Energetic and Exergetic Optimization of a Combined Cycle Power Plant with Dual-Pressure HRSG, Compressed Air Cooling, Steam Injection and Vapor Extraction Systems

An energetic and exergetic optimization is conducted on a combined cycle power plant of net power of 400 MW. This power plant is equipped with dual-pressure heat recovery steam generator, compressed air cooling, steam injection and vapor extraction systems. Energy and exergy balances are established on the different components of the cycle. The selected objective function is to increase the energy efficiency and reduce the exergy losses. A code is established using the Engineering Equation Solver software in order to perform the required calculations. The effects of the main operating conditions, such as ambient temperature, pressure ratio, air excess ratio, steam injection ratio and extracted steam pressure, on the power plant performances are analyzed taking into consideration the local environmental conditions. Obtained results show that the ambient temperature affects significantly the overall thermal efficiency of the combined cycle. The maximum energy efficiency is obtained for an optimal pressure ratio of about 14, medium and low pressure of 45 and 4 bars, respectively. The air excess and the steam injection ratios should be carefully defined to better the control of cycle performances. The combustion chamber is a major contributor of exergy destruction followed by the heat recovery steam generator. Compared to a standard combined cycle, the selected design characteristics have enabled the overall energy and exergy efficiencies to be enhanced about 11.7 and 10%, respectively.