Open-circuit Operation Research Articles

Bioelectrode-based approach for enhancing nitrate and nitrite removal and electricity generation from eutrophic lakes

Nitrate and nitrite contamination of surface waters (e.g. lakes) has become a severe environmental and health problem, especially in developing countries. The recent demonstration of nitrate reduction at the cathode of microbial fuel cell (MFC) provides an opportunity to develop a new technology for nitrogen removal from surface waters. In this study, a sediment-type MFC based on two pieces of bioelectrodes was employed as a novel in situ applicable approach for nitrogen removal, as well as electricity production from eutrophic lakes. Maximum power density of 42 and 36 mW/m2 was produced respectively from nitrate- and nitrite-rich synthetic lake waters at initial concentration of 10 mg-N/L. Along with the electricity production a total nitrogen removal of 62% and 77% was accomplished, for nitrate and nitrite, respectively. The nitrogen removal was almost 4 times higher under close-circuit condition with biocathode, compared to either the open-circuit operation or with abiotic cathode. The mass balance on nitrogen indicates that most of the removed nitrate and nitrite (84.7 ± 0.1% and 81.8 ± 0.1%, respectively) was reduced to nitrogen gas. The nitrogen removal and power generation was limited by the dissolved oxygen (DO) level in the water and acetate level injected to the sediment. Excessive oxygen resulted in dramatically decrease of nitrogen removal efficiency and only 7.8% removal was obtained at DO level of 7.8 mg/l. The power generation and nitrogen removal increased with acetate level and was nearly saturated at 0.84 mg/g-sediment. This bioelectrode-based in situ approach is attractive not only due to the electricity production, but also due to no need of extra reactor construction, which may broaden the application possibilities of sediment MFC technology.

Advanced electrolyte-free fuel cells based on functional nanocomposites of a single porous component: analysis, modeling and validation

Recently, a fuel cell device constructed with only one layer composited of ceria-based nanocomposites (typically, lithium nickel oxide and gadolinium doped ceria (LiNiO2–GDC) composite materials), called an electrolyte-free fuel cell (EFFC), was realized for energy conversion by Zhu et al. The maxium power density of this single-component fuel cell is 450 mW cm−2 at 550 °C when using hydrogen fuel. In this study, a model was developed to evaluate the performance of an EFFC. The kinetics of anodic and cathodic reactions were modeled based on electrochemical impedance spectroscopy (EIS) measurements. The results show that both of the anodic and cathodic reactions are kinetically fast processes at 500 °C. Safety issues of an EFFC using oxidant and fuels at the same time without a gas-tight separator were analyzed under open circuit and normal operation states, respectively. The reaction depth of anodic and cathodic processes dominated the competition between surface electrochemical and gas-phase reactions which were effected by the catalytic activity and porosity of the materials. The voltage and power output of an EFFC were calculated based on the model and compared with the experimental results.

PEMFC MEA and System Design Considerations

Proton exchange membrane fuel cells (PEMFCs) are being developed and sold commercially for multiple near term markets. Ballard Power Systems is focused on the near term markets of backup power, distributed generation, materials handling, and buses. Significant advances have been made in cost and durability of fuel cell products. Improved tolerance to a wide range of system operation and environmental noises will enable increased viability across a broad range of applications. In order to apply the most effective membrane electrode assembly (MEA) design for each market, the system requirements and associated MEA failures must be well understood. The failure modes associated with the electrodes and membrane degradation are discussed with respect to associated system operation and mitigating approaches. A few key system considerations that influence MEA design include expected fuel quality, balance-of-plant materials, time under idle or open circuit operation, and start-up and shut-down conditions.

Pyrolytic graphite electrode modified with a thin film of a graphite/diamond nano-mixture for highly sensitive voltammetric determination of tryptophan and 5-hydroxytryptophan

We have prepared a pyrolytic graphite electrode (PGE) whose surface is covered with a thin film of a nano-mixture of graphite/diamond (NGD). The electrode is shown to be capable of electrochemically sensing of tryptophan (TRP) and 5-hydroxytryptophan (HTRP). The presence of the NGD film resulted in a remarkable increase in the peak currents and sharpness of the waves so that submicromolar concentrations of TRP and HTRP become detectable. Potential scan rates, the pH of the solution, the accumulation conditions and the amount of the modifier were optimized via cyclic voltammetry. Linear sweep voltammetry, under optimized accumulation time and in open circuit operation, was applied to the determination of TRP and HTRP with detection limits (S/N = 3) of 30 nM (TRP) and 6 nM (HTRP). The electrode can be easily prepared, displays high sensitivity, sharp peaks, long-term stability, and remarkable voltammetric reproducibility and repeatability. These properties make the sensor suitable for the trace analysis of TRP and HTRP in pharmaceutical and clinical preparations.

