Electrification in Maritime Vessels: Reviewing Storage Solutions and Long-Term Energy Management

Abstract

Electric and hybrid marine vessels are marking a new phase of eco-friendly maritime transport, combining electricity and traditional propulsion to boost efficiency and reduce emissions. The industry’s advancements in charging infrastructure and strict regulations help these vessels lead the way toward a sustainable and economically viable future in shipping. In this review, electric and hybrid marine vessels are discussed, including past applications and trend demonstrations. This paper systematically analyzes maritime vessels’ energy management and battery systems, highlighting advances in lithium-based and alternative battery technologies. Additionally, the review examines the impact of these technologies on sustainability and operational efficiency in the maritime industry. This paper contributes to the field by presenting a holistic view of the challenges and solutions associated with the electrification of maritime vessels, aiming to inform future developments and policymaking in this dynamic sector. Unlike many existing reviews that focus exclusively on battery chemistries or energy management algorithms, this manuscript integrates multiple aspects of maritime electrification—including propulsion types, charging infrastructure, grid systems (MVDC), EMS, BMS, and AI applications—into one cohesive systems-level review. This cross-sectional integration is particularly rare in the literature and enhances the practical value of the review for designers, policymakers, and shipbuilders.

1. Introduction

1.1. Rise of Electric Vessels and Their Power

Traditional fossil-fuel-based marine propulsion systems significantly contribute to greenhouse gas emissions and environmental degradation, prompting increased scrutiny of the maritime industry. In response to global climate concerns, the International Maritime Organization (IMO) has implemented stricter emissions standards, aiming to reduce shipping-related emissions by at least 50% by 2050 compared to 2008 levels [1]. Another critical goal is to decrease the average CO₂ emissions per unit of transport work in international shipping by at least 40% by 2030, and 70% by 2050, relative to 2008 levels. These regulations promote lower sulfur fuel use, improved energy efficiency, and the adoption of cleaner propulsion technologies like electricity, hydrogen, and ammonia. This shift is driving innovation and investment in sustainable maritime solutions, accelerating the industry’s transition to a low-carbon future. Accordingly, countries around the world have begun to conduct comprehensive studies and invest in the development of electric marine vehicles as part of their broader strategies to combat climate change and promote sustainable transportation. These efforts include research into advanced battery technologies, energy storage systems, electric propulsion designs, and charging infrastructure tailored for maritime use. Norway’s launch of its first fully electric ferry, the Ampere, in 2015 marked a pivotal step toward sustainable maritime transport. Building on this success, the country has rapidly expanded its electric ferry fleet, with 70 additional vessels demonstrating both environmental and economic viability. This momentum underscores Norway’s strong commitment to cutting emissions and advancing green transportation, made possible by its plentiful renewable energy resources—especially hydropower—which power these ferries efficiently and cleanly [2]. The country is rapidly moving toward a future where the majority of its ferry fleet will be powered by battery-electric systems or hydrogen fuel. By 2030, most ferries in Norway are expected to operate with zero or near-zero emissions, further solidifying the nation’s leadership in sustainable maritime transportation and demonstrating how policy can effectively support environmental goals [3]. In 2017, China launched the world’s first fully electric cargo ship in Guangzhou, marking a significant milestone in sustainable maritime transport. Measuring 70 m in length, the vessel is capable of carrying up to 2000 tons of cargo and is powered by a 2.400 kWh battery system, giving it a range of approximately 80 km on a single charge [4]. Despite the irony that it primarily transports coal along the Pearl River, one of the busiest waterways in the world, the ship itself produces zero emissions during operation. This pioneering move highlights China’s commitment to reducing carbon emissions in key industrial sectors [5].

By eliminating the use of fossil fuels in its propulsion system, the electric cargo ship reduces air and water pollution, contributing to cleaner urban environments. Moreover, it sets a precedent for the global shipping industry, encouraging further innovation in electric-powered vessels and signaling a shift toward more eco-friendly logistics solutions in the face of growing environmental challenges. Launched in 2019, the Ellen is one of the world’s largest all-electric ferries and a groundbreaking example of sustainable marine transportation. Operating between the Danish islands of Ærø and Fynshav, Ellen covers a distance of 22 nautical miles (approximately 40 km) on a single charge, making it ideal for regional routes [6]. Powered entirely by electricity, the ferry has eliminated the need for fossil fuels, resulting in a reduction of approximately 2000 tons of CO₂ emissions annually. With its quiet operation, zero-emission design, and efficient performance, Ellen not only enhances the quality of life for local communities but also serves as a powerful model for green innovation in the maritime industry. Its success has inspired interest from other regions around the world looking to transition toward cleaner, more sustainable transportation solutions on water [7].

In 2018, Port Liner launched fully electric barges in the Netherlands, revolutionizing inland shipping with a cleaner, more sustainable alternative to diesel-powered vessels. Capable of carrying up to 280 containers, these barges reduce CO₂, nitrogen oxide, and particulate emissions, while also minimizing noise pollution [8]. Their introduction supports public health, aligns with climate goals, and positions the Netherlands as a leader in green maritime innovation, inspiring similar efforts across Europe. The Sea Change, launched in 2021, is the first hydrogen-electric passenger ferry in the US, operating in San Francisco Bay. Powered by hydrogen fuel cells, it produces zero emissions and only emits water vapor, reducing air and noise pollution. The project demonstrates the viability of hydrogen as a clean energy source for maritime transport and supports efforts to decarbonize the industry, setting a precedent for future hydrogen-powered vessels [9]. The Yara Birkeland, launched in 2021, is the world’s first fully electric and autonomous cargo ship, operating in Norway to transport fertilizer with zero emissions. Powered by a large battery system and advanced AI navigation, it eliminates the need for fossil fuels and crew, reducing road traffic, operational costs, and emissions [10]. The vessel marks a major advancement in sustainable and automated maritime transport, setting the stage for future innovations in the shipping industry. Japan’s e5 Project aims to decarbonize the maritime industry by developing electric and hybrid vessels, such as the e5 Tanker launched in 2022. Powered by lithium-ion batteries and diesel generators, the hybrid tanker is designed for coastal shipping, reducing emissions, fuel use, and noise [11]. The project also promotes smart maritime technologies, positioning Japan as a leader in sustainable shipping and offering a model for global adoption. Movitz, an electric ferry operated by Green City Ferries, began service in Stockholm in 2023 to support the city’s low-emission public transport goals [12]. Powered by a 1.000 kWh battery, it carries up to 150 passengers and offers a clean, quiet alternative to diesel ferries. Movitz reduces emissions, improves air quality, and serves as a model for sustainable urban transit, inspiring similar initiatives globally. Undoubtedly, the key factor driving the progress of these developments, as well as the size and range of the ships produced, is the advancement of battery technology. Batteries are a fundamental component of electric vessels, serving as the primary energy source that powers both the propulsion system and onboard electrical systems [13]. Their performance, capacity, and efficiency directly influence the vessel’s range, speed, and operational reliability. Among the most commonly used battery types are lithium-ion (Li-ion) and lithium–polymer (Li-polymer) batteries, favored for their high energy density, long cycle life, and relatively compact size [14]. These characteristics make them especially suitable for marine applications where space and weight are critical factors. Li-ion batteries are widely adopted due to their proven performance in a variety of electric mobility solutions, offering a balance between energy storage and durability. Li-polymer batteries, while similar in chemistry, are lighter and can be manufactured in flexible shapes, making them ideal for custom or space-constrained vessel designs. Ongoing advancements in battery technology are also leading to the development of next-generation batteries, such as solid-state and lithium–sulfur batteries, which promise even greater energy density and safety [15].

Additionally, integrating battery management systems (BMSs) is crucial for monitoring performance, ensuring safety, and optimizing charging and discharging cycles. As electric marine technology evolves, the role of batteries will become even more central, driving improvements in vessel efficiency, operational range, and overall environmental impact. However, considering the current limitations in the range and capacity of electric batteries, hybrid systems are often preferred over purely electric ones, especially in the early stages of the transition to sustainable maritime transportation. While all-electric vessels offer clear environmental benefits, the technology behind electric propulsion is still evolving, particularly in terms of energy density, charging speed, and the capacity to support long-distance voyages or large cargo loads [16]. Hybrid systems, which combine traditional fuel-powered engines with electric propulsion, provide a practical solution to these challenges [17]. They allow vessels to benefit from the efficiencies and environmental advantages of electric power while still relying on conventional fuel when needed to extend range or ensure operational flexibility. Hybrid vessels can operate in purely electric mode during short trips, in urban areas, or on emission-restricted routes, while seamlessly switching to fossil fuels for longer journeys or when battery capacity is low. This dual-power approach allows for gradual integration of electric technology into the maritime sector, while still meeting the demands of industries such as cargo shipping, ferry transport, and offshore operations, where long distances and high-power requirements are typical. Additionally, hybrid systems help mitigate the risk of relying solely on electric power during the current phase of battery technology development [18]. As battery technology continues to improve, hybrid systems will likely evolve, eventually leading to fully electric vessels capable of handling longer distances and larger loads without compromising on performance or sustainability [19]. In the short term, hybrid vessels serve as a practical bridge between conventional and fully electric maritime transport, enabling industries to reduce emissions and fuel consumption without sacrificing operational reliability [20]. This approach paves the way for future advancements in electric vessel technology, allowing the maritime industry to progressively move toward a cleaner and more sustainable future [21].

