- Second Century
- ✈️ #18: Why the jury is still out on hydrogen
✈️ #18: Why the jury is still out on hydrogen
The fact that hydrogen can store large amounts of energy per kilo makes it a promising technology for zero emission aviation.
Nonetheless, apart from the considerable advantage, employing hydrogen in airplanes presents several drawbacks.
As a result, it remains uncertain if hydrogen will be the cornerstone of sustainable aviation in the future.
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There are three energy sources that are fueling the development of cleaner, or even emission free, aviation:
Sustainable Aviation Fuels (SAF)
After several newsletter issues on batteries and SAF, now it’s time to discuss hydrogen. There are many ways to discuss hydrogen and how to use it to fuel car and aircraft engines. I will stick to discussing how the characteristics of hydrogen affect aviation operations.
A sweet spot for sustainable aviation
An increasing number of companies have been investing in hydrogen technologies for aviation purposes in the last decade. The latest results are ZeroAvia flying a 19-seat Dornier 228 with hydrogen-powered electric engines last January and Universal Hydrogen flying a 55-seat De Havilland Canada Dash 8-300 with one electric engine running on hydrogen last March.
The reason why companies are investing in this technology is that it has a higher energy density than batteries and jet fuel - it can store more energy per unit of weight - while at the same time, it does not leave any harmful emissions. Making it a promising technology for sustainable aviation.
There are two ways to use hydrogen to power an engine:
As a fuel cell, where it generates electricity through an electrochemical reaction between hydrogen and oxygen. Hydrogen fuel is supplied to the anode, where it is split into protons and electrons.
When mixed with oxygen and exposed to heat, it can be burned like kerosene or gasoline, with only water leaving the exhaust.
In normal conditions, hydrogen is a gas and needs to be transformed into a liquid to be able to burn or store electrical energy. That is why hydrogen is stored in large tanks that keep the liquid at a temperature below -252°C. Because these tanks need to be taken on board the plane, this technology is unfit for small aircraft. Yes, there are relatively small cars that run on hydrogen, but cars consume less energy per kilometer and don't have the same requirements for range as aircraft do.
However, regional airliners, like the Dornier 228 and Dash 8 mentioned earlier, are big enough to carry hydrogen tanks on board. And because of the high energy density, it beats batteries when it comes to weight, making it the ideal fuel for zero-emission regional airliners.
So that's why this type of operation, often the feeder flights to and from large airport hubs, is often mentioned as the main business case for this technology.
A sweet spot for competition
Despite the unique selling point of hydrogen, the other two types of sustainable energy sources are gaining traction.
The Dutch company Maeve Aerospace presented the image below during their reveil of the Maeve 01, their first fully electric regional airliner. The message they want to communicate is that hydrogen is facing fierce competition from both batteries and SAF as preferred energy source.
While batteries (red-orange in the bottom left) are currently only suitable for small General Aviation (GA) and Urban Air Mobility (UAM) applications, developments in battery technology will make it possible for battery electric propulsion systems to be used in regional jets as well in the (near) future. At the same time, SAF is going to infiltrate the regional airline market by becoming cheaper and more widely available.
Image by Maeve Aerospace
On the one hand, it is true that both battery electric aircraft and SAF-fueled aircraft will become more suitable for regional aviation, making it difficult for hydrogen to grow its market share. On the other hand, hydrogen technologies are developing as well, making it a stronger alternative.
The structure of the infrastructure
A downside of hydrogen is the extensive supply chain it requires before it can be placed inside an aircraft. In many ways, the logistics of hydrogen are comparable to the logistics of fossil fuels. Production is centralized around a relatively small number of large facilities, and it is shipped by land or water to the place where it is loaded into a mode of transportation.
Aviation hydrogen infrastructure as imagined by Universal Hydrogen
An important side effect of this supply chain is the low well-to-wheel efficiency of hydrogen vehicles. This is the total energy efficiency of a vehicle, accounting for both the energy used to produce and distribute fuel, as well as the energy used by the vehicle itself. For hydrogen, this number is around 20%-25%. This means that during the production and transportation of hydrogen, around 75% of the initial available amount of energy is lost. This number is comparable to fossil fuel engines, while battery electric engines have a well-to-wheel efficiency of 60%-70%.
The logistics to put energy in a battery electric aircraft has the potential to be a lot simpler. Currently, most electric energy is generated in large power plants, often far away from airports. However, airports around the world have plans to become more self-sufficient when it comes to energy production by generating solar and wind energy at their own facilities. This would make the supply chain of electric energy extremely short since the energy is produced at the same location as where the aircraft is recharged and where it will take off.
On the one hand, an upside of using these tanks is that when they are almost empty, at the end of the flight, they can be quickly replaced with new ones. This speeds up the turnaround time, especially when compared to recharging a battery, and makes it possible to execute more flights per day.
On the other hand, the size of the tanks is a major downside for the economics of the flight. Hydrogen can store a lot of energy per unit of weight; however, since hydrogen is the lightest element on earth, it requires a high volume to store a large amount of energy. The image below perfectly shows the impact of transporting high-volume tanks on a large passenger jet.
A visual interpretation of a hydrogen powered passenger jet by Airbus.
As you can see, a significant part of the cabin, which is usually reserved for paying passengers or cargo, is used to store hydrogen. This significantly reduces the profitability of the flight.
You probably think: but kerosene is also a liquid that is stored on board. And that's true. However, since kerosene is not being cooled and pressurized, it is possible to store it in the wings, an otherwise empty space. Even though it is not yet the case in current battery electric aircraft, a large number of future aircraft will have the batteries in the wings as well. Leaving more space for payload.
Is it too early to tell?
Even though I have used more words to describe the downsides than the upsides of hydrogen, it is still a technology that is considered part of the future of sustainable aviation. It has a higher energy density than batteries, making it more suitable for longer flights and it is without any harmful emissions, which cannot be said about SAF.
So, even though the use of hydrogen in aviation is developing rapidly, it is still the question whether the upsides of using this element outweighs the downsides. Especially considering the competition from batteries and SAF.
Thank you for getting all the way down to the end of this issue of Airline Food for Thought! If you have any questions or suggestions you can contact me via [email protected] or send me a message via LinkedIn.
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