LH₂ vs the ‘competition’

LH₂ vs GH₂

Whilst liquid hydrogen (LH₂) is the focus of this article, it is important to note that another form of hydrogen fuel, gaseous hydrogen (GH₂), shows promise for aviation use.

Regardless of how hydrogen is stored (ie. as a liquid or gas), it must be used (either in a turbojet engine or fuel cell) in its gaseous form. Compared to LH₂, GH₂ is an arguably more ‘straightforward’ fuel source, as it only requires confinement in tanks, and not the cryogenic conditions required to keep hydrogen liquid.

GH₂ could be considered an LH₂ precursor – as liquid hydrogen is the result of concentrating (or densifying) gaseous hydrogen through the liquefaction process. By liquifying hydrogen, we get the same mass of H₂, but reduced volume, increasing density from 0.09 Kg/m³ at 0°C and 1.013 bar to 70.8 kg/m³ at -252.9°C and 1.013 bar. The liquefaction process is energy intensive, and, as mentioned, requires cryogenic conditions to maintain its state. As a result, LH₂ is more expensive (but more energy dense) than GH₂.

To that end, we could foresee gaseous hydrogen used for light aviation or shorter ranges, and LH₂ for longer (and more expensive) journeys, or more power-intensive uses (eg. space) – ones in which GH₂ lacks the necessary energy density.

Similarly, private initiatives are also underway. International Airlines Group (IAG) has committed to a 10% SAF target by 2030 as part of its strategy to achieve net zero emissions by 2050. Part of SAF's global challenge will be the availability of resources to supply such a fuel volume.

LH₂ vs SAF

As sustainable fuels for aerospace, hydrogen's main competitor is the sustainable aviation fuels (SAF) category. SAF can be divided into biofuels (created from natural conversion pathways, utilizing feedstocks like animal fat or bio oils) and synfuels - the generation of which requires CO₂ and hydrogen, and is not sustainable without a decarbonized source of hydrogen.

Synfuel's production incorporates the cost to catch CO₂, the cost to decarbonize hydrogen, and the cost to combine CO₂ and H₂ through a production process like Fischer-Tropsch. E-fuels (sometimes referred to as ‘power to liquid’), are a subset of synfuels.

Compared to LH₂, SAF benefits from compatibility with existing fuel infrastructure for storage and transport. SAF can also integrate seamlessly with current aircraft technologies, accommodating ‘drop in’ blends of up to 50% with conventional fuels.

SAF is also gaining momentum. In Europe, for example, the EU’s ReFuelEU Aviation Regulation requires aviation fuel suppliers to gradually increase the share of SAF blended into the conventional aviation fuel supplied at EU airports. Its target is a 2% share of SAF in EU airports from 2025, and a 70% share of SAF in all EU airports from 2050 [5][6].

LH₂'s combustion process (which produces NOx but no CO₂) pollutes differently to SAF's, which produces CO₂ and much lower quantities of NOx than LH₂, due to hydrogen burning at higher temperatures. However, when LH₂ is used in fuel cell systems, it emits no CO₂ or NOx.

It is important to note that water vapor is emitted in all three cases (ie. combusted SAF, combusted LH₂ and fuel cell LH2), but the scientific community has not yet reached a consensus regarding the contribution of these water emissions to global warming.

Calculations by FlyZero for global warming potential over 100 years for aircraft by fuel and propulsion type predict LH₂ emissions through a gas turbine at around 18 units, SAF with carbon capture at about 25 and - suggesting that LH₂ is more favorable than SAF from an emissions perspective, with fuel cells being the best option.

Those same calculations show that hydrogen combustion could potentially reduce emissions in flight by 75-85% compared to Jet A1 aviation fuel, whereas SAF is estimated to achieve a smaller reduction, of 10-20% without carbon capture.

Yes, SAFs offer a renewable alternative to conventional jet fuels, but still emit CO₂ when burned. They also emit soot, which is not a concern with liquid hydrogen, and NOx, which are not produced at all through hydrogen fuel cell use. Furthermore, some biofuel feedstocks compete with other industries, like agriculture, for land and resources.

Liquid hydrogen, on the other hand, avoids these issues – especially when derived from renewable sources; it can be produced from various sustainable processes, including water electrolysis using solar or wind power. Liquid hydrogen is also noted for its excellent energy-to-weight ratio, with a higher energy content per kilogram, compared to traditional jet fuels.

Emissions: a quick comparison

Emissions from synthetic fuels and LH₂ are very different.

Pricing

The cost-effectiveness of hydrogen production depends largely on the energy source used to produce it. Currently, the most economical methods for hydrogen production leverage fossil fuels, with costs estimated around €1.50 per kilogram of H2 (if derived from fossil fuels as ‘grey’ hydrogen) [7][8].

