Thermodynamic Optimization And Experimental Study On Organic Rankine Cycle For Low-and Medium-grade Geothermal Power Generation

Nowadays, the energy gap is becoming larger and larger, making thecontradiction between energy supply and need becoming more and more serious.Moreover, the coal-dominated energy structure results in serious environmentalpollution. The renewable energy can only rely to realize the coordinated developmentof economic, resource, and environment. Geothermal resources are abundant andwidely distributed in china, and they are not subject to the environmental temperatureand climatic change. Organic Rankine cycle (ORC) lowers geothermal watertemperature for geothermal power generation, increasing the available globalresources for geothermal power generation.This paper focuses on90-120℃geothermal water. In order to improve thethermal efficiency of low-and medium-temperature geothermal power plant, theparallel and series multi-evaporator regenerative organic Rankine cycle (PMRORC,SMRORC) were proposed. Energetic and exergetic analyses combined with theentropy and entransy anlyses were used to optimize the RORC, PMRORC, andSMRORC.25pure fluids and19zeotropic mixtures were screened. The experimentalperformances of the screw expander and ORC were obtained. The conclusions can besummarized as follows:(1) Exergetic and entropy analyses are from the point of the available work andsystematic irreversibility, respectively. However, the entransy analysis focuses on theheat transfer performance. Although the reaearch perspectives are different, the resultsare consistent. The entransy analysis is also an effective way in studying ORC.(2) For RORC, the entransy loss rate is proportional to the net power output, andthe turning point of the rate of change of the entropy generation rate corresponds tothat of the entransy dissipation rate. For PDRORC, the net power output isproportional to the entransy loss rate, the thermal efficiency, the exergetic efficiency,and it is inversely proportional to the entropy generation rate. For SDRORC, the netpower output is proportional to the entransy loss rate, the thermal efficiency, theexergetic efficiency, and it is corresponding to the minimum entropy generation ratein decreasing regions.(3) RORCs outperform ORCs, and the degree of improvement depends on the effectiveness of regenerator. The recommended value of the effectiveness ofregenerator is60%in engineering applications. SDRORCs precede RORCs, as themerit of SDRORCs is more obvious as the temperature difference between inlet andoutlet becomes larger and larger.(4) The binary zeotropic mixtures, such as R365mfc/R245fa, isopentane/R245fa,neopentane/R245fa, isohexane/R245fa and pentane/R245fa, show excellent cycleperformances. The ternary zeotropic mixtures, such as R245fa/pentane/isohexane andR245fa/isopentaneiso/isohexane, have higher net power output.(5) The scroll expander shows the highest isentropic efficiency under thespecified expansion ratio. The heat transfer coefficient is proportional to refrigerantcharge. The pump discharge pressure, the working fluid flowrate, the heating power ofthe electrically heated boiler, and the net power output are also proportional with theinlet temperature of the conducted oil. The optimal refrigerant charge is different withthe evaluation target. Using the thermal efficiency as the evaluation target, thedimensionless volume ratio (DVR) for R123、R245f, and R245/Isopentane are0.38,0.38, and0.41, respectively.(6) The direct heat transfer enhances the available heat source temperature, thusimproving the system performance. The series geothermal poly-generation system ispreferable for low geothermal water inlet temperature and high heat source inlettemperature for gathering and transportation heat tracing, whereas it is just reverse forthe parallel geothermal poly-generation system. Moreover, replacing oil boilers withgeothermal water can save8160tons of crude oil and simultaneously reduce a largenumber of pollutant emissions.(7) Absorption refrigeration system can be used as the auxiliary cold source ofORC in summer, or residential buildings for air-conditioning. Geothermalpoly-generation system can also satisfy258000m2residential buildings for heating inwinter and47300persons for domestic hot water supply in summer. The earning foroil accounts for over74%. The earning for the power generation is low, but it isindispensible, because geothermal power generation can relieve the contradictionbetween the energy consumption and production. The payback period of is less than4years, so the geothermal poly-generation system is worth being popularized.