Nonuniformity Of Current Density Distribution Research Articles

(Invited) Current Density Distribution In a Pilot-Scale Electrodialysis Unit - 3D Mathematical Modeling and Experimental Investigation

Electrodialysis (ED) represents a matured and well-established desalination and membrane separation technology. In order to satisfy application demands, there is a trend for increasing dimensions of not only the industrial ED, but also of the other electrochemical membrane units, like electrolyzers, fuel cells etc.), capable of high process intensity and efficiency. These activities are driven mainly by cost reduction. This is also the case of reverse electrodialysis as an allied process, currently meeting rapidly growing popularity as a promising green energy source. However, experiences in a development of larger scale membrane electrochemical devices document scale-up from laboratory or pilot system to industrial ones not to be straightforward. The scale-up based on an direct increase of smaller devises dimensions, like e.g. increasing membrane active area or number of membranes without appropriate modifications of entire unit design, often leads to pronounced nonuniformity of local current density, mass and heat distribution inside the system. Consequently, the performance, reliability and lifetime of industrial scale apparatuses is significantly exacerbated. A uniformity of the above-mentioned fluxes distribution strongly depends on numerous aspects, particularly on the geometry of electrodes, manifolds and fluid distributors. Due to these facts, there is an urgent need of fundamental understanding of the relations between the system geometry and uniformity of the fluxes, especially on an industrial scale.The presented work focuses at investigation on local current density distribution in the pilot-scale ED unit consisting of 200 vertically orientated membrane pairs with working area of 0.64 × 0.32 m2 per membrane. Planar-plate electrodes are horizontally segmented into the 6 identical and electrically insulated segments allowing determination of current distribution along main direction of the solutions flow. The current distribution is investigated at different operating conditions (salt concentration, applied current, solution flow rate, electrode compartment solutions properties etc.). In parallel, a macrohomogeneous (volume-averaged) 3D mathematical model is validated and employed to calculate and visualize the local fields of electric potential, current density and salt concentrations. A nonuniformity in the current distribution mainly due to the by-pass current through the manifold is observed. The qualitative relationships between the nonuniformity of current density distribution and process performance and reliability observed for pilot- or industrial-scale can be generalized to be valid also for other similar electromembrane systems with plate-and-frame design.

Multifunctional Energy Storage Composites: Analysis and Optimization of Current Density Distribution

The introduction of electric vehicles (EVs) has posed many technical challenges to the automotive industry; in particular, the state-of-the-art lithium-ion (Li-ion) batteries add massive weight to vehicles both in the form of battery weight and supporting systems, immensely hindering vehicle performance and efficiency. A primary source of this challenge is that the battery packs serve only one purpose – energy storage, as current EV batteries do not carry loads or absorb collision impact energy. Recently introduced by the authors, a hybrid energy storage design, called the Multifunctional Energy Storage Composites (MESC), possesses the unique feature of simultaneously carrying tremendous structural loads and providing energy-storage capabilities (1-3). It has been demonstrated that the concept potentially leads to significant weight and volume savings in electric vehicle and system designs. In brief, the MESC involves a unique integration technique for embedding Li-ion battery materials in structural carbon-fiber-reinforced-polymers (CFRP). The crux of the multifunctional design is the incorporation of three-dimensionally interlocking “rivets” (1-3). The electrode layers, which make up the battery stack, contain a rectangular array of perforations (with diameter on the order of millimeters), instead of being solid, continuous, planar films. The perforations in each layer of the stack line up to fit the interlocking rivets, which three-dimensionally interlock the electrodes and rigidly anchor onto the stiff CFRP faceplates. The interlocking rivets exploit the intrinsic mechanical properties of the Li-ion battery electrodes, allowing them to carry mechanical loads. Despite the non-conventional cell construction, the MESC has demonstrated cycling and cycle-life performance tantamount to commercial pouch cells, with similar mechanical load carrying capabilities to automotive structural materials (2). Even though the rivets are indispensable in the MESC design and offer immense mechanical benefits, they present a slight electrochemical challenge. The presence of the perforations influences the current flow path, current density distribution, and depth of discharge (DoD) uniformity. Therefore, in this work, special attention is given to the numerical modelling and optimization of current density distribution in the electrodes, with the presence of a perforation array. Standard non-perforated graphite/NMC pouch cells are used to determine the DoD dependency of the intrinsic open-circuit voltage, electrochemical conductance, and resistance. The governing equations for charge conservation, polarization, and current density are solved numerically in COMSOL simulation (4-6). The perforations are included as boundary conditions on the electrode domains. The model is parameterized to capture various geometrically feasible perforation patterns, array densities, and diameters. Under different constant-current profiles, the non-uniformity of current density distribution and DoD is quantified. The increase in cell impedance, due to the local constriction and spreading of electrical current, is also evaluated. Complete functional relationships between perforation parameters (e.g. pattern, density, and hole diameter) and the electrochemical performance (current density distribution, DoD distribution, cell impedance, etc.) are established. Counter-intuitively, our results confirm minimal electrochemical impact in MESC cells with current path disruption sufficient for mechanical stabilization. Compared to unperforated cells of the same dimensions, less than a 10% increase in non-uniformity of current distribution and less than a 5% increase in cell impedance are observed. Beyond these indicative results, the model provides meaningful physical insights into the current flow path around the holes, as well as near the current collector tabs. Together, these allow the MSEC electrode design to be optimized in the electrochemical-mechanical tradeoff. Additionally, towards a broader context, it is believed this study will shed light upon electrode design and the optimization of conventional high-energy, high-power Li-ion batteries. To this end, a first step has already been taken to generalize the model and employ advanced optimization schemes e.g. genetic algorithms (GA). References P. Ladpli, R. Nardari and F.-K. Chang, Multifunctional Energy Storage Composites, WO patent application 2016127122, published 2016-08-11.P. Ladpli, R. Nardari, R. Rewari, H. Liu, M. Slater, K. Kepler, Y. Wang, F. Kopsaftopoulos and F.-K. Chang, Multifunctional Energy Storage Composites: Design, Fabrication, and Experimental Characterization, in ASME 2016 Energy Storage Forum, Charlotte, NC (2016).P. Ladpli, R. Nardari, Y. Wang, P. A. Hernandez-Gallegos, R. Rewari, H. T. Kuo, F. Kopsaftopoulos, K. D. Kepler, H. A. Lopez and F. Chang, Multifunctional Energy Storage Composites for SHM Distributed Sensor Networks, in International Workshop on Structural Health Monitoring 2015, Stanford, CA (2015).U. S. Kim, C. B. Shin and C.-S. Kim, Journal of Power Sources, 189, 841 (2009).K. H. Kwon, C. B. Shin, T. H. Kang and C.-S. Kim, Journal of Power Sources, 163, 151 (2006).J. Newman and W. Tiedemann, Journal of The Electrochemical Society, 140, 1961 (1993). Figure 1

