Polymeer Elektrolyt-Membraan Brandstofcellen voor Onbemande Luchtvaartuigen: Karakterisatie en Modellering onder Variërende Omgevingscondities

Fuel cells (FCs) integrated into unmanned aerial vehicles (UAVs) are regarded as a promising option for enabling long-endurance missions with electric propulsion. This thesis first reviews the current state-of-the-art in the design of energy systems for UAVs utilising fuel cells, for which polymer electrolyte membrane fuel cells (PEMFCs) are considered the most suitable candidate. The power profiles of UAVs are highly dynamic with potential power surges. This requires PEMFC hybridisation with other energy systems, which is also reviewed. Additionally, the focus is on PEMFCs in general, with particular attention to commercially available PEMFCs for UAVs. Moreover, the management of operating conditions is discussed. Because of mass constraints, PEMFCs for UAVs rely on a simplified balance of plant (BoP) and are therefore referred to as PEMFCs employed with simplified operating condition management strategies (SOCMS). Consequently, the literature suggests that PEMFCs for UAVs may experience variable performance and shorter lifetimes owing to increased exposure to ambient conditions and anticipated higher degradation rates. This highlights the need for further research, as these effects could limit the commercial deployment of PEMFC-powered UAVs. Furthermore, it could be beneficial to consider these effects when designing PEMFC UAV hybrid propulsion systems. Besides durability and resilience to ambient factors, clear knowledge gaps in the literature stem from a lack of guidelines specifically focused on the selection and dimensioning of hybrid propulsion systems for PEMFC UAVs. A stepwise approach is proposed to assess the commercial feasibility of PEMFC UAVs for realistic, real-world missions, focusing on applicability, performance, and lifetime to advance beyond the demonstration phase. This thesis focuses on the initial steps of this approach and aims to experimentally assess the steady-state performance of a 250 W PEMFC open-cathode system designed for small UAVs. The testbench, equipped with a climate chamber, exposed the fuel cell to six ambient conditions, with temperatures ranging from 0 to 35°C and relative humidity (RH) ranging between 9% and 81%. The stack net power, temperature, and fan power were evaluated, and the gross power output was determined as a function of ambient conditions. A novel methodology was developed to characterise the discussed performance characteristics of PEMFCs employing SOCMS, since the presence of the hybrid system and transients such as purging and short-circuiting require a specific approach. The highest net power corresponds to the 9 A setting and was observed at 35°C and 75% RH, while the lowest occurred at 35°C and 9% RH with an absolute difference of 13 W. The fan power influenced performance, particularly for the setpoints at 35°C, where requested fan power reached 10 W at 5 A and increased further to 15 W at 9 A for the 9% RH condition. In contrast, the 12°C and 0 °C conditions stayed below 2 W fan power for all currents. This indicates that, from a gross power perspective, the 35°C and 9% condition is not the worst performing, but rather the 12°C and 36% condition with a gross power difference at 9 A of 15.7 W. The stack temperature dropped below 15°C at currents below 2 A for the coldest inlet ambient condition setting at 0°C and 64% RH, which corresponds to a difference of over 20°C compared to the target temperature. In the case of 35°C, slight overheating compared to the target temperature of around 3°C is observed for most current setpoints. Overall performance deviations remained below 6%, confirming moderate robustness to ambient conditions. These insights can support the development and design of more resilient control strategies and hybrid systems to integrate PEMFCs with SOCMS in UAVs. Furthermore, the developed methodologies can serve as a standardised approach for characterising the performance of these PEMFCs in both industry and academic contexts. Building on the observed results, models were developed that analyse the impact of ambient temperature and relative humidity on the performance of the PEMFC. This is considered helpful for valorisation and to support applications. Two complementary approaches were followed: a data-driven regression based on a fourth-order current-voltage model and a physics-informed hybrid model relying on polarisation curve theory. The development process was stepwise, initially focusing on the current-voltage relationship and then incorporating the effects of ambient conditions. Significance testing, along with model accuracy assessments and metrics such as information criteria and cross-validation, was used to prevent overfitting. The best-performing hybrid model achieved the highest accuracy while remaining interpretable and physically consistent, even when extrapolated to other PEMFCs for UAVs, indicating its potential for PEMFC UAV design and control. To conclude, the pathways for future adoption of PEMFC UAVs are explored based on the research performed. The focus is on insights from quantified impacts of ambient conditions, the use of models that account for these conditions, and the development of tools and knowledge-sharing to facilitate the broader adoption of PEMFC UAVs.