Ship Energy Efficiency and Maritime Sector Initiatives to Reduce Carbon Emissions

Abstract

With stricter IMO regulations on CO₂ taking effect in 2023 and ambitious goals to reduce carbon intensity by 2030, the maritime industry is scrambling to clean up its act. Conventional methods and equipment are now being reevaluated, upgraded or completely replaced. The difference between a short-term fix and a long-term sustainable option is how flexible vessels will be to use new energy sources or technology as they become viable. The review discusses the recent literature on renewable energy sources, technical and operational strategies for new and existing ships, technology maturity, and alternative fuels. It is found that the IMO’s targets can be met by combining two or three technologies, or via a radical technology shift which can provide innovative, high-efficiency solutions from an environmental and economic standpoint. It has also been noted that policies and enforcement are essential management instruments for mitigating the unfavourable environmental effects of marine transportation and directing the maritime industry toward sustainability on a regional, national, and international scale.

1. Introduction

Oceanic transport is the spine of universal exchange and the worldwide economy. Over 85% of the volume of worldwide exchange in merchandise is carried by ocean, and the rate is indeed higher for most developing countries [1,2,3]. In 2020, global seaborne commerce volumes were predicted to have topped 11 billion tonnes [4]. Marine transportation covers both cargo-carrying (e.g., merchant marine vessel) and non-cargo commercial shipping (e.g., ferries). Military vessels, tugboats, and fishing ships moreover may have critical environmental consequences. As of 1 January 2021, around 15,000 of the approximately 55,000 merchant ships traveling internationally were general cargo ships. As a result, general cargo ships accounted for around 27% of the global commerce fleet [5].

Air pollution from maritime shipping has an influence on both the environment and human health. 33% of all emissions resulting from the burning of fossil fuels in trade, including 3.3% of the world’s carbon dioxide (CO₂) emissions, are attributable to maritime transport [1,2,3,6]. The kind of fuel, engine, and engine efficiency all affect emissions (Pham & Nguyen, 2015). Marine diesel oil (MDO), marine fuel oil (MFO), and heavy fuel oil are examples of fuels (HFO). Marine transportation pollutants have grown over the past 50 years, although they are difficult to measure [3]. The primary source of greenhouse gases (GHGs) and other conventional pollutants that contribute to the greenhouse effect is the combustion of fuel (

Table 1. Ship-produced gaseous pollutants and their effects on the environment according to [7].

Consequences	Pollutants
	SOX	NOX	CO2
Greenhouse effect	-	-	✓
Ozone-ground level	-	✓	-
Acid rain	✓	✓	-
Linked-up with:
Fuel combustion	✓	✓	✓
Cargo handling	-	-	-
Ships equipment	-	-	-
Incinerators	✓	✓	✓

). Around 70% of emissions of traditional contaminants and GHGs take place within 400 kilometers of the land.

Global anthropogenic air pollution is significantly influenced by GHG emissions from sea transportation, which include CO₂, methane (CH₄), and nitrous oxide (N₂O) [3]. Total shipping pollutants reached 961 million tonnes of CO₂ equivalent in 2012, up from 816 million tonnes of CO₂ equivalent in 2007. According to [3,6,8], maritime transport generated 0.022 Gg of N₂O-N annually in 2010 and is predicted to increase by 20% by 2030.

Since shipping accounts for 3% of total worldwide greenhouse gas (GHG) emissions [6,7,8,9,10], severe environmental rules governing oxide of nitrogen (NOx), sulfur oxide (SOx), and carbon dioxide (CO₂) emissions are anticipated to drive significant technological advancement in the industry. With regard to this, Liquefied Natural Gas (LNG) can boost performance, while methane slip reduces the benefits [2,11,12]. Other fills and/or innovations such as biofuels, carbon capture and storage, nuclear, and hydrogen have the potential to decarbonize the industry. Still, each has significant economic, resource, and social acceptability constraints [2]. In addition, many efficiency upgrades (such as hull design, propeller design, cleaning, etc.) can reduce fuel usage. Numerous issues/problems must undoubtedly be addressed to accomplish the carbon depletion of the shipping industry. As a result, “there is no single route,” and a comprehensive reaction is needed from several industry segments. Furthermore, shipping demand is expected to increase over the next three decades [9]. To understand current industry trends, Shell and Deloitte performed a market survey [13]. In their study, more than 90% of shipping sector respondents said decarbonization was an essential or top priority for their companies. Decarbonization’s importance has “grown greatly over the previous 18 months,” according to 80% of the respondents. Given the sea sector’s 3% contribution to GHG emissions, profound decarbonization will require budgetary motivating forces and approaches at the worldwide and territorial levels. For this, technical and operational are the two primary types of maritime emission and reduction measures [14,15]. According to [15], alternative fuels, ship electrification, and renewable energy sources are all recognized as different ways to decarbonization. At the same time, these ways may be considered to come within the technical measure’s category.

There are many strategies to reduce emissions in the maritime area. To the best of the authors’ knowledge, there are a number of review studies on emission reduction, some of which concentrate on the use of alternative energy [10,11,12,13,14,15], and others of which concentrate on other decarbonization strategies [1,3,6]. This study establishes a strong background on ship energy efficiency and emission reduction studies to achieve deep decarbonization by 2050, as stated by the IMO objectives, by providing a thorough review of the overall energy efficiency and emission reduction literature rather than concentrating on just one approach. For instance, ignoring other strategies that have the potential to lower ship emissions, Lindstad, E. et al. [10] and Issa et al. [6] published a bibliometric literature assessment on alternative marine fuels. Additionally, Pariotis, E.G. et al. [16], concentrated on analysing potential technical and operational steps to decarbonize the maritime sector. This research attempted to provide a wider range of techniques for enhancing energy efficiency and lowering emissions in shipping, in contrast to the studies previously mentioned. Hence, this study undertakes the following research objectives: (1) to cover the widest range of information regarding research work in terms of energy efficiency and emission reduction on board merchant ships, (2) to examine this research through in-depth content analysis, (3) to derive pathways for future research. The results of this study have a variety of effects for academics and industrial stakeholders. The remainder of this paper is structured as follows: Section 2 presents the research methodology. Current Legislation and Policies to meet the IMO’s 2050 emission targets are presented in Section 3. Section 4 highlights sustainable energy sources such as wind, solar, and biofuels available within the maritime industry. Alternative fuels for shipping are discussed in Section 5. The relative benefits and drawbacks of alternative fuels will also be discussed. Section 6 examines the maturity of current technologies that can assist the shipping sector in achieving deep decarbonization. Internal combustion engines, fuel cells, batteries, supercapacitors, and nuclear energy will be among the technologies showcased. Section 7 goes through several CO₂ abatement solutions, strategies, and procedures for reducing fuel usage, such as vessel and propeller design and waste heat recovery. Section 8 illustrates future directions, and finally, the review will end with a summary and recommendations.

2. Research Methodology

As shown in Figure 1, the following procedure was used in this investigation.

Step 1-

Extensive Literature Review: Data compiled using a structured literature review; the final product included 329 studies taken from Google Scholar and the Scopus database system.

Step 2-

Quantitative Analysis of Citations: The authors used the open source bibliometrix package in the R software to conduct a citation-based analysis based on the reference list data of the 329 studies to establish the annual publications, standings of journals, institutions, and writers by amount of literature, current rank of articles, and ranking of countries.

Step 3-

Content Analysis: The VOSviewer program was used to assess the 329 research papers that were previously extracted in Step 1. An interactive tool called VOSviewer is used to create and display citations maps. However, by integrating the overall citation and total link strength, two bibliographic features in the VOSviewer, the 37 most pertinent publications were found. The structured literature review matrix approach was used to conduct content methods that rely on these 37 papers that were the most pertinent. The review matrix is a spreadsheet or database with rows and columns used to extract collected data about each published paper, book report, as well as other materials involved in the study of the literature. The matrix technique is described as a structure and a methodology for examining the literature.

Step 4-

Perspective for Future Research: The possible research areas for upcoming investigations were determined based on the synthesis of publications in each category.

This study examined maritime industry research on fuel efficiency and emission reductions that was published between 2002 and 2021. The median number of citations per document was 2.5 as the writers analyzed 329 research papers that had been written by 908 authors and published in 134 different publishing sources.

Approximately 26 publications were single-authored papers, whereas the majority of the research had 859 authors in total.

3. Current Legislation and Policies to Meet the IMO’s 2050 Emission Targets

The International Maritime Organization (IMO) is a United Nations (UN) organization whose objective is to increase safe, secure, ecologically sound, efficient, and viable shipping through partnership [16]. The IMO defined the Initial Strategy in April 2018, intending to reduce GHG emissions from shipping by at least 50% by 2050 compared to 2008 [17]. The Initial Strategy’s primary goals are as follows:

• By 2030, diminish carbon level by 40% relative to 2008.
• By 2050, we choose to reduce carbon emissions by 70%.
• Diminish GHG emissions from worldwide shipping as a minimum 50% by 2050 compared to 2008.
• To reach zero GHG emissions by 2100.

