Download Thermoelectric material and the conductor rods

Basic Principle, Selection, Properties and
Applications of Thermoelectric Materials
Abraham Shibu¹, Ashish Noble Jacob V², Jom Abraham John³, Viswanath Reddy S⁴
¹ ² ³ &⁴Department of Mechanical Engineering, Karunya University, Coimbatore, India
ARTICLE INFO
Abstract history:
14 November 2016
Keywords
Smart materials,
thermoelectric materials,
ZT value, seebeck
coefficient,
thermoelectric
generators, thermal
insulating box
ABSTRACT
Smart materials have properties that react to changes in their environment. This means
that one of their properties can be changed by an external condition, such as
temperature, light, pressure or electricity. This change is reversible and can be repeated
many times. Thermoelectric materials are used to build devices that convert temperature
differences into electricity and vice versa. The working principle of thermoelectric
material is based on three effects, discovered by Seebeck, Peltier and Thomson. The
amount of power generated using thermoelectric material depends on a dimensionless
figure of merit called ZT, which depends upon seebeck coefficient, thermal conductivity,
temperature, electrical conductivity. Here we discuss about SnSe (thermoelectric
materials) and its behaviour at various temperatures .This paper will be explaining how
and where this material is used in our day to day practices. For the application of the
thermoelectric material a hot and cold reservoir is necessary. A set up of consisting of an
thermal insulating box in which the thermo electric material is placed in the middle in
such a way that there is no heat transfer between the two regions. Thermal conductive
metal such as aluminium is used to transfer the heat from the two reservoirs to the
thermo electric material.
INTRODUCTION
Thermoelectric material is the most essential
and primary input for almost all activities now
a days. One can’t even imagine a day without
electricity. As we know, electricity can be
generated through mechanical energy, wind
energy, nuclear energy, etc. Now a day, energy
is produced through non renewable resources
like coal, which causes pollution to the
environment. There are so many other energy
sources which can be converted into electrical
energy. Energies are inter-convertible; like
mechanical energy can be converted into
electrical energy and wind energy can be
converted into electrical energy and vice versa.
Similarly, we can also convert the energies
that are in the form of light, sound, heat, etc
into electrical energy. Light energy can be
converted into electrical energy by solar
panels. Sound energy and heat energy can also
be converted into electrical energy by using
smart materials. Heat energy is getting wasted
in most of conventional power plants, vehicles,
home appliances, etc. Here we discuss about
the harnessing of waste heat energy by using
smart materials. Thermal energy can be
directly converted into electrical energy by
using a smart material called thermo-electric
material.
BASIC PRINCIPLE
Thermoelectric materials are used to build
devices that convert temperature differences
into electricity and vice versa. The working
principle of thermoelectric material is based on
three effects, discovered by Seebeck, Peltier
and Thomson.
A thermocouple is a device used extensively for
measuring temperature. The working principle
of thermocouple is based on three effects,
discovered by Seebeck, Peltier and Thomson.
They are as follows:
1) Seebeck effect: The Seebeck effect states that
when two different or unlike metals are joined
together at two junctions, an electromotive
force (emf) is generated at the two junctions.
The amount of emf generated is different for
different combinations of the metals.
2) Peltier effect: As per the Peltier effect, when
two dissimilar metals are joined together to
form two junctions, emf is generated within the
circuit due to the different temperatures of the
two junctions of the circuit.
3) Thomson effect: As per the Thomson effect,
when two unlike metals are joined together
forming two junctions, the potential exists
within the circuit due to temperature gradient
along the entire length of the conductors within
the circuit. In most of the cases the emf
suggested by the Thomson effect is very small
and it can be neglected by making proper
selection of the metals. The Peltier effect plays
a prominent role in the working principle of
the thermocouple.
The basic theory behind this Thermo Electric
Generator is "seebeck effect". Seebeck effect
was discovered by Thomas Seebeck in 1821.
