Systematic Overview of Newly Available Technologies in the Green Maritime Sector

Abstract

The application of newly available technologies in the green maritime sector is difficult due to conflicting requirements and the inter-relation of different ecological, technological and economical parameters. The governments incentivize radical reductions in harmful emissions as an overall priority. If the politics do not change, the continuous implementation of stricter government regulations for reducing emissions will eventually result in the mandatory use of, what we currently consider, alternative fuels. Immediate application of radically different strategies would significantly increase the economic costs of maritime transport, thus jeopardizing its greatest benefit: the transport of massive quantities of freight at the lowest cost. Increased maritime transport costs would immediately disrupt the global economy, as seen recently during the COVID-19 pandemic. For this reason, the industry has shifted towards a gradual decrease in emissions through the implementation of “better” transitional solutions until alternative fuels eventually become low-cost fuels. Since this topic is very broad and interdisciplinary, our systematic overview gives insight into the state-of-the-art available technologies in green maritime transport with a focus on the following subjects: (i) alternative fuels; (ii) hybrid propulsion systems and hydrogen technologies; (iii) the benefits of digitalization in the maritime sector aimed at increasing vessel efficiency; (iv) hull drag reduction technologies; and (v) carbon capture technologies. This paper outlines the challenges, advantages and disadvantages of their implementation. The results of this analysis elucidate the current technologies’ readiness levels and their expected development over the coming years.

1. Introduction

Global economic growth is the most important factor for global energy demand, and this is also true for the shipping industry. International maritime trade increases rapidly as well as the number of vessels [1]. Since the 2008 crisis, the demand for container ships increased slowly, but the overall growth was limited due to the slow growth of the global economy and the oversupply of shipping capacity [2]. Freight rates reached historic lows in 2016, and shipping companies struggled to turn a profit [3]. Since then, the situation has improved, but major players in maritime transport are searching for innovative solutions to resolve the turmoil in the shipping market [4]. In 2019, the percentage increase in global trade was 18% compared to 2016 and this trend will lead to a 50% increase in fuel consumption in shipping between 2012 and 2040 [5]. Fossil fuels with a high sulfur content, particularly heavy fuel oil (HFO), dominate this market. Over one million metric tons of greenhouse gases (GHG) and carbon dioxide (CO₂) were created by shipping worldwide in 2018, an increase of 9.6% and 9.3%, respectively, from 2012 [6]. Consequently, the contribution of shipping to global anthropogenic emissions has increased to 2.9%. This poses a serious threat to the global environment and human health from shipping-related emissions of greenhouse gases [7], sulfur oxides (SOx) [8], nitrogen oxides (NOx) [9], and particulate matter (PM) [10]. This emissions situation of the maritime sector jeopardizes important global emissions commitments such as the Kyoto Protocol, the Paris Agreement, etc. One of the most effective solutions to this problem was suddenly provided by the coronavirus pandemic, drastically reducing total emissions, especially PM, by up to 38% [11]. Of course, that is not a long-term solution; therefore, the International Maritime Organization (IMO) and the entire shipping industry have a role to play in reducing their emissions. IMO has introduced and proposed stricter regulations for ship operators and owners in the maritime sector to address these issues [12]. The IMO has set a goal of reducing CO₂ intensity by 40% by 2030 and reducing total GHG emissions by at least 50% by 2050, both compared to 2008 levels [13]. It is estimated that at least 70% of current marine fuels will need to be modified to meet these regulations. By combining energy efficiency measures with a shift to low or zero carbon energy sources, there is an excellent opportunity to achieve very low and eventually zero GHG emissions from shipping. DNV’s maritime forecast for 2050 includes several predictions about how energy sources will be distributed and how much savings can be achieved with new technologies [14]. Figure 1 shows the GHG emission reduction potential of technologies that can contribute to the decarbonization of shipping. Some of the technical and operational solutions identified in the literature to improve energy efficiency and reduce GHG emissions include: implementing waste heat recovery systems, improving hull design and performance, installing or retrofitting energy-efficient engines, reducing vessel speed, and improving routing and scheduling [15]. Various efforts have been made to improve existing diesel engine-based technologies, such as using liquefied natural gas (LNG) in dual-fuel or gas engines. Although NOx, SOx, and particulate emissions can be significantly reduced with LNG, the GHG savings from LNG are limited to a maximum of 21%, and methane emissions could potentially negate the benefit. There are a number of potential biofuels that can replace fossil fuels to reduce emissions in shipping, such as biodiesel [16], bio-LNG or bio-methane [17], hydrogenated vegetable oil (HVO), synthetic diesel [18], and bio-methanol [19]. Additionally, as a transition solution, post-combustion carbon capture could reduce CO₂ emissions from the maritime industry in the short term, buying needed time until zero-emission technologies are fully developed and implemented. In addition to alternative fuels, there are also solutions that involve entirely new propulsion systems, such as all-electric and hybrid propulsion systems. In hybrid solutions, the main propulsion unit can be a traditional diesel engine, but it can also be a fuel cell system. Proton exchange membrane fuel cells (PEMFC) powered by H₂ have undergone rapid development in recent decades, leading to improved performance and lower costs, and have been adopted by parallel industries [20,21]. Lithium batteries are a rapidly evolving technology that offers great potential for harnessing renewable energy and improving the performance of existing power solutions. Batteries can be used as single components or in hybrid configurations in marine propulsion systems. When used in a hybrid configuration, batteries can optimize the load on other power sources [22]. Auxiliary energy sources in the ship’s hybrid system can be solar [23] or wind energy. A number of wind-assisted marine propulsion systems have been developed and tested to harness wind energy on modern ships. The most common wind-assist technologies are rotors, towing kites, suction wings, rigid and soft sails, wind turbines, and hull sails [24]. However, implementing these solutions requires overcoming a number of challenges: the availability of sustainable non-food biofuels, the high cost of batteries (making electric ships uncompetitive for long voyages), the availability and cost of synthetic fuels, and the adaptation of internal combustion and bunkering systems to these fuels [25]. Finally, in addition to propulsion, various technologies that affect the structure of the ship itself can lead to significant improvements. Technologies such as hull coating [26] and hull air lubrication [27] reduce drag and thus improve ship overall efficiency. The maritime sector does not appear to have any definitive solutions for reducing GHG emissions as of yet. With review of literature the following issues were also found:

(1) Different measures for reducing emissions and ship fuel consumption are not properly categorized and internationally standardized.

(2) There is a lack of consistency across the published studies that quantifies the possible drop in GHG emissions using different metrics, which suggests that the data sources may not be trustworthy.

(3) There is a lot of uncertainty regarding the impact of combining different reduction measures because the claimed emission reduction potential is primarily used for individual measures and few in-depth studies of the interdependence of different measures have been published.

Figure 1. GHG emission-reduction potential of technologies that can contribute to shipping decarbonization.

Figure 1. GHG emission-reduction potential of technologies that can contribute to shipping decarbonization.

Due to the mentioned gaps, the shipping industry still needs a thorough examination of this subject. Since this topic is very broad and interdisciplinary, the goal of this article is to offer a systematic overview of state-of-the-art available technologies in green maritime transport with a focus on the following subjects: (i) alternative fuels; (ii) hybrid propulsion systems and hydrogen technologies; (iii) the benefits of digitalization in the maritime sector aimed at increasing vessel efficiency; (iv) hull drag reduction technologies; and (v) carbon capture technologies. This article also outlines the challenges, advantages and disadvantages of their implementation. The results of this analysis elucidate the current technologies’ readiness levels and their expected development over the coming years with the most promising decarbonization pathways. This will be our starting base for future research in the field of green technologies applied to marine energy systems.

