Thermoelectric cooling


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A Project Report on "Thermoelectric Effect"...

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Thermoelectric cooling

  1. 1. THERMOELECTRIC REFRIGERATIONA PROJECT REPORT- by- DEBASIS RAY Page 1CERTIFICATEThis is to certify that Mr. DEBASIS RAYbearing roll no-…………of +3 final year science (Physics Honors) of BJB AutonomousCollege, Bhubaneswar, has successfully completed the dissertationentitled “THERMOELECTRIC EFFECT” for the degree examinationof 2010-2013, which is based on actual report and under my guidanceand supervision.Date:Dr. INDIRA MISHRAHEAD OF DEPARTMENTDEPARTMENT OF PHYSICSBJB AUTONOMOUS COLLEGE
  2. 2. THERMOELECTRIC REFRIGERATIONA PROJECT REPORT- by- DEBASIS RAY Page 2ACKNOWLEDGEMENTI owe a great many thanks to a great many people whohelped and supported me during the writing of this book.My deepest thanks to Lecturer, Dr. (Mrs.)Indira Mishra, HODPhysics Dept. BJB Autonomous College the Guide of the projectfor guiding and correcting various documents of mine withattention and care. He has taken pain to go through the projectand make necessary correction as and when needed.My deep sense of gratitude to Dr. Arundhati Mishra(Lecturer),PhysicsDept. forhervaluablesupportandguidance.Thanks and appreciation to the helpful people at Department OfPhysics,BJBAUTONOMOUSCOLLEGE, fortheirsupport.I would also thank my Institution and my faculty memberswithout whom this project would have been a distant reality. Ialso extend my heartfelt thanks to my family and well wishers.Debasis Ray………………………..Physics Dept. BJB (A) College
  4. 4. THERMOELECTRIC REFRIGERATIONA PROJECT REPORT- by- DEBASIS RAY Page 4INTRODUCTIONThermoelectric are based on the Peltier Effect, The Peltier Effectis one of the three thermoelectric effects; the other two are knownas the Seebeck Effect and Thomson Effect. Whereas the last twoeffects act on a single conductor, the Peltier Effect is a typicaljunction phenomenon. Thermoelectric coolers are solid state heatpumps used in applications where temperature stabilization,temperature cycling, or cooling below ambient are required. Thereare many products using thermoelectric coolers, including CCDcameras (charge coupled device), laser diodes, microprocessors,blood analyzers and portable picnic coolers. This article discussesthe theory behind the thermoelectric cooler, along with the thermaland electrical parameters involved.
  5. 5. THERMOELECTRIC REFRIGERATIONA PROJECT REPORT- by- DEBASIS RAY Page 5Historical BackgroundAlthough commercial thermoelectric modules were not available untilalmost 1960, the physical principles upon which modern thermoelectriccoolers are based actually date back to the early 1800s. The firstimportant discovery relating to thermoelectricity occurred in 1821 whenGerman scientist Thomas Seebeck found that an electric current wouldflow continuously in a closed circuit made up of two dissimilar metals,provided that the junctions of the metals were maintained at twodifferent temperatures. Seebeck did not actually comprehend the scientificbasis for his discovery, however, and falsely assumed that flowing heatproduced the same effect as flowing electric current.In 1834, a French watchmaker and part-time physicist, Jean Peltier,while investigating the Seebeck Effect, found that there was an oppositephenomenon where by thermal energy could be absorbed at one dissimilarmetal junction and discharged at the other junction when an electriccurrent flowed within the closed circuit. Twenty years later, WilliamThomson (eventually known as Lord Kelvin) issued a comprehensiveexplanation of the Seebeck and Peltier Effects and described theirrelationship. At the time, however, these phenomena were still consideredto be mere laboratory curiosities and were without practical application.In the 1930s, Russian scientists began studying some of the earlierthermoelectric work in an effort to construct power generators for use atremote locations throughout their country. This Russian interest inthermoelectricity eventually caught the attention of the rest of the worldand inspired the development of practical thermoelectric modules. Todaysthermoelectric coolers make use of modern semiconductor technology inwhich doped semiconductor material takes the place of the dissimilarmetals used in early thermoelectric experiments. The Seebeck, Peltier andThomson effects, together with several other phenomena, form the basis offunctional thermoelectric modules.
