Back in 2022 when I wrote about new developments in wind-powered shipping, my article attracted quite a lot of comments. Ships are big polluters and I described some of the amazing innovations using wind to power such vessels. But some people seemed to think wind-powered shipping was a retrograde step if we want to make shipping greener.

Many of the objectors repeated the memes that prompted me to write that original article in the first place – essentially mocking the notion that wind-powered ships could possibly be the solution. But shipping is cleaning up its act, with plenty of recent developments. Most exciting of all is that several novel “clean” ships have been launched in the last few months.

Shipping is the lifeblood of the global economy, with about 90% of trade being seaborne. More than 90,000 ships crossed the oceans in 2018, burning two billion barrels of the dirtiest fuel oil. Making up 2–3% of global greenhouse-gas emissions, they also belch out sulphur dioxide, nitrogen oxides and particulate matter, endangering human health, especially along key shipping routes.

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But shipping – like aviation – isn’t covered by the Paris Agreement on climate change, which seeks to limit the global temperature rise to 2 °C this century via emissions reductions. Instead, it is up to International Maritime Organization (IMO) – an industry body – to negotiate cuts. It wants to halve emissions from the sector by 2050, which would otherwise rise six-fold by then if nothing were done.

Most ships today still burn bunker fuel – a “heavy” oil that’s mostly what’s left over after crude oil is refined. Literally, it’s the dregs: when burned, it spews out about 3500 times as much sulphur as road diesel fuels. Since 2019, however, the IMO has said that all ships over 5000 tonnes – which emit 85% of all maritime greenhouse-gases – must collect fuel-oil consumption data.

In 2020 the IMO also banned the sale of high-sulphur fuels. In the past, such fuels were allowed to contain as much as 3.5% sulphur. But that limit has now been capped to 0.5%. Ships must also use “scrubbers” to clean up exhaust gases. These mandatory measures from the IMO are a good first step, but they’re not going to solve the problem long term or let us hit targets.

It’s a gas

So how can we cut emissions from ships? It’s a complex question that depends on the energy density of a fuel, how much space you have on a ship, and how far it needs to travel on a regular basis. One of the main contenders is liquid natural gas (LNG), which has an energy density of 55 MJ/kg compared to 45 MJ/kg for heavy oil.

Many see liquid natural gas as the only realistic short-term solution if we are to cut shipping emissions by 40% by 2030

Indeed, many see LNG as the only realistic short-term solution if we are to meet the IMO’s interim target, which it introduced last year, of cutting emissions by 40% by 2030. LNG is 25% less carbon intensive than heavy oils and doesn’t emit as much nitrogen and sulphur oxide. It’s also a mature technology, with many ships already running on LNG.

Green hydrogen, which has energy density of 120 MJ/kg, is a longer-term goal but economics are currently not on its side. Ammonia (18.5 MJ/kg) is another contender even if it’s toxic and, like hydrogen, has to be stored under high pressure. Still, the shipping industry is used to moving it around the globe as it’s a feed stock for many industrial processes and a good method of storing hydrogen.

Batteries, though, aren’t realistic for many applications. Apart from their tiny energy density (0.4 MJ/kg) you’d need loads of room on a ship taking up valuable space if range is required. Nuclear is not considered an option either. Despite its massive fuel energy densities (79,390,000 MJ/kg), it has many restrictions, huge operational costs and lots of geopolitical issues at play.

There are now over 469 LNG-powered ships in operation, according to the latest data in the Alternative Fuels Insight (AFI) platform from risk-management experts DNV. Whilst this is still in the “burning things to make things move” category, LNG is still much better environmentally than bunker fuel and a step in the right direction. Taking the current order book into account, DNV reckons there will be more than 1000 LNG ships by 2027.

Batteries, ammonia and hydrogen

Among other recent developments, we’ve seen two battery-powered container ships completed last year by Shanghai-based COSCO Shipping. Battery prices continue to fall due to volume production and the new ships use swappable shipping-container-sized batteries, which is a neat idea. The two ships were built by CHI in Yangzhou and are the world’s first 700TEU electric container ships.

In what will make SI unit purists squirm, a TEU (or twenty-foot equivalent unit) is a measure of volume, where 1 TEU is a container 20 feet (6.1 m) long. Large container ships can typically transport more than 18,000 TEU, while some can even carry more than 21,000 TEU. The COSCO ships use battery-powered electric motors that can move a ship at a top speed of 19.4 km/h, with each battery having a total capacity of over 50,000 kWh.

COSCO expects each such ship could reduce carbon dioxide emissions by almost 3000 tonnes per year. Named COSCO Shipping Green Water 01 and COSCO Shipping Green Water 02, respectively, it has been reported that the former will be used in the Yangtze River shipping goods from Jiangsu to Shanghai.

Work is still progressing on ammonia-powered fuel-cells for ships. Although conventional fuel oil has a higher energy density, ammonia is easily stored in bulk as a liquid at modest pressures (10–15 bar) or refrigerated to –33 °C. Ammonia also benefits from an existing near-global distribution network, in which it’s stored in large, refrigerated tanks and transported around the world by pipes, road tankers and ships.

The world’s first clean, ammonia-powered contained ship was announced in November 2023

The world’s first clean, ammonia-powered container ship was announced in November 2023 in a joint venture between Yara Clean Ammonia, North Sea Container Line, and Yara International. A 1400 TEU ship named Yara Eyde, the vessel will – when it starts operation in 2026 – be the first to sail emission-free on a route between Norway and Germany.

As for hydrogen, 2021 saw the launch of the MF Hydra – the first commercial passenger and car ferry running on liquid hydrogen. Powered by two 200 kW fuel cell modules, the ship – built by Norled AS – can carry up to 295 passengers, eight crew members and 80 vehicles. It currently sails on a triangular route between Hjelmeland, Skipavik and Nesvik. In fact, Norway has ruled that all ferries, tourist boats and cruise ships travelling on its World Heritage fjords must be zero-emission by 2026.

Another company at the forefront of developments is Samskip, which was awarded a contract worth NKr149m ($14m) from Norway’s government to develop two SeaShuttle hydrogen fuel-cell vessels in 2022. The company claims that the pair will be the first green hydrogen-powered container vessels of their size for short-sea journeys. The 135 m-long 500 TEU vessels run on 3.2 MW fuel cells and will also have diesel backup generator sets. Delivery is due next year.

Green future

The shipping industry is building up valuable experience in these new technologies and does not seem to be shying away from the challenge. But will these technologies scale up, how long will it take – and at what cost? Those are the big questions given the huge number of vessels currently sailing on the world’s oceans and the fact that they still have very long operational lives.

It will take a long time to build new ships or refurbish existing vessels to make them greener. But the direction of travel for shipping is becoming clearer.