A hydrogen aircraft does not extract its energy from the usual fossil fuel (kerosene, a petroleum product) but from hydrogen. These planes are environmentally friendly when the hydrogen is obtained by electrolysis of water and the electricity required for this is also environmentally friendly. A conventional jet or piston engine of an aircraft runs without much adjustment to hydrogen, but the efficiency of the combustion engine is thermodynamically limited. Basically, it is more efficient to convert hydrogen fuel with a fuel cell into electricity and to use an electric motor, but the theoretical maximum efficiency is still far from being approached. Hydrogen vs. kerosene
The amount of energy (combustion) per kg of hydrogen is three times higher than kerosene. That's beneficial because weight is important with airplanes. A problem with hydrogen is the tank volume. Strong compressing decreases the volume, but that moves the problem to the weight of the high pressure tank. Even from liquid hydrogen, the volume is four times larger than kerosene with the same energy content; The pressure is then low, but the hydrogen must be stored at extremely low temperature (20 K = -253 ° C). Many other ways to store hydrogen efficiently, chemically bound or as metal hydride are still in research.
The greatest danger with both substances is of course a fire in an accident. In hydrogen, the ignition temperature is 550 ° C; that of kerosene is about 300 ° C. Hydrogen burns quickly and almost completely vertically while outflowing kerosene can form large burning puddles. Hydrogen quickly forms an explosive mixture with air (explosion limits between 4 and 76 volume%) that can explode by the smallest spark while the vapor pressure of kerosene is so low that it is difficult to ignite at room temperature and only burns with a pit .
A tank with compressed hydrogen is thick, like a lpg tank. Also a liquid water tank is not flat to keep the evaporation (determined by the surface volume ratio) low, and by the insulation. Therefore, the hydrogen is not as common with kerosene in the wings but in the hull. Pilot projects
In the Soviet Union, a hydrogen-powered Tupolev-155, a modified Tu-154, ran from 1988 until the disintegration of the USSR in 1991. The liquid hydrogen was in the trunk. >
In the CRYOPLANE project of the European Union, Airbus has researched hydrogen companies with other companies. This was terminated in 2002 but started again in 2007.
Small planes were built, eg in 2008 the Boeing Fuel Cell Demonstrator, with compressed hydrogen, but the action radius was low.
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