2. Recent Available Technologies for Green Maritime Sector

The selection criterion for these topics is based on DNV’s maritime forecast [14] and ITF analysis [28], according to which technologies are classified into different categories with varying potentials to reduce greenhouse gas emissions. Figure 2 shows that new fuels have the greatest potential for decarbonizing shipping, but greater energy efficiency and improved logistics are also needed to fully realize their benefits. Additionally, it can be concluded that the digitalization of various systems can lead to significant efficiency gains in a number of critical areas, including better design, energy-efficient operation, and improved fleet utilization. While any individual step can significantly reduce CO₂ emissions, it is doubtful that any single measure will be the most cost-effective approach for decarbonizing shipping; instead, a combination of measures that provide distinct decarbonization pathways is necessary. This chapter presents the reader with five subchapters on recent technological advances in the marine sector in the areas of alternative fuels: hybrid, battery and fuel cell systems; the benefits of digitalization with the aim of increasing ship efficiency; hull drag reduction technologies; and the latest carbon capture systems. Each subchapter will aim to showcase the latest and most promising articles in its field.

2.1. Alternative Fuels

The original IMO policy was to phase out fossil fuels as soon as possible in this century while continuing efforts to lower the carbon intensity of international shipping by 70% by 2050 and the overall yearly greenhouse gas emissions by at least 50% compared to 2008 levels. Therefore, the common practice for SOx emissions reduction was to start using low sulfate heavy fuel oil (LSHFO), marine diesel oil (MDO), marine gas oil (MGO) along with exhaust gas cleaning systems (EGCS). Additionally, with engine technology advancements being more limited with time and EGCSs for NOx emissions from ships in their infancy, liquefied natural gas (LNG, mostly methane), liquefied petroleum gas (LPG) and methanol have emerged as the primary possibilities for meeting IMO NOx emission regulations. Recent studies [29,30] have explored the possible decarbonization pathways, and the CO₂ avoidance potential of alternative marine fuels such as LNG, methanol, biofuels, hydrogen, and ammonia is estimated at 20–100% (Figure 2).

Xing et al. [31] examined and explained a variety of potential marine fuels with the aim of attaining low-carbon shipping by 2050. The combined decrease of SOx, NOx and CO₂ emissions was taken into consideration along with key physicochemical characteristics, feedstocks, transportation, manufacturing methods and emission performance. The examined fuels were hydrogen, renewable natural gas (RNG), ammonia, methanol and biodiesel. Finally, the following conclusions were reached:

• It is recommended to replace primary marine fuels such as LSHFO and MDO because replacing them would take time. Prior to the start of a new age of alternative marine fuels, the choice of the most promising route is extremely crucial. Given the modest potential for carbon reductions and the significant infrastructure expenditure, LNG or LPG should be used with caution as a marine fuel.
• In some areas, hydrogen and ammonia are the best fuels for coastal shipping since they are carbon-free fuels with already promising futures in the road transportation sector. The present cost structure is still excessive, and the infrastructure is insufficient, so based on local low-carbon development objectives, encouragement for the implementation of national and regional incentives should be given. Additionally, they are not advised for deep-sea application due to their low volumetric energy density and limited potential.
• The unreliable fuel supply for biofuels, including so-called carbon-neutral fuels such as RNG, biogenic methanol, bioethanol and biodiesel, is a result of the varied geographic distribution of land and water resources as well as competition from other energy-consuming industries. As a result, the future of biofuels in international shipping is not encouraging. However, there could be advantages to using them in local and regional shipping, but that needs further investigation. Liquid biofuels could start to be used more frequently in shipping and aviation as a result of the growing electrification of road transportation. RNG and methanol may be produced using fossil and renewable feedstocks, as well as with renewable and fossil energy combined with captured CO₂.
• The most viable alternative fuel for international transport is methanol, both current fossil methanol and future renewable methanol. Therefore, the international shipping community must come to an agreement and establish a united plan of action given the lengthy timelines connected with marine fuel replacement.
• The combination of alternative marine fuels suggested in this study includes hydrogen, ammonia, and biodiesel produced from renewable energy sources for short-sea shipping, and methanol for worldwide shipping. It is recommended that LSHFO and MDO are gradually phased out and it is suggested that the development of vessels powered by LNG and LPG should be carried out cautiously. Consequently, future research and development efforts should place a high priority on fuel cell devices that run on hydrogen, ammonia, and methanol.
• When building infrastructure, it is important to consider cogeneration (cooling, heating, and electricity) while utilizing raw and recycled materials in the process of creating alternative fuels. This would be a significant step in lowering production costs, which are still a major barrier to the broad use of alternative marine fuels.

In Table 1, the usual physicochemical characteristics, emissions, and qualitative evaluation of marine fuels are discussed. In summary, carbon-neutral options, such as biofuels, have great promise as future fuels and may be utilized as drop-in fuels, which reduces the need to change existing technologies, particularly biodiesel [32], with its energy-efficient manufacturing methods. The switch to methanol is less encouraging since it requires a lot of energy to produce and has greater CO₂ emissions throughout the production stage. To fulfill the IMO 2050 objective, CO₂ emissions can only be reduced significantly by utilizing bio-methanol. All hydrogen and ammonia paths result in fewer CO₂ emissions. However, switching to hydrogen or ammonia through the blue pathway will not result in zero carbon consumption in the marine industry unless a green production method is adopted. Electrical power supply from batteries, followed by biodiesel, is the alternative fuel with the highest energy efficiency. However, because of the poor energy density and expensive batteries, it cannot be used for long-distance delivery.

To better understand the characteristics of the literature on clean alternative marine fuels, a bibliometric method was applied in [33]. The alternative maritime fuel that has received the greatest research is liquefied natural gas (LNG). Recent developments imply that scientists are now more focusing on methanol, ammonia, and hydrogen fuels. As was evident from an analysis of the frequently used keywords and relevant articles, the research community has primarily focused on the potential of different alternative fuels as substitutes for conventional marine fuels to limit emissions from the shipping sector from an environmental and economic perspective.

Table 1. Typical physicochemical properties and emissions of marine fuels [31] and their qualitative assessment [34,35].

Fuel	Chem. Formula	Well-to-Wake Energy (rel. to HFO)	Well-to-Wake Cost (rel. to HFO)	Scalability	Regulations	Technology Readiness	ICE Combustion Emissions
							CO2	NOx	SOx	PM
LSHFO	C8-C25	100%	100%	Scalable	IGF Code	Commercialized	100%	100%	100%	100%
LNG	CH4	101%	110%	Scalable	IGF Code	Commercialized	92%	7%	0%	4%
Blue H2	H2	138%	306%	Scalable	Require amendment of IGF Code	Small scale	17%	100%	0%	0%
Blue H2 (FC)	H2	112%	469%	Scalable	Require amendment of IGF Code	Small scale	14%	0%	0%	0%
Blue methanol	CH3OH	177%	221%	Scalable	Require amendment of IGF Code	Small scale	129%	19%	0%	0%
Bio-methanol	CH3OH	134%	384%	Challenging	Require amendment of IGF Code	Small scale	15%	108%	11%	26%
Biodiesel	C18-C18	100%	190%	Challenging	ISO 8217:2017 standard	Small scale	1%	108%	11%	26%
Ammonia	NH3	178%	371%	Scalable	IGF code not approved	Small scale	34%	100%	0%	0%
Ammonia (FC)	NH3	145%	521%	Scalable	IGF code not approved	Small scale	27%	0%	0%	0%

Table 1. Typical physicochemical properties and emissions of marine fuels [31] and their qualitative assessment [34,35].

