thermo- electricity) refers to a class of phenomena in which a temperaturedifference creates an electric potentialor 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 enginesthat are used to generate electricityand 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 bismuthand 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.
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
Currently, some power plants utilize a method known as
cogenerationin 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]
Thermoelectric devices applied to refrigeration utilizing the
Peltier effectcould 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
lack's proposal: Phonon-Glass, electron-crystal (PGEC) behavior
Notably, in the above equation,
Thermal Conductivityand Electrical Conductivityare 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, phononswhich are responsible for thermal conductivity must experience the material as they would in a glass (experiencing a high degree of phononscattering--lowering the thermal conductivity) while electronsmust 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:
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)]
skutteruditematerials 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 groupIm3. 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.
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]
Thermoelectric cooling/ Peltier device
Pyroelectric effect- the creation of an electric field in a crystal after uniform heating
* [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
* [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]
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