The soldering of TECs in Thermo-electric Cooling System (II)

(c) Use proper pre tin in the soldering area of the surface of the heat sink. Thus, the melting point of the solder shall be lower or equal to the maximum operating temperature of the thermoelectric cooler. When plating tin on the heat sink, we shall keep the temperature of the heat sink in a proper temperature, which could melt the solder but not exceed the maximum operating temperature of the TEC .

(d) Cover the solder flux on the hot end of the TEC, and then place the TEC on the pre tin area of the heat sink. Keep the liquid solder to be float and then rotate the TEC to make the solder and the TEC contact each other. If the TEC tends to sink in the solder, it indicates that the solder is not enough. At this time, take the TEC down and add more solder on the heat sink.

(e) After several seconds of step d, the surface of the TEC has been soaked fully. Put a heavy object on the TEC or use a chucking to be fastened in the required position. Remove the heat sink from the heat resource and cool down the TEC. After fully cooling. Then remove the residual flux by deoiling the TEC.

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The soldering of TECs in Thermo-electric Cooling System (I)

When proper measures have been adopted to protect the TECs from being overheated, we can solder the TEC with metalized surface to thermoelectric components. In order to prevent the TEC from excessive mechanical force, solder one surface (usually hot end) to a component with rigid structure. It’s worth noted that when we solder the TEC to a rigid structure component, other components or small circuits shall be soldered in the cold end of the TEC, which will not connect to the external structure rigidly. During soldering, in order to prevent the TEC from excessive heat, we need to control the temperature precisely. In the meantime, the temperature during the process will not exceed the operating temperature of TECs. Since the coefficients of thermal expansions of ceramic plates, heat sink and the object are different, it’s not recommended to use soldering method in over 15×15 mm2 TECs.

There are several steps in soldering process

(a) Flat the installing surface of the TEC by lathe machine or polishing method. Although it doesn’t need high flatness when using epoxy resin, but usually the surface should be flat enough. In addition, the surface of the heat sink is a weldable surface, such as copper or copper plating material.

(b) Clean the TEC and all the installing surfaces in order to remove grease and heavy oxide layer. Make sure there are no rags or foreign matters on the soldering surface.

To be continued….

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Comparing TEC with vapor compression

Thermoelectric cooler (TEC) is built due to the Peltier effect, which both have the abilities of heating and cooling. When comparing with the traditional cooling method, vapor compression cooling, TEC has the following features:

1) TECs need no refrigerant, so there is no leakage or pollution.

2) Because of no mechanical rotation parts, when TEC works, there are no noise, no wear, long service life, with high reliability and convenient maintenance.

3) The cooling speed and temperature can be adjusted arbitrarily by changing the working current.

4) The exchange of cold and hot side can be realized by changing electrical polarity, so it is very suitable for high temperature thermostat and other occasions

5) TEC has a compact size. For example, a four-stage TEC that can reach the low temperature of 100℃ has the same size with a cigarette case does.

6) The cooling capacity can be very small. It can as small as 1W or even smaller.

7) TEC needs to be connected with a DC power supply.

8) The cost of TEC is very expensive.

The main disadvantage is that when the cooling capacity is large, the efficiency will be declined and the power consumption is about twice as much as that of the vapor compression refrigeration. However, the smaller the cooling capacity of vapor compression is, the lower the efficiency will be. But the efficiency of TEC will remain unchanged. Therefore, the efficiency of TEC is higher than that of other cooling methods for the occasion with tens of watt.

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The basic principle of TEC (II)

The cooling principle of TEC is shown in figure 1. P and N type semiconductors are sandwiched between metal plates, which can form a pathway. Under the action of external electric field, the hole of P type in the point(a) needs to absorb certain energy to improve its own potential energy and then goes into P type semiconductor, so the temperature of the junction can be decreased and the cold contact point is formed. But the hole in the point(b) needs to release the excess energy to enter thesheet metal, and then the junction temperature will rise to form a hot contact.

