Today, aviation is responsible for around 2.5 % of anthropogenic CO2 emissions. However, aviation represents a special case, as the share of human-induced warming is not identical to its share of CO2 emissions here. Experts estimate that aviation as a whole is responsible for around 3.5% of global warming. The reason for this is that the emission of aircraft exhaust gases and components contained therein, such as water vapor, nitrogen oxides or soot particles, at high altitudes has an additional warming effect on the climate. Strong global growth in air traffic is forecast over the next few decades. Despite it's currently still relatively small contribution to global warming, a significant reduction of aviations climate impact is required.

 

The use of hydrogen as a fuel for aircraft is currently the subject of intense debate. Hydrogen aircraft would only affect the climate by emitting water vapor at high altitudes. However, this impact would likely be little compared to the current practice of burning fossil kerosene. The construction of hydrogen aircraft is currently being driven forward by Airbus in particular. The company plans to bring the first hydrogen aircraft in commercial service by 2035. 

 

Current concepts envisage storing the hydrogen on board of the aircraft in liquid form. Other forms of hydrogen storage have either too low volumetric energy densities (cGH2) or too low gravimetric energy densities (LOHC, methanol, ammonia). Even for aircraft fueled with liquid hydrogen, the tank volume will increase three to four times due to the lower volumetric energy density compared to kerosene. For this reason, it is currently assumed that hydrogen aircraft will only be used for short- and medium-haul flights in the foreseeable future. However, synthetic kerosene (e-fuel) appears to be the most suitable option for reducing greenhouse gas emissions from long-haul flights. 

 

Two completely different approaches are currently being considered for the design of the propulsion system of hydrogen-powered aircraft. On the one hand, it would be possible to equip the aircraft with hydrogen turbines so that the necessary thrust could be generated by direct combustion of the hydrogen (“Turbofan” concept). On the other hand, the use of fuel cells is also conceivable. In this case, the hydrogen would first be converted into electrical energy. The electrical energy would drive several propellers via an electric motor, which in turn would provide the thrust (“Turbopro” concept). Which of the two drive concepts will be used in the future, or if even both variants will be realized, is unclear as of today.