The Investigations On The Interfacial Heat Transfer And The Temperature Regulation In A Photovoltaic-thermoelectric Hybrid System

With the intensified problems of environment pollutions and the shortage of fossil energy,the photovoltaic(PV)technology that can directly utilize green and renewable solar energy becomes more and more attractive.However,photovoltaic cells can only convert a part of solar energy into electricity;the remaining solar energy is converted into heat that results in the temperature rise and the efficiency decrease of PV cells,which leads lots of solar energy wasted.Therefore,in order to realize the full utilization of solar spectrum energy,a photovoltaic-thermoelectric(PV-TE)hybrid system is proposed,in which the thermoelectric(TE)module is able to convert waste heat energy into electricity.Nonetheless,due to the different electricity generation principles of PV cells and TE modules,the requirements of the system temperature for these two devices are totally different.For PV cells,a lower system temperature can bring more photovoltaic conversion efficiency;but for the TE module,due to that its cold-side temperature is limited to the ambient temperatures and cooling methods,a higher system temperature is more conducive to TE modules to have a better thermoelectric conversion efficiency.Therefore,in order to maximize the efficiency of PV-TE system,it is very important to regulate and match the operating temperatures of PV cells and TE modules.In addition,because that the existence of interfacial thermal contact resistance greatly hinders the energy transfer within the system,which affects the temperatures and efficiencies of PV cells and TE modules,it is very necessary to analyze the characteristics of interfacial heat transfer and find ways to reduce the thermal contact resistance.Therefore,in order to realize the high solar energy utilization efficiency of PV-TE system,the interfacial heat transfer and the temperature regulation in a photovoltaic-thermoelectric hybrid system are investigated in this paper.The main contents are as follows:1.Research on the temperature characteristics of PV-TE hybrid systemThe theoretical model of photovoltaic-thermoelectric coupling system is established.Based on the temperature characteristics of PV cells and the temperature characteristics of TE module,the temperature characteristics of PV-TE hybrid system are studied.The relationship between system operating temperature and system efficiency is explored.The main factors that affect the operating temperature and power generation efficiency of PV-TE system are analyzed.The importance of regulating the operating temperature of the whole PV-TE system and the interfacial temperature difference between the system components is brought forward.2.Research on the characteristics of interfacial heat transfer through a multi-scale simulation methodBased on the microscopic lattice Boltzmann method and the macroscopic FD method,a multi-scale simulation method of interface thermal contact resistance(TCR)is established.The problems of energy transfer,boundary condition,and the coupled method at the micro contact interface are solved.By comparing with the experimental data and the empirical formula of TCR,it is found that the average error of the multi-scale simulation method is less than 8%,which is better than the traditional empirical formula.The effects of surface roughness,surface flatness,hardness,contact pressure,thermal conductivity of material and micro/nano scale effect on the interfacial heat transfer are investigated.The results show that the surface topography parameters are the significant factors to the TCR at a low contact pressure.Reducing surface roughness is benefit to decrease TCR.However,it is inappropriate to analyze the influences of the surface topography on TCR just through surface roughness.The flatness of the surface and the matching between two contact rough surfaces are also need to be considered.When contact pressure increases,the influences of pressure and material hardness on TCR are increased.In addition,a higher thermal conductivity is able to reduce TCR,and the heat transfer at the microscale contact point can be enhanced if the size of contact point is close to or below the mean free path of hot carriers.3.Research on the characterization and application of rough surface morphologyThe relationship between surface roughness and the characterization parameters of regular machined rough surface is studied,based on the experimental data of rough surfaces processed by turning and end-milling processes.The turning and end-milling surface topography generation methods are established.The error between the measured surface and the simulated surface is less than 9%.By using the multi-scale simulation method and the experimental results of TCR,the influence of surface generation methods on the prediction of TCR is investigated.Moreover,the influences of the surface characterization parameters on the regular machined rough surface are investigated.The results show that a short periodic wavelength of the.rough surface profile is favorable to enhance the interfacial heat transfer.A larger plane angle of the turning surface is un-benefit to the interfacial heat transfer.4.Research on the enhancement of interfacial heat transfer characteristicsA novel graphene pressure-sensitive thermal interface material with a high thermal conductivity is investigated.The effects of the surface modification method and the doping density of graphene on the thermal conductivity and the hardness of the composites are investigated.The TCR of the interface connected by the graphene pressure-sensitive thermal interface material is measured.The results show that the thermal conductivity and the hardness of the composites are both increased with the increases of the graphene doping density.When the graphene doping density is 10wt%,the thermal conductivity of the surface-modified graphene pressure-sensitive interface increases to 5.6W/mK,which is 18.2 times higher than that of the pressure-sensitive adhesive,however,the material's Shore hardness is also increased by 2.8 times.When the graphene doping density is 6 wt%and the contact pressure is 1 MPa,the TCR is reduced to the minimum value of 31 mm 2K/W.In addition,a higher temperature and pressure can further enhance the interfacial heat transfer of the composite.When temperature is 60 ?,the contact pressure is 5 MPa,the TCR reaches to only 11 mm2K/W.5.Theoretical and experimental study on phase change material regulating the temperature of the photovoltaic-thermoelectric hybrid systemPhase change material(PCM)is introduced into the PV-TE hybrid system to form a novel photovoltaic-phase change material-thermoelectric(PV-PCM-TE)hybrid system,which significantly realizes the match of the temperatures of PV cells and TE modules and regulation of the system temperature.The numerical model of the PV-PCM-TE system is established.Based on the matched temperatures of PV cells and TE modules,the properties of PCM and the structural parameters of each component in this system are optimized.The influences of the optical concentration,the thermal concentration,the volume mass of PCM,the flow velocities of water,thermal contact resistance,and other factors on the hybrid system are analyzed.Then,an experimental PV-PCM-TE hybrid system is built according to the optimized results of theoretical simulation.The performances of the experimental hybrid system with different optical concentrations and cooling methods are studied and compared to the pure PV system.The results show that the performance of PV-PCM-TE system is better than the pure PV system,in which one can found that the efficiency of air-cooled PV-PCM-TE hybrid system is 2.95%higher than the air-cooled pure PV system.Besides,the influence of thermal contact resistance on the hybrid system is analyzed,which shows that the efficiency of the hybrid system is decreased with the increases of thermal contact resistance.