The free electrons flow in the N type semiconductor and its direction is contrary to the direction of holes. At the point of c, they absorb heat and then enter the N type semiconductor.At the point of d, they release heat and then enter the sheet metal. So at a and c, the external heat can be absorbed and at the b and d, the extra heat can be released, which is cooling principle of TEC. Obviously, by changing the current direction, the cooling and heating can be switched.

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Basic principle of TEC (I)

In the closed circuit with two different metals, if the temperature of the two contact points keeps different, electric potential difference will generate and at the same time, there will be current through the closed circuit, which is called Seebeck effect. Conversely, in the same closed circuit, when the direct current is supplied, one contact point becomes cold and the other one gets hot, which is called Peltier effect or thermoelectric effect. This phenomenon can directly lead to the invention of coolers, but it is not TEC. Until in the 1950s, TEC began to be used in practical applications with the development of semiconductor technology.

Actually, in the loop with any two different conductors, there are always endothermic phenomenon and exothermic phenomenon at the contact point. Why is semiconductor material used to make TEC? This is because in the metallic conductor, the average energy of the free electron participating in the conduction is very small and the effect of endothermic and exothermic is weak. However, because of the internal structure of semiconductor, it can generate temperature difference phenomenon much more obviously than other metals do. That is why semiconductor material is generally used for making TEC.

The important feature of semiconductor is that after a certain amount of some impurity seeping into semiconductor, it can not only improve the electrical conductivity greatly, but also produce many semiconductors with different nature and different uses, depending on the type and quantity of impurity. After seeping impurity into semiconductor, free electrons are released, which is called N type semiconductor. Correspondingly, P type semiconductor is conductive, which relies on “hole”. The free electrons in the N type semiconductors and something all involved in the conduction in the P semiconductor are collectively called “carriers”. Carriers are unique to semiconductorsdue to the incorporation of impurities.

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The advantages and disadvantages of TEC and its development status (II)

With the rapid development of science and technology, technical difficulties about material system and manufacturing technology have been solved. At home, there has been ceramic technology to produce the TEC cooling materials and the standard of the materials can be in the international advanced level. What’s more, the merit of figure of high efficient TEC with China’s independent intellectual property rights has improved a lot and can be over 13×10-3K. Now when in the temperature difference of 50℃，the cooling coefficient of high efficient TEC is more than 3 and its efficiency can also be over that of compressor refrigeration.

Nowadays, the advantages of TEC have come out. Thermoelectric cooling is a method of solid cooling. Compared with the compressor cooling system, TEC has no mechanical rotation parts, no noise, no refrigerant, no pollution, high reliability and long service lifetime. Additionally, it can also be controlled easily.

But the outstanding advantages of TEC are that the micro cooling can be realized, the power can be below 1Watt it can be extremely light minima (<50mm 3,<1g), which the traditional mechanical compressor cooling cannot replace. For example, the thermoelectric cooling technology has been applied into semiconductor lasers and CPU of personal computers. Furthermore, the TEC can be used to make many other small household electrical appliances, such as small air conditioner, automotive air conditioning, ice machine, cold and hot water dispenser, heating mattress, air-conditioning chair, etc.

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The advantages and disadvantages of TEC and its development status (I)

Thermoelectric cooler (TEC) is an ancient technology. As early as in the 1950’s, TEC has once set off a boom. When it is connected to a power supply, it can become cold, which is very popular among home appliance manufacturers. However, because of the poor performance of components, it failed to be applied in actual life. The key, which determines the rise of decline of materials and process of TEC, is to improve figure of merit and the cooling effect of the hot and cold end.

In order to meet the cooling requirement, TEC materials not only need N and P type semiconductor properties, but also need thermoelectric potential, electric conductivity and heat conduction rate of semiconductor added impurity. Production of traditional TEC materials mainly use zone-melting method and powder metallurgy method, but the production cost is very high and the equipment process is also very complicated. At home, the common used materials are on the basis of Bi2Te3, P type is Bi2Te3-Sb2Te3 and N type is Bi2Te3-Bi2Te3. The vertical zone melting method is used for extracting crystal materials. The optimization coefficient of thermoelectric elements produced in this way is only 3×103K-1, which can result in low cooling efficiency. When the temperature difference is 50℃, the cooling coefficient is only 0.06. Only if the optimization coefficient is up to l3×103K-1, the cooling efficiency can be the same with the compression refrigeration efficiency.

