Finite Time Thermodynamic Analysis And Optimization Of Brayton Cogeneration Cycle

Energy conservation through improving the utilization efficiency of unrenewable energy resource is the feasible and effective solution for energy and environment sustainable development at present. Gas turbine-based combined heating and power or combined cooling, heating and power, which operates Brayton cogeneration cycle in principle, is an energy efficient and environment friendly energy utilization technology and thus will has wide application prospects in China. It is significant to analyze and optimize the Brayton cogeneration cycle from the viewpoint of thermodynamics, which will be helpful for understanding and penetrating it in theory and for improving its performance through optimally designing a cogeneration system in practice.On the basis of understanding and summarizing the current research fruits, the present dissertation is to analyze and optimize various Brayton cogeneration cycles using finite time thermodynamics (FTT). Some relatively idealized models, but which can fully represent the basic characteristic of the practical processes, will be used to analyze the cogeneration cycles. Design parameters of various Brayton cogeneration cycles will be optimized, factors which have effect on cogeneration cycle performance will be analyzed, and the performances of various cogeneration cycles will be compared. The layout of this dissertation is as follows:Chapter 1 is to narrate the background and significance of the present investigation. FTT and its application in optimization of cogeneration cycle will be reviewed in detail. A brief introduction about the current development of combined heating and power will also be presented. Four performance indexes, including total useful energy rate (TUER), cogeneration efficiency, total exergy output rate (TEOR), and exergy efficiency, will be defined.In Chapters 2, 3 and 4 will, respectively, study the endoreversible simple opened Brayton cogeneration cycle, irreversible simple opened Brayton cogeneration cycle, and irreversible recuperative opened Brayton cogeneration cycle. With the thermodynamics models of various cogeneration cycles, their effective ranges of pressure ratio parameter will be determined. The formulas of various performance indexes, including efficiency, total useful energy rate, exergy efficiency, and total exergy output rate, will be derived and the results of these performance indexes at some special cases will also be analyzed. The various cogeneration cycles will be optimized through finding the optimal pressure ratio parameter. The regularities of various performance indexes and their optimal results varying with pressure ratio parameter and the ratio of power to heat of cogeneration cycles will be numerically analyzed.To find the effective methods for improving the cogeneration cycle performance, Chapter 5 concentrates on the effects of various factors, including the cycle temperature ratio, the user temperature ratio, the various internal irreversibilities, and the recuperation efficiency, on cogeneration cycle performance indexes and their optimization results. Performance comparisons between various power and cogeneration cycles, reversible, endoreversible, and irreversible cycles, simple and recuperative cycles are presented in Chapter 6. The differences among various cycles and the most suitable conditions of various cycles are also analyzed in this chapter. Energy saving and greenhouse gas reduction are the purpose of cogeneration. InChapter 7, an analysis on the energy saving and CO2 emission reduction potentials of various opened Brayton cogeneration cycles will be presented. The analytic formulas of fuel energy saving ratio and CO2 emission saving ratio will be deduced for various cogeneration cycle. A brief analysis on the relationships of fuel energy saving ratio and CO2 emission saving ratio with various cycle design parameters will also conducted in Chapter 7. Some practical engineering issues about gas turbine-based cogeneration system, such as the energy saving potential, the condition for energy saving, and the optimal design region etc., will be studied by using the research results in theory. A kind of optimization design diagram, which can be used as a tool for cogeneration system optimization design, will be introduced in this chapter. Moreover, this chapter will also discussed the optimal configurations of microturbine- based building cooling, heating and power system in order to help engineers in designing this kind of cogeneration system.Closed Brayton cycle is often used in nuclear energy utilization system and solar energy thermal power system and thus in Chapters 8 and 9 the author will, respectively, discuss the endoreversible simple closed Brayton cogeneration cycle and the irreversible simple closed Brayton cogeneration cycle. The finite time thermodynamics models of closed Brayton cogeneration cycles will be built and the formulas for exergy output rate and exergy efficiency of closed cogeneration cycle will be duduced in these two chapters. Besides, the regularities of the exergy output rate and the exergy efficiency varying with the cycle design parameters will be analyzed and the optimal pressure ratio parameters will be determined. Different from opened Brayton cogeneration cycle, there exists an optimal thermal conductance allocation among the heat resource-side, the heat sink-side, and the heat user-side heat exchangers for closed Brayton cogeneration cycle. So, the optimal thermal conductance allocation and the dual optimization for closed Brayton cogeneration cycle will also be discussed in Chapters 8 and 9. In addition, a comparison between the research results of this dissertation and these given by reference will be presented in Chapter 8 and the difference between the two results and the reason which caused this disagreement will be discussed.Finally, main conclusions and innovations of this dissertation are presented in Chapter 8 and possible future work is discussed as well.