When a temperature difference is recognized
between the hot and cold junctions of two
dissimilar
materials
(metals
or
semiconductors) a voltage is generated, this
voltage is called Seebeck voltage. Indeed this
phenomenon is applied to thermocouples that
are extensively used for temperature
measurements. When a Thermoelectric
material
(Thermoelectric
Module
or
Thermocouple) held in-between temperature
gradient it generate some voltage . In fact, this
phenomenon is applied to thermocouples that
are extensively used for temperature
measurements. Base on this Seebeck effect,
thermoelectric devices can act as electrical
power generators.
Temperature. Mostly Ceramics material for this
which is Al2O3. It also transfer temperature to
the modules from hot side. It should be thick.
Thermoelectric Power Generator (TEG) is a
solid state device which converts Heat Energy
into Electrical Energy. All the exciting
conventional power generators convert
Thermal Energy into Mechanical Energy then
to Electrical Energy. So here no mechanical
work ( no moving parts). So it produce less
noise and no pollution when compare to
conventional power generators. TEG is working
by Thermo Electric Effect (seebeck) effect.
When TEG held between temperature
gradients (Hot end, Cold end) it produce some
voltage this voltage is called seebeck
voltage.TEG
has
Modules
which
is
semiconductors (p,n). Here electrons acting as
a thermoelectric power fluid (working
medium). Pair of p-type semiconductor and ntype semiconductor is called as a Module.
These semiconductors highly doped by
pollutants in order to increase the Electric
conductivity.TEG has shield it avoid modules
damaging due to high temperature. The
efficiency of TEG and voltage generated by TEG
is directly proportional to semiconductor
material and temperature gradients. So
selections of semiconductor based on electric
conductivity of the material and try to increase
the temperature difference value. This
semiconductor is coupled by copper electrode.
Increasing no of modules and no of stages and
coupling no of TEG increase overall efficiency
and voltage output. Exciting efficiency of TEG is
4.2% to 6%. When using stages it increases the
efficiency to7%.
The
thermoelectric
property
of
a
thermoelectric material depends on the
dimensionless figure of merit called ZT.
Thermal Fin It is used here for increase the
thermal gradient value. When we increase the
Thermal gradient value it increase the seebeck
voltage generated by TEG. This FIN also
transfers the heat from Thermoelectric Module.
It is made by Aluminum metal. When we
include Thermal fin it increase the efficiency of
the TEG.
Important Parameters taken into account are:
Seebeck Coefficient is a measure of the
magnitude of an induced thermoelectric
voltage in response to a temperature difference
across that material. Thermal conductivity
(often denoted k, λ, or κ) is the property of a
material to conduct heat.Electrical conductivity
or specific conductance is the reciprocal of
electrical resistivity, and measures a material's
ability to conduct an electric current.
SEEBECK EFFECT
Open circuit voltage, V = T α (Th – Tc)
Seebeck coefficient, α = dV /dT
units: V/K
Seebeck coefficient = 1/q x entropy Q /T
MEASURING SEEBECK COEFFICIENT
Components of TEG :
1. Thermoelectric Module
2. Thermoelectric shield
3. Thermal Fin
4. Copper electrode
Thermoelectric Module It is semiconductor
which is highly doped by pollutants to increase
the electric conductivity of the semiconductor.
Good semiconductor has electric conductivity
in between 200µV/K - 300µV/K. When
choosing semiconductor it has to withstand
that much high operating temperature. Some of
the
good
thermoelectric
module
semiconductors are Bi2Te3, CaMnO, Ca3Co4O9,
Sb2Te3, and PbTe
Thermoelectric Shield It is a material which
protects the modules damage due to high
Physically heat one side of sample,
Thermocouples top and bottom to measure ΔT,
Cold sink at the other side of sample and
4 terminal electrical measurements
DIMENSIONLESS FIGURE OF MERIT
The ability of a given material to efficiently
produce thermoelectric power is related to its
dimensionless figure of merit given by:
Z T = (σ S²T)/λ
which depends on the Seebeck coefficient S,
thermal conductivity λ, electrical conductivity
σ, and temperature T.
THERMOELECTRIC
EFFICIENCY
POWER
GENERATING
The typical efficiency of TEGs is around 5–8%.