Figure 2 illustrates the IMO plan for vessel enhancements from 2013 to 2050.

In [18], Bouman et al. discuss six major categories for achieving the most significant potential CO₂ emission reductions. Namely, these are propulsion and power, including energy-saving devices, hull design, economy of scale, speed, weather forecasting and planning, fuels, and alternative energy sources. According to their findings, all six reduction scenarios in the shipping industry will need to be executed to achieve almost net-zero CO₂ emissions.

3.1. Energy Efficiency Design Index (EEDI)

MARPOL was updated in 2001 to include the EEDI. The EEDI is a monitoring tool that ship owners and operators can reference to measure the possible impact of any management changes they make and hence weigh up the possibilities from a more informed position [19], and it is applies to new ships. As a result, the EEDI reduces CO₂ emissions through technological efficiency improvements [2]. The EEDI [20] is the first global rule establishing CO₂ emission requirements. However, the International Council of Clean Transportation (ICCT) anticipates that not all ships (globally) will fully comply with the EEDI requirements by 2040–2050 [20], depending on the year of adoption.

3.2. Energy Efficiency Existing Ship Index (EEXI)

The IMO approved revisions to the International Convention for the Prevention of Pollution from Ships (MARPOL) Annex VI in November 2020, introducing a new metric for existing ships, the Energy Efficiency Existing Ship Index (EEXI) [21]. By 2023, the EEXI will be in effect and “will apply to any boat above 400 GT that comes under MARPOL Annex VI” [21]. The EEXI is effectively seen as an extension of the EEDI, and the “necessary EEXI is practically in agreement with requirements” for new-build ships. The EEXI “determines the standardized CO₂ emissions associated with installed engine power, transport capacity, and ship speed” [22], and it “describes CO₂ emissions per cargo tonne and mile”. In other terms, the EEXI sets a CO₂ emission limit per unit of transportation supply [22]. It’s important to note that the EEXI is a technical (or design) index, not an operational index. As a result, there are no observed values for previous years, and no onboard measurements are necessary. The EEXI, in essence, simply relates to the ship’s design. Figure 3 shows an example allowing multiple options for design improvement according to EEXI. “Below is a brief overview:”

According to Figure 3, the shipowner will have the option of reducing cruise speed to reduce carbon emissions, switching to low sulfur density fuel, or using clean fuel (such as LNG, biofuels, etc.) in order to maximize the energy efficiency of an existing vessel. The ideal option is to replace the vessel with a new (orange colour), energy-efficient one if the other two options are too expensive or lead to technical complication.

3.3. Energy Efficiency Operational Index (EEOI)

The Energy Efficiency Operational Indicator (EEOI) is an IMO-proposed index for measuring the efficiency of current ships on a voluntary basis. The EEOI is the mass of CO₂ emitted per unit of transportation work (or “capacity mile”). The index considers fuel consumption (and CO₂ emissions) as all fuel consumed at sea and in ports including both main and auxiliary engines, boilers and incinerators [23].

3.4. Ship Energy Efficiency Management Plan (SEEMP)

The Ship Energy Efficiency Management Plan (SEEMP) is a management plan for new and existing ships that optimizes fuel efficiency through operational changes. SEEMP is a legal requirement for current ships. The Ship Energy Efficiency Management Plan can be applied in various ways, including improving the vessel’s speed, changing direction to avoid rough weather, hull cleaning in dry dock, and adding heat recovery methods, among others. All these strategies aid in boosting the efficiency of the ship and optimizing its functioning. Figure 4 illustrates the key features of SEEMP.

3.5. Carbon Intensity Index (CII)

There are intentions to require ships with a gross tonnage of more than 5000 GT to publish their annual operational carbon intensity indicator (CII) [24]. The CII calculates the annual reduction factor required to maintain continuous improvement of the ship’s operating carbon intensity within a given rating level,” according to the rating scheme [24,25]. Unlike the EEXI, the CII is a short-term operational indicator. The ship’s SEEMP will keep track of the CII performance.

3.6. Short, Medium, and Long-Term Actions

In October 2018, the IMO Intersessional Working Group (ISWG) developed three-term measures to follow up on the IMO initial strategy for reducing GHGs caused by ships. Below is a brief overview:

3.6.1. Short-Period Movements: 2018–2023

The International Maritime Organization has established the EEXI measurements, which Japan is promoting to meet the IMO 2030 objectives, and the CII. These measures were developed as a result of the ISWG-GHG 7 meeting, and they effectively implemented new measures from various groupings of countries in two categories:

• Technical: In the case of existing ships, EEXI is a system that takes EEDI and applies it to current ships.
• Operational: Introduction of a mandatory CII with a ranking method ranging from A to E. The rule applies to all current vessels that meet a particular size requirement.

3.6.2. Medium-Period Movements and Long-Period Movements

Short-Period movements will be developed further by medium- and long-period movements. The means will also consider adopting market-based methods and the supply of emission-reduction incitement. To meet the objective of GHG reductions by 2050, this necessitates synergies of technical, political, and infrastructure solutions.

3.7. Market-Based Measures (MBM)

Given the shipping industry’s projected growth, it is believed that operational and technical measures alone will not be sufficient to meet the IMO’s targets [26]. As a result, there is a general consensus that market-based measures (MBM) as part of a comprehensive package of measures will assist in meeting the IMO’s targets. MBM are predicted to be operative in the medium term, according to the Marine Environment Protection Committee (MEPC) [26]. However, like CO₂ emission reduction strategies, talks have been impeded by divergent viewpoints among stakeholders [27,28]. MBM measurements are based on economic indicators and/or tax levies, and they are used for two reasons:

• Financial motivations for the marine division to diminish its fuel use by contributing to more fuel-efficient ships, technology, and ship operations.
• Expanding emissions related to the maritime industry are being offset in other sectors.

In the near term, MBM may play a representative role in IMO’s strategy. On the other hand, MBM must be managed by “global norms,” as Jorgensen [29] correctly points out, to avoid paying a charge on carbon dioxide emissions. Another concern is that the literature models are based on a short-period movement. As a result, it is fundamentally complex for a shipping firm to estimate CO₂ emission reductions because present models cannot determine how much capacity investment and effort are required to enhance fuel efficiency [30,31].

4. Degree of Ambition and Renewable Energy in Shipping

The early strategy outlines various degrees of desire for the international shipping industry, stressing that technical advancement and the international introduction of alternative fuels and/or renewable energies for international shipping will be crucial to achieving the overall aim. Latest carbon projections, international shipping emission reduction strategies, and Intergovernmental Panel on Climate Change (IPCC) reports should all be included in reviews. The Initial Approach is guided by the following levels of vision:

• The ship’s carbon intensity will decrease as more Energy Efficiency Design Index (EEDI) phases are implemented for new ships;
• To examine the energy efficiency design standards for vessels with the intention of strengthening them, with the required percentage change for every phase for each ship class;
• Reduction in the carbon intensity of international shipping;
• To cut emissions of carbon dioxide per transport job by at minimum 40% by 2030 for all shipping, with a goal of 70% by 2050;
• For emissions of greenhouse gases from global shipping to cease and drop.

Renewable energy can make green fuels or be employed directly for propulsion. However, in recent years, the excess of fossil-fuel-powered transportation and the reduced investment market have slowed the evolution of renewable energy systems for shipping [32]. The following are the main obstacles to expanded selection of renewable energy solutions for shipping:

• Weakness of marketing and commercial feasibility of such technologies.
• The low motivation for clean energy deployment is due to a gap in interests between charterers and operators.

Figure 5 summarizes different renewable energy technology and its potential for the shipping industry according to [32].

Renewable energy can be implemented in shipping in one of two ways: (1) as retrofits for current fleets or (2) as part of new vessel designs. In terms of new ship concepts, most renewable energy technology will deliver electricity for auxiliary and additional uses, regardless of ship size.

4.1. Biofuels, Natural Gas, and Hydrogen in Ship Propulsion

The use of biofuels, hydrogen and natural gas as ship propulsion fuels has been proposed [11,32]. However, biofuels from food plants compete at once with agriculture for soils, potentially causing more difficulties than they solve in the long run. Cellulosic ethanol is also troublesome since it extracts supplements from crop soils [33,34] and will contest with methane and olefins generation for crop misuse in a post-carbon economy [35]. Algae-based biofuels could be deployed in the next ten years, but they must first overcome several unsolved challenges that make their adoption uncertain as a widely available fuel [36]. The European Biofuels Technology Platform defines, first, second and third generation biofuels as follows: (Figure 6):

• First Generation: Sugar, fat, or starch directly derived from a plant serve as the biofuel’s carbon source. The crop is thought to conflict with food, either directly or indirectly.
• Second Generation: Carbon for biofuel comes from cellulose, hemicellulose, lignin, or pectin. Examples include agricultural, forestry residues, or purpose-grown non-food feedstocks.
• Third Generation: Aquatic autotrophic organisms provide the carbon for biofuel (e.g., algae). The feedstock is made from light, carbon dioxide, and nutrients, which “extend” the carbon resource accessible for biofuel synthesis.

In the transportation industry, biofuels are now the foremost pertinent elective for supplanting or blending with gasoline or diesel [37]. However, in the shipping business, utilization and experience are limited [38]. The challenge with biofuels in the maritime sector is a lack of experience and understanding in handling and implementing biofuels as a part of their energy source. Another challenge is the vast quantity of biofuels must-have to feed the maritime sector [39]. Consequently, sustainable biofuel production is restrained by food costs, land availability, and societal considerations [40]. Furthermore, there are concerns about biofuel storage and oxidation steadiness, thus further investigation and research are required [11,39]. Nonetheless, a mix of legislation, regulations, encouragements, and technological and root upgrades could assist the shipping industry to build a considerable market for biofuels.

Hydrogen propulsion in ships is technically possible, but it requires large-scale implementation of hydrogen infrastructures for production, shipping, storage and port services, which is at present yet to be determined. Furthermore, as hydrogen has a very low flaming temperature, burning hydrogen–air mixtures near stoichiometric composition can result in chaotic pre-ignition practices and high combustion temperatures, resulting in substantial NOx emissions [41]. Moreover, hydrogen is more appropriated as a fuel for a spark-ignition engine than a compression-ignition engine, which could be a problem for the shipping market.

For diesel ignition engines, natural gas could attain climate uniformity in 30 years [37], but it must be produced responsibly in a future 100% sustainable economy. Natural gas might be created viably by fermenting farm and urban waste and using the Sabatier process to combine electrolytic H₂ with CO₂. The first means will also be the origin of methane needed for the manufacturing of ammonia in a post-carbon economy, at least during the conversion to totally organic agriculture. As a result, the second means would be better in the long run. If the heat generated in the reaction and CO₂ emitted by industrial operations are repurposed, it might achieve efficiencies of 55–56% [42,43].

4.2. Wind Energy for Shipping Applications

Sails dominated the high seas before the invention of the steam engine, propelling comparably tiny ships with huge sailor numbers. Ultimately, wind is a well-known, widely accessible, albeit fluctuating, sustainable energy source. The most significant drawbacks are variations in wind force and the difficulty in harnessing the entire propulsion capability when sailing into or near the wind. Current actions involve using various renewable energy sources, both as primary and auxiliary propulsion, on various ship types ranging from small ships to big freight carriers.

4.2.1. Soft Sails, and Fixed Wings

Soft sails mounted on tall spar are a tried-and-true technique used for primary or secondary propulsion [38]. Soft sails are simple to adapt to existing ships or include in new ship designs. The Oceanbird (Figure 7) and the Seagate delta wing sail are two examples.

According to a 2015 European Commission study for the periodic 1-Seagate Sail [44], a hybrid wind plus motor cruising mode resulted in 20% fuel savings for commercial vessels. As a result, prices and polluting emissions are reduced by 20%. In response to the sociological and environmental issues posed by developing marine legislation, Seagate Sail and cruise control will provide a reliable and advantageous solution for commercial vessel owners to lower fuel expenses and minimize emissions.

Several Japanese ships were modified with fixed wings in the 1980s to reduce fuel consumption. The oil crisis of the 1970s, which resulted in oil deficits and skyrocketing oil prices, was a major driving force behind this. However, the crisis ended, and when costs plummeted in the 1980s, the cost-effectiveness of stiff sails was questioned. Despite this, Japanese ships equipped with solid sails, such as the Shin Aitoku Maru and Usuki Pioneer, demonstrated that solid sails reduced fuel consumption. Fuel savings of 10-30% have been reported on ships equipped with JAMDA sails [45].

4.2.2. Rotors

Flettner Rotors use the Magnus effect for propulsion, which occurs when wind passes through a pivoting cylinder [46,47,48]. It was first shown on several ships in the 1920s. However, the technique was mainly forgotten until the early 1980s when French Captain Jacques Cousteau and his research team integrated the turbo-sail, a non-rotating fan-driven variant, on their research vessel. In 2010, the German company Enercon tested the 12,800 dwt E-SHIP 1 on board its ship, which has four Flettner rotors powered by the exhaust gases from of the principal turbine generator [49] (Figure 8). Adapting Flettner rotors to tankers and bulkers is being looked into, even though board space use for various ship classes is vital. According to [49], the E-ship 1 with Flettner rotors consumes up to 25% less fuel than similar traditional merchant ships.

4.2.3. Kite Sails

Kite sails are linked to the ship’s bow and perform at high altitudes to take advantage of strong wind speeds, (Figure 9). The kite sail can help shipowners save 10–35% on annual fuel costs [50]. The MS Michael A. was the first container ship that used a kite sail in part [38,45].

4.2.4. Solar Energy

Photovoltaic (PV) cells use the photoelectric effect to generate electricity directly from sunlight [51]. PV cells suffer two fundamental challenges, even though this is a rapidly expanding technology with rapid progress: (1) insufficient space aboard a vessel and (2) the need for battery-based energy storage [38,52]. PV cells are further constrained by the risk of corroding due to saltwater [2,53] and by intermittency concerns. Finally, it’s worth noting that solar energy’s potential for reducing emissions is limited.

Smith et al. [54] assume a “0.1–3%reduction in auxiliary engine fuel use,” while Bouman et al. [18] predict a CO₂ reduction potential of 0.2–12%. Based on the investigations of Smith et al. [54] and Bouman et al. [18], an average improvement of 1.5% in auxiliary fuel saving and 6% in possible CO₂ cutting is considerable and encouraging. On the other hand, PV systems aboard vessels need more research and development because of the corrosive nature of the marine environment. Their potential is therefore lower aboard vessels than it is on land.

4.2.5. Solar-Hybrid Systems in Shipping

Solar PV cells offer potential when it comes to recharging shore battery systems. This is only significant for especially short shipping [55]. It can also supplement other forms of electricity for most shore-side infrastructures. Yuan, Y. et al. [56], claim that a ship’s fuel consumption and CO₂ emissions can be reduced by 4% and 8,5%, respectively, by adopting hybrid solar energy power.

5. Alternative Fuels

When employed for ship propulsion, alternative fuels may have the ability to reduce or eliminate carbon intensity. Alternative fuels are becoming more and more common, including hydrogen and LNG [57].

Table 2. Eventual CO₂ emission reductions by using alternative fuels according to [8].

Types of Alternative Fuels	CO2 Emissions Reductions
LNG	0–20%
Ammonia	0–100%
Biofuels	25–100%
Hydrogen	0–100%
Fuel cells	2–20%
Wind	1–32%
Solar	0–12%
Nuclear	0–100%
Electricity	0–100%

shows how alternative fuels and energy sources can help reduce CO₂ emissions [15].

LNG has a 40% lower volumetric energy density than diesel, according to DNV GL [58]. However, when the storage system is considered, the volumetric energy density of LNG is around one-third that of diesel. Liquid hydrogen, ammonia, and methanol have volumetric energy densities that are 40 to 50% lower than LNG, according to DNV GL [58]. Therefore, according to the report, biodiesel is the only fuel that matches the energy density of diesel.

In the medium term, techno-economic research by Lloyd’s Register and UMAS found that biofuels are marginally more cost-effective than renewable energy or natural gas with carbon capture and storage [59]. Conversely, biofuels face hurdles in sustainability and supply, making “any biofuel pathway uncompetitive and susceptible to restrictions or increased pricing” in the mid–long term. As a result, compared to hydrogen or ammonia produced from natural gas or renewable energy, biofuels are not necessarily more affordable. [59].

5.1. Hydrogen as Marine Fuel

The fundamental advantage of hydrogen is that it has the opportunity to be an emission-free fuel when made from renewable sources [60]. Additionally, the predicted energy transition to land-based renewable power is compatible with the projected increase in hydrogen production capacity [61]. However, on a volumetric basis, liquid hydrogen may require four times the space of marine gas oil (MGO) or around two times the space of LNG for a comparable amount of transported energy due to its lower volumetric energy density. Considering consumer energy efficiency is also crucial when comparing fuel energy and needed volumes. For all maritime fuels, additional fuel may be required to account for efficiency losses between the tank and the output shaft power. To liquefy hydrogen, the temperature may be below −253 °C. The volume needed to store liquid hydrogen could be significantly higher when layers of materials or vacuum insulation for cryogenic storage, as well as other structural arrangements, are taken into account because of the extremely low temperature. Furthermore, due to its volatility and flammability, hydrogen safety is a significant issue [62].

