waste water utilization

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Contents

1. Introduction to Project.

2. Engine.

3. Engine operation.

4. Thermoelectric Effect.

5. Basic on which project is based.

6. Theory of the project.

7. Thomson effect.

8. Figure of merit.

9. Types of thermocouple.

10. Advantages and Disadvantages.


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What new to do with waste heat in Automobiles?

If you have a lot of heat, then you can do what Powerplants do you can use the heat to generate steam, and

use the steam to spin a turbine. The turbine can drive agenerator, which produces electricity. This setup is verycommon, but it requires a fair amount of equipment andspace.

If you would like to generate electricity from heat in asimple way that has no moving parts, this usuallyinvolves thermocouples.

Thermocouples take advantage of an electrical effectthat occurs at junctions between different metals. Forexample, take two iron wires and one copper wire. Twistone end of the copper wire and one end of one of the ironwires together. Do the same with the other end of thecopper wire and the other iron wire. If you heat one ofthe twisted junctions (perhaps with a match) and attachthe two free ends to a volt meter, you will be able to

measure a voltage. Similarly, if you hook the two ironwires to a battery one junction will get hot and the otherwill get cold.


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ENGINE

The internal combustion engine is an engine inwhich the combuston of fuel and an oxidizer(typically air) occurs in a confined space called acombustion chamber. This exothermic reactioncreates gases at high tempreture and, pressure

which are permitted to expand. Internalcombustion engines are defined by the useful workthat is performed by the expanding hot gasesacting directly to cause the movement of solidparts of the engine.

The term Internal Combustion Engine (ICE) is oftenused to refer to an engine in which combustion is

intermittent, such as a Wankel engine or areciprocating piston engine in which there iscontrolled movement of pistons, cranks, cams, orrods. However, continuous combustion enginessuch as jet engines, most rockets, and many gasturbines are also classified as types of internalcombustion engines. This contrasts with externalcombustion engines such as steam engines and

stirling engines that use a separate combustionchamber to heat a separate working fluid whichthen in turn does work, for example, by moving apiston or a turbine.


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A huge number of different designs for internalcombustion engines exist, each with differentstrengths and weaknesses. Although they're usedfor many different purposes, internal combustion

engines particularly see use in mobile applicationssuch as cars, aircraft, and even handheldapplications: all where their ability to use anenergy-dense fuel (especiallyfossil fuels) to delivera high power-to-weight ratio is particularlyadvantageous.

Applications

The motion of internal combustion engines isusually performed by the controlled movement ofpistons, cranks, rods, rotors, or even the entireengine itself.

Internal combustion engines are most commonly

used for mobile propulsion in vehicles and portablemachinery. In mobile equipment, internalcombustion is advantageous since it can providehigh power-to-weight ratios together withexcellent fuelenergy density. Generally using fossilfuel(mainly petroleum), these engines haveappeared in transport in almost all vehicles(automobiles, trucks, motorcycles, boats, and in a

wide variety of aircrafts and locomotives). Thesevehicles, when they are not hybrid, are called All-Petroleum Internal Combustion Engine Vehicles(APICEVs) or All Fossil Fuel Internal CombustionVehicles (AFFICEVs).


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Internal combustion engines appear in the form ofgas turbines as well where a very high power isrequired, such as injet aircraft, helicoptores, and

large ships. They are also frequently used forelectric generator and by industry.

Operation


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Four-stroke cycle (or Otto cycle)1.Intake2.compression3.power4. exhaust

Basic process

Internal combustion engines have 4 basic steps:

Intake

Combustible mixtures are emplaced in thecombustion chamber

Compression

The mixtures are placed under pressure

Combustion/Expansion

The mixture is burnt, almost invariably adeflagration, although a few systems involve
http://en.wikipedia.org/wiki/Four-stroke_cyclehttp://en.wikipedia.org/wiki/Image:4-Stroke-Engine.gifhttp://en.wikipedia.org/wiki/Four-stroke_cycle


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detonation. The hot mixture is expanded, pressingon and moving parts of the engine and performinguseful work.

Exhaust

The cooled combustion products are exhausted

Many engines overlap these steps in time, jetengines do all steps simultaneously at differentparts of the engines. Some internal combustionengines have extra steps.

