| With the increasing demand of traditional energy and electricity,to find renewable energy,which alternatives to coal,petroleum or nature gas,and effective ways to improve the efficiency of energy utilization have been the global urgent affair.In recent years,Organic Rankine Cycle has attracted wide attention of researchers in related fields,because of its unique advantage in low-grade heat recovery.It can recover not only industrial waste heat,but also biomass energy,solar energy,geothermal energy,waste incineration energy,engine exhaust heat,even ocean temperature difference energy with lower temperature.However,low efficiency in the conversion of low-grade heat into electric and mediocre output power are the bottlenecks of this technology,which is limited by the low energy level of low-grade heat.Previous researches mostly focused on the effect of single parameter on system output characteristic or the transverse work capacity comparison among organic working fluids.To bridge this research gap,this thesis puts forward a comprehensive model of ORC power generation system.Helmholtz Free Energy Equation is ultilized to solve the organic working fluid properties.We conducts numerical simulation to study the affecting factors on output performance of low-grade heat ORC generation system respectively from three angles-thermal parameters,key equipments and cycle modes.The details are summarized as follows:1.Influence of cycle thermodynamic parameters on the performance of ORC system.Modeling of ORC devices is carried out.Net power output,thermal efficiency,exergy efficiency and UPR value are chosen as evaluation parameters from the angle of energy,exergy and finite dimension thermodynamics.Through the simulation and analysis for model of ORC,effects made by evaporating temperature,degree of superheat,condensing temperature,degree of supercooling,heat source temperature,heat flow and temperature of cold source on low-grade heat ORC generation system performance are found out.During that process,the available range of organic working fluid evaporating temperature is induced.Under the rated conditions,R245 fa does do more work among the 13 working fluids.Through the further study that evaporating temperature changing with the heat source temperature and the maximized exergy efficiency as the output performance evaluation standard,the appropriate heat source temperature for each of the 13 working fluids is given,which will guides the system design and the working fluid application.Study on the organic mixture indicated that the matched-degree of heat transfer temperature difference in condenser and thermodynamic property of the system are the best when the inlet and outlet temperature difference of cold source approaches the temperature glide of the mixture in condenser.Furthermore,the net power output and thermal efficiency of mixture are higher than those of pure working fluids.2.Influence of key devices on the performance of ORC system.Targeted studies of three key devoices—evaporator or condenser,screw expander and internal heat exchange(IHE)are carried out.The effects of the pinch point temperature difference in evaporator or condenser,the builtin and actual expansion ratio difference of expander,the internal heat exchange and acid dew point of heat source on system performance are studied.The uncertainty of pinch point location in heat exchanger and possible errors are explained.A method of finding the pinch point location applicable to various cycle modes has been given out.The correction factor of built-in and actual expansion ratio difference is deduced,which reveals that slight over-expansion is acceptable,while under-expansion should be fought off and expander with a smaller built-in expansion ratio than the actual one should be used to yield better net power output.Although IHE cannot improve the net power output and exergy efficiency of the system,it can improve thermal efficiency.When the acid dew point of heat source is considered,the adjustable range of working condition can be expanded by IHE,which means that the net power output and exergy efficiency of the system are improved.3.Influence of cycle modes on the performance of ORC system.Three low-grade heat power generation cycle modes,transcritical ORC,trilateral cycle(TLC)and organic flash cycle(OFC)are realized based on the basic(subcritical)ORC.Furthermore,OFC has been improved and OFC-M cycle put forwarded.Meanwhile,the main factors effecting system output performance in each cycle are studied.In order to demonstrate the advantages of each cycle more explicitly,three heat source temperature at 150oC,200oC and 250oC are considered in this thesis and the above four cycles are compared with subcritical ORC.The results can be summarized into three conclusions.In the first place,under three kinds of given conditions,OFC-M and triangle loop TLC keep better output performance.In the second place,OFC is not the reasonable cycle mode.Transcritical ORC shows no obvious advantages over subcritical ORC in this example,for the uncertainty of evaporating pressure and fluid critical pressure.The system output performance can be improved by wider screening of working fluids.In this thesis,relatively complete studies on the affecting factors of ORC output performance are carried out from various angles.The results provide important theoretical basis for the design,optimization and application of ORC.Meanwhile,they are helpful for the further development of the cycle models. |