Unlike many existing reviews that focus solely on battery chemistries or energy management algorithms, this manuscript offers a comprehensive systems-level review of maritime electrification. It integrates various aspects, including propulsion types, charging infrastructure, medium-voltage direct current (MVDC) grid systems, energy management systems (EMSs), battery management systems (BMSs), and artificial intelligence (AI) applications. Such cross-disciplinary integration is uncommon in the literature and significantly enhances the practical value of the review for designers, policymakers, and shipbuilders. The manuscript adopts a maritime-vessel-centric approach, structuring its discussion around the operational needs and systems of real-world vessels, such as hybrid versus electric propulsion, EMS/BMS functions, charging strategies, and case studies of maritime technologies. A particularly novel aspect is the in-depth examination of AI, machine learning (ML), and deep learning (DL) applications within EMS and BMS frameworks, specifically designed for naval environments, an area typically limited to land-based vehicles or grid applications in existing literature. Furthermore, the review incorporates real-world maritime projects—such as Ellen, Ampere, MF Tycho, Sea Change, Yara Birkeland, and the e5 Project—supported by comparative tables detailing vessel characteristics and battery configurations, thereby providing empirical context often absent in theory-heavy reviews. It also presents a comparative analysis of various lithium-ion battery chemistries (e.g., NMC, LFP, LCO, and NCA) and emerging technologies like solid-state and lithium–air batteries, with a particular emphasis on marine-specific performance criteria, including safety, thermal stability, and energy density. Moreover, the manuscript highlights practical challenges, such as corrosion in saltwater environments, and explores the potential of wireless charging as a robust, maintenance-friendly solution—an aspect frequently overlooked in reviews of land-based electric vehicle infrastructure.

1.2. The Benefits and Challenges of Hybrid Marine Vessels

All-electric marine vessels rely solely on electric power for propulsion and onboard systems, eliminating direct emissions and significantly reducing the environmental impact [22]. These vessels use electric motors powered by batteries to drive the propellers or water jets. Common battery types include Li-ion and Li-polymer batteries, which store the electrical energy necessary for propulsion and other systems. Charging these vessels is accomplished through shore power connections or onboard generators, making the development of charging infrastructure at ports essential for their operation. The advantages of electric marine vessels are numerous: they produce zero emissions at the point of use, have lower operating costs due to reduced fuel and maintenance expenses, operate more quietly, and can integrate with renewable energy sources. However, they also face challenges such as limited range due to battery capacity, longer charging times compared to traditional refueling, and the need for significant initial investment in charging infrastructure [23].

Additionally, hybrid marine vessels combine internal combustion engines (typically diesel) with electric propulsion systems, offering greater operational flexibility and efficiency improvements. These vessels can operate in different modes: pure electric mode, hybrid mode (using both engines and electric motors), or internal combustion engine mode. This adaptability allows for optimized fuel efficiency and reduced emissions. Hybrid vessels also have batteries that store energy from regenerative braking, surplus engine power, or shore charging. The advantages of hybrid systems include improved fuel efficiency, reduced emissions, silent operation in electric mode, and the provision of backup power in case of engine failure, which also reduces wear and tear on the main engines. However, hybrid systems are more complex to design and maintain compared to traditional power systems, and the initial investment cost is higher due to the integration of multiple technologies [23,24].

Leading maritime companies and shipyards are now investing in electric and hybrid propulsion systems, exploring their application across various vessel types, from small ferries and leisure boats to large cargo ships and tankers. Pilot projects and commercial deployments demonstrate the viability and advantages of electrification, paving the way for broader adoption in the coming years [25,26]. However, this shift towards electrification presents several challenges, including the need for substantial infrastructure development, high initial costs, and technological hurdles related to energy storage and power management. Despite these challenges, ongoing research and development efforts are actively addressing these issues, ensuring a smooth and effective transition to a more sustainable future for the maritime industry [27]. The electrification of marine vessels is not just a critical component but an urgent necessity in the maritime industry’s evolution towards sustainability and efficiency. As technological advancements continue and the regulatory landscape evolves, electric propulsion systems are poised to play a central role in the future of marine transportation, significantly contributing global efforts to reduce environmental impact and promote sustainable development [28].

Table 1. Definitions and features of electric and hybrid marine vessels.

Electric Marine Vessels: Electric marine vessels are those that rely solely on electric power for propulsion and onboard systems. They have no direct emissions, making them environmentally friendly options for maritime transportation
Propulsion	Electric marine vessels use electric motors powered by batteries to drive the propellers or water jets. This propulsion method eliminates the need for fossil fuels and reduces emissions
Batteries	Batteries are a fundamental component of electric vessels. They store electrical energy that is used to power the vessel’s systems and propulsion. Common battery types include Li-ion and Li-polymer batteries
Charging	Electric vessels are charged through shore power connections or onboard generators that produce electricity. Charging infrastructure in ports is essential for keeping electric vessels operational
Advantages	Electric vessels offer zero emissions at the point of use, reduced operating costs (lower fuel and maintenance expenses), quieter operation, and potential for integration with renewable energy sources
Challenges	Limited range due to battery capacity, longer charging times compared to refueling, and initial investment in charging infrastructure are challenges for electric vessels
Hybrid Marine Vessels: Hybrid marine vessels combine multiple power sources, typically an internal combustion engine (often diesel) and an electric propulsion system. This setup offers operational flexibility and efficiency improvements
Propulsion Modes	Hybrid vessels can operate in different modes, such as pure electric mode, hybrid mode (using both engines and electric motors), or internal combustion engine mode. This adaptability allows vessels to optimize fuel efficiency and emissions
Energy Storage	Hybrid vessels have batteries that store energy for electric propulsion. This energy can come from regenerative braking, surplus engine power, or shore charging
Advantages	Hybridization improves fuel efficiency, reduces emissions, enables silent operation in electric mode, and provides backup power in case of engine failure. It also reduces wear and tear on the main engines
Challenges	Hybrid systems are more complex to design and maintain compared to traditional power systems. The initial investment cost can be higher due to the integration of multiple technologies

summarizes the key characteristics and advantages of electric and hybrid marine vessels, highlighting their potential to transform maritime transportation into a more sustainable and efficient industry [29].

This paper focuses on electric and hybrid marine vessels, highlighting their promising advancements in maritime technology. Electric vessels lead the charge in emissions reduction, offering a sustainable solution, while hybrid vessels strike a balance with enhanced efficiency and flexibility. The ongoing development and adoption of these technologies are crucial for the future of maritime transportation, aligning the industry with global sustainability goals and mitigating its environmental impact. To address the accelerating interest and advancing technologies in this field, this paper consolidates current knowledge, explores emerging trends, and emphasizes the imperative for further research and development. In the Section 2, the electrification of marine vessels is examined, encompassing environmental considerations, charging systems, energy management, battery management, and the integration of artificial intelligence. The Section 3 delves into energy storage systems for marine power, while the Section 4 provides a comprehensive summary of battery applications in marine contexts. Finally, the Section 5 focuses on the grid system structure for marine vehicles and the Section 6 concludes the study with final comments and discussions.

In addition to their benefits, hybrid-powered vessels face key challenges in managing dual power sources, particularly in synchronizing transitions between electric and fuel-based propulsion, maintaining battery stability under fluctuating demands, and optimizing energy distribution. Recent advancements in AI-powered EMS/BMS, modular battery architectures, and regenerative systems help mitigate these challenges, paving the way for more reliable and efficient hybrid marine operations.

2. Electrification of Marine Vessels

2.1. Environmental Aspects

Climate change is recognized as a global emergency that demands international cooperation and solutions at all levels. At the UN Climate Change Conference (COP21) in Paris on 12 December 2015, world leaders agreed to combat climate change and its negative effects through the Paris Agreement [30]. The agreement aims to limit the global temperature increase to 2 °C this century by reducing greenhouse gas emissions. Furthermore, the agreement sets a more ambitious target of limiting the temperature increase to 1.5 °C. To achieve this, it is necessary to reduce emissions by 45% by 2030 and achieve net zero by 2050 [31]. The agreement includes commitments from all major emitting countries to reduce pollution and strengthen these commitments. It also provides avenues for developed countries to assist developing countries in climate mitigation and adaption efforts. It establishes a framework for transparent monitoring, reporting, and accelerating the goals of individual countries and their collective climate goals [30,31].

