Modelling and numerical optimization of dual- and triple-pressure combined cycles

Optimization of the combined-cycle efficiency by optimizing the gas or steam cycle efficiencies does not guarantee an optimum combined cycle. In this paper, triple-pressure and dual-pressure non-reheat combined cycles are modelled and optimized for 50 and 300 cases respectively. The layout for the triple-pressure cycle was chosen to reduce NOx emission. Constraints are set on the pressure of the feed water heater, the minimum temperature difference for pinch points, the superheat approach temperature difference, the steam turbine inlet temperature and the dryness fraction at the steam turbine outlet. The combined-cycle efficiency is optimized using two different methods: the direct search method and the variable metric method. The effects of the inlet temperature of both the gas turbine and the steam turbine and of the minimum pinch point temperature difference on the performance of the optimum cycles are presented and discussed. The optimization results indicate that the layout for the triple-pressure cycle redefines the temperature differences for pinch points and adds a constraint on the intermediate pressure of the steam generator, which reduces the optimum efficiency by 2-3 percent when compared with the dual-pressure cycle under the same conditions. For the dual-pressure cycle, the optima of the most efficient cases were found to be 0.5-2 percent higher in efficiency than the optima of the regular design case, which is the usual design case for a commercial combined cycle. The optimum dual-pressure combined cycle was found to be almost as efficient as the most efficient commercially available triple-pressure reheat combined cycle, when compared at the same firing temperature.