thermoelectricity

This page is about devices that use the thermoelectric effect. For the physics underlying the thermoelectric effect, see thermoelectric effect.

Thermoelectricity (thermo-electricity, abbreviated as TE) refers to a class of phenomena in which a temperature difference creates an electric potential or an electric potential creates a temperature difference. In modern technical usage, the term almost always refers collectively to the Seebeck effect, Peltier effect, and the Thomson effect. Analyzing the word thermoelectricity by its etymological components, it might be taken to refer generically to all heat engines that are used to generate electricity and all electrically powered heating devices, for which there is an almost arbitrary number of conceivable techniques, but in practice such a broad use of the term is seldom encountered.

In recent years, thermoelectricity sees rapidly increasing usages in applications like portable refrigerators, beverage coolers, electronic component coolers, metal alloy sorting devices etc. One of the most commonly used material in such application is Bismuth telluride (Bi₂Te₃), a chemical compound of bismuth and tellurium.

Motivation for research

Currently there are two primary arenas in which thermoelectric devices can lend themselves to increase energy efficiency and/or decrease pollutants: conversion of waste heat into usable energy and refrigeration.

Power generation

In the transportation sector, although very common as a means of powering vehicles, internal combustion engines are highly inefficient in energy use (using only 20-25% of the energy generated during fuel combustion)^[1]. Furthermore, the electricity requirement in vehicles is increasing due to the demands of enhanced performance, on-board controls and creature comforts^[2] (stability controls, telematics, navigation systems, electronic braking, etc.). In order to gain fuel efficiency, it may be possible to shift energy draw from the engine (in certain cases) to the electrical load in the car, e.g. electrical power steering or electrical coolant pump operation.^[3] Thermoelectric devices are thus being investigated to convert waste-heat into usable energy using the Seebeck Effect.

Currently, some power plants use a method known as cogeneration in which in addition to the electrical energy generated, the heat produced during the process is used for alternative purposes. Thermoelectrics may find applications in such systems or in solar thermal energy generation.^[4]

Refrigeration

Thermoelectric devices applied to refrigeration using the Peltier effect could reduce the emission of ozone-depleting refrigerants into the atmosphere. Hydrochlorofluorocarbons (HCFCs) and chlorofluorocarbons (CFCs) are known ozone depleting substances (ODSs); however, these chemicals have long been at the heart of refrigeration technology. Recently, there has been legislation regulating the use of such chemicals for refrigeration; current international legislation mandates caps on HCFC production and will prohibit their production after 2020 in developed countries and 2030 in developing countries.^[5] These mandates as well as the environmental mindedness of consumers is leading to an increased effort in developing effective thermoelectric refrigeration units. Such units could reduce the use of such harmful chemicals and would operate more quietly (since they are solid state and do not require noisy compressors.) Vapor compression refrigerators are still more efficient than peltier refrigerators, but they are larger, and require more maintenance.

Materials selection criteria

Figure of merit

The primary criterion for thermoelectric device viability is the figure of merit given by:

,

which depends on the Seebeck coefficient, S, thermal conductivity, λ, and electrical conductivity, σ.

Slack's proposal: Phonon-Glass, electron-crystal (PGEC) behavior

Notably, in the above equation, thermal conductivity and electrical conductivity are typically intertwined. G. A. Slack^[6] proposed that in order to optimize the figure of merit, phonons which are responsible for thermal conductivity must experience the material as they would in a glass (experiencing a high degree of phonon scattering--lowering the thermal conductivity) while electrons must experience it as a crystal (experiencing very little scattering--maintaining the electrical conductivity). It is through the adjustment of each these properties independently of the other that the figure of merit can be improved.

Materials of interest

There are a number of materials being researched for thermoelectric device applications and temperature ranges. Some such materials include:

Bismuth chalcogenides

These materials involve Bi₂Te₃ and Bi₂Se₃ and comprise some of the best performing thermoelectrics at room temperature with a temperature-independent figure of merit, ZT, between 0.8 and 1.0.^[7] Nanostructuring of these materials to produce a layered superlattice structure of alternating Bi₂Te₃ and Bi₂Se₃ layers produces a device within which there is good electrical conductivity but perpendicular to which thermal conductivity is poor. The result is an enhanced ZT (approximately 2.4 at room temperature for p-type).^[8] Note that this high value has not entirely been independently confirmed.

Skutterudite thermoelectrics

Recently, skutterudite materials have sparked the interest of researchers in search of new thermoelectrics^[9] These structures are of the form (Co,Ni,Fe)(P,Sb,As)₃ and are cubic with space group Im3. Unfilled, these materials contain voids into which low-coordination ions (usually rare earth elements) can be inserted in order to alter thermal conductivity by producing sources for lattice phonon scattering and decrease thermal conductivity due to the lattice without reducing electrical conductivity.^[10] Such qualities make these materials behave with PGEC behavior.

Oxide thermoelectrics

Due to the natural superlattice formed by the layered structure in homologous compounds (such as those of the form (SrTiO3)_n(SrO)_m--the Ruddleson-Popper phase), oxides are also being considered for high-temperature thermoelectric devices.^[11] These materials exhibit low thermal conductivity perpendicular to these layers while maintaining electrical conductivity within the layers. The figure of merit in oxides is yet relatively low (~0.34 at 1000K),^[12] but the enhanced thermal stability, as compared to conventional high-ZT bithmuth compounds, makes the oxides superior in high-temperature applications.^[13]

Nanomaterials

In addition to the nanostructured Bi₂Te₃/Bi₂Se₃ superlattice thin films that have shown a great deal of promise, other nanomaterials have also shown potential in improving thermoelectric materials. One example involving PbTe/PbSeTe quantum dot superlattices was shown to provide an enhanced ZT (approximately 1.5 at room temperature) that was a great deal higher than the bulk ZT value for either PbTe or PbSeTe (approximately 0.5).^[14] More recently, two research groups have shown that individual silicon nanowires can act as efficient thermoelectric materials. Each group found ZT values approaching 1.0 for their structures, even though bulk silicon is known to be a poor thermoelectric material (approximately 0.01 at room temperature) because of its high thermal conductivity.^{[15] [16]}

See also

• Thermoelectric effect
• Thermopower
• Batteryless radio
• Joule's law
• Heat transfer
• Thermoelectric cooling/ Peltier device
• Pyroelectric effect
• Thermogenerator
• Thermionic emission
• Bismuth telluride

References

External links

• Thermoelectricity diagrams
• Analyzer for Seebeck Effect and Electric Resistivity
• How kerosene refrigerator works
• Image:Kerosene_radio.jpg
• History of thermoelectric devices
• International Thermoelectric Society
• Thermoelectric News
• Global Thermoelectric
• Tellurex thermoelectric module
• Amerigon
• RMT Ltd
• TE Modules Application Tips and Hints
• TEC Microsystems GmbH
• Thermoelectric (TE) systems that provides the heating, ventilation, and air conditioning (HVAC) in vehicles, USDOE.