Thermoelectric Cooling Ravi

8/3/2019 Thermoelectric Cooling Ravi

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Essentials of Thermoelectric (TE)

CoolingWith an Emphasis in Thermal Control of

Electronics -- Widah Saied

http://www.tetech.com/modules/


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OutlineIntroduction and PurposeWhy use TE coolersDisadvantagesWhich industries use TE cooling and their applications.

Basic PrinciplesSemiconductor P and N Type DopingTE Design

Figure of` meritThermoelectric materials

CondensationTE performanceDesign methodologyImproving performance

TE Electronics Cooling Applications

TE Cooling Alternatives


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Why are TE Coolers Used for Cooling?No moving parts make them very reliable; approximately 10 5hrs of operation at 100 degrees Celsius, longer for lowertemps (Goldsmid,1986). Ideal when precise temperature control is required.Ability to lower temperature below ambient.Heat transport controlled by current input.Able to operate in any orientation.Compact size make them useful for applications where size or

weight is a constraint.Ability to alternate between heating and cooling.Excellent cooling alternative to vapor compression coolersfor systems that are sensitive to mechanical vibration.

(TE Tecnology, Inc., http://www.tetech.com/techinfo/)


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DisadvantagesAble to dissipate limited amount of heat flux.Lower coefficient of performance than vapor-

compression systems.Relegated to low heat flux applications.More total heat to remove than without a TEC.

(Simons and Chu, 2000)


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Which Industries Use TE Cooling?ElectronicMedical

AerospaceTelecommunications



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What are Some Applications?Cooling:Electronic enclosuresLaser diodesLaboratory instrumentsTemperature bathsRefrigeratorsTelecommunications equipmentTemperature control in missiles and space systemsHeat transport ranges vary from a few milliwatts to severalthousand watts, however, since the efficiency of TEdevices are low, smaller heat transfer applications are more

practical.(TE Tecnology, Inc.,http://www.tetech.com/techinfo/)


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Basic Principles Peltier Effect- when a voltage or DC current is appliedto two dissimilar conductors, a circuit can be created thatallows for continuous heat transport between theconductors junctions. The Seebeck Effect - is the reverseof the Peltier Effect. By applying heat to two differentconductors a current can be generated. The Seebeck Coefficient is given by:

where is the electric field.

The current is transported through charge carriers(opposite the hole flow or with electron flow).Heat transfer occurs in the direction of charge carriermovement. (Tellurex, www.tellurex.com)

dxdT x

/


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Basic Principles

Applying a current (e - carriers) transports heatfrom the warmer junction to the cooler junction.

(Tellurex, www.tellurex.com)


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Basic PrinciplesA typical thermoelectric cooling component is shown on thenext slide. Bismuth telluride (a semiconductor), is sandwichedbetween two conductors, usually copper. A semiconductor(called a pellet) is used because they can be optimized forpumping heat and because the type of charge carriers withinthem can be chosen. The semiconductor in this examples Ntype (doped with electrons) therefore, the electrons movetowards the positive end of the battery.

The semiconductor is soldered to two conductive materials,like copper. When the voltage is applied heat is transportedin the direction of current flow.



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Basic Principles



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Basic PrinciplesWhen a p type semiconductor (doped with holes) isused instead, the holes move in a direction oppositethe current flow. The heat is also transported in adirection opposite the current flow and in the directionof the holes. Essentially, the charge carriers dictate thedirection of heat flow.



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Method of Heat TransportElectrons can travel freely in the copper conductors but not so freely inthe semiconductor.As the electrons leave the copper and enter the hot-side of the p-type, theymust fill a "hole" in order to move through the p-type. When the electronsfill a hole, they drop down to a lower energy level and release heat in theprocess.Then, as the electrons move from the p-type into the copper conductor onthe cold side, the electrons are bumped back to a higher energy level andabsorb heat in the process.Next, the electrons move freely through the copper until they reach thecold side of the n-type semiconductor. When the electrons move into then-type, they must bump up an energy level in order to move through thesemiconductor. Heat is absorbed when this occurs.Finally, when the electrons leave the hot-side of the n-type, they can movefreely in the copper. They drop down to a lower energy level and releaseheat in the process.

http://www.tetech.com/techinfo/#1


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Basic PrinciplesTo increase heat transport, several p type or n typethermoelectric(TE) components can be hooked up inparallel.

However, the device requires low voltage and therefore,a large current which is too great to be commerciallypractical.



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Basic PrinciplesThe TE components can be put in series butthe heat transport abilities are diminished

because the interconnectings between thesemiconductor creates thermal shorting.



