In order to compare the power output of the carbon dioxide transcritical cycle and the R125 transcritical cycle for a low-grade heat source of about 100 degrees C. the two cycles were optimized for power output using a simulation method. In contrast to conventional approaches, each working fluid's heat transfer and pressure drop characteristics within the heat exchangers were taken into account by using a discretized heat exchanger model. To fairly compare the power output of the cycles by using different working fluids, the inlet temperatures and the flow rates of both the heat source and the heat sink were fixed. The cycle minimum temperature was not given, but was determined by the heat sink conditions and the working fluid's heat transfer and pressure drop characteristics, as it is in actual practice. The total heat transfer area was fixed, whereas the allocation of the heat-exchanger area between the vapor generator and the condenser was optimized in the simulation. The R125 transcritical cycle produced 14% more power than did the carbon dioxide transcritical cycle. Even though the carbon dioxide cycle shows better heat transfer and pressure drop characteristics in the heat exchangers, the high pumping power required to manage the large pressure head degrades the cycle's power output. Based on this study, the R125 transcritical cycle is recommended for heat sources of about 100 degrees C. (C) 2010 Elsevier Ltd. All rights reserved.