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Unformatted text preview: (Il ustration source: G. Jeffrey Snyder and Eric S. Toberer, Complex Thermoelectric Materials, nature materials, Vol. 7, February 2008) The TE principle When heat is absorbed on one side of a TEG (red arrow) the movable charge carriers begin to diffuse, resulting in a uniform concentration distribution in the TEG along the temperature gradient, and producing the difference in the electrical potential on both sides of the TEG. To maximize the power generation output, p-bars and n-bars (see circles) are connected together in a cell. Due to the thermoelectric effect, electrons ow through the n-type element to the colder side while in the p-type elements, the positive charge carriers ow to the cold side. This illustrates how connecting the p-bar and the n-bar augments the voltage of each bar and the voltage of each unit cell. These unit cells are assembled in long sequences to eventually build a TEG. Interestingly enough, the thermodynamic principle can be reversed; by forcing voltage through a TEG a cooling effect (Peltier cooler) is achieved. In the future, todays compression- based refrigeration systems could be replaced with such higher effi ciency, solid-state Peltier coolers. The type of solid-state energy conversion represented by a TE generator has great appeal in terms of its simplicity. As these generators have no moving parts, they are silent, reliable and scalable, making them ideal for small, distributed power generation. For example, satellites and spaceships have been using TE generators for power for many years, such as on deep space probes such as Voyager . Solid-state Peltier coolers, which reverse the thermoelectric principles to create a cooling effect, provide precise thermal management for opto-electronics and passenger seat cooling in automobiles. Efforts are already underway to replace the alternator in cars with a thermoelectric generator mounted on the drivetrain to improve fuel-burning effi ciency (see The Case for Thermoelectrics in Cars). Another advantage of thermoelectric generators is their scalability waste heat and co-generation sources can be as small as a home water heater or as large as industrial or geothermal sources. In a smaller version, the TEG can be used for energy harvesting to build up independent power supplies, which is important for the newest sensor technology. Advances in TE designs could also enable the replacement of compression-based refrigeration systems with solid-state Peltier coolers. Building the ideal TE generator To maximize power-generation effi ciency, the temperature differential between the hot and cold sides of a TE generator should be as large as possible. As material properties vary with the temperature, they exhibit optimum performance over a relatively narrow temperature range. As a result, in order to maximize the effi ciency of power generation modules, individual TE elements are usually formed from two and sometimes three different TE materials laminated together in the...
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This note was uploaded on 05/21/2010 for the course MS Thermoelec taught by Professor Snyder during the Spring '10 term at Caltech.
- Spring '10