Jul 22 2021

Hydrogen Aircraft

When it comes to cars, the technology competition between batteries and hydrogen has been won. The future of cars appears to be battery technology. There are some hydrogen-fueled cars, but they are a tiny slice of the market. The bottom line is that batteries are more efficient than hydrogen, and they are only going to get better. Volkwagen pretty much declared a victor with this statement:

 “The conclusion is clear” said the company. “In the case of the passenger car, everything speaks in favour of the battery and practically nothing speaks in favour of hydrogen.”

But the key phrase there is “in the case of the passenger car.” Hydrogen may still find a niche when it comes to other types of vehicles, such as trucks or trains. White battery technology has the advantage in efficiency (the total percentage of energy that get transferred to momentum), hydrogen has other advantages. One is that hydrogen (being the lightest element) has a very high specific energy (energy per mass). The specific energy for hydrogen is three times that of jet fuel, and more than 200 times that of current lithium-ion batteries.

In fact, hydrogen has the highest specific energy of any practical fuel. The two recent commercial suborbital flights, by Virgin Galactic and Blue Origin, both used hydrogen fuel, which combines with oxygen to form water, and no CO2. Hydrogen is likely to be the fuel of spaceflight for the foreseeable future (until we perfect nuclear engines, but that’s another article). So for anything that needs to fly, where weight is the primary concern, hydrogen is a great fuel.

Hydrogen, therefore, sounds like a perfect candidate for jet fuel. And, it pretty much is. But there are, of course, limitations. One is that the technology for hydrogen fueled jets is not as well developed as current jet technology based on fossil fuel. But the technology is being developed, with prototypes being tested right now. Airbus has announced its intentions to have zero-carbon jets by 2035, and those jets will by hydrogen-fueled.

Another potential limitation of hydrogen is that it has a very low energy density (energy per volume) in its native gas form. Therefore it has to be highly compressed, to 5,000 or even 10,000 psi, in order to have sufficient energy density to serve as a fuel. Rockets solve the problem by using liquid hydrogen, which has an energy density of about a third of fossil-fuel based rocket fuel. Practical use of hydrogen fuel, therefore, partly depends on the ability to safely carry around tanks of highly compressed explosive gas.

This is why one of the primary goals of the “hydrogen revolution” two decades ago was developing a way to safety store hydrogen with high specific energy and energy density in a non-combustible form that could be quickly released as needed. A lot of this research focused on ceramics, but the requisite breakthrough just never came. The delay allowed battery technology to get to the point where it simply won the technology race (again, for cars). The same problem exists for hydrogen jets. Liquid hydrogen could work, as well as compressed hydrogen. Either way, the entire system needs to be sufficiently light so that the weight advantages of hydrogen are not lost.

The final challenge of hydrogen is production. There is essentially no free hydrogen on Earth. Hydrogen is not, therefore, an energy source, it is an energy storage medium. We would need to make enough hydrogen that it would be readily available at all airports around the world. Today “more than 70 million tonnes of hydrogen are produced every year,” and most of this is stripped off of fossil fuels at high energy cost. This obviously is not sustainable, and also defies the whole purpose of transitioning to hydrogen.

Ideally hydrogen would come from electrolysing water – separating it into hydrogen and oxygen. Then the hydrogen can be burned back with the oxygen to reform water, in a sustainable cycle. This is why so many researchers are working on methods to efficiently make hydrogen from water, and steady progress is being made. The goal would be to have factories making massive amounts of hydrogen from water using clean energy, such as solar cells or even nuclear power. If you burn coal to make hydrogen, then again the advantage is lost.

It does seem likely that hydrogen has a role to play in the future of our energy infrastructure. It definitely will remain important to the rocket industry. It will very likely be important to the large transportation industry, such as trains and aircraft. But we need to see some steady advances in hydrogen production and storage. Hopefully incremental advances will add up the way they have for battery and solar technology.

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