The Themodynamic Research Of Partial Evaporation Organic Rankine Cycle With Zeotropic Mixture Working Fluids

In the context of the global shortage of fossil energy,the recycling of industrial waste heat and the large-scale development of renewable energy have become an important research flied concerned by governments and scientists.In recent years,the waste heat power generation technology based on the organic Rankine cycle has once again attracted everyone's attention.This technology can not only recover a large amount of industrial waste heat,engine exhaust waste heat and waste incineration heat,but also can be applied to the development of renewable energy,such as geothermal energy,biomass energy,solar energy and ocean temperature difference energy.However,due to the low temperature or low grade of these heat sources,the power generation efficiency and actual net output power of the organic Rankine cycle power generation technology have not been very high,which is also a bottleneck restricting the further development of the technology.How to increase the power generation of the organic Rankine cycle system as much as possible is an important focus of research in this field.Therefore,a new type ORC system with zeotropic mixture working fluid R227ea/R245 fa,partial evaporation and twin-screw total-flow expander is proposed in this paper.The advantages of this system are illuminated and the scientific problems related to this system are discussed.The specific research contents and conclusions are as follows:First of all,the influence of zeotropic working fluids and partial evaporation on the ORC system are studied thermodynamically.The thermodynamic model of each device in the organic Rankine cycle power generation system was constructed numerically.The first law of thermodynamics,the second law of thermodynamics and the thermal economic evaluation index were applied to analyze and discuss the performance of the system.It has been found that the zeotropic mixture working fluid and the partially evaporation cycle architecture can draw more heat from the heat source than the conventional pure working fluids superheated cycle to obtain a larger net output power.Under the working condition of heat source temperature 120oC and evaporation temperature 80oC,the net power output can be improved by 44.0% at most.The principles of zeotropic mixture working fluid and partial evaporation cycle to improve the system performance are clarified.The zeotropic mixture working fluids improves the temperature matching between the working fluid and the cold/heat source in the condenser/evaporator,and reduces the exergy loss in the condenser/evaporator.The partial evaporation cycle architecture mainly enhances the temperature matching between the working fluid and the heat source in the evaporator,and reduces the exergy loss in the evaporator.However,the traditional zero-dimentional thermodynamic model cannot reveal the influence of “composition shift” characteristic of zeotropic working fluids on the ORC system.Aiming at the "composition shift" characteristic,the discretized model of evaporator and condenser was established by using composition conservation and energy conservation.The calculation method of circulating composition was given,and the formation mechanism of "composition shift" phenomenon and its influence on the performance of the ORC system were analyzed.The study on the formation mechanism of “composition shift” founds that increasing pressure,reducing the length of the two-phase zone,reducing the difference between the gas-liquid two-phase flow rate and increasing the quality of the filling can reduce the degree of "composition shift";It also been found that the phenomenon of "composition shift" of zeotropic mixtures always causes the low-boiling component to account for more mass fraction of the circulating composition,which increases the evaporation pressure and condensation pressure of the system.This phenomenon will decrease the system net power output when the mass fraction of R227 ea is among 50%-70%;Composition shift also affects the heat transfer temperature difference in the heat exchanger.The average heat transfer temperature difference is slightly reduced in the evaporator,while the average heat transfer temperature difference is increased in the condenser.In addition,the gas-liquid two-phase expansion performance in the expander must be considered when partial evaporation is adopted.A simple geometric model of a twin-screw expander was constructed using the 3D modeling software pro/E.The data of the inlet area,the working chamber volume and the leakage channel area varying with the rotation angle of the anode rotor were measured.The working process of the screw expander working chamber was analyzed by mass conservation and energy conservation.The thermodynamic model of gas-liquid two-phase expansion in the expander was established based on the geometric data of the expander.The model was solved by Matlab and the performance of the expander was analyzed.The main conclusions are as follows: the lower the inlet dryness of the sream,the more severe the pressure loss when entering the steam,the higher the pressure at the end of the expansion,the higher the relative internal efficiency of the expander and the lower the delivery rate.Under full-expansion condition,the isentropic efficiency increases by 10 percent point when the inlet dryness of the stream decreases from 0.9 to 0.1.At last,the dynamic characteristics of a pratical ORC system under variable heat source and heat sink conditions need to be studied.Firstly,the governing equations in the evaporator and the condenser are deduced by the moving boundary method according to the mass conservation and energy conservation,and the main variables are determined.Then combined with the control equation of the working fluid pump and the expander as well as the coupling relationship between the components,the state equation of the whole system is established.Finally,the state equations of the system are solved and analyzed by Matlab and ode15 s solver.It is found that the 10oC increase of the heat source temperature will increase the net power output of 12.3% and thermal efficiency of 3.8%.The subcooling section proportion of the condenser will not change much,which can ensure the degree of subcooling.The 0.1kg/s increase of the heat source flow rate will lead to a 4.7% small increase of the evaporation pressure and a 12.7% large increase of the condensation pressure.Therefore,an increase in heat source flow rate may reduce the net output power of the system,which needs to be paid attention.The research content provides the important theoretical basis for the design,optimization and application of the partial evaporation organic Rankine cycle generation system with zeotropic fluids.It also provides some help for the further development of the new efficient organic Rankine cycle power generation system.