Thermodynamic analysis of a power cycle using a low-temperature source and a binary NH 3–H 2O mixture as working fluid

Abstract

Two Rankine cycles, one with and one without a regenerator, both using a NH 3–H 2O mixture as the working fluid, have been analyzed for fixed source and sink inlet temperatures. A fixed mass flow rate of a hot gaseous stream is providing the thermal energy input at the heat recovery boiler (HRB). The methodology used in this work is divided in four steps: energy analysis, exergy analysis, finite size (or finite time) thermodynamics (i.e. thermodynamic calculations in the context of reasonable temperature differences in the heat exchangers) and calculation of the heat exchangers' areas. The results show that the range of evaporation pressures satisfying some basic conditions increases with the source inlet temperature and with the ammonia concentration. They also show the existence of an optimum evaporation pressure for each of the four analyses. In the first two analyses, an optimum evaporation pressure of approximately 3.2 MPa maximises the thermal and exergetic efficiency, at respectively 11% and 73%. In the last two analyses, the optimum pressure of 2.5 MPa minimises the heat exchangers' areas. The results also show that the net power output generated from a limited energy source doesn't influence the results of the energy analysis. However, an increase of the net power output decreases the exergetic efficiency while at the same time it increases the heat exchangers' surface.