Fuel	Chem. Formula	Well-to-Wake Energy (rel. to HFO)	Well-to-Wake Cost (rel. to HFO)	Scalability	Regulations	Technology Readiness	ICE Combustion Emissions
							CO2	NOx	SOx	PM
LSHFO	C8-C25	100%	100%	Scalable	IGF Code	Commercialized	100%	100%	100%	100%
LNG	CH4	101%	110%	Scalable	IGF Code	Commercialized	92%	7%	0%	4%
Blue H2	H2	138%	306%	Scalable	Require amendment of IGF Code	Small scale	17%	100%	0%	0%
Blue H2 (FC)	H2	112%	469%	Scalable	Require amendment of IGF Code	Small scale	14%	0%	0%	0%
Blue methanol	CH3OH	177%	221%	Scalable	Require amendment of IGF Code	Small scale	129%	19%	0%	0%
Bio-methanol	CH3OH	134%	384%	Challenging	Require amendment of IGF Code	Small scale	15%	108%	11%	26%
Biodiesel	C18-C18	100%	190%	Challenging	ISO 8217:2017 standard	Small scale	1%	108%	11%	26%
Ammonia	NH3	178%	371%	Scalable	IGF code not approved	Small scale	34%	100%	0%	0%
Ammonia (FC)	NH3	145%	521%	Scalable	IGF code not approved	Small scale	27%	0%	0%	0%

Hansson et al. [36] choose marine fuel based on their relative performance on ten criteria that addressed economic and environmental issues, as well as the relative significance of these factors based on the preferences of Swedish stakeholders. LNG is ranked top among ship owners, fuel suppliers, and engine makers, followed by HFO and fossil methanol, and then biofuels (in varying order). These rankings are the result of the fact that for these stakeholders, economics, particularly gasoline price, is by far the most decisive factor. In contrast, according to government organizations, renewable hydrogen comes out on top, followed by renewable methanol and HVO. This is due to the fact that for these players, GHG emissions and the potential to meet regulations are the most key determinants. Regarding the possibility of various biofuels for shipping, this article [36] does not offer any conclusive findings. Although none of the stakeholder groups place biofuels first, the shipping sector may nevertheless be interested in them. Since most biofuels can be used in existing engines (or with minor modifications, unlike hydrogen), some biofuels are already commercially available and offer greater CO₂ reduction potential than LNG, they may be an appealing solution in situations where there is a pressing need to switch to fuels with low GHG emissions. There are various active projects for the use and production of alternative marine fuels [37]. Most efforts and operations are on a pilot or test size, with the exception of LNG and electric propulsion in short-sea transport. It is therefore still a long way off before new marine fuels are introduced that are also commercially viable, especially for deep-sea transportation. Another intriguing idea for future maritime transportation is electro-fuels, which are created utilizing renewable energy from CO₂ and water [38].

According to Svanberg et al. to [39], producing renewable methanol from biomass for use in the shipping sector is a theoretically possible way to lessen the environmental effect of marine transportation. Using methanol from fossil feedstocks as a supplement can facilitate the switch from fossil fuels to renewable methanol. Additionally, some transportation and storage infrastructures are already in place or are easily adaptable to current circumstances. Methanol burned in a diesel engine may be lower in purity than the high-grade methanol used by consumers in the chemical industry. In experiments [40] and with more detail in [41], methanol with a purity as low as 90% has demonstrated that it functions effectively as a fuel. Additional studies on the reductions in manufacturing costs and environmental impact of using less distilled methanol would be beneficial, as would work that provides more details about the type of methanol that would be suitable for a wider range of methanol engine designs. Regarding large marine engines, successful dual-fuel engine conversions [42] and new dual-fuel engine production [43] using methanol have been carried out with good results, although only in a few cases. Over the years, methanol has undergone considerable testing and has been utilized in spark-ignited automobile engines with little modification needed. The types of compression engines used on the majority of ships have undergone little testing since they need an ignition assist, which necessitates additional modifications. Methanol was tested in a diesel engine using pilot diesel fuel to ignite the methanol in [44], and they were able to achieve good engine performance, low fuel consumption, and decreased emissions of nitrogen oxides and particulates. In order to retrofit the engines of the Stena Germanica ferry, another study was conducted by marine engine manufacturer Waertsila in which the researchers researched various methanol combustion methods and ultimately settled on one in which a small quantity of diesel pilot fuel is used to ignite the methanol. The engines converted according to this concept have been operating successfully since 2015. The long-term supply and availability of fuel is a crucial concern for the shipping sector, particularly for a new fuel, such as methanol, and necessitates considerable investment from shipowners. Only 200,000 tons of bio-methanol were generated worldwide in 2013, according to the International Renewable Energy Agency, making it a relatively limited resource at the moment. Several feedstocks, including as solid and liquid biomass, as well as solid waste from waste incineration, are utilized to create renewable methanol. The potential yearly output of bio-methanol in Sweden was calculated at 40 TWh in [45]. To put it in the perspective of shipping, this may be compared to the yearly methanol need for the 69-vessel Swedish National Road Ferries fleet.

Whether ammonia can actually aid in lowering marine activities’ carbon footprint was the subject of research in [46]. Eight various ammonia production techniques were integrated with the study, including steam methane reforming, photovoltaics, wind electrolysis, underground coal gasification (UCG) with carbon capture and storage (CCS), UCG without CCS and the three-step Cu-Cl cycle. As seen in Figure 3, the American Bureau of Shipping (ABS) has discovered some noteworthy conclusions. They contrast the LSHFO well-to-wake emissions with those of green, blue, gray, and orange ammonia. Electricity produced from renewable resources is used to create green ammonia. Natural gas is used to produce gray ammonia. The source of blue ammonia is the same as gray, but carbon emissions from the conversion process are captured by a carbon capture system. Gray and green ammonia are divided up to create orange ammonia. Compared to LSHFO, green ammonia had 83% fewer life cycle carbon emissions, blue ammonia had 57% fewer, orange had 17% fewer and gray had 48% more.

As for ammonia-based engines, manufacturers such as MAN recognized the need to usher in a new era in marine engines. By building the first 2-stroke ammonia engine based on liquid gas injection (ME-LGI) by 2024, MAN has recognized how important carbon-efficient ammonia could be for the industry [47]. In particular, this dual-fuel engine makes ammonia a viable choice for any ship that must fulfill IMO decarbonization regulations by 2050 in the maritime sector. The ME-LGI engines are dual-fuel engines that run on diesel and methanol or LPG, according to the manufacturers; however, MAN said that this technology is actually more suited for ammonia than methanol. The ME-LGI may be converted into an ammonia-fueled engine with a few minor technical adjustments to the fuel delivery system, such as delivering ammonia at 70 bar and injecting it into the cylinder at 600–700 bar. More details can be found in [48]. From an environmental standpoint, using ammonia as a marine fuel is not as safe as it would appear. Ammonia might leak during combustion if the exhaust valve malfunctions. Because of its toxicity, unburned ammonia can escape and present a major risk to not only the materials it comes into touch with but also to both people and the environment. The ME-LGI system, which adds the ammonia fuel later in the compression stroke via a high-pressure direct injection system, offers a solution to this issue. Ammonia leakage will be less of an issue as a consequence. Along with ammonia slip, NOx generation during combustion must also be considered. Recent studies have shown that, while eliminating CO₂, ammonia may produce approximately as much NOx as LSHFO. As a result, NOx management is essential, and the Selective Catalytic Reduction (SCR) system should be used. The study came to the conclusion that the 3-stage Cu-Cl cycle [49] is the most promising pathway for the use of ammonia as a fuel in the maritime sector since it produces the least amount of carbon. The study’s findings also indicated that the environmental indicators suggested in this article have the potential to advance the sector and are anticipated to benefit long-term stakeholders.

