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Thermoelectric devicesThe simplest TE device consists of a TE couple, two dissimilar semiconductors, a p-type and n-type [5]. It is connected electrically in series and thermally in parallel. To get a thermoelectric generator (Fig. 1, left), heat is pumped into one side of the couple and rejected from the opposite side. An electric current is produced in proportion to the temperature gradient between the hot and cold junctions. Electric current is propagated by electrons in the n-type semiconductor and by holes travelling in the opposite direction in the p-type semiconductor. If a voltage is applied to the TE couple across in the right direction (Fig. 1, right), electron/hole pairs are created near the p-n junction. Electrons will flow away from the junction in the n-type material, as will holes in the p-type material, absorbing heat in the process and cooling the junction. Electron and hole recombine at the opposite end, generating heat. Thus it will effectively pump heat from the cold to the hot junction. The cold junction will rapidly drop below ambient temperature provided heat is removed from the hot side. The temperature gradient will vary according to the magnitude of current applied. Figure 1 Thermoelectric device Left, generator; right, cooler A typical TE module consists of many TE couples connected electrically in series and thermally in parallel (Fig. 2). Figure 2. A thermoelectric module Improving the efficiency of TE devicesTE devices are easy to construct and robust in operation. The main drawback is the low efficiency. The dimensionless ‘figure of merit’ zT expresses the efficiency of the semiconducting materials that make up the TE couple (see Box 1). In today’s best commercial TE cooling/heating modules, zT is about 1.0 [9], which gives an efficiency one-quarter that of a conventional air-conditioning systems using gas/liquid two-phase fluids, where the heat rejection and cooling parts of the system can be widely separated, and large temperature differences do not lead to the high heat backflow that destroys the efficiency of TE systems. Ideal TE system efficiency increases nonlinearly with zT, so to double efficiency, zT has to increase to about 2.2, and to achieve fourfold increase to equal efficiency of today’s two-phase refrigeration, zT would have to increase to more than 9.2. In 1993, the US government’s Office of Naval Research and Defense Advanced Research Project asked researchers to propose improvements of zT for cooling and heating applications. This has resulted in major efforts dedicated to making new semiconducting materials and in the design of the modules. Only three efforts have produced zT values in excess of 2 [9], but they are still in the laboratory. In 2008, researchers have boosted the zT of bulk semiconducting material to 1.5, (see Box 2), which is beginning to make thermoelectric devices commercially viable, particularly at the small scale, and in combination with other design improvements (see below). Enhancing efficiency of thermoelectric semiconductorsThe efficiency of thermoelectric energy converters is limited by the figure of merit zT (see Box 1). One promising approach to increasing zT involves creating materials with nanometer-scaled morphology to dramatically lower the thermal conductivity k by scattering phonons. Quantum-dot superlattices have reported values of zT > 2, and silicon nanowires have such a reduced k that zT approached that of commercial materials. But these are still in the laboratory and not in the bulk material that can be used commercially. In 2008, two research teams reported significant improvements in zT of bulk semi-conducting material that are much closer to the market. The research team led by US scientists Gang Chen at MIT Cambridge, and Zhifeng Ren at Boston College Chestnut Hill, in Massachusetts, significantly boosted zT simply by ball-milling crystalline ingots of p-type bismuth antimony telluride BixSb2-xTe3, and hot pressing the nanopowders into solid discs [10]. When tested, these solids gave a peak zT of 1.4 at 100 C. The enhanced zT is the result of a significant reduction in thermal conductivity due to strong phonon scattering by interfaces in the nanostructures. Apart from the peak zT of 1.4 at 100 C, the material has a zT of about 1.2 at room temperature and 0.8 at 250 C. In comparison, conventional Bi2Te3-based materials commercially used have a peak zT of about 1 at room temperature and about 0.25 at 250 C. The high zT of the present material in the 25 C to 250 C range makes the material attractive for cooling and low-grade waste-heat recovery applications, such as (suggested elsewhere) harvesting body heat to run electrical devices [11]! The research team led by Joseph Heremans at Ohio State University, Columbus, in the United States took another approach, working on the p-type material lead telluride PbTe. Its minimum thermal conductivity is already quite low, and they decided that progress must come from other sources, in the numerator of eq. (1), such as the Seebeck coefficient S. The