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Basic PrinciplesThe most efficientconfiguration is where a p and nTE component is putelectrically in series but

thermally in parallel . Thedevice to the right is called acouple.One side is attached to a heatsource and the other a heat sink that convects the heat away.The side facing the heat sourceis considered the cold side andthe side facing the heat sink thehot side.

(TE Tecnology, Inc. , http://www.tetech.com/techinfo/)


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Basic PrinciplesBetween the heat generating device and the conductor mustbe an electrical insulator to prevent an electrical short circuitbetween the module and the heat source.

The electrical insulator must also have a high thermalconductivity so that the temperature gradient between thesource and the conductor is small.Ceramics like alumina are generally used for this purpose.(Rowe, 1995).


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Basic PrinciplesThe most common devices use 254 alternating p and ntype TE devices.The devices can operate at 12-16 V at 4-5 amps. Thesevalues are much more practical for real life operations.



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Basic PrinciplesAn entire assembly:

(Tellurex., http://www.tellurex.com/12most.html)


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Semiconductor Doping: N TypeN doped semiconductors have an abundant number of extra electrons to use ascharge carriers. Normally, a group IV material (like Si) with 4 covalent bonds(4 valence electrons) is bonded with 4 other Si. To produce an N typesemiconductor, Si material is doped with a Group V metal (P or As) having 5

valence electrons, so that an additional electron on the Group V metal is freeto move and are the charge carriers (Wikipedia, http://en.wikipedia.org/wiki/Semiconductor ).

http://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Semiconductor


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Semiconductor Doping: P TypeFor P type semiconductors, the dopants are Group III (In, B) which have3 valence electrons, these materials need an extra electron for bondingwhich creates holes. P doped semiconductors are positive chargecarriers. Theres an appearance that a hole is moving when there is a

current applied because an electron moves to fill a hole, creating a newhole where the electron was originally. Holes and electrons move inopposite directions. (Wikipedia, http://en.wikipedia.org/wiki/Semiconductor ).

http://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Semiconductor


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Figure of MeritThe figure of merit represents the quality of performance of athermoelectric material, sometimes it is multiplied bytemperature. It is defined as:

Where is the electrical resistivity, k is the thermal conductivity,and is the Seebeck Coefficient.

Note: low electrical resistivity and thermal conductivity arerequired for high high figure of merit. These values aretemperature dependent therefore, the figure of merit istemperature dependent. P and N type material have differentfigures of merit and are averaged to determine a materialsoverall quality.

k Z

(Nolas et al., 2001)


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Thermoelectric MaterialsSemiconductors are the optimum choice of material tosandwich between two metal conductors because of theability to control the semiconductors charge carriers, as well

as, increase the heat pumping ability.



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Thermoelectric MaterialsThe most commonly used semiconductor for electronicscooling applications is Bi 2Te 3 because of its relatively highfigure of merit. However, the performance of this material is

still relatively low and alternate materials are beinginvestigated with possibly better performance.Alternative materials include:

Alternating thin film layers of Sb 2Te 3 and Bi 2Te 3.

Lead telluride and its alloysSiGeMaterials based on nanotechnology

(Sales, 2002)


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Thermoelectric MaterialsA plot of various p-type semiconductor figures of merit timestemperature vs. temperature are shown. Within the temperatureranges concerned in electronics cooling (0-200 C) Bi 2Te 3 performsthe best.

(Snyder, J. http://www.its.caltech.edu/~jsnyder/thermoelectrics/science_page.htm)

zT for p-type thermoelectric materials


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Thermoelectric MaterialsSimilar results are shown for n-typesemiconductors

(Snyder, J. http://www.its.caltech.edu/~jsnyder/thermoelectrics/science_page.htm)


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Bi2Te

3Properties

Below is a plot of the figure of merit (Z), Seebeck coefficient,electrical resistivity, and thermal conductivity, as a functionof temperature for Bi 2Te 3. Carrier concentration will alter the

values below.

(Yazawa, 2005)


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Bi2Te

3Properties

Bi2Te 3 figure of merit as a function of tellurium concentration.

(Nolas et. Al, 2001)


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Thermoelectric MaterialsMetals are used to sandwich thesemiconductor. Because the TE performance

is also dependent on these materials, anoptimal material must be chosen, usuallycopper.


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CondensationA common problem with TE cooling is thatcondensation may occur causing corrosion anderoding the TEs inherent reliability.

Condensation occurs when the dew point is reached.The dew point is the temperature to which air mustbe cooled at constant pressure for the water vapor tostart to condenseCondensation occurs because the air loses the abilityto carry the water vapor that condenses. As the airstemperature decreases its water vapor carryingcapacity decreases.