Older devices used bimetallic junctions and
were bulky. More recent devices use highly
doped semiconductors made from bismuth
telluride (Bi2Te3), lead telluride (PbTe),
calcium manganese oxide (Ca2Mn3O8), or
combinations
thereof,
depending
on
temperature. These are solid-state devices and
unlike dynamos have no moving parts, with the
occasional exception of a fan or pump.
greater difference in temperature greater is the
energy produced, so as this material can not
with stand higher temperatures the second was
selected which is not upto the mark but it gives
a better ZT value compared with the first one.
The third material taken for testing was tin
selenide which showed a better performance
than all the other materials. It gives a ZT value
of 2.6 at a temperature of 923K which is a good
value compared with the others.
Tin selenide, also known as stannous selenide,
is an inorganic compound with the formula
(SnSe), where Tin has a +2 oxidation state.
Tin(II) selenide is a narrow band-gap (IV-VI)
semiconductor and has received considerable
interest for applications including low-cost
photovoltaics and memory-switching devices.
Tin(II) selenide is a typical layered metal
chalcogenide ; that is, it includes a Group 16
anion (Se2−) and an electropositive element
(Sn2+), and it is arranged in a layered structure.
Tin(II) selenide exhibits low thermal
conductivity as well as reasonable electrical
conductivity, creating the possibility of it being
used in thermoelectric materials. In 2014, a
team at Northwestern University has
established the world record performance for
thermoelectric material efficiency.
Name and identifiers
Other names -Tin(II) selenide
CASR Number- 1315-06-6
Properties
Chemical formula – SnSe
Molar mass - 197.67 g/mo
Appearance - steel gray odourless powder
Density - 6.179 g/cm3
Melting point - 861 °C (1,582 °F; 1,134 K)
Where Tc is the sink temperature and Th is the
heat source.
Thermodynamic efficiency: the competition- ZT
of 4 start to become seriously competitive
Solubility in water - insoluble
Band gap - 0.9 eV (indirect), 1.3 eV (direct)
Structure
SELECTION OF MATERIALS
Crystal
The below mentioned are some of the materials
selected for testing:
Related compounds
1. β-Zn4Sb3 - ( The ZT value is 1.4 at
400°C)
2. Ge0.55Pb0.45Te – ( ZT of 1.55 at 723 K)
3. SnSe – ( ZT of 2.6 ± 0.3 at 923 K) *
The first material has a ZT value of 1.4 at 673K,
so for the thermodynamic efficiency to be
competitive the ZT value must really reach
upto 4 as mentioned above, and this ZT value is
not given at a higher temperature. Therefore
the first material could not give sufficient
energy which we required because as there is a
structure-
Orthorhombic,
Dangerous for the environment (N) .
Toxic(T),
Other anions
Tin(II) oxide,Tin(II) sulphide ,Tin telluride
Other cations
Carbon monoselenide, Silicon monoselenide
Germanium selenide,Lead selenide.
THERMOELECTRIC SETUP
PRACTICAL LIMITATIONS
The proposed thermoelectric mainly consists of
few components:
Besides low efficiency and relatively high cost,
practical
problems
exist
in
using
thermoelectric devices in certain types of
applications resulting from a relatively high
electrical output resistance, which increases
self-heating, and a relatively low thermal
conductivity, which makes them unsuitable for
applications where heat removal is critical, as
with heat removal from an electrical device
such as microprocessors.
Thermoelectric
materialTin(II)selenide,
conductor rods (Aluminium or copper rods),
fibre glass box.
This proposed model has been modelled using
pro-e. The two long conductor rods are
attached to the two opposite sides of the
thermoelectric material. This combined
material has been inserted in a cuboid with two
holes at the two faces , hole dia little greater
than that of the conductor rods.
The one end of the rod is connected to the heat
producing agent and the other end to the
colder side, this temperature difference will
make the thermoelectric material produce
electricity. Greater the temperature difference
greater is the voltage developed.
Thermoelectric material and the conductor
rods
Fibre glass box
Assembled view of Product
High generator output resistance: In order to
get voltage output levels in the range required
by digital electrical devices, a common
approach is to place many thermoelectric
elements in series within a generator module.
The element's voltages add, but so do their
individual output resistance. The maximum
power transfer theorem dictates that maximum
power is delivered to a load when the source
and load resistances are identically matched.
