725
1 INTRODUCTION
In a blue maritime economy perspective [1], to reduce
greenhouse gases (GHG), the International Maritime
Organization (IMO) has indicated the goal of net-zero
emissions by 2050 [2]. In recent years, both researchers
and the shipping industry have increasingly explored
new fuel types—renewable and low-carbon fossil
fuels—as potential energy sources for marine engines,
including both propulsion and onboard electricity
generation [3]-[14]. Alternative fuels, for drop-in
solutions and retrofitting strategies, seem to be an
interesting solution for drastically reducing emissions
[3]-[25]. These alternative marine fuels are available in
two primary physical states under ambient conditions:
liquid and gaseous. Liquid alternative fuels include
methanol, ethanol, as well as biological and synthetic
liquid fuels. The main gaseous alternative fuels are
natural gas, propane, hydrogen, ammonia, and
biological and synthetic gas fuels [9], [11], [17]. As well
known, in marine applications, emissions are
categorized into GHG, including Carbon Dioxide
(CO2), methane (CH4), and nitrous oxide (N2O), which
contribute to global warming, and non-GHG
pollutants, such as sulfur oxides (SOx), nitrogen oxides
(NOx), and particulate matter (PM), which impact air
quality and human health. The IMO MARPOL Annex
VI set limits on both types to reduce the environmental
footprint of shipping. In this framework of non-GHG
pollutants production, SECA (Sulfur Emission Control
Areas) and NECA (Nitrogen Emission Control Areas)
are designated maritime zones where stricter limits on
sulfur oxide (SOx) and nitrogen oxide (NOx) emissions
apply, respectively. Established by the IMO under
MARPOL Annex VI, these zones currently include
regions such as the Baltic Sea, North Sea, North
American coast, and the U.S. Caribbean. On the other
hand, as part of its strategy to reduce GHG emissions
from ships, the IMO has introduced two key regulatory
measures: the Energy Efficiency Design Index (EEDI),
A Technical, Environmental and Economical Comparison
Among Traditional and Unconventional Marine Fuels
M. Acanfora
1
, M. Altosole
1
, F. Balsamo
1
, L. Mocerino
1
, F. Scamardella
1
& U. Campora
2
1
University of Naples Federico II, Naples, Italy
2
University of Genoa, Genoa, Italy
ABSTRACT: The study presents a comprehensive comparison among traditional marine fossil fuels and seven
alternative unconventional solutions: fossil-based fuels (natural gas (NG) and methanol), biological (HVO, bio-
NG, and bio-methanol), and synthetic fuels (NG and methanol). The comparison evaluates technical, economic,
and environmental aspects. The case study focuses on the mechanical propulsion system performance of a small
luxury passenger ship, simulated by using a mathematical model developed in a Matlab-Simulink environment.
In the simulation, the ship's original engines have been replaced with alternative engines of comparable power
and RPM, which are compatible with the fuels under consideration. In addition to GHG evaluation, the study
uses International Maritime Organization parameters such as the EEXI and the CII to assess the environmental
impact of fuels. From an economic perspective, the vessel’s propulsion system has been evaluated based on
CAPEX and OPEX indices, providing a holistic view of the feasibility and sustainability of each fuel option.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 19
Number 3
September 2025
DOI: 10.12716/1001.19.03.04
726
Energy Efficiency Existing Ship Index (EEXI), and the
Carbon Intensity Indicator (CII) [1]-[2]. As part of the
mid-term measures to reduce greenhouse gas (GHG)
emissions, the International Maritime Organization
(IMO) has introduced several mandatory frameworks
targeting ship energy efficiency. The Energy Efficiency
Design Index (EEDI), implemented in 2013, applies to
newly built ships above 400 gross tonnage (GT). It
measures energy efficiency at the design stage and
aims to reduce CO₂ emissions by promoting improved
vessel design. The IMO introduced the Energy
Efficiency Existing Ship Index (EEXI) for existing ships
in 2021. Like the EEDI, the EEXI targets ships above 400
GT, but focuses on ensuring existing vessels meet a
minimum energy efficiency standard. Compliance
became mandatory in 2023, and ships may need to
undergo modifications or retrofitting to meet the
required performance levels. In addition, the Carbon
Intensity Indicator (CII) came into force in January
2023. It applies to ships above 5,000 GT and evaluates
their operational carbon intensity. Vessels receive
annual ratings from A to E indexes, with the system
designed to incentivize ongoing improvements in
energy efficiency during actual operation [19]. A more
detailed description of the calculation methods for
both indices will be presented later in the paper.
