The low ambient pressure at high altitudes poses a central challenge for fuel cells in aircraft. The ambient air must be highly compressed to achieve an operating pressure of more than one bar. Researchers at the German Aerospace Center (DLR) have now systematically investigated which air supply systems are best suited for different types of aircraft.
The team led by Matthias Schröder from the DLR Institute of Technical Thermodynamics analyzed six different topologies. These differ in compressor configuration – single-stage, two-stage serial, or two-stage parallel – as well as in the optional use of a turbine stage for energy recovery from the fuel cell exhaust.
Turbine stage offers weight advantage despite additional components
For a regional aircraft with 70 passengers and ten fuel cell systems, a single-stage compressor with a turbine proved to be the best solution. This configuration is expected to reduce the total mass of the fuel cell system and hydrogen tank by 4.5 percent. To avoid condensation in the turbine, an additional heat exchanger is required. Nevertheless, according to the study, the advantages of energy recovery outweigh the disadvantages.
For larger aircraft with higher altitudes, two-stage compressors are more advantageous. In a studied narrow-body aircraft with 250 passengers, a serial two-stage compressor with a turbine is expected to reduce the mass by 8.1 percent. A parallel two-stage compressor with turbines improves efficiency during cruise by 10.9 percentage points according to the calculations.
Specific power must quadruple
The EU aims for a power density of 2.0 kilowatts per kilogram for fuel cell systems in aviation by 2030. The current state of the art is 0.5 to 0.75 kilowatts per kilogram. According to the authors, the study shows a way in which tailored air supply systems can contribute to achieving this goal.
The research was funded by the Federal Ministry for Digital and Transport. The results have been published in the journal Energy Conversion and Management.