An analysis of the manufacturing processes, techno-economic performance, storage and safety was conducted by Atilhan et al. [50] in order to analyze the possible use of green hydrogen in the maritime industry. Benchmarking was also conducted for the maritime sector versus current gray and blue production pathways. By reforming fossil fuels, gray hydrogen is created [51]. More than 95% of the hydrogen used in the world is created by the reforming of fossil fuels, with natural or shale gas making up nearly half of this amount [52]. This process generates just a smaller amount of emissions than black or brown hydrogen, which uses black (bituminous) or brown (lignite) coal in the hydrogen-making process. The most harmful type of hydrogen for the environment is black or brown hydrogen since neither the CO₂ nor the carbon monoxide produced during the process is captured. Hydrogen is deemed to be blue when carbon emissions are caught, stored or utilized (for instance, dry reforming [53]). Turquoise hydrogen refers to a way of creating the element through a process called methane pyrolysis, which generates solid carbon. As such, there is no need for carbon capture, and the carbon can be used in other applications, such as tire manufacturing or as a soil improver. Its production is still in the experimental phase. Green hydrogen primarily relates to the conversion of renewable feedstock using renewable energy. All hydrogen production pathways are shown in Figure 4.

The carbon footprint of gray Liquid Hydrogen (LH₂) is significant (120–155 gCO_2eq/MJ) when the life cycle of hydrogen in the tank is considered. This figure is substantial given that the carbon footprint of heavy fuel oil is about 90 gCO_2eq/MJ [54]. Blue LH₂ has a smaller carbon footprint that varies between 40 to 90 gCO_2eq/MJ depending on the composition of the source of sequestered CO₂, the technology used to capture it, and the amount and route of use. Reduced GHG emissions are shown in green LH₂ (4.6, 11.7, and 43.3 g CO_2eq/MJ from wind, solar, and grid, respectively). There are benefits and drawbacks to utilizing LH₂ when it comes to safety and health concerns. Although non-toxic, it can cause choking hazards. From a maritime standpoint, LH₂’s high energy density (2.8 times that of HFO on a MJ/kg basis) is beneficial, but when paired with its broad flammability range, lower boiling point, and lower flame temperature, it can increase the danger of fire and explosion. Many nations have conducted extensive studies on the combustion of hydrogen in internal combustion engines. Engine knock is a major issue when using hydrogen in internal combustion engines, much like with natural gas. For an engine to run without knocking, the air-fuel ratio and intake temperature have been recognized as the key causes of the issue [55]. Most crucially, in order to build a supply chain and meet the challenging CO₂ reduction objectives in the marine industry, there must be a global usage and demand for hydrogen fuel. These prices are anticipated to decline as renewable energy and hydrogen production technologies progress.

2.2. Alternative Propulsion and Auxiliary Systems

A promising solution for enhancing the economic and environmental performance of ships is hybrid propulsion, which combines mechanical and electric propulsion [56]. One of the main disadvantages of the complete replacement of conventional propulsion systems with electric ones in merchant ships is limited energy storage capacity [57]. In the simplest type of hybrid propulsion, the propeller may be powered physically by an internal combustion engine or electrically by an electric motor that can also serve as an electric generator. It is a hybrid propulsion system with a hybrid power supply when the electric motor is powered by a hybrid power source, such as diesel generator(s), natural gas generator(s), fuel cells, and/or batteries [58]. The modes of operation of a hybrid propulsion system include power take-off (PTO), slow power take-in (PTI), and boosted power take-in [59]. Lower fuel consumption, fewer CO₂ emissions and other pollutants, the flexibility to operate and cruise in port and coastal regions with zero emissions, increased redundancy, decreased noise, and lower maintenance needs are all benefits of a hybrid propulsion system [60]. However, due to their various operational characteristics, many vessel types may benefit from hybrid propulsion in different ways. Hybrid propulsion systems are typically favored for ships performing the majority of their operational hours at less than 40% of their maximum speed. Contrarily, smart ships equipped with DC power systems have significantly reduced fuel usage and emissions (down to 20% from conventional designs) [61]. In order to establish which types of ships may benefit from hybrid propulsion, the operating profiles of eight distinct ship categories, including tankers, bulk carriers, general cargo ships, container ships, Ro-Ro ships, offshore ships, and passenger ships, were examined in [62]. Figure 5 demonstrates that these particular ship types are the most prevalent in terms of both the quantity in the European fleet and the volume of CO₂ emissions released [63]. Hybrid propulsion systems are typically used on naval vessels, tugs, offshore vessels, and passenger ships including ferries.

Some of the most recent advancements in ecologically friendly hybrid ships are addressed by Nguyen et al. with flywheel storage [64] and by Koumentakos et al. with shaft generators [65]. The shaft generator was one improvement that was highlighted. The energy Efficiency Design Index (EEDI), which was introduced in 2011, encourages the use of technologies such as shaft generators to reduce the use of auxiliary generator sets [66]. This reduced consumption lowers the amount of fuel oil used, which, as was already indicated, lowers the amount of CO₂ emissions. The shaft generator compels the diesel engine to operate in a load range relatively near the ideal fuel oil consumption point [67]. Aside from the aforementioned benefits, the frequency converter may be utilized to adapt to various shore power voltages and frequencies after it is installed as a component of the shaft generator system without the need for extra panels in the main switchboard. A selective control and optimization system is suggested in the study [68] that enables the shaft generator to work in Power Take Off (PTO) or Power Take In (PTI) mode, guaranteeing that the main engine always operates in the optimal fuel consumption range and ensuring the lowest CO₂ emissions.

Model Predictive Control (MPC) is used to investigate the topic of energy management methods in hybrid diesel-electric marine propulsion systems in [69]. Many possible hybrid configurations have been proposed [58]. The top focus with such hybrid systems is their control strategy or the choice of how to divide the drivetrain’s power based on a variety of factors. The physical and operational limitations of the hybrid system have been considered in the design of the controllers. To assess the capabilities of the suggested control mechanism, a number of MPC designs were taken into account [70]. The air-fuel equivalence ratio λ was chosen because it is a readily available metric that can characterize instantaneous engine emissions. Additionally, precise restrictions on fuel oil use and NOx concentration were provided. On the hybrid diesel-electric test rig, the effectiveness of three controllers was experimentally confirmed under actual ship operating circumstances. The findings demonstrated that the system could be effectively regulated during transient operation to lower gas emissions and improve fuel efficiency while successfully regulating the input and output conditions for hybrid propulsion. The MPC controllers reduced NOx emissions by up to 42% while reducing fuel usage by 33%.

According to research published by Yoo et al. [71], a hybrid marine energy system made up of dual-fuel generators, a fuel cell, and vanadium redox flow batteries (VRFB) was presented. The VRFB stack unit was modeled to calculate both reversible and irreversible capacity deterioration. Due to its advantages over other extended storage systems (ESSs), this study concentrates on the usage of VRFBs as an ESS in the marine industry. In terms of energy and power capacity, VRFBs are independently expandable [72]. The millisecond response speed enables the use of VRFBs for load balancing and as an uninterruptible power supply [73]. The ability of a VRFB to use a liquid electrolyte allows it to be recharged on site, which is very attractive for mobile use [74]. VRFBs still have a lower power density than lithium-ion batteries (LIBs); however, this has been raised to 557 mWcm⁻² [75]. A VRFB’s redox coupling is what restricts its energy density, although advances have been made by boosting the amounts of appropriate additives that support electrolyte stability [76].

