Thermoelectric generator

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Thermoelectric generator is a kind of static Solid state device It has no rotating parts, small size, long service life, no noise when working, and no maintenance. It has become a hot spot in space power research and development, which greatly stimulated Thermoelectric technology Development.
Chinese name
Thermoelectric generator
Sexuality
A static solid state device
Hot spot
Become space power R & D
Special points
No maintenance is required
Reason
Temperature difference of semiconductor emf more

Thermoelectric generator summary

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Because of the temperature difference between semiconductors emf It is a new type of electronic device. It has the characteristics of no noise, no pollution and high efficiency of energy conversion, which indicates the beginning of a refrigeration technology revolution, Thermoelectric power generation Because there are too many "waste heat" around us, such as waste steam heat, waste water heat, waste fire heat, solar heat, and so on; in today's energy shortage, our desire for thermoelectric power generation is even stronger. It appears to be arbitrary Phase state It is possible to intelligentize, digitize and program the temperature of any local environment. If you want to be cold, you will be cold; if you want to be hot, you will be hot. Temperature control, for us to do what we want.

Thermoelectric generator History of development

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1. In 1821, the German scientist T.J. Seebeck discovered it Seebeck effect It has been nearly 200 years. After the discovery of semiconductor materials at the end of the Second World War, exploration was launched thermoelectric materials And device craze, promoted Thermoelectric The development of theory and technology. In the late 1950s and early 1960s, with the rapid development of space technology, a kind of long-life and anti-jamming technology is urgently needed irradiation Power supply.
2. In the early 1960s, there were some Radioisotope thermoelectric generator It has been successfully applied in space and ocean. In 1963, snap3, an isotope thermoelectric generator with an output power of 2.7W was used in the United States Military navigation satellite Go ahead. From 1969 to 1972, the Americans used five snap27 isotope thermoelectric generators [1]   Successfully placed on the lunar surface as a lunar surface scientific instrument Power supply
3. Conventional thermoelectric generator Thermoelectric conversion The efficiency is less than 10%. And other chemistry and Physical power supply Compared with power supply, the efficiency of thermoelectric generator is indeed lower. However, the thermoelectric generator has the advantages that other power sources do not have, such as long service life, unlimited application environment and heat source, especially it can use the so-called low-level thermal power generation, such as industrial waste heat and garbage Heat of combustion It has been attracting people's attention, such as geothermal energy, exhaust pipe of the ocean and so on. Since 1990, for the needs of environmental protection and sustainable economic development, many governments and companies have invested in development Thermoelectric technology The research and development of this kind of green power supply has once again set off a heat wave around the world.
4. RTG is the preferred power source for lunar surface and deep space spacecraft. RTG can also be used as the power supply of buoys and sonar at sea, military concealed power supply and early warning system power supply in polar and border areas. Natural gas fuel thermoelectric generator has been applied in oil and gas pipelines and communication networks in many countries in the world.

Thermoelectric generator Test system

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 The structure of gphs-rtg The structure of rths-gpg
1、 Thermoelectric performance test system of thermoelectric generator , involving Applied Electronic Technology And thermal technology field. The utility model is characterized in that it contains an electric heating heat source unit which is close to the hot surface of the thermoelectric generator and heats the hot surface of the thermoelectric generator; and it is close to the cold surface of the thermoelectric generator to cool the cold surface of the thermoelectric generator Circulating cooling water loop Heat sink Unit: collect the temperature of the hot and cold surface of the thermoelectric generator, the output current and voltage of the thermoelectric generator, the flow rate and liquid temperature of the heat sink unit of the circulating cooling water circuit, process and analyze the above data, and output the control signal to the data acquisition and data processing unit of the electric heating heat source unit.
2. The system measures the thermoelectric performance of the whole thermoelectric generator. The measured data directly reflect the overall performance of the thermoelectric generator. It also has the characteristics of simple structure and strong adaptability.
3. The thermoelectric performance test system of thermoelectric generator is characterized in that it contains: electric heating heat source unit: close to the thermal end face of thermoelectric generator to heat the thermal end face of thermoelectric generator; Circulating cooling water loop Heat sink Unit: close to the cold end face of thermoelectric generator to cool the cold end face of thermoelectric generator; Data acquisition and data processing unit: collect the temperature of the hot end face and cold end face of the thermoelectric generator, the output current and voltage of the thermoelectric generator, the flow rate and liquid temperature of the heat sink unit in the circulating cooling water circuit, process and analyze the above data, and output the control signal to the electric heating heat source unit.

