This article addresses hydrogen-powered propulsion, its brief history, and what its limitations are, and how they will be used to make aviation more climate friendly.
Other than COVID, hydrogen-powered aircraft have been in the spotlight this year in aerospace and aviation news. Like all new technologies it brings both promise and hype. Is this hype justified?
One thing is certain – aviation needs radical new solutions to meet the ICAO goals of achieving net-zero emissions target by 2050. Hydrogen just might be the missing piece of achieving this. Recently published McKinsey study on hydrogen powered aviation, amongst other things, concluded:
“H2 combustion could reduce climate impact in flight by 50 to 75 percent, and fuel-cell propulsion by 75 to 90 percent.”
Read on to find out how H2 combustion and fuel-cell propulsion work, resulting in such a massive impact.
Hydrogen as a Fuel
Hydrogen exists as a gas, Freeze it to below –252 °C (cryogenic temperatures), it becomes a liquid. Liquid hydrogen is used for propulsion for rockets and spacecraft, as well as other non-aerospace related applications. Gaseous hydrogen is also often blended with other fuels such as natural gas for power production.
Here’s how hydrogen compares with Jet-A, a typical aircraft fuel:
(Liquid) Hydrogen delivers THREE times more energy while being ten times LESS dense!
Hydrogen is used extensively – as raw material for chemicals and fertilizers, to refine diesel and power rocket engines.
Although it is manufactured at a scale of tens of millions of tons per year, hydrogen production would need to be immensely scaled up to be used in aircraft commercially.
Today, 95% of hydrogen is produced using a steam reforming process, which utilises methane or natural gas. A clean method of producing hydrogen is electrolysis – this is hugely energy intensive and therefore expensive.
If we want to ensure green hydrogen production, the electricity used for electrolysis should be supplied from renewable sources. Can aviation industry really call itself ‘net-zero’ or ‘green’ if the hydrogen production is not?
A hydrogen combustion engine is much the same as a conventional one – mix fuel and air to create explosion, use that explosion to drive a turbine to extract this energy, and transfer it to a fan or propeller. The fuel in this case would be hydrogen instead of conventional Jet-A fuel.
A hydrogen gas turbine would work the same as a conventional gas turbine shown below.
Seems easy right? Not exactly.
There are some very important differences between the two energy sources that cannot be ignored. Here are five differences to give you an idea:
Burning H2 creates no CO2 emissions whereas jet fuel does. Air is largely made up of nitrogen and oxygen and at the high temperatures of the combustion chamber, they atomise and react to produce NOx – a greenhouse gas. Hydrogen combustion still creates NOx emissions, and a lot more water vapour (contrails).
A H2 burning engine can operate at much higher air-to-fuel ratios than typical jet fuel – this means a leaner mixture and lower NOx emissions. However, this also means lower power output.
H2 is much more flammable than jet fuel. A hydrogen-air mixture will require a less ignition energy than a jet fuel-air mixture.
H2 has a much higher flame speed than jet fuel. This can lead to combustion instabilities such as flashback or blow off due to differences is local flow velocities and flame velocities. It ends up damaging or reducing the life of the combustor or turbine components.
Since H2 is a lighter than conventional jet fuel, it forms a more uniform mixture in the combustion chamber.
Hydrogen combustion engines for aircraft have been tested and flown before.
A well-known example is NASA’s Bee Project – a modification of a B-57 to run one engine on liquid hydrogen for high altitude tests. (image (a) above)
Another is the Soviet era Tupolev Tu-155 (image (b) above). This was a Tu-154 modified to have one of its turbojet engines run on liquid hydrogen. Only five flights were completed before bring converted to utilise liquefied natural gas (LNG) since infrastructure for hydrogen storage in aviation was lacking.
Gas turbines are also used for power generation. Fuel mixes with a high percentage of hydrogen content are increasingly being used to limit emissions. Gas turbine power generation with 100% hydrogen is likely to be achieved by 2023. Even though the application and operating conditions are different, knowledge gained from these can be transfer over to hydrogen combustion for aviation.
Hydrogen Fuel Cell
A fuel cell generates electricity through a chemical reaction, much like cells or batteries. There are different types of fuel cells, but I’m going to focus on the Proton Exchange Membrane Fuel Cell – PEMFC, since they deliver fairly high power to weight density and operate at relatively low temperatures.
Here’s how a PEM fuel cell works.