research team succeeded in doing just that, by doping PbTe with thallium [12]. At a level of doping of 2 percent thallium, zT was increased to 1.5 at 773 K, and still increasing. It appears that thallium distorts the electron density of the material, increasing it over a narrow range, probably due to the valence band of the host semiconductor resonating with one energy level of a localized thallium atom in the semiconductor matrix. The temperature range where the TlPbTe material exhibits high zT values is particularly appealing for power generation from waste heat sources such as the automobile exhaust. Apart from increasing material zT, it is possible to making significant design improvements for thermodynamic efficiency and to reduce parasitic losses. Some examples are given by Lon Bell [9] at BSST, a company based in Irwindale, California. Using a counter flow pattern, heat transported from one fluid to the other is modified by the TE engines. In the cooling/heating mode (Fig.3 top), the TE elements boost the heat quality so that one of the opposing fluid streams is heated and the other cooled, the efficiency can be about double that of a single module operating with all the elements at the same temperature. In the power-generation mode (Fig. 3 bottom), the efficiency gains about 30 percent compared with TE heat engines in which the incoming working fluid is combusted without being preheated by the waste side of the array. This design is yet another example of the circular thermodynamics of organisms I referred to earlier [4]. The cycles can be combined with higher zT materials to compound the performance gains. .Figure 3. Thermodynamic cycles to optimize the performance of TE engines Top, cooling mode; bottom, power generation mode A further improvement is to reduce the amount of material used in construction. In real devices, system performance degrades as the device is made smaller, because the relative impact of parasitic electrical and thermal loss mechanisms increases as size decreases. Also, manufacturing tolerances and electrical isolation requirements place practical constraints on device size. An alternative, stacked configuration for TE elements, for example, reduces parasitic losses from the electrical connections, and better still, if used in combination with a reduction in the electrical resistance at the TE material/shunt interfaces. Under many practical operating conditions, the weight of TE material used can be reduced by a factor of 6 to a factor of 25. The stacked configuration, for example, reduces material costs considerably for high-capacity system. For TE cooling and heating systems, the traditional configuration is cost-competitive up to about 400 thermal watts output, but increases to about 4 000 W with the stacked design. TE future within reachAverage zT in the range from 1.5 to 2 would enable substantial waste-heat harvesting as well as primary power-generation applications. Various government-sponsored programmes are underway in the US and Japan to increase vehicle mileage by converting a fraction of the heat in the exhaust of trucks and cars to electric power (see above). The power would be available for power steering, brakes, water pumps, turbo chargers and replace other vehicle subsystems such as the alternator, thereby reducing the weight carried. The energy target of 10 percent fuel reduction is within reach [9]. Gains of 5 to 10 percent would be possible in small scale diesel-powered cogenerators that are becoming widely used in developed countries and for 5000 to 20 000W primary generators in developing countries. In another proposed co-generator concept, the solar spectrum is split into shorter wavelengths that yield high photovoltaic conversion efficiency and longer wavelengths that heat a TE generator. Another potential application, I suggest, is to use heat from low energy nuclear reaction (also referred to as condensed matter nuclear reactions) [13, 14] (see From Cold Fusion to Condensed Matter Nuclear Science, SiS 36; Low Energy Nuclear Reactions for Green Energy , SiS 41) to generate electricity with TEGs Industrial waste-heat recovery in aluminium smelting, glass manufacture, and cement production is practical at a zT of 2. At the same zT, it may also be possible to replace small internal combustion engines such as those used in lawn mowers, blowers, and small outboard motorboats with external combustion TE engines, These engines would be very quiet and nearly vibration free. They would burn a wide spectrum of fuels, such as propane, butane, natural gas (including biogas methane [15] The Biogas Economy Arrives, SiS 40), and free us from fossil fuels. If the average zT reaches 2, room, home, and commercial TE heating ventilating and air-cooling systems become practical, replacing R-134a. Because of their ruggedness, portability, and ready ability to be electrically powered, TE systems should provide more-efficient and better performing temperature control in vehicles of many types including cars, trucks, trains and aircraft.
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(1)
