Peltier

TEC-Thermo Electric Cooler

PELTIER

Peltier modules :

Thermoelectric refrigerator is used based on Peltier effect. The given effect was called after a french watchmaker (1785-1845.), who discovered it in 1834.

If you put a drop of water in the hollow on the joint of 2 semiconductors Sb and Bi, and switch on the current, the drop would freeze (with the reverse direction of the current the drop would melt ). This is how Peltier effect works.

Unlike the Joule heat which is proportional to the current strength squared (Q=R·I·I·t), the Peltier is proportional to the current strength and changes the sign (-/+) if the current changes the direction. The Peltier heat equals:

Qp = P · q
q=I·t, P is a Peltier factor that depends on contacting materials and temperature.

Peltier heat is considered positive in case of dissipation, and negative in case of absorption.

Fig. 1. The scheme of the experiment of Peltier heat measuring, Cu, Bi.

In this case the Joule heat in both calorimeters is the same (since R = R(Cu)+R(Bi)). But the Peltier heat differs in the sign. So, this experiment allows to calculate the Peltier factor.

In the table below you can see some Peltier factors for different pairs of metals.

Peltier factors for different metal pairs
Fe-constantan		Cu-Ni		Pb-constantan
T, K	P, mV	T, K	P, mV	T, K	P, mV
273	13,0	292	8,0	293	8,7
299	15,0	328	9,0	383	11,8
403	19,0	478	10,3	508	16,0
513	26,0	563	8,6	578	18,7
593	34,0	613	8,0	633	20,6
833	52,0	718	10,0	713	23,4

Usually, a Peltier factor is calculated this way:

P = a · T
P - Peltier factor, a - Tomson factor, T - absolute temperature.

In theory, the Peltier effect is explained the following way: electrons speed up or slow down under the influence of contact potential difference. In the first case the kinetic energy of the electrons increases, and then, turns into heat. In the second case the kinetic energy decreases and the joint temperature falls down.

In case of usage of semiconductors of p- and n- types the effect becomes more vivid. On the scheme you can see how it works.

COLD

Heat

Fig. 2. Usage of semiconductors of p- and n-type in thermoelectric coolers.

Combination of many pairs of p- and n-semiconductors allows to create cooling units - Peltier modules of relatively high power (see the scheme below).

Fig. 3. Structure of a Peltier module

A Peltier module consists of semiconductors mounted successively, which form p-n- and n-p-junctions. Each junction has a thermal contact with radiators. When switching on the current of the definite polarity, there forms a temperature difference between the radiators: one of them warms up and works as a heatsink, the other works as a refrigerator.

Fig. 4. Peltier module

A typical module provides a temperature difference of several tens degrees Celsius. With forced cooling of the hot radiator, the second one can reach the temperatures below 0 Celsius. For more temperature difference the cascade connection is used.

Fig. 5. An example of cascade connection of Peltier modules

The cooling devices based on Peltier modules are often called active Peltier refrigerators or Peltier coolers.

Peltier module's power depends on its size. The modules of low power might not be efficient enough. But the usage of the modules of too high power might cause moisture condensation, what is dangerous for electronic circuits. The distance between conductors on the modern printed circuit boards constitutes parts of a millimeter. Nevertheless, they were powerful Peltier modules and additional cooling systems which helped KryoTech and AMD companies to overclock AMD processors up to 1 GHz. We should notice here, that the systems work was stable and reliable enough. Similar experiments were made with Intel Celeron, Pentium II, Pentium III, which achieved tremendous performance growth.

We should point out that Peltier modules dissipates a lot of heat. That's why it's necessary to use not only a powerful fan in the cooler, but also other different fans inside the case.

Fig. 6. An outward appearance of a cooler with a Peltier module

Operating features :

Peltier modules are very reliable; they haven't got any moving parts, unlike refrigerators constructed according to the traditional technology.

But despite all the mentioned advantages, Peltier modules have some specific features, which must be taken into account when using as a part of a cooling unit. The most important characteristics are:

     1.The modules, dissipating much heat, require the relative fans and heatsinks which would manage to carry off the heat effectively. We should notice, that the thermoelectric modules have a quite low performance factor and they are themselves a powerful source of heat. The usage of these modules might cause overheating of the other components inside the system block. That's why it's necessary to install additional cooling systems inside the block. Besides, the modules consume a lot of energy, and in this connection a power supply unit capacity shouldn't be less that 250 W. Though there are sometimes Peltier refrigerators with its own power supply unit.
     2.In case of the module's failure the cooler becomes isolated from the cooled element. It might lead to fast overheating of the latter.
     3.Low temperatures might cause moisture condensation. This might lead to short circuits between the elements. That's why you should use the modules of the optimal power. Moisture condensation depends on the temperature inside the system block, the temperature of the cooled device and air moisture. The warmer air is and the more moisture is, the condensation is more probable. The table below shows the dependence between a condensation temperature on a cooled element and a temperature and moisture of the surrounding air.

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