Additionally, the recent FuelEU Maritime Regulation,
taking effect from 1 January 2025, sets well-to-wake
GHG emission intensity requirements for energy used
on board ships over 5,000 GT transporting cargo or
passengers for commercial purposes in the EU or
European Economic Area (EEA) [20]. The International
Maritime Organization (IMO) has, indeed, introduced
the concepts of Well-to-Wake (WTW) and Tank-to-
Wake (TTW) in its guidelines for assessing GHG
emissions across the entire lifecycle of marine fuels.
These concepts were formalized in Resolution
MEPC.391(81) (2024), adopted during the 81st session
of the IMO’s Marine Environment Protection
Committee (MEPC) [20]. The guidelines cover
emissions from WTT, TTW, and WTW for all potential
marine fuels and energy sources used onboard ships.
For marine applications, WTW is crucial for comparing
alternative fuels like LNG, biofuels, ammonia, and
hydrogen, as it provides a more holistic view of their
sustainability and carbon footprint. The TTW approach
only considers the direct emissions from fuel
combustion onboard the ship. It does not account for
emissions produced during fuel production and
transportation. TTW is often used for regulatory
compliance, such as IMO regulations, but it does not
fully reflect the overall environmental footprint of
different fuels. The WTW approach evaluates the entire
lifecycle emissions of fuel used in marine applications.
This includes Tank-to-Wake and Well-to-Tank, which
cover fuel extraction, production, processing, and
transportation to the vessel. WTW is the most
comprehensive metric for assessing the total
environmental impact of fuel, considering both
upstream and downstream emissions. In a WTW
approach, according to the IMO, the required
reduction in GHG intensity is set at 2% by 2025, 6% by
2030, 14.5% by 2035, 31% by 2040, 62% by 2045, and
80% by 2050 [2]. These GHG intensity targets will
gradually force ships sailing into EU/EEA ports to shift
fuels, including adopting new technologies, to reduce
GHG emissions. In the framework of alternative fuels,
LNG stands out as the most balanced choice, offering
high energy density, well-established technology, and
strong safety. Its fuel supply infrastructure is relatively
advanced, though its availability as a green fuel
remains limited. Methanol also performs well in
several aspects, particularly in energy density and
technological maturity. However, it does not reach the
same level of safety as LNG and faces challenges with
fuel supply infrastructure. The biggest limitation for
methanol is its low availability in green form, making
it less attractive from a sustainability perspective.
Demonstration projects have already been in progress
since before 2025, and commercial availability is
expected to grow steadily into the 2030s. Just for
example the Stena Germanica, a 1500-passenger ferry
[26], [27], from 2015 is a notable demonstration project
in methanol practical use in shipping. In 2018, also the
tanker vessel (50000 deadweight tonnage) engine was
converted to methanol use as fuel [28]. Ammonia and
Hydrogen share similar challenges. Both have
moderate energy density and are still developing in
terms of technological maturity. Safety concerns are
more pronounced for Ammonia, which poses
significant risks compared to the other fuels.
Additionally, both Ammonia and Hydrogen face major
obstacles in fuel supply infrastructure, making
widespread adoption difficult. Their availability as
green fuels is also quite limited, further restricting their
feasibility as sustainable alternatives. For Ammonia,
the first demonstration projects for both ICE and fuel
cell applications begin around 2025, with commercial
applications expected to expand significantly after
2030 [22]. Hydrogen follows a similar pattern, with its
first demonstration projects occurring before 2025.
