Frequency Response Techniques Research Articles

Diagnosis of Polymer Electrolyte Fuel Cell: Should We Rely Only on Electrical Quantities?

Polymer electrolyte fuel cells (PEMFC) will play a central role in the establishment of a de-carbonized society. They are seen as a key technology to empower electric heavy-duty vehicles. However, the lifespan is still below the fixed standards for their commercialization. A way to increase their durability is to complement them during operation with a mitigation strategy enabling to avoid the arise of faulty states accelerating degradation processes. Nowadays, most of the proposed fault identification and isolation architectures rely on monitoring of the performance through electrochemical impedance spectroscopy (EIS). Nevertheless, the analysis of impedance data is often complicated, since the contribution of many processes in the EIS spectra are coupled with each other and, for this reason, difficult to identify. This constitutes a limiting factor for its employment as diagnostic tool. In the last years, in order to overcome such issues, the use of nonelectrical periodic inputs and/or outputs has been advanced. The idea motivating this research direction is the possibility to detect the contribution of selected processes on the performance of PEMFCs separating them from the others. Novel frequency response techniques involving back pressure, temperature and partial pressure have been developed [1].In this contribution, the so-called concentration frequency response analysis (CFRA) is considered [2, 3]. It consists on perturbing the cathode side of the cell with a feed characterized by a periodic partial pressure of oxygen and water, and the detection of the electric response, voltage or current. A test station has been designed and constructed in order to apply this technique and investigate its capabilities. A model based analysis of the measured CFRA spectra indicates the possibility to decouple the contributions of the water transport into Nafion and oxygen mass transport on the performance of the cell depending on the chosen concentration input and electrical output. In this way, the analysis and identification of the related power losses is simplified with respect to classic EIS.Moreover, the performance of a fault identification and isolation architecture employing CFRA as monitoring technique have been evaluated and compared to EIS. For such purpose, an identifiability analysis of parameters related to faulty states and obtained by model fitting to spectra of each technique has been performed. The results clearly show that the employment of CFRA based on water pressure input offers the most reliable parameter estimation and, consequently, constitutes the most preferable diagnosis method among the others analyzed.

Assessment of frequency response techniques in diagnosing polymer electrolyte membrane fuel cells

In this study, we evaluated the effectiveness of various frequency response analysis (FRA) techniques for identifying fault states in the diagnosis of polymer electrolyte membrane fuel cells (PEMFCs). To this end, an identifiability analysis was conducted to determine the reliability of parameters obtained by fitting a previously validated PEMFC model to the spectra from different methods. Specifically, we focused on electrochemical impedance spectroscopy (EIS) and the newly introduced concentration frequency response analysis (CFRA). The identifiability analysis revealed that CFRA, when applied with water pressure as the input and voltage as the output, provides the most accurate parameters estimates related to mass transport in the cathode electrode and the Nafion electrolyte, yielding physically meaningful insights. Consequently, employing this input for PEMFC diagnosis emerges as a promising approach. Furthermore, our findings underscore the importance of meticulously evaluating the quality of parameter estimation, even when utilizing well-established techniques such as EIS.

Switchable Frequency Response Based on Electropermanent Magnet Actuator for Wide-Range Operation of Electromagnetic Devices

This paper proposes a linear oscillatory actuator (LOA) that can switch its degree of freedom (DOF) for the purpose of improving its frequency response. Electropermanent magnet (EPM) is selected as an energy-saving solution to switch the DOF by absorbing and desorbing secondary mass with primary mass. First, motion equations for 1- and 2-DOF systems are derived and The corresponding frequency response curves are numerically obtained. Second, finite element analysis coupled with a numerical simulator reveals that transient absorption and desorption motions are successfully performed with a help of EPM. Finally, measurement using a prototype shows that the proposed switchable frequency response technique is experimentally realized and the measured result ares in good agreement with the numerical ones.

