ORC (organic Rankine cycles) are promising systems for conversion of low temperature geothermal energy to electricity. The thermodynamic performance of the ORC with a wet cooling system is analyzed here using hydrocarbon working fluids driven by geothermal water from 100 degrees C to 150 degrees C and reinjection temperatures not less than 70 degrees C. The hydrocarbon working fluids are butane (R600), isobutane (R600a), pentane (R601), isopentane (R601a) and hexane. For each fluid, the ORC net power output first increases and then decreases with increasing turbine inlet temperature. The turbine inlet parameters are then optimized for the maximum power output. The ORC net power output increases as the condensation temperature decreases but the circulating pump power consumption increases especially for lower condensation temperatures at higher cooling water flow rates. The optimal condensation temperatures for the maximum plant power output are 29.45-29.75 degrees C for a cooling water inlet temperature of 20 degrees C and a pinch point temperature difference of 5 degrees C in the condenser. The maximum power is produced by an ORC using R600a at geothermal water inlet temperatures higher than 120 degrees C, followed by R245fa and R600 for reinjection temperatures not less than 70 degrees C. R600a also has the highest plant exergetic efficiency with the lowest turbine size factor. (C) 2013 Elsevier Ltd. All rights reserved.