Today, 96% of the bunker fuels used to power ships are made of fossil fuels, making shipping responsible for 2.8% of global GHG emissions or 1036 Mt of CO2
e per annum [1
]. As reported in the Fourth IMO Greenhouse Gas Study, GHG shipping emissions will increase by up to 50% to 1500 Mt CO2
following the rise in seaborne trade activities [1
]. Currently, speed reduction and installation of energy saving technologies are the main approaches for shipping decarbonisation. Speed reduction can potentially save up to 34% of energy [2
], however, it was demonstrated in recent studies [3
] that the classical cubic law for fuel consumption-speed curve only holds near the vessel design speed and that the energy saving was anticipated to be lower. In addition, Berthhelsen et al. [4
] suggested that the effect of slow steaming in carbon reduction has been overestimated. Energy saving technologies, for example the Flettner rotor, air cavity lubrication system, flex tunnel and hull vane, were proven effective in energy saving, however, the energy saving is between 10–15% [2
]. Hence, these approaches are not sufficient to achieve IMO’s 2050 goal and the use of low-carbon alternative fuels and carbon capture appear to be necessary to reach reductions in carbon emissions up to 50%.
The maritime industry has already started to operate vessels with fuels different than the conventional HFO [5
], for example LNG and biofuels. Both LNG and biodiesel can be burned in an ICE that gives at least 40% efficiency, and since the conversion of existing vessels to biodiesel and LNG requires minor retrofits, these alternative fuels emerge as an interesting solution for shipowners, at least in the short term. Methanol (denoted for convenience as MeOH in this paper) is another alternative fuel that can be burned easily in an ICE and is stored in the liquid phase, and so MeOH also gets attention. Maersk will reportedly invest USD 1.4 billion on eight ocean-going vessels with capacity to carry 16,000 containers, claimed to be ready to set to sail from early 2024 [6
]. Apart from ICE, some vessels may be fitted with higher efficiency propulsion systems such as fuel cells and electrical motors. Battery-based ships and hybrid ships have been commercialized for short trips and small-scale vessels, driven by their higher energy efficiency and robustness of the battery system [7
]. Although the fuel cells available in the market may still require optimization, there are a few projects for vessels. In 2014, the HySeas project conducted a commercial study for a hydrogen fuel cell-powered vessel that later integrated PEM fuel cells to a sea-going ferry [8
]. Toyota developed a fuel cell system for the world first hydrogen-powered vessel, the Energy Observer [9
]. In 2021, Shell in collaboration with Sembcorp Marine Ltd. developed a plan to install PEM fuel cells on an existing Ro-Ro vessel in Singapore [10
]. This shows that fuel cells are getting increasing attention, with hydrogen and ammonia as the key energy carriers. In addition, installation of onboard CCS to capture CO2
from vessel’s exhaust has also been implemented; for instance, Nordica is the first vessel operated with onboard CCS [12
Although the above demonstration projects show the practical interest in decarbonisation technologies, there is still need for significant research on the technologies themselves and on system-level studies to determine the optimum solutions for the short or long term. LNG has been suggested as the fuel with the most potential for shipping [13
]. In 2018, DNV.GL examined the implication of alternative fuels to replace conventional marine fuels and identified LNG, LPG, methanol, and biofuels as the most promising solutions [15
], which are capable of meeting the emission limits with cost comparable to oil-fuelled systems with a scrubber [16
], DNV.GL also forecasted the demand of LNG to be increased exponentially [17
]. Law et al. [18
] consistently compared various low-carbon alternatives against HFO and ranked onboard CCS for fossil-fuelled ship and biofuels as the best low-carbon solution, in terms of energy and financial cost, when expressed per Joule of propeller work. Those calculations did not include ship design and operating condition, and therefore the energy, GHG, and financial cost per ton of cargo and distance travelled, which are important ratios in practice, could not be evaluated. However, a fleet-level assessment would be an insightful way to determine alternative fuel potential.
