Hydrogen supply in proton exchange membrane fuel cell systems

Proton exchange membrane fuel cells (PEMFCs) are a fuel cell type that operate at low temperature and are commonly fueled with hydrogen gas. A PEMFC is seen as a promising power source for various applications including road vehicles, marine vessels, backup power, and grid balancing. Commercial PEMFC products are already available. The main factors limiting their widespread use are 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, and cooling. This set of components is called the balance of plant (BoP). The PEMFC and the BoP together form a PEMFC system. The BoP contributes to a substantial fraction of the total system price and considerably affects system performance and durability. This thesis examines the hydrogen supply in PEMFC systems with electrical power in the range 5 to 50 kW. In particular, components and methods for realizing hydrogen purge, hydrogen humidification, and hydrogen recirculation are evaluated theoretically and experimentally. Effort is 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 dead-end anode. In this work, methods for determining PEMFC membrane permeability, fuel purity, as well as the amount and composition of purged gas are developed and demonstrated. These methods can be used as inidicators of fuel supply or PEMFC system malfunctioning. Further, the effects of hydrogen purge on an 8 kW PEMFC system performance are studied by varying the cathode inlet humidity. Results show that PEMFC stack efficiency improves by 0.7% when increasing cathode inlet dew point temperature from 52 °C to 58 °C. The role of the purge shifts at 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, to increase the PEMFC efficiency and durability. In this work, a bubble humidifier for a 50 kW PEMFC pilot plant using PEMFC stack waste heat is modelled and characterized. One commonly cited disadvantage of a bubble humidifier is the high hydrostatic pressure drop. The modelling results 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 to mechanical 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 is validated against experimental data using three different turbulence models. In addition, a discrete control system for ejector is developed. Finally, the low-price and robust combination of a single fixed geometry ejector and a discrete control system is tested with a 5 kW PEMFC system by performing load transients from 2 kW to 4 kW within a fraction of a second.