Green Technologies for Environmental Air and Water Impact Reduction in Ships: A Systematic Literature Review

Abstract

This study reviews various green technological strategies integrated into vessels to mitigate environmental impact, focusing on atmospheric pollution and marine environment protection. The research is based on a systematic review of academic literature published between 2019 and 2024, using the Scopus and Web of Science databases and applying PRISMA criteria. The findings reveal that the main environmental issues in the naval sector include greenhouse gas emissions, harmful discharges, and invasive species that affect marine biodiversity. The analysis is framed within international regulations such as those established by the IMO and classification societies, where the most relevant indicators identified are the EEDI and EEXI. However, the results of this review emphasize that, while these regulations are fundamental, it is necessary to analyze further the technical and economic barriers affecting the widespread implementation of these technologies and develop incentive mechanisms that facilitate their adoption across different vessel types and sizes. Promising solutions include alternative fuels, new propulsion systems, and emission-reduction technologies. The conclusion underlines that although the sector is transitioning toward sustainability, economic and widespread implementation challenges remain.

1. Introduction

Adopting green technologies is now critical to protecting oceans for future generations. The naval sector faces mounting pressure to reduce its environmental impact and ensure operational sustainability, driven by international regulations targeting the adverse effects of maritime activities. In 2023, the Marine Environment Protection Committee (MEPC 80) of the International Maritime Organization (IMO) approved a strategy to cut greenhouse gas (GHG) emissions, aiming for net-zero emissions by 2050. This strategy also emphasizes the adoption of alternative fuels with zero or near-zero GHG emissions by 2030 [1]; additionally, it introduces indicative checkpoints for progress in 2030 and 2040. A key example of such efforts is the MARPOL Convention, established by the IMO in 1973, which laid out rules and protocols to minimize marine pollution, from accidental substance spills to emissions. The 2015 Paris Agreement, as well, has provided a global framework that integrates all economic sectors, including the naval industry, in a collective effort to limit global temperature rise [2,3]. Pursuing sustainability and competitiveness has become imperative in this context and permeates all spheres of human activity. Consequently, public and private sectors and civil society must align their operations and strategies with the stated goals, fostering a transition towards more responsible and resilient development models.

Studies on green technologies in the naval industry highlight advancements and challenges in mitigating environmental impacts. Gu et al. [4] show how global sulfur emission limits and SECAs have led shipping companies to adopt sulfur scrubbers and fuel-switching technologies. Coppola et al. [5] analyze fuel cell systems powered by hydrogen, emphasizing hydrogen’s potential for decarbonization, with Agarwala [6] noting that its viability as a zero-emission fuel depends on advances in renewable production methods and economic incentives; both studies demonstrate the promising role of hydrogen in supporting the shipping industry’s transition to alternative fuels while addressing key challenges related to cost and feasibility.

Wan et al. [7] stress the significant role of global ports in reducing greenhouse gas emissions. At the same time, Kanchiralla [8] evaluates strategies like e-fuels, biofuels, battery-electric propulsion, and carbon capture technologies for decarbonization. Du et al. [9] highlight the effective use of green technologies in fleet deployment to reduce emissions and operating costs simultaneously. Sun et al. [10] identify transformative opportunities such as integrated hydrogen systems and hybrid power solutions combining ammonia and hydrogen for greener maritime fuels. Finally, Hu et al. [11] explore the role of Compressed Air Energy Storage (CAES) as a pivotal technology in advancing large-scale renewable energy, highlighting its eco-friendly nature, durability, and significant storage capacity.

Technology must undergo incremental change to reduce the naval industry’s environmental footprint and achieve the environmental objectives set by the Paris Agreement and the IMO regulations above.

In addition to analyzing the environmental impacts of the maritime industry, another focus of research in green technologies literature is the existing regulations by organizations such as the IMO, classification societies, etc. These set the standards required for vessels to make them safer, more reliable, and sustainable. Authors Prensarman et al. [12] complement this by analyzing emissions in non-ECA areas, including strategies like adopting low-sulfur fuels and cleaner technologies. Sheng et al. [13] focus on determining the optimal vessel speed and fleet size to comply with ECA regulations using a mixed-integer convex cost-minimization model. Psaraftis et al. [14] evaluate short-term measures and challenges to meet carbon emission reduction targets for 2030 and 2050, highlighting the need for energy-saving technologies and alternative fuels and Market-Based Measures (MBMs) to incentivize development and reduce emissions.

Conducting a comprehensive literature mapping on the aforementioned topics is essential to enable result comparisons and ensure the subject remains current. In this regard, prior reviews provide valuable compilations that have addressed various aspects of maritime sustainability through significantly fragmented approaches, revealing a complexity that extends beyond individual sectoral analyses, such as the flow of ship materials throughout their useful life through LCA [15], while Tan et al. [16] focused on greenhouse gas emission conversion strategies through bioelectrocatalysis, recognizing operational limitations due to sensitive parameters like environmental considerations. Zhang et al. [17] explored digital port transformation towards greener models and highlighted insufficient integration of economic, social, and environmental assessments.

The primary limitation of these investigations is their inability to establish significant connections between different components of the maritime ecosystem. In this regard, Zhou et al. [18] delved into the impact of micro-nano plastics in natural ecosystems; in that line, Baley et al. [19] comprehensively examined polymeric compounds and alternative materials in the marine industry, emphasizing the need for more sustainable practices and pointing out the insufficiency of regulations that promote lower-impact alternatives. Similarly, Ning et al. [20] reviewed decarbonization strategies with a technical approach, identifying emerging technologies to reduce CO₂ emissions and store carbon in oceanic subsoil but acknowledging the limited infrastructure for their massive deployment.

The complexity is further evidenced in works such as Pei et al. [21], which highlights the trend towards intelligent and ecological vessels in countries like Japan, Korea, and China, or Qi et al. [22], which evaluated the environmental impacts of maritime transport in the Arctic while pointing out the scarcity of quantitative research on direct and synergistic impacts. Pinar et al. [23] addressed the bilge water challenge, exposing its resistant composition and threat to coastal ecosystems and highlighting the limitations of conventional treatment technologies. Hojnik et al. [24] have analyzed transnational projects focused on environmental quality and eco-innovation, developing an analytical framework to evaluate EU programs such as Interreg, Horizon, and Life. Paipa et al. [25], in turn, explored eco-innovation as a conceptual approach in the manufacturing sector’s transition towards sustainable practices; their study also underscored the need for a greater focus on specific ecological technologies, materials, and manufacturing processes for naval construction. Ma et al. [26] offer a comprehensive perspective, examining the evolution of maritime vessel design that identifies global research patterns, highlighting contributions from countries like China, the USA, and South Korea in sustainable technologies, safety, and structural integrity. Fofack et al. [27] also addressed France’s strategy for energy transition towards marine renewable energies, focusing on floating wind energy. While Xiao et al. [28] explored the accelerated growth of research on digital technologies for decarbonization in the maritime sector, specifically regarding autonomous ships, the research underscored the need for refinement in route planning, energy efficiency, communication systems, and ecological design.

Our review addresses existing gaps by employing synergistic approaches, analyzing indicators, and assessing environmental impacts. It aims to establish connections between various technological strategies, evaluate recent advances, and provide a comprehensive perspective beyond partial interpretations. Evaluating the performance of green technologies across different ship systems is crucial, as these innovative solutions reduce human activities’ environmental and societal impacts while promoting sustainability.

By analyzing their efficiency in propulsion, waste treatment, and emission reduction systems, this research offers critical insights into their benefits and areas for improvement [29], enhancing decision-making for meeting environmental objectives while ensuring economic viability. The study explores green technologies in the naval industry and assesses their compliance with international environmental standards and indicators. Ultimately, it seeks to understand their role in advancing sustainability in alignment with global regulations and frameworks, guiding the investigation through carefully formulated research questions.

Q1:

What are the main environmental challenges for adopting green technologies in the maritime sector?

Q2:

What key regulations drive the implementation of green technologies in the naval industry?

Q3:

What are the most relevant environmental indicators used in the naval industry to measure the impact of green technologies?

