The UK government has enshrined net zero into law, but if it is to meet its targets then big strides need to be made towards decarbonising land-based transport, aviation, and shipping. Unlike most land-based transport, aviation and shipping are considered ‘difficult to decarbonise’. Their long-lived infrastructure, ongoing reliance on energy dense fuels, and issues around governance of their emissions, create particular challenges. In seeking viable and sustainable alternative fuels, hydrogen is being promoted as an option that might resolve some of the challenges around decarbonising transport. In this article, Professor Alice Larkin and Dr Amanda Lea-Langton consider if hydrogen really is the answer to decarbonising transport, if there are other alternatives, and how low carbon hydrogen use should be prioritised.

• Demand for hydrogen in transport is expected to increase from 1-4 TWh in 2030 to 75 -165 TWh by 2050 in the UK, according to government predictions.
• With a limited supply of green hydrogen currently available, it would be prudent to consider where its use is both most likely and appropriate.
• Policymakers should consider the availability of green hydrogen, who will benefit from green infrastructure policies and other initiatives to cut emissions in the short-term to ensure effective and just net-zero transport policies.

Hydrogen: a suitable alternative for heavy-duty vehicles?

There are two methods to use the energy stored in hydrogen to power vehicles, namely through fuel cells or conventional internal combustion engines (ICE). Fuel cells use an electrochemical reaction between hydrogen and air across a membrane. The most common is called a PEM (proton exchange membrane) fuel cell. ICEs instead generate power through the burning of hydrogen or ammonia produced from hydrogen, in a similar way to a petrol engine. Both forms of propulsion produce water vapour as an emission, whilst ICEs also emit nitrogen oxides as byproducts.

Though produced by manufacturers, very few hydrogen-powered light duty vehicles – like cars, vans and taxis – have been sold in the UK. Instead, the preference is for electric battery-powered vehicles, particularly as costs fall and range improves. Hydrogen, on the other hand, faces poor refilling infrastructure, high fuel costs (around twice that of conventional fuel) and the inconvenience of needing large onboard storage tanks, reducing space for passengers and goods.

With electrification of private vehicles expanding rapidly, a more likely application for hydrogen is in heavy duty vehicles, such as buses and heavy goods vehicles (HGVs), either as hydrogen only or electric/hydrogen hybrids. HGVs have space for multiple storage tanks, and routes could incorporate hydrogen filling stations. Depending on fleet size, it can be feasible to install small scale hydrogen generation and storage at vehicle base stations. Indeed, hydrogen could be preferred on longer routes due to its higher range than batteries, or where vehicle downtime for battery recharging isn’t viable. However, using hydrogen in HGV fleets remains stymied by a lack of infrastructure, technology development, and costs.

Climate impacts of aviation and shipping

Shipping and aviation release a large proportion of their emissions over international airspace and waters, complicating governance arrangements. Globally, their levels of emissions are of a similar scale to a top ten emitting country and both are framed as ‘difficult to decarbonise’. Yet as our research has shown, when exploring options to cut emissions, they are different in almost every way, and in the case of shipping, perhaps not so ‘difficult to decarbonise’, after all.

When it comes to climate impacts, where in the atmosphere the emissions are released, matters. Emissions of CO2 are long-lived, becoming well mixed in the atmosphere. However, ships and planes create warming and cooling through their other pollutants, too. Measures to minimise sulphur emissions from ships by enforcing cleaner fuels near coasts are in place and likely to be strengthened; the UK government is currently consulting on the matter, but the challenge around aviation is more problematic. Aircraft release a large proportion of their emissions into sensitive regions of the atmosphere causing chemical reactions. The CO2 impacts from combusting kerosene are understood, however other emissions, including nitrous oxides, soot and water vapour have doubled the amount of climate warming from aviation, when compared with the CO2 alone. The timeframes that some of these impacts are sustained are short and geographically they do not become ‘well mixed’. This means that they haven’t been subject to policies to curb them, unlike sulphur from ships. So, any future alternative fuel choice must also minimise these harmful emissions, as well as CO2. Combustion of hydrogen would also produce water vapour and nitrous oxides, and the relative non-CO2 climate impacts are not well understood.

