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Hydrogen is amazing. It exists under standard temperature and pressure as a gas of diatomic molecules (H2) and is simultaneously the simplest, lightest and most abundant element, comprising approximately 75% of the total mass in the universe.
Conversely, here on Earth, there is very little pure hydrogen in the atmosphere because of its low density, which allows easy escape of earth’s gravitational pull. Hydrogen is highly reactive, so most of the hydrogen on earth is stored in compounds, commonly as water (H2O) and hydrocarbons such as methane (CH4) and propane (C3H8).
Extracting hydrogen is the biggest hurdle to widespread usage today, though that challenge is being gradually overcome. Hydrogen can be obtained in several ways, such as extraction from hydrocarbon-containing materials like fossil fuels and biomass; or produced from the electrolysis of water. Ideally, to minimize environmental impact, extraction can be powered by green energy sources (e.g., solar, wind, hydro), and there are a range of methods for producing low-carbon-intensity H2.
Importantly, hydrogen is one of the few energy sources that does not release CO2 when used in a fuel cell to generate electricity or even when burned to create mechanical energy.
Hydrogen has many advantages as a fuel source because it has the highest specific energy (or energy per mass) of any fossil or hydrocarbon fuel, making it a fantastic energy carrier. At 120 megajoules per kilogram (MJ/kg), hydrogen is around 3X more energy dense than commonly used fuels like gasoline (44 MJ/kg). Compared to lithium-ion batteries, a widely used energy carrier in electrified transportation, hydrogen has a significantly higher energy-to-mass ratio or “gravimetric density,” storing over 100X more MJ of energy per kg.
However, it is important to consider the additional weight of the hydrogen management system (tanks) that will be required when using hydrogen as a fuel. Studies looking at electric vehicles show that – even taking the hydrogen management system into account – the specific energy of a PEM (Proton Exchange Membrane) fuel cell and compressed gaseous hydrogen system is still at least 4X that of lithium-ion batteries plus the associated battery management system, and is even higher when using liquid H2. Hence, hydrogen is being explored as a fuel in a variety of sectors, particularly where weight is an important factor. e
Hydrogen as a fuel source has many applications in the transport sector, including:
In January 2023, the largest aircraft propelled by hydrogen-electric technology (at that time) was flown by ZeroAvia in the UK. Later that year, an even larger aircraft was flown in the U.S.
Hydrogen-electric engines like those built by ZeroAvia use hydrogen flowing through fuel cells to generate electricity, which is then used to power electric motors to turn the aircraft’s propellers. Hydrogen molecules are first split into their constituent subatomic particles (electrons and protons), the flow of electricity generated is then used to power the motor with the only byproduct of this reaction being water vapor. Hydrogen can also be combusted in adapted gas turbine engines; however, this process still emits nitrogen oxides (NOx), a source of warming, and particulates like soot, which help persistent contrails form in the atmosphere. This is why ZeroAvia is pursuing hydrogen-electric propulsion.
Whilst hydrogen has a remarkably high gravimetric density (as discussed above), it has a relatively low volumetric density, which is the amount of available energy in a given volume. This means storing hydrogen onboard an aircraft presents spatial challenges. In its gaseous form, hydrogen must be compressed by bringing the molecules closer together under high pressures – nearly 120X that of the average car tire pressure. It is then held in high-pressure composite tanks with specialized seals. Alternatively, hydrogen can be condensed into its liquid form by cooling it to cryogenic temperatures (-253 degrees C/-425F). The advantage of storing hydrogen as a liquid is that its volumetric density is increased, allowing more fuel to be stored in the same volume. Therefore, longer journeys can be achieved. We will address this challenge further in a future “Hydrogen 101” series post.
If you’re thinking, “What about batteries?”, you are not alone. Advances are being made in battery energy density that are unlocking new concepts like electric Vertical Take-off and Landing (eVTOL, or air taxis) and drone applications. General and private aviation offer further battery opportunities, though range, energy density, distributed charging infrastructure and weight limitations will make batteries impractical for most commercial aviation. Because of this, scaling electrification for the bulk of commercial air travel will require hydrogen fuel cells.
As with all types of aviation fuel, using hydrogen comes with challenges. These include:
With green and other low-carbon hydrogen on track to be more cost-effective than jet fuel in the coming years, it’s encouraging to see airports, operators, lessors and manufacturers actively working toward the transition. Using hydrogen in its liquid form will also be necessary to overcome the challenges with the low volumetric density of gaseous hydrogen.
Stay tuned to future posts in this series as we cover hydrogen’s role in aviation. Up next: now that we’ve explained hydrogen as a fuel source, Part 2 of this series will explain how fuel cells work with regard to electric propulsion.
Click here for more information about hydrogen as aviation fuel.
