Thermal Performance Study Of Organic Flash Cycle With Ejector Driven By Geothermal Water

Low and medium temperature geothermal energy(<200℃)has the advantages of non-pollution,abundant reserves,not being affected by climate change.The development of power generation from geothermal energy can reduce fossil fuel consumption and the greenhouse gas emissions.At present,the organic flash cycle(OFC)is a suitable technology for the generation of low and medium temperature geothermal energy.However,the high irreversibility of the low-pressure throttle in the throttling process leads to the low efficiency of OFC.The ejector can effectively avoid the large throttling loss and improve the system efficiency.In this paper,the OFC with ejector system was studied,the thermal performance model of the OFC with ejector system driven by geothermal water was established,and the simulation analysis was carried out with MATLAB2019 a.Main contents and conclusions are included as follows:(1)The structure and working principle of traditional OFC system and OFC with ejector system were introduced.The key mechanism of the ejector was introduced and analyzed,the one-dimensional "constant pressure mixing" ejector model was established.On this basis,according to the first and second law of thermodynamics,the thermal performance model and exergy analysis model of the OFC with ejector system were established,and the variables of the system model and the calculation formula of the system performance parameters were determined.(2)R601 was selected as working fluid.The effects of the key cycle parameters,the mass flow of geothermal water and the efficiency of each section of the ejector on the performance of the OFC with ejector were studied.The results show that the net power output,thermal efficiency and exergy efficiency of system increase firstly and then decrease,and the total exergy destruction decreases firstly and then increase with increasing flash pressure.The performance of the system can be improved by increasing the endothermic temperature or reducing the condensation temperature.The change of geothermal water flow rate does not affect the thermal efficiency and exergy efficiency.The efficiency of each section of the ejector has little effect on system performance.(3)R601,R600,R600 a,R227ea,R236 fa,R236ea,R1234 ze and R1234 yf were selected as working fluids for geothermal water between 100 and 200℃.The flash pressure was optimized with net power output as the optimization objective,and the variation of the optimal flash pressure of different working fluid systems with geothermal water inlet temperature was analyzed.On this basis,the relationship between the thermophysical properties of different working fluids and the geothermal water inlet temperature,and the variations of the thermal performance of different working fluid systems with the geothermal water inlet temperature were investigated.The results show that there is a characteristic temperature whose value is the saturated liquid temperature corresponding to the upper limit of the endothermic pressure plus the pinch point temperature difference.When the geothermal water inlet temperature is higher than the characteristic temperature,the increment of the net power output decreases,the increment of the total exergy destruction increases,and the thermal efficiency and the optimal flash pressure remain unchanged.(4)The influence of flash pressure on the net power output and total exergy destruction of traditional OFC and OFC with ejector was compared using R601 as working fluid.Under the optimal flash pressure,the comparative analysis on performances of two systems was conducted.The results show that when the geothermal water inlet temperature is constant,the net power output of OFC with ejector system is larger and the total exergy destruction is smaller than that of traditional OFC system under the same flash pressure,and the gap increases with increasing flash pressure.In addition,the performance of the OFC with ejector is maximal for the geothermal water between 100 and 200℃,the relative increments of the net power output are 1.67-21.98% compared with the traditional OFC,and it decreases with increasing geothermal water inlet temperature.