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Royal Academy of Engineering study examines future lower-carbon ship powering options

International shipping contributes an estimated 3% of global emissions of CO2. Although the industry has reduced its consumption of fossil fuels by a number of measures such as using increasingly thermally efficient diesel engines in recent decades, the current total fuel oil consumption is in excess of 350 million tonnes per year (about 98.5 billion gallons US).

A new study by an expert working group at the UK’s Royal Academy of Engineering examines a wide range of possibilities for future, lower-emitting and cost-effective ship powering options. The report reviews a range of short-, medium- and long-term technologies and examines the advantages and limitations of systems from solar and wind power, through fuel cells to nuclear propulsion. One of the key takeaways is that an integrated systems engineering approach is required to achieve effective improvements in efficiency and reductions in emissions for ships. This integrated approach must embrace all of the elements of naval architecture, marine and control engineering alongside operation practices.

International agreements on the need to combat climate change, the fluctuating but generally rising costs of marine fuels which account for a large proportion of the running costs of a ship, and developments on a number of other fronts have led many in the industry to question whether the present methods of ship propulsion are sustainable. These concerns are enhanced by the introduction of environmental regulations intended to reduce the impact of climate change—primarily MARPOL Annex VI and the Energy Efficiency Design Index regulations together with the possible introduction of carbon taxes.
This report embraces a number of conventional propulsion methods and fuels and also addresses the newer options of biofuels, liquid natural gas and hydrogen...There are other factors that affect the emissions from shipping. Avoiding poor weather by using weather-routing technologies offers important fuel consumption benefits. Similar benefits are also realisable if ship speed is optimized during voyages and the crew are trained to understand the implications of the decisions and actions they take. Furthermore, the condition of a ship’s machinery has a significant influence on fuel consumption and emissions performance. There is, therefore, good reason to keep machinery well-maintained and operated by well-motivated crews.
...a systems approach must include all of the stakeholder requirements to achieve a sustainable and optimal design solution. With any propulsion option it is essential that the overall emission profile of the propulsion method and the fuel used is properly assessed, so that reductions in exhaust emissions from ships are not at the cost of increasing harmful emissions in land-based sectors that produce either the propulsion machinery or the fuel.
—“Future Ship powering options”

Further work is needed in two directions, the report finds: the adaptation of current technologies from the maritime industries and elsewhere to broader application in different types of ship; and the research and development of innovative technologies specifically for maritime propulsion.

The report identifies a range of short-, medium- and long-term propulsion options:

  • Short-term options. The diesel engine is currently the most widespread of marine prime movers; however, diesel engines will continue to produce CO2 emissions as well as NOx, SOx, volatile organic compounds and particulate matter. Liquid natural gas (LNG) can be used as an alternative fuel in reciprocating engine propulsion systems and is a known technology with classification society rules for the fuel systems already in existence.

    Service experience with dual fuel and converted diesel engines has been satisfactory, the report notes, and currently LNG is considerably cheaper than conventional fuels. LNG, while not free of harmful emissions, has benefits in terms of CO2, NOx, and SOx emissions, given that methane slip is avoided during the combustion and fueling processes.

    Gas turbines have been successfully used in niche areas of the marine market and represent a proven high power density propulsion technology. However, the fuel for aero-derivative gas turbines is expensive when compared to conventional marine fuels and gas turbine thermal efficiencies are lower than for slow-speed diesel engines of similar power. Renewable energy, principally derived from wind and solar origins, is considered as an augment to the main propulsion and auxiliary power requirements of a ship.

  • Medium- to long-term options. Biofuels are potential medium-term alternatives to conventional fuels for diesel engines. Synthetic fuels based on branch-chain higher alcohols and new types of E-coli as well as algae and other microorganisms are medium- to long-term possibilities, but further work is necessary to examine their storage, handling, and impacts on health, safety and the environment.

    Dimethyl ether shows some potential as an alternative fuel; however, there are presently disadvantages which need resolution in terms of lubricity and corrosion together with the creation of sufficient production and supply networks.

    Fuel cells offer potential for ship propulsion with good experience gained in auxiliary and low-power propulsion machinery. For marine propulsion, the high-temperature solid oxide and molten carbonate fuel cells show most promise, while for lower powers the low temperature proton exchange membrane fuel cells are more suitable. While hydrogen is the easiest fuel to use in fuel cells, this would require a worldwide infrastructure to be developed for supply to ships.

    Nuclear ship propulsion has the advantage during operation of producing no CO2, NOx, SOx, volatile organic or particulate emissions. The conventional methods of design, planning, building and operation of merchant ships would, however, need a complete overhaul since the process would be driven by a safety case and systems engineering approach. Issues would also need to be addressed in terms of international regulation, public perception and acceptability, financing the initial capital cost, training and retention of crews, setting up and maintenance of a global infrastructure support system, insurance and nuclear emergency response plans for ports.

    Battery technology is developing rapidly, offering some potential for propulsion. However, full ship battery propulsion requires further technical development and is likely to be confined to relatively small ships. Batteries may offer a potential hybrid solution in conjunction with other modes of propulsion for some small- to medium-sized ships provided that their recharging does not increase the production of other harmful emissions from land-based sources or elsewhere.

    Superconducting electric motor technology has been successfully used in demonstrator applications, with low electrical losses resulting in a more efficient motor. Depending upon the type of prime mover deployed, exhaust emissions will be lower, the machine can run for some time after a coolant failure, and further advantages may accrue from their smaller size.

    Hydrogen, compressed air and liquid nitrogen (the last two as energy storage) are likely to be long-term propulsion considerations.

John Carlton FREng, Professor of Marine Engineering at City University London, said there was no obvious single winner in terms of technology in the medium- to long-term, and that operational issues were also an important contributor to power choices.

The Royal Academy of Engineering working party says that research and funding is needed to take some of the ideas forward, and broader techno-economic work is also needed to identify realistic targets in terms of emissions from the shipping industry.

The report is intended as a technical aid to the shipping industry which is under pressure internationally to improve its environmental record and increase efficiency. Sir John Parker, President of the Royal Academy of Engineering and himself formerly identified closely with the UK shipbuilding industry, said the report was both broad in its application and informed by the expertise that the working party had assembled.

Shipping is vital to the world economy. It is a critical part of international import and export markets and supports the global distribution of goods. As for all industries, concerns about climate change require the reduction of greenhouse gas emissions from the shipping sector. This entails higher fuel prices for low sulfur fuels. It means that the industry must prepare for the new future and investigate alternative, more economic ship propulsion systems.
—Sir John Parker




"And all I ask is a tall ship and a star to steer her by"

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