Application of Thermoelectric Technology

The earliest thermoelectric generator was successfully developed by the former Soviet Union in 1942, with a power generation efficiency of 1.5%~2%. Later, the demand for power in some special fields greatly stimulated the development of thermoelectric technology. Since the 1960s, a number of thermoelectric generators have been successfully used in space shuttles, military and ocean exploration. In recent years, with the continuous progress of science and technology, thermoelectric generators are gradually broadening their application fields, not only in military and high-tech fields, but also in civilian fields. With the approaching energy and environmental crisis, scientists have increased their research efforts in the use of low-grade and waste energy for power generation, and some research results have entered industrialization.

2.1 Remote Space Exploration
Since the successful landing of the Apollo spacecraft on the moon in 1969, human exploration of space has been going on in depth. With the expansion of space exploration, people have set their sights on more distant planets and even remote space outside the solar system. In a dark, cold and empty world far away from the sun, the amount of solar radiation is extremely small, and solar cells are difficult to function. The use of radioisotope thermoelectric power generation systems with stable heat sources, compact structures, reliable performance and long life has become an ideal choice. Using thermoelectric technology, a coin-sized radioisotope heat source can provide continuous electricity for more than 20 years, which is unmatched by any other energy technology. The National Aeronautics and Space Administration (NASA) has used thermoelectric power generation devices using various radioisotopes as heat sources on its Apollo lunar module, Pioneer, Viking, Voyager, Galileo and Ulysses spacecraft. Among them, the Voyager 1 spacecraft needs to conduct scientific investigations in space for 25 years, and all the electricity on the spacecraft is provided by thermoelectric conversion modules. Its power generation system includes 1,200 thermoelectric generators, which are provided with heat energy by the neutron decay of the radioactive fuel Pu-238. The power system has been operating safely for 21 years and is expected to continue to work for 15 to 20 years.
Compared with solar cells, the radioisotope thermoelectric power generation system not only has the advantages of long life and reliable performance, but also has attractive specific volume and specific weight. If the Ulysses spacecraft is designed according to the structure of solar cells, the weight of the solar panels it carries will reach 550 kg, which is twice the weight of the spacecraft itself, which is difficult for the launch vehicle to bear. When the thermoelectric power generation system is adopted, the weight of the generator is only 56 kg, which can fully meet all the power requirements of the spacecraft in navigation, communication and scientific instrument use1). Figure 2 is the appearance of the radioisotope thermoelectric power generation system, and Figure 3 is its cross-section.

Figure 3 Cross-section of radioisotope thermoelectric power generation system
2.2 Military
In addition to playing an important role in the aerospace field, the navy is its second largest user. As early as the early 1980s, the United States completed the development of 500~1000W military thermoelectric generators and officially included them in the military equipment in the late 1980s. Its greatest advantage is that it is silent, vibration-free and concealed, and has important applications in submarines and long-distance signal transmission. The thermoelectric generator is placed in the deep sea to power the radio signal forwarding system. This system is a component of the US missile positioning system network. It is designed to work at a depth of 10 kilometers, with a working power of more than 1W and a service life of more than 10 years. Recently, Hi-Z has developed a high-performance micro-thermoelectric power generation module based on the quantum dot principle for the military [14], which is used to power a variety of wireless sensors on board. These sensors are responsible for monitoring fractures, corrosion, impact damage, temperature drift and other tasks. Only thermoelectric generators can meet their extremely high requirements for power supply size, weight, leakage and life.
In order to meet the special requirements of the Army for power supply systems - light, flexible, easy to charge, etc., the U.S. Department of Energy launched the "Energy Harvesting Science and Technology Project" in 19991). The project studies the use of thermoelectric power generation
modules to collect soldiers' body heat for battery charging. Its short-term
goal is to achieve a minimum output of 250 watt-hours of electricity for a 12-hour combat mission. At present, the research project has achieved many research results.