Design and Selection of Inductor Current Feedback for the Sliding-Mode Controlled Hybrid Boost Converter

The hybrid step-up converter is a fifth-order system with a dc gain greater than the traditional second-order step-up configuration. Considering their high order, several state variables are accessible for feedback purposes in the control of such systems. Therefore, choosing the best state variables is essential since they influence the system’s dynamic response and stability. This work proposes a methodical method to identify the appropriate state variables in implementing a sliding-mode (SM) controlled hybrid boost converter. A thorough comparison of two SM controllers based on various feedback currents is conducted. The frequency response technique is used to demonstrate how the SM method employing the current through the output inductor leads to an unstable response. The right-half s-plane poles and zeroes in the converter’s inner-loop transfer function, which precisely cancel one another, are what is causing the instability. On the other hand, a stable system may result from employing a SM controller with the current through the input inductor. Lastly, some experimental outcomes using the preferred SM control method are provided.

A study of kinetic air separation at low temperatures: Oxygen, nitrogen, and argon in carbon molecular sieve

Carbon molecular sieves (CMS) are among the most promising materials for industrial gas separations, including their utilization for the kinetical removal of O2 from N2 and Ar in commercial pressure swing adsorption processes. However, little is known about the characteristics of these kinetics processes at lower temperatures. In this article, a kinetic study was carried out at three temperatures (-25, 0, and 25 °C) for pure O2, N2, and Ar using a commercially available Shirasagi MSC-3K-172. A pressure-swing frequency response technique was applied to determine the dominating mass transfer resistances, as well as corresponding temperature dependences, revealing that a combined resistance model of surface barrier and micropore diffusion best describes the behavior of all three gases. In addition, this MSC-3K-172 exhibited higher kinetic rates but lower kinetic selectivities as compared to Shirasagi MSC-3R-162 and −170, which showed only surface barrier resistances for N2 and Ar. When the temperature was reduced from 25 °C to −25 °C, the equilibrium capacities for three gases increase similarly, but the kinetic rates decrease to varying extents, resulting in an unexpected increase in kinetic selectivity – the value near doubled (from 17 to 33) for O2/N2 and more than tripled (from 24 to 88) for O2/Ar. The observed kinetic selectivities were found to correlate well with the difference of the activation energies between the gas pairs. These activation energies are 36, 29, and 21 kJ/mol for Ar, N2, and O2, respectively, greater than the isosteric heats of adsorption, which follow the reverse order. Together, these results indicate a delicate balance among kinetic selectivities, controlling mass transfer resistances and rate parameters, and equilibrium capacities, and can be utilized for the design of kinetic separations with CMSs, especially when extended to sub-ambient temperatures.

Analytic Matrix Method for Frequency Response Techniques Applied to Nonlinear Dynamical Systems II: Large Amplitude Oscillations

This work is the second in a series of articles that deal with analytical solutions of nonlinear dynamical systems under oscillatory input that may exhibit harmonic frequencies. Frequency response techniques of nonlinear dynamical systems are usually analyzed with numerical methods, because in most cases analytical solutions such as the harmonic balance series solution turn out to be difficult, if not impossible, as they are based on an infinite series of trigonometric functions with harmonic frequencies. The key contribution of the analytic matrix methods reported in the present series of articles is to work with the invariant submanifold of the problem and to propose the solution as infinite power series of the oscillatory input; this procedure is a direct one that speeds up the computations compared to traditional series solution methods. The method reported in the first contribution of this series allows for the computation of the analytical solution only for small and medium amplitudes of the oscillatory input, because these series may diverge when large amplitudes are applied. Therefore, the analytic matrix method reported here, which is a reconfiguration of the method proposed in the first contribution in this series, allows the solving of problems in the regime of large-amplitude oscillations where the contributions of the high order harmonics affect the amplitudes of the low order harmonics, leading to amplitude- and frequency-dependent coefficients for the infinite series of trigonometric function expansion.