For low impedance loads near zero ohms, as
the generator resistance rises the power
delivered to the load decreases. To lower the
output resistance, some commercial devices
place more individual elements in parallel and
fewer in series and employ a boost regulator to
raise the voltage to the voltage needed by the
load.
Low thermal conductivity: Because a very
low thermal conductivity is required to
transport thermal energy away from a heat
source such as a digital microprocessor. The
relatively high thermal conductivity of a
generator module relative to copper and
aluminum thermal conductors used in heat
sinks means the thermoelectric generator
impedes the waste heat removal causing the
silicon device temperature to rise significantly.
Cold-side heat removal with air: In aircooled thermoelectric applications, such as
when harvesting thermal energy from a motor
vehicle's crankcase, the large amount of
thermal energy that must be dissipated into
ambient air presents a significant challenge. As
a thermoelectric generator's cool side
temperature rises, the device's differential
working temperature decreases. Ast the
temperature rises, the device's electrical
resistance increases causing greater parasitic
generator self-heating. In motor vehicle
applications a supplementary radiator is
sometimes used for improved heat removal,
though the use of an electric water pump to
circulate a coolant adds an additional parasitic
loss to total generator output power.Water
cooling the thermoelectric generator's cold
side, as when generating thermoelectric power
from the hot crank case of an inboard boat
motor, would not suffer from this disadvantage.
Water is a fare easier coolant to use effectively
in contrast to air.
APPLICATION
For the application of the thermoelectric
material a hot and cold reservoir is necessary.
A set up of consisting of an thermal insulating
box in which the thermo electric material is
placed in the middle in such a way that there is
no heat transfer between the two regions.
Thermal conductive metal such as aluminium is
used to transfer the heat from the two
reservoirs to the thermo electric material.
Water heater
of primary energy which is equal to 42.9% of
total primary energy consumption. About 70%
of India's electricity generation capacity is from
fossil fuels. India is largely dependent on fossil
fuel imports to meet its energy demands.
During the fiscal year 2014-2015, the per
capita electricity generation in india was 1,010
kWh.
The quantity of waste heat is nearly equal to
the electric power generated. Therefore, when
thermoelectric power generation is applied to
all the cogeneration systems, the energy
recovered will be 1,010 kWh x15% = 151kWh.
Assuming
that
thermoelectric
power
generation is introduced to all the new
cogeneration systems in 2018, it will be applied
2,34,000 MW. This corresponds to the saving of
approximately 3,41,800kl of crude oil per year
and the reduction of approximately 9,20,000
tons of CO2 per year. Saving the government
crores in the generation of electricty.
CONCLUSION
The proposed product is attached to the top of
the water heater one end of rod connected to
hot and the other to the cold side.
Air-Conditioner
By harnessing the wasted heat energy and
converting it into electricity, we can drastically
reduce the cost and increase the power
generation.More than 19000 villages in our
country doesn’t have access to electricity. By
installing this setup we can generate enough
electricity for powering almost all the places in
our country.`
REFERENCES
Thermoelectric materials: energy conversion
between heat and electricity by Xiao Zhang, LiDong Zhao.
Waste heat energy harvesting using thermo
electric generator by A Jack delightus peter,
Balaji D, D Gowrishankar.
One end of the rod in the proposed model will
be connected to the evaporator, that is cold
side and the other end connected with the
condenser which eject heat outside. This
temperature difference can produce the
electricity. And after getting the required
voltage the voltage has to be stepped up using
a step up transformer and through this
electricity is given to different plug points from
where current can be drawn.
STATISTICAL ANALYSIS
Concerning the energy-saving effect of
thermoelectric power generation, we shall
study the application of a high-efficiency
thermoelectric system to a heat based power
plant for cogeneration. We estimate the overall
effect in India as follows.
India's net imports are nearly 144.3 million
tons of crude oil, 16 million tons of LNG and 95
million tons coal totalling to 255.3 million tons
Role of thermo electric generator in recovery of
waste heat of automobiles by Om Prakash,
Mukesh Pandey, Anurag Gour, Savita Vyas.