Abstract: Nowadays, repowering is considered as the most common methods for improving status of current power plants. Repowering is the transformation of an existing steam power plant into a combined cycle system by adding one or more gas turbines and heat recovery capacity. It is a cost-effective way to improve performance and extended unit lifetime while adding capacity, reducing emissions and lowering heat rejection and water usage per kW generated. Each methods of repowering from "para repowering" to "full repowering" shall probably be the best choice for special national and economical power plant. In this paper different repowering methods have been introduced. The design concept consists in adding a gas turbine to the combined cycle, integrated by steam injection into the existing gas turbine. The steam is produced in a simplified heat recovery steam generator fed by the additional turbine's exhaust gas. A 156MW steam cycle power plant has been chosen as a case study. Two repowering scenarios have been utilized for this case. Thermodynamics code has been supplied for combined cycle and STIG combined cycle and compare with each others. The exergy and exergoeconomic analysis method was applied in order to evaluate the proposed repowered plant. Also, computer code has been developed for exergy and exergoeconomic analysis. It is anticipated that the results provide insights useful to designers into the relations between the thermodynamic losses and capital costs, it also helps to demonstrate the merits of second law analysis over the more conventional first law analysis techniques.The efficiency of the STIG repowered plant compares favourably with repowered combined cycle
