Thermoelectric Peltier effect

Electrolytic type sensors Uxt thick film techniques, e.g. capacitor coated in gl bonded on to a ceramic disc mounted on a thermoelectric (Peltier effect) cooler. Control is by a platinum resistance thermometer which adjusts the temperature of the cooler to regain equilibrium after a change in capacitance due to moisture deposit. Range depends on technique. Capable of high precision. Limitations are similar to those for AIjO) sensor. Capable of being direct mounted. Relatively cheap. Suitable for on-line use. 

But there is no compressor noise associated with an automobile refrigerator, so what is happening here Something called the thermoelectric Peltier effect, which can also be explained in terms of thermodynamics, as follows. 

In 1857, Thomson (Lord Kelvin) placed the whole field on firmer footing by using the newly developing field of thermodynamics (qv) to clarify the relationship between the Seebeck and the Peltier effects. He also discovered what is subsequently known as the Thomson effect, a much weaker thermoelectric phenomenon that causes the generation or absorption of heat, other than Joule heat, along a current-carrying conductor in a temperature gradient. 

The other primary thermoelectric phenomenon is the Peltier effect, which is the generation or absorption of heat at the junction of two different conductors when a current flows in the circuit. Whether the heat is evolved or absorbed is determined by the direction of the current flow. The amount of heat involved is determined by the magnitude of the current, I, and the Peltier coefficients, 7T, of the materials ... 

Some heat pumps, called thermoelectric heat pumps, employ the Peltier effect, using thermocouples. The Peltier effect refers to the evolution or absorption of heat produced by an electric current passing across junctions of two suitable, dissimilar metals, alloys, or semiconductors. Presently, thermoelectric heat pumps are used only in some specialized applications. They have not been developed to a point to make them practical for general heating and cooling of buildings. 

Kohlrausch s theory leaves quite unexplained the fact that no thermoelectric current is set up in a homogeneous wire along which a current of heat is flowing, whilst the theory of Lord Kelvin is difficult to reconcile with the fact that thermoelectric currents cannot be set up in a circuit of liquid metals, although these show the Thomson effect. The latter seems, therefore, to be to a certain extent independent of the Peltier effect. Theories intended to escape these difficulties have been proposed by Planck (1889), and Duhem, in which the conception of the entropy of electricity is introduced. 

Peltier effect - (TELLURIUM AND TELLURIUM COMPOUNDS] (Vol 23) - [THERMOELECTRIC ENERGY CONVERSION] (Vol 23)... 

The thermoelectric power tensor A is not symmetric because A = a 1f3, and although the ORRs require a to be symmetric, this is not true of the off-diagonal 71(2) 8. Equation (24) shows that when there is no temperature gradient, a flow of electric current produces heat (the Peltier effect), the magnitude of which is determined by A8... 

There are also two well-known thermoelectric effects resulting from the joining of dissimilar materials (forming a junction) the Seebech effect, on which thermocouples are based, and the Peltier effect, used for thermopiles. The Seebech effect results when the two junctions of the dissimilar materials are held at different temperatures. The Seebech coefficient, e, is defined as the open-circuit voltage generated per unit temperature differential of the two junctions ... 

In a nonisothermal system, an electric current (flow) may be coupled with a heat flow this effect is known as the thermoelectric effect. There are two reciprocal phenomena of thermoelectricity arising from the interference of heat and electric conductions the first is called the Peltier effect. This effect is known as the evolution or the absorption of heat at junctions of metals resulting from the flow of an electric current. The other is the thermoelectric force resulting from the maintenance of the junctions made of two different metals at different temperatures. This is called the Seebeck effect. Temperature measurements by thermocouples are based on the Seebeck effect. 

The Thomson effect is small in comparison with the Peltier effect, and may be neglected to a first approximation. The thermoelectric current is therefore determined by the potential difference E — E2=J(T —f(T. In the simplest case we may assume that is a linear function of the temperature, so that E = E - -aT. We have then... 

The following tables give the values of the Peltier effect and the thermoelectric e.m.f. of various metals for a temperature difference of 1 degree ... 

