Pathways to Decarbonization of Deep-Sea Shipping: An Aframax Case Study

: Deep-sea decarbonization remains an enigma as the world scrambles to reduce global emissions. This study looks at near-term decarbonization solutions for deep-sea shipping. Pathways are defined, which are appealing to ship owners and major world economies alike. The economic and environmental viability of several of the most advanced near-term technologies for deep-sea decar-bonization are revealed. The environmental analysis suggests the necessity of new emission intensity metrics. The economic analysis indicates that the carbon tax could be a great motivator to invest in decarbonization technologies. Standalone decarbonization technologies can provide a maximum of 20% emissions reduction. Hence, to meet IMO 2050 targets of 50% emissions reduction, several solutions need to be utilized in tandem. This study reaches the conclusion that alternative fuels are the crucial step to achieve a net zero carbon economy, although bunkering, infrastructure, and economic hurdles need to be overcome for the widespread implementation of carbon-neutral fuels.


Introduction
The global economy relies on the shipping industry, with approximately 90% of the world's trade being carried out via deep-sea shipping [1].Consequently, in 2018, 2.89% of the world's anthropogenic CO 2 emissions originated from deep-sea shipping, amounting to almost one billion metric tons of CO 2 emissions.In the final report released by the International Maritime Organization (IMO) in 2020, it was estimated that without repercussive measures, marine Green greenhouse gas (GHG) emissions would increase by 130% by 2050 [2].With the drastic rise in marine GHG emissions, the IMO passed a resolution in 2018, aiming to reduce GHG emissions from the marine sector by at least 50% by the year 2050, relative to 2008 [3].With stringent regulations already in place for NO x and SO x emissions, the IMO is pushing hard towards energy-saving technologies and alternative fuels to counter CO 2 emissions as well.In 2021, the EU released its aggressive "fit for 55" policy for maritime decarbonization, which aims to cut 55% GHG emissions, relative to 2008, by 2030 and a fully carbon-neutral maritime industry by 2050 [4].
The shipping industry is currently dominated by heavy fuel oil (HFO), which is the cheapest, most polluting, and widely available fuel in the marine sector [5,6].HFO combustion releases CO 2 , NO x , SO x , and particulate matter (PM) emissions, which are the basis for the application of global environmental policies against the marine sector.The ramifications of HFO emissions are concerning, with almost 400,000 premature deaths in Europe caused by PM as a direct result of shipping industry HFO combustion [7].
Racing against the repercussions of inevitable global policy tightening, the marine industry is seeking commercially deployable decarbonization technologies to reduce the while SOFCs can convert ammonia into H 2 and N 2 during operation.Wu et al. [35] performed a comparative study on ammonia SOFC and diesel ICE and found that the GHG emissions could be reduced to zero with fully electric propulsion powered by green ammonia fuel cells.Kim et al. [36] found that while a reduction of 83-91% could be achieved in GHG emissions, the cost of running a vessel on ammonia SOFC would be 5.2 times higher than conventional ICEs.It is notable that hydrogen and ammonia are only viable when produced from renewable energy since the production of blue/gray hydrogen and ammonia is very carbon intensive [37,38].However, renewable hydrogen and ammonia production is currently uneconomical [39,40].This, along with low volumetric energy densities [41], cryogenic storage temperatures, and high-pressure requirements [42], has severely limited the development of bunkering infrastructure, hindering the use of hydrogen and ammonia in the shipping industry.
Comparative studies are an important indicator to determine the relative performance of alternative fuels.In a review conducted by [43] on alternative fuels, they discovered that from the year 2000, the research on alternative fuels has increased by an annual growth rate of 15.8%.The current trend of research suggests that while LNG has been the focus of most researchers, there has been a shift towards cleaner alternative fuels, such as hydrogen, ammonia, and methanol.However, the inefficiencies regarding the production of alternative fuels, the lack of bunkering infrastructure, and cost parity with HFOs needs to be addressed for alternative fuels to be adopted industry-wide.

Looking beyond Fuel: A Near-Term Solution
Deep-sea decarbonization might be the most difficult to achieve, owing to high energy requirements for vessels.These energy requirements may, in part, be met by alternative energy sources such as wind and solar.Solar energy has been found to be inconsequential in decarbonizing long-haul vessels [44].On the other hand, wind-assisted ship propulsion (WASP) technologies provide proven 1-30% fuel savings [9].From our intensive literature review, it was found that the most efficient WASP technology currently available is the Flettner rotor, providing CO 2 reductions of up to 25-30% whilst having low investment costs and high return on investment (ROI) [45][46][47][48].
In terms of hydrodynamics, many studies have been conducted on hull design and optimization, the use of lightweight materials, hull coatings, and vessel size [49][50][51][52][53].Air lubrication is a new technology in this regard, which works by using compressors and blowers to blow air bubbles underneath the hull, effectively reducing the viscosity friction with the water.Commercially available air lubrication systems [54] provide fuel savings of up to 8%, depending on ship speed.
The electrification of deep-sea vessels is another method to reduce emissions and may include complete or partial electrification.With the UK's renewable electricity production nearing 45% in 2022 [55], batteries could reduce marine GHG emissions.However, battery size constraints pose an issue in deep-sea shipping [56].Kersey et al. [57] conducted a study using the most technologically advanced battery systems currently available and concluded that battery-operated deep-sea vessels would only be cost effective up to a 2000 km voyage length.This study [57] reaffirmed the stance of Minnehan et al. [58] that battery-powered vessels do not have enough volumetric energy density to support long-haul voyages and are only suitable for short-duration trips.
Another technology that could potentially be used to reduce GHG emissions is the carbon capture and storage (CCS) technology.Although widely used in land-based applications (coal power plants, oil refineries, steel industry, etc.), CCS technologies are a new concept in the marine industry.Several studies concluded that the main hindrances to CCS technologies onboard a marine vessel are the cost and size requirements [59][60][61].Recently, Wärtsilä [62] announced the building of a research center in Norway, where maritime CCS technology would be developed on a pilot scale up to 2024.They expect to release the CCS technology by 2025.