Q4:

What strategies does the naval industry implement to comply with leading environmental indicators, and which have proven most effective?

Answering these questions is crucial for comprehensively evaluating green technologies in the maritime sector. Each question addresses a different aspect of adopting and implementing these technologies, providing a holistic view of their impact and effectiveness. By identifying the main challenges, understanding key regulations, recognizing relevant environmental indicators, and evaluating effective compliance strategies, we can assess maritime sector green technologies through regulatory frameworks and performance metrics.

The solution to these research questions is based on a literature review. The document is structured as follows: Section 2 describes the methodology used, detailing the criteria for selecting sources, the search strategy, and the analysis process. Section 3 presents the results obtained, summarizing the key findings from the reviewed literature. Section 4 discusses the results, analyzes the findings, and highlights study limitations. Finally, Section 5 presents conclusions and suggestions for future work, offering recommendations based on the review and identifying potential areas for future research. This structured approach ensures a thorough and systematic evaluation of the research questions, contributing to advancing knowledge in the field.

2. Methodology

This research employs the PRISMA methodology, an internationally recognized framework for conducting systematic reviews and meta-analyses to ensure transparency, accuracy, and comprehensiveness in presenting literature review studies [30]. This methodology was chosen for its structured approach, which enables rigorous evaluation of existing literature, essential for ensuring complete and reliable results.

2.1. Selection Criteria

The following selection criteria were adopted to ensure the relevance, timeliness, and quality of the sources used in this study (See

Table 1. Document Selection Criteria.

Criterion	Description
Time Range	Documents published between 2019 and 2024 to ensure updated information aligned with recent trends and advancements.
Academic Nature and Accessibility	Academic documents were selected, including peer-reviewed scientific articles and literature reviews, recognized scientific proceedings, and official documents issued by regulatory organizations such as the IMO and other entities in the sector.
Thematic Relevance	Documents directly related to the research topic, specifically green technologies applied to vessels.
Language	Documents are available in Spanish or English.
Quality and Reputation of Sources	Consideration is given to the impact factor and reputation of sources, prioritizing internationally recognized journals and publishers in the field.

). These criteria were carefully chosen to ensure that the documents included were relevant and contributed significantly to the research objectives:

After searching the central databases, documents from 2019 to 2024 were processed using the RStudio (version 2023.12.0) tool to organize the data into a database and remove duplicates. Subsequently, the bibliographic information was extracted in Excel format using the Bibliometrix extension. The specific timeframe of 2019–2024 was selected because it represents the most recent five-year period at the time of the search (March 2024), ensuring that the literature review captures the latest developments and modern technologies in the field. This recency is particularly crucial in the rapidly evolving domain of green maritime technologies, where innovations and regulatory changes occur frequently. The March 2024 search date represents a deliberate effort to include the most up-to-date research available.

However, it is important to note that this literature review must be continuously updated to maintain its relevance as new studies, technological breakthroughs, and regulatory frameworks emerge.

With this data matrix, the following inclusion criteria were applied:

• By Title and Abstract: A first evaluation was carried out by reading the titles and abstracts of the articles to exclude those that did not explicitly mention the topic of interest, in this case, green technologies applied to vessels.
• By Full Reading: In this stage, the documents were thoroughly reviewed to assess their relevance and alignment with the research objectives. Although some articles mentioned the keywords specified in

  Table 2. Keywords used for the search.

  Group	Keywords
  Group 1	eco-innovation, ecoinnovation, green innovation, environmental innovations, green innovations, ecological innovations, ecological innovation
  Group 2	ecodesign, eco-design, green design, sustainable design, ecoproduct, ecoproducts, environmental technology, green technology, green technology innovation, eco-friendly product, sustainable products, sustainable technology, eco-product, eco-products
  Group 3	naval industry, shipbuilding, shipbuilding industry, shipyard, shipyards, ships, ship, boats, boat, vessels, vessel, naval vessels, fluvial, maritime, watercraft, small craft, yacht, sailing, pushboat, barge, ferry, riverboat

  , not all met the performance and quality evaluation criteria for the technologies and, thus, were excluded.

As Table 2 shows, Group 1 of keywords was selected to encompass innovative technologies within an ecological context. Therefore, terms such as “eco-innovation” and its variants were included.

Group 2 refers to the type of technology sought and its characteristics. In this case, the focus is on technologies designed and developed with environmental impact in mind. Consequently, keywords such as “green technology”, “eco-design”, etc., were incorporated.

Finally, Group 3 consists of terms related to the sector or product where these technologies are intended to be applied. As a result, words such as “ships”, “sailboats”, and “shipyards” were included.

2.2. Databases

A bibliographic search was conducted using various databases, following a strategy designed to identify the most relevant studies. For this purpose, a systematic computerized search was carried out using 2 data sources, as shown in

Table 3. Data sources.

Data Source	URL
Scopus	www.scopus.com	27 March 2024
Web of science	www.clarivate.com	27 March 2024

: SCOPUS and Web of Science. This search led to journals such as Elsevier, MDPI, and IEEE. The search strategy was developed to locate the relevant literature, adhering to the PRISMA guidelines in all process stages.

This systematic review was registered in the Open Science Framework (OSF), and the protocol is publicly available at the following link: (https://osf.io/zktp7, accessed on 7 April 2025). No amendments were made to the protocol during the development of the review.

2.3. Search Strategy

In this phase, the search terms, the operators to be used, and their application were defined to generate a search equation that would yield technical results. The equation was constructed considering three groups of keywords:

• Group 1: Terms related to eco-innovation or green innovation.
• Group 2: Words associated with eco-products, eco-design, and green technologies.
• Group 3: Terms describing the naval sector, including vessels and shipbuilding industry.

The three groups were combined into one equation, ensuring that the results included elements from all three (1, 2, and 3) and guaranteeing that the findings focused on eco-innovations or eco-products within the naval industry context (see

Table 4. Search Algorithms.

Digital Library	Group	Algorithm Search
Scopus	1 o 2 y 3	(TITLE-ABS-KEY (“eco-innovation” OR “ecoinnovation” OR “Green Innovation” OR “Environmental Innovations” OR “Green Innovations” OR “Ecological Innovations” OR “Ecological Innovation”) OR TITLE-ABS-KEY (“Ecodesign” OR “Eco-design” OR “Green Design” OR “ecoproduct” OR “ecoproducts” OR “Environmental Technology” OR “Green Technology” OR “Green Technology Innovation” OR “eco-friendly product” OR “Sustainable Products” OR “Sustainable Technology”) AND TITLE-ABS-KEY (“naval industry” OR “Shipbuilding” OR “Shipbuilding Industry” OR “shipyard” OR “Ships” OR “Ship” OR “Boats” OR “boat” OR “Vessels” OR “Vessel” OR “Naval Vessels” OR “fluvial” OR “Maritime” OR “Watercraft” OR “Small craft” OR “yatch” OR “sailing” OR “Pushboat” OR “barge” OR “ferry” OR “riverboat”)) AND PUBYEAR > 2018 AND PUBYEAR < 2025
Web of science	1 o 2 y 3	“eco-innovation”OR”ecoinnovation”OR”Green Innovation”OR”Environmental Innovations”OR”Green Innovations”OR”Ecological Innovations”OR “Ecological Innovation” OR “Ecodesign”OR”Eco-design”OR”Green Design”OR”ecoproduct”OR”ecoproducts”OR”Environmental Technology”OR”Green Technology” OR ”Green Technology Innovation”OR”eco-friendly product”OR”Sustainable Products”OR”Sustainable Technology” (All Fields) and “naval industry” OR”Shipbuilding”OR”Shipbuilding Industry”OR”shipyard”OR”Ships”OR”Ship”OR”Boats”OR”boat”OR”Vessels”OR”Vessel”OR”Naval Vessels” OR “fluvial” OR “Maritime” OR “Watercraft” OR “Small craft” OR “yatch” OR “sailing” OR “Pushboat” OR “barge” OR “ferry” OR “riverboat” (All Fields) and 2019 or 2020 or 2021 or 2022 or 2023 or 2024 (Publication Years)

: Search Algorithms).