However, the timeline for commercial adoption
appears to be slightly more extended, indicating
potential technological or infrastructure challenges
before full-scale implementation [22]. Batteries, as a
different form of energy storage, offer advantages in
sustainability but struggle in other key areas. Their
energy density is much lower than traditional fuels,
limiting their suitability for applications requiring
long-range power. While battery technology is
developing, it is not as mature as other alternatives
[29]. Despite being considered a safer option, batteries
still face infrastructure challenges. However, their
greatest strength lies in their strong potential for green
energy use, making them an attractive option for future
sustainability. Overall, LNG remains the most viable
fuel, balancing efficiency, safety, and infrastructure,
while methanol presents a reasonable alternative with
some limitations. Ammonia and hydrogen still have
significant barriers to overcome, particularly in safety
and availability, whereas batteries, despite their green
energy potential, currently lack the necessary energy
density and infrastructure to be a dominant energy
solution. According to DNV [22] the vast majority of
the world’s fleet still relies on conventional fuel, with
only 0.50% of ships in operation using alternatives such
as Methanol (0.01%), LNG (0.19%), and Battery
technology (0.30%). However, the trend is shifting, for
the ships in order category. The percentage of ships
relying on conventional fuel decreases to 88.16%, while
alternative fuel adoption grows significantly to 11.84%.
Within this group, Battery-powered ships account for
3.85%, LNG for 6.10%, and Methanol for 0.30%, with
smaller shares for Hydrogen (0.06%) and Ammonia
(0.02%) [29], [30]. This suggests an increasing interest
in cleaner energy sources, particularly LNG and
727
Battery technology, as the shipping industry moves
toward sustainability. Recently, Hydrotreating
Vegetable Oil (HVO), a biological fuel [29], has begun
to be used to power fast marine engines [31]. HVO is
a renewable diesel produced through the
hydroprocessing of vegetable oils or animal fats.
Unlike conventional biodiesel FAME (Fatty Acid
Methyl Esters), HVO has a chemical structure nearly
identical to fossil diesel, which allows it to be used as a
drop-in replacement without engine modifications.
The use of HVO contributes to lower CO2 lifecycle
emissions, reduced particulate matter, and improved
air quality, especially in coastal and port areas. Its
compatibility with existing marine diesel engines and
infrastructure makes HVO a practical interim solution
on the pathway to full maritime decarbonization. In
recent studies, the authors have developed
thermodynamic simulation models for four-stroke
marine engines fuelled by Heavy Fuel Oil (HFO),
Natural Gas (NG), and methanol [32],[33]. This paper
integrated these engine models into a previously
developed simulation model tested on a small luxury
passenger ship [34]. Using the three versions of the
ship's propulsion model and considering the vessel’s
actual service route, this study compares three
alternative marine fuels (NG, methanol, and HVO)
against a conventional marine fuel (HFO). For a
comprehensive analysis, NG and methanol are
considered in their fossil, bio-based, and synthetic
forms [3], [4], [22], [24], [25], [35]-[37] while HVO is
assessed as a biofuel [29]. The fuel comparison covers
technical aspects (lower heating value, onboard storage
requirements, engine efficiency), environmental
impact (GHG emissions), and economic factors
(specific fuel costs, engine and onboard storage
expenses, carbon tax). Beyond the ship’s in-service
GHG emissions, the study also evaluates the fuel-
related environmental impact using the EEXI and CII.
Finally, CAPEX (Capital Expenditure) and OPEX
(Operational Expenditure) indicators are applied for
the economic assessment.
2 FUELS
The fuels considered in this study include
conventional, alternative, and renewable options for
maritime applications. The reference fossil-based fuel
is Ultra Low Sulphur Fuel Oil (ULSFO) [38], which
complies with emission regulations by reducing sulfur
content. Liquefied natural gas has been analyzed in
three forms: fossil-based Liquefied Natural Gas
(LNG), bio-derived Liquefied Biological Gas (LBG),
and Liquefied Synthetic Methane (LSM), produced
using renewable energy. Methanol has been
considered in its fossil (Methanol), biological (bio-
Methanol), and synthetic (e-Methanol) forms.
Additionally, the renewable biofuel, Hydrotreating
Vegetable Oil (HVO) [38], [39] has been included as a
drop-in alternative since it is compatible with existing
marine engines. These fuels are compared in terms of
technical feasibility, environmental impact, and
economic viability for maritime propulsion. Table 1
presents the main parameters of the considered fuels
[38], [39].