2.3 Long-distance communication, navigation and equipment protection
The stable performance and maintenance-free nature of thermoelectric technology make it play an important role in remote areas where power generation and transmission are difficult [15]. It has been used in microwave relay station power supplies, remote automatic radio receiving devices and automatic weather forecast stations, unmanned navigation lights, and cathodic protection of oil pipelines in uninhabited areas such as polar regions, deserts, and forests. Figure 4 shows a thermoelectric generator for pipeline monitoring, data acquisition, communication, and corrosion protection manufactured by Global Thermoelectric Inc., the world's largest thermoelectric generator manufacturer in the United States. The output power can reach 5000W. Since the late 1960s, the former Soviet Union has manufactured more than 1,000 radioisotope thermoelectric motors, which are widely used in lighthouses and navigation signs, with an average service life of more than 10 years. This type of generator uses Sr90 as a heat source and can stably provide an output of 7~30V and 80W.

2.4 Low-power power
supply Small size, light weight, no vibration, no noise make the thermoelectric generator very suitable for use as a low-power power supply (less than 5W). Thermoelectric technology can play a unique role in the short-term microwatt and milliwatt power required by various unmanned sensors, micro short-range communication devices, and micro-generators, sensor circuits, logic gates and various error correction circuits for medical and physiological research [16,17]. Figure 5 is a micro-battery with coordinated load manufactured by Hi-Z, with an output power of up to 2.5W and an output voltage of 3.3V4).

Research on an integrated universal thermoelectric microbattery system with a size of cm2[18]. After three years of project development, some products have entered the practical stage.

Seiko Instruments of Japan has developed a micro battery for watches that uses human body temperature to generate electricity[19]. The battery uses BiTe block material, the battery size is 2 mm×2 mm×1.3 mm, and it consists of 50 pairs of elements connected in series. A temperature difference of 1K can generate a voltage of 20 mV and an output power of 1 μW.
DTS of Germany is a world leader in the production of thin-film thermoelectric generators with an output power of 10~40 μW1).
2.5 Thermoelectric sensors
Recently, based on the Seebeck effect of thermoelectric conversion materials, many new thermoelectric sensors have been successfully developed and used for low-temperature temperature measurement[20], single-pixel infrared and X-ray detection[21], hydrogen and other flammable gas leak detection[22], etc.
Scientists from the National Institute of Advanced Industrial Science and Technology of Japan used magnetron sputtering technology to prepare a thin-film thermoelectric hydrogen sensor[23]. Its working principle is to coat a catalyst on half of the surface of the thermoelectric thin film material. When hydrogen is present, the temperature of the thermoelectric conversion material coated with the catalyst increases, Then, a potential difference is established at both ends of the device. The measurement of the voltage signal can not only sense hydrogen leakage, but also be used to estimate the hydrogen concentration. Traditional hydrogen sensors have the disadvantages of large size, heavy weight, complex structure, poor gas selectivity (often with a broad spectrum response to combustible gases), and long response time, and are increasingly unable to meet the requirements of use.
In addition, the sensitivity of traditional sensors to gas can only reach a peak when the temperature is strongly proportional (200~400℃), which not only consumes additional heating power, but also easily causes fire. Thin film sensors made of thermoelectric conversion materials can operate near room temperature, with small size, good selectivity and short response time. A 1% hydrogen content can output a 2 mV voltage signal with a response time of 50 s (Figure 6). This type of sensor has broad application prospects in hydrogen fuel cell systems, hydrogen refueling stations, micro-aircraft, etc.,
usually at higher temperatures .
Based on the 235 thermoelectric module developed by the German DTS company, a micro infrared sensor [24] was successfully developed for non-contact temperature measurement, monitoring of household and factory equipment, etc. It has the characteristics of small size (mm3), light weight (mg), no filter window, fast response, not affected by environmental heat conduction and heat convection, and stable operation under high thermal radiation. Figure 7 shows its F-type thermoelectric infrared sensor, which is 5.6 mm×3.1 mm×0.08 mm in size and weighs 19 mg2).
2.6 Low-grade and waste heat power generation
For a long time, due to the limitations of production costs and conversion efficiency, the application of thermoelectric technology has been limited to high-tech, military, and aerospace fields. Recently, due to the decreasing amount of fossil energy and the approaching environmental degradation caused by the combustion of fossil energy, people have realized the importance of using low-grade and waste heat to generate electricity in solving environmental and energy problems [25]. In addition, the wide range and cheapness of available heat sources have greatly enhanced the commercial competitiveness of thermoelectric power generation. We know that the cost of power generation is mainly composed of operating costs and equipment costs. Operating costs depend on conversion efficiency and raw materials, and equipment costs are determined by the device that generates rated output power. Although the cost of thermoelectric conversion modules is high, the cost of raw materials for generating electricity using low-grade and waste heat is very low, The operating cost is almost zero, and the operating cost is very low, so the total cost of power generation is reduced, making thermoelectric power generation commercially competitive with existing power generation methods. In recent years, Japan has launched a series of government programs entitled "Solid Waste Combustion Energy Recovery Research Program" to study waste heat power generation technology for solid waste incinerators, combining turbine generators and thermoelectric generators to achieve the maximum utilization of waste incineration heat of different scales, making waste a truly usable resource [26]. Following Japan, in November 2003, the US Department of Energy announced funding for Pacific Northwest National Laboratory, Michigan Technological University, Pittsburgh PPG Process Co., Ltd. and other units, focusing on supporting their development of high-performance thermoelectric conversion materials and application technologies. Its main application objects are exhaust heat in industrial production and waste heat and residual heat utilization in other components 3).

250 billion yuan in benefits1). However, the marketization, specialization and industrialization of waste-to-energy in China have just started. In order to mobilize more social forces to participate in the waste-to-energy business, the State Council has recently formulated a series of preferential policies for comprehensive resource utilization, hoping to promote the development of this technology.
(3) Waste heat from automobiles
As people's living standards continue to improve, cars, as an important means of transportation for modern families, have begun to enter the homes of ordinary people. Cars not only bring convenience to people's lives, but also promote the continuous progress of social economy. However, with the continuous increase in the popularity of cars, people's demand for energy, especially oil and natural gas, is increasing, which further accelerates the deterioration of global energy problems. At the same time, the pollution of automobile exhaust to the environment has also brought certain impacts on the world environment. The energy loss caused by automobile exhaust, cooling water, lubricating oil and thermal radiation accounts for a large proportion of the energy of gasoline combustion. For example, the energy loss of an ordinary family car at normal speed is 20~30 kW. Scientists have been working hard to apply thermoelectric technology to environmentally friendly cars, using the waste heat of automobile exhaust and the waste heat of the engine to provide auxiliary power for the car. In this way, It can not only greatly improve the overall performance of the car and reduce engine energy consumption, but also reduce the emission of pollutants in the exhaust gas, achieving three goals at one stroke. Theoretical research shows that if thermoelectric technology can be applied to automobiles, it is expected to save 20% of fuel, which is enough to provide the electrical energy of a medium-sized car [29]. Japan has developed a small thermoelectric generator that uses automobile exhaust to generate electricity. The power is 100W and can save 5% of fuel [30]. The United States has also recently announced the successful trial production of a 1000W motor based on large truck exhaust [31,32]. Figure 10 shows a thermoelectric generator installed on a Mack diesel engine in the United States. From the appearance, it looks like a vertical muffler.
(4) Natural heat
Solar radiation heat, ocean temperature difference heat, geothermal heat and other natural heat are all inexhaustible and ideal power sources given to humans by nature. Traditional natural thermal power generation methods use heat engines, generators or steam
turbines as prime movers. Such systems can only achieve good technical and economic indicators in large-capacity power generation. The international community is now turning its attention to direct power generation devices (such as thermoelectric conversion modules) that have no moving parts, are silent, and do not require maintenance. Using them to replace the above energy conversion components will greatly simplify the structure of the energy conversion components of the existing natural thermal power generation system and achieve considerable economic benefits. Professor Stevens of Mississippi State University in the United States has conducted research on the use of the temperature difference between the surface and the underground to generate electricity [33]2) (as shown in Figure 11). This method has the characteristics of stable performance, long life, no sound radiation, invisible, and continuous operation at night and in harsh environments. It can be widely used in small long-distance sensing and communication devices that are not manned for a long time. Its initial design power is 100 mW.
(5) Other dispersed heat sources
Recently, Professor Rowe of Cardiff University in the United States demonstrated the use of residual heat from the remaining water in the bathtub after a person takes a bath to generate electricity, which can keep a color TV working continuously for 1 hour. If the system can operate for three years, the cost of producing electricity will be equivalent to the cost of electricity generated by conventional energy power companies.