5.2. Ammonia as Marine Fuel

Contrary to hydrogen, ammonia is a prime mover that may be used in a variety of engines and fuel cells. Liquefied hydrogen and LNG can be kept at substantially lower pressures and/or temperatures than ammonia. Since ammonia is one of the top three compounds carried each year, it has previously been shipped, and there is a global infrastructure for storage and delivery [63]. However, ammonia’s main drawbacks are its toxicity and effect on the ecosystem. If inhaled, ammonia is poisonous and can cause severe skin burns and eye damage [64].

5.3. Electricity Stored in Batteries

Batteries provide zero-emission propulsion and have an efficiency that is up to double that of a diesel generator [65]. They produce lower noise levels than traditional propulsion systems. In comparison to conventional fuels, the OpEx may be reduced [66]. Battery prices are continually falling, and performance is significantly improving. The weak energy density per mass unit (approximately 150 times) with the low volumetric density of batteries are the most significant disadvantages (100 times lower than diesel). The production of batteries consumes a lot of energy, and the capital spending for a large battery system is much more than for a conventional propulsion system.

Table 3. Principal benefits and drawback.

Alternative Fuels	Advantages	Disadvantages
LNG	Prices are competitive Infrastructure technology that are available	Insulated tanks are necessary for storage Unable to meet the 50% CO2 reduction target
Hydrogen	Enable the zero-emission option with fuel-cell Can be made from electrolysis near ports	Fuel prices are extremely high There is not a piston engine or infrastructure available Very low storage temperature
Ammonia	Can be employed for engines and fuel cells. Can be stored at a high temperature and low pressure.	Toxicity and environmental impact when leaked When utilized in internal combustion engines, hydrogen must be added.
Biofuels	It is possible to be carbon-free Compatibility with existing engine	Price Narrow product volume
Electricity stored in batteries	Efficiency Enable zero-emission	Small energy density of mass and volumetric density Prohibitive CapEx

summarizes significant benefits and drawbacks of alternate fuels.

5.4. Risk to Humans and Commerical Value

Another significant factor when considering alternate marine fuels is the risk to humans onboard. As previously stated, hydrogen gas is highly explosive, and ammonia is far more toxic than other sources. In addition, human lives are also at risk onboard due to fire and gas explosions from the batteries. Therefore, when choosing alternative marine fuels, we must evaluate the risk to humans to avoid sacrificing human lives in order to lower GHG emissions.

According to [67], efforts to increase environmental performance can occasionally reduce human safety. Increased lack of propulsion along the California coastline is an example of this dilemma. The California Air Resource Board enacted a new sulfur pollution policy in summer 2009 that mandated fuel switching close to the California coastline [68]. However, the main engine could turn off if the fuel switch is not properly arranged and accomplished [69]. As a result, there was a sharp rise in propulsion loss events within a few years. These accidents pushed the United States Coast Guard to deliver a Maritime Safety Alert titled “Fuel Switching Safety” in 2011 [70]. The fire and explosion of the MF Ytteryningen, a diesel-electric hybrid passenger ferry, is another example [71]. On 10 October 2019, a fire broke out in the battery space. The ferry’s crew and passengers were evacuated to the shore so that it could make its own way back to port. Overnight, however, a significant gas explosion happened. Twelve firefighters were rushed to the hospital after being exposed to dangerous fumes emitted by the batteries. The outcome was that the Norwegian Maritime Authority warned all ship owners with battery installations.

On the other hand, the cost is the principal barrier to operating ships on alternative fuels. Biofuels often require no additional CapEx, but their OpEx is more significant than HFO. Hydrogen and ammonia might have drastically different CapEx and OpEx based on the fuel’s propeller and manufacturing process. For illustration, since we need to eliminate high NOx emissions while burning ammonia, both CapEx and OpEx can be increased. Although the OpEx of batteries may be comparable to that of a traditional propulsion system in some circumstances, the CapEx is higher [66]. Another factor to consider while using alternative fuels is fuel availability. The main problems of running vessels on biofuels are narrow product volume and fuel supply [72].

6. Technological Maturity

6.1. The Role of Internal Combustion Engines (ICEs) in Decarbonization

Today, ICEs are the most used maritime propulsion technology. However, if ICEs play a role in the decarbonization of shipping, vendors, authorities, cargo owners, and engine makers must concur on workable substitutes for carbon-based fuel.

Marine ICEs, which make up over 98% of all commercial ships, dominate the global maritime fleet [73]. ICE engines nowadays offer the highest levels of efficiency in the industry. Furthermore, the ICE has become highly appealing for shipping due to the reduction in energy costs through heavy fuel oil [73]. Furthermore, their environmental performance has been greatly improved as well as the adoption of new control system technologies. Since ICE engines are rather mature and have been in use for close to a century, there is a plethora of knowledge and expertise in this area. As a result, the operation of ICEs is straightforward, contemporary, and robust, with a long lifespan that is occasionally equivalent to a ship. The shipping industry will be able to meet its emissions goals by enhancing ICEs. This is because ICEs can adapt to future alternative fuels, it dominates the shipping sector, and they can be modified and updated utilizing smart control technology [74].

6.2. Alternative Fuels in ICEs

6.2.1. Ammonia in ICEs

The lack of carbon and sulfur atoms in ammonia’s chemical formula makes it an attractive fuel. Because ammonia and hydrogen have a greater octane rating than gasoline, they may be used at a higher compression ratio [75], making them excellent for diesel engines. Although ammonia has a high auto-ignition temperature, a dual-fuel method in a diesel turbocharged multicylinder engine [76] might be the way to go. The usage of ammonia as a fuel in ICEs has recently piqued the market’s interest. However, ammonia has a lot of qualities that need to be investigated further before it can be used commercially [77]. These are the following:

• Compared to other fuels, it has weak ignition and a slow flame propagation speed.
• Toxic and corrosive.
• Increased NOx emissions unless the combustion process is optimized, or after-treatment is used.
• It will be necessary to design regulations and policies for its usage as a marine fuel.

6.2.2. Hydrogen in ICEs

For the extensive decarbonization of the maritime sector, green hydrogen is a viable option. However, there are considerable obstacles that prevent the usage of hydrogen as an ICE fuel. For example, because hydrogen has a relatively low minimum ignition temperature, thus burning hydrogen–air mixtures near stoichiometric composition might result in significant NOx emissions [78].

The shipping sector may face difficulties as a result of hydrogen’s high auto-ignition temperature, which makes it better suitable as a fuel for spark-ignition engines than compression-ignition engines [78]. However, since it creates fewer emissions while only slightly reducing engine performance, using hydrogen as a supplementary fuel in compression-ignition engines is preferred. [79]. The 80-passenger ferry HydroBingo by CMB is an example of a boat that uses hydrogen (Figure 10). This hydrogen-powered ferry is an environmentally friendly vessel that emits much less CO₂ and other hazardous emissions. Compared to a standard diesel engine, CO₂ emissions are 50% lower [80].

Due to the underdeveloped hydrogen infrastructure needed for all hydrogen-powered vessels (including both fuel cell and combustion engines), the widespread deployment of hydrogen combustion engines into serial manufacturing has yet to start [82]. It is now being discussed if and how hydrogen-powered internal combustion engines can complement fuel cells to help contribute to CO₂-free propulsion systems in light of the construction of hydrogen infrastructure that is planned for and already underway in some markets. The extensive use of current vessel architecture and production capacity, as well as their current low investment costs, are some arguments in favour of internal combustion engines that run on hydrogen.

6.2.3. Methanol in ICEs

Marine compression-ignition and spark-ignition engines, the latter of which operates in dual-fuel mode, both accept methanol as a fuel [83]. Without further precautions in marine engines, methanol may immediately comply with the IMO 2020 global sulfur cap, resulting in zero particulate matter (PM) emissions. According to [82], installing methanol in an existing boat is much less expensive than installing LNG. Methanol refit costs for 10–25 MW engines range from 25% to 35% of the corresponding LNG conversion expenditures. Andersson and Salazar demonstrate the required retrofitting (and quantity of pipes) to introduce methanol into the combustion cylinder [84]. The retrofits were made for the Stena Germanica (Figure 11), which used a four-stroke medium-speed engine. The ferry cuts emissions of SOX by 99%, NOX by up to 60%, and PM by 95% when using methanol as its primary fuel instead of conventional marine gasoline. A two-stroke engine for new-build tankers has also been developed and tested by MAN Diesel and Turbo [84,85]. The cost of changing a ship’s engine from “diesel fuel to dual-fuel methanol/diesel fuel” has been estimated at €250–350/kW for large engines (10–25 MW).