According to the requirements of the Paris Agreement, the IMO has also set a goal to decrease greenhouse gas emissions from global shipping by at least 50% by 2050 compared to 2008 [32]. Given their lengthy lifespan, this will require significant modifications to marine vessels that will be constructed in the coming years. Achieving this goal will necessitate integrating existing technologies in novel ways, developing new inventions, and introducing alternative fuels to the global market. The potential of electric energy storage systems in maritime has been overlooked despite the popularity of vehicle electrification for over a decade [33]. However, research on optimizing storage systems and charging protocols for different applications has accelerated recently.

2.2. 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 3 illustrates a ship with a DC BUS and a hybrid power system linked to a DC shore charging station. After passing through a transformer, the energy from the grid is converted to DC via a converter and then directly connected to the ship’s main DC BUS. Like other charging stations, this battery-supported station matches its voltage to the ship using a DC-DC converter. This charging station design can offer a weight advantage for ships [38,39].

Figure 4 demonstrates that the ship’s bus system is configured to operate with an AC setup. This configuration indicates that the ship is designed to work with an alternating current (AC) bus without requiring any alterations or adjustments to the existing charging station. The charging station remains unchanged, maintaining its original design and functionality. This seamless integration suggests compatibility between the ship’s AC bus system and the charging infrastructure, highlighting the convenience and efficiency of the setup [40].

These four solutions can be chosen based on various factors, including the size of the ships, the distances they need to travel, and the associated costs. Each solution offers its own set of advantages and disadvantages that must be carefully considered when planning. For instance, all the charging stations mentioned above connect to the ships via standard sockets. However, using socket-based charging stations in salty water environments introduces specific risks. The corrosive nature of salty water can lead to significant corrosion, which, in turn, can cause safety issues and increase maintenance costs for the charging stations. An alternative approach is to address these challenges using wireless charging stations. Wireless charging eliminates the need for direct contact points, thereby reducing the risk of corrosion and enhancing the overall safety and reliability of the charging process in harsh maritime environments. This approach can provide a more robust and maintenance-friendly solution for charging ships operating in saltwater conditions [41].

To transform this charging station into a wireless charging facility, specific components tailored for wireless charging need to be integrated, all without altering the ship’s existing charging equipment or the fundamental structure of the charging station. This approach, while convenient in eliminating the need for physical cables, accentuates the critical importance of the charging station’s placement. Accurate calculations become paramount to ensure optimal alignment for seamless docking, thus maximizing charging efficiency. Within this setup, the charging station and the ship are outfitted with transmitter and receiver coils positioned at corresponding heights, respectively. The interaction between these coils, characterized by a high magnetization current, leads to reactive power consumption within the charging station’s coil. To address this, capacitive compensation networks are employed on both ends to generate the necessary reactive power (Figure 5) [42].

The energy the charging station supplies to these networks follows a square wave pattern, oscillating at a frequency typically in the kilohertz range. Upon reaching the ship, a converter transforms these waves into direct current (DC). Here, the charging station’s converter acts as a power supply converter, while the ship’s converter functions akin to a diode-based current rectifier. Notably, unlike conventional AC BUS charging stations, this system employs capacitive compensation networks and coils for the transfer of reactive power to the ship. This makes the inductive charging system particularly advantageous for scenarios where ships frequently dock at the shore for short intervals, facilitating time-saving operations. Nevertheless, the technology’s adoption comes with challenges, including alignment difficulties, the necessity for larger coils to manage higher power demands, elevated temperatures due to larger coil sizes, and various other considerations that may limit this charging station’s effectiveness [43]. The integration of electric charging infrastructure in port environments presents significant financial and logistical challenges. Retrofitting existing port facilities to support wired, wireless, or battery swapping systems often requires substantial investment in grid upgrades, spatial reconfiguration, and equipment procurement. Moreover, ports must consider the impact of installation on daily operations, as well as long-term maintenance costs and compatibility with diverse vessel types. Regulatory compliance, safety standards, and coordination with utility providers further complicate the transition, making it essential for port authorities to conduct comprehensive feasibility studies before implementation.

2.3. Energy Management System (EMS) for Marine Vessels

The energy management system (EMS) is designed to monitor, control, and optimize the distribution, production, and consumption of electrical energy onboard. Its primary goal is to improve energy efficiency, reduce fuel consumption, and minimize environmental impact. The EMS achieves this by managing various power sources and loads, optimizing generator operations, and controlling the charge and discharge cycles of energy storage systems. Additionally, it monitors energy consumption, manages electrical loads, and integrates potential renewable energy sources, such as solar panels and wind turbines, into the ship’s power systems.

With these essential functions, the EMS has become a critical component in various maritime platforms. On cruise ships, it ensures efficient power generation and load supervision. For container ships, it effectively distributes power to refrigeration units and other storage systems. Ferries benefit from the EMS by maximizing the use of renewable energy sources and battery systems, ensuring minimal environmental impact. On naval vessels, the EMS provides stable power for mission-critical systems while minimizing overall energy use [44].

Artificial intelligence (AI) integration enhances the capabilities of the EMS, enabling smarter decision making and predictive functionalities. Through AI, the EMS can learn from historical data and real-time input to optimize operations even further. For example, AI-driven algorithms can continuously adjust power distribution in response to changing conditions such as weather, sea currents, and operational requirements. AI also enables more precise forecasting of energy demand, providing better load balancing and reducing waste.

There are several main use cases of EMSs for maritime vessels. Firstly, an AI-powered EMS can optimize engine and propulsion systems to minimize fuel consumption based on a wide range of variables, including routes, sea conditions, and weather factors [45]. Machine learning algorithms analyze historical data and real-time inputs to recommend optimal routes and speeds, further enhancing fuel efficiency. Secondly, by supervising the energy flow and engine operation processes, the EMS contributes to decreasing emissions and ensuring compliance with international regulations, such as those set by the International Maritime Organization (IMO) [46]. The integration of AI allows EMSs to proactively adjust operations to meet emissions standards in real time [47].

Another key use case is the EMS’s ability to integrate renewable energy sources, such as solar panels and wind turbines, with conventional diesel engines [47]. AI algorithms can predict and optimize the contribution of renewable sources based on weather forecasts and ship operational conditions. This seamless integration not only maximizes energy efficiency but also minimizes reliance on fossil fuels, reducing the overall environmental footprint. AI-based predictive analytics can also monitor the current state of equipment, forecast potential failures, and schedule maintenance proactively, thereby preventing unexpected breakdowns and reducing costly downtime [26].

Lastly, EMSs, integrated with AI, can manage the efficient distribution of power to different onboard systems, particularly in complex naval ships with high energy demands [48]. AI allows the system to dynamically adjust power distribution to ensure critical systems are always supplied with energy, while non-essential systems can be powered down or reconfigured as necessary, ensuring optimal resource allocation.

An EMS offers numerous benefits for maritime vessels, such as fuel cost reduction through optimized routes and speed configurations, increased operational safety by observing real-time data on equipment status and energy systems, and assistance in meeting strict emission regulations by supporting sustainable operations. An AI-driven EMS further enhances these advantages by minimizing operational costs, improving equipment lifespan through predictive maintenance tailored to maritime environments, and enabling data-driven decision making through real-time monitoring. As AI continues to evolve, the integration of advanced machine learning techniques will unlock even greater efficiencies, allowing for more sustainable, reliable, and cost-effective maritime operations.

