Properties of Hydrogen
Being an alternative to traditional jet fuel, hydrogen has a higher energy density per unit mass but a lower energy density per unit volume, and containing the hydrogen at high pressure would require a heavy container. In aircraft heavy containers are not an option, and therefore regular carbon fibre tanks are often used, which can only sustain a pressure of about 350 bar. This is significantly lower compared to steel hydrogen containers (used in cars and ships) which can sustain 500 to 700 bar. This limitation decreases the amount of energy that can be spent on the propulsion by about half. Alternatively, as with some rockets, cryogenic liquid hydrogen could be employed.
If hydrogen were available in quantity from renewable energy sources, its use in aircraft would produce fewer greenhouse gases (water vapor and a small amount of nitrogen) than current aircraft. Currently, however, very little hydrogen is produced using renewable energy sources, and there are several serious obstacles to the use of hydrogen in aircraft and other vehicles. Due to the way it is produced, and the relatively inefficiencies of its production given current technology, hydrogen is a much more expensive fuel than fossil fuels.
Liquid hydrogen is one of the best coolants used in engineering, and it has been proposed to use this property for cooling intake air for very high speed aircraft, or even for cooling the vehicle's skin itself particularly for scramjet-powered aircraft.
Read more about this topic: Hydrogen-powered Aircraft
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