Adsorption of trichloroethylene on common indoor materials studied using a combined inverse gas chromatography and frequency response technique

Building materials can act as sinks and sources of volatile organic compounds (VOCs) which are indoor air contaminants. A knowledge of the dynamics of VOC sorption processes on building materials is needed in order to fully understand how these compounds can influence indoor air quality, and thus, their potential for influencing human health. In the current work, a combination of classical inverse gas chromatography (IGC) and frequency response (FR) technique was used to investigate the sorptive partition and diffusion coefficients of trichloroethylene (TCE) on building materials. This is a compound of considerable interest in many indoor air environments, particularly those impacted by vapor intrusion processes, and the TCE also serves as a model VOC for demonstrating the method. Six typical indoor materials (carpet, cotton, cinderblock, printer paper, polyethylene, drywall) were selected to demonstrate the technique. A selected building material was packed into a stainless-steel column and exposed to a low-concentration TCE flow applied in a sinusoidal temporal pattern at room temperature (22 ℃). In this case, cinderblock showed the highest sorption uptakes (6209 ng TCE/g material-ppbv TCE) and the slowest sorption rates (7.3 × 10−10 m2/s) among tested materials. The results from the FR-IGC method are compared to other conventionally obtained results and agree well.

Analytic Matrix Method for Frequency Response Techniques Applied to Nonlinear Dynamical Systems I: Small and Medium Amplitude Oscillations

This is the first on a series of articles that deal with nonlinear dynamical systems under oscillatory input that may exhibit harmonic and non-harmonic frequencies and possibly complex behavior in the form of chaos. Frequency response techniques of nonlinear dynamical systems are usually analyzed with numerical methods because, most of the time, analytical solutions turn out to be difficult, if not impossible, since they are based on infinite series of trigonometric functions. The analytic matrix method reported here is a direct one that speeds up the solution processing compared to traditional series solution methods. In this method, we work with the invariant submanifold of the problem, and we propose a series solution that is equivalent to the harmonic balance series solution. However, the recursive relation obtained for the coefficients in our analytical method simplifies traditional approaches to obtain the solution with the harmonic balance series method. This method can be applied to nonlinear dynamic systems under oscillatory input to find the analog of a usual Bode plot where regions of small and medium amplitude oscillatory input are well described. We found that the identification of such regions requires both the amplitude as well as the frequency to be properly specified. In the second paper of the series, the method to solve problems in the field of large amplitudes will be addressed.

Homogeneous-like Alkyne Selective Hydrogenation Catalyzed by Cationic Nickel Confined in Zeolite

The selective hydrogenation of alkynes to their corresponding alkenes is an important type of organic transformation, which is currently accomplished by modified palladium catalysts. Herein, we rep...

Smart frequency control in low inertia energy systems based on frequency response techniques: A review

Integrated energy systems are considered as an indispensable part of the pathway towards a low-carbon sustainable future, as well as secure and reliable systems, characterised with a high level of flexibility and resilience. Increased penetration of renewable energy sources into energy systems is contributing to the reduction of carbon emission, thereby reducing the level or air pollution, climate changes and supporting the quality of life on the Earth. In this context, energy conversion systems realized by wind turbine generators have been and are still in the focus of extensive research on different system aspects, from planning, exploitation, monitoring, control, or protection perspective. To maximize the utilisation of available wind energy, modern wind turbine generators are connected to the main grid over power electronics (e.g. Type-3, or Type-4 wind generators). Consequently, they are electromagnetically disconnected from the rest of the power system and they provide little or no inertia, contrary to conventional synchronous generators, synchronously connected to the grid and synchronously operated to each other. This synchronism is necessary to ensure a stable system operation. The reduction of the system inertia imposes serious technical challenges on preserving system frequency stability. As it is known, inertia is one of key factors determining the robustness of power systems against sudden active power imbalances caused by different types of frequency events (generator disconnection, or load connection). The reduction of synchronous power reserves further intensifies this problem by reducing the system ability to maintain frequency within a permissible range following frequency events. Consequently, grid operators demand renewable energy sources, which are also referred to as nonsynchronous generators, to emulate the behaviour of synchronous generators to some extent and to participate in (fast) frequency control upon need. In general, countermeasures applied to these sources to contribute to frequency support are classified into two main categories: a) temporary and b) persistent energy reserve-based approaches. This paper presents a review on latest research findings and developed mechanisms for frequency control using wind energy conversion systems as the most frequently deployed renewable energy sources in modern power systems. Relying on lessons learned from the past two decades, in this paper current and future challenges, feasible solutions and subsequent research prospects are detailed. Some key principles that should underlie future changes of wind integrated energy systems are suggested and further research directions are addressed.

