# Electrification in Maritime Vessels: Reviewing Storage Solutions and Long-Term Energy Management
## 2 Electrification of Marine Vessels
### Charging Infrastructure of Marine Vessels  
A charging system for marine vessels operates similarly to one designed for electric vehicles; however, maritime electrification requires more robust charging infrastructures. This is particularly true as the maritime industry expands, demanding higher energy capacities and more efficient, widespread charging solutions to support larger vessels and increased traffic. From a charging power system perspective, solutions for providing electrical power from shore involve several key components. These include interfacing with the main grid through a step-down transformer, integrating an onshore energy storage system, using power electronics converters for both AC-DC and DC-DC conversion, employing transformers for galvanic isolation and voltage-level adjustment, and incorporating circuit breakers and cable management systems [34]. There are three types of charging systems: (1) classical wired charging, which is further categorized into AC and DC charging systems, (2) wireless charging, which is based on capacitive and inductive charging technology, and (3) battery swap mechanisms, which have not yet been applied to ferries due to physical challenges, such as the need for large cranes to move the huge battery packs. Figure 1 demonstrates a ship with an AC BUS and a hybrid power system connected to an AC shore charging station, a scaled version of the slow and inexpensive charging stations commonly seen in electric vehicles. Although a single BUS is shown in the figure, multiple BUSs can be built to increase charging speed. Having a hybrid power unit allows for energy storage during off-peak times, when electricity is cheaper, providing cost savings and reducing the load on the grid. The energy flow in this system is as follows: energy from the grid passes through a transformer and is integrated with the ship's voltage. To allow storage from the AC BUS, the energy is sequentially converted by AC-DC and DC-DC converters within the charging station, charging the batteries. Energy is transferred from the AC BUS to the ship and converted to suit the ship's DC BUS using an AC-DC converter. The main grid pathway within the ship (DC BUS) can be converted and used in any desired area with DC-AC converters. DC-AC and DC-DC converters are used sequentially to charge the batteries and also supply AC loads on the ship [35]. Figure 2 shows a ship with a DC BUS and a hybrid power system connected to an AC shore charging station, similar to the one in Figure 1. It is important to note that the ship's onboard power system voltage must be reduced to 0 V before making a land connection. To safely establish a connection between the ship's onboard power system and the land-based grid, it is essential to reduce the ship's onboard power system voltage to 0 V. This is necessary to avoid the risk of inrush currents and electrical shock, as well as to ensure proper synchronization of the ship's voltage, frequency, and phase with that of the grid. Achieving a 0 V state requires a controlled de-energizing process, which typically involves disconnecting power sources, discharging capacitors, and managing transient voltages to  
prevent any abrupt electrical surges during the connection. This prevents severe sudden currents and synchronizes the ship's voltage, frequency, and phase with the grid [36]. The energy flow in this system is as follows: energy from the grid passes through a transformer and is integrated with the ship's voltage. To allow storage from the AC BUS, the energy is converted sequentially by AC-DC and DC-DC converters within the charging station, charging the batteries. Energy is transferred from the AC BUS of the grid to the ship and matched with the AC load/auxiliary load used on the ship. The main grid pathway within the ship (DC BUS) can be used directly in any desired area. AC-DC and DC-DC converters are used sequentially to charge the batteries on a ship [37]. Figure 1: Schematic diagram of a hybrid power system (full-electric ship) connected to an AC shore charging station.