Oxygen Enriched Fuel Cell Propulsion in Regional Aircraft

Kurzfassung

The aviation sector is making several efforts to reduce its climate impact as the demand for air travel is expected to rise. These efforts include the development of novel propulsion systems that would reduce the emissions produced by aircraft. Proton exchange membrane fuel cells are considered a promising system for reducing emissions. However, their integration into an aircraft brings many challenges. Fuel cells have a significantly lower power density than conventional internal combustion engines. Their low operating temperature results in heavier heat exchange systems and increased drag, and substantially increases the system complexity due to the auxiliary system required and hydrogen storage. One way to improve fuel cell performance is to operate it with oxygen enriched air. This thesis evaluates the impact of such an operation at aircraft level. This is achieved by performing the preliminary design of a regional aircraft where a developed fuel cell propulsion system model is implemented. This model includes the fuel cell stack and several balance-of-plant components assuming the state-of-the-art technology. The modelling allowed for the system's sizing and simulation under different flight scenarios and varying levels of oxygen percentage of the air supplied to the fuel cell. The integration and sizing of the oxygen tanks used to increase the oxygen percentage were conducted while considering regulations and the required oxygen content. Results show that implementing a fuel cell propulsion system significantly increases the aircraft's maximum take-off mass (MTOM) compared to a conventional regional aircraft. The oxygen percentage variation showed that a reduction in MTOM is possible, reaching a maximum of 4%. This reduction resulted from a significant improvement in fuel cell propulsion system mass, with a maximum reduction of 45% while using pure oxygen, with the cruise part of the flight limiting any further improvements. Although MTOM was reduced, the downsizing of the fuel cell system resulted in a higher hydrogen consumption, increasing the required hydrogen mass by up to 20%. The reduction in fuel cell system mass is opposed by the added mass of oxygen and its tank system. Several sensitivity analyses showed that design choices can affect the system mass. Although that is verified, the effect of oxygen enrichment does not change, and the same trends are verified. The possibility of future improvements in fuel cell power density was also evaluated, where it was verified that such an improvement results in no reduction of MTOM with higher oxygen percentages. Under the conditions of the selected propulsion architecture, design choices and technology level, oxygen enriched fuel cell air supply operation is considered to have no positive impact on aircraft level performance.