| Thermoelectric power generation(TEG)systems often confront with mismatch between the supply and demand of thermal energy in waste heat recovery.The phase change heat storage that based on phase change materials(PCMs)have advantages of high heat storage density and good thermal stability.When phase change heat storage is coupled with TEG system,it not only coordinates the heat storage and transport,but also effectively reduce the heat loss.However,the heat storage and release,thermoelectric conversion,and heat transfer processes are influenced by various factors such as thermophysical properties of PCM,thermoelectric parameters and interfacial thermal conduction etc.during operation of TEG power generation system coupled with PCMs(TEG-PCM).Once the heat supply is lost,the emphasis on heat dissipation alone will results in the limited amount of phase change heat storage cannot meet with demand of thermoelectric power generation.Therefore,TEG-PCM system still require further synergistic optimization in phase change heat storage,thermoelectric conversion,interfacial thermal conduction and cooling performance of heat exchanger in its dynamic thermal equilibrium adjustment.In this thesis,the heat transfer optimization of TEG-PCM system is taken as the entry point.Theoretical analyses and experimental tests are used for evaluating the influence of various factors on the TEG-PCM system,which provides theoretical guidance and scientific basis for component adaptation and optimum design.The main research contents and conclusions are as follows:1.A new type of TEG-PCM device is designed by embedding the PCM directly into the hot side of thermoelectric generator.Meanwhile,the influence of thermophysical properties and geometry of PCM on the TEG-PCM system was also investigated.The results show that this method not only reduces the thermal contact resistance between PCM and thermoelectric generator but also increases the heat transfer area.Moreover,the effective length of contact layer and thermoelement are optimized through varying the length-to-diameter ratio of the PCM vertically and horizontally,as well as increasing the heat storage capacity.Thus,it promotes the thermoelectric generator make full use of phase change heat storage and convert it into electrical energy efficiently.In addition,the use of composite PCM with high thermal conductivity can further accelerate the phase transition and shorten the thermal response time,which is beneficial to the TEG-PCM system generating more electrical energy and prolong the duration of power generation after the loss of heat supply.2.In the case of TEG-PCM system lost heat supply,the interface thermal resistance needs to be optimized to coordinate thermoelectric conversion and heat transfer in order to ensure that the waste heat stored in the PCM can be appropriately allocated for thermoelectric power generation.In this study,on the basis of temperature-dependent nonlinear equations of phase change heat storage and thermoelectric conversion,the heat transfer theoretical model of TEGPCM system is constructed based on the thermal resistance principle.Meanwhile,the matching relationship of thermal resistance of thermoelectric generator and interfacial thermal resistance is investigated and analyzed in detail in conjunction with experiments.The results show that when the ratio of the thermal resistance of thermoelectric generator to the sum of interfacial thermal contact resistances at hot and cold sides is (?)(where (?) is the average temperature on two sides of thermoelectric generator),the TEG-PCM system can obtains the maximum electrical energy at cooling platform.At the same time,the electrical energy obtained by TEG-PCM system in full cooling region shows a monotonic increase with the enhancement of interfacial thermal conduction.Besides,the proportion of electrical energy stably obtained by TEG-PCM system at cooling platform to the total electrical energy obtained in full cooling region is also reduced significantly.3.Due to the limited heat storage capacity of TEG-PCM system,enhancing the cooling performance of heat exchanger alone will inevitably lead to rapid heat loss.It is incompatible with the use of PCMs to store waste heat for prolong the duration of power generation and harvesting additional electrical energy.In this study,the cooling performance of heat exchanger was adjusted by improving the heat exchanger structural design,applying nanofluid as the heat transfer medium and regulating the mass flow rate to fit TEG-PCM system.The results show that the W-type heat exchanger facilitates turbulent formation,under this condition the TEGPCM system using nanofluid with excellent heat transfer performance at high mass flow rate is conducive to generate more electrical energy in full cooling region.In contrast,when TEGPCM system using heat transfer medium with mediocre heat transfer performance under low mass flow rate in conventional pipeline-type heat exchanger.It still has advantage of making full use of phase change heat storage and convert it into electrical energy at cooling platform.4.The synergistic action between Thomson effect and the non-equilibrium heat transport of carriers can induce local heat absorption or release of thermoelement.In view of optimization in favor of thermoelectric power generation,it is necessary to clarify the physical mechanism and influence law from the microscopic perspective.Then it will provide theoretical guidance for improving the power generation of TEG-PCM system.In this study,the influence of Thomson effect on the output power,conversion efficiency and temperature distribution of Ptype and N-type thermoelements(bismuth telluride and skutterudite)were comprehensively investigated by theoretical calculation,finite-element modeling analysis and experimental tests,respectively.The systematic analysis of genesis and physical mechanisms of the Thomson effect producing changes in heat absorption and release is also carried out.The results show that P-type and N-type thermoelements present opposite endothermic and exothermic directions at the temperature conditions corresponding to different Thomson coefficients.At the microscopic level,the overall direction of heat transfer between carriers within P-type and Ntype thermoelectric materials are opposite.This originates from thermal transport imbalance caused by the migration of carriers above the excited Fermi level and the Thomson effect used in concert to maintain the dynamic heat transport equilibrium.Hence,the positive or negative effects on the power generation performance of thermoelement show opposite patterns.At the macro level this in turn determines the temperature suitability in coupling of thermoelectric power generation and phase change heat storage.Hence,the Thomson effect should be taken into account for further improving the power generation performance of TEG-PCM system in the future.5.Based on the adaptation of components along the heat transfer path which restrained from each other through temperature changes in TEG-PCM system,this study summarizes the matching principle between PCMs,heat storage unit,thermoelectric generator and heat exchanger cooling performance.For PCM,appropriate phase transition temperature,high specific heat capacity or enthalpy and good thermal conductivity are required to ensure reliable heat storage.For heat storage unit,the thermal resistance of heat transfer zone and the area of heat preservation zone should be reduced as much as possible,while the thermal resistance of heat preservation zone and the area of heat transfer zone should be increased to meet the need of stable heat storage or release.For thermoelectric generator,it should have high conversion efficiency as much as possible under low temperature difference,and the thermal resistance of thermoelectric generator also needs to match with interfacial thermal contact resistance in order to reduce heat loss and prolong power generation duration.For cooling performance of heat exchanger,the use of nanofluids with superior heat transfer performance should be given priority under the premise of controllable cost,and liquid-cooling should be complete in the heat exchanger with good cooling capacity at a suitable heat exchange rate.In this way,it is promising to improve the power generation performance of TEG-PCM system after the loss of heat supply. |
