| All natural processes are irreversible.Finite-time thermodynamics is an important theory to investigate the performance of thermodynamic systems that contain irreversible processes.Based on this theory,people can construct more actual thermodynamic models,explore change rules of various thermodynamic parameters under finite-time or finite-size constraint,and derive theory results with more application value.In addition,the special analytical method included in finite-time thermodynamics is very helpful to study the performance characteristics of different energy conversion systems and optimally design the corresponding parameters.Therefore,finite-time thermodynamics becomes an important subfield of the modern thermodynamics and is widely used in many research fields including national defense,economy,energy technology,industrial and agricultural production,chemical industry,etc.On the other hand,with the continual innovation of science technology and rapid development of human society,energy issue and environment pollution are getting worse and worse.There have been all sorts of solutions proposed by people.Among them,the most critical part is increasing energy utilization rate,which leads to some hot research subjects like searching more effective energy conversion systems,seeking possible ways to reduce energy losses and finding more suitable materials.This dissertation will use the analytical method of finite-time thermodynamics to carry out some studies.The thermodynamic performance of two energy conversion models involving low-dissipative generalized Carnot cycles and a molten carbonate fuel cell hybrid system under irreversible process effects will be investigated and their characteristic parameters will be optimally analyzed,which may bring some optimum design criteria and specific optimization theories.Main research contents are divided into two parts below.The first part is focus on investigating the performance of low-dissipative generalized Carrot cycles.By considering the energy leakage losses between two potential energy reservoirs,theoretical expressions of the power output and efficiency are obtained.With the help of Lagrangian function,optimal relations between the power output and efficiency can be further derived.The figures are drawn to analyze the performance characteristics of these cycles.The optimal working regions and the performance limits are also determined.In some extreme conditions,the analytical expressions of the power output and efficiency at maximum power output and efficiency can be further derived.The results obtained here not only include previous conclusions,but also can be directly applied to discuss the performance characteristics of other low-dissipative models.The second part is focus on investigating the performance of a new hybrid system constructed by a thermionic generator and a molten carbonate fuel cell.By considering the irreversible heat transfer losses,the energy equilibrium equation can deduce the relations among the current density of molten carbonate fuel cell,the voltage output of thermionic generator,and the area ratio of two subsystems.The effects of these parameters on power output and efficiency are also discussed,from which some optimal ranges of corresponding parameter values and optimal working regions of the system can be determined.The results show that the maximum power output of this new hybrid system can be increased by 33%compared to that of a single fuel cell,and is larger than those of other molten carbonate fuel cell hybrid systems reported in literatures.The research in this dissertation not only enriches the theory of finite-time thermodynamics,but also constructs a new fuel cell hybrid system.The results obtained here can guide the optimal design and operation of practical systems.It is also significant to the theoretical guidance of improving the energy utilization rate and developing new energy conversion devices with low-carbon and environment protection. |
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