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 degrees C to 160 degrees C, and cooling air at ambient conditions of 25 degrees 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 degrees C to 36 degrees 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. Copyright (c) 2015 John Wiley & Sons, Ltd.