Thomson heat

Voltage measurement have been made at very low temperatures using a superconductor as one leg of a thermocouple. Eor a superconductor, S is zero, so the output of the couple is entirely from the active leg. The Thomson heat is then measured at higher temperatures to extend the absolute values of the Seebeck coefficients (7,8). The Thomson heat is generally an order of magnitude less than the Peltier heat and is often neglected in device design calculations. 

Taking all of the above into account, it can be shown that the efficiency, Tj, of a thermoelectric power generator, neglecting Thomson heat, is given by... 

Fig. 6.12. Simplified scheme of a Joule-Thomson heat exchanger for dilution refrigerator.

Specific resistance, Thermal conductivity, Peltier heat against lead, Thomson heat,... 

The most familiar thermoelectric effect is the generation of Ohmic heat due to current flow, discussed in Section 12.3.2. Less well known is the Thomson heat, produced or absorbed when a current flows in a temperature gradient. The Thomson coefficient, a, can be defined as... 

Give a microscopic explanation for the Seebech effect and for Thomson heat generation. 

Thomson coefficient r — is the Thomson heat (Qth) of the transport of one mole of charged particles in an homogeneous phase from a high temperature site to another one at lower temperature with a temperature difference of one degree [i]. The unit of the Thomson coefficient is VK 1. The relationship of Qth and r is given by the following expression ... 

Thomson heat — corresponds to the heat produced by the transport of a charged particle in an electric conductor through a temperature gradient in steady state. Particles in an homogeneous phase are transported from... 

Figure 7.5 shows a composed of a bimetallic couple metal wires a and b with one junction maintained at temperature T and the other maintained at T+ dT. An electromotive force E causes a current / to pass through the wires. A Peltier heat qpe(T + dT) per unit current will be absorbed at the warm junction and an amount of heat qpe(T) will be given off at the cool junction. To maintain a temperature gradient, Thomson heat (q l h i)(dT) must be supplied to the metal a, and an amount of heat (r/Th h)(c/7 j must be removed from b, since the current is in the opposite direction in metal wire b. In a closed work cycle, the electric energy is fully converted to heat. Therefore, the energy balance per unit current by the first law of thermodynamics is... 

Figure 7.4. System for the Thomson heat demonstration. The uniform wire is at 373 K at the middle point. At the end points, temperatures are held at 273 K. After passing a current (/), the temperature at points a and b are measured.

Figure 7.5. A bimetallic couple of metals a and b, the two junctions (points 2 and 3) are held at different temperatures Tand T + dT. qPe and g,-h show the Peltier and Thomson heats respectively, while Eis the electromotive force.

This relation represents the Thomson heat with specific entropies of transfer of individual metals a and b... 

Poynting and Thomson, Heat, 1911, 307 cf. Parker, Elementary Thermodynamics, Cambridge, 1891, 273 Thermo-dynamics with Elementary Mathematics, 1894, 145 Jellinek, Kurzes Lehrbuch der physikalischen Chemie, Deventer, 1938, 110. 

Helically finned tubes coiled around mandrels have been used for the compact Joule-Thomson heat exchangers. The high-pressure gas flows inside the tubes and the low pressure gas flows outside the tubes, in combined counter- and cross-flow. Heat exchanger designs were based on test data, some of which have been previously reported [6]. 

There is one exception to this. The liquid-nitrogen bath is parallel with the Joule-Thomson heat exchanger in order to reduce the unites over-all height. However, the bath and heat exchanger are insulated from each other, and all available space is filled so that convection does not occur in the gas column. 

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