(Ulm) – Hybrid systems that combine fuel cells and batteries are considered possible innovative solutions for environmentally friendly aircraft propulsion. “Not only do they achieve significantly higher ranges than purely electric aircraft, but they also offer the technological potential for upscaling to larger performance classes,” explains a research team from Ulm University. As part of the “EnaBle” research network, the scientists are pursuing the goal of further developing and optimizing highly innovative hybrid drive systems. The heart of the system is a 250 kilowatt electric powertrain module that uses compressed air-fed fuel cells.
In order to accelerate the path to industrial production and commercial use of this technology, the Federal Ministry for Economic Affairs and Energy (BMWi) is funding the EnaBle research network with eight million euros. The consortium includes the companies Diehl Aerospace and MTU Aero Engines, two industrial companies in the aviation sector, as well as the German Aerospace Center (DLR), the DLR spin-off H2Fly and the University of Ulm.
“We are working together on the development of a hybrid-electric drive consisting of a fuel cell, battery, power electronics and power management system,” explains Ronny A. Knepple, responsible engineer in the energy systems department at Diehl Aerospace. “The specific goal is the timely industrial implementation of light aircraft with up to 19 seats.”
This is how hybrid systems work
“The fuel cell produces electricity from hydrogen and thus ensures the energetic basis of the propeller drive. “Lithium-ion batteries provide additional power during takeoff or climb that is needed to reach cruising altitude,” explains Caroline Willich, scientist from the university’s Institute for Energy Conversion and Storage, how the hybrid systems work. 
The engineer leads the Ulm sub-projects together with her institute colleague Christiane Bauer. Among other things, the air supply module for the fuel cells is to be developed. “The fuel cells used here are operated with compressed air. Pressure charging makes the fuel cells more efficient and enables higher performance, explains Caroline Willich. “This is of particular interest in aircraft because they travel at high altitudes and therefore in the low-pressure area.”
The Ulm researchers are also responsible for developing and optimizing the performance management system. This must ensure “precisely, quickly and fail-safely that the battery provides additional energy for the drive when high power is required and can be recharged during the flight”. The system should be able to “react precisely and in an application-oriented manner to the requirements of different flight profiles”. The unique selling point at the Ulm fuel cell research site is a test stand that is integrated into an air-conditioned negative pressure chamber. Powertrain systems could be characterized and tested under realistic, flight-relevant conditions. The University of Ulm receives 1,8 million euros from the research network for its task.
Scaling is crucial for series production
By modularizing the drive train, the partners want to increase the scalability of the system, “which is ultimately crucial for a prototype to go into industrial production.” A modular concept also makes maintenance and repairs easier.
While the University of Ulm is particularly concerned with the module for the compressed air fuel cell, the fail-safe power management and the testing of the new hybrid overall drive train in the test facility, Diehl Aerospace provides a so-called Integrated Modular Avionics (IMA), an electronics unit made up of standardized components and Interfaces that ensure that the various systems in the aircraft can communicate with each other.
The Institute of Technical Thermodynamics at DLR is working on the development of the fuel cell and battery system. The engine manufacturer MTU Aero Engines ensures the integration of the developments into aircraft from the 19 to 80 seater class. The DLR spin-off H2Fly is dedicated to safety requirements and approval issues.
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Insights into the test bench: You can see electronic parts of a performance management system for hybrid systems that is currently being tested. / © Ulm University, Elvira Eberhardt
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The test stand is housed in a fully air-conditioned negative pressure chamber. Hybrid drive systems are also tested here under realistic conditions. / © Ulm University, Elvira Eberhardt