2.4. Selection Process

Document search was conducted using keywords detailed in Table 2 and the search algorithm defined in Table 4. Files from both databases were consolidated using RStudio. Relevant bibliographic information was extracted into Excel with columns for title, abstract, authors, document type, DOI, and publication date. Two exclusion filters were applied: first based on abstracts, then after a full-text review.

Document types were filtered to literature reviews, chosen initially for their comprehensive overview of multiple articles, enabling efficient research landscape assessment. Complete documents were reviewed and filtered by publication year, and final documents were selected after applying inclusion criteria. The annual distribution of selected documents is shown in Figure 1.

Selected documents were initially classified by technological approach and associated impact to identify predominant issues in the sample. Regulations and agreements mentioned in the documents were then extracted. An analysis was conducted on how regulations influence alternative technology adoption, considering energy efficiency, environmental impact, and economic viability. Finally, identified technologies were organized into strategic categories based on application areas and common approaches.

3. Results

Figure 2 serves as a visual representation of the bibliographic selection process according to the PRISMA methodology. The initial search in the Scopus and Web of Science databases, using the predefined search strings and applying the corresponding filters, yielded 822 references. After eliminating 60 duplicates, an initial set of 762 unique documents was obtained. Subsequently, a screening based on the analysis of titles, abstracts, and keywords was carried out, resulting in the selection of 295 potentially relevant documents. The eligibility assessment, which involved a partial reading of the texts, identified 85 documents that fully met the inclusion criteria established for this study. This final corpus constitutes the documentary base upon which the present analysis is founded.

3.1. What Are the Main Environmental Challenges for Adopting Green Technologies in the Maritime Sector?

The analysis of various statistics includes data from years prior to 2019 that reveals significant trends and developments in the emission of polluting gases and the contamination of the marine environment, encompassing both habitat and species. Figure 3 illustrates the primary environmental challenges discussed, highlighting air, water, and underwater acoustic pollution within the maritime sector. Generally, the available information predominantly covers air and water pollution due to their considerable impact on marine ecosystems and public health. Even so, there is a growing concern about underwater acoustic pollution, despite limited data availability. It is estimated that ship-generated noise emissions globally are doubling every 11.5 years at the current rate, where the COVID-19 pandemic temporarily interrupted this growing trend. However, noise emissions are expected to increase again as the global economy recovers [32]. Underwater acoustic pollution represents another challenge within the sustainable transition of the maritime industry, with navigation being the most pervasive and continuous source [22].

3.1.1. Emission of Polluting Gases

The contemporary naval industry has transformed into a significant source of pollution, generating emissions and waste that considerably impact marine ecosystems and air quality. Regarding maritime-sourced air pollution in the EU, EEA [33] shows that SOx emissions have decreased by approximately 70% since 2014, largely due to the introduction of SOx Emission Control Areas (SECA) in Northern Europe. The Mediterranean SECA, which will come into effect on 1 May 2025, is expected to replicate this positive effect in this region. Additionally, countries in the Northeast Atlantic are also studying the possibility of establishing an ECA potentially by 2027 [33]. On the other hand, Nitrogen Oxide (NOx) emissions have increased significantly in the period 2015–2023, averaging 10% across the EU. The increase in maritime traffic in the Arctic, driven by sea ice loss and resource extraction, has intensified emissions of nitrogen oxides (NOx) and sulfur dioxide (SO₂). Between 2013 and 2023, NOx emissions rose by 115% and SO₂ emissions by 68%, alongside a 61% increase in the distance traveled by vessels along the Northern Sea Route [34].

In response to these environmental challenges, the IMO has established specific industry targets, including a reduction in carbon intensity of emissions to zero by 2050 [35,36]. The limited implementation of energy efficiency technologies on ships results in greater consumption of fossil fuels and an increase in polluting emissions. It is estimated that ship energy consumption and CO₂ emissions could be reduced by up to 75% by applying operational measures and implementing existing technologies [37]. Annex VI of MARPOL establishes mandatory technical and operational energy efficiency measures to reduce greenhouse gas emissions from ships [3].

3.1.2. Pollution from Oil Spills, Contaminated Water, and Chemical Substances

NASEM [38] reports that during the 2010s, oil spills accounted for between 0.3% and 4.4% of the total annual hydrocarbon inputs into North America’s marine environment, depending on whether the Deepwater Horizon spill is considered. In the 1990s, approximately 4.1% of these inputs came from oil spills (NRC 2003). While this reflects a reduction in spill contributions over time, oil spills continue to cause severe localized environmental and socioeconomic harm.

Etkin [39] further observed that the frequency of oil spills across all oil industry sectors has decreased over the last five decades, and reductions in spillage rates per amount of oil produced and transported indicate steady progress. However, despite the marine environment’s adaptation to chronic inputs such as natural seeps and land-based runoff, the acute impacts of oil spills underscore the necessity for continuous and intensified efforts to eliminate them and protect marine ecosystems due to the fact that crude oil contains Volatile Organic Compounds (VOCs), which are organic substances that evaporate during loading. Approximately 0.08–0.15% of the crude oil is released as VOCs, equating to 50–200 tons per loading; it also has compounds with nitrogen, sulfur, oxygen, and metals [40], which represent severe impacts on marine organisms and ecosystems [22]. Despite the measures initially created, such as OILPOIL, these emissions significantly impact marine ecosystems. The IMO has created regulations to control and limit the discharge of hydrocarbons and harmful chemicals from ships, as these discharges seriously affect seawater quality and threaten aquatic life. Standards and limits have also been established to regulate these emissions, such as MARPOL [3]. Like chemicals, ballast water from ships can represent a risk factor for marine ecosystems, as it can transport invasive species and microorganisms that are then released into foreign environments, causing an imbalance in the underwater fauna. Ballast water also contains elevated levels of heavy metals like mercury, lead, and cadmium, posing significant risks to marine ecosystems [41]. These heavy metals can bioaccumulate in marine organisms, entering the food chain and negatively impacting biodiversity and ecosystem functioning [42].

The naval industry contributes to generating marine litter and plastic waste from vessels, which represents a serious environmental problem for marine ecosystems. Waste, such as microplastics (particles less than 5 mm), comes from daily-use products and the degradation of larger products. These can cause physical damage to organisms and act as pollutants and pathogens [43]. Growing concern exists about the concentration of dredged sediments from estuaries, ports, bays, and coastal areas. Additionally, wastewater from vessels, both grey and black water and oily water, must be treated to prevent ecosystem damage.

According to Harsha et al. [44], ballast water contains hydrocarbons, specifically benzene, toluene, ethylbenzene, and xylene (BTEX), as well as emerging contaminants like dissolved hydrocarbon oxidation products (HOPs). Studies have also detected pollutants such as copper, arsenic, semi-volatile and volatile organic compounds, and nonylphenol and octylphenol ethoxylates. In addition to operational discharges, the materials used in the hull, such as antifouling paints, can also contain microplastics, tributyltin, and copper, affecting marine fauna [22].

3.2. What Key Regulations Drive the Implementation of Green Technologies in the Naval Industry?

The current landscape of the maritime sector is marked by growing regulatory pressure. Regulatory pressure plays a significant role in driving the shipping sector to adopt environmentally friendly practices and comply with international standards. This pressure encourages firms to focus on minimizing the environmental impact of their operations [45]. Far from being a mere obstacle, this regulatory environment has become a catalyst for the sector’s transformation.

Shipping companies face the challenge and opportunity of integrating advanced green technologies into their operations. This adoption allows them to comply with increasingly strict environmental regulations and opens the door to significant improvements in efficiency and cost savings [46]. Paradoxically, what could initially be perceived as a regulatory burden is driving companies to explore innovative solutions that, in many cases, prove beneficial to their bottom line. Technological solutions must be made aware of whether the high initial costs will return in profit [7].