At the moment, 2030 price predictions for SAFs are at €0.121 per liter, with liquid hydrogen looking to be between €8 to €9.50 per kg, and €1.21 to €3.26 per kg of gaseous hydrogen - from green sources. Fossil fuels can be used to produce SAF for significantly less, but this, obviously, is unrealistic in the context of sustainable aviation.

Cost per kWh calculations

SAF: Cost per kWh

Energy density: 35 MJ/L
Energy per liter in kWh: 35 MJ/L ÷ 3.6 MJ/kWh ≈ 9.72 kWh/L
Cost: €1.21/L
Cost per kWh of SAF: €1.21/L ÷ 9.72 kWh/L ≈ €0.1245/kWh

Gaseous Hydrogen: Cost per kWh

Energy density:120 MJ/kg
Cost in EUR/kg (low):1.3 × 0.93 ≈ €1.21/kg
Cost in EUR/kg (high):3.5 × 0.93 ≈ €3.26/kg
Cost per kWh of Gaseous Hydrogen (€1.21/kg):€1.21/kg ÷ 33.33 kWh/kg ≈ €0.036/kWh
Cost per kWh of Gaseous Hydrogen (€3.26/kg):€3.26/kg ÷ 33.33 kWh/kg ≈ €0.098/kWh

Liquid Hydrogen: Cost per kWh

Energy density: 120 MJ/kg
Energy per kilogram in kWh = 120 MJ/kg
÷ 3.6 MJ/kWh ≈ 33.33 kWh/kg
Cost: €8.00/kg to €9.50/kg
Cost per kWh of Liquid Hydrogen (€8.00/kg): €8.00/kg ÷ 33.33 kWh/kg ≈ €0.240/kWh
Cost per kWh of Liquid Hydrogen (€9.50/kg): €9.50/kg ÷ 33.33 kWh/kg ≈ €0.285/kWh

Summary of Costs per kWh

SAF: Approximately €0.1245/kWh
Gaseous Hydrogen:

GH₂ @ €1.21/kg: €0.036/kWh
GH₂ @ €3.26/kg: €0.098/kWh

Liquid Hydrogen:

LH₂ @ €8.00/kg: €0.240/kWh
LH₂ @ €9.50/kg: €0.285/kWh

Sources

International Journal of Hydrogen Energy, “The hydrogen storage challenge: Does storage method and size affect the cost and operational flexibility of hydrogen supply chains?” https://doi.org/10.1016/j.ijhydene.2023.06.269
International Journal of Hydrogen Energy, “Liquefied hydrogen value chain: A detailed techno-economic evaluation for its application in the industrial and mobility sectors” - https://doi.org/10.1016/j.ijhydene.2023.10.107
Sustainable Energy & Fuels, “Future costs of power-to-liquid sustainable aviation fuels produced from hybrid solar PV-wind plants in Europe”, 10.1039/D3SE00978E https://pubs.rsc.org/en/content/articlelanding/2024/se/d3se00978e
International Journal of Hydrogen Energy, “Impact of hydrogen refueling configurations and market parameters on the refueling cost of hydrogen” https://doi.org/10.1016/j.ijhydene.2017.05.122
IEA, “Global Hydrogen Review 2021” https://www.iea.org/reports/global-hydrogen-review-2021
US DOE, “Hydrogen Delivery and Dispensing Cost” https://www.hydrogen.energy.gov/docs/hydrogenprogramlibraries/pdfs/20007-hydrogen-delivery-dispensing-cost.pdf?Status=Master

Figure 1: Cost per kWh by fuel type.

SAF: 0.0857 kg/kWh, 0.1072 L/kWh
Gaseous Hydrogen: 0.03 kg/kWh, 333.89 L/kWh
Liquid Hydrogen: 0.03 kg/kWh, 0.4237 L/kWh

Liquid hydrogen, in terms of its mass to energy ratio (kg/kWh), is favorable when compared to other fuels – but the volume the hydrogen occupies (l/kWh) is still significant, due to its very low density.

Using the figures above, we can see that liquid hydrogen requires ~4x the space that SAFs would to provide the same amount of energy – but the mass for that same amount of energy needed, compared to SAFs, is almost a third.

This means that, with sufficiently voluminous storage tanks, we could truly capitalize upon LH2’s mass to energy density. However, such large tanks create separate challenges – due to the weight of the tanks themselves (minus propellant), and their aerodynamic implications.

Mass to energy density

Figure 2. Comparison of energy density for different fuel sources
Source: https://www.energy.gov/eere/fuelcells/hydrogen-storage