In this paper, a comprehensive analysis of a combined Rankine (water) cycle and an organic Rankine cycle (two-stage organic Rankine cycle) is presented, in which the low-temperature flue gas waste heat of a 123.5 MW combined cycle power plant unit, together with the refrigeration energy of liquefied natural gas, can be effectively recovered and utilized. A two-stage organic Rankine cycle is proposed to recover the low-temperature waste heat of the flue gas of a combined cycle power plant.

Based on thermodynamic mathematical models, the effects of key thermodynamic design parameters on system performance are examined from both thermodynamic and economic perspectives. The considered two-stage system shows a significant power output of 49.6 MW from the Rankine cycle and 3.8 MW from the organic Rankine cycle. A detailed exergy analysis shows that the condenser is the main site of exergy destruction, accounting for 3254 kW in the Rankine cycle and 714 kW in the organic Rankine cycle, which raises concerns about operating costs, estimated at $0.8 and $0.25/h, respectively. The highest exergy efficiencies recorded are 86% for the Rankine cycle superheater and 85% for the organic Rankine cycle reheater, contributing to an overall cycle efficiency of approximately 80% and an exergy efficiency of 72%.