Governments, for their part, face the challenge of maintaining regulatory frameworks that evolve at the pace of technological advances. This requires a more agile and proactive approach to policymaking. Incorporating emerging technologies into the regulatory process is gaining ground, allowing for more accurate risk assessments and better-informed decisions. In this context, the role of regulators goes beyond mere surveillance and sanction. They are becoming facilitators of innovation, working to remove barriers that may hinder the sector’s digital transformation. This new dynamic between regulators and the regulated creates a more collaborative ecosystem where sustainability and efficiency become shared goals. However, more than technical and operational measures are required to meet these objectives. The price differences between implementing some alternatives can lead owners and shipowners to choose based on economic availability. Therefore, it is necessary to combine the influence of the market on adopting these technologies [47]. For this, there are market-based measures (MBM) that serve as a cornerstone for regulating emissions and enhancing energy efficiency, playing a key role in the strategies of both the International Maritime Organization (IMO) and the European Commission (EC) [48]. In other words, the approach must be dynamic between cost and effectiveness. The need for this holistic framework is reflected in the studies aimed at evaluating the profitability of emissions; the one presented by Salman [49] identifies key differences in efficiency measures when adjusting for environmental impacts versus purely economic factors. It emphasizes that container shipping excels in economic efficiency, while bulk shipping demonstrates higher cargo efficiency.

In short, the current regulatory framework is redefining the competitive landscape of the maritime sector. Companies that manage to align their technological strategies with these regulatory imperatives will not only ensure legal compliance but will also position themselves at the forefront of a sector undergoing a green revolution.

3.3. What Are the Most Relevant Environmental Indicators Used in the Naval Industry to Measure the Impact of Green Technologies?

In order to evaluate the environmental performance of vessels, various indices have been developed to quantify it. Some of these indices, such as the EEDI, are designed for new vessels, while others, such as the EEXI, are applied to older vessels. There are also specific indicators for operation, like the EEOI. These indicators complement the broader objectives proposed by organizations such as the IMO and the UN and are part of the ongoing effort to be aware of environmental impact and mitigate it.

3.3.1. Impact on Air

In the current context of maritime transport decarbonization and the reduction of polluting emissions, it is essential to consider the objectives and indicators established by international organizations. In July 2023, the International Maritime Organization (IMO) adopted a more ambitious revised strategy, setting the goal of achieving net-zero greenhouse gas emissions by around 2050, replacing the previous target of a 50% reduction compared to 2008 levels. To monitor progress toward this goal, indicators such as the Carbon Intensity Indicator (CII) have been introduced, which consider operational parameters such as hull cleaning to reduce the ship’s hydrodynamic resistance and, consequently, lower fuel consumption. The optimization of speed and routes plays a fundamental role in the search for the most efficient balance between travel time and energy consumption. Likewise, measures such as installing low-consumption light bulbs and incorporating auxiliary solar or wind power systems for onboard accommodation services contribute to reducing the electrical demand on board [37,50]. Additionally, the IMO has pointed out that using alternative fuels, such as hydrogen, could reduce greenhouse gas (GHG) emissions by up to 91.4% compared to diesel. On the other hand, the United Nations Sustainable Development Goals include the reduction of total GHG emissions (SDG 13) and the intensity of CO₂ emissions from manufacturing industries relative to value added (SDG 9), which applies to the shipping and shipbuilding sectors [51]. These goals and indicators represent coordinated global efforts to mitigate the environmental impact of maritime activity and promote more sustainable practices.

There are various initiatives and regulations aimed at promoting the implementation of innovative technologies and establishing energy efficiency parameters. In this sense, the following indicators and objectives stand out: First, the percentages of efficiency in the implementation of technologies such as Compressed Air Energy Storage (CAES) are evaluated, a promising solution for large-scale energy storage where the IMO has been one of the drivers of this technology in the maritime sector [52]. In addition, the IMO has established the Energy Efficiency Index for Existing Ships (EEXI), which sets a minimum level of energy efficiency for ships in service and a limit on CO₂ emissions per unit of transport supply to reduce greenhouse gas emissions from maritime transport [50], as well as the Energy Efficiency Design Index (EEDI), which consists of a monitoring tool to measure the possible impact of management changes. The latter was also found in the development and application of a new methodology applied to ships with hybrid propulsion systems; this is a monitoring and regulation tool aimed at new ships, unlike the EEXI [53], although both are design indices. Regarding operational indices, there is the Energy Efficiency Operational Indicator (EEOI), which is quantified through the mass of CO₂ per unit of transport work [54]. On the other hand, within the framework of the United Nations Sustainable Development Goals (SDGs), the reduction of energy intensity, measured in Terajoules per billion dollars in 2005, has been set as a target as a key indicator to assess progress in energy efficiency at the global level, as well as a maximum concentration of 0.5% sulfur content in marine fuels [51].

3.3.2. Impact on Water

It is important to establish strict limits and controls to mitigate the negative impacts of marine pollution, wastewater, and hydrocarbon discharges. The following is a detailed analysis of the regulations and objectives established around these issues. The International Maritime Organization has established various international agreements aimed at protecting the marine environment and preventing pollution caused by dredging activities and the disposal of dredged sediments at sea. These agreements include the Barcelona Convention for the Protection of the Mediterranean Sea against Pollution, the Protocol of the London Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter, the Helsinki Convention on the Protection of the Marine Environment of the Baltic Sea, and the Oslo-Paris Convention for the Protection of the Marine Environment of the North-East Atlantic [55]. Conventions such as the International Convention on the Control of Harmful Anti-Fouling Systems on Ships, adopted on 5 October 2001 and in force since 17 September 2008, regulate and manage the use of anti-fouling systems on ships to reduce environmental harm, specifically addressing the impact of harmful substances like tributyltin (TBT) in marine ecosystems [56]. To manage wastewater, it is important to ensure the quality of water bodies and protect aquatic ecosystems. Conventions such as the BWM for ballast water and its management establish that all ships in international traffic must manage their ballast water and sediments according to a specific standard and plan specific to each ship [42].

Finally, accidents and chemical spills into water bodies are also considered. The IMO has established average concentration limits of hydrocarbons discharged from ship bilges to minimize marine pollution from oil and derivative spills [3].

3.3.3. Technologies Associated with Indicators and Regulatory Instruments

Following the analysis of the main environmental impacts on air (Section 3.3.1) and water (Section 3.3.2), specific technologies were identified to mitigate these effects and comply with established international standards. These technologies have been classified according to their functional category and linked to the environmental indicators they aim to influence or the regulations they are designed to meet.

Table 5. Applicable technologies for each regulation.

Category	Applicable Technologies	Regulation/Standard	References
Emission Reduction	Lubricating Hull Coatings	EEXI, EEDI	[57,58,59]
	Propeller and Hull Shape Optimization	EEXI, EEDI	[20,60,61,62,63]
Clean Energy Technologies	Liquefied Natural Gas (LNG)	EEXI, EEDI, EEOI	[20,57,59,64,65]
	Liquefied Petroleum Gas (LPG)	EEXI, EEDI, EEOI	[20,66,67]
	Biofuels	EEXI, EEDI, EEOI	[50,57,59,68,69,70,71]
	Hydrogen	EEXI, EEDI, EEOI	[50,57,72,73,74,75]
	Nuclear Energy	EEXI, EEDI, EEOI	[50,59,72,76]
Propulsion System Optimization	Water Injection	EEXI, EEDI	[77,78]
	Conversion Systems	EEXI, EEDI	[16,69,78,79,80,81]
	Propulsion System Optimization	EEXI, EEDI	[50,77,82,83]
	Heat Recovery Systems	EEXI, EEDI	[84,85,86,87,88,89,90]
Assisted Propulsion	Wind-Assisted Propulsion	EEXI, EED	[50,69,91,92,93]
Electric and Storage Systems	Electric Propulsion Systems with Fuel Cells	EEXI, EEDI, EEOI	[59,72,87,90,91,94,95,96,97]
	Hybrid Electric Propulsion Systems	EEXI, EEDI, EEOI	[5,98,99]
	Renewable Electric Propulsion	EEXI, EEDI, EEOI	[100]
	Electric Propulsion (IC Engine + Batteries)	EEXI, EEDI, EEOI	[95,101]
Solar Power and Energy Storage	Solar Panels	EEXI, EEDI	[91,102,103]
	Lithium-Ion Batteries	EEXI, EEDI	[95,98,104,105]
	Energy Storage and Supply Systems	EEXI, EEDI	[18,20,87,98,106,107,108,109,110,111]
Pollution Control and Environmental Protection	Anti-fouling Surfaces	International Convention on the Control of Harmful Anti-fouling Systems on Ships	[112,113,114]
	Hull Materials	International Convention on the Control of Harmful Anti-fouling Systems on Ships	[113,115,116,117]
Water Management	Ballast Water Systems	International Convention for the Control and Management of Ships’ Ballast Water and Sediments	[118]
Water Management and Pollution	Oily Water Separation Method	MARPOL Annex I	[119,120,121]
	Greywater Separation Methods	MARPOL Annex IV	[42,77,112]

presents this classification, providing an integrated view of how each group of technologies relates to international regulatory frameworks such as EEXI, EEDI, EEOI, MARPOL, or the Ballast Water and Anti-fouling Systems Conventions.

