Thermoelectric cooling

THERMOELECTRIC REFRIGERATION

CERTIFICATE

This is to certify that Mr. DEBASIS RAY bearing roll no-…………

of +3 final year science (Physics Honors) of BJB Autonomous

College, Bhubaneswar, has successfully completed the dissertation

entitled “THERMOELECTRIC EFFECT” for the degree examination

of 2010-2013, which is based on actual report and under my guidance

and supervision.

Date:

Dr. INDIRA MISHRA

HEAD OF DEPARTMENT

DEPARTMENT OF PHYSICS

BJB AUTONOMOUS COLLEGE

A PROJECT REPORT- by- DEBASIS RAYPage 1


ACKNOWLEDGEMENT

I owe a great many thanks to a great many people who helped and supported me during the writing of this book.

My deepest thanks to Lecturer, Dr. (Mrs.)Indira Mishra, HOD Physics Dept. BJB Autonomous College the Guide of the project for guiding and correcting various documents of mine with attention and care. He has taken pain to go through the project and make necessary correction as and when needed.

My deep sense of gratitude to Dr. Arundhati

Mishra (Lecturer),Physics Dept. for her valuable support and guidance.

Thanks and appreciation to the helpful people at Department Of Physics, BJB AUTONOMOUS COLLEGE, for their support.

I would also thank my Institution and my faculty members without whom this project would have been a distant reality. I also



extend my heartfelt thanks to my family and well wishers.

Debasis Ray ……………

………….. Physics Dept. BJB (A) College

CONTENT INTRODUCTION HISTORICAL BACKGROUND

Why are TE Coolers Used for Cooling? Disadvantages Which Industries Use TE Cooling? What are Some Applications?

BASIC PRINCIPLES Seebeck Effect Peltier Effect Thomson Effect

THERMOELECTRIC PRINCIPLE OF OPERATION METHOD OF HEAT TRANSPORT



THERMAL ANALYSIS & PARAMETER NEEDED Figure Of Merit

MOISTURE AND VIBRATION Condensation Shock & Vibration

THERMOELECTRIC PERFORMANCE COMPARISON: CONVENTIONAL REFIGERATION SUMMARY BIBLIOGRAPHY

INTRODUCTIONThermoelectric are based on the Peltier Effect, The Peltier Effect is one of the three thermoelectric effects; the other two are known as the Seebeck Effect and Thomson Effect. Whereas the last two effects act on a single conductor, the Peltier Effect is a typical junction phenomenon. Thermoelectric coolers are solid state heat pumps used in applications where temperature stabilization, temperature cycling, or cooling below ambient are required. There are many products using thermoelectric coolers, including CCD cameras (charge coupled A PROJECT REPORT- by- DEBASIS RAYPage 4


device), laser diodes, microprocessors, blood analyzers and portable picnic coolers. This article discusses the theory behind the thermoelectric cooler, along with the thermal and electrical parameters involved.

Historical BackgroundAlthough commercial thermoelectric modules were not available until almost 1960, the physical principles upon which modern thermoelectric coolers are based actually date back to the early 1800s. The first important discovery relating to thermoelectricity occurred in 1821 when German scientist Thomas Seebeck found that an electric current would flow continuously in a closed circuit made up of two dissimilar metals, provided that the junctions of the metals were maintained at two A PROJECT REPORT- by- DEBASIS RAYPage 5


different temperatures. Seebeck did not actually comprehend the scientific basis for his discovery, however, and falsely assumed that flowing heat produced the same effect as flowing electric current.In 1834, a French watchmaker and part-time physicist, Jean Peltier, while investigating the Seebeck Effect, found that there was an opposite phenomenon where by thermal energy could be absorbed at one dissimilar metal junction and discharged at the other junction when an electric current flowed within the closed circuit. Twenty years later, William Thomson (eventually known as Lord Kelvin) issued a comprehensive explanation of the Seebeck and Peltier Effects and described their relationship. At the time, however, these phenomena were still considered to be mere laboratory curiosities and were without practical application. In the 1930s, Russian scientists began studying some of the earlier thermoelectric work in an effort to construct power generators for use at remote locations throughout their country. This Russian interest in thermoelectricity eventually caught the attention of the rest of the world and inspired the development of practical thermoelectric modules. Today's thermoelectric coolers make use of modern semiconductor technology in which doped semiconductor material takes the place of the dissimilar metals used in early thermoelectric experiments. The Seebeck,



Peltier and Thomson effects, together with several other phenomena, form the basis of functional thermoelectric modules.Why are TE Coolers Used for Cooling?

No moving parts make them very reliable; approximately 105 hrs. of operation 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 or weight is a constraint.

Ability to alternate between heating and cooling.

Excellent cooling alternative to vapor compression coolers for systems that are sensitive to mechanical vibration.

