Regenerative gas turbines and steam injection for repowering combined cycle power plants: Design and part-load performance

The fast increase in electricity demand and the need to meet the carbon dioxide emissions reduction targets are fostering the role of natural-gas based technologies for both distributed and centralized power generation. Due to their reliability and high-efficiency, combined cycle gas turbines (CCGTs) are regarded as a short to mid-term "bridge" technology towards a secure and cleaner electricity system, characterized by the full deployment of renewable technologies. In this context, it is important to explore potential options to upgrade the capacity of CCGTs, provided that the design of new facilities currently faces challenges related to insufficient financial resources and the lack of suitable sites for their installation. This study investigates a novel concept for repowering CCGTs. The solution proposed relies on the integration of a cogeneration unit based on a regenerative gas turbine (RGT), providing additional power capacity and superheated steam to be injected into the existing CCGT. Compared to traditional repowering techniques, the option proposed in this study is able to enhance the power production without drastically reducing the efficiency of existing CCGT. The simulation study investigates the effect of RGT operating conditions on techno-economic performance of the integrated power plant (IPP), referring to the marginal and the overall power productions. With the aim to assess the plant flexibility, the part-load operation of IPP is also analyzed, assuming an inlet guide vane (IGV) and turbine inlet temperature (TIT) control schedule. Simulation results found out that the repowering approach proposed allows for a maximum power increase of more than + 70%, at the expense of a penalty on net efficiency of around 1% pts compared to the existing CCGT. The repowered CCGT can also be operated at part-load, while providing remarkable energy and economic performances.