Investigation On The Performance Optimization Of Thermodynamic Systems Of Solar Energy And Low-grade Heat Energy Utilization

With the increasing environmental pollution and energy crisis,people pay more attention to the utilization of renewable energy sources and the improvement of energy conversion efficiency-Solar energy is one of the renewable energy sources under currently active exploitation and arouses great concern due to its clean,renewable,and nearly inexhaustible.The energy utilization rate can be further improved by harvesting low-grade heat energy.Thus,the efficient utilization of solar energy and the recovery of low-grade heat energy have been becoming one of the hot topics in current researches.In this thesis,four kinds of the updated or new thermodynamic systems of solar energy utilization and low-grade heat energy recovery,including the concentrating photovoltaic(PV)cell-thermoelectric generator(TEG)system with two-band spectrum splitting,concentrating PV cell-heat engine system with three,band spectrum splitting,concentrating solar thermionic converter(CSTIC)with multi-graphene layer(MGL)cathode,far-field thermalphotovoltaic cell,and the recovery system of automobile exhaust waste heat by using thermophotovoltaic(TPV)cell,were investigated through numerical calculation.The performance characteristics of these systems are evaluated.Furthermore,parametric optimal designs are conducted.The research contents of the thesis mainly include three parts as follows:The first part contains Chapters 2 and 3.The subjects of this section are the updated concentrating PV-TEG system with two-band spectrum splitting and concentrating PV-heat engine system with three-band spectrum splitting.The updated analytical expressions of the total efficiencies of two systems are developed to evaluate the performance and accurately predict the maximum total efficiencies of these systems.Based on the irreversible models of the PV cell and TEG,the model of a concentrating PV-TEG system with two-band spectrum splitting is established.The low-frequency photons are converted into heat to be used by the TEG,so as to improve the utilization rate of solar energy.The optimum values including the cut-off energy,solar collector temperature,and cell voltage are determined through the numerical calculation.Importantly,the influence of the area ratio of the solar collector to the PV cell on the performance of the system is discussed.The expressions of the upper and lower bounds of the area ratio in the reason able working region were derived.The performances of the concentrating PV-TEG system with two-band spectrum splitting and the single concentrating PV cell are compared.Because the two-band spectrum splitting system cannot effectively mitigate the thermalization loss in the PV cell,the concentrating PV-heat engine system with three-band spectrum splitting was further investigated.The relationship between the area ratio and the maximum total efficiency is discussed.The performances of the single concentrating PV cell,solar powered heat engine,and concentrating PV-heat engine system with two-band spectrum splitting in comparison with that of the concentrating PV-heat engine system with three-band spectrum splitting are conducted.The results show that the area ratio dramatically affects the performance of the system.The discussion about the area ratio would be helpful to the economic analysis of the system.When the area ratio is optimized,the maximum total efficiency of the system can be further enhanced.The spectrum splitting systems can more efficiently utilize the whole solar spectrum than the single concentrating PV cell and solar powered heat engine,especially at low concentrations.Moreover,the concentrating PV cell-heat engine system with three-band spectrum splitting is able to choose more conveniently the materials in the PV cell than the one with two-band spectrum splitting.The second part includes Chapter 4.According to the theory of the thermionic emission from the MLG with two kinds of stacking orders,a new model of the CSTIC with an MGL-based cathode is put forwarded.The influences of the work functions of cathode and anode,graphene layer number,and solar concentration level on the performance characteristics of the CSTIC are discussed.The performances of the CSTIC using the MLG-based cathode with ABA and ABC stacking in comparison with that of the one using the single layer graphene(SLG)-based cathode is conducted-It is concluded that the the maximum efficiency of the CSTICs with the MLG-based is higher than that of the one with the SLG-based cathode and the cathode temperature of the former is lower than that of the latter.Moreover,the advantages of the CSTIC using MLG-based cathode with the ABA stacking are even more obvious than those of the one using MLG-based cathode with the ABC stacking.The third part includes Chapter 5.The model of the far-field TPV cell is established,in which both internal and external irreversible losses are considered.The performance characteristics of the far-field TPV cell are discussed and the optimally operating regions of several key parameters are determined.Furthermore,based on the far-field TPV cell model,the model of the automobile exhaust waste heat recovery system by using the cylindrical TPV cell is established.By means of the variational principle and the modified Lagrangian formulation,the optimum distributions of the temperatures of the system are determined.The maximal power output and the corresponding optimum regions of parameters are achieved through numerical calculation.It is found that the TPV cell has potential as an efficient bottoming cycle to dramatically harvest the automobile exhaust waste heat through optimum design.In this thesis,the results obtained can enrich the research contents of these systems and enhance the solar energy utilization and low-grade heat energy recovery and they may provide some reference for the optimal design and practical operation.