Finite-element Analysis And Multi-parameter Optimization Design Of Thermoelectric Conversion Devices

In a prosperous human society,the problems of energy shortage,resource waste and environmental deterioration are becoming more prominent in recent years,thus people all over the world urgently seek and exploit clean,environment-friendly,and economical energy technology.Thermoelectric power generators,as novel environment-friendly power generations,have the ability of recovering and reusing waste heat without mechanical moving parts,and the advantages of no noise,no vibration,no pollution,rapid response and long lifespan.Therefore,thermoelectric power generators have been widely concerned by all walks of life in various countries,which have been applied to many fields such as aerospace,military,automobile and Internet of Things.Traditionally,n-type and p-type thermoelectric legs in a thermoelectric generator are connected by electrodes and electrical insulating substrates electrically in series and thermally in parallel.The maximum theoretical efficiency of a thermoelectric generator is depended by material properties.However,due to the structural construction(such as device construction,legs shape,and legs size),interface design(such as interfacial thermal resistance,interfacial electrical resistance,interfacial diffusion,and interfacial reaction),and thermal management(such as heat radiation,heat convection,and heat collection/heat dissipation integration),the actual efficiency of about 10%is lower than the theoretical value greatly.Moreover,owing to the difference of thermoelectric properties,heterogeneous interface,and complex working environment(oxygen,moisture,high temperature,mechanical vibration,mechanical pressure),many problems such as material deterioration,interface fracture,and structural damage will occur on thermoelectric generators during long-term service,leading to performance deterioration and even device failure.In addition,compared with traditional energy generators,thermoelectric power generators have long and complex R&D chain and production chain from materials to devices to systems,so their cost performance is relatively low.Thus,although thermoelectric generators have excellent energy conversion characteristics,the large-scale industrial development is restricted by their limited output performance,unstable service performance and low cost performance severely.To optimize module performance and reduce cost,many numerical simulation methods are proposed.Finite-element analysis as a numerical computation method can optimize and research the structural design,contact design and service performance efficiently.It has the advantages of improving module performance,shortening the studying time,and saving the research cost.All in all,in this paper,the finite-element method is used to optimize the multi-parameter integrated design of thermoelectric generators,and the effects of interface contact,geometric size,structural construction and thermal stress on output performance and service performance are systematically studied.Following achievements have been obtained:1.The effect of interfacial resistance on module performance has been simulated systematically,and then the module performance is improved by optimizing metallized layer,surface roughness and pressure.As a result,higher interfacial thermal resistance will result in heat loss at the contact interface and reduce actual temperature difference;high interfacial electric resistance will impede current transmission and increase inner resistance.High interfacial thermal and electric resistances lead to a swift deterioration in conversion efficiency.Consequently,maximum efficiency(?max)of rough contacted thermoelectric module is 23%of that of ideal contacted module at ?T=600K.Then the metallized layer is added between electrodes and thermoelectric materials to optimize interfacial design.The results show that the metallized layer is beneficial to decrease the microgaps,strengthen electric and thermal transport,and reduce interfacial resistances.Among these modules,the module with metallized Ag layer shows the highest ?max of 1.7%at ?T=600K,which is about 2.4 times that of rough contacted module.Following that,the surface roughness slope(m)and pressure(p)have great influence on interfacial resistances.At m=0.8 or p=100kPa.the efficiency will reach 90%of that of the ideal contacted module.2.The relationship between module performance and multiple coupled geometric parameters has been simulated basing on a PbTe-based thermoelectric module.The module is composed of eight pairs of p-type PbTe-8%SrTe legs and n-type PbTe0.998I0.002-3%Sb legs.The results show that ideal contacted module can obtain the maximum output power(Pmax)of 8.45W and ?max of 15.17%at ?T=500K.Contact resistance greatly degrades module performance.When the contact resistance is half of inner resistance,the ?max of module is only 60%of the ideal value.In this case,the geometric structure is further optimized,and the effect of structural parameters on module performance is studied.It is found that the cross-sectional area ratio(Ap/An)and legs height(H)show a strong