Land Widths Research Articles

Influence of Anode Flow-Field Channel Distance on the Performance of Polymer Electrolyte Membrane Water Electrolyzers

Polymer electrolyte membrane water electrolysis (PEMWE) is well suited for flexible and efficient hydrogen production based on electricity from renewable energy sources [1]. Of all PEMWE stack components, flow-fields (FF) or bipolar plates (BP) are currently among the most expensive parts: In an analysis from 2020, FF/BP are reported to account for more than half of current PEMWE stack costs with medium potential for cost reduction towards the next-generation [2]. Another recent study attributes only 28% of current PEMWE stack costs to FF/BP, but claims that next-generation bipolar plates will be 3 times less expensive [3]. In any case, the optimization of flow-fields represents an important lever to improve the economic competitiveness of PEMWE. Although sophisticated designs are reported in the literature, commercial flow-fields tend to follow simpler approaches [4]. For classic design patterns like serpentine or parallel channels, studies compared different channel patterns or assessed the influence of the channel width [5]. Nevertheless, systematic investigations on the influence of the channel distance (FF land width) on the PEMWE performance have not been reported so far. Recently, an optimal FF land width of 4 times the thickness of the porous transport layer (PTL) was postulated [6]. However, it remains unclear if this ratio is universally valid, because only one specific FF configuration and only one PTL type (with variable PTL thickness) was investigated in the study.In this work, we analyze the influence of the anode side flow-field channel distance (land width) on the cell performance of 5 cm2 PEMWE single-cells. Different single-channel serpentine flow fields with land widths ranging from 0.7 mm to 6.2 mm and a constant channel width of 1 mm are characterized as a function of PTL type (powder-sintered- as well as fiber-based titanium PTL). For all FF/PTL combinations, polarization curves up to 6 A/cm2 are performed with electrochemical impedance spectroscopy (EIS) measurements at each current step. The anode proton sheet resistance is evaluated by EIS fitting during blocking conditions. On one side, a voltage loss analysis reveals that mass transfer losses in PEMWE cells are very small for the best suited FF/PTL combinations (<15 mV at 6 A/cm²). On the other side, for large land widths in combination with certain PTL types, severe mass transport losses are observed at high current densities (increased cell as well as iR-free cell voltage, cf. figure 1a), accompanied by a partial dry-out of the membrane that is reflected by an increased high-frequency resistance (HFR, cf. figure 1b). For this case, the remaining water content in the membrane will be estimated. Porosity and pore structure of the PTL used in this study are analyzed ex-situ through mercury intrusion porosimetry (MIP). The PTL samples cover a broad range of pore sizes (≈15-60 µm) and porosities (≈30-70 %). Further analysis will be conducted to elucidate the contribution of PTL bulk properties such as their in-plane permeability to the observed mass transport losses for large flow-field land widths.

Computational Modeling of Tubular Flow Fields for PEM Fuel Cells

The bipolar plates of polymer electrolyte membrane fuel cells (PEMFCs) are of focus for improvement since they are responsible for most of the fuel cell stack weight and volume. Traditionally these flow fields are machined or stamped out of a graphitic or metallic material. Recent research has shown that additively manufactured metal plates show comparable performance to those made with conventional method. Due to the layer-by-layer nature, there is a great deal of design flexibility for additively manufactured parts. This allows for far more freedom in flow field designs, such as a tubular flow field with internal channels.In recent years there have been developments of tubular PEMFCs. Numerical simulation reported in literature have shown that tubular PEMFCs provide higher power density [1], more uniform reactant gas velocity distribution [2], and a smaller pressure gradient [3].The goal of the tubular model in our study was to replicate in size a standard AAA battery , so the total dimensions of the fuel cell were limited at a height of 40.5 mm and a diameter of 10.5 mm, with a cell active area of 987 mm2. The tubular channel length was approximately 400 mm. For reference, a conventional (planar) single serpentine fuel cell with a comparable 982 mm2 active area was used, replicating a scaled down commercially available design from Fuel Cell store. This planar design had square channels with a depth of 1.016 mm and a channel width of 0.7874 mm. The length of the channel was approximately 550 mm. For both designs, the land widths were 0.7874 mm to maintain a 1:1 channel-land width ratio. The gas diffusion layer (GDL), microporous layer (MPL), catalyst layer, and membrane had respective thicknesses of 360 μm, 50 μm, 10 μm, and 50 μm. The channel geometry for the tubular fuel cell was changed from rectangular to a semicircle, as the additive manufacturing process is not adept at building overhangs with supports. To maintain an accurate comparison, the cross-sectional area of the planar and tubular channels was kept constant at 0.79 mm2. This flow field would be incorporated into the outer anode flow field of the final tubular design, so hydrogen at 80°C was used as the fluid. A volumetric flow of 0.24 L/min was used as the inlet boundary condition.Initially, basic fluid flow simulations were performed on several tubular serpentine flow field designs (Figure 1). The double corkscrew Tubular v4 design had the greatest pressure drop, which was due to the fact the cross-sectional area was divided into two smaller sections. Tubular v2 and v3 had similar results being the most alike geometrically. Tubular v1, which split off into two symmetrical flow channels, exhibited the lowest pressure drop as the half channels cut down on the distance the hydrogen would have to travel inlet to outlet.With the chosen design Tubular v1, this geometry was used for the PEMFC module. Various changes were made to the tubular model to simulate the new design. Our proprietary membrane electrode assembly (MEA) design had no GDL present in the cathode layer, with oxygen being pumped directly through the cathode catalyst layer. This design assured that the supplied O2 concentration is much higher since its transport is by convection instead of diffusion. The ANSYS PEMFC model will not run if there is no defined GDL between the gas channel and MPL, so this was circumvented by directly changing the properties of the catalyst layer. To model this, the mass fraction of the oