Temperature difference power generation technology and its application in exhaust heat utilization of automobile engine exhaust
Product and technology temperature difference power generation technology and its application in the exhaust heat utilization of automobile engine exhaust Zhang Zheng, Zeng Meiqin Si Guangshu (South China University of Technology, Guangzhou 510640, Guangdong), the structure of the thermoelectric generator for automobile exhaust heat recovery power generation, etc. An example of a thermoelectric generator for vehicles in the United States and Japan was also published, and the development trend of related technologies was also analyzed. The automobile industry is one of the pillar industries of China's national economy. With the development of China's automobile industry, the energy consumed by vehicles is increasing day by day, and the energy conservation of vehicles is receiving more and more attention. However, based on the existing internal combustion engine index, about 60% of the energy in the fuel is not effectively utilized, and most of it is discharged into the atmosphere in the form of waste heat, causing huge economic losses and serious environmental pollution. Therefore, using engine waste heat to generate electricity is a good way to save energy. Due to the compact structure of the car and the small amount of engine exhaust, the utilization of the residual heat of the vehicle engine is more difficult than the waste heat recovery of large industrial equipment. Since the 1970s, scholars from some industrialized countries have proposed a thermoelectric generator (TEG) to solve the above problems. TEG is based on the principle of direct conversion of thermoelectricity. It has the characteristics of simple structure, no moving parts, no noise, etc. It has a unique effect in the utilization of low-grade heat energy. It is installed on the exhaust pipe of internal combustion engine, which can directly convert the residual heat of internal combustion engine into Electrical energy. Thermoelectric power generation research includes two aspects of thermoelectric devices and generators, and is an important field of thermoelectrics. 1 Thermoelectric conversion materials and components 1.1 Thermoelectric materials research Thermoelectric conversion devices are the basic components of thermoelectric generators. Their function is to convert thermal energy directly into electrical energy. The efficiency depends on the performance of the thermal electrode material and the design and manufacturing level of the device. Since the 1950s academician Ioffe of the former Soviet Academy of Sciences proposed the theory of semiconductor thermoelectrics, the thermoelectric materials used for thermoelectric power generation are all semiconductor materials, such as Bi2Te3 and its solid solution alloy for low temperature (below 300C), medium temperature (300°). C~ 1000C) Thermoelectric materials SiGe, MnTe, SiRe2, CeS, etc. The index for measuring the pros and cons of thermoelectric materials is the rate, which is the thermal conductivity, and the dimension of Z is K-). ZT values ​​are also often used in research (called dimensionless merit, T is absolute temperature). Materials that can be used for thermoelectric generation have both a high aa value and a low X value. This is a very harsh and contradictory condition, so that the highest value of the thermoelectric material ZT at room temperature is about 1. Failure to break through. Therefore, the search for high-value thermoelectric materials has always been an important part of thermoelectric research. These studies mainly include: research on new thermoelectric materials, such as rare earth sulfides, selenides, boron-rich solids, and skutterudite compounds. . These studies have shown that it is effective to solve the contradiction between good conductance and thermal insulation by controlling the optimum carrier concentration or by solid solution doping. Research on new structures of thermoelectric materials, including gradient materials, composite materials and thermoelectric materials with quantum well structures. The gradient structure of the thermoelectric material includes a gradient of the carrier concentration of the material and a gradient of the junction surface of the laminated thermoelectric material. A reasonable gradient structure allows the material to adapt to changes in the internal temperature gradient, allowing the best material to be used in the most reasonable temperature range to improve overall conversion efficiency. The research on new processes for the preparation of thermoelectric materials, such as melt growth and powder metallurgy, and vapor phase growth using micro-semiconductor thermoelectric devices. The perfection of the preparation method and process has a great influence on the performance of the material. The powder metallurgy method is suitable for large-scale production, and the waste of raw materials is small, and the obtained material has good mechanical properties, and is a promising process suitable for practical use. 1.2 Modification of thermoelectric conversion components The conversion power of a single thermoelectric conversion device is very small, and it is necessary to make a conversion module through serial/parallel combination, and realize standardization and serialization of products. A thermoelectric module developed for the vehicle waste heat conversion by HZ Company of the United States, which is connected by a thermocouple coupling of 71 pairs of bismuth telluride, fixed on a frame called “egg frameâ€; the module outputs 2.38V/19W when the temperature difference is 200C. Current. The company has developed four different specifications of products, forming a series, output electric power from HZ-20 thermoelectric conversion module 2 thermoelectric generator structure 2.1 structural form thermoelectricity generator structure depends on heat source characteristics, heat dissipation method and temperature distribution, And the performance and arrangement of the thermocouples used in the generator. At present, the thermoelectric generators are mainly of the flat type and the drum type. The thermoelectric conversion module of the flat thermoelectric generator is suitable for tiling on a rectangular channel, and a thermocouple array similar to a solar array is used to flow heat through the channel, and the heat is transmitted through the wall facing the conversion module. Thermocouples on the surface of drum-type thermoelectric generators have a certain degree of curvature and are generally used on larger structures. A typical gantry, the diesel engine exhaust pipe channel is fixed with a thermoelectric generator experimental device, which can be seen in its rectangular structure and connecting wires. 