Performance Study Of Organic Flash Cycle System Based On Liquid-separation Condensation

Medium and low temperature geothermal energy（< 200℃）has many advantages,such as clean and environmental protection,abundant reserves,not affected by climate and so on.Efficient development of medium and low temperature geothermal power generation can reduce fossil fuel consumption and greenhouse gas emissions.Current research shows that organic flash cycle（Organic Flash Cycle,OFC）is one of the technologies to effectively utilize medium and low temperature geothermal energy for power generation.The organic flash cycle（OFC）system has many advantages,but the high investment cost has always been a problem faced by many scholars,among which the condenser has an important impact on the economic performance of the OFC system.Liquid-separation condensation technology can significantly improve the condensation heat transfer coefficient.In this paper,liquid-separation condensation is applied to organic flash cycle,and the organic flash cycle system using liquid-separation condensation technology（Liquid-separation Condensation Organic Flash Cycle,liquid-separation condensation OFC）is studied.The optimization model of organic flash cycle system based on liquid-separation condensation is established,and the thermal performance of the system is optimized by multi-parameters.R600,R601,R245 fa,R134a,R236 ea,R227ea,R1234 ze and R1234 yf are used as cycle refrigerants.The influence of different system parameters on the heat transfer area of the condenser under the optimal working condition is studied.The simulation analysis is carried out by using MATLAB 2019 a.The main contents and conclusions of this paper are as follows:（1）The calculation model of（OFC）system using liquid-separation condensation organic flash cycle is constructed.Firstly,the working principle of liquid-separation condensing OFC system is introduced,and then according to the operation rules of constant output mode under the law of thermodynamics,the numerical model of fixed output liquid-separation condensing organic flash cycle system is established,and then the numerical thermal performance calculation model of OFC subsystem of liquid-separation condenser is established,including evaporator,turbine,working medium pump and liquid-separation condenser and other main components calculation model.（2）Under the condition of R600,R601,R245 fa,R134a,R236 ea,R227ea,R1234 ze and R1234 yf,the effects of heat source temperature and flash pressure on the net output work,efficiency,heat transfer area of heating section,heat transfer area of condenser,heat absorption,pressure drop,mass flow and steam dryness of the system were studied,and the related mechanisms were discussed.The results show that with the increase of the inlet temperature of the heat source,the maximum net output power of the eight refrigerant systems increases gradually,and the maximum net output power is 47.69 k W,77.47 k W,51.32 k W,14.34 k W,43.51 k W,20.00 k W,21.28 k W and 11.17 k W,respectively.The maximum efficiency of the system will first increase and then decrease,and the maximum efficiency is 30.2%,42.8%,32.5%,11.2%,28.7% and15.7%,respectively.15.9%,9.1%.With the increase of flash pressure,the net output work of the eight refrigerant systems increases at first and then decreases,and the maximum net output work is 30.87 k W,31.23 k W,31.87 k W,9.69 k W,29.42 k W,13.53 k W,14.38 k W and 7.55 k W,respectively.The maximum efficiency of the system also increases at first and then decreases,and the maximum efficiency is 30.07%,30.41%,31.04%,9.43%,28.65%,13.17%,14.00% and 7.35%,respectively.（3）Taking the net output power of the system as the objective function,the flash pressure is optimized,and the variation of the optimal flash pressure with the inlet temperature of the heat source is discussed.There is always an optimal flash pressure at the same heat source temperature to maximize the net output power of the system.Under the optimal working conditions,the inlet dryness of the condenser increases with the increase of the inlet temperature of the heat source,but it will remain unchanged after reaching the critical temperature.It is found that the higher the thermal conductivity of the working fluid is,the better the heat transfer performance is,and the smaller the heat transfer area of the condenser is.The heat transfer area of the condenser increases with the increase of the inlet temperature of the heat source.And the heat transfer area of the condenser decreases at first and then increases with the increase of liquid-separation dryness,that is,there is an optimal liquid-separation dryness at different heat source inlet temperature to minimize the heat transfer area of the condenser.There is a maximum relative reduction between the heat transfer area of the condenser with the optimal liquid dryness of R600,R601,R245 fa,R134a,R236 ea,R227ea,R1234 ze and R1234yf8 and that of the traditional non-liquid condenser.In the range of 100 ～ 200 °C,the maximum relative reduction of the condenser heat transfer area is 9.64 mm2,13.73 mm2,18.03 mm2,27.80 mm2,20.04 mm2,28.17 mm2,25.74mm2,29.92 mm2,respectively.（4）For the liquid-separation condensing organic flash cycle system studied in this paper,when the working fluid is different,the variation trend of most of the loss rate with the inlet temperature of the heat source is basically the same.The exhaust gas loss rate at the outlet of the heat sink and the condenser loss rate decrease with the increase of the inlet temperature of the heat source,while the emission loss rate of the evaporator first decreases,and then increases with the increase of the heat source inlet temperature.The exhaust loss rate of turbine and working fluid pump increases at first and then decreases with the increase of heat source inlet temperature.In addition,when the inlet temperature of the heat source is 100 ℃ to 200 ℃,the working medium with the lowest total loss rate of the system is R601,with a maximum of 81.61% and a minimum of 55.35%.