INTRODUCTION
Thermoelectric cooler (TEC) is a solid-state heat pump which offers no-moving parts, ease of miniaturization, high reliability and flexibility in design. It makes use of Peltier effect of semiconductor material to provide a temperature difference when solid state P-N materials are connected electrically in series and thermally in parallel. The heating and cooling is almost instantaneous and the temperature can easily be controlled by an appropriate control circuit with proper heat sink for heat dissipation. Due to flexibility in design and fabrication, it is possible to integrate into devices that require precise temperature control
WORKING PRINCIPLES
The working principle of a TEC is based on two effects:-
1. SEEBECK EFFECT
The Seebeck effect is the conversion of temperature differences directly into electricity. This effect was first discovered, accidentally, by the German-Estonian physicist Thomas Johann Seebeck in 1821, who found that a voltage existed between two ends of a metal bar when a temperature difference ΔT existed in the bar.
The effect is that a voltage, the thermoelectric EMF, is created in the presence of a temperature difference between two different metals or semiconductors. This causes a continuous current to flow in the conductors if they form a complete loop. The voltage created is of the order of several microvolts per degree difference.
In the circuit,
SA and SB are the Seebeck coefficients (also called thermoelectric power or thermopower) of the metals A and B, and T1 and T2 are the temperatures of the two junctions. The Seebeck coefficients are non-linear, and depend on the conductors' absolute temperature, material, and molecular structure. If the Seebeck coefficients are effectively constant for the measured temperature range, the above formula can be approximated
thus, a thermocouple works by measuring the difference in potential caused by the dissimilar wires. It can be used to measure a temperature difference directly, or to measure an absolute temperature, by setting one end to a known temperature. Several thermocouples in series are called a thermopile.
This is also the principle at work behind thermal diodes and thermoelectric generators (such as radioisotope thermoelectric generators or RTGs) which are used for creating power from heat differentials.
The Seebeck effect is due to two effects: charge carrier diffusion and phonon drag.
Charge carrier diffusion :
Charge carriers in the materials (electrons in metals, electrons and holes in semiconductors, ions in ionic conductors) will diffuse when one end of a conductor is at a different temperature than the other. Hot carriers diffuse from the hot end to the cold end, since there is a lower density of hot carriers at the cold end of the conductor. Cold carriers diffuse from the cold end to the hot end for the same reason.
If the conductor were left to reach thermodynamic equilibrium, this process would result in heat being distributed evenly throughout the conductor (see heat transfer). The movement of heat (in the form of hot charge carriers) from one end to the other is called a heat current. As charge carriers are moving, it is also an electrical current.
In a system where both ends are kept at a constant temperature relative to each other (a constant heat current flows from one end to the other), there is a constant
diffusion of carriers. If the rate of diffusion of hot and cold carriers in opposite directions were equal, there would be no net change in charge. However, the diffusing charges are scattered by impurities, imperfections, and lattice vibrations (phonons). If the scattering is energy dependent, the hot and cold carriers will diffuse at different rates. This creates a higher density of carriers at one end of the material, and the distance between the positive and negative charges produces a potential difference; an electrostatic voltage.
This electric field, however, opposes the uneven scattering of carriers, and equilibrium is reached where the net number of carriers diffusing in one direction is canceled by the net number of carriers moving in the opposite direction from the electrostatic field. This means the thermopower of a material depends greatly on impurities, imperfections, and structural changes (which often vary themselves with temperature and electric field), and the thermopower of a material is a collection of many different effects.
Typical thermoelectric devices are structured as alternating p-type and n-type semiconductor elements connected by metallic interconnects as pictured in the figures below. Current flows through the n-type element, crosses a metallic interconnect, and passes into the p-type element. If a power source is provided, the thermoelectric device may act as a cooler, as in the figure to the left below. Electrons in the n-type element will move opposite the direction of current flow and holes in the p-type element will move in the direction of current flow, both removing heat from one side of the device. If a heat source is provided, the thermoelectric device may function as a power generator, as in the figure to the right below. 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 then be used to power a load, thus converting the thermal energy into electrical energy.
Phonon drag :
Phonons are not always in local thermal equilibrium; they move along 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 already present thermoelectric field. This contribution is most important in the temperature region where phonon-electron scattering is predominant. This happens for
where θD is the Debye temperature. At lower temperatures there are fewer phonons available for drag, and at higher temperatures they tend to lose momentum in phonon-phonon scattering instead of phonon-electron scattering.
This region of the thermopower-versus-temperature function is highly variable under a magnetic field.
1. Peltier effect:
The Peltier effect is the reverse of the Seebeck effect; a creation of a heat difference from an electric voltage.
It occurs when a current is passed through two dissimilar metals or semiconductors (n-type and p-type) that are connected to each other at two junctions (Peltier junctions). The current drives a transfer of heat from one junction to the other: one junction cools off while the other heats up; as a result, the effect is often used for thermoelectric cooling. This effect was obser
Where Π is the Peltier coefficient, ΠAB of the entire thermocouple, and ΠA and ΠB are the coefficients of each material. P-type silicon typically has a positive Peltier coefficient (though not above ~550 K), and n-type silicon is typically negative, as the names suggest.
The Peltier coefficients represent how much heat current is carried per unit charge through a given material. Since charge current must be continuous across a junction,
the associated heat flow will develop a discontinuity if ΠA and ΠB are different. This causes a non-zero divergence at the junction and so heat must accumulate or deplete there, depending on the sign of the current. Another way to understand how this effect could cool a junction is to note that when electrons flow from a region of high density to a region of low density, they expand (as with an ideal gas) and cool.
The conductors are attempting to return to the electron equilibrium that existed before the current was applied by absorbing energy at one connector and releasing it at the other. The individual couples can be connected in series to enhance the effect.
