Effect Peltier

The application discussed in this chapter relates to the thermoelectrical effects observed with the formalism of linear thermodynamics of irreversible processes, or TIP, also known as classic irreversible thermodynamics (CIT — see [PRU 12]), in volume (the phenomenological relations for a metal are recapped in Appendix A1.2), and then considering the junction as an interface. 

The Peltier effect is well known. It is encountered at the joint between two metals. It is generally spoken of at the same time as the Thomson effect, because these two effects both cross-link between the thermal gradient and intensity of the current. 

De Groot and Mazur introduce the Peltier effect using the following reasoning the conductor is composed of two pure materials, A and B, and the joint between the two is composed of an alloy of A and B. In this joint, of volume, l3dng between two sections 2 and 2j, the thermod3mamic values vary continuously. The temperature 

The phenomenological relations enable us to write the entropy variation in the form  

On the right-hand side of this equation, we recognize the heat conduction term (AVT)  

When a current flows through the boundary between twa different metals, heat is either absorbed or set free at the boundary according to the nature of the pair of metals. This phenomenon is called the Peltier effect, after its discoverer. 

When the current is reversed, the sign of the heat evolution is also reversed. The heat evolved is again proportional to the temperature and to the duration of the current. We shall write -f q for the heat evolved by 1 ampere in 1 second, g is a function of the temperature and of the nature of the two metals. 

The Peltier effect has been shown to be reversible, and is. therefore subject to Le Chatelier s principle, like all reversible processes. Thus, when the boundary between two metals is heated, an electromotive force must be produced. The direction of this E.M.F. must be such as to oppose a current which would produce a positive Peltier effect (that is, a heating effect) at the boundary. Thus a current will flow in a closed circuit made up of two different metals when the two boundaries are at different temperatures. No current will flow when the two boundaries are at the same temperature, even when the temperature at other parts of the circuit is not uniform. In the first case the E.M.F. in the circuit is E = — E2+E — E2, where E and 2 

According to the pioneering experiments of Seebeck, who discovered the thermoelectric potentials in 1823, we may arrange the metals in a series such that each metal becomes positively charged at the hot join with respect to the preceding metal of the series. The same summation law holds for the thermoelectric potential difference between any two metals as in the voltaic sequence, namely that it is equal to the sum of the potential differences between the intermediate members of the series. Thus, in the series A, B, C, A etc., the potential difference AD between A and D is equal at the same temperature to AB BC+CD. The thermoelectric series is approximately as follows  

Different observers do not always agree as to the exact order of the metals in the series, as the value of the thermoelectric potential difference is affected in a marked degree by the temperature and the purity of the metal. 

If the temperature values of the same chain s two junctions A/B and B/A are identical, and we have a current i, there is a power transfer W from one junction to the other, which results in a difference in temperature between the two junctions. 

---

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. 

The passage of an electric current through junctions of dissimilar metals causes a fall in temperature at one junction and a rise at the other, the Peltier effect. Improvements in this method of cooling have heen made possible in recent years hy the production of suitable semiconductors. Applications are limited in size, owing to the high electric currents required, and practical uses are small cooling systems for military, aerospace and laboratory use (Figure 2.13). 

The reversible reaction heat of the cell is defined as the reaction entropy multiplied by the temperature [Eq. (15)]. For an electrochemical cell it is also called the Peltier effect and can be described as the difference between the reaction enthalpy AH and the reaction free energy AG. If the difference between the reaction free energy AG and the reaction enthalpy AH is below zero, the cell becomes warmer. On the other hand, for a difference larger than zero, it cools down. The reversible heat W of the electrochemical cell is therefore ... 

We first assume that the Peltier effects are the only reversible heat effects in the circuit. Then if 7Ti, 7t2 are the Peltier effects at the hot and cold junctions ... 

The Peltier effect at a single junction is therefore equal to the absolute, temperature of the junction multiplied by the rate of... 

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. 

The heat of formation of a substance iji a voltaic cell may therefore be calculated from the measured Peltier effects and the electromotive force. 

Pawlewski s rule, 407 Peltier effect, 450, 460 Perpetuuin mobile, 51, 70 Phase, 20 rule, 169, 388, 446 Phosphorescence, 35 Physically small, 38, 69 Plait point, 244... 

The Peltier Effect The evolution of heat caused by the flow of an electric current across an isothermal junction of two materials. 

Pelletizing, in pyrometallurgy, 16 140 Pelouze reaction, 17 227 Peltier effect, 21 555 24 428 Pelton wheel turbine, 26 85 Pemanent Red 2B, Strontium Salt, pigment for plastics, 7 366t PEM fuel cell (PEMFC), 12 202-203 PEN, 10 222. See also Poly(ethylene 2,6-naphthalenedicarboxylate) (PEN) Penaeid shrimp, aquacultural chemical needs, 3 209... 

The reverse of the Seebeck effect is called the Peltier effect and results from flowing an electric current through the circuits of figure 9.1. If the junctions are initially at the same temperature, a temperature gradient will be developed for instance, in the case of figure 9.1a, one of the junctions will cool and the other will warm. Associated with this electric current there will also be a Joule (resistive) effect, so that the net power (P) produced at each junction is given by... 

Notes Conductive ovens are noted. Peltier effect systems in bold. Information from manufacturers literature. 

Figure 2.3 depicts an apparatus at constant uniform temperature. The battery drives an electrical current around the circuit. Heat is absorbed at one A/B junction and emitted at the other. Explain this phenomenon, known as the Peltier effect, in terms of relevant forces and fluxes. 

Figure 2.3 Peltier effect apparatus composed of metal wires A and B.

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

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. 

---