1. Introduction
The shipping sector is crucial for international trade (~80–90% of the global trade occurs through shipping) and hence vital to the world economy [
1]. Due to the scale of the sector, shipping represents ~3% of the total global green house gas (GHG) emissions [
2], therefore strict environmental regulations around NO
x, SO
x and CO
2 emissions are set to cause major technological changes in the industry [
1,
3]. For example, Liquefied Natural Gas (LNG) can improve the performance, and on the other hand, with methane slip, the benefits are reduced [
1]. Other fuels and/or other technologies such as biofuels, hydrogen, nuclear and carbon capture and storage (CCS) could all decarbonize the industry, but each have significant barriers regarding cost, resources and social acceptability [
1]. In addition, fuel consumption can be improved by various efficiency improvements (such as hull design and cleaning, and propeller design, to name a few). It is obvious that numerous problems/issues must be tackled in order to achieve deep decarbonization of the shipping industry. Thus, there is “no single route and a multifaceted response is required” [
1] from different sectors of the industry.
Moreover, demand for shipping is likely to grow over the next three decades [
2]. Shell and Deloitte conducted a market survey in order to understand the current market trends [
4]. In their survey, more than 90% of respondents from the shipping industry considered decarbonization an important or top priority for their organizations. Eighty percent of the respondents also noted that the importance of decarbonization has “increased considerably over the past 18 months” [
4]. This, therefore, shows that the market and industry are considering decarbonization as part of their business strategy over the coming decades, in line with the International Maritime Organization’s ambition to reduce CO
2 emissions from shipping by at least 50% by 2050 compared with a 2008 baseline [
5].
Deep decarbonization will require financial incentives and policies at an international and regional level given the maritime sector’s ~3% contribution to GHG emissions [
1,
6]. Maritime emission and reduction measures are commonly divided into two main categories: technical (ship size, ship–port interface, etc.) and operational (lower-speeds, waste heat recovery, etc.) [
7,
8]. The International Transport Forum recognizes additional separate routes to achieve decarbonization, which is the use of alternative fuels (sustainable biofuels, hydrogen, ammonia), electrification of ships and wind assistance, albeit these could be argued to fall under the technical measures’ category [
8].
The scope of this review paper is to survey the literature related to the shipping industry, based on research articles and grey literature, and discuss the potential routes to achieve deep decarbonization by 2050, as set by the International Maritime Organization’s (IMO) targets.
Section 2 discusses alternative fuels that can be used in shipping. The section will discuss the relative advantages and disadvantages of alternative fuels.
Section 3 discusses renewable energy sources that are available within shipping, such as wind, solar and biomass.
Section 4 discusses the maturity of technologies currently available that can help the shipping industry achieve deep decarbonization. Various technologies will be presented, such as Internal Combustion Engines, fuel cells, batteries, supercapacitors and nuclear energy.
Section 5 discusses the various CO
2 abatement options; in essence, strategies and techniques that can help reduce fuel consumption such as vessel and propeller design and waste heat recovery. Finally, the review will conclude with a summary and future recommendations.
3. Alternative Fuels for Shipping
Compared to other sectors such as road transportation and aviation, the shipping sector uses less refined and/or processed fuels [
33]. The primary fuel used in the shipping sector to power marine diesel engines is heavy fuel oil (HFO). HFO has very high viscosity and contains large amounts of sulfur [
33], which when combusted releases harmful SO
x emissions. The shipping sector uses other fuels with lower viscosity levels and lower sulfur content, such as the marine gas oil (MGO) and marine diesel oil (MDO), the former being used for smaller vessels [
33].
Alternative fuels may have the potential to lower or have zero net emissions when used for ship propulsion [
8]. Currently, the use of alternative fuels, such as LNG and hydrogen, are gaining traction [
34]. Alternative fuels can also be used as “drop-in” fuels (such as biodiesel) but are still applied in shipping on an experimental basis [
34].
Table 1 lists possible CO
2 emissions reductions from alternative fuels and energy sources ([
8], p. 32), which are further discussed in this section. Note that Japan is working on four pilot projects (identified in
Section 1), which are based on its report on “Roadmap to Zero Emission from International Shipping” [
20]. The report mentions that pilot concepts are based on two possible emission reduction pathways: (1) LNG, provided that LNG transitions to carbon-recycled methane, and (2) adoption of hydrogen/ammonia as fuel [
20].
DNV GL [
35] illustrate the various energy densities for different fuels. The energy density partly illustrates “how applicable the fuel is for certain ship types and ship operations” [
35]. DNV GL [
35] show that LNG has around “40% lower volumetric energy density than diesel, roughly the same as LPG” [
35]. However, when the storage system is taken into account, LNG has roughly a third of the volumetric energy density as diesel. DNV GL [
35] also mention that for liquid hydrogen, ammonia and methanol, their volumetric energy density is even lower, ~40–50% of LNG. Note that biodiesel “is the only fuel which is close to matching the energy density of diesel” [
35].
In the same study DNV GL [
35] compare the fuel cost/technology pathways. The main takeaways are [
35]:
LNG, methanol and LPG are competitive in terms of energy costs, while HVO is significantly more expensive.