2.2 Heat source and heat source The heat source of the temperature difference generator is the residual heat taken by the engine exhaust and cooling water. The temperature of the former can reach 800C, and the latter is generally below 100C. The form of the thermoelectric generator cold source includes four kinds of natural convection heat dissipation, forced ventilation, water cooling and circulation cooling. The thermocouple arm currently designed is only 3 ~ 10mm long, and the hot and cold end spacing is very small. Therefore, the proper cold source form is one of the key factors for obtaining a large temperature difference. 2.3 Fixing method At present, the structure trend of the thermoelectric generator Generalization and componentization, the thermoelectric module is fixed on the outer wall of the generator, and the fixing method mainly includes a bonding method and a mechanical fixing method. As an example of mechanical fixing, the heat source is a rectangular pipe, and a plurality of thermoelectric modules are arranged on the outer wall thereof, and a cooling water channel is arranged on the outer side of the module, and is pressed by a bracket and a spring. Compared with the bonding method, this method is easy to replace and repair, but the structure is complicated and the contact thermal resistance is relatively large. 2.4 Partitioning Structure Another measure to make full use of the temperature difference is to operate each thermoelectric module in the most appropriate temperature range. Since the temperature of the hot fluid in the exhaust pipe is decreasing along its flow direction, the channel is divided into several temperature zones in the direction of flow, and thermoelectric modules of different materials are disposed in different temperature zones to operate at an appropriate temperature. Next, to get the maximum conversion efficiency and output power. 3 Temperature difference of vehicle engine waste heat generation In recent years, the vehicle engine residual heat temperature difference power generation technology has developed rapidly. Many universities, research institutes, military, related enterprises and automobile companies at home and abroad have reported the research results: 45. Some test devices have been After laboratory bench tests, road tests, and durability tests, the conversion scale was between several hundred watts and 1,500 watts. The thermoelectric generator developed by Nissan Automobile Research Center of Japan, its external dimensions are 440mmX180mmX170mm, connected to the middle of the exhaust pipe of 3000cc gasoline engine; the thermocouple used is 20mm in diameter and 9.2mm in height, and each 8 pairs form a mold ± every The modules output 1.2W of electrical energy, and a total of 72 modules are laid on the rectangular outer wall of the inner channel. The outside of the thermoelectric generator is water cooled, and the maximum temperature difference is 563K. In the road experiment, when the car climbs at a speed of 60km/h, the generator can convert 11% of the heat in the exhaust. The paper on the design of 1.5kW thermoelectric generator has studied the temperature difference power generation of the domestic liberation 141 automobile exhaust heat: the original vehicle generator output power is 350W, the rated voltage is 14V, and the rated current is 25A. The calculation shows that when using 960 deuteration Lead thermocouple, high temperature heat source is 500K low temperature heat source is 100 ° 0, as long as there is 16610 / s exhaust heat can be converted to the same amount of power as the generator output. The bench test shows that the temperature and flow rate of the exhaust gas of the engine at the lowest speed have met these requirements, indicating that the temperature difference of the exhaust heat is practical. The Institute of Physics and Energy Engineering of the Russian Federation Science Center conducted research on direct power generation and heating of truck engines in alpine regions. In this respect, advanced design methods and multidisciplinary basic theories are integrated, such as: optimization design research. The excellent value of the thermocouple is related to the electrode material and also to the cross-section and length of the electrode. The electrodes of different resistivity and thermal conductivity should have different geometrical dimensions, and only the optimal size can be obtained to obtain the best device figure. At the same time, the performance and structure of the thermoelectric generator load should be comprehensively considered in the design. The optimization design, optimization and simulation methods such as CAD and other modern design methods are the important topics for future research. Higher reliability has always been one of the most important advantages of semiconductor thermoelectric devices, but due to factors such as design, manufacturing and use environment, the stability and service life of the device are significantly different. Analysis and experiments on the failure mechanism show that the main cause of failure of the semiconductor thermoelectric device is the migration of substances at the junction of the thermocouple. The solder at the joint or the atoms of the baffle enter the electrode material through the form of solid diffusion, which becomes an additional blend. The hetero atom, as well as the precipitation of impurity atoms incorporated in the material, etc., change the material properties and the like. These issues remain to be resolved. Research on multi-field co-translators. The research group of South China University of Technology proposed a new thermoelectric generator structure based on the heat transfer form of the heat flow in the exhaust pipe and the basic law of the thermocouple circuit. The research uses a built-in axial mesh thermoelectric conversion module and a multi-stage conversion structure. The cold source is a separate circulating water cooling system and is directly connected to the water cooling system of the engine. In this way, a large temperature difference, a high degree of thermocouple integration, a lower requirement for the A value of the thermoelectric material, and a coupling of the velocity field, the flow field, the temperature field, and the electric field are enhanced, and the energy of the residual heat flow is fully utilized. The research has completed theoretical research and confirmatory experiments. The 5 small junction temperature difference generator has a wide application prospect for the waste heat utilization of the vehicle engine. From the research results, the current temperature difference of the design structure is not high, the total number of thermocouples is less than 1000, and there are problems such as low efficiency, high cost, and insufficient structure. Before the breakthrough in thermoelectric materials research, the development of high-power / high-performance / high-economic conversion devices and conversion systems using enhanced thermoelectric direct conversion technology will be the development trend of this topic. Connecting Link Coupling,Connecting Link,Safety Hook,Lifting Hook Shandong Shenli Rigging Co.,LTD. , https://www.shenliriggingcn.com