Microturbines have become popular among small-scale distributed energy systems. This paper focuses on a two-shaft arrangement where high efficiency is obtained through intercooling, reheat and recuperation. An optimized method for controlling the part-load performance via variable speed control of the generator shaft in addition to the turbine inlet temperature reduction is presented. The studied methods to reduce the power output were variable speed control of the generator shaft in combination with independent turbine inlet temperature control of both turbines. Optimization was performed by using a differential evolutionary algorithm to find a sufficient number of points at steadily reducing power settings to determine the optimal control curves for the three control parameters. In the microturbine model the operating values of the engine were obtained by solving the system of nonlinear equations formed by the governing relations. As a result an optimal part-load control method was found which provides better part-load efficiency than any of the studied control methods alone or in simple combinations could have provided. The optimal control strategy and the relative change of part-load electric efficiency were shown to be fairly independent of the design-point specifications for the turbomachinery and recuperator.

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