Thermoelectric generator Main categories

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1. According to the classification of heat source used, thermoelectric generator can be divided into radioactivity isotope Thermoelectric generator nuclear reactor Thermoelectric generator, hydrocarbon fuel thermoelectric generator, low-level thermal thermoelectric generator, etc.
2、 Radioisotope thermoelectric generator RTG (RTG) is a thermoelectric generator that converts the decay heat of radioisotopes (such as Pu-238, Sr-90, Po-210, etc.) into electrical energy.
3. The thermoelectric generator of nuclear reactor is to transfer atomic energy reactor A thermoelectric generator in which the heat generated by fuel fission is directly converted into electrical energy.
4. Hydrocarbon fuel thermoelectric generator, combustion gas hydrocarbon The thermal energy generated by the fuel or liquid of electric appliances is converted directly into thermal energy.
5. Low level thermal thermoelectric generators will be of various forms Low temperature heat energy Thermoelectric generator (including waste heat and waste heat) directly converted into electric energy.
6. According to the working temperature, thermoelectric generator can be divided into three categories: high temperature thermoelectric generator, medium temperature thermoelectric generator and low temperature thermoelectric generator. High temperature thermoelectric generator, its hot surface working temperature is generally above 700 ℃, the use of typical thermoelectric materials It is a silicon germanium alloy (SiGe); the medium temperature thermoelectric generator has a hot surface working temperature of 400 ℃ ~ 500 ℃. The typical thermoelectric materials used are Lead telluride (PbTe); low temperature thermoelectric apparatus, whose hot surface working temperature is generally below 400 ℃, the typical thermoelectric materials used are Bismuth telluride (Bi2Te3)。

Thermoelectric generator working principle

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1. Thermoelectric generator is used Seebeck effect To convert heat energy directly into electricity
 Structure diagram of angel RTG Structure diagram of angel RTG
A kind of power generation device. A p-type Thermoelectric The element and an n-type thermoelectric element are connected by metal conductor electrode at the hot end, and the cold end electrode is connected respectively at the cold end to form a thermoelectric unit or couple. An external load with resistance RL is connected to the open circuit end of the thermoelectric unit. If the hot side of the thermoelectric unit inputs heat flow and establishes a temperature difference between the hot end and the cold end of the thermoelectric unit, there will be current flowing through the circuit and the load will get electric power Thus, i2rl directly converts heat energy into electricity.
2. When the generator works, in order to maintain a certain temperature difference between the hot joint and the cold joint, the hot joint should be continuously supplied with heat, and heat should be continuously discharged from the cold joint. Part of the heat supplied by the hot joint is taken as Peltier The heat is absorbed and the other part is transferred to the cold joint through heat conduction. The heat discharged shall be the sum of Peltier heat released from the cold joint and the heat transferred from the hot joint. For the above joints heat balance In addition, Thomson heat and joule Hot. Generated in the system Joule heat Half of i2ri is transferred to the hot end, and the other half is released from the cold end heat transfer The thermal Pt migrated to the cold end and Joule heat returned to the heat source are composed of three parts Thermoelectric Monomeric Thermoelectric conversion Efficiency is the ratio of the useful power to the heat consumed by the heat source. You want to get a good value thermoelectric materials Only by improving its seebeck Sum of coefficients conductivity To reduce the Thermal conductivity However, Seebeck coefficient, conductivity and thermal conductivity are all dependent on carrier concentration and thermal conductivity mobility They are related to each other.

