Thermoelectricity

Thermoelectricity (thermo-electricity) 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, 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 (utilizing only 20-25% of the energy generated during fuel combustion) [Yang, International Conference on Thermoelectrics: 2005, pp. 155.] . Furthermore, the electricity requirement in vehicles is increasing due to the demands of enhanced performance, on-board controls and creature comforts [Fairbanks, J., Thermoelectric Developments for Vehicular Applications, U.S. Department of Energy: Energy Efficiency and Renewable Energy. Presented on: August 24, 2006.] (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. [Yang, International Conference on Thermoelectrics: 2005, pp. 155.] Thermoelectric devices are thus being investigated to convert waste-heat into usable energy utilizing the Seebeck Effect.

Currently, some power plants utilize a method known as cogeneration in which in addition to the electrical energy generated, the heat produced during the process is converted to useful heat. Thermoelectrics may find applications in such systems or in solar thermal energy generation. [Tritt et al., "Thermoelectrics: Direct Solar Thermal Energy Conversion," MRS Bulletin: April 2008, Vol. 33, pp. 366-8]

Refrigeration

Thermoelectric devices applied to refrigeration utilizing the Peltier effect could reduce pollutants into the atmosphere. Hydrochlorofluorocarbons (HCFCs) and chlorofluorocarbons (CFCs) are known ozone depleting substances (ODSs); however, they 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. [NOAA: Earth System Research Laboratory, Hydrochlorofluorocarbon measurements in the Chlorofuorocarbon Alternatives Measurement Project, http://www.esrl.noaa.gov/gmd/hats/flask/hcfc.html.] 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 might run more quietly (since they do not require noisy compressors).

Materials selection criteria

Figure of merit

The primary criterion for thermoelectric device viability is the figure of merit given by::$Z\; =\; \{sigma\; S^2\; over\; lambda\}$,which depends on the Seebeck coefficient, $S$, thermal conductivity, $lambda$, and electrical conductivity, $sigma$.

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

Notably, in the above equation, Thermal Conductivity and Electrical Conductivity are typically intertwined. G. A. Slack [Slack GA., CRC Handbook of Thermoelectrics, ed. DM Rowe, Boca Raton, FL: CRC Press (1995)] 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. [D.Y. Chung et al., Complex Bismuth Chalcogenides as Thermoelectrics, 16th International Conference on Thermoelectrics (1997), pp. 459-462] 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). [Venkatasubramanian et al., Nature, 413, 597 (2001)]

kutterudite thermoelectrics

Recently, skutterudite materials have sparked the interest of researchers in search of new thermoelectrics [Caillat, T., Borshchevsky, A., and Fleurial, J.-P., In Proceedings of 7th International Conference TEs,K. Rao, ed., pp. 98 – 101. University of Texas, Arlington, 1993.] 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. [Nolas et al., J. Appl. Phys ., 79 (1996), 4002-8] 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. [K. Koumoto,I. Terasaki, T. Kajitani, M. Ohtaki, R. Funahashi “Oxide Thermoelectrics”; Section 35: pp.1-14 in Thermoelectrics Handbook: Macro to Nano, Edited by D.M. Rowe, CRC Press: New York (2006).] These materials exhibit low thermal conductivity perpendicular to these layers while maintaining electrical conductivity within the layers providing relatively high figure of merit of ~0.34 at 1000K. [W. Wunderlich, S. Ohta, and K. Koumoto, 24th International Conference on Thermoelectrics, 2005, pp. 252-255]

See also

* Thermoelectric effect
* Thermopower
* Batteryless radio
* Joule's law
* Heat transfer
* Thermoelectric cooling/ Peltier device
* Pyroelectric effect - the creation of an electric field in a crystal after uniform heating
* Thermogenerator
* Thermionic emission
* Bismuth telluride

References

External links

* [http://www.electriccircuits.net/book,6,chapter,319,lesson,1470,thermoelectricity.aspx Thermoelectricity diagrams]
* [http://www.cam.net.uk/home/StKilda/electrolux.html How kerosene refrigerator works]
* [http://blog.modernmechanix.com/2005/11/18/kerosene-radio/
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* [http://www.dself.dsl.pipex.com/MUSEUM/POWER/thermoelectric/thermoelectric.htm History of thermoelectric devices]
* [http://www.its.org/ International Thermoelectric Society]
* [http://www.zts.com/ Thermoelectric News]
* [http://www.tellurex.com/cthermo.html Tellurex thermoelectric module]
* [http://www.amerigon.com/thermoelectrics.php Amerigon]