented over the yearswith the properties of gases. Cne of his mainstay enterprises was the highDvolume production ofhydrogengas. 'e also held several patents on ice making machines. 'is #"ompression $ceEachine# would revolutioni!e the cold storage industry. $n )6*, he and other investorspurchased an old steamship onto which they loaded one of %oweJs refrigeration units and beganshipping fresh fruit from ew Kork to the (ulf "oast area and fresh meat from (alvestonTexas back to ew Kork. 7ecause of %oweJs lack of knowledge about shipping the businesswas a costly failure and it was difficult for the public to get used to the idea of being able toconsume meat that had been so long out of the packing house.

+omestic mechanicalrefrigeratorsbecame available in the 3nited States around ),)).

N""D OF R"FRI#"R$TION%& Wi'esprea' co!!ercial (se

%oading blocks of factoryDmade ice from a truck to an #ice depot# boat in the fishing harbor ofGhuhai "hina

7y the )64Bs breweries had become the largest users of commercial refrigeration units thoughsome still relied on harvested ice. Though the iceDharvesting industry had grown immensely bythe turn of the @Bth century pollution and sewage had begun to creep into natural ice making it aproblem in the metropolitan suburbs. /ventually breweries began to complain of tainted ice. This

7/22/2019 191419188 Thermoelectric Refrigeration System

5/39

raised demand for more modern and consumerDready refrigeration and iceDmaking machines. $n)6,5 (erman engineer "arl von %indeset up a largeDscale process for the production of li9uidair and eventually li9uid oxygen for use in safe household refrigerators.

Refrigerated railroad cars were introduced in the 3S in the )6=Bs for the shortDrun transportationof dairy products. $n )6*4 8.7. Sutherland of +etroit Eichigan patented the refrigerator car

designed with ice tanks at either end of the car and ventilator flaps near the floor which wouldcreate a gravity draft of cold air through the car.

7y ),BB the meat packinghouses of "hicago had adopted ammoniaDcycle commercialrefrigeration. 7y ),)= almost every location used artificial refrigeration. The big meat packers-rmour Swift and 2ilson had purchased the most expensive units which they installed on traincars and in branch houses and storage facilities in the more remote distribution areas.

$t was not until the middle of the @Bth century that refrigeration units were designed forinstallation on tractorDtrailer rigs trucks or lorries>. Refrigerated vehicles are used to transportperishable goods such as fro!en foods fruit and vegetables and temperatureDsensitivechemicals. Eost modern refrigerators keep the temperature between D=B and L@B :" and have amaximum payload of around @= BBB kg. gross weight in /urope>.

)& Ho!e an' cons(!er (se

2ith the invention of synthetic refrigerants based mostly on a chlorofluorocarbon"0">chemical safer refrigerators were possible for home and consumer use. 0reon is atrademark ofthe +upont "orporationand refers to these "0" and laterhydrochlorofluorocarbon '"0"> andhydrofluorocarbon '0"> refrigerants developed in the late ),@Bs. These refrigerants wereconsidered at the time to be less harmful than the commonly used refrigerants of the timeincludingmethyl formate ammonia methyl chlorideand sulfur dioxide. The intent was toprovide refrigeration e9uipment for home use without danger& these "0" refrigerants answeredthat need. 'owever in the ),4Bs the compounds were found to be reacting with atmospherico!one an important protection against solarultraviolet radiation and their use as a refrigerantworldwide was curtailed in the Eontreal Protocolof ),64.

*etho's of refrigeration

Eethods of refrigeration can be classified as nonDcyclic cyclic and thermoelectric.

Non+cclic refrigeration

$n nonDcyclic refrigeration cooling is accomplished by melting ice or by subliming dry icefro!en carbon dioxide>. These methods are used for smallDscale refrigeration such as inlaboratories and workshops or in portablecoolers.

$ce owes its effectiveness as a cooling agent to its constant melting pointof B :" ?@ :0>. $norder to melt ice must absorb ???.55 k8Mkg approx. )== 7tuMlb> of heat. 0oodstuffs maintainedat this temperature or slightly above have an increased storage life.

Solid carbon dioxide has no li9uid phase at normal atmospheric pressure so sublimesdirectlyfrom the solid to vapor phase at a temperature of D46.5 :" D)B,.? :0> and is therefore effectivefor maintaining products at low temperatures during the period of sublimation. Systems such asthis where the refrigerant evaporates and is vented into the atmosphere are known as #total lossrefrigeration#.

Cclic refrigeration

7/22/2019 191419188 Thermoelectric Refrigeration System

6/39

This consists of a refrigeration cycle where heat is removed from a lowDtemperature space orsource and rejected to a highDtemperature sink with the help of external work and its inverse thethermodynamic power cycle. $n the power cycle heat is supplied from a highDtemperature sourceto the engine part of the heat being used to produce work and the rest being rejected to a lowDtemperature sink. This satisfies the second law of thermodynamics.

- refrigeration cycle describes the changes that take place in the refrigerant as it alternatelyabsorbs and rejects heat as it circulates through arefrigerator.$t is also applied to'F-"Rworkwhen describing the #process# of refrigerant flow through an 'F-"R unit whether it is apackaged or split system.

'eat naturally flows from hot to cold. 2orkis applied to cool a living space or storage volumeby pumping heat from a lower temperature heat source into a higher temperature heat sink.$nsulationis used to reduce the work and energy re9uired to achieve and maintain a lowertemperature in the cooled space. The operating principle of the refrigeration cycle was describedmathematically by Sadi "arnotin )6@= as aheat engine.

The most common types of refrigeration systems use the reverseDRankinevaporDcompressionrefrigerationcycle although absorption heat pumpsare used in a minority of applications.

"yclic refrigeration can be classified as&

). Fapor cycle and

@. (as cycle

Fapor cycle refrigeration can further be classified as&

1. FaporDcompression refrigeration

2. FaporDabsorption refrigeration

1. ,apor+co!pression ccle

The vaporDcompression cycle is used in most household refrigerators as well as in many large

commercial and industrial refrigeration systems. 0igure ) provides a schematic diagram of thecomponents of a typical vaporDcompression refrigeration system.