The article [77] uses secondary data analysis to explore the development of wind-assisted ship propulsion (WASP) in marine transport, provides the possible influence on fuel economy, and highlights the crucial elements that affect the technology’s operational efficiency. Different commercial WASP technologies are now on the market; hence, a quick summary of these technologies is given below [78]:

• Rotors: yhese rotating cylinders, which are positioned on deck and use the Magnus effect to provide forward propulsion, are also known as Flettner rotors since Anton Flettner was the person who first patented them.
• Towing kites: by generating lift from high-altitude winds, towing kites propel ships.
• Suction wings: similar to the wings of an airplane, suction wings provide an upward lift force.
• Rigid sails/wing sails: rigid sails or wing sails are foils that may be modified to produce aerodynamic forces.
• Soft sails: soft sails are traditional sails with modern characteristics. The DynaRig, which is presently employed mostly on big sailing yachts, serves as an example [79].
• Wind turbines: these are turbines mounted on a ship’s deck that produce thrust or power used for propulsion.
• Hull sails: hull sails are ship hulls that use the relative wind with their symmetrical hull foils to generate aerodynamic lift.

As demonstrated in

Table 2. Comparison of different WASP technologies.

Study	Dimensions of the Technology	Ship Type	Route	Fuel Savings Found
[78]	2 Flettner rotors: h = 22 m, d = 3 m 3 Flettner rotors: h = 48 m, d = 6 m	5k dwt Tanker 90k dwt Tanker	Worldwide trade of each ship type according to AIS data	5–7% 9–13%
[85]	1 kite: a = 320 m2, l = 300 m	50k dwt Tanker	N.A.	10–50%
[86]	1 wingsail (rigid): h = 50 m, w = 20 m	Aframax Tanker	Cape Lopez–Point Tupper Angra dos Reis–Rotterdam	8.8% 6.1%
[86]	1 Dynarig (soft): area = 1000 m2	Aframax Tanker	Cape Lopez–Point Tupper Angra dos Reis–Rotterdam	5.6% 4.2%
[78]	1 wind turbine: height (h) = 20 m, diameter (d) = 38 m	5k dwt Tanker	Worldwide trades of each ship type according to AIS data	1–2%

, the findings of previous research consistently demonstrate that WASP technologies have the potential to assist ships in achieving large fuel savings under a range of circumstances. It should be noted that the available studies use different parameters in their models depending on the technology’s quantity, size, and other technical features, the vessels’ type and speed, the wind conditions and the routes. Studies that directly compared the two technologies under the same conditions found that Flettner rotors saved more fuel than DynaRigs (soft sails). In [80], it was discovered that the power output of Flettner rotors was less susceptible to geographic locations and weather circumstances than that of kites since the latter naturally create less propulsive power throughout a larger range of wind directions. However, ref. [81] discovered that kites had several benefits over traditional sails, including the ability to absorb stronger winds at greater altitudes and a lower point of connection to the ship, which reduces the rolling moment. They also occupy less room on the deck. For the models identified in the available literature, fuel efficiency is significantly influenced by wind speed and wind direction. Generally speaking, it has been discovered that the higher the wind speed, the bigger the energy production of the WASP technology, and the better the fuel savings that occur. On the other hand, increased wind speeds frequently result in larger wave heights, which is detrimental to vessel performance. To generate more precise forecasts of fuel consumption while predicting the performance of ships using WASP technologies, more complex models that account for lateral forces and yawing moments should be utilized [82]. According to [83], route optimization increases fuel savings by WASP technology from 14–36% to 28–53%. Smith et al. [84] showed that by deviating from the Great Circle and using the wind and waves, an extra 5–10% in fuel savings may be gained.

An established model-based design technique for coastal hybrid ships was put out in [87]. The power source sizing problem is solved using constrained mixed-integer multi-objective optimization at the external layer. The globally optimal energy management strategies for an averaged operational profile were determined by deterministic dynamic programming in the inner layer, considering the degradation of the energy sources in the sizing algorithm. In order to examine the viability and possible advantages of the hybrid PEMFC and battery propulsion system in Matlab, the suggested technique was applied to a coastal ferry. In order to assess the specific hydrogen consumption of the fuel cell system and the rates of fuel cell degradation under various operating circumstances, a system-level PEMFC model was constructed and calibrated in accordance with [88]. PEMFCs usually have substantially shorter working lives than marine diesel engines; the pace of deterioration can be influenced by elements such as power transients, cycling, and charging conditions. To obtain an overall optimal cost performance given the high cost of PEMFC manufacture, degradation characteristics must be taken into account throughout both the ship design and operating periods [89]. An equivalent circuit was created and calibrated using experimental data from [90] in order to depict battery performance over the whole capacity range. For ordinary vessel operation, the two basic modes of operation are sail mode and port mode. The battery acts as an energy buffer while the ship is in sailing mode, which includes cruising or maneuvering, to improve fuel cell use and lessen PEMFC power peaks. The shore connection powers the ship’s electrical loads and charges the battery while the PEMFC is turned down or removed from the grid when the ship is at anchor (port mode). By keeping track of the power demand, fuel cell power level, battery state of charge (SOC) and shore power availability, the energy management system controls the power-sharing amongst the power sources. It then decides the fuel cell power change for the following time step. The plug-in hybrid design with fuel cell and battery propulsion can reduce emissions for the two cases analyzed with use of two different green H₂ sources by at least 65%.

2.3. Digitalization

Cross-industry research has shown that there have been significant improvements in industries that are in pursuit of digitalization, where these improvements have led to better economic performance [91]. Although digitalization in the shipping industry has been relatively slow compared to other industries, large companies such as Rolls Royce and Wärtsilä have established research and development centers to explore remote and autonomous shipping. In Figure 6, some of the motivations and challenges of further implementing digitalization in marine systems are presented. Machinery built for ships usually does not last the expected lifetime. This may be due to a lack of maintenance or an inability to detect faults earlier to prevent catastrophic damage [92]. To mitigate such risks, predictive maintenance that includes equipment health assessments through regular inspections (i.e., Big Data Analysis (BDA)) and continuous monitoring of equipment health (i.e., IoT) enables a much more efficient maintenance process by enabling the remote diagnosis of marine machinery. When faults are detected immediately, further engine damage can be prevented, reducing fuel consumption. This in turn reduces the amount of greenhouse gas emissions and results in 10–35 percent less costly operation [93]. According to a report [94], predictive maintenance can reduce unexpected failures by 55% and is estimated to reduce maintenance costs by 25% to 30%. Companies also use advanced technologies such as weather routing, which gives ships enough time to avoid bad weather [95]. In addition, technology ensures that ships’ gas emissions and cargo temperatures are monitored from shore, reducing maintenance costs and the risk of failure due to negligence [96]. To stay ahead in a constantly evolving environment, the maritime industry is increasingly emphasizing the use of digital technologies, such as BDA and Machine Learning (ML) to create digital twins [97]. This allows large amounts of data to be collected, stored, and processed so that many aspects of ship operations can be handled efficiently and effectively via digital platforms. One example of the use of a digital twin was by Matulic et al. [98] where a digital twin was developed and could simulate multiple engine failures simultaneously, for example, injector and exhaust valve leakage, etc. The model might be applied on board the ship for the development of diagnostic software, software testing in a loop scenario, or complete engine optimization. Another example of an expert system was offered by Rudzki et al. [99]. In reality, operators of ships with variable pitch propellers choose the commands based on their own knowledge and readily available outside information. Their choices are frequently irrational or flawed. Due to this, a two-objective optimization model-based decision support system was created. It allows the continuous dialogue between the decision-maker and the computer, where the decision-maker takes the appropriate decisions, and the computer processes the collected data and makes available a proposal of possible upcoming outputs.