Methanol has performed admirably in laboratory and field experiments from a technical standpoint. However, methanol requires further large-scale demonstration tests in its next development phase. The conversion of the Stena Germanica is the only large-scale methanol deployment in a marine scenario (ro-pax ferry, 24 MW). In the eyes of investors, the relative absence of a track record raises the technology risk. Additionally, methanol as a marine fuel currently has no specialized market [88]. As a result, the methanol sector should raise awareness through targeted marketing activities directed at the shipping industry.

6.2.4. LNG in ICEs

Indeed, LNG has a higher C:H ratio and lower CO₂ emissions, and diesel-LNG engines have numerous advantages. Furthermore, LNG is derived from methane, allowing for diversification of fossil fuel supplies. LNG is a gas at conventional temperatures and pressures. Due to its superior vaporization, mixing, and combustion compared to diesel, it emits fewer harmful pollutants into the atmosphere [89,90].

On the other hand, low-pressure engines produce less NOx than high-pressure engines, necessitating selective catalytic reduction in the second scenario [90]. Engine conditions could affect the amount of CO and unburned HC [91]. In their investigation, Li et al. [91] converted a marine diesel engine with new control systems to analyze its performance with LNG and found that the “dual fuel management system could run steadily over a lengthy period.” Because of its high flammability, LNG poses an additional hurdle when employed in dual-fuel engines.

6.3. Fuel Cell Systems for Maritime Applications

Fuel cells are a low-cost method of producing low-carbon electricity and critical technology for enabling the use of forthcoming alternative fuels [92]. This is because fuel cells, which are adequate and emit minor perilous compounds, could be a viable alternative [93]. Fuel cells turn fuel within electricity and are a validated land-based power source that could someday replace ICEs [94]. Although ships use electricity for auxiliary purposes, recent movements have proven that it can also be used for propulsion [93]. Fuel cells may boost energy conversion efficiency to over 60%, and if waste heat is utilized, it can reach an impressive 80% [94,95]. On a $/kW basis, however, fuel cells are currently more costly than ICEs [96]. On the other hand, Ballard shows a 70–80% cost decrease for vehicles that use fuel cells [97], which might be attributed to the shipping sector. In addition, it’s worth noting that once fuel cells are deployed, they have fewer moving parts than ICEs. Therefore, their operating and maintenance expenses are lower [96,98].

The FCS Alsterwasser (Figure 12), the first entirely proton-exchange membrane fuel cell (PEMFC)-powered ship, was created in 2008 by the Zemships project and was powered by two 48 kW hydrogen fuel cell units [99]. It was a 100-passenger inland waterway craft, followed by several other small ferries and riverboats.

In 2012, Germanischer Lloyd approved design approaches for emissions-free power generation with a mixed battery and fuel cell system using liquid hydrogen as a fuel. However, the long-term viability of hydrogen production remains a significant concern [38].

Up to 2016, van Biert [93] provided an analysis of notable maritime fuel cell utilization exploration initiatives and lessons learned.

Table 4. List of successful projects according to Xing et al. [98].

Fuel Cell Type	Project/Vessel Name	Fuel	Capacity
Solid Oxide Fuel Cell	Felicitas subproject 2	LNG	250 kW
	METHAPU Undine	Methanol	20 kW
	SchIBZMS Forester	Diesel	100 kW
High Temperature Proton Exchange Membrane Fuel Cells (HT-PEMFC)	RiverCell	Methanol	250 kW
	Pa-X-ell MS Mariella	Methanol	2 × 30 kW
	RiverCell ELEKTRA	H2	3 × 100 kW
	MF Vågen	H2	12 kW
Molten Carbonate Fuel Cell (MCFC)	MC WAP	Diesel	150/500 kW
	FelloShip Viking Lady	LNG	320 kW
	US SSFC	Diesel	625 kW
Alkaline Fuel Cell (AFC)	Hydra	Metal hybride	6.9 kW
	HydroCell Oy	Metal hybride	30 kW
Low-temperature proton exchange membrane fuel cell	Elding	H2	10 kW
	Zemship Alsterwasser	H2	96 kW
	Hydrogenesis	H2	12 kW
	SF-BREEZE	H2	120 kW
	Cobalt 233 Zet	H2	50 kW
	US SSFC	Diesel	500 kW

summarizes a list of fuel cell projects in maritime utilizations since 2000, as provided by Xing et al. [99].

There is a considerable diversity of fuel cells. However, PEMFC, MCFC, and SOFC are the most auspicious solutions for maritime utilizations. According to Xing et al. [99], “once energy efficiency, power capacity, and sensitivity to fuel impurities are considered.” Xing et al. [99] suggest that coupling fuel cells with batteries, modularization, and enhanced management and operating techniques are the approach for fuel cell utilization in the maritime sector. Due to the low-grade temperatures, several fuel cell types use waste heat recovery systems to boost global efficiency, which may be combined with organic Rankine cycles [98,100].

LT-PEMFC has shown the ultimate accelerated advancement of fuel cell technologies, resulting in grand power densities and positive transient performance. However, because LT-PEMFCs function at low temperatures, a platinum catalyst is required to activate the electrochemical reaction [102]. Because of the holes in the gas diffusion layer, EMFC membranes necessitate a wet membrane to allow proton transfer, complicating water management, especially for LT-PEMFCs [103]. Due to the obvious high surface adsorption by the catalyst at low temperatures, LT-PEMFCs have limited endurance for contaminants which disband the catalyst [104,105].

MCFCs are commercially available and somewhat mature high-temperature fuel cells, but they still have to contend with big costs, lifespan, and low power density [106,107]. Low-temperature SOFCs have electrical performances of over 60% and are utilized alone, whereas high-temperature SOFCs have performances of over 70% and are used in conjunction with gas turbines [108,109,110]. Mechanical flaws, high costs, and limited adoption characterize SOFCs.

6.3.1. Ammonia as Fuel for Fuel Cells

Ammonia-based power plants operate similarly to hydrogen-based power plants in terms of generating electricity. Ammonia is first conveyed into a fission reactor, where it is separated into nitrogen and hydrogen. Hydrogen accounts for 75% of the gas. A minor amount of ammonia is leftover in the gas stream and is not transformed. Additionally, nitrogen and hydrogen are fed into the cell, and therefore the air is brought in, allowing the hydrogen to set on fire and form water. This produces voltage, Figure 13 [111,112].

The attraction to using ammonia for fuel cells in maritime sectors is increasing [111]. Fuel Cells and Hydrogen Joint Undertaking received a €10 million grant from the EU’s Horizon 2020 research and innovation program to install maritime fuel cells that run on green ammonia. The project will utilize Viking Energy, an offshore ship operated by Eidesvik (Figure 14) [113].

6.3.2. Hydrogen as Fuel for Fuel Cells

The operating principle is based on the fact that the anode receives hydrogen while the cathode receives air. Protons and electrons from hydronium ions are split apart by a catalyst at the anode of a hydrogen fuel cell, and they then proceed in separate directions to the cathode. The movement of the electrons across the external device results in the electricity flowing. In the cathode, where they mix with oxygen and electrons to form water and heat, protons travel through the electrolyte. The employment of hydrogen in fuel cells has attracted a lot of interest in the business world. A Danish–Norwegian project to develop and test a hydrogen-fueled boat, for instance, just secured funding from the EU for the Europa Seaways, managed by DFDS Ferries. By 2027, the project hopes to have Europa Seaways up and running. Several significant companies in the transportation and energy sectors have teamed up on this project to create a ferry capable of transporting 1800 passengers (Figure 15) [114].

6.4. Electric and Hybrid Propulsion

Batteries, flywheels, and supercapacitors can be used as energy storage for zero-carbon electric propulsion [115,116,117]. Electric motors can provide propulsion; however, when coupled with batteries’ weight and storage on vessels, it can be an overpriced choice [8]. Under several scenarios, Lloyd’s Register [118] evaluated electric boats in comparison to other fuels such as hydrogen, ammonia, and biofuels and concluded that electric ships are the slightly more advantageous technology.

Hybrid marine engines are agreeable as they can be powered by a fuel cell, batteries, or an electric motor and they can run on diesel, LNG, or hydrogen [119]. Hybridization can save consumers 10–40% on gas and has a “payback time as short as one year” [38,120].

6.5. Nuclear Propulsion

Nuclear ship propulsion has existed from 1955, primarily for naval and armed forces uses [15]. Nuclear power is appealing because of its high power density, consistent value, and small GHG emissions. An additional significant benefit of nuclear propulsion is its capability to perform for extended times without refueling, which promotes independence and protects against fuel price swings [2,15,121]. In terms of both technicalities and finances, nuclear propulsion has proven to be essential for missions in the Russian Arctic, especially for icebreakers [121], where high-power demand is necessary. [122].