Outline
The applicability of decarbonization technologies in the marine industry has a rich literature basis.Although researchers have performed comparative analysis with different technologies and alternative fuels with the baseline case using HFOs, little regard has been given to ship owners who recently purchased ships and will not be willing to invest more money.There seem to be no clear-cut guidelines from either the IMO or the EU regarding the implications that come with the application of the proposed carbon tax, whether a financial levy on non-conforming ships or a blanket ban on ships that do not meet GHG requirements.The novelty of this study lies in the use of alternative fuel technologies and alternative energy technologies, readily available or available in the near term, to ascertain the best way forward to achieve deep-sea decarbonization.Most previous studies have not considered ammonia and hydrogen fuel cells in long-haul deep-sea shipping, and the economic analysis lacks consideration of the fuel storage tank.To the best of the authors' knowledge, no such studies are available that perform a comparative study on alternative fuels, fuel cells, batteries, CCS, and WASP technologies.Additionally, the existing literature lacks in providing a direct link between the effect of environmental impact on economic variables.We use eleven scenarios (using real ship operational profiles and technology performance data) to encompass a wide range of deep-sea decarbonization technologies, including alternative fuels (LNG, hydrogen, and ammonia) being utilized in engines and fuel cells, as well as alternative energy technologies (batteries, WASP, and air lubrication) and emission abatement technologies (CCS).

Methodology
This section examines the system configuration, which then serves as the foundation for the performance, environmental, and economic models.The establishment of a baseline model is vital for long-haul ships, and the route plays an important part.In this regard, the first section defines the case ship and the route specifications.Equally important is the ship power demand profile, which has also been specified in this section.This is shown in the first section of Figure 1.The second section defines the scenarios and outlines the system configuration of each scenario.This is shown as scenario identification in Figure 1, and in this section, Siemens Simcenter 2022 and Simulink 2022 were used to predict technology partial load performances.The third section outlines the performance, economic, and environmental models.Here, MATLAB 2022 was used to build the fuel economy, environmental, and economic models with spreadsheet data input from Microsoft Excel.The methodology is concluded with the fourth section, which details the assumptions and key input variables necessary for model functionality.Figure 1 shows the concept methodology (Sections 1-3) and the analysis stemming from it (Sections 4-6).

Case Ship and Route Specifications
The case ship is an Aframax tanker, built in 2021, measuring 250 m in length and weighing 115,000 DWT.The considered ship has undergone sea trials and is now in service; one of the most common routes it travels on is from Rotterdam, Netherlands, to Newark, New Jersey, United States [63].It is one of the most common routes for transatlantic trade; hence, it is chosen for this case study.The route details can be accessed via [64] by setting a service speed of 13 knots.The main ship parameters and engine data were provided by [65].The main engine for the baseline ship is the MAN B&W 6G60ME-C10.5 (TIER III), designed by MAN Energy Solutions.Three sets of auxiliary engines are installed on the ship, each capable of producing 1200 kW of power [66].The engine performance curves and the ship power demand profile are shown in Figure 2. Figure 2a shows the specific fuel oil consumption (SFOC) and the exhaust gas specifications, and it can be seen that the engine has the highest efficiency between 60 and 70% engine load (8000-9000 kW).The highest efficiency is achieved at the lowest SFOC.That is why the vessel runs at this engine load for the majority of the voyage, as shown in Figure 2b.The data provided by [65] correspond to the engine performance data found in the literature [67].

Case Ship and Route Specifications
The case ship is an Aframax tanker, built in 2021, measuring 250 m in length and weighing 115,000 DWT.The considered ship has undergone sea trials and is now in service; one of the most common routes it travels on is from Rotterdam, Netherlands, to Newark, New Jersey, United States [63].It is one of the most common routes for transatlantic trade; hence, it is chosen for this case study.The route details can be accessed via [64] by setting a service speed of 13 knots.The main ship parameters and engine data were provided by [65].The main engine for the baseline ship is the MAN B&W 6G60ME-C10.5 (TI-ER III), designed by MAN Energy Solutions.Three sets of auxiliary engines are installed on the ship, each capable of producing 1200 kW of power [66].The engine performance curves and the ship power demand profile are shown in Figure 2. Figure 2a shows the specific fuel oil consumption (SFOC) and the exhaust gas specifications, and it can be seen that the engine has the highest efficiency between 60 and 70% engine load (8000-9000 kW).The highest efficiency is achieved at the lowest SFOC.That is why the vessel runs at this engine load for the majority of the voyage, as shown in Figure 2b.The data provided by [65] correspond to the engine performance data found in the literature [67].Energies 2023, 16, 7640 6 of 26

