Thermoelectric generator

A static solid device

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Thermoelectric generator is static Solid state device It has no rotating parts, small size, long service life, no noise during operation, and no maintenance. It has become a hot spot in space power research and development, greatly stimulating Thermoelectric technology Development of.

Chinese name: Thermoelectric generator
Nature: A static solid device

Hot spots: Become a space power R&D
Features: No noise during operation and no maintenance
Principle: Temperature difference of semiconductor emf more

catalog

six performance
▪ Open circuit voltage
▪ Output power
▪ Efficiency of thermoelectric generator
▪ Life and power degradation rate
▪ Weight specific power

Product Introduction

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Due to the temperature difference of semiconductor emf Because of its large size, most of them use it to make thermoelectric generators. It is a new type of electronic device, featuring no noise, no pollution, and efficient energy conversion, which indicates the beginning of a refrigeration technology revolution, Thermoelectric generation Because there are too many "waste heat" available around us, such as waste steam heat, waste water heat, waste fire heat, solar heat, etc; In today's increasingly tense energy, our desire for thermoelectric power generation is even stronger. Its appearance makes arbitrary Phase state It is possible to intelligentize, digitize and program the control of the material and temperature of any local environment. If you want to be cold, you are cold; If you want to be hot, you are hot. The control of temperature is free for us.

Development history

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1. 1821 German scientist T.J. Seebeck Found Seebeck effect It has been nearly 200 years. the Second World War After the discovery of semiconductor materials, exploration was launched thermoelectric materials And devices Thermoelectric The development of theory and technology. At the end of the 1950s and the beginning of the 1960s, space technology developed rapidly, and a long-life, anti irradiation Power supply of.

2. In the early 1960s, there were a number of Radioisotope thermoelectric generator (Radioisotope Thermoelectric Generator, RTG for short) has been successfully applied to space, ground and ocean. In 1963, an isotope thermoelectric generator Snap3 with output power of 2.7W was used in Military navigation satellite On. From 1969 to 1972, the Americans introduced five Snap27 isotope thermoelectric generators^[1] Successfully placed on the lunar surface as a scientific instrument Power supply 。

3. Conventional thermoelectric generator Thermoelectric conversion The efficiency is less than 10%. With other chemistry and Physical power supply Compared with power supply, the efficiency of thermoelectric generator is really low. However, the thermoelectric generator has the advantages that other power sources do not have, such as long service life, unlimited application environment and use of heat sources. In particular, it can use the so-called low-grade heat to generate electricity, such as industrial waste heat and garbage Heat of combustion The waste heat of automobile exhaust pipe, solar heat, geothermal heat and ocean heat have always attracted people's attention. Since 1990, for the need of environmental protection and sustainable economic development, governments and companies in many countries have invested in development Thermoelectric technology , another hot wave of research and development of this kind of green power supply has been set off worldwide.

4. RTG is the preferred power supply for lunar surface and deep space spacecraft. RTG can also be used as the power supply for sea buoys and sonar, or the power supply for military concealed power supply and early warning system in polar regions and borders. The natural gas fuel thermoelectric generator has been applied to oil and gas pipelines and communication networks in many countries in the world.

Test system

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Structure of GPHS-RTG

1、 Thermoelectric performance test system of thermoelectric generator , involving Applied Electronic Technology And thermal technology. The utility model is characterized in that it comprises an electric heating heat source unit which is closely attached to the hot surface of the thermoelectric generator and heats the hot surface of the thermoelectric generator; Closely attached to the cold side of the thermoelectric generator to cool the cold side of the thermoelectric generator Circulating cooling water loop Heat sink Unit; Collect the temperature of the hot and cold surfaces of the thermoelectric generator, the current and voltage output by the thermoelectric generator, the flow 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, and the measured data directly reflects the overall performance of the thermoelectric generator, which also has the characteristics of simple structure and strong adaptability.

3、 Thermoelectric performance test system of thermoelectric generator The utility model is characterized in that it comprises: an electric heating heat source unit which is closely attached to the hot end surface of the thermoelectric generator to heat the hot end surface of the thermoelectric generator; Circulating cooling water loop Heat sink Unit: close to the cold end face of the thermoelectric generator to cool the cold end face of the thermoelectric generator; Data acquisition and data processing unit: collect the temperature of the hot end and cold end of the thermoelectric generator, the current and voltage output by the thermoelectric generator, the flow 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 electric heating heat source unit.

Main categories

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1. According to the classification of heat sources used, thermoelectric generators can be divided into radioisotope thermoelectric generators nuclear reactor Thermoelectric generator, hydrocarbon fuel thermoelectric generator, low-grade thermal thermoelectric generator, etc.

