Can it be green?
Environmental Friendly High Speed Transportation
The aim of this research axis was to analyse the impact of high speed emissions on the atmosphere and on the long term effects on the climate, while taking into account production and distribution issues as a function of the type of fuel.
The impact of human activity on the environment is a very complex one to determine, all the more when dealing with emissions of particles such as CO2, H2O, and NOx at high altitude, which is the main concern of high-speed transport.
High speed flight is associated to high altitude. For example, a Mach 4-5 aircraft would have an optimized cruise altitude in the range 25 to 30 km. At such altitude, the atmosphere is more stratified, dryer, and the consequences of mixing effects resulting from large scale transport and turbulence might deeply modify the mechanical and chemical processes need to be studied.
More specifically, in order to reduce environmental impact of jet engines, hydrogen fuelled jet engines are under investigation. Carbon dioxide (CO2) could be eliminated with the engine but water vapor (H2O) would be emitted from the engine and the environmental impact should be estimated. H2O has a large greenhouse effect but the amount of H2O emitted from an aircraft is very small compared to the global water circulation.
Consequently, the effect should be estimated by a simplified analytical model. The greenhouse effect of H2O at high altitude was also not quantitatively clear. Research on these subjects has been performed in Japan (JAXA), Germany and France for example.
Beyond this, the use of advanced biofuel, consuming massively CO2 for their production could be of interest depending of their actual global effect on climate (captured on ground and released at high altitude).
The main objective was to predict the impact of high altitude high speed flight emissions on the atmosphere on the long term evolution of the climate, while taking into account production and distribution issues, as a function of the type of fuel.
The outcome of this study provided a database in order to enable a complete comparison and trade-off analysis of the global effect of fuel choice on environment.
Example of emission distribution for HS fleet
Technology wise, the hydrogen fuel, though providing excellent range and cooling capacity, might not be the only alternative to consider, for other fuels (bio liquid hydrocarbons) might provide better overall climate/performance characteristics. Hence, the use of hydrogen is not an obvious greener solution than hydrocarbon fuels, and the latter are still good candidates for the high altitude / high speed application. An investigation of alternative designs using other fuels (e.g. LNG) is thus recommended.