During the search, information was also found regarding monitoring and control systems, such as big data-based technologies for maritime traffic analysis, real-time systems for environmental management, and web-based models for environmental data storage, which reflect an innovative character and a commitment to environmental impact reduction. However, their impact is not measurable through the indices identified, and thus they were excluded from Table 5.

3.4. What Strategies Does the Naval Industry Implement to Comply with Leading Environmental?

Figure 4 provides a comprehensive overview of technologies and strategies to reduce the environmental impact of maritime vessels, focusing on two main areas: Air Impact and Water Impact. The Air Impact section (green) covers various approaches to reducing emissions and improving efficiency. It includes alternative fuels such as biofuels and hydrogen, engine emission reduction technologies, and propulsion systems like wind-assisted and electric propulsion for enhanced energy efficiency. The diagram also highlights monitoring and control systems to optimize operational efficiency and reduce emissions, as well as energy acquisition and storage solutions for sustainable operations.

The Water Impact section (blue) addresses technologies designed to minimize the negative effects of ships on marine ecosystems, including ballast water systems to prevent the spread of invasive species by treating ballast water before discharge, hull optimization techniques to reduce water resistance and enhance fuel efficiency, and anti-fouling paints and materials to prevent the growth of marine organisms on ship hulls, thereby reducing drag and improving fuel efficiency.

3.4.1. Technologies Impacting Air

Considering the emission reduction targets proposed by the IMO, the maritime industry has developed strategies to create increasingly cleaner and more environmentally friendly vessels: the engineering applied in manufacturing processes has brought strategies such as the optimization of the hull-propeller structure, the use of alternative fuels with a lower environmental footprint, the implementation of systems that optimize conventional engines, and the application of alternative propulsion systems that make use of clean resources such as wind energy.

Hull Optimization and Resistance Reduction

Hull optimization and resistance reduction are fundamental aspects of improving the hydrodynamic efficiency of ships [122]. These advanced naval engineering technologies focus on modifying the hull geometry and implementing innovations that minimize friction with water, resulting in more efficient vessel displacement.

Hull optimization involves refining its shape through computational modeling techniques and computational fluid dynamics (CFD) analysis, which allow the design of more efficient hydrodynamic profiles. This can include modifying the bow, stern, and waterlines to reduce wave formation and turbulence. Coatings, usually made of different material, represent a film around the hull that allows the vessel to be efficiently displaced.

• Hull lubricating coatings: The resistance to a ship’s advance is mainly composed of hydrodynamic resistance in the water and aerodynamic resistance in the air. Advanced hull coatings play a crucial role in optimizing both aspects. As mentioned by McCarney [57], in the aquatic environment, these coatings improve the hydrodynamics of the ship, reducing the friction between the hull and the water, which allows for more efficient displacement. This application has been recognized by entities such as the Italian Navy, which, in its emissions reduction initiative, has implemented release coatings to lubricate the hull [58]. In parallel, in the air environment, the potential of “biomimetic air-retaining surfaces” that act as slipping agents on the hulls, based on the Salvinia Effect, providing a reduction of up to 20% in aerodynamic resistance, is highlighted [59]. This holistic approach to coating design demonstrates how a single technology can simultaneously address resistance to advance in the aquatic and aerial environments, contributing significantly to the vessel’s overall efficiency.
• Propeller and hull optimization: These strategies range from innovative hull-cleaning coatings to intelligent control systems and the optimization of key components, showcasing integrated approaches within the maritime industry. As mentioned in the work by Ning et al. [20], propeller optimization plays a fundamental role in improving vessel efficiency through enhanced design. For instance, Nasso et al. [60] presented a comprehensive design process for a fully electric wooden passenger boat. They also highlighted hull form design as a critical aspect to ensure efficient navigation in shallow-water lagoons. On the other hand, Tu H. et al. [61] established a method for predicting optimal trim using machine learning models, demonstrating its ability to rapidly identify the best trim settings for any container ship under varying operational conditions. Likewise, Ghadimi et al. [62] conducted a study focused on the implementation of hydrofoils to improve the hydrodynamic efficiency of trimarans, optimizing total resistance through an automated design model based on Computational Fluid Dynamics (CFD) and the Particle Swarm Optimization (PSO) algorithm. In addition to hydrofoils, research has been carried out on how spray rails enhance the hydrodynamic performance of high-speed boats. Results showed that these structural elements efficiently reduce viscous resistance by deflecting the water flow and minimizing the wetted surface area in the stagnation zone where water meets the hull [63].

Alternative Fuels

Fuels are the energy source in engines due to the chemical reaction from combustion. The heat and gas emissions from these will depend on their composition. Those traditionally used due to their ease of access and development, such as fossil fuels, oil derivatives, natural gas, and coal, are considered conventional. However, these sources are finite and often have negative environmental impacts. For this reason, it is necessary to consider using non-conventional or alternative fuels that come from cleaner and renewable sources, such as biofuels, hydrogen, ammonia, etc. Each alternative presents advantages in emission reduction but also faces challenges related to costs, supply infrastructure, on-board storage, and necessary modifications to the propulsion system [69].

Alternatives with lower greenhouse gas emissions have been evaluated in the last five years.

• LNG: Among the range of alternative fuels to petroleum-derived fossil fuels, liquefied natural gas (LNG) emerges as one of the main alternatives [57,64]. An increasing number of countries view LNG as an opportunity to diversify their energy imports [65]. LNG offers 20 to 30 percent reductions in CO₂ emissions, minimizes SOx emissions and other pollutants, and achieves a 75 to 90 percent reduction in NOx compared to heavy fuel oil [59]. However, despite the advantages presented, liquefied natural gas has disadvantages, such as methane emissions, which are also considered GHGs, and much space is required for storage [20].
• LPG: On the other hand, Yeo et al. [66] argue that liquefied petroleum gas (LPG) is among the replacement options as a fuel with greater ecological efficiency and a lower price than other marine fuels. LPG has advantages in transportation, storage, and implementation costs and lower CO emissions [20], but it requires additional stability measures, and its application in medium and small engines has not been studied [66]. A recent study reinforces these claims, demonstrating through simulation that LPG engines in small fishing vessels emit significantly less CO₂, CO, and soot than gasoline and diesel engines. However, they produce more NO than gasoline engines [67]. In addition, the economic feasibility analysis of the same study indicates that the conversion to LPG can reduce significant fuel costs in the life cycle of the vessels, with savings that vary depending on the type of vessel and the substitute fuel.
• Biofuels: Biofuels, whether first generation obtained from plants, second generation produced using non-food waste, or third generation derived from algae [50], can significantly contribute to the reduction of NOx, SOx, and greenhouse gas emissions [57,68]. It has been estimated that all types of biofuels could comply with upcoming maritime emissions regulations, offering equivalent or even lower life-cycle emissions compared to heavy fuel oil [59]. Examples include biogas and biomethane [69,70]. Biodiesel production from microalgae has also been evaluated, demonstrating significant advantages in terms of effectiveness and sustainability compared to other biodiesel sources. Biomethane, in particular, is produced through the anaerobic digestion of biomass and stands out due to its renewable nature and economic feasibility [71]. This reflects the great potential of these as alternative energy sources. Similarly, it was found that hydrogen could be produced through methods such as dark fermentation, photofermentation, biophotolysis, and microbial electrolysis cells, giving rise to a variant known as biohydrogen [71].
• Hydrogen: Hydrogen is another alternative that plays a vital role in decarbonization [57,72]. It is considered less sustainable than biohydrogen because its production mostly comes from fossil fuels. However, like its predecessor, its application in fuel cells and lower emissions during its use is its advantage. It is useful in adapted engines producing water vapor as an emission or through an electrochemical reaction in fuel cells generating electricity [50]. Addressing economic barriers is crucial for adoption, which requires holistic policies across different sectors and industries [73], such as hydrogen-methanol propulsion systems with pre-combustion carbon capture on board that could significantly reduce emissions compared to conventional technologies [74]. Within this framework, recent research has focused on implementing hydrogen technologies in the maritime sector, particularly on fishing vessels. The study addresses risk-based inspections (RBI) and maintenance planning to reduce safety-related uncertainties and optimize associated operations [75].
• Nuclear energy: On the other hand, Balcombe et al. [59] mention that nuclear energy produces very low greenhouse gas and air pollutant emissions at competitive prices. It is an attractive option for its stable value and high power density, obtained by heating steam with an on-board nuclear reactor [50]. Likewise, according to Kistner et al. [72], synthetic fuels emerge as a viable and promising alternative to decarbonize maritime transport, requiring a careful analysis of the associated technologies, economic aspects, and the necessary infrastructure for their successful implementation. As proof of its effectiveness, Kotrandenko et al. [76] point out that nuclear energy has been used successfully for more than 60 years in Arctic icebreakers, an Arctic container ship, and many naval vessels without any known significant accidents. More than 15% of existing icebreakers designed to operate in the Arctic and Antarctic are nuclear-powered.

