The method of specific entropy generation (SEG) is employed to show how the thermal efficiency of a combined cycle power plant can be improved. SEG is defined as the total entropy generation rate associated with the operation of a power plant per unit flowrate of the fuel burnt in the combustor. In a recent article published in Journal of Energy Resources and Technology, it is shown that the thermal efficiency of a gas turbine cycle inversely correlates with SEG. In this work, we extend the analysis to show that the same relation between the thermal efficiency and SEG is also valid for a combined cycle. The topping cycle consists of a compressor, a combustor and a gas turbine, whereas the bottoming cycle includes a heat recovery steam generator, a steam turbine, a condenser, a deaerator, a condensate pump and a feed water pump.
It is shown that the minimization of SEG is identical to the maximization of thermal efficiency. An illustrative example is presented using the SEG method to improve the efficiency of the combined cycle. The results reveal that 89% of the inefficiencies takes place in the gas turbine cycle. A modified design is then proposed to reduce the efficiency losses in the topping cycle. In the modified design, the thermal energy of the flue gases is first used in a heat exchanger to preheat the air before the combustor. The flue gases leaving the heat exchanger is then directed to the HRSG for producing steam. With this modification, the thermal efficiency and the power output of the combined cycle increase 2.7 percentage points and 20.9 kW per unit molar flowrate of the fuel. Recovering the thermal energy of the flue gases for both preheating the air and producing the steam appears to be more efficient than just producing the steam. Despite the net power production of the bottoming cycle decreases in the modified design, the overall efficiency of the combined cycle increases due to the improvement in the efficiency of the topping cycle.