Liquid Hydrogen Fuel System Shines Light on Renewable Energy in Commercial Aviation

­Germany’s MTU Aero Engines has just taken a major step towards decarbonizing one of the most stubborn industries on the planet with a successful multi-week testing of a liquid hydrogen fuel system for its “Flying Fuel Cell.”

To be fair, commercial aviation’s infatuation with fossil fuels isn’t entirely their fault. While air travel encompasses a comparatively large 3% of total global emissions, jet fuel is insanely efficient, with a specific energy around 12,000 Wh/kg.

Air travel is a constant balancing act between being light enough to fly and heavy enough to function properly (and carry passengers). Jet fuel hits that sweet spot perfectly, which is why commercial aviation has lagged behind other industries – like the automotive world, which doesn’t have to worry about aerodynamics.

Alternatives just aren’t remotely comparable, at least yet. The few examples of lithium-ion electric planes produce dramatically fewer pollutants (if not emissions-free) and they’re a lot cheaper – about four to six dollars per operating hour, with jet fuel running a comparable $1000-$2000.

But batteries also have comparatively low energy density, and as MIT Tech Review points out, modern batteries can only power the smallest planes, “and even for those, the trip will be about as far as a long bike ride.”

Future advances could change that, especially with the theoretical energy density of 2,700 Wh/kg for lithium-sulfur batteries (Li-S). Indeed, Li-S could solve many of the issues with battery-powered air travel.

Researchers think Li-S batteries could reach nearly double the energy density as li-ion, which could help solve the weight issue. But we’re not there yet.

The other viable energy source for commercial aviation is hydrogen, and with a specific energy of about 39,500 Wh/kg, it’s extremely promising, not to mention greener, emitting only water vapor and heat.

But, liquid hydrogen – despite its high specific energy – has about ¼ of the energy by volume as jet fuel, making the storage tank prohibitively large. The required storage temperature – about -250°C – also creates issues.

The immediate solution is to employ a hybrid system, or, in the case of MTU Aero Engines, designing a complete liquid-hydrogen fuel system from the ground up, consisting of tanks, sensors, heat exchangers, valves, safety systems, and controls.

And the first test of this hydrogen fuel system was a runaway success, with Dr. Günther Schullerer, Director of Future Technologies, MT Aerospace, remarking “Now that we have fully confirmed the functionality, we can focus our shared skills on lightweight engineering, integration concepts, and optimized space utilization.”

The company will begin testing a product-specific full-system Flying Fuel Cell beginning in 2026, and we’ll see just how close we are to overhauling commercial aviation with renewable energy.