Flying Sustainably: Towards Hydrogen-Powered Aviation
Even after the pandemic, the annual passenger growth is expected to be at a rate of 3% [2]. There is no surprise that there will be an inherent rise in the number of airplanes to cater to such a high passenger volume in the coming years. But this will come at the cost of increased emissions.
Amongst all the by-products emitted by a kerosene jet engine operating on conventional aviation fuel i.e., essentially kerosene, carbon dioxide (CO2) is the one which when released in large amounts can be ecologically harmful. This is because of the fact that CO2 has the highest “radiative forcing” amongst all the anthropogenic greenhouse gases. Although CO2 has a dominant impact on global warming, other emissions from kerosene powered aircraft such as nitrogen oxides (NOx), soot, and water vapour have significant contributions as well. The preceding statement is important to understand how different propulsion technologies are rated and chosen.
This esoteric term called “radiative forcing” is fairly a straightforward concept and is essentially the net change in the energy balance of the Earth system due to perturbation. As the energy from the sun reaches the Earth, a part of it is absorbed by the Earth and the rest is radiated by to the space as heat. The balance between the incoming energy and outgoing energy dictates the Earth’s average surface temperature and when this difference is anything other than zero, leads to an imbalance indicating warming or cooling.
At present, aviation contributes towards 2.1% of all human-induced CO2 emissions and this number will potentially rise with increased air transport in the future[4]. However, it is worth mentioning that over the period of 60 years, aircraft flying today are 80% more fuel-efficient. Fuel efficiency can essentially be translated to lower fuel burn, thus lower emissions. The increments in fuel efficiency have been achieved by improving the jet engines and aircraft designs, but as seen in Figure 3, these potential increments have started to plateau out (up to 2015). Further innovation in existing designs such as increased wingspan and larger fans, can only offset fuel benefits by increased weight.
Given the above-stated issue in front of the world of aviation to adhere to the strict environmental regulations, especially the goal of net-zero carbon by 2050, a lot of innovative and revolutionary technologies are being studied and tested. Research institutes and aerospace manufacturers are not only targeting the airframe designs that would be different from the current tube and wing configuration but also exploring the power source itself- electric, hybrid, hydrogen, sustainable aircraft fuel (SAF) etc. In the series of articles, I will talk about potential revolutionary technologies that are currently being explored on the path to decarbonizing aviation. These can broadly be categorized into- airframe designs, fuel, and propulsion technologies.
Based on the recent developments towards sustainable aviation, this blog talks about the potential use of liquid hydrogen as a fuel to power the aircraft highlighting the motivation, brief history, testing, and challenges involved in its application.
Airbus’s new commitment towards hydrogen-powered combustion
At the time of writing this article, a giant leap has been taken towards the decarbonization journey of aviation by Airbus. In an agreement with the engine manufacturer CFM International (a 50/50 partnership company between GE and Safran), Airbus will collaborate on a hydrogen-powered flight demonstration called the ‘ZEROe’ demonstrator [6].
This comes a year after Airbus presented its three conceptual designs of aircraft powered by hydrogen combustion; varying based on the propulsion system and aerodynamic design- Turbofan, Turboprop, and Blended Wing Body (BWB) [7]. The former two are based on the conventional tube and wing, while the latter is a blended body which is something different from the ones flying today. It is worth mentioning that Airbus seems to align towards the hydrogen combustion, after putting a halt to its hybrid-electric flight demonstrator, called E-Fan X [8].
But, why hydrogen?
Hydrogen seems a lucrative option to replace the existing hydrocarbon-based conventional jet fuel because -
- It essentially emits no carbon on being burned, thus making it a “carbon zero” solution (but this is conditional! Read more about it under Challenges).
- It is the lightest known element as well as it is the most abundant element in the universe.
- Hydrogen’s energy density based on mass (gravimetric energy density) is three times that of jet fuel and much superior to lithium-ion batteries; making it an attractive option as a fuel for aircraft. However, hydrogen has a lower volumetric density than kerosene.
A brief history of hydrogen as a fuel-
The first application of hydrogen as a fuel dates back to 1783 when its property of being “lighter than air” was used to uplift a large balloon. Thereafter, the most famous application utilizing hydrogen as fuel was the Zeppelin airship which burned hydrogen in its internal combustion engine to uplift the airship. This is stated to be the first commercial aircraft to be powered by hydrogen. However, the infamous ‘Hindenburg accident’ on 6th May 1937, where an airship burst into flames, sent a signal that hydrogen was not a safe fuel to be used for commercial flight [9].
When it comes to fixed-wing flights, US Air Force successfully tested liquid hydrogen to fly B-57 twin-engine bomber back in 1957. After initial failures, the first successful flight of hydrogen-powered aircraft was made during this project. Even though the flight was successful, it revealed one of the shortcomings — that was the low flight endurance [9]. The aircraft could only operate for 20 mins until the hydrogen engine ran out of fuel. Another significant observation was the condensation trail (contrails) emitted by the hydrogen engine, which could impact the climate.