Thermodynamic relationships between thermoelectric E.M.F., Thomson effect, and Peltier effect. 

The Peltier effect q is therefore determined by the first derivative, and the difference of the Thomson effects by the second derivative of the thermoelectric e.m.f. 

Thermoelectric materials have been used extensively for thermal sensing, energy conversion (heat to electricity), and for cooling (Peltier effect). In general, thermoelectric devices consist of n-type and p-type semiconductor constituents connected electrically in series and thermally in parallel [151]. While energy conversion efficiencies for thermoelectric devices are not yet competitive with conventional refrigerator or power generation systems, it is possible to achieve enhanced efficiencies... 

To measure the Seebeck coefficient a, heat was applied to the sample which was placed between the two Cu discs. The thermoelectric electromotive force (E) was measured upon applying small temperature difference (JT <2 E) between the both ends of the sample. The Seebeck coefficient a of the compound was determined from the E/JT. The electrical resistivity p of the compound was measured by the four-probe technique. The repeat measurement was made rapidly with a duration smaller than one second to prevent errors due to the Peltier effect [3]. The thermal conductivity k was measured by the static comparative method [3] using a transparent Si02 ( k =1.36 W/Km at room temperature) as a standard sample in 5x10 torr. 

The measurement of thermoelectric properties employed the DC method with high speed and high resolution[2] to remove fully errors occurring by Peltier effect. The thermo-electromotive force o was measured as a function of temperature difference AT at both ends of a specimen. The thermoelectric power a was obtained from a slope of o-A Tcurve and expressed as an absolute value. 

A solar ammonia-carbon ice maker produced 500 kg ice per day.184 Electricity from photovoltaic cells can also be used to run refrigerators.185 Photovoltaic cells were also used to power a thermoelectric device using the Peltier effect (which involves passing a current through a junction of two dissimilar metals) to reach —3°C.186 The system has no moving parts and is small, lightweight, reliable, noiseless, portable, and is potentially low-cost when mass-pro-... 

Because the dark current properties of SPD and MCP-SPD arrays are low and much of the dark current has thermal origins, the dark current can be substantially reduced by detector cooling. Most devices are equipped with Peltier-effect thermoelectric cooling systems coupled to cold liquid coolant systems to achieve temperatures as low as -40 to -80°C in some designs. 

An elementary thermocouple circuit is shown in Fig. 16.16. The EMF generated in this circuit is a function of the materials used and the temperatures of the junctions. It is useful to describe briefly the basic thermoelectric phenomena or effects that are related to the Seebeck effect and are present in thermocouple measurements. They include two well-known irreversible phenomena—Joule heating and thermal conduction—and two reversible phenomena—the Peltier effect and the Thompson effect. 

Temperature measurement using thermocouples is based on the thermoelectric effect. Two dissimilar metals are joined together at a junction where an electromotive force (emf) is generated according to the Seebeck effect. The emf level depends on the junction temperature. The Peltier effect causes an emf to be generated when the dissimilar metals are connected to an electrical circuit. 

A third emf is produced if there is a temperature gradient along either of the materials. The emf generated by the Peltier effect at the junction can be used to measure temperature, knowing the thermoelectric properties of the metals. 

There are three thermoelectric phenomena that result from correlation between propagation of heat through a conductor and displacement of the current carriers in the conductor. The Seebeck effect (Ref 1) consists of formation of an electric current in an electrical circuit formed by two dissimilar conductors if the contacts between the conductors are held at different temperatures. A reverse phenomenon, the Peltier effect (Ref 2), consists of formation of a temperature difference between the contacts in a circuit of this type if an electric current is created in the circuit by an external current source to which the circuit is connected. W. Thomson (Lord Kelvin), who explained both effects (Refs. 3,4), predicted and experimentally confirmed the existence of another thermoelectric phenomenon, named the Thomson effect, which consists of absorption or release of heat in a uniform conductor with a current passing through it when a temperature gradient (positive or negative) is present along the current direction. 

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