7/22/2019 191419188 Thermoelectric Refrigeration System

7/39

0igure )& Fapor compression refrigeration

The thermodynamicsof the cycle can be analy!ed on a diagram as shown in 0igure @. $n thiscycle a circulating refrigerant such as0reonenters the compressor as a vapor. 0rom point ) topoint @ the vapor is compressed at constant entropyand exits the compressor as a vapor at ahigher temperature but still below the vapor pressureat that temperature. 0rom point @ to point ?and on to point = the vapor travels through the condenserwhich cools the vapor until it startscondensing and then condenses the vapor into a li9uid by removing additional heat at constantpressure and temperature. 7etween points = and 5 the li9uid refrigerant goes through theexpansion valvealso called a throttle valve> where its pressure abruptly decreases causing flashevaporationand autoDrefrigeration of typically less than half of the li9uid.

0igure @& TemperatureN/ntropy diagram

That results in a mixture of li9uid and vapor at a lower temperature and pressure as shown atpoint 5. The cold li9uidDvapor mixture then travels through the evaporator coil or tubes and iscompletely vapori!ed by cooling the warm air from the space being refrigerated> being blownby a fan across the evaporator coil or tubes. The resulting refrigerant vapor returns to thecompressor inlet at point ) to complete the thermodynamic cycle.

The above discussion is based on the ideal vaporDcompression refrigeration cycle and does nottake into account realDworld effects like frictional pressure drop in the system slight

thermodynamic irreversibilityduring the compression of the refrigerant vapor or nonDideal gasbehavior if any>.

2. ,apor absorption ccle

$n the early years of the twentieth century the vapor absorption cycle using waterDammoniasystems was popular and widely used. -fter the development of the vapor compression cycle thevapor absorption cycle lost much of its importance because of its low coefficient of performanceabout one fifth of that of the vapor compression cycle>. Today the vapor absorption cycle is

7/22/2019 191419188 Thermoelectric Refrigeration System

8/39

used mainly where fuel for heating is available but electricity is not such as in recreationalvehicles that carry %P gas. $tAs also used in industrial environments where plentiful waste heatovercomes its inefficiency.

The absorption cycle is similar to the compression cycle except for the method of raising thepressure of the refrigerant vapor. $n the absorption system the compressor is replaced by an

absorber which dissolves the refrigerant in a suitable li9uid a li9uid pump which raises thepressure and a generator which on heat addition drives off the refrigerant vapor from the highDpressure li9uid. Some work is re9uired by the li9uid pump but for a given 9uantity ofrefrigerant it is much smaller than needed by the compressor in the vapor compression cycle. $nan absorption refrigerator a suitable combination of refrigerant and absorbent is used. The mostcommon combinations are ammonia refrigerant> and water absorbent> and water refrigerant>andlithium bromideabsorbent>.

#as ccle

2hen the working fluid is a gas that is compressed and expanded but doesnAt change phase therefrigeration cycle is called a gas cycle.-iris most often this working fluid. -s there is no

condensation and evaporation intended in a gas cycle components corresponding to thecondenser and evaporator in a vapor compression cycle are the hot and cold gasDtoDgas heatexchangersin gas cycles.

The gas cycle is less efficient than the vapor compression cycle because the gas cycle works onthe reverse 7rayton cycle instead of the reverseRankine cycle. -s such the working fluid doesnot receive and reject heat at constant temperature. $n the gas cycle the refrigeration effect ise9ual to the product of the specific heat of the gas and the rise in temperature of the gas in thelow temperature side. Therefore for the same cooling load a gas refrigeration cycle will re9uirea large mass flow rate and would be bulky.

7ecause of their lower efficiency and larger bulk air cycle coolers are not often used nowadaysin terrestrial cooling devices. The air cycle machineis very common however on gas turbineD

powered jet aircraftbecause compressed air is readily available from the enginesA compressorsections. These jet aircraftAs cooling and ventilation units also serve the purpose of pressuri!ingthe aircraft.

Ther!oelectric refrigeration

Thermoelectric cooling uses thePeltier effect to create a heatfluxbetween the junction of twodifferent types of materials. This effect is commonly used in camping and portable coolers andfor cooling electronic components and small instruments.

*agnetic refrigeration

Eagnetic refrigeration or adiabatic demagneti!ation is a cooling technology based on themagnetocaloric effect anintrinsicproperty of magnetic solids. The refrigerant is often a

paramagneticsaltsuch as ceriummagnesiumnitrate. The active magneticdipolesin this caseare those of the electron shellsof the paramagnetic atoms.

- strong magnetic field is applied to the refrigerant forcing its various magnetic dipoles to alignand putting these degrees of freedom of the refrigerant into a state of loweredentropy.- heatsink then absorbs the heat released by the refrigerant due to its loss of entropy. Thermal contactwith the heat sink is then broken so that the system is insulated and the magnetic field isswitched off. This increases the heat capacity of the refrigerant thus decreasing its temperaturebelow the temperature of the heat sink.

7/22/2019 191419188 Thermoelectric Refrigeration System

9/39

7ecause few materials exhibit the re9uired properties at room temperature applications have sofar been limited to cryogenics and research.

Other !etho's

Cther methods of refrigeration include the air cycle machineused in aircraft; thevortex tubeused for spot cooling when compressed air is available; and thermoacoustic refrigerationusing

sound waves in a pressuri!ed gas to drive heat transfer and heat exchange. Eany Stirling cycleheat engines can be run backwards to act as a refrigerator and therefore these engines have aniche use in cryogenics.

Ther!oelectric effect

The thermoelectric effect is the direct conversion of temperature differences to electric voltageand vice versa. - thermoelectric device creates a voltage when there is a different temperature oneach side. "onversely when a voltage is applied to it it creates a temperature difference known

as the Peltier effect>. -t atomic scale specifically charge carriers> an applied temperaturegradient causes charged carriers in the material whether they are electronsor electron holes todiffuse from the hot side to the cold side similar to a classical gasthat expands when heated;hence the thermally induced current.

This effect can be used to generate electricity to measure temperature to cool objects or to heatthem or cook them. 7ecause the direction of heating and cooling is determined by the polarity ofthe applied voltage thermoelectric devices make very convenient temperature controllers.