Alongside the development of B-57 aircraft, the US Air Force started a secret project code-named ‘Suntan’ to develop a high-flying reconnaissance aircraft with liquid hydrogen, having superior performance than U-2. Although the project was cancelled, it led directly to the development of the first rocket engine using liquid hydrogen [11]. Hydrogen has been a choice of fuel to power rockets for space exploration, especially after NASA mastered the technology to use this light, yet extremely powerful propellant in the late 1950s. This fuel was used to lift off the astronauts to the moon during Apollo missions on the Saturn V rocket. Similarly, it had been used in NASA’s Space Shuttle program. Since then, it has been actively been used by other space agencies as a propellant worldwide.
Soviets had their own experience with hydrogen by converting Tupolev Tu-155 by fitting it with a liquid hydrogen-powered gas turbine back in 1988. This research by the Soviet Union was motivated by the oil crisis in the 1970s, which led them to explore liquid hydrogen and liquid natural gas as an alternative fuel. In this research, the Tu-154 aircraft was modified to have one engine operating on hydrogen. But even this project couldn’t see the light of the day after its first successful flight, potentially due to the fall of the Soviet Union. European Commission’s project called “CRYOPLANE” is another renowned project that was initiated to research the application of liquid hydrogen to commercial aircraft in light of increasing environmental concerns, in 2000.
Pathways for hydrogen-powered aircraft
There are two pathways for using hydrogen in the propulsion systems for aircraft-
- Hydrogen fuel cells- The fuel cells are used to convert the energy possessed in the hydrogen atoms along with oxygen to produce electrical power using an electrochemical reaction, which in turn is used to power the electric motor that drives the ducted fan or propeller. An aircraft operating with a hydrogen fuel cell doesn’t emit any emissions except water vapor.
- Hydrogen combustion- As the name suggests, this process involves the combustion of hydrogen instead of kerosene in a ‘modified gas turbine engine’. In contrast to the hydrogen fuel cell, this process emits oxides of nitrogen (NOx) and water vapor.
Challenges-
Based on the research carried out in the past, the researchers and manufacturers faced a number of challenges while studying the potential of hydrogen to power aircraft. Below are the challenges which were known, while some of them are a result of studies carried out in the past -
- Low volumetric density- As aforementioned, hydrogen has a lower volumetric density than kerosene. Since the storage of hydrogen as gas would require high-pressure tanks as well as larger space (exceeding the size of a whole aircraft!), it is only feasible to liquify it. Liquification of hydrogen requires it to be stored in cryogenic temperatures since hydrogen has a boiling point of -253° C. Still it is estimated that for a given range and mission, hydrogen would require space that is 4 times larger than that of kerosene. This presents a challenge for airframe designers as they would have to make significant changes to the conventional airframes for incorporating hydrogen technology. Figure 7 shows some of the potential designs of hydrogen aircraft with varying tank configurations.
- Emissions- Even though the application of hydrogen would limit carbon emissions, water vapor is another greenhouse gas that is produced by the combustion of hydrogen and still has a significant radiative forcing. A study showed that hydrogen combustion produces 2.6 times more water vapor than kerosene-based combustion [5]. Nevertheless, the carbon-free emission offsets the effects of water vapor.
- Hydrogen availability and cost- Hydrogen is not accessible in nature naturally and has to be manufactured through either of two methods: a) splitting water molecules into H2 and O2 (green hydrogen method); or b) mixing hydrocarbons in steam to produce H2 and CO2 (grey and blue hydrogen method). To this day, the majority of hydrogen production (~95%) is dependent on hydrocarbons, which releases carbon dioxide. It is done because of the low cost. If such infrastructure is used for hydrogen production for aviation use, it would kill the whole point of flying sustainably. But, if the industry tends to use the former method i.e. using renewable sources for electricity to split water molecules, no carbon is emitted and therefore, this method is called ‘green hydrogen’. But, this is is currently a far more expensive method to produce hydrogen.
- Infrastructure- There is a need for overhauling existing infrastructure for the transportation of hydrogen from the hydrogen production plants to the airports if hydrogen-powered aircraft is to fly.
Airbus and CFM’s ZEROe demonstrator programme
Now coming to Airbus and CFM’s plan of equipping an Airbus A380 platform with a hydrogen-powered engine for cruise phase testing [6]. They plan on using the flight testbed which will test a direct combustion engine fueled by hydrogen, thus taking the path of hydrogen combustion in this case. A modified version of the GE Passport engine will be mounted on the top of an A380, connected to the 4 liquid hydrogen fuel tanks located in the aft of the aircraft. A liquid hydrogen distribution system is used for conditioning and supplying the H2 into the combustor of the engine.
The engineers at CFM International would have to perform a complete overhaul of the combustor, fuel system and control system to make it compatible with liquid hydrogen. This is necessary due to the fact that the combustion of hydrogen is very different from that of kerosene. Burning hydrogen produces higher temperature and flame speeds which could lead to a lot of stress. Moreover, since hydrogen is stored at extremely low cryogenic temperatures, it would have to be preheated before entering the combustor so that it is fully vaporized.
