Ship weight reduction and efficiency enhancement through combined power cycles

This work addresses the problem of configuration of gas and steam turbine combined cycles for ships by simultaneously considering increased efficiency and reduced weight as design objectives. The performed analysis provides basic information to produce systems with simultaneous advantage in both aspects. The combined cycle considered, with total constant power of 20 MW, is modeled as a gas turbine in standard configuration coupled to a simple Rankine cycle. Calculation of system's weight includes the machinery as well as the fuel required to guarantee a given time at sea. To estimate the machinery components weight, some scaling relations have been developed and used. The results presented include an analysis of the predicted weight and efficiency of the combined cycle respect to varying design parameters such as amount of heat recovered, time at sea, steam turbine exit quality, steam generator pinch point, and gas turbine performance. When compared against gas turbines in simple cycle mode, combined cycles produce a fuel requirement reduction that can overcome, in terms of weight, the size increase experienced by the plant. However, it is in general observed that minimum weight and maximum efficiency configurations do not necessarily coincide, as both objectives compete at intermediate values of heat recovery. Therefore, the particular choice of the final design depends on the relative importance assigned to each objective for the considered system. Notably, minimum weight and maximum efficiency solutions are very different for short trip periods but become basically the same for very long ones. Regarding gas turbine operation parameters, they have a strong influence on both total weight and efficiency. An interesting consequence is that a low efficiency gas turbine could produce better results than a high efficiency one, given a large enough temperature for the exhaust gases.