Hybrid ship

Hybrid ships take to the high seas

Image credit: Hurtigruten

Hybrid ships that voyage across the deep blue could very well be the key to unlocking environmental sustainability.

The dream is alluring. A sleek ship on a sparkling sea, set against a sky of the purest blue, cleanliness and healthiness embodied in luxury. The reality is painfully different. Shipping is a major contributor to pollution and greenhouse gas emissions, responsible for an estimated 15 per cent of nitrogen oxide (NOx) and 8 per cent of sulfur gas emissions, said the Independent in February 2018. According to figures originally published in the Guardian in 2009, one giant container ship can emit almost the same amount of cancer- and asthma-causing chemicals as 50 million cars, and around 50,000 premature deaths in Europe have been attributed to international shipping.

To counteract the environmental and health impacts of ships’ air pollution, the International Maritime Agency (IMO) established Emission Control Areas (ECA) for waters around North America and Europe, in which sulfur standards for marine fuel are less than 0.1 per cent and, from 2020, a global 0.5 per cent sulfur limit in fuel will apply to all shipping.

Stricter emissions targets have encouraged ship owners to choose from a number of options including use of low-sulfur marine diesel, marine gas oil, ultra-low-sulfur heavy fuel oil and exhaust-cleaning devices such as scrubbers or alternative sources of energy including liquefied natural gas (LNG). But, for some ship owners, installing a hybrid propulsion system to existing or new vessels is seen as a rewarding way to meet emission targets.

In ECA waters and particularly around Norway, ferries, platform support vessels and cruise liners are pioneering the use of hybrid marine propulsion systems. Børre Gundersen, R&D manager for ABB’s marine activities in Norway, states that “hybrid propulsion systems significantly reduce both fuel consumption and emissions”. Gundersen’s colleague Sindre Sæter said: “Hybridisation of 230 offshore supply vessels operating in Norwegian waters could reduce CO2 emissions by 400,000 tonnes.”

Another attraction of hybrid marine engines is that they can be fuelled by diesel, LNG or hydrogen, and use a fuel cell, batteries or an electric motor. This capability makes hybrids particularly suitable for ferries in coastal or enclosed waters, explained DNV GL’s director of battery services and products, Narve Mjøs, who has seen increasing interest from cruise ferries between Portsmouth and Santander. UK shipbuilder Ferguson Marine has built Catriona, a £12.3m diesel-electric-battery-power hybrid ferry for CalMac to use on its Clyde and Hebridean routes. Also in the UK, Wightlink’s new flagship ferry for the Fishbourne-Portsmouth route will use diesel-electric hybrid batteries.

Hybrid systems are currently a niche segment worth just US$2,653.4m in 2015 but forecast to nearly double to $5,252.5m by 2024, according to Transparency Market Research report ‘Marine Hybrid Propulsion Market – Global Industry Analysis, Size, Share, Growth, Trends, and Forecast, 2016-2024’.

Design issues

There are a variety of operational and safety factors affecting the design of vessels using hybrid propulsion systems (see diagram).


Any vessel that needs large amounts of power at a moment’s notice to meet varying loads, such as a tug, ferry, offshore support vessels or naval craft, could benefit from retrofitting a battery or energy storage system (ESS) to provide hybrid power. The advantage, according to Marintek research manager Anders Valland, is that, “the battery can absorb peak loads, while the internal combustion engine can continue to operate at its optimal level”.

Significant gains have been demonstrated by the installation of an Orca ESS energy storage solution to a platform support vessel by Corvus Energy, which reduced fuel consumption by 15-20 per cent, CO2 emissions by 15-20 per cent, and NOx emissions by 25-35 per cent. The Port of London Authority (PLA) has just ordered a pioneering hybrid pilot cutter, for transferring pilots to and from vessels in Gravesend Reach, from Goodchild Marine Services. Managing director Alan Goodchild said the vessel can “achieve 15 knots under hybrid power”.  

Interoperability issues

Interoperability of component parts and control systems, for example genset, inverter, power management system and control software for all components, is a key design challenge. Nowadays, testing control software of individual components and the entire system, including faults and under all operating conditions, is performed with virtual digital twin models such as Typhon HIL, whose testbed examines every aspect of the propulsion system from concept to operation.

Digital twin ‘what if?’ models are being adopted by operators and ship designers. In addition, to manage the shift from one power mode to another, companies such as Rolls-Royce supply the ACON mode-shift system, which allows the captain on the bridge to shift modes with a single keystroke.