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CondensationSince TE coolers can cool to low and even belowambient temperatures, condensation is a problem.The most common sealant employed is siliconrubber (Nagy, 1997).

Research has been performed to determine the mosteffective sealing agent used to protect the chip fromwater.


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CondensationFour sealants were used to seal a TE cooling device and the weight gaindue to water entering the device measured. The best sealants should havethe lowest weight gain. The epoxy has virtually no weight gain.

(Nagy, 1997)


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CondensationAccording to the previous results, it seems that the epoxyis the best sealant. These results are verified by thepublished permeability data showing the epoxy having the

lowest permeability (vapor transmission rate) of all thesealants.

(Nagy, 1997)


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Thermoelectric PerformanceTE performance depends on the following factors:

The temperature of the cold and hot sides.Thermal and electrical conductivities of the devices materials.

Contact resistance between the TE device and heat source/heat sink.Thermal resistance of the heat sink.

(Chein and Huang, 1994)


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Thermoelectric PerformanceThe current yielding the maximum COP is givenby:

The maximum COP is:

]1)1[(

))((2 / 1

m

chn p

ZT R

T T I

]1)ZT1)[(TT(]T / T)ZT1[(T

WQ

2 / 1m12

122 / 1

m1

in

cma x

(Goldsmid,1986).

Where T m= (T H+T C)/2


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Thermoelectric PerformanceThe COP corresponding to the maximum heatpumping capacity is:

The current corresponding to the maximum

heat pumping capacity is:

c H

c H cq T ZT

T T ZT )(2 / 1 2

R

T I cn pq

)(

(Goldsmid,1986)


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Coefficient of PerformanceA typical AC unit has a COP of approximately 3. TE coolers usually have

COPs below 1; 0.4 to 0.7 is a typical range.


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Coefficient of PerformanceBelow are COP values plotted versus the ratio of input current tothe modules Imax specification. Each line corresponds with aconstant DT/DTmax (the ratio of the required temperaturedifference to the module's max temperature difference

specification).



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Thermoelectric Performance

(Nolas et al.,2001)


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Thermoelectric PerformanceA simplified way of determining the voltage andthe heat load are given by:

Where V is the voltage and Q c is the heat load, N is the number of couples, and L is the element height.

A IRL

T T N V ch )(2


R I T T K IT Qc c H cn p 22 / 1)()(


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Design MethodologyChein and Huang (1994) suggest thefollowing method to design and analyze a TE

cooler with a heat sink.


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Design MethodologyThere are various ways to design a TE cooler. Each companyhas a different methodology to design one that meets a givenspecification. Most companies have performance, current,

temperature etc. data for their specific coolers which can beused for design. For example, the following websites byTellurex ( www.Tellurex.com ) and Mollar(http://www.marlow.com/TechnicalInfo/themoelectric_cooler_selection_p.htm ) have examples of their methodolgies. Thenext several sides displays Mollars design method. Note: or performance curve (or Figure 2) refers to the figure on thenext slide.
http://www.tellurex.com/http://www.marlow.com/TechnicalInfo/themoelectric_cooler_selection_p.htmhttp://www.marlow.com/TechnicalInfo/themoelectric_cooler_selection_p.htmhttp://www.marlow.com/TechnicalInfo/themoelectric_cooler_selection_p.htmhttp://www.marlow.com/TechnicalInfo/themoelectric_cooler_selection_p.htmhttp://www.tellurex.com/


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Mollar Design Method

http://www.marlow.com/TechnicalInfo/themoelectric_cooler_selection_p.htm
http://www.marlow.com/TechnicalInfo/themoelectric_cooler_selection_p.htmhttp://www.marlow.com/TechnicalInfo/themoelectric_cooler_selection_p.htm


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Note: maximum Q refers to the maximum heat the thermoelectric device can pump which will occur when DT=0.


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Mollar Design MethodFor this example, let us assumemaximum efficiency is desired.Thus, the 5.6 amp, 8.2 voltcooler is selected, becausebetween these two potential

TECs, its Qmax (30 watts) isclosest to the optimum Qmax(36 watts).


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TE Technology, Inc.Some companies such as TE Technology, Inc. will offersoftware (sometimes online applets) that allow the user toinput certain design criteria and it will output the kind of TEcooler that will satisfy the criteria.TE Technology, Incs. software (shown in the next couple of slides), allows the user to input the internal and externaltemperatures, as well as, the dimensions of the heatgenerating device and its heat production.

Among other things, the program will output the amount of power input to the TE cooler, its dimensions and the type of TE device that the company has available that will fulfill thedesign requirements.