In Ref. [100], the author provided an overview of autonomous ship development projects, as well as pointed out the benefits of autonomous ships based on environmental, economic, and social perspectives. Furthermore, innovative, autonomous ship applications were proposed for short-sea shipping, Arctic shipping, and conventional shipping, as well as the discussion of the potential business models from the autonomous ship manufacturer’s perspective. The adaptation of autonomous ships can significantly reduce operating costs while reducing the shipping carbon, CO₂ and NOx emissions for all considered contexts. The feasibility of autonomous ships has already been established [101]. However, despite the many potential benefits of autonomous ships, there are some challenges. For short-sea shipping, these challenges include new port operational capabilities, training of dedicated crew, fleet inefficiencies due to lower vessel speed, as well as operational risks related to cyberpiracy [102]. From the perspective of Arctic shipping, autonomous ships can be the solution for many challenges in commercializing the Arctic route, but insuring that autonomous ships sail on the Arctic route would be expensive. Although the study [102] offers a thrall analysis of the considered concept, the study is still conceptual, and therefore many future research possibilities are left open. For example, optimization models [103] can facilitate the design of potential short-sea shipping service networks, and multi-criteria decision models [104] can support decision making in navigating the Arctic route, etc.

The fuel consumption, as expected, represents a great share of the overall maritime shipping operating costs, and therefore, vessel voyage optimization has become vital, not only for the financial benefits but from the environmental perspective as well. However, vessel voyage optimization is a challenging task, as it is influenced by many complicated business and time-varying factors [105]. Virtual arrival is one of the new promising vessel voyage optimization approaches regarding emission control countermeasures in maritime shipping. Zis et al. [106], Du et al. [107] and Schwartz et al. [108] have pointed out that ship emissions can be reduced significantly by reducing the ship’s passage speed, which could be achieved through virtual arrival and shorter anchorage time. However, Poulsen and Sampson [109] highlighted the difficulties associated with virtual arrival implementation and explained why shipping is unlikely to achieve the associated mitigation potential, as assumed in previous studies. First and foremost, cargo owners have other commercial interests besides fuel saving, and for them, long anchorage time is not caused by irrational behavior. These results are consistent with the study on greening shipping from the cargo owners’ perspective [110]. In [111], it was suggested that the IMO and the EU should consider policies aimed at forcing operators to adopt virtual arrival in shipping. However, the contrary results suggest that the commercial benefits (to cargo owners) of quick access to cargo outweigh the fuel savings benefits by several orders of magnitude. The study also points out the risks of unintended negative consequences associated with virtual arrival. In particular, the risks of seafarer fatigue must be considered [112].

Emissions from ships do not affect the quality of life equally throughout their entire operational cycle. The highest concentration of released pollutants is located closest to the general population, in ports [113] and shipyards [114]. The time that ships spend in ports accounts for a sizeable portion of the CO₂ emissions from shipping. Emissions from ships are almost 10 times larger than those from the ports’ own activities, making them the single biggest cause of port-related pollution. By implementing incentive programs that encourage fuel savings within the port area and supporting systems and technology, port authorities may reduce GHG emissions from ships [115]. Los Angeles and Long Beach (CA, USA) were the first ports to implement port-level initiatives, with the Alternative Maritime Power program requiring the shutdown of auxiliary diesel engines at-berth in 2004 and the Vessel Speed Reduction Incentive Program requesting ships to reduce speeds as they approach the port in 2008. Similar initiatives have been carried out by Gothenburg and Mediterranean Ports, typically as a result of incentives or voluntary participation [116].

Styhre et al. [117] in their study developed a way to quantify ships’ GHG emissions for four geographically divided ports (Gothenburg, Sydney, Osaka and Long Beach) and discussed the potential GHG emission reduction strategies. Reduced speed in fairway channels, onshore power supply, shorter turnaround times at the dock and alternative fuels were all evaluated as reduction measures used to inform the case studies. A model for calculating emissions from ships in ports that was developed by the Swedish Environmental Research Institute was used [118].

Comparisons across ports can be made only in the context of ship traffic characteristics, such as type of shipping, ship types and sizes. Additionally, each port’s geographic borders have an impact on the emissions. To illustrate, the fairway channels in Sydney and the Port of Gothenburg are longer than those in the other two ports. The primary sources of GHG emissions are the liquid bulk tankers and ferry/RoRo vessels for the Port of Gothenburg, liquid bulk tankers and container ships for Long Beach, container ships and ferry/RoRo for the Port of Osaka, and container ships and liquid bulk tankers for Sydney Ports. Emissions calculated per every port call (the time when the vessel loads/unloads cargo or embarks/disembarks passengers) and total emissions are shown in

Table 3. Calculated tons of equivalent CO₂ per port call in the four different ports.

Ports	Port Calls	Tonnes of CO2 Equivalent Per Port Call						Total (Tonnes of CO2 Equivalent)
		Container	Dry Bulk	Liquid Bulk	General Cargo	Ferry/RoRo	Cruise
Gothenburg	5999	33.34	2.33	31.89	0.23	23.21	41.67	150,000
Long Beach	2806	73.95	35.64	134.76	34.93	28.00	67.32	240,000
Osaka	12,399	8.33	7.46	3.69	3.89	10.57	0.00	97,000
Sydney	1370	76.70	21.90	70.48	20.07	0.00	54.01	95,000

. for every port. The location of emissions within the port is also crucial since different operational modes (such as “in fairway channels”, “at anchor”, “in port basins”, “maneuvering” or “at berth”) are targeted by different measures. Figure 7 displays how the emissions of five operational modes are allocated.

The “at berth” mode for the ports is where the majority of the emissions come from. Gothenburg (25%) and Osaka (16%) have greater GHG emissions in “the fairway channel” as stated as a percentage than Sydney (4.5%). Long Beach cannot be compared since fairway emissions were not included in the study’s inventory. With this information, the following conclusions were drawn:

• Steps toward stricter legislation and regulations linked to alternative fuels and ship design need to be adopted at the international level in order to achieve sustainability targets for the shipping sector and reduce GHG emissions.
• The port can still help with the process of decreasing GHG emissions, though, by providing alternative fuel supplies in the port and employing ecologically differentiated port taxes, for example.
• Measures that particularly target emissions from the at-berth mode, such as decreased time at berth and on-shore power supply, are also beneficial, particularly for terminals with a high volume of ships in liner service.
• The findings also show that it is very challenging to perform accurate comparisons of ship emissions in ports from the standpoint of conducting international benchmarking studies. Given the different conditions each port faces, it is likely that emission reduction strategies should be customized for each port.

