Applications of Thermoelectricity

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These are in the measurement of temperature and in thermoelectric generators and refrigerators.

Thermoelectric thermometer

This provides the most accurate and conveinient measurement of temperature.

One junction of the thermocouple is maintained at 32^o F (cold junction). The portions of the wires near the hot junction are well insulated from each other by using asbestos beads. The wires of the thermocouple are passed through mica-discs D-D fitted in a porcelain tube, P. The ends of these wires are connected to terminals T₁ and T₂. Compensating lead wires L₁ and L₂ of the same material as the thermocouple are connected to T₁ and T₂ so that the cold junction can be shifted to any convenient place. The hot junction is placed in contact with the object whose temperature is to be measured. The instrument is calibrated by keeping the hot junction at different known temperatures and the emf generated is measured by an accurate digital voltameter (large internal resistance 107-108 ohm).

Advantages

• It can be used to measure temperature accurately because of its electrical nature and a wide range of temperature difference(0.001K - 2000K)

• It absorbs a very small amount of heat from the object and therefore does not change the temperature to be measured.

• As the thermal capacity is low, it attains the temperature to be measured quickly; this is useful in measuring changing temperatures.

• It is cheap and can be easily constructed.

• It can be used to measure temperature the of remote objects, e.g., a furnace, and of small objects like a small cavity.

Disadvantages

• It is not a direct reading instrument, so cannot be used for calorimetric purposes.

• For different ranges of temperature, different thermo-couples have to be used.

• Each thermo-couple has to be calibrated individually.

Thermoelectric Detector

The thermopile is a sensitive device used for detection and measurement of intensity of heat or high radiation. It is based on Seebeck effect.

A thermo-pile consists of a large number of thermo-couples of Sb-Bi, connected in series. One set of junctions (H) at one end is coated with lamp-black while the second set of junctions ( C ) at the other end of the thermopile is well polished . The whole arrangement is kept in a funnel or horn-shaped vessel. The ends T₁ and T₂ of the thermo-pile are connected to a sensitive galvanometer G.

When heat radiation is allowed to fall on the set of junctions H, which are coated with lamp-black, they are absorbed quickly. The temperature of these junctions rises. The other set of junctions C, reflect these radiations and therefore remains cold. A thermo-emf is thus developed in each of the thermo-couples and since all the thermo-couples are connected in series, the current flows in the same direction (i.e., from Sb to Bi through the cold junction.) These currents add up and produce a large deflection in the galvanometer.

Thermocouple current-meter

Thermoelectric effects are also used to measure current. A thermocouple current-meter consists of a resistance R, a thermocouple and a sensitive galvanometer, as shown in the below figure.

The current to be measured passes through the resistor where heat is generated in accordance with I²Rt heating. This heat warms one junction of the thermocouple while the other junction remains at room temperature. This results in a thermoelectric current that passes through the sensitive galvanometer. The thermoelectric current is roughly proportional to the average rate at which heat is generated, i.e., to the mean value of the square of the alternating current. Thus, the galvanometer reading is roughly proportional to the square of the current being measured.

Thermoelectric generator

Thermo-couples can be used to generate electric power using Seebeck effect, in remote areas. This can be done by heating one junction of a thermo-couple in a flame (e.g., of kerosene lamp) and keeping the other junction in air at room temperature. The thermo-emf so generated can be used to operate radio sets (receivers) or even radio transmitters.

Metallic thermocouples have low efficiencies of only 1%, as against 26% efficiency for gasoline or diesel - powered electric generators.

For a thermocouple to be useful as a generator, it must have high thermoelectric power, high electrical conductivity, and low thermal conductivity. Known metals do not fulfill these conditions. Semiconductor thermocouples offer thermoelectric power upto 0.25 mV per K (metallic ones give only 0.1 mV per K).

Bismuth Telluride (Bi₂Te₃) alloys (semiconductors) thermocouples have an efficiency of about 7% with nearly 550 K temperature difference between the two junctions. Recently using semiconductor alloys, electrical power of upto 5 kW has been produced.

Thermoelectric Refrigerator

It is based on the Peltier effect. If a current is passed through a thermo- couple, heat is absorbed (or removed) at one junction and is evolved at the other junction of the thermo-couple. If the first junction is in a closed region, it can cool it. Such a refrigerator uses no motor or compressor and is noiseless. However, its efficiency is small as compared to a conventional refrigerator. It is very useful, if the space to be cooled is very small and noise is not acceptable.

The thermo-couple used in the refrigerator should have the following characteristics:

• It should have low resistivity so that the loss of energy in the form of joule heating is minimum.

• It should have low thermal conductivity. It will help in maintaining a large temperature difference between the two junctions.

• It should have high thermo-electric power. So far, the best-suited material is the semiconductor Bismuth telluride (Bi₂Te₃). Temperatures as low as 160K have been achieved.