Hydrogen and ammonia are also far more expensive.
The large cost range indicates a significant uncertainty in terms of pricing.
Lloyd’s Register and University of Maritime Advisory Services (UMAS) [
36] performed a technoeconomic study of carbon fuels with the potential of becoming a zero-carbon emitter. Lloyd’s Register and UMAS [
36] concluded that in the short term, biofuels look marginally more competitive than fuels derived from renewable electricity or from natural gas with carbon capture and storage. However, biofuels have challenges in terms of sustainability and availability, which, thus, in the mid–long-term, “any biofuel pathway is uncompetitive and prone to restrictions or higher prices” [
36]. Therefore, it does not necessarily make biofuels more competitive than hydrogen or ammonia that are derived from natural gas or renewable energy [
36]. Lloyd’s Register and UMAS also mention that, although some fuel pathways may be more resilient than others, fuel price is the dominant factor that determines the total cost of operation (TCO), therefore a fuel derived from natural gas or from a renewable energy source “may offer longer term benefits” as opposed to biofuel considering the future growth in energy global demand and the aforementioned sustainability and availability issues of biofuels [
36].
From a technology readiness level (TRL) viewpoint, methanol, LNG and diesel are more mature compared to hydrogen and ammonia as “rules and regulations currently exist and there are vessels already using these fuels” [
36]. There is also little difference for onboard technologies, “for example, between using bio-methanol, e-methanol or NG-methanol; the same applies to LNG (bio-LNG, fossil-LNG and e-LNG)” [
36].
3.1. Hydrogen
Hydrogen has the advantage that no CO
2, particulate matter (PM) or SO
x are released while burning, however, its availability and low volumetric energy density require significant additional infrastructure and system design [
37]. Although, NO
x emissions can be generated at significant levels, if the flame temperatures are greater than 1700 K and air is the oxidant. Nearly all hydrogen is produced from fossil fuels, in fact 6% of CH
4 and 2% of coal are used to generate hydrogen [
38]. Hydrogen can be used as a fuel in fuel cells or as a blend in existing conventional diesel fuels commonly used in the shipping industry, such as heavy fuel oil (HFO) [
8]. However, burning hydrogen as a “drop-in” fuel in marine diesel engines is possible at low levels of blending without significant risks of engine damage [
39].
The IEA show that the global demand for pure hydrogen for the period 1975–2018 has increased [
40], and this illustrates the fact that demand for hydrogen is growing, hence the current “hype” for hydrogen is here to stay for the long run. The cost of producing hydrogen is the greatest challenge, especially for green hydrogen (hydrogen produced from renewable energy): “A study by the Hydrogen Council determined that, using an import price of
$3 per kilogram of hydrogen to power turbines, the electricity produced would cost about
$140/MWh. By comparison, a 2019 estimate of electricity’s levelized cost suggests the unsubsidized cost of natural gas combined-cycle electricity generation is between
$44/MWh and
$68/MWh” [
41]. Furthermore, hydrogen does not have a standardized design and fueling procedure for ships and its bunkering infrastructure [
42]. Currently, there are two techniques that are used to produce hydrogen: by steam methane reforming and water electrolysis [
43].
Hydrogen production, storage, delivery and utilization are important issues for a sustainable Hydrogen Economy [
44]. In addition, to achieve wider acceptance for hydrogen as a viable fuel, safety and reliability issues must be addressed, bearing in mind that public opinion may have a direct impact on policies [
44]. Therefore, hydrogen safety, due to its volatility, is a major issue to be considered [
8]. Hydrogen has high capital cost of storage, and onboard a ship there is a loss of cargo-carrying capacity [
36]. Therefore, significant improvements are needed to first reduce capital cost of storage and secondly to resolve the onboard storage issues which are necessary to improve hydrogen’s competitiveness compared to other alternative fuels [
36].
3.1.1. Hydrogen Storage Technologies
Hydrogen storage applications can be divided into two main categories: (1) stationary (for on-site storage and stationary power generation), and (2) mobile (transportation of hydrogen to final destination) [
44]. In this review paper, the discussion will focus on hydrogen storage technologies applicable to the shipping industry, because maritime applications have different challenges compared to stationary or automotive applications [
45]. Hydrogen under standard conditions has a low volumetric energy density, hence the need for efficient storage, i.e., hydrogen storage requires (1) high pressures, and/or (2) low temperatures and/or (3) material that can store hydrogen in physical carriers (via adsorption) or chemical carriers [
44,
45].
Van Hoecke et al. [
45] present the potential applicability of hydrogen storage methods for the shipping industry, namely, these are:
Compressed hydrogen is currently the most accepted and used storage method in the shipping industry. Such system is being used onboard ships, such as the FCS Alsterwasser inland passenger ship, which has been sailing since 2008 [
46,
47]. Liquid hydrogen has several complexities and currently there is no large-scale demand for the shipping industry (apart from a pilot project by Kawasaki Heavy Industries discussed later). Ammonia, nitrogen-based storage and CO
2-based storage are used in other industries and hence have lots of potential for knowledge transfer. Metal hybrides is quite promising, but still, further research and development is required to reach commercialization, but they have been used in submarine applications [
47,
48].