Nuclear propulsion is accomplished by heating steam with an onboard nuclear reactor, which drives steam turbines and generators. Nuclear propulsion, as previously stated, is improbable owing to “political considerations,” such as port authorities’ unwillingness to allow overseas vessels with a nuclear reactor on board [123]. Licensing, certification, and security against accidents, terrorist acts, and propagation are further difficulties for nuclear propulsion [9]. Another difficulty with nuclear propulsion is the radioactivity and disposal of natural uranium, which poses significant health and environmental problems. This will necessitate a “total rethink” of merchant ship design, as safety will take precedence over efficiency [15]. With the current focus on decreasing GHG emissions, nuclear power may refocus in the future on providing hydrogen to ships [124,125].

6.6. Batteries and Supercapacitors

Battery technology for electric cars is fast declining in price, suggesting that this technology may “become a more feasible and readily available choice also for other transport sectors such as shipping” [15]. The average annual cost for the 2017–2020 period was $137/cents per kilowatt hour, which was lower than anticipated [126,127]. Lithium-ion battery bank prices will reach $73/cents per kilowatt hour by 2030 [15]. In their different transition pathways analysis, Lloyd’s Register and UMAS [59,128] state that “batteries play a minimal role as a principal energy store/source onboard ships” given their high costs and low energy volumetric density. In fact, according to [128], “the cost of batteries (cost of storage system) appears to be prohibitive relative to other zero-carbon choices” in the majority of the situations they reviewed. The Zerocat 120, a 120-car boat with a capacity of up to 360 passengers for brief excursions (20 min), is depicted in Figure 16 as the first lithium battery-powered vessel in the world. It has a very quick battery charge time [129].

Ships can also use supercapacitors to generate power, but they can hold and release huge amounts of energy even more quickly than batteries. [15]. For example, Ar Vegan Tredan [130], a net-zero emissions passenger boat driven by supercapacitors, was designed as part of the Ecocrizon research and development initiative. The supercapacitors may be charged at the dock (it takes four minutes), but “this can only be deemed a renewable energy-powered vessel” if the electricity is generated by sustainable sources [38].

6.7. Carbon Capture and Storage (CCS)

CCS is a viable method of lowering CO₂ emissions. Innovative ideas and ongoing research are being developed to convert captured CO₂ into energy, such as methane. For instance, efforts are being made to create a novel photocatalyst that effectively imitates photosynthesis by converting CO₂ into methane [130]. Another alternative is to use molten carbonate cells to simultaneously create energy and capture CO₂ [132].

7. Technical and Operational Measures to Reduce Greenhouse Gases (GHGs) and Fuel Consumption

7.1. Technical Measures to Reduce GHG Emissions Using Pre-Treatment, Internal-Treatment, and Post-Treatment Solutions

Figure 17 shows the GHG emissions reduction for the diesel engine (DE) based on pre-treatment, internal-treatment, and post-treatment solutions [3,6,7,11].

According to [6,7,11,133], a pre-treatment solution is the easiest and fastest way to comply with the IMO’s emissions regulations. However, low-sulfur replacement fuels such as methanol and LNG have several hurdles in terms of flexibility on board. They necessitate dual-fuel engines and additional unique fuel storage tanks in terms of engineering [3,134].

7.1.1. Emulsified Fuel

Emulsified fuel relies on adding water to the fuel to lower the temperature in the combustion chamber. Emulsified fuel allows for better atomization and dispersion of the fuel inside the combustion chamber, resulting in a complete combustion. Emulsified fuel has the advantage of lowering nitrogen oxide and particulate matter emissions. However, it also causes engine component corrosion and the short-term occurrence of oil-water separation phenomena [135,136,137].

7.1.2. Fuel Switching

Wärtsila and MAN B&W have developed DEs that can run on natural gas, marine diesel oil (MDO), or heavy fuel oil (HFO). This is referred to as fuel switching or dual-fuel technology [11,138]. The dual-fuel technology offers significant advantages to shipowners and operators. The engine complies with IMO Tier III requirements [11] in gas mode without secondary exhaust gas purifying equipment. In addition, dual-fuel technology reduces SOx and CO₂ emissions while also allowing for smokeless running in gas mode. On the other hand, fuel switching allows the operator to choose the type of fuel to use based on market price fluctuations.

7.1.3. Direct Water Injection (DWI)

With DWI, water is immediately fed into the combustion cylinder, resulting in a high NO_x reduction by up to 50% [139] as peak temperatures are reduced. DWI consumes less water than other processes such as humidification and water in fuel emulsions, and water quality is less of a concern. However, this method consumes more fuel and produces more smoke, especially at low loads.

7.1.4. Miller Cycle and Scavenge Air Cooling

The Miller cycle is a cold cycle that reduces NOx emissions by up to 40–60% while increasing engine efficiency [140]. The Miller cycle can also fulfill low scavenge air temperature on four-stroke marine diesel engines [141]. Combustion temperatures and NOx are minimized by lowering the scavenge air temperature. According to Holtbecker, M. [142], nitrogen oxide lowers by approximately 1% for every 3 °C reduction. Internal engine technology, such as Exhaust Gas Recirculation (EGR), also results in lower combustion temperatures and lower NOx composition. It is regarded as the most important technology for reducing NOx from DE.

7.1.5. Exhaust Gas Recirculation (EGR)

Exhaust gas recirculation (EGR) reduces NOx emissions by returning exhaust gases into the combustion cylinder. Because of their high specific heat capacity, exhaust gases lower peak temperatures, reducing NOx emissions. The lower O₂ level in the engine also causes a reduction in temperature [143,144]. Compared to a typical diesel engine, EGR can reduce NOx by up to 50% [142]. EGR used in conjunction with scrubbers can reduce SOx and PM emissions while protecting the EGR cooler from clogging [3,6]. With increasing fuel consumption, however, EGR may raise installation difficulties and costs [3]. Furthermore, EGR produces effluent, which must be considered while constructing such a system.

7.1.6. Selective Catalytic Reduction (SCR)

A catalyst is used in an SCR to lower the activation energy required for NOx reduction. Compared to other de-NOx technologies, SCR systems offer the highest NOx removal effectiveness [3,145,146]. In the exhaust gases, a reductant (typically ammonia by injecting aqueous urea) is added, and NOx is reduced to nitrogen (N₂) with the help of a catalyst. According to the review study of Lu et al. [147], SCR technology is the most promising technology for fulfilling the IMO NOx Tier III restrictions.

7.1.7. Non-Thermal Plasma (NTP)

The exhaust gas is ionized with the help of electricity in the NTP (AFTER-TREATMENT). Highly powerful electrons in the plasma convert oxygen to oxygen radicals. The oxygen radicals subsequently react with NO, resulting in the formation of NO₂. The NO₂ is transformed into N₂ with the help of a catalyst. It’s crucial to remember that plasma could be produced by either an electrostatic discharge or an e-beam [148,149,150,151]. NTP can, according to [150], cut NOx and SOx emissions from marine engines by 60% and 80%, respectively.

7.1.8. Scrubber

Scrubbers are the most promising technology for providing SOx and PM emission solutions that comply with IMO requirements [3,6,7]. Wet scrubbers are the most common technologies today [152]. There are also hybrid aqueous scrubber systems, which combine open-loop and closed-loop systems. Finally, dry flue gas scrubbers are available. These methods can help reduce SOx and PM emissions by 95% for particles larger than 5μm and up to 80% for particles from 3 to μm [153]. They are, nonetheless, intricate, weighty, and substantial installations.

7.2. Operational Measures to Reduce Fuel Consumption

7.2.1. Cargoship Speed

Fuel consumption can be decreased by lowering a boat’s average speed [154]. However, the trip takes longer as a result.

Slow steaming has become a commonly used method to cut bunker costs [154] because marine boats are more fuel-efficient at low speeds. Many emission reduction options exist (such as the kite sails mentioned before), and slow steaming can be “an immediate solution for carriers to improve their environmental impacts” [155,156,157].

The benefits of slow steaming depend on “ship type, tonnage, itineraries, and activities” [157]. As has already been mentioned, slow steaming lengthens journeys; nonetheless, “a 10% decline in speed may result in a total average emissions reduction of 19% [158]”. Slow steaming, however, because it runs at part load for extended periods of time, may potentially harm the engine [158,159].

7.2.2. Vessel Design

Hydrodynamic optimization is a robust and reliable design method that plays an important role in hull optimization [160]. The robust and trustworthy design approach of hydrodynamic optimization is crucial for hull improvement [160]. A common tool for hydrodynamic simulations of clear, foul-free hull forms is computational fluid dynamics (CFD). Hull resistance can be reduced by using the best hull designs, less ballast management, lighter weight, low profile ships apertures, interceptor trim plates, skeg shape-trailing edge, and bulbous bow.