7/22/2019 191419188 Thermoelectric Refrigeration System

10/39

Traditionally the term thermoelectric effect or thermoelectricity encompasses three separatelyidentified effects the Seebeck effect the Peltier effect and the Thomson effect. $n manytextbooks thermoelectric effect may also be called the PeltierNSeebeck effect. This separationderives from the independent discoveries of 0rench physicist 8ean "harles -thanase Peltier and/stonianD(erman physicist Thomas 8ohann Seebeck. 8oule heating the heat that is generatedwhenever a voltage is applied across a resistive material is somewhat related though it is notgenerally termed a thermoelectric effect and it is usually regarded as being a loss mechanismdue to nonDideality in thermoelectric devices>. The PeltierNSeebeck and Thomson effects can inprinciple be thermodynamically reversible whereas 8oule heating is not.

-eebec. effect

The Seebeck effect is the conversion of temperaturedifferences directly into electricity.

Seebeck discovered that a compassneedle would be deflected when a closed loop was formed oftwo metals joined in two places with a temperature difference between the junctions. This isbecause the metals respond differently to the temperature difference which creates a current

loop which produces a magnetic field.Seebeck however at this time did not recogni!e therewas an electric current involved so he called the phenomenon the thermomagnetic effectthinking that the two metals became magnetically polari!ed by the temperature gradient. The+anish physicist 'ans "hristian Orstedplayed a vital role in explaining and conceiving the term#thermoelectricity#.

The effect is that a voltage the thermoelectric/E0is created in the presence of a temperaturedifference between two different metals or semiconductors. This causes a continuous current inthe conductors if they form a complete loop. The voltage created is of the order of severalmicrovoltsper kelvindifference. Cne such combination copperDconstantanhas a Seebeckcoefficient of =) microvolts per kelvin at room temperature.

$n the circuit&

which can be in several different configurations and be governed by the same e9uations> thevoltage developed can be derived from&

7/22/2019 191419188 Thermoelectric Refrigeration System

11/39

S-and S7are the Seebeck coefficients also called thermoelectric powerorthermopower> of the

metals - and 7 as a function of temperature and T)and T@are the temperatures of the twojunctions. The Seebeck coefficients are nonDlinear as a function of temperature and depend onthe conductorsA absolute temperature material and molecular structure. $f the Seebeckcoefficients are effectively constant for the measured temperature range the above formula canbe approximated as&

The Seebeck effect is commonly used in a device called a thermocouple because it is made froma coupling or junction of materials usually metals> to measure a temperature difference directlyor to measure an absolute temperature by setting one end to a known temperature. - metal of

unknown composition can be classified by its thermoelectric effect if a metallic probe of knowncomposition kept at a constant temperature is held in contact with it. $ndustrial 9uality controlinstruments use this Seebeck effect to identify metal alloys. This is known as thermoelectricalloy sorting.

Several thermocouples connected in series are called athermopile which is sometimesconstructed in order to increase the output voltage since the voltage induced over each individualcouple is small.

This is also the principle at work behind thermal diodesand thermoelectric generatorssuch asradioisotope thermoelectric generatorsor RT(s> which are used for creating power from heatdifferentials.

The Seebeck effect is due to two effects& charge carrier diffusion and phonon drag describedbelow>. $f both connections are held at the same temperature but one connection is periodicallyopened and closed an -" voltageis measured which is also temperature dependent. Thisapplication of the 1elvin probeis sometimes used to argue that the underlying physics onlyneeds one junction. -nd this effect is still visible if the wires only come close but do not touchthus no diffusion is needed.

Ther!opo/er

The thermopower thermoelectric power or Seebeck coefficient of a material measures themagnitude of an induced thermoelectric voltage in response to a temperature difference acrossthat material. The thermopower has units of FM1> though in practice it is more common to use

microvolts per kelvin. Falues in the hundreds of FM1 negative or positive are typical of goodthermoelectric materials. The term thermopower is a misnomer since it measures the voltage orelectric field induced in response to a temperature difference not the electric power. -n appliedtemperature difference causes charged carriers in the material whether they areelectronsorholes to diffuse from the hot side to the cold side similar to a classical gas that expands whenheated. Eobile charged carriers migrating to the cold side leave behind their oppositely chargedand immobile nuclei at the hot side thus giving rise to a thermoelectric voltage thermoelectricrefers to the fact that the voltage is created by a temperature difference>. Since a separation of

7/22/2019 191419188 Thermoelectric Refrigeration System

12/39

charges also creates an electric potential the buildup of charged carriers onto the cold sideeventually ceases at some maximum value since there exists an e9ual amount of charged carriersdrifting back to the hot side as a result of the electric field at e9uilibrium. Cnly an increase in thetemperature difference can resume a buildup of more charge carriers on the cold side and thuslead to an increase in the thermoelectric voltage. $ncidentally the thermopower also measures theentropyper charge carrier in the material. To be more specific the partial molar electronicentropy is said to e9ual the absolute thermoelectric power multiplied by the negative of 0aradayAsconstant.

The thermopower of a material represented by Sor sometimes by Q> depends on the materialAstemperature and crystal structure. Typically metals have small thermopowers because most havehalfDfilled bands. /lectrons negative charges> and holes positive charges> both contribute to theinduced thermoelectric voltage thus canceling each otherAs contribution to that voltage andmaking it small. $n contrast semiconductorscan be doped with excess electrons or holes andthus can have large positive or negative values of the thermopower depending on the charge ofthe excess carriers. The sign of the thermopower can determine which charged carriers dominatethe electric transport in both metals and semiconductors.

$f the temperature difference Tbetween the two ends of a material is small then thethermopower of a material is defined approximately> as&

and a thermoelectric voltage Vis seen at the terminals.

This can also be written in relation to the electric fieldEand the temperature gradient bythe approximate e9uation&

$n practice one rarely measures the absolute thermopower of the material of interest. This isbecause electrodes attached to a voltmeter must be placed onto the material in order to measurethe thermoelectric voltage. The temperature gradient then also typically induces a thermoelectricvoltage across one leg of the measurement electrodes. Therefore the measured thermopowerincludes a contribution from the thermopower of the material of interest and the material of themeasurement electrodes.