Thermoelectric generator performance

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The main performance parameters of thermoelectric generator include: open circuit voltage, output power, efficiency, power attenuation rate, weight, volume, weight ratio, power and reliability, etc.

Thermoelectric generator Open circuit voltage

The open circuit voltage of thermoelectric generator refers to the voltage at the output terminal of thermoelectric generator when the load of thermoelectric generator is open circuit. Symbol ε, unit V

Thermoelectric generator Electrical output power

Output power of thermoelectric generator [2]   Is equal to the product of the voltage on the load and the loop current.

Thermoelectric generator Efficiency of thermoelectric generator

It refers to the thermoelectric conversion efficiency, defined as the ratio of output electric power to input thermal power of thermoelectric generator.

Thermoelectric generator Lifetime and power degradation rate

Thermoelectric generator is a long life power supply. Its life span can reach several years to more than ten years. The life of thermoelectric generator is defined as the time from normal operation of thermoelectric generator to the time when the output power drops below the rated power value.
The power degradation rate of thermoelectric generator refers to the percentage of output power decline of thermoelectric generator in unit time.

Thermoelectric generator Weight ratio power

The weight ratio power of thermoelectric generator is defined as the ratio of output power of thermoelectric generator to total weight of thermoelectric generator.

Thermoelectric generator Preparation process

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thermoelectric materials In principle, the usual single crystal Growth process. However, the single crystal process needs precise equipment, complicated operation and high cost. Thermoelectric materials in practice [3]   Polycrystalline or oriented polycrystalline materials are often used. Generally, the preparation method of thermoelectric materials is Powder metallurgy as well as Zone melting Law. Thermoelectric materials prepared by powder metallurgy usually have lower Thermal conductivity , higher mechanical strength But it's down conductivity Relatively speaking, the thermoelectric materials with high conductivity can be prepared by zone melting method, but the thermal conductivity of the materials is also improved. powder metallurgy Technology, conventional Cold pressing method In recent years, Ma, pies, extrusion and hot pressing were developed Spark plasma sintering Method (SPS method). In general, PbTe and SiGe alloys are prepared by powder metallurgy process, and Bi2Te3 and its alloys are used Zone melting It can also be prepared by hot pressing or extrusion. The following is just an introduction Powder metallurgy It was prepared by melt melting method thermoelectric materials Technology.

Thermoelectric generator Main applications

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Thermoelectric generator Space

Since 1961, the United States has used isotope thermoelectric generators as power sources in more than 20 space missions. The output of these isotope thermoelectric generators electric power From 2.7W to 300W, the mass from 2kg to 34kg, the maximum efficiency has reached 6.7%, the maximum mass specific power has reached 5.2w/kg, and the design life is 5 years. Apollo, the famous outer planet flyers Viking Mars Lander Galileo The spacecraft and so on have used the isotope thermoelectric generator. In October 1997, the United States successfully launched the probe Saturn Of Cassini The interplanetary spacecraft has three isotope thermoelectric generators as its power supply. In January 2006, a new horizon spacecraft was launched to explore Pluto, using a RTG as power supply. Some of them have been used for more than 19 years [4]  

Thermoelectric generator Daily aspects

The application of isotope thermoelectric generator in surface and marine development is also increasing. The power range of the isotope thermoelectric generator is from several milliwatts to hundreds of watts and more than kilowatts. It is mainly used for lighthouse, navigation mark, submarine sonar, submarine microwave relay station, etc automatic weather station And seismic test station power supply.

Thermoelectric generator military phase

The U.S. military has developed a liquid fuel thermoelectric generator with high mobility, silent, light weight, and long-term operation without maintenance, which is used for night vision devices radar Navigation equipment, radio station and command system. This kind of electric appliance can be used from one kilowatt to several kinds of portable fuel. Global Canada Thermoelectric Many national gas transmission companies have obtained the application of temperature difference pipeline and gas transmission network in the world.

