Abstract:
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 110oC to 160oC, and
cooling air at ambient conditions of 25oC and 101.3 kPa reference temperature and
atmospheric pressure, respectively. A thermodynamic optimization process and irreversibility
analysis were performed to maximize the power output while minimizing the overall exergy
destruction and improving the First- and Second-law efficiencies of the cycle. Maximum net
power output was observed to increase exponentially with the geothermal resource
temperature to yield 16-49 kW per unit mass flow rate of the geothermal fluid for the nonregenerative
ORCs, as compared to 8-34 kW 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 IHE. 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 -24oC to 36oC, 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.