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TE Technology, Inc. InputsTE Technology Incs. program can be reached athttp://www.tetech.com/design/3081.shtml

The figure on the left explains the numbers referred to in thefigure on the right.


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TE Technology, Inc. OutputsHere are some outputs from thecompanys program


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Improving TE performanceVarious methods have been used to improvethe performance of TE coolers which are its

major drawback.Examples: thin film coolers or multistage(bulk) coolers.


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Thin Film CoolersThin films are material layers of about 1 micrometer thickness.Alternating layers of Sb 2Te 3 and Bi 2Te 3 are used to produce thin filmTE coolers. An example is shown below where the highest powercomponents are mounted on a diamond substrate which would be the

top or cold side substrate of a thin film TE cooler. Power densitieswere reported to be above 100W/cm 2.

(Simons and chu,2000)


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Thin Film CoolersThin film coolers considerably reduce the size of TEdevices. Because the cooling density of a Peltier cooler isinversely proportional to its length, scaling to smaller sizeis desirable. A comparison of sizes are shown below.

(Lasance and Simmons, 2005, http://www.electronics-cooling.com/html/2005_nov_article2.html )
http://www.electronics-cooling.com/html/2005_nov_article2.htmlhttp://www.electronics-cooling.com/html/2005_nov_article2.htmlhttp://www.electronics-cooling.com/html/2005_nov_article2.htmlhttp://www.electronics-cooling.com/html/2005_nov_article2.html


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Multistage ModulesWhen the desired temperature differential between thecold and hot side cannot be obtained with a single stagemodule, or when the cold side temperature must belower than a one stage cooler will allow, a multistagemodule may need to be applied.Multistage modules are essentially single stage modulesstacked up in a vertical pyramid-shaped array (see nextslide).As the number of stages increases, the minimum coldside temperature will decrease (Rowe, 1995) .Also, increasing the number of stages increases thecoefficient of performance for a given cold sidetemperature (Nolas et al.,2001).


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Multistage Modules

Increasing the number of stages increases the coefficientof performance for a given coldside temperature, as seen in the

figure on the right (Goldsmid,1986).

(Goldsmid,1964)


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Multistage ModulesThe coefficient of performance of amultistage module is given by:

Where is the coefficient of performance of one stage of the module and N is the numberof stages (Goldsmid,1986).


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Comparison of Various TE CoolersThe Figure below compares the three types of coolers bulk (multistage), thin film, and current.

(Simons and chu,2000)


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Improving PerformanceMore exotic TE devices are being researchedthat could result in better performance such

as, superlattice structures, quantum wires andquantum wells, thin films using SiGe/Si, andthermionic cooling. However, research inthese are preliminary and are not inwidespread use.


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Temperature stabilityTE cooling provides high degrees of temperaturestability because the amount of cooling it provides isproportional to the applied current.The reported temperature stability of a TE device hasbeen .0003 degrees celcius but considerable efforthad to be used for this level of stability.Several factors are involved in the temperaturestability:

The controller and its resolution.The response time of the specific cooling assemblyThe response time of the object being cooled.



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TE Cooling of ElectronicsTE cooling devices are favorable in electronics coolingsystems because of their high reliability, flexibility inpackaging and integration, low weight and ability to maintaina low junction temperature, even below ambient temperature.

Also, other cooling devices that can fit the tiny spacesrequired for electronics cooling, such as, a capillary loop heator a miniature scale vapor compression refrigerator are notcommercially available.

Disadvantages of these devices are the limit to their coolingcapacity limit and coefficient of performance which may berestrictive in the future when heat transfer demands becomemuch larger.

(Chein and Huang, 1994)


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TE Cooling of ElectronicsTypical TE cooling schemes have a TE deviceattached to a heat source (the cold side) that

transports heat to a heat sink (the warm side).



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TE Cooling of ElectronicsWithout a heat sink it is difficult to get anadequate T but with a good airflow the heat

sink size can be reduced.

A DC power supply is needed for the TE

cooler.


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TE Cooling of ElectronicsIBM has used a Multi Chip Module(MCM) todetermine under what conditions TE coolingenhances performance.

(Simons and chu, 2000)


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TE Cooling of ElectronicsUnder 300 W TE aircooled MCM performbetter (lower chip

temperature) thanw/out a TE cooler.

Under 400W TE water

cooled MCM performbetter than w/our a TEcooler.



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TE Cooling of ElectronicsIBM used a device shown below to cool a wafer. ATE module is placed below the wafer and heat isexpelled to liquid flowing below. The TE module isable to precisely control the temperature of thewafer.