The main objective of this demonstration would be to exhibit the hydrogen propulsion system for the given requirements of a commercial aircraft, wherein the safe storage capability of hydrogen in tanks is monitored along with the efficient distribution of hydrogen fuel into combustors for a range of characteristics. The monitoring of condensation trails would be another significant outcome of this research.
Burning questions -
On analyzing the current developments and decisions made by Airbus to choose hydrogen combustion for its testbed, one of the questions crops up is whether Airbus and its engine partners have ditched the idea of using hydrogen fuel cells? Or will they be able to modify the same demonstrator to perform the hydrogen fuel cell-based propulsion? It would be interesting to see the comparison between the two systems under similar conditions.
Another question that comes to my mind is- What is Boeing’s take on the step taken by Airbus, since Boeing had decided to part its way from the idea of using hydrogen as a potential alternative fuel? Would Boeing stick to its plan of exploring SAF or give hydrogen another try [14]?
For a wide-scale operation of hydrogen-powered aircraft, a large scale infrastructure for hydrogen production using the ‘green hydrogen method’ as well as its transportation to aircraft has to be set up. Such an initiative would not only require cross-industry collaboration but investment and commitment from the governments as well. Thus, it would be interesting to see how different industry players around the world will collaborate and sign agreements to make hydrogen-powered aviation a reality.
It is certainly an interesting time to be alive to see such innovations taking place in the field of aviation!
References-
[1] Airbus.com. 2022. Airbus and CFM International to pioneer hydrogen combustion technology. [online] Available at: <https://www.airbus.com/en/newsroom/press-releases/2022-02-airbus-and-cfm-international-to-pioneer-hydrogen-combustion> [Accessed 8 March 2022].
[2] Endseurope.com. 2022. Waypoint 2050 — Balancing growth in connectivity with a comprehensive global air transport response to the climate emergency — ATAG. [online] Available at: <https://www.endseurope.com/article/1696594/waypoint-2050-balancing-growth-connectivity-comprehensive-global-air-transport-response-climate-emergency-atag> [Accessed 8 March 2022].
[3] US EPA. 2022. Climate Change Indicators: Climate Forcing | US EPA. [online] Available at: <https://www.epa.gov/climate-indicators/climate-change-indicators-climate-forcing#:~:text=Of%20the%20greenhouse%20gases%20shown,in%20radiative%20forcing%20since%201990.> [Accessed 8 March 2022].
[4] Atag.org. 2022. Facts & figures. [online] Available at: <https://www.atag.org/facts-figures.html#:~:text=The%20global%20aviation%20industry%20produces,to%2074%25%20from%20road%20transport.> [Accessed 8 March 2022].
[5] www.iata.org. 2022. Liquid hydrogen as a potential lowcarbon fuel for aviation. [online] Available at: <https://www.iata.org/contentassets/d13875e9ed784f75bac90f000760e998/fact_sheet7-hydrogen-fact-sheet_072020.pdf> [Accessed 8 March 2022].
[6] Airbus.com. 2022. The ZEROe demonstrator has arrived. [online] Available at: <https://www.airbus.com/en/newsroom/stories/2022-02-the-zeroe-demonstrator-has-arrived> [Accessed 8 March 2022].
[7] Airbus.com. 2022. ZEROe. [online] Available at: <https://www.airbus.com/en/innovation/zero-emission/hydrogen/zeroe> [Accessed 8 March 2022].
[8] Airbus.com. 2022. Our decarbonisation journey continues: looking beyond E-Fan X. [online] Available at: <https://www.airbus.com/en/newsroom/stories/2020-04-our-decarbonisation-journey-continues-looking-beyond-e-fan-x> [Accessed 8 March 2022].
[9] Lei, H. and Khandelwal, B., 2021. Hydrogen fuel for aviation. Aviation Fuels, pp.237–270.
[10] History.nasa.gov. n.d. Liquid Hydrogen as a Propulsion Fuel 1945–1959, NACA Research on Hydrogen for High-Altitude Aircraft. [online] Available at: <https://history.nasa.gov/SP-4404/ch6-4.htm> [Accessed 8 March 2022].
[11] History.nasa.gov. n.d. Liquid Hydrogen as a Propulsion Fuel 1945–1959, Suntan. [online] Available at: <https://history.nasa.gov/SP-4404/ch8-1.htm> [Accessed 8 March 2022].
[12] Tupolev [online] Available at: <https://www.tupolev.ru/en/about/bibliography/?sphrase_id=52830> [Accessed 8 March 2022].
[13] Khandelwal, B., Karakurt, A., Sekaran, P., Sethi, V. and Singh, R., 2013. Hydrogen powered aircraft : The future of air transport. Progress in Aerospace Sciences, [online] 60, pp.45–59. Available at: <https://www.sciencedirect.com/science/article/pii/S0376042112000887?via%3Dihub> [Accessed 8 March 2022].
[14] Simple Flying. 2022. Why Boeing Isn’t Focusing On Hydrogen As A Fuel. [online] Available at: <https://simpleflying.com/boeing-no-hydrogen-focus/> [Accessed 8 March 2022].
