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Stuttgart-based H2Fly and Stuttgart Airport in Germany recently announced the launch of a joint project to construct a Hydrogen Aviation Center, scheduled to open in late 2024.
The center will be managed by H2Fly and will provide a central facility where businesses and scientific institutes can develop state-of-the-art concepts in zero-emission hydrogen-electric aviation, and test them on the ground and inflight in a real-world environment.
For eVTOL companies, hydrogen technology could offer an alternative energy source to its current battery technology, opening up new markets for regional air mobility or other applications.
California-based Joby Aviation sees potential in the technology, reportedly acquiring the German startup in April 2021. While the eVTOL developer is currently working on getting its S4 eVTOL aircraft certified for urban air mobility mission, the acquisition shows that Joby has one foot in the door on hydrogen development.
Vertical reached out to Josef Kallo, founder and CEO of H2Fly, to discuss the status of the company’s progress and how it can contribute to advanced air mobility (AAM).
This interview has been edited for length and clarity.
Alex Scerri: Josef, what is your background and what was the vision for H2Fly?
Josef Kallo: I am an electrical engineer by training and a passionate recreational pilot. I want to be able to fly 30 years from now and to do that, we need a new fuel. From the moment I started flying, I was conscious of the impact of burning fossil fuel and there was also the issue of aircraft noise. This opportunity with hydrogen fuel produced from renewable energy sources is like living a dream, where we have the possibility to develop a clean technology for aircraft propulsion that also helps reduce the acoustic impact.
In 2015, we founded H2Fly after my time at Ulm University and at the German Aerospace Center (DLR). I always had an entrepreneurial spirit, starting my first companies when I was 18 and concurrently holding multiple positions. H2Fly was started with my own funds and we made rapid progress in hydrogen fuel cell technology.
We developed a couple of generations of powertrains together with DLR and Ulm University, also driven by us. Crucially, we have all the required technology bricks from liquid hydrogen storage, the hydrogen fuel cells themselves, fuel cell cooling technology, the inverter, and the electric motor. This is very important as we have a complete powertrain solution, which differentiates us from some of our competitors. Today, our target is a 40-seater aircraft with hydrogen fuel cell propulsion, using liquid hydrogen.
Alex Scerri: Can you briefly explain the science behind hydrogen fuel cells?
Josef Kallo: Hydrogen is the fuel, and we have oxygen from ambient air. We use these in an electrochemical reaction where, with the help of a catalyst and an electrolyte membrane, we combine these two elements that results in electric energy as an output, with the only waste product being water. As long as you have a hydrogen and oxygen supply, it is like an inexhaustible battery, also with no moving parts.
The big advantage is that the fuel is not burned with oxygen as in a traditional thermodynamic engine to increase pressure and heat, which then needs complex machinery and electronics to generate electricity. Therefore, with a hydrogen fuel cell, electric energy production is much more efficient.
Alex Scerri: There are other projects that are doing flight testing with similar technology. How would your technology compare to these and what would be the differentiator?
Josef Kallo: The advantage is that we have an aircraft that has been flying for at least four years that has the blueprint of the architecture needed to build a 40- to 50-seat aircraft. We have logged more than 100 takeoffs and landings and are now on the verge of transitioning from gaseous to liquid hydrogen storage that will double our range.
We are not in the business of building demonstrators as we have completed that step. Our aircraft — the HY4 — has flown several cross-country flights, and for the past 2.5 years, we are focusing on maturing the aviation-grade development of the technology.
Alex Scerri: Earlier, you mentioned the advantage of using a hydrogen fuel cell over burning it in, for example, a microturbine which may be a lighter component than the fuel cell pack.
Josef Kallo: When hydrogen is burned in a turbine, this results in very high temperatures with a very high flame speed. That combination leads to the production of nitrous oxide (NOx). If you want to eliminate NOx emissions, then you need to find a way to reduce the temperature.
Considering the Carnot cycle, a lower temperature will result in a lower efficiency, so effectively, the Carnot efficiency is lower than that of an electrochemical fuel cell. While it is true that the fuel cell is heavier, you then save much more weight at the storage side of the whole system, for the same range.
Alex Scerri: One of your focus points now is to transition to liquid hydrogen storage.
Josef Kallo: We started the project together with Air Liquide in 2020. In January 2023, we started implementing hydrogen liquid storage, which was tested on ground. This week, I am flying to Grenoble, France, to witness the first refueling using liquid hydrogen. We will continue ground testing until May and go into flight testing during the summer. This will also be on our HY4 platform. It will help us to advance toward the 40-seater Dornier (Do)-328 where we started to develop and integrate the megawatt (MW) scale cooling and fuel cell system.
Alex Scerri: Can you share any timeline of when we could see the hydrogen fuel cell powered Do-328 flying?