  6. 6. THERMOELECTRIC REFRIGERATIONA PROJECT REPORT- by- DEBASIS RAY Page 6Why are TE Coolers Used for Cooling? No moving parts make them very reliable; approximately 105 hrs. ofoperation at 100 degrees Celsius, longer for lower temps(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,zero gravity and high G- levels Compact size makes them useful for applications where size orweight is aconstraint. Ability to alternate between heating and cooling. Excellent cooling alternative to vapor compression coolers forsystems that are sensitive to mechanical vibration. Maintenance Free. Sub-ambient coolingDisadvantages Able to dissipate limited amount of heat flux. Lower coefficient of performance than vapor-compressionsystems. Relegated to low heat flux applications. More total heat to remove than without a TEC.
  7. 7. THERMOELECTRIC REFRIGERATIONA PROJECT REPORT- by- DEBASIS RAY Page 7Which Industries Use TE Cooling? Electronic Medical Aerospace TelecommunicationsWhat are Some Applications? Electronic enclosures Laser diodes Laboratory instruments Temperature baths Refrigerators Telecommunications equipment Temperature control in missiles and space systems Heat transport ranges vary from a few milliwatts to severalthousand watts, however, since the efficiency of TE devices are low,smaller heat transfer applications are more practical. Thermoelectric Blankets used in medical for ICUs and NICUs
  8. 8. THERMOELECTRIC REFRIGERATIONA PROJECT REPORT- by- DEBASIS RAY Page 8Basic PrinciplesSeebeck EffectThe conductors are two dissimilar metals denoted as material A andmaterial B. The junction temperature at A is used as a reference andis maintained at a relatively cool temperature (TC). The junctiontemperature at B is used as temperature higher than temperatureTC. With heat applied to junction B, a voltage (Eout) will appearacross terminals T1 and T2 and hence an electric current would flowcontinuously in this closed circuit. This voltage as shown in Figure.1, known as the Seebeck EMF, can be expressed asEout = α (TH – TC) ….(1)Where:• α = dE / dT = α A – α B• α is the differential Seebeck coefficient or (thermo electric powercoefficient) between the two materials, A and B, positive whenthe direction of electric current is same as the direction of thermalcurrent, in volts per oK.• Eout is the output voltage in volts.• TH and TC are the hot and cold thermocouple temperature,respectively, in oK.
  9. 9. THERMOELECTRIC REFRIGERATIONA PROJECT REPORT- by- DEBASIS RAY Page 9Figure 1 Seebeck effectPeltier EffectPeltier found there was an opposite phenomenon to the SeebeckEffect, whereby thermal energy could be absorbed at one dissimilarmetal junction and discharged at the other junction when anelectric current flowed within the closed circuit. In Figure .2, thethermocouple circuit is modified to obtain a differentconfiguration that illustrates the Peltier Effect, a phenomenonopposite that of the Seebeck Effect. If a voltage (Ein) is applied toterminals T1 and T2, an electrical current (I) will flow in thecircuit. As a result of the current flow, a slight cooling effect (QC)will occur at thermocouple junction A (where heat is absorbed),and a heating effect (QH) will occur at junction B (where heat isexpelled). Note that this effect may be reversed whereby a changein the direction of electric current flow will reverse the direction ofheat flow. Joule heating, having a magnitude of I2 x R (where Ris the electrical resistance), also occurs in the conductors as a result
  10. 10. THERMOELECTRIC REFRIGERATIONA PROJECT REPORT- by- DEBASIS RAY Page 10of current flow. This Joule heating effect acts in opposition to thePeltier Effect and causesa net reduction of the available cooling.The Peltier effect can be expressed mathematically asQc or QH = β x I … (2)= (α T) x IWhere:• β is the differential Peltier coefficient between the twomaterials A and B in volts.• I is the electric current flow in amperes.• QC and QH are the rates of cooling and heating, respectively, inwatts.Figure 2 Peltier effectPeltier coefficient β has important effect on Thermoelectric cooling asfollowing:a) β <0 ; Negative Peltier coefficientHigh energy electrons move from right to left.Thermal current and electric current flow in opposite directionsb) β >0 ; Positive Peltier coefficient
  11. 11. THERMOELECTRIC REFRIGERATIONA PROJECT REPORT- by- DEBASIS RAY Page 11High energy holes move from left to right.Thermal current and electric current flow in same direction.a) -ve Peltier coefficient b)+ve Peltier coefficientFigure3. Effect of Peltier coefficient on cooling ProcessThomson EffectWilliam Thomson, who described the relationship between the twophenomena, later issued a more comprehensive explanation of theSeebeck and Peltier effects. When an electric current is passedthrough a conductor having a temperature gradient over its length,heat will be either absorbed by or expelled from the conductor.Whether heat is absorbed or expelled depends on the direction ofboth the electric current and temperature gradient. Thisphenomenon is known as the Thomson Effect.
