Study On The Performance Of Organic Flash Cycle Using Zeotropic Working Fluid Driven By Low-and Medium-temperature Thermal Energy

The low-and medium-temperature thermal energy（solar energy,geothermal energy,industrial waste heat）below 200℃ widely and abundantly exists in nature.The efficient utilization of this thermal energy is of great practical significance to reduce environmental pollution,carbon emissions and alleviate energy crisis.However,its exploitation and utilization are difficult because the conventional power cycle systems are not effective enough.Focusing on the heat-power conversion of geothermal energy at 100-200℃,this paper takes the organic flash cycle as the realization object.The zeotropic working fluid can effectively improve the temperature match between the cycle and the cold source,the organic flash cycle（OFC）were combined with the zeotropic mixtures in this paper.The thermodynamic performance and economic model of the organic flash cycle system using zeotropic working fluid was constructed.The effects of variable heat-source temperatures and zeotropic working fluid mole fraction on the thermodynamic performance and economic performance of the system were studied;the change of flash pressure on the net power output and thermal efficiency of the system was reveal.In addition,the heat-power conversion efficiency of the organic flash cycle system is limited due to the exergy loss during the heat transfer processes of the system.Therefore,the organic flash-Rankine cycle（OFRC）system using zeotropic working fluid was studied.The effect of the zeotropic working fluid mole fraction on the thermodynamic performance and economic performance of the system was analyzed under the conditions of stable heat-source.The main conclusions are as follows:（1）In the present paper,the heat-power conversion superiority of organic flash cycle system was confirmed for the low-and medium-temperature heat energy.When R600a/R601a mixture is introduced into the organic flash cycle system,the heat-source inlet temperature is less than or equal to 180℃,the thermodynamic performance of the mixture system is well compared with the pure working fluid system,and the relative increments in the net power outputs of the mixture system are 24.89%and 51.98%compared with the pure R600a system and R601a system,respectively.The thermal efficiency of the system is not necessarily better than that of the R601a system.（2）For the thermodynamic perfection of the R600a/R601a mixture OFC system,when the heat-source inlet temperature is greater than or equal to 180℃,the exergy efficiency and external exergy efficiency of the R600a/R601a mixture system are greater than that of the pure working fluid system;When the heat-source inlet temperature is greater than 180℃,the exergy efficiency and the external exergy efficiency of the R601a system are maximal,the exergy efficiency and external exergy efficiency of system can reach to 35.97%and 91.91%;When the heat-source inlet temperature is 110-180℃,the internal exergy efficiency of the R600a/R601a mixture system is greater than that of the pure working fluid system;when the heat-source inlet temperature is 100℃,190℃ and 200℃,the internal exergy efficiency of the R601a system is maximal,which can reach to 40%.（3）For the economic performance of OFC system,when the heat-source inlet temperature is 100-200℃,the electricity generation costs（EGC）of R600a/R601a mixture system is smaller than that of the R600a system,and the maximum relative decrement can reach to 37.84%;When the mixture working fluid mole fraction is constant,the EGC of system decreases with increasing heat-source inlet temperature.For the total cost of the system,when the heat-source inlet temperature is 180℃,the relative decrements of the R600a/R601a（0.9/0.1）system compared with that of the pure R601a systems reach maximum（24.83%）.（4）The zeotropic working fluid can effectively improve the heat-power conversion efficiency of the OFRC system for the low-and medium-temperature heat sources.The relative increment in the net power output of the R600/R245fa mixture OFRC system compared with that of R600 system and R245fa system are 10.27%and 5.57%,and the thermal efficiency can be increased by 3.12%and 1.1%,respectively.The thermodynamic performance of the OFRC system is better than that of the OFC system.For the pure working fluid R600,the net power output and thermal efficiency of the OFRC system are increased by 5.50%and 57.48%compared with the OFC system,respectively.（5）The structural optimization of the OFC system can effectively reduce the total cost and the EGC.The maximum relative decrement of the total cost and EGC of the OFRC system can reach to 10.08%and 14.25%,respectively.In addition,compared with the pure working fluid,the zeotropic working fluid was introduced into OFRC system,and it was significant to improve the economic performance.The EGC of the OFRC system using R600/R245fa（0.3/0.7）mixture can decrease by 3.35%and 0.39%compared with the R600 system and the R245fa system.