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WEEK 03: GENERATION: ALTERNATIVE SOURCES: THERMOELECTRIC & THERMIONIC CONVERSION Sections: Introduction | Principles | Facts | Future Thermoelectricity1 is electricity generated by the direct action of heat. The subject also includes the study of heat generated by electricity, but not in the usual manner. Thermoelectric effects are the result of interactions between mobile electric charges and thermal conditions. These effects occur in liquids and solids, which may be metals, semimetals, semiconductors, or ionic conductors. None of the major thermoelectric effects have been found in insulators or superconductors. The three major thermoelectric effects, which are discussed below, are the Seebeck effect, the Peltier effect, and the Thomson effect. Seebeck Effect If a homogeneous material having mobile charges has temperature T(1) at one end and T(2) at the other end while it is in an open circuit, then a difference in electric voltage will occur between the two ends. This voltage is directly proportional to the temperature difference T(1) - T(2). If the material is homogeneous, the voltage depends only on T(1) and T(2) and is independent of the detailed temperature conditions between the two ends. The existence of a voltage difference caused by a temperature difference was first reported to the Prussian Academy of Sciences by Thomas Seebeck in 1822. Seebeck failed to understand the basic nature of his discovery, because in subsequent experiments he used closed circuits of two dissimilar materials and claimed that the resulting deflections of nearby magnetic-compass needles proved that heat currents produce the same effect as electric currents. Ohm's law, stated in 1827, showed that Seebeck's use of closed circuits had inadvertently produced electric currents. Peltier Effect Unlike the Seebeck effect, which occurs in a single material in the presence of a temperature difference without an electric current, the Peltier effect only occurs at the junction of two dissimilar materials when electric current flows. Heat, called the Peltier heat, is either emitted or absorbed at the junction, depending on the direction of current flow. This effect was discovered by the French physicist Jean C. A. Peltier in 1834. Once again the basic nature of the effect was at first misunderstood. Peltier believed that he had discovered a violation of Ohm's law. It was not until a few years later, in 1838, that Heinrich Lenz demonstrated the true nature of this effect when he used a bismuth-antimony junction and froze a drop of water when passing electric current in one direction (absorbing heat). He then melted the drop by reversing the current (emitting heat). Thomson Effect In 1854, William Thomson (later to become Lord Kelvin for his contributions in laying the first transatlantic cable) used thermodynamic arguments to relate the Peltier and Seebeck effects. In the process he predicted a third effect--namely, that an electric current flowing through a homogeneous material that also has a temperature difference will cause the emission or absorption of heat in the body of the material. The direction of the electric current relative to the sense of the temperature difference (that is, flowing toward higher or lower temperature) determines whether heat is emitted or absorbed. This effect was subsequently discovered and called the Thomson effect. Sections: Introduction | Principles | Uses | Future Principle The Seebeck and Peltier effects readily lend themselves to qualitative description. Temperature differences produce a force of diffusion that causes the mobile charges to deviate from a uniform distribution. This redistribution causes electric forces. The final result is a steady-state situation where diffusive and electric forces balance each other and thus cause no net motion (no electric current) but have effected a nonuniform distribution of the mobile charges. This distribution is related to the temperature difference and causes the Seebeck voltage. In the Peltier effect a net motion exists, and thus the charges transport energy. The energy associated with each charge differs in the two materials. When a charge moves from one material to the other, at the junction it emits or absorbs this difference in energy and causes the Peltier effect. The Thomson effect is too subtle for such a qualitative description. Sections: Introduction | Principles | Uses | Future Uses3 Thermoelectricity is practically applied in two general areas-- the use of thermal energy to generate electric energy, and the use of electric energy for heating or refrigeration. Electrical generation is based on the Seebeck effect.2 Metals show small Seebeck effects, but material properties dictate their use in thermometers.5 The thermocouple, an open circuit using two dissimilar metals, is a widely used thermometer. A semiconductor shows much larger Seebeck effects, and it is commonly used to generate electric power. Because any source of the heat energy is acceptable, a variety of methods are in use: heat from kerosene lamps and firewood (in remote areas), heat from nuclear decays (in space and in floating weather stations), and heat from direct sunlight (in space) have all been combined with semiconductors to generate electric power ranging from a few watts up to several hundred watts. The Peltier effect4 is used in refrigeration and heating; a modern single-stage Peltier cooler can reduce temperatures to nearly 70 C degrees (125 F degrees) below room temperature. In comparison to more conventional systems, Peltier coolers have the advantages of very local heat transfer (occurring only at the junction) and no moving parts; they also have the disadvantages of lower efficiencies and higher costs. Numerous special circumstances have justified the use of a Peltier cooler. Important factors are its durability, resulting from the absence of moving parts, and its versatility--by reversing the current direction, it can be changed from a cooling to a heating system.4 A number of electronic systems require very local cooling or heating to obtain optimum performance; Peltier systems are ideal in these instances and have been used with transistors, lasers, microwave amplifiers, and light detectors. Peltier coolers have also been used to maintain biological samples during periods of storage and transfer. The largest Peltier-effect systems built so far are air conditioners used on U.S. Navy submarines. 1. C. L. Foiles. The 1998 Grolier Multimedia Encyclopedia, Grolier Interactive Inc.: 1997
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