An alternative and hybrid propulsion for merchant ships: current state and perspective

ABSTRACT

The maritime industry is becoming increasingly aware of the global environmental impact of ships and is being forced, by international legislation, to gradually reduce its emissions. International Maritime Organization conventions and energy efficiency standards set challenges to shipping sector, to ship owners and ship designers, and to offer propulsion concepts that will effectively reduce or completely eliminate emission rates and increase energy efficiency with acceptable technological costs and adjustment time. New concepts include environmental-friendly fuels in existing propulsion architecture, hybrid propulsion, and all-electric propulsion architecture with the possible application of renewable energy sources, which are reviewed in this paper. Each concept has advantages and disadvantages regarding adjustment time, implementation cost and energy storage system capacity. One of the main disadvantages for complete replacement of conventional propulsion systems is the limited energy storage capacity of existing storage devices, which would decrease the operational ability of the shipping sector as a cheap transportation solution on the global market. Such problems and unsolved environmental issues regarding the production and recycling of energy storage devices are highlighted in this paper and must be further developed. The main motivation for the paper's work is to offer a detailed review of possible solutions for ship propulsion and to offer direction for further research. This is obtained by applying a literature review method to compare the advantages and disadvantages of each proposed solution. The results are discussed in the last section of the paper with the final conclusion that the internal combustion engines will not be completely replaced in the shipping sector in terms of the next few decades but will be able to use environmental-friendly fuels or fuels without a global carbon footprint. This conclusion is the result of analyzing especially large ocean-going vessels with very strong internal combustion engines. Such high power and energy demand are not very easy to be replaced with alternative energy sources or with all-electric ship solution without decreasing all other advantages of merchant shipping sector. Forecasts of ambitious decarbonization scenarios predict wide usage of carbon-neutral fuels in the late 2030s or mid-2040s and Green House Gases reduction (in the range between 50% and 100%) in 2050, which can be obtained by using enviromental-friendly fuels in existing infrastructure. It is still hard to identify which carbon-neutral fuel will be dominant, but e-ammonia, blue ammonia, bio-methanol, and hydrogen are the most promising carbon-neutral fuels in the decarbonization path.

KEYWORDS:

• Environmental impact
• hybrid-propulsion
• pure electric propulsion
• renewable energy
• energy storage systems

NOMENCLATURE

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Acknowledgments

This work has been fully supported by Croatian Science Foundation under the project IP.2020-02-6249.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Notes on contributors

Maro Jelić

Maro Jelić, Professor, Univesity of  Dubrovnik, Maritime department, Croatia. Field of interest: thermodynamics, marine enginereeng, steam turbines, renewable energies, alternative propulsion systems.

Vedran Mrzljak

Vedran Mrzljak, Assistent professor, University of Rijeka, Faculty of engineering, Croatia. Field of interest: steam turbines, internal combustion engines, energy conversion.

Gojmir Radica

Gojmir Radica, Professor, University of Split, Faculty of electrical engineering, mechanical engineering and naval architecture, Croatia. Field of interest: internal combustion engines, renewable energies, hybrid propulsion of ships, alternative fuels for ships.

Nikola Račić

Nikola Račić, Professor, University of Split, Maritime faculty, Croatia. Field of interest: Internal combustion engines, hybrid propulsion of ships, alternative fuels for ships.