Systems study for fuel cell powered more electric aircraft

In response to the increased demand for aircraft electrification and growing interest in fuel cell technology, a comprehensive study has been carried out to assess the suitability of fuel cells for a range of aircraft. Fuel cell systems, whether they are fuelled by gaseous hydrogen or a liquid alcohol must always be treated as a ‘system of systems’ and comprise four well defined, interlinked subsystems: fuel cell stack, fuel, oxidant and thermal management.

The key objectives of this work were to: define a methodology to predict the electrical requirements, propulsive or otherwise of any aircraft based on the highest level design information; critically analyse existing fuel cell technologies and down-select to two technologies; assess the required system of systems for the down-selected fuel cell technologies; and produce and evaluate a dynamic fuel cell system sizing model to assist aircraft designers during an aircraft's preliminary design phase.

Fifteen aircraft categories have been defined based on the aircraft's primary function and propulsion method. A model was then developed which can predict the electrical generation capability and propulsive requirements. Validating the categorisation model against real aircraft data showed a good correlation between the real and modelled data. Generally, an error of less than 5% was obtained by the model. The output of this model was used in the sizing of an appropriate fuel cell system.

A unique challenge to the integration of fuel cell systems in aircraft, the atmosphere was investigated. Three atmospheric models were presented and their usefulness discussed. The challenges to fuel cell system design are primarily ambient temperature and total pressure at altitude. In the field of electrochemistry it is usual to denote the partial pressure of oxygen in the cathode stream as a limiting factor. In reality, it is a combination of both the concentration of oxygen and the total pressure that influence performance. This important distinction is made as these variables can be controlled independently.

Six commercially available fuel cell technologies were reviewed for use in aeronautical applications. Hydrogen fed polymer electrolyte membrane and liquid fed direct methanol fuel cells were down-selected for further study. For each technology, an experimentally validated fuel cell stack model was created to describe the electrochemical reactions between their fuels and oxygen.

Different storage methodologies for molecular hydrogen, methanol and molecular oxygen were compared and optimum solutions in terms of storage efficiency were deduced based on aircraft mission length. A case study was carried out to investigate the system mass variation with altitude. Key variables included the performance derating of the fuel cell as well as the choice of either a compressor based air-breathing design or an air-independent alternative. It was found that an air-breathing solution is preferable for longer mission durations.

Primary thermal management strategies were compared for both fuel cell technologies. For hydrogen fed fuel cells the choice between air-cooling and liquid-cooling is based on the heat generation rate of the fuel cell. If the heat generation rate is less than 4 kW, an air-cooling strategy offers both system mass and volume benefits. For higher power systems, liquid-cooling should be used. Direct methanol fuel cells were shown to offer reduced system complexity from a thermal management perspective as the heat can be rejected to the unused fuel solution in the exhaust.

Four primary submodels, each representing a subsystem of the overall fuel cell system, were combined into a single, dual function dynamic fuel cell sizing model. The first function of this model was to physically size a fuel cell system based on primary design information and a flight profile. Secondary functionality was a dynamic representation of the fuel cell system response to the input current and altitude profiles. Case studies were carried out using the model Skywalker X8 and General Atomics MQ-1 Predator aircraft.