Reliability of thermoelectric cooler

As thermoelectric cooler (TEC) is built on the basis of solid state, TEC has very high reliability. In most conditions of practical applications, TEC can provide a relatively long service lifetime. Currently, in many specific cases, the continuous work time of TEC has been over 20 years and is longer than other relative devices. However, it is still difficult to obtain specific reliability of TEC because failure and application environment are closely related. In some steady cooling conditions, a DC power supply put on the TEC is very stable and not basically interrupted, and at the same time, the reliability of TEC can be very high. The mean time before failures (MTBFs) is generally over 2000000 hours, which is usually regarded as an industrial standard. When in the application conditions of the hot and cold cycling work, MTBFs can be greatly shortened, especially when the temperature of TEC increases to a high one in the process of cycling.

Generally speaking, it is quite difficult to announce the reliability data of TECs because in practical application, there are application conditions and work state, which have effect on the final results. Therefore, the reliability data is valid only for the application environment similar to the test environment.

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A summary of TEC technology

Seebeck effect, Peltier effect and Thomson effect found the basis of thermoelectric theory ofthermoelectric coolers (TEC). In the view of the history of TEC (picture below), it has roughly gone through three stages of development.

Physical picture of TEC

The first stage refers to thermoelectric generation and thermoelectric cooling studied by Seebeck and Peltier respectively. Because of bad thermoelectric performances of metallic materials at that time, the efficiency of energy conversion was very low and there was no practical value. Therefore, thermoelectric effect can’t be used in practical application.

The second stage is from the beginning of 1950s to 1980s, by using semiconductor materials, it was found that semiconductor materials have good thermoelectric performances, which has improved the efficiency of thermoelectric effect a lot. So thermoelectric generation and thermoelectric cooling entered into engineering practice.

The third stage is from 1980s to now, the main purpose is to improve the thermoelectric performance of TEC and further developthermoelectric cooling application areas.

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Application of TEC in home appliances

With the development of science and technology, TEC has attracted more and more attention and it has been widely used in our life. Now the following is about the application of TEC in home appliances.

Physical picture of TEC

1. Refrigerator

A lot of manufacturers have launched TEC refrigerator. For example, car refrigerator is also a kind of refrigerator, which is suitable for drive out use. On the current market, the size of this car refrigerator is from 3L to 33L. With the increase of car ownership, there will be more and more car refrigerators.

However, it is noteworthy that the function of TEC refrigerator is refrigeration but not freezing, and when comparing with ordinary steam compressor refrigerator, it has lower efficiency, small volume and high power consumption. TEC refrigerator is usually appropriate for special occasions or requirements.

2. Water dispenser

Water dispenser is a new kind of household electrical appliance and its sales and production figures are impressive. When comparing with traditional teakettle, water dispenser has many advantages of beautiful appearance, simple operation, and safe use and can also provide hot water and cold water at the same time, so water dispenser is very popular among more and more families and offices.

There are also many other household devices using TEC. If you want to know more, please contact us.

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Recent prospect of thermoelectric materials

Thermoelectric materials have three effects: Seeback effect, Peliter effect and Thomson effect.

Thermoelectric material has many advantages: (1) It has compact size, light weight and no noise in work; (2) The temperature can be controlled within ±0.1℃;(3)There is no CFC used in the thermoelectric material, which can pollute the environment; (4) As heat source can be recycled and transformed into electric energy (save energy), it has long service life and is easy to control. Therefore, thermoelectric material is a kind of material with wide application prospect. At present, thermoelectric materials have been mainly used for thermoelectric generator (TEG) and thermoelectric cooler (TEC). But they are also used in the military, medical physics and some other aspects.

With the further research of thermoelectric material, its performance will be further improved and it will also become a new hotspot in the new material research field.The focus of the future research of thermoelectric materials is the following aspects:

1. Based on traditional semiconductor energy band theory and modern quantum theory, materials with different crystal structures are calculated the Seeback coefficient, electrical conductivity and thermal conductivity, so that the new thermoelectric materials with higher thermal figure of merit can be found in the bigger range.

