Thermoelectric Devices

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1 Outline MAE 493R/593V- Renewable Energy Devices Thermoelectric effects Operating principle of thermoelectric generator Applications of thermal electric generator Thermoelectric cooling devices 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 temperature difference between two different metals or semiconductors. This causes a continuous current in the conductors if they form a complete loop 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 The voltage developed can be derived from: S A and S B 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 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

2 Thermoelectric Generators The figure of merit for thermoelectric devices is defined as ZT Range of σ - electrical conductivity κ - thermal conductivity S - Seebeck coefficient or thermopower, in μv/k. Dimensionless figure of merit, ZT Where T = (T 2 + T 1 ) / 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 Phonon Drag 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. 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. 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. The magnitude of this effect is typically appreciable only at low temperatures (<200 K). Phonon Drag 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 θ D 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. Dimensionless figure of merit: Thermal conductivity Materials 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, κ electron < κ phonon, so it is easier to decouple κ and σ in a semiconductor through engineering κ phonon. S S 2 σ For high ZT Materials: Low thermal conductivity High electric conductivity D.G. Cahill, et al. Phys. Rev. B, 46 (1992), 6131

3 Dimensionless figure of merit: σ, electrical conductivity: For Metals : As temperature increases, τ decreases, thereby decreasing σ. ZT for p-type thermoelectric materials For Semiconductors : Carrier mobility decreases with increasing temperature, but carrier density increases faster with increasing temperature. Overall, the electrical conductivity in semiconductors correlates positively with temperature Bi 2 Te 3 performs the best (Snyder, J. ZT for n-type thermoelectric materials (Snyder, J. How to Improve the ZT of thermoelectric materials Papers on Improvement of Electrical Conductivity Improvement in Thermal Resistance Operating at High Temperature Range Reducing Manufacturing Cost, Phenomena, and Applications: A Bird's Eye View: T. M. Tritt, M. A. Subramanian, MRS Bulletin, March, Recent Developments in Bulk : G.S. Nolas, M. Kanatzidis, MRS Bulletin, March, Properties of Nanostructured One-Dimensional and Composite: : A. M. Rao, X. Ji, and T. M. Tritt, MRS Bulletin, March, Source: B. S. Yilbas Superlattice (2D) Nanowire (1D)

4 Thermoelectric Generators Efficiency of thermoelectric generators The efficiency (η) is defined as Thermoelectric Generators Power of thermoelectric generators P = ηq Q net heat adsorbed η efficiency T H T C _ zt - the temperature at the hot junction - the temperature at the surface being cooled - 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 Advantages of Thermoelectric generator Electric Power Harvested from Waste Heat Direct Energy Conversion No Moving Parts No Working Fluids Maintenance-free Durability Noiseless Operation No moving parts Waste Heat Released from Vehicles 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

5 Thermoelectric generator for Vehicles Configuration of Thermoelectric generator Thermoelectric generator for Vehicles GM s Thermoelectric Generators BMW Series 5, Model Year 2010, 3.0 Liter Gasoline Engine w/ 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)

6 Thermoelectric generator for Portable Devices Thermoelectric Cooling Devices Seiko Thermic, a wristwatch powered by body heat using a thermoelectric generator; Left: the watch, Right: cross-sectional diagram (Seiko Instruments Incorporated) Thermoelectric Cooling Devices Tellurex PK1 Cold Plate Cooler Al 2 O 3