Comparative thermo-economic analysis of regenerative Rankine cycles with two feed water heaters

Abstract

Five configurations of regenerative Rankine cycle with two feed water heaters are compared in this paper. One configuration has two open feed water heaters (OFWH). The other four configurations have an open feed water heater placed before or after a closed feed water heater, which may be either closed feed water heater with drain pumped forward (FFWH) or closed feed water heater with drain cascaded backward (BFWH). The five regenerative Rankine cycles are OFWH-OFWH, OFWH-FFWH, OFWH-BFWH, FFWH-OFWH, and BFWH-OFWH cycles. The optimum extracted steam fractions that result in the maximum thermal efficiency can be determined for each cycle. For a case study of Rankine cycle with the maximum steam pressure, temperature, and flow rate of 6 MPa, 450 °C, and 50.46 kg/s, respectively, the maximum efficiency of OFWH-OFWH cycle is 34.64%, and the net power output is 39.74 MW. The maximum efficiencies of the other four cycles increase with feed water heater area. They reach 34.64% when feed water heater areas of OFWH-FFWH, OFWH-BFWH, FFWH-OFWH, and BFWH-OFWH cycles are, respectively, 94, 138, 137, and 198 m2. The installation costs of five cycles are also compared so that the condition under which each cycle has the lowest installation cost can be identified.