Solutions
There is now a range of alternative energy systems available for shipping and aviation, but which are
the most sustainable?
Decarbonising shipping and aviation?
Most shipping and aviation is currently powered by fossil fuels, heavy fuel oil for ships and kerosene in planes. Unlike sectors like road transport, household heating or power generation, long-haul shipping and aviation is not easy to electrify – but it is not impossible.
Battery-electric solutions for short air and sea journeys are being developed, while synthetic fuels – or “e-fuels” – where all the inputs are derived from renewable electricity can decarbonise long-haul voyages.
Even though these solutions can achieve nearly zero lifecycle carbon emissions, some people are still pushing for non-solutions that have limited environmental benefits – and in some cases may even be more damaging than the fossil fuels they’re seeking to replace.
The most sustainable solutions
Zero carbon emission ships and planes
As is the case across all sectors, the most sustainable energy system is one that directly uses electricity that has been generated from renewable sources.
Today, only small planes and marine vessels can be powered by electrical propulsion systems. Larger craft can also be powered by hydrogen converted to electricity in a fuel cell. If the hydrogen is produced via water electrolysis using renewable energy then these will also have zero associated carbon emissions across their lifecycle.
To reach commercial scale, policies need to be put in place that incentivise their uptake and the rollout of necessary fuelling and charging infrastructure.
Read our policy briefing on accelerating zero carbon emission flight.
Our members and knowledge partners
E-fuels
For long-haul journeys, the most sustainable alternative to fossil fuels is e-fuels.
For shipping, there are two main marine e-fuels, e-ammonia and e-methanol, which can both be near zero-emission fuels if produced sustainably.
E-ammonia is made of hydrogen combined with nitrogen. Since it contains no carbon, it does not release CO2 emissions when burnt. If hydrogen is produced using renewable electricity, then e-ammonia has no associated emissions further up the supply chain either.
However, hydrogen and nitrous oxide are themselves greenhouse gases that may be emitted at various points across the supply chain, while ammonia is an environmental and human health hazard and nitrogen oxides contribute to air pollution. This means rolling out e-ammonia will need to be accompanied by stringent regulations to minimise the negative effects of these gases.
Using e-ammonia at scale will also require new infrastructure and engines on ships.
E-methanol is made of hydrogen and carbon. This means that it does release carbon emissions when burnt, but if the carbon it is produced with comes from a sustainable source then it will have net zero carbon emissions across its lifecycle. Methanol is largely compatible with existing infrastructure, and is being used already on some ships whose engines have been modified to accommodate it.
For aviation, e-kerosene is the most sustainable drop in fuel to replace fossil kerosene for long-haul flights. Like e-methanol it requires a sustainable source of carbon and renewable hydrogen. While it has net zero carbon emissions, it still produces negatively impactful non-CO2 emissions.
Given e-fuels are more expensive that fossil fuels, policy is needed to narrow the price gap between them and conventional fuels, and to fund innovation to see costs reduce.
Our members and knowledge partners
Non-solutions
Biofuels
Biofuels are made from biomass, and have a wide range of environmental impacts depending on where the material they are produced from.
The most easily accessible kind of biofuels are produced using crops that would otherwise be eaten. These have high associated carbon emissions from land use change (in some cases more than from fossil fuels), and dire consequences for biodiversity and food systems.
Advanced biofuels are produced with waste products such as used oils or agricultural waste. While these may have significantly lower carbon emissions compared to fossil fuels, truly sustainable feedstocks are severely limited in supply, meaning they are not a reliable energy source for decarbonising shipping and aviation.
Read our report on the biodiversity impacts of biofuels.
Fossil liquified natural gas (LNG)
Fossil LNG has been touted as a sustainable “transition” fuel – but it is itself a fossil fuel. While LNG does produce less carbon dioxide at the point of combustion, it consists mainly of methane, which is a greenhouse gas around 30 times more potent than carbon dioxide over a 100-year period. Methane is prone to slip or leak across the LNG lifecycle. For this reason, e-LNG, an e-fuel produced using renewable hydrogen, still causes huge climate damage regardless of its production process.
Read Opportunity Green’s report on the use of LNG as a shipping fuel.