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Outline MAE 493R/593V- Renewable Energy Devices • Thermoelectric effects • Operating principle of thermoelectric generator Thermoelectric Devices • Thermoelectric Materials • Applications of thermal electric generator • Thermoelectric cooling devices http://www.flickr.com/photos/royal65/3167556443/ Thermoelectric Devices Thermoelectric Devices Thermoelectric Effects Thermoelectric Effects Thermoelectric effect is the direct conversion of temperature differences to electric power and vice-versa. Seebeck effect is the conversion of temperature differences directly into electricity. The effect is that a voltage, the thermoelectric EMF, is orignated from Thermopower, thermoelectric power, or Seebeck coefficient of a material measures the magnitude of an induced thermoelectric voltage in response to a temperature difference across that material temperature difference between two different metals or semiconductors. This causes a continuous current in the conductors if they form a complete loop The voltage developed can be derived from: SA and SB are the Seebeck coefficients (also called thermoelectric power or thermopower) of the metals A and B as a function of temperature. The Seebeck effect is commonly used in a device called a thermocouple. Source: Wikipedia Source: Wikipedia Thermoelectric Devices Thermoelectric Effects L. Onsager, Physical Review 37, 405 (1931) Thermoelectric Generators Hot carriers diffuse from the hot end to the cold end. Cold carriers diffuse from the cold end to the hot end for the same reason. Metallic junctions are common in temperature measurement. Semiconductor junctions are common in power generation devices. If a heat source is provided, the thermoelectric device may function as a power generator. The heat source will drive electrons in the n-type element toward the cooler region, thus creating a current through the circuit. Holes in the p-type element will then flow in the direction of the current. The current can power a load, thus converting the thermal energy into electrical energy. Charge flows through the n-type element, crosses a metallic interconnect, and passes into the p-type element. Source:: Akram Boukai Source:: Wikipedia Thermoelectric Generators The figure of merit for thermoelectric devices is defined as Thermoelectric Materials ZT Range of Thermoelectric Materials σ - electrical conductivity κ - thermal conductivity S - Seebeck coefficient or thermopower, in μV/K. Dimensionless figure of merit , ZT Where T = (T2 + T1) / 2 Greater values of ZT indicate greater thermodynamic efficiency ZT = 3~4 are considered to be essential for thermoelectrics to compete with mechanical generation and refrigeration in efficiency To date, the best reported ZT values have been in the 2–3 range A. Majumdar, Science, 303, (2004), 777 Source:: Wikipedia Thermoelectric Materials Thermoelectric Materials Phonon Drag Phonon Drag ¾ Phonon drag is an increase in the effective number of conduction electrons or valence holes due to interactions with the crystal lattice in which the electron moves. ¾ As an electron moves past atoms in the lattice its charge distorts or polarizes the nearby lattice. This effect leads to a decrease in the electron (or hole) mobility, which reduces conductivity. Phonons move against the thermal gradient. They lose momentum by interacting with electrons (or other carriers) and imperfections in the crystal. If the phonon-electron interaction is predominant, the phonons will tend to push the electrons to one end of the material, losing momentum in the process. This contributes to the thermoelectric field. This contribution is most important in the temperature region where phononelectron scattering is predominant. This happens for ¾ However, as the magnitude of the thermopower (Seebeck coefficient) increases with phonon drag. It may be beneficial in a thermoelectric material for direct energy conversion applications. ¾ The magnitude of this effect is typically appreciable only at low temperatures (<200 K). θD is the Debye temperature. At lower temperatures there are fewer phonons available for drag, and at higher temperature they tend to lose momentum in phonon-phonon scattering instead of phonon-electron scattering. Source:: Wikipedia Source:: Wikipedia Thermoelectric Materials Dimensionless figure of merit: According to the Wiedemann–Franz law, the higher the electrical conductivity, the higher κ electron becomes. Therefore, it is necessary to minimize κ phonon. In semiconductors, κelectron < κphonon, so it is easier to decouple κ and σ in a semiconductor through engineering