Hydrogen is a renewable energy source that can be used to completely eliminate all CO2 emissions during flight and during the whole energy lifespan. Its use in fuel cells enables emission-free (including NOx and particle) propulsion. Moreover, fuel cell propulsion could reduce climate impact in flight by 75-90%, compared to 30-60% for synfuels, according to “Hydrogen-powered aviation” study. However, before its full potential can be realized, a number of technical challenges must be overcome.
The low volumetric energy density of hydrogen, about a quarter that of jet kerosene, together with the need to keep cryogenic hydrogen at low temperature, calls for new aircraft design. Fuel tanks may be located in the fuselage rather than in wings to meet flying range requirements, which would reduce the space available for passengers. Existing programmes prioritize cryogenic hydrogen over pressurized hydrogen at ambient temperature, allowing for longer ranges. However, Liquid hydrogen systems are expected to be practical for large commercial air transport applications, but the "in-tank" temperature must be kept at -253 degrees Celsius, making liquefying and storing liquid hydrogen difficult. Significant research and development challenges are brought about by the effects on tanks, fuel/distribution systems, replenishment, and overall system design, dependability, and safety. The availability of hydrogen, its distribution, the requisite recharging/refuelling infrastructure, and renewable generation are essential to the overall success of this approach.
There are two aircraft designs that use hydrogen as an energy carrier; hydrogen can either be used to operate a fuel cell on an electric aircraft or used as combustion fuel to power a jet engine. Neither produces direct CO2 emissions during the operation of the aircraft (although the upstream emissions depend on the technologies used to produce and transport the hydrogen). Combustion of hydrogen produces water vapour, which can contribute to global warming.
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).
Hydrogen aircraft are at an early development stage and commercial application in small regional jets is only expected in the long term. An early application of hydrogen in aircraft will be in auxiliary power units (APU) for non-propulsion applications such as lighting, HVAC, or cabin pressurization.