  12. 12. THERMOELECTRIC REFRIGERATIONA PROJECT REPORT- by- DEBASIS RAY Page 12Thermoelectric Principle of OperationThe typical thermoelectric module is manufactured using two thinceramic wafers with a series of P and N doped bismuth-telluridesemiconductor material sandwiched between them as shown inFigure. 4. The ceramic material on both sides of the thermoelectricadd rigidity and the necessary electrical insulation. The N typematerial has an excess of electrons, while the P type material has adeficit of electrons. One P and one N make up a couple, as shownin Figure 5. The thermoelectric couples are electrically in series andthermally in parallel. A thermoelectric module can contain one toseveral hundred couples.Figure.4 TEC Principle of operation
  13. 13. THERMOELECTRIC REFRIGERATIONA PROJECT REPORT- by- DEBASIS RAY Page 13Figure .5 Cross section of a thermoelectric coolerAs the electrons move from the P type material to the N typematerial through an electrical connector, the electrons jump to ahigher energy state absorbing thermal energy (cold side).Continuing through the lattice of material; the electrons flow fromthe N type material to the P type material through an electricalconnector dropping to a lower energy state and releasing energy asheat to the heat sink (hot side).Thermoelectric can be used to heatand to cool, depending on the direction of the current. In anapplication requiring both heating and cooling, the design shouldfocus on the cooling mode.Using a thermoelectric in the heating mode is very efficientbecause all the internal heating (Julian heat) and the load from thecold side is pumped to the hot side. This reduces the power neededto achieve the desired heating.
  14. 14. THERMOELECTRIC REFRIGERATIONA PROJECT REPORT- by- DEBASIS RAY Page 14Method of Heat TransportFigure.6- Method Of Heat TransportApplying a current (e- carriers) transports heat from thewarmer junction to the cooler junction. Bismuth telluride (asemiconductor),is sandwiched between two conductors, usuallycopper. A semiconductor (called a pellet) is used because theycan be optimized for pumping heat and because the type ofcharge carriers within them can be chosen. The semiconductor inthis examples N type (doped with electrons) therefore, theelectrons move towards the positive end of the battery. Thesemiconductor is soldered to two conductive materials, likecopper. When the voltage is applied heat is transported in thedirection of current flow.