The measured thermopower is then a contribution from both and can be written as&

Superconductorshave !ero thermopower since the charged carriers produce no entropy. Thisallows a direct measurement of the absolute thermopower of the material of interest since it is

7/22/2019 191419188 Thermoelectric Refrigeration System

13/39

the thermopower of the entire thermocouple as well. $n addition a measurement of the Thomson

coefficient of a material can also yield the thermopower through the relation&

The thermopower is an important material parameter that determines the efficiency of a

thermoelectric material. - larger induced thermoelectric voltage for a given temperature gradientwill lead to a larger efficiency. $deally one would want very large thermopower values since onlya small amount of heat is then necessary to create a large voltage. This voltage can then be usedto provide power.

Charge+carrier 'iff(sion

"harge carriersin the materials electrons in metals electrons and holes in semiconductors ionsin ionic conductors> will diffuse when one end of a conductor is at a different temperature to theother. 'ot carriers diffuse from the hot end to the cold end since there is a lower density of hotcarriers at the cold end of the conductor. "old carriers diffuse from the cold end to the hot end

for the same reason.$f the conductor were left to reach thermodynamic e9uilibriumthis processwould result in heat being distributed evenly throughout the conductor .The movement of heatin the form of hot charge carriers> from one end to the other is called a heat current. -s chargecarriers are moving it is also anelectric current.$n a system where both ends are kept at aconstant temperature difference a constant heat current from one end to the other> there is aconstant diffusion of carriers. $f the rate of diffusion of hot and cold carriers in oppositedirections were e9ual there would be no net change in charge. 'owever the diffusing chargesare scatteredby impurities imperfections and lattice vibrations phonons>. $f the scattering isenergy dependent the hot and cold carriers will diffuse at different rates. This creates a higherdensity of carriers at one end of the material and the distance between the positive and negativecharges produces a potential difference; an electrostatic voltage.This electric field howeveropposes the uneven scattering of carriers and an e9uilibrium is reached where the net number of

carriers diffusing in one direction is canceled by the net number of carriers moving in theopposite direction from the electrostatic field. This means the thermopower of a material dependsgreatly on impurities imperfections and structural changes which often vary themselves withtemperature and electric field> and the thermopower of a material is a collection of manydifferent effects.

/arly thermocouples were metallic but many more recently developed thermoelectric devicesare made from alternating pDtype and nDtype semiconductor elements connected by metallicinterconnects as pictured in the figures below. Semiconductor junctions are especially commonin power generation devices while metallic junctions are more common in temperaturemeasurement. "harge flows through the nDtype element crosses a metallic interconnect andpasses into the pDtype element. $f a power source is provided the thermoelectric device may act

as a cooler as in the figure to the left below. This is the Peltier effect described below. /lectronsin the nDtype element will move opposite the direction of current and holes in the pDtype elementwill move in the direction of current both removing heat from one side of the device. $f a heatsource is provided the thermoelectric device may function as a power generator as in the figureto the right below. The heat source will drive electrons in the nDtype element toward the coolerregion thus creating a current through the circuit. 'oles in the pDtype element will then flow inthe direction of the current. The current can then be used to power a load thus converting thethermal energy into electrical energy.

7/22/2019 191419188 Thermoelectric Refrigeration System

14/39

Phonon 'rag

Phononsare not always in local thermal e9uilibrium; they move against the thermal gradient.They lose momentum by interacting with electrons or other carriers> and imperfections in thecrystal. $f the phononDelectron interaction is predominant the phonons will tend to push theelectrons to one end of the material losing momentum in the process. This contributes to thealready present thermoelectric field. This contribution is most important in the temperatureregion where phononDelectron scattering is predominant. This happens for

where +is the +ebye temperature.-t lower temperatures there are fewer phonons available fordrag and at higher temperatures they tend to lose momentum in phononDphonon scatteringinstead of phononDelectron scattering.

This region of the thermopowerDversusDtemperature function is highly variable under a magneticfield.

-pin -eebec. "ffect an' *agnetic 0atteries

Physicists have recently discovered that heating one side of a magneti!ed nickelDiron rod causeselectrons to rearrange themselves according to their spins. This soDcalled #spin Seebeck effect#

could lead to batteries that generate magnetic currents rather than electric currents. - source ofmagnetic currents could be especially useful for the development ofspintronics devices whichuse magnetic currents in order to reduce overheating in computer chips since unlike electriccurrents steady magnetic currents do not generate heat.

7/22/2019 191419188 Thermoelectric Refrigeration System

15/39

Peltier effect

The Peltier effect bears the name of 8eanD"harles Peltier a 0rench physicist who in )6?=discovered the calorific effect of an electric current at the junction of two different metals. 2hena current is made to flow through the circuit heat is evolved at the upper junction at T@> andabsorbed at the lower junction at T)>. The Peltier heat absorbed by the lower junction per unit

time is e9ual to

where is the Peltier coefficient U-7of the entire thermocouple and U-and U7are thecoefficients of each material. pDtype silicon typically has a positive Peltier coefficient thoughnot above V55B 1> and nDtype silicon is typically negative.

The Peltier coefficients represent how much heat current is carried per unit charge through agiven material. Since charge current must be continuous across a junction the associated heatflow will develop a discontinuity if U-and U7are different. This causes a nonD!ero divergence atthe junction and so heat must accumulate or deplete there depending on the sign of the current.-nother way to understand how this effect could cool a junction is to note that when electronsflow from a region of high density to a region of low density this #expansion# causes cooling aswith an ideal gas>.

The carriers are attempting to return to the electron e9uilibrium that existed before the currentwas applied by absorbing energy at one connector and releasing it at the other. The individualcouples can be connected in series to enhance the effect.

-n interesting conse9uence of this effect is that the direction of heat transfer is controlled by thepolarity of the current; reversing the polarity will change the direction of transfer and thus thesign of the heat absorbedMevolved.

- Peltier coolerMheater or thermoelectric heat pump is a solidDstate active heat pump whichtransfers heat from one side of the device to the other. Peltier cooling is also called thermoDelectric cooling T/">.

Tho!son effect

The Thomson effect was predicted and subse9uently experimentally observed by 2illiamThomson%ord 1elvin> in )65). $t describes the heating or cooling of a currentDcarryingconductor with a temperature gradient.

-ny currentDcarrying conductor except for a superconductor> with a temperature difference

between two points will either absorb or emit heat depending on the material.$f a current densityJis passed through a homogeneous conductor heat production per unitvolume is&

where

7/22/2019 191419188 Thermoelectric Refrigeration System

16/39

is theresistivityof the material

dTMdxis the temperature gradient along the wire

is the Thomson coefficient.