Combustion

All internal combustion engines depend on theexothermic chemical process ofcombustion: thereaction of afuel, typically with oxygen from theairalthough other oxidizers such as nitrous oxidemay be employed. The combustion processtypically results in the production of a great

quantity of heat, as well as the production ofsteam and carbon dioxide and other chemicals atvery high temperature; the temperature reached isdetermined by the chemical make up of the fueland oxidisers .

The most common modern fuels are made up ofhydrocarbons and are derived mostly fromfossilfuels. Because of this, vehicles that uses this

energy are called All-Fossil Fuel InternalCombustion Engine Vehicles (AFFICEVs). Fossilfuels include dieselfuel, gasoline and petrolieumgas, and the rarer use ofpropane. Except for thefuel delivery components, most internalcombustion engines that are designed for gasoline


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use can run on natural gas or liquefied petroleumgases without

major modifications. Liquid and gaseousbiofuels

such as ethanol and biodiesel (a form of dieselfuel that is produced from crops that yieldtriglycerides such as soybean oil), can also beused. Some engines with appropriatemodifications can also run on hydrogengas.

All internal combustion engines must achieveignition in their cylinders to create combustion.Typically engines use either a spark ignition(SI)method or a compression ignition(CI)system. In the past, other methods using hot tubesor flames have been used.

Gasoline Ignition Process

Gasoline engine ignition systems generally rely ona combination of a lead-acid battery and an

induction coil to provide a high-voltage electricalspark to ignite the air-fuel mix in the engine'scylinders. This battery is recharged duringoperation using an electricity-generating devicesuch as an alternator or generator driven by theengine. Gasoline engines take in a mixture of airand gasoline and compress it to not more than 185psi, then use a spark plug to ignite the mixture

when it is compressed by the piston head in eachcylinder.

Diesel Ignition Process

Diesel engines and HCCI engines, rely solely onheat and pressure created by the engine in its


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compression process for ignition. The compressionlevel that occurs is usually twice or more than agasoline engine. Diesel engines will take in aironly, and shortly before peak compression, a small

quantity of diesel fuel is sprayed into the cylindervia a fuel injector that allows the fuel to instantlyignite. HCCI type engines will take in both air

and fuel but continue to rely on an unaided auto-

combustion process, due to higher pressures andheat. This is also why diesel and HCCI engines aremore susceptible to cold-starting issues, althoughthey will run just as well in cold weather oncestarted. Light duty diesel engines in automobilesand light trucks employ glow plugs that pre-heatthe combustion chamber just before starting toreduce no-start conditions in cold weather. Most

diesels also have a battery and charging system;nevertheless, this system is secondary and isadded by manufacturers as a luxury for the ease ofstarting, turning fuel on and off (which can also bedone via a switch or mechanical apparatus), andfor running auxiliary electrical components andaccessories. Most new engines rely on electricaland electronic control systems that also control

the combustion process to increase efficiency andreduce emissions.

Measures of engine performance


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Engine types vary greatly in a number of differentways:

Energy effeciency fuel/propellant consumption (brake specific

fuel consumtion for shaft engines, thrustspecific fuel consumption for jet engines)

power to weight ratio thrust to weight ratio torque curves(for shaft engines)

Energy Efficiency

Once ignited and burnt, the combustion productshot gaseshave more available thermal energythan the original compressed fuel-air mixture(which had higherchemical energy). The availableenergy is manifested as high tempreture andpressure that can be translated into work by theengine. In a reciprocating engine, the high-pressure gases inside the cylinders drive theengine's pistons.

Once the available energy has been removed, theremaining hot gases are vented (often by openinga valve or exposing the exhaust outlet) and thisallows the piston to return to its previous position(top dead center, or TDC). The piston can thenproceed to the next phase of its cycle, which varies


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between engines. Any heat that isn't translatedinto work is normally considered a waste productand is removed from the engine either by an air orliquid cooling system.

Engine efficiency can be discussed in a number ofways but it usually involves a comparison of thetotal chemical energy in the fuels, and the usefulenergy abstracted from the fuels in the form ofkinetic energy. The most fundamental and abstractdiscussion of engine efficiency is thethermodynamic limit for abstracting energy from

the fuel defined by a thermodynamic cycle. Themost comprehensive is the empirical fuel economyof the total engine system for accomplishing adesired task; for example, the miles per gallonaccumulated.