3.1.2. Hydrogen Safety
Hydrogen is a small molecule and can thus easily leak, especially when stored in a compressd state [
45]. Due to its low ignition energy, it can lead to explosions which can be detrimental on a ship. The effects of low ignition energy are discussed later when hydrogen is used in internal combustion engines leading to knock events. In general, storage tanks for compressed hydrogen can withstand high pressures and have generally high safety factors (for the interested reader, Moradi and Groth [
44] and van Hoecke et al. [
45] discuss the different types of compressed storage tanks—Type I–IV). Materials under cryogenic conditions must withstand extremely low temperatures (−252.9 °C) otherwise they become brittle. In addition to the risk of hydrogen explosion, spills under cryogenic conditions onboard are particularly hazardous because they can damage the hull of a ship by cold fracture [
45,
49]. Storage via chemical carriers poses the risk due to the handling of toxic materials (depending on the chemical storage method) such as ammonia and formic acid [
45].
3.1.3. Green Hydrogen
Given the large scale of the shipping industry, which emits nearly 3% of all GHG emissions, green hydrogen may be one of the ways forward [
50]. Green hydrogen is produced from renewable sources such as biomass, solar and wind through the process of electrolysis. It is worthwhile mentioning that the costs to produce green hydrogen are projected to fall significantly in the next decade due to economies of scale, technological improvements as well as renewable deployment. These could make green hydrogen price-competitive compared to grey and blue hydrogen (other colors for hydrogen are defined in the next section) [
51]. Note that costs for producing green hydrogen have dropped by 50% since 2015 and could be further reduced by 30% by 2025 due to the benefits of increased scale and standardized manufacturing, among other factors. Currently, green hydrogen costs €0.1–0.15/kWh, whereas grey hydrogen (produced by reforming methane from natural gas, which releases CO
2 in the atmosphere) costs €0.045/kWh [
38]. However, green hydrogen is “already cost competitive” in niche applications, provided that recent market trends continue, and current policy mechanisms are maintained [
52].
3.1.4. Other “Colors” for Hydrogen (Brown, Grey, Yellow, Blue)
Yellow hydrogen is produced from nuclear power, but unlikely to be used due to “political reasons”. Port authorities will be unwilling to accept foreign ships with a nuclear reactor onboard [
53]. Grey hydrogen is produced from methane reformation, autothermal reformation, partial oxidation, low-temperature plasma reformation and reformer, electrolyzer and purifier, whereas brown hydrogen is produced from coal. These emit GHG emissions. Blue hydrogen is produced from natural gas, but CO
2 emissions are captured (CCS). The ideal color is green, whereby hydrogen is produced from renewable energy sources (wind, solar, hydropower, etc.).
3.1.5. Liquid Hydrogen (LH2)
According to Shell [
54], liquid hydrogen (LH
2) has many advantages compared to other potential zero-emission fuels for shipping, therefore giving a higher likelihood of success. On the other hand, storage of LH
2 is complicated and costly, with a large number of safety issues due to the requirement of cryogenic storage (i.e., at high pressures and low temperatures: −252.9 °C). An additional major issue with LH
2 is the fueling process that occurs at low temperatures. Specific insulation materials are required for the tank materials in order to avoid evaporation of LH
2 and hence avoid large heat fluxes into the tank [
45,
55]. Studies on novel insulation systems under cryogenic conditions, such as the one proposed Zheng et al. [
55], can bring benefits to the shipping industry if LH
2 is the way forward.
Apart from the first pilot project by Kawasaki Heavy Industries transporting LH
2 in a tanker ship [
45,
56], no large-scale shipping exists for liquid hydrogen because of the storage complexities and the unavailability of a global market. In the pilot study by Kawasaki Heavy Industries [
56], it was found that it is technically and economically possible to transport and store LH
2 from Australia to Japan.
3.2. Ammonia
Ammonia is produced commercially via the Haber-Bosch process, which combines hydrogen and nitrogen with the help of high temperatures and a catalyst [
8]. Green ammonia can be produced by employing renewable energy sources such as solar, wind or hydropower, “which gives ammonia a comparative advantage compared to the production of HFO” [
8]. However, producing green ammonia is not yet cost-competitive compared to conventional ammonia, where “90% of its production relies on fossil fuels such as natural gas” [
8]. There are several global initiatives to produce green ammonia, such as the ammonia producer Yara (one of the world’s leading ammonia producers) is planning to build a demonstration plant for the next-generation green ammonia synthesis plant that will utilize solar energy by 2025 [
57,
58].
Ammonia, compared to hydrogen, allows more hydrogen storage in liquid form without the need to use cryogenic storage (−33.4 °C for ammonia compared to −252.9 °C for hydrogen), thus making NH
3 a suitable hydrogen carrier [
8]. The capital cost required to store hydrogen is far more expensive compared to ammonia, despite the fact that the energy density of both fuels is similar [
8]. Similar to hydrogen, ammonia can be used as drop-in fuel in diesel engines, gas turbines and as a primary fuel in fuel cells, thus making ammonia a very attractive and competitive option [
8]. Regarding the latest developments and ongoing projects for ammonia in Internal Combustion Engines (ICE) and fuel cell applications, the reader is directed to
Section 5.
