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

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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.

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Abbreviations

e :

Specific exergy (kJ/kg)

E hf,max :

Maximum available exergy from geothermal source (kW)

h :

Specific enthalpy (kJ/kg)

GWP:

Global Warming Potential

m hf :

Mass flow rate of geothermal fluid (kg/s)

m wf :

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

W ACC :

Electrical power consumption by air cooling condenser (kW)

W net :

Net electrical power output (kW)

W p :

Electrical power consumption by the working fluid pump (kW)

W ph :

Electrical power consumption by geothermal fluid pump (kW)

W t :

Turbine power output (kW)

x 1-2 :

Dry fraction of working fluid in turbine expansion process

η hf :

Isentropic efficiency of geothermal fluid pump

η ph-e :

Product of mechanical efficiency and motor efficiency of geothermal fluid pump

η p :

Pump isentropic efficiency

η p-e :

Product of mechanical efficiency and motor efficiency of working fluid pump

η t :

Turbine isentropic efficiency

η t-e :

Product of turbine mechanical efficiency and generator efficiency

η sym :

System exergy efficiency

Δp :

Increased pressure in geothermal fluid pump (kPa)

ΔT 1,oh :

Turbine inlet overheating degree (K)

ΔT pp,e :

Pinch point temperature difference in evaporator (K)

ΔT ap,c :

Approach point temperature difference in condenser (K)

ΔT pp,c :

Pinch point temperature difference in condenser (K)

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

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Acknowledgements

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

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Authors and Affiliations

  1. Tianjin Key Laboratory for Advanced Mechatronic System Design and Intelligent Control, Tianjin, China

    Chao Zhang & Wencheng Fu

  2. National Demonstration Center for Experimental Mechanical and Electrical Engineering Education (Tianjin University of Technology), Tianjin, China

    Chao Zhang & Wencheng Fu

  3. Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing, China

    Jinglun Fu

  4. School of Electrical and Electronic Engineering, University of Tianjin Technology, Tianjin, China

    Jun Kang

Authors
  1. Chao Zhang
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  2. Jinglun Fu
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  3. Jun Kang
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  4. Wencheng Fu
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Correspondence to Chao Zhang.

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Technical Editor: Jose A. dos Reis Parise.

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Zhang, C., Fu, J., Kang, J. et al. Performance optimization of low-temperature geothermal organic Rankine cycles using axial turbine isentropic efficiency correlation. J Braz. Soc. Mech. Sci. Eng. 40, 61 (2018). https://doi.org/10.1007/s40430-018-0996-9

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  • DOI: https://doi.org/10.1007/s40430-018-0996-9

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