Internal combustion engines are primarily heatengines and as such the phenomenon that limitstheir efficiency is described by

thermodynamic cycles. None of these cyclesexceed the limit defined by the Carnot cycle whichstates that the overall efficiency is dictated by thedifference between the lower and upper operatingtemperatures of the engine. A terrestrial engine isusually and fundamentally limited by the upperthermal stability derived from the material used tomake up the engine. All metals and alloyseventually melt or decompose and there is


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significant researching into ceramic materials thatcan be made with higher thermal stabilities anddesirable structural properties. Higher thermalstability allows for greater temperature difference

between the lower and upper operatingtemperaturesthus greater thermodynamicefficiency.

The thermodynamic limits assume thatthe engine is operating in ideal conditions. Africtionless world, ideal gases, perfect insulators,and operation at infinite time. The real world is

substantially more complex and all thecomplexities reduce the efficiency. In addition, realengines run best at specific loads and rates asdescribed by their power curve. For example, a carcruising on a highway is usually operatingsignificantly below its ideal load, because theengine is designed for the higher loads desired forrapid acceleration. The applications of engines are

used as contributed drag on the total systemreducing overall efficiency, such as windresistance designs for vehicles. These and manyother losses result in an engines' real-world fueleconomy that is usually measured in the units ofmiles per gallon (or kilometers per liter) forautomobiles. The miles in, "MPG" represents ameaningful amount of work and the volume ofhydrocarbon implies a standard energy content.

Most steel engines have a thermodynamic limit of37%. Even when aided with turbochargers andstock efficiency aids, most engines retain anaverage efficiency of about 18%-20%.


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There are many inventionsconcerned with increasing the efficiency of IC-Engines. In general, practical engines are alwayscompromised by trade-offs between different

properties such as efficiency, weight, power, heat,response, exhaust emissions, or noise. Sometimeseconomy also plays a role in not only in the cost ofmanufacturing the engine itself, but alsomanufacturing and distributing the fuel. Increasingthe engines' efficiency brings better fuel economybut only if the fuel cost per energy content is thesame.

Seebeck effect

The See beck effect is the conversion oftemperature differences directly intoelectricity.

Seebeck discovered that acompass needle would be deflectedwhen a closed loop was formed of twometals joined in two places with atemperature difference between thejunctions. This is because the metalsrespond differently to the temperaturedifference, which creates a current loop,which produces a magnetic field.Seebeck, however, at this time did not


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recognize there was an electric currentinvolved, so he called the phenomenonthe thermomagnetic effect, thinking that

the two metals became magneticallypolarized by the temperature gradient.The Danish physicist Hans Christiarsted played a vital role in explainingand conceiving the term"thermoelectricity".

The effect is that a voltage, thethermoelectric EMF, is created in thepresence of a temperature differencebetween two different metals orsemiconductors. This causes acontinuous current in the conductors ifthey form a complete loop. The voltagecreated is of the order of several

microvolts per kelvin difference. Onesuch combination, copper-constantan,has a Seebeck coefficient of 41microvolts per kelvin at roomtemperature.


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In the circuit:

(which can be in several different

configurations and be governed by thesame equations), the voltage developedcan be derived from:

SA and SB are the Seebeck coefficients(also called thermoelectric powerorthermopower) of the metals A and B as afunction of temperature, and T1 and T2are the temperatures of the twojunctions. The Seebeck coefficients

are non-linear as a function oftemperature, and depend on the
http://en.wikipedia.org/wiki/Image:Seebeck_effect_circuit_2.png


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conductors' absolute temperature,material, and molecular structure. If theSeebeck coefficients are effectively

constant for the measured temperaturerange, the above formula can beapproximated as:

The Seebeck effect is commonly used ina device called a thermocouple (becauseit is made from a coupling or junction ofmaterials, usually metals) to measure atemperature difference directly or tomeasure an absolute temperature bysetting one end to a known temperature.Several thermocouples when connected

in series are called a thermopile, whichis sometimes constructed in order toincrease the output voltage since thevoltage induced over each individualcouple is small.

