Performance optimization of low-temperature geothermal organic Rankine cycles using axial turbine isentropic efficiency correlation

Technical Paper


Present study deals with parametric optimization and performance evaluation of an air-cooled organic Rankine system for the low-temperature geothermal source, especially considering the effects of turbine isentropic efficiency. Turbine isentropic efficiency is predicted with turbine size parameter and volume ratio, using the well-known correlation for single-stage axial turbine. Optimal performances with the objective of maximizing system exergy efficiency are compared with the common used working fluid R245fa and two environmental friendly working fluids R1234ze(E) and R1234ze(Z). Highest turbine isentropic efficiency is achieved for working fluid R1234ze(Z). The optimal turbine inlet vapor is overheating for Working fluid R1234ze(E) with the limitation of allowable minimum geothermal brine reinjection temperature. Due to the influence of turbine isentropic efficiency, optimal system exergy efficiency for working fluid R1234ze(E) is 0.4576 for the 100 kg/s geothermal source, which is slightly higher than the value of 0.4487 for the 10 kg/s geothermal source.


Organic Rankine cycle Performance optimization Axial turbine isentropic efficiency System exergy efficiency 

List of symbols


Specific exergy (kJ/kg)


Maximum available exergy from geothermal source (kW)


Specific enthalpy (kJ/kg)


Global Warming Potential


Mass flow rate of geothermal fluid (kg/s)


Mass flow rate of working fluid (kg/s)


Organic Rankine cycle


Pressure (kPa)


Turbine size parameter (m)


Temperature (K)


Volumetric flow rate (m3/s)


Volume ratio


Electrical power consumption by air cooling condenser (kW)


Net electrical power output (kW)


Electrical power consumption by the working fluid pump (kW)


Electrical power consumption by geothermal fluid pump (kW)


Turbine power output (kW)


Dry fraction of working fluid in turbine expansion process


Isentropic efficiency of geothermal fluid pump


Product of mechanical efficiency and motor efficiency of geothermal fluid pump


Pump isentropic efficiency


Product of mechanical efficiency and motor efficiency of working fluid pump


Turbine isentropic efficiency


Product of turbine mechanical efficiency and generator efficiency


System exergy efficiency


Increased pressure in geothermal fluid pump (kPa)


Turbine inlet overheating degree (K)


Pinch point temperature difference in evaporator (K)


Approach point temperature difference in condenser (K)


Pinch point temperature difference in condenser (K)



Turbine inlet


Turbine outlet


Turbine outlet with isentropic expansion


Saturated vapor state in condenser


Pump inlet


Pump outlet


Pump outlet with isentropic expansion


Saturated liquid state in evaporator


Saturated vapor state in evaporator


Geothermal brine injection


Geothermal brine reinjection


Cooling air inlet


Cooling air outlet


Condensing state


Critical point


Upper limit


Lower limit


Saturated state



The authors would like to acknowledge the financial support from Tianjin Science & Technology Pillar Program (Grant No. 16YFZCGX00090).


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Copyright information

© The Brazilian Society of Mechanical Sciences and Engineering 2018

Authors and Affiliations

  1. 1.Tianjin Key Laboratory for Advanced Mechatronic System Design and Intelligent ControlTianjinChina
  2. 2.National Demonstration Center for Experimental Mechanical and Electrical Engineering Education (Tianjin University of Technology)TianjinChina
  3. 3.Institute of Engineering ThermophysicsChinese Academy of SciencesBeijingChina
  4. 4.School of Electrical and Electronic EngineeringUniversity of Tianjin TechnologyTianjinChina

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