Abstract: The decarbonisation of the shipping sector is undoubtedly one of the biggest challenge
the maritime industry is currently facing. The International Maritime Organization (IMO) aims to
attain a 50% reduction in GHG emissions by 2050 compared to 2008 levels, with the end goal of
ultimately phasing them out by the end of the century. To meet such targets, different
Introduction:
According to the American Meteorological Society's State of the Climate 2020 report,
greenhouse gas emissions continued to rise in 2020. [1]. Similarly, CO2 levels were 47 percent
higher than they were before the industrial revolution. [2]. Such contaminants have adverse effects
on human health as well as wildlife. Furthermore, it increases the risk of flooding, severe
temperatures, and rising sea levels, devastating crop harvests and livelihoods in the process. [3].
Therefore, it is paramount that significant thought be given to reducing greenhouse gases, carbon
emissions, and other hazardous pollutants such as nitrous (NOx) and sulphur (SOx) oxides in order
to develop future sustainable transportation.
Climatic impact of shipping:
Along with other modes of transportation, waterborne transportation is one of the major
contributors towards the adverse climatic condition. It is estimated that shipping activities accounts
for 3 to 5% of global CO2 emissions and over 5% of global SOx emissions [4]. According to the
third International Maritime Organization, shipping emitted about 1000 Mt of CO2 per year during
2007-2012, which equals to approximately 3.1% of annual global CO2 emissions [7]. While
automotive industries have become more advanced in their technology, with the aim to curtail the
increased climatic severity and hence decarbonize successfully, mainly by commercializing
electric vehicles, the development of a sustainable propulsion system for ships has lagged behind,
primarily due to the lack of stringent regulations imposed on the waterborne transportation to
reduce adverse climatic impact [5, 6]. Hence, if other sectors were to decarbonize successfully
except shipping, its emission is estimated to increase by up to 120% by 2050 [7].
Clean shipping future:
Vision:
The decarbonization of shipping sector is perhaps the biggest challenge the maritime industry has
been facing over the past few years. However, the International Maritime Organization (IMO)
envision a future where the ships are driven by fuels which do not produce the levels of greenhouse
gases as seen by the currently used diesel engines. In 2018, the IMO embraced an initial strategy
to reduce the GHG emissions from the shipping by setting up the key ambition of achieving a 50%
reduction in GHG emissions by 2050 compared to 2008 levels, with the end goal of ultimately
phasing them out by the end of the century [8]. This strategy along with other previous regulations,
such as, the Energy Efficiency Design Index (EEDI), Energy Efficiency Existing Ship Index
(EEXI), Existing Vessel Design Index (EVDI), Carbon Intensity Indicator (CII) and the enhanced
Ship Energy Efficiency Management Plan (SEEMP), are essential for the development of
sustainable waterborne transport.
Meanwhile, the UK Government introduced a Clean Maritime plan in 2019, with the end goal of
attaining zero-emission shipping by 2050, while expecting that all vessels operating in UK waters
will be capable of zero-emissions by 2025 [9]. Similarly, Mission Innovation, co-lead by Denmark,
USA and Norway, issued a joint zero-emission shipping mission statement in 2021, with the
ambitious goal of introducing commercially viable vessels capable of zero-emission into the
ocean-going global fleet by 2030, increasing production of zero-emission fuels, and establish
global port infrastructure to support vessels operating on zero-emission fuels [10]. Hence, it is
evident that many countries want to address the primary challenge of zero-emission shipping,
currently being faced by the maritime sector.
Zero emission shipping technology options:
It is anticipated that future waterborne transportation will rely solely on sustainable means with no
reliance on fossil fuels; however, such lofty goals strongly depend on the commercial viability and
technology readiness level of the zero-emission technology options available. Zero-emission
technologies, such as, ammonia, hydrogen, and batteries emit zero greenhouse gases, given that
electricity produced to supply the propulsion system originates from a green renewable energy
source, including but not limited to solar, wind, or tidal. In contrast, the zero-emission technology
which utilizes alternative fuels such as methanol do not proliferate the total anthropogenic carbon
balance in the atmosphere but uses the CO2 from the air for production [11]. However, each
candidate considered for the purpose of delivering zero-emission shipping has its own advantages
and disadvantages.
Batteries are a feasible solution for decarbonization, given that renewable energy sources are used
to produce electricity. It is alone sufficient to drive a small light craft travelling short distances;
however, for large cargo vessels travelling long distances, a standalone battery system is not
feasible [13].
Methanol is an attractive alternative fuel and has been around for several years since the conversion
of RoPax ferry Stena Germanica [14]. Along with low emission of NOx and particulate matter
during combustion, methanol does not contain sulphur hence resulting in zero SOx emissions.
Moreover, having a readily available infrastructure and mature technology for safe storage
deployment makes methanol an excellent candidate to achieve the desired sustainable shipping
future [12]. However, due to it being toxic towards humans and having a higher explosive range,
in addition to its non-carbon free nature, it is hard to anticipate methanol as the core to
revolutionize the shipping industry towards an emission-free future [19].
