Power Optimization Strategies for a Converter-Free Multi-Stack Fuel Cell System in Near MW-Range

Abstract

Hydrogen-electric propulsion based on polymer electrolyte membrane fuel cells is considered a promising technology for zero-emission propulsion in small and regional aircraft. Achieving megawatt-class power levels requires multi-stack fuel cell systems, in which several fuel cell modules are electrically interconnected. In converter-free architectures, electrical interactions between stacks can significantly influence the achievable system performance. This study experimentally investigates power optimization strategies for a near-megawatt multi-stack fuel cell system at the BALIS test facility of the German Aerospace Center. The investigated system consists of eight identical polymer electrolyte membrane fuel cell modules (~100 kW each) arranged in a 2-series/4-parallel configuration with master-slave control within each series pair. Two approaches to enhance overall system power are examined. First, module pairing is optimized based on experimentally determined polarization characteristics to minimize voltage deviations between parallel strings. Reconfiguration of the modules reduces current imbalance and increases the maximum system current from 1450 A to 1560 A, resulting in a 7 % increase in peak power from 790 kW to 848 kW. Second, the influence of reversible stack degradation on system performance is analyzed using standardized polarization curve measurements and four adapted recovery procedures. Voltage Pulsing was the only recovery procedure, which led to a significant performance increase of around 0.4 %. The low impact of the recovery procedures is assumed to result from facility constraints, fuel cell controller interventions, and low reversible degradation of the fuel cells. Nevertheless, the proposed procedures can serve as an indication of how a targeted reduction of reversible degradation can be achieved in future aviation systems and thus improve the performance of converter-free architectures in application-relevant long-term operation. Future studies should focus on degraded operation to demonstrate the potential of the recovery procedures and the comparison of fuel cell system architecture variants, also considering the integration of DCDC converters.