System Configuration
Each scenario case is summarized in Table 1.The Aframax tanker comes equipped with a two-stroke diesel engine, which is LNG-ready, according to [65].Scenario A and B employ 10% biodiesel blend and LNG fuel, respectively.For scenarios C and D, one auxiliary engine in each case was replaced with batteries and six hydrogen PEMFCs, respectively.Due to battery size constraints, the battery is utilized during port operations.Scenario E and F make use of alternative energy in the shape of Flettner rotors, with each case making use of two rotors on board.The Flettner rotors aid in ship propulsion, causing a reduction in fuel consumption via less load on the main engine.CCS adsorption technology is applied for scenarios G and H.In scenario G, an additional ammonia refrigeration loop is applied for CO 2 liquification for storage, whereas in scenario H, LNG cryogenic energy is utilized for this purpose.The system design and space requirements of the CCS system can be found in [60].This results in a lower cost for the CCS system in scenario H. Scenario I works in a similar fashion to scenarios E and F, reducing the load on the main engine with a reduction in viscosity drag between the vessel's hull and the water by introducing a film of air bubbles as lubricant.Scenario J and K make use of solid oxide fuel cells (SOFCs) by replacing one auxiliary engine, respectively.The battery pack, PEMFC, and SOFCs are sized to meet the power requirements of one auxiliary engine.

Mathematical Models 2.3.1. Fuel Consumption Calculations
The fuel consumption model employed in this study uses actual sea trial and engine data, including the specific fuel oil consumption (SFOC) (g/kWh), engine power with respect to time spent sailing at this power, and the time (sailing hours at specific power rating, port operations, manoeuvring, and berthing).Equation ( 1) is used for the fuel consumption calculation for each case.
Here, M(fuel) is the annual fuel consumption, P is the power, and dt is the time.For the fuel cell scenarios, annual fuel consumption (M(fuel) PEMFC,SOFC ) is calculated based on the requirements of baseline auxiliary engine power (P baseline ):

GHG Calculations
To analyze the complete effect of the vessel on the environment, it was necessary to calculate the GHG emissions based on the WTW lifecycle [72].Several factors have been developed by researchers to correctly predict the WTW GHG emissions.These factors are based on the fuel and engine type being utilized by the vessel.For this study, the baseline engine can be classed as a slow speed diesel, the LNG DF engine as a diesel-cycle LNG, and the auxiliary engines running on diesel and LNG can be classed as a medium speed diesel and medium speed DF LNG, respectively.The factors are shown in Table 2 [73].Equations ( 3) and ( 4) were used to calculate the GHG emissions.For scenario cases G and H, since there are no fuel savings, the GHG emissions are calculated by subtracting the annual CO 2 saved due to the CCS technology.
Here, F is the GHG factor.

EEDI and CII Calculations
The Energy Efficiency Design Index (EEDI) is an-IMO approved method to calculate the mass of CO 2 emitted per transport work for newly built ships [74].EEDI only considers the CO 2 emitted by vessels and not the other GHG emissions.The carbon intensity indicator (CII) is an operational efficiency indicator that measures carbon intensity over time [75].
Energies 2023, 16, 7640 8 of 26 CII has not yet been finalized by the IMO, but several equations have been provided by the IMO to calculate the CII.The equation for calculating the EEDI and the definition of each term can be obtained from [76].The equation for the CII is given below [75]: For the calculation of the EEDI for scenario cases C to I, there is no regulation from the IMO for adjustments to the EEDI formula.Hence, a factor was developed to calculate the EEDI for each case.
Then, the new EEDI can be calculated as The same factor was used to calculate the CII for each case.Hence, the CII would become

Economic Calculations
The economic implications of decarbonization technologies stem from the total vessel powertrain cost, which includes engine cost, the operating costs (including fuel, maintenance, etc.), and the additional cost of the decarbonization technologies.The product lifetime was assumed to be 25 years, equal to the average lifetime expectancy of the vessel.
The capital investment is first converted to the amortized annual investment cost (C inv,a ), expressed as [77] C inv,a = i(Cap) where i is the annual interest rate, Cap is the capital cost, and z is the engine lifetime.
The annual operating and maintenance cost (C O&M,a ) depends on each scenario case and is calculated using Equation (10) [77]: Some decarbonization technologies, such as batteries and CCS systems, have a shorter lifespan than the main engine lifespan.Hence, the replacement costs must be accounted for in the economic calculations.The annualized replacement cost (C rep,a ) is defined as follows [77]: Here, t is the year of replacement.
To calculate the relative annual revenue generated, it was assumed that the power produced by the vessel would be converted to electricity and sold to the national grid at the current cold ironing prices.Hence, the annual revenue, R t j , is given by [77] as follows: where p E is the unit price of electricity ($/kWh), while P Ej is the annual energy production (kWh) by the vessel's powertrain.According to Duan and Zhang [78], carbon tax application in the marine industry is imminent and needs to be accounted for when calculating the net annual income N AI j [77]: Energies 2023, 16, 7640 Here, j is the year in the vessel's lifetime.The discounted payback period (DPP) is defined as the year the initial investment is covered and the product starts to generate profit and is calculated as [77] The levelized cost of electricity (LCOE) is the cost of producing one unit of electricity via the vessel's powertrain.It can be calculated using the sum of total annualized costs and the annual electricity production and is expressed as follows [77]: Here, W e is the annual electricity production.