3.3. Methanol
Methanol may have a potential as it offers simpler handling and lower investment costs [
59]. Methanol is an attractive fuel because it is low in carbon and sulfur-free. The methanol international market is going through a phase of huge expansion [
60]. Methanol can be produced from methane and it could serve as a possible future marine fuel [
8]. Methanol can offer ~25% CO
2 emissions reduction potential compared to HFO [
8]. In addition, methanol can reduce SO
x, NO
x and PM by 99%, 60% and 95% respectively, “however, methanol can also be produced from renewable energy resources, such as CO
2 capture, industrial waste, municipal waste or biomass”, in this way, the GHG effect can be significantly reduced [
8].
Methanol has a relatively low flash point, is toxic and its vapor is denser than air [
61]. The risk and safety analysis in the SPIRETH project have contributed to the development of ship classification society rules for methanol as a ship fuel. The work has also contributed to IMO’s draft IGF code and class rules [
62]. Methanol is plentiful, available globally, readily miscible in water, biodegradable and it can be 100% renewable. The life-cycle environmental footprint of bio-methanol is “greener” compared to LNG [
63].
The Effship project evaluated different technical solutions and marine fuels that will be able to satisfy the SO
x and NO
x reductions regulations “in the short term (2015–2016), GHG reduction targets in the medium term (2030) and long term”. It was concluded that methanol was the “best alternative fuel, taking into account prompt availability, use of existing infrastructure, price and simplicity of engine design and ship technology” [
64,
65].
The first ever methanol-powered ship is Stena Germanica, a project supported by the EU Motorways of the Seas program. Stena Germanica is a large passenger and car ferry. The project “converted a RoPax vessel into a methanol-powered vessel and provided bunkering as well as other necessary facilities in ports” [
8].
3.4. Liquefied Natural Gas (LNG)
Liquefied Natural Gas (LNG), compared to its gaseous state, takes up 600 times less space for storage and transportation [
66], hence natural gas is liquefied by cooling it to −162 °C. Currently, LNG is the cleanest available fuel for shipping which can be produced in meaningful volumes [
54] and can comply with the SO
x and NO
x requirements while reducing CO
2 emissions (20–30%) [
1]. In particular, LNG significantly reduces pollution from NO
x and particulate matter (PM) compared to conventional marine fuels, while cutting emissions of SO
x by more than 90% [
1,
54,
67]. LNG is also a cost-effective fuel.
On the other hand, LNG can offer moderate environmental benefits due to methane slip (leaking of methane gas into the atmosphere), which varies across engine types [
1]. Current LNG engines have a methane slip of 2–5% [
68,
69]. In this way, LNG may have short-term promise with minor policy intervention [
1]. There are several lifecycle studies that analyze GHG emissions per kWh for LNG as shipping fuel. Balcombe et al. [
1] have analyzed the various lifecycle studies [
69,
70,
71,
72] and estimated that the average total GHG emissions, which includes combustion and upstream, is 650 gCO2
e/kWh. Note that upstream denotes resource extraction, processing and liquefaction and transportation, and downstream emissions occur from combustion and methane slip.
There are four main technologies that use LNG as a fuel with different characteristics, efficiencies and emissions profiles [
73]. The engines/turbines in use in the shipping sector today are [
73]:
Lean-burn spark ignition,
Low-pressure dual fuel (4- and 2-stroke),
High-pressure dual fuel and
Gas turbine.
The AIDAnova is the world’s first cruise ship powered by LNG. Shell LNG worked with the Carnival Corporation PLC to achieve reduced emissions, quieter engines and no visible emissions [
54], and it is an example of a successful project powered by LNG. The AIDAnova has achieved a 95–100% reduction in SO
x, up to 85% reduction in PM and NO
x and up to 22% tank to wake greenhouse gas emissions reduction compared to marine diesel oil [
54]. Tom Strang, Senior Vice President Maritime Affairs at Carnival said: “LNG is currently the most cost-effective solution that offers emission reductions today. What we can achieve today with fossil LNG plus an increasing introduction in the percentage of renewable liquefied methane—either from bio or synthetic sources—could see ships being effectively carbon-neutral in the future” [
54].
Lindstadt et al. [
74] quite notably mention “overall, in the public debate, LNG seems to be treated as a bipolar issue”. They further mention that some studies argue that LNG is beneficial for both GHG and local air quality, however, other studies argue against LNG due to its high methane slip through the whole well-to-tank chain lifecycle analysis [
74]. LNG contains carbon in its molecular formula, and hence it cannot decarbonize the shipping industry but can help the shipping industry to achieve decarbonization as a transition fuel [
74].
Table 2 summarizes the advantages and disadvantages of LNG as a transition fuel for the shipping industry.