This is also the principle at work behind

thermal diodes and thermoelectricgenerators (such as radioisotopethermoelectric generators or RTGs)which are used for creating power fromheat differentials.


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The Seebeck effect is due to two effects:charge carrier diffusion andphonon drag(described below). If both connections

are held at the same temperature, butone connection is periodically openedand closed, an AC voltage is measured,which is also temperature dependent.

This application of the Kelvin probe issometimes used to argue that theunderlying physics only needs onejunction. And this effect is still visible ifthe wires only come close, but do not

touch, thus no diffusion is needed.

Thermopower

The thermopower, or thermoelectricpower, or Seebeck coefficient of a

material measure the magnitude of aninduced thermoelectric voltage inresponse to a temperature differenceacross that material. The thermopowerhas units of (V/ K), though in practice it


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is more common to use microvolts perkelvin. Values in the hundreds of V/K, negative or positive, are typical of good

thermoelectric materials. The termthermopower is a misnomer since itmeasures the voltage or electric fieldinduced in response to a temperaturedifference, not the electric power. Anapplied temperature difference causescharged carriers in the material, whetherthey are electrons or holes, to diffusefrom the hot side to the cold side,similar to a classical gas that expandswhen heated. Mobile charged carriersmigrating to the cold side leave behindtheir oppositely charged and immobile

nuclei at the hot side thus giving rise toa thermoelectric voltage (thermoelectricrefers to the fact that the voltage iscreated by a temperature difference).Since a separation of charges alsocreates an electric potential, the buildupof


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charged carriers onto the cold sideeventually ceases at some maximumvalue since there exists an equal amount

of charged carriers drifting back to thehot side as a result of the electric fieldat equilibrium. Only an increase in thetemperature difference can resume abuildup of more charge carriers on thecold side and thus lead to an increase inthe thermoelectric voltage. Incidentallythe thermopower also measures theentropy per charge carrier in thematerial. To be more specific, the partialmolar electronic heat capacity is said toequal the absolute thermoelectric powermultiplied by the negative of Faraday's

constant.

The thermopower of a material,represented by S (or sometimes by ),depends on the material's temperature

and crystal structure. Typically metalshave small thermopowers because mosthave half-filled bands. Electrons(negative charges) and holes (positivecharges) both contribute to the induced


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thermoelectric voltage thus cancelingeach other's contribution to that voltageand making it small. In contrast,

semiconductors can be doped with anexcess amount of electrons or holes andthus can have large positive or negativevalues of the thermopower depending onthe charge of the excess carriers. Thesign of the thermopower can determinewhich charged carriers dominate theelectric transport in both metals andsemiconductors.

If the temperature difference Tbetween the two ends of a material issmall, then the thermopower of amaterial is defined (approximately) as:

and a thermoelectric voltage Vis seenat the terminals.

This can also be written in relation to theelectric field E and the temperature


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gradient , by the approximateequation:

In practice one rarely measures theabsolute thermopower of the material ofinterest. This is because electrodesattached to a voltmeter must be placedonto the material in order to measure

the thermoelectric voltage. Thetemperature gradient then also typicallyinduces a thermoelectric voltage acrossone leg of the measurement electrodes.Therefore the measured thermopowerincludes a contribution from thethermopower of the material of interestand the material of the measurementelectrodes.

The measured thermopower is then acontribution from both and can bewritten as:


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Superconductors have zero

thermopower since the charged carriersproduce no entropy. This allows a directmeasurement of the absolutethermopower of the material of interest,since it is the thermopower of the entirethermocouple as well. In addition, ameasurement of the Thomson

coefficient, , of a material can alsoyield the thermopower through the

relation:

The thermopower is an importantmaterial parameter that determines the

efficiency of a thermoelectric material. Alarger induced thermoelectric voltage fora given temperature gradient will lead toa larger efficiency. Ideally one wouldwant very large thermopower valuessince only a small amount of heat is thennecessary to create a large voltage. This

voltage can then be used to providepower.

Charge-carrier diffusion


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Charge carriers in the materials(electrons in metals, electrons and holesin semiconductors, ions in ionic

conductors) will diffuse when one end ofa conductor is at a different temperaturethan the other. Hot carriers diffuse fromthe hot end to the cold end, since thereis a lower density of hot carriers at thecold end of the conductor. Cold carriersdiffuse from the cold end to the hot endfor the same reason.