Recombination Dynamics as a Key Determinant of Open Circuit Voltage in Organic Bulk Heterojunction Solar Cells: A Comparison of Four Different Donor Polymers

Transient photovoltage and charge extraction analyses are employed to analyzes charge carrier densities and bimolecular recombination dynamics in organic polymer: fullerene solar cells under open circuit operating conditions, employing four different donor polymers. An equation is derived which allows us to calculate the device VOC from these kinetic measurements. This equation allows us to calculate voltage output of devices within ± 25 meV of directly measured values. This analysis thus allows us to relate device open circuit voltage directly to the kinetics of bimolecular recombination, and thereby the influence of nanomorphology upon device voltage output. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Positive anodic poised potential regulates microbial fuel cell performance with the function of open and closed circuitry

Positive influence of poised potential on microbial fuel cell (MFC) performance was observed with increase in the applied potential up to 600 mV and decreased thereafter. Higher power output (79.33 mW/m 2) was observed at 600 mV poised potential under open circuit operation (OC). Closed circuit operation (CC) showed almost negligible power output due to continuous electron discharge against an external load (100 Ω). However, CC operation resulted in the higher substrate (chemical oxygen demand (COD)) degradation [61.23% (control); 70.46% (OC; 600 mV); 74.15% (CC; 600 mV)] and total dissolved solids (TDS) removal [29.17% (control); 43.75% (OC; 600 mV); 72.92% (CC; 600 mV)] efficiencies compared to OC. Electron discharge and energy conversion efficiency was also observed to be higher with 600 mV poised potential. Poising potential showed additional redox couples (−0.29 ± 0.05 mV) on cyclic voltammetry. Application of poised potential during startup phase will help to enrich electrochemically active consortia on anode resulting in improved performance of MFC.

Degradation location study of proton exchange membrane at open circuit operation

In order to investigate the location of the radical-initiated membrane degradation at open circuit operation, ceria (CeO 2) nanoparticles are firstly placed at different locations of the membranes to scavenge free radicals generated there. Scanning electron microscopy (SEM) is used to characterize cross-sectional morphology of membrane before and after the open circuit voltage (OCV) test. OCV decay rate is used as an indicator of membrane degradation rate. Composite membranes with CeO 2 nanoparticles facing anode or cathode show improved membrane durability than that of plain membrane in terms of OCV decay rate and cross-sectional morphology. CeO 2–Nafion composite membrane with the same thickness is subsequently proposed and it obtains the best durability. It is concluded that the chemical degradation at open circuit operation occurs at both sides of anode and cathode.

Effects of open-circuit operation on membrane and catalyst layer degradation in proton exchange membrane fuel cells

Durability issues have been attracting a great deal of attention in hydrogen/air proton exchange membrane (PEM) fuel cell research. In the present work, membrane electrode assembly (MEA) degradation under open circuit (OC) conditions was carried out for more than 250 h. By means of several on-line electrochemical measurements, the performance of the fuel cell was analysed at different times during the degradation process. The results indicate that structural changes in the PEM and catalyst layers (CLs) are the main reasons for the decline in performance during OC operation. The results also show that degradation due to platinum oxidation or catalyst contamination can be partially recovered by a subsequent potential cycling process, whereas the same cycling process cannot recover the membrane degradation.

Multislice Inter-Bar Model for Large Synchronous Machines With Skewed Stator Slots

This paper describes an implementation of the time-stepped multislice finite-element method including a rotor inter-bar model in simulations of large skewed synchronous machines. The general form of the system equations is developed with an improved definition of inter-bar loop currents in order to enhance the numerical stability at large inter-bar resistances. In order to reduce simulation time, finite-element analysis (FEA) algorithms are implemented to exploit multiple processing cores. The simulations predict the flux density distributions and induced voltage harmonics at various inter-bar resistances for a synchronous generator under open-circuit operation. The simulated voltage harmonics are compared to test results and results obtained using conventional two-dimensional FEA and traditional skew factors.

Preferential Thinning Behaviors of the Anode-Side of the PEM Under Durability Test

Polymer electrolyte membrane fuel cell (PEMFC) have attracted much interest as energy conversion systems, especially in automobiles and portable applications, in response to mounting environmental problems caused by fossil fuels. Since PEMFC durability is currently an important issue, it is of great importance to closely analyze the membrane of PEMFCs for sign of degradation. In this work, we have performed accelerated degradation tests of a PEMFC using over 100 hours of open circuit operation with both dry and humidified inlet gases. It was of interest to determine which side of the membrane presented the most severe degradation since this indicates the type of degradation mechanism that was occurring. After disassembly of the cell, cross sectional images of the membrane were obtained with scanning electron microscopy. It was found that the anode side showed more significant deterioration than the cathode side in terms of overall membrane thinning.