Emission Reduction Technologies in Engines

The main reason why polluting gases such as carbon monoxide, hydrocarbons, and sulfur oxides are generated is incomplete or poor combustion in internal combustion engines [77]. This is why the alternatives to reduce emissions in conventional engines aim to optimize the process of these through systems that capture and treat the released contaminants. Emission reduction technologies can be classified into primary methods related to engine modifications and secondary methods or retrofits. Primary methods include advanced turbocharging strategies combined with Miller timing, combustion adjustments, water injection methods, use of alternative fuels, and exhaust gas recirculation (EGR) [78].

• Water injection: Water injection reduces peak temperatures and NOx formation while optimizing parameters such as fuel injection, air/fuel ratio, and chamber geometry, decreasing emissions [78]. The optimization of the electronically controlled high-pressure common rail fuel injection technology improves fuel control and accumulation, improving efficiency in the combustion process and reducing the release of harmful substances [77].
• Conversion systems: Regarding secondary methods, Lion et al. [78] describe selective catalytic reduction (SCR) systems with or without plasma burners to improve their efficiency [79] that convert NOx into nitrogen and water through catalysts and wet treatment systems that wash the exhaust gases with seawater to remove SOx, particles, and NOx. Onboard capture is an attractive solution for short-term maritime decarbonization while carbon-free alternative fuels are being developed. Tan and Nielsen [16], Farrukh et al. [69], and Negri et al. [80] highlight the potential of bioelectrocatalysis, which employs microorganisms or enzymes as biological catalysts to fix and transform CO₂ into fuels and chemicals, with advantages such as lower environmental impact and higher selectivity and stability. However, due to the large existing fleet still using sulfur fuels, ref. [81] proposes strategies such as the installation of scrubbers (SI) to clean the exhaust gases of these.
• Propulsion system optimization: Tang et al. [83] mention the optimization of hybrid propulsion systems using artificial intelligence techniques, an efficiency factor, and a correction function to maintain the energy balance of the battery, achieving fuel savings of up to 80%. Issa et al. [50] mention speed reduction, hull design optimization, air lubrication, propeller/rudder improvements, and waste heat recovery in the field of operational measures. Gritsenko et al. [82] experimentally verified that deactivating cylinders in diesel engines reduces consumption when the load is low. Xiaoliang et al. [77] describe optimizing the diesel combustion engine and improving high-pressure boost technology to reduce energy consumption. However, it is important to continue improving turbocharging systems with methods such as exhaust bypass and variable cross-section. The author also describes a new gas combustion technology that uses the pre-combustion of homogeneous materials, improving efficiency and reducing nitrogen oxides. High-pressure combustion and compression technologies in diesel engines ensure controlled combustion.
• Heat recovery systems: For their part, Li et al. [84], Ouyang et al. [85], and Theotokatos et al. [86] study the residual heat recovery systems for ship engines, which improve thermal efficiency and are recommended to be installed in medium and large vessels optimized for their current operating profile, where fuel cells combined with residual heat recovery are explored [84,87]. In turn, a device has been developed for small boats that takes advantage of the residual heat from the exhaust gases of diesel engines through the vortex tube effect. This innovative technology allows for energy-efficient seawater desalination with a compact design, and it is also applicable to medium-sized vessels; likewise, turbo-generator technologies equipped with bidirectional pulsed turbines have been evaluated [88]. The authors examine this technology in the context of marine thermoacoustic waste heat recovery systems (WHRSs) [89]. In addition to capturing the gases, certain technologies seek to recirculate them through two configurations: (a) Recirculation of the boiling gas from a cryogenic separation unit. Moreover, (b) Recirculation of the gas separated by a semipermeable membrane between the MCFCs and the cryogenic separation unit [90].

Alternative Propulsion Systems

In the search for more sustainable propulsion and power generation options, proposals for alternative propulsion systems that avoid using fossil fuels in their process have been evaluated. Many of these alternative propulsion systems are at different stages of development and adoption. Their implementation depends on factors such as the type of vessel, navigation route, environmental regulations, and economic viability.

• Wind-assisted propulsion: Wind-assisted propulsion through rotors, kites, suction wings, Flettner rotors, and rigid sails [69] also represents a viable option that could reduce CO₂ emissions by up to 20% [50,91]. The research was also found on a self-adaptive sail based on real-time adjustment of the angle of attack and sail attitude to track wind direction during navigation [92]. It is important to note that these technologies are primarily developed as auxiliary systems to reduce fuel consumption and not as a total replacement [93], possibly due to wind not always being constant throughout navigation. The study made by [93] proposes an agent-based simulation model to analyze the adoption of three specific wind-assisted propulsion technologies (WPT): Wingsail (rigid sail), Ventifoil, and a third unspecified option. This model considers two stages in the adoption process, technological awareness and utility-based decision, and uses real data from vessels and WPT options to evaluate different policy interventions that could incentivize the adoption of these technologies in the maritime retrofitting industry.
• Electric propulsion systems with fuel cells: Another technology under development is fuel cells, which are electrochemical devices that directly convert the chemical energy of a fuel, usually hydrogen, into electrical energy. There are different types of fuel cells: proton exchange membranes (PEMFC), which operate at low temperatures, have high power density, and are suitable for portable and small-scale applications; high-temperature PEMFCs, which operate up to 200 °C and are less sensitive to impurities; molten carbonate (MCFC), which operate at high temperatures, can use various fuels, and are efficient but slow to start; and solid oxide (SOFC), which operate between 500 and 1000 °C, are flexible in terms of fuel and highly efficient but also slow to start [94]. They function through a reaction between hydrogen and oxygen, producing electricity and water as the only byproduct [95]. These systems offer benefits such as low noise emission, high efficiency, and lower environmental impact, making them suitable as a main source in maritime applications. In this regard, hybrid energy systems are proposed that combine solar panels, wind rotors, fuel cells, batteries, and diesel generators for large vessels [91]. Specifically for hydrogen cells, excellent environmental performance is evident, with 45–72% reductions in global warming potential compared to conventional technologies, facilitating the energy transition in maritime transport [96]. Using renewable hydrogen, greenhouse gas emissions are reduced by 91.4% compared to equivalent diesel vessels, demonstrating their viability for coastal research vessels [97]. According to Balcombe et al. [59], fuel cells produce low-carbon electricity, generating little noise and vibration compared to internal combustion engines. Specifically, solid oxide fuel cell (SOFC) technology is economically viable for maritime applications despite higher capital and maintenance costs due to its high energy efficiency and the use of LNG [72,90]. Projects with designs for comprehensive waste heat utilization systems in ships propelled by hydrogen fuel cells are highlighted, which improve energy efficiency, making this option even more appealing [87].
• Hybrid electric propulsion systems: In the field of naval electric propulsion, various studies have been conducted. Hayton [98] proposes implementing electric propulsion systems independently or in conjunction with internal combustion engines to take advantage of renewable sources and reduce fossil fuel consumption. Additionally, research has been conducted on the challenges these systems face due to load changes in adverse maritime conditions. The application of energy storage systems, such as batteries and supercapacitors, is explored to mitigate transients caused by these load changes, keeping electrical parameters within acceptable limits. These storage systems in intelligent hybrid power marine energy management could reduce operating costs, meet emission standards, and improve performance [5]. Also, Gelver et al. [99] propose an electric propulsion system for ships with a particular focus on a configuration that uses common DC buses, which is presented as an improvement over existing systems.
• Renewable energy electric propulsion system: However, sometimes the sources of this energy, such as solar panels, may not be as efficient during nighttime hours. This leads to considering more complete options such as hybrid systems with solar panels and fuel cells during the night [100].
• Electric propulsion system with energy generation from internal combustion engines and batteries: On the other hand, Candelo et al. [95] analyze a more complex system that combines generation by internal combustion engines and batteries. In the latter, the electric motor obtains energy from thermoelectric generators and a battery bank, also incorporating internal combustion generators, power converters, and DC and AC distribution bars, representing a more comprehensive approach to electric propulsion of vessels. Entirely electric ships also lead to considering and linking aspects beyond their propulsion, such as real-time digital twin applications and Power Hardware-in-the-Loop (PHIL) platforms in developing and validating advanced power electronics systems [101].

