Hydrogen aircraft are only green if the hydrogen is being produced cleanly. Credit: Scharfsinn via Shutterstock.

Aircraft have a reputation for pollution. Aviation makes up 2% to 3% of global carbon dioxide emissions. Some think that hydrogen is the solution. Hydrogen power has been in development for many years, with the first simple hydrogen fuel cell invented in 1842 by Sir William Grove and the first functional hydrogen-powered car developed in 1860. However, throughout the 20th century, hydrogen took a backseat to fossil fuels. The major development in hydrogen power was the invention of the hydrogen fuel cell. In 1939, a 5kW fuel cell was developed, but it took several years for it to become powerful enough to be useful. In the 1960s, General Electric provided Nasa with 1.5kW hydrogen fuel cells for the space program, which not only provided the Apollo missions with hydrogen as fuel but also drinking water as a useful byproduct. Toyota introduced a hydrogen-powered car in 2014. With the technology now advancing at a more rapid pace, how could this apply to aircraft?

A hydrogen fuel cell works by an electrochemical reaction between hydrogen and oxygen. The hydrogen is oxidised to a single proton as it passes over a platinum anode, while the oxygen is reduced as it passes over a platinum cathode. The protons pass through an electrolyte membrane and react with the oxygen at the cathode, producing only water and energy. This makes it environmentally friendly, as the water can be recycled into the natural water system. Despite hydrogen seems to be the solution to green aviation, it also posits challenges for infrastructure and safety. As hydrogen is a minuscule molecule, leaking is common. This is not only a major efficiency concern but could become unsafe as hydrogen is highly flammable. Recently, Nasa’s launch of Artemis 1 was delayed due to a liquid hydrogen leak. Although as a comparison to Jet A fuel, it is less flammable with a flammability concentration limit of 4% versus Jet A’s 0.7%. However, Jet A is significantly less volatile and has a lower minimum ignition energy of between five millijoules and one joule, whereas Hydrogen’s is 0.02 millijoules. This could cause problems certifying hydrogen aircraft, as the widely accepted guideline is ten times higher at 0.2 millijoules. Hydrogen diffuses quickly and has a higher auto-ignition temperature, so this may not become an issue.

Despite hydrogen having around three times more energy per kilogram than conventional jet fuel, Hydrogen only has 0.0028kWh per litre and jet fuel has 9.52, as jet fuel is a liquid under atmospheric conditions. In order to produce the same energy as one litre of jet fuel, you would need 3,200 litres of hydrogen at the same pressure. This makes jet fuel much more efficient. Under standard conditions, and with its tanks full of hydrogen, a Boeing 787-9 would only have enough energy to fly for 21 seconds. This is why developing more efficient methods of storing liquid hydrogen will be vital for its viability as aviation fuel. Even as a liquid, hydrogen will need four times the volume to deliver the same power as jet fuel. Hydrogen is stored in big spherical tanks, so it doesn’t boil off. It has a boiling point of 20 kelvin (-252.9°C). This is a huge barrier due to the challenges of storing large quantities cheaply. There are also issues in storing hydrogen on the aircraft. Normally, jet fuel is loaded into the wings of passenger aircraft. Hydrogen won’t be able to do that effectively due to the size and shape of the tanks required, so would need to fit in the fuselage, decreasing the payload and passenger volumes by up to 40% on conventional designs. Although blended wing bodies may be able to overcome this issue, they are yet to be built and won’t fit in current airports.

Hydrogen aircraft are only green if the hydrogen is being produced cleanly. While hydrogen is the most abundant element on Earth, it is locked up in other molecules. Steam Methane Reforming (SMR) produces 99.9% of the world’s hydrogen annually (19 million tons) and for every one kilogram of hydrogen it produces, nine kilograms of carbon dioxide are produced – this is a challenging overhead for environmentally sustainable goals. Green hydrogen can be produced by electrolysis, but only makes up less than 0.04% of all hydrogen produced globally. It is much more expensive at $5.10 -$23.27 per kilogram for solar electrolysis, whereas SMR-produced hydrogen costs less than $1 per kilogram.

Despite the many challenges, large corporations have decided to launch projects. Airbus’ ZEROe project has three hybrid-hydrogen designs for a turbofan, turboprop and blended wing body aircraft and Boeing has conducted six hydrogen technology demonstrations. Some startups are attempting innovations too. ZeroAvia’s mission is ‘a hydrogen-electric engine in every aircraft.’ The challenges of hydrogen-powered aircraft providing the answer to environmentally sustainable aviation are only beginning to be answered, but much more investment will be needed to develop this technology for it to be the cheapest and most practical option.

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