2、 Radioisotope thermoelectric generator (RTG) Yes radio isotope (such as Pu-238, Sr-90, Po-210, etc.).

3. The nuclear reactor thermoelectric generator is to convert atomic energy reactor A thermoelectric generator in which the heat energy generated by fuel fission is directly converted into electric energy.

4. Hydrocarbon fuel thermoelectric generator, a thermoelectric generator that directly converts the heat generated by burning gaseous hydrocarbon fuel or liquid hydrocarbon fuel into electrical energy.

5. Low level thermal thermoelectric generator Low temperature heat energy (including waste heat and waste heat) thermoelectric generator directly converted into electric energy.

6. According to the working temperature, thermoelectric generators can be divided into three categories: high-temperature thermoelectric generators, medium temperature thermoelectric generators and low-temperature thermoelectric generators. The working temperature of the hot surface of the high-temperature thermoelectric generator is generally above 700 ℃ thermoelectric materials It is silicon germanium alloy (SiGe); The working temperature of the hot surface of the medium temperature thermoelectric generator is generally 400 ℃~500 ℃, and the typical thermoelectric materials used are Lead telluride （PbTe）; The working temperature of the hot surface of low-temperature thermoelectric appliances is generally below 400 ℃, and the typical thermoelectric materials used are Bismuth telluride （Bi2Te3）。

working principle

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Angel RTG structure diagram

1. Thermoelectric generator uses Seebeck effect A power generation device that directly converts heat energy into electrical energy. Place a p-type Thermoelectric The element and an n-type thermoelectric element are connected with metal conductor electrodes at the hot end, and the cold end electrodes are respectively connected at the cold end to form a thermoelectric monomer or single couple. Connect an external load with resistance RL at the open circuit end of the thermoelectric unit. If the hot surface of the thermoelectric unit inputs heat flow and a temperature difference is established between the hot end and the cold end of the thermoelectric unit, current will flow through the circuit and the load will get electric power I2RL, thus a generator that directly converts heat energy into electrical energy is obtained.

2. When the generator is working, in order to maintain a certain temperature difference between the hot connector and the cold connector, the hot connector should be continuously heated and the cold connector should be continuously discharged. Part of the heat supplied by the hot joint is taken as Peltier Heat is absorbed, and the other part is transferred to the cold joint through heat conduction. The heat emitted shall be the sum of Peltier heat emitted from the cold joint and heat transmitted from the hot joint. For the above joints heat balance , Thomson heat and heat released by conductor shall be added joule Heat. Set in the system Joule heat Half of I2Ri is transferred to the hot end and the other half is discharged from the cold end. The heat consumed by the heat source is Peltier heat Ph heat transfer The hot PT transferred to the cold end and the Joule heat returned to the heat source are composed of three parts, namely Thermoelectric Monosomic Thermoelectric conversion Efficiency is the ratio of useful power to heat consumed by the heat source. To get high value thermoelectric materials , only to improve its seebeck Coefficients and conductivity , lower it Thermal conductivity 。 However, Seebeck coefficient, conductivity and thermal conductivity all depend on carrier concentration and mobility , are related to each other.

performance

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

Open circuit voltage

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

Output power

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

Efficiency of thermoelectric generator

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

Life and power degradation rate

Thermoelectric generator is a long-life power supply. Its service life can generally reach several to ten years. The service life of the thermoelectric generator is defined as the time from normal operation to output power decline to a moment below the rated power value.

The power degradation rate of thermoelectric generator refers to the percentage of output power degradation of thermoelectric generator in unit time.

Weight specific power

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

Preparation process

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thermoelectric materials In principle, common single crystal Growth process. However, the single crystal process requires precise equipment, complicated operation and high cost. Thermoelectric materials in practice^[3] Polycrystalline or oriented Polycrystalline material 。 Generally, the method for preparing thermoelectric materials is Powder metallurgy as well as Regional melting Law. Thermoelectric materials prepared by powder metallurgy usually have lower Thermal conductivity , higher mechanical strength, but lower conductivity 。 Relatively speaking, zone melting can prepare thermoelectric materials with high conductivity, but it also improves the thermal conductivity of materials. powder metallurgy Process, conventional Cold pressing method In recent years, mechanical alloy (MA), comminution mixed sintering (PIES), extrusion and Spark plasma sintering Method (SPS method). Generally, PbTe and SiGe alloys are prepared by powder metallurgy process, and Bi2Te3 and its alloys are Regional melting It can also be prepared by hot pressing process or extrusion process. The following only describes Powder metallurgy And zone melting thermoelectric materials Technology.

