Thermodynamic analysis of combined cycle under design/off-design conditions for its efficient design and operation

To achieve a highly efficient design and operation of combined cycles, this study analyzed in detail the off-design characteristics of the main components of three combined cycles with different compressor pressure ratios (PRs) based on real units. The off-design model of combined cycle was built consisting of a compressor, a combustor, a gas turbine, and a heat recovery steam generator (HRSG). The PG9351FA unit is selected as the benchmark unit, on the basis of which the compressor is redesigned with two different PRs. Then, the design/off-design characteristics of the three units with different design PRs and the interactive relations between topping and bottoming cycles are analyzed with the same turbine inlet temperature (TIT). The results show that the off-design characteristics of the topping cycle affect dramatically the combined cycle performance. The variation range of the exergy efficiency of the topping cycle for the three units is between 11.9% and 12.4% under the design/off-design conditions. This range is larger than that of the bottoming cycle (between 9.2% and 9.5%). The HRSG can effectively recycle the heat/heat exergy of the gas turbine exhaust. Comparison among the three units shows that for a traditional gas-steam combined cycle, a high design efficiency results in a high off-design efficiency in the usual PR range. The combined cycle design efficiency of higher pressure ratio is almost equal to that of the PG9351FA, but its off-design efficiency is higher (maximum 0.42%) and the specific power decreases. As for the combined cycle with a design PR of 12.73, the decrement of the efficiency under the design/off-design conditions is in the range of 0.20-0.39%, however, its specific power increases. Thus, for the efficient design of a combined cycle, its optimal efficiency and maximum specific power, instead of that of the topping cycle, should be considered. For the operation strategy, the performance of the topping cycle should be kept at a high level first (the turbine inlet temperature should be as high as possible), followed by the high setting of the turbine exhaust temperature.