Some fleet-level assessment addressed the impact of fuel transition on cargo attainment. For example, Ref. [19
] suggested that a bulk carrier with approximately 82,000 DWT experienced a reduction in cargo capacity with zero carbon fuels. Horvath et al. [20
] showed that hydrogen fuel would result in a higher cargo space loss of up to 13.3% for a short-sea vessel, which was contradicting to the 5% cargo loss reported by the International Council on Clean Transportation [21
]. Imhoff et al. showed that ammonia fuel could result in cargo loss between 4–9% [22
]. In addition, some studies compared the performance of ships powered by different fuels. Smith et al. [23
] concluded that the liquefied H2
carrier would have to be 1.7 times larger than an LNG carrier by volume, whereas Kim et al. [24
] concluded that an ammonia-based ship requires 1.6–2.3 times more volume and 1.4–1.6 times more weight than a conventional HFO-based ship. These studies give some implications on the impact of fuel transition on cargo loss, however, there is lack of consistency possibly due to the different approaches or assumptions in the assessment, which make it difficult to systematically compare the potential of different alternative fuels. In addition, the previous research also neglected the specific characteristics of the ship such as size, deadweight and ship type, which could result in variation in cargo capacity. Hence, more detailed studies still need to be done to cover a wider range of ships, so that the result could reliably reflect the potential of fuel transition. Since the performance of a future decarbonised ship should not be concluded based on the cargo loss only, an assessment from different perspectives, especially energy and cost, should be included, for example the techno-economic assessment of advanced fuel done by Korberg et al which compared the performance of fossil-free ship from energy and cost perspectives [25
There are many types of ships in operation, which have different size, carry different types of cargo and travel for different distances. The novelty of this research is the inclusion of a wide range of ships by considering four ship design factors, namely fuel type, ship type, cargo type, and voyage distance. Three types of ships are included: a cargo ship, tanker, and containership, hence covering the majority of current vessels. The type of cargo determines the stowage factor (volume per mass), and here the stowage factor was varied from 0.5 m3
/ton to 4 m3
/ton, covering most types of cargo today [26
]. Based on the voyage range of all ships derived from AIS [27
], the vessels were assessed for voyage distances between 2000 and 20,000 nautical miles. Eight fuel pathways were selected including HFO as reference fuel, HFO with CCS installation, LNG with CCS, bio-MeOH, biodiesel, NG-hydrogen, NG-ammonia, and NG-electricity. These were the top-ranked scenarios with more than 50% of lifecycle carbon reduction as discussed in our previous work [17
Apart from the broad coverage of ships, the addition of an analysis of the ship performance using various performance indicators (CAR, ES
, and CII) also make this study useful for various stakeholders in maritime decarbonization. Among all, CAR is the most common performance indicator used in fleet-level assessment to express the cargo loss [20
], studied the influence of refuelling stops and cargo loss allowance on the CAR [27
], whereas ES
are specific indicators defined in this study to quantify the TTW energy and TTW cost to transport cargo per nautical mile and ship deadweight. CII which is a rating system developed by IMO to measure the CO2
emission per cargo capacity and nautical mile, which is also included as one of the performance indicators for quantification of the potential of carbon reduction of alternative fuels [28
]. Usage of these numerical indicators allows a quantitative comparison between ships. By using the same assumption in calculation of the ship performance indicators (per unit of ship deadweight and voyage distance), different ships can be compared consistently. This study demonstrates the variation in CAR for different ships, whereby different ships are designed by varying the design parameters (ship type, cargo type, voyage distance, fuel type) of the ship. The outcome emphasizes the significant of ship type and mission towards the suitability of different alternative fuels to replace HFO. The last part of the study also demonstrates the application of the outcome to justify the design of the ship, so that the ship could be powered by selected alternative fuels. Shipowners and other stakeholders in the maritime industry can use the results and the present methodology to take consistent decisions in fuel transition. Hence, the results obtained from this work are an important step for shipping decarbonization via fuel transition.
In the next section, the research methodology is presented. Section 3
includes the results of exploration of sensitivity of CAR towards ship design factors (Section 3.1
). A case study for deep-seagoing containership powered by alternative fuels is presented in Section 3.2
, and another case study for ship propulsion with alternative fuels with upper limits for cargo loss is presented in Section 3.3
. Section 4
includes a discussion for the results, while Section 5
summarizes the conclusions from this work.
From a Well-to-Wake perspective, the results of Ref. [17
] compared 22 different pathways of primary energy source, fuels, and propulsion systems. The comparison was made on the basis of work at the propeller, i.e., how much energy is needed and how much carbon is emitted on a lifecycle basis to deliver a given amount of work. However, more refined data may be needed by policy makers and ship operators to decide routes and decarbonization strategies, and such an effort was presented in this paper. Seven alternative fuels have been selected regardless of the primary energy source and the analysis was done from a Tank-to-Wake perspective. The present results demonstrate that cargo must be displaced to account for the alternative fuel and its propulsion system, and each decarbonization alternative will come with a range of energy and economic penalties depending on the cargo stowage factor and voyage length. In addition, performance indices must be carefully defined: for instance, should they be per deadweight or per cargo weight? Here, the CAR and the relative ES,
, and CII were obtained for a range of ship types and alternative fuels.
The effect of ship type, cargo type (stowage factor), voyage length, and fuel type on CAR are summarized in Table 2
. The conclusion is that cargo type, fuel type, and voyage distance are significant design factors in determining ship performance, while the ship type has shown minor impact on CAR. Detailed estimates of CAR as a function of fuel type, ship type, stowage factor, and voyage length are shown in Figure 5
, Figure 6
and Figure 7
. In this research, cargo type is represented by the stowage factor (m3
/ton), and here CAR was calculated by reference to a given HFO ship (type, stowage factor, distance travelled) or by referring to the same ship with the same cargo type but burning HFO. The first type of comparison assists sector-wide estimates, while the latter is helpful for assessments of individual ship operations. The CAR of alternative fuels depends on whether the cargo is high-density or low-density. A high-density cargo makes a ship weight critical, and then a fuel with high LHV, such as hydrogen, gives the highest CAR. In contrast, a volume-critical ship has the highest CAR with a fuel of high volumetric density. Importantly, the definition of high- or low-density cargo depends on the available VC
of a ship, which was discussed in Section 2.1
. Next, voyage distance is closely related to the quantity of fuel needed to complete the voyage. In general, a longer voyage is accompanied by a lower CAR due to the larger
. However, there is an exceptional case for a weight-critical ship powered by hydrogen, which has a higher LHV value than the reference HFO fuel. Different fuel types result in variation in CAR due to the differences in weight and volume of fuel (
) and energy converter (
), as well as the additional weight and volume of the CCS system (
). The ship type yields only minor differences in CAR, suggesting that decarbonisation solutions would have similar effectiveness across all ships.
