How can we make shipping cleaner?

By EnPowered - August 05, 2021

Maritime shipping is a major polluter and many vessels use dirty fuels like bunker oil, so what greener options could they run on instead? EnPowered simplifies energy management.


  • International shipping is playing catch up as dirty fuels, a lack of regulation, and industry constraints have stymied efforts to reduce emissions

  • Electrifying shipping is still economically unfeasible, as battery technology is not advanced enough to provide sufficient energy density without taking up too much space

  • Ammonia and nuclear power are both alternatives that could help offset the use of diesel and bunker fuel in the coming decades, although each has its own drawbacks

Electrification is the name of the game in recent years, and as we ramp up the transition to electric cars, you may be wondering about whether we should electrify ships as well. The amount of cargo being shipped worldwide has quadrupled since 1970 and is only set to increase as cargo ships keep growing in size. The circa 50,000 container and cargo ships around the world pump out 900 million tonnes of CO2 each year (around 3% of total global emissions). And the 15 largest ships alone emit more nitrogen oxide and sulphur oxide (both potent greenhouse gases) than all the world’s cars combined.

What is the shipping industry doing to become greener?

A single container ship can have an 80MW engine running on some of the lowest grade and dirtiest fuels. Many ships use bunker oil, a low grade fuel left over from the oil refining process. Using diesel emits fewer emissions, but is still not an ideal solution. And even ships with industry-leading efficiency, like the G-Class (which produces 60% fewer emissions than traditional ships) used by OOCL still consume 79,500 litres a day (enough to fill up around 800 F-150s.)

Left unchecked, shipping could account for 17% of global emissions by 2050, as trade increases and emissions are reduced in other sectors of the economy, according to a 2015 report by the EU parliament. The global shipping industry has been a laggard when it comes to climate change, with the International Maritime Organization (IMO) only adopting a greenhouse gas (GHG) reduction strategy in 2018. In 2020, the IMO mandated a reduction in sulfur content in ship fuel from 3.5% to 0.5%. While this is a laudable improvement, it still dwarfs the sulphur content in low-sulphur diesel for cars (0.05%) as mandated by the EPA.

Looking further ahead, the IMO also aims to bring about a 40% reduction in CO2 per transport work compared to 2008 levels by 2030, and a 50% reduction by 2050. One easy way to reduce emissions is to decrease ship speeds: reducing speed by 20% cuts emissions and fuel costs by 24-34%, according to the World Economic Forum. Similarly, non-fossil fuels are projected to constitute roughly 50% of shipping fuel by 2050 according to Energy Technology Perspectives. Other industry efforts include a proposal for a $2 per fuel tonne tax to create a $5 billion research and development pool for clean fuel innovation. And in June 2021, Maersk - the world’s largest shipping group - suggested a $50 per CO2 tonne carbon tax by 2027, to eventually rise to over $150 per tonne.

Alongside industry GHG reduction plans, governments are increasingly looking to legislate shipping emissions, including the EU which is expected to propose extending its emissions trading system to include maritime transport. With all these disparate efforts, a key issue will be ensuring a clear and level playing field to prevent carbon leakage; namely, companies moving to regions with lax carbon regulations. According to Anne Steffensne, chief executive of Danish Shipping, “the worst thing for the global shipping industry is to have a patchwork of regional schemes.”

Poor quality fuels, a lack of international environmental regulation on the high seas, profit margins, demand for lower prices, and industry norms all contribute to shipping’s poor climate credentials. Looking ahead, ship builders and owners are now faced with the following dilemma, according to a spokesperson for DNV, an accredited registrar:

“The challenge for a ship built today is that [these changes] will take place within its lifespan. Failure to account for foreseeable regulatory and technology developments may render a ship built today uncompetitive at best; in the worst case it may end up being prohibited from operating altogether.”

Out at sea means out of sight and often out of mind, so a lack of public awareness about shipping pollution also contributes to a lack of past progress by the industry to tackle emissions. As things stand, we are literally powering ships with the dregs of the fossil fuel industry, so surely there has to be a better solution?


On the surface, electrification looks like a good proposition; container ships are huge and weigh hundreds of thousands of tonnes when fully loaded, so the weight and space requirements of battery banks should have plenty of room, right? To date only smaller vessels have been converted to (or purpose built) full-electric, like the 60 metre E5 vessel by Asahi Tanker in Japan in 2020, which ferries diesel fuel to other ships along the coast line. 2020 also saw the launch of the largest full-electric container ship, the 80 metre Yara Birkeland, built by Norwegian fertilizer company Yara. The ship sails along Norway’s coast carrying 2,900 tonnes of cargo and eliminating emissions from 40,000 diesel truck trips.

As of July 2019, there were 356 all electric or electric hybrid vessels in operation or under construction around the world, up from none just 10 years ago. While this is encouraging, around 8,000 merchant ships were built between 2015-2020. And the limitations of current battery technology means that only smaller vessels like the Yara and regional ferries can use electrical propulsion as a practical choice. If you think it’s bizarre to describe a vessel carrying almost 3,000 tonnes of cargo as small, remember that the largest cargo ships are 400 metres long and can carry more than 23,000 twenty-foot equivalent (TEU) cargo containers.