Model Assumptions and Key Inputs
Several input variables were required to be put into the model.Table 3 shows the fuel price and properties.Since there are no fuel savings for the CCS system, neither does it contribute to the powertrain; it is assumed that the captured CO 2 would be sold at the same rate as the carbon tax.Only a fixed amount of exhaust gas would pass through the CCS system so that the system was completely self-sufficient in terms of thermal energy requirements and to keep the size of the system as compact and small as possible.The CCS system was based on the studies conducted by [60,61].For the battery scenario, it is assumed that the battery is charged with 100% renewable electricity.For the fuel cell scenarios, the fuel cells are containerized as 100 kW × 6 modules.The SOFC efficiencies are set at 50%, and PEMFC efficiency is set at 43% [79].Figure 3 shows the historical fuel and electricity prices.It is interesting to note that the fuel prices do not show a specific trend but are rather heavily influenced by the geopolitical climate.It is pertinent to note that since these are input variables, the validity of the model and the methodology would still be upheld, even in the case of drastic changes to fuel prices.Additionally, this model can be adapted to any marine vessel and any route if the engine data, fuel consumption, propulsion power demand, and exhaust gas analysis are provided.

Results and Discussion
The above-presented methodology was applied to the case of an Aframax tanker.Different scenario cases were considered, as shown in Table 1.The environmental benefits of applying different decarbonization technologies are discussed based on reduced fuel consumption and reduced GHG emissions.The economic implications of the decarbonization technologies are discussed on a vessel basis rather than a standalone technology basis.This provides a better overview of the overall cost repercussions on a system level.

Fuel Consumption Analysis
When evaluating a vessel's fuel consumption, it is important to consider both the fuel volume and the fuel mass.Each fuel has different volumetric energy densities and hence different storage volumes for the same amount of energy supplied.Both have economic implications, as the fuel is sold on a mass basis, whereas the storage tank size and cost are decided on a volume basis.Another way to look at the fuel consumption of any vessel would be to determine the overall annual energy consumption of the vessel.The lower calorific value (LCV) is the amount of energy available in per kilogram of a fuel.With LNG having the highest LCV, it is only logical that the mass fuel consumption of the DF scenario case is less than that of the VLSFO scenario case.However, the volumetric energy density of LNG is lower than VLSFO, hence requiring more storage volume as compared to VLSFO. Figure 4 shows the comparison of different fuels for each scenario case in terms of both fuel mass and fuel volume.An observation that can be made is that the Flettner rotor provides the highest fuel savings, whereas the battery and fuel cells do not provide significant fuel savings.Hence, from a purely fuel consumption standpoint, the Flettner rotor is the front runner in the race for the most efficient decarbonization technology.Of course, in the case of the CCS technology, no fuel savings were expected, as the CO2 capture is initiated post-combustion.However, it is important to note that the storage of the captured liquid CO2 must be considered during the design stage, as it would be stored at a high pressure and low temperature with a different standard compared to LNG storage tanks.
A takeaway from this analysis is that when considering the use of alternative fuels, the volumetric energy density should be of the highest concern.With discussions sur-

Results and Discussion
The above-presented methodology was applied to the case of an Aframax tanker.Different scenario cases were considered, as shown in Table 1.The environmental benefits of applying different decarbonization technologies are discussed based on reduced fuel consumption and reduced GHG emissions.The economic implications of the decarbonization technologies are discussed on a vessel basis rather than a standalone technology basis.This provides a better overview of the overall cost repercussions on a system level.

Fuel Consumption Analysis
When evaluating a vessel's fuel consumption, it is important to consider both the fuel volume and the fuel mass.Each fuel has different volumetric energy densities and hence different storage volumes for the same amount of energy supplied.Both have economic implications, as the fuel is sold on a mass basis, whereas the storage tank size and cost are decided on a volume basis.Another way to look at the fuel consumption of any vessel would be to determine the overall annual energy consumption of the vessel.The lower calorific value (LCV) is the amount of energy available in per kilogram of a fuel.With LNG having the highest LCV, it is only logical that the mass fuel consumption of the DF scenario case is less than that of the VLSFO scenario case.However, the volumetric energy density of LNG is lower than VLSFO, hence requiring more storage volume as compared to VLSFO. Figure 4 shows the comparison of different fuels for each scenario case in terms of both fuel mass and fuel volume.An observation that can be made is that the Flettner rotor provides the highest fuel savings, whereas the battery and fuel cells do not provide significant fuel savings.Hence, from a purely fuel consumption standpoint, the Flettner rotor is the front runner in the race for the most efficient decarbonization technology.Of course, in the case of the CCS technology, no fuel savings were expected, as the CO 2 capture is initiated post-combustion.However, it is important to note that the storage of the captured liquid CO 2 must be considered during the design stage, as it would be stored at a high pressure and low temperature with a different standard compared to LNG storage tanks.
rounding hydrogen or ammonia-fueled DF engines, the biggest concern should be fuel tank spacing considerations.This is because ammonia has half the volumetric energy density of LNG, while hydrogen has one-third [43].The same goes for methanol, which has two-thirds the volumetric energy density of LNG.For reference, the volumetric energy densities of VLSFO, LNG, liquid hydrogen, liquid ammonia, and methanol are 40 MJ/L, 22.5 MJ/L, 8.5 MJ/L, 11.5 MJ/L, and 16 MJ/L, respectively [88].A takeaway from this analysis is that when considering the use of alternative fuels, the volumetric energy density should be of the highest concern.With discussions surrounding hydrogen or ammonia-fueled DF engines, the biggest concern should be fuel tank spacing considerations.This is because ammonia has half the volumetric energy density of LNG, while hydrogen has one-third [43].The same goes for methanol, which has two-thirds the volumetric energy density of LNG.For reference, the volumetric energy densities of VLSFO, LNG, liquid hydrogen, liquid ammonia, and methanol are 40 MJ/L, 22.5 MJ/L, 8.5 MJ/L, 11.5 MJ/L, and 16 MJ/L, respectively [88].