Table 1. Fuels main parameters
Fuel parameters
ULSFO
HVO
LNG,
LBG, SM
Foss-Meth, bio-
Meth, e-Meth
Ambient conditions state
liquid
liquid
gas
liquid
Density [kg/m3]
890
780
0.717
781
Carbon contents [wt%]
86.88
-
75
37.5
Sulphur content [wt%]
0.1
0
0
0
Lower Heating Value
(LHV) [kJ/kg]
42700
43700
49000
19600
Stoichiometric air fuel
ratio
14.5
14.9
17.5
6.5
3 INPUT AND METHODOLOGY
3.1 Engine
To the scope, two four-stroke Rolls-Royce marine
Bergen engines have been selected (C25:33L8P with
2665 kW at 1000 rpm and C26:33L9PG with 2430 kW at
1000 rpm); these engines are normally fuelled both
with diesel fuel (ULSFO) and LNG [15]. According to
the manufacturer, the main design characteristics of
the two engines are reported in Table 2.
Table 2. Main data and performance (at MCR) for the
selected engines and fuels
Engine parameters
C25:33L8P
C26:33L9PG
fuel type
ULSFO-HVO
NG/simulated
METHANOL
cylinders number
8L
9L
bore [mm]
250
260
stroke [mm]
330
330
brake power [kW]
2665
2430
(b.m.e.p.) [bar]
24.7
18.5
speed [rpm]
1000
1000
efficiency [%]
46.3
47.7/46.2
charge air pressure [bar]
4.2
2.8/4.2
charge air temperature [°C]
55÷60
55/56
Authors have already developed and validated the
thermodynamics simulator of the Bergen engines
fuelled with diesel and NG [32], [33]. The same engine
has been fuelled with methanol in simulation
environments (see Table 2). The engine's efficiency
(Eeff), reported in Figure 1 and Table 2, are determined
by eq (1):
E
eff
f
P
E
M FLHV
=
(1)
where: PE is the engine-delivered power; Mf and FLHV
are the fuel mass flow rate and its lower heating value
respectively. In particular, the data in Table 2 shows a
very similar diesel and methanol engine efficiency,
while that of the NG engine is about 1.5 percent higher.
Figure 1 reports the working engine curves versus the
ship’s speed, in the figure the black curves are
pertinent to the propulsion power only, while the blue
one considers also the shaft generator's mechanical
power. To define this last curve, 10% of sea margin and
15% of the engine margin are considered. According to
manufacturer data, Figure 1a and Figure 1b display the
efficiency curves for diesel and NG configurations on a
power-speed chart. Figure 1c presents the same
information for the methanol configuration based on
simulation results [32], [33]. The diesel engine
simulator maintains almost the same MCR efficiency
[32], whether fed with ULSFO or HVO (the difference
in MCR engine efficiency is less than 0.1 %); this is due
728
to the very similar characteristics of these two fuels (see
Table 1). Based on this consideration, the constant
efficiency curves for the diesel engine reported in
Figure 1a are considered valid for both fuels.
Regarding NG and methanol, the use of fuel from
fossil, biological, or synthetic origin does not show any
difference in the simulation model and results, as the
characteristics of these fuels are the same regardless of
their origin (see Table 1).
a)
b)
c)
Figure 1. Diesel (a), NG (b), and methanol (c) engines
efficiency
3.2 Engine
A small cruise ship, called Spirit of Oceanus owned by
the Cruise West company, has been chosen as a case
study vessel. The main data of the ship and the original
propulsion plant are reported in Table 3. According to
Figure 2, the vessel's actual nine-day cruise to Alaska
starts in Seattle and includes stops at the ports of
Ketchikan, Juneau, and Skagway, along with a low-
speed cruise through Endicott Arm Fjord, before
returning to Seattle. The cruise schedule is reported in
Figure 3. A harbor departure and arrival time of 1h is
considered, with the vessel operating at an average
speed of 3 knots during these segments. Despite the
whole route being within an ECA area [40] all the
tested fuels can be used as conventional fossil ULSFO
has a sulfur content of less than 0.1% while the other
tested fuels contain no sulfur. Finally, a total of 30
cruises/ per year are considered. The vessel propulsion
plant diagram is shown in Figure 4 [41]; according to
this figure, the ship has two shaft lines, each equipped
with a four-stroke diesel engine (MAIN-DE) that drives
a Controllable Pitch Propeller (CPP) through a
reduction gear (G). In the navigation configuration, the
vessel's hotel electric load is supplied by a Shaft Electric
Generator (SEG), which is directly connected to each
engine shaft. The two diesel-electric generators (DE-
EG) are used only when the ship is at the harbor. A bow
thruster (364 kW electric load) is used for port
maneuvers.