The first term Jis simply the 8oule heating which is not reversible.

The second term is the Thomson heat which changes sign whenJchanges direction.$n metals such as !inc and copperwhich have a hotter end at a higher potential and a cooler endat a lower potential when current moves from the hotter end to the colder end it is moving froma high to a low potential so there is an evolution ofheat. This is called the positive Thomsoneffect.

$n metals such as cobaltnickel and iron which have a cooler end at a higher potential and ahotter end at a lower potential when current moves from the hotter end to the colder end it ismoving from a low to a high potential there is an absorption of heat. This is called the negativeThomson effect.

The Thomson coefficient is uni9ue among the three main thermoelectric coefficients because it

is the only thermoelectric coefficient directly measurable for individual materials. The Peltierand Seebeck coefficients can only be determined for pairs of materials. Thus there is no directexperimental method to determine an absolute Seebeck coefficient i.e. thermopower> orabsolute Peltier coefficient for an individual material. 'owever as mentioned elsewhere in thisarticle there are two e9uations the Thomson relations also known as the 1elvin relations seebelow> relating the three thermoelectric coefficients. Therefore only one can be considereduni9ue.

$f the Thomson coefficient of a material is measured over a wide temperature range includingtemperatures close to !ero one can then integrate the Thomson coefficient over the temperaturerange using the 1elvin relations to determine the absolute i.e. singleDmaterial> values for thePeltier and Seebeck coefficients. $n principle this need only be done for one material since allother values can be determined by measuring pairwise Seebeck coefficients in thermocouplescontaining the reference material and then adding back the absolute thermoelecric powerthermopower> of the reference material.

$t is commonly asserted that lead has a !ero Thomson effect. 2hile it is true that thethermoelectric coefficients of lead are small they are in general nonD!ero. The Thomsoncoefficient of lead has been measured over a wide temperature range and has been integrated tocalculate the absolute thermoelectric power thermopower> of lead as a function of temperature.

3nlike lead the thermoelectric coefficients of all known superconductors are !ero.

The Tho!son relationships

The Seebeck effect is actually a combination of the Peltier and Thomson effects. $n fact in )65=Thomson found two relationships now called the Thomson or 1elvin relationships between thecorresponding coefficients. The absolute temperature T the Peltier coefficient U and Seebeckcoefficient Sare related by the second Thomson relation

7/22/2019 191419188 Thermoelectric Refrigeration System

17/39

which predicted the Thomson effect before it was actually formali!ed. These are related to theThomson coefficient by the first Thomson relation

ThomsonAs theoretical treatment of thermoelectricity is remarkable in the fact that it is probablythe first attempt to develop a reasonable theory of irreversible thermodynamics nonDe9uilibriumthermodynamics>. This occurred at about the time that "lausius Thomson and others wereintroducing and refining the concept of entropy.

Fig(re of !erit

The figure of meritfor thermoelectric devices is defined as

where W is the electrical conductivity X is the thermal conductivity and S is the Seebeckcoefficient or thermopower conventionally in FM1>. This is more commonly expressed as thedimensionless figure of merit GT by multiplying it with the average temperature T@L T)> M @>.(reater values of GT indicate greater thermodynamic efficiency subject to certain provisionsparticularly the re9uirement that the two materials of the couple have similar G values. GT istherefore a very convenient figure for comparing the potential efficiency of devices usingdifferent materials. Falues of GTY) are considered good and values of at least the ?N= range areconsidered to be essential for thermoelectrics to compete with mechanical generation andrefrigeration in efficiency. To date the best reported GT values have been in the @N? range.Euch research in thermoelectric materials has focused on increasing the Seebeck coefficient and

reducing the thermal conductivity especially by manipulating the nanostructure of the materials.

Device efficienc

The efficiency of a thermoelectric device for electricity generation is given by defined as

and

where T'is the temperature at the hot junction and T"is the temperature at the surface being

cooled. is the modified dimensionless figure of merit which now takes into consideration thethermoelectric capacity of both thermoelectric materials being used in the power generatingdevice and is defined as

7/22/2019 191419188 Thermoelectric Refrigeration System

18/39

where Z is the electrical resistivity is the average temperature between the hot and coldsurfaces and the subscripts n and p denote properties related to the nD and pDtype semiconductingthermoelectric materials respectively. $t is worthwhile to note that the efficiency of athermoelectric device is limited by the "arnot efficiencyhence the T'and T"terms in max>since thermoelectric devices are still inherently heat engines.

The "CPof current commercial thermoelectric refrigerators ranges from B.? to B.* only aboutoneDsixth the value of traditional vaporDcompression refrigerators.

Uses

Thermocouplesand thermopilesare commonly used to measure temperatures. They use theSeebeck effect. Eore precisely they do not directly measure temperature they measure

temperature differences between the probe and thevoltmeterat the other end of the wires. Thetemperature of the voltmeter usually the same as room temperature can be measured separatelyusing #cold junction compensation# techni9ues.

Ther!oelectric cooling

Thermoelectric cooling uses thePeltier effect to create aheatflux between the junction of twodifferent types of materials. - Peltiercooler heater or thermoelectric heat pump is a solidDstateactive heat pumpwhich transfers heat from one side of the device to the other side against thetemperature gradient from cold to hot> with consumption ofelectrical energy.Such aninstrument is also called a Peltier device Peltier heat pump solid state refrigerator orthermoelectric cooler T/">. 7ecause heating can be achieved more easily and economically by

7/22/2019 191419188 Thermoelectric Refrigeration System

19/39

many other methods Peltier devices are mostly used for cooling. 'owever when a single deviceis to be used for both heating and cooling a Peltier device may be desirable. Simply connectingit to a +" voltage will cause one side to cool while the other side warms. The effectiveness ofthe pump at moving the heat away from the cold side is dependent upon the amount of currentprovided and how well the heat can be removed from the hot side.

- Peltier cooler is the opposite of athermoelectric generator.$n a Peltier cooler electric power isused to generate a temperature difference between the two sides of the device; while in athermoelectric generator a temperature difference between the two sides is used to generateelectric power. The operation of both is closely related both are manifestations of thethermoelectric effect> and therefore the devices are generally constructed from similar materialsusing similar designs.

