Hybrid and fully electric cars are now beginning to garner wider acceptance and are becoming an increasingly common sight on our roads. But what about the use of electric power for propulsion in the shipping industry?
Whilst recent studies have largely debunked many sensationalist claims comparing shipping emissions to those of cars (claims such as “A large container ship is just as polluting as 50 million cars”), the fact remains that global shipping is a major and growing contributor of greenhouse gas emissions. A study by the International Maritime Organisation (IMO) found that in 2012 global shipping emitted around 940 million tonnes of CO2 and was responsible for about 2.5% of global greenhouse gas emissions. The same study estimated that if left unchecked, shipping emissions could increase by between 50% and 250% by 2050.
Since the 1960s, most ocean-going vessels such as oil tankers, cargo vessels and container ships have burned heavy diesel oil which is derived as a residue from crude oil distillation and is high in sulphur. And they have a staggering thirst – consuming some 300 million tonnes each year. Whilst lighter and cleaner marine diesel is used by smaller inland vessels, it is much more expensive than heavy fuel oil and nevertheless is still a fossil fuel.
In 2019 the IMO adopted a strategy on reducing greenhouse gas emissions, which has set targets of reducing such emissions by at least 50% by 2050, and reducing the carbon intensity of emissions by 40% by 2030 and 70% by 2050, relative to 2008 levels.
So the shipping industry is beginning to turn to alternative energy sources. Sceptics might draw a supertanker analogy, but the changes are beginning to happen. And electricity is playing a role.
A true hybrid system
The use of electricity to propel ships is not new. Many ships already use diesel-electric systems in which diesel generators produce electricity which is then used to power an electric motor which in turn drives the ship’s propellers. But whilst this type of relatively common drive system comprises both an internal combustion engine and an electric motor, this is not what is meant by the term ‘hybrid drive’. A true hybrid system can propel the ship purely electrically, without the operation of any internal combustion engines, at least for limited periods.
Whilst the ultimate goal for shipping is considered by many to be fully electric drives, these are presently only viable for small inland vessels such as short-crossing ferries and pleasure craft. Vessels of this type are well-suited to current battery technology because they cover only relatively short distances, and importantly their operational requirements involve regular docking, thereby affording the opportunity to recharge their batteries regularly from shore-side electricity supplies before every voyage.
Leading the (Nor)way
Norway boasts the world’s first fully electric car and passenger ferry. The ‘Ampere’ is powered by lithium-ion batteries and was originally developed as a submission to a Norwegian Ministry of Transport competition to develop the most environmentally friendly ferry possible. The prize was a 10-year licence to operate the 6 kilometre Lavik-Oppedal route, which ‘Ampere’ duly began to serve in 2015. This 80m long catamaran carries up to 120 cars and 360 passengers across a six kilometre stretch of water 34 times a day, and is recharged in approximately 10 minutes at each end from a large shore-side battery whilst unloading. The shore-side batteries are necessary to avoid blackouts in the local villages, and each is recharged from clean hydropower after the ferry departs, to be ready again when she next returns. It is estimated that a conventional diesel ferry operating the same schedule would burn approximately 1 million litres of diesel fuel each year.
The ‘Ampere’s batteries have a total output of 1000KWh and weigh 10 tonnes, and the vessel’s design includes some interesting aspects to optimize her for this battery power. Weight saving is an important aspect of the design, because today’s lithium-ion batteries still can’t compete with diesel in terms of energy density. A large proportion of the weight saving comes from the fact that the catamaran’s two hulls are made exclusively from aluminium instead of steel. The double-ended design also plays a role because this avoids the need for the vessel to turn around at each end of its short route, an operation which would require a large amount of power. Propulsion is provided by Rolls-Royce thrusters, and in particular so-called Azipull thrusters – steerable thruster units whose propellers pull through the water instead of push, and whose trailing housings are hydrodynamically optimised with a wide chord to provide a rudder effect, whilst being shaped to increase available thrust by recovering some of the energy which would normally be lost to the swirl produced by conventional propellers. One thruster is located at each end of the hull, but only the aft thruster is powered during transit, to provide steerage and all of the thrust required, whilst the propeller of the forward thruster is feathered to reduce drag. Both thrusters are powered when accelerating or manoeuvring, and as the ‘Ampere’s double-ended design means she doesn’t need to turn round at either end of the route, the thrusters then swap functions for the return trip.
Whilst fully electric drive is viable with today’s battery technology for vessels such as the ‘Ampere’, it remains a distant goal for larger vessels such as cargo ships which must carry much greater weight over much greater distances, and which often have unpredictable operating requirements. Hybrid drives, similar in principle to those we see in cars, offer a solution for some ships.