Therefore, because of early technology and process constraints, the TEC efficiency is too low, so TEC haven’t received enough attention and application.

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Practical application of TECs

CPU of computer is a high density heating element. Based on the cooling principle of thermoelectric cooler (TEC), the cold end of TEC is in contact with the display chip and the hot end is in contact with the heat sink. When connecting a power supply, the temperature difference of the cold and hot end will come out and moves from the cold end to the hot end through lattice energy. Only if the hot end heat can be dissipated effectively, the cold end will be cooling constantly and the cooling effect will be pretty good. In fact, TEC is a heat pump, which can transfer the heat from chips to heat sinks, so only if the heat of the TEC hot end is being dissipated continually, the cold end can be cooling and the display chips can be in a relatively constant temperature state. However, in the hot end of TEC, the heat sink and fan are still needed. In the process of using TEC, there are some other points to consider.

1 External power supply

When comparing with general air cooling, the power of TEC is very high and it can be up to 36W~40W, so at least the power supply 12V, 3A is needed.

2 Condensation phenomenon

The condensation phenomenon is a killer for TECs. In the high humidity environment, if the temperature is low, the water vapor in the air condenses into droplets on the surface of TEC, which is condensation phenomenon. If the droplets flow to the motherboard or display chip, the consequences will be terrible.

3 TEC size

According to the area of chips, the nearest size should be selected, but not greater than the chip itself.

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Some other heat-dissipation methods of TEC

1.Using endothermic phenomenon of material melting

When a material is melting, it will absorb the heat from the hot end of thermopile in the isothermal condition. The isothermal characteristic is the necessary condition of maintaining the cold end constant. The time for the melting stage is the temperature constant time of the cold end.

Because of different material thermal conductivities, the heat transfer temperature difference is also different, the thermal conductivity is low and the temperature difference is high. It is suitable for TECs only in a relatively short working time or intermittent work situations.

2.Using endothermic phenomenon of the heat capacity

The material with great heat capacity can absorb the exhausted heat from the hot end and make the temperature increase. At this point, the heat transferring is unstable. With the time going by, the temperature of the endothermic material becomes high and the hot end temperature also become high. At the same time, the amount of heat change between the endothermic material and the air around also changes with time.

3.Using the latent heat of vaporization from materials

Using the big latent heat of vaporization from a lot of liquid, the hot end can be dissipated. In order to make TEC reach the necessary low temperature level, the insulating materials can be used as the outside packing of TEC working space, such as foam, etc.

4.The derived heat dissipation of heat conduction strips

Basically, components with the cooling requirements can contact with the heat conduction strips directly or indirectly. The heat conduction strips can be directly used as cooling devices if the heating power is not high or the environment permits. The advantage of this way is low cost, high reliability, but the disadvantage is the requirement of high temperature gradient for heat dissipation.

5.The heat dissipation of thermo-syphon

In the small temperature gradient, the thermo-syphon is a heat transfer element, which can transfer heat from one place to another. The thermo-syphon belongs to “two times heat change” and its heat transfer ability mainly depends on thermal resistance between heat medium pipes and working medium, if the total thermal resistance is bigger than conventional heat sink, the heat ability will decrease. The remaining non condensable gas and appropriate saturated vapor pressure have great effects on the performance of thermo-syphon radiator. What’s more, the working temperature is determined by the boiling point of liquid because the temperature is the necessary condition of selecting thermo-syphon.

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Installation of TECs in Thermo-electric Cooling System (II)

In some specified applications, the main method is to use a special high thermal conductive epoxy resin binder. Since the coefficients of thermal expansion of the ceramics of TECs, heat sink and the cooled object, it’s not recommended to use epoxy resin binder in large TECs. The detailed steps are as follows:

(a) Flat the installing surface of the TEC by lathe machine or polishing method. Although it doesn’t need high flatness when using epoxy resin, but usually the surface should be flat as much as possible.

