Multi-objective Optimization Of Organic Rankine Cycle Based On Variable Turbine Efficiency

With the increasing depletion of fossil energy,the recycling of low-grade energy by ORC system has become a hot spot in current research.Through working fluid screening and parameter optimization,the overall performance of the ORC system can be improved effectively.Comparing with single objective optimization,multi-objective optimization could make up the limitation.This paper focuses on multi-objective optimization problems of ORC system and two problems are researched,which are the variation of turbine efficiency in the turbine and pressure drop in the evaporator.Two new criteria are proposed,which are related to the processes above.Models are established to predict and optimize the ORC system.Multi-objective gray wolf algorithm are introduce to conduct optimization and B point are verified by algorithm to be the optimal point in the closed region.Firstly,multi-optimization model is combined with turbine model from thermoeconomic aspect.Pentane,hexane,heptane,cyclohexane,benzene and toluene were selected as candidate working fluids and the variation of turbine efficiency is analyzed with respect to evaporation temperature and condensation temperature.The relationship between the turbine efficiency and volumetric flow ratio(VFR)are analyzed and.The results of multi-objective optimization with variable turbine efficiency and that with constant turbine efficiency were compared and the sensitivity analysis of the optimal working fluid is conducted.The results showed that in the multi-optimization with constant turbine efficiency,toluene and cyclohexane are the optimal working fluids,whereas with variable turbine efficiency,benzene is the optimal working fluid.In the sensitivity analysis,the optimal exergy efficiency shows opposite trends for the multi-objective optimization with constant and variable turbine efficiency.Secondly,multi-optimization model is combined with turbine model from exergoeconomic aspect.R245 fa,R123,R114,R245 ca,pentane were chosen as candidate working fluids.According to the idea of variables-control,only the influence of evaporation temperature on dynamic turbine efficiency during the multi-objective optimization process is studied.The working fluid(R245fa)with the most severe trending and that(R123)with the most gentle trending are taken as examples The results of multi-objective optimization with variable turbine efficiency and that with constant turbine efficiency were compared and the sensitivity analysis of the optimal working fluid is conducted.The results showed that in given conditions,pentane is the optimal working fluid.The dynamic turbine efficiency affects the distribution of decision variables in the Pareto frontiers for R245 fa,while the effect for R123 can be ignored which only leads to a little deviation of optimal results.Finally,based on the non-constrained system design method,the plate length,the plate width and the plate space were selected for multi-objective optimization design with given evaporation temperature and condensation temperature.The plate heat exchanger is selected as the evaporator and the evaporator area and the evaporator pressure drop were selected as performance indicators.The results show that with the plate length increasing,the total pressure drop of the evaporator increases and the area of the evaporator decreases.With the plate width and the plate spacing increasing,the total pressure drop of the evaporator decreases and the area of the evaporator increases.