dependent relationship on the optimization of module performance.For this PbTe-based module,the Pmax of 5.36W is achieved at Ap/An=1.78 and H=0.65mm at ?T=500K.In the meantime,?max=14%is achieved at Ap/An=1.64 and H=13mm.The Pmax and ?ma)x are 62%and 65%higher than the initial values.In other words,the higher Ap/An and shorter H are beneficial for enhancing the power output.Moreover,the conversion efficiency is increased at an optimal Ap/An and a larger H Thus,the results demonstrate that geometry optimization can enhance module performance effectively,and different practical devices should be designed according to exact performance requirement.3.A novel(Bi,Sb)2Te3-based multilayer composite structured thermoelectric module(MCTEM)with high output power is proposed,its performance is modelled and simulated for the first time.Parallel heat transfer and electrical parallel connection are simultaneously achieved in an n-type single-leg MCTEM which composed of several alternately stacked thermoelectric slices and inner electrodes.The extremely low inner resistance(Rin)and high current(I)are 0.03 and 12.7 times of those measured in a single-leg traditional module,respectively,then a Pmax of 5.8 mW is achieved at?T-35?.Following that,the numbers of thermoelectric slices of n-and p-type MCTEM are optimized to balance the transfer between heat flux and current,and consequently improve power output.And the optimal Pmax for n-and p-type single-leg MCTEMs have been achieved when using 3 slices.Based on the optimized single-leg MCTEMs above mentioned,a ?-type MCTEM has been designed to improve the output voltage(V)and further enhance Pmax.The ?-type MCTEM achieves a high Vof 3.1 mV while keeping a low Rin of 0.94 m? and large I of 3.3 A,and then a Pmax of 10.5 W is obtained which is 4.2 times of traditional module at ?T=350C.4.A high-temperature CaMnO3-based U-type unileg thermoelectric module with excellent service performance is simulated,and the effect of rounded corners and structural size on stress distribution and service performance is investigated.This module is consisted by a unileg structure and pn-junction,and three left legs are connected by short-circuited right legs electrically in series and thermally in parallel.The novel design is beneficial to avoid the device failure due to different thermal expansion coefficients and high temperature gradients.Moreover,the novel module eliminates the need for hot-side electrodes,and keeps working even if the hot-side electrodes and substrates are broken.The maximum output power,maximal thermal stress(?max,TEM)and fatigue life(N)are 6.6mW,3.31GPa and 41686 cycles at 6W and 300K,respectively.The ?max,TEM and N are 46%and 132%of those of traditional modules,respectively.Following that,the output and mechanical performance are further improved by optimizing radius of rounded corners(ru,n),length of right legs(LR)and Ag layer thickness(HAg).It has been found that larger ru and n are suitable to relieve the local stress concentration,moreover,the ?max,TEM position is shifted from rounded corners to the bottom corner.Finally,the lowest ?max,TEM is achieved at(ru,r1)=(0.1,0),which is 7%lower than that of the initial structure.Furthermore,larger LR improves mechanical performance by decreasing the peak stress and dispersing the high stress regions,but deteriorates the module performance.Thin HAg of 35?m can achieve optimal Pmax and lower ?mmax,TEM simultaneously.To summarize,the U-type thermoelectric module has high thermal and mechanical stability under high temperature and thermal cycling conditions,which would be highly beneficial for commercializing high-temperature thermoelectric devices with excellent mechanical strength and long lifespan.5.Pure Bi2Te3 powder with 100nm grain size has been synthesized successfully by microwave hydrothermal method.Moreover,the Bi2Te3-Polyimide and Bi2Te3-Al2O3 thick films have been fabricated using a screen-printing method and measured preliminarily,and the effect of grain size and substrate material has been studied.The results show that the Bi2Te3-Al2O3 thick film has stronger adhesion and lower inner resistance,while the Bi2Te3-Polyimide film has the characteristics of flexibility and bending.In addition,the optimum mass fraction is 70%,and the optimal grain size is about 1-10?m.Lager mass fraction or grain size will result in brittle and fragile film,on the contrary,smaller mass fraction or grain size will lead to higher inner resistance and poor performance.Finally,the Bi2Te3-Polyimide film has the output voltage of 3.7mV at ?T=33?.In this dissertation,performance of thermoelectric modules is analyzed,investigated and optimized by the finite element method.Though analyzing the heat and electric transfer characteristics,module construction,geometric size,interfacial resistance and stress distribution,the optimum design can be obtained to meet different performance requirement and adapt varies operating environments.This dissertation is beneficial to obtain high-performance thermoelectric generators with high stability and cost performance,which will accelerate the large-scale and industrial application of thermoelectric technology.