Hydrogen fuel cell aircraft propulsion

In electric aircraft designs with fuel cell technology, batteries are typically used to regulate power output or as back-up fuel source, but not as primary energy storage. The reduced battery capacity requirements, together with the high energy density of hydrogen per unit mass, alleviate a major constraint of electric flying (related to the low energy density of current batteries that prohibit electric flights beyond short-haul flights).
Major manufacturers are exploring fuel cell electric aircraft designs and several small aircraft have been tested.
The low volumetric energy density of pressurised hydrogen, about one-eight that of jet kerosene even at pressures of around 700 bar, means that cryogenic hydrogen (hydrogen cooled to below -250°C such that it becomes a liquid and increases its energy density to around one quarter that of kerosene) is likely more practical for longer distance flights and is preferred by existing programmes. This calls for new aircraft design with fuel tanks potentially located in the fuselage rather than in wings to meet flying range requirements, which would reduce the space available for passengers.
Hydrogen aircraft are at an early development stage and commercial application in small regional jets is only expected in the 2030s, at the earliest.