Thermodynamic analysis and performance optimization of organic rankine cycles for the conversion of low-to-moderate grade geothermal heat
International Journal of Energy Research
Department of Mechanical Engineering, University of Cape Town, Private Bag X3, Rondebosch, South Africa; Department of Mechanical and Aeronautical Engineering, University of Pretoria, Private Bag X20, Pretoria, Hatfield, South Africa
The present study considers a thermodynamic analysis and performance optimization of geothermal power cycles. The proposed binary-cycles operate with moderately low temperature and liquid-dominated geothermal resources in the range of 110°C to 160°C, and cooling air at ambient conditions of 25°C and 101.3kPa reference temperature and atmospheric pressure, respectively. A thermodynamic optimization process and an irreversibility analysis were performed to maximize the power output while minimizing the overall exergy destruction and improving the First-law and Second-law efficiencies of the cycle. Maximum net power output was observed to increase exponentially with the geothermal resource temperature to yield 16-49kW per unit mass flow rate of the geothermal fluid for the non-regenerative organic Rankine cycles (ORCs), as compared with 8-34kW for the regenerative cycles. The cycle First-law efficiency was determined in the range of 8-15% for the investigated geothermal binary power cycles. Maximum Second-law efficiency of approximately 56% was achieved by the ORC with an internal heat exchanger. In addition, a performance analysis of selected pure organic fluids such as R123, R152a, isobutane and n-pentane, with boiling points in the range of -24°C to 36°C, 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 non-regenerative cycles. The regenerative ORCs, however, require organic fluids with lower vapour specific heat capacity (i.e. isobutane) for an optimal operation of the binary-cycle. © 2015 John Wiley & Sons, Ltd.
Atmospheric pressure; Atmospheric temperature; Bins; Boiling point; Exergy; Geothermal fields; Optimization; Paraffins; Rankine cycle; Specific heat; Thermoanalysis; Thermodynamic properties; Thermodynamics; Boiling-point temperature; Internal heat exchanger; Irreversibility analysis; Organic Rankine cycles; Organic rankine cycles (ORCs); Performance optimizations; Thermo dynamic analysis; Thermodynamic optimization; Geothermal energy