The lumbering cargo vessels that ply the seas use the equivalent of dozens of GWh, and you’d have to use up a large part of the ship just to house enough batteries to power it. The key issue is energy density; diesel and bunker oil offer more energy per kilogram than batteries. For example, an 18,000 TEU ship uses 4,650 tonnes of fuel during a 31 day trip. One kilogram of said fuel has an energy density of 11,700 watt-hours per kilogram, while lithium-ion batteries only provide 300 watt-hours per kilogram. That means you would need 100,000 long tons (or 40% of the ship’s cargo capacity) just to carry enough batteries to power the ship. And while battery pack prices have declined from $670 per kWh in 2013 to $140 in 2020 alone, the efficiency of lithium ion batteries has only tripled in the last 30 years of commercialization.

Using 40% of the ship just to carry batteries is obviously not going to make economic sense, although using batteries to replace fossil fuel generation for ship systems and lights is one way to incorporate electrification and reduce emissions. The shipping industry could also look to the cruise ship sector, which has stricter air quality rules, with most cruise ships diesel electric (instead of diesel combustion engines), which means using diesel to power an electric generator and then running ship systems electrically. Diesel electric systems also make integrating batteries and fuel cells easier, since only the initial energy source needs to be changed - the rest of the power system is already electric.


While the potential for biofuels to replace fossil fuels has been given ample media attention, comparatively little has been written about the potential of ammonia. At first sight, ammonia seems a strange alternative, as 80% of ammonia produced today is used in fertilizer. Lloyd's Registry reports that the shipping industry expects ammonia to make up 7% of fuel by 2030 and 20% by 2050: Energy Technology Perspectives even has ammonia supplying more than 50% of shipping’s energy needs by 2070. If these projections play out then global ammonia production will have to nearly double to supply (only) 30% of shipping fuel in the coming decades.

Ammonia already has an international production and transport network, which could mean substantial savings

Ammonia has several advantages that make it attractive to the shipping industry. Firstly, it produces no CO2 when burned in a combustion engine, and its energy intensity is higher than hydrogen and similar to methanol, which makes onboard storage viable (unlike batteries). Ammonia can also be stored at a far higher temperature than hydrogen (-33C vs -253C) which makes storage and transport far simpler.

And unlike hydrogen, ammonia already has an international production and transport network, which could mean substantial savings. A 2020 study by University Maritime Advisory Services of the IMO’s 2050 carbon reduction goals estimated that up to $1.4 trillion will be needed to meet said goals. Of this amount, land-based infrastructure and production facilities for low-carbon fuels account for 87%. So being able to leverage existing ammonia distribution networks could save international shipping billions of dollars.

Now this isn’t to say that ammonia is without drawbacks, as most production is powered by natural gas, so its lifecycle emissions must be factored in. Ammonia is also highly toxic, with airborne concentrations as low as 0.25% liable to cause fatalities. This means that port services and authorities have hitherto been reluctant to bunker ammonia, and current regulations preclude ammonia for fuel use. And while no CO2 is released when ammonia is burned, nitrous oxide is, which is a far more potent GHG, so additional filtering and emissions mitigation will be required if ammonia is widely adopted.


Using ammonia to fuel ships may be commonplace in the coming decades, but another alternative to fossil fuels - nuclear power - has been powering submarines and surface vessels for decades. The IMO is considering the use of small or modular nuclear reactors to power shipping vessels, a technology most notably used by nuclear submarines and the US Navy’s aircraft carriers. IMO rules adopted in 1981 already allow for nuclear merchant ships, and the first nuclear merchant ship - the NS Savannah - operated from 1964 to 1972. And Russia / former USSR has operated dozens of nuclear powered icebreakers (and continues to do so).

Nuclear energy produces no GHG emissions and has by far the densest energy intensity per kilogram of fuel of any energy source. Switching to nuclear could save the international shipping industry 5 billion barrels of oil each year, while nuclear powered ships are 50% faster than fossil fuel powered vessels of comparable size. A potent example of the advantages of nuclear power can be seen in the fates of the USS John F. Kennedy and USS Dwight D. Eisenhower. Both are aircraft carriers, the former launched in 1967 and powered by oil, the latter launched in 1974 and nuclear powered. Both were of similar length, but the Eisenhower’s displacement (weight) is almost 20,000 tons more than the Kennedy’s.

Nuclear energy produces no GHG emissions and has by far the densest energy intensity of any energy source

The USS Kennedy became the most expensive vessel in operation in the US Navy, since at full operation and launching planes from the deck, the ship got less than 9cm per litre of fuel - an utterly abysmal fuel economy. This meant that, at full steam, the Kennedy had to use over 3,700 litres just to move its own length, and 473 million litres to circumnavigate the Pacific once. On the other hand, the USS Eisenhower can sail for 20 years without refuelling on a piece of uranium the size of a grapefruit: the Kennedy was mothballed in 2007 while the Eisenhower is still on active duty with the US Navy.

The drawbacks of nuclear include expensive upfront and initial fuel costs, especially compared to fossil fuels. Many ports and countries also currently refuse (whether due to public opinion, geopolitics, or insufficient infrastructure) to allow nuclear powered vessels to access their facilities. Nuclear powered cargo vessels would therefore - in the absence of legislative changes - be relegated to a circumscribed list of destinations. Concerns about the proliferation of nuclear technology could also hamper the adoption of nuclear shipping, as leaders in modular nuclear reactor technology such as the United States and Russia may be unwilling to provide wider access to said technology.

No single fuel source will be able to completely replace fossil fuels in shipping in the foreseeable future, so we need to be open to harnessing various fuel sources where they make sense. Combining electric, ammonia, biofuels, and eventually ammonia and perhaps nuclear can instil resilience and flexibility into international logistics networks, preventing disruptions from fuel bottlenecks or production decreases. There is no coup de grâce in the battle against climate change; it’s a battle of a thousand small cuts.

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