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Hydrogen is a paradox. Abundant throughout the universe, but priced as a relatively scarce commodity on earth. Colourless, but available in many different colours. A zero-emissions energy source that is today derived from emissions-intensive hydrocarbons.
At ZeroAvia, we’ve long believed hydrogen fuel cell technologies are the future of zero-emissions flight – and now we’re making it happen. We’re hard at work building hydrogen-electric powertrains for 40 to 80-seater regional aircraft, with first deployment expected in 2027.
Let’s now look at where the hydrogen to fuel these aircraft will come from. Because it’s essential that we get hydrogen right to make the future of aviation truly clean.
The large majority of all hydrogen in use today is known as ‘grey hydrogen.’ This is produced by splitting hydrogen atoms from carbon atoms within natural gas (which contains methane) by heating the gas with steam in the presence of a catalyst, a process known as steam methane reforming1. The grey hydrogen process releases significant volumes of carbon dioxide. Worldwide greenhouse gas emissions from hydrogen production are estimated at up to 1,300 million tonnes of carbon dioxide equivalent every year, and every 1 kilogramme of hydrogen produced generates up to 13.5 kilogrammes of greenhouse gases2.
‘Blue hydrogen’ is identical to grey hydrogen with one important difference; the carbon dioxide emissions generated are captured and stored underground, pumped at high pressure into depleted oil and gas reservoirs or other suitable geological formations in the deep subsurface. The problem with blue hydrogen is that carbon capture and storage (CCS) is complex, costly and very difficult to achieve effectively. The right geology for CCS may be located hundreds or thousands of kilometres away from the plants generating the emissions, which means building very long (and expensive) pipelines. There is also the risk of leakage from pipelines and at the well injection site. Additionally, some geological formations may allow the carbon dioxide stored within them to leach upwards over time, eventually reaching the atmosphere3.
‘Black hydrogen’ derived from coal and ‘brown hydrogen’ from lignite are about as polluting as their colour names would suggest. They are about as far from a clean energy source as you can imagine.
Neither grey nor blue hydrogen are viable options for a sustainable zero-emissions hydrogen future, and black and brown hydrogen are the precise opposite of what’s needed. We need to keep travelling across the hydrogen rainbow to find what we need.
‘Turquoise hydrogen’ uses natural gas as feedstock, heating it to very high temperature (around 10,000OC) until the methane reaches a plasma state and the carbon atoms are then split from the hydrogen atoms. The output is pure hydrogen and solid carbon (which is a valuable and useful commodity in its own right). Turquoise hydrogen production is not operating at scale. It relies on natural gas supplies, and the energy required to heat the gas may also come from fossil fuel energy sources. However, overall emissions intensity is significantly lower than grey hydrogen, estimated at around 0.45 kilogrammes of greenhouse gas emissions per kilogramme of hydrogen produced4 versus 13.5 kg of emissions for grey hydrogen. Emissions intensity would improve even further if biomethane is used in place of natural gas and if renewable energy sources power the methane pyrolysis process. Turquoise hydrogen could be a promising new source of zero-emissions hydrogen.
All of the processes above use heat to release hydrogen atoms from hydrocarbon resources. A much cleaner alternative approach is to use electricity to split water into hydrogen and oxygen atoms. Water is fed into an electrolyser chamber containing an anode and a cathode separated by an electrolyte. As electricity flows from anode to cathode, the oxygen and hydrogen atoms are separated.
‘Green hydrogen’ is produced using electrolysers powered by electricity from renewable energy sources. It has captured the imagination of policymakers worldwide (for example, green hydrogen is a key aspect of the EU’s clean energy strategy5) and is one of the main targets for both public6 and commercial sector7 investment in zero-emissions energy systems.
‘Pink hydrogen’ is identical to green except the electrolysis process is powered by nuclear energy instead of renewable sources. Confusingly, pink hydrogen is also known as ‘purple’ or ‘red’ hydrogen, but the underlying process is the same. Nuclear reactors produce significant heat which could be used for steam methane reforming (as used in grey hydrogen production) in parallel with generating the electricity required for electrolysis, which further increases production efficiency.
However, there are two challenges to overcome with both green and pink hydrogen.
The first is that both processes require large volumes of freshwater, approximately 9 litres of water for every kilogramme of hydrogen produced8. Water demand may not be a problem in locations with access to abundant freshwater resources. But it is an important factor to consider in a warming world where significant population centres are already at risk from extreme water scarcity9.
The second challenge is energy inefficiency. Hydrogen electrolysis generates fewer watts of usable energy output than if you’d used the electricity generated as a power source directly.
That’s less of a factor in zero-emissions aviation where sustainable aviation fuel derived from fuel crops and food waste will not be viable at scale, and lithium-ion batteries aren’t sufficiently energy dense to support longer flights. Hydrogen-electric is the only way to fly any kind of distance with zero emissions. But addressing the water consumption and energy efficiency challenges of green (and pink) hydrogen are as critical for aviation as much as for any other sector.
The good news is that a combination of smart engineering and emerging science – and appropriate investment in those streams – promises a shift in the hydrogen equation. Electrolyser efficiency is one area of focus with some important developments just over the last year10, and research into the use of seawater instead of freshwater11 in electrolysers could also lead to significant breakthroughs in the near future. Never underestimate human ingenuity and the passion of engineers to solve seemingly intractable problems. The aviation industry doubled average fuel efficiency rates over two generations12, and other industries – from power generation to the automotive sector – have achieved even greater gains in that time.
There is one other colour in the hydrogen rainbow that could also make a critical difference. The colour is gold, and the hydrogen in question lies far below our feet. ‘Gold’ hydrogen (also known as ‘white’ hydrogen) occurs naturally in geological formations, and there may be large deposits of it in multiple locations worldwide, including parts of the US13.
Geological hydrogen extraction is in its infancy. But if it scales over time, including by leveraging many of the infrastructure skills of the oil and gas industry, it could provide humanity with an additional (and abundant) source of zero-emissions fuel to power the next generation of aviation.
Learn more about how ZeroAvia is approaching the ecosystem for hydrogen production, storage and distribution for aviation and airports – Hydrogen Fuel – ZeroAvia