7.2.3. Cleaning

Maintaining the ship’s intended fuel consumption may benefit from routine hull cleaning. Algae, shellfish, and mussels are drawn to the bacteria on the hull’s surface, which increases the drag coefficient [161]. It is important to remember that the average surface roughness of a typical ship hull “grows by 40 m/year,” translating to an increase in fuel demand of 1%. [162].

7.2.4. Bulbous Bow

The bulb’s generated wave system interacts with the ship’s wave system. As a result, a bulbous bow, a below-waterline extension of the bow, forms a crest ahead of the ship and enhances water flow around the hull [163]. This minimizes drag for big vessels running within commercial speed ranges, resulting in up to a 20% reduction in fuel usage [23].

7.2.5. Low Profile Hull Openings

Sea chest apertures with low-profile hull openings can minimize resistance and hence fuel usage by decreasing the impacts of turbulence from bow thruster tunnels. These holes can be designed and optimized to reduce power demand by up to 5% [164].

7.2.6. Reduced Ballast Operation

Lesser wetted hull surfaces and lower resistance result from lighter displacement. Ballast must be adequate to maintain stability, handling, and propeller immersion at the proper depth. The highest fuel consumption reduction varies per design, although it can be as high as 7%, according to [23].

7.2.7. Air Lubrication

Air lubrication is a novel design (Figure 18) that uses air as a lubricant to reduce hull friction. “A layer of air is created between the vessel’s carefully designed underside and the water surface, allowing it to glide through the water effectively and reduce drag by 5–15%” ([165], p. 20). With commercial computer fluid dynamics (CFD) software, Fotopoulos and Margaris [166] computationally analyzed two distinct geometries, looking at the influence of air lubrication on fuel consumption, and estimated that it might be decreased by 8% [167]. Air lubrication can be carried out in three ways.

7.2.8. Propeller Design

Reduction of hull resistance can be achieved [168,169,170,171,172,173] via:

• Propeller efficiency management (1–2%)
• Pulling thruster (<10%)
• Constant vs. variable speed operation (<5%)
• Propeller tip winglets (<4%)
• Propeller nozzle (<5%)
• Advanced propeller blade sections (<2%)
• Wing thrusters (<10%)
• Counter-rotating propeller (<12%)
• Propeller-rudder interactions (<4%)

7.2.9. Waste Heat Recovery System (WHRS)

About half of the energy is lost due to irreversibility [173,174,175,176] and heat rejection to satisfy the Second Law of Thermodynamics. However, WHRS can recover some of this energy from exhaust gases, resulting in decreased emissions and fuel consumption [6,7,176]. The following sources of potential energy generation are: (1) jacket water (5.2%), (2) air cooler (16.5%), and (3) exhaust gases (25.5%) [177,178].

The heat from the exhaust fumes is used to power steam turbines that generate energy for auxiliary power generation in a WHRS. As a result, the auxiliary engines save fuel. Therefore, WHRS is best suited to ships that generate a lot of waste heat and use a lot of energy. Various estimates of the reduction emission potential have been reported in the literature; hence, the IMO reports an 8–10% potential [178,179]. According to other studies, 4–16% fuel savings are possible [7,179,180,181].

7.2.10. Interceptor Trim Plates

A vertical subsea projection at the rear of the hull called an interceptor trim plate directs high-pressure flow behind the propellers downward, producing a lifting effect. This is an option for ships that move at a reasonable speed, including RoRos and cruise liners. For a normal ferry, the improvement is 4%, whereas it is 1–5% for a minor increase in fuel efficiency [164].

7.2.11. Peak Shaving

Diesel–electric propulsion is a viable option for vessels that must navigate in sensitive areas or with prudence on the water (such as on urban waterways or at a variety of speeds). Ships that provide quick river or maritime connections as well as ships with dynamic positioning systems can both use diesel–electric power. Energy efficiency can be greatly increased by using a diesel–electric propulsion system in conjunction with a battery system. [181].

This is made feasible through a method called peak shaving. This makes it possible to absorb load peaks while releasing the necessary power as required. Installing a storage system that maximizes the power production required for extremely brief periods is advised in order to avoid employing massive diesel engines. The approach is modified to handle load peaks that are very changeable without emitting pollutants such as carbon dioxide (CO₂), nitrogen oxides (NOx), and sulfur oxides (SOx). Fuel, operating, and maintenance costs are also decreased when the installed capacity is decreased [181,182,183]. According to [184], a ship should expect to save up to 20% on fuel usage.

7.2.12. Ship Operational Considerations–Weather Routing

Since the fastest path is not always the quickest or most efficient in the event of bad weather, the fundamental goal of meteorological navigation is to determine the best course for long-journey travel [185,186]. This is due to the fact that sustaining speed in storms or bad weather creates additional resistance from the wind and waves, the amount of which is based on the severity and direction of the weather relative to the ship, despite the possibility of causing damage. This results in increased fuel use [187]. Alternately, if speed is decreased due to unfavourable conditions, predicted times of arrival may be increased, which will affect the availability of docking slots. Therefore, the main concept is to analyze current weather forecast data and choose the optimum route across quieter sea with the most downwind tracks using predictive and optimizing techniques. Such approaches are based on understanding the ship’s resistance in calm water and in turbulent sea [32,186,187,188].

7.2.13. Potential for Combined Carbon Removal

The several technical and operational solutions to decarbonize international shipping and the challenges surrounding each have been described in the preceding sections. The potential for carbon mitigation is outlined in this section, along with the possibility of employing fuels and efficiency improvements to help achieve the IMO’s 50% decarbonization goal. Based on a review of studies, Figure 19 and Figure 20 summarize the carbon savings provided by various fuels when compared to heavy fuel oil (HFO) or marine diesel oil (MDO) as well as other alternatives measures that lower overall fuel consumption. The analysis in the figures incorporates findings from a systematic study by Bouman et al. [18] and two industry reports [15,189].

Given purchase price and accessible infrastructures, LNG is undoubtedly the most practical short-term solution to reduce carbon dioxide emissions. Compared to HFO, LNG is anticipated to deliver a relatively moderate improvement, typically resulting in a 10% decrease in air pollutants. The integration of LNG and biofuel technology (bio-LNG) may offer up to 90% in the reduction of carbon dioxide emissions, assuming that the bio-LNG distribution network shows modest social and environmental implications. Biofuels have a huge variety of decarbonization possibilities but typically a decrease above 70% [190]. According to this assessment, nuclear power reduces carbon emissions by about 100%, while using grid power depends on the regional generating mix.

Measures that improve efficiency could lessen effects by, on average, 5 to 30%. Each method may yield modest efficiency advantages, but slow steaming accounts for the lion’s share of them (up to 60%). Furthermore, significant advantages are also provided by the use of solar and wind assistance (up to 32%) and advancements in ship design (near to 24%). The benefits are multiplied if combined because none of these options are necessarily restrictive, either among them or in combination with the fuel options.

In conclusion, a combination of multiple approaches may be used to implement specific technological and operational steps that would satisfy the maritime industry’s criteria for decarbonization. A new fleet with worldwide supportive laws and regulations may certainly accomplish this, but the existing fleet might need pricey retrofitting techniques to make these solutions possible. Therefore, efficient, internationally enforced rules must be used to enable a mixture of technology, fuels, and operational measures.

7.3. Government Policies Systems in Maritime Shipping

At several levels of government, advancements regarding shipping’s emissions have occurred [189]. Black carbon (BC) and diesel emissions are sometimes part of the intended emissions. In other circumstances, a reduction in emissions, including those from BC, is the express aim. Of course, one restriction of fuel efficiency measures is that improvements in efficiency must outweigh increases in traffic volume. This is an issue in both aviation and marine transportation.

Locally, Los Angeles, Long Beach, Oakland, and Vancouver on the Pacific coast of North America have all enacted measures to cut BC emissions. Combined, Los Angeles and Long Beach cut their emissions of particulate matter, including black carbon, by 81% between 2005 and 2013. Their scheme has an important distinction between using positive incentives that consist of cash compensation for each vessel entering the ports that complies with voluntary emission norms and using negative sanctions for non-compliance with required restrictions [190]. However, it is more complicated at the national EU level. In order to warn the IMO that the EU would start including shipping in its Emission Trading System in 2023 unless the IMO agrees to a global system that would cut shipping’s carbon, there is support in both the Commission and the Parliament. The EU’s split national member governments must yet vote to approve the idea. In any case, new EU regulations went into effect on 1 January 2018, requiring ports to gather information on exhaust emissions. This is only the first of several restrictions that will be implemented over the coming years [191].