If the conductor were left to reach

thermodynamic equilibrium, this processwould result in heat being distributedevenly throughout the conductor. Themovement of heat (in the form of hotcharge carriers) from one end to theother is called a heat current. As chargecarriers are moving, it is also an

electrical current.

In a system where both ends are kept ata constant temperature difference (aconstant heat current from one end to


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the other), there is a constant diffusionof carriers. If the rate of diffusion of hotand cold carriers in opposite directions

were equal, there would be no netchange in charge. However, the diffusingcharges are scattered by impurities,imperfections, and lattice vibrations(phonons). If the scattering is energydependent, the hot and cold carriers willdiffuse at different rates. This creates ahigher density of carriers at one end ofthe material, and the distance betweenthe positive and negative chargesproduces a potential difference; anelectrostatic voltage.

This electric field, however, opposes theuneven scattering of carriers, and anequilibrium is reached where the netnumber of carriers diffusing in onedirection is canceled by the net numberof carriers moving in the oppositedirection from the electrostatic field.

This means the thermopower of amaterial depends greatly on impurities,imperfections, and structural changes(which often vary themselves withtemperature


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andzelectric field), and the thermopowerof a material is a collection of manydifferent effects.

Early thermocouples were metallic, butmany more recently developedthermoelectric devices are made from

alternating p-type and n-typesemiconductor elements connected bymetallic interconnects as pictured in thefigures below. Semiconductor junctionsare especially common in powergeneration devices, while metallic

junctions are more common intemperature measurement. Charge flowsthrough the n-type element, crosses ametallic interconnect, and passes intothe p-type element. If a power source isprovided, the thermoelectric device mayact as a cooler, as in the figure to the

left below. This is the Peltier effect,described in the next section. Electronsin the n-type element will move oppositethe direction of current and holes in thep-type element will move in the direction


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of current, both removing heat from oneside of the device. If a heat source isprovided, the thermoelectric device may

function as a power generator, as in thefigure to the right below. The heatsource will drive electrons in the n-typeelement toward the cooler region, thuscreating a current through the circuit.Holes in the p-type element will thenflow in the direction of the current. Thecurrent can then be used to power aload, thus converting the thermal energyinto electrical energy.
http://en.wikipedia.org/wiki/Image:Thermoelectric_Generator_Diagram.svghttp://en.wikipedia.org/wiki/Image:Thermoelectric_Cooler_Diagram.svg


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Much research in thermoelectric materialshas focused on increasing the Seebeck

coefficient and reducing the thermalconductivity, especially by manipulatingthe nanostructure of the materials Peltiereffect

This effect bears the name of Jean-Charles Peltier(a french physicist) who discovered in 1834, thecalorific effect of an electrical current at thejunction of two different metals. When a current I ismade to flow through the circuit, heat is evolved atthe upper junction (at T2), and absorbed at thelower junction (at T1). The Peltier heat absorbed bythe lower junction per unit time, is equal to

Where is the Peltier coefficient AB of the entirethermocouple, and A and B are the coefficients ofeach material. P-type silicon

typically has a positive Peltier coefficient (thoughnot above ~550 K), and n-type silicon is typicallynegative, as the names suggest.


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The Peltier coefficients represent how much heatcurrent is carried per unit charge through a givenmaterial. Since charge current must be continuous

across a junction, the associated heat flow willdevelop a discontinuity ifA and B are different.This causes a non-zero divergence at the junctionand so heat must accumulate or deplete there,depending on the sign of the current. Another wayto understand how this effect could cool a junctionis to note that when electrons flow from a region ofhigh density to a region of low density, theyexpand (as with an ideal gas) and cool.

The conductors are attempting to return to theelectron equilibrium that existed before the currentwas applied by absorbing energy at one connectorand releasing it at the other. The individual couplescan be connected in series to enhance the effect.

An interesting consequence of this effect is thatthe direction of heat transfer is controlled by thepolarity of the current; reversing the polarity willchange the direction of transfer and thus the signof the heat absorbed/evolved.