Emissions Reduction Potential of Deep-Sea Decarbonization
CO 2 , NO x , SO x , and PM are common byproducts of fossil fuel combustion, while additional methane emissions have to be considered in the case of LNG engines.With all other combustion byproducts causing greater environmental damage than CO 2 [73], it is imperative to monitor all pollutants individually rather than just using CO 2 reduction as the benchmark for decarbonization.Equally important is to note the effect each pollutant has over a specific time period.While some pollutants cause greater short-term harm, their effects mellow out over a longer period of incubation.Methane, in this regard, is particularly noteworthy, being 87 times more potent than CO 2 over 20 years while being 36 times more lethal over a hundred-year period [2,73].This is evident from the results of this study, shown in Figure 5a, where the disparity between the baseline scenario and the DF scenario cases stems from the emission of methane in the case of GWP20.The outlook improves over the hundred-year incubation period and hence raises the question as to which time horizon and climate metric be adopted as a standard for decarbonization technology evaluation.

Cost Parity with HFOs
The economic analysis of every system stems from the capital investment and the annual operational and maintenance costs of the system.A breakdown of the CAPEX for each decarbonization technology, inclusive of the powertrain, is shown in Figure 6.Due to  Whilst the results are self-explanatory, it may be surprising for some to see that the battery adds little value to the GHG reduction objective.This is contradictory to the research conducted by Ref. [57], who achieved a 51% decrease in CO 2 emission intensity if the UK grid carbon intensity is considered.However, Ref. [57] considered a fully electric vessel with a maximum voyage length of 2000 km for it to be cost effective.In this study, the battery was only utilized during port operations.Normally, the battery pack is used to make sure the engine runs at peak efficiency, with the battery providing the rest of the required propulsion [89].The battery sizing was performed based on the available space for this case ship.Comparatively, the fuel cells show promising reductions in GHG emissions even with a partial load application.One of the most important observations that can be made from this life cycle analysis study is that the Flettner rotor provides almost the same environmental benefit to the baseline scenario as does switching to LNG as the fuel.This is incredibly important, as LNG is the most carbon-intensive fuel among all alternative fuels [9].This signifies that less carbon-intensive alternative fuels will always have a higher deep-sea decarbonization potential, at least from an environmental perspective, than alternative energy technologies.
The current carbon intensity metric for deep-sea vessels adopted by the IMO is the EEDI, while the CII regulations, reported to be an upgrade to the EEDI [75], are yet to be determined.Whilst the authors believe that the EEDI is an outdated method to determine a vessel's environmental impact since it only considers CO 2 emissions, the analysis was performed to reaffirm the authors' opinions.In Figure 5b, the horizontal red lines show the IMO-specified EEDI targets for ship owners.Let us consider the DF and the VLSFO-RT scenario cases.While the GWP100 outlook is slightly better for the VLSFO-RT case, the EEDI is higher than the DF case due to higher CO 2 reduction in the case of DF.This inherent lack of consideration on the part of other pollutants results in an inaccurate depiction of a vessel's carbon intensity.These observations are in line with those made by Attah et al. [74], who concluded that the EEDI could not accurately predict the environmental impact of LNG-fuelled vessels and recommended that the IMO include methane slip into the calculation of the EEDI.Lindtsad et al. [90] passed a similar judgement while investigating the use of batteries for deep-sea shipping.They discovered that the EEDI always exaggerated the decarbonization potential of technologies due to shortcomings such as the assumption of a uniform engine load during the entire voyage.Based on the results of this analysis and that conducted by other researchers, the authors recommend the IMO adopt a GHG screening policy, which accounts for all post-combustion pollutants.The same stands true for the CII, although the IMO may yet provide certain considerations in the calculations for the CII.The bottom line is that there need to be regulations put forward by the IMO, which provide a vessel's emission levels in terms of complete GHG emissions.