Table 3. Case study
Dimensions
Overall length
90.6 m
Breadth
15.3 m
Draught
3.9 m
Gross tonnage (GT)
4200 t
Speed
Maximum speed
(design draught)
18.5 kn
Propulsion and
electric load
Propulsion diesel
engines MCR power
2x2289 kW at 1050
rpm
Shaft electric
generators
2x800 kWe
Diesel generators
2x1050 kWe
Hotel electric load
889 kWe
Manoeuvre electric
load
1637 kWe
Harbors stop electric
load
698 kWe
Passenger and crew
Passenger
127
Crew
85
Figure 2. Actual ship route for the Alaska cruise
Figure 3. Travel segments for the Alaska cruise
729
Figure 4. Ship propulsion plant
The original ship propulsion plant MCR engine
power and speed (Tab. 3) are similar to those of the
Bergen selected engines to fuels comparison (Tab. 2),
then, in the propulsion system simulator the two
original propulsion engines are replaced with the
Bergen engine (in different versions fed with ULSFO or
HVO (both diesel fuels), NG or Methanol, depending
on the tested fuel). Since the diesel and NG-methanol
engine's different torque limit curves (see Figure 1),
diverse propeller P/D laws versus shaft speed are
considered, see (Figure 5).
Figure 5. Diesel, NG, and methanol engines propeller P/D
laws vs ship speed
For the harbour condition, for the diesel-electric
generators engine (DE-EG blocks in Figure 4), in the
case of ULSFO or HVO fuels, the four-stoke high-speed
MTU 8V 4000 M 63 diesel engine has been adopted (1
MW MCR power at 1800 rpm and 727 kW mechanical
power at 1500 rpm [42]) to satisfy the electric load at 50
Hz, with 41% of engine efficiency. In the case of NG
fuel, the same electric energy is satisfied by the four-
stoke high-speed MTU 8V 4000 GS engine (1.065 MW
MCR power at 1500 rpm, 35% of efficiency) [43]. Due
to a lack of information, an efficiency of 34% has been
assumed for methanol configuration according to the
engine data used for NG, with the correct efficiency
value based on the data of the Bergen engines reported
in Figure 1 (MTU 8V 4000 GS).
4 RESULTS AND COMPARISONS
As declared before, the considered fuels are compared
between them from different points of view: technical
(engine efficiency, and fuel consumption), GHG
emissions (CO2eq, EEXI, and CII index), and economical
(CAPEX, OPEX, AK).
4.1 Technical aspect
Figure 6 reports the efficiency of the engine versus the
ship’s speed referred to the propeller plus shaft
generator electric power (blue curve in Figure 1).
Results show that the diesel and methanol engine
efficiency are similar at a certain speed, except for 13
knots, where the methanol engine efficiency is about
1.5% higher than the diesel engine. NG engine is
characterized by about 1.5-2.5% greater efficiency
compared to the other two engines, for all ship speeds
considered. In addition, Figure 7a shows the overall
fuel consumption, distinguishing between navigation,
maneuvering, and stay in port; overall annual fuel
consumption, based on 30 cruises/y, is reported in
Figure 7b; the lower HVO consumption, compared to
the ULSFO one, is due the ULSFO less lower heating
value (see Table 1), referring the HVO one, assuming
equal the engine efficiency. Finally, Figure 7b shows
the higher methanol consumption, due to its very low
LHV compared to the other fuels.
Figure 6. Engine efficiency (propeller and electric load) as a
function of ship speed for the different fuels.
a)
b)
Figure 7. Ship fuel consumption: cruise (a) and annual (b).
SEG
CPP
MAIN-DE
hotel el. load
EG
DE
G
SEG
MAIN-DE
G
CPP
EG
DE
P
o
P
p
P
B
P
H/2
Th
730
4.2 GHG emissions comparison
Table 4 reports the fuel Carbon factors (CF) in tCO2eq/tfuel
for the considered fuel and Life Cycle Assessment
(WTT, TTW, and WTW) [22],[38]. In the same table, for
some fuels, are reported also the TTW CF value
reported in the IMO report [44].
