Hydrogen power seems increasingly likely to be a key part of the puzzle to power the aircraft of the future, with the use of fuel cells looking to be a strong contender in the mix. To learn more, we sat down with Matthieu Pettes-Duler, deputy head of powertrain and flight test at Beyond Aero, for the latest in our ab initio introductory primer series.
“Hydrogen fuel cells boast a rich history of applications, dating back to their pioneering use during the Apollo missions, where they served as electrical generators utilising liquid hydrogen,” Pettes-Duler tells us. “In the automotive industry, hydrogen fuel cells made their debut in 2008 when Toyota introduced the first fuel cell car, followed by the Toyota Mirai. These fuel cells have also gained prominence in the transportation of heavy-duty vehicles, including trucks and trains.”
There’s a key difference within the world of using hydrogen to power aircraft between direct hydrogen combustion — in essence, burning hydrogen as a fuel much as today we would use jet kerosene and tomorrow we will use more sustainable fuels, in the form of fossil kerosene substitutes — and using fuel cells to convert stored hydrogen to electric power for what is in essence then an electric aircraft.
“It’s crucial to highlight that the efficiency of hydrogen utilisation varies depending on the propulsion system,” Pettes-Duler explains. “Direct combustion of hydrogen is less efficient, converting approximately thirty percent of the hydrogen source into mechanical power. In contrast, hydrogen fuel cells can achieve a higher efficiency rate of around forty percent. Additionally, fuel cells are well-known, but the potential for improvement is significantly greater compared to conventional direct-combustion engines. This efficiency advantage underscores the considerable potential of hydrogen fuel cells in various applications.”
A key advantage of hydrogen fuel cells is that their efficiency is markedly higher than hydrogen combustion. In the context of the challenges in sourcing, storing and delivering hydrogen to the airport environment, this is a real point in fuel cells’ favour.
“We have a variety of potential hydrogen sources at our disposal, including H2 gas and liquid hydrogen, and we are even exploring the use of cold hydrogen gas,” Pettes-Duler says. This diversity allows us to tailor our approach to meet specific project requirements. Additionally, it’s worth noting that the combustion of hydrogen emits NOx [oxides of nitrogen], which can have environmental implications. In contrast, our focus on fuel cell technology avoids this issue, further highlighting the environmental benefits and efficiency of our approach.”
Challenges are both common to hydrogen combustion and unique to fuel cells
This advantage speaks back to the most immediate challenge of hydrogen power, whether the gas is used for direct combustion or to power fuel cells: the physical fact that it is less power dense than the kerosene it will replace at standard operating temperatures.
Solutions to this issue will likely involve substantially redesigning the shape and form of commercial aircraft, as well as potentially supercooling the hydrogen to reduce its volume. These are not, however, straightforward challenges.
On the fuel cell side, “the cooling of the fuel cell poses a significant challenge due to the increased drag caused by the integration of the heat exchange,” Pettes-Duler notes, highlighting that it is also “crucial to consider the high-pressure requirements of the fuel cell and the associated challenges of managing high voltage within the system, especially under high-altitude conditions. These are essential aspects of our design that require meticulous attention to detail to ensure the safe and efficient operation of the powertrain.”
Conceptualising the powertrain within the constraints of the fuel cells and aircraft available in the short to medium term is a challenge as well.
“Ensuring redundancy in our system is of utmost importance, and one approach to achieving this is by carefully determining the number of components to be employed,” Pettes-Duler says. “The integration of a 1MW fuel cell could be highly advantageous in this context. Currently, such a high-capacity fuel cell may not be readily available, which could necessitate the use of multiple components. Paradoxically, this situation can enhance safety by providing backup emergency options in case of component failures.”
All this will likely mean that aircraft powered by fuel cells look markedly different to the current day. As a result, during the development of fuel cell powertrains and aircraft, it will be especially important to balance their capabilities, missions and potential operational roles.
“The performance of an aircraft powered by this technology will inevitably differ from that of a conventional aircraft, primarily due to the previously discussed challenges,” Pettes-Duler says. “This translates to reduced endurance and a lower payload capacity. However, it’s essential to note that despite these limitations, the utilisation of fuel cell technology offers distinct advantages in terms of efficiency, as we have previously highlighted. This efficiency factor opens up significant opportunities for improvement, thereby potentially enhancing the overall range and capabilities of such aircraft in the future.”
Hydrogen production and storage remains a key question
Getting the hydrogen to where it’s needed will be a continental-scale challenge, with any number of infrastructure questions to be overcome.
“When it comes to hydrogen production, regardless of the specific propulsion system, we must prioritise considerations such as electricity pricing and the quantity of energy required to produce hydrogen competitively for various applications,” Pettes-Duler says. “Shifting our focus to distribution, we are exploring two primary approaches. The first involves the reuse of conventional distribution methods, while the second contemplates the creation of new distribution lines without disrupting current airport activities. Concerning storage at airports, numerous possibilities are under evaluation. The primary challenge revolves around space allocation, taking into account market requirements and the technologies utilised for refuelling these future aircraft.”
Regulatory and certification hurdles will need to be cleared too, some of which are linked: to what extent can airports not routinely providing hydrogen fuelling capabilities be used as alternates? How can the storage and security questions for what is a famously volatile gas be resolved? These questions are not unique to fuel cells — but they affect the technology and its implementations nonetheless.
Author: John Walton
Published 11th January 2024
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