2.4. Battery Management Systems (BMSs) for Marine Vessels

All-electric ships have become a standard configuration in marine propulsion and energy systems. The strategy for managing battery operations significantly impacts energy efficiency and marine vessels’ safety. Battery management systems (BMSs) are essential for battery packs’ safe and efficient operation, particularly crucial in maritime applications where substantial energy storage capacities are required. These BMSs are not just systems but meticulously engineered solutions designed to perform several critical functions precisely. These functions include continuous monitoring of battery health, precise management of charging and discharging cycles, and ensuring optimal conditions for battery systems’ safety, stability, and longevity [48,49]. Similar to an EMS, a BMS is tasked with a suite of critical functions. These include state of charge (SoC) estimation, which predicts the current capacity of the battery to prevent issues such as overcharging and deep discharging. State of health (SoH) prediction involves continuous monitoring of battery health to identify problems such as cell imbalance or gradual battery degradation. Thermal monitoring is conducted to check the battery’s temperature and implement precautions against overheating. Safety protection mechanisms are in place to monitor and mitigate risks such as overvoltage, undervoltage, overcurrent, and short circuits. Additionally, balancing control is crucial for maintaining uniform charge levels across cells, enhancing the battery’s stability, and prolonging its lifespan [50]. BMSs play a pivotal role in naval platforms and systems deployed on vessels such as hybrid and electric ships, where they oversee extensive battery clusters used in propulsion and auxiliary systems to ensure optimal safety and stability. Additionally, BMSs are integral in managing backup power systems, such as uninterruptible power supplies (UPSs) on ships, and maintaining critical operations during power outages. They also optimize the integration of renewable energy sources into the battery systems to enhance environmental safety and extend the lifespan of the batteries. Furthermore, a BMS is crucial in emergency systems, providing reliability and stability during critical scenarios, such as sudden power failures or lighting issues [51,52]. BMSs are also deployed across various use cases in maritime vessels. A critical application is within hybrid propulsion systems, where a BMS meticulously monitors and manages the interaction between battery usage and conventional engines. This allows for an optimized balance that minimizes fuel consumption and emissions [53]. In scenarios where high power demand arises due to challenging maritime conditions, a BMS can provide supplemental power from the battery components, alleviating the generator load and reducing fuel consumption [54]. A BMS also ensures that battery components are consistently ready to supply additional power in diverse emergency scenarios within maritime environments, thus enhancing vessel safety [55]. Additionally, for maritime vessels equipped with regenerative braking technology, a BMS efficiently manages and stores the energy captured during braking operations, making it available for future use [56]. In conclusion, a BMS significantly contributes to maritime vessels by enhancing energy efficiency through optimized usage of battery components, tailored to varying maritime environmental conditions. They also provide an additional layer of safety and reliability by continuously monitoring the state of charge of each battery component to prevent overcharging and deep discharge situations. Consequently, a BMS presents several critical benefits including extended battery life through management of operating conditions suited to maritime environments, reduced fuel consumption in hybrid propulsion systems by significantly decreasing reliance on diesel engines, improved operational efficiency by dynamically allocating battery power where it is most needed, and aiding maritime vessels in complying with environmental regulations. In maritime hybrid propulsion systems, the battery management system (BMS) plays a vital role in ensuring the safety, reliability, and efficiency of the energy storage system under harsh sea conditions. Unlike stationary or automotive environments, maritime settings present unique challenges such as constant vibrations, significant temperature fluctuations, and high humidity with salt-laden air. To withstand mechanical shocks and vibrations caused by wave activity and engine operations, BMS hardware must be ruggedized, often requiring advanced testing standards like MIL-STD-810G. Temperature variations, ranging from cold ambient conditions to high internal heating, necessitate real-time temperature sensing, thermal management integration, and protective logic to prevent thermal runaway. Humidity and corrosive environments demand sealed enclosures with high IP ratings (typically IP65 or above), conformal coatings on PCBs, and corrosion-resistant connectors. Beyond environmental durability, the BMS ensures operational safety by continuously monitoring cell voltages, currents, temperatures, and states of charge (SoCs), while also enabling emergency shutdowns in case of faults or thermal anomalies. Furthermore, the BMS enhances efficiency by supporting predictive algorithms to estimate state of health (SoH) and remaining useful life (RUL), enabling optimized switching between diesel and electric modes. It must also be interoperable with the ship’s energy management and automation systems via protocols like NMEA 2000 or CANOpen to provide real-time data for decision making. In this context, the maritime BMS is not just a supervisory unit but a core component that supports the safe, efficient, and intelligent operation of hybrid propulsion systems at sea.

2.5. Artificial Intelligence and Applications in Marine Vessels

AI, ML, and deep learning (DL)-based approaches can significantly leverage and automatize EMS and BMS platforms for maritime platforms and minimize human staff supervision for monitoring, maintenance, and repair process costs. More specifically, AI applications in maritime energy systems introduce significant practical benefits, especially in fuel optimization and predictive maintenance. For instance, AI-powered route optimization systems can use real-time weather data, ocean current patterns, and vessel load information to suggest fuel-efficient navigation paths. A specific example is Wärtsilä’s Navi-Planner, which employs AI approaches for dynamically adjusting speed and route, consequently reducing fuel consumption by noticeable levels on long-haul voyages. Similarly, predictive maintenance in battery systems can be further leveraged by machine learning models that analyze voltage fluctuations, temperature variations, and historical degradation patterns. As a particular instance, the usage of recurrent neural network (RNN) models in BMS platforms enables early detection of internal short circuits or abnormal SoC drops, which provides timely intervention before a potential system failure. Consequently, these real-world applications indicate how AI not only enhances energy efficiency but also ensures the reliability and safety of hybrid and electric marine vessels, which points to a critical step toward autonomous maritime operations [55,56].

Generally, within the following main factors, these approaches drastically contribute to EMSs and BMSs [44,51,52].

Optimization of energy consumption: AI, ML, and DL-based approaches can estimate and supervise a ship’s or naval platform’s energy requirement more efficiently. These approaches can analyze patterns from past data to estimate future energy requirements and optimize power distribution across different systems on naval platforms. Consequently, this process provides better energy management, diminishes waste, and concurrently increases the efficiency of operations within EMSs and BMSs [57,58].

Predictive maintenance: AI, ML, and DL-based methods can foresee potential failures and errors in energy systems before they arise. Especially for BMSs, these methods can estimate battery life degradation or predict the cells that might fail beforehand. Additionally, for EMSs, these approaches can heuristically detect machinery failures that potentially give rise to escalated energy consumption. Consequently, this predictive feature greatly contributes to planning maintenance and management tasks proactively, hence minimizing downtime and increasing the life of equipment dramatically [58].

Real-time decision making: Learning (a specific domain of ML) and AI-based approaches provide real-time decision making by quickly processing and analyzing great volumes of data. Especially in environments such as naval operations and other maritime-based domains including multiple variable and complex factors, DL models can make immediate decisions about power allocation and system configurations for adapting to changing conditions, such as different operational modes based on naval environments, weather conditions, or critical system malfunctions [44]. For example, modern maritime systems utilize real-time AI techniques to continuously monitor ship performance, fuel consumption, and weather conditions and consequently adjust operational parameters to increase energy efficiency and safety. Additionally, in specific maritime systems, reinforcement-learning-based systems are deployed for real-time autonomous navigation and power allocation under dynamic marine conditions. These techniques combine real-time inputs from radar, GPS, battery sensors, and engine telemetry to make quick decisions about optimal energy distribution and propulsion settings. In hybrid vessels, DL models such as convolutional neural network (CNN)–long short-term memory (LSTM) structures are utilized to instantaneously assess battery load profiles and switch between diesel and electric propulsion in response to sudden environmental changes, which ensures both performance continuity and fuel savings.

Advanced safety and reliability: With constant monitoring of the health and different status conditions of batteries and other energy systems, AI and ML approaches can dramatically improve the stability, safety, and reliability of naval systems. These approaches can predict anomalies pointing to various safety risks, such as overheating batteries or electrical errors, and this way, they can enhance the trigger automatic safety protocols to alert human operators beforehand [52,59,60].

Integration with renewable energy sources: AI and ML approaches can integrate renewable energy sources like solar and wind into EMSs and BMSs for naval environments. These approaches can forecast renewable energy availability, handle its integration with traditional power sources, and consequently utilize stable and efficient energy supply even in varying environmental conditions and factors [53].

Economic processes: These approaches can conduct cost–benefit analyses in real time to decide the most economically efficient direction for managing various systems onboard, including energy and battery resources. Hence, this provides remarkable cost savings in fuel and maintenance for EMSs and BMSs in naval environments [58].

Adaptation and learning: These methods can learn from new incoming data and adapt their learning strategies over time. This ability enables EMSs and BMSs to become much more efficient and effective. Because of that, they collect more operational data over time from the naval environment, and they can continuously improve energy and battery management strategies in maritime operations [59].

In summary, applications based on AI, ML, and DL approaches in EMSs and BMSs have the potential to drastically advance efficiency, stability, and safety and leverage naval platforms and ships to operate more autonomously and robustly in challenging environments.

Studies [45,52,60,61,62,63,64,65] have shown that we can divide the AI and ML- and DL-based approaches for EMSs and BMSs into three main branches, shown below (Figure 6). Additionally, these approaches are especially utilized for accurate estimation of critical battery parameters such as state of energy (SoE), state of charge (SoC), and remaining useful life (RUL) for BMSs.