4. Renewable Energy Sources
Renewable energy can either be used to generate green fuels or used directly for propulsion. “The development of renewable energy solutions for shipping has been hampered by over-supply of fossil fuel-powered shipping in recent years and the related depressed investment market” ([
39], p. 4). The main barriers to increased penetration of renewable energy solutions for shipping remain: (1) the lack of commercial viability of such systems, and (2) limited motivation for deployment of clean energy solutions due to split incentives between ship owners and operators ([
39], p. 4). Mofor, Nuttal & Newell ([
39], pp. 10–11) tabulate a summary of various renewable energy applications, pathways, as well as their potential application for the shipping industry.
There are numerous potential renewable energy sources for shipping applications, namely: wind (soft sails, fixed wings, rotors, kites and conventional wind turbines), solar photovoltaics, biofuels, wave energy, batteries and supercapacitors, where the latter two are charged with renewable energy [
39,
77,
78,
79,
80,
81,
82,
83]. Renewable energy may be introduced in shipping via the following two routes: (1) as retrofits for the existing fleet, or (2) inclusion into new designs. Regarding new design concepts, and independent of ship size, most of the renewable energy will supply power for auxiliary and ancillary applications. However, very few designs will target 100% renewable energy or zero-emissions for primary propulsion (examples include B9, Ecoliner, Greenheart, Orcelle) [
39]. Solar and wave energy will provide the energy source to produce green fuels such as hydrogen and ammonia.
4.1. Wind
Wind propulsion can be categorized into the following types [
26,
39]:
Soft-sail,
Fixed-sail,
Flettner rotor,
Kite-sail, and
Turbine technologies.
The major drawback of wind is its intermittency and difficulty in maximizing the full power potential while sailing into the wind [
39]. Wind can be used for primary and auxiliary propulsion. If wind is to be used for primary and auxiliary propulsion in existing ships, the structural stability has to be taken into account. It may even be the case that for existing ships, retrofitting some technologies is impossible, such as Flettner rotors.
Of course, wind can also be used (onshore or offshore) to electrolyze water in order to generate green hydrogen. There has been renewed interest in the production of green hydrogen by electrolysis of seawater by using electricity generated by an offshore wind turbine, an example is the joint press release of Siemens Gamesa and Siemens Energy announcing a “total investment of ~€120 million over five years in developments leading to a fully integrated offshore wind-to-hydrogen solution” [
84].
4.1.1. Soft-Sails
Soft-sails attached on masts are a proven and mature technology, which can be deployed for either primary or auxiliary propulsion [
39]. Soft-sails can be easily retrofitted into existing ships or included as part of a new ship design [
39]. Examples are the Ecoliner, and Seagate delta wing sail, which is a patented folding delta wing sail [
39]).
4.1.2. Fixed-Sails
Fixed sails are rigid sails attached on a rotating mast. Different fixed-sail designs have been used in ships such as the UT wind challenger and Ecofoil.
4.1.3. Flettner Rotors
Flettner rotors take advantage of the Magnus effect (propulsion when wind flows over a rotating cylinder). Flettner rotors were installed on E-Ship and Viking Grace. Deck space for different ship types is the best candidate for retrofitting Flettner rotors to bulkers and tankers up to VLCC class, which is being actively considered [
39].
4.1.4. Kite Sails
Kite sails are attached on the bow of a ship and operate at altitudes so that they can utilize high wind speeds. The kite sail can help reduce annual fuel costs by 10–35% [
85]. The MS Beluga Skysail, which was the first ever commercial container ship partially powered by a kite sail [
39,
86].
4.1.5. Wind Turbines
Apart from producing electricity (either onshore or offshore), wind turbines have not received much attention for ship propulsion. This is due to the fact that wind turbines suffer from inherent issues of vibration, stability and low energy efficiency conversion [
39]. However, if wind turbines find their way for ship propulsion or for auxiliary power, their main advantage would be the harnessing of wind power even when the ship is sailing directly into the wind [
39].
4.2. Solar
Photovoltaic (PV) cells produce electricity directly from the sunlight via the photoelectric effect. This is a fast-evolving technology with fast-paced progress, however, PV cells face two fundamental issues: (1) the lack of sufficient area on a ship to deploy the PV cells, and (2) the energy storage required, in the form of batteries [
39]. Additional constraints of PV cells are the potential erosion of solar panels due to the presence of salt in the seawater [
1] and intermittency issues. It is worthwhile mentioning that potential emissions reduction from solar energy is limited [
8]. Smith et al. [
11] assume a “reduction of around 0.1–3% of auxiliary engine fuel consumption”. Bouman et al. [
17] report the potential in CO
2 reduction in the range of 0.2–12%. Even though there has been tremendous progress on batteries, ship propulsion via solar PV, on the other hand, requires further research and development. PV applications are likely to be confined to relatively small ships [
81]. A solar ship that integrates PV cells into its power system is one of the most promising and fastest developing green ship systems [
87].
Assuming an average improvement of the order of 1.5% on auxiliary fuel consumption and 6% on CO
2 potential reduction (based on the studies of Smith et al. [
11] and Bouman et al. [
17]) is quite significant and promising. Given the latest progress of PV systems in general (for example the continuous improvement in efficiency) and their rapid growth shows that they are the most promising technology utilizing renewable energy. However, PV systems on a ship require further R&D in terms of materials due to the corrosive nature of salty water, hence the potential of PV systems on ships is lower compared to on land applications.
