Study On Influence Of Thermal Resistance Optimization And Phase Change Material Coupling On Performance Of Thermoelectric Generator

In the background of increasing energy and environmental problems,thermoelectric generator（TEG）which can convert the thermal energy directly into electrical energy is widely used in the fields of aerospace,solar power generation,waste heat recovery,wearable equipment and marine energy power generation because of its advantages such as no moving parts,low maintenance cost,no pollution,no noise,small size and light weight.It is one of the most promising new technology methods in energy utilization technology.However,the relatively low conversion efficiency of thermoelectric conversion in nowadays makes the heat cannot be effectively used,which limits its future wider application.Except the influence of the thermoelectric materials,there are many factors in the device level that cause the low conversion efficiency of TEG,such as interface contact resistance,geometry of device,unstable heat source,interface contact thermal resistance,etc.Among these factors listed above,the interface contact thermal resistance is one of the important factors that affects the heat transfer and thus the performance of the TEG.In recent years,a lot of works focus on the effect of total thermal contact resistance on the performance of the TEG.However,the matching problem between thermal contact resistances at different interfaces needs further study.In addition,the temperature fluctuation at the hot-end of TEG is another important factor that affects the performance of the TEG.Phase change material（PCM）has been used to stabilize the temperature of heat source with the advantages of high latent heat,high density and the ability to absorb and release a large amount of heat under approximate constant temperature.However,the matching problem of PCM and thermoelectric properties of thermoelements and the size optimization of PCM in PCM-TEG coupling system are important issues,which needs further study.In this thesis,the model of the TEG with thermal contact resistance included and the model of PCM-TEG coupling system are established.The Enthalpy equation is applied to describe the PCM and the three-dimensional thermoelectric coupling equation is used to describe the TEG.We studied the influence of the phase change temperature and the size of PCMs on the performance of the TEG in detail.The results are addressed as follows:（1）The optimal output power and conversion efficiency of TEGs can be reached when the TCR at the interfaces of p-thermoelement equals to its counterpart at the interfaces of n-thermoelement.Furthermore,the optimization of the TEG performance by matching the TCR becomes much more obvious when the total TCR is smaller.We then applied the model to the TEG with p-Bi_0.2Sb_1.8Te₃ and n-Bi₂Te_2.6Se_0.4 thermoelectric materials,whose thermoelectric properties are temperature-dependent,and found that enhanced TEG performance can also be obtained by TCR matching.The study presented in this work could be useful for guiding the searching for high efficiency TEGs.（2）The output electric energy of a PCM-TEG system can be optimized by the phase change temperature of PCM.The optimized phase change temperature of PCM is determined by the thermoelectric properties of the thermoelectric legs.Moreover,the value of optimized phase change temperature of PCM varies with the temperature when the figure of merit（ZT）of the thermoelectric material reaches maximum linearly.Our work provides a method to optimize the performance of the PCM-TEG system on the view of matching phase change material and the thermoelectric materials,which could be helpful to realize further high efficiency TEG for intermittent heat sources.（3）The length of the thermoelement wrapped by PCM will affect the heat transfer from the PCM to the TEG,thus affecting the performance of the TEG.The results show that the application of PCM can greatly improve the utilization of thermal energy in the TEG,and the length of the thermoelectric leg wrapped by PCM has an optimal value when the volume of PCM remains unchanged.