Roberto Ferrero

Single PEM Fuel Cell Analysis for the Evaluation of Current Ripple Effects

In this paper, the results of an experimental analysis performed on a polymer electrolyte membrane fuel cell (FC) for the evaluation of the current ripple effects are reported. The polarization curve (PC) and the impedance spectrum measured for a single FC with a Nafion membrane are presented discussing some advantages and disadvantages of the two adopted techniques from the measurement point of view. The ohmic and activation resistance values provided by the PC are compared to the frequency limits of the equivalent resistance estimated by the spectroscopic analysis to evaluate the additional losses and the system efficiency reduction introduced by the current ripple. The presence of a voltage–current hysteretic behavior is discussed throughout the paper, and some comments on how such a relationship can impact on the identification of the indicated parameters are included.

Characterization of Inductance Gradient and Current Distribution in Electromagnetic Launchers

We reapproach the characterization of the inductance gradient and the current distribution in electromagnetic rail launchers with multiple brush armatures. A measurement procedure based on the use of a loop system mutually coupled with the launcher is illustrated for the case of a two-brush armature. We show how to estimate the current distribution between the brushes following the basic rules of the partial-inductance method. To test the proposed procedure, we performed an experimental analysis on a rail launcher prototype supplied by a high-voltage high-power capacitor bank. Experimental results were obtained under the static condition of the armature, and they are in agreement with theoretical calculations.

Diagnosis of PEM Fuel Cell Drying and Flooding Based on Power Converter Ripple

This paper discusses the possibility to use the current ripple introduced by switch mode power converters for low-cost monitoring of polymer electrolyte membrane (PEM) fuel cell (FC) state-of-health, suitable for commercial applications that cannot afford dedicated instrumentation. In more details, an estimate of the ohmic resistance, which is a good indicator of the membrane water content, can be obtained from the high-frequency ripple response by data processing in the frequency domain, while lower frequency ripple at 100/120 Hz (when present) is in the typical frequency range of activation processes. All the available impedance estimates, together with the dc voltage measurement, can be used to promptly detect FC drying and flooding, that are the two opposite failure modes as far as water balance is concerned. The proposed diagnostic approach is tested on a single PEM FC in drying and flooding conditions, by emulating three-phase and single-phase inverter ripples by means of an electronic load.

Analysis and Compensation of PEM Fuel Cell Instabilities in Low-Frequency EIS Measurements

This paper analyzes possible humidity-related instabilities of polymer electrolyte membrane (PEM) fuel cell (FC) operation that affect electrochemical impedance spectroscopy measurements. The compensation of ohmic resistance variations due to such instabilities, achieved using the well known multisine excitation technique with properly chosen high-frequency harmonic components, is proposed here as a new approach to improve low-frequency measurements. Additionally, this method allows a more accurate determination of equivalent circuit parameters identified from impedance spectra and their uncertainties taking into account correlation between impedances at different frequencies, and thus avoiding uncertainty overestimation of low-frequency parameters (such as transport and activation equivalent resistances) that is likely to occur with classic measurement techniques. The proposed method is experimentally validated on a single PEM FC.

A model predictive protection system for actuators placed in hostile environments

In most applications where motors control several degrees of freedom of a mechanical system there is the necessity to implement a protection which is able to stop the motors when the mechanical system reaches the extreme positions of its working space. When the mechanical system is located in an hostile environment and/or when the limit positions define a complex surface this simple protection strategy is not always viable, since its not possible to place sensors and limit switches in the proximity of the limit positions. We will present in this paper a solution for the actuator protection problem based on a model predictive algorithm capable to estimate in real time the actuator state variables (positions and speeds). The idea of the model predictive protection scheme is to implement a model of the mechanical system forced by the emergency braking actions in order to predict, using as initial condition the current observed state of the system, the final rest position together with its uncertainty. If the probability that the predicted rest position lays outside the physical boundaries of the working space exceeds a predefined threshold the protection trips. With this approach we will also reach the maximum exploitation of the systems working space, since it is adapted to the current system state. We will show the application of this concept to the protection of a set of steering microwave antennas placed in an hostile environment (hard vacuum condition, strong neutron and gamma radiation, strong magnetic fields) inside a nuclear fusion reactor.