2.5.1. Rule-Based Approaches

These approaches focus on implementing predefined rules and logic to efficiently handle and optimize energy usage. These approaches are instrumental in naval environments with presumable, consistent, and explainable decisions. These approaches can be further divided into two main branches:

Fuzzy-Logic-Based Approaches

Fuzzy logic is a type of rule-based AI dealing with uncertain and imprecise states and conditions. This nature makes fuzzy logic particularly useful for handling complex systems like EMSs and BMSs in naval-based electric vehicles (EVs) and hybrid electric vehicles (HEVs). Unlike classical binary logic, which separates input samples into strict true or false categories, fuzzy-logic-based approaches provide varying degrees of truth. Hence, these approaches can manage the approximate information suitable for real-world scenarios, such as fluctuating energy levels or ambiguous system statuses [54,66,67,68,69].

Deterministic-Based Approaches

Deterministic-based approaches operate under a clear, predictable framework where specific sample inputs give rise to specific outputs based on predefined rules. These approaches are highly structured, providing precision and consistency for the decisions made, which is crucial for the reliability and safety of naval operations. Although they offer important advantages to simplicity and consistency, their process can be limited by their rigidity, and manual updates may be needed to adapt to new scenarios or changing conditions [70,71,72,73,74].

2.5.2. Optimization-Based Approaches

These approaches rely on constantly increasing energy usage distribution efficiency and productivity to satisfy specific operational purposes and constraints. These types of approaches deploy various algorithms to find the best solutions under given constraints for optimizing one or multiple objectives, such as minimizing energy consumption, maximizing operational range, or prolonging battery life. Dynamic programming (DP), particle swarm optimization (PSO), genetic algorithms (GAs), and simulated annealing methods can be given as examples of these types of approaches [72,74,75,76,77].

2.5.3. ML-Based Approaches

Machine learning (ML)-based approaches introduce dramatic advancements to EMSs and BMSs in naval-based electric vehicles (EVs) and hybrid electric vehicles (HEVs). By utilizing data-driven techniques, these approaches provide adaptive, predictive, and highly efficient energy and battery management solutions that can dynamically respond to changing conditions and complex operational demands on naval environments. These types of approaches can be further divided into three main branches:

Prediction-Based Approaches

These approaches are particularly deployed for constructing informed decisions about energy usage, maintenance schedules, and operational strategies based on predictive models. Generally, specific-sized historical data are given as input into these models, and consequently, the models are optimized to properly capture underlying patterns of the input samples collected from the specific naval environment and create a prediction based on learned patterns from the historical data. Specific methods in these types of approaches are decision trees, artificial neural networks (ANNs), support vector machines (SVMs), random forests, and ensemble-based ML [74,75,76,77,78,79,80,81,82,83,84,85,86,87,88]. Additionally, specially designed neural network (NN) structures such as convolutional neural networks (CNNs) [87] for handling spatial and temporal information and recurrent neural networks (RNNs) [89], gated recurrent units (GRUs) [90], and long short-term memory (LSTM) [91] are particularly utilized for handling multidimensional sequential data within BMSs and for leveraging battery state estimation, determining optimal charging and discharging plans, and proper error detection in naval/ship-based EV batteries depending on SoE, SoC, and RUL factors.

Learning-Based Approaches

Learning-based approaches, especially reinforcement learning (RL) and deep reinforcement learning (DRL) approaches, are highly potent for obtaining adaptive strategies in complex and dynamic environments similar to the EMS and BMS for naval-based environments. These approaches focus on training algorithms to make a sequence of decisions to maximize a cumulative reward over time. This makes these approaches well-suited for optimizing long-term operational objectives. Unlike other approaches, samples to be used as an input to the model are not obtained from a fixed-size observation but as constant feedback from the dynamic and changing environment itself (in our case, a naval environment). More specifically, for the model’s every choice of action from an action set, the environment provides the model with the current state for itself (observation) and reward value for the optimization process. Approaches such as deep Q-network (DQN), proximal policy optimization (PPO), deep deterministic policy gradient (DDPG), and double-delayed Q-learning (DDQL) are specific examples of these types of approaches [91,92,93,94,95,96,97].

Linear Programming Approaches

Recent advancements have integrated artificial intelligence (AI) and machine learning (ML) with linear programming (LP) to enhance the optimization capabilities of energy management systems (EMSs) and battery management systems (BMSs) in naval environments. AI algorithms are now employed to predict changing conditions, such as variations in weather, sea currents, and power demands, enabling these systems to dynamically adjust optimization models in real time. This integration leads to more adaptive and responsive energy distribution strategies, improving efficiency and performance under challenging maritime conditions [98]. In complex and dynamic naval power systems, ML models analyze historical data to fine-tune EMSs and BMSs, predicting optimal operational parameters for energy storage and consumption. This optimization reduces the need for manual calibration, enhancing system autonomy, and reducing reliance on constant human intervention [99]. Furthermore, AI-based heuristics are applied to solve large-scale, complex LP problems encountered in EMSs and BMSs, such as optimizing power distribution across multiple subsystems or managing battery charging cycles. These heuristics avoid the exhaustive search methods required in traditional LP techniques, improving efficiency for real-time decision making. This approach is particularly crucial in naval environments, where balancing mission-critical systems, fuel consumption, and environmental impact is essential. Integrating AI with LP in EMSs and BMSs overcomes the limitations of traditional optimization methods. These advanced techniques provide flexible and scalable solutions, optimizing energy use while meeting constraints related to non-linearity, battery performance, and operational requirements. In naval vessels, this results in improved energy management, enhanced mission readiness, extended battery life, and reduced environmental impact, contributing to more sustainable and efficient naval operations [100].

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.

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, 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. The active materials in both electrodes possess a crystalline structure that accommodates the lithium ions through an intercalation mechanism, allowing the ions to embed within the electrode materials during charging and release during discharging. During the charging phase, lithium ions decouple from the cathode (positive electrode) and migrate through the electrolyte to intercalate into the anode (negative electrode) [106,107,108,109]. Concurrently, electrons flow externally from the cathode to the anode to balance the charge. Conversely, lithium ions move from the anode back to the cathode during discharge, with electrons traveling through the external circuit in the same direction, thereby delivering power to the connected device. This bidirectional flow of ions and electrons, essential for the battery’s operation, is visually detailed in Figure 10.

Moreover, the reaction that takes place on the positive side during charging is given in the following Equation (1):

$$LiM{O}_2\to x.{Li}^{+}+x.e^{-}+{Li}_{1-x}M{O}_2$$

(1)

Equation (2) demonstrates that the reaction that takes place in the negative electrode during discharging:

$$Li{C}_6\to {Li}_{1-x}{C}_6+{x.Li}^{+}+x.e^{-}$$

(2)

The labels of Li-ion batteries in the market are created by using the chemical composition of the cathode active materials because of descriptive characteristics of the performance and behavior of the battery. Cathode active materials consist of metal oxides (mixed metal oxides) or phosphate (especially LiFePO₄) in Li-ion batteries. The most suitable cathode active materials for the shipping industry on the battery market are lithium nickel manganese cobalt oxide (LiNi_xMn_yCo_zO₂) (longer life cycle), lithium iron phosphate (reduced risk of thermal runaway), lithium nickel cobalt aluminum oxide (LiNi_xCo_yAl_zO₂), lithium cobalt oxide (LiCoO₂), and lithium manganese oxide (LiMn₂O₄) (higher charging rates and thermal stability) [27,110,111]. These cathode active materials have superior features in terms of energy density, power density, safety, stability, and price. Material that responds well to all requirements has not yet been developed. Whichever requirement is important for users, they should choose the battery that gives the best results in that specific feature.

On the other hand, graphite, hard carbon, graphene, etc. materials made of carbon sources are generally used as anode active materials. Besides these, LiTiO₂-based titanite compounds (LTO) are also anode active materials. This material is applicable for marine vessels that require high power or very large amounts of cycling.

3.1. Lithium Nickel Manganese Cobalt Oxide (LiNi_xMn_yCo_zO₂, NMC)

Lithium nickel manganese cobalt oxide (NMC), representing a significant advancement in cathode materials, has gained prominence over lithium cobalt oxide (LCO) in large-scale consumer electronics applications due to its superior performance characteristics [112,113]. NMC batteries exhibit a specific energy range of 150–220 Wh/kg, making them highly effective for demanding applications. These batteries are increasingly favored in electric vehicles and the maritime sector, attributed to their versatile power density, robust safety features, extended lifecycle, and ample energy density [27,114]. The unique composition of NMC allows for fine-tuning of its properties to meet specific application requirements, further enhancing its applicability across various high-energy demand settings. LiNi_xCo_yMn_1−x−yO₂ is synthesized by modifying the proportions of transition metals—increasing Ni and Mn while reducing Co in the conventional LCO cathode formulation. This adjustment in the NMC cathode material optimizes its characteristics: cobalt enhances current density, nickel boosts capacity, and manganese contributes to structural stability. Such a balanced attribute profile renders the NMC cathode material highly versatile and extensively utilized in Li-ion battery applications. Additionally, the energy density of NMC is supported by its stability at high cut-off voltages exceeding 4.5 V [115]. The stoichiometric variations of these elements, including common ratios like (1:1:1), (4:3:3), (5:3:2), (6:2:2), and (8:1:1), lead to divergences in energy and power properties, which are well-documented in the literature [111,114]. Furthermore, the configuration of these elemental ratios directly influences the battery’s cost, capacity, and thermal stability, making specific NMC composition a critical factor in battery design [111]. Thus, Li-ion batteries remain the dominant choice for marine applications due to their high energy density and design flexibility. Recent developments include high-nickel NMC formulations (e.g., NMC811), which offer enhanced capacity and improved performance at higher voltages—important for high-power maritime operations. Additionally, LTO-based chemistries are gaining attention in fast-charging, high-cycle ferry systems where safety and durability under frequent docking are critical.