‘It will be an evolution and a changeover in the population of ships over many decades.’

Scott Samuelsen, National Fuel Cell Research Centre, USA


Alternative fuel solutions

Ships generally use one of three types of fuel: heavy fuel oil (HFO), low-sulfur fuel oil (LSFO) or diesel oil. For example, in the Baltic Sea and other land-enclosed waters including the English Channel, the Mediterranean, and the Great Lakes of America, vessels must use LSFO. To reduce their impact on the environment, ship operators are also considering alternative energy sources such as LNG (liquefied natural gas) and hydrogen for both hybrid and dual-fuel propulsion systems.

Liquefied natural gas

There are currently around 120 LNG-powered ocean-going ships, with another 130 on order. Most sources put the future market share of LNG-powered vessels at between 5 and 8 per cent, but DNV GL, a maritime classification society, forecasts that by 2050 no less than 30 per cent of vessels will be LNG fuelled. The EU and the Port of Rotterdam have backed an LNG masterplan for the Rhine-Main-Danube river system.

Some new bulk carriers, tankers, pure car and truck carriers (PCTCs) and cruise ships are being built with dual-fuel engines using oil and LNG. This natural gas helps ships to meet the strict SOx requirements in an ECA while offering much reduced particulate matter (PM) emissions compared to most oil fuels (one of the US Environmental Protection Agency’s stated goals for the ECA is PM reduction). Beyond its clean burning credentials, LNG has the added benefit of being cheaper than low-sulfur diesel oil or other alternative fuels under consideration, such as methanol.

In terms of ship layout, the ship and engine room will look much the same except for a few unusual looking tanks and specially coloured pipes and boxes in the engine room. However, more so than with other fuels, the storage and handling of LNG is complex since it is highly flammable in its gaseous state and has to be stored at cryogenic temperatures. These characteristics require more planning and greater engineering for the construction and operation of LNG-fuelled vessels than for just oil-fuelled vessels. In comparison with oil fuels, the major impacts of LNG fuel on ship design concern storage, tank location, size of tank, hazard protection and boil-off of fuel.

Adopting a hybrid propulsion system is useful in a world that does not enjoy an extensive network of alternative fuel bunkering facilities. According to SEA\LNG in October 2018, there are just 31 LNG bunkering facilities for ships. Most are located at major ports in London, Rotterdam, New York, Hong Kong and Singapore.

LNG fuel storage

LNG has greater volume and lower density than oil, but it contains more energy (heat value) in each kilogram, which partially compensates for its lower density. Therefore, LNG requires four times more volume storage on-board than oil for the same range, making it more problematic for ship design. There is also the need for back-up provision by another source of fuel, which also reduces available cargo space. In contrast to LNG fuel, oil can easily be stored in hull structural tanks with few restrictions on location.

Tank location

Safety requirements for LNG constrain both the size and location of storage tanks. Because each cubic metre of LNG storage has a high cost, there is a tendency to minimise the extent of LNG storage volume on board, which reduces the vessel’s range and increases the need for bunkering. The choice of storage tank type depends on the vessel’s size, type, route and gas usage profile – no single type is best for all. Portable containerised tanks and IMO Type C pressure tanks are convenient for retrofitting short-range ships because they are built off- site and are easy to drop into position. Long-range vessels require large volume tanks with built-in prismatic IMO type A and B tanks or membrane tanks, the latter providing greater volume in the same space and lower cost per cubic metre.

Hazard protection

In order to avoid potential damage to LNG storage tanks from collisions or groundings, they are required to be positioned away from the side and bottom of the ship. The requirements for LNG fuel storage tanks are more severe than for LNG cargo tanks on LNG carriers because of the concern that cargo ships using LNG fuel will not get the special attention that LNG carriers do, and thus will be as prone to collisions and groundings as any other ship.

Boil-off gas

Unlike oil fuels, which can stay in a tank for many years, LNG cannot. Due to its cryogenic nature, it is evaporating or boiling-off every minute that it is in the tank. If LNG can evaporate without restraint or relief, the pressure can build up enough to damage tanks and fittings, as the volume ratio of natural gas to LNG at atmospheric pressure is 600 to 1. The best way to tackle this problem is to develop a boil-off gas (BOG) plan at the design stage. The plan should demonstrate how the BOG would be handled in all the LNG tanks and piping systems, how bunkering will take place, and how the tanks will be used in service. The BOG plan should also address contingencies such as what happens after the ship has fully bunkered and the machinery plant or hull is damaged, forcing the ship to stay tied up at the dock with no consumption from the main propulsion engine.