Electrochemical modification of the catalytic activity of TiO2/YSZ supported rhodium films

The electrochemical activation of ethylene oxidation was studied over rhodium catalysts of different thickness (40, 100 and 160 nm) sputtered on top of a thin layer of TiO2 deposited on YSZ. The strong relationship between catalytic activity and oxidation state of rhodium was confirmed. Under open-circuit operation the catalyst potential appears as a suitable indicator of the surface oxidation state of rhodium allowing a prediction of the catalytic behavior from solid electrolyte potentiometric measurements. Under closed-circuit conditions the catalyst potential was used as a tool to tune the catalytic activity of rhodium which showed increasing promotional efficiency with decreasing catalyst film thickness.

Degradation Mechanism of PEMFC under Open Circuit Operation

We present a durability test of polymer electrolyte membrane fuel cell (PEMFC) in open circuit condition. Such a condition enhances the deterioration of the membrane electrode assembly. ac impedance spectroscopy measurements and scanning electron microscopy observation suggest that the degradation occurred at the cathode. Direct gas mass spectroscopy of the cathode outlet gas indicated the formation of HF, H 2 O 2 , CO 2 , SO, SO 2 , H 2 SO 2 , and H 2 SO 3 . A kinetic model is presented assuming that the H 2 gas cross leakage from the anode caused the cathode degradation. The model determines the rate of degradation using the permeability across the electrolyte membrane measured for crossover H 2 gas.

Transient response analysis of a Rosen-type piezoelectric transformer and its applications

This paper investigates the characteristics of voltage transient response, such as the maximum voltage, direct current (DC) time constant, alternating current (AC) time constant and oscillation frequency, and their applications for a Rosen-type piezoelectric transformer (PT). The transient response is induced immediately after an AC voltage connected to the PT is switched off. For the applications, the maximum voltage is used to elucidate how to cause an electrical shock for users of the PT under open-circuit operation. The DC time constant and the AC time constant are used to estimate the equivalent resistance of the mechanical loss and the dielectric resistance of the dielectric loss, respectively. Also, the AC time constant is used to estimate the quality factor of the PT. Additionally, the oscillation frequency is used as an antiresonant frequency of the PT. In order to verify the above characteristics and applications, both an equivalent circuit with initial conditions and a drive system with a control switch interposed between the PT and its AC voltage source are adopted to derive the transient response and measure its characteristics. Effects of the load resistance of the PT and the switching-off time of the voltage source on the transient response are measured and discussed.

Finding Electrical Characteristics of a Piezoelectric Transformer through Open-Circuit Operation

In this paper we investigate the electrical transient responses of a piezoelectric transformer (PT) under open-circuit operation and its response times. The transient responses are induced on both sides of the PT immediately after the AC voltage applied to the PT is switched off. An equivalent circuit with initial conditions is expressed first. Then, the equivalent circuit is used to derive an input voltage transient response and its response times. Moreover, an AC power supply system with a control switch is used to determine the transient responses, and an oscilloscope with a smoothing-and-filtering function is used to measure the transient responses and their DC components. Furthermore, the effects of load resistance, switching-off time and operating frequency on the transient responses are measured and discussed. From the measured results, the DC component of each output voltage transient response is almost zero and the AC response time of the output transient response is greatly decreased from 11.5 ms under an output open to 3.0 ms by adopting a 1 MΩ load resistance.

Modeling and analysis of hydrogen permeation in mixed proton–electronic conductors

