Denise A. McKay

Measurement of Liquid Water Accumulation in a PEMFC with Dead-Ended Anode

2active area, Nafion 111-IP membrane, and carbon cloth gas diffusion layer. Even though dry hydrogen is supplied to the anode via pressure regulation, accumulation of liquid water in the anode gas distribution channels was observed in most tested conditions. Moreover, the accumulation of liquid water in the anode channels is followed by a significant voltage drop. Anode purges and cathode surges are also used as a diagnostic tool for differentiating between anode and cathode water flooding. The rate of accumulation of liquid water, and its impact on the rate of cell voltage drop is shown for a range of temperature, current density, cathode inlet RH, and air stoichiometric conditions. Operating the fuel cell under dead-ended anode conditions offers the opportunity to observe water dynamics and measured cell voltage during large and repeatable transients.

Model-Based Detection of Hydrogen Leaks in a Fuel Cell Stack

Hydrogen leaks are potentially dangerous faults in fuel cell systems that are fed with hydrogen-rich gas mixtures. This brief presents an approach to hydrogen leak detection and, thus, complements direct detection using hydrogen sensors. It relies on simple mass balance equations of an anode filling volume after taking into account the natural leak of the stack. A hydrogen mass flow, anode pressure, and relative humidity sensor are employed. Hydrogen leak detection without the use of relative humidity sensors is considered by employing adaptive alarm thresholds to eliminate false alarms. The validity of the method is also discussed in terms of common hydrogen supply system configurations. The detection method is validated using a 1.25-kW polymer electrolyte membrane fuel cell stack in a laboratory facility where leaks could be introduced in a controlled manner.

Modeling, Parameter Identification, and Validation of Reactant and Water Dynamics for a Fuel Cell Stack

This paper describes a simple two-phase flow dynamic model that predicts the experimentally observed temporal behavior of a proton exchange membrane fuel cell stack and a methodology to experimentally identify tunable physical parameters. The model equations allow temporal calculation of the species concentrations across the gas diffusion layers, the vapor transport across the membrane, the degree of flooding in the electrodes, and then predict the resulting decay in cell voltage over time. A nonlinear optimization technique is used for the identification of two critical model parameters, namely the membrane water vapor diffusion coefficient and the thickness of the liquid water film covering the fuel cell active area. The calibrated model is validated for a 24 cell, 300 cm2 stack with a supply of pressure regulated pure hydrogen.Copyright

Modeling and validation of fuel cell water dynamics using neutron imaging

Using neutron imaging, the mass of liquid water within the gas diffusion layer and flow channels of an operating polymer electrolyte membrane fuel cell (PEMFC) is measured under a range of operating conditions. Between anode purge events, it is demonstrated that liquid water accumulates and is periodically removed from the anode gas channels; this event is well correlated with the dynamic cell voltage response. The estimation of flooding and cell performance is achieved by a spatially distributed (through-membrane plane), temporally-resolved, and two-phase (liquid and vapor) water model. Neutron imaging techniques have never before been applied to characterize flooding with a dead-ended anode and elucidate important issues in water management as well as provide a means for calibrating and validating a dynamic lumped parameter fuel cell model.

Model based detection of hydrogen leaks in a fuel cell stack

Hydrogen leaks are potentially dangerous faults in fuel cell systems. The paper presents an approach to detect hydrogen leaks. The method is applicable during startup and shutdown as well as normal operating conditions. The method relies on simple mass balance equations but takes into account the natural leak of the stack and humidity. Hydrogen leak detection without using relative humidity sensors is specially studied. In that case, adaptive alarm thresholds are given so that false alarms due to the lack of humidity sensors are eliminated. The validity of the method is also discussed in terms of common hydrogen supply system configurations. The detection method is validated on an real fuel cell laboratory rig where leaks could be introduced in a controlled manner.