Table 4. Fuels carbon factors
Carbon
factors
[tCO2eq/tfuel]
ULSFO
HVO
LNG
LBG
LSM
foss-
Meth
bio-
Meth
e-Meth
WTT
0,564
0.607
0.636
0.049
0
0.630
0.265
0
TTW
3.390
(3.114
IMO)
0.679
3.621
(2.750
IMO)
0.897
0.897
1.430
(1.375
IMO)
0.050
0.050
WTW
3.954
1.286
4.527
0.946
0.897
2.060
0.315
0.050
Based on the carbon factors reported in Table 4 and
the fuel consumption data presented in Figure 7, the
well-to-wake (WTW) estimated CO₂eq emissions for
navigation, maneuvering, and port stays have been
calculated (Figure 8a). The results show that fossil fuels
(ULSFO, LNG, and fossil methanol) have significantly
higher CO₂eq emissions compared to bio-based and
synthetic fuels. Among the non-fossil fuels, HVO has
the highest emissions, but still shows approximately
75% lower CO₂eq emissions than fossil methanol, and
about 66% lower than ULSFO and LNG. Synthetic
methanol (e-methanol) has the lowest CO₂eq emissions
of all the fuels considered—only 2.4% of the emissions
from fossil methanol, which is the most carbon-
intensive option.
Figure 8b presents the annual well-to-tank (WTT)
and tank-to-wake (TTW) CO₂eq emissions for the
various fuels, based on 30 cruises per year. LSM and e-
methanol have zero WTT emissions; e-methanol total
emissions are much lower than those of LBG and bio-
methanol, while are similar to LSM case (refer to the
corresponding carbon factors in Table 4).
a
b
Figure 8. Ship CO2eq emissions Cruise (a) and annual (b)
4.3 EEXI and CII
According to the IMO regulation, the attained EEXI
(EEXIatt) value must be lower than the required EEXI
(EEXIreq), both expressed in gCO2/t/nm. For ships
without energy recovery systems the EEXIatt index can
be calculated according to the following formula [45]:
( ) ( ) ( ) ( )
( )
j FME FAE
ME i ME i AE i ME i
att
c ref
f P C SFC )+ P C SFC
EEXI =
f Capacity V
(2)
where: PME and SFCME are the main engines with 75%
MCR power and specific fuel consumption; PAE and
SFCAE are the auxiliary engine power and its specific
fuel consumption [45]; CFME and CFAE are the main
engines and auxiliary engine TTW fuel Carbon factor
respectively; Capacity is the ship capacity defined as
function of ship type [46] (GT for passenger's vessel);
Vref is the vessel speed (at 75% MCR); fj and fc are
constant-coefficient set to 1 for passenger ships [45].
The required index (EEXIreq) can be determined as [46]:
0.214
1 170.84
100
req
y
EEXI GT
−
= −
(3)
Both the reduction factor (y=0) and the EEDI
reference value (square brackets in the eq. 3),
depending on vessel type, can be obtained from an
exponential regression based on GT value [46]. In the
same way, for the CII calculation customized for
passenger's vessels, to meet the CII regulations, the
attained values (see eq.4 for CIIatt) [46]-[49] must be
lower than the required one (see eq.5 for CIIreq):
2
att
CO annual mass
CII =
GT D
(4)
where D is the ship annual distance travelled (nm)
-0.383
req
z
CII = 1- ·930 ·Capacity
100
(5)
where: z is the reduction factor relative to 2019, its
values are: 5% to 2023, 7% to 2024, 9% to 2025, and 11%
to 2026.
As a result, Figure 9a shows that for all fuels, the
attained EEXIatt is always lower than the required
EEXIreq, with a significantly larger margin for HVO,
biological and synthetic fuels.
731
a)
b)
Figure 9. EEXI (a) and CII (b) results
Similar trends can be identified for the CII
evaluation. For all fuels, the attained CIIatt values are
lower than the required CIIreq values, with a much
larger margin for HVO, biological and synthetic fuels
compared to fossil ones.
4.4 Economical comparison
To make the economic comparison between the
considered fuels, the ship's Annual Costs (AC) are
determined. AC is the sum of Capital Expenditure
(CAPEX, see eq. 6) and Operational Expenditure
(OPEX, see eq. 7) [50]-[54]
( )
( )
n
n
1+R
CAPEX=IC R
1+R -1
(6)
where IC is the Investment Cost determined based on
the specific cost of the propulsion plant's main
components (see Table 5) [23], [51], [52]; R is the
discount rate, set at 12 [53]; n is the investment lifetime
set at 20 years.