Solar-Hybrid Systems
PV cells can be combined with fixed sails so that solar and wind energy are utilized at the same time [
39]. Such an example is the OCIUS Technology’s SolarSailor design [
88]. Another example is Japan-based Eco Marine Power which is developing a large solar-sail Aquarius Marine Renewable Energy (MRE) for tankers and bulkers [
89].
Batteries can also be used in conjunction with PV cells, an example is the Greenheart design that uses solar energy to “charge lead-acid batteries for auxiliary propulsion for its primary sail rig” [
39]. However, solar (along with wave energy and wind turbines) can provide energy to produce green hydrogen by electrolysis of water. The hydrogen produced can then be later used in fuel cells [
39] or ammonia production for later use in fuel cells.
4.3. Biofuels
Biofuels are produced from organic waste such as plant and animal waste [
8]. At the moment, the main sources of biofuels are from plant-based sugars and oils, such as from palm, soybean and rapeseed [
33]. Hsieh & Felby provide in a form of a flowchart an overview of the different feedstock conversion routes for marine biofuels, showing conventional and “advanced” biofuels ([
33], p. 42).
Table 3 refers to first, second and third generation biofuels, the latter two known as “advanced biofuels”, which depends on the source of carbon used [
39]. The European Biofuels Technology Platform defines first, second and third generation biofuels as follows [
39]:
First Generation: “The source of carbon for the biofuel is sugar, lipid or starch directly extracted from a plant. The crop is actually or potentially considered to be in competition with food.”
Second Generation: “The biofuel carbon is derived from cellulose, hemicellulose, lignin or pectin. For example, this may include agricultural, forestry wastes or residues, or purpose-grown non-food feedstocks (e.g., Short Rotation Coppice, Energy Grasses).”
Third Generation: “The biofuel carbon is derived from aquatic autotrophic organisms (e.g., algae). Light, carbon dioxide and nutrients are used to produce the feedstock, “extending” the carbon resource available for biofuel production.”
Advanced biofuels are excellent candidates for the shipping sector and the most viable option as a renewable energy source [
39]. Advanced biofuels “have very low sulfur levels and low CO
2 emissions, as such they are a technically viable solution to low-sulfur fuels meeting either the very low sulphus fuel oil (VLSFO) or ultra low sulphur fuel oil (ULSFO) requirements” [
33]. Biofuels are currently the most relevant alternative for replacement or blending with gasoline (blended with bioethanol) or diesel (blended with biodiesel) in the transport sector [
39]. However, usage and experience in the shipping industry is limited [
39].
The challenge with biofuels in the shipping sector is that there is little experience and knowledge “on handling and applying biofuels as part of their fuel supply. Another challenge is that the volumes of biofuels required to supply the shipping sector are large” [
33]. Hence, sustainable biofuel production is limited taking into account food price, natural resources (such as availability of land) and social conditions [
8]. There are also concerns regarding the storage and oxidation stability of biofuels and further research is required [
33].
Hsieh and Felby ([
33], p. 74) present a SWOT analysis regarding marine biofuels. Compared to fossil fuels, biofuels have a higher cost, and this is expected to remain at least for the short- and medium-term. However, a combination of policies, regulations (reduced sulfur levels in marine fuels and reduction of GHG emissions), incentives and technology and infrastructure improvements may help create a healthy market for biofuels in the shipping industry [
33].
4.3.1. Liquid Biofuels
Liquid biofuels can be combusted in a spark or compression ignition engine and are applicable to any vessel type [
39]. A spark ignition engine can combust up to 10% bioethanol (E10) and a compression ignition engine can combust up to 20% biodiesel (B20) without significant adaptations. For blends of fuels between E10 and E85, the engine must be retrofitted with a fuel sensor in order to determine the proportion of ethanol in the blend and hence optimize the engine’s performance. For proportions greater than E85, the engine performance is rather poor ([
90], p. 507). On the other hand, compression ignition engines can combust blends in the range of B20 and B100 (i.e., 100% biodiesel) without too much retrofitting or degradation in engine performance. However, special care must be taken regarding the fuel due to its viscosity ([
90], p. 507).
There have been tests on third generation biofuels in shipping. An example is the Maersk Kalmar, where Maersk partnered with the US navy to test algal biofuels [
91]. The test included testing fuel blends (from 7% to 100%) in the auxiliary engine through a ~6500 nautical mile journey from Northern Europe to India. The test used Solazyme’s 100% algal derived advanced biofuel, Soladiesel
RD, and proved that it is possible to substitute an advanced renewable fuel for diesel [
92].
4.3.2. Biogas
Biogas can be either methane or hydrogen. Biogas is produced from anaerobic digestion within an enclosed environment which consists of microbes that break-down organic material ([
90], p. 470). Biogas can be processed in order to remove impurities, hydrogen sulfide and moisture [
39]. Methane produced from anaerobic digestion of organic waste can be liquefied to from Liquified Bio-Methane (LBM) [
39]. The shipping industry favors LNG as a transitional fuel for decarbonization [
39], however, LBM is seen as an option for deep decarbonization in the long-term because it is produced from renewable sources. Hydrogen can be either used in a fuel cell to generate electricity for propulsion or blended with the fuel for combustion in a gas turbine or internal combustion engine to achieve reduction in emissions.