Parameterization of fuel cell stack voltage: Issues on sensitivity, cell-to-cell variation, and transient response

We present here a calibrated and experimentally validated lumped parameter model of fuel cell polarization for a hydrogen fed multi-cell, low-pressure, proton exchange membrane (PEM) fuel cell stack. The experimental methodology devised for calibrating the model was completed on a 24 cell, 300 cm2 stack with GORE™ PRIMERA® Series 5620 membranes. The predicted cell voltage is a static function of current density, stack temperature, reactant partial pressures, and membrane water content. The maximum prediction error associated with the sensor resolutions used for the calibration is determined along with a discussion of the model sensitivity to physical variables. The expected standard deviation due to the cell-to-cell voltage variation is also modelled. In contrast to other voltage models that match the observed dynamic voltage behavior by adding unreasonably large double layer capacitor effects or by artificially adding dynamics to the voltage equation, we show that a static model can be used when combined with dynamically resolved variables. The developed static voltage model is then connected with a dynamic fuel cell system model that includes gas filling dynamics, diffusion and water dynamics and we demonstrate the ability of the static voltage equation to predict important transients such as reactant depletion and electrode flooding. It is shown that the model can qualitatively predict the observed stack voltage under various operating conditions including step changes in current, temperature variations, and anode purging.Copyright

Model and experimental validation of a controllable membrane-type humidifier for fuel cell applications

For temperature and humidity control of fuel cell reactants, a gas humidification apparatus was designed and constructed. We then developed a low-order, control-oriented model of the humidification system thermal dynamics based on first principles. A simple and reproducible methodology is then employed for parameterizing the humidification system model using experimental data. Finally, the system model is experimentally validated under a wide range of operating conditions. It is shown that a physics based estimation of the air- vapor mixture relative humidity leaving the humidifier system (supplied to the fuel cell) is possible using temperature and pressure measurements. This estimation eliminates the need for a bulky and expensive humidity sensor and enables the future application of temperature feedback control for thermal and humidity management of the fuel cell reactants.

A Controllable Membrane-Type Humidifier for Fuel Cell Applications—Part I: Operation, Modeling and Experimental Validation

For temperature and humidity control of proton exchange membrane fuel cell (PEMFC) reactants, a membrane based external humidification system was designed and constructed. Here we develop and validate a physics based, low-order, control-oriented model of the external humidification system dynamics based on first principles. This model structure enables the application of feedback control for thermal and humidity management of the fuel cell reactants. The humidification strategy posed here deviates from standard internal humidifiers that are relatively compact and cheap but prohibit active humidity regulation and couple reactant humidity requirements to the PEMFC cooling demands. Additionally in developing our model, we reduced the number of sensors required for feedback control by employing a dynamic physics based estimation of the air-vapor mixture relative humidity leaving the humidification system (supplied to the PEMFC) using temperature and pressure measurements. A simple and reproducible methodology is then employed for parameterizing the humidification system model using experimental data.

Correlating Nitrogen Accumulation With Temporal Fuel Cell Performance

The permeability or crossover characteristics of a typical polymeric perfluorosulfonic acid membrane are used for the temporal and spatial estimation of nitrogen concentration along the anode channels of a fuel cell stack. The predicted nitrogen accumulation is then used to estimate the impact of local fuel starvation on stack voltage through the notion of apparent current density. Despite simplifying assumptions on membrane hydration levels, the calibrated model reasonably predicts the response of a 20-cell stack whenever there is no significant liquid water accumulation in the dead-ended anode. Specifically, the predicted voltage decay and estimated anode outlet gas composition are experimentally validated using stack-averaged voltage and a mass spectrometer. This work shows that the crossover of nitrogen and its accumulation in the anode can cause a considerable stack voltage decay and should be considered under high hydrogen utilization conditions.Copyright

A Controllable Membrane-Type Humidifier for Fuel Cell Applications—Part II: Controller Design, Analysis and Implementation

A membrane-based gas humidification apparatus was employed to actively manage the amount of water vapor entrained in the reactant gas supplied to a fuel cell stack. The humidification system utilizes a gas bypass and a series of heaters to achieve accurate and fast humidity and temperature control. A change in fuel cell load induces a reactant mass flow rate disturbance to this humidification system. If not well regulated, this disturbance creates undesirable condensation and evaporation dynamics, both to the humidification system and the fuel cell stack. Therefore, controllers were devised, tuned, and employed for temperature reference tracking and disturbance rejection. Two heater controller types were explored: on-off (thermostatic) and variable (proportional integral), to examine the ability of the feedback system to achieve the control objectives with minimal hardware and software complexities. The coordination of the heaters and the bypass valve is challenging during fast transients due to the different time scales, the actuator constraints, and the sensor responsiveness.