The advantages and applications of thermoelectric cooling technology

Until 1950’s, with the rapid development of semiconductor thermoelectric mateials with good performances, the efficiency of thermoelectric effect was improved so greatly that the thermoelectric refrigeration and the thermoelectric generation entered into the field of engineering practice. When comparing with the existing compression refrigeration or absorption refrigeration, the thermoelectric cooler (TEC) depends on the heat transfered by electrons to achieve refrigeration. It has the following advantages:

1）no refrigerant and no pollution;

2）no mechanical moving parts, simple structure, no noise, no friction and high reliability ;

3）the cooling speed and temperature is adjusted by changing the working current, which is flexible control;

4）thermopile can be arbitrary distributed and the size and shape can be changed according to requrement. So TEC can be widely uesd in the defense industry, agriculture, commerce, medical and some other fields. For example, it not only can be used for small travel and refrigerators, water dispensers and other household appliances, but also can be used for low temperature medical apparatus. However, the most important application is in the field of information technology, which can be used as cold source of electronic componets (infrared detector, laser diode, transistor, resistor or other CPU components and computer chip).

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The performances and characteristics of TEG (I)

Based on the “thermoelectric effect”, a thermoelectric generator (TEG) converts heat directly into electricity. Because of no moving parts, it is very silent and highly reliable.

1 The classification of TEG

According to the hot end temperature of TEG, it can be divided into three categories: high temperature TEG (more than 700℃), medium temperature TEG (between 400 and 700℃) and low temperature TEG (under 400℃). According to the usage heat source, there are four types of TEG: 1) radio isotope TEG, which converts the decay heat of radioactive isotopesdirectly into electricity; 2) nuclear reactor TEG, which converts the heat produced by nuclear fission into electricity; 3) hydrocarbon fuel TEG, which includes natural gas, liquefied petroleum gas and other gas TEG; 4) low heat TEG, which converts available low-grade thermal energy into electricity.

2  The main structure of TEG

The structure of TEG mainly include the following sections: 1) heat source; 2) thermoelectric devices (including thermo electric materials); 3) electrode, which is used for connecting two different types of thermoelectric devices; 4) thermal insulation materials; 5) heat sinks; 6) radiation shielding and safety protection device; 7) voltage converting and power adjusting device. To measure the overall electrical performance index is the thermoelectric conversion efficiency, output power, output voltage, working lifetime, weight, volume, cost and reliability respectively.
The left characteristics of TEG will be introduced in the next article.

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Heat Sink in Thermoelectric Cooling System

Thermoelectric cooler is like a heat pump when it’s working, transferring heat from one point to another. This is not a common heat absorption process or a magic heat consumption process. When it’s powered on, one end of the TEC module will become cold while the other end hot. The cooled down end transfers the heat to the hot end, which conforms to the thermodynamics law. In order to complete a heat transfer process, the thermoelectric cooler needs to be connected to a proper heat sink, which is used to release the heat from the cold end and produced during the operation of the components.

A heat sink is an indispensible part of the thermoelectric cooling system. Since all the performances of thermoelectric cooler are closely related to the temperature of the heat sink. An ideal heat sink can absorb unlimited heat with the capability to increase the temperature of itself. However, in actual application, it’s impossible. Therefore, designers should find a heat sink, whose temperature rise is in an acceptable range after it absorbs heat from TEC module. Although this temperature rise is related to different application environment, it’s good to reduce it since the refrigerating capacity of TEC module is decreased with the increase of the temperature difference.

To be continued….

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Research progress of thermoelectric refrigeration technology from thermoelectric material point of view (I)

The material figure of merit Z is an important parameter to measure the cooling capacity of TEC and its expression is. In order to improve Z value, conductivity σ and Seebeck coefficient α should be improved and thermal conductivity λ should be reduced. λ consists of phonon thermal conductivity λp (about 90%) and electronic conductivity λe. These three parameters are not independent of each other, which all are the function of carrier concentration n and temperature T. In the condition of different environment temperature, the value of Z is different. In general, the dimensionless factor ZT is used to describe material’s performance. Bi2Te3 is the most widely used thermoelectric material and the value of ZT can be up to 0.9. However, the cooling efficiency of TEC made by Bi2Te3is only 30% of compression refrigeration.

In recent fifty years, the performance of thermoelectric materials has been increased by roughly 20%. At present, there are many kinds of developing thermoelectric materials, such as non-oxide semiconductor thermoelectric materials, oxide thermoelectric materials, low dimensional thermoelectric materials, super lattice thermoelectric materials and so on.

Semiconductor thermoelectric materials of the earliest and most mature is (Bi,Sb) 2(Te, Se) 3, which is a solid solution material. Most of the current TEC are using this material, which can increase α and reduce λ.