Maintenance Free. Sub-ambient cooling

Disadvantages



Able 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.

Which Industries Use TE Cooling? Electronic Medical Aerospace Telecommunications

What 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 several thousand 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

Basic PrinciplesSeebeck EffectThe conductors are two dissimilar metals denoted as material A and material B. The junction temperature at A is used as a reference and is maintained at a relatively cool temperature (TC). The junction temperature at B is used as temperature higher than temperature TC. With heat applied to junction B, a voltage (Eout) will appear across terminals A PROJECT REPORT- by- DEBASIS RAYPage 9


T1 and T2 and hence an electric current would flow continuously in this closed circuit. This voltage as shown in Figure .1, known as the Seebeck EMF, can be expressed as

Eout = α (TH – TC) ….(1)

Where:• α = dE / dT = α A – α B• α is the differential Seebeck coefficient or (thermo electric power coefficient) between the two materials, A and B, positive when the direction of electric current is same as the direction of thermal current, in volts per oK.• Eout is the output voltage in volts.• TH and TC are the hot and cold thermocouple temperature, respectively, in oK.



Figure 1 Seebeck effect

Peltier EffectPeltier found there was an opposite phenomenon to the Seebeck Effect, whereby thermal energy could be absorbed at one dissimilar metal junction and discharged at the other junction when an electric current flowed within the closed circuit. In Figure .2, the thermocouple circuit is modified to obtain a different configuration that illustrates the Peltier Effect, a phenomenon opposite that of the Seebeck Effect. If a voltage (Ein) is applied to terminals T1 and T2, an electrical current (I) will flow in the circuit. As a result of the current flow, a slight cooling effect (QC) will occur at thermocouple junction A (where heat is absorbed), and a heating effect (QH) will A PROJECT REPORT- by- DEBASIS RAYPage 11


occur at junction B (where heat is expelled). Note that this effect may be reversed whereby a change in the direction of electric current flow will reverse the direction of heat flow. Joule heating, having a magnitude of I2 x R (where R is the electrical resistance), also occurs in the conductors as a result of current flow. This Joule heating effect acts in opposition to the Peltier Effect and causes a net reduction of the available cooling. The Peltier effect can be expressed mathematically as

Qc or QH = β x I … (2)

= (α T) x IWhere:• β is the differential Peltier coefficient between the two materials A and B in volts.• I is the electric current flow in amperes.• QC and QH are the rates of cooling and heating, respectively, in watts.



Figure 2 Peltier effect

Peltier coefficient β has important effect on Thermoelectric cooling as following:a) β <0 ; Negative Peltier coefficient High energy electrons move from right to left. Thermal current and electric current flow in opposite directionsb) β >0 ; Positive Peltier coefficient High energy holes move from left to right. Thermal current and electric current flow in same direction.

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



Figure3. Effect of Peltier coefficient on cooling Process

Thomson EffectWilliam Thomson, who described the relationship between the two phenomena, later issued a more comprehensive explanation of the Seebeck and Peltier effects. When an electric current is passed through a conductor having a temperature gradient over its length, heat will be either absorbed by or expelled from the conductor. Whether heat is absorbed or expelled depends on the direction of both the electric current and temperature gradient. This phenomenon is known as the Thomson Effect.

Thermoelectric Principle of Operation



The typical thermoelectric module is manufactured using two thin ceramic wafers with a series of P and N doped bismuth-telluride semiconductor material sandwiched between them as shown in Figure. 4. The ceramic material on both sides of the thermoelectric add rigidity and the necessary electrical insulation. The N type material has an excess of electrons, while the P type material has a deficit of electrons. One P and one N make up a couple, as shown in Figure 5. The thermoelectric couples are electrically in series and thermally in parallel. A thermoelectric module can contain one to several hundred couples.

Figure.4 TEC Principle of operation



Figure .5 Cross section of a thermoelectric cooler

As the electrons move from the P type material to the N type material through an electrical connector, the electrons jump to a higher energy state absorbing thermal energy (cold side). Continuing through the lattice of material; the electrons flow from the N type material to the P type material through an electrical connector dropping to a lower energy state and releasing energy as heat to the heat sink (hot side). Thermoelectric can be used to heat and to cool, depending on the direction of the current. In an application requiring both heating and cooling, the design should focus on the cooling mode.A PROJECT REPORT- by- DEBASIS RAYPage 16


Using a thermoelectric in the heating mode is very efficient because all the internal heating (Julian heat) and the load from the cold side is pumped to the hot side. This reduces the power needed to achieve the desired heating.