Mitigation measures for aviation and shipping

With both sectors needing to move away from fossil fuels, hydrogen has been gaining interest, either directly in a fuel cell or engine, or to create other fuels. Hydrogen has been muted as a ‘solution’ to decarbonising aviation for over two decades, but with its lower volumetric energy density, using it requires a redesign of aircraft to allow for a much larger fuel tank. With a slow turnover rate of the fleet, and constraints on airport infrastructure for handling aircraft of different designs, this route is unlikely to make huge inroads over the coming couple of decades. Hydrogen fuel cells for propulsion are even less likely, except for small planes, given the extra power required to uplift the fuel cells onboard.

Where hydrogen use is interesting is in creating e-fuels, a form of synthetic fuel that can be used as a fossil fuel alternative, using renewable energy. Methanol and ammonia can both be produced as e-fuels and offer benefits in terms of their energy density compared with hydrogen, although methanol requires carbon capture technology to be developed in tandem as a source of CO2. Crucially, both require sufficient renewable capacity to create the hydrogen in the first place. Furthermore, if fuel continues to be combusted in the aeroengine, the impact of other emissions released still need to be mitigated. Nevertheless, with few options to curb its CO2 emissions technologically, hydrogen will likely remain a focus.

For shipping, the picture is less clear. Although badged as ‘difficult to decarbonise’, there is already a suite of available options to cut ship emissions. Some are operational – such as slowing down or using different contracts and practice, while others, as illustrated by research from the Tyndall Centre for Climate Change Research, relate to technologies that can be retrofitted such as wing-sails, harnessing wind power and cutting fuel consumption. Even on the fuel side, with such diversity of ship sizes and types, shore-power with battery-electric propulsion is being adopted for some markets. And as it is cumulative emissions that dictate future climate change, these benefit from being able to deliver cuts now – unlike the prospect of large-scale green alternative fuels. Ammonia though, produced from green hydrogen, is a fuel of interest to the sector. Ships already transport ammonia for the fertiliser industry, and some ships are already ‘ammonia ready’ for when there is a steady green supply. Nevertheless, there are safety issues to overcome if this is to become widely adopted. Green methanol, too, is gaining interest – with some ships already combusting methanol – albeit at a small scale. It also seems unlikely that large amounts of green ammonia will be available to the industry for some time – not least because the biggest ammonia consumer – the agriculture sector, must ‘green’ its fertiliser too.

Recommendations and conclusions

Demand for hydrogen in transport is expected to increase from 1-4 TWh in 2030 to 75-165 TWh by 2050 in the UK, according to government predictions. This scenario includes use across HGVs, buses and coaches, as well as domestic and international aviation and shipping.

With a limited supply of green hydrogen currently available, it would be prudent to consider where its use is most likely and appropriate. Aviation has few technological alternatives, so could be prioritised for hydrogen use, given its limited supply. Yet with huge inequities in who uses aviation and for what, travel demand management must also be part of the picture, particularly when other sectors that are more equitably used by the population, need hydrogen too. For shipping, with its multiple options available to deliver near-term cuts, is hydrogen really the big deal it appears to be at first sight? Well perhaps in the longer term – and most likely to produce green ammonia – but scaling up production where there is a plentiful renewable resource will need to accelerate rapidly.

In light of these conclusions, the following recommendations should be considered.

• As short-term cuts in emissions fit with the climate science, the alternatives to using hydrogen that are likely to cut emissions most quickly – including demand management – should first be considered.
• Supply of green hydrogen should be prioritised for hard to abate sectors that underpin key functions of society, with electrification promoted for transport modes if technically and practically viable.
• An essential requirement to producing low carbon hydrogen is the greater availability of renewable electricity, a priority that would also support concurrent electrification.
• The UK government must revisit the previous administration’s plan for a 2024 zero emission HGV and coach infrastructure strategy to ensure it considers recent battery developments and demand, to plan any investment in refuelling infrastructure.
• Before investing in materials for storage and hydrogen transport airside at airports, who benefits from green infrastructure should be reviewed.
• Prioritise ship energy efficiency, including wind-assist retrofit, and support for shore-power, while the safety and environmental implications of using ammonia onboard ships remains poorly understood.
• Research investment is needed to find alternative hydrogen energy systems less reliant on critical rare elements, compared to current PEM fuel cells.