The steam-augmented gas turbine (SAGT) differs from commercial steam-injected gas turbines where steam flow may be considerably less than 15% of air flow. SAGT combustors may operate near stoichiometric combustion conditions with steam flow as high as 50% of air flow, thus achieving specific powers exceeding 555 hp-sec/lb.
A previous simulation study of the steam-augmented gas turbine, which did not include compressor and turbine maps, examined the applicability of the concept in the Navy’s DDG-51-class ship environment. In this re-examination, component maps were employed to establish credible off-design engine performance, and to confirm estimates of overall ship fuel requirements based solely on anticipated component efficiencies. Also, the present simulation employs a heat-exchanger sub-program fully integrated into the main software program. The re-examination has led to several revisions and refinements of previous conclusions, which are discussed in the text.
The SAGT engine concept described herein, dispenses with intercoolers, but adds a low-pressure reheat combustor. The low-pressure combustor flame temperature exceeds 2700° F, which analyses show to be stable. Exhaust gas temperatures are not permitted to fall below 450° F, and the heat recovery steam generator is designed to hold feedwater temperatures close to 300° F to avoid the gas-side acid dewpoint.
At the most efficient operating points, the efficiency of this new reheat SAGT engine exceeds 44.5% with a 2200° F turbine inlet temperature, at an ambient 100°-F temperature. Moreover, it exhibits a 23% reduction in overall system volume. Simulation data show that the maximum efficiency of the SAGT engine peaks at engine powers required for cruising speeds, in contrast to the efficiency of the LM2500, which peaks at full-throttle. Since Navy ships operate near cruise conditions for the majority of their mission time, a SAGT plant uses 29% less fuel than the baseline LM2500 plant. Moreover, employing conservative cost estimates, the SAGT plant is quite competitive on a first-acquisition cost basis with gas turbines currently in the fleet.