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Publication Title | THERMODYNAMIC ANALYSIS AND PERFORMANCE OPTIMIZATION OF ORGANIC RANKINE CYCLES FOR THE CONVERSION OF LOW-TO-MODERATE GRADE GEOTHERMAL HEAT

Organic Rankine Cycle

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THERMODYNAMIC ANALYSIS AND PERFORMANCE OPTIMIZATION OF ORGANIC RANKINE CYCLES FOR THE CONVERSION OF LOW-TO-MODERATE GRADE GEOTHERMAL HEAT

Yekoladio, P.J., Bello-Ochende, T. and Meyer J.P.

Department of Mechanical and Aeronautical Engineering, University of Pretoria, Pretoria Private Bag X20, Hatfield 0028, South Africa.

Abstract

The present study considers a thermodynamic analysis and performance optimization of small binary-cycle geothermal power plants operating with moderately low-temperature and liquid-dominated geothermal resources in the range of 110oC to 160oC. The paper consists of an analytical and numerical thermodynamic optimization of selected Organic Rankine Cycles (ORC) to maximize the cycle power output. The optimization process and Entropy Generation Minimization (EGM) analysis were performed to minimize the exergy loss of the power plant. Optimal operating conditions were determined for maximum cycle power output per unit mass flow rate of the geothermal fluid. The maximum cycle power output was observed to increase exponentially with the geothermal resource temperature, whereas the optimal turbine inlet temperature increased almost linearly with the increase in the geothermal heat source. In addition, a performance analysis of selected organic working fluids, namely refrigerants R123, R152a, isobutane and n-pentane, was conducted under saturation temperature and subcritical pressure operating conditions of the turbine. Organic fluids with higher boiling point temperature, such as n-pentane, were recommended for the basic type of ORCs, whereas those with lower vapour specific heat capacity, such as butane, were more suitable for the regenerative ORCs.

Keywords: Geothermal energy, Organic Rankine Cycles, Optimization, Exergy analysis, binary cycle.

Nomenclature

Alphabetic symbols Subscripts

Specific heat capacity, J/kg.K Exergy rate, W

Enthalpy, kJ/kg

Exergy destruction, W

Mass flow rate, kg/s

Pressure, Pa

Heat transfer rate, W

Specific entropy, J/kg.K

T emperature, oC

Power output, W

Abbreviations

Hydrocarbons

Hydrochlorofluorocarbons Hydrofluorocarbons

Greek symbols

ƞ First Law efficiency, %

ƞ Second Law efficiency, %

Effectiveness, %

Vapour

ψ Specific exergy, W/kg

0 Reference state 1 15 Thermodynamic states Condenser

Critical value

Cooling system

Destruction

Evaporator

Geothermal fluid

Heat exchanger

Inlet

Maximum

Minimum

Optimum

Outlet

Circulation pump

Pinch-point

Reinjection

Isentropic

Turbine

Thermal

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