Hydrogen supply in proton exchange membrane fuel cell systems: Dissertation

Proton exchange membrane fuel cells (PEMFCs) are a fuel cell type that operate at low temperatureand are commonly fueled with hydrogen gas. A PEMFC is seen as a promising power source forvarious applications including road vehicles, marine vessels, backup power, and grid balancing.Commercial PEMFC products are already available. The main factors limiting their widespread useare the lack of an extensive hydrogen distribution network, their price, and their durability.To work efficiently, a PEMFC requires a set of components for fuel supply, oxidant supply, andcooling. This set of components is called the balance of plant (BoP). The PEMFC and the BoPtogether form a PEMFC system. The BoP contributes to a substantial fraction of the total systemprice and considerably affects system performance and durability.This thesis examines the hydrogen supply in PEMFC systems with electrical power in the range5 to 50 kW. In particular, components and methods for realizing hydrogen purge, hydrogenhumidification, and hydrogen recirculation are evaluated theoretically and experimentally. Effortis put on examining solutions that improve system efficiency and durability while decreasing cost.Hydrogen purge is a widely used approach for removing impurities and liquid water from a deadendanode. In this work, methods for determining PEMFC membrane permeability, fuel purity, aswell as the amount and composition of purged gas are developed and demonstrated. Thesemethods can be used as inidicators of fuel supply or PEMFC system malfunctioning. Further, theeffects of hydrogen purge on an 8 kW PEMFC system performance are studied by varying thecathode inlet humidity. Results show that PEMFC stack efficiency improves by 0.7% whenincreasing cathode inlet dew point temperature from 52 °C to 58 °C. The role of the purge shiftsat these high-humidity conditions from impurity removal towards liquid water removal.A humidifier can be employed to increase the anode inlet gas humidity and, consequently, toincrease the PEMFC efficiency and durability. In this work, a bubble humidifier for a 50 kWPEMFC pilot plant using PEMFC stack waste heat is modelled and characterized. One commonlycited disadvantage of a bubble humidifier is the high hydrostatic pressure drop. The modellingresults suggest that efficient humidification is achieved with only 5 mbar hydrostatic pressure drop.Hydrogen recirculation is commonly applied to increase the gas flow velocity in a PEMFC.Ejectors have attracted attention because of their low price and high durability compared tomechanical pumps. However, ejector sizing and control still lack established methods. In this work,a 2-dimensional (2D) computational fluid dynamics (CFD) modelling approach for ejectors isvalidated against experimental data using three different turbulence models. In addition, a discretecontrol system for ejector is developed. Finally, the low-price and robust combination of a singlefixed geometry ejector and a discrete control system is tested with a 5 kW PEMFC system byperforming load transients from 2 kW to 4 kW within a fraction of a second.