Exactly a century ago, in 1923, the eminent biologist J.B.S. Haldane wrote an essay called “Science and the Future”.

Four hundred years in the future, he predicted, England would run on hydrogen. A network of windmills would produce energy that would split the oxygen from the hydrogen in water; the hydrogen would be liquified and kept as fuel. That fuel could be used for industry, heating, lighting, and transportation. In particular, he mentioned “its use in aeroplanes”.

Haldane didn’t foresee an event just 14 years later in 1937 that dealt a near-fatal blow to the possibility of using hydrogen for aviation. The swastika-emblazoned German airship LZ-129 Hindenburg – the biggest ever such vehicle, filled with 140,000 cubic metres of highly-flammable hydrogen – dramatically exploded as it attempted to dock in the US state of New Jersey, killing 36 people.

Of course, the Hindenburg was using all that hydrogen – which is lighter than air – to help it float, not as jet fuel; its engines were powered by diesel). But the infamous photograph of the explosion had a serious chilling effect on the idea of stocking flying machines with the notoriously volatile gas.

But are we finally looking at a new advent for hydrogen-powered flight? This week, the US company Universal Hydrogen posted a video on Twitter of its new passenger plane, fuelled using ‘green hydrogen’, successfully taking off and landing at an airport in Washington state. Other companies, including some in the UK, have also recently demonstrated their hydrogen planes – and in February, the UK government announced £113m funding for “hydrogen and all-electric flight technologies”.

Hydrogen is normally obtained from reactions like the burning of coal or methane, which produces carbon dioxide and other pollution. But the ‘green’ kind is so-called because it comes from breaking apart water molecules in exactly the way Haldane suggested, with his vision of windmills back in 1923. Producing the hydrogen using renewable energy isn’t so damaging to the environment, nor is burning it in the plane’s engine: the main thing produced there is water, too.

Paul Eremenko, chief executive of Universal Hydrogen, is bullish about the prospect of hydrogen planes. In his company’s video, which quickly went viral, he stated that “as early as 2025” passengers will be able to take a “guilt-free”, environmentally-friendly, “hydrogen regional flight”. And in his accompanying tweet he added: “No, it’s not the Hindenburg.”

Hydrogen Regional Airliner test flight Universal Hydrogen Image via Kristen Georgette
The Universal Hydrogen test flight (Credit: Universal Hydrogen Image via Kristen Georgette Copyright: Francis Zera)

Technically these recent advances aren’t the first examples of hydrogen-powered flight: in the late 1980s, the USSR flew an experimental airliner – larger than that used by Universal Hydrogen or any of the new companies – called the Tupolev Tu-155, which used hydrogen fuel. But it only ran a few test flights to trial different types of fuel (it later flew using natural gas), and the collapse of the Soviet Union put paid to any further development.

Could we really get to the point where planes are using entirely clean fuel to take us on a hydrogen holiday? The first part of the answer is probably not for a while. What wasn’t mentioned in Universal Hydrogen’s video was that their plane flew for only fifteen minutes – time to do a couple of circles of the airport, but hardly enough to take you to Mallorca.

The main reason for that comes down to the physics of hydrogen. Compared with kerosene, the standard kind of jet fuel, hydrogen contains much more energy by mass. So for the same weight of hydrogen, you get a lot more energy than you do with kerosene. But here’s the problem: it has much lower energy by volume – something like a quarter of the energy than you’d get for the same volume of kerosene. That means you have to have a very high volume of hydrogen on your plane to get anywhere, which, in turn, means you need a lot of space for fuel storage.

And to add another difficulty: hydrogen fuel needs to be stored as a liquid, and to get it to be a liquid, it needs to be cooled to -253 degrees Celsius. All that refrigeration equipment is heavy, weighing down your plane and meaning – in addition to the space taken up by the hydrogen itself – it carries fewer passengers.

Michael Liebreich, a clean energy expert and adviser to the UK Government, estimates in a sceptical essay that enough hydrogen for a long-haul flight would take up as much space as the entire fuselage of a passenger plane. He thinks this makes the idea “a non-starter”. For short-haul flights, he reckons the hydrogen would take up about a third of the fuselage – and since this means fewer passengers, he thinks we can expect “a doubling or tripling of prices”. That’s in sharp contrast to Universal Hydrogen’s Paul Eremenko, who described the hydrogen flights as “more affordable” – although he didn’t provide any reasoning.

To add insult to injury, getting enough hydrogen to the airport is also something of a logistical nightmare. There isn’t technology to keep pipelines cooled to the required temperature, Liebreich argues – which means we’d have to rely on tankers and trucks full of liquid hydrogen on our roads in very large numbers, increasing the likelihood of an accident. To quote an explainer from NASA, “even small amounts of liquid hydrogen can be explosive when combined with air, and only a small amount of energy is required to ignite it”. We might not be talking about the Hindenburg, but the same questions about safety apply.

“The bottom line,” Liebreich writes, “is that liquid hydrogen could perhaps end up powering a few executive jets… but not aviation as we know it.”

Even “green hydrogen” loses its lustre upon closer inspection. The process of electrolysis that’s used to produce hydrogen from water is currently expensive – and that might explain why only 1 per cent of the world’s hydrogen supply is produced this way. The Financial Times reported in 2021 that when it comes to green hydrogen, there is “scepticism over its efficiency and whether enough can be made using renewable electricity at a commercially viable price”.

J.B.S. Haldane didn’t think we’d have useful hydrogen-powered planes until the year 2323. If companies like Universal Hydrogen are to be believed, we could beat his projection by about three centuries. But the arguments of the hydrogen sceptics are difficult to ignore: they’re based on the basic science of how hydrogen works – and the inescapable fact that you need an awful lot of hydrogen to get your plane any appreciable distance. Maybe we won’t have to wait three hundred years for a hydrogen holiday – but it could still be a lot longer yet.

Jet fuel of the future

Other than hydrogen, here are the major candidates:

  • Biofuels: These can be produced from crops like soybeans or rapeseed oil, from waste cooking oils, or even from sewage. They’re less carbon-intensive than fossil fuels, but if they come from crops, they need lots of farmland and water.
  • Synthetic e-Fuels: Hydrocarbons can come from a sustainable process that starts with reacting carbon dioxide with water. But at the moment, it’s a difficult process: synthetic e-fuels need more energy to produce than they make when burned.
  • Ammonia: Ammonia can be made with renewable hydrogen, but is much less flammable than hydrogen itself. But that also means it contains a lot less energy. It also produces more pollution. Ammonia can be stored as a liquid at -33°C, so would need a lot less cooling than hydrogen.

Sources: Royal Society; Irish Aviation Authority

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