Aviation is very energy-hungry. While hydrogen is quite efficient in terms of energy per mass (141.8 MJ/kg), it is awful in terms of energy per volume. Aircraft today use their wings for fuel storage, which helps to reduce bending loads and uses a volume unsuitable to transport payload. This will not work for hydrogen.
The biggest problem is storage, and none of the solutions is really attractive:
- Kryogenic: Storing the hydrogen at a temperature where it is liquid (below 20°K) greatly reduces volume demands, but now you need to add insulation, and even then the energy per volume is only a quarter compared to hydrocarbons. This is used in rockets, because they can rely on extensive ground support before launch and use up the stuff in a few minutes. Airliners, however, need to store their fuel over tens of hours, so their hydrogen tanks will be much heavier and bulkier than those of rockets.
- Compressed: This avoids the energy input for liquefaction, but now you need a strong and heavy pressure vessel. Current gaseous hydrogen storage uses up to 700 bar pressure, but even then the volume demand is higher than that of kerosene for the same energy content.
- Hydrides: Some metals like magnesium or sodium can bind hydrogen and release it when heated. However, even in the most efficient compounds like sodium tetrahydroaluminate, only 7.4% of mass is useable hydrogen, making this exquisitely unsuitable for mass-critical applications like aviation.
Currently a mixture of cryogenic and compressed storage looks the most attractive and has been tested widely.
The first (partially) hydrogen powered aircraft was the LZ-127 Zeppelin which used a mixture of gasses of equal mass per volume as air (Blaugas) to power its engines.
Tupolev designed and flew the Tu-155, the first hydrogen-powered heavier-than-air aircraft, in 1988. The results showed that the principle worked, but could not compete with kerosene.
Currently, several demonstrators are in operation, but none with the hope of widespread application - as it stands, binding hydrogen chemically to carbon will still give the best way of storage (liquid and unpressurized) for aviation.
Hydrogen becomes the best source of energy only when combustion speed is critical, such as in supersonic combustion ramjets (scramjets).