Next, two examples were analysed to compare the performance of various alternative fuels for a deep-seagoing containership with cargo stows at 1.25 m3
/ton. The various indices were given equal weight in an effort to provide a single metric. The performance of various fuels for both case studies was compared against the reference HFO-fuelled containership described in Table 1
. From the first case study, the CAR, relative ES
, relative CS
, relative CII, and transportation cost of a unit of cargo for 10,000 n.m. for various fuels are compared, and the results are summarized in Figure 8
and Figure 9
. It was found that biodiesel offers the most potential as alternative fuel, followed by ammonia, hydrogen, bio-MeOH, HFO with CCS, and LNG with CCS, and electricity has the lowest potential based on its low CAR and exceptionally high CS
(mostly due to the capital cost of the batteries). More R&D is required to improve the energy density of the batteries so that longer distances can be achieved, and less cargo displaced. However, electricity can still be a potential alternative marine fuel for short-sea shipping, suggested by the increasing number of hybrid and electrical-propulsion vessels over the years.
Case study 2 was then carried out to study the feasibility to use all types of alternative fuels, especially electricity for ships. The case study showed that all alternatives can replace HFO as marine fuel including electricity, but with a shorter voyage distance if we are to ensure that a 3% cargo loss limit is not exceeded (assuming this is a reasonable value for acceptable cargo loss). Refuelling can also be planned to extend the voyage distance of the vessel.
The results in Section 3.1
are independent of policy priorities and are based only on estimates of propulsion system efficiencies, fuel cost, and ship design methodologies. Therefore, the evaluation of the various performance indices (CAR, CS
, and CII) is relatively robust. The results in Section 3.2
and Section 3.3
depend on the weighting factors given to the various indices, which depend on the stakeholder’s priorities. Here, an equal weight is given as an example, but obviously the ranking between decarbonization options can be different if priorities are different. The present results and modelling can assist with the corresponding quantitative estimates.
This study carried out a fleet-level assessment to determine the suitability of seven selected alternative fuels to replace HFO as marine fuel. The fleet-level assessment was started by building a model based on empirical design rules with four selected design factors, namely ship type, cargo type, fuel type, and voyage distance. The exploration of these design factors gave the variation of CAR and is summarized in Table 2
with the individual results presented in Figure 5
, Figure 6
and Figure 7
. In short, cargo type is the most sensitive factor and determines if a ship is weight- or volume-critical, followed by voyage distance, fuel type and, lastly, ship type, which is the least sensitive factor in terms of CAR. Whether a ship is weight- or volume-critical affects the amount of cargo that must be replaced to allow for the different mass and volumetric densities of the low-carbon fuel and propulsion system. For very dense cargos, hydrogen offers the best performance, but for the usual values of the stowage factor, all fuels perform worse than the current HFO, suggesting decarbonization will come at a severe cost.
Three additional performance indicators, i.e., specific energy ES
, specific cost CS
, and the CII rating for various fuels were calculated. For a reference HFO ship and operation, the ES
, and CII are 55.92 kJ/dwt.nm, $
0.00049/dwt.nm, and 3.78 gCO2
/dwt.nm, respectively. Each alternative, when compared to the conventional HFO reference ship, ranks differently for each of these indices, and the relative values of these indices showed that fossil fuels with CCS lead to about 90% CII reduction with about 20–30% economic and energy penalty, while the other alternatives reach zero TTW CII but with Cs greater than 3. The individual indices can be combined into one by selection of appropriate weightings, reflecting the user’s priorities. If equal weighting is given in each of these indices, then a normalized scoring system can be formed, as shown in Figure 10
. Biodiesel has the highest normalized score (80%), followed by ammonia, hydrogen, bio-MeOH, HFO (CCS), LNG (CCS), and finally electricity, with 33%. Further, considering voyage distance and refuelling, the CAR of various alternative fuels were studied, and it was shown that the various decarbonisation alternatives could potentially achieve any desired CAR via voyage planning and adjustment on voyage distance.
In summary, all alternative fuels have potential for decarbonization of the shipping industry but give very different performance as a function of cargo density and voyage distance. The present results can help an operator define an optimal decarbonisation strategy depending on their requirements (their fleet, cargo types, and voyage lengths) and priorities (carbon emission vs. fuel cost).