When water is created at cathode, heat is also released because the reaction is exothermic. For fuel cell stacks, thermal management plays a key part, and thermal management systems can get heavy for stacks that generate several hundred kilo-Watts of power.
Fuel cells are expensive in part due to materials required as well as the stringent manufacturing requirements for them. Bipolar plates are typically made from carbon-based composites or metals, gas diffusion layers from carbon-fiber based porous paper, catalyst layers from platinum and carbon black, and PEM from a synthetic polymer, commonly a brand name called Nafion.
Recently published research paper provides an excellent review of PEMFC materials, manufacturing, and limitations.
Fuel cells have been around for a while now. NASA’s Apollo Command Module had a set of 3 fuel cells as a source of primary power (and drinking water). Space Shuttle also had 3 fuel cells, and astronauts were able to consume the water produced by the fuel cell reaction.
A PEMFC propulsion system produces electricity to run electric motors, much the same as a fully-electric aircraft. ZeroAvia is one of several start-ups, who only recently demonstrated their propulsion system in a flight test using a modified Piper M-Class aircraft.
Alaka’I Technologies is another interesting company building a fuel cell powered VTOL called Skai. Here's a very recent update from them in this article by New Atlas, which mentions they are at a tethered flight stage.
Hydrogen Combustion vs. Fuel Cell
Several factors dictate what type of propulsion system to use. However, they mainly revolve aircraft requirements and operating conditions.
With today’s technology, large aircraft flying longer routes would generally be better off with hydrogen combustion based gas turbines than fuel cells. This is because fuel cells currently do not have the energy density required (addressed in McKinsey study), and power requirements mean complex thermal management systems.
For smaller regional, or short-haul aircraft where power requirements are not as high, fuel cells (or hybrid systems) would be more suitable.
Efficiency is also an important consideration. There are several variables like operating altitude, temperature, etc. which affect efficiencies of a propulsion system. I will cover this in detail later.
Limitations of hydrogen
Here are a select few limitations, which will pose a challenge for hydrogen-powered aircraft.
Storage of hydrogen
Hydrogen exists as a gas at standard temperature and pressure. To store hydrogen in a reasonable amount of space, it needs to be stored in pressurized tanks.
The higher the pressure (and lower the temperature), the more hydrogen can be fitted in the tank. Spherical or cylindrical shapes work best. Since hydrogen is flammable, small leaks can be disastrous.
All of this means that it is easiest to store hydrogen in the fuselage, making aircraft longer and heavier.
For liquid hydrogen, this becomes even more of a limitation due to added mass of thermal insulation required to keep the tank at cryogenic temperatures throughout the flight. For hydrogen combustion engines, heat exchangers may also be needed to vapourise the liquid hydrogen before being used for combustion.
Leaks can be a problem with hydrogen since it is a very light gas. It can explosively combust by mixing with air in the right conditions, so precautions need to be taken. Combined with embrittlement (see next point), this can be dangerous especially given the massive temperature range that commercial aircraft need to be certified to.
You’ve likely not heard of this, but this is important. Hydrogen embrittlement is a process through which metallic materials lose their ductility, crack, and fail. Hydrogen atoms interact with the cohesion of the material lattice and weakens it. So the hydrogen tank or fuel system is susceptible to this.
NACE, a leading association of corrosion engineers describe this as a corrosion-like process.
This mostly applies to fuel cells. Where the hydrogen ion, electron, and oxygen combine to form water, it is an exothermic reaction (i.e. release of heat). In larger stack of fuel cells, heat accumulates and therefore requires careful management. Thermal management systems add weight to the overall propulsion system and can become complex for large aircraft.
Some Final Thoughts
It is easy to see why hydrogen, especially in its liquid form, is attractive as a fuel – it provides three times more energy than jet fuel while being ten times less dense, and burning it produces no CO2 emissions.
Yet, hydrogen doesn’t offer a 100% clean solution, since the production of water vapour, which leads to contrails, is a contributor to warming. Hydrogen combustion will also not completely get rid of NOx emissions – another greenhouse gas. Hydrogen shows promise for more climate friendly aviation, as showcased in the late 20th century with hydrogen combustion, and today with fuel cells.
How much of the hype today translates into reality depends on overcoming the limitations of hydrogen, culmination of unconventional aircraft configurations, and ultimately the cost-benefit of hydrogen compared to synfuel, biofuel, and electric aircraft. This will especially be true for commuter and regional / short-haul routes.
That’s it for now! I will be updating and improving this post over time with more detailed information.
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