Table 5. The main propulsion plant component's specific
costs
Component
Specific cost
[USD/kW]
Diesel Internal Combustion Engine (fossil, bio)
(2,665-1 MW)
370 - 450
LNG, LBF, LSM ICE (2,43-1 MW)
515 - 630
Methanol ICE (fossil, bio, synth) (2,43-1 MW)
405 - 490
LNG tank and equipment
430
Electric generator – shaft generator
140 - 180
CPP and shaft line
320
AC/AC converter
195
OPEX = AFC + ALC + AMC (+CTAX) (7)
where AFC is the annual fuel cost; ALC and AMC are
the annual lubrication oil and maintenance costs,
respectively, which are assumed to be equal for all the
considered fuels. Consequently, their contribution is
not included in the fuel comparison. CTAX represents
the carbon tax (if considered). The European Union has
included shipping in the Emissions Trading System
(ETS) [54], [55] starting in 2025, which involves the
payment of a carbon tax based on a vessel's CO₂
emissions. The purpose of this tax is to discourage the
use of high-carbon fuels and to reduce CO₂ emissions
[56], [57]. For shipping, the carbon tax will come into
force in 2025 (the year of entry into the ETS). In the first
year, operators will be required to pay for 40% of their
emissions. This percentage will rise to 70% in the
second year (2026) and to 100% in the third year (2027)
[55], [58]. To include the carbon tax in the OPEX
calculation, a second economic analysis was carried
out, considering the cost of CO₂ emissions. Currently,
there are no precise indications regarding the cost per
ton of emitted CO₂ (Carbon Tax Price – CTP).
Therefore, an average value of 80 USD/t was assumed
based on a review of the literature [56]-[62]. The CTAX
value was obtained by multiplying the CTP (80 USD/t)
by the yearly fuel consumption, as reported in Figure
7b. The specific costs of the fuels considered are
reported in Table 6.
Table 6. Fuel-specific cost
Fuel
type
ULSFO
HVO
LNG
LBG
LSM
foss-
Meth
bio-
Meth
e-Meth
Specific
cost
[USD/t]
637
[63]
640
[64]
706
[39]
1758
[35]
7175
[14]
[65]
488
[66]-
[67]
574
[66]
2298
[14]
Starting from the data reported in Tables 5 and 6,
the overall propulsion system investment cost (IC) is
determined and reported in Table 7, where the ratio
relative to the plant cost (using ULSFO as fuel) is also
shown.
Table 7. Propulsion plants overall IC
Propulsion plant type
Plant cost
[MUSD]
Ratio plant/ref.
plant
Diesel ICE (foss, bio) (ref.
plant)
4.94
1.00
LNG, LBF, LSM ICE
7.96
1.56
Methanol ICE (foss, bio,
synth)
5.03
1.02
Comparisons show that the system employing
biological diesel fuel (HVO) has a cost equal to the
reference one and that the systems powered by
methanol (fossil, biological, and synthetic) have only a
small surcharge (2%) compared to the reference plant.
On the contrary, the plant using natural gas (NG)
(fossil, biological, and synthetic) involves a 56%
surcharge; this significant difference is due to increased
costs related to the engine and the fuel tank (see Table
5), which, in the case of diesel and methanol fuels, have
been neglected (since they are similar to the reference
one).
As reported above, a second economic comparison
of fuels is carried out, also considering the effects of the
carbon tax (although the vessel’s route is not in the
application area of this tax [56]). Figure 10 shows the
difference between the reference fuel (ULSFO) Annual
Cost (AC) minus the considered fuel's AC, all
multiplied by the ship's service lifetime (n = 20 years).
732
In the graphs, at t = 0, the starting points of the lines
are given by the ULSFO Investment Cost (IC) value
minus that of the considered fuel (all IC values are
reported in Table 7). As is normally done in this type of
analysis, over the 20 years considered, the fuel and
system component costs are assumed constant [50],
[63].
