Application of an integrated lumped parameter-CFD approach to evaluate the ejector-driven anode recirculation in a PEM fuel cell system

Abstract

Proton exchange membrane fuel cells (PEMFCs) are considered a promising candidate to replace internal combustion engines. The unconsumed hydrogen, in PEMFC systems, is released at the anode exit and a recirculation can be applied to increase the fuel utilization; to this end, a recirculation system is needed and ejector technology is a promising technique, considering its many advantages. Unfortunately, ejectors are characterized by extremely complex fluid dynamic phenomena and a small deviation from the optimum operating condition might drastically lower the performances of the ejector itself and, consequently, of the whole ejector-based system. For this reason, multi-scale models—taking into account both the “local-scale” and the “component-scale” fluid dynamics phenomena—should be applied to evaluate the performance of ejector-based systems. In this paper, we contribute to the existing discussion concerning multi-scale modeling techniques and we propose an integrated lumped parameter- Computational Fluid Dynamics model to investigate the performance of convergent-nozzle ejectors for the anode recirculation in PEMFC systems. The integrated approach is based on a lumped parameter model (able to estimate the “component-scale” performance) with variable ejector component efficiencies, provided by Computational Fluid Dynamics simulations (able to predict the “local-scale” phenomena). Computational Fluid Dynamics simulations have been used to investigate the ejector “local-scale” fluid dynamic phenomena and to formulate correlations for ejector component efficiencies, thus linking ejector component efficiencies to the “local-scale” phenomena. In the first part of the paper, the integrated lumped parameter-Computational Fluid Dynamics approach has been formulated, validated and compared with different constant efficiency models, showing better performance and a wider range of applicability. In the second part of the paper, the integrated approach has been included in a complete PEMFC system model (considering both electro-chemical and pressure-drop effects). It has been demonstrate that a small deviation from the optimum operating condition of the ejector lower the performances of the whole system. The Integrated lumped parameter model-Computational Fluid Dynamics approach, because of the variable ejector component efficiencies, has been able to correctly consider the off-design performance of an ejector based system; conversely, constant ejector component efficiency models cannot correctly predict the performance of the PEMFC system. In conclusion, the use of variable ejector component efficiency models is needed in order to (a) provide a realistic model of the system and (b) analyze the performance in both for on- design and off-design performance. In addition, the proposed paper also provides a demonstration for the implementation of modeling involving both fluid dynamics and electro-chemical analysis in the context of fuel cells.