Three alternative power sources enabling green shipping

Three alternative power sources enabling green shipping

With the IMO committing to cut the industry’s greenhouse gas emissions by 50% from the 2008 levels by 2050, and a cross-industry coalition attempting to have net zero-emission vessels sailing by 2030, the race is on the find alternative power sources for the 50,000 ships sailing the world’s oceans. While operational efficiencies and waste reduction can make a big dent in the emission reduction targets, the industry needs to find completely new ways of meeting its energy requirements if it is to have any chance of hitting them.

Here we take a brief look at three of those alternatives; wind, batteries, and hydrogen.


Wind energy has powered the world’s fleet for centuries, it is only in the last 200 years that fossil fuels have taken over as the dominant propulsion system for ships. The problem is, in that time we have become used to the reliability of combustion propelled ships; they don’t get stuck in the doldrums, and they can power through oncoming wind much more quickly than a sailing ship can beat into it. But it would be foolhardy not to realise the benefits of harnessing this ancient source of energy in modern shipping.

When combined with forecasting and routing systems and with other forms of propulsion, wind becomes a reliable power source. There are a number of companies working on bringing wind back into shipping. Some, like Timbercoast, are operating small scale 100% wind energy powered vessels, others like Wind+Wing Technologies are developing solid polymer wings that act like sails and operate autonomously to provide extra propulsion whenever possible.

In addition to more traditional sail and wing ideas, we are also seeing a resurgence of Flettner rotors. In 1926, German aviation engineer Anton Flettner set sail across the Atlantic. Instead of using traditional sails, his ship was powered by large rotating cylinders on deck. The cylinders made use of the ‘Magnus effect’ which applies a force to rotating balls and cylinders perpendicular to the spin. It is the same reason backspin on a tennis ball will lift the ball, making it harder to hit. The rotors were more efficient than sails, able to function closer into the wind, and by producing more thrust, they also required less crew to operate.

Until recently, however, they were never deployed commercially at sea because the price and availability of fossil fuels meant they weren’t an economically viable alternative. Today though, Finish technology company Norsepower has deployed rotor sails to three vessels, a RoRo, a cruise ferry, and a product tanker. After 45,000 hours of operation, their solution has saved an estimated 1,700 tons of fuel and stopped 5,000 tons of CO2 entering the atmosphere. Norsepower claims that with rotors installed to a ship, it is possible to reduce fuel consumption by 5%-20% without reducing speed.

Norsepower rotor on board a product tanker.
Norsepower rotors fitted to a product tanker. Credit: Norsepower

Another innovator operating in this space is Magnuss. Recognising that bulk carriers need to make their decks free from obstructions when they are loading and discharging cargo, Magnuss has developed a retractable Flettner rotor. When a ship is in port, the rotors retract into the deck making it easier for cranes to access the cargo for loading and discharge.


Electric propulsion has existed at sea for many years. Usually, this takes the form of diesel generators powering an electric drive train. Diesel-electric ships are often used when a vessel needs to be able to quickly increase power consumption for manoeuvring, this is how most ferries and offshore support vessels operate.

If the diesel generator was swapped out for a large enough battery pack, it would be theoretically possible to power the ship with no emissions. There are a number of limitations to this, however, including the size and weight of a battery pack with enough power to reliably support the ship’s propulsion, and the ability to quickly charge the battery while the vessel is in port. A final point is that the battery would need to be charged by a renewable power source ashore, otherwise the emissions problem is being moved rather than solved.

In December 2019, Maersk will be installing a 600kwh battery housed inside a 40ft container to one of their ships; the Maersk Cape Town. This is the equivalent of having the power from six Tesla Model S cars available for use onboard. Rather than for propulsion, the battery pack will be used to improve the efficiency of the generators that power the onboard support systems. Additionally, it will be used to support rapid changes in electrical load such as using bow thrusters and to provide additional redundancy in case of power failure.

Maersk marine battery
Maersk’s containerised marine battery. Credit: Maersk

Taking this concept one step further is Dutch startup Skoon. They offer containerised large battery packs to act as a sustainable alternative to diesel generators on land and at sea. Users can buy Skoon batteries for use in their own operations, and rent them out to other users as part of a power-sharing service. The logistical flexibility that comes with housing batteries in containers, means they can supply power anywhere a container can be delivered and offer power on demand to ships in port and at sea as an alternative to using diesel generators.

One company working to make batteries a viable source of power for propulsion is Phinergy Marine. Rather than Lithium, which powers most batteries in existence. Phinergy Marine builds aluminium air batteries. Up to 70% of the weight of a traditional battery is made up of a cathode that bounds the oxygen required to release energy in a metal anode. Metal air batteries use oxygen from ambient air rather than requiring it to be bound in another element (ie lithium). This makes it possible to build batteries that are significantly smaller and more powerful than current battery systems. Phinergy Marine has developed a battery system that offers 7.2MWh of power stored in two twenty-foot equivalent shipping containers. One container contains the metal-air battery system, and the second is a tank container containing electrolyte. That’s the equivalent of having the power from 75 Tesla Model S cars in the space of a 40ft container. To scale the solution, they propose storing electrolyte in bunker tanks and swapping out container-based batteries for charging whenever a ship is in port.

Hydrogen Methanol and Ammonia

Hydrogen is an abundant source of energy and the waste product from its combustion is water. That said, it is dangerous to handle and 95% of the world’s industrial hydrogen supply is created through a process that requires fossil fuel inputs and emits CO2. Renewable sources of hydrogen are in development, though we are yet to see production at the scale that would be required to make it a viable alternative for vessel propulsion.

One initiative looking at exploring the use of hydrogen as a ship’s fuel is Hymethship. This EU funded project is working towards building a system that uses a carbon capture to turn hydrogen into methanol, making it safer to transport and handle. Before combustion, the methanol is reformed into hydrogen and CO2. Waste CO2 is captured and stored in tanks onboard the vessel which can then be discharged ashore to be used to create more methanol. Though not suitable for all ship types, Hymethsip claims that by using CO2 in a closed-loop, this system makes the most of the current combustion engine technology that exists in the world’s fleet and creates an economically viable way for ships to undertake long ocean passages with 97% less greenhouse gas emissions.

Hymethsip combines a closed loop CO2 system with a renewable hydrogen production system to power a marine engine.
Hymethsip combines a closed-loop CO2 system with a renewable hydrogen production system to power a marine engine. Credit:

Hydrogen ammonia is another promising method for fuelling ships on long distance voyages. Ammonia can be created by fusing nitrogen and hydrogen using renewable energy sources. The fertiliser industry has been using the compound for well over a century so we have a good understanding of how to handle and transport it. It can be stored in a liquid state under pressure or at a relatively straightforward -33.4°C. When it needs to be used as a fuel it can be split by a catalyst into nitrogen and hydrogen before combustion, with the resultant emissions are nitrogen and water. This again makes the most of existing combustion technology, and the world’s ammonia supply chain already exists because of the fertiliser industry.


We are a long way off having access to clean energy at sea, but with the Poseidon Principles gaining traction we are likely to see significant investment made in alternative energy sources for the industry in the next five years. Wind is an obvious example of a clean and renewable energy source, but it is not reliable enough to support the industry alone. The likely answer will be a combination of all of the above and more, with hybrid vessels becoming a common sight in the coming years.

As we begin to prove and disprove the viability of particular technologies, a critical question will become how we go about quickly scaling them to the world fleet. Developing the infrastructure to support changing the industry’s energy supply will likely be a greater challenge than developing the energy sources themselves.