The analysis of a subcritical Rankine cycle with superheating, operating between a constant flowrate low-temperature heat source and a fixed temperature sink, according to the principles of classical and finite size thermodynamics, is presented. The results show the existence of two optimum evaporation pressures: one minimizes the total thermal conductance of the two heat exchangers, whereas the other maximizes the net power output. A comparison of such results for five working fluids leads to the selection of R141b for a system generating 10% of a reference power which depends on the specified source and sink characteristics; for the conditions under consideration this reference power is 6861?kW. The results for this particular system show that the minimum total thermal conductance of the two heat exchangers is 1581?kW?K-1; the corresponding thermal efficiency is 12.6% and the total exergy losses are 13.8% of the source's exergy. Slightly more than 50% of the exergy destruction occurs in the vapor generator. Copyright (C) 2011 John Wiley & Sons, Ltd.