Air travel plays a critical role in the interconnectedness of our world, but emissions from aviation have doubled since the 1980s, with flying responsible for more than a billion tonnes of CO2 pumped into the atmosphere every year. If we’re to avoid the worst effects of climate change and keep the global temperature increase below 1.5°C, something needs to change.
Many ‘greener’ alternatives have been explored, but few present a feasible long-term solution to the problem at hand. Batteries work in theory, but their range and heavy weight make them unsuitable for long journeys, therefore limiting their application to distances that could easily be covered by other modes of transport, such as trains. Solar-powered planes have also been trialled, but they can’t generate enough power based on the efficiency of current solar cells alone, so they’re also reliant on batteries.
Other alternatives include synthetic and biofuels, which come with their own sustainability issues. Burning these still produce emissions, for example, and in the case of biofuels – which are derived from crops – huge swathes of farmland and forest would be needed to ensure an adequate supply.
“Using hydrogen as a fuel produces zero carbon emissions – the only by-product is water vapour”
As the world increasingly turns its attention to the climate crisis, it’s not enough for the aviation industry to make things more efficient; it needs a completely new way of doing things. And hydrogen could be part of the answer.
“Using hydrogen as a fuel produces zero carbon emissions – the only by-product is water vapour,” explains Valeska Ting, Professor of Smart Nanomaterials at the University of Bristol. “And you only need a power source and water to create it. Every country has access to a power source of some kind, whether it’s solar, wind, geothermal or waste heat, so it’s a very equitable fuel, too.” Hydrogen is also extremely light, and contains three times as much energy per kilogram than jet fuel, making it a potentially attractive proposition for long haul flights, she adds.
Valeska’s research looks at the best way to turn hydrogen into a feasible aviation fuel source, and there are some challenges involved. Hydrogen is the lightest known chemical fuel, and exists as a gas at room temperature, so it takes up a lot of space and it can’t be simply poured into a tank like traditional jet fuel. As such, she and her team are working on developing nanoporous materials that could hold the gas inside a plane’s fuel chamber.
“Using nanoporous material as a storage mechanism means you don’t necessarily have to have one tank to hold the fuel”
“Nanoporous materials are essentially just materials with pores so small they can’t be seen by the naked eye. They work like a sponge – the hydrogen comes in and is sucked into the tiny pores of the material and the hydrogen is therefore compressed into a smaller space. And since the hydrogen is now stored inside a ‘solid’ material, it can be stored using lower pressures, making it safer.” The interaction between the hydrogen and the material is sensitive, Valeska explains, so the gas can be released from the material simply by increasing the temperature, or by lowering the pressure.
This different style of powering a plane comes with a host of design opportunities, too. “Hydrogen fuel tanks would typically have to be larger, and because hydrogen is such a small molecule and can therefore escape quite easily you’ll need different linings,” says Valeska. “But using nanoporous material as a storage mechanism means you don’t necessarily have to have one tank to hold the fuel. You could use composite materials to build the storage materials into the fuselage, for example. Planes wouldn’t necessarily need to be a tube with wings sticking out of it – there’s the potential for doing something quite radical.”
Indeed, Airbus has revealed a number of concept designs for its hydrogen-powered aircraft, which it hopes to be operational by 2035. One includes a futuristic ‘blended-wing body’ design, comprising an ultra-wide triangular fuselage – not unlike a giant samosa – capable of holding up to 200 passengers.
But while the technology is making rapid progress, there are some wider challenges at play. For a start, we don’t yet have enough infrastructure to deliver the volumes of hydrogen needed to support our current aviation requirements. “At the moment, the way we produce hydrogen is largely as a by-product of the fossil fuel industry, so we’d need a concerted effort to increase the supply of truly sustainable hydrogen, through renewable sources.”
This would mean greater investment in renewable technology by governments around the world, although there are some specific opportunities for hydrogen production in this way. “Let’s take wind power,” Valeska says. “If a company is generating energy when no-one is using it they can’t store it easily, so they would opt to turn the turbines off. In those instances, we could pass the electricity through an electrolyser to create hydrogen which could be stored, and so we’d be using electricity that would otherwise be wasted.”
“A lot of people think it’s unsafe, but it is no more so than many of the fuels we already use every day”
There are also some issues around public perception of hydrogen. “A lot of people think it’s unsafe, but it is no more so than many of the fuels we already use every day,” says Valeska. “There are just different risks involved.” Indeed, hydrogen is highly flammable, for example, but in the event that there is an ignition hydrogen will flare for a short burst before it burns away. Traditional jet fuel, by comparison, burns slowly and may continue to burn for hours. “It’s a different problem, but one that represents opportunities for even safer air travel if we’re able to redesign systems from the ground up.”
But despite the clear benefits of hydrogen, Valeska says that the aviation industry is facing something of a ‘chicken and egg’ dilemma. “There needs to be a lot of investment to support hydrogen-based aviation, but that won’t happen without demand, and the demand won’t come until it’s been a proven technology.”
However, fuelled by the increasingly urgent climate conversation, the tide is slowly turning. “Some countries like France and Germany are really leading the charge on heavy investment in this area,” says Valeska. “Companies like Airbus are showing what’s possible, and research like we’re doing at Bristol backs it all up. I think hydrogen-powered commercial aviation could become a reality in 10 years or so.”
Prof Valeska Ting is a Professor of Smart Nanomaterials in the Department of Mechanical Engineering, and has a background in materials synthesis, characterisation and physical properties testing. She currently holds a prestigious EPSRC Research Fellowship in Energy Materials and sits on the EPSRC Strategic Advisory Committee for the cross-council Energy Theme.