Patent-Invent: Thermoelectricity (Peltier-Seebeck Effect)

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Thermoelectricity
(Peltier-Seebeck Effect)

Thermoelectricity Patents
Invention	Inventor	Year	Patent No.
Heat-to-electrical energy converter	Clarence W. Hansell	1950	US2,510,397
Thermal electric alternator	Lyndon A. Durant	1959	US2,881,384
Conversion of thermal energy into electrical energy	G. N. Hatsopoulos	1959	US2,915,652
Conversion of heat to electricity	John E. Creedon	1965	US3,175,105

The Peltier-Seebeck effect, or thermoelectric effect, is the direct conversion of heat differentials to electric voltage and vice versa. Related effects are the Thomson effect and Joule heating. The Peltier, Seebeck, and Thomson effects are reversible; Joule heating is not, and cannot be, under the laws of thermodynamics.

The conducting material is not limited to solids with electrons as charge carriers. Such effects can be observed in conductors where the carriers are ions, or in semiconductors where the carriers are holes or electrons.

1 Seebeck effect
2 Peltier effect
3 Thomson effect
4 Applications
5 See also
6 External links

Seebeck effect

The Seebeck effect is the conversion of heat differences directly into electricity.

This effect was first discovered, accidentally, by the Estonian physicist Thomas Johann Seebeck in 1821, who discovered that a voltage existed between two ends of a metal bar when a temperature gradient ${\nabla}$ T existed in the bar.

He also discovered that a compass needle would be deflected when a closed loop was formed of two metals with a temperature difference between the junctions. This is because the metals respond differently to the heat difference, which creates a current loop, which produces a magnetic field.

A voltage, the thermoelectric EMF, is created in the presence of a temperature difference between two different metals or semiconductors. This usually causes a continuous current to flow in the conductors. The voltage created is on the order of several μV per kelvin (or degree Celsius) of difference.

In the circuit:

(which can be in several different configurations and be governed by the same equations), the voltage developed can be derived from:

$V = \int_{T_1}^{T_2} \left( S_B(T) - S_A(T) \right) \, dT$

S_A and S_B 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 as:

$V = (S_B - S_A) \cdot (T_2 - T_1)$

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, thermoelectric generators (such as a radioisotope thermoelectric generator (RTG)) which are used for creating power from heat differentials.

This is due to two effects: charge carrier diffusion and phonon drag.

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. This effect was observed 13 years after Seebeck's initial discovery in 1834 by Jean Peltier.

When a current I is made to flow through the circuit, heat is evolved at the upper junction (at T₂), and absorbed at the lower junction (at T₁). The Peltier heat absorbed by the lower junction per unit time, $\dot{Q}$ is equal to

$\dot{Q} = \Pi_{AB} I = \left( \Pi_B - \Pi_A \right) I$

Where Π is the Peltier coefficient Π_AB of the entire thermocouple, and Π_A and Π_B 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.

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.

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 Converters (TEC).

Thomson effect

Thomson effect, named for William Thomson, 1st Baron Kelvin, describes the heating or cooling of a current-carrying conductor with a temperature gradient.

Any current-carrying conductor, with a temperature difference between two points, will either absorb or emit heat, depending on the material.

If a current density J is passed through a homogeneous conductor, heat production per unit volume is

$q = \rho J^2 - \mu J dT/dx \,$

where

ρ is the resistivity of the material

dT/dx is the temperature gradient along the wire

μ is the Thompson coefficient.

The first term ρJ is simply the Joule heating, which is not reversible.

The second term is the Thomson heat, which changes sign when J changes directions.

The Peltier and Seebeck coefficients are related by the Thomson relation

$\Pi = S \cdot T$

which predicted the Thomson effect before it was actually formalized. It can also be written

$\mu = T dS/dT \,$

where T is the absolute temperature of the metal.

In metals such as zinc and copper, which have a hotter end at a higher potential and a cooler end at a lower potential, when current moves from the hotter end to the colder end, it is moving from a high to a low potential, so there is an evolution of energy. When it moves from the colder to the hotter end, there is an energy absorption. This is called the positive Thomson effect.

In metals such as cobalt, nickel, and iron, which have a cooler end at a higher potential and a hotter end at a lower potential, when current moves from the hotter end to the colder end, it is moving from a low to a high potential, there is an absorption of energy. When it moves from the colder to the hotter end, there is an energy evolution. This is called the negative Thomson effect.

In lead, there is zero Thomson effect.

The Seebeck effect is actually a combination of the Peltier and Thomson effects.

Applications

Thermoelectricity is the principle behind heat engines, heat pumps, thermocouples, thermal diodes, and solid-state refrigerators, etc.

It can be used to electrically measure temperature, or to generate power from a heat source.

The heat from radioactive decay has been used to electrically power several space probes, in the form of radioisotope thermoelectric generators.

Thermoelectric power sometimes refers to this direct conversion, but usually just refers to a power plant which converts heat into electricity, through the use of steam turbines or similar devices.

Thermoelectricity was widely used in the remote parts of the Soviet Union from the 1920s to power radios. The equipment comprised some bimetallic rods, one end of which could be inserted into the fireplace to get hot with the other end left out in the cold.

Another way of achieving the same function is a Clockwork radio.

External links

General

Semiconductors

Metals

The origin of the thermoelectric potential

This article is licensed under the GNU Free Documentation License. It uses material from Wikipedia Encyclopedia article "Peltier-Seebeck Effect"

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