  15. 15. THERMOELECTRIC REFRIGERATIONA PROJECT REPORT- by- DEBASIS RAY Page 15Figure.6- Heat flow in a N-type ConductorWhen a p-type semiconductor (doped with holes) is usedinstead,the holes move in a direction opposite the current flow.The heat is also transported in a direction opposite the currentflow and in the direction of the holes. Essentially, the chargecarriers dictate the direction of heat flow.Figure.7- Heat Flow in a P-Type ConductorElectrons can travel freely in the copper conductors but not sofreely in the semiconductor. As the electrons leave the copperand enter the hot-side of the p-type, they must fill a "hole" inorder to move through the p-type. When the electrons fill a hole,they drop down to a lower energy level and release heat in the
  16. 16. THERMOELECTRIC REFRIGERATIONA PROJECT REPORT- by- DEBASIS RAY Page 16process.Then, as the electrons move from the p-type into thecopper conductor on the cold side, the electrons are bumped backto a higher energy level and absorb heat in the process. Next, theelectrons move freely through the copper until they reach thecold side of the n-type semiconductor. When the electrons moveinto the n-type, they must bump up an energy level in order tomove through the semiconductor. Heat is absorbed when thisoccurs. Finally, when the electrons leave the hot-side of the n-type, they can move freely in the copper. They drop down to alower energy level and release heat in the process. To increaseheat transport, several p type or n type thermoelectric (TE)components can be hooked up in parallel.However, the devicerequires low voltage and therefore, a large current which is toogreat to be commercially practical.The TE components can be put in series but the heat transportabilities are diminished because the interconnecting between thesemiconductors creates thermal shorting.
  17. 17. THERMOELECTRIC REFRIGERATIONA PROJECT REPORT- by- DEBASIS RAY Page 17The most efficient configuration is where a p and n TEcomponent is put electrically in series but thermally in parallel.The device to the right is called a couple.One side is attached toa heat source and the other a heat sink that convects the heataway.The side facing the heat source is considered the cold sideand the side facing the heat sink the hot side.Between the heat generating device and the conductor must bean electrical insulator to prevent an electrical short circuitbetween the module and the heat source. The electrical insulator
  18. 18. THERMOELECTRIC REFRIGERATIONA PROJECT REPORT- by- DEBASIS RAY Page 18must also have a high thermal conductivity so that thetemperature gradient between the source and the conductor issmall. Ceramics like alumina are generally used for this purpose.The most common devices use 254 alternating p and n type TEdevices. The devices can operate at 12-16 V at 4-5 amps. Thesevalues are much more practical for real life operations.
  19. 19. THERMOELECTRIC REFRIGERATIONA PROJECT REPORT- by- DEBASIS RAY Page 19Thermal Analysis and ParametersNeeded:The appropriate thermoelectric for an application, depends on atleast three parameters. These parameters are the hot surfacetemperature (Th), the cold surface temperature (Tc), and the heatload to be absorbed at the cold surface (Qc).The hot side of the thermoelectric is the side where heat is releasedwhen DC power is applied. This side is attached to the heat sink.When using an air cooled heat sink (natural or forced convection)the hot side temperature and its heat transferred can be found byusing Equations 3 and 4.Th = Tamb + θ Qh ….(3)Where:Th = the hot side temperature (°C).Tamb = the ambient temperature (°C).θ = Thermal resistance of heat exchanger (°C/watt).AndQh = QC + Pin … (4)COP = QC / Pin … (5)Where:Qh = the heat released to the hot side of thethermoelectric (watts).
  20. 20. THERMOELECTRIC REFRIGERATIONA PROJECT REPORT- by- DEBASIS RAY Page 20QC = the heat absorbed from the cold side (watts).Pin = the electrical input power to the thermoelectric(watts).COP = coefficient of performance of the thermoelectricdevice, typically is between 0.4 and 0.7 for single stageapplications.Estimating QC, the heat load in watts absorbed from the cold sideis difficult, because all thermal loads in the design must beconsidered. Among these thermal loads are:1. Active:i. I2R heat load from the electronic devicesii. Any load generated by a chemical reaction2. Passive:i. Radiation (heat loss between two close objects with differenttemperatures)ii. Convection (heat loss through the air, where the air has adifferent temperature than the object)iii. Insulation lossesiv. Conduction losses (heat loss through leads, screws, etc.)v. Transient load (time required to change the temperature of anobject)By energy balance across the hot and cold junction it producesQh = (α Th) x I – C (Th – Tc) + I2 R/2 …(6)QC = (α Tc) x I – C (Th – Tc) - I2 R/2 …(7)R = RA + RBC = (kA+ kB) (A/L)
  21. 21. THERMOELECTRIC REFRIGERATIONA PROJECT REPORT- by- DEBASIS RAY Page 21To get the max the heat absorbed from the cold side (QC); bydifferentiate the Qc to the electric current I,d Qc /d I = 0Then it producesIopt. = α Tc /RSubstitute for Iopt. In Equation 17.7 to get the max the heatabsorbed from the cold sideQC (max) = [(Z Tc2)/2 – (Th – Tc)] C ….(8)Where:Z = Figure of merit for the material A and B= α2 / R CThe cold side of the thermoelectric is the side that gets cold whenDC power is applied. This side may need to be colder than thedesired temperature of the cooled object. This is especially truewhen the cold side is not in direct contact with the object, such aswhen cooling an enclosure. The temperature difference across thethermoelectric (ΔT) relates to Th and Tc according to Equation 9.ΔT = Th - Tc …(9)The thermoelectric performance curves in Figures 6 and 7 show therelationship between ΔT and the other parameters.
