A direct fuel-cell-based hybrid system for airborne applications
Creators: name: Willich, Caroline
Publisher: name: Universität Ulm
Year: 2024
Subjects: subject: Fuel cell hybrid
subject: Aviation
subject: Direct hybrid
subject: DDC 620 / Engineering &amp; allied operations
subject: Fuel cells
subject: Brennstoffzelle

description: The work presented in this book concerns the development of a direct hybrid based on a fuel cell and a battery. It examines its behaviour under different operating conditions and various other relevant aspects for its application as a power train for an electric aircraft. At first the fundamentals of fuel cells and batteries are explained on cell as well as on system level and different fuel cell and battery types are introduced. For both technologies the pressure dependency of the respective technology is addressed. The next section introduces the concept of hybridisation in general as well as various possible architectures. The concept of the direct hybrid and its characteristic performance and current-voltage relationship is explained. The next chapters then examine the influence that hybrid ratio, battery state of charge as well as the operation at low ambient pressure have on the current-voltage characteristics of the direct hybrid. A newly developed power management module is presented next, that enables to control the direct hybrid by choosing operating modes in which the power required for propulsion either comes from the fuel cell, the battery, or both at the same time when working together as a hybrid. When switching from one mode to the other certain limitations and procedures for transitioning from one mode to the other must be defined and observed during operation, which is explained in a separate chapter. The examined dependencies of the hybrid behaviour have to be considered when determining the size of the fuel cell and battery for the power requirement of a given flight profile. A method for optimising the sizes of both the fuel cell and the battery is presented in chapter 4.5, which takes into account the impact of fuel cell and battery behaviour that were examined in the previous chapters. The second part of the book introduces the concept of pressurising the fuel cell at high altitudes, to mitigate the lower power output from the fuel cell at low ambient pressures. Possible compression technologies are compared and assessed for their suitability for the target application. Since electrical turbo-compressors currently show the highest potential for the application their functioning principle and characteristics are explained in more detail. Since the output pressure of turbo-compressors varies with the inlet conditions (temperature and pressure), the performance of a Rotrex EK10AA compressor at low inlet pressures and varying temperature is examined in detail and the influence of low inlet pressures on a power limit that is imposed by the inverter of the electric motor driving the compressor is described. When pressurising a fuel cell its power output increases, but the energy required for compression in an electrical turbocharger increases almost linearly with its pressure ratio. Therefore, the optimum pressure, above which a further increase in pressurisation does not provide any additional benefit and would only make the system larger and heavier, is examined, as well as the influence of altitude (low inlet pressures) and pressure losses on this value. When combining a fuel cell and an electric turbo-compressor the dynamic control of the system is also a challenge. To ensure a stable and safe operation, instable operating conditions, like compressor surge, must be avoided and a strategy for surge avoidance is introduced. At the end an outlook for further research challenges and opportunities is given.