Research On Optimization And Applicability Of Compound Power Cycles For Enhanced Geothermal Systems

A system using geothermal energy from a hot dry rock resource(HDRR)for power generation is internationally known as enhanced geothermal system(EGS).There are abundant hot dry rock resorces in chnia,so it is very important to carry out a study on the most suitable power generation system matching with a given HDRR.In this study,detailed thermodynamic analysis and system optimization were performed for different power generation systems,with consideration of five commonly used working fluids in organic Rankine cycles(ORC).Three maps showing the application scopes for different compound power generation systems were then generated.Four power generation systems analyzed in this study are: single-flash(SF),double-flash(DF),flash-ORC(FORC),and double-flash-ORC(DFORC).Here,the SF system is used as the basic model.The net power output increase of using a compound power generation system is calculated by comparing its net power with that of the SF.Four thermodynamic models of SF,DF,FORC and DFORC are established,and R123,R152 a,isobutane,pentane and R245 fa are used as ORC working fluids respectively.The optimization processes of the following parameters are described and shown in detail: second stage flash temperature in DF system,ORC evaporation temperature in FORC system,and ORC evaporation temperature in DFORC system.In this study,the system net power output is taken as the objective function.The variation patterns of the optimized parameters were analyzed under different geofluid conditions.The maps showing the application scopes of DF,FORC and DFORC systems under different geofluid conditions are plotted based on a goal that 20% net power output increase can be reached.Under a given condition of geothermal fluid temperature and dryness,the optimization results of a compound power generation system show that using the net power output as an objective function is practical to effectively obtain the optimum parameters.Under different geofluid conditions(temperature = 80 ~ 260℃;dryness = 0 ~ 0.4),the optimization results showed that the optimum values of flash temperature and ORC evaporation temperature increase with an increasing geofluid temperature or dryness,which is more obvious when the geofluid temperature is higher.The application scope of each compound power generation system is a function of both geofluid temperature and dryness.It is shown that the net power output and its growth rate of each compound power generation system increase with an increasing geofluid temperature.The less value the geofluid dryness,the more advantage of using a compound power generation system.Techno-economic analysis shows that there is an obvious advantage of using the DF power generation system.The engineering economy of the FORC system is better than that of SF system,but is limited by the critical temperature of ORC’s working fluid,and the cost of electricity generation under the condition of a lower geofluid temperature.Compared with the SF system,the techno-economy of the DFORC system has no obvious advantage.However,since the DFORC system has a greater net power output,its techno-economy will become better if the life of the power plant is longer.