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MAE 493R/593V- Renewable Energy Devices

Thermoelectric Devices

Outline

• Thermoelectric effects

• Operating principle of thermoelectric generator

• Thermoelectric Materials

• Applications of thermal electric generator

• Thermoelectric cooling devices

Thermoelectric Devices

Source: Wikipedia

Thermoelectric EffectsThermoelectric 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

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.

Thermoelectric Devices

Source: Wikipedia

Thermoelectric Effects

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

L. Onsager, Physical Review 37, 405 (1931)

Thermoelectric Devices

Source:: Akram Boukai

Thermoelectric Effects

Thermoelectric Generators

Source:: Wikipedia

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.

Thermoelectric Generators

Source:: Wikipedia

The figure of merit for thermoelectric devices is defined as

σ - electrical conductivityκ - thermal conductivityS - Seebeck coefficient or thermopower,

in μV/K. Dimensionless figure of merit , ZT

Where T = (T2 + T1) / 2 Greater values of ZT indicate greater thermodynamic efficiencyZT = 3~4 are considered to be essential for thermoelectrics to compete with mechanical generation and refrigeration in efficiencyTo date, the best reported ZT values have been in the 2–3 range

Thermoelectric MaterialsZT Range of Thermoelectric Materials

A. Majumdar, Science, 303, (2004), 777

Thermoelectric MaterialsPhonon Drag

Phonon drag is an increase in the effective number of conductionelectrons 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).

Source:: Wikipedia

Thermoelectric MaterialsPhonon Drag

Phonons move against the thermal gradient. They lose momentum byinteracting 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 phonon-electron scattering is predominant. This happens for

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

Thermoelectric Materials

Source:: Wikipedia

Dimensionless figure of merit:

S

S2σ

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 κphonon.

Thermoelectric MaterialsThermal conductivity Materials

D.G. Cahill, et al. Phys. Rev. B, 46 (1992), 6131

For high ZT Materials:Low thermal conductivityHigh electric conductivity

Thermoelectric Materials

Source:: Wikipedia

Dimensionless figure of merit:

σ, electrical conductivity:

For Metals : As temperature increases, τdecreases, thereby decreasing σ.

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

Thermoelectric MaterialsZT for p-type thermoelectric materials

(Snyder, J. http://www.its.caltech.edu/~jsnyder/thermoelectrics/science_page.htm)

Bi2Te3 performs the best

Thermoelectric MaterialsZT for n-type thermoelectric materials

(Snyder, J. http://www.its.caltech.edu/~jsnyder/thermoelectrics/science_page.htm)

Thermoelectric Materials

By courtesy of M. A. Subramanian, Oregon State

Superlattice (2D) Nanowire (1D)

Improvement of Electrical ConductivityImprovement in Thermal ResistanceOperating at High Temperature RangeReducing Manufacturing Cost

Thermoelectric Materials

How to Improve the ZT of thermoelectric materials

Source: B. S. Source: B. S. YilbasYilbas

Papers on Thermoelectric Materials

By courtesy of M. A. Subramanian, Oregon State

Thermoelectric Materials

Thermoelectric Materials, Phenomena, and Applications: A Bird's Eye View: T. M. Tritt, M. A. Subramanian, MRS Bulletin, March, 2006.

Recent Developments in Bulk Thermoelectric Materials: G.S. Nolas, M. Kanatzidis, MRS Bulletin, March, 2006.

Properties of Nanostructured One-Dimensional and Composite: Thermoelectric Materials: A. M. Rao, X. Ji, and T. M. Tritt, MRS Bulletin, March, 2006.

Thermoelectric Generators

Source:: Wikipedia

Efficiency of thermoelectric generators

The efficiency (η) is defined as

TH - the temperature at the hot junctionTC - the temperature at the surface being cooled

- the modified dimensionless figure of merit _Tz

ρ is the electrical resistivity, 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

_T

Thermoelectric Generators

Source:: Wikipedia

Power of thermoelectric generators

Q – net heat adsorbedη – efficiency

P = ηQ

the efficiency of a thermoelectric device is limited by the Carnot efficiency

Thermoelectric Devices

Source: John W. Fairbanks

Advantages of Thermoelectric generator

Direct Energy Conversion

No Moving Parts

No Working Fluids

Maintenance-free Durability

Noiseless Operation

No moving parts

Electric Power Harvested from Waste Heat

By courtesy of M. A. Subramanian, Oregon State

Application of Thermoelectric Generator

Waste Heat Released from Vehicles

Source: Yang et.al, Journal of Electronic Materials, 38, 1245, 2009

Application of Thermoelectric Generator

There is strong need to develop highly efficient thermoelectric devices for recovering waste heat from vehicles

Increasing Electrical Power Requirements for Vehicles

Source: Juhui Yang, GM

Increased electrical power needs are being driven by advanced Engines for enhanced performance, emission controls, and creature comforts

Application of Thermoelectric Generator

Source: John W. Fairbanks

Thermoelectric generator for Vehicles

Application of Thermoelectric Generator Thermoelectric Devices

Source: John W. Fairbanks

Configuration of Thermoelectric generator

Application of Thermoelectric GeneratorThermoelectric generator for Vehicles

Source: John W. Fairbanks

GM’s Thermoelectric Generators

Source: John W. Fairbanks

Application of Thermoelectric Generator

BMW Series 5 , Model Year 2010, 3.0 Liter Gasoline Engine w/ Thermoelectric Generator

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 GeneratorThermoelectric generator for Micro-devices

Application of Thermoelectric Generator

Microfabricated thermoelectric elements Micropelt).

Completed device (RTI) next to a penny (Copyright RTI.)

Thermoelectric generator for Portable Devices

Application of Thermoelectric Generator

Seiko Thermic, a wristwatch powered by body heat using a thermoelectric generator; Left: the watch,Right: cross-sectional diagram(Seiko Instruments Incorporated)

Thermoelectric Effects

By courtesy of M. A. Subramanian, Oregon State

Thermoelectric Cooling Devices

Thermoelectric Cooling Devices

Al2O3

http://www.tellurex.com

TellurexPK1 Cold Plate Cooler