| The earth has abundant reserves of low-medium temperature heat energy.If these low-medium temperature heat energy below 200℃are used,the consumption of fossil fuels can be reduced and the energy crisis can be alleviated.However,due to the low temperature of medium and low temperature heat energy,the conventional power cycle can not make effective use of it.The organic flash cycle（OFC）is a power generation technology suitable for medium and low temperature thermal energy.However,the traditional OFC system will produce huge losses.Therefore,this paper studies the double organic flash cycle（double flash OFC）system.The thermal performance and economy model of the double flash OFC system is constructed,the influence of different working fluids on the thermal performance and economy of the double flash OFC under variable heat source is analyzed,and the maximum net power output of traditional OFC system and double flash OFC is compared.At the same time,because the zeotropic working fluid has the characteristics of variable temperature phase transition compared with the pure working fluid,which can improve the temperature matching between the system and the cold source,the zeotropic working fluid is introduced into the double flash OFC to analyze the impact of the change of working fluid composition on the thermal performance and economy of the double flash OFC under different heat source inlet temperatures.The thermal performance and economy of the double flash OFC with zeotropic working fluid and pure working fluid are compared.The main conclusions are as follows:（1）The double flash OFC system model was built,and the thermal performance of the system was studied with the goal of maximum net power output.R601 system and R601a system collectively produce more net output power than other systems.Compared with conventional OFC system,the net power output of the double flash OFC system is increased.Among them,the maximum relative increment is working fluid R600a,and its value is 106.28%.The minimum relative increment is working medium R601,and its value is 14.03%.（2）In terms of thermal performance,the thermal efficiency and exergic efficiency of double flash OFC system with pure working fluid was studied,and the thermal efficiency and exergic efficiency of R601system and R601a system were higher than those of other working fluid systems as a whole.The maximum thermal efficiency of R601 system and R601a system are 11.88%and 11.02%,respectively,and the maximum exergic efficiency is 43.06%and 41.13%,respectively.The thermal efficiency and exergic efficiency of R227ea system are the lowest overall,with the maximum values of 2.92%and 14.35%,respectively.（3）The economic model of double flash OFC system was established,and the heat exchange area and power generation cost of the system were analyzed.The heat transfer area of heat exchanger of R601system is up to 752.84%compared with that of R227ea system,and the heat transfer area of condenser of R601 system is up to 143.43%compared with that of R227ea system.The total cost of R123 system is the largest,which is 54.51k$.R601 and R601a had the lowest electricity generation costs（EGC）of0.07$·k W-1·h-1and 0.07$·k W-1·h-1,respectively.（4）Combining zeotropic working fluid with the double flash OFC system,The effect of heat source temperature and molar composition on net power output was studied.When the heat source inlet temperature is less than 190℃,the net power output of R600a/R601a mixture system is greater than that of R601a system.Among them,the net power output of R600a/R601a（0.1/0.9）system is the best,and the relative increase rate is up to 7.90%compared with R601a system,but the thermal efficiency of R600a/R601a mixture system is not necessarily better than that of R601a system.（5）For exergy of the system,the effects of heat source temperature and molar composition on system exergy efficiency and exergyloss were studied.when the heat source inlet temperature was 180℃and the ratio of R600a mole was 0.1,the exergic efficiency of the system was the maximum,which was 41.08%.When the heat source inlet temperature was greater than 190℃,the exergic efficiency of R601a system was the highest,41.13%.When the heat source inlet temperature was less than 190℃,the exergic loss of the R600a/R601a mixture system was less than that of the R601a system.When the heat source inlet temperature was greater than 190℃,exergic loss of R600a/R601a mixture system was greater than that of R601a system.（6）For the economy of the double flash OFC system,the total cost and EGC of zeotropic working fluid system and pure working fluid system were compared.The heat exchange area of R600a/R601a mixture system is larger than that of R600a system and R601a system.When the heat source inlet temperature is less than 150℃or 200℃,the total cost of R600a/R601a（0.9/0.1）system is the largest,and when the heat source inlet temperature is 150-190℃,the total cost of R600a/R601a（0.1/0.9）system is the largest.The EGC of R600a/R601a（0.1/0.9）system was lower than that of R600a system,and the maximum relative reduction was 40.18%and the minimum was 10.71%. |
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