In this study, the change of electrochemical parameters during the gradual water accumulation of a polymer electrolyte fuel cell (PEFC) are estimated using electrochemical impedance spectroscopy ...(EIS) measurements and an impedance model based on electrode theory during a two‐step oxygen reduction reaction (ORR). EIS measurements were carried out in a 5 cm2 H2/O2 PEFC operated under a dead‐ended configuration of the gas reactants and during gradual accumulation of water. Kramers‐Kronig evaluation demonstrated that the EIS measurements comply with stability and linearity properties and the inductive loops featured at low frequencies are attributed to electrochemical mechanisms within the PEFC and not to instability during gradual water accumulation. The impedance model has been reported in a previous study and simulates inductive loops at low frequencies which are attributed to platinum oxide formation during the ORR. The estimated parameters obtained from this EIS‐modeling analysis can provide an insight into the decrease in PEFC performance during flooding conditions. A further qualitative analysis considering the effect of long‐term water accumulation on the oxygen‐reduction charge transfer resistance is discussed. It is possible to have an insight into the physical mechanisms of the PEFC, by combining different experimental techniques and fundamental theory in a complimentary manner.
In this study, an analysis of the current distribution and oxygen diffusion in the Polymer Electrolyte Fuel Cell (PEFC) Cathode Catalyst Layer (CCL) has been carried out using Electrochemical ...Impedance Spectroscopy (EIS) measurements. Cathode EIS measurements obtained through a three-electrode configuration in the measurement system are compared with simulated EIS data from a previously validated numerical model, which subsequently allows the diagnostics of spatio-temporal electrochemical performance of the PEFC cathode. The results show that low frequency EIS measurements commonly related to mass transport limitations are attributed to the low oxygen equilibrium concentration in the CCL–Gas Diffusion Layer (GDL) interface and the low diffusivity of oxygen through the CCL. Once the electrochemical and diffusion mechanisms of the CCL are calculated from the EIS measurements, a further analysis of the current density and oxygen concentration distributions through the CCL thickness is carried out. The results show that high ionic resistance within the CCL electrolyte skews the current distribution towards the membrane interface. Therefore the same average current density has to be provided by few catalyst sites near the membrane. The increase in ionic resistance results in a poor catalyst utilization through the CCL thickness. The results also show that non-steady oxygen diffusion in the CCL allows equilibrium to be established between the equilibrium oxygen concentration supplied at the GDL boundary and the surface concentration of the oxygen within the CCL. Overall, the study newly demonstrates that the developed technique can be applied to estimate the factors that influence the nature of polarization curves and to reveal the effect of kinetic, ohmic and mass transport mechanisms on current distribution through the thickness of the CCL from experimental EIS measurements.
► Cathode impedance measurements gathered with a three-electrode configuration. ► Comparison between measured and simulated data using a validated impedance model. ► Electrochemical and diffusion mechanisms calculated from impedance measurements. ► Analysis of current density and oxygen distributions in the cathode catalyst layer. ► Effect of kinetic, ohmic and mass transport mechanisms on current distribution.
In this study, an assessment of the performance of an industrial gas turbine during compressor fouling conditions is undertaken. The assessment is carried out through performance diagnostic and ...prognostic methods. The diagnostic technique identifies and isolates faults in the components of the gas turbine, and considers the health parameter of the compressor (efficiency). The remaining useful life of the compressor is estimated by a degradation model that also considers the compressor efficiency health index. Compressor efficiency is estimated using a gas turbine real-time executable digital-twin. The digital-twin is integrated within a gas turbine system operating in the field. Data processing of the compressor efficiency is required to obtain a meaningful trend of the compressor health index. Results demonstrate that the diagnostic and prognostic methods applied to the problem of compressor health parameter estimated by the digital-twin can be a powerful tool to initiate maintenance actions on associated axial compressors prior to significant performance deterioration.
In this study, a sensor fault diagnostic system to detect/isolate and accommodate faults in sensors from an industrial gas turbine has been developed. The sensor fault diagnostic module is integrated ...with a gas turbine real-time executable digital-twin (RT xDT) reported in a previous study. The sensor fault diagnostic module of the digital-twin considers analytical sensor redundancy using a reference engine model to provide redundant estimates of measured engine variables. A Software-in-the-Loop (S-i-L) architecture and Hardware-in-the-Loop (H-i-L) facility are constructed to assess the sensor diagnostic module (fault detection/ fault isolation) during failure in sensors from the engine. The results demonstrated that if the discrepancy between virtual measurement (provided by digital-twin) and sensor measurement exceeds the prescribed tolerance levels, the sensor fault diagnostic logic determines the state of switching between the virtual and engine sensor measurements in a dual lane control configuration of the gas turbine control system. The sensor fault detection system implemented in the gas turbine RT xDT can be deployed onto a distributed control system of industrial gas turbines to diagnose sensor deficiencies and ensure continuous and safe operation of the gas turbine. Consequently, the developed system will increase engine availability and reliability by diagnosing engine operational deficiencies before severe failure.
