Showing papers by "Raghunathan Rengaswamy published in 2015"

Classification of High-Temperature PEM Fuel Cell Degradation Mechanisms Using Equivalent Circuits

Abstract: This paper presents the evaluation of a high-temperature proton exchange membrane (HTPEM) fuel cell for different degradation mechanisms using equivalent circuit analysis. Specific consideration is given to the variation of phosphoric acid content in the polybenzimidazole membrane and the effect on the equivalent circuit. The importance of the cell assembly and operating conditions on acid migration are discussed, and it is shown how it affects performance both in the short-term and the long-term operation of the HTPEM cell. A new method is developed, whereby acid leaching can be greatly accelerated in order to quantify performance loss. The change in system parameters as a function of membrane electrode assembly acid content is investigated using electrochemical impedance spectroscopy and compared with the changes that take place for catalyst degradation, CO poisoning, and reactant starvation. It is shown, by using the equivalent circuits, that the drop in performance relating to the individual degradation mechanisms in the cell can be investigated and isolated for fault classification in online diagnostic systems.

Electrical Circuit Analysis of CO Poisoning in High-Temperature PEM Fuel Cells for Fault Diagnostics and Mitigation

Abstract: High-temperature proton exchange membrane (HTPEM) fuel cells are able to withstand a substantial amount of CO poisoning while still maintaining stable output. High concentrations of CO will however degrade performance and can lead to instability. This paper presents a detailed study on the impact of CO poisoning on the performance of High-temperature PEM fuel cells and proposes a new method for mitigating the long term effects by using natural current profiles. A dedicated test setup was constructed to perform both steady state and dynamic analysis on the fuel cell under a wide range of operating conditions and variations in CO content. The drop in performance captured in the polarization curves is modeled using a simple circuit model with a dedicated fault element. The captured impedance spectra from the electrochemical impedance spectroscopy tests provide insight to the changes in the electrochemical circuit parameters that can be used to diagnose the extent of CO poisoning. Possible load control strategies that can reverse the CO poisoning is explored and the optimal profile is experimentally determined along with the long term effects.

Optimal Sensor Placement for Fault Diagnosis Using Magnitude Ratio

Abstract: Major performance losses occur in process industries due to failures that are not identified at the incipient stage. Early detection of such faults is also critical for the safety of the equipment, operating personnel, and other resources. When a fault occurs in a system, it can propagate and affect several process variables. Variables that need to be measured in order to detect and diagnose the faults have to be identified and chosen economically. An algorithmic approach for identifying the optimal number, type, and location of the sensors for fault detection and diagnosis is useful, particularly for large-scale, chemical process plants. In this work, previous algorithms for sensor placement that use signed directed graph (SDG) models for the process are enhanced to include magnitude ratio (MR) information to identify more promising sensor locations. Further, we also study the combination of fault evolution sequences (FES) already introduced in the literature and the MR information for effective fault di...

Online Diagnostics of HTPEM Fuel Cells Using Small Amplitude Transient Analysis for CO Poisoning

Abstract: Recent developments in materials have allowed PEM fuel cells to operate at higher temperatures and alleviate some of the problems that occur during operation. High-temperature PEM fuel cells are still under development, and very little has been done to study transient conditions, specifically the application of small amplitude load transients for diagnostic purposes. This paper presents the evolution of the fuel cell voltage transient for small current pulses over a range of operating conditions. A fault mechanism in the form of CO poisoning is introduced to further study and evaluate the transients for diagnostic purposes. A new two-stage diagnostic method is proposed based on the voltage transient. The first stage makes use of the discrete S-transform for fault marker identification and provides fast estimations on the fuel cell state of health. The second stage makes use of a population-based incremental learning (PBIL) algorithm for equivalent circuit parameter extraction, required for detailed diagnostics. The method is evaluated for both the healthy and the faulted CO poisoning condition in order to verify performance.

An integrated approach for oscillation diagnosis in linear closed loop systems

Abstract: In industrial plants with non-oscillatory set points, oscillation detection and diagnosis are key steps to improve plant performance and safety. Oscillations in linear closed loop systems can occur due to one or more of the following reasons: (i) changes in process/controller settings, (ii) stiction in control valves, (iii) external oscillatory disturbances, (iv) quantization effects, and (v) presence of saturation and hysteresis in closed loop systems. Though there are techniques to address oscillation diagnosis problem, there are gray areas such as the identification of multiple sources that cause oscillations in the process output. In this work, this problem is addressed through the development of an algorithm to identify multiple sources of oscillations in Single Input Single Output (SISO) loops. Further, an integrated approach to diagnose both single/multiple root causes in SISO loops is presented. Simulation and industrial case studies are provided to show the applicability of the proposed algorithms.

On-line performance monitoring of PEM fuel cell using a fast EIS approach

Abstract: The Polymer Electrolyte Membrane Fuel Cell is a widely researched fuel cell, and a highly potential candidate for alternate power generation. However, technical issues such as membrane flooding and drying prevent its deployment in many applications. Electrochemical Impedance Spectroscopy (EIS) is a very powerful technique that is used to isolate flooding and drying of the fuel cell from operating data. Such information about the state of operation of the cell is critical to deciding necessary control actions to maintain the health and performance of the cell. However, the time taken in obtaining measurements in EIS can be large enough to allow the cell to flood or dry beyond irreparable damage, rendering it a mere postmortem technique. Moreover, after long durations of perturbation, the cell takes a considerable amount of time to return to its regular operation. A new technique is proposed that uses the concept of EIS, but is computationally faster and gives results comparable with those of traditional EIS. This technique is based on perturbing the cell with a small chirp signal containing large number of frequencies instead of series of small sinusoids at different frequencies. Simulation results on isolation of flooding and drying based on Fast EIS are illustrated and future work directions are indicated.

Investigating Arrangement of Composite Drops in Two-Dimensional Microchannels Using Multiagent Simulations: A Design Perspective

Abstract: Drops can self-organize in two-dimensional (2D) microchannels to form ordered arrangements. Design of microchannels that result in a particular pattern of drops for a specific purpose is difficult and nonintuitive due to the inherent complexity of the drop dynamics. We address the problem of understanding the arrangement of composite drops inside a microchannel. A multiagent modeling strategy that was recently proposed by us is employed to understand this complex design problem. We consider the design of a drop–drop contactor that results in an equal mix of A and B. We seek to find the inlet sequence of drops A and B that would result in the maximum contact between A and B in the ordered arrangement. We find that intuition-based results work well only for a single layer arrangement of drops. We attribute this anomalous behavior to the symmetry breaking instability of the drop patterns in these 2D microchannels. From the dynamic simulations, we understand why certain inlet sequences perform better than oth...

A New Measure To Improve the Reliability of Stiction Detection Techniques

Abstract: A variety of methods have been developed to identify the presence of stiction in linear closed-loop systems. One of the commonly used approaches is based on identification of a Hammerstein model (linear dynamic model preceded by static nonlinear element) between the controller output and process output. These techniques utilize the fact that control valve stiction introduces nonlinearities in the otherwise linear feedback system. However, the present work shows that these techniques could provide ambiguous results depending on the frequency response of the controller and the process of interest. Therefore, in this work, a reliability measure to validate the results from Hammerstein model-based stiction detection approaches is proposed. This measure of reliability is important from an industrial perspective because (i) it helps in reducing false alarms and (ii) it improves the computational speed by guiding the selection of search space in the linear model identification step. The applicability of this rel...