The CE Delft [
93] study investigated the availability and costs of LBM and Liquified Synthetic Methane (LSM), also known as e-methane. Note that in the CE Delft [
93] study, it is assumed that LSM is produced from synthesis of CO
2 and H
2, where hydrogen is produced from renewable energy sources. The CO
2 is from “recycled CO
2”, where it is either captured from industrial processes (e.g., flue gas) or the air [
93]. The study determined that the future maximum conceivable sustainable supply of LBM and LSM exceeds the energy demand from the shipping sector, provided that biomass will be used to produce methane and sufficient investments are made in renewable electricity production [
93]. In addition, the production costs of LSM and LBM may not be significantly higher and could even be comparable to the production costs of other low- and zero-carbon fuels [
93]. If the costs of bunkering infrastructure and ships are comparable as well, then LSM and LBM fuels may be viable candidates to achieve decarbonization in the shipping sector [
93].
Table 3.
Internal combustion engine types, manufacturer and engine model, as presented in Reference [
94].
Table 3.
Internal combustion engine types, manufacturer and engine model, as presented in Reference [
94].
Engine Type | Manufacturer | Engine Model |
---|
Four-stroke | Anglo Belgian Corporation (Gent, Belgium) | VDZC Series; DL36 Series |
| Akasaka diesels (Tokyo, Japan) | AX 28 Series |
| Caterpillar Inc (Deerfield, MA, USA) | 3500 Series; 3600 Series |
| MaK Motoren GmbH and Co. (Peoria, IL, USA) | M601; M20C, M25C, M32C and M43C Series |
| Daihatsu Motor Co. Ltd. (Ikeda, Japan) | DK-28 Series; DC-17Ae Series |
| Deutz AG (Cologne, Germany) | TCD 2015 V Series |
| Doosan Engine Co. Ltd. (Changwon, Korea) | V222TI; L136TI Series |
| Hanshin Diesel (Kobe City, Japan) | LA 30; LA 34 Series |
| Hyundai Heavy Industries Co. Ltd. (Ulsan, Korea) | HiMSEN Series H17/28; H21/32; H32/40 |
| MAN Diesel and Turbo SE (Augsburg, Germany) | L23/30; L16/24; L21/31; L27/38; L32/40; L40/54; L48/60 CR; L58/64; L32/44 CR; D2876 |
| Mitsui and Co. Ltd. (Tokyo, Japan) | ADD30V |
| MTU Friedrichschafen GmbH (Friedrichshafen, Germany) | MTU V 4000 |
| Paxman (Colchester, UK) | VP185 |
| Rolls-Royce Group Plc (London, UK) | B32:40R; C25:33L |
| Ruston and Hornsby (Lincoln, UK) | RK 270; RK 280 |
| Pielstick (Augsburg, Germany) | S.E.M.T. Pielstick series PC2.6B |
| Sulzer Brothers Ltd. (Winterthur, Switzerland) | Z40; ZA40S |
| Wärtsilä NSD (Helsinki, Finland) | L20; L/V32C; L/V46C; 64C |
| Yanmar Co. Ltd. (Osaka, Japan) | 6N21; EY18L; EY26L; AYM |
Gas four-stroke | MAN Diesel and Turbo SE (Augsburg, Germany) | V35/44G |
| Mitsubishi Heavy Industries (Tokyo, Japan) | KU30G |
| Rolls-Royce Bergen (Hordvik, Norway) | C26:33LPG |
Gas-diesel dual-fuel four-stroke | MAN Diesel and Turbo SE (Augsburg, Germany) | 35/44DF |
| Caterpillar Inc (Deerfield, MA, USA) | M46DF |
| Wärtsilä NSD (Helsinki, Finland) | L50DF |
Two-stroke ship low-speed crosshead | MAN Diesel and Turbo SE (Augsburg, Germany) | MC, MC-C, ME, ME-C and ME-B series |
| Mitsubishi Heavy Industries (Tokyo, Japan) | UEC LSH-Eso; LSE-Eco |
| Wärtsilä-Sulzer (Helsinki, Finland) | RTA; RT-Flex Series |
| WinGD (Winterthur, Switzerland) | W-X Series |
Gas diesel two-stroke ship low-speed (low pressure) | WinGD (Winterthur, Switzerland) | X-DF Series |
Gas diesel two-stroke ship low-speed (high pressure) | MAN Diesel and Turbo SE (Augsburg, Germany) | ME-GI Series |
7. Future Trends, Challenges and Conclusions
This review paper examined the possible pathways and possible technologies available that will help the shipping sector achieve the IMO’s deep decarbonization targets by 2100. There has been increased interest from the shipping sector’s important stakeholders regarding deep decarbonization. However, deep decarbonization will require financial incentives and policies at an international and regional level given the maritime sector’s 3% contribution to GHG emissions [
1,
6].