Peltier element schematic& Thermoelectric legs are thermally in parallel and electrically in series.

Peltier element (16x16 mm)

Thermoelectric junctions are generally only around 5N)B[ as efficient as the ideal refrigerator"arnot cycle> compared with =BN*B[ achieved by conventional compression cycle systemsreverse Rankinesystems using compressionMexpansion>. +ue to the relatively low efficiencythermoelectric cooling is generally only used in environments where the solid state nature no

moving parts maintenanceDfree> outweighs pure efficiency.

Peltier thermoelectric> cooler performance is a function of ambient temperature hot and coldside heat exchanger heat sink> performance thermal load Peltier module thermopile>geometry and Peltier electrical parameters.

7/22/2019 191419188 Thermoelectric Refrigeration System

20/39

Uses

Peltier devices are commonly used incampingand portable coolers and for cooling electroniccomponents and small instruments. Some electronic e9uipment intended for military use in thefield is thermoelectrically cooled. The cooling effect of Peltier heat pumps can also be used toextract water from the air in dehumidifiers.

Peltier elements are a common component inthermal cyclersused for the synthesis of +- bypolymerase chain reaction P"R> a common molecular biological techni9ue which re9uires therapid heating and cooling of the reaction mixture for denaturation primer annealing anden!ymatic synthesis cycles.

The effect is used in satellites and spacecraft to counter the effect of direct sunlighton one sideof a craft by dissipating the heat over the cold shaded side whereupon the heat is dissipated bythermal radiation into space.

Photon detectors such as ""+sin astronomical telescopes or very highDend digital cameras areoften cooled down with Peltier elements. This reduces dark counts due to thermal noise. - darkcount is the event that a pixel gives a signal although it has not received a photon but rather

mistook a thermal fluctuationfor one. Cn digital photos taken at low light these occur asspeckles or #pixel noise#>.

Thermoelectric coolers can be used to cool computer components to keep temperatures withindesign limits without the noise of a fan or to maintain stable functioning whenoverclocking.$nfiber optic applications where the wavelength of a laser or a component is highly dependent ontemperature Peltier coolers are used along with a thermistor in a feedback loop to maintain aconstant temperature and thereby stabili!e the wavelength of the device. - Peltier cooler with aheat sinkor waterblock can cool a chip to well below ambient temperature.

A US !"#ered $e%era&e c""ler

Peltier devices are used in 3S7 drink coolersMchillers one of the latest addition to 3S7gadgetsMtoys. These devices are powered directly from the 3S7 port and are said to keep drinks

chilled some can even keep drinks warm. The effectiveness of these devices however is highly9uestionable. The available power from a 3S7 socket is very limited maximum of 5BB m- at 5F+" for most situations although highDpower ports providing ) amp or more do exist> socooling or heating will be minimal 5D)B[ of @.5 2>.

7/22/2019 191419188 Thermoelectric Refrigeration System

21/39

Ther!oelectric !aterials

Thermoelectric materials show the thermoelectric effectin a strong andMor convenient form. Thethermoelectric effect refers to phenomena in which atemperaturedifference creates an electricpotentialor electric potential creates a temperature difference& Specifically the Seebeck effecttemperatureD\current> Peltier effectcurrentD\temperature> and Thomson effectconductorheatingMcooling>. 2hile all materials have a non!ero thermoelectric effect in most materials it istoo small to be useful. 'owever low cost materials that have a sufficiently strong thermoelectriceffect and other re9uired properties> could be used for applications includingpower generationrefrigerationand a variety of other applications.

- commonly used thermoelectric material in such applications is 7ismuth telluride7i@Te?>.

$pplications

Po/er generation

-pproximately ,B[ of the worldJs electricity is generated by heat energy typically operating at?BD=B[ efficiency losing roughly )5 terawattsof power in the form of heat to the environment.Thermoelectric devices could convert this waste heat into useful electricity. Thermoelectricefficiency depends on the figure of merit GT. There is no theoretical upper limit to GT althoughno known thermoelectrics have a GT\). @B)B devices serve application niches where efficiencyis less important than reliability light weight and small si!e.

$nternal combustion engines capture @BD@5[ of the energy released during fuel combustion.$ncreasing the conversion rate can increase mileage and provide more electricity for onDboardcontrols and creature comforts stability controls telematics navigation systems electronicbraking etc.> $t may be possible to shift energy draw from the engine in certain cases> to theelectrical load in the car e.g. electrical power steering or electrical coolant pump operation.

"ogenerationpower plants use the heat produced during electricity generation for alternativepurposes. Thermoelectrics may find applications in such systems or in solar thermal energygeneration.

7/22/2019 191419188 Thermoelectric Refrigeration System

22/39

Refrigeration

Peltier effectdevices could reduce the emission of o!oneDdepleting refrigerants into theatmosphere. 'ydrochlorofluorocarbons '"0"s> and nowDobsolete chlorofluorocarbons"0"s>deplete the o!one layer. "0"s were replaced by '"0"s however the latter also impact theo!one and are being phased out. $nternational legislation caps '"0" production and prohibitsproduction after @B@B in developed countries and @B?B in developing countries. Thermoelectricrefrigeration units could reduce the use of such harmful chemicals and reduce noise levelsbecause they do not re9uire compressors.> "ommon vapor compression> refrigerators remainmore efficient than peltier refrigerators but they are larger and re9uire more maintenance. -GT\? about @BD?B[ "arnot efficiency> is re9uired to replace traditional coolers.

*aterials selection criteria

Fig(re of !erit

The primary criterion for thermoelectric device viability is thefigure of meritgiven by&

which depends on the Seebeck coefficient Sthermal conductivity] andelectrical conductivityW. The product GT> of G and the use temperature T serves as a dimensionless parameter toevaluate the performance of a thermoelectric material.

Phonon+#lass1 electron+crstal behavior

otably in the above e9uationthermal conductivity andelectrical conductivityintertwine. (. -.

Slackproposed that in order to optimi!e the figure of meritphonons which are responsible forthermal conductivity must experience the material as they would in a glass experiencing a highdegree ofphononscattering^lowering thermal conductivity> whileelectronsmust experience itas a crystal experiencing very little scattering^maintainingelectrical conductivity>. The figureof merit can be improved through the independent adjustment of these properties.

