Thermodynamic analysis and optimization of power cycles using a finite low-temperature heat source

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.