The thermoelectric temperature difference characteristic of semiconductor is used to promote heat transfer from low temperature to high temperature.The whole refrigeration device is composed of a cold plate, a connecting piece, a stack, a radiator, etc.[1]
Thermoelectric refrigeration uses Peltier effect to achieve the purpose of refrigeration.
Thermoelectric cooling
1834 J-C. Peltier found that when two different metals are connected and connected with current, one connector absorbs heat and the other releases heat.This phenomenon was later called the Peltier effect.
The Peltier effect has not been applied to the manufacture of refrigerators for a long time, because the Peltier effect of metal is very weak.Until the 1950s, yessemiconductorThe research of materials is further deepened, and people useN-type semiconductorandP-type semiconductorThe Peltier effect is far stronger than the metal thermocouple.From then on, the thermoelectric cooler gradually entered the practical stage.
working principle
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A thermoelectric couple composed of N-type semiconductor and P-type semiconductor connects the two semiconductors with copper sheets.When the current I flows from the N-type semiconductor to the P-type semiconductor, the connector absorbs Peltier heat α IT1, where α=α 1 - α 2, α 1 and α 2 are respectivelyThermoelectric electromotive forceThe thermoelectric electromotive force rate of P type semiconductor is positive, while that of N type semiconductor is negative.
Thermoelectric cooling
At the other connector, the current isP-type semiconductorflow toN-type semiconductorThere is heat released.The heat at the hot end is taken away by various heat dissipation methods, and the cold end can maintain a lower temperature.Under the condition of heat balance, the temperature that the cold end can reach depends on the thermoelectric characteristics of semiconductor materials, the thermal load of the cold end, and the design of devices (working state, heat dissipation conditions, etc.).
The actual cooler has two working conditions: maximum temperature difference and maximum cooling efficiency.
① Maximum temperature difference state (i.e. maximum cooling capacity state): when the current is, the temperature difference between the two ends of the refrigerator reaches the maximum value, where R is the sum of N-type and P-type semiconductor resistance, and T1 is the cold end temperature.If the thermal load Q0=0, the maximum temperature difference between the two ends of the cooler can reach: where T0 is the temperature of the hot end, Z is the quality factor of the semiconductor material, where x1 and x2 are respectivelyP-type and N-type semiconductorsρ 1 and ρ 2 are resistivity respectively.Using this method can save semiconductor materials, but the refrigeration efficiency is low.
② Maximum cooling efficiency state: when the current is, the cooling efficiency ε (the ratio of the thermal load Q0 to the consumed electric power 0 can reach the maximum value ε 0. This working mode can save electric energy consumption, but compared with the previous mode, it requires more semiconductor materials.
It can be seen from the expressions of (T0-T1) max and ε 0 that the larger the quality factor Z of the semiconductor material is, the greater the maximum temperature difference and cooling efficiency can be achieved. This requires that the thermoelectric electromotive force rate α of the material be as large as possible, so that the absorbed Peltier heat α IT1 is as large as possible.
At the same time, it is required that the resistivity and thermal conductivity of the material should be as small as possible, so that the generated Joule heat and the heat conduction from the hot end to the cold end should be as small as possible, because these two parts of heat will reduce the cooling efficiency.The best cooling materials with this characteristic are Bi2Te3 and its solid solution Bi2Te3-Bi2Se3, Bi2Te3-Sb2Te3, Bi2Te3-Bi2Se3-Sb2Te3, etc.
In addition, the thermoelectric electromotive force rate and resistivity areCarrier concentrationAs the carrier concentration increases, the resistivity decreases, but the thermoelectric electromotive force also decreases.Theoretical calculation shows that the optimal carrier concentration is 1019/cm3. Therefore, the purity of raw materials for thermoelectric cooling is only 99.999%, so the price is low.
