Thermodynamic Optimization of a Triple Combined Cycle Power Plant: Integrating a Bottoming Organic Rankine Cycle into a 500 MWe Natural-Gas Combined Cycle

Combined cycle power plants (CCPPs) are the most efficient fossil-fuelled technology available today, yet a significant fraction of the fuel energy still leaves the plant with the heat recovery steam generator (HRSG) exhaust gases at temperatures too low to be exploited by a conventional steam cycle. This work assesses the thermodynamic benefit of recovering that low-grade heat by coupling a subcritical Organic Rankine Cycle (ORC) to the stack of a 500 MWe natural-gas CCPP, producing a triple combined cycle. The plant was modelled in EBSILON Professional following a two-stage methodology. The Brayton–Rankine plant was first optimized over the compressor pressure ratio (5–25), the gas turbine exhaust temperature (400–600 °C) and the HRSG live-steam pressure (60–140 bar), reaching a net efficiency of 60.69% with a stack temperature of 175 °C. Six dry hydrocarbon working fluids were then screened at evaporation temperatures between 90 and 150 °C under an explicitly enforced 10 K evaporator pinch-point constraint. Above a stack temperature of about 100 °C, the six candidates deliver almost identical net power, so that the selection is governed by stack temperature, turbine size and evaporator surface rather than by cycle efficiency. Two design points are therefore reported. At maximum power, n-butane evaporating at 110 °C adds 9.78 MWe at unchanged fuel input, raising the net efficiency to 61.88% (+1.19 percentage points) but cooling the stack to 74.4 °C. Constraining the stack above 130 °C selects cyclopentane at 140 °C, which adds 5.68 MWe (+0.69 percentage points) at a stack temperature of 132.2 °C, reduces the rejected thermal power by 24.7% and lowers the specific CO2 emissions by 1.12%.