Important policy challenges in Asia are related to China’s and India’s long-term aspirations to expand their seaports. India intends to build 10 brand-new mega-ports, modernize twelve already-existing government-owned ports, and build brand-new private ports. Regarding China, Chinese investors hold or have a stake in nearly two-thirds of the 50 largest container ports around the globe. Six ports in China and another 15 or so ports, notably seven in Africa and four in Europe, are part of its Maritime Silk Road (MSR) strategy [190,191]. Others are currently in advanced phases of development, while others are still being proposed. Up to 73 nations have expressed interest in joining the MSR.

Given the quantity and scope of these Chinese and Indian developments, emission control regulations at their facilities will naturally have an impact on local, regional, and even worldwide BC emission levels. If Chinese MSR port developments abroad are required to abide by local regulations, this might include EU regulations as well as regional and national regulations in cities such as Athens and Venice.

National Chinese Emission Control Zones (ECZs) are being developed in China, and international regional Emission Control Areas (ECAs) have been formed in North American and Northern European waterways. According to estimates from [192], the North American ECAs will mostly be responsible for fuel switching to avert a tripling of particulate matter less than 2.5 μm (PM_2.5) by 2030. BC is the subject of a proposal for a regional accord for the Arctic [193,194]. It might be patterned after some of the current ECAs or even added as an extension to one of them.

Many private attempts to further the regulation of maritime emissions have been made [195]. These include the International Association of Ports and Harbors’ World Ports Climate Initiative, the Sustainable Shipping Initiative, and the Clean Shipping Index (IHPH). Besides the diesel engine BC emissions of cars, railroads, and machinery in the harbour areas, the latter in particular has raised worries about ships BC [196].

Shipping Industry Climate Risk and Opportunity

Since the Paris Accord was ratified, investors and companies have begun to see climate change as a source of financial risk [197]. These risks may appear in the shipping industry in the following ways. The IMO emission objectives are currently quasi aspirational indicators for shipping firms, with most of the specifics on the methods to meet these goals still being defined. Increased regulation in this area is not only feasible, but also likely, given the IMO’s recent regulatory activity aimed at energy efficiency [198]. Additionally, shipping businesses may feel immense pressure from their business clients to provide low-carbon transportation choices. These businesses are motivated to address GHG emissions in their supply and chains by legislative and commercial constraints. On the other hand, the move to a clean maritime industry creates a variety of business opportunities, including the creation and implementation of new alternative fuel technologies and supply chain infrastructure.

Banks, investment firms, and other participants in the financial sector are also directly impacted by these kinds of risks and opportunities, in addition to shipping businesses and their corporate clients. Through the factors which contribute, property holdings, or credit investments, these players may be exposed to climate risks, which could result in bad loans, declines in asset prices, and decreased investment income [199].

Climate change is framed as a financial risk concern. The guidelines of the Taskforce on Climate-Related Financial Disclosures (TCFD), a voluntary industry standard, are swiftly becoming best practices for climate risk disclosure in several jurisdictions [200]. The TCFD advises firms and investors to structure their business strategies in accordance with the possible financial consequences and possibilities associated with various scenarios for mitigating climate change and implementing an energy transition. Institutional investors’ pressure is one of the main factors influencing these trends in investor-owned businesses [201]. Through their regulation of conventional financial reporting, business authorities are progressively examining the disclosure of financial risks associated with the climate in numerous jurisdictions [202].

Despite the increasing international focus on the value of financial stakeholders (particularly banks and stockholders) in society’s relation to climate change, which is a result of their significant influence over the accumulation of funds and resources, there is also a growing recognition of climate change as a financial risk issue. The advancement of European policy in this area serves as a template for comparable projects worldwide [203]. A program for deployment has already started after the European Commission released a plan of action on Sustainable Finance in 2018 that outlines methods for building a sustainable finance system. The EU-wide classification, which presents a list of economic activity categorized according to its commitment to sustainability policy objectives, is of particular significance. The taxonomy’s primary objective is to “allow financial system to recognize and respond to investment options that contribute to environmental policy objectives,” either by capital allocation to these initiatives or by enabling financial interested parties to interact with businesses to influence their operations in accordance with sustainability standards [204,205,206,207,208,209]. One of the industries where a lot of effort has already been made in identifying sustainability initiatives and proposing technological screening criteria to direct investment and funding decisions is transportation. The focus has primarily been on land transportation up to this point, but in the upcoming phase, specific sustainability standards for maritime shipping will be developed. These criteria are likely to include recommendations for low direct emission ships and efficiency improvements [205].

8. Perspective for Future Research

MBMs are a collection of indicators that were initially referred to as mid–term measurement methods by the IMO. MBMs require ship owners to make up for their pollution by internalizing the external environmental cost through a variety of means, including carbon tax and cash benefits. Initial measures, including EEDI and SEEMP, were intended as benchmarks for newly constructed and existing ships that took into account the addition of energy-efficient equipment in ship equipment and operational processes, such as optimization and weather forecasting, to improve performance while at sea. However, these actions have some limitations because they cannot be implemented to all fleets, which significantly lowers the emissions less than previously anticipated. The study on sustainable shipping by Shi et al. [206] categorizes these initiatives according to their technical, operational, or market-based characteristics and comes to the conclusion that long-term research should concentrate on zero-carbon technology and alternative energy sources. Additionally, considering that a rise in technological complexity leads to higher operational efficiency and enhanced vessel performance, new technologies such as sensors and big data analytics, that are already extensively used in other industries, are becoming more significant in the shipping sector. Ships currently generate enormous volumes of data from numerous sensor systems, and this volume of data has become a critical challenge for the sector. However, using big data analytics could help in tracking pollutants, forecasting a ship’s performance, and reducing its fuel usage. Currently, there aren’t many studies that concentrate on examining the possibilities of big data analytics and how they might assist in the mitigation of climate change caused by ships [187], thus making this a subject that merits further study but also a subject that has a wide range of advantages.

9. Trends, Challenges, and Conclusions

In order to prevent the negative environmental consequences of marine transportation and to direct the maritime industry, regulations and enforcement have been implemented on a national, regional, and worldwide level. This assessment examined the possibilities for a wide range of choices, covering fuels, energy efficiency technology, operations, and legislation, to decarbonize international shipping. Since there is no one way to completely decarbonize the maritime sector, a holistic approach is necessary. Decarbonization could be supported by long-term, continuous, and effective policy to allow the industry to significantly decrease carbon, even though it is based in a complicated international regulatory framework.

The primary substitute for MDO and HFO is LNG, which might reduce CO₂ emissions cost-effectively while also satisfying SOx and NOx emission standards. Although LNG is now less expensive than the standard maritime fuels, infrastructure must be improved to gain market share. LNG must be supplemented with efficiency techniques such as slow steaming, wind assistance, or even blend with bio-LNG in order to achieve a 50% decrease in GHG emissions.

On the other hand, biofuels appear to be slightly more expensive than sustainable energy. In addition, biofuels face issues in terms of durability and accessibility. As a consequence, due to ecological issues and price fluctuations, they could become unprofitable in the mid–long term [59].

Green hydrogen and green ammonia are particularly attractive fuels for fuel cells since they lessen or completely remove GHG emissions and pollutants. Although fuel cells face several obstacles in shipping uses, work is being carried out to test and confirm this technology by 2027. It was also examined how advanced the technology is currently that can help the maritime industry achieve profound transition to a low carbon future. They are not anticipated to be changed anytime soon, though, given the ICE’s hegemony in the maritime industry. Accordingly, studies and industry trends show that combusting blended fuels in ICE marine diesel engines is the way to go, at least in the short future.

Different efficiency measures may bring greater decarbonization possibilities even with conventional fuels. Fuel consumption and CO₂ emissions are cut by 20–30%, and even up to 60%, when steaming slowly. Antifouling coatings can be used to prevent biofouling and minimize drag, but further research is required to determine their cost–benefit ratio and potential impact on fleet emissions. Fuel savings of between 4 and 16% may be achieved through WHRS.

Evidently, there is a trade-off between price and footprints, with the most cost-effective sources, such as LNG, now only offering minor reductions in carbon footprint. In order to reduce expenses, a compromise between affordable fuels and enhanced efficiency measures is crucial. With LNG-fueled vessels, all categories of efficiency improvements must be applied in order to have a 50% chance of reducing GHG emissions by 50%.

To this end, it is crucial that the path to decarbonization includes a mix of fuels, technologies, and legislation, and that the different combinations of each serve to provide both short- and long-term solutions. A mix of incentives and port charges can successfully hasten the introduction of LNG because it is technologically sound, financially viable, and guarantees near-term environmental advantages. To promote the usage of nuclear power, renewables, and hydrogen in the long run, additional thought is still required. Energy efficiency programs, both technological and policy-related, can complement both strategies; however, it is crucial that a comprehensive policy be established in the near future to promote the swift and fair decarbonization that this significant sector desperately needs. Despite numerous studies on the technical and financial viability of several alternative energy sources for shipping, unanimous consensus is lacking.