RAWAvCon Fuel Cell Model Development

Various organisations are currently exploring the potential of PEM fuel cells and hydrogen for small and regional aircraft. Airframe companies Airbus and Embraer are studying clean-sheet designs to improve their understanding of fuel cell propulsion systems' technical challenges and their potential benefits. The UK's ATI FlyZero project and the GKN led H2Gear project also studied new aircraft designs with fuel cell-driven propulsors.

The Project Fresson consortium and ZeroAvia aim to install fuel cell-based propulsion solutions initially as retrofit or line-fit options for existing small and regional aircraft, replacing gas turbine power plants. Both target mid-decade entry into service. Indeed, various aircraft operators placed commitments for these solutions during the last year.

Hence, the need exists to extend my RAWAvCon aircraft design suite to match these market developments. The ultimate objective targets a simple switch in the model to switch propulsion types from gas turbine to fuel cell power generation, although each propulsion option initially needs optimising.

To avoid any doubt with my current FlyZero colleagues, this model uses public domain methods and information – no proprietary FlyZero data.

Fuel cell performance and weight are complicated due to the various options associated with the 'balance of plant' (BoP), i.e. all fuel cell components external to the fuel cell stack itself.

The fuel cell schematic diagram below illustrates the system layout adopted. The modelling includes power demand or generation, weight and drag attributes for each element, with various optional BoP components selected by the user, e.g. inlet air compressor, inlet cooling, humidification system, turbine.

The fuel cell model currently works successfully in two primary modes:

• At a rated power, i.e. take-off and climb, defined by the lowest of the rated fuel cell delivery power capability or any other power limit linked to the BoP and propeller, or by user-defined limits. The analysis process varies for each limit definition and includes aircraft secondary power requirements.

• At part power flight conditions, e.g. cruise and approach, the required propulsive power is an input from the mission model. The BoP and aircraft systems power requirements, and drive train losses are added to determine the total fuel cell load and associated H2 fuel flow.

Various public domain methods define the mass and volume attributes for all components when used.

The model also calculates a drag value associated with the cooling air needed for the FC stack cooling heat exchanger and the optional intake air intercooler. A future development will consider the option to apply the 'Meredith' effect thrust recovery to the FC outlet gases or the stack cooling flows, although the effect may not significantly exceed a 'mouse fart'.

The FC model is now fully functional as a standalone model working in the two modes described previously.

The next step involves plugging the FC model into the mission model, the inlet drag, exhaust thrust recovery and external tank drag into the drag model, and the FC and H2 tank/fuel system weights into the OEW.

The mission performance calculation remains unchanged to the gas turbine model. The process calls the fuel cell model in the appropriate mode for each flight phase, checking against various limits before returning thrust and H2 fuel flow values. The H2 fuel can be gaseous or liquid, with user-defined appropriate tank gravimetric efficiency values.

The model is not yet perfect, but the fundamentals are in place to model fuel usage through entire missions. Further improvements will occur as time allows or customers demand.

Adapting existing RAWAvCon turboprop aircraft models will be relatively straightforward, e.g. various ATR and DHC-8 models, Do 328, and Beech 1900 variants, and others. The capability already includes various compressor scheduling options (if one is included) and options of minimum fuel cell stack mass, e.g. minimal aircraft mass, or minimum H2 usage on the economic mission. However, each aircraft model requires an optimisation process to consider the most appropriate fuel cell system set up, and positioning around the airframe.

I shall be generating some of my own models for internal technology studies.

External organisations wishing to access this capability can engage with RAW Aviation Consulting Ltd to model their systems confidentially. The baseline model IPR always remains with RAW Aviation Consulting Ltd; any customer-specific system arrangements, novel features and component attributes remain the customer's property unless the knowledge to the public domain (not through me).