April 15, 2022, ©. Leeham News: Last week, we examined different airliner types’ power requirements and the importance of their size classes in the market.
Now we look at what propulsion system alternatives are available when using hydrogen as the energy source and their principal advantages and disadvantages.
We listed the typical power levels for different airliner classes last week, Figure 2.
We also looked at the expected new deliveries per market segment based on a market study from the JADC.
Figure 1 shows the contribution of CO2 emissions by market segment. We see the main emissions (43%) come from the 166 to 250 seat segment, with long-range over 250 seat second (30%) and short-range (81-165 seats) third at 24%. The segments below 80 seats represent less than 4% of CO2 emissions.
Consequently, our primary need for a green propulsion system is in the 10 to 20 MW range on an aircraft level. Propulsion systems below 5 MW on the aircraft level are not helping us reach 2050 emission goals.
We have seen in earlier articles that battery-based or hybrid-electric aircraft are not viable solutions to help with Greenhouse gas emissions from air transport.
Sustainable Aviation Fuel, SAF, is the easiest and most straightforward solution. Still, as its volume production path (the biomass path is too limited) is additional processing of green hydrogen, its cost will be higher than hydrogen fuel.
A hydrogen propulsion system can burn H2 in a modified gas turbine or generate electric energy in a fuel cell that drives an electric motor to drive the fan/propeller.
We will deep dive into both alternatives in subsequent Corners. Let’s examine on a high level how these alternatives can meet our powerplant objectives for our airliner segments.
As I’ve outlined before, the world’s first jet engine, von Ohaine’s S-1, started on hydrogen in 1937 as he had difficulty getting it to run on kerosene. Hydrogen is in many respects a better fuel for gas turbines than our kerosene-based jet fuel:
Hydrogen as a fuel for gas turbines has been used in ground-based gas turbines for decades and has been trialed several times in airborne test aircraft.
The advantages of a gas turbine as a hydrogen propulsion system can be summarized as:
But the LH2 gas turbine alternative also has disadvantages:
A fuel cell system combines H2 with air in a fuel cell and forms heat, electricity, water, and oxygen-depleted air as outputs. The electric output then drives an electric motor to turn the propeller/fan.
Fuel cells for ground use are established and mature technology. For airborne use at the power levels we need, they pose thermal and electric management problems combined with high mass and volumes. Recent trials with small fuel cell propelled aircraft (H2FLY, MAHEPA) confirm their advantages and problem areas.
The fuel cell alternative’s advantages can be summarized as:
But there are also disadvantages:
We will go through these alternatives in more depth in subsequent Corners.