  22. 22. THERMOELECTRIC REFRIGERATIONA PROJECT REPORT- by- DEBASIS RAY Page 22ΔT (OC)Figures .6 Performance curve (ΔT vs. Voltage)ΔT (OC)
  23. 23. THERMOELECTRIC REFRIGERATIONA PROJECT REPORT- by- DEBASIS RAY Page 23Figures 7 Performance curve (ΔT vs. QC)Figure of MeritThe figure of merit represents the quality of performance of athermoelectric material, sometimes it is multiplied bytemperature. It is defined as:Where ρ is the electrical resistivity, k is the thermalconductivity, and is the Seebeck Coefficient.Note: Low electrical resistivity and thermal conductivity arerequired for high figure of merit. These values are temperaturedependent therefore; the figure of merit is temperaturedependent. P and N type material have different figures ofmerit and are averaged to determine a material’s overall quality.
  24. 24. THERMOELECTRIC REFRIGERATIONA PROJECT REPORT- by- DEBASIS RAY Page 24Thermoelectric MaterialsSemiconductors are the optimum choice of material to sandwichbetween two metal conductors because of the ability to controlthe semiconductors’ charge carriers, as well as, increase the heatpumping ability.The most commonly used semiconductor for electronics coolingapplications is Bi2Te3 because of its relatively high figure ofmerit. However, the performance of this material is stillrelatively low and alternate materials are being investigatedwith possibly better performance.A plot of various p-type semiconductor figures of merit time’stemperature vs. temperature is shown. Within the temperatureranges concerned in electronics cooling (0-200 C) Bi2Te3performs the best.
  25. 25. THERMOELECTRIC REFRIGERATIONA PROJECT REPORT- by- DEBASIS RAY Page 25Similar results are shown for n-type semiconductors:
  26. 26. THERMOELECTRIC REFRIGERATIONA PROJECT REPORT- by- DEBASIS RAY Page 26Moisture and Vibration Effect:Condensation: A common problem with TE cooling is thatcondensation may occur causing corrosion and eroding the TE’sinherent reliability. Condensation occurs when the dew point isreached. The dew point is the temperature to which air must becooled at constant pressure for the water vapor to start tocondense Condensation occurs because the air loses the ability tocarry the water vapor that condenses. As the air’s temperaturedecreases its water vapor carrying capacity decreases.Since TEcoolers can cool to low and even below ambient temperatures,condensation is a problem. The most common sealant employed issilicon rubber.Shock and Vibration:Thermoelectric modules in varioustypes of assemblies have for years been used in differentMilitary/Aerospace applications. Thermoelectric devices have beensuccessfully subjected to shock and vibration requirements foraircraft, ordinance, space vehicles, shipboard use and most othersuch systems. While a thermoelectric device is quite strong in bothtension and compression, it tends to be relatively weak in shear.When in a sever shock or vibration environment, care should betaken in the design of the assembly to insure "compressive loading"of thermoelectric devices.