(b) Clean the TEC and all the installing surfaces in order to make sure there are no burr, dust or grease dirt. Pretreatment to the surfaces shall be made based on the requirements of the epoxy resin manufacturer.

(c) Overlay a thin epoxy resin on the hot end of the TEC, and put the hot end in a proper position of the heat sink. Press and rotate the TEC repeatedly till some resistance can be felt.

(d) Put a heavy object on the TEC or use a chucking to be fully cured. If you want to cure it in a roaster, make sure the heating temperature will not exceed the operation temperature of the TEC.

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Selection and installation of TECs

In general, selecting the size of TECs depends on the area of chips, but not bigger than that. Additionally, TEC has a certain thickness, which is put between chip and heat sink. If it is too thick, it will be hard to fix.

Fig. 2. Physical photo of TEC

Generally speaking, the operating temperature of TEC is about under 50℃, that is: CPU should work in the range of operating temperature. As the maximum temperature difference can be up to 65℃, if the hot end temperature is not over 65℃, in the limit case, the temperature of the cold end can reach 0℃ or even under 0℃.

As shown in fig. 2, the TEC should be fixed according to the order of heat sink-TEC-CPU. Interface between TEC and the other two parts should be smeared with thermal grease, which can not only be useful to transmit heat, but also to buffer.

Fig. 2. Installation diagram of TEC

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Installation of TECs in Thermo-electric Cooling System (I)

n thermo-electric cooling system, the installation of TECs is very critical. Basic principles should be abided by during installation, or it may result in bad performance and reliability. There are a few factors to be considered in designing and installing the thermo-electric cooling system, as below.

TECs have a high mechanical strength, but a relatively low shear strength. Therefore, do not place TECs in a mechanical system that plays a major supporting role.

All the interfaces in the system should be in parallel, smooth and clean, in order to reduce the thermal resistance. High thermal conductivity materials should be used to guarantee good contact among surfaces.

Hot end and cold end of thermo-electric cooler can be told by the positions of the leads. The lead is usually soldered on the surface of the hot end, which is connected to the heat sink. For those TECs using insulated leads, red lead and black lead are connected to the anode and cathode of DC power supply. Heat flows from the cold end of the TEC to the overall TEC and then to the heat sink. For those TECs using bare leads, when the lead is facing the observer and the base of the leads is downward, the anode is connected to the right side of the TEC, and the cathode to the left side.

When the temperature is lower than the ambient temperature, the object shall be cooled with air insulated as far as possible, in order to reduce heat loss. Moreover, in order to reduce convection loss, blow the air directly to the object instead of installing fans. Meanwhile, it’s necessary to avoid the direct contact between the cooled object and external structure unit, which helps reduce convection loss.

To be continued….

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Air cooling of TEC

In this paper, the air cooling method of TEC is mainly introduced.

1.Air national convection cooling

In many small TECs, heat exchange system with air national convection cooling is usually used, which needs a certain heat sink as the heat exchanger. The heat moved by cooling medium is absorbed by heat exchange of the cold end heat absorber and air around. Then based on the heat conduction of connection pieces and insulation layers, the heat is taken away by thermopile cold end and after that the heat is transferred to the hot end heat sinks by the thermal conductivity of each layer. Based on air convention, heat sinks dissipate the heat into the environment, to achieve the cooling purpose. There are many kinds of heat sinks for air cooling, such as heat sink with ribs, acicular heat sink and so on. It is noteworthy that in the process of heat dissipation, except for heat convection, there is also radiation heat transfer. Studies show that under different surface temperature of heat sinks, the radiation heat transfer of the heat sink surface also plays a very important role.

2.Forced-convection heat transfer

When comparing with natural ventilation, the forced-convection heat transfer has improved the heat transfer coefficient a lot, and in the same power dissipation, its radiating area is reduced by many times. However, when using the forced ventilation heat sink, the calculations of additional conditions must be considered, such as the structure size of heat sinks, surface roughness, air velocity, and so on.