Method of Heat Transport

Figure.6- Method Of Heat Transport

Applying a current (e- carriers) transports heat from the warmer junction to the cooler junction. Bismuth telluride (a semiconductor), is sandwiched between two conductors, usually copper. A semiconductor (called a pellet) is used because they can be optimized for pumping heat and because the type of charge carriers within them can be chosen. The semiconductor in this examples N type (doped with electrons) therefore, the electrons move towards the positive end of



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

Figure.6- Heat flow in a N-type Conductor

When a p-type semiconductor (doped with holes) is used instead,the holes move in a direction opposite the current flow. The heat is also transported in a direction opposite the current flow and in the direction of the holes. Essentially, the charge carriers dictate the direction of heat flow.

Figure.7- Heat Flow in a P-Type Conductor



Electrons can travel freely in the copper conductors but not so freely in the semiconductor. As the electrons leave the copper and enter the hot-side of the p-type, they must fill a "hole" in order 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 process. Then, as the electrons move from the p-type into the copper conductor on the cold side, the electrons are bumped back to a higher energy level and absorb heat in the process. Next, the electrons move freely through the copper until they reach the cold side of the n-type semiconductor. When the electrons move into the n-type, they must bump up an energy level in order to move through the semiconductor. Heat is absorbed when this occurs. Finally, when the electrons leave the hot-side of the n-type, they can move freely in the copper. They drop down to a lower energy level and release heat in the process. To increase heat transport, several p type or n type thermoelectric (TE)



components can be hooked up in parallel. However, the device requires low voltage and therefore, a large current which is too great to be commercially practical.

The TE components can be put in series but the heat transport abilities are diminished because the interconnecting between the semiconductors creates thermal shorting.



The most efficient configuration is where a p and n TE component is put electrically in series but thermally in parallel. The device to the right is called a couple. One side is attached to a heat source and the other a heat sink that convects the heat away. The side facing the heat source is considered the cold side and the side facing the heat sink the hot side.

Between the heat generating device and the conductor must be an electrical insulator to prevent an electrical short circuit between the module and the heat source. The electrical insulator must also have a high thermal conductivity so that the temperature gradient between the source



and the conductor is small. Ceramics like alumina are generally used for this purpose.

The most common devices use 254 alternating p and n type TE devices. The devices can operate at 12-16 V at 4-5 amps. These values are much more practical for real life operations.



Thermal Analysis and Parameters Needed:

The appropriate thermoelectric for an application, depends on at least three parameters. These parameters are the hot surface temperature (Th), the cold surface temperature (Tc), and the heat load to be absorbed at the cold surface (Qc).The hot side of the thermoelectric is the side where heat is released when DC power is applied. This side is attached to the heat sink. When using an air cooled heat sink (natural or forced convection) the hot side temperature and its heat transferred can be found by using Equations 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 the thermoelectric (watts).QC = the heat absorbed from the cold

side (watts).Pin = the electrical input power to the

thermoelectric (watts).COP = coefficient of performance of

the thermoelectric device, typically is between 0.4 and 0.7 for single stage applications.

Estimating QC, the heat load in watts absorbed from the cold side is difficult, because all thermal loads in the design must be considered. Among these thermal loads are:1. Active:i. I2R heat load from the electronic devicesii. Any load generated by a chemical reaction2. Passive:i. Radiation (heat loss between two close objects with different temperatures)ii. Convection (heat loss through the air, where the air has a different temperature than the object)



iii. Insulation lossesiv. Conduction losses (heat loss through leads, screws, etc.)v. Transient load (time required to change the temperature of an object)By energy balance across the hot and cold junction it produces

Qh = (α Th) x I – C (Th – Tc) + I2 R/2 …(6)

QC = (α Tc) x I – C (Th – Tc) - I2 R/2 …(7)R = RA + RBC = (kA+ kB) (A/L)To get the max the heat absorbed from the cold side (QC); by differentiate the Qc to the electric current I,

d Qc /d I = 0

Then it produces

Iopt. = α Tc /R

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



QC (max) = [(Z Tc 2)/2 – (Th – Tc)] C ….(8)

Where:Z = Figure of merit for the material A and

B = α2 / R C

The cold side of the thermoelectric is the side that gets cold when DC power is applied. This side may need to be colder than the desired temperature of the cooled object. This is especially true when the cold side is not in direct contact with the object, such as when cooling an enclosure. The temperature difference across the thermoelectric (ΔT) relates to Th and Tc according to Equation 9.

ΔT = Th - Tc …(9)

The thermoelectric performance curves in Figures 6 and 7 show the relationship between ΔT and the other parameters.



ΔT (OC)

Figures .6 Performance curve (ΔT vs. Voltage)

ΔT (OC)

Figures 7 Performance curve (ΔT vs. QC)



Figure of MeritThe figure of merit represents the quality of performance of a thermoelectric material, sometimes it is multiplied by temperature. It is defined as:

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

Note: Low electrical resistivity and thermal conductivity are required for high figure of merit. These values are temperature dependent therefore; the figure of merit is temperature dependent. P and N type material have different figures of merit and are averaged to determine a material’s overall quality.