3.4.2. Monitoring Systems

• Monitoring and Control Systems: Studies have found that present technologies for assessing maritime traffic density are based on big data from vessel trajectories. This technology uses data from the Automatic Identification System (AIS), which is preprocessed to reflect actual trajectories, as explained by Dai et al. [123]. Other authors, such as Yang [124], have proposed the design of an energy parameter monitoring system that can monitor these parameters in real-time. These systems can also be applied to marine environmental monitoring, such as the one proposed by Hu et al. [125], which uses multiple data sources to provide real-time information and early warnings to support decision-making in marine environment management and protection [126]. In [127] is highlighted the use of “advanced control technologies, such as Linear Matrix Inequalities (LMI) and Model Predictive Control (MPC) paradigms, to control the movements of an autonomous ship at different stages of its journey”, tested in full-scale experiments. Regarding environmental monitoring, studies have advanced in a web development model based on the MVC (Model-View-Controller) software architecture, which facilitates data storage and visualization applied to tanks [125]. Additionally, research on electric propulsion for ships highlights the use of permanent magnet synchronous motors (PMSM) and proportional-integral (PI) scheme controllers to optimize the speed and torque of the propeller, demonstrating the effectiveness of these technologies in simulations [127]. The implementation of unmanned systems has also gained relevance, such as the case of the teleoperation of remotely operated vehicles, which leverage mixed reality technologies, sensory augmentation, and closed-loop control to intuitively and immersively visualize complex underwater environmental data [87].

3.4.3. Energy Acquisition and Storage Systems

• Solar panels: Huang et al. [91] and Chen et al. [102] highlight that photovoltaic solar panels are emerging as a promising alternative, although their main challenge lies in how to efficiently use this system to provide a stable energy supply to ships. A recent study on a short-route ferry in Turkey confirms this potential, demonstrating a reduction in emissions of 3 × 10⁶ kg CO₂ equivalent during its lifetime, with an investment recovery period of only 3 years. However, the study emphasizes the need to improve the energy conversion efficiency of solar panels to maximize benefits, especially on short routes [103].
• Lithium-ion batteries: Lithium-ion batteries are presented as a promising alternative for short-distance navigation [98], requiring a comprehensive analysis of their technical and safety characteristics for their applicability [104,105]. Recharging systems for electric propulsion powered by rechargeable lithium-ion batteries as an energy source have been proposed, focused on small vessels. The authors analyze these systems, highlighting that they generate no noise, vibrations, or pollution, providing a quality experience for passengers [95]. Additionally, to reinforce safety measures for the use of these technologies, an advanced fire detection system for ship batteries has also been developed [104].
• Energy storage and supply systems: Regarding energy storage systems for supply, studies have been conducted on innovative technologies that improve efficiency and reduce costs, such as automatic energy coupling and charging systems [98,106]; batteries on offshore platforms [20]; and compressed air energy storage (CAES) systems with piston machines in parallel mode [87]. In particular, Zhang et al. [107] highlight the need to mitigate the effects of unbalanced condensation in CAES technologies, as suspended particles in the air, both soluble and insoluble, alter the flow structure and increase flow resistance. Additionally, prototypes of integrated and off-grid mobile power supplies have been developed to provide power to ships docked in river anchorages [18]. This technology, also known as “cold ironing”, allows power to be supplied from land to ships. At the same time, they are docked in port, providing an opportunity to suspend the operation of auxiliary engines that emit pollution [108]. Combining these systems leads to the creation of microgrids; on-board microgrid systems, which integrate energy and communication networks, offer a solution to efficiently manage multiple energy sources and loads [109]. DC microgrids equipped with solid-state technologies and energy storage systems offer vessel owners greater flexibility in system energy management, reduced fuel consumption, and compliance with environmental regulations in ports [110]. Hybrid microgrids, which combine renewable energies with battery storage and diesel generators, demonstrate potential for optimizing costs and energy production [111].

3.4.4. Regarding Technologies with an Impact on Water

This consists of technologies directly impacting the marine environment that marine biota inhabits. The composition and purpose of these technologies influence such organisms. Among the main strategies are water treatment systems; vessels use this fluid to displace and carry out tasks on board that involve using this resource. Properly managing waste that may be contained in fluids discharged into the sea, such as ballast water or retained as grey and oily waters, is crucial to avoid contamination. On the other hand, hull materials and coatings are always in direct contact with water, which exposes them to a corrosion process and the marine environment to particles or toxic substances released from the materials. In response, alternatives have been designed for materials and coating paints, such as antifouling paints, which are worth evaluating when creating increasingly cleaner vessels.

Anti-Fouling Paints and Eco-Friendly Materials

• Anti-fouling Surfaces: Recent studies by Maliowsi et al. [113] have demonstrated the good performance of a biocide-free silicone coating (FRC) in the Baltic Sea region under static conditions and high fouling pressure. This technology is part of bio-inspired antifouling surfaces, which mimic natural biological systems to prevent the accumulation of microorganisms and biological fouling (biofouling) on surfaces submerged in aquatic environments [112]. On the other hand, authors such as Felicia et al. [114] have studied the use of xerogel coatings, as well as Aquafast and Aquafast Pro, which are mechanically more stable and durable.
• Hull Materials: Materials used in vessel hulls often contain polluting agents, making it necessary to study alternative materials that have lower contamination without compromising material strength. For example, Zhou et al. [115] studied the bulk and in-situ mechanical properties of rubber-wood-plastic composites (RUBWPC) and their correlations, while Malinowski et al. [113] focused on polybutylene adipate-co-terephthalate (PBAT) modified with coconut fiber (CF) to analyze changes in selected properties of the modified polymer. Iwuozor et al. [116] proposed plant-based biomass composites (PBCs) for their feasibility and impact in the maritime sector. The authors explored the use of these materials in the manufacture of various naval components such as boats, hulls, decks, canoes, surfboards, load ropes, and oars. Similarly, Minillo et al. [117] studied through finite element analysis (FEA) the use of hybrid composites with natural fibers such as curaua, which is extracted from bromeliaceous plants and whose fiber diameter does not exceed 56 μm, and which can be applied in nautical products such as cleats.

Water Treatment Systems

Among the findings, it was discovered that water treatment systems, whether for oily or bilge water, constituted a significant part of academic production. Marine oily wastewater treatment systems typically employ a combination of physical, chemical, and biological methods.

