Physical bases of freezing point measurement using ... - Boschung

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Proceedings of the 9 th SIRWEC Conference 15-17 March 1998, Luleå, Sweden 284 Appendix A - Peltier cooling In the active probe, the freezing of the solution is artificially created by a TEC, which is a solid state heat pump that utilizes the Peltier effect. It consists of a number of p- and n-types pairs (couples) of a semiconductor material 8 [Goldsmid 1986] connected electrically in series and sandwiched between two ceramic plates, i.e. connected thermally in parallel. As shown schematically in figure A.1, when powered by a DC power supply, current i causes heat to move from one side of the TEC to the other. This creates of course a cold side and a hot side 9 . The rejected heating power Qh, greater than the heat Qc pumped in the cold side, is given by : Qh = Qc + Qin = Qc + U ⋅ i . (A1) Tc and Th are the temperatures of the cold and hod side of the TEC. U i TEC Figure A1 : schematic representation of the principle of the Peltier pumping. Qc does not increase linearly with the current. The yield of the TEC is defined by the coefficient of performance (COP) : Qc Qc COP = = Q U ⋅i in Qc Qh Tc Th . (A2) For the same TEC and temperatures Tc and Th than those considered in § 2.2, figure A2 shows the COP versus the current. The current giving the maximum coefficient of performance COPmax is named the optimal current iopt. 8 The best semiconductor for thermoelectric refrigeration at ordinary temperatures is bismuth telluride, Bi2Te3. 9 If the direction of the current is inverted, the cold and hot sides are also inverted. It is then possible to cool or to heat the solution at the surface of the probe.
REFERENCES Proceedings of the 9 th SIRWEC Conference 15-17 March 1998, Luleå, Sweden 285 COP 1.0 0.8 0.6 0.4 0.2 0.0 0 0.5 1 1.5 2 2.5 3 3.5 4 i [A] Figure A2 : dependence of COP on current for a commercial TEC 10 , Tc = - 19.1 °C and Th = 5.9 °C. Coordinates of the maximum are (iopt = 1.52 A, COPmax = 0.805). Barthel, J., Buchner, R., and Münsterer, M., 1995, Electrolyte Data Collection - Part 2 : Dielectric Properties of Water and Aqueous Electrolyte Solutions, DECHEMA. Franks, F. (ed.), 1972, Water - A Comprehensive Treatise, Vol. 1 : The Physics and Physical Chemistry of Water, Plenum Press. Franks, F. (ed.), 1973, Water - A Comprehensive Treatise, Vol. 3 : Aqueous Solutions of Simple Electrolytes, Plenum Press. Franks, F. (ed.), 1982, Water - A Comprehensive Treatise, Vol. 7 : Water and Aqueous Solutions at Subzero Temperatures, Plenum Press. Goldsmid, H. J., 1986, Electronic Refrigeration, Pion Limited. Hobbs, P. V., 1974, Ice Physics, Clarendon Press. Horvath, A. L., 1985, Handbook of Aqueous Electrolyte Solutions : Physical Properties, Estimation and Correlation Methods, Ellis Horwood Limited. Jaccard, C., 1959, Etude théorique et expérimentale des propriétés électriques de la glace, Diss. Naturwiss. ETH Zürich Nr. 2871, Helv. Phys. Acta 32 (2), 88-129. Rogers, P. S. Z., and Pitzer, K. S., 1982, Volumetric Properties of Aqueous Sodium Chloride Solutions, Journal of Physical and Chemical Reference Data 11 (1), 15-81. Ruepp, R., 1973, Electrical Properties of Ice Ih Single Crystals, in Whalley, E., Jones, S. J. and Gold, L. W. (eds), Physics and Chemistry of Ice, Royal Society of Canada, Proc. of the Fourth International Symposium on the Physics and Chemistry of Ice, Ottawa, August 14-18, 1972, 179-86. 10 MELCOR CP 1.0-31-05L.
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