Thomson effect A phenomenon discovered in 1854

by William Thomson, later Lord Kelvin.

Showed that heat power (Qτ) is absorbed or evolved along the length of a material rod whose ends are at different temperatures.

The direction of the current determines whether the Thomson heat is evolved or absorbed.

The Thomson heat is proportional to the current I, time t, and temperature drop T2 – T1; that is, Q = τ(T2 –T1)It. The proportionality factor τ is known as the Thomson coefficient.

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Thermoelectric Applications

Contents

Thermoelectric cooler/heater1

2

Temperature measurement3

Thermoelectric generator

Peltier Effect- when a voltage or DC current is applied to two dissimilar conductors, a circuit can be created that allows for continuous heat transport between the conductor’s junctions.

Thermoelectric cooler/heater“Peltier effect”

A solid-state active heat pump which transfers heat from one side of the device to the other side against the temperature gradient , with consumption of electrical energy.

When direct current runs through it, heat is moved from one side to the other.

Basic applications of thermoelectric coolers and assemblies:

Radio electronics - miniature coolers for electronic instruments

Medicine - portable thermostats, medical instruments and equipment

Air-conditioning systems - thermoelectric conditioners, devices for temperature stabilization of electronic units, etc.

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The heart of the PC AirCon is a thermoelectric cooler that operates on the Peltier effect. The incoming room air is cooled by the TEC and then blown into the PC.

Thermoelectric generators

Are devices that either convert heat directly into electricity or transform electrical energy into thermal power for heating or cooling.

In a Peltier cooler, electric power is used to generate a temperature difference between the two sides of the device, while in a thermoelectric generator, a temperature difference between the two sides is used to generate electric power.

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• A heat source provides the high temperature, and the heat flows to a heat sink (maintained at a temperature below that of the source).

• The temperature differential across the two interface produces direct current (DC) to a load (RL) having a terminal voltage (V) and a terminal current (I).

• There is no intermediate energy conversion process. For this reason, thermoelectric power generation is classified as direct power conversion. The amount of electrical power generated is given by I2RL, or VI.

If the load resistor is removed and a DC power supply is substituted, the

thermoelectric device can be used to draw heat from the “heat source” element and

lower its temperature. In this configuration, the reversed energy-conversion process of thermoelectric devices is invoked, using

electrical power to pump heat and produce refrigeration.

Major Types of Thermoelectric Generators

(uses natural gas, propane, butane, kerosene, jet fuels, and wood as heat sources) These are used in remote areas in applications such as navigational aids, data collection and communications systems, and cathodic protection, which prevents electrolysis from corroding metallic pipelines and marine structures.

Have been used with some success to power small irrigation pumps in remote areas and underdeveloped regions of the world. Solar thermoelectric generators have been designed to supply electric power in orbiting spacecraft, though they have not been able to compete with silicon solar cells, which have better efficiency and lower unit weight.

The decay products of radioactive isotopes can be used to provide a high-temperature heat source for thermoelectric generators. Because thermoelectric device materials are relatively immune to nuclear radiation and because the source can be made to last for a long period of time, such generators provide a useful source of power for many unattended and remote applications. Ex. Radioisotope thermoelectric generators (used in spacecraft)

Fossil Fuel Solar Source Nuclear fuel

Temperature measurement

Thermocouples and thermopiles are devices that use the Seebeck effect to measure the temperature difference between two objects, one connected to a voltmeter and the other to the probe.

Thermocouple• Junction between two different

metals that produces a voltage related to a temperature difference.

• Widely used type of temperature sensor for measurement and control and can also be used to convert heat gradient into electricity.

• Inexpensive and interchangeable and can measure a wide range of temperatures.

• The main limitation is accuracy: system errors of less than one degree Celsius (C) can be difficult to achieve.

Thermopile • An electronic device that converts

thermal energy into electrical energy.

• Composed of several thermocouples connected usually in series or, less commonly, in parallel.

• Thermopiles do not respond to absolute temperature, but generate an output voltage proportional to a local temperature difference or temperature gradient.

• They are also used widely in heat flux sensors (such as the Moll thermopile and Eppley pyrheliometer) and gas burner safety controls

Advantages and Disadvantages

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One of the benefits of TEC is that you can switch the direction of heat pumping by simplyreversing the polarity of the applied voltage—you get heating with one polarity, cooling with the other.

Thermoelectric Cooler/HeaterAdvantages

Thermoelectric coolers have no moving parts, which results in less vibrations, a more silent operation and virtually maintenance free operations.

Thermoelectric coolersAdvantages

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Thermoelectric wine refrigerators are more environmentally friendly because they do not release ozone depleting chemicals such as chlorofluorocarbons or other gases.

Thermoelectric coolers' solid state design allows them to take up less floor space, plus they are more lightweight.

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TEs can be used in any orientation and in zero gravity environments. Thus they are popular in many aerospace applications.

With a TE cooler it is possible to cool one specific component or area only, thereby often making it unnecessary to cool an entire package or enclosure.

Thermoelectric coolersAdvantages

Disadvantages:

It uses a lot of electricity and creates much more heat than it transports, which can easily lead to overheating if the extra heat isn't compensated for.

Thermoelectric generatorsAdvantages

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Exploiting Unclaimed Resources

Thermoelectric generators help tap an unclaimed resource [heat] now considered waste.

Durability

The thermoelectric modules that make up thermoelectric generators have solid-state constructions, which make the generators highly durable.

Thermoelectric generatorsDisadvantages

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Cost

One of the main disadvantages of thermoelectric generators, which has prevented their adoption on a wider scale, lies in their cost. A single thermoelectric module capable of producing 14 watts of electrical power costs approximately $100.

Efficiency

Most thermoelectric generators have an average efficiency of 4 percent, which means the generators cannot pass on 96 percent of the energy they obtain from heat sources. A thermoelectric generator will only operate efficiently when supplying electrical current to a device that has a similar electrical resistance.

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