High-voltage electrode optimization towards uniform surface treatment by a pulsed volume discharge

In this study, the shape and material of the high-voltage electrode of an atmospheric pressure plasma generation system were optimised. The research was performed with the goal of achieving maximum uniformity of plasma treatment of the surface of the low-voltage electrode with a diameter of 100 mm. In order to generate low-temperature plasma with the volume of roughly 1 cubic decimetre, a pulsed volume discharge was used initiated with a corona discharge. The uniformity of the plasma in the region of the low-voltage electrode was assessed using a system for measuring the distribution of discharge current density. The system's low-voltage electrode - collector - was a disc of 100 mm in diameter, the conducting surface of which was divided into 64 radially located segments of equal surface area. The current at each segment was registered by a high-speed measuring system controlled by an ARM™-based 32-bit microcontroller. To facilitate the interpretation of results obtained, a computer program was developed to visualise the results. The program provides a 3D image of the current density distribution on the surface of the low-voltage electrode. Based on the results obtained an optimum shape for a high-voltage electrode was determined. Uniformity of the distribution of discharge current density in relation to distance between electrodes was studied. It was proven that the level of non-uniformity of current density distribution depends on the size of the gap between electrodes. Experiments indicated that it is advantageous to use graphite felt VGN-6 (Russian abbreviation) as the material of the high-voltage electrode's emitting surface.

Effect of Geometry Change on Armature Behavior

Diffusion-controlled erosion of armatures, which is accentuated by the velocity skin effect, gives rise to nonuniform current conduction and consequent nonuniform heating. Earlier computational studies have highlighted the role played by armature geometry in current density and temperature distribution. Recently, we designed modified C-shaped armatures, called saddle-shaped armatures, to study the effect of geometry change on armature erosion and its consequent effect on rail deposits. The trailing edge and the throat of the armature were curved to better align with the magnetic field, thus reducing the nonuniformity of current density distribution. A front tab was also provided to reduce current concentration and heating at the leading edge of the contact area. Low-speed tests, in which armatures were recovered and compared with recovered conventional C-shaped armatures, were conducted. Similar experiments at speeds above 2 km/s were conducted and compared with corresponding C-shaped armature experiments. It was found that in conformation with our hypothesis, the saddle-shaped armatures recovered from low-speed shots showed less erosion both on the contact surface and in the throat area. The high-speed tests showed that the contact voltage for saddle-shaped armatures was significantly lower than that for conventional C-shaped armatures. This implies less energy loss at the rail-armature interface and potentially less lateral force generated during contact arcing

Distribution of cathode current density and breaking capacity of medium voltage vacuum interrupters with axial magnetic field

A high-speed photographic technique incorporating computer-aided reconstruction of the cathode current density was applied to butt solid contacts with an external axial magnetic field (AMF) at relatively low current density j<2 kA/cm/sup 2/. In the present work, we have further developed this technique. This advanced technique is applicable for the analysis of the current density distribution on the complicated electrodes of commercial vacuum interrupters (VIs) carrying currents up to I=50 kA at current density up to j/spl sim/4 kA/cm/sup 2/. The experiments have been carried out for three types of electrode designs generating AMFs with different configurations. The results obtained by this new technique proved a previously derived conclusion that current density tends to distribute evenly across that part of the contact surface where the AMF induction fits the inequality B/sub z//sup (1)/<B/sub z/<B/sub z//sup (2)/, where B/sub z//sup (1)/, B/sub z//sup (2)/ are characteristic induction fields defined in our previous work. Results of the analysis of erosion tracks on the electrodes are also in agreement with this conclusion. Slots on the electrode surface lead to substantial nonuniformity of current density distribution even if the AMF is properly configured. Comparison of the breaking capacity and distribution of current density for different electrode designs proves that interruption failure occurs at peak current density /spl sim/3.5 kA/cm/sup 2/. This value exceeds by at least 50% previous estimates of maximum current density interruptible by a commercial VI. We have also found that changing the configuration of the AMF may substantially improve breaking capacity. In our experiments, a slight modification of the AMF distribution provides a 12% increase of the breaking capacity.