Transcript Thermoelectric Applications

Thomson effect
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A phenomenon discovered in 1854
by William Thomson, later Lord
Kelvin.
Showed that heat power (Qτ) is
absorbed or evolved along the
length of a material rod whose ends
are at different temperatures.
The direction of the current
determines whether the Thomson
heat is evolved or absorbed.
The Thomson heat is proportional to
the current I, time t, and
temperature drop T2 – T1; that is,
Q = τ(T2 –T1)It.
The proportionality factor τ is
known as the Thomson coefficient.
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Thermoelectric Applications
Contents
1
Thermoelectric cooler/heater
2
Thermoelectric generator
3
Temperature measurement
Thermoelectric cooler/heater
“Peltier effect”
A solid-state
active heat pump
which transfers
heat from one
side of the device
to the other side
against the
temperature
gradient , with
consumption of
electrical energy.
When direct
current runs
through it, heat is
moved from one
side to the other.
Peltier Effect- when a voltage or
DC current is applied to two
dissimilar conductors, a circuit can
be created that allows for
continuous heat transport between
the conductor’s junctions.
Basic applications of thermoelectric
coolers and assemblies:
Radio electronics - miniature coolers for
electronic instruments
Medicine - portable thermostats,
medical instruments and equipment
Air-conditioning systems - thermoelectric
conditioners, devices for temperature
stabilization of electronic units, etc.
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The heart of the PC AirCon
is a thermoelectric cooler
that operates on the Peltier
effect. The incoming room
air is cooled by the TEC and
then blown into the PC.
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Thermoelectric generators
Are devices that either
convert heat directly into
electricity or transform
electrical energy into
thermal power for
heating or cooling.
In a Peltier cooler, electric power is used to
generate a temperature difference between
the two sides of the device, while in a
thermoelectric generator, a temperature
difference between the two sides is used to
generate electric power.
If the load resistor is removed and a DC
power supply is substituted, the
thermoelectric device can be used to draw
heat from the “heat source” element and
lower its temperature. In this configuration,
the reversed energy-conversion process of
thermoelectric devices is invoked, using
electrical power to pump heat and produce
refrigeration.
•A heat source provides the high
temperature, and the heat flows to a heat
sink (maintained at a temperature below that
of the source).
•The temperature differential across the two
interface produces direct current (DC) to a
load (RL) having a terminal voltage (V) and a
terminal current (I).
•There is no intermediate energy conversion
process. For this reason, thermoelectric
power generation is classified as direct power
conversion. The amount of electrical power
generated is given by I2RL, or VI.
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Major Types of Thermoelectric Generators
Fossil Fuel
(uses natural gas, propane,
butane, kerosene, jet fuels,
and wood as heat sources)
These are used in remote
areas in applications such
as navigational aids, data
collection and
communications systems,
and cathodic protection,
which prevents electrolysis
from corroding metallic
pipelines and marine
structures.
Solar Source
Have been used with
some success to power
small irrigation pumps in
remote areas and
underdeveloped regions
of the world. Solar
thermoelectric generators
have been designed to
supply electric power in
orbiting spacecraft, though
they have not been able to
compete with silicon solar
cells, which have better
efficiency and lower unit
weight.
Nuclear fuel
The decay products of
radioactive isotopes can be
used to provide a hightemperature heat source for
thermoelectric generators.
Because thermoelectric
device materials are relatively
immune to nuclear radiation
and because the source can
be made to last for a long
period of time, such
generators provide a useful
source of power for many
unattended and remote
applications. Ex. Radioisotope
thermoelectric generators
(used in spacecraft)
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.
Thermocouple
•Junction between two different metals
that produces a voltage related to a
temperature difference.
•Widely used type of temperature
sensor for measurement and control
and can also be used to convert heat
gradient into electricity.
•Inexpensive and interchangeable and
can measure a wide range of
temperatures.
•The main limitation is accuracy:
system errors of less than one degree
Celsius (C) can be difficult to achieve.
Thermopile
•An electronic device that converts
thermal energy into electrical energy.
•Composed of several thermocouples
connected usually in series or, less
commonly, in parallel.
•Thermopiles do not respond to
absolute temperature, but generate an
output voltage proportional to a local
temperature difference or temperature
gradient.
•They are also used widely in heat
flux sensors (such as the Moll
thermopile and Eppley pyrheliometer)
and gas burner safety controls
Advantages and Disadvantages
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Thermoelectric Cooler/Heater
Advantages
One of the benefits of TEC is that you
can switch the direction of heat
pumping by simply
reversing the polarity of the applied
voltage—you get heating with one
polarity, cooling with the other.
Thermoelectric coolers have no
moving parts, which results in
less vibrations, a more silent
operation and virtually
maintenance free operations.
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Thermoelectric coolers
Advantages
Thermoelectric wine refrigerators
are more environmentally friendly
because they do not release ozone
depleting chemicals such as
chlorofluorocarbons or other gases.
Thermoelectric coolers' solid
state design allows them to take
up less floor space, plus they are
more lightweight.
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Thermoelectric coolers
Advantages
TEs can be used in any
orientation and in zero gravity
environments. Thus they are
popular in many aerospace
applications.
With a TE cooler it is possible
to cool one specific component
or area only, thereby often
making it unnecessary to cool
an entire package or
enclosure.
Disadvantages:
It uses a lot of electricity and creates much
more heat than it transports, which can
easily lead to overheating if the extra heat
isn't compensated for.
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Thermoelectric generators
Advantages
Exploiting Unclaimed
Resources
Thermoelectric generators help
tap an unclaimed resource
[heat] now considered waste.
Durability
The thermoelectric modules that make up
thermoelectric generators have solid-state
constructions, which make the generators
highly durable.
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Thermoelectric generators
Disadvantages
Cost
One of the main disadvantages of
thermoelectric generators, which
has prevented their adoption on a
wider scale, lies in their cost.
A single thermoelectric module
capable of producing 14 watts of
electrical power costs
approximately $100.
Efficiency
Most thermoelectric generators have
an average efficiency of 4 percent,
which means the generators cannot
pass on 96 percent of the energy they
obtain from heat sources.
A thermoelectric generator will only
operate efficiently when supplying
electrical current to a device that has a
similar electrical resistance.
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