A rigorous model for hydrogen permeation through dense mixed conductors was derived using the formalism of non-equilibrium thermodynamics for various operating modes and process conditions. The concentrations of charge carriers were rigorously included in this model through defect equilibria with the chemical environment at each membrane surface and through balance equations and a virtual pressure formalism within the membrane. Hydrogen permeation rates through proton–electron–hole mixed conductors were simulated using this framework under open-circuit, short-circuited, and applied potential operating modes. The sensitivity of H 2 permeation rates to the reduction–oxidation potentials at each side of the membrane and to the membrane properties (e.g. electron/hole diffusivity, oxygen binding energy) was examined in terms of the mobility and concentration of each charge carrier in order to identify rate-limiting steps for H 2 transport. These simulations showed that electronic transport controls H 2 permeation rates in proton–electron–hole mixed conductors typically used for H 2 permeation, especially when hydrogen chemical potentials are significantly different in the two sides of the membrane. These electronic conduction limitations arise from a region of very low electronic conductivity within the membrane, caused by a shift in the predominant charge carriers from electron to holes with decreasing hydrogen chemical potential. Under these asymmetrical conditions, H 2 permeation rates increase more markedly when an external electron-conducting path is introduced than at lower chemical potential gradients. Such interplay between rate-controlling variables leads to complex effects of H 2 chemical potential gradients on permeation rates. The effects of intrinsic membrane properties on H 2 permeation were examined by systematic changes in the defect equilibrium constants. A decrease in oxygen binding energy, manifested in a stronger tendency for reduction of the oxide membrane material, leads to higher electron concentrations and to higher rates for open-circuit operation, during which electron conduction limits H 2 transport rates.

HTS electrical machines with BSCCO/Ag composite plate-shaped rotor elements

This work relates to the investigation of electrical machines having advanced BSCCO/Ag-sheathed elements which are being produced by the well-known technology “powder in tube”. That foliate HTS material is being considered to be used in three types of electric machines: reluctance and hysteresis motors and synchronous machines with “trapped magnetic flux”. Depending on the thickness, quantity of BSCCO layers and filling factor they are being produced as thin and thick ones. Several small-scale experimental models of the above-mentioned types were developed and tested in liquid nitrogen. The output power rating of reluctance motors with thin and thick HTS elements are 3 and 4 kW respectively. “Trapped field” machine was tested in generator mode for open circuit operation. The value of obtained magnetic flux density in the air gap is 0.8 T.

Theory of noise equivalent power of a high-temperature superconductorfar-infrared bolometer in a photo-thermoelectrical mode ofoperation

We present a theory of noise equivalent power (NEP) and relatedparameters for a high-temperature superconductor (HTSC) bolometer in whichtemperature and resistance are the noise sources for open circuitoperation and phonon and resistance are the noise sources for voltage-biasedoperation of the bolometer. The bolometer is designed to use aphoto-thermoelectrical mode of operation. A mathematical formulation for theopen circuit operation is first presented followed by an analysis of theheterodyne case with a bias applied in constant voltage mode. For the firsttime electrothermal (ET) and thermoelectrical (TE) feedback aretreated in the heat balance equation simultaneously. A parallel resistancegeometry consisting of thermoelectric and HTSC material legs has been chosenfor the device. Computations for the ET-TE feedback show that the responsetime improves by three orders of magnitude and the responsivity becomes doublefor the same TE feedback. In the heat balance equation we have included amongthe heat transfer processes the temperature dependence of the thermalconductance at the bolometer-substrate interface for the dynamic state.

A 3D finite-element computation of eddy currents and losses in the stator end laminations of large synchronous machines

Eddy current losses due to axial fluxes are computed in the stator end laminations of a salient-pole synchronous machine under open-circuit operating conditions. The calculation is carried out with the aid of a 3D finite-element package which uses a linear T-/spl phi/ formulation. The domain spans a full pole pitch of the machine. The flux densities computed in the end region at points outside the stator core are compared with experimental measurements. The results and the limitations of the model are discussed.

On the significance of investigations of transformer core models

Many investigations of transformer core models have been reported in literature. However, low value has been set on the significance of the results for full size cores. It is the intention of this paper to collect experiences concerning model core experiments either from literature or from own investigations. The occurrence of stray fields and leakage fluxes during open-circuit operation prove to be the most serious effect influencing the significance of model core measurements. Stray fields are mainly caused by low numbers of lamination and narrow sheet widths respectively, especially in cases of cores including yokes without magnetization coils. In addition, investigations can be considerably impaired by longitudinal air gaps resulting from bad preparation or arrangement of the sheet samples.

Topological transformations of electrical networks

AbstractThis paper deals with effects of modifications of network structure that may be studied without reference to the type of devices present in the network. We introduce and make systematic use of the notion of a generalized minor of a vector space. This operation generalizes the usual short and open circuit operations for a graph. Using the generalized minor operation we show how to make the equations of a given network appear to be the ‘bordered version’ of the equations of some other specified network. We also consider the decomposition of a network into several ‘multiports’ and a ‘port connection diagram’, and study the properties of a minimal decomposition (with port connection diagram having a minimum number of edges). In each case we present efficient algorithms wherever appropriate. Although the paper makes use of ideas from elementary matroid theory it is entirely self‐contained and requires no more than the knowledge of elementary linear algebra from the reader.