Performance Analysis Of Organic Rankine Cycle System With Mixture Working Fluids And Matching Between Working Fluids And Heat Sources

With the large consumption of fossil fuels, environmental pollution and energy shortages have become increasingly outstanding. In China, it has a large number of low-grade thermal energy, such as industrial waste heat, geothermal energy, solar energy,etc. Converting low-grade energy to power effectively is a good method to reduce environmental pollution. Organic Rankine Cycle(ORC) has a broad application prospects in low-grade heat into electricity and it has become an effective way to improve energy efficiency.Because zeotropic mixtures have physical properties of non-isothermal phase change relative to pure working fluids, it provides a better thermal match between working fluids and heat source or cold source. Then the system performance may be improved and irreversibility loss in heat exchangers can be reduced. Therefore, ORC with zeotropic mixtures has gradually received attention. In this paper, the ORC system with zeotropics and matching problem are selected as research objects. The thermodynamic properties of basic characteristics are analyzed and theoretical expressions of net output power is given in subcritical ORC systems with zeotropics.Parameters of the near-critical state continuity and transition from subcritical to supercritical state thermodynamic parameters are analyzed in the ORC systems with zeotropic mixtures. A new ORC system heat matching model is established, then the relationship of thermal efficiency and exergy efficiency is researched. “Area analysis” is used to analyze the new model. The main contents are as follows:(1) For subcritical ORC system with zeotropics, basic thermodynamic parameters with ratio of the components are researched. Different types of refrigerants: dry refrigerants, isentropic working fluids, wet refrigerants are selected. Those parameters:system thermal efficiency, exergy efficiency, the net output power, irreversible loss,expander size parameters(SP), refrigerant mass flow rate, etc. are analyzed. The results show that with the increasing ratio of the components, there is a maximum working fluid in a component of net output power. The net output power is larger when the evaporation temperature of the system is closer to the critical temperature ofthe working fluid. The system thermal efficiency vary widely with different working fluids with increasing ratio of the components. The thermal efficiency has a maximum value when using dry and wet refrigerant working fluids is mixed while the rest of the portfolio was gradually reduced. It also provides a selection way of working fluids. SP value is an important filter criteria when using different working fluids. When the quality of the working fluid is mixed with the wet work, the expander size is smaller,and other ways is bigger. In addition, from basic thermodynamic theory, the theoretical expression of net power output is given in subcritical ORC system with zeotropic mixtures. According to the basic thermodynamic principles, expression of the ratio between exergy efficiency and thermal efficiency is given, and the expression is related only with the heat source inlet temperature and temperature drop.(2) Near-critical state parameters are selected as objects for the study. Parameters in mixed refrigerants ORC system from subcritical to supercritical state are analyzed.The results show that all the parameters are not the same with the expander inlet pressure changes in contrast. When the transition from subcritical to supercritical state,the thermal efficiency of the system is continuously changing, while exergy efficiency,clean power, irreversible loss of other parameters are not continuous. In the near-critical state, the system thermodynamic parameters mutate. When the the net output power is selected as the optimization goal, near-critical state parameters will be more valuable. In addition, net output power of mixtures are greater than pure fluids.(3) A new matching model is established and matching problem between system and heat source or cold source are researched. “Area analysis” is used and irreversible losses are divided into three parts: matching effectively irreversible loss,preheating effectively irreversible loss and ineffective irreversible loss. Influences of heat source outlet temperature, evaporator and condenser pinch point temperature difference are respectively calculated. The results show that evaporator and condenser invalid irreversible loss of mixtures are less than pure refrigerants. Heat source outlet temperature has bigger effect than evaporator and condenser pinch point temperature difference. Among them, pinch point of the evaporator temperature difference has little influence to condenser irreversible losses, while pinch point of the condenser temperature difference has barely influence to evaportor irreversible losses.