Cost Parity with HFOs
The economic analysis of every system stems from the capital investment and the annual operational and maintenance costs of the system.A breakdown of the CAPEX for each decarbonization technology, inclusive of the powertrain, is shown in Figure 6.Due to the commercial sensitivity of such data, the analysis was conducted compared to the cost of the powertrain of the VLSFO scenario.As discussed in Section 3.1, the volumetric energy density of each fuel is different, and some technologies provide fuel savings, as was evident from Figure 4. Thus, alternative fuels add extra cost in terms of fuel storage, while the HFO tank size remains constant as newly built ships are disregarded.Engine modifications lead to an increase in CAPEX for the DF scenario cases, with each decarbonization technology piling on top.As described in Section 2.4, the CAPEX of the CCS system depends on the choice of fuel and mode of CO 2 liquification.With LNG cryogenic energy available for CO 2 liquification in the DF-CCS scenario case, the CAPEX is significantly reduced compared to the VLSFO-CCS scenario case, in which the additional ammonia refrigeration loop leads to a high CAPEX.For the hydrogen PEMFC and SOFC, hydrogen storage cost dominates the CAPEX.For the purpose of a detailed economic analysis, the DPP and the LCOE are used as the economic indicators.The application of a carbon tax within the marine industry is imminent, especially with renewed efforts from IMO regarding its 2050 decarbonization targets [3].The effect of carbon tax inclusion results in a negative performance arc for the economic indicators, which increase across the board from Figure 7 to Figure 8.The carbon tax levy would increase the economic burden on shipowners, and with increasing fuel costs worldwide, this would lead to a domino effect, with the financial stress ultimately landing on the consumers.This would favour the use of alternative fuels, especially biofuels, which do not have the same storage problems that ammonia and hydrogen have and have a higher volumetric energy density [88].However, achieving the IMO targets of 2050 Table 4 shows the CAPEX and OPEX of the different technologies used.The CAPEX includes the cost of the prime mover, installation costs, fuel tank, and auxiliary costs.For the cases where the lifetime of the technology is less than the vessel's, the replacement cost (REPLEX) needs to be accounted for when doing a technology lifetime economic analysis.
While the capital investment analysis is extremely useful, it is equally important to calculate the total cost incurred over the vessel's lifetime, which includes the CAPEX, OPEX (including fuel costs) and the REPLEX.Figure 6b clearly shows that the overall cost of a vessel is OPEX dominated, with the majority of it being fuel surcharge.Hence, in this regard, the technologies that achieved the highest fuel savings (Figure 4), the Flettner rotor and the air lubrication technology, actually provide the most cost-effective solution.These findings are in accordance with other studies conducted on Flettner rotors, which emphasise the major effect of fuel savings on deep-sea vessel economics [45,46].Flettner rotors for deep-sea shipping would always provide economic benefits unless the fuel price drops to uncharacteristically low values, which is highly unlikely to happen.Without the application of a carbon tax, the CCS system would be a financial black hole.From Figure 5, the highest CO 2 -reducing technologies would automatically have the lowest carbon tax levy, with the Flettner rotor providing dual savings of both fuel cost and carbon tax.
In summation, considering the fuel savings, the GHG emissions and the economic impact, the Flettner rotors are leading in the race for decarbonization, with CCS coming in a close second.With the inevitable levy and increase in carbon tax in the coming years, as predicted by [91], the overall economic effectiveness of both these technologies would significantly improve.For the purpose of a detailed economic analysis, the DPP and the LCOE are used as the economic indicators.The application of a carbon tax within the marine industry is imminent, especially with renewed efforts from IMO regarding its 2050 decarbonization targets [3].The effect of carbon tax inclusion results in a negative performance arc for the economic indicators, which increase across the board from Figure 7 to Figure 8.The carbon tax levy would increase the economic burden on shipowners, and with increasing fuel costs Energies 2023, 16, 7640 16 of 26 worldwide, this would lead to a domino effect, with the financial stress ultimately landing on the consumers.This would favour the use of alternative fuels, especially biofuels, which do not have the same storage problems that ammonia and hydrogen have and have a higher volumetric energy density [88].However, achieving the IMO targets of 2050 would require increasing the percentage of biofuel mixtures, in addition to increasing worldwide production.Governments around the world would need to incentivize the use of biofuels, which can be achieved by decreasing and regulating the biofuel costs and subsequently increasing the carbon tax.From Figure 6b, an overall cost increase of 10% is seen in the scenario case of BD-10, whilst a decrease of almost 10% is seen in the VLSFO-RT scenario case.A possible solution could be utilizing biofuels in tandem with the Flettner rotor, which could offset the additional fuel costs by decreasing fuel consumption, as showcased by comparing the scenario cases DF and DF-RT.
the decision to invest in a deep-sea vessel.Of course, freight profits heavily play a part in determining the profit margins [97].However, making less carbon-intensive deep-sea vessels which exhibit a payback period solely based on the powertrain, less than the intended investment maturity period in the shipowners' mind, would attract a higher investor confidence.Comparing Figures 7 and 8, the carbon tax levy condemns several scenario cases as unfavourable (>10 years).However, it is interesting to note how the retrofitting of Flettner rotors decreases the payback period of the DF case by 78%, even with the carbon tax.
While the fuel price plays a pivotal role in determining the payback period, equally important is the unit price of electricity in determining cash inflow (revenue).According to [57,98,99], electricity prices are dependent on wholesale fuel prices, which are subject to change based on demand and geopolitics.While the claim that electricity prices are fuel cost-dependent seems to be true, the increment does not seem to be relative, as the price of LNG increases almost 100% from 2021 to 2022, yet the electricity price increase is only 10% (Figure 3).The payback period calculations are highly sensitive to the instantaneous fuel and electricity prices because while the increase in electricity price may be small, the annual energy generated by the vessel's engines is much higher than the annual fuel consumption (for reference, the main engine for the VLSFO case produces 44.4 GWh of energy annually, while consuming 7.3 million kg of fuel).Hence, a system of calculations may need to be performed annually since a dynamic system of calculations may be very complex, with fuel prices changing daily.It is currently unknown as to how the carbon tax may be implemented, whether regulated or future market-based.If future market-based, the payback period calculations would need to include the instantaneous carbon tax price, along with the fuel and electricity prices.

