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

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.