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In the context of fuel cell stack control a mayor challenge is modeling the interdependence of various complex subsystem dynamics. In many cases, the states interaction is usually modeled through several look-up tables, decision blocks and piecewise continuous functions. Many internal variables are inaccessible for measurement and cannot be used in control algorithms. To make significant contributions in this area, it is necessary to develop reliable models for control and design purposes. In this paper, a linear model based on experimental identification of a 7-cell stack was developed. The procedure followed to obtain a linear model of the system consisted in performing spectroscopy tests of four different single-input single-output subsystems. The considered inputs for the tests were the stack current and the cathode oxygen flow rate, while the measured outputs were the stack voltage and the cathode total pressure. The resulting model can be used either for model-based control design or for on-line analysis and errors detection.

Presentado al Fuel Cells Science & Technology: Scientific Advances in Fuel Cell Systems celebrado en Copenague del 8 al 9 de octubre de 2008., This work proposes an experimental methodology for modeling water diffusion transport in a PEMFC, which is applied to a commercially available fuel cell stack. The effective water vapor diffusion coefficient and its dynamical variation for the stack under no load conditions will be determined for the purpose of isolating the transfer of water across the 5 layer MEA (which include the two gas diffusion layers the two catalysts layers and the membrane) in relation to temperature and membrane resistance. This coefficient is dependent on the material properties of the components in the fuel cell, as well as on the membrane temperature and water content. The dynamics of the membrane temperature and water content will be established experimentally. The controlled variables in this experiment are the water vapor partial pressures at the inlets and the temperature of the fuel cell. Experimental procedure is to independently vary the water vapor partial pressures and fuel cell temperature to obtain the water transport across the membrane, membrane resistance, and total resistance. The experimental setup for this study is based on a 21 cell PEMFC stack with a forced flow open cathode. An environmental chamber controls the fuel cell temperature and the inlet dew point temperature of the cathode reactant. The anode reactant inlet water vapor partial pressure is maintained by a membrane based humidifier which is controlled with a dew point sensor. The measured variables are the outlet dew point temperatures, membrane resistance through continual high frequency electrochemical impedance spectroscopy (EIS), and total resistance through full frequency spectrum EIS at steady state conditions. The study determines the in situ effective diffusion coefficient and membrane resistance from 10ºC to 70ºC and 10% to 100% relative humidity. The dynamics of the membrane temperature and water content will be used to determine the movement of water in the 5 layer MEA., This work was supported by the project 'Avances en el modelo y diseño de controladores para sistemas basados en pila de combustible PEM' (4800).