Alternative fuel technologies can bridge the gap

Many alternative fuel technologies are available for reducing the GHG emissions of shipping. For alternative fuels and power sources, the technical applicability and commercial viability will vary greatly for different ship types and trades, where deep-sea vessels have fewer options compared with the short-sea segment. It is important to find technically feasible and cost-effective solutions for the deep-sea segment, accounting for more than 80% of world fleet CO2 emissions. Currently, the only technically applicable alternatives for this are liquefied natural gas (LNG) and sustainable advanced biofuels. 

Apart from biofuels, efforts to substitute fossil fuels with carbon-neutral fuels depend heavily on access to non-combustible renewable energy sources. The term carbon-neutral refers to a variety of energy sources or energy systems that have no net GHG emissions or carbon footprint. Apart from biofuels, electricity from renewables (or from zero-carbon sources like nuclear) used in maritime battery applications is currently the only commercially available alternative for carbon-free shipping. This is presently limited to short trades up to approximately one hour; in practical terms this also means for (very) small ships. For the majority of global shipping, battery applications do not provide enough energy to cover the entire length of voyages. 

An alternative energy carrier is hydrogen (H2) produced from carbon-neutral energy resources, such as electricity from renewables. Alternatively, carbon-neutral H2 can be produced from natural gas (with carbon capture and storage) or from nuclear energy. Using compressed or liquefied H2 in fuel cells is a realistic option for the short-sea shipping segment in the medium term. 

Hydrogen can itself be the basis for different electrofuels. Electrofuels, sometimes referred to as e-fuel, is an umbrella term for synthetic fuels such as diesel, methane and methanol when they are produced from H2 and CO2 (carbon-based fuels), or from H2 and nitrogen (nitrogen-based fuels), and when renewable electricity powers the production. 

Biofuels and carbon-based electrofuels are drop-in fuels requiring only limited or no modification to engines and fuel systems to replace or blend with traditional fuels used by internal combustion engines. Nitrogen-based electrofuels such as ammonia can also be produced from H2; but they require more moderate modification to engines, and to fuel storage and supply systems, to replace traditional fuels. While electrofuels have clear advantages with regards to technical application and GHG-footprint, producing them is currently expensive and energy intensive. For biofuels, the challenges are related to price and sustainable production in sufficient volumes. 

Widespread adoption of low-emission and carbon-neutral fuels could potentially take a long time, factoring in the time needed to properly develop low-carbon fuels, production capacity and infrastructure and to scale this. This study therefore introduces ‘bridging technologies’ that can facilitate and ease the transition from traditional fuel, via fuels with lower-carbon footprints, to carbon-neutral fuels. The bridging philosophy is built on three flexibility pillars, as illustrated in the figure below. Fuel-flexible energy converters are essential as bridging technologies. However, but fuel-flexible arrangements for onboard storage and supply systems (allowing fuel switching), as well as flexible shore-side fuel infrastructure, are also needed.