Main applications

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Aerospace

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 is 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. For example, the famous Apollo moon landing plan Tourists flying to outer planets Viking Mars lander Galileo The spacecraft and others have used isotope thermoelectric generators. In October 1997, the United States successfully launched a probe Saturn Of Cassini The interplanetary spacecraft has three isotope thermoelectric generators for power supply. In January 2006, the New Horizons spacecraft to explore Pluto was launched, using one RTG as the power supply. These isotope thermoelectric generators have a service life of more than 19 years, and some have been working for more than 30 years^[4] 。

Daily aspects

Isotopic thermoelectric generators are increasingly used in surface and marine development. The power range of isotope thermoelectric generators in use now is from a few milliwatts to hundreds of watts and kilowatts. It is mainly used for lighthouses, navigation aids, undersea sonar, undersea microwave relay stations automatic weather station And seismic test station power supply.

military phase

The US military has developed a liquid fuel thermoelectric generator with high mobility, soundless, light weight, and can be operated for a long time without human maintenance, which is used in the front position to provide night vision devices radar , navigation equipment, radio station and command system. The generator can use diesel, gasoline and other liquid fuels, with power ranging from tens of watts to one kilowatt, and can be portable or used as an auxiliary power supply for vehicles. Universal Canada Thermoelectric The gas temperature difference generator produced by the company has been applied to oil and gas pipelines and communication networks in many countries in the world.

Environmental protection

In terms of low-grade heat utilization, thermoelectric generators are also promising. Low level heat, including industrial waste heat and garbage Heat of combustion Waste heat of automobile exhaust pipe, solar heat, geothermal ocean heat The temperature range of the heat source is wide. use Thermoelectric generation The technology of large-scale utilization of low-grade heat can develop clean energy with simple structure, less maintenance and no pollution. Many experts believe that thermoelectric generators^[5] These heat energy can directly generate low voltage and large current. For example, it is the best low peak for hydrogen production by electrolysis of water Energy storage One way.

Use and maintenance

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1. Maintenance and control of the temperature of the hot surface, such as the temperature of the hot surface of the thermoelectric generator is too high, welded joint Easy to fall off, some thermoelectric materials The sublimation rate of is also increased sharply, which is very easy to cause the failure of thermoelectric generator. Therefore, the hot surface temperature of the thermoelectric generator should be controlled and maintained below the rated temperature. After adding isotope fuel to RTG, the temperature of hot surface will reach a certain value uncontrollably. Therefore, the design of RTG should ensure that the hot surface temperature will not be higher than the allowable temperature value when it is operated with fuel. Due to the temperature difference inside the generator Temperature field Very sensitive to changes in ambient temperature. When the ambient temperature rises, 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 the cold surface temperature to a reasonable temperature range. For example, before the launch and landing of RTG, as well as the lunar mission, especially the day and month, the heat dissipation problem should be fully considered and solved.

2、 Power controller For a certain power thermoelectric generator, due to process reasons, Thermoelectric The size of the element cannot be very thin and long, and the thermoelectric unit in the thermoelectric transducer logarithm It is also impossible to increase arbitrarily. Therefore, in general, the thermal generator Open circuit voltage Low. If the consumer needs high voltage power supply, it is necessary to design and use a matching booster. The volt ampere characteristic of the thermoelectric generator is linear solar cell 、 Chemical battery They are different, and their output characteristics are very sensitive to changes in ambient temperature. The design of the power controller should fully consider these factors. Peltier effect can consume the input heat of thermoelectric generator and reduce its hot surface temperature. Therefore, the thermoelectric generator for space application is in a short circuit state before landing, that is, launch, orbit change, orbit and other stages.

Development trend

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Schematic diagram of regional smelting method

1、 Thermoelectric technology field^[6] Most efforts are made to improve the Thermoelectric conversion Efficiency. For space applications, it is very important to improve its weight ratio and power. The main way to improve thermoelectric conversion efficiency is to improve thermoelectric materials The superior value of. 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 excellent value of thermoelectric materials and increase the Thermoelectric conversion Efficiency, the final result, reduces the cost of the thermoelectric generator (especially the cost of RTG), and improves the weight ratio power of the thermoelectric generator. NASA（ NASA ）Developed space nuclear innovation plan, developed advanced radioisotope power supply system and space nuclear reactor Power supply system. Later, the plan was renamed as the Plumetheus Nuclear Power and Propulsion Plan. The goal of the plan is to develop nuclear power in the face of various new space missions such as the future Mars science station network, small electric thrusters, small deep space exploration vehicles with complex mobility, and greatly expand the ability of humans to move laboratories and deep space exploration on the surface of the planet or moon.

3. The Prometheus plan requires the isotope power supply system developed to increase the specific power while reducing the amount of isotopes to reduce flight costs. Target: increase the specific power by 2 times, that is, from 4.5We/kg of RTG to 8-10We/kg, and increase the efficiency from 5.7% to 2 to 4 times.

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