An Analytical Study on the Performance and Feasibility of Converting a Combined Gas or Gas Propulsion System to a Combined Gas Turbine–Electric and Steam System for a Type 22 Frigate

Abstract

A Type 22 Broadsword class of frigate uses a combined gas or gas (COGOG) propulsion system that utilizes four different gas turbines, depending on the mode of operation. Its twin ST40M cruise gas turbines are operational most frequently, and an increase in their efficiency would significantly impact fuel usage and ship range. This study evaluates the option to upgrade the cruise gas turbines to a combined gas turbine–electric and steam (COGES) system, which utilizes a steam Rankine cycle to recover energy from the exhaust gases of the gas turbine. An alternative system using hot air is also analyzed for performance comparison. The analysis contains calculations for the energy extraction and dimensions of heat exchangers, for the power extracted from the additional steam or air turbine and for pressure losses of the exhaust gas. Different configurations for the heat exchangers were investigated, as well as various parameters for the steam and hot air. The size and mass of the system were an important aspect of the analysis. It was concluded that an auxiliary system operating at a maximum steam pressure of 20 bar could generate an additional 771 kW of power while adding a mass of 7.4 tons to the frigate. These findings suggest that upgrading to a COGES system could cover the electricity needs of the entire ship and thus reduce the overall fuel consumption, resulting in lower operational costs and less emissions.