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Heat-dissipating methods of TEC (I)

In practical applications, TEC can sustain work through continuous heat change of heat exchanger and cold and heat source. When the system works, the heat density of cold and hot ends can be up to 104W/m2, so effective operation is strongly dependent on heat transfer performance of cold and hot end. The heat dissipation of the cold end is more important than the hot end’s. Suppose the heat dissipating capacity of the cold end is q1, the heat dissipating capacity of hot end is q2 and the electric power consumption of the system is w0. After the TEC is connected to a power supply, the cold end continuously absorbs q1, which is cooling capacity. This part of the heat and electro-thermal consumption need to be dissipated from the hot end together, which can ensure the normal work of TEC and make TEC play an effective role.

Obviously, 

Efficiency of TEC: 

Thus, in order to improve the cooling efficiency, it mainly depends on heat dissipation and transfer cooling capacity. So the design of heat sink is very important.

Currently, there are some common dissipating methods:

1.Air cooling. Heat transfer coefficient is about3~6 W/(m2•K) (natural convection) or 2 6 ~ 3 0 W/(m2•K) (forced convection).

2.Liquid cooling. Heat transfer coefficient is about 100~1000W/(m2•K).

There are also some other dissipating methods, such as using melting heat, dissolution heat and thermal capacity.

In the next paper, the specific heat-dissipating methods will be introduced.

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The characteristics of TEC technology (Ⅱ)

Although the two cooling systems comply with the same thermal dynamics, a TEC has many obvious advantages: simple structure, small volume, light weight, high reliability, long service lifetime, no noise, small power consumption, low cost and so on. What’s more, there is no need for TECs to fill chemical consumables, which the mechanical cooling method needs. Because of no moving parts, there is no wear and tear for TECs. Thus TEC is an ideal cooling method, which can gradually take the place of the mechanical cooling method when cooling many components (such as CCD).

TEC modules not only have the cooling function, but also have the heat function when the current direction is changed. This characteristic can make TEC control temperature more ideally, heat or cool the media according to the work needs, so it is easy to achieve intelligent temperature control. Studies show that if a thermal management system is established, the precise temperature control（±0.1℃）can be achieved by using TEC modules andone module can both heat and cool.

Our company (ATI) is specialized in designing and manufacturing TECs. We not only have off-the-shelf standard products, but also have custom designed and manufactured ones. We can become your one stop company for designing and building your TEC based on thermal systems.

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The characteristics of TEC technology(I)

Based on the Peltier effect, thermoelectric cooler (TEC) is an energy conversion process, which uses electric energy as power. When the direct current is through the loop made of two different conductive materials, the endothermic phenomenon will be generated in the node, and when the current is in the opposite direction, the exothermic phenomenon will be generated.

As the Peltier effect of semiconductor materials is particularly significant, currently, the TEC module is usually composed of semiconductor materials in the field of TEC. In practical application, there are generally many couples of semiconductors N type and P type in series. At the same time, the hot end needs to be connected to the heat sink and the cold end is directly connected to the cooled object through cold storage tablets. By changing the direct current, the absorbed and liberated heat at both ends of TEC can be changed, so the cooling capacity of the cold end and the heat of the hot end can meet the practical demand. The direction of the cold and hot end can be changed by changing the current direction. In practical application, when the one-level TEC cannot satisfy the required operating temperature, the two-level of multi-level TEC can be used. The TEC modules are solid electronic components. In all of cooling system, the TEC system is the only one made of one component.Traditional mechanical cooling unit include two parts of compressor and circulation system.

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Application prospect of thermoelectric cooling technology in temperature control of composite materials

In practical application, the thermoelectric modules are often placed inside of devices, and even they can be directly combined with devices needing refrigeration into a whole. For example, in optical application, the thermoelectric module can be a part of a detector or diode; in a semiconductor device, it can become a whole with the ceramic package. This lays the foundation for developing composite materials with temperature control-structure integration or even intelligent temperature control composite materials.

Especially with the further miniaturization of the refrigeration module structure, the adverse effects of internal contact layer resistance, heat conduction and thermal radiation will be more significant. Therefore, the thin film integrated structure with strong compatibility will become the mainstream development direction of microelectronic cooling technology.

