Optimising Liquid Hydrogen Refuelling: A Feasibility Study into Subcooled Methods -Our intern Julian examines a novel approach to liquid hydrogen refuelling.

Hydrogen refuelling in aviation isn’t just a logistical hurdle, it’s an engineering puzzle layered with fluid dynamics, cryogenics and data systems. What began as a continuation of earlier research into refuelling procedures quickly evolved into something far more unexpected. Starting with a focus on real-time monitoring between aircraft and ground systems, our intern Julian Winterdal followed the thread left by his predecessors and explored an alternative: subcooled liquid hydrogen. Borrowed from the heavy-duty transport world, this approach could potentially sidestep complexity altogether and transform how aircraft are refuelled. In this interview, we spoke with Julian about his research, his pivot and the potential impact of sLH₂ on the future of sustainable aviation.

How did you narrow down from the broader topic of the DutcH₂ Aviation Hub to your specific research focus?
I was initially tasked to continue research on Jelmer’s findings, which concerned the process of refuelling of liquid hydrogen for aviation, which is a very relevant topic within the DutcH₂ Aviation Hub. At the time, the next step for me was to dive deeper into his research. This next step was to specifically look into designing a data exchange system that monitors and communicates the pressure and temperature between the aircraft and the refuelling unit. Such a data system complicates the design significantly, especially when you compare it to current refuelling systems for Jet A, which are very simple in comparison. That’s how the process went for me initially to come up with the topic of the research.
 

How does your approach relate to the work done by Maurice and Jelmer? What did you take away from their research?
Maurice initially researched the operations and logistics that were required on the airport itself for the introduction of hydrogen into aviation. Jelmer then continued on this research, by narrowing down the research done by Maurice. As explained before, Jelmer really dove into the aspect of refuelling technology specifically. He laid out the procedure and an initial concept for a general liquid hydrogen refuelling system to be used in aviation. 

Jelmers research did not consider the data communication system, which was then a logical next research topic for the next intern. This was eventually the starting point for my research. I used the knowledge obtained by his research to start my literature research.
 

Where and why did you choose to take a different direction and what insights, challenges or new opportunities did that lead to?
After several weeks of reading scientific literature on the subject, I found that very recently a more efficient refuelling method has been demonstrated in the heavy duty industry. The refuelling station was designed and manufactured by Linde and Daimler Truck. They called it the Subcooled Liquid Hydrogen (sLH₂) Refuelling Station. 

Their technology is based on initially increasing the pressure of the liquid, to a point where evaporation losses during refuelling are eliminated. Since evaporation losses are eliminated, the flow inside the system can now be evaluated as a single-phase flow, which greatly simplifies the process. This then eliminates the need for this data communication system. My first question was then immediately if this could be a feasible method for refuelling in aviation as well. After several convincing discussions with my supervisors Loes and Stephen, we finally agreed that the best way forward for my research was to focus on analyzing the feasibility of this method of refuelling for aviation.
 

In what way does your research contribute to accelerating the transition to sustainable aviation?
Refuelling of liquid hydrogen has been a challenge for decades, where usually complex sensor and communication systems are required, accompanied with low mass flow rates and substantial losses in the refuelling process. This makes refuelling liquid hydrogen a pain point in the entire supply chain, specifically when you consider large aircraft. My research brings attention to a new method, where these problems are minimized. 

Because ideally, the refuelling procedure has to be as similar to current jet A refuelling practices as possible. sLH₂ refuelling seems to be a promising solution for this, ensuring minimal extra training requirements for refuelling operators. At the same time, using subcooled liquid hydrogen refuelling, similar turnaround times and minimum losses in the process can be achieved.
 

How could your work be translated into a concrete product or service within this ecosystem?
The model could form the basis for an actual subcooled liquid hydrogen refuelling station, to be manufactured and used in the future to refuel liquid hydrogen aircraft. However, to design and develop an actual prototype would require for example a lot more research, gathering of experimental data, design, data verification, validation and certification. However, for now, the possibilities and advantages are highlighted by my research, and will hopefully spark a lot of interest among researchers and designers at universities and companies to start designing and manufacturing an actual sLH₂ refuelling system for aircraft refuelling.
 

Looking back at the start of your internship, what would you approach differently now?
I look back on the internship as a great learning experience, where I even got involved in several other projects that were running in the company at the moment. There are almost no things that I regret or would have done differently. The only thing that could have been interesting, was to seek more engagement with actual researchers in this field. It could have definitely added to the experience and the results, had there been an opportunity to arrange for example some interviews with experts in the field, to get an insight into their view on the concept. Their insights could have been very helpful into directing the research and helping with modelling the system.
 

What do you see as the next steps for the topic you’ve been working on?
There is still a lot to do before hydrogen powered aircraft can truly take off at any airport in the world. The next critical steps for fully enabling this, are green hydrogen production, efficient hydrogen transportation methods, liquefaction plants, airport infrastructure requirements, loss-free storage methods, efficient refuelling technologies, compact and light hydrogen fuel systems and of course, efficient conversion into propulsion inside the aircraft. Work into any of these topics can be seen as critical steps into enabling liquid hydrogen aircraft in the future.
 

In your view, what would be a valuable angle for a future intern or researcher to explore?
There are many aspects of the model that can be improved. One instance would be to provide more detailed models to provide more accurate physics. The downside is that two-phase flow is intrinsically complex, and most often requires costly CFD calculations. 

One example where the model could be improved easily, is by implementing more in-depth calculations of the heat flux for the airport storage tank, fuel hose and the aircraft tank. This could provide better predictions of boil-off and evaporation in the refuelling system. Multiple models for conductive, radiative and convective heat transfer need to be produced for this. 

Another aspect could be a more detailed design for the pump, to accurately predict cavitation and performance of the pump, in terms of pressure increase, temperature increase, and mass flow rate. A detailed model for evaporation in the pipe, using several detailed models for each corresponding evaporation zone, is another aspect in the model that can be improved.

What’s the next step for you?
The next step for me is to finish my Master’s degree in Aerospace Engineering at the Delft University of Technology. I will start my thesis project on liquid hydrogen fuel systems in aircraft, to hopefully be able to further contribute towards making hydrogen aircraft a reality in the near future. 

And what are the next steps regarding your research at Unified?
As I said, there are many aspects of this project that are still to be improved and require additional research. Unified International is looking for a new intern to continue this research project in liquid hydrogen refuelling. At Unified International, we believe in guiding interns to develop the talents of tomorrow and help shape the future of the workforce. 

Looking for an internship at Unified?

If you are looking to do an internship project on finding solutions to one of the most challenging problems for realising sustainable, emission-free aviation, please send an email to loes@unifiedinternational.net.