Is hydrogen the fuel of the future?

As the global aviation industry reeled from the impact of Covid-19, with thousands of jets grounded and passenger numbers down by 60 per cent, a bold initiative began at the Aerospace Technology Institute (ATI) in Cranfield, England. Early in 2021, a team of 100 experts from across the UK launched a one-year intensive research project to investigate the best route to zero-emission commercial flight.

The FlyZero project examined key aspects of developing zero-emission aircraft, ranging from design challenges to operational requirements, and compared the potential of electric batteries, hydrogen and ammonia for the task. It found green liquid hydrogen (produced using electricity from renewables) was the most viable zero-​emission fuel for commercial aviation and concluded that hydrogen aircraft have the potential to replace all short-haul flights and 93 per cent of long-haul ones.

FlyZero is one of several cutting-edge research programmes pointing to green hydrogen as a potential solution for hard-to-decarbonise transport modes that are highlighted in 'Destination Net Zero', a joint report by the IET and the Engineering and Physical Sciences Research Council. The report, which reflects the views of leading experts on the research challenges that need to be addressed for the UK to achieve net-zero carbon emissions by 2050, is due to be published to coincide with COP27 – the 27th UN Climate Change Conference, which opens on 6 November in Sharm El Sheikh, Egypt.

The UK was the first major industrialised country to set legally binding targets for cutting greenhouse gas (GHG) emissions. The government introduced secondary legislation in 2019 requiring all sectors of the economy to achieve net zero by 2050. This ambitious target poses an extraordinary challenge to all economic sectors but especially transport, which is the single biggest contributor to domestic GHG emissions in the UK, contributing 27 per cent of the total in 2019. With road vehicles accounting for more than 90 per cent of all transport emissions, electric vehicles (EVs) and charging systems have been a major focus of decarbonisation efforts to date, and electric cars and vans are now at a fairly advanced stage.

But in modes where the technological pathway to net zero remains unresolved, such as aviation, maritime transport, road freight and other large, land-based vehicles, the challenge is huge and urgent, prompting intensive R&D ranging from fundamental science to demonstrator projects and 'living labs'. For hard-to-decarbonise transport modes, experts advocate a two-pronged approach, with lower-carbon drop-in fuels used in the short term, while zero-emission technologies are identified and developed for the longer term.

The Department for Transport (DfT), which is leading the drive to slash emissions, is taking a 'technology neutral' approach to such modes in its masterplan 'Decarbonising Transport: A Better, Greener Britain', which was published in July 2021. The strategy involves pursuing promising avenues for R&D across all types of fuel and technology, and then picking a potential winner based on results. Following this approach, the DfT launched zero-emission road freight trials of three options: battery electric, hydrogen fuel cell and overhead catenary, as part of a £377m funding programme for decarbonisation R&D in 2021.

However, Phil Blythe, professor of intelligent transport systems at Newcastle University, argues that the government needs to give stronger leadership and support for R&D in hydrogen technology as a key option for decarbonising large vehicles. He maintains that the UK is lagging behind other industrialised countries – such as Australia, Japan, China and many European states – in hydrogen technology because the investment risks are too high.

"If they gave that clear steer, I'm convinced the innovators in hydrogen would be there," says Professor Blythe, a former chief scientific officer at the DfT. "So, we need more certainty that hydrogen will have a big role to play, not just in transport but in big, industrial, energy-intensive processes.

"If we think about large vehicles such as buses and lorries and off-road vehicles, a lot of those won't be suitable for batteries because they require massive amounts of torque to run the service, so we have got to look at other things. One of the key options for decarbonising transport for those larger vehicles is to look at hydrogen."

With the notable exception of short-range electric vertical take-off and landing aircraft, batteries have limited potential for zero-emission aviation, says Mark Scully, the ATI's head of technology, advanced systems, and propulsion. "With the vertical take-off battery system, you are looking for power density, because you need an enormous amount of power for take-off and landing," he says. "They are relatively short duration flights, so the energy density is less important, although it is a limiting factor on range and payload. It is not going to be a mass transport service, in my view, because they are relatively small scale, relatively niche.

"But for larger, longer-range aircraft, carrying more passengers, we need a lot more energy density. Typically, today, we are seeing something like 250Wh/kg as a battery energy density, while kerosene is greater than 12kWh/kg.

"We see hydrogen with its significantly higher energy density (33.6kWh/kg) offering an opportunity, in terms of fuel cells being used to convert the hydrogen to an electric propulsion system, and we are supporting the development of this technology. But fuel cells only get you part of the way on this journey. For larger aircraft, fuel cells and the associated systems become less competitive, and aircraft will need to burn the hydrogen in gas turbines to realise the benefits."

For rail, extending electrification, which currently accounts for 38 per cent of track nationwide, is "likely to be the main way of decarbonising the majority of the network", according to the DfT's masterplan. Both battery and hydrogen trains are being trialled for low-use routes, where electrification is not cost-effective.

Stuart Hillmansen, professor of sustainable traction systems at the University of Birmingham, says hydrogen trains are the best solution for low-use sections of track because of the topology of the UK's rail network.

Birmingham was the first university to show that hydrogen is a viable fuel and energy source for rail applications in certain environments, and its Centre of Excellence in Rail Decarbonisation (CERD) developed the prototype HydroFLEX train, a hydrogen battery train with a pantograph, in partnership with rolling stock company Porterbrook. The train's second version, which was showcased at COP26, can run for about 300 miles on hydrogen alone.

"Ultimately, it is about finding the right solution for the particular part of the railway system that you are interested in," says Professor Hillmansen, who is a lead engineer at CERD. "There are battery electric multiple units that are being developed by major manufacturers, and they would work really well with gaps of about 50 to 80km, so if you have those sorts of distances between electrification, they can work really well.

