Thermoelectric conversion_finalized.ppt

Thermoelectric Energy Conversion

Introduction
Thermoelectric Generators (also called thermo-generators)
Thermoelectric Generators (also called thermo-generators)
are devices which convert heat (temperature
differences) energy directly into electrical energy
through semiconductor or conductor. The
conversion is based on the "See-beck Effect " (or
"thermoelectric effect").

The thermoelectric effect refers to phenomena by which either
a temperature difference creates an electric potential or an
electric potential creates a temperature difference.
These phenomena are known more specifically as :
Seebeck effect (converting temperature to current)
Peltier effect (converting current to temperature or
conversion of electrical energy into temperature difference
between two ends when current is passed between two
dissimilar metals)
Thomson effect (heating/cooling of a homogenous
conductor resulting from the flow of an electric current in the
presence of a temperature gradient)

Thermoelectricity
 Thermoelectricity: Electricity produced from heat
 First observed by Seebeck in 1822
Thomas
Seebeck
Replica of
the apparatus
Hot End Cold End
TH Tc
i
Wire
V = S.T

Seebeck discovered that a compass needle would be deflected by a
closed loop formed by two metals joined in two places, with a
temperature difference between the junctions. This was because the
metals responded differently to the temperature difference, creating a
current loop and a magnetic field. He also observed that the magnitude of
the deflection is proportional to the temperature difference and depend
upon the conducting material.
Seebeck did not recognize that there was an electric current involved, so
he called the phenomenon the thermomagnetic effect. Danish physicist
Hans Christian Qrsted rectified the mistake and coined the term
"thermoelectricity".

Seebeck Effect
 In 1821, Thomas Seebeck found that an electric current would
flow continuously in a closed circuit made up of two dissimilar
metals, if the junctions of the metals were maintained at two
different temperatures.
The thermo emf, V produced by the device is given by :
V = S .  T;
S is the Seebeck Coefficient with units of Volts per Kelvin
S is positive when the direction of electric current is same as
the direction of thermal current

Peltier Effect
 The Peltier effect is the presence of heating or cooling at an electrified
junction of two different conductors and is named for French physicist
Jean Charles Athanase Peltier, who discovered it in 1834. When a
current is made to flow through a junction between two conductors A
and B, heat may be generated (or removed) at the junction.
p <0 ; Negative Peltier coefficient
High energy electrons move from
right to left.
Thermal current and electric
current flow in opposite
directions.
(electronic)

Peltier Cooling
p >0; Positive Peltier coefficient
High energy holes move from
left to right.
Thermal current and electric
current flow in same
direction.
qp= p . I,
where qp is thermal current density and I is electrical current density

Thomson Effect
 Discovered by William Thomson (Lord Kelvin) in 1851.
 When an electric current flows through a conductor, having a
temperature gradient, heat is evolved at a rate approximately
proportional to the product of the current and the temperature
gradient. This phenomenon is called Thomson effect.
dQT / dx =  I dT/dx
I = Electric current
 is the Thomson coefficient in Volts/Kelvin
Seebeck coeff. S is temperature dependent
dT
dS
T


Relation given by Kelvin:

Thomson relations:
In 1854, Lord Kelvin found relationships between the three coefficients, implying that
the Thomson, Peltier, and Seebeck effects are different manifestations of one effect. All
thermoelectric effects are closely related to each other and relations between different
coefficients are called the Kelvin relations (uniquely characterized by the
Seebeck coefficient).
The firstThomson relation is between Seeback effect andThomson effect by relation as:
where T is the absolute temperature,  is theThomson coefficient and S is the Seebeck coefficient.
The secondThomson relation is between Seeback effect and Peltier effect
where  is the Peltier coefficient and S is the Seebeck coefficient.

 TheThomson coefficient is unique among the three main thermoelectric
coefficients because it is the only one directly measurable for individual
materials. The Peltier and Seebeck coefficients can only be easily
determined for pairs of materials; hence, it is difficult to find values of
absolute Seebeck or Peltier coefficients for an individual material.
 If the Thomson coefficient of a material is measured over a wide
temperature range, it can be integrated using the Thomson relations to
determine the absolute values for the Peltier and Seebeck coefficients.
This needs to be done only for one material, since the other values can
be determined by measuring pairwise.

