Pressurizing the Fuel Cell in a Direct Hybrid System for Airborne Applications
Authors: Graf Tobias , Fonk Robin , Frank Daniel , Bauer Christiane , Willich Caroline 
Published: December 2024

The push towards achieving emission-free aviation is intensifying. The number of demonstrator aircraft like the HY4 form H2FLY or the ATR72 from Universal Hydrogen taking flight and gaining experience in electric aviation is increasing. One existing approach for the realization of an all-electric aircraft is a direct hybrid system comprising a fuel cell and a battery. During high-power phases of a mission, such as takeoff and climb, both the battery and the fuel cell contribute power. However, during cruise, power is solely supplied by the fuel cell to leverage the high energy density of hydrogen. In a direct hybrid system, the power share of the fuel cell and the battery is determined by the size and the operating conditions of both.
The low ambient pressures at high altitude negatively affect the performance of fuel cells. This is not only important in fuel cell only propulsion systems but also when operating a direct hybrid system. The impact of low pressures on the behavior of a direct hybrid system, linking a fuel cell and a battery without a DC/DC converter, is demonstrated. The effects of a non-pressurized PEM-fuel cell in a direct hybrid with a lithium-ion battery in a low-pressure environment was analyzed previously (see attached figure).
However, since weight and volume are crucial factors in an aircraft, the fuel cell reactants can be pressurized to increase the power density of the system. Pressurization improves the fuel cells performance and therefore reduces weight and volume of the system, but the pressurization of the ambient air to the desired inlet pressure of the fuel cell cathode requires energy. The pressurization of a fuel cell also changes its voltage power behavior, which in turn affects the operation of the direct hybrid system.
In order to examine and analyze the influence of pressurization on the overall system performance, an open-source two-phase model of a fuel cell is used and compared to experimental measurement data to gain fuel cell data at different pressure levels.
The behavior of the direct hybrid system at different operating conditions will be presented, where the net power of the pressurized PEM-fuel cell system is combined with a battery to form a direct hybrid system. At lower state of charge (SOC) levels, the share of the fuel cell increases due to the lower voltage level of the battery over all power levels. In a pressure-driven fuel cell system, we take advantage of the fact that the pressure of the reactants can be controlled. By adjusting the pressure, the power share of the fuel cell and battery can be actively controlled. Lower operating pressure results in a higher battery share, due to the lower fuel cell voltage. By increasing the pressure, the voltage level provided by the fuel cell increases and therefore the power share of the fuel cell also increases.
In addition, a realistic flight mission is analyzed with a constant compression ratio for the fuel cell air supply resulting in variable operating pressures of the fuel cell during flight, due to the different environmental pressures. The performance of the pressurized fuel cell direct hybrid under those varying ambient conditions will be presented.

Figure 1