• Oily Water Separation Method: Physically, methods such as gravity are used to separate oil from water by density difference, coagulation to increase the size of oil particles and facilitate their flotation, and filtration through porous media to retain oil droplets. Chemically, processes such as disinfection, electrochemical treatment, and advanced oxidation processes are applied to remove organic contaminants. Regarding biological processes, “activated sludge systems with suspended microbial flocs, membrane bioreactors, and integrated ecological treatments that leverage microbial metabolism” are employed [118].
• Greywater Separation Methods: Two main methods are commonly used: adsorption on porous and high surface area materials and electrochemical oxidation through redox reactions. In addition, other technologies include physical separation systems for grey and black water, flocculation-precipitation, membrane separation, and high-temperature and pressure treatments [119]. Moreover, Cecconet et al. [120] demonstrate that microbial fuel cells are an effective and viable option for greywater treatment, suggesting local reuse of the treated water while simultaneously reducing energy demand. Similarly, Kalnina et al. [121] studied technologies and methods for identifying microplastics in wastewater. These include Bengal rose staining to facilitate particle separation and identification, optical microscopy for visual recognition, and scanning electron microscopy (SEM) to analyze particle surfaces. Finally, to monitor contamination from ship discharges, “online systems based on Automatic Identification System information” have been designed [119].
• Ballast Water Systems: Ballast water systems often transport invasive organisms, which is why technology development begins with the study of these organisms to understand how to prevent contamination by alien species [112]. Addressing this issue, Wright et al. [42] and Ghita et al. [112] propose technologies for assessing organism populations in ballast water through (a) high-throughput sequencing, a DNA-based molecular technique that maps entire populations, and (b) conventional microscopy, which involves visual counting. Additionally, there are alternatives such as the modification of a retrofit ballast water system that includes three main processes: filtration, electrolysis, and neutralization. The renovated system is reliable and capable of treating turbid water with only 1–2% of the total flow passing through electrolysis [77].
• Desalination Systems: Regarding consumable water, it was found that residual heat from industrial steam boilers is used to distill saltwater for consumption. Saltwater separation methods are divided into two main categories: temperature-based and membrane-based [128]. In addition to producing freshwater, this design also provides hot water for daily use, ensuring high efficiency in waste heat recovery. Regarding membrane methods, recent research has achieved significant advances. One study highlights the incorporation of carbon nanotubes into polyamide (PA) membranes for seawater desalination. This innovation improves membrane robustness and anti-fouling performance [129].

4. The Ways Forward

Addressing the proposed questions required additional consultation of the regulations referenced in the found articles to gain a deeper understanding of each one and its scope. The responses were structured to first introduce the most frequent issues, then the proposals to address these, how they influence the adoption of strategies, and what strategies exist in this field. Each analysis laid a foundation for understanding the rationale behind the next and ultimately interpreting them on a global level.

The broad range of studies aimed at evaluating the environmental impact of the naval industry has served as a guide for establishing regulatory frameworks and indicators that standardize environmental requirements and optimize assessments of vessels. Likewise, robust regulations drive the creation or modification of systems within ships, reflecting a reduction in environmental impacts. Despite the extensive and varied literature, there are still understudied topics, such as acoustic pollution, which has limited research on the effects of underwater noise on marine ecosystems, and proposals for turbines that produce less acoustic pollution for wave energy generation, such as the biradial turbine and the Wells turbine [130].

In this context, future studies should focus on potential topics such as ship engines, exhaust systems, hydraulic pumps, and electric generators. Due to the nature of these systems, it is also important to consider standards such as ISO 3746 [131].

Solid biofuels, such as pellets, present a sustainable alternative to reduce emissions in sectors like the maritime industry by replacing fossil fuels in adapted propulsion systems and boilers, as well as carbon capture and storage (CCS) [132], thereby lowering carbon footprints. The biofuels high energy density and ease of transportation make them efficient options, although their production requires careful evaluation to mitigate secondary impacts, including resource overuse and processing-related emissions. In the context of Bioenergy with Carbon Capture and Storage (BECCS), pellets can be integrated to achieve negative emissions under specific conditions, although their effectiveness varies depending on the chosen pathway [133]. Further research is essential to analyze the full lifecycle of pellets and assess their potential in diverse sectors to ensure their effective contribution to sustainability and their integration into innovative systems.

Similarly, key aspects of ship design, such as habitability and weaponry in warships, lack extensive studies related to the integration of environmentally sustainable technologies. It is essential that the purpose of these systems be considered when evaluating their effectiveness, as the requirements differ for commercial fleet vessels and military/naval ships.

In reviewing technological approaches, it was found that existing evaluations of alternative solutions fail to provide a comprehensive analysis of their lifecycle impact, which is crucial for understanding their overall environmental benefits. It is recommended to expand these investigations into stages such as design and construction since certain fuels used in propulsion systems emit fewer pollutants, but their production entails other significant impacts.

Additionally, indices such as EEXI, EEDI, EEOI, and CII, though technically robust, may be insufficient to achieve environmental goals if applied independently without considering market dynamics. Future research faces a significant challenge in integrating not only technical considerations but also economic ones, as the willingness of shipyards and shipowners to adopt these measures depends on such factors.

Finally, it is emphasized that innovations in shipbuilding should prioritize eco-friendly technologies, including sustainable materials for various systems. Lifecycle assessments of alternative solutions are essential to provide a complete perspective of their environmental contributions. Effectively addressing environmental challenges in the maritime sector requires a unified and comprehensive strategy. Expanding the scope of existing indicators to more thoroughly assess vessel-level impacts is essential, and incorporating market dynamics ensures their meaningful application. It is also important to consider lifecycle analyses of emerging technologies to fully understand their environmental contributions and guide their adoption. Overcoming economic and technical hurdles is crucial to fostering the integration of green technologies, while carefully crafted policies can encourage their widespread implementation in a cost-effective and impactful manner.

5. Conclusions and Future Research

This study has demonstrated that the maritime industry faces significant environmental challenges related to greenhouse gas emissions, water pollution, and the spread of invasive species. Through a systematic review of recent academic literature (2019–2024), the most relevant green technologies currently being assessed and implemented to mitigate these impacts were identified, along with the regulatory frameworks and key indicators such as EEDI, EEXI, EEOI, and CII that are guiding this transition toward sustainability.

A clear conclusion that emerges from the analysis is that, although international regulations have played a crucial role in driving technological improvements, they are not sufficient on their own. The effective adoption of clean technologies in the maritime sector also requires economic incentive mechanisms, more flexible international cooperation frameworks, and comprehensive life cycle assessments that consider not only the operational phase but also the design, construction, and end-of-life stages of maritime systems.

It is recommended that policymakers, shipyards, and maritime operators work in a coordinated manner to expand the coverage of existing indicators, prioritize environmentally low-impact technologies with economic feasibility, and promote research in underexplored areas such as underwater noise pollution and the effects of using alternative materials in hulls.

Despite the scope achieved in this review, it is important to acknowledge certain limitations. One such limitation is the language criterion, as only documents in English or Spanish were included. This may have excluded relevant studies published in other languages, especially considering the global nature of the maritime industry.

Moreover, although the review prioritized high-quality academic sources such as Scopus and Web of Science, this focus excluded technical reports, documents from maritime organizations, grey literature, or recent innovations not yet published in indexed scientific journals. On the other hand, while the 2019–2024 period allowed the capture of emerging technologies and regulations, the review represents a snapshot in time. Given the fast pace of technological advances in the maritime sector, regular updates of this review will be necessary to maintain its relevance and applicability.

Looking ahead, it is essential that research on green ship technologies move toward a more integrated and specific pathway. It is recommended to conduct applied case studies that analyze the real-world implementation of clean technologies across various types of vessels, especially those with complex or highly specific operational requirements, such as warships, research vessels, or inland waterway transport ships. Additionally, further investigation is needed into underexplored technologies, such as low-noise propulsion systems or the use of solid biofuels in adapted engines.

Another promising line of inquiry is the full life cycle assessment of each technology from production to disposal or dismantling, which would enable a more accurate evaluation of environmental impacts. It is also proposed to expand the study of economic incentive mechanisms, such as market-based instruments, and their relationship with shipowners’ actual willingness to adopt these technologies.