A Peltier cooler/heater or thermoelectric heat

pump is a solid-state active heat pump whichtransfers heat from one side of the device to theother. Peltier cooling is also called thermo-electriccooling (TEC).


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Thomson effect

The Thomson effect was predicted andsubsequently experimentally observed by William

Thomson (Lord Kelvin) in 1851. It describes theheating or cooling of a current-carrying conductorwith a temperature gradient.

Any current-carrying conductor (except for asuperconductor), with a temperature differencebetween two points, will either absorb or emit heat,depending on the material.

If a current densityJ is passed through ahomogeneous conductor, heat production per unitvolume is:

where is the resistivity of the material

dT/dxis the temperature gradient along the wire

is the Thomson coefficient.


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The first term J is simply the Joule heating, whichis not reversible.

The second term is the Thomson heat, which

changes sign whenJ changes direction.

In metals such as zinc and copper, which have ahotter end at a higher potential and a cooler end ata lower potential, when current moves from thehotter end to the colder end, it is moving from ahigh

to a low potential, so there is an evolution of heat.This is called the positive Thomson effect.

In metals such as cobalt, nickel, and iron, which

have a cooler end at a higher potential and ahotter end at a lower potential, when currentmoves from the hotter end to the colder end, it ismoving from a low to a high potential, there is anabsorption of heat. This is called the negativeThomson effect.

The Thomson coefficient is unique among the threemain thermoelectric coefficients because it is theonly thermoelectric coefficient directly measurablefor individual materials. The Peltier and Seebeckcoefficients can only be determined for pairs ofmaterials. Thus, there is no direct experimental


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method to determine an absolute Seebeckcoefficient (i.e. thermopower) or absolute Peltiercoefficient for an individual material. However, as

mentioned elsewhere in this article there are twoequations, the Thomson relations, also known asthe Kelvin relations (see below), relating the threethermoelectric coefficients. Therefore, only one canbe considered unique.

If the Thomson coefficient of a material ismeasured over a wide temperature range,

including temperatures close to zero, one can thenintegrate the Thomson coefficient over thetemperature range using the Kelvin relations todetermine the absolute (i.e. single-material) valuesfor the Peltier and Seebeck coefficients. Inprinciple, this need only be done for one material,since all other values can be determined by

measuring pairwise Seebeck coefficients inthermocouples containing the reference materialand then adding back the absolute thermoelecricpower (thermopower) of the reference material.

It is commonly asserted that lead has a zeroThomson effect. While it is true that thethermoelectric coefficients of lead are small, theyare in general non-zero. The Thomson coefficient oflead has been measured over a wide temperature


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range and has been integrated to calculate theabsolute thermoelectric power (thermopower) oflead as a function of temperature.

Unlike lead, the thermoelectric coefficients of allknown superconductors are zero.

The Thomson relationships

The Seebeck effect is actually a combination of thePeltier and Thomson effects. In fact, in 1854

Thomson found two relationships, now called theThomson or Kelvin relationships, between thecorresponding coefficients. The absolutetemperature T, the Peltier coefficient andSeebeck coefficient S are related by the firstThomson relation

which predicted the Thomson effect before it wasactually formalized. These are related to theThomson coefficient by the second Thomsonrelation

Thomson's theoretical treatment of

thermoelectricity is remarkable in the fact that it isprobably the first attempt to develop a reasonabletheory of irreversible thermodynamics (non-equilibrium thermodynamics). This occurred atabout the time that Clausius, Thomson, and others


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were introducing and refining the concept ofentropy.

Figure of merit

The figure of meritfor thermoelectric devices isdefined as

,

where is the electrical conductivity, is thethermal conductivity, and S is the Seebeckcoefficient or thermopower (conventionally in

V/K). This is more commonly expressed as the dimensionless figure of meritZTby multiplying it

with the average temperature ((T2 + T1) / 2).Greater values of ZT indicate greaterthermodynamic efficiency, subject to certainprovisions, particularly the requirement that thetwo materials of the couple have similarZvalues.ZTis therefore a very convenient figure forcomparing the potential efficiency of devices using

different materials. Values ofZT=1 are consideredgood, and values of at least the 34 range areconsidered to be essential for thermoelectrics tocompete with mechanical generation and