The Physics
When a wire is heated electrons and phonons diffuse
 Electrons
 Higher electron diffusion  more current (good)
 Phonons
 Collide with other phonons and increase heat flow (bad) or
 Either transfer their momentum to electrons (good) or
 Lose their momentum due to boundary collisions (good)
p
e
p p
e
e
p
e
p p
e
e
e
e e
p
e
Phonons: heat flow
Electrons: current flow
Hot end Cold end

Traditional Materials
Constant Metals Insulators Semiconductors
Seebeck Small High Acceptable
Electrical High Very Low Variable
Thermal High X MediumHigh
Ideally for large thermoelectric current
 Low phonon flow
 Const temperature difference  Low thermal conductivity
 Many high energy electrons
 Small resistance  High electrical conductivity
 Many phonon electron collisions
 Large voltage per unit temperature difference  High Seebeck constant

Properties of Thermoelectric Material
Properties of Thermoelectric Material
 The metal should have high Seeback coeffcient. Many
semiconductors have much larger values of Seeback
coefficient of the order of 100  V/K.
 It should be inexpensive and stable at high temperature
 The material should act as a thermal insulator. Low thermal
conductivity helps to retain heat at the junctions and maintain
a large temperature gradient.
 The material should have high resistance to corrosion
Bismuth telluride is the best bulkTE material
Bismuth telluride is the best bulkTE material

Generator Efficiency

Efficiency =
Th - Tc
Th

1 M  1
1 M 
Tc
Th










Maximum theoretical efficiency of any generator
Temperature Difference Max. efficiency of a Bismuth
Telluride Generator
50 7.1%
25 3.7%
Chip temperatures
 Cold end (Tc)
 27°C
 Hot end (TH)
 77° C, 52 ° C
 M for BismuthTelluride
 6x better

Thermoelectric Power Generation Efficiency
R
I
T
k
IT
S
S
IR
T
S
S
I
q
w
c
H
2
1
2
1
2
5
.
0
)
(
]
)
[(





























 

h
c
m
m
h
c
h
T
T
zT
zT
T
T
T
/
1
1
1
max

where: w is the power delivered to the external load and QH is the positive
heat flow from source to sink

Applications
 Thermoelectric generators
The Seebeck effect is used in thermoelectric generators, which function like heat engines, but
are less bulky, have no moving parts, and are typically more expensive and less efficient.
They have a use in power plants for converting waste heat into additional electrical power
(a form of energy recycling), and in automobiles as automotive thermoelectric generators
(ATGs) for increasing fuel efficiency. Space probes often use radioisotope thermoelectric
generators with the same mechanism but using radioisotopes to generate the required heat
difference.
 Peltier effect
The Peltier effect can be used to create a refrigerator which is compact and has no
circulating fluid or moving parts; such refrigerators are useful in applications where their
advantages outweigh the disadvantage of their very low efficiency.
 Temperature measurement
Thermocouples and thermopiles are devices that use the Seebeck effect to measure the
temperature difference between two objects, one connected to a voltmeter and the other to
the probe.The temperature of the voltmeter, and hence that of the material being measured
by the probe, can be measured separately using cold junction compensation techniques.
 Used in Space shuttles and rockets for compact source of power.

Advantages
Advantages
 Highly reliable
 Free from noise due to the absence of moving part
 Compact and durable
 Minimum maintenance
 It is portable as it can be used any location
 Uses low grade thermal energy

Disadvantages
Disadvantages
 Low Output
 Low efficiency (5 – 10 %)
 High Cost

Limitations
Limitations
 The thermal conductivity of the material should be low as
possible.
 One of the arm should consist of a purely hole type and the
other of a purely electron type semiconductor.

Thermoelectric conversion_finalized.ppt

More Related Content

Similar to Thermoelectric conversion_finalized.ppt (20)

Recently uploaded (20)

Thermoelectric conversion_finalized.ppt

Editor's Notes