Thermoelectric MaterialsSemiconductors are the optimum choice of material to sandwich between two metal conductors because of the ability to control the semiconductors’ charge carriers, as well as, increase the heat pumping ability.

The most commonly used semiconductor for electronics cooling applications is Bi2Te3

because of its relatively high figure of merit. However, the performance of this material is still relatively low and alternate materials



are being investigated with possibly better performance.

A plot of various p-type semiconductor figures of merit time’s temperature vs. temperature is shown. Within the temperature ranges concerned in electronics cooling (0-200C) Bi2Te3 performs the best.

Similar results are shown for n-type semiconductors:



Moisture and Vibration Effect:

Condensation: A common problem with TE cooling is that condensation may occur causing corrosion and eroding the TE’s inherent reliability. Condensation occurs when the dew point is reached. The dew point is the temperature to which air must be cooled at constant pressure for the water vapor to start to condense Condensation occurs because the air loses the ability to carry the water vapor that condenses. As the air’s temperature decreases its water vapor carrying capacity decreases. Since TE coolers can cool to low A PROJECT REPORT- by- DEBASIS RAYPage 31


and even below ambient temperatures, condensation is a problem. The most common sealant employed is silicon rubber.Shock and Vibration: Thermoelectric modules in various types of assemblies have for years been used in different Military/Aerospace applications. Thermoelectric devices have been successfully subjected to shock and vibration requirements for aircraft, ordinance, space vehicles, shipboard use and most other such systems. While a thermoelectric device is quite strong in both tension and compression, it tends to be relatively weak in shear. When in a sever shock or vibration environment, care should be taken in the design of the assembly to insure "compressive loading" of thermoelectric devices.

Thermoelectric Performance:TE performance depends on the following factors:



The temperature of the cold and hot sides.

Thermal and electrical conductivities of the device’s materials.

Contact resistance between the TE device and heat source/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)/2

The COP corresponding to the maximum heat pumping capacity is:



The current corresponding to the maximum heat pumping capacity is:

A simplified way of determining the voltage and the heat load are given by:

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

Thermoelectric Multistage (Cascaded) Devices:A multistage thermoelectric device should be used only where a single stage device does not fill the need. Given the hot side temperature, the cold side temperature and



the heat load, a suitable thermoelectric can be chosen. If ΔT across the thermoelectric is less than 55 °C, then a single stage thermoelectric is sufficient. The theoretical maximum temperature difference for a single stage thermoelectric is between 65 °C and 70 °C. If ΔT is greater than 55 °C, then a multistage thermoelectric should be considered. A multistage thermoelectric achieves a high ΔT by stacking as many as six or seven single stage thermoelectric on top of each other. The two important factors are ΔT and C.O.P. should effect on selection of the number of stages. The following Figure 17.9 depicts ΔT, vs. C.O.P. max, vs. Number of stages at Th = 35 oC.

Figure 17.9 ΔT vs. C.O.P. Max as a function of stages



There is another very significant factor that must always be considered and that is the cost. Usually, as the number of stages increase, so does the cost. Certain applications require a trade-off between C.O.P. and cost.

Comparison: Conventional Refrigeration:Because thermoelectric cooling is a form of solid-state refrigeration, it has the advantage of being compact and durable. A thermoelectric cooler uses no moving parts (except for some fans), and employs no fluids, eliminating the need for bulky piping and mechanical compressors used in vapor-cycle cooling systems. Such sturdiness allows thermoelectric cooling to be used where conventional refrigeration would fail. In a current application, a thermoelectric cold plate cools radio equipment mounted in a fighter jet wingtip. The exacting size and weight requirements, as well as A PROJECT REPORT- by- DEBASIS RAYPage 36


the extreme G-forces in this unusual environment, rule out the use of conventional refrigeration. Thermoelectric devices also have the advantage of being able to maintain a much narrower temperature range than conventional refrigeration. They can maintain a target temperature to within ±1° or better, while conventional refrigeration varies over several degrees. Unfortunately, modules tend to be expensive, limiting their use in applications that call for more than 1 kW/h of cooling power. Owing to their small size, if nothing else, there are also limits to the maximum temperature differential that can be achieved between one side of a thermoelectric module and the other. However, in applications requiring a higher ΔT, modules can be cascaded by stacking one module on top of another. When one module's cold side is another's hot side, some unusually cold temperatures can be achieved.

Summary



Although there are a variety of applications that use thermoelectric devices, all of them are based on the same principle. When designing a thermoelectric application, it is important that all of the relevant electrical and thermal parameters be incorporated into the design process. Once these factors are considered, a suitable thermoelectric device can be selected based on the guidelines presented in this project.

BIBLIOGRAPHY Springer Introduction to Thermoelectricity. Higher Education Enhancement Project

Fund’s – Electronic Cooling, Cairo University.

Wikipedia, the free encyclopedia




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