Session: 06-08 Hydrogen for Aviation

Paper Number: 121340

121340 - Evaluation of Air Supply Conditioning Architectures for PEM Fuel Cell Systems in Aviation 

Electric propulsion systems for aviation based on polymer electrolyte membrane fuel cells (PEMFC) are promising concepts for reducing the climate impact of the aviation sector due to their benefits of zero carbon dioxide and nitrogen oxide emissions and high efficiencies. However, the performance of a PEMFC is highly dependent on the air supply to the cathode side, which is usually provided from the ambient. To ensure efficient and reliable operation, a conditioning system for the supply air is necessary, resulting in a parasitic power consumption for the fuel cell. A compressor is required to provide the fuel cell with an operating pressure of 1.5-3 bar, and a heat exchanger to adjust the inlet temperature to 340-360 K. Humidification is also fundamental to prevent the fuel cell membrane from drying out. According to the literature various cooling and humidification strategies are conceivable.

Membrane humidifiers use the humid exhaust gas to humidify the air, so neither liquid water nor additional energy are needed. As the inlet temperature is limited to approximately 370 K, a heat exchanger upstream of the humidifier is essential. Spray humidifiers as well as compressors with water injection combine humidification and cooling, while the latter leads to an increase in compressor efficiency. Nevertheless, these concepts are constrained due to the required temperature rise to ensure evaporation and also demand additional power for water pumps and risk flooding of the fuel cell. Another possibility to reduce compressor work and outlet temperature is to insert an intercooler between subsequent compressor stages.

The main focus of this paper is the investigation of feasible system architectures for the cathode gas path in the context of aviation, including the presented humidification methods. A turbine downstream of the fuel cell is also considered, as it is valuable to use the enthalpy contained in the exhaust gas to reduce the parasitic load. The objective is to determine the operating parameter space during the flight mission of a medium-range aircraft and identify limitations and critical conditions.

For configuration studies, the thermodynamic cycle calculation module of the in-house software ASTOR (AircraftEngine Simulation for Transient Operation Research) is used to calculate the possible operating range of the system at critical operating points of the aircraft. Parameters such as operating pressure, temperature, cathode relative humidity and air number are sampled using a latin hypercube method. Based on the design, the performance, efficiency and waste heat of the fuel cell, the parasitic load of the air supply, and the sizing of heat exchangers and humidifiers can be evaluated for the different system architectures. Since the requirements of the air supply system are specified in terms of compressor mass flow and pressure ratio, the calculation can also be seen as a starting point for a more detailed design of turbo components. General characteristics from the literature can be reproduced. Wet compression reduces compressor work, but leads to larger heat exchangers due to the diminishing temperature difference. The intercooled architecture also has increased weight and volume due to additional heat exchangers. Therefore, the membrane humidifier proves to be the most promising humidification strategy.

In order to determine an appropriate design point for the air supply system, it is also essential to consider off-design performance. On the one hand, an iterative design point selection is necessary to cover all operating points within the compressor map. On the other hand, the off-design analysis is used to derive operating strategies, for example to minimise the fuel consumption for an entire flight mission. It is shown that the turbo components do not have the same design point as the air supply components. Therefore, heat exchangers and humidifiers must be redesigned for the harshest environmental conditions. The lowest fuel consumption and waste heat are located at low air numbers. However, operation with lower air numbers is limited by insufficient water content of the fuel cell membrane at cruise and top of climb operating points.

Presenting Author: Patrick Meyer Institute of Jet Propulsion and Turbomachinery, Technische Universität Braunschweig

Presenting Author Biography: The author has studied aerospace engineering at Technische Universität Braunschweig. He has written his master's thesis on the investigation of the cathode gas supply in a fuel cell system for a medium-range aircraft. Currently he is working as a research assistant at the Institute of Jet Propulsion and Turbomachinery at TU Braunschweig. He is participating in the project “Design Space Evaluation of the Air-, Heat- and Power-Management of Fuel Cells for Aviation” (DEFCA) of the Cluster of Excellence “Sustainable and Energy-Efficient Aviation” (SE2A).

Authors:

Patrick Meyer Institute of Jet Propulsion and Turbomachinery, Technische Universität Braunschweig
Sebastian Lück Institute of Jet Propulsion and Turbomachinery, Technische Universität Braunschweig
Jan Goeing Institute of Jet Propulsion and Turbomachinery, Technische Universität Braunschweig
Jens Friedrichs Institute of Jet Propulsion and Turbomachinery, Technische Universität Braunschweig