Design of a variable passive ejector for hydrogen recirculation of a PEM fuel cell system

In order to increase the overall efficiency of the Polymer Electrolyte Membrane (PEM) fuel cell system the recirculation of excess hydrogen from the fuel cell outlet to the inlet can be conducted by a passive hydrogen ejector instead of an electric hydrogen pump. The disadvantage of a hydrogen ejector with a fixed layout is that it does not cover the necessary range of the fuel cell operating conditions from low to high power. Therefore, in this study a new approach for thoroughly analyzing the influence of variable geometric parameters on the recirculation performance of the passive hydrogen ejector is investigated by Computational Fluid Dynamics (CFD) simulations. Based on a sensitivity analysis the nozzle radius has been identified as geometric parameters which has the largest effect on the mass flow and mass flow ratio. A moving needle concept has been selected as most suitable to vary the nozzle cross section. Compared to a passive recirculation unit with a fixed ejector geometry for a fuel cell operation limited to 100 kW the needle concept allows an operation from 17 kW to 100 kW of fuel cell power. An additional optimization with a two-step needle can further reduce the sensitivity of the fuel cell pressure drop from 50 mbar to 70 mbar on the necessary mass flow at low fuel cell power. System simulations of the hydrogen ejector confirm that the variable recirculation unit can be operated also under dynamic operation of the fuel cell system by maintaining the necessary mass flow and mass flow ratio. • A concept for a variable passive hydrogen recirculation unit is developed. • Influence of ejector geometries on recirculation characteristics is evaluated. • Variable recirculation unit maintains required stoichiometry ratios.