Interference Mitigation Among Indoor Phone Subscribers In Lte Based Heterogeneous Networks Using Fast Response Frequency Reuse Technique

Mobile phone users both for voice and data have increased tremendously in the last decade thereby making it necessary for service providers in the telecommunication industry maintain quality of service as much as possible. The huge complaint from indoor mobile users concerning signal coverage and capacity has made it a necessity to conduct this research. The aim of this research is to minimize interference among indoor phone subscribers in long term evolution (LTE) based heterogeneous networks using fast frequency reuse technique. Femtocells are deployed in varying number within the macrocell coverage area. For communication between femto and macro users within the network, a fast frequency response technique was proposed and parameters such as path loss, SINR and channel capacities were determined to ascertain the level of performance of the system when compared with a situation with random and unplanned deployment. Results show that a high SINR results when the bit error rate is low, this consequently improves communication and throughput in the network. The high values of throughputs for edge macro-femto users compared to inner edge-macro users are due to distance of macro and femto users from the macrocell. As femtocells increase for edge macro-femto users within the coverage area, throughputs reduced gradually

Use of the Frequency Response Technique to Analyze the Impact of Improving the Power Factor in the Electrical Networks

Current electrical systems tend to have low power factors that are often accompanied by high levels of harmonic distortion of voltage and current, which have been growing by the increase of the loads no line in the electrical systems, infringing the limits established by the norms and regulations in force. These operating conditions affect the stability of the network and the bad functioning of the equipment, presenting high costs of billing and sanctions imposed by the state. Therefore, the need to analyze the electrical systems before the ingredient of non-linear loads arises in order to guarantee the correct operation and the quality of the power according to the normative requirements. This article presents a typical case study; The installation of capacitor banks to improve the power factor, which fulfil their main task, but serious problems such as the generation of resonances in parallel in the natural response of system, deformation of the waveforms of tension and current, and increase of the harmonic distortion individual and total of tension and current. for which a frequency response or frequency-scanning analysis is presented using ETAP as an additional analysis tool.

Rapid CO2 capture from ambient air by sorbent‐containing porous electrospun fibers made with the solvothermal polymer additive removal technique

Direct air capture (DAC) of CO2 is an emerging technology in the battle against climate change. Many sorbent materials and different technologies such as moisture swing sorption have been explored for this application. However, developing efficient scaffolds to adopt promising sorbents with fast kinetics is challenging, and very limited effort has been reported to address this critical issue. In this work, the availability and kinetic uptake of CO2 in sorbents embedded in various matrices are studied. Three scaffolds including a commercially available industrial film containing ion‐exchange resin (IER), IER particles embedded in dense electrospun fibers, and IER particles embedded in porous electrospun fibers are compared, in which a solvothermal polymer additive removal technique is used to create porosity in porous fibers. A frequency response technique is developed to measure the uptake capacity, sorbent availability, and kinetic uptake rate. The porous fiber has 90% IER availability, while the dense fibers have 50% particle accessibility. The sorption half time for both electrospun fiber samples is 10 ± 3 min. Our experimental results demonstrate that electrospinning polymer/sorbent composites is a promising technology to facilitate the handleability of sorbent particles and to improve the sorption kinetics, in which the IER embedded in porous electrospun fibers shows the highest cycle capacity with an uptake rate of 1.4 mol CO2 per gram‐hour. © 2018 American Institute of Chemical Engineers AIChE J, 65: 214–220, 2019

Pipe network blockage detection by frequency response and genetic algorithm technique

Pipe network blockage detection by frequency response and genetic algorithm technique

Insulation Assessment of Oil Impregnated Paper Condenser Bushings using Dielectric Frequency Response Technique

Objectives: Investigation of insulation to judge the health of the condenser bushing with different techniques has been the area of interest for the researchers in recent past. Power transformers and their bushings are manufactured with oil impregnated paper insulation but their insulation integrity is comprised due to absorbed, residual or liberated moisture/ water which leads to further deterioration and catastrophic failure. Methods: Conventional measurement technique taken at rated power frequency only gives relative indication of specific dielectric properties and health of the condenser bushings. This article introduces a new approach for insulation assessment of the condenser bushings where OIP insulation as a key component has been investigated in a wide frequency response spectrum and to confirm the validity of this approach, measurements were taken on several condenser bushings of same type and age. Findings: The advance technique is a nondestructive method which evaluates the water concentration of the test object. The resulting curve is highly determined by the behavior of solid insulation and it is also sensitive to oil conductivity and insulation geometry of the bushing. Furthermore, the interpretation scheme related to dielectric response is explained. Applications: The signatures achieved helps in reliable data analysis and can be used in qualitative comparisons of bushings which compliments the available quality assurance techniques in the factory. This approach can also be used for frequent monitoring and effective moisture management at site.