Sensitivity Analysis
Keeping in mind the concluding remarks of Section 3.3, it seemed logical to perform a sensitivity analysis to determine the validity of the methodology and how rapid changes in prices would affect ship owners.Fuel and energy prices are at a historical peak right now, showing high volatility [80]; hence, an electricity and fuel price sensitivity analysis was carried out.We decided to perform the sensitivity analysis separately, keeping the fuel price constant for the electricity price analysis and vice versa.A summary of the variable parameters is shown in Table 5.The results achieved were expected, as an increase in electricity price would increase revenue, reducing the payback period, while an increase in fuel price increases the OPEX, effectively reducing the revenue stream, which negatively impacts the payback period.Notably, from Figure 9b, there seems to be a "break-out" price for fuel.If the price of fuel increases above that break-out threshold, the annual revenue generation would be less than the OPEX, essentially turning the vessel into a nonprofitable endeavour.
With the introduction of a carbon tax, the economic liability would increase, leading to a decrease in revenue.Thus, there would be a "break-out" cost of the carbon tax as well, and the regulating authorities would need to be careful with the implementation of the carbon tax, keeping it just high enough to incur a sizeable financial penalty but not high enough to make deep-sea shipping an unlucrative venture.Figure 10 shows that if a car- Highlighting the importance of the payback period on the psyche of a ship owner, Stott [96] performed a market-based study in which he highlighted that only 20% of deepsea vessel owners utilized a ship for the entirety of its lifetime, whereas a majority of shipowners preferred to sell a vessel ten years after purchase.Subsequently, the market for second-hand purchases of ships is on the rise, with the payback period deeply influencing the decision to invest in a deep-sea vessel.Of course, freight profits heavily play a part in determining the profit margins [97].However, making less carbon-intensive deep-sea vessels which exhibit a payback period solely based on the powertrain, less than the intended investment maturity period in the shipowners' mind, would attract a higher investor confidence.Comparing Figures 7 and 8, the carbon tax levy condemns several scenario cases as unfavourable (>10 years).However, it is interesting to note how the retrofitting of Flettner rotors decreases the payback period of the DF case by 78%, even with the carbon tax.
While the fuel price plays a pivotal role in determining the payback period, equally important is the unit price of electricity in determining cash inflow (revenue).According to [57,98,99], electricity prices are dependent on wholesale fuel prices, which are subject to change based on demand and geopolitics.While the claim that electricity prices are fuel cost-dependent seems to be true, the increment does not seem to be relative, as the price of LNG increases almost 100% from 2021 to 2022, yet the electricity price increase is only 10% (Figure 3).The payback period calculations are highly sensitive to the instantaneous fuel and electricity prices because while the increase in electricity price may be small, the annual energy generated by the vessel's engines is much higher than the annual fuel consumption (for reference, the main engine for the VLSFO case produces 44.4 GWh of energy annually, while consuming 7.3 million kg of fuel).Hence, a system of calculations may need to be performed annually since a dynamic system of calculations may be very complex, with fuel prices changing daily.It is currently unknown as to how the carbon tax may be implemented, whether regulated or future market-based.If future market-based, the payback period calculations would need to include the instantaneous carbon tax price, along with the fuel and electricity prices.

Sensitivity Analysis
Keeping in mind the concluding remarks of Section 3.3, it seemed logical to perform a sensitivity analysis to determine the validity of the methodology and how rapid changes in prices would affect ship owners.Fuel and energy prices are at a historical peak right now, showing high volatility [80]; hence, an electricity and fuel price sensitivity analysis was carried out.We decided to perform the sensitivity analysis separately, keeping the fuel price constant for the electricity price analysis and vice versa.A summary of the variable parameters is shown in Table 5.The results achieved were expected, as an increase in electricity price would increase revenue, reducing the payback period, while an increase in fuel price increases the OPEX, effectively reducing the revenue stream, which negatively impacts the payback period.Notably, from Figure 10b, there seems to be a "break-out" price for fuel.If the price of fuel increases above that break-out threshold, the annual revenue generation would be less than the OPEX, essentially turning the vessel into a non-profitable endeavour.

Conclusions
This paper investigated the environmental and economic impact of the implementation of decarbonization technologies via a detailed scenario study in the deep-sea shipping industry.We show that the applicability of decarbonization technologies can be considered a high-yield investment rather than a regulated penalty by considering fuel savings and  With the introduction of a carbon tax, the economic liability would increase, leading to a decrease in revenue.Thus, there would be a "break-out" cost of the carbon tax as well, and the regulating authorities would need to be careful with the implementation of the carbon tax, keeping it just high enough to incur a sizeable financial penalty but not high enough to make deep-sea shipping an unlucrative venture.Figure 9 shows that if a carbon of $90/ton were to be introduced today, only a handful of scenario cases would be economically viable.In this regard, renewable energy technologies (such as Flettner rotor) would show the highest performance since they would provide dual economic benefits: fuel and carbon tax savings.