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refrigeration in efficiency. To date, the bestreportedZTvalues have been in the 23 range.

thermocouples

In electrical engineering and industry,thermocouples are a widely used type oftemperature sensor and can also be usedas a means to convert thermal potentialdifference into electric potentialdifference. They are cheap and

interchangeable, have standardconnectors, and can measure a wide rangeof temperatures. The main limitation isaccuracy; System errors of less than oneKelvin (K) can be difficult to achieve.[ [


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Principle of operations

In 1821, the GermanEstonian physicist ThomasJohann Seebeck discovered that when anyconductor (such as a metal) is subjected to athermal gradient, it will generate a voltage. This isnow known as the thermoelectric effect or Seebeckeffect. Any attempt to measure this voltagenecessarily involves connecting another conductor

to the "hot" end. This additional conductor will thenalso experience the temperature gradient, anddevelop a voltage of its own which will oppose theoriginal. Fortunately, the magnitude of the effectdepends on the metal in use. Using a dissimilarmetal to complete the circuit creates a circuit inwhich the two legs generate different voltages,

leaving a small difference in voltage available formeasurement. That difference increases withtemperature, and can typically be between 1 and70 microvolts per degree Celsius (V/C) for themodern range of available metal combinations.Certain combinations have become popular asindustry standards, driven by cost, availability,convenience, melting point, chemical properties,


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stability, and output. This coupling of two metalsgives the thermocouple its name.

Thermocouples measure the temperature

difference between two points, not absolutetemperature. In traditional applications, one of thejunctionsthe cold junctionwas maintained at aknown (reference) temperature, while the otherend was attached to a probe.

Having available a known temperature cold

junction, while useful for laboratory calibrations, issimply not convenient for most directly connectedindicating and control instruments. Theyincorporate into their circuits an artificial coldjunction using some other thermally sensitivedevice, such as a thermistor or diode, to measurethe temperature of the input connections at theinstrument, with special care being taken tominimize any temperature gradient betweenterminals. Hence, the voltage from a known coldjunction can be simulated, and the appropriatecorrection applied. This is known as cold junctioncompensation.

Additionally, a device can perform cold junction

compensation by computation. It can translatedevice voltages to temperatures by either of twomethods. It can use values from look-up tablesorapproximate using polynomial interpolation.


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A thermocouple can produce current, which meansit can be used to drive some processes directly,without the need for extra circuitry and power

sources. For example, the power from athermocouple can activate a valve when atemperature difference arises. The electric powergenerated by a thermocouple is a conversion of theheat energy that one must continuously supply tothe hot side of the thermocouple to maintain theelectric potential. The flow of heat is necessarybecause the current flowing through thethermocouple

tends to cause the hot side to cool down and thecold side to heat up (the Peltier effect).

Thermocouples can be connected in series witheach other to form a thermopile, where all the hotjunctions are exposed to the higher temperatureand all the cold junctions to a lower temperature.The voltages of the individual thermocouples addup, allowing for a larger voltage and increasedpower output, thus increasing the sensitivity of theinstrumentation. With the radioactive decay oftransuranic elements providing a heat source thisarrangement has been used to power spacecrafton missions too far from the Sun to utilize solarpower.


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Voltagetemperature relationship

The relationship between the temperaturedifference and the output voltage of athermocouple is nonlinear and is approximated bypolynomial:

The coefficients an are given for N from zero tobetween five and nine.

To achieve accurate measurements the equation isusually implemented in a digital controller orstored in a look-up table.[4] Some older devices useanalog filters.

Types Of Thermocouple

Type E

The Type E thermocouple is suitable for use attemperatures up to 900C (1650F) in a vacuum,inert, mildly oxidizing or reducing atmosphere. At
http://en.wikipedia.org/wiki/Thermocouple#cite_note-Baker2000-3http://en.wikipedia.org/wiki/Thermocouple#cite_note-Baker2000-3


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cryogenic temperatures, the thermocouple is notsubject to corrosion. This thermocouple has thehighest EMF output per degree of all the commonlyused thermocouples.

Type J

The Type J may be used, exposed or unexposed,where there is a deficiency of free oxygen. Forcleanliness and longer life, a protecting tube isrecommended. Since JP (iron) wire will oxidizerapidly at temperatures over 540C (1000F), it isrecommended that larger gauge wires be used tocompensate. Maximum recommended operatingtemperature is 760C (1400F).

