# Electrification in Maritime Vessels: Reviewing Storage Solutions and Long-Term Energy Management
## 3 Energy Storage System for the Marine Power System  
Battery usage is divided into two main categories in the maritime industry. These are all-electric and hybrid marine vessels. In all-electric vessels, batteries serve as the primary energy source for both propulsion and auxiliary systems, analogous to the role of diesel engines in conventional ships. Thus, all the power (for propulsion and auxiliaries) comes from batteries on an all-electric ship. The batteries in the hybrid vessel supplement the other fuel(s) to operate the system optimally [99]. Hybridization of ships can be performed as plug-in hybrid and conventional hybrid ships. Plug-in hybrid ships need shore power to charge their batteries when in a dock or terminal [101]. Therefore, these ships are rarely used for extensive long-distance operations [102]. The ships can operate their batteries during certain sections of their route, maneuver in port, and perform stand-by operations [103]. On the other hand, a conventional hybrid ship uses power derived from the main engine or the auxiliary engines to recharge its batteries by utilizing generators, alternators, and other means without the need for the shore [104]. A typical hybrid propulsion system for marine applications is illustrated in Figure 7[105]. This diagram illustrates the integration of various renewable energy sources, including wind energy and photovoltaic (PV) arrays, which feed into the electrical grid and an energy storage system (ESS). The energy is then used to charge batteries at the port, which powers the vessel's electric motor alongside a conventional fuel-powered generator. Additionally, the system captures renewable energy directly from surface waves using a floating device, further contributing to the ship's hybrid energy mix. This comprehensive approach optimizes energy use and enhances marine vessels' operational efficiency, demonstrating a sophisticated balance of traditional and renewable energy sources. Figure 7: Integrated hybrid propulsion and renewable energy system for marine vessels. The Maritime Battery Forum reported in January 2023 that large-scale battery usage has recently accelerated in the maritime industry. In 2016, while the number of electrified ships worldwide was only 106, this number reached 552. Moreover, the Maritime Battery Forum's Ship Register stated that 194 ships were on order. The distribution of all-electric, plug-in hybrid, and hybrid ships in operation is given in Figure 8. The graph shows that hybridization is the preferred option with a higher percentage instead of an all-electric option. More than half of the electric ships in 2010 were all electric. The trend has changed to the majority of electrified ships being plug-in hybrid and hybrid ships, with a small percentage of ships being all-electric. This distribution is demonstrated in Figure 9. The currently available battery technology caused this situation to occur because hybrid solutions are considered more suitable for most of the ships in the world [106]. Li-ion batteries are the most common type used as a secondary battery for marine energy storage systems. They have high energy density, reliability, and safety. Furthermore,  
Figure 8: Percentages of battery-powered Ships [7]. Figure 9: The percentage of pure electric, hybrid, and plug-in hybrid ships by year. Li-ion batteries can be adjusted to meet the specific power needs of different ships [26]. The following section will be composed of current (the most preferred Li-ion battery) and future battery technologies suitable for marine applications. Li-ion batteries are advanced rechargeable energy storage systems where lithium ions use redox reactions to store electrical energy. These batteries comprise four essential components facilitating these reactions: cathode active materials, anode active materials, electrolytes, and a separator. The separator acts as a porous membrane that enables the transfer of lithium ions between the anode and cathode through an electrolyte containing lithium salts.