August 7, 2020, ©. Leeham News: In our series on Hydrogen as an energy store for airliners we now look at the emission targets one is chasing and then discuss for what type of airliner does a hydrogen propulsion system make sense.
The CO2 emission target as expressed by Air Transport Target Group (ATAG) is shown in Figure 1. The graph is from the EU report on Hydrogen Powered Aviation.
Figure 1 shows we were on route to increase the air transport CO2 emissions to about double the 2018 value by 2050 if the traffic had developed without the COVID pandemic. This assumes a 2% improvement of airliner efficiency per year (the present trend is about 1.5%/year).
The target as defined by ATAG is to halve the emissions level from 2005 by 2050. This is a sporty target as we only have 30 years to achieve it.
We need to prioritize the first moves to focus our innovation and development resources where they bring maximum effect.
We shall focus the first airliner development for a segment that suits the constraints of a hydrogen powered airliner (the energy will take more space) at the same time as we achieve a maximum effect on the emission curve in Figure 1.
Figure 2 shows the hot spot of emissions to be in the segment between 81-165 seaters and the next segment 166-250 seaters.
The reality is easier than that. The Boeing 737-800/8 and Airbus A320ceo/neo dominates this segment, and these have 150 to 180 seats dependent on when a cabin update was made. They represent 11,000 of the 25,000 airliners that fly in our skies (in normal times). The other members of these airliner families represent a much smaller part of the segment.
So we should focus a hydrogen-powered airliner at the segment of 165 seats plus any increase in seating that follows from now and to 2035 when it shall be introduced. In fact, we shall size it for its first ten years in service, not the year of entry into service.
We can also see the range sweet spot is between 500nm and 1500nm. This is important as LH2 has three times higher energy content per kg (which is good) but it takes four times the volume of jet fuel (which is problematic).
It will help us if we let the over 2,000nm range segment be flown with the jet-fuel powered aircraft in the segment. This range segment is not the center of emission and it will skew our aircraft to an inefficiently large LH2 storage tank if we try to cover these routes.
For the development of a hydrogen airliner that shall start bending the CO2 curve down by 2035, we shall focus the seating segment that represents the 165 seat aircraft of today. Would it be 180 seats (one seat up per year)? Perhaps, with the damping effect of the pandemic, we might land there.
We see from Figure 2, there is no point in going for the easier to achieve regional hydrogen airliner. We burn our powder without bending the emission curve appreciably.
As for range, we shall go for the sweet spot there as well. A practical 1,500nm range which equals a maximum trip length of three and a half hours, means we need a nominal range of around 2,000nm (for in-service deterioration and ATC/weather margins).
By avoiding going after the routes over 1,500nm we make the sizing of the hydrogen propulsion system easier and we wouldn’t gain much in emission suppression by sizing for longer range for the first project.