A comparison between exergetic and economic criteria for optimizing the heat recovery steam generators of gas-steam power plants

• Two methods for optimizing the HRSG of gas-steam power plants are compared.
• Decision variables are HRSG operating parameters and heat exchange sections layout.
• Gas-steam power plants with single or multi-pressure HRSG are optimized.
• Results of exergoeconomic optimization differ from those of COE minimization.
• COE minimization allows to establish a proper ranking of CCGT configurations.

CCGT (Combined-cycle gas turbines) are gaining an increasingly important role in power generation thanks to their high thermal efficiency, low installed cost and ready availability. Increasing natural gas prices, the optimization of CCGT operating parameters is becoming a topic of growing interest.In this paper two different methodologies for optimizing CCGTs are compared. The first aims to minimize the cost per unit of electricity generated, the second to minimize an objective function based on exergoeconomic principles accounting for the costs related with thermodynamic inefficiencies.Optimization results have been obtained considering different CCGT configurations, with single or multi-pressure HRSG (heat recovery steam generators), and varying the gas turbine technology, fuel price and plant capacity factor. A modular approach has been adopted to design a highly effective and flexible HRSG layout, in terms of number of pressure levels and arrangement of heat exchange sections along the flue gas path, together with the corresponding energy, exergy and cost balances, using an “interaction matrix”, with nodes between elementary components and towards the surrounding environment.