-e!icon'(ctors

Semiconductors are ideal thermoelectric devices because of theirband structureand electronicproperties at high temperatures. +evice efficiency is proportional to GT so ideal materials have alarge G value at high temperatures. Since temperature is easily adjustable electrical conductivityis crucial. Specifically maximi!ing electrical conductivity at high temperatures and minimi!ingthermal conductivity optimi!es GT.

Ther!al con'(ctivit

X Y X electronL X phonon

-ccording to the 2iedemannN0ran! lawthe higher the electrical conductivity the higher X electronbecomes. Therefore it is necessary to minimi!e X phonon. $n semiconductors X electron_ X phonon so itis easier to decouple X and W in a semiconductor through engineering X phonon.

7/22/2019 191419188 Thermoelectric Refrigeration System

23/39

"lectrical con'(ctivit

Eetals are typically good electrical conductors but the higher the temperature the lower theconductivity given by the e9uation for electrical conductivity&

WmetalY ne@`Mm

n is carrier density e is electron charge

` is electron lifetime

m is mass

-s temperature increases ` decreases thereby decreasing Wmetal. 7y contrast electricalconductivity in semiconductors correlates positively with temperature.

W semiconductorY ne

n is carrier density

e is electron charge

is carrier mobility"arrier mobility increases with increasing temperature thereby increasing W semiconductor.

-tate 'ensit

The band structure of semiconductors offers better thermoelectric effects than the band structureof metals.

The 0ermi energyis below the conduction bandcausing the state density to be asymmetricaround the 0ermi energy. Therefore the average electron energy is higher than the 0ermi energymaking the system conducive for charge motion into a lower energy state. 7y contrast the 0ermienergy lies in the conduction band in metals. This makes the state density symmetric about the0ermi energy so that the average conduction electron energy is close to the 0ermi energy

reducing the forces pushing for charge transport. Therefore semiconductors are idealthermoelectric materials.

*aterials of interest

Strategies to improve thermoelectrics include both advancedbulk materialsand the use of lowDdimensional systems. Such approaches to reduce lattice thermal conductivityfall under threegeneral material types&

). -lloys& create point defects vacancies or rattling structures heavyDionspecies with largevibrational amplitudescontained within partially filled structural sites> to scatter phonons withinthe unit cellcrystal.

@. "omplex crystals& separate the phononDglass from the electron crystal using approachessimilar to those for superconductors. The region responsible for electron transport would be anelectronDcrystal of a highDmobility semiconductor while the phononDglass would be ideal tohouse disordered structures and dopantswithout disrupting the electronDcrystal analogous to thecharge reservoir in highDTc superconductors.

?. Eultiphase nanocomposites& scatter phonons at the interfaces of nanostructured materials bethey mixed composites or thin film superlattices.

Eaterials under consideration for thermoelectric device applications include&

7/22/2019 191419188 Thermoelectric Refrigeration System

24/39

0is!(th chalcogeni'es

Eaterials such as 7i@Te?and 7i@Se?comprise some of the best performing room temperaturethermoelectrics with a temperatureDindependent thermoelectric effect GT between B.6 and ).B.anostructuring these materials to produce a layered superlattice structure of alternating 7i@Te?and 7i@Se?layers produces a device within which there is good electrical conductivity but

perpendicular to which thermal conductivity is poor. The result is an enhanced GTapproximately @.= at room temperature for pDtype>. ote that this high value has not entirelybeen independently confirmed.

-.(tter('ite ther!oelectrics

Recently skutterudite materials have sparked the interest of researchers in search of newthermoelectric. These structures are of the form "oi0e>PSb-s>?and are cubic with spacegroup$m?. 3nfilled these materials contain voids into which lowDcoordination ions usuallyrareearth elements> can be inserted in order to alter thermal conductivity by producing sources forlattice phonon scattering and decrease thermal conductivity due to the lattice without reducingelectrical conductivity. Such 9ualities make these materials exhibit P(/" behavior.

O2i'e ther!oelectrics+ue to the natural superlattice formed by the layered structure in homologous compounds suchas those of the form SrTiC?>nSrC>m^the RuddlesonDPopper phase>oxideshave potential forhighDtemperature thermoelectric devices. These materials exhibit low thermal conductivityperpendicular to these layers while maintaining electrical conductivity within the layers. Thefigure of merit in oxides is still relatively low VB.?= at )BBB1>but the enhanced thermalstability as compared to conventional highDGTbismuthcompounds makes the oxides superiorin highDtemperature applications.

Nano!aterials

$n addition to the nanostructured 7i@M7i@Se?superlattice thin films that have shown a great deal

of promise other nanomaterials show potential in improving thermoelectric materials. Cneexample involving PbTeMPbSeTe 9uantum dotsuperlattices provides an enhanced GTapproximately ).5 at room temperature> that was higher than the bulk GT value for either PbTeor PbSeTe approximately B.5>. $ndividual siliconnanowires can act as efficient thermoelectricmaterials with GT values approaching ).B for their structures even though bulk silicon is a poorthermoelectric material approximately B.B) at room temperature> because of its high thermalconductivity.

7/22/2019 191419188 Thermoelectric Refrigeration System

25/39

TH"R*O"3"CTRIC COO3"R 0$-IC-

Intro'(ction

-lthough thermoelectric T/> phenomena was discovered more than )5B years agothermoelectric devices T/ coolers> have only been applied commercially during recent decades.0or some time commercial T/"s have been developing in parallel with two mainstreamdirections of technical progress N electronics and photonics particularly optoelectronics and lasertechni9ues. %ately a dramatic increase in the application of T/ solutions in optoelectronicdevices has been observed such as diode lasers superluminescent diodes S%+> variousphotodetectors diode pumped solid state lasers +PSS> chargeDcoupled devices ""+s> focalplane arrays 0P-> and others.

The progress in applications is provided by advantages of T/ coolers N they are solid state haveno moving parts and are miniature highly reliable and flexible in design to meet particularre9uirements.

Histor

The effect of heating or cooling at the junctions of two different conductors exposed to thecurrent was named in a honor of the 0rench watchmaker 8ean Peltier )465N)6=5> whodiscovered it in )6?=. $t was found that if a current passes through the contacts of two dissimilar

7/22/2019 191419188 Thermoelectric Refrigeration System

26/39

conductors in a circuit a temperature differential appears between them. This briefly describedphenomenon is the basis of thermoelectricity and is applied actively in the soDcalledthermoelectric cooling modules.

