Variable speed operation of turbogenerators to improve part-load efficiency

Our investigation of variable speed operation of turbogenerators, including both single-shaft and twin-shaft variants, shows significant opportunities to improve part-load efficiency in those certain electrical power generation applications that permit variable speed operation. Efficiency improvement increases as load decreases and the improvement is larger for single-shaft engines than for twin-shaft engines. For example, when operating at 20% loading, adjusting the engine speed can improve fuel efficiency by 14% for single-shaft gas turbines, and by 2% for twin-shaft gas turbines. In addition, we present a semi-theoretical analysis that provides a procedure to obtain the gas turbine optimal efficiency and its corresponding optimal speed as a function of shaft load. Simulation results of part-load variable speed modeling of gas turbines further confirmed the theoretical analysis. This has important practical implications. An analysis of fuel consumption by a gas turbine that operates with a load profile representative of a typical propulsion profile for a DDG51 ship, shows a 15% reduction in fuel consumption when variable speed operation is used, as compared to fixed speed operation. In addition, the analysis presented in this paper provides a general method to evaluate the steady-state performance of gas turbines operating with variable speed.