Currently, the semiconductor oxide material is one of oxide thermoelectric materials, which has many advantages of no pollution, stable performance and can work for a long time at high temperature. This material mainly includes Na-Co-O and Ca-Co-O.

The left research progress of thermoelectric refrigeration technology will be introduced in the next article.

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Research progress of thermoelectric refrigeration technology from thermoelectric material point of view (II)

Low dimensional thermoelectric (0~3D) material is a new research in recent years. Because of the function of the quantum well and quantum wire, low dimensional material has extraordinary thermoelectric property. The low dimension can increase α and phonon boundary scattering of the potential well surface. Terry M Tritt obtained the different dimension curve of Bi2Te3 with the change of quantum welldW(quantum wire thickness). The result shows that three dimensional material do not change with the change of dW; however, in the condition of the dimension reduction, the value of ZT will increase sharply when dW reduces, which shows that low dimensional material can improve ZT value of thermoelectric materials.

Superlattice material is formed by periodically alternating growth of two or more kinds of materials with thin layers, which was researched by Hicks and Dresselhaus. The structure of superlattice material can obviously improve the energy transfer of phonon and electron and reduce λe, which is a very potential way to improve the thermoelectric figure of merit. Currently, it is just used for electronic devices.

Thermoelectric refrigeration technology should achieve a breakthrough in materials, and through doping, low dimensional and superlattice structure, the figure of meritcan be significantly improved, which is the future development direction of thermoelectric materials.

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The performances and characteristics of TEG (Ⅱ)

3 Thermoelectric materials

In the view of the research and development of thermoelectric materials, lead telluride is used as a thermoelectric material in the early days, and then TAGS alloy is developed for P-type element (N-type is still PbTe). The temperature of the hot end can be up to 600℃. After using new thermoelectric materials in hundred watt thermoelectric generator and universal thermoelectric generator, the hot end temperature can reach 1000℃ and its mechanical properties and oxidation resistance are very good. In the condition of high temperature, this material is not easy evaporated and can work in vacuum and in air.

In recent ten years, because of the need for environment protection and military application, the research of thermoelectric materials again attracts people’s attention. Currently, the dimensionless figure of merit ZT of the best bulk thermoelectric materials (BiTe) is approximately equal to 1.0. In order to complete with other energy conversion systems, ZT must be increased to 1.5～3.There are three ways: firstly, researching new materials; secondly, studying function gradient materials; thirdly, reducing the material’s dimension.

There are many kinds of high-quality TEG in our company (ATI). We have such a professional design team and production workshop that we can be your one stop company for designing and building your TEG based on the “thermoelectric effect”.

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Characteristic analysis of TEC refrigeration cycle (III)

From (3), (13) and (14), it is shown that when Th –Tc>△Tmax, the cooling capacity qc<0. At the same time, TEC cannot cool, but start heating, which is not allowed in practical work. So the working current should be controlled in range of I0≤I≤Isat, which can ensure the cooling capacity qc≥0. Although based on the analysis of the cooling capacity qc, the general range of working current can be determined, but in practical work, the refrigeration coefficient is still an important indicator. From (9), when the working current of TEC satisfies the following relations, the TEC can reach the maximum refrigeration coefficientβmax:

Where, Iopt is the working current in the condition of meeting the optimal refrigeration coefficient; Z is the figure of merit,

The relationship among current I, cooling capacity qc and β is shown in the figure 3. WhenIsh<I<0 andW<0, TEC is in the power generation mode. When I<Ish、0<I<I0 and I>Isat, the cooling capacity of TEC qc<0 and it will heat the cold load, which should be avoided as far as possible in practical work. When 0<I<Isat, the amount of heating load is small because there exists the Peltier effect reducing the impact of heat resistance in the cold end. Only when the current meetsI0<I<Isat, TEC can work normally in the cooling mode; when I=I0 or I=Isat, it cannot cool. The cooling capacity qc is a quadratic function about current I, so qc is symmetric in the two interval, I0<I<Isat and Imax<I<Isat. However, when considering that the refrigeration coefficient β increases with the current increasing in the range of I0<I<Iopt and decreases with the current increasing in the range of Iopt<I<Isat, so he refrigeration coefficient should be considered at the same time with improving cooling capacity and the working current in Imax<I<Isat is not selected in general. Obviously, TEC has two extreme ways of working, the maximum refrigeration coefficient operation mode and the maximum cooling capacity operation mode. The former is the operation mode of variable voltage and variable operating current; the latter is the operation mode of constant working voltage and working current. But in the actual work, the hot and cold end temperature of thermoelectric cooleris changing, and it generally works between the maximum refrigeration coefficient and the maximum refrigeration capacity, so the current needs to be regulatedaccording to the actual situation.

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