"If you had a railway similar to Germany, then a battery electric unit with a pantograph plus a battery would be the ideal solution. Whereas in the UK, we don't have that – we've got electrification and it all spans out from London. The routes that we are looking at in the UK may have electrification at the start of them... but then the vehicles may have to run several hundred miles without using electrification – and that is where hydrogen comes in."

Both hydrogen and ammonia are in early trials as fuels for maritime transport, which is arguably the hardest mode to decarbonise due to its international nature, economic model, and wide variety of vessel types. But Alice Larkin, professor of climate science and energy policy at the University of Manchester, says there is no leading candidate as the optimum zero-emission fuel for the longer term, with uncertainty both over which fuel it will be and whether it will be one fuel or multiple options.

Professor Larkin says transporting fossil fuels accounts for about 15 per cent of maritime cargo, while ships have generally burnt "the fuel that nobody else wants" – dirty, heavy fuel oil. The replacement of fossil fuels with renewables leaves the shipping industry with hard questions about whether it will be "lowest down the pecking order again" in the choice of fuel or will have a role in "actually leading a market in what the new fuel might be", she says.

With ammonia used to transport hydrogen, and green ammonia produced from hydrogen through a modified version of the Haber-Bosch process, the two fuels are deeply intertwined and their potential role in shipping is reliant on green hydrogen production levels, which are currently very low worldwide.

Manchester University is conducting a study for the International Chamber of Shipping looking at how much potential there is for an international trade in hydrogen. Such potential might enable the shipping industry to take a leading role in developing the market. "This is where the interest in hydrogen and ammonia lies, because it only works for the shipping sector, if there is a trade in hydrogen to transport as ammonia," says Larkin.

Alongside transport, green hydrogen is being investigated for other potential uses including manufacturing, heating and energy storage, and together they are expected to form an integrated hydrogen economy. Low-carbon hydrogen production is being stepped up to support growth of the hydrogen economy and the government has set a capability target of 5GW by 2030 in the UK Hydrogen Strategy. The DfT aims to create regional hydrogen transport hubs – typically close to ports and airports – that encompass energy production and industrial uses to serve as 'living labs' for zero-emission transport. The approach is being spearheaded by the Tees Valley Multi-Modal Hydrogen Transport Hub, which received £3m from the DfT in 2021.

There are many R&D barriers to be overcome before vehicles powered with green hydrogen can become a reality across modes. The high energy consumption – and hence cost – of producing green hydrogen needs to be reduced. Production, storage, transmission and refuelling infrastructure will need to be developed, along with safety measures and standards. The efficiency of hydrogen fuel cells needs to be improved, and problems with the heat exchanger resolved, while manufacturing capability needs to be developed. But the scale – and stage – of the R&D challenges vary hugely between transport modes.

For rail, the main barriers lie in the deployment stage, says Hillmansen. "The technology is ready, and it can be deployed," he says. "The main barrier is getting the technology to be trialled. Basically, the Department for Transport needs to commission a hydrogen railway somewhere. A route needs to be identified and there would be an order for a fleet of hydrogen trains and then they would operate over a route for 10 years or something. That would give industry a guarantee that they would have a customer for the vehicles for a decent amount of time."

For aviation, however, there are fundamental scientific challenges to solve before development can begin. Scully points to two major knowledge gaps: the potential impact of water vapour and other emissions from hydrogen aircraft on the atmosphere and materials characterisation for cryogenic hydrogen in contact with other materials used in aircraft components.

"What we don't know is, if we adopt hydrogen, whether... it will have an impact in terms of the contrails – or condensation trails – that we see in the sky," he says. "We are putting that water into a different part of the atmosphere to ground transport. On the ground, it might be a local climate science problem, whereas we are putting it into an area where it is a lot more difficult to model."

Scully says there are lots of challenges with burning hydrogen and the non-CO2 emissions that you get from it, although this will be addressed by an intensive research and technology (R&T) campaign. "There is some work to be done in terms of understanding what happens in terms of nitrous oxides and things like that," he says.

"And there are other fundamental challenges in terms of how you store liquid hydrogen and get the cryogenic hydrogen from the fuel tank to the engine. It's everything from the metals and composites to sealing materials and insulation systems. Globally, our knowledge of the interaction of hydrogen with materials is quite limited."

The government's ambition is for the UK to become a global leader in the production of green transport technology, exporting carbon-reducing products and know-how to the rest of the world, so cutting-edge R&D into hydrogen transport continues to garner support.

A further £685m in government funding for FlyZero was announced in April 2022 along with the establishment of a new expert Zero Emission Flight Delivery Group to examine the technology, infrastructure and regulation needed to make zero-​emission flight a reality.

As the world faces the growing effects of climate change and the dramatic changes in energy supply, technology and infrastructure needed to tackle it, a resolute focus on innovation aimed at zero-emission transport could yet produce some radical solutions that pay unexpected long-term dividends.

Newcastle University is involved in a demonstration project at RAF Leeming in Yorkshire to convert an airside tug from diesel to hydrogen with industrial partner ULEMCo, which proved the concept of 100 per cent hydrogen combustion with zero emissions in 2019. Evaluation of this initial finding is ongoing.

"We are putting all the sensors on the tailpipe to see if there are any emissions when that happens," says Newcastle University's Blythe. "If you can get that right, you could enable millions of vehicles with diesel engines to be converted to work on hydrogen, which is a real prize because it means that you are not having to build new vehicles – you can retrofit old ones thereby saving on new embedded carbon."