How does shipping go green?

The maritime world wants – and needs – to get greener. This is more than an outworking of the Paris Climate Accord – it's also the stated aspiration of many shipping companies, shipyard operators and skippers. But what waters have to be navigated to get there?

“The key enabler for greener shipping is the fuel,” emphasizes Dr. Daniel Chatterjee, Director of Technology Management & Regulatory Affairs at Rolls-Royce Power Systems. E-fuels such as e-hydrogen, e-methane, e-methanol, e-diesel or e-ammonia, which are produced from renewable energy sources and subsequently refined, can be converted into propulsion power in ways that are climate-neutral.  

The search for the right Power-to-X fuel

In recent years, a team of Rolls-Royce experts has been working with customers and industry partners to examine the entire fuel system – from fuel infrastructure, the cost of creating it and vessel range, to the propulsion system and its integration in the vessel. Their goal has been to identify the perfect fuel for marine use. The experts expect that as early as 2035 over half of all the world's ferries and tugs will no longer be powered by conventional diesel engines.

“We want to make methanol the standard”

Rolls-Royce experts currently reckon methanol is shaping up best to be the marine fuel of the future. As a result, they've already begun developing a high-speed, four-stroke engine optimized for methanol combustion. “We want to be a pioneer here and set new standards with a methanol engine. After all, our customers need planning certainty, and we urgently need to work with them to create specific solutions to meet climate targets,” said Denise Kurtulus, Vice President, Global Marine at Rolls-Royce Power Systems.  

The benefits of methanol are its high energy density compared to other sustainable fuels, and its liquid state which means it's easy to store and refuel at ambient temperatures – indeed existing infrastructure can continue to be used in many cases. Unlike ammonia, methanol is environmentally safe and not highly toxic. The combustion of methanol can be shown to be climate-neutral with significantly reduced nitrogen oxide emissions, thereby obviating the need for complex SCR exhaust gas aftertreatment. On board, methanol tanks can be arranged flexibly in vessel designs and have significantly lower safety requirements than hydrogen or ammonia. In addition to safety aspects, methanol tank systems are less complex and require lower investment outlay.

Methanol for fuel cells too

Another benefit of methanol is that it can be used not just in diesel and spark-ignition combustion engines, but also in conjunction with zero-emission fuel cells: using a reformer, hydrogen will be produced from methanol and then used in fuel cells to generate electricity.   This is an especially attractive proposition aboard ships, as hydrogen tanks would take up more space, and space is a premium commodity on any vessel (and simply not available at all on some).  

Hydrogen propulsion – when range is not an issue

Even so, developers at Rolls-Royce Power Systems also have their eye on other options and are working on a hydrogen combustion engine for use aboard ferries and tugboats in particular, as these tend to be used on short, well defined routes. They are able to refuel frequently and can also rely on this fuel being available at their port of call.  

Range is an important consideration, especially for yachts which are often underway in exotic locations where novel fuels like hydrogen or methanol may not always be available. This is where diesel – and its net-zero cousin e-diesel – remain an important option. It will continue to be available at most ports and has a much higher energy density than hydrogen. E-diesel can also be used in today's existing engines.

E-ammonia is also an attractive option as a marine fuel, although for safety reasons Rolls-Royce experts do not currently foresee it being used in coastal shipping, i.e. in ferries, tugs or yachts.  

“We'll need to commit to a certain path in three to five years' time.”

“Whereas we're now focusing our attention on methanol, we remain open to other technologies and are keeping our eye on the other fuels as well,” said Chatterjee. Nevertheless, he is convinced: “We'll need to commit to a certain path in three to five years' time at the latest in order to achieve our overriding goal of making shipping climate-neutral by 2050.”

Yet this goal still seems so far away. However, a simple calculation shows that preparations for this must begin now: Ships have service lives of anywhere up to 40 years. If the entire shipping industry is to be climate-neutral in 2050, the vessels in service then must be planned today. Any vessel entering service in 2030 must have climate-neutral propulsion. If this is to be built in 2027 – assuming a three-year construction period – the decision on the propulsion system, and thus the fuel, will have to be made in the years leading up to this.  

Vessel architectures with hybrid systems

But it is not just the choice of fuel that is crucial: the architecture of propulsion systems is also changing. Vessels powered solely by combustion engines are becoming less commonplace. The combustion engines are increasingly being incorporated in hybrid systems where batteries supply electric motors and the engines only come into play when battery power is no longer available. “Our focus is on vessel architectures involving hybrid systems that are able to use combustion engines and fuel cells. Combustion engines are still being powered by diesel, but in the medium term they'll be running on methanol,” said Kurtulus.  

Political support called for

Rolls-Royce studies show that the huge demand for e-fuels – 20,000  TWh of energy per year, the equivalent of around two trillion liters – cannot be met before 2030. The technology is certainly available, but not yet on an industrial scale. “Advancing this and creating the framework for these fuels to find a market need the support of political decision-makers,” urges Chatterjee.