Conclusions
This paper investigated the environmental and economic impact of the implementation of decarbonization technologies via a detailed scenario study in the deep-sea shipping industry.We show that the applicability of decarbonization technologies can be considered a high-yield investment rather than a regulated penalty by considering fuel savings and carbon tax levy.The baseline scenario case of an Aframax tanker using VLSFO as fuel is scrutinized, while subsequent scenario cases utilize different decarbonization technologies, including alternative fuels, alternative energy, energy saving, and carbon capture and storage technologies.The common variable between each scenario case is that engine modification is not considered, as the case ship is fitted with an LNG-ready dual fuel engine.
Based on the IMO 2050 GHG emissions reduction policy, an action timeline was generated to provide an overview of how the decarbonization technologies would meet the targets, shown in Figure 11.This action timeline has been developed keeping in mind the needs of existing ships, where complete engine replacement to accommodate other fuels may prove to be very costly.The compliance can be extended by retrofitting different technologies to work in tandem.By 2050, the price of green hydrogen and ammonia is expected to be competitive with traditional fuels [93], and a complete transition to alternative fuels would become a real possibility.With the inevitable advent of alternative fuels, the fuel volumetric energy densities should be of the highest concern since that would be a driving factor in determining the initial fuel storage CAPEX.Our results indicate that several technologies, especially the Flettner rotors, will help in the reduction in fuel consumption of a vessel.The Flettner rotors reduce fuel consumption by up to 11%.While the fuel cells increase the cost of fuel storage capacity, their emissions reduction potential cannot be ignored.Partial electrification might be a viable solution for decarbonization, especially considering that renewable wind electricity is now cheaper than fossil fuel electricity [100].However, technical constraints from battery storage requirements would need to be addressed in this regard.The environmental study results stem primarily from fuel savings.In the case of alternative fuels, the relative potency of different pollutants needs to be considered individually.This is especially true for LNG, which has high methane emissions and thus counteracts the reduced CO2 emissions.One of the most important indications from this study is that the best-performing alternative energy technology, the Flettner rotor, provides the same environmental benefits to the baseline scenario as switching to LNG as the alternative fuel.Switching to LNG reduces GHG emissions by up to 9.2%, whereas retrofitting Flettner rotors to the baseline scenario reduces GHG emissions by up to 10.3%.This is an important revelation, as LNG is the most carbon-intensive alternative fuel, implying that alternative fuels will always have a higher decarbonization potential.The CAPEX and OPEX, domi- With the inevitable advent of alternative fuels, the fuel volumetric energy densities should be of the highest concern since that would be a driving factor in determining the initial fuel storage CAPEX.Our results indicate that several technologies, especially the Flettner rotors, will help in the reduction in fuel consumption of a vessel.The Flettner rotors reduce fuel consumption by up to 11%.While the fuel cells increase the cost of fuel storage capacity, their emissions reduction potential cannot be ignored.Partial electrification might be a viable solution for decarbonization, especially considering that renewable wind electricity is now cheaper than fossil fuel electricity [100].However, technical constraints from battery storage requirements would need to be addressed in this regard.The environmental study results stem primarily from fuel savings.In the case of alternative fuels, the relative potency of different pollutants needs to be considered individually.This is especially true for LNG, which has high methane emissions and thus counteracts the reduced CO 2 emissions.One of the most important indications from this study is that the best-performing alternative energy technology, the Flettner rotor, provides the same environmental benefits to the baseline scenario as switching to LNG as the alternative fuel.
Switching to LNG reduces GHG emissions by up to 9.2%, whereas retrofitting Flettner rotors to the baseline scenario reduces GHG emissions by up to 10.3%.This is an important revelation, as LNG is the most carbon-intensive alternative fuel, implying that alternative fuels will always have a higher decarbonization potential.The CAPEX and OPEX, dominated by fuel cost, are important factors when considering the economic implications of decarbonization technologies.The inclusion of the carbon tax makes alternative energy technologies highly desirable since they provide fuel and carbon tax savings.The economic analysis is highly sensitive to the fuel and electricity prices (for revenue generation).Fuel cells currently exhibit the highest capital cost: up to three times higher than the baseline scenario.Innovative business models and financial incentives are needed to address these high capital costs.The environmental impact of deep-sea shipping is of a much higher magnitude than the propulsion costs.Hence, financial incentives to build bunkering infrastructure and lower capital costs would accelerate the move towards carbon-neutral deep-sea shipping.

Figure 3 .
Figure 3. Historical fuel and electricity price data (in the month of July every year) [87].

Figure 3 .
Figure 3. Historical fuel and electricity price data (in the month of July every year) [87].

Figure 6 .
Figure 6.(a) CAPEX breakdown for each scenario case; (b) vessel lifetime cost without carbon tax.

Figure 6 .
Figure 6.(a) CAPEX breakdown for each scenario case; (b) vessel lifetime cost without carbon tax.

Energies 2023 , 29 Figure 10 .
Figure 10.Effect of carbon tax price change on DPP.

Figure 9 .
Figure 9.Effect of carbon tax price change on DPP.

Figure 10 .
Figure 10.Effect on DPP of (a) electricity price; (b) fuel price.

29 Figure 11 .
Figure 11.Action timeline for ship owners to comply with IMO regulations.

Figure 11 .
Figure 11.Action timeline for ship owners to comply with IMO regulations.

Table 1 .
Scenario descriptions based on different decarbonization technologies.

Table 5 .
Sensitivity analysis parameters.To view the numerical value of the constant, refer to Section 2.4.

Table 5 .
Sensitivity analysis parameters.To view the numerical value of the constant, refer to Section 2.4. ).