An interesting consequence of this effect is that the direction of heat transfer is controlled by the polarity of the current; reversing the polarity will change the direction of transfer and thus the sign of the heat absorbed/evolved.
A Peltier cooler/heater or thermoelectric heat pump is a solid-state active heat pump which transfers heat from one side of the device to the other. Peltier coolers are also called thermo-electric coolers (TEC).
COMPARISON OF COOLING TECHNOLOGY:
(TECs Vs COMPRESSORS)
The flow of heat with the charge carriers in the thermoelectric device is very similar to the way the compressed refrigerant transfer’s heat in the mechanical system. The circulating fluids in the compressor system carry heat from the thermal load to the evaporator, where the heat can be dissipated. With TE technology, on the other hand, the circulating direct current carries heat from the thermal load to some type of heat sink that can effectively discharge the he
TEC MATERIALS AND PROPERTIES
MATERIALS:
The materials that are of interest for TE applications are poor thermal conductors but at the same time good electrical conductors, i.e., they maximize the TE figure-of-merit, Z = S2/ρκ, where S, ρ, and κ denote thermoelectric power, electrical resistivity, and thermal conductivity, respectively. There is a large need for higher performance materials than those that currently exist.
Early TE materials were Bi2Te3 and Si-Ge systems. More recently, the focus on new materials development was shifted to skutterudites, superlattice structures and low-dimensional and disordered systems. Currently the best TE materials are artificial multilayered semiconducting alloys with low phonon thermal conductivity and large electronic mobility. The misfit-layered oxides like Ca3Co4O9 accomplish a similar effect in naturally assembled crystals that play a dual role of being a “phonon glass” and an “electron crystal”. They attract now interest as candidates for high-temperature TE applications.
Peltier Effect coolers are almost always constructed with Bismuth Telluride (Bi2Te3) and used around room temperature and below. Seebeck Effect power generators are often constructed of PbTe or, SiGe as well as Bi2Te3 and are used at much higher temperatures.
THERMOELECTRIC PARAMETERS: Imax, Vmax, dTmax and Qmax
As current flows through a material, heat is generated. Thermoelectric material is no different. There is a point where the heat generated internally offsets the TECs ability to pump heat. Each TEC has a limit on how much heat that it can pump. This limit is referred to as Qmax. The current associated with Qmax is referred to as Imax. The corresponding voltage across the coolers is referred to as Vmax. If a TEC is completely insulated and isolated from the environment and running at Imax, it will produce its maximum temperature difference, dTmax. At this point it will also be pumping no heat whatsoever. As heat is applied to the cold side of the TEC, the temperature differential is suppressed. Effectively, one trades temperature differential for heat pumping. As such, if the temperature differential is 0, the corresponding heat load is Qmax. The coefficient of performance (COP) is defined as the amount of heat pumping one gets for each unit of electrical power supplied.
RATE OF TEMPERATURE CHANGE...
Peltier device cooling & heating speed - they can change temperature extremely quickly, but to avoid damage from thermal expansion control the rate of change to about 1 degree C per second.
POWER SUPPLY REQUIREMENTS...
A simple DC supply is fine if the AC ripple is not more than about 10% or 15%.
TEMPERATURE CONTROL...
Varying the power supply voltage works. Pulse width modulation can be used, but a frequency above 1 KHz (Marlow) or 2 KHz (Tellurex) is recommended (watch out for EMI!) It's best to use some kind of temperature sensor feedback (thermistor or solid-state sensor) and a closed-loop control circuit.
HEATSINK REQUIRED!...
Peltier devices don't cool by making heat magically disappear! They move the heat from one side to the other where you must remove it with a heatsink.
ADVANTAGES AND LIMITATIONS :
Advantages:
1. No moving parts
2. Small size and weight
3. Ability to cool below ambient temperature
4. High reliability
5. Ability to generate electrical power
6. Environment friendly
Limitations:
· High cost
· Can be applied on small areas only.
· Less efficient
· Moisture effect
TEC APPLICATIONS
1. Used in automobiles as coolers, heaters as well as generator applications in various ways to increase efficiency of the vehicle.
2. Used in satellites and spacecraft to counter the effect of direct sunlight by dissipating the heat over the cold shaded side
3. Photon detectors such as CCDs in astronomical telescopes or expensive digital cameras are often cooled down with Peltier elements.
4. Thermoelectric coolers can be used to cool computer components to keep temperatures within design limits without the noise of a fan, or to maintain stable functioning when overclocking.
at into the outside environment.
ved in 1834 by Jean Peltier, 13 years after Seebeck's initial discovery.
POTENTIAL AUTOMOTIVE APPLICATIONS:
1. Heating, ventilation and air conditioning (HVAC) systems.
2. Seats
3. Cup holders
4. Arm rests
5. Enable hot or cold appliances (ACs and Warmers)
6. Electrical power generation from waste heat conversion.
BARRIERS FOR AUTOMOTIVE APPLICATIONS:
1. High cost
2. No clear acceptance of a specific thermoelectric technology.
3. Material issues(thermal stress and temperature limitations).
4. Lack of volume production capability.
CONCLUSION:
A thermoelectric material has paved the way for many innovative applications in the field of opto-electronics and automobiles. Thermoelectric technology has made its mark in the field of automobiles, but it needs to be prove and tested thoroughly. If both domestic and industrial uses switch to thermoelectric coolers from the conventional air conditioners, we can prevent the emission of chloro-fluo-carcons into the earths atmosphere and thereby depletion of the ozone layer. With pollution increasing at an alarming rate, thermoelectric coolers have come to the rescue as these are environment friendly, compact and affordable.
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