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

Technical Paper

Abstract

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.

Keywords

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

List of symbols

e

Specific exergy (kJ/kg)

Ehf,max

Maximum available exergy from geothermal source (kW)

h

Specific enthalpy (kJ/kg)

GWP

Global Warming Potential

mhf

Mass flow rate of geothermal fluid (kg/s)

mwf

Mass flow rate of working fluid (kg/s)

ORC

Organic Rankine cycle

p

Pressure (kPa)

SP

Turbine size parameter (m)

T

Temperature (K)

V

Volumetric flow rate (m3/s)

VR

Volume ratio

WACC

Electrical power consumption by air cooling condenser (kW)

Wnet

Net electrical power output (kW)

Wp

Electrical power consumption by the working fluid pump (kW)

Wph

Electrical power consumption by geothermal fluid pump (kW)

Wt

Turbine power output (kW)

x1-2

Dry fraction of working fluid in turbine expansion process

ηhf

Isentropic efficiency of geothermal fluid pump

ηph-e

Product of mechanical efficiency and motor efficiency of geothermal fluid pump

ηp

Pump isentropic efficiency

ηp-e

Product of mechanical efficiency and motor efficiency of working fluid pump

ηt

Turbine isentropic efficiency

ηt-e

Product of turbine mechanical efficiency and generator efficiency

ηsym

System exergy efficiency

Δp

Increased pressure in geothermal fluid pump (kPa)

ΔT1,oh

Turbine inlet overheating degree (K)

ΔTpp,e

Pinch point temperature difference in evaporator (K)

ΔTap,c

Approach point temperature difference in condenser (K)

ΔTpp,c

Pinch point temperature difference in condenser (K)

Subscripts

1

Turbine inlet

2

Turbine outlet

2s

Turbine outlet with isentropic expansion

3

Saturated vapor state in condenser

4

Pump inlet

5

Pump outlet

5s

Pump outlet with isentropic expansion

6

Saturated liquid state in evaporator

7

Saturated vapor state in evaporator

8

Geothermal brine injection

9

Geothermal brine reinjection

10

Cooling air inlet

11

Cooling air outlet

cond

Condensing state

cr

Critical point

max

Upper limit

min

Lower limit

sta

Saturated state

Notes

Acknowledgements

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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