In this study, an assessment of degradation and failure modes in the gas-path components of twin-shaft industrial gas turbines (IGTs) has been carried out through a model-based analysis. Measurements ...from twin-shaft IGTs operated in the field and denoting reduction in engine performance attributed to compressor fouling conditions, hot-end blade turbine damage, and failure in the variable stator guide vane (VSGV) mechanism of the compressor have been considered for the analysis. The measurements were compared with simulated data from a thermodynamic model constructed in a Simulink environment, which predicts the physical parameters (pressure and temperature) across the different stations of the IGT. The model predicts engine health parameters, e.g., component efficiencies and flow capacities, which are not available in the engine field data. The results show that it is possible to simulate the change in physical parameters across the IGT during degradation and failure in the components by varying component efficiencies and flow capacities during IGT simulation. The results also demonstrate that the model can predict the measured field data attributed to failure in the gas-path components of twin-shaft IGTs. The estimated health parameters during degradation or failure in the gas-path components can assist the development of health-index prognostic methods for operational engine performance prediction.
In this study, the inter-stage dynamic performance of a multistage axial compressor is simulated through a semi-empirical model constructed in the Matlab Simulink environment. A semi-empirical 1-D ...compressor model developed in a previous study has been integrated with a 0-D twin-shaft gas turbine model developed in the Simulink environment. Inter-stage performance data generated through a high-fidelity design tool and based on throughflow analysis are considered for the development of the inter-stage modeling framework. Inter-stage performance data comprise pressure ratio at various speeds with nominal variable stator guide vane (VGV) positions and with hypothetical offsets to them with respect to the gas generator speed (GGS). Compressor discharge pressure, fuel flow demand, GGS and power turbine speed measured during the operation of a twin-shaft industrial gas turbine are considered for the dynamic model validation. The dynamic performance of the axial-compressor, simulated by the developed modeling framework, is represented on the overall compressor map and individual stage characteristic maps. The effect of extracting air through the bleed port in the engine center-casing on transient performance represented on overall compressor map and stage performance maps is also presented. In addition, the dynamic performance of the axial-compressor with an offset in VGV position is represented on the overall compressor map and individual stage characteristic maps. The study couples the fundamental principles of axial compressors and a semi-empirical modeling architecture in a complementary manner. The developed modeling framework can provide a deeper understanding of the factors that affect the dynamic performance of axial compressors.
In this study, an impedance model represented as an equivalent electrical circuit (EEC) and comprised of a transmission line circuit and a frequency dispersion Warburg component is developed for the ...study of the electrochemical impedance spectroscopy (EIS) of Li-ion batteries. The model considers the impedance response of a porous cathode electrode comprised of solid particles surrounded by solution. Theory of diffusion of charge carriers in the solid and solution phases of the cathode composite electrode has been considered in the impedance model. The simulated impedance response predicted by the impedance model is compared with EIS measurements carried out in a LiPo battery pack. In addition, a theoretical comparison between the simulated EIS response from the developed model with the simulated impedance response of an EEC representing a Li-ion battery and reported in the literature is carried out as well. The effect of increasing and decreasing the cathodic diffusion distance of lithium ions across the solution and solid phases on the Nyquist impedance complex-plot is simulated. The impedance model is able to decouple diffusion mechanisms in the solid and solution phases of the cathode electrode represented at different frequencies of the Nyquist complex-plot. The results show that the impedance model can provide an insight into the different electrochemical mechanisms of the Li-ion battery represented in the Nyquist complex impedance plot at different frequencies. The model could assist the understanding of the different phenomenological processes in the electrode during the decrease of state of charge of Li-ion batteries using EIS.
•Development of an impedance model to analyse EIS in Li-ion batteries.•Impedance model is developed from fundamental electrochemical theory.•Experimental and theoretical validation of the impedance model is carried out.•Analysis of diffusion mechanisms in the cathode composite electrode is carried out.
•Electrochemical impedance model for estimation of oxygen diffusion in PEFC.•Development of a cathodic impedance model comprising the CCL and GDL.•EIS and polarisation curves measured in PEFC ...operated with nitrox and heliox.•Estimation of oxygen diffusion resistance from EIS in PEFC with nitrox and heliox.•Analysis of oxygen diffusion resistance in catalyst layer and gas diffusion layer.
In this study, a new electrochemical impedance model to predict oxygen diffusion resistance in the cathode catalyst layer (CCL) and in the gas diffusion layer (GDL) of polymer electrolyte fuel cells (PEFCs) is proposed. The impedance model of the PEFC is validated with electrochemical impedance spectroscopy (EIS) measurements carried out in a single PEFC operated with oxygen diluted in nitrogen (nitrox) and diluted in helium (heliox) at different relative humidity (RH). The PEFC impedance model combines the impedance model for the CCL reported in the authors’ previous study with a diffusion impedance model for the GDL reported in the literature. The EIS measurements in the PEFC represent lumped parameters (total oxygen diffusion resistance RO2_PEFC) from the different layers comprising the PEFC. The impedance model ignores the effect of oxygen transport in the flow field plates on EIS measurements, as this effect is apparent on EIS measurements carried out in PEFCs with low oxygen stoichiometry. The results from this study demonstrate that the estimated oxygen diffusion resistances in the CCL and GDL are distributed in the total oxygen diffusion resistance RO2_PEFC=RO2_CCL+RO2_GDL. This is demonstrated by analysing the EIS measurements carried out in the PEFC with nitrox and heliox as cathode gas reactants and neglecting oxygen transport resistance in either the CCL RO2_PEFC≈RO2_GDL or GDL RO2_PEFC≈RO2_CCL from the PEFC impedance model. It is possible to obtain a deeper understanding of the influence of the electrochemical mechanisms from the different layers comprising the PEFC on EIS measurements by combining fundamental theory and experimental measurements in a complimentary manner.
PDF