3.2. Lithium Iron Phosphate (LiFePO₄, LFP)

LiFePO₄, with its unique phosphorus olivine structure, offers a cathode chemistry that is distinct from the layered metal oxides of NMC. This structural arrangement significantly reduces the risk of thermal runaway due to its inherent oxygen deficiency [111]. LiFePO₄ presents a discharge potential of 3.4 V and a theoretical capacity of 170 mAh.g⁻¹ [114,115]. The lower discharge voltage reduces specific energy value [111]. However, its key advantage lies in the stability of the cathode during charge–discharge cycles, effectively safeguarding the battery against thermal runaway. Additionally, LiFePO₄’s longer cycle life compared to LiCoO₂ is a testament to its durability despite its lower energy density [116].

3.3. Lithium Nickel Cobalt Aluminum (NCA)

An NCA battery is composed of approximately 80% nickel, 15% cobalt, and 5% aluminum. While its chemistry is similar to NMC’s, NCA distinguishes itself by substituting manganese with aluminum. This modification contributes to a specific energy range of 200–260 Wh/kg. The inclusion of aluminum not only elevates the energy density but also imparts additional stability to the cathode material. Despite these advantages, NCA cathodes face reduced safety profiles and increased production costs. Nonetheless, due to its high energy density, this material is considered suitable for maritime applications where energy efficiency is critical [27]. In maritime environments, where fire suppression and ventilation are more challenging than in land-based settings, these risks are magnified. As a result, NCA batteries require more robust battery management systems (BMSs), active cooling mechanisms, and thermal insulation to be used safely on ships. Despite these drawbacks, NCA cells are still considered for marine use when high energy density is a priority—especially in space-constrained vessels—but only when paired with appropriate safety measures and system-level engineering.

3.4. Lithium Cobalt Oxide (LiCoO₂, LCO)

First commercialized by Sony in 1991, LiCoO2 is known for its straightforward synthesis route and high specific energy, ranging from 150–240 Wh.g⁻¹ [45]. Despite these advantages, LiCoO₂ has drawbacks, including a shorter cycle life, toxicity, high cost, and the limitation of utilizing only about 50 percent of its theoretical capacity [111,116]. Furthermore, LiCoO₂ is prone to rapid capacity fading when subjected to high voltage values (>4.4 V) and high current densities. In response to these challenges, researchers have developed LiM_yCo_1−yO₂ (M: metals), a modified cathode formulation that incorporates various metals such as Ni, Al, Mg, Fe, Mn, Ti, Zn, Cr, B, Rh, and other transition metals. This synthesis aims to create cathode materials that are cost-effective, high-energy, longer-lasting, and safer [110]. When evaluated for marine vessels, although it has high energy density (it is widely used in older consumer electronics), it is appropriate for some maritime applications because of its short life cycle, limited power rates, and safety concerns [27,107].

3.5. Lithium Manganese Oxide (LiMn₂O₄, LMO)

LMO is recognized for its high power capacities, rapid charging capabilities, and thermal stability. It is a viable option for applications requiring quick energy delivery and robustness under thermal stress. Despite these benefits, LMO exhibits some limitations, including a lower energy capacity than LiCoO₂ batteries and a reduced lifespan when operated at elevated temperatures. These attributes lead to a relatively shorter cycle life, reducing its suitability for specific marine applications that require long-term durability and higher energy densities to be essential [27].

3.6. Lithium Titanate Oxide (LTO)

LTO, when used as an anode active material in Li-ion cells, significantly enhances both the power output and cycle life of the battery. These attributes are not just beneficial but crucial in maritime applications. However, it is important to note that LTO is characterized by a relatively low specific energy, typically ranging from 50–80 Wh.g⁻¹, attributed to its low cell voltage. This lower energy density necessitates using more batteries to fulfill maritime vessels’ energy requirements, consequently leading to increased battery sizing and elevated costs [27]. Li-ion cells typically have an open circuit voltage ranging from 3.2 to 3.9 V. To meet specific voltage and current requirements, these cells are combined to form modules, which are assembled in series and/or parallel configurations to create complete battery packs. The battery management system (BMS) and the thermal management system (TMS) are crucial components of maritime battery systems. The BMS plays a key role in linking the battery cells with the power management system (PMS) to ensure optimal performance. It carefully monitors and controls each cell’s electrical, mechanical, and thermal parameters, managing charge and discharge activities to maximize the efficiency and longevity of the battery pack [110,117,118].

3.7. Future Battery Technologies for Marine Vessels

Solid-state batteries, comprising a solid-state electrolyte along with an anode and cathode, utilize materials similar to those found in Li-ion batteries, such as NMC and carbon/graphite. Unlike Li-ion batteries, which employ a liquid electrolyte, solid-state batteries feature a solid electrolyte that significantly enhances safety by eliminating the flammability risks associated with liquid electrolytes. These batteries also boast a long cycle life and a high energy density, with specific energies ranging from 200 to 400 Wh.g⁻¹. Due to these characteristics, solid-state batteries are increasingly regarded as an up-and-coming technology for marine applications, offering substantial safety and energy density benefits. However, challenges such as electrolyte conductivity and structural integrity at the electrode interfaces remain to be fully addressed. Overcoming these issues could dramatically improve the performance and extend the operational capabilities of all-electric vessels [27]. Li-air batteries are also emerging as a potentially viable energy storage system for marine vessels. They boast a theoretical specific energy of up to 3500 Wh.g⁻¹, although practical achievements are capped at approximately 950 Wh.g⁻¹ to date. Like fuel cells, these batteries encounter several challenges, such as sluggish oxygen reactions that hinder performance. Additionally, they depend on advanced high-power battery technologies for operational support, which is critical for optimizing their functionality and reliability in marine applications [119]. Flow batteries, such as vanadium redox systems, are well-suited for large-scale energy storage due to their scalability, long cycle life, and ability to maintain performance over extended periods. While their size and complexity may limit their use aboard smaller vessels, they are increasingly being explored for offshore platforms, shore-side energy storage, and port-based charging infrastructure.

Table 2. Batteries regarding various aspects [110,120].

Battery Type	Cost (USD/kWh)	Energy Density (Wh.g−1)	C-Rate	Degradation Rate	Cycle Life	Safety	Applications
Lithium-Ion (Li-Ion)	150–300	150–250	1–5 C	Moderate	500–1500	Moderate (thermal runaway risk)	Hybrid and fully electric propulsion systems
Lithium Iron Phosphate (LiFePO4)	100–200	90–120	1–3 C	Low (better stability)	1500–3000	High (stable, safer)	Electric vehicles, grid storage, maritime
Lead–Acid	100–150	30–50	0.1–0.5 C	High (rapid capacity loss)	300–500	Low (can be hazardous)	Emergency backup, auxiliary power systems
Nickel–Cadmium (Ni-Cd)	200–300	40–60	1–3 C	Moderate to high	1000–1500	Moderate (toxic cadmium)	Marine applications, aerospace
Nickel–Metal Hydride (NiMH)	250–500	60–120	0.5–2 C	Moderate	500–1000	Moderate (less toxic than Ni-Cd)	Hybrid vehicles, medical equipment
Solid-State	500–1000	250–500	1–5 C	Very low (due to solid electrolyte)	3000–5000	High (no flammability)	Emerging tech, future maritime applications
Flow Batteries	200–400	20–40	0.2–1 C	Low	5000+	High (liquid-based, scalable)	Large-scale energy storage, offshore platform
Hydrogen Storage	400–600	33000 (by weight)	-	-	3000–5000	High (flammable)	Long-range cargo, ferries, backup systems
Fuel Cells (PEMFC)	1000–5000	800–1500 (system level)	-	-	5000–10,000	High (but controlled)	Passenger ships, cruise ships, auxiliary power

provides a comparison of various battery types across several key aspects. To better assist in evaluating the applicability of energy storage technologies for maritime vessels, Table 2 presents a quantitative comparison of lithium-ion batteries, hydrogen storage, and fuel cells. The table includes key metrics such as energy density, cost per kWh, cycle life, safety, and typical marine applications. This matrix is intended to guide selection based on vessel type, operational range, and performance requirements.