κ phonon. Thermoelectric Materials Thermal conductivity Materials For high ZT Materials: Low thermal conductivity High electric conductivity S S2σ D.G. Cahill, et al. Phys. Rev. B, 46 (1992), 6131 Source:: Wikipedia Thermoelectric Materials Thermoelectric Materials ZT for p-type thermoelectric materials Dimensionless figure of merit: σ, electrical conductivity: For Metals : As temperature increases, τ decreases, thereby decreasing σ. Carrier mobility decreases with increasing temperature, but carrier density increases faster with increasing temperature. Overall, the electrical conductivity in semiconductors correlates positively with temperature For Semiconductors : Source:: Wikipedia Thermoelectric Materials Bi2Te3 performs the best (Snyder, J. http://www.its.caltech.edu/~jsnyder/thermoelectrics/science_page.htm) Thermoelectric Materials ZT for n-type thermoelectric materials (Snyder, J. http://www.its.caltech.edu/~jsnyder/thermoelectrics/science_page.htm) Thermoelectric Materials How to Improve the ZT of thermoelectric materials By courtesy of M. A. Subramanian, Oregon State Thermoelectric Materials Papers on Thermoelectric Materials ¾ Thermoelectric Materials, Phenomena, and Applications: A Bird's Eye View: T. M. Tritt, M. A. Subramanian, MRS Bulletin, March, 2006. Improvement of Electrical Conductivity Improvement in Thermal Resistance Operating at High Temperature Range Reducing Manufacturing Cost ¾ Recent Developments in Bulk Thermoelectric Materials: G.S. Nolas, M. Kanatzidis, MRS Bulletin, March, 2006. ¾ Properties of Nanostructured One-Dimensional and Composite: Thermoelectric Materials: A. M. Rao, X. Ji, and T. M. Tritt, MRS Bulletin, March, 2006. Superlattice (2D) Source: B. S. Yilbas Nanowire (1D) By courtesy of M. A. Subramanian, Oregon State Thermoelectric Generators Thermoelectric Generators Efficiency of thermoelectric generators Power of thermoelectric generators The efficiency (η) is defined as P = ηQ Q – net heat adsorbed η – efficiency TH - the temperature at the hot junction TC_ - the temperature at the surface being cooled z T - the modified dimensionless figure of merit the efficiency of a thermoelectric device is limited by the Carnot efficiency _ ρ is the electrical resistivity, T is the average temperature between the hot and cold surfaces, and the subscripts n and p denote properties related to the n- and p-type semiconducting thermoelectric materials, respectively Source:: Wikipedia Thermoelectric Devices Source:: Wikipedia Application of Thermoelectric Generator Electric Power Harvested from Waste Heat Advantages of Thermoelectric generator ¾ Direct Energy Conversion ¾ No Moving Parts ¾ No Working Fluids ¾ Maintenance-free Durability ¾ Noiseless Operation ¾ No moving parts Source: John W. Fairbanks Application of Thermoelectric Generator Waste Heat Released from Vehicles By courtesy of M. A. Subramanian, Oregon State Application of Thermoelectric Generator Increasing Electrical Power Requirements for Vehicles Increased electrical power needs are being driven by advanced Engines for enhanced performance, emission controls, and creature comforts There is strong need to develop highly efficient thermoelectric devices for recovering waste heat from vehicles Source: Yang et.al, Journal of Electronic Materials, 38, 1245, 2009 Source: Juhui Yang, GM Application of Thermoelectric Generator Thermoelectric generator for Vehicles Thermoelectric Devices Configuration of Thermoelectric generator Source: John W. Fairbanks Application of Thermoelectric Generator Source: John W. Fairbanks Application of Thermoelectric Generator GM’s Thermoelectric Generators Thermoelectric generator for Vehicles Source: John W. Fairbanks Application of Thermoelectric Generator BMW Series 5 , Model Year 2010, 3.0 Liter Gasoline Engine w/ Thermoelectric Generator Source: John W. Fairbanks Application of Thermoelectric Generator Thermoelectric generator for Micro-devices Completed device (RTI) next to a penny (Copyright RTI.) Microfabricated thermoelectric elements Micropelt). The selected vehicle is a state-ofthe-art BMW sedan with a 3 liter displacement engine (BMW 530i, MY 2006, automatic transmission) Source: John W. Fairbanks Application of Thermoelectric Generator Thermoelectric generator for Portable Devices Thermoelectric Cooling Devices Thermoelectric Effects Seiko Thermic, a wristwatch powered by body heat using a thermoelectric generator; Left: the watch, Right: cross-sectional diagram (Seiko Instruments Incorporated) By courtesy of M. A. Subramanian, Oregon State Thermoelectric Cooling Devices Tellurex PK1 Cold Plate Cooler Al2O3 http://www.tellurex.com