Green ammonia is another promising candidate of zero-emission technology particularly because
it has no carbon content, hence no carbon emissions are released during the combustion process.
However, the combustion process can lead to significant NOx emissions. Furthermore, ammonia
is highly toxic [15]. Hence, the large-scale use of ammonia as an alternative fuel for shipping can
result in adverse effects on the human health as well as wildlife, thereby making it not the most
attractive technology to be used in the long-run to support the transition to zero-emission shipping.
Green hydrogen presents an attractive solution to fulfill the ambitions of decarbonisation of the
shipping industry. Besides, being one of the most readily available elements on earth, green
hydrogen is essentially completely carbon-free if produced from renewable energy sources, with
water being the only by-product [16]. However, to determine whether hydrogen as an alternative
fuel is the most attractive and viable contributor within a future clean shipping vision, certain
technical, ecological, and economic aspects are needed to be taken into consideration [19].
Technical Consideration:
Over the past few years, hydrogen-based ship propulsion systems have gained a lot of popularity
as the strongest potential candidate to achieve the clean shipping future. According to analysis,
hydrogen fuel cell is capable of achieving an efficiency of 65% in contrast to the approximate 40%
efficiency of internal combustion engine with no heat recovery system [17]. However, using
hydrogen as a fuel presents its own challenges. Firstly, at atmospheric pressure, hydrogen has a
low energy density; hence, in order to increase the energy density so as to reduce the required fuel
storage space, hydrogen must be stored either as a highly compressed gas, as a liquid or through
metal hydrides, each having its own set standards to be met, hence increasing the difficulty.
Moreover, selection of storage material must be given careful consideration as to avoid leakage of
hydrogen through the walls [19]. Secondly, fuel cell can experience starvation, a condition in
which high voltage drop occurs in the fuel cell as a result of transient loads required by the vessel;
this can result in reduced performance and lifetime of the fuel cell [17]. Though it is possible to
address this predicament by using a hybrid (battery/fuel cell)-powered propulsion system where
the batteries can help in supplying the sudden quick spikes in power demand, this hybridization
still requires further research to completely understand their load delivering capability [18, 20].
Lastly, with the implementation of strict regulations over the use of fossil fuel while promoting
the use of environmental-friendly yet costly alternative fuel, the use of energy-saving devices
(ESDs) have become more important. However, the applicability of existing ESDs used for
traditional fossil fuel driven vessels have not been accessed for hydrogen-powered ships. Hence,
more challenges and technical aspects are needed to be considered for their design and operation.
Ecological Consideration:
Green-hydrogen is the cleanest potential alternative fuel which can help the maritime sector realize
their dream of attaining the zero-emission technology [19]. Since pure hydrogen is entirely carbonfree, it only emits water vapor and heat, producing no harmful emissions what-so-ever. Moreover,
fuel cells offer a quiet operation since they have only few moving parts; hence based on these,
hydrogen-based propulsion system can be perceived as a viable contributor towards a clean
shipping future. However, since hydrogen is colorless and odourless, any leak is hard to detect.
Furthermore, it is highly flammable and can cause explosions if not handled properly.
Economical Consideration:
The potential use of hydrogen-based propulsion systems for the transition to cleaning shipping is
also impacted by several economic factors. Firstly, the cost for the production of green hydrogen
- at least in the short-term – is higher compared to traditional fossil fuels since to utilize hydrogen
as fuel, processes like electrolysis are required to split hydrogen from water [16] which have
significant installation and operational cost. Secondly, since the volumetric energy density for
hydrogen, in all forms, is relatively less than those for the conventional fuels, the storage space
required for the same amount of fuel when compared to the conventional fuels increases by several
folds. Hence, the manufacturing costs for the hydrogen storage tank increases significantly. Lastly,
with the goal of implementing hydrogen-based propulsion systems on various different types of
waterborne transportation, a huge investment is required to establish a viable hydrogen supply
chain and bunkering infrastructure. Hence, aforementioned economic barriers are needed to be
overcome to meet the ambitious target of zero-emission shipping.
Conclusion:
This essay did a comprehensive review of the climatic effects of shipping, as well as the envision
of zero-emission shipping and the possible candidates available for the zero-emission technology,
was conducted. This was achieved by first identifying the contribution of shipping in the total
greenhouse gases, CO2, and other toxic emissions all over the globe. Then, the plans and strategies
introduced by IMO and several other countries to achieve clean shipping were reviewed, followed
by various zero-emission technologies, namely batteries, methanol, ammonia, and hydrogen,
which can contribute to the transition to zero-emission. This review highlighted hydrogen,
produced from renewable sources, to be the best possible zero-emission technology. To check the
validity of this claim, the technical, ecological, and economic aspects and challenges of hydrogenbased propulsion systems were taken into consideration. The conclusion drawn from the
aforementioned data is that although there are several technical, economical, and ecological
barriers that are needed to be overcome to adopt hydrogen-based propulsion technology fully, it
also has the potential to be the strongest candidate that can help achieve the ambition of a clean
shipping future.