2. Since based on the theoretical and experimental research, the microstructure and preparation process of material have effects on its thermoelectric performance, the thermoelectric performance can improved further.

3. Theoretical and experimental studies on the high performance materials should be done to achieve the stable high thermoelectric performance.

4. The study on preparation technology should be paid more attention on so that the industrialization of thermoelectric materials can be realized.

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Thermoelectric cooler applied in the LED packaging

LED is a kind of solid semiconductor lighting devices. Similar with traditional light source, during work, the solid light source will generate heat because of current flowing in the diodes. As long as the current flows continuously, the heat will continue to be produced. The node temperature of LED must be below 125℃. The high node temperature will bring many adverse effects, such as the quantum efficiency of LED reducing, the luminousefficiency declining, the peak wavelength shifting and service lifetime severely shortening.

For LED devices with different power, the density of heat flow rate generated by chips is of all sizes. In order to control their node temperatures, LED needs a certain dissipation method to realize good thermal management. There are two ways of dissipation, thermoelectric cooler (TEC) and heat sink, which can meet the cooling requirements of LED devices with different power. Next, TEC is applied in the LED packaging will be mainly introduced.

Fig 1. Schematic diagram of thermoelectric cooler

TEC is made of semiconductor thermocouples with obvious thermoelectric effect and high cooling efficiency. As shown in the figure 1, a P-type semiconductor element and an N-type semiconductor are connected into thermocouples and after connecting a DC power supply, at the joint, there will generate the temperature and heat transfer. On the top of the joint, the current will flow from N to P and the temperature will reduce, which is called cold side; at the junction below, the current direction is from P to N, which is called hot side.

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The heat resistance theory of heat sink

As shown figure 1, the heat flow ∅_0 goes through the bottom of heat sink and then goes into its root. In the root, the heat flow will be divided into two shares ∅_1and∅_2, and then through the root and environment air, start convection and heat exchange. Finally, the heat flow goes into the environment by heat exchange with fins. The heat resistance network model is shown in figure 1.

Fig 1. The heat flow schematic diagram of heat sink dissipation

Fig 2. The heat resistance network model of fins

As the lateral area of heat sink base is much smaller than the root cross-sectional area, the heat dissipated from side can be omitted. The whole heat flow enters the bottom and side of heat sink by heat conduction. The heat transfer thermal resistance  on the surface of fins is much larger than the heat conduction thermal resistance  in the fins. So on the cross section at the constant height, the fin temperature can be considered uniform. The top of the fins can be regarded as insulation. The heat conduction coefficient , the surface heat transfer coefficient h and the cross section area Ac along the fin height are all constant.

According to the above simplification, the model established is a one-dimensional steady state heat conduction model and the equivalent thermal resistance of the rib fin is

Where, Rr— heat resistance of the heat dissipation part at the root, K / W;
Rf—heat resistance of the heat dissipation part in the side, K / W.

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The structure of thermoelectric cooler

Thermoelectric cooler (TEC) is built on the Peltier effect. Due to the internal structure features of semiconductor material, it has more obvious thermoelectric effects than other metals do. Therefore, semiconductor material is usually used for making TEC.

The cooling capacity of a pair of couple, which consists of P-type and N-type semiconductor, is fairly small. For example, the cooling capacity of the couple (Φ6mm× 7mm) is only 0.92~1.16W. In order to obtain more cooling capacity, many pairs of couple can be formed into thermopile by using series way, which is named single-stage thermopile. However, the single-stage thermopile can only get the temperature difference of about 50℃. In order to get lower temperature in the cold end, the methods of series, parallel or series and parallel can be used to constitute multi-stage thermopile, as shown in the following figures. The hot end of upper thermopile is attached to the cold end of next thermopile, which usually plays a role of heat sink  for the upper thermopile.

Picture of multi-stage thermopile.

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