Figure 10a shows that, compared to ULSFO fuel,
HVO entails a saving of 0.9 million USD, and when
considering the carbon tax (CTAX), the saving
increases to 9.2 million USD.
a)
b)
c)
Figure 10. Fuels AC comparison versus reference fuel
(ULSFO) with and without the Carbon tax
Due to its higher investment cost (IC), LNG
consistently results in greater operating costs
compared to ULSFO. Over a 20-year period, the
additional cost of LNG amounts to USD 9.6 million,
regardless of the carbon tax (CTAX), since the CO₂-
equivalent emissions of LNG and ULSFO are nearly
identical (see Figure 8b). Bio-NG (LBG, green dotted
line in Figure 10b) and e-NG (LSM, blue dotted line)
are associated with even higher 20-year costs
compared to ULSFO—USD 45.5 million and USD 58.8
million, respectively. When CTAX is considered, these
additional costs are reduced to USD 35.9 million for
LBG and USD 50.0 million for LSM (solid lines). The
fossil methanol (foss-Meth) option (red dotted line in
Figure 10c) shows additional costs of USD 22.5 million
without CTAX and USD 24.9 million with CTAX (solid
red lines), both relative to ULSFO. In this case, the
application of CTAX leads to greater economic loss, as
fossil methanol produces higher CO₂-equivalent
emissions than ULSFO (see Figure 8b). Biological
methanol (bio-Meth) (green lines in Figure 10c)
involves additional costs of USD 26.7 million without
CTAX and USD 16.8 million with CTAX (dotted and
solid green lines, respectively). Finally, e-methanol,
due to its high specific cost (see Table 6), results in a
substantial cost increase of USD 180.2 million over 20
years (blue dotted line in Figure 10c), only slightly
mitigated by CTAX, which reduces the cost to USD
168.4 million (solid blue line). As a final observation,
Figures 8b and 10 illustrate that fuels with lower CO₂-
equivalent emissions than the reference fuel (ULSFO)
consistently benefit more economically from the
application of CTAX.
5 CONCLUSIONS
In this paper, a comprehensive comparison of eight
marine fuels was conducted, evaluating their technical
performance, GHG emissions, and economic impact.
The analysis was applied to a small cruise ship used as
a case study. The comparison between non-
conventional fuels and the reference fossil fuel
(ULSFO) led to the following key findings:
− The ship powered by traditional fuel (ULSFO),
HVO, and methanol (fossil, biological, and
synthetic) exhibits very similar engine efficiencies
across the entire operating range. In contrast, when
fueled with natural gas (NG—fossil, biological, or
synthetic), the same engines operating under
identical conditions achieve higher efficiency—
ranging from 1.5% to 2.5% more.
− In terms of GHG emissions, the ranking of fuels
from best to worst is as follows: e-methanol, bio-
methanol, LBG, LSM, HVO, ULSFO, LNG, and
fossil methanol. LNG's emissions are nearly
equivalent to those of ULSFO, while fossil methanol
emits approximately 15% more. HVO emissions
amount to just 31.2% of those from ULSFO, while e-
methanol—by far the best performer—achieves
emissions equal to only 2.9% of ULSFO’s. These
results are largely corroborated by the EEXI and CII
index evaluations.
− From an economic standpoint, and excluding the
carbon tax (CTAX), the fuels rank from most to least
cost-effective as follows: ULSFO, HVO, LNG, fossil
methanol, bio-methanol, LBG, LSM, and e-
methanol. Among the synthetic fuels—generally
the most expensive—LSM shows an annual cost
(AC) 2.5 times higher than ULSFO, while e-
methanol’s cost is 5.74 times higher.
The inclusion of CTAX does not significantly alter
the ranking, with the notable exception of HVO: its cost
ratio relative to ULSFO improves from 0.97 (without
CTAX) to 0.82 (with CTAX) (see also Fig. 9a).
In terms of GHG emissions, e-methanol represents
the most environmentally favorable solution, albeit
with an annual fuel cost 827% higher than that of
733
ULSFO (excluding CTAX). HVO, emitting only 31.2%
of ULSFO's GHGs and with a fuel cost 3.48% lower
(without CTAX), emerges as the best compromise
between emissions and economic sustainability. This
advantage becomes even more pronounced when
CTAX is applied.
ACKNOWLEDGMENTS
This research was developed within the “CN-MOST – Spoke
3 (Waterways)” research activity, funded under the National
Recovery and Resilience Plan (NRRP) of the Italian Ministry
of University and Research (MUR); funded by the European
Union – NextGenerationEU. The authors wish to gratefully
acknowledge the Rolls-Royce England technical office and
engineer Andrea Cerutti of Rolls-Royce Italy for the received
support.
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