Shell and Deloitte conducted a market survey in order to understand the current market trends [
4] and it has been realized that the majority of the stakeholders considered decarbonization an important or top priority for their organizations [
4]. This, therefore, shows that the market and the industry are considering decarbonization as part of their business strategy. For example, Maersk (the world’s largest shipping container company) has announced its intentions to be net-zero carbon by 2050, with carbon-neutral vessels commercially viable by 2030 [
199]. The IMO has introduced measures for new and existing ships (SEEMP, EEDI and the fairly recent EEXI), which combine operational and technical measures to help ship operators achieve reductions in emissions.
Section 2 of this review paper discussed alternative fuels that can be used in shipping. The relative advantages and disadvantages of alternative fuels, in terms of cost, technical difficulties and maturity, were presented. Shell’s view on future pathways involves a “poly-fuel scenario”, in other words, the use of different fuels [
54]. Japan’s report on “Roadmap to Zero Emission from International Shipping” mentions that pilot concepts are based on two possible emission reduction pathways: (1) LNG, provided that LNG transitions to carbon-recycled methane, and (2) adoption of hydrogen/ammonia as fuel [
20]. Although hydrogen/ammonia fuels are a very promising solution to achieve deep decarbonization, there are still issues to be resolved in order for them to be a commercially viable solution. Issues of storage, transportation, safety, toxicity and cost are the prime inhibitors for these alternative fuels. LNG is a very promising solution to achieve short-term decarbonization, with existing ships already deploying LNG (mostly as a drop-in-fuel, blended with existing fuels in marine sector).
Biofuels look marginally more competitive than fuels derived from renewable electricity or from natural gas with carbon capture and storage. However, biofuels have challenges in terms of sustainability and availability, thus, in the mid–long-term, they may be uncompetitive due to sustainability restrictions and price volatility [
36]. Because fuel price is the dominant factor that determines the total cost of operation, a fuel derived from natural gas or from a renewable energy source “may offer longer term benefits” compared to biofuels if the future growth in energy global demand and the aforementioned sustainability and availability issues of biofuels are taken into account [
36].
Section 3 discussed the various renewable energy sources that are available or currently under development for shipping. The technologies discussed were wind (via the use of kite sails, Fletnner rotors, fixed sails), solar (PV) and biomass. There are existing ships as well as promising technologies that will help the shipping sector reduce the CO
2 emissions, however, the technologies are still not mature enough to achieve deep decarbonization on their own.
Section 4 discussed the maturity of technologies currently available that can help the shipping industry achieve deep decarbonization. Marine diesel ICEs are the dominant technology to provide a ship power for propulsion and for its ancillary and auxiliary needs. Given the ICEs dominance in the shipping sector, they are not expected to be replaced any time soon. Hence, it is evident from the research and market trends that the use of combusting blended fuels (e.g., blending hydrogen/ammonia/biofuel with marine diesel fuel) in ICE marine diesel engines is the way forward, at least in the short term. There are existing ships that use hydrogen as a drop-in-fuel, such as Hydroville by CMB. The technology is a hybrid engine that allows it to run on diesel and hydrogen [
107]. In addition, the CMB technology is building several hydrogen-powered vessels.
Fuel cells with hydrogen or ammonia as fuels is a promising technology, especially green hydrogen and green ammonia, because not only do they reduce/eliminate GHG emissions, but they also eliminate pollutants (NO
x and SO
x). However, fuel cells in shipping applications have lots of issues to resolve, but there are exciting projects underway to test and validate this technology. Projects such as ShipFC, a project funded by Fuel Cells and Hydrogen Joint Undertaking (FCH JU) under the EU’s Horizon 2020 research and innovation program, will install a maritime fuel cell that will be powered by green ammonia [
147]. The use of hydrogen in fuel cells has attracted lots of commercial interest. Recently, a Danish–Norwegian project aimed to build and test a ferry, the Europa Seaways operated by DFDS ferries, powered by a hydrogen fuel cell. The project recently gained EU funding and aims to have Europa Seaways operational by 2027. Note that this project attracted several key players in the shipping and energy sectors, who joined forces to build a ferry that will be able to transport 1800 passengers [
145].
Section 5 discussed the various CO
2 abatement options, which in essence discussed strategies and techniques that can help reduce fuel consumption, such as vessel speed, vessel and propeller design, waste heat recovery and even cleaning and coating of the hull’s surface. These techniques reduce fuel consumption by either reducing the ship’s resistance, improving propulsion or utilizing the wasted heat energy from the ship’s engine for auxiliary and ancillary power needs. With WHR technologies, the most promising concepts are to reduce fuel consumption and thus CO
2 emissions. Steam and organic Rankine Cycles seem to be the most promising technologies, as they are mature. Other technologies, such as the Kalina Cycle, require further research, especially in terms of techno-economic feasibility if this technology is to compete with the more mature steam and organic Rankine Cycles. Note that the techniques discussed in
Section 5 are short-term measures to achieve reduction of CO
2 emissions, but will not achieve complete decarbonization on their own because the main source of energy is fossil-based fuels.
The IMO’s targets will be achieved via a radical technology shift together with the aid of social pressure, financial incentives and regulatory and legislative reforms at the local, regional and international levels.