Thermoelectric generator Environmental protection

In the aspect of low-level heat utilization, thermoelectric generator also has a bright future. Low grade heat, including industrial waste heat and garbage Heat of combustion Waste heat of automobile exhaust pipe, solar heat, geothermal energy ocean heat The temperature range of the heat source is wide. use Thermoelectric power generation Using low-grade heat on a large scale, the technology can develop clean energy with simple structure, less maintenance and pollution-free. Many experts believe that thermoelectric generators [5]   Using these heat energy, low voltage and large current can be directly generated. For example, it is the best low peak for hydrogen production from water electrolysis Energy storage One way.

Thermoelectric generator Use and maintenance

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1. Maintenance and control of hot surface temperature, for example, the hot surface temperature of thermoelectric generator is too high, welded joint Easy to fall off, some thermoelectric materials The sublimation rate is also increased rapidly, which is easy to cause the failure of thermoelectric generator. Therefore, the hot surface temperature of thermoelectric generator should be controlled and maintained below the rated temperature. The temperature of the fuel will not necessarily reach the temperature after adding the RTG isotope. Therefore, the design of RTG should ensure that the temperature of hot surface will not be higher than the allowable temperature during fueling operation. Due to the temperature difference inside the generator Temperature field It is sensitive to changes in ambient temperature. When the ambient temperature increases, both the hot surface temperature and the cold surface temperature will increase. Measures must be taken to create favorable heat dissipation conditions to reduce the hot surface temperature and cold surface temperature to a reasonable temperature range. For example, before the launch and landing of RTG, as well as the lunar mission, especially in the daytime, the heat dissipation problem should be fully considered and solved.
2、 Power controller For a certain reason, due to the temperature difference of the generator, Thermoelectric It is possible that the thermoelectric element is very small in size logarithm It is impossible to increase it at will. Therefore, generally speaking, thermoelectric generator Open circuit voltage Relatively low. If the electrical appliances need high voltage power supply, the corresponding booster must be designed and used. The volt ampere characteristic of thermoelectric generator is linear solar cell Chemical battery In addition, the output characteristics are sensitive to the change of ambient temperature, which should be fully considered in the design of power controller. The Peltier effect can consume the input heat of the thermoelectric generator and reduce its hot surface temperature. Therefore, it is more advantageous for space application thermoelectric generator to be in short circuit state before landing, i.e. launch, orbit transfer and other stages.

Thermoelectric generator Development trend

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1、 Thermoelectric technology field [6]   Most efforts have been made to improve the performance of thermoelectric generators Thermoelectric conversion effect
 Schematic diagram of zone melting method Schematic diagram of zone melting method
Rate. For space applications, it is very important to improve the weight ratio power. The main way to improve the efficiency of thermoelectric conversion is to improve the efficiency thermoelectric materials The value of merit. Specifically, it is to improve the thermoelectric properties of existing thermoelectric materials, research new thermoelectric materials, develop functional gradient thermoelectric materials, and reduce the dimension of thermoelectric materials.
2. Improve the merit value of thermoelectric materials, increase the Thermoelectric conversion Efficiency, the final result, reduces the cost of thermoelectric generator (especially the cost of RTG), and improves the weight ratio power of thermoelectric generator. NASA, NASA( NASA )A space nuclear innovation program was developed to develop advanced radioactivity isotope Power system and space nuclear reactor Power system. Later, the program was renamed the Prometheus nuclear power and propulsion program. The goal of the plan is to develop nuclear power in the face of various new space missions, such as the Mars Science station network, small electric thrusters, and small deep space exploration vehicles with complex maneuverability, so as to greatly expand the ability of human mobile laboratory and deep space exploration on the surface of planets or the moon.
3. The Prometheus project requires the development of the isotope power supply system to increase the specific power, while reducing the isotope quantity, in order to reduce the flight cost. Objective: to increase the specific power by 2 times, from 4.5we/kg of RTG to 8-10we / kg, and increase the efficiency from 5.7% to 2-4 times.
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