The move away from fossil fuels towards a greener, more sustainable future is not just for land-based vehicles. An estimated 11 billion tons of cargo is moved by sea every year across the globe, shipping everything from mined ores to food.
However, the impact of fossil fuels on climate change means that shipping has a big impact on the environment. In 2023, the International Maritime Organization released its strategy aiming to reduce the amount of greenhouse gases from ships burning fuel.
What currently powers cargo ships?
The days of sailing ships for commerce and cargo are long gone, replaced by steam engines in the 19th century. With advances in steam turbine and petrochemical technology, ships started running on oil or diesel rather than coal-fired steam.
More efficient than coal, it revolutionised the way maritime cargo and travel was done in the early 20th century, with faster speeds and more reliable timekeeping. Today’s ships are powered by huge two-stroke diesel engines producing immense torque to push their loads.
However, over a century after the first diesel engine on the Russian vessel Vandal was launched, maritime companies are looking to other sources of fuel to propel them across the waves.
What fuels could be used to power ships?
Concerns about climate change and fuel supplies are forcing shipping companies to rethink how their ships travel. Reducing fossil fuel emissions and greenhouse gases is a priority, and alternative fuels and technologies can offer a solution.
Liquefied Natural Gas
Liquefied Natural Gas, or LNG, reduces emissions significantly. It offers shipping companies a way to align their assets with International Maritime Organization regulations on greenhouse gases and sulphur.
LNG is a very efficient fuel choice, allowing longer range, and the infrastructure for refuelling is already being put in place. Safe storage, handling, and transfer of fuel are concerns for the industry.
Methanol
Methanol is a highly flammable yet readily available fuel that has potential as a clean energy source for shipping. Used in a similar way to diesel and natural gas, it requires combustion to release energy but gives off fewer emissions than traditional fossil fuels.
The fuel is easier to store and transfer than other alternative fuels such as ammonia. However, the need for a low flash point means additional safety valves and suppression technology would need to be added to comply with regulations.
Ammonia
Often used in dual-fuel engines, alongside traditional oil-based fuels, ammonia has a high energy density, which means less is needed for the same power output. However, refuelling is difficult due to lack of fuel storage and infrastructure, plus specialist facilities are needed.
Hydrogen
Hydrogen is a zero-emission option for alternative maritime fuels, producing only water vapour and powering electric engines. Although maritime engineers and organisations are still in the early days of experimentation and adoption, it shows potential.
Storage is a concern both for on board and bunkering, as much larger containers are needed, requiring a redesign of the ship and fuelling stations. It can be stored as both gas in pressure tanks or as cryogenic liquid or absorbed into a solid state material.
Biofuels
A potential winner for alternative fuels is biofuels such as hydrotreated vegetable oils, biomass to liquid and other plant-based fuels. They are sustainable and produce fewer emissions than diesel and heavy oils.
Biofuels do require specialist equipment for safe storage, including tanks and pressure valves that can withstand being at sea. Thankfully, these technologies already exist; however, the quantities of biofuels needed might be a hindrance to wide adoption.
Nuclear
Although predominantly used for naval ships and submarines, there are some examples of civilian ships that utilise nuclear steam generation.
The power source gives a much longer range than other fuel types, allowing them to stay at sea for longer, but they are expensive to build and maintain. This, combined with regulations, insurance and inspections, makes nuclear engines unsuitable for cargo.
How can ships be adapted to use alternative fuels?
For some alternative fuels such as LNG and biofuels, existing ships can be retrofitted to accommodate the new fuel type and onboard storage. However, for others, particularly nuclear fuel, it requires an entirely new vessel to be designed and built.
The lower cost of entry for retrofitting a combustion engine, over building a new vessel, means certain fuels will have an economic advantage and are more likely to be adopted widely.
Are there concerns about using alternative fuels for shipping?
There are several concerns regarding alternative fuels for maritime cargo transport.
- Fuel range – some fuels are currently only suitable for short distances
- Safety – regulations and safety measures need to be put in place to protect the crew, cargo and vessel, particularly with high-pressure and flammable fuels
- Bunkering, storage and infrastructure – these require investment and safety measures, particularly for gases stored in high-pressure tanks
- Supply and demand of alternative fuels
- Cost of conversion and new vessels
While technology, investment and adoption of alternative fuels is still in its infancy, there is hope that switching from fossil fuels can give shipping a more sustainable future.