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TE Cooling of ElectronicsIn this application, amemory array is beingcooled using a heat

exchanger with sixthermoelectric modulessandwiched between twoparallel plate fin heat sink assemblies. Therequirements for thissystem was a cooling airtemperature of 30 3 C.



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TE Cooling of ElectronicsLaser modules are used as the transmitters infiber-optic telecommunications networks. TEcoolers are used to maintain the laser chip at aconstant temperature typically, 25degreesCelsius (Rowe,1995).


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Alternative Method of TE CoolingThe heat produced by a computer chip can beused to provide the electricity to run a fan thatcools the chip. The fan uses a TE device

operating on the Seebeck Effect to convert theheat to electricity.

When a laptop is running on batteries, theelectricity used to power the fan comes from thebattery. Therefore, to conserve battery life, athermoelectric power generator is a good

alternative.(Bar-Cohen et al., 2005)


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Alternative Method of TE CoolingA design such as the one below may be used.

(Bar-Cohen et al., 2005)


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ReferencesBar-Cohen, A., Solbrekken G. L., and Yazawa, K. (2005). Thermoelectric Powered Convective Cooling of Microprocessors. IEEE Transactions of Advanced Packaging, 28(2).Chein, R. and Huang, G. (2004). Thermoelectric cooler application in electronic cooling. Applied Thermal Engineering , 24 (14-15),pp. 2207-2217.Goldsmid H. (1986). Electronic Refrigeration. London:Pion.Goldsmid H.(1964). Thermoelectric Refrigeration. New York:Plenum. Lasance, C.J.M., and Simmons, R.E. (2005) Advances In High-Performance Cooling For Electronics. Electronics Cooling. RetrievedMay2006. http://www.electronics-cooling.com/html/2005_nov_article2.html Mollar(2003). Themoelectric Cooler Selection Procedure. Retieved June 2006.http://www.marlow.com/TechnicalInfo/themoelectric_cooler_selection_p.htm Nagy, J. (1997). The Effectiveness of Water Vapor Sealing Agents When Used in Application With Thermoelectric Cooling Modules.16 th International Conference on Thermoelectrics.Nolas, G.S. Goldsmid H., and Sharp J. (2001). Thermoelectrics : basic principles and new materials developments. New York:Springer.Rowe, D.M. (1995). CRC Handbook of Thermoelectrics. Boca Raton, FL: CRC Press.Sales, Brian. (February 2002). Thermoelectric Materials: Smaller is Cooler.. Science (Vol. 295. no. 5558, pp. 1248 1249). RetrievedApril 2006. http://www.sciencemag.org/cgi/content/full/295/5558/1248 .Simons, R. E. and Chu, R. C. (2000) Application of thermoelectric cooling to electronic equipment: A review and analysis. Annual

IEEE Semiconductor Thermal Measurement and Management Symposium, pp1-9.Snyder, J. The Science and Materials behind Thermoelectrics. Caltech-JPL Thermoelectrics Website. Retrieved April 2006.Tellurex. (2002). Retrieved May 2006. http://www.tellurex.com Tellurex. (2002). The 12 Most Frequently Asked Questions About Themoelectric Cooling. Retrieved May 2006.http://www.tellurex.com/12most.html TE Technology, Inc. (2005) Cold Plate/Solid. Free Design Service. Retrieved June 2006. http://www.tetech.com/design/3081.shtml TE Technology, Inc. Retrieved May 2006. http://www.tetech.com/techinfo/ TE Technology, Inc. (2005). Thermoelectric Modules. Retrieved April 2006. http://www.tetech.com/modules/
http://www.electronics-cooling.com/html/2005_nov_article2.htmlhttp://www.marlow.com/TechnicalInfo/themoelectric_cooler_selection_p.htmhttp://www.sciencemag.org/cgi/content/full/295/5558/1248http://www.tellurex.com/http://www.tellurex.com/12most.htmlhttp://www.tetech.com/design/3081.shtmlhttp://www.tetech.com/techinfo/http://www.tetech.com/modules/http://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Semiconductorhttp://www.tetech.com/modules/http://www.tetech.com/techinfo/http://www.tetech.com/design/3081.shtmlhttp://www.tellurex.com/12most.htmlhttp://www.tellurex.com/http://www.sciencemag.org/cgi/content/full/295/5558/1248http://www.marlow.com/TechnicalInfo/themoelectric_cooler_selection_p.htmhttp://www.electronics-cooling.com/html/2005_nov_article2.htmlhttp://www.electronics-cooling.com/html/2005_nov_article2.htmlhttp://www.electronics-cooling.com/html/2005_nov_article2.html

Thermoelectric Cooling Ravi

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