The thermoelectric effect is phenomenon exhibited by thermocouple that convert temperature differences to electric voltage and vice versa. A thermocouple is an electrical device with two dissimilar conductors forming electrical junctions at differing temperatures. The electrical output of which is an directly proportional to the temperature sensed.
Thermocouple
A thermocouple is a type of active transducer that do not require an external power supply.This is because at the atomic level, the materials exhibiting thermoelectric effect have charge carriers in the material that diffuse from the hot side to the cold side.
The heat that is generated in a resistive material when current passes through it is not thermoelectric effect. It is joule heating which is not a reversible phenomenon like thermoelectric effect.
Classification of Thermoelectric effect
Thermoelectric effect can be classifeid into three differerent effects which are more or less related to each other. They are Seebeck effect , Peltier effect , and Thomson effect .
1. Seebeck effect
The Seebeck effect is the production of EMF (electromotive force) at the junctions of two different conductors. Two dissimilar materials connected back to back at two different temperatures TH (hot) and TC (Cold)generates an EMF (V) across their junction
The maginitude of thermo-electric emf , proportional to the temperature difference between the junctions,depends upon the nature of the two metals.
and the direction of the thermoelectric current can be reversed by switching the hot and cold junctions.
Thermo-electric series It is a set of elements used in thermocouples and which participate in the Seebeck effect and Peltier effect. The series is as follows
Si,Bi,Ni,Co,Pt,Cu,Mn,Hg,Pb,Sn,Au,Ag,Zn,Cd,Fe,As,Sb,Te
Two materials farther apart used in a circuit gives higher thermo - emf .
Variation of thermo-emf with temperature
In a thermo-couple of two dis-similar metals A and B with a cold junction at temperature 0 C and the temperature of the hot junction raised gradually, thermo emf varies with the temperature of the hot junction.
The thermoemf is zero when both the junctions are at the same temperature 0 C and gradually increases as the temperature of the hot junction increases The temperature of the hot junction at which thermoemf in the thermocouple becomes maximum is called neutral temperature (Tn) for that thermocouple which is fixed for a given pair. Further increase in the temperature of the hot junction,after Tn, decreases the thermoemf and becomes O at temperature of inversion Ti. Beyond Ti ,the thermo-emf starts to increase the in reverse direction Mathematically,
Tn-Tc = Ti -Tn Tn = (Ti+Tc)/2
Hence neutral temperature is the mean of the temperature of inversion and temperature of the cold junction Thermo-emf of number of thermocouples is given by the simple relation
E=αT+βT2
where T is the temperature difference between the two junction and α and β are the parameters of the material used The rate of change of thermo-emf with temperature i.e dE/dT is called thermo-power or seeback coefficent S .
S=dE/dT
Application of seebeck effect
1. Temperature Sensor , As a thermocouple they can sense temperatures by having one junction at a constant temperature and sensing through the other junction. More than one thermocouples are connected in series to form a thermopile which is very sensitive and can detect minute temperature differences.
2. Thermoelectric generators Used as less bulky heat engines, but are more expensive and less efficient.
- In power plants for converting waste heat into additional electrical power (energy recycling)
- automotive thermoelectric generators (ATGs) for increasing fuel efficiency in automobiles.
- As portable energy generation applications like body heat powered electronic appliances
2. Peltier Effect:
When an electric current is passed through thermo-couple ,heat is evolved at one junction and absorbed at the other end i.e one end become hot while other become cold, essentially a reverse Seebeck effect. Being reversible, hot and cold junctions are determined by the current flow.
A simple thermocouple circuit when closed, will generate current by seebeck effect which drives the peltier effect to take place with heat transferred from hot to cold junction. This gives a direct relation ship between the Seebeck and Peltier coefficient ∏.
Applications of peltier effect
1. Thermoelectric cooling
Construction:
One n-type and one p-type semiconductors, with different electron densities are placed parallel to each other and connected electrically to be in series mounted on a thermally conducting plate on each side. When a voltage is applied to the free ends of the two semiconductors there is a flow of DC current across the junction of the semiconductors causing a temperature difference. The side with the cooling plate absorbs heat that is transferred to the other side into the heat sink at ambient temperature. More connections increases the cooling ability of the system.
Commonly used materials are bismuth telluride, lead telluride, silicon germanium, and bismuth-antimony alloys.
2. Temperature controller
Identification of TECs Thermelectric cooling devices have specifications printed onto the cold side.
3. Thomson Effect:
Similar in working to Seebeck effect, but here only one conductor is used.
According to this effect, if a conductor has placed in varying temperature along its length and current is passed through it then it will absorb or evolve heat which depends on the direction of the current flow. Basically heat evolves , ie the side becomes hotter, when current passes from high potential to lower potential. Depending on the material they exhibit either positive or negative thomson effect.
| Positive Thomson effect | Negative Thomson effect |
|---|---|
| Hot end is at high potential | Hot end is at low potential |
| cold end is at low potential | cold end is at higher potential |
| Heat is evolved when current is passed from hotter end to the colder end | Heat is evolved when current is passed from colder end to the hotter end |
| heat is absorbed when current is passed from colder end to hotter end | heat is absorbed when current flows from hotter end to colder end |
| Cu, Sn, Ag, Cd, Zn… etc | Fe, Co, Bi, Pt, Hg… etc |