True hybrid cruise ships are already in operation, with others in the planning and build stages. Again we can turn to Norway for an example, with Hurtigruten’s expedition cruise ship ‘MS Roald Amundsen’ entering service in 2019, having been designed by Rolls-Royce to operate off the Norwegian coast and in polar waters. Although still relatively small in a world of truly enormous ships, this ship is considerably larger and more comprehensively equipped than the ‘Ampere’, having a gross tonnage some thirteen times greater, and luxurious accommodation spread over nine decks. Yet her battery-based diesel-electric hybrid drive system permits purely electric running for around 45 minutes, using Rolls-Royce’s first permanent magnet Azipull thrusters, and allowing silent and emission free operation in Norway’s precious fjords and when entering ports and vulnerable areas. The drive system comprises low emission diesel generators, the electric motors in the thrusters, and large batteries. The generators produce electricity which is used to power the propulsive motors in the manner of a conventional diesel-electric set-up, with surplus energy being stored in the batteries to provide supplemental propulsive power under peak load conditions, to support spikes in the ship’s electrical ‘hotel load’, and to propel the ship silently without the diesel generators when conditions allow or dictate.
Peak shaving and Dynamic positioning
While the ‘MS Roald Amundsen’s party trick of completely electric running has grabbed headlines, battery-based hybrid drive systems are also providing benefits on other types of vessels to supplement diesel engines via so-called peak shaving, and during dynamic positioning.
Peak shaving involves the use of electrical energy stored in the hybrid system’s batteries to bridge intermittent peaks in load on the ship’s propulsion system. This permits the internal combustion engine to operate at its most efficient speed during average load conditions, with the hybrid system’s batteries providing additional power when required to handle peak load conditions. This in turn allows smaller combustion engines to be specified at the design stage. Peak shaving in this manner has been found to reduce fuel consumption by around 20% for small vessels such as oilfield supply vessels which are required to accelerate and manoeuver frequently.
Dynamic positioning involves the use of a computer-controlled system to maintain a vessel’s heading, and often its exact position, using the vessel’s thrusters, and is common in supply vessels, offshore drilling ships, oceanographic research vessels and some cruise ships (the MS Roald Amundsen included). Not only does dynamic positioning involve large fluctuations in power demand, particularly in heavy seas, but its operation often requires strict redundancy measures to be in place in case of power loss due to equipment failure such as an engine breakdown. These requirements make battery hybrid systems ideal for dynamic positioning, because electrical energy stored in the batteries can be brought online instantly to meet increases in power demand, and can be used as a ‘spinning reserve’ in case the main engine should fail, thereby avoiding the need to run an offline diesel engine just in case of failure.
We are familiar with the concept of regenerative braking in electric and hybrid cars. The car’s motor/generator switches from motor mode to generator mode when the driver lifts off the throttle, to generate electricity which is sent to the battery for charging, whilst magnetic friction contributes to slowing the vehicle. Whilst this concept can be used to provide very useful fuel savings in road vehicles which frequently slow and stop to negotiate other traffic, junctions and traffic lights, potential fuel savings from storing braking energy from propulsion are much more limited in the marine world because ships simply don’t have to slow or stop as quickly or as frequently as cars. But that does not mean that hybrid or electric ships of the future cannot charge their batteries using regenerated energy from braking electric motors – ships have other motors that are better suited to the task than those used for propulsion. A good example is an offshore drilling ship which uses a heave compensation system to keep its drill string stationary relative to the seabed whilst the ship is heaving in waves. Significant energy can be regenerated from motor braking in such systems, and stored in a hybrid drive battery. Of course drilling ships of this type are also heavily dependent on dynamic positioning systems to prevent movement in sway and surge relative to the wellbore, so they are very well suited to hybrid systems. Ships with numerous large cranes are also good candidates for energy regeneration – using the cranes’ motors to generate charge.
The future of electric marine propulsion
Whilst we are now seeing increased development in the use of electricity for marine propulsion, there are significant obstacles which must be overcome before we will see widespread adoption of fully electric or even hybrid drives. The main challenge is that of battery efficiency. The energy density of current battery technology is still too low for a large proportion of the shipping industry, meaning that today’s batteries are unable to store sufficient energy in relation to their size and weight to make them a viable source of propulsive energy for large ships.
The need for new battery technologies to underpin the goal of cleaner shipping is clear, but as covered in earlier blogs by Eleanor Maciver, Callum McGuinn and Melodie Richardson, research into the development of batteries with greater energy density is proceeding at pace.
Although it may be some years yet before we see fully electric ships crossing the oceans on a single charge, in the meantime we can expect to see significant development of marine hybrid drive systems to maximise the potential of rapidly evolving battery technology.
Simon has also explored how electric cars are driving the battery advances needed to power big ships in TradeWinds. Access the article.
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