Development History
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Thermoelectric cooling
Thermoelectric refrigeration originated from the Polarier effect discovered more than 150 years ago.By the 1950s, the rapid development of solid state physics has made people realize that semiconductor materials will produce more obvious Polarier effect than metals[2]。
Based on a large number of theoretical and experimental studies by Iofoe and Goldsmid, it was finally found that
Based on semiconductor thermoelectric materials with high merit, thermoelectric refrigeration has entered a real practical stage since then.
In the next 30 years, although the progress in improving the performance of materials has been slow, the manufacturing process of conventional refrigeration devices has developed quite maturely, and the application fields have been increasing.At present, thermoelectric refrigeration has been involved in electronics, biology, medicine, special military applications, and even some civil fields.
Cooler structure
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In practical applications, it is often necessary to connect several pairs of thermocouples in series or parallel to form a thermoelectric stack, and the cooler is composed of several thermoelectric stacks.thermoelectric refrigerator Also calledSemiconductor coolerOr electronic cooler, which works by Peltier thermoelectric effect.When direct current passes through a circuit connected by N-type and P-type semiconductors, energy transfer occurs, absorbing heat on one connector and releasing heat on the other.[3]
When the thermal load is zero, the maximum temperature difference of semiconductor cooling materials can reach about 80 ℃, and the temperature difference with thermal load is less than this value.In order to obtain lower temperature or higher refrigeration efficiency, multi-stage structure can be used.Good thermal contact is required between the hot end of the upper cooler and the cold end of the lower cooler.
thermoelectric refrigerator
Thermoelectric cooler is a device that uses electric energy to transfer heat from cold end to hot end.
Therefore, how to take heat away from the hot end continuously is one of the main contents of refrigerator design.Heat dissipation mainly includes natural convection of air, strongForced passageAir and water cooled.The air natural convection type is simpler, but the heat dissipation effect is slightly poor. The water cooling type has the best heat dissipation effect, but it needs a lot of water.It can also be cooled by circulating water and then dissipated by natural convection of air through fins.
Refrigeration application
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Compared with commonly used compressor refrigeration, when the refrigeration capacity is large, thermoelectric refrigeration is not only expensive but also consumes about twice as much electricity.
Thermoelectric cooling
The advantages of thermoelectric cooler are: no mechanical rotating parts, no noise, no refrigerant corrosion and pollution, convenient maintenance, long service life, variable cooling capacity, large design flexibility, its shape can be arbitrarily changed, can be miniaturized and miniaturized, easy to control temperature, as long as the current direction is changed, cooling can be changed into heating.According to these characteristics, thermoelectric refrigeration is suitable for making various small refrigerators and thermostats, as well as for special occasions requiring no noise and no pollution.
The main uses are:
① Small refrigerator and thermostat for instruments, such as photoelectricMultiplier tube, infrared detectors andsemiconductor laser Refrigerant, semiconductor zero meter, dew point meter, standard frequency oscillator and thermostat for electronic devices, petroleumSolidification point testerEtc.
② Medical applications include microsection freezing table for pathological sections, cataract extractor, skin disease cryotherapy device, refrigeration cap, etc.
③ There are also portable cold and hot boxesSemiconductor refrigerator, semiconductor air conditioner, etc.The efficiency of a large capacity thermoelectric cooler is not as high as that of a compressor, because the quality factor Z of semiconductor cooling materials is not large enough, while the characteristics of a small capacity thermoelectric cooler are better than that of a compressor cooler.The cooler adopts two stacks, and each stack is composed of 31 pairs of semiconductor elements.
The element size is 2.2 × 2.2 × 2.5 mm3.The cooler weighs 2.7 kg and has an electric power of 55 watts. It uses water cooling for heat dissipation.The cooler can guaranteePhotomultiplier tubeThe temperature of the photocathode is 40 ℃ lower than the ambient air, which greatly reduces the noise of the photomultiplier tube.The thermoelectric couple of the photomultiplier tube cooler adopts a primary structure.
If the two-stage structure is adopted and the electric power is increased to 200W, the temperature of the photocathode can be guaranteed to be 60 ℃ lower than the ambient air.