Type K

Due to its reliability and accuracy, Type K is usedextensively at temperatures up to 1260C(2300F). It's good practice to protect this type of

thermocouple with a suitable metal or ceramicprotecting tube, especially in reducingatmospheres. In oxidizing atmospheres, such aselectric furnaces, tube protection is not alwaysnecessary when other conditions are suitable;however, it is recommended for cleanliness andgeneral mechanical protection. Type K willgenerally outlast Type J because the JP (iron) wire

rapidly oxidizes, especially at highertemperatures.


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Type N

This nickel-based thermocouple alloy is usedprimarily at high temperatures up to 1260C(2300F). While not a direct replacement for TypeK, Type N provides better resistance to oxidationat high temperatures and longer life inapplications where sulfur is present.

Type T

This thermocouple can be used in either oxidizingor reducing atmospheres, though for longer life aprotecting tube is recommended. Because of itsstability at lower temperatures, this is a superiorthermocouple for a wide variety of applications inlow and cryogenic temperatures. It's recommendedoperating range is -200 to 350C (-330 to660F), but it can be used to -269C (-452F)

(boiling helium).

Types S, R and B

Maximum recommended operating temperature forType S or R is 1450C (2640F); Type B isrecommended for use at as high as 1700C(3100F). These thermocouples are easilycontaminated. Reducing atmospheres are

particularly damaging to the calibration. Noblemetal thermocouples should always be protectedwith a gas-tight ceramic tube, a secondary tube ofalumina and a silicon carbide or metal outer tubeas conditions require.


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W-5 Percent Re/W-26 Percent Re (Type C*)

This refractory metal thermocouple may be used attemperatures up to 2315C (4200F). Because it

has no resistance to oxidation, its use is restrictedto vacuum, hydrogen or inert atmospheres.

ThermopileA thermopile is an electronic device thatconverts thermal energy into electricalenergy. It is composed of thermocouplesconnected usually in sfries, or less commonlyin parllel. Thermopiles do not measure theabsolute temperature, but generate anoutput voltage proportional to a localtempreture difference or temperaturegradient. Thermopiles are the keycomponent of the infrared thermometersthat are widely used by medical professionals to measure body temperaturevia the ear. They are also used widely in heat

flux sensors and gas burner safety controls.The output of a thermopile is usually in therange of tens or hundreds of millivolts. Aswell as increasing the signal level, the device


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may be used to provide spatial temperatureaveraging.


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Basics of minor :

Above discussed topis as :

Thermopile

Thermocouple

and the principles of Seeback and Peltier

are the basic concept of the minor project.


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Minor project:

This project is the basic of utilising the heatand colness of the engine or of the vehicle,so thet a thermocouple can be made ythusproducing the electricity

Now the question arises how to genrate

electricity?

Electricity can be produced using thermopilemaking a thermocouple.genuinely maximumtemp. of the vehicle lies in the engine butcant be used at all, so then utilising the heatof the exhoust manifold which is round1500`c and then the internal minimum temp.of the vehicle is round 70`c that is ofradiating fluid in the radiator.

Now making a junction in this hot and coldpart of the vehoicle as shown in the dia. Thisjunction is made by the us4e of thethermopile by using a thihck copper wire

which is to carry the lightest ev of current.This finally contributes to the thermocouplewhich genrates electricity acting as athermogenrator.and finally this electric


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current is shifted to battery in order to get itcharged .

When the battery gets charged then it comesto reduce the load on the engine and rathervehicle also.this is done by reducing the sizeof the alternator which derives motion fromthe engine. Thus increasing ingines thermalefficiency and also the fuel efficiency.

Advantages and Disadvantages:

Advantages:

1. Increase in the efficiency of the engine.

2. Decrease in load appearance on engine.

3. Decrease in gross weight of anautomobile.

4. Effective electronic and electrical systemof an automobile.


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5. Increase in fuel efficiency of the engine.

Disadvantages:

1. Difficulties in utilizing low volts ofcharge generated.

2. Proper metallic connections arerequired.

3. Complex connection of wires involved in

engine

waste water utilization

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