Thermodynamics of regenerative Rankine cycle power plants

Regenerative Rankine cycles are usually used to improve the performance of the power plants. These cycles are conventionally analyzed by an energy-balance approach. Although the energy balance approach to system analysis produces correct results that the improvement in performance of a power plant is very much coupled with the number of feedwater heaters, it fails on its own of locating sources of therodynamic losses in a simple versus regenerative Rankine cycle. In the present paper second-law analysis in terms of irreversibility analysis is carried out for a regenerative Rankine cycle power plant. Based on this analysis, the distributions of irreversible losses in various components of a regenerative cycle are presented. The results of the overall cycle performance based on the irreversibility analysis are compared to those of the conventional energy balance approach. The main parameters on which the calculations are based are the number of feedwater heaters, the feedwater enthalpy rise and the turbine efficiency. The results show, for example, that the incorporation of feedwater heating results in a reduction of total irreversibility rate of the cycle by 20%. The corresponding improvement in efficiency is 13%. These two figures are augmented to 23% and 16%, respectively, by the incorporation of reheat in addition to regeneration. It is also found that 20% drop in the turbine efficiency causes an increase in total irreversible losses of the system by 50%.