Performance Analysis Of A Coupled Transcritical And Subcritical Organic Rankine Cycle System

Owing to the serious consumption of energy, recovering waste heat becomes an effective way to alleviate the energy problem. The organic Rankine cycle(ORC) can convert the low grade waste heat to high grade of power output. At present, the ORC becomes the research hotspot of the waste heat recovery because of its simple structure, high flexibility, low operating cost and high recovery efficiency of waste heat.In order to improve the efficiency of the ORC system, this thesis presents a novel coupled ORC system, which couples a transcritical organic Rankine cycle with a subcritical organic Rankine cycle. On the basis of the basic law of thermodynamics and thermo-economic theory, the detailed performance analysis of the coupld ORC system is conducted. At the same time, the comparison of the coupled ORC system with the transcritical ORC is carried on. Finally, the effects of the critical temperature of working fluid and the working fluid type on the performance of the coupled ORC system are analyzed. The following conclusions can be drawn.① Thermodynamical performance and thermo-economic performance are all improved with the increase of the inlet temperature of the flue gas. With the increase of the outlet temperature of the flue gas, the net power output of the coupled ORC system reduces, while the thermal efficiency increases. Moreover, the exergy efficiency decreases slightly, then increases, and the electricity production cost and the payback period have the opposite variation trend. With the increase of the flue gas temperature at the evaporator outlet, thermodynamical performance of the coupled ORC system reduces, while the thermo-economic performance increases.② With the increase of the inlet pressure of the expander No.1, the net power output, thermal efficiency and the exergy efficiency of the coupled ORC system all increase, while the thermo-economic performance decreases, that means that the electricity production cost and the payback period all increase.③ With the increase of the pinch point temperature difference of the internal heat exchanger, the net power output, thermal efficiency and the exergy efficiency of the coupled ORC system all decrease, while the electricity production cost and the payback period decrease firstly and then increase slightly. There exists a critical value of the pinch point temperature difference, which makes the performance of the coupled ORC system higher than that of the transcritical ORC. The range of the critical pinch point temperature difference is 0~20℃. In addition, there has an optimal value of the pinch point temperature difference which corresponds to the optimal thermo-economic performance, and the range of the optimal pinch point temperature difference is 7~9℃.④ The coupled ORC system has a better performance when the critical temperature of the working fluid is higher than 150℃. In addition, the performance of the coupled ORC system with isentropic working fluid is better than that with dry working fluid. It is hinted that the isentropic working fluid has positive effect on improve the system performance. The coupled ORC system with R123/R141 b obtains the best thermodynamical performance, and that with R123/n-hexane has the best thermo-economic performance.⑤ For the coupled ORC system, the main sources of the exergy loss are the evaporator, internal heat exchanger and preheater, and then are the condenser and expanders. The exergy loss of the pump is the last. The exergy loss of each component varies with the variation of the operation conditions.