Download Thermoelectric Devices

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
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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