Thermoelectrics can convert thermal energy into electrical energy or use electrical energy to move heat . Thermoelectric generators are solid-state power sources that utilize the Seebeck effect, while thermoelectric coolers are solid-state heat pumps utilizing the Peltier 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)
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 .
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.
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.
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.
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
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.
 CRC Handbook of Thermoelectrics, Introduction, Edited by D.M. Rowe, Ph.D., D.Sc., CRC Press, 1995.