Simultaneous Performance Optimization Of Organic Rankine Cycle(ORC) System For Recovering Low Temperature Waste-heat From Flue Gas

Energy shortage and environmental pollution are two major problems faced by the world today,and the Organic Rankine Cycle(ORC)can effectively utilize the low-grade waste heat resources,which is an effective way to improve the utilization efficiency of energy and reduce environmental pollution.However,how to get the maximum return with the least investment cost is the most concerned issue for investors.Electricity Production Cost(EPC)is one of the major economic evaluation indicators that concerned both the investment cost and the net power output of ORC system.The evaporation temperature,condensation temperature,the pinch point temperature differences(PPTDs)of evaporator and condenser have a significant influence on the economic performance of ORC system,so they are often used as the optimization parameters of the ORC system.The parameter optimization results are different while the external operation conditions(e.g.the heat source inlet temperature,the mass flow rate of heat source,the cooling source inlet temperature,the mass flow rate of the cooling source,the heat transfer coefficients of evaporator and condenser)are different.In order to improve the economic performance of the ORC system,the EPC was chosen as the optimization objective.The multiple parameters were optimized simultaneously for ORC that using different refrigerants as working fluid when the external operation parameters are variable.The multiple parameters include evaporation temperature,condensation temperature,the PPTDs of evaporator and condenser.Through the analysis of the optimization results,we can get the following conclusions:(1)For the selected 19 working fluids,the optimal evaporation and condensation temperatures increase with the increase of the inlet temperature of the heat source and cooling source,the mass flow rate of the heat source and the heat transfer coefficient of the evaporator.Moreover,it decreases with the increase of the mass flow rate of the cooling source and the heat transfer coefficient of the condenser.The optimal PPTD of evaporator increases with the increase of the inlet temperature and the mass flow rate of the heat source,and the heat transfer coefficient of the evaporator.Moreover,it decreases with the increase of the inlet temperature and mass flow rate of the cooling source,and the heat transfer coefficient of the condenser.The optimal PPTD of condenser increases with the increase of the inlet temperature of heat source,the mass flow rate of the cooling source and the heat transfer coefficient of the evaporator.Moreover,it decreases with the inlet temperature of the cooling source,the mass flow rate of the heat source and the heat transfer coefficient of the condenser.The minimum EPC decreases with the increase of the mass flow rate of the heat source and cooling source,the heat transfer coefficients of heat exchangers and the inlet temperature of the heat source.Moreover,it increases with the increase of the inlet temperature of the cooling source.(2)Variable condition analysis was conducted near the optimal rated point.The results showed that with the increment of the critical temperature of the refrigerant,the minimum EPC and its corresponding net power output,the optimal evaporation pressure and the optimal PPTD of evaporator decrease,while the optimal expansion ratio,the optimal PPTD of condenser and the optimal outlet temperature of the heat source increase.The optimal condensation temperature is invariable with the change of the critical temperature of the refrigerant.The permit maximum inlet temperature of the heat source is limited by the PPTD of evaporator,evaporation pressure and expansion ratio.(3)When the rated operating point is determined,EPC decreases with the increase of the inlet temperature and mass flow rate of the heat source.When the inlet temperature of the cooling source is greater than the design value or the mass flow rate of the cooling source is less than the design value,EPC increases with the increase of the inlet temperature of the cooling source or the decrease of the mass flow rate of the cooling source.Contrarily,EPC is almost constant.The order of the external operating parameters’ impact on EPC is: the inlet temperature of the cooling source’s > the inlet temperature of the heat source’s > the mass flow rate of heat source’s > the mass flow rate of the cooling source’s.Considering economy and security,the selection of the design value of the inlet temperature of the heat source should be according to the heat source annual average inlet temperature.Then the working fluid with better economic performance is selected from the refrigerants whose critical temperatures are close to the design value of the heat source inlet temperature.(4)By fitting the simultaneous optimization results,the fitting formulas of the optimal evaporating temperature,condensation temperature,the optimal PPTDs of evaporator and condenser were obtained.Their scope of application was given in this thesis.The fitting formula can be used for engineering design.