Thus, thermoelectric cooler (TEC) with the integrated design and integration technology of aerospace vehicles in the aspects of function and structure satisfies the future development trend of thermoelectric cooling technology, especially in the application of aerospace. Based on composite materials of TEC temperature control and intelligent temperature control, the organic combination of TECs, structural materials and functional materials is a major development direction of future application of thermoelectric cooling technology.

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The miniaturization of thermoelectric cooler (TEC)

Based on the Piltier effect, thermoelectric cooling is an energy conversion process, using electric energy as power, that is: when the direct current is through two different conductive materials, nodes will have endothermic phenomenon (when the current is in the opposite direction, exothermic phenomenon will be generated). As the Peltier effect of semiconductor materials is very obvious, the thermoelectric modules are mostly made up of semiconductor materials at home and aboard.

In the practical application, thermoelectric cooling is generally made up of only one element (i.e. TEC), which is suitable for dot cold. With the continuous development of electronic device miniaturization, structureintegration and function diversification, thermoelectric cooling will become the mainstream technology of cooling electronic components. Therefore, the TEC miniaturization is the direction of development. What’s more, due to the lightweight requirements of the aerospace application, the slim and large thermoelectric cooling film is the another development direction. In order to solve the heat dissipation problems generated by device miniaturization of aerospace vehicles, nanotechnology will be used to develop TECs, which ensure thermoelectric materials of TEC can meet the requirements of high conductivity and low thermal conductivity. Maybe nanotechnology can restrain thermal conductivity obviously and at the same time, electron transfer capability cannot be reduced. Additionally, it can also effectively improve the contact thermal conduction between TEC and other devices.

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The principle of improving material performances of thermoelectric cooler （II）

In the ion conduction type conductor, the carrier may be positive ion or negative ion. In the electronic conduction type conductor, carrier is electron and current can be formed by electron movement. With the increasing of carrier concentration, thermoelectric coefficient α will reduce and thermal conductivity σ will increase. In the thermal coefficientλ, the lattice thermal conductivity λP (about 90%) is irrelevant to the carrier concentration, while the electron thermal conductivity λe is proportional to the carrier concentration. So the appropriate carrier concentration n is selected to make  get the maximum difference. Figure of merit z and other three parameters α, ρ, λ have a certain relationship with carrier concentration n. When n is close to 1019cm-3, the value of Z can be the maximum.

Though controlling the carrier concentration n, thermal conductivity σ and thermoelectric coefficient α can be improved and thermal conductivity λe can be reduced. So the current investigators of TEC materials should try to reduce thermal conductivity λp. One of the effective ways to reduce the lattice thermal conductivity is to increase phonon scattering factors, but grain boundary structure, grain boundary content and the interaction of the grain boundary and the phonon need to be further studied.

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The principle of improving the material performance of thermoelectric cooler (I)

Although thermoelectric cooling technology has been developed very fast, there are still many problems that need resolution. Because of the material constraints, the cooling capacity of thermoelectric cooler (TEC) is limited and its cooling efficiency is too low. With the increase of the required temperature difference, the more the TEC series is, the lower the cooling efficiency is. So TEC cannot completely replace the traditional cooling technology, which can only be used in the small cooling capacity field. Only if the cooling capacity is improved, TEC can get more extensive application. The key is that better materials should be developed based on the cooling principle of TEC.

When the direct current I is through couple, the cooling capacity of couples should be the difference value of Piltier’s cooling capacity, Joule heat on the cold end and the heat of couple on the hot end.

When couple is working, the total power consumption is the summation of the resistance consumption and the consumption of overwhelming the thermoelectric potential:

So the cooling coefficient is 

If the corresponding thermal coefficients λ, thermal conductivity σ, and thermoelectric coefficient α of couple arm are equal, the maximum temperature difference between the hot end and the cold end can be expressed as:

Obviously, the maximum temperature difference of TEC depends on the cold end temperature Tc and the figure of merit:

So in order to improve the thermoelectric properties of TEC, the figure of merit Z can be improved from considering three aspects, increasing thermal coefficients λ and thermal conductivity σ and reducing thermoelectric coefficientα.

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