| As a kind of high quality,clean and efficient energy,natural gas has been used more and more,and the recovery and utilization of residual energy in the natural gas industry has also been widely concerned for the wide distribution and large temperature region.It comes not only from the low-temperature cold energy in the process of LNG gasification,but also from the low-temperature wet flue-gas discharged by natural gas boiler,and also from the high-temperature exhaust of natural gas vehicle.Multi-grade residual energy power generation is considered to be one of the most effective utilization modes.Because of the merits such as no chemical reaction and no mechanical moving parts,the thermoelectric generator can be used flexibly and conveniently in various low-temperature unsteady waste-heat power generation.In this paper,the thermoelectric generation technology is applied to the recovery of multi-grade residual energy of natural gas.According to the characteristics of different grade residual energy,the research on thermoelectric conversion law and the development of high-efficiency thermoelectric conversion technology are the main contents of this paper.In the aspect of LNG cold energy thermoelectric power generation,a novel air-heated vaporizer combining with thermoelectric generator is designed.A mathematical model is established by coupling the complex heat transfer process of natural gas with thermoelectric conversion process,and the heat transfer and power generation characteristics of the new tube are obtained.The results show that the outer wall temperature of the new tube increases obviously compared with the traditional gasification pipe.The generating efficiency can be maintained between 1.57% and 2.12% when the fluid is in liquid-phase and two-phase regions.And in the gas-phase region,the generating efficiency decreases gradually with the increase of natural gas temperature.In addition,it is found that there is an optimal length of tube to maximize the output power.The optimal tube length and maximum output power almost increase linearly with the increase of flow rate.According to the above model,a cold energy thermoelectric generation system is designed and built with liquid nitrogen as cold source and air as heat source.The thermoelectric performance of cold energy thermoelectric generator is experimentally researched,and the influence of heat transfer process of liquid nitrogen on thermoelectric performance is obtained.The results show that with the gasification of liquid nitrogen,the temperature difference between hot and cold sides and the open-circuit voltage increase gradually in the liquid-phase and two-phase regions.In the liquid-phase region,both decrease first and then increase,and the open-circuit voltage increases greatly because of the influence of material properties.In addition,with the increase of flow rate of liquid nitrogen,the maximum output power increases obviously.Moreover,it is pointed out that the less temperature difference between the hot and cold sides than that between the hot and cold sources,and the poor performance of the module under low temperature are the main restraining factors for thermoelectric generator.For the large quantity of water vapor present in the wet flue-gas,not only the latent heat is huge,but the heat transfer coefficient is also increased significantly when the flue-gas condenses.Aiming at this characteristic,a mathematical model of wet flue-gas thermoelectric generation is established.It is found that the characteristics curves can be divided into a sensible heat generation region and a mixed power generation region,with the power generation performance of both regions having different characteristics.The power generation performance jumps when the wet flue gas condenses,which leads to the inflection point of output power curve.By discussing the influence of gas temperature on generation performance,it is found that there exists a critical value of water vapor content that can directly determine whether it is necessary to recover the latent heat for power generation.Considering that the generation performance can be significantly increased when wet flue-gas condenses,gas humidification is proposed to improve the performance of thermoelectric generator.Through the energy and exergy analysis of the novel system,it is found that although energy destruction is not caused by flue gas humidification,the exergy destruction of system is increased.The large decrease of convective exergy destruction at the hot-side leads to increased output power.Moreover,an increase of the heat transfer coefficient could also reduce the module area.For high-temperature wet flue-gas,an intermediate humidified flue-gas thermoelectric power generation system is proposed,which can not only fully utilize high-temperature flue gas and also improve the power performance at lower temperature.An optimum intermediate humidification temperature that results in maximum output power at the smallest corresponding module area is also determined.For high-temperature automobile exhaust,in order to avoid excessive pressure loss caused by heat transfer enhancement of exhaust channel,the intermediate fluid exhaust thermoelectric generator is proposed in this paper.The intermediate fluid first heat transfer directly with the exhaust,then the intermediate fluid after heating is used for the thermoelectric generation.Therefore,the relative separation of the exhaust heat exchange process and the thermoelectric generation process is realized.Compared with the traditional thermoelectric generation system,in a certain condition of the exhaust heat exchange area,not only is the maximum output power increased slightly,but the optimal module area can also be reduced by 68.9%.In addition,the temperature distribution along the module of the intermediate fluid thermoelectric generation system is more uniform,which is beneficial to prolong the service life of the module.Considering the extra power consumption caused by intermediate fluid circulation in the intermediate fluid exhaust thermoelectric generator,a phase change exhaust thermoelectric generator system is proposed in this paper.The waste exhaust heat is transferred through boiling and condensation of the intermediate fluid,which makes a higher heat flux on the hot-side of module.A mathematical model is also established to analyze the power generation characteristics of the novel system.Compared with the traditional thermoelectric generator system,the peak output power is increased by 32.6%,and the optimal thermoelectric module area is reduced by 73.8%,indicating that the phase change thermoelectric generator has better power generation performance. |
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