References:
1. Shein, K.A. State of the climate in 2005. Bull. Am. Meteorol. Soc. 2006, 87, S1–S102
2. Friedlingstein, P.; Jones, M.W.; O’sullivan, M.; Andrew, R.M.; Hauck, J.; Peters, G.P.;
Peters, W.; Pongratz, J.; Sitch, S.; Le Quéré, C.; et al. Global carbon budget 2019. Earth
Syst. Sci. Data 2019, 11, 1783–1838.
3. McMichael, A.J.; Woodruff, R.E.; Hales, S. Climate change and human health: Present
and future risks. Lancet 2006, 367, 859–869.
4. Miola, A.; Ciuffo, B.; Giovine, E.; Marra, M. Regulating Air Emissions from Ships: The
State of the Art on Methodologies, Technologies and Policy Options; Publications Office
of the European Union: Luxembourg, 2010.
5. Eyring, V.; Köhler, H.W.; Lauer, A.; Lemper, B. Emissions from international shipping:
2. Impact of future technologies on scenarios until 2050. J. Geophys. Res. Atmos. 2005,
110.
6. Gerard, D.; Lave, L.B. Implementing technology-forcing policies: The 1970 Clean Air Act
Amendments and the introduction of advanced automotive emissions controls in the United
States. Technol. Forecast. Soc. Chang. 2005, 72, 761–778.
7. Third IMO GHG Study (2014)
8. International Maritime Organization (IMO). IMO Action to Reduce GHG Emissions From
International Shipping; International Maritime Organization: London, UK, 2019.
9. Department for Transport. Clean Maritime Plan; Department of Transport: London, UK,
2019; pp. 1–60.
10. Mission Innovation. Mission Statement for Zero-Emission Shipping Mission; Mission
Innovation, 2021.
11. Clark, A.; Ives, M.; Fay, B.; Lambe, R.; Schiele, J.; Larsson, L.; Krejcie, J.; TillmannMorris, L.; Barbrook-Johnson, P.; Hepburn, C. Zero-Emissions Shipping: Contracts-forDifferenceas Incentives for the Decarbonisation of International Shipping; Smith School
of Enterprise and the Environment, University of Oxford: Oxford, UK, 2021.
12. Sims, R.; Schaeffer, R.; Creutzig, F.; Núnez, C.X.; Agosto, D.M.; Dimitriu, D.; Figueroa
Meza, M.J.; Fulton, L.; Kobayashi, S.; Lah, O. Transport climate change: Mitigation of
climate change. In Contribution of Working Group III to the Fifth Assessment Report of
the Intergovernmental Panel on Climate Change; Cambridge University Press: Cambridge,
UK; New York, NY, USA, 2014.
13. Carlton, J.; Smart, R.; Jenkins, V. The Nuclear Propulsion of Merchant Ships: Aspects of
Risk and Regulation; Lloyd’s Register Technology Days: London, UK, 2010.
14. Srivastava, A.; Ölçer, A.I.; Ballini, F. Decision Framework for Shipowners to Comply with
Air Emission Reduction Measures: A Case Study of Methanol as A Fuel. In Trends and
Challenges in Maritime Energy Management; Springer: Cham, Switzerland, 2018; pp.
415–431.
15. Al-Aboosi, F.Y.; El-Halwagi, M.M.; Moore, M.; Nielsen, R.B. Renewable ammonia as an
alternative fuel for the shipping industry. Curr. Opin. Chem. Eng. 2021, 31, 100670.
16. Atilhan, S.; Park, S.; El-Halwagi, M.M.; Atilhan, M.; Moore, M.; Nielsen, R.B. Green
hydrogen as an alternative fuel for the shipping industry. Curr. Opin. Chem. Eng. 2021, 31,
100668.
17. Tronstad, T.; Atrand, H.; Haugom, G.; Langfeldt, L. Study on the Use of Fuel Cells in
Shipping; EMSA European Maritime Safety Agency: Lisbon, Portugal, 2017.
18. Boekhout, M. Hydrogen Powered Ship Propulsion for High-Speed Craft the
Implementation of Fuel Cell Battery Propulsion Systems. Master’s Thesis, Delft University
of Technology, Delft, The Netherlands, 2020.
19. McKinlay, C.J.; Turnock, S.R.; Hudson, D.A. Route to zero emission shipping: Hydrogen,
ammonia or methanol? Int. J. Hydrog. Energy 2021, 46, 28282–28297.
20. van Biert, L.; Godjevac, M.; Visser, K.; Aravind, P.V. A review of fuel cell systems for
maritime applications. J. Power Sources 2016, 327, 345–364.