Solid-state battery technology, though still in early stages of commercialization, has drawn considerable interest in maritime sectors due to its enhanced safety and energy density. Prototypes using solid-state electrolytes such as sulfide-based or ceramic-based conductors have shown promise in reducing flammability risks—particularly important in enclosed shipboard environments. Current research focuses on improving interfacial stability and scaling production for larger battery modules. These advancements suggest solid-state cells may soon transition from concept to deployment, especially in naval and passenger cruise applications where safety margins are non-negotiable.

4. Maritime Battery Projects

Battery technologies’ use in automotive transportation as a source of energy has paved the way for their applicability to the marine industry. Battery technology research and development have generally taken place in the consumer electronics and automotive sectors due to dominance in the market. The maritime industry has evaluated and integrated these developed technologies into a marine environment [27].

Battery system design must be performed before being implemented on a marine vessel. All batteries can be used for all types of ships; some of them are more appropriate than others. The suitability depends on the weight, volume, and costs of batteries. Furthermore, the characteristics required by vehicles should not be overlooked when evaluating their suitability. Maritime battery systems are categorized as depicted in Figure 11, based on their energy density and maximum C-rates for discharging, which shows how quickly they discharge. Thus, the properties of the battery system may vary depending on the operating conditions of marine vessels [119].

Table 3. The required features of battery systems as applications [114].

Products/Vessels	Character
Mobile phone	High energy, short life
Nissan Leaf	High energy, medium life
Tesla Model S100d	High energy, medium life
Ampere—the first modern electric car ferry	Medium power, long life
Aurora and Tycho Brahe—the world’s largest electric vessels	Medium power, long life

demonstrates the change in a broad picture of the variation of expectations in the battery systems according to the requirements of the vehicles (customer devices, automotive, or ships). While vessels and cars need batteries having high energy and medium life, Ampere as the first modern electric car ferry and Aurora and Tycho Brahe, the world’s largest electric vessels, were designed with medium-power and long-life batteries.

4.1. Major Market Players of Energy Storage Systems for Maritime Industry

The large-scale use of batteries in hybrid and all-electric marine vessels requires professionalism on points of overall vessel performance, reliability, and safety. There are some key players, such as Corvus Energy, Leclanche, and EST-Floattech, leading providers of high-quality energy storage solutions for the maritime industry in Europe. These companies have extensive amounts of experience in the field of marine vessels’ storage systems. Corvus Energy has pioneered with more than 750 projects across almost every maritime segment [121,122]. Likewise, EST-Floattech has also supplied more than 200 projects [123]. All mentioned companies produce a scalable and modular rack-type battery module [108,123,124]. The rack-type battery module system requires battery room for installation. The rack system consists of Li-ion battery cells and a safe battery management system for a longer system lifetime and high performance. Specifically, Leclanche shares that its cells used in the marine industry are manufactured from high-energy G/NMC and high-power LTO for battery systems. These materials have the advantage of providing high energy and long cycle life. In addition, all companies produce modules available at different heights to enable batteries to fit all battery room sizes according to the vessel type [108,123,125].

4.2. Battery Projects of Hybrid and All-Electrical Vessels in the World

Lithium-ion (Li-ion) batteries are the most widely used energy storage systems in maritime applications, primarily due to their high energy density, long cycle life, and relatively fast charging capabilities. These characteristics make them particularly well-suited for both hybrid and fully electric propulsion systems. Their compact size and performance efficiency contribute to their dominance in the industry. Lithium iron phosphate (LiFePO₄) batteries, a subtype of lithium-ion technology, have gained popularity for their exceptional safety, thermal stability, and long service life. While they offer slightly lower energy density than other lithium-ion chemistries, their resistance to overheating and thermal runaway makes them a preferred choice for applications where safety is paramount, such as passenger vessels or ferries. Lead–acid batteries, though considered a more traditional technology, continue to be used in auxiliary power systems and emergency backup applications. Despite being heavier and less energy-dense compared to lithium-based alternatives, their lower initial cost and proven reliability make them a viable option in certain contexts, particularly where cost-effectiveness is a key consideration. Nickel-based batteries, including nickel–cadmium (NiCd) and nickel–metal hydride (NiMH), are utilized in specific maritime applications that demand durability and performance under extreme environmental conditions. Their ability to function efficiently across a wide range of temperatures, combined with a robust cycle life, makes them suitable for specialized vessels and equipment operating in harsh climates.

Hybrid marine vessels offer a more environmentally friendly and energy-efficient transportation option by using various types of batteries.

Table 4. Some Hybrid Marine Vessels in the World [45,109,125].

Name	Country	Battery (kWh)	Battery Type	Year	Passengers	Cars	Operational Profile	Speed (Knots)	Length (m)
M/S Sjovagen	Sweden	500	Li-ion	2014	150	-	Short-range commuter routes	8.5	24.5
Movitz	Sweden	120	NiMH	2014	100	-	Quiet, low-speed passenger service	9	22.8
MV Ampere	Norway	1000	Li-ion	2015	360	-	Regular fixed route, medium range	10	79
Aditya	India	50	Li-ion	2016	75	-	Short distances, low power demand	7.5	21
MF Tycho	Denmark	4100	Li-ion	2017	1250	240	Medium to high load, moderate range	14.5	111
Zhongtiandianyum 001	China	2400	Li-ion	2017	-	-	Port-to-port logistics in coastal waters	7	70
Ellen	Denmark	4300	Li-ion	2019	200	-	Long-distance route for a ferry	21	60
MF Moldefjord	Norway	1582	Li-ion	2020	390	125	Frequent docking, reliable scheduling	-	122.67
MF Hella	Norway	1582	Li-ion	2020	300	80	Short- to mid-range ferry services	-	84.2
NZK Pont 104	Netherlands	678	Li-ion	2021	400	20 cars, 4 trucks	High turnaround, frequent stops	-	41
Randsfjordferja Elrond	Norway	678	Li-ion	2021	48–65	16	Intermittent usage, short routes	-	33.7
OP Stroom	Belgium	396	Li-ion	2022	150	75 Bikes	Urban mobility for mixed traffic	-
Norddalsfjord	Norway	1808	Li-ion	2023	248	80	Medium-load trips with regular intervals	-	84

indicates the variety and features of hybrid marine vessels available in various countries. The table allows us to compare the battery types, production years, passenger capacities, vehicle carrying capabilities, speeds, and sizes of these vehicles. The differences in speed and size in the table show that these vehicles depend on the purposes and routes for which they are used. Ships used for tourist tours generally have slow speeds and large sizes, indicating the capacity to carry more passengers, while faster and smaller ships are especially suitable for short-distance voyages. Thus, it reflects the efforts of various countries to develop and use these environmentally friendly vessels. This can be seen as a step towards the evolution of maritime transport towards a sustainable future.

Table 5. Some Fully Electrical Marine Vessels in the World [45,109,126].

Name	Country	Battery (kWh)	Battery Types	Year	Passengers	Cars	Speed (Knots)	Length (m)
M/S Sjovagen	Sweden	500	Li-ion	2014	150	-	8.5	24.5
Movitz	Sweden	120	NiMH	2014	100	-	9	22.8
MV Ampere	Norway	1000	Li-ion	2015	360	-	10	79
Aditya	India	50	Li-ion	2016	75	-	7.5	21
MF Tycho	Denmark	4100	Li-ion	2017	1250	240	14.5	111
Zhongtiandianyum 001	China	2400	Li-ion	2017	-	-	7	70
Ellen	Denmark	4300	Li-ion	2019	200	-	21	60
MF Moldefjord	Norway	1582	Li-ion	2020	390	125	-	122.67
MF Hella	Norway	1582	Li-ion	2020	300	80	-	84.2
NZK Pont 104	Netherlands	678	Li-ion	2021	400	20 cars, 4 trucks	-	41
Randsfjordferja Elrond	Norway	678	Li-ion	2021	48–65	16	-	33.7
OP Stroom	Belgium	396	Li-ion	2022	150	75 bikes	-
Norddalsfjord	Norway	1808	Li-ion	2023	248	80	-	84

presents the battery specifications of various electric ferries. The types and capacities of batteries used on ferries vary significantly. For example, large ferries such as MV Ampere and Ellen have Li-ion batteries ranging from 1000 kWh to 4300 kWh. On the other hand, Movitz is a smaller ferry and uses 120 kWh nickel–metal hydride (NiMH) batteries. Conversely, Aditya has a Li-ion battery of only 50 kWh, indicating a smaller capacity. These battery characteristics provide important information about how far each ferry can travel and how many passengers or vehicles it can carry.

