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Cargo ships could switch to renewable fuels, but it ain’t cheap (yet)

Using Europe as an example, the technical feasibility looks promising.

Scott K. Johnson – | 166
Credit: Darren Hillman

The transition to clean energy involves some markets where we have solutions that are no-brainers (more wind and solar, please) but also some head-scratchers. Maritime shipping falls in the latter category. Barring a return to sailing, these vessels will continue to require fuels with relatively high energy density to cheaply move goods over long distances. So how do we clean up an industry that runs on heavy fuel oil?

A new study led by Boris Stolz and Maximilian Held at ETH Zürich analyzes plausible options for the fleet of cargo ships that operate within Europe. The researchers' idea was to take real shipping data from 2018 and calculate the impact of changing out each ship's propulsion system. For ships carrying heavy loads, Stolz and Held set a benchmark of ditching no more than 3 percent of the ships' cargo in order to install potentially emissions-free propulsion systems. From there, the researchers found out how many voyages could still be completed—and at what cost.

Renewable options

The team evaluated propulsion systems using hydrogen, ammonia, methane, methanol, and diesel—all made from renewable feedstock and using clean electricity. That includes sourcing the carbon for methane, methanol, or diesel from captured atmospheric CO2. The researchers also considered internal combustion engines and two kinds of fuel cells (proton-exchange membrane or solid oxide) with electric motors for turning those fuels into motion. Lithium-ion batteries were included, as well.

These scenarios are speculative to varying degrees, given that most rely on immature technologies at some step. The speculation is particularly strong when it comes to their price tags. For example, atmospheric carbon capture is in its infancy, with only a handful of demonstration plants operating around the world, making it quite expensive.

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The study attempts to account for the full supply chain rather than just the equipment needed on the ship. That means that the amount of electricity required to produce each fuel has an impact on cost, as well as the requirements for storage and transport (which is trickier for hydrogen and methane).

Allowing for a 3 percent cargo reduction and assuming ships would only carry as much fuel as they need, the number of 2018 shipping voyages that could be handled (without stopping to recharge/refuel) varied. Current lithium-ion batteries could only cover a small number of voyages—though possible battery advances could push that number closer to half.

The chemical fuels are considerably more energy dense than batteries, so liquefied hydrogen was suitable for about 93 percent of those voyages. Ammonia, methane, methanol, and diesel each cleared 99 percent. And if you stretch to allow a 6 percent cargo reduction, even hydrogen can hit 99 percent of shipping.

data figure
Energy densities of each option (bottom) and the percentage of 2018 voyages covered (top). The orange curve shows the benefit of only loading as much fuel as is required for the trip.
Credit: Stolz et al./Nature Energy
Energy densities of each option (bottom) and the percentage of 2018 voyages covered (top). The orange curve shows the benefit of only loading as much fuel as is required for the trip. Credit: Stolz et al./Nature Energy

A matter of price

The basic technical feasibility is pretty good, with manageable tradeoffs. The real obstacle is cost. Many of these technologies haven't scaled yet, so any careful analysis will show them to be far more expensive than currently available fuel and engines. The researchers estimate the near-term total cost of ownership to be around two to six times greater. Absent generous public subsidies, this wouldn't be a financial investment.

The high cost isn't set in stone, though—if prices go down as the technologies mature, cost isn't necessarily the showstopper it might seem.

Among the fuel options, ammonia stands out as the cheapest, while renewable diesel is the most expensive. Although it's not the only factor in the cost, there are significant differences in the amount of electricity required to produce each—some fuels allow relatively efficient conversions of electrical energy into chemical energy.

Hydrogen made by splitting water looks good on that measure, but this bit of economy is outweighed by the cost of storing and transporting this fuel. Ammonia is similar in efficiency—it would be produced from hydrogen and atmospheric nitrogen—and is much easier to transport. The researchers highlight ammonia and methanol as the two most promising options in this analysis.

If these fuels were fully adopted, a substantial amount of electricity would be needed to produce enough of them. The analysis shows that this new industry would increase electricity consumption in Europe by about 4 to 8 percent. (For comparison, the shipping industry is currently responsible for about 3 percent of global greenhouse gas emissions.)

Building more renewable energy generation to meet this demand is straightforward enough. But these fuel technologies must develop into mature industries with much lower costs. If that happens, doing away with fossil heavy fuel oil may not be such a heavy lift.

Nature Energy, 2020. DOI: 10.1038/s41560-021-00957-9 (About DOIs).

Listing image: Darren Hillman

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Scott K. Johnson Associate Writer
Scott has written about geoscience and energy at Ars as a freelancer since 2011.
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