Effects of electric batteries charge and discharge strategy on modern Ro-Ro vessels

The maritime sector plays a crucial role in global trade and economic growth by transporting massive amounts of cargo and containers across the seas. However, the environmental impact of this industry poses a significant sustainability challenge, with the shipping industry accounting for 2-3% of global CO2 emissions. To address this issue, various energy solutions and strategies can be implemented on modern ships, including the use of electric battery storage systems. This paper proposes a novel approach to managing the charging and discharging phases of electric battery systems onboard Ro-Ro vessels and cargos. This strategy is implemented in a suitable dynamic simulation model purposely developed for assessing energy, economic, and environmental impact of different energy ship-plant system layouts including many different technologies. The strategy for charging /discharging the batteries is based on the following criteria. During the ship's navigation phase, it is necessary to activate all diesel generators onboard to meet the propulsion electrical load. However, these generators typically operate under partial load conditions, which are suboptimal and result in higher specific fuel consumption during electricity production. In this study, we propose a strategy for charging the ship's batteries during navigation by generating excess electricity from the diesel generators, which can be used to operate the generators under full load conditions with the lowest specific fuel consumption. This approach ensures that the diesel generators are charged with an optimal partial load factor, resulting in the production of electricity with minimum specific fuel consumption, as well as an excess production of electrical energy to recharge the batteries. Once the batteries are charged during navigation, they are discharged when the ship is docked at the port. In ports where diesel generators can be operated and no electric batteries are equipped onboard (e.g., non-ECA area), the electrical load can cause significant partialization of the generators, resulting in higher specific fuel consumption during electricity production compared to nominal conditions. Conversely, in areas where diesel generators are not allowed to be operated (e.g., ECA area), the electricity load is covered by electricity purchased at the port through shore-connection configuration. Therefore, in both scenarios, the proposed strategy can achieve significant energy savings by utilizing the electricity stored in batteries to meet (and/or partially meet) the ship's electricity demands. Of course, this strategy also reflects on waste heat availability and allow obtaining different and new scenarios to be optimize. The investigation is conducted by means of the dynamic simulation approach; a simulation model is developed in TRNSYS environment. To prove the energy and environmental potential of the proposed strategy and to demonstrate the advantages offered by this approach, a suitable case study referred to a Ro-Ro vessel is considered. The energy systems of a such a representative ship are modelled and dynamically simulated to investigate the impact of the implementation of batteries on the overall ship’s energy performance. Promising and preliminary energy savings are obtained from this study.