  27. 27. THERMOELECTRIC REFRIGERATIONA PROJECT REPORT- by- DEBASIS RAY Page 27Thermoelectric Performance:TE performance depends on the following factors:The temperature of the cold and hot sides.Thermal and electrical conductivities of the device’smaterials.Contact resistance between the TE device and heatsource/heat sink.Thermal resistance of the heat sink.The current yielding the maximum COP is given by:The maximum COP is:Where Tm= (TH+TC)/2The COP corresponding to the maximum heat pumping capacity is:
  28. 28. THERMOELECTRIC REFRIGERATIONA PROJECT REPORT- by- DEBASIS RAY Page 28The current corresponding to the maximum heat pumping capacityis:A simplified way of determining the voltage and the heat load aregiven by:Where V is the voltage and Qc is the heat load, N is the number ofcouples, and L is the element height.Thermoelectric Multistage (Cascaded) Devices:A multistage thermoelectric device should be used only where asingle stage device does not fill the need. Given the hot sidetemperature, the cold side temperature and the heat load, asuitable thermoelectric can be chosen. If ΔT across thethermoelectric is less than 55 °C, then a single stage thermoelectricis sufficient. The theoretical maximum temperature difference for asingle stage thermoelectric is between 65 °C and 70 °C. If ΔT isgreater than 55 °C, then a multistage thermoelectric should beconsidered. A multistage thermoelectric achieves a high ΔT bystacking as many as six or seven single stage thermoelectric on topof each other. The two important factors are ΔT and C.O.P. should
  29. 29. THERMOELECTRIC REFRIGERATIONA PROJECT REPORT- by- DEBASIS RAY Page 29effect on selection of the number of stages. The following Figure17.9 depicts ΔT, vs. C.O.P.max, vs. Number of stages at Th = 35oC.Figure 17.9 ΔT vs. C.O.P. Max as a function of stagesThere is another very significant factor that must always beconsidered and that is the cost. Usually, as the number of stagesincrease, so does the cost. Certain applications require a trade-offbetween C.O.P. and cost.
  30. 30. THERMOELECTRIC REFRIGERATIONA PROJECT REPORT- by- DEBASIS RAY Page 30Comparison: Conventional Refrigeration:Because thermoelectric cooling is a form of solid-state refrigeration, it hasthe advantage of being compact and durable. A thermoelectric cooler usesno moving parts (except for some fans), and employs no fluids, eliminatingthe need for bulky piping and mechanical compressors used in vapor-cyclecooling systems. Such sturdiness allows thermoelectric cooling to be usedwhere conventional refrigeration would fail. In a current application, athermoelectric cold plate cools radio equipment mounted in a fighter jetwingtip. The exacting size and weight requirements, as well as theextreme G-forces in this unusual environment, rule out the use ofconventional refrigeration. Thermoelectric devices also have the advantageof being able to maintain a much narrower temperature range thanconventional refrigeration. They can maintain a target temperature towithin ±1° or better, while conventional refrigeration varies over severaldegrees. Unfortunately, modules tend to be expensive, limiting their use inapplications that call for more than 1 kW/h of cooling power. Owing totheir small size, if nothing else, there are also limits to the maximumtemperature differential that can be achieved between one side of athermoelectric module and the other. However, in applications requiring ahigher ΔT, modules can be cascaded by stacking one module on top ofanother. When one modules cold side is anothers hot side, some unusuallycold temperatures can be achieved.
  31. 31. THERMOELECTRIC REFRIGERATIONA PROJECT REPORT- by- DEBASIS RAY Page 31SummaryAlthough there are a variety of applications that usethermoelectric devices, all of them are based on the same principle.When designing a thermoelectric application, it is important thatall of the relevant electrical and thermal parameters beincorporated into the design process. Once these factors areconsidered, a suitable thermoelectric device can be selected basedon the guidelines presented in this project.BIBLIOGRAPHYSpringer Introduction to Thermoelectricity.Higher Education Enhancement Project Fund’s – ElectronicCooling, Cairo University.Wikipedia, the free encyclopedia