Currently, hydrogen fuel hybrid vessels are confined to tourist and commuter boats in Hamburg, San Francisco and Shanghai. However, cruise ship operator Royal Caribbean is pioneering a hybrid LNG and hydrogen fuel propulsion system for its cruise ship being built by Meyer Turku in Finland. The hydrogen fuel cell will help power the ship’s hotel functions. Elsewhere, Viking Line has unveiled tentative plans for a liquid-hydrogen-fuelled cruise ship some 230m long for 900 passengers. She will be similar in design to the cruise line’s Viking Sun. Several tender ships to carry the fuel to the cruise ship are also part of the project.

These pioneering projects face operational and regulatory challenges as noted by Swiss Hydrogen CEO Alexandre Closet, who said that when handling hydrogen, “it must be done in a very secure way. It’s a challenge to be able to clearly respond to every point given by a certification body, and at the same time integrate everything on the ship.”

In the case of energy-hungry ocean-going container ships, hydrogen fuel cells are seen as a good option because hydrogen fuel cells get their charge not by plugging into the wall as batteries do, but from on-board hydrogen stores, allowing fuel cells to produce sufficient power for long trips. The Emma Maersk, a mega-container ship with an 81MW diesel propulsion engine on a route between Malaysia and Egypt, could support enough fuel cell modules and hydrogen tanks to allow the ship to make the long journey without the need to refuel, according to a 2017 Sandia Report.  

A fuel cell will convert the hydrogen to electricity for propulsion and electric power on-board. A fuel cell power pack consists of a fuel- and gas-processing system and a stack of fuel cells that convert the chemical energy of the fuel to electric power through electrochemical reactions. The process can be described as like that of a battery, in which electrochemical reactions occur at the interface between the anode or cathode and the electrolyte membrane, but with continuous fuel and air supplies. There are several different fuel cell types on the market characterised by the materials contained in the membrane. ABB Marine & Ports managing director Juha Koskela observed: “Fuel cells have been the next big thing for 25 years, but now they are reality.”

The main obstacles of hydrogen fuel cells include high investment costs, reduced ship space due to their volume and weight, and their expected longevity. Consideration must also be given to storage of hydrogen on ships in order to ensure safe operations, says a DNV-GL 2014 study, ‘LNG as Ship Fuel’.

The technology is very costly because fuel cells are not mass-produced. On a pound-per-kilowatt-hour basis, the electricity cost from a fuel cell is roughly double or triple that from a diesel generator, explained Ryan Sookhoo, Hydrogenics’ director of business development. Moreover, in comparison with numerous oil bunker ports and LNG bunkering opportunities, hydrogen-fuelled ships’ access to bunkering is extremely limited. Los Angeles and San Francisco in the US, and Kirkwall in Orkney, are currently at various stages of developing hydrogen-fuelling capacity. In the case of Orkney, EMEC takes surplus tidal and wind energy from turbines on the island of Eday to produce the hydrogen.

A 500kW electrolysis machine splits water into its component parts – hydrogen and oxygen – with an electric current. The hydrogen is transhipped to Kirkwall, where some of it is used to fuel the port’s fuel cell, which reverses the electrolysis process to produce electricity for the port as shore-to-ship power, known as ‘cold ironing’. This has the advantage of reducing pollution in the surrounding area. This facility is seen as a foundation for developing Kirkwall’s capacity to provide visiting ships of the future with hydrogen-bunkering facilities.

Hydrogen fuel cell technology is still at the infant industry stage in shipping and will take time to be adopted and competitive. In the medium term, this technology, like solar and wind power technology before it, should benefit from economies of scale and the learning curve, thereby lowering the cost of fuel cells and ships’ operating costs, especially when oil either becomes too expensive or is prohibited for use in shipping.

As Scott Samuelsen, director of the National Fuel Cell Research Centre at the University of California, Irvine, predicts: “It will be an evolution and a changeover in the population of ships over many decades.”

The future of hybrid propulsion systems in shipping looks promising, although it remains to be seen which energy sources and combinations of power modes will prove popular.

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