Thermoelectric cooling

A Peltier element is an electrothermal transducer, which generates based on the Peltier effect ( by Jean Peltier, 1785-1845 ) in the case of current flow, a temperature difference or temperature difference between a current flow ( the Seebeck effect). A common abbreviation for Peltier and Peltier cooler TEC (English thermoelectric cooler ).

Principle, basics

Based on the Peltier effect, the contact of two semi-conductors, which have a different power level (either p-type or n -type) of the conduction bands. On passing a current through two consecutive points of contact of these materials, it must be included on the one point of contact heat energy so that the electron enters the higher energy conduction band of the adjacent semiconductor material, therefore it comes to cooling. On the other contact point, the electron falls from a higher to a lower energy level, so that this energy is released as heat.

Since n-type semiconductor having a lower energy level of the conduction band, the cooling takes place at the point where the electrons from the n-type into the p -type semiconductor transition (technical current flow that is p- doped to n-doped semiconductor).

, The effect also occurs with metals on, here, however, is very low and is almost completely overlapped by the Joule heat and the high thermal conductivity.

A Peltier element is made of two or more small blocks each composed of p- and n- doped semiconductor material ( bismuth telluride, silicon germanium ) which are alternately connected upward and downward to each other by metal bridges. The metal bridges also form the thermal contact surfaces and are isolated by an overlying film or a ceramic plate. Always two different cubes are joined together so that they form a series circuit. The supplied electric current flows through all the rectangles in sequence. Depending on current speed and direction, the upper connection points cool, while the lower heat up. The current thus pumps heat from one side to the other, and produces a temperature difference between the plates.

The most common form of Peltier elements usually consist of two square plates of aluminum oxide ceramic having an edge length of 20 mm to 90 mm and a distance of 3 mm to 5 mm, between which the semi-conductor cubes are soldered. The ceramic surfaces are for this purpose provided, at their facing surfaces with solderable metal surfaces.

On cooling the hot side eg by means of a mounted heatsink with fan, the cooling side will be even colder. The temperature difference between the two sides, depending on the element and power, be in single-stage elements to about 70 Kelvin.

The inverse of the Peltier effect is the Seebeck effect. It is possible to generate by forming a temperature differential between the two sides of a Peltier element electricity (see thermoelectric generator ).

Pros and Cons

The biggest advantages of a Peltier element are the small size, low weight, avoiding any moving parts, gases and liquids; a chiller, however, always requires a refrigerant and in most cases a compressor.

By reversing the direction of current is possible with Peltier elements for both cooling and heating. Thus, a thermostatic control of components can be achieved, when the ambient temperature is above or below the target temperature.

A disadvantage of the Peltier elements is the low efficiency of about 1/10 of the Carnot efficiency, which results in high electrical power consumption at relatively low cooling power and temperature difference. Further elements are larger than 60 mm × 60 mm rarely available. Nevertheless Peltier elements are used for many applications.

Use

Peltier elements can be used wherever cooling is required with small temperature difference or without the need for efficiency. Peltier elements are used for example in coolers, where the use of a chiller for reasons of space prohibits or would not be profitable because the required cooling power is low.

Further Peltier elements are used to cool the CCD chip in a digital photo camera. This ( in astrophotography for example ) the image noise is reduced when long exposure times significantly. Multistage Peltier elements are used for cooling of the radiation receivers in infrared sensors.

Increasingly, Peltier elements and input in a variety of laboratory instruments, where the temperature is an important parameter, such as in density meters, viscometers, rheometers or refractometers.

Taupunktspiegelhygrometern typically cool in one or more serially connected Peltier - elements from the mirror up to -100 ° C. This makes use of the fact that you can control the cooling capacity of Peltier elements electrically quickly.

Also diode lasers are often cooled with Peltier elements and thermostated, that is kept at a constant temperature to keep their emission wavelength and / or efficiency constant. Also, optical elements downstream of the diode and the other lasers are often thermostated with Peltier elements.

Peltier elements can here both for cooling and - are used for heating - with current reversal.

Likewise, Peltier elements are used in isolated cases as part of CPU coolers. The Peltier - element enables here, the CPU cooling to temperatures below the body inside temperature, resulting in either the over-clocking of the CPU without any loss of stability allows or enhances the lifetime of the processor. The element is thereby installed at the bottom of a heatsink with fan and powered by the power supply to the power required. To date, such solutions have but because of their additional energy consumption ( the electrical energy used is given off as waste heat into the cabinet! ) Not enforced, but could in the near future for high-powered processors with thermal power dissipation of 150 watts and more become interesting again.

Photodiodes, eg for reading out scintillators can be cool due to their low surface with Peltier elements, making it possible to the noise and the dark current dramatically decrease.

In diffusion cloud chamber Peltier elements are used to maintain the temperature difference between the base and lid.

The in molecular biology, now part of the basic equipment thermocycler using Peltier elements to heat and cool, which is, for example, the polymerase chain reaction necessary samples quickly.

Satellite DXler can relatively inexpensive improve the signal -to-noise ratio by cooling their packed between two Peltier elements LNB. In this way, you can also receive a bit too small satellite dish television programs outside of the actual illumination zone.

Peltier elements are also used in dehumidifiers sometimes. Here flows to the humid air through the cooling element through the cooling of the contained water which is collected in a collecting vessel condensed.