This paper presents the latest developments of a methodology for the initial design of the water/steam cycle in combined-cycle power plants, which aims at delivering optimal designs from an operator’s perspective. To this end, an evolutionary algorithm optimization toolbox is coupled to a process model of the water/steam cycle. The process model requires the definition of a number of boundary conditions (like GT type and ambient conditions) and the selection of the cycle configuration (number of pressure levels, single or double reheat, supplementary firing, heat integration with GT coolers, fuel gas preheating, steam extraction from the steam turbine and type of cold end, among others). Based on a number of thermodynamic parameters assigned by the optimizer, the process model derives an initial dimensioning and/or selection of the key components and systems from the OEM’s portfolio: HRSG (full, geometry-based technical dimensioning), piping, steam turbines, condenser and generator, among others. For each of those, realistic designs are ensured by checking and enforcing the component design rules. Finally, performance and cost are derived. In the latest development, the process model computes the plant performance in a number of off-design conditions, specified in a plant operating profile. These may include different ambient conditions, GT loads, power augmentation (e.g. supplementary firing, inlet fogging and evaporative cooling) and steam exports (e.g. to district heating, desalination plant, carbon capture system) or imports (e.g. from a solar field). The cost of electricity (CoE), net present value (NPV) or average efficiency of the plant design in the given operating profile is the feedback to the optimization algorithm. This guides the process towards the definition of a plant design that gives the best thermo-economic performance under the specified economic boundary conditions and operating scenario. In a typical example, an air-cooled peaking plant needs to be optimized to maximize NPV in an operating scenario characterized by large spikes of the electricity price in hot summer days, during which the plant operator wants to use supplementary firing to boost power production. The described methodology is applied to find the most advantageous dimensions of the supplementary firing to be installed and the right HRSG design pressure at design conditions, ensuring that all design rules and technical limits are respected in all operating conditions. In this way, an optimal point is found in the trade-off between amount of supplementary firing and dimensions of HRSG and air-cooled condenser, delivering the highest possible benefit to the plant operator.
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