How feasible is retrofitting aircraft to run on hydrogen, compared to other alternatives?
The central point is not a question of should we do it, but that we have to do it. There are a lot of components that already exist. Rather than developing an entirely new aircraft, let’s target only the pieces we must change. That means the combustor up to the fuel system and fuel tank, and that’s pretty much it. Retrofitting is one key driver of the research we’re doing at C-PARC [Combustion and Propulsion for Aviation Research Center], we have a path and a plan to get there.
What is the long-term plan for producing aircraft that are outfitted with hydrogen propulsion?
It is a question of budget and timing. What I can tell is that for the premix combustion mode that we are researching—with potential for up to no NOX and no CO2—right now we are at a TRL level of 1 for several components, so it’s going to take time. The point is there are several stakeholders, everyone feels the momentum on hydrogen, and it can accelerate, but the progress is going to be step-by-step for safety and robustness. To overcome the current scientific gaps and demonstrate the technology we’re developing— for example in the testbed that Airbus is putting together called ZEROe Demonstrator— I will say 10 years to get a baseline design for fly test before it then can be deployed.
What’s the timeline for seeing the first aircraft retrofitted for hydrogen?
As of today, Airbus and CFM (GE/Safran) and a few other major (Rolls Royce, for example) and startup companies are working on testing H2 propulsion within the next decade. This is great. These technologies are not using premixed combustion. C-PARC works now on designing the next generation of engine, fuel system, and tank, addressing the knowledge gaps for premixed combustion/combustors. The premixed hydrogen engine will be a breakthrough having the power from combustion and the environmental impact of a fuel cell. We shouldn’t stop there. Those technologies need to be deployed worldwide and not stop at the demonstration.
What’s the benefit of using hydrogen-based propulsion over, say, biofuels or so-called sustainable aviation fuels?
Hydrogen-based propulsion is cleaner. If you look at the CO2 footprint of the narrow-body aircraft as of today, those aircrafts emit half of total aviation CO2. If you target retrofitting those narrow-body aircrafts with H2 propulsion, you’ll reduce half the footprint. When you look at the data, the narrow-body aircraft flies, on average 850 miles, which is roughly less than a quarter of its maximum Breguet range. Similar data hold for wide-body aircrafts. That means, onboard, you do not need as much [H2] fuel as we think to mitigate strongly the CO2 footprint. When you do combust H2 with air, nitrogen from the air can disassociate and make some NOx, a product not necessarily bad from a climate change perspective, but bad for health, like respiratory. C-PARC develops onboard concepts for N2/O2 separation to reach zero NOx.
What are the limitations to using hydrogen propulsion?
The main disadvantage is you won’t get the maximum range of the narrow-body aircraft—perhaps it will be half—but as I said, you reduce most of the CO2 emissions. So airlines and airports may need to modify aircraft refueling schedule, avoiding carrying unused fuel for the most flown averaged distance. Whereas sustainable aviation fuel (SAF) is a drop-in fuel, implying no changes to infrastructures, hydrogen fuel transport and storage will need to be implemented.
What have been some of the challenges in developing this type of propulsion for aircraft?
For SAF, it was really to have the nearly identical fuel properties with kerosene to be able to drop-in those in current engines so that we don’t change anything. To shift to H2 requires changing some of the infrastructures on airports. Many of the technologies exist infrastructure-wise, but they are just not used in the aviation and aeronautic sectors today. In order to modify existing infrastructures, there is a lot of support and commitments from public agencies in the U.S. and other countries. But what’s missing today is all airframe and engine manufacturers getting into the race. For example, right now, Boeing is investing in SAF, as a strategic direction. If Boeing gets into the race for hydrogen propulsion, in my opinion, that will help the aviation community.