PEM Fuel Cell Drying and Flooding Diagnosis With Signals Injected by a Power Converter

In this paper, a low-cost approach for polymer electrolyte membrane fuel cell (FC) drying and flooding diagnosis based on power converter ripple is presented, suitable to be implemented in commercial applications. If proper signal processing algorithms are employed, the high-frequency ripple inherently produced by switch-mode converters allows one to monitor the FC ohmic resistance, which is a good indicator for membrane drying, while an ad hoc switching control of the converter allows one to monitor also the low-frequency impedance, which is an indicator for cell flooding. This technique is tested with a dc/dc boost converter directly connected to a single cell, discussing how different FC operation modes (such as constant current or constant voltage) may affect the sensitivity required to the diagnostic algorithm to correctly recognize drying and flooding occurrences.

Analytical Study of Impulse Current Measuring Shunts With Cage Configuration

In this paper, an analytical study of high-impulse current measuring shunts with cage configuration is presented. Through the use of the partial mutual inductance method, an equivalent first-order model is developed, and a discussion of the minimization of the time constant taking into account physical constraints for two different materials is presented. The transfer function of the measuring circuit is provided, and the possibility of compensating the inductance via the use of rectangular loops formed by the sensing wires and placed inside the cage is shown. The derived model is tested on a homemade shunt prototype with application to an electromagnetic rail launcher. The experimental results confirm the validity of the model and its robustness with respect to the geometrical parameter uncertainty.

Uncertainty Analysis of Local and Integral Methods for Current Distribution Measurements

This paper provides an uncertainty analysis for current distribution measurements based on local magnetic field measurements (e.g., via Hall probes) or on magnetic flux measurements using large pick-up loops. The fundamental difference between the two approaches is first pointed out in the ideal case of an infinite straight wire. Then, a more complex geometry composed by parallel identical conductors connected between two bars is considered. The mathematical models employed for the current reconstruction according to the two approaches are derived, and their uncertainties are evaluated. An experimental validation of the model for pick-up loops is presented with application to an actual rail launcher prototype. From this analysis, the critical aspects of the two methods are highlighted, showing the different effect that the uncertainty of the sensor position has on the reconstructed current uncertainty. Finally, a case study aimed at evaluating the current distribution uncertainty under simplified hypotheses is described.

Low-Cost PEM Fuel Cell Diagnosis Based on Power Converter Ripple With Hysteresis Control

This paper deals with a low-cost diagnostic technique for polymer electrolyte membrane (PEM) fuel cells (FCs), which exploits the ripple produced by power converters to monitor the equivalent ohmic resistance. While the available literature on this topic is focused on constant-frequency control of the power converter (such as pulsewidth modulation), this paper discusses the measurement issues that arise when hysteresis current control is employed for a dc/dc boost converter, which represents the simplest solution from the implementation point of view, and therefore particularly suitable for low-cost applications. The classic frequency-domain analysis for ohmic resistance identification, based on the Fourier transform, is compared with a time-domain analysis based on a simple identification algorithm, and a real-time implementation of the latter is presented. The experimental results are obtained on a single PEM FC, but the extension to FC stacks for commercial applications is also discussed.

Classification and analysis of failures of ultracapacitors in rail launchers applications

This paper deals with the classification and analysis of failures of ultracapacitors to be used in rail launchers applications with the aim of investigating the influence of strong impulsive electromagnetic fields on their performance. Considering the very harsh environment present in rail launchers applications, a reliability analysis of components of the launch mass is very important to assess the failure modes and the relevant effects. To this purpose, we introduced a Failure Modes and Effects analysis methodology (FMEA) to ensure an optimal design and material choice as well as to evaluate the impact of all possible failure mechanisms and causes on the ultracapacitors. This approach allows to reduce or possibly eliminate the effects of potential failure modes before the launch mass design completion and before failures occur during the launch.