Wan et al. [119] developed a quantitative model to identify the exhaust emissions from the key energy-consuming components in the container port zone of the Shekou Container Terminal. The utilization of shore power, switching to low-sulfur fuel oil and improving quay crane efficiency were examined as GHG emission reduction strategies. The combination of shore power and greater quay crane efficiency provides the maximum potential for emission reduction when the emission reduction measures are implemented in pairs, according to simulation data. The greatest emission reduction benefit is achieved by combining all three strategies, as no negative correlations were found between the three measures. Based on their findings and discussions, the authors proposed policy implications. First, if conditions allow, combined measures are advised to reduce emissions from ships in port regions. Second, compared to reductions in SO₂ and NOx, the benefit of lowering CO₂ for all three measures combined is modest. This is mostly due to the dominance of thermal power in China’s power system, which means that using shore power still limits how much CO₂ may be reduced. It is projected that these three measures taken together will result in considerable reductions in CO₂ in the Senkou Terminal if China strengthens its power structure and utilizes more clean energy sources, such as nuclear and solar energy.

Using Automatic Identification System (AIS) data, Toscano et al. [120] looked at how ship emissions affected the city of Naples’ air quality in 2018. Based on the findings, 5418, 193, and 602 tons of NOx, SO₂, and Particulate Matter below 10 μm (PM10) emissions were projected to be produced annually overall. The yearly emission findings were compared with the regression research published by Toscano et al. [121]. The comparison demonstrates that the emissions calculated in this study for NOx and PM10 are respectively 1.3 times higher and 3.2 times higher. About 95% of all emissions occur during the berthing phase, and just 5% occur when the ship is being navigated in port. Ninety percent of the total emissions were from passenger and commercial ships. The contributions to the concentrations of pollutants throughout the winter (December, January and February) and summer (June, July and August) were also examined. The findings indicate that the contribution of ship emissions is greater in the summer than in the winter. In reality, the contribution to NO₂ at specific air quality receptors was, on average, 13% in the winter and 27% in the summer. Average contributions for total PM10 were 3% in winter and 7% in summer, while SO₂ average contributions were 24% in winter and 36% in summer.

2.4. Hull Optimization

During navigation, the ship must overcome resistances based on the Froude theory, such as friction resistance, air resistance, etc., so the ship will consume energy in order to overcome these resistances. Therefore, reducing the overall ship resistance during navigation will reduce the overall fuel consumption and thus reduce the CO₂ emissions [122]. The overall ship resistance is greatly affected by the hull hydrodynamics. One way of reducing the overall hydrodynamics resistance, and thus the fuel consumption, is by building more slender vessels [123]. However, this would also require great investments in overall maritime infrastructure changes. Apart from hull-form optimization, regular periodic hull maintenance also reduces the overall hydrodynamic resistance. Adland et al. [124] performed an analysis of multiple years’ worth of actual data for a fleet of identical oil tankers and concluded that periodic underwater hull cleaning reduces fuel consumption by 9%, while dry-dock hull cleaning reduces fuel consumption by 17%. In [125], the biofilm impact on the ship’s overall fuel consumption and CO₂ emissions were assessed using the deterministic method. The authors concluded that the adequately optimized periodic hull maintenance schedule could provide both environmental and financial benefits. The different hull maintenance strategies can be compared using the Hull Maintenance Strategies for Emission Reduction (HullMASTER) tool, which allows the user to conduct the hull maintenance strategy comparison based on the operation cost, health, and environment [126]. Farkas et al. [127] performed a detailed analysis of low-roughness antifouling coating applications for CO₂ emission reduction. The study resulted in significant operational cost reduction and CO₂ emission reduction when this type of coating was applied. Furthermore, the authors also concluded that the shipowners should start using non-biocidal coatings instead of the currently used biocidal coatings, as the latter poses higher roughness characteristics and therefore has unfavorable financial and environmental effects.

Regarding the air lubrication systems for ship hulls, the evaluation of the propulsion system equipped with the Air Lubrication System was published in [128]. The ship moves at the boundary between two fluids, namely, air and water—which oppose motion by causing hydrodynamic and aerodynamic forces that create resistance to motion. To reduce viscous friction, the area of the hull’s wet surface must be reduced. This can be achieved by separating the underwater portion of the hull’s surface from the water by means of a layer of air [129]. The general term used to describe this phenomenon is “air lubrication of the hull” (AL). High-power blowers are used to generate air bubbles that flow at a constant velocity under the bottom of the hull. The Air Lubrication System method (ALS) can be applied during the design phase and incorporated into a new ship, as well as installed on the ship after a certain period of operation. The introduction of ALS on an in-service ship is a complicated process and requires extensive analysis, calculations, measurements, and usually computer simulations [130]. There are several companies specializing in the development and installation of ALS on ships, and each company calls this system differently, e.g.,: Mitsubishi Co.-Mitsubishi Air Lubrication System (MALS), R&D Engineering -Winged Air Induction Pipe System (WAIP), Samsung Heavy Industries-SAVER System (SAVER Air), Silver-stream-Silverstream System, Foreship-Foreship Air Lubrication System (Foreship ALS) and others [131]. Based on the conducted analyses of available literature and records of operational data, it can be concluded that:

• The benefit of using ALS seems doubtful (only in the ship design phase the application of this system improves the EEDI value, which is interesting for ship designers and ship owners)
• The use of ALS for the entire speed range of the ship is not beneficial. There are minimum and maximum speeds beyond which the use of the system does not lead to the assumed savings.
• The equipment included in the structure of the ALS, including the main blowers, requires high investment costs and high operating costs.

2.5. Carbon Capturing Technologies

The CO₂ emissions can also be reduced by using carbon capture and storage (CCS) technologies. Carbon capture has mainly focused on chemical absorption technologies using amine-based solvents. However, a significant amount of thermal energy is required for solvent regeneration when used in large amounts. This high thermal energy requirement problem encouraged the development of new CCS technologies, such as calcium loops, algae-based capture, membranes, direct capture from air, catalyzed sorbents and liquefaction [132]. However, most of these technologies are at a lower stage of the development process due to the different challenges they face. The low-temperature CO₂ capture technologies, often referred to as cryogenic carbon capture (CCC), rely on a phase change where the CO₂ is extracted from the gas as a solid or liquid form [133]. Although CCC is regarded as a universally inapplicable technology, there are still viable applications where cryogenic capture can be proven more favorable compared to other technologies, and shipping is one of those applications. In Ref. [29], it was concluded that in order to achieve the GHG 50% emissions reduction goal, a combination of efficiency measures for the decarbonization process will be required when using LNG. Furthermore, it was also concluded that the bio-based fuels with a lower sustainable source availability would be efficient measures dependent on achieving the consumption reduction. Therefore, engine modifications could be avoided by exhaust gas treatment via carbon capture. The exhaust gases emitted by the ship’s propulsion and auxiliary engines could be processed by an onboard carbon capture and storage system (OCCS) without any modifications to or replacement ship engines, while the stored CO₂ could be used for synthetic natural gas production [134]. Even though amine-based solvents require a high amount of thermal energy for the regeneration process, the majority of marine carbon capture studies are still related to carbon capture based on chemical adsorption technology, while only a few onboard CCC systems have been analyzed in the literature, so far. Font-Palma et al. [135] found the feasibility of advanced cryogenic carbon capture (A3C) process incorporation into newly built or retrofitted ships. The A3C process is based upon the moving packed cold bed material principle that captures CO₂ by freezing CO₂ on the bed material surface without using multiple circulating packed beds. After the layer carrying the CO₂ frost is transported via the screw conveyor to the sublimation unit, the CO₂ frost is sublimated and collected for later storage while the layer material is recirculated. Willson et al. [136]. performed the A3C performance evaluation, where the A3C process affected the fuel consumption increase of 17% and 24% for LNG and HFO, respectively, with a 90% carbon capture rate. However, the authors concluded that, regardless of the fuel consumption increase, the onboard A3C process could cost up to 50% less compared to the full zero-carbon fuel ship conversion. In Ref. [137], OCCS implementation on LNG-fueled ships is analyzed due to the possible intensification of the process through extensive heat integration. The heat energy supply for the stripper reboiler and CO₂ liquefaction cooling capacity requirements are available from the exhaust gasses and a heat sink (LNG), respectively. The OCCS systems were developed using a 30 wt% aqueous solution of monoethanolamine (MEA) and the proposed high-pressure solvent for the captured CO₂ compression cost reduction. Furthermore, the 30 wt% aqueous piperazine (PZ) was used to compare results with the pilot scale demonstration study [138]. The results showed that the 90% carbon capture rate was possible for the 3000 kW LNG-fueled ship using 30 wt% MEA and piperazine with the OCCS cost of 120 EUR/ton of CO₂ and 98 EUR/ton of CO₂, respectively. However, a more recent study by Einbu et al. [139] opposes the results and conclusions made by Feenstra et al. [137], as their study suggests that the engine exhaust thermal energy is insufficient for achieving the carbon capture rate above 50% for the MEA based carbon capture unit. Furthermore, the required thermal energy demands could only be satisfied using the fuel afterburner, which would increase the fuel consumption by 6–9% and 8–12% for LNG and diesel fuel, respectively.