Josef Kallo: We are very conservative on this because we are not aiming for a demonstrator, which we have done already with smaller aircraft in eight test campaigns. Our target for this aircraft is the next generation of improved, aircraft standard, high-performance fuel cells rather than state-of-the-art. That is why we are saying 2025 for a pre-production aircraft, initially flying with a permit to fly, then by 2028, go into the certification phase.
Alex Scerri: You have been quoted that you can expect to have an aircraft certified to European Union Aviation Safety Agency (EASA) CS-25 standard in about seven to eight years. EASA Special Condition SC E-19 – Electric/Hybrid Propulsion System still does not include hydrogen as an energy option. When do you expect that you will have that regulation in place to give you the tools for certification?
Josef Kallo: This may take some time and for the moment, we are using our internal standards to do all the testing needed to get to the permit to fly stage. We pay close attention to the regulations that are already in place, and supplement these with our own standards and means of compliance.
Work is also being done jointly by EUROCAE Working Group WG-80 and SAE AE-7AFC to develop the standards for fuel cell systems, as well as other working groups looking into other aspects, such as hydrogen storage, etc. It is a continuous learning process from which the whole industry will benefit, but of course, we have the advantage of hands-on experience and we carefully protect our intellectual property (IP).
Alex Scerri: Some critics of hydrogen sometimes bring up the Hindenburg accident. How do you counter that?
Josef Kallo: The main factor in that accident was the design and material of the envelope, which was made of rubber. There were also issues with the aluminum and iron oxide coating. A small static electricity discharge is thought to have started a fire that led to failure of the containment structure. The hydrogen leak and fire were therefore secondary elements, not the root cause.
Nearly one century later, we now store hydrogen in very strong, rigid aluminum tanks with plastic and carbon fiber coatings. We also have the benefit of nearly 100 years of standards development on how to store, handle and use hydrogen. In the last 30 years, we have made giant strides in the quality of the materials and supporting regulations and standards. This is confirmed by the absence of serious incidents even if hydrogen is used in a wide variety of applications and industries.
Alex Scerri: You are working closely with the European Union (EU) that appears very determined in promoting hydrogen as a fuel.
Josef Kallo: When you see the amount of renewable energy needed to fulfill the ambitious emission reduction targets for air transportation in the EU, you really need the most efficient fuel available which is hydrogen. This fuel has only one conversion step — no additional chemical conversion steps, no additional heat, no reactors, etc.
The required investment to produce renewable energy is the lowest for hydrogen when compared with synthetic fuel (synfuel). If we only needed, say, a two to 5% reduction of fossil fuel use, we could go for battery-powered electric systems. If we needed 8%, we could use synfuel. But if we really want to transform the whole energy supply sector to rapidly replace fossil fuels that nature provided us in a process that took millions of years in just a few decades, I have no doubt that the most efficient path is hydrogen.
Alex Scerri: What do you see as the most sustainable way to produce the amounts of hydrogen that we will need for aviation, including eVTOL development?
Josef Kallo: Definitely wind and solar energy but this must be at large scale — not gigawatt hours (GWh), but terawatt hours (TWh) and even petawatt hours (PWh) — in dedicated plants housing electrolyzers running continuously, as long as there is cheap wind and solar energy available. This is the only way to do the required radical transformation.
Alex Scerri: What are the scalability challenges from going from the HY4, roughly equivalent to an eVTOL, to something like the Do-328?
Josef Kallo: Basically, we have two challenges. One challenge is solved in the HY4 where we have a redundant multi-engine architecture. This means we have two segregated storage systems, four independent fuel cells, two inverters, two cooling systems and even two motors which are implemented in one hull. We know how to control this redundant system all the way from the fuel tanks to the propeller. That is the blueprint that we are using to design the Do-328, so the system architecture challenge is solved.
The bigger challenge is going from 120-150 kilowatt (kW) per module and upscaling that to 500-700 kW, or even up to 1MW per module. This means high voltage, high currents, breakers that must be developed, excellent insulation, electric field strength issues, etc. We have a very good idea how to do that even considering electromagnetic interference (EMI) and radio frequency interference (RFI) protection requirements in aviation. That is what we are working on now. Beyond that, we are also considering upscaling to 6-10 MW which needs even higher voltage and power density.
Alex Scerri: What can you share about your relationship with Joby Aviation?
Josef Kallo: We do not have much we can share on this. We are working together, and we at H2Fly are happy to be part of this. Joby will break new ground in electric aircraft certification.
Alex Scerri: Is there a message you would like to share with the AAM community?
Josef Kallo: If we really want to do the transformation toward an emission-free future, we have to think about the full supply chain — from providing renewable energy, storing it, and finally, using this precious resource efficiently without wastage. We can each focus on our individual business cases of specific links in the chain, but we must be always aware of the complete ecosystem, to grow successfully together.