$n contrast to the 8oule heating which is proportional to the s9uare of the current&

the Peltier heat p> varies as a linear function of the current and changes its sign with it&

where 9 is the charge that passes through the junction 9Y$t>; P is the Peltier coefficient whosevalue depends on the contacting materialsJ nature and the contact temperature. The common wayof presenting the Peltier coefficient is the following&

'ere Q D alpha is the Seebeck coefficient defined by both contacting materialsproperties and their temperature. T is the junction temperature in 1elvins.

Ther!oelectric *o'(le Constr(ction

- T/ module is a device composed of thermoelectric couples and PDtype semiconductor legs>that are connected electrically in series in parallel thermally and fixed by soldering sandwichedbetween two ceramic plates. The latter form the hot and cold thermoelectric cooler T/"> sides.The configuration of thermoelectric coolers is shown in 0igures .

7/22/2019 191419188 Thermoelectric Refrigeration System

27/39

"ommonly a T/ module consists of the following parts&

). Regular matrix of T/ elements N Pellets. 3sually such semiconductors as bismuthtelluride 7iTe> antimony telluride or their solid solutions are used. The semiconductorsare the best among the known materials due to a complex optimal T/ performance andtechnological properties. 7iTe material is the most typical for T/ cooler.

2."eramic plates N cold and warm and intermediate for multiDstage coolers> ceramic layersof a module. The plates provide mechanical integrity of a T/ module. They must satisfystrict re9uirements of electrical insulation from an object to be cooled and the heat sink.

The plates must have good thermal conductance to provide heat transfer with minimalresistance. The aluminum oxide -l@C?> ceramics is used most widely due to the optimalcostMperformance ratio and developed processing techni9ue. Cther ceramics types suchas aluminum nitride -l> and beryllium oxide 7eC> are also used. They have muchbetter thermal conductance N five to seven times more than -l@C? N but both are moreexpensive. $n addition 7eC technology is poisonous.

?. /lectric conductors provide serial electric contacting of pellets with each other andcontacts to leading wires. 0or most of the miniature T/ coolers the conductors are madeas thin films multilayer structure containing copper "u> as a conductor> deposited ontoceramic plates. 0or large si!e highDpower coolers they are made from "u tabs to reducethe resistance.

=. Solders provide assembling of the T/ module. The most standard solders used include%eadDTin PbDSn> -ntimonyDTin SnDSb> and (oldDTin -uDSn> alloys. The soldersmust provide good assembling of the T/ module. The melting point of a solder is the oneof limiting factors for T/ "ooler reflow processes and operating temperature. %eadingwires are connected to the ending conductors and deliver power from a direct current+"> electrical source.

7/22/2019 191419188 Thermoelectric Refrigeration System

28/39

A sin&le'sta&e m"dleconsists of one matrix of pellets and a pair of cold and warm sides see0igure )>.A mlti'sta&e m"dlecan be viewed as two see 0igure @> or more single stagesstacked on top of each other. The construction of a multiDstage module is usually of a pyramidaltype N each lower stage is bigger than the upper stage. Cnce the top stage is used for cooling thelower stage re9uires greater cooling capacity to pump heat that is dissipated from the upperstage.

Fig&%SingleDstage Thermoelectric "ooler "onstruction Fig& )TwoDstage Thermoelectric "ooler "onstruction

T"C$D -OFTW$R"

$t is not a simple task to select a T/ module that fits the need most optimally. $t is also fre9uentlynecessaryto give some operational characteristics of a T/ module for a range of conditions and in the graphic form.To assist in it the company RET has developed the software T/"cad.T/"cad is available in two version&

%& T"Cca' 3ite)& T"Cca' Pro

H"$T -IN4

- heat sink is a term for a component or assembly that transfers heat generated within a solid

material to a fluid medium such as air or a li9uid. /xamples of heat sinks are the heatexchangers used in refrigeration and air conditioning systems and the radiator also a heatexchanger> in a car. 'eat sinks also help to cool electronic and optoelectronic devices such ashigherDpower lasers and light emitting diodes %/+s>.

- heat sink uses its extendedsurfaces to increase the surface area in contact with the coolingfluid the air for example. The term is not meant literally as a heat sink does not have a #magicalability to absorb heat like a sponge and send it off to a parallel universe#. 'eat transfer theoryhelps explain practical aspects of how heat sinks work and can also help to clear up common

7/22/2019 191419188 Thermoelectric Refrigeration System

29/39

misconceptions and design mistakes. -pproach air velocity choice of material fin or otherprotrusion> design and surface treatment are some of the design factors which influence thethermal resistance i.e. thermal performance of a heat sink. Cne engineering application of heatsinks is in the thermal management of electronics often computer "P3 or graphics processors.0or these heat sink attachment methods and thermal interface materials also influence theeventual junction or dietemperature of the processors>. Theoretical experimental and numericalmethods can be used to determine a heat sinkAs thermal performance.

5(alit of *o(nting a Coole' Object onto a T"!o'(le an' a T"

*o'(le onto a Heat Rejecting -ste!

The efficiency of T/ subDmount as a whole depends on the 9uality of mounting an object to becooled onto a T/ module and a T/ module onto heat rejecting elements. 7ased on RETAs andother manufacturersA for example the company Eelcor> data the highest extent of T/ modulesfailure is due to an improper process of installation of objects on the module and the module onthe appropriate header. $t is necessary to take into account that at installation some problems aresolved namely&

Strong mechanical design steady at mechanical influence of various kinds while in servicevibration impacts>;

(ood thermal contact which is important both foran object located on the T/ module cold sideand for effective heat rejecting system.There are three widely applied methods that can be used for mounting&&)> Eechanical mounting@> Soldering?> -dhesive bonding.

7/22/2019 191419188 Thermoelectric Refrigeration System

30/39

/ach of the listed methods has its own areas of applications both advantages and short coming.2hen choosing an optimal way of mounting a T/ module and cooled objects it is necessary tobe guided by features of the methods.

*echanical 6Co!pression *etho'7

Description.- T/ module is placed betw