New development in flow‐through pressure‐swing frequency response method for mass‐transfer study: Ethane in ZIF‐8

A new pressure‐swing frequency response (PSFR) method has been developed to study mass transfer in adsorption systems as a function of temperature and pressure, from −70 to 180°C, and up to 7 bar. New in‐phase and out‐of‐phase functions have been derived for the PSFR in a general way to allow information extracted from it independent of whether the system is operated in a batch volume swing or a flow‐through pressure swing mode. A new mathematical model that considers distribution of diffusion rates has been introduced to account for diffusive transport in heterogeneous samples. Numerical simulation results have shown that a single rate diffusion model works well when heterogeneity can be described by a normal distribution, but not for asymmetrically bimodal distributions. As a test reference system, the transport of ethane in ZIF‐8 was investigated at different pressures and temperatures using the new PSFR method. The mass transfer was found to be dominated by micropore diffusion. Diffusivity was found to be weakly dependent on pressure or loading, but quite strongly dependent on temperature. The results agree very well with our independent batch volume frequency response technique experiments. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1077–1090, 2017

Effects of debonding length on the fatigue and vibration behaviour of sandwich composite

The aim of this study is to investigate the effects of debonding length on the fatigue and vibration behaviour of sandwich material. The sandwich material used in this study is constructed with glass fibre laminates as skins and with PVC closed-cell foams as core. The tests were conducted using impulse frequency response technique and cyclic fatigue loading in three-point bending with various debonding lengths. The vibration test was used to study the effects of debonding length on modal parameters (frequency, loss factor). The effects of debonding lengths on the stiffness, hysteresis loops and damping were studied during fatigue tests. The different relative change in frequency, loss factor and maximum load is determined. These parameters are compared for vibration and fatigue tests.

Development and application of a standstill parameter identification technique for the synchronous generator

This work presents the development of an offline standstill estimation technique, where the synchronous machine is locked at an arbitrary (but known) angle and is excited over a short period of time. The proposed time domain method requires few seconds of captured data in contrast to the well-known standard Standstill Frequency Response (SSFR) technique that could take more than 6h to conduct. This is based on nonlinear least squares estimation and algebraic elimination theory. The resulting algorithm is non-iterative where the data is used to construct polynomials that are solved for a finite number of roots which determine the electrical parameter values. Experimental results are presented showing the efficacy of the technique in furnishing the parameters of a salient pole synchronous machine.

Effect of flow rate variation on the frequency response in slot coating process with different upstream sloped die geometries

The sensitivity of slot coating process with different sloped upstream die configurations has been investigated using the viscocapillary model for a coating liquid with constant viscosity (Newtonian) through frequency response technique. Amplitude ratios of coating thickness with respect to a sinusoidal disturbance at flow rate are compared under different input frequency and upstream die lip angle conditions. Amplitude ratio values decrease as the upstream die lip angle increases, implying that the coating system is less sensitive to the given disturbance due to the larger space condition in the upstream die region. Amplitude ratio curves obtained from various conditions can be usefully unified in a single one with the help of new dimensionless time between fluid time scale in upstream region and perturbed time scale by input frequency.

Gas pipeline modelling and control

The distributed parameter modelling of the gas flow through long pipelines is considered. Procedures which incorporate the gas stream energy storage, the pipeline frictional resistance and pressure attenuation characteristics are introduced. The pipeline input–output, transfer function, pressure and volume flow representations are formulated. An optimum, least effort, closed loop regulation strategy is proposed. Frequency response techniques are invoked enabling the derivation of simple, robust control algorithms. Confirmation of the results obtained, from the transient response computation of the outputs following input reference and load disturbance changes, are presented. The accuracy and novelty of the approach presented is commented upon.