5. Grid System of Marine Vehicles

The implementation of medium-voltage direct current (MVDC) systems is increasingly pertinent in ship power systems due to the presence of pulsed and other dynamic electrical loads. A robust management system is essential to regulate the MVDC bus voltage, ensuring stable operation amidst the sporadic introduction of impact loads. An advanced intelligent coordination algorithm has been proposed to mitigate the effects of these pulsed loads and optimize power distribution among storage units. The effectiveness and accuracy of these algorithms have been rigorously tested and validated within a MATLAB/Simulink environment [26,47,48,49,55,56,57,58]. Unlike traditional AC systems, which are often bulky and inflexible, DC systems alleviate synchronization issues and eliminate the need to manage reactive power. This makes DC systems particularly suitable for isolated power systems and specialized applications such as shipboard microgrids. Given these advantages, the MVDC system is regarded as the optimal solution for fulfilling the complex demands of onboard power systems. Consequently, addressing the control challenges associated with MVDC is crucial, especially considering the variety of power sources and the presence of transient shock loads. Control techniques must be tailored to these diverse conditions to ensure the system’s effective and safe operation [44,50,51,52,53,54,55,56]. Current literature highlights that gas-based generators in hybrid ships often fall short of delivering the peak currents required by the network. There is a critical need to enhance naval platforms’ flexibility, efficiency, and performance. Research findings suggest several promising advancements facilitated by medium-voltage direct current (MVDC) systems. The University of Texas has developed a megawatt-level MVDC/high-frequency alternating current (HFAC) power system testbed. This system can operate at clock speeds up to 40 MHz on an FPGA board, enabling real-time visualization of power system dynamics. Additionally, a real-time digital simulation module is employed for conducting hardware-in-the-loop (HIL) testing, further augmenting the development and validation of advanced naval power systems [56,57].

6. Conclusions

In the future, hybrid and electric vessels are expected to play a pivotal role in transforming the maritime industry by significantly reducing greenhouse gas emissions (GHGs) during operation. These vessels will produce little to no emissions, helping to mitigate air pollution and contribute to the achievement of global climate goals [119]. Electric propulsion systems are anticipated to be quieter than traditional diesel engines, drastically reducing noise pollution and minimizing the harmful impact on marine ecosystems, which will be crucial for the preservation of marine life. Additionally, electricity is expected to become even greener, with a growing share of renewable sources in the grid, while also being more affordable and stable in price compared to fossil fuels, offering substantial long-term savings for vessel operators.

The design of electric and hybrid systems will continue to evolve, with fewer moving parts and greater efficiency and power, resulting in lower maintenance costs and reduced downtime. Hybrid vessels will become increasingly effective in optimizing energy use by combining internal combustion engines with electric propulsion systems, enhancing fuel efficiency and minimizing energy waste. The future will likely see the widespread adoption of regenerative technologies, such as energy capture during braking or deceleration, which will further boost the efficiency of these vessels.

As global emissions regulations, like the IMO’s Carbon Intensity Indicator, become more stringent, hybrid and electric vessels will provide a practical and sustainable solution for compliance, helping to avoid penalties and fostering a greener maritime sector. This transition will drive continued innovation in battery technology and energy storage systems and the integration of renewable energy sources, benefiting not only the maritime industry but also sectors like transportation and energy [1].

Companies and governments that adopt green technologies will enhance their reputation and meet the growing consumer demand for sustainable practices, positioning themselves as leaders in the green transition. Furthermore, electric vessels powered by onboard renewable energy sources such as solar, wind, or hydrogen will reduce reliance on fossil fuels, contributing to greater energy security and promoting the widespread adoption of renewable energy across industries. The future of hybrid and electric vessels holds immense promise, with the potential to create a cleaner, more sustainable, and more efficient maritime industry while contributing to global environmental goals [126].

Future ships are expected to become increasingly robotic and autonomous, marking a significant shift in the maritime industry. With advancements in artificial intelligence, machine learning, and sensor technologies, ships will be able to navigate without human intervention, responding dynamically to changing environmental conditions and obstacles in real time. These autonomous vessels will use advanced algorithms to optimize routes, reduce fuel consumption, and enhance safety by minimizing human error. Robotics will also play a major role, with automated systems handling tasks such as cargo loading, maintenance, and repairs, further reducing the need for onboard crews [127]. The maritime industry is experiencing a significant transformation driven by innovative electric and hybrid marine vessels utilizing cutting-edge technologies. Electric vessels rely solely on electricity, and hybrid vessels, which cleverly combine electric and internal combustion systems, showcase the industry’s dedication to sustainable and efficient solutions. Electric ships ensure emissions-free operations, cost savings, and the potential for seamless integration with renewable energy sources. Meanwhile, hybrid models offer operational flexibility, increased fuel efficiency, and reduced emissions. With the advancement of technologies, strict regulatory compliance, and the development of charging infrastructure, these vessels are set to revolutionize maritime transportation, heralding a new era of environmentally conscious and high-performance fleet operations.

As the maritime industry continues its transition toward sustainability, future research should focus on several promising directions. Emerging storage technologies such as solid-state batteries, lithium–sulfur systems, and hybrid lithium–supercapacitor solutions offer the potential for higher energy density, improved safety, and lower cost. Advancing hybrid energy architectures that combine batteries with renewable sources and auxiliary generators can optimize power flexibility and operational efficiency. Additionally, integrating AI-driven energy management systems, modular power electronics, and standardized interfaces will be key to enabling scalable, interoperable solutions across vessel types. Exploring these directions will be essential to meeting long-range decarbonization goals and aligning with IMO emissions targets.

This review provides a comprehensive overview of energy storage technologies for hybrid and fully electric marine vessels, with a particular focus on lithium-ion batteries and their role in decarbonizing maritime transport. We have categorized and compared various battery chemistries, analyzed their performance in real-world vessel applications, and outlined the operational implications of integrating energy storage systems onboard ships. Our analysis highlights several critical research gaps that remain unaddressed:

• Thermal management and safety mechanisms for high-energy-density chemistries (e.g., NCA, NMC) in enclosed marine environments.
• Durability and degradation modeling of batteries under marine-specific duty cycles (e.g., frequent docking, vibration, salinity exposure).
• Standardization of battery integration and modularity for ship retrofitting and multivessel fleets.
• Hybrid architectures that combine batteries with alternative fuels or renewable inputs for longer voyages.

Future research should also explore the application of solid-state batteries, AI-enhanced energy management systems, and circular battery design for second-life and recycling strategies. Addressing these issues will be key to scaling battery-powered propulsion systems and achieving compliance with global emission targets.

In conclusion, the increasing reliance on battery systems in hybrid and all-electric marine vessels emphasizes the paramount importance of professionalism, with a particular focus on reliability and safety. Leading the industry, companies such as Corvus Energy, Leclanché, and EST-Floattech are recognized as top providers of high-quality energy storage solutions within the European maritime sector. Their extensive expertise in developing advanced storage systems for maritime applications demonstrates a solid commitment to enhancing the field, reflecting both innovation and rigorous standards in energy management. These companies provide scalable and modular rack-type battery modules expertly engineered to accommodate various battery room dimensions across various types of vessels. Each module is composed of advanced Li-ion battery cells integrated with a sophisticated battery management system (BMS). This system is crucial for prolonging the battery’s lifetime and ensuring peak performance under maritime operating conditions. The design’s modularity facilitates customization to specific ship requirements and enhances ease of maintenance and scalability in maritime energy solutions. Future research in battery solutions for energy storage, particularly within the maritime industry, should focus on addressing key challenges such as enhancing the energy density of Li-ion cells to accommodate the increasing demand for storage capacity without significantly increasing the size and weight of the modules. Additionally, improving the longevity and robustness of battery management systems (BMSs) in extreme environmental conditions, such as high humidity and fluctuating temperatures, will be crucial. Long-term energy management solutions must also consider the degradation mechanisms of batteries over time, with a focus on optimizing the charging and discharging cycles to extend battery life. Moreover, integrating advanced predictive algorithms for real-time monitoring and maintenance, coupled with efficient recycling methods for end-of-life batteries, will play a vital role in creating sustainable, long-term maritime energy solutions.

Ultimately, the selection of Li-ion battery compositions is determined by specific operational requirements and priorities, as each material presents a distinct profile of benefits and limitations. In the maritime industry, critical factors such as energy density, safety, and longevity are paramount in deciding the optimal battery technology for different types of vessels. These considerations ensure that the chosen battery technology not only meets the energy demands of the vessel but also adheres to rigorous safety standards and delivers a sustainable lifecycle performance, aligning with the industry’s operational and environmental objectives.