3. Conclusions

A systematic overview of newly available technologies in the green maritime sector, conducted in this work, results in some key findings and directions for the advancement of research in this field. By comparing the results of research in state-of-the-art articles on topics of the maritime application of (i) alternative fuels, (ii) hybrid propulsion systems and hydrogen technologies, (iii) the benefits of digitalization in the maritime sector aimed at increasing vessel efficiency, (iv) hull drag reduction technologies and (v) carbon capture technologies, it can be concluded that the findings can be categorized into three strategies: (a) short-term, (b) transitory and (c) long-term solutions.

The short-term strategy can be implemented immediately to meet the requirements of the current and future regulations for the foreseeable future. They are made evident by implementing different strategies to increase the overall energy efficiency design index of the vessel and by implementing digitalization to increase the operational efficiency of such vessels and satisfy the harmful emissions requirements. These strategies are short-term because they can be implemented on existing ships in service immediately, but they will only meet the requirements to a certain extent in the near future, i.e., with stricter ecological regulations, they will most likely become obsolete. It can be expected that digitalization will gain more and more traction in order to optimize the operating conditions to achieve maximum operational efficiency for prescribed routes and will definitely be a standard component for long-term solutions. There is great potential for fuel savings with some technological (lower design speed, improved ship hull form, propulsion efficiency devices, hybrid propulsion, etc.) and operational (slow steaming and trip optimization) methods. The fuel-saving potential of identical technological and operational solutions stated by various studies varies significantly, indicating that the majority of these solutions are very case-sensitive. Therefore, the context must be taken into consideration when planning further research.

Transitory strategies, such as the implementation of hull drag reduction technologies and exploitations of wind energy via kites and Flettner rotors, require the design of new vessels and cannot be reliably implemented on the existing vessels. Due to their modest improvement of performance (5–15%) and disadvantages such as high economic cost, unreliability, increased system complexity and maintenance requirements, for the foreseeable future, they could, in the most optimistic view, gain traction in niche areas with specific ship types and ship routes, while their implementation for large-scale systems is unlikely. Solar energy can be utilized as an auxiliary power source in some ship types; however, a major substitution of marine power by solar energy seems unrealistic. Carbon capture technologies are still in the early stages of development for marine applications, and their chances for the future depend on reasonable technological advancement along with supportive legislation. The research’s findings are still mostly hypothetical. For instance, the results in [137] showed that a 90% carbon capture rate was feasible at a cost of 120 EUR/ton of CO₂, whilst the recent results in [139] revealed that a carbon capture rate beyond 50% is not feasible for using engine exhaust thermal energy. While still in the early stages of research, new technologies such as cryogenic carbon capture (CCC) partially address the issue of the high thermal energy requirement. Willson et al. [136] performed the CCC process evaluation and the fuel consumption increased 17% and 24% for LNG and HFO, respectively, with a 90% carbon capture rate. These systems can also be considered transitory, with the advantage of the possibility of implementation on the existing ships in service. Nevertheless, the carbon capture systems will also become obsolete once the transition to new fuels is realized; therefore, it can be expected that carbon capture systems will have limited applications in the long-term future.

A long-term strategy is the implementation of clean fuels, such as hydrogen. This is currently very difficult and unreasonable due to impracticality and very high economic costs. Further investigation of which solutions work best for which ship types and applications and the techno-economic barriers to the wide adoption of hydrogen as a marine fuel are needed. Still, hydrogen, ammonia and fully electric ships are predicted to have a significant presence in coastal and short-sea shipping in contrast to deep-sea shipping. The transition to the long-term solution constitutes the gradual implementation of what we today consider alternative fuels with the complementary aid of current and new digitalization technologies. The literature that is currently accessible has certain gaps. The first research gap is due to the lack of studies covering the regulatory and policy frameworks that ease the switch from traditional to alternative fuels. Second, when comparing alternative fuels to fossil fuels, the majority of studies exclusively consider emissions. There are, however, relatively few studies comparing the essential performance and combustion characteristics of these alternative fuels to those of traditional marine fuels, such as brake-specific fuel consumption (BSFC) and thermal efficiency (BTE), heat release rate, combustion duration, ignition delay, etc. Additionally, there are several studies on advanced controlled emissions [140] and combustion measurements of biofuel engines for road transport [141,142]; however, there is not enough information on maritime transport. It can be seen that implementing alternative fuels results in significantly different mechanics of internal combustion, namely, reduced power output and efficiency, and requires adaptations and the retrofitting of the existing systems in order to meet the requirements of satisfactory performance. The overall environmental impact evaluations of methanol and ammonia as future fuels are needed. The most promising alternative transition fuel is currently LNG, as is reasonably matured technology that has recently started to have commercial uses in the marine industry. However, as the maritime industry transitions to low or zero-carbon shipping, it is likely that LNG will not make up a significant fraction of the marine fuel mix in the future. Biofuel’s future is mostly influenced by the availability of feedstock and the fuel’s final cost. For now, the cost of biofuel production is still 1.9 (for biodiesel) and 3.84 (for bio-methanol) times more expensive than HFO.

By considering the state-of-the-art available technologies in green maritime transport, it can be seen that the future is inconclusive and that it will primarily depend on the government’s environmental incentives. It can also be noted that zero-emission laws cannot be implemented in the short-term future due to very disruptive consequences on the global economy. Due to the same reason, the current situation and the evidently fluctuating prices of fossil fuels, in the short term, it is unreasonable to expect the introduction of radically different new technologies. For now, the most feasible and reasonable solutions are to implement the methodologies for increasing the energy efficiency design index, developing dual fuel systems and increasing the implementation of digitalization until the global situation stabilizes for the implementation of long-term solutions. This analysis has a forward-looking outlook, not only assessing options based on their current technology readiness level but also considering their expected development over the coming years. Consequently, the results of this report should not be seen as a recommendation of the best available solutions today but rather as a projection of the long-term viability of different alternative fuels and technology options implemented in the future maritime industry. Our future research will propose a possible hybrid solution for a marine energy system applied on a ferry along with energy management control tailored for its particular application to increase efficiency and reduce emissions compared to conventional systems.