Managing power demand in Diesel-electric multi-engine vessels for improving engine robustness: An approach for an evaluation tool

摘要

Use of diesel generators (DGs) for main ship propulsion through use of electrical drives is increasingly common, particularly in high performance/naval applications, work boats, cruise liners and multi-role ships. Previous studies and operational examples of the benefits to a Diesel-electric drive system with and without hybridisation with use of an energy storage device (ESD) have shown the benefits of such systems in regards to fuel consumption gains by utilisation of more efficient modes of engine operation, no-emissions operation for the ship in sensitive areas and overall reduction of emissions out through a typical duty cycle. However, within this operating environment the number of stop/start events for these generators can be very high, potentially leading to a lack of product reliability. To manage the demand of power from the DGs the use of an energy store to reduce the number of stop/start events for improved durability may be implemented. In order to understand the potential impact of such hybridisation systems, Ricardo have developed a rapid assessment tool for evaluation of a ship and set of operating duty cycles to determine whether advanced engine control and hybridisation with or without an ESD can provide a benefit to the performance and reliability of the vessel. Current engine management on vessels with multi-DG units will utilise control algorithms of varying levels of complexity for managing the engine operating conditions, the more simple strategies typically switching an additional unit on when other units pass a threshold load (for example, 80% load) or switching off a unit when load drops below a specified level (for example 20%). For standard cruise and base load operations this does not tend to result in excessive cycling of the engines, however, during manoeuvring and active operations, for example, naval exercises, the DG units may be switching on and off multiple times per day, resulting in a large cumulative number of on/off events. Experience with engine mechanical development and warranty investigation programmes at Ricardo has highlighted the damaging nature of these events on high speed Diesel engines, with excessive bearing and piston system damage common despite the implementation of pre-lubrication prior to engine start. For this study a simulation activity based on typical usage profiles for a mid-weight vessel with multiple DG sets has been performed using the evaluation tool described. This has assessed how an energy storage capability may be used to smooth the power demand on the engines and hence provide a reduction of engine on/off events. As a further benefit the fuel consumption may be reduced by operating the engines at more efficient load points when running. The simulation has been setup with a primary target to reduce the on/off cycles by 50% over a typical set of cycles, providing as an output the required size of the energy storage to meet this target. A secondary target of minimising fuel consumption over the cycle is also implemented. The further benefits of the energy storage have been assessed in regards to efficiency and quiet running time for the vessel.