Study On Matching Characteristic Between The Organic Fluids And Geothermal Fluid In Organic Rankine Cycle

Geothermal energy is abundant and widespread in China and development of geothermal power generation is an efficient approach to adjust energy consumption structure and reduce pollutant emission.But over 70% of the geothermal source is medium-low enthalpy geothermal water at temperatures lower than 150°C.Organic Rankine cycle(ORC)is one of the main technologies for medium-low enthalpy geothermal power generation.The thermal efficiencies of the geothermal ORCs are still relatively lower,and improving matching characteristic between the working fluid and the geothermal water is an important approach to improve the thermodynamic performance of geothermal ORCs.This study optimizes the parameters for both of single-pressure and dual-pressure ORCs with 100-150°C geothermal water.Working fluid is selected by analyzing the temperature matching characteristic between the working fluid and the geothermal water.The thermodynamic performance of single-pressure ORC under off-design is also studied.The effects of reinjection temperature on matching characteristic between the working fluid and the geothermal water for single-pressure ORC are investigated,and the exergetic destruction distribution laws of the evaporation process and thermodynamic cycle are revealed.The evaporation process exergy efficiency and the net power output of ORC are maximal using isobutane(R600a)compared to other studied working fluids.The effects of production well depth on net power output,power consumed by production/reinjection pump and economic performance of ORCs are analyzed.The area per unit power output and the levelized cost of electricity significantly decrease with increasing production well depth.The effects of evaporator pinch point temperature difference(PPTD)on the matching characteristic between the working fluid and the geothermal water are analyzed,and the PPTD is selected based on the optimal thermo-economic performance.The evaporation temperatures of dual-pressure ORCs are optimized by maximizing the net power output,and the effects of the permissible reinjection temperature on matching characteristic between the working fluid and the geothermal water are studied to analyze the geothermal exergy utilization efficiency,the evaporation process exergy efficiency and the cycle exergy efficiency.Themodynamic performance is compared between the single-pressure and the dual-pressure ORCs,and the law of power improvement by dual-pressure evaporation is revealed.The effects of internal heat recovery on dual-pressure ORC thermodynamic performance are studied.Internal heat recovery improves the matching characteristic between the working fluid and the geothermal water,so the plant exergy efficiencies relatively increase by 1.2-3.7%.The temperatures of geothermal water and cooling water are subject to change for different environment temperatures and the single-pressure ORC operation characteristics under off-design conditions are studied.