Exergoeconomic Optimization For Irreversible Direct And Reverse Carnot And Brayton Cycles

Carnot cycle model, as a theoretical model, has played an all-important role since the establishment of classical thermodynamics. Brayton cycle is a common cycle, whose engine cycle is extensively used in regions of nuclear energy, solar power, military affairs, etc. and the use of air as refrigerant in its refrigeration cycle has gained more and more application value in today when environmental protection is focused on. Thus, it is very necessary to subject the above two cycles to finite time thermodynamic analysis and optimization to make them closer to engineering.On the basis of understanding and summarizing the achievements of the finite time thermodynamic studies on Carnot and Brayton cycles, this dissertation focuses on the exergoeconomic optimizations of irreversible Carnot heat engine cycles, irreversible Carnot refrigeration cycles, irreversible Brayton heat engine cycles and irreversible Brayton refrigeration cycles using the methods of mathematical modeling, theoretical analysises and numerical calculations, respectively. The results obtained herein have important theoretical significances and application values.When analyzing the Carnot cycles, the optimal finite time exergoeconomic performances of the generalized irreversible Carnot heat engine and refrigerator are derived respectively, in which the heat transfer law between the working fluid and the heat reservoirs is assumed to obey various heat transfer laws including)(n??TQ and n??TQ)(, considering several irreversibilities such as heat resistance, heat leakage and other undesirable irreversible factors. The fundamental optimal relations between the profit rate and other thermodynamic coefficients as well as its performance bound at maximum profit are derived for various thermodynamic cycles with various heat transfer law, various loss types. A series of characteristic curves obtained by numerical calculations of Carnot heat engine and refrigerator are helpful for profoundly understanding the effects of heat transfer law, heat-resistance, heat leakage and internal irreversibility on the finite time exergoeconomic performance.When analyzing the Brayton cycles, considering the irreversible effects of heat exchanger, compressor and expander, the performance analyses and optimizations are carried out respectively for irreversible simple and regenerated direct and reverse Brayton cycles coupled to both constant- and variabletemperature heat reservoirs by using finite time exergoeconomic analysis method. The analytical expressions of the profit rate are derived. The relationships between profit rate and pressure ratio, between profit rate and the distribution of heat conductance as well as between profit rate and the ratio of heat capacity rates have been analyzed by numerical examples. For direct and reverse Brayton cycles coupled to constant- temperature heat reservoirs, a double maximum profit rate can be achieved by simultaneously optimizing the pressure ratio and the distribution of heat conductance. For Brayton refrigeration cycles coupled to variable- temperature heat reservoirs, a double maximum profit rate can be achieved by simultaneously optimizing the distribution of heat conductance and the ratio of heat capacity rates between the heat reservoir and working fluid. For Brayton heat engine cycles coupled to variable- temperature heat reservoirs, a triple maximum profit rate can be achieved by simultaneously optimizing pressure ratio, the distribution of heat conductance and the ratio of heat capacity rates between the heat reservoir and working fluid. Besides, the influences of other major parameters on the finite-time exergoeconomic performances of irreversible direct and reverse Brayton cycles are analyzed. By selecting particular price ratios, the optimization of profit rate can be converted to the optimizations of power, cooling load, entropy generation rate and ecological function objectives.