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Load levelling through Storage System for Hybrid Diesel Electric Ship Propulsion in Irregular Wave Conditions

This paper concerns the identification of the most suitable electrical energy storage system, devoted to propulsion power leveling of ship sailing in irregular wave conditions. Power irregularity originates from different flow conditions at the propeller disk that, in a quite complex way, causes torque and shaft speed variations. In this paper, the more adequate storage system is identified by relating storage performances with the period of the sea waves that suggests the employment of a threshold frequency, which allows choosing between two electrical energy storage technologies, namely supercapacitor and battery.This study is carried out for a Ro-Pax ferry powered by an electrical motor. Numerical simulations of ship dynamics in irregular seas, combined with propeller behaviour in waves, allow the estimation of the power propulsion spectra. The analysis of these numerical outcomes discloses that the power fluctuations due to irregular sea are characterized by frequencies higher than the threshold frequency. Therefore, these results indicate the suitability of supercapacitor in minimizing the irregularity of electrical power requested to the electrical power system.