Introduction
Thermoelectrics can convert thermal
energy into electrical energy or use electrical energy to move heat [1].
Thermoelectric generators are solid-state power sources that utilize the Seebeck
effect, while thermoelectric coolers are solid-state heat pumps utilizing the
Peltier effect.
Seebeck Effect
In 1821 Thomas
Johann Seebeck found that a circuit made from two dissimilar metals, with
junctions at different temperatures would deflect a compass magnet. Seebeck
initially believed this was due to magnetism induced by the temperature
difference. However, it was quickly realized that it was an electrical current
that is induced, which by Ampree's law deflects the magnet. More specifically,
the temperature difference, produces and electric potential (voltage) which can
drive an electric current in a closed circuit. Today, this is known as the
Seebeck effect.
The voltage produced is
proportional to the temperature difference between the two junctions. The
proportionality constant (a) is known as the Seebeck
coefficient, and often referred to as the thermoelectric power or thermopower.
The Seebeck voltage does not depend on the distribution of temperature along the
metals between the junctions. This is the physical basis for a thermocouple,
which is used often for temperature measurement.
 Thomas Johann Seebeck |
 V = a(Th - Tc) The voltage
difference, V, produced across the terminals of an open circuit made from
a pair of dissimilar metals, A and B, whose two junctions are held at
different temperatures, is directly proportional to the difference between
the hot and cold junction temperatures, Th - Tc [2].
|
Peltier
Effect
In 1834, a French watchmaker and part time physicist, Jean Charles
Athanase Peltier found that an electrical current would produce heating or
cooling at the junction of two dissimilar metals. In 1838 Lenz showed that
depending on the direction of current flow, heat could be either removed from a
junction to freeze water into ice, or by reversing the current, heat can be
generated to melt ice. The heat absorbed or created at the junction is
proportional to the electrical current. The proportionality constant is known as
the Peltier coefficient.
|
Thomson
Effect |
 William Thomson (Lord Kelvin)[4] |
Figure of
Merit
Not until about 1910 was an adequate description of the figure of
merit given by Altenkirch, with the modern theory provided by Ioffe in 1949. For
both power generation and cooling the thermoelectric material needs to have high
Seebeck coefficient (a), high electrical conductivity
(s) and low thermal conductivity (k). It can be shown that the efficiency of a thermoelectric
material depends primarily on the thermoelectric figure of merit, defined as
a2s/k. Materials with high thermoelectric figures of merit are
typically heavily doped semiconductors, the best known are the tellurides of
antimony and bismuth. With the advent of semiconductors the efficiency of
thermoelectric generators greatly increased. In the 1950's, generator
efficiencies had reached 5% and cooling from ambient to below 0 C was
demonstrated.
 Thermoelectric Module |
Thermoelectric
Module A thermoelectric converter consists of a number of alternate n- and p- type semiconductor thermoelements, which are connected electrically in series by metal interconnects, sandwiched between two electrically insulating but thermally conducting ceramic plates to form a module. Provided a temperature difference is maintained across the module, electrical power will be delivered to an external load and the device will operate as a generator. Conversely, when an electric current is passed through the module, heat is absorbed at one face of the module and rejected at the other face; thus, the device operates as a refrigerator. |
| Thermoelectric Generator
for Space For Space Exploration missions, particularly beyond the planet Mars, the light from the sun is too weak to power a spacecraft with solar panels. Instead, the electrical power is provided by converting the heat from a Pu238 heat source into electricity using thermoelectric couples. Such Radioisotope Thermoelectric Generators (RTG) have been used by NASA in a variety of missions such as Apollo, Pioneer, Viking, Voyager, Galileo and Cassini. With no moving parts, the power sources for Voyager are still operating, allowing the spacecraft to return science data after over 25 years of operation. |
 Radioisotope Thermoelectric Generator (RTG) Used on Voyager 1 & 2 |
Future of
Thermoelectrics
Thermoelectric coolers are finding new applications in
such diverse areas as optoelectronics and automobiles. Thermoelectric generators
could eventually be used to waste heat, such as that produced by combustion in
an automobile, to electricity. Many new applications depend on improving the
efficiency of thermoelectric materials. Recent success has been achieved by
examining new compounds and engineering structures on a nanometer scale.
REFERENCES
[1] CRC Handbook of Thermoelectrics, Introduction, Edited by D.M. Rowe,
Ph.D., D.Sc., CRC Press, 1995.
[2]
http://chem.ch.huji.ac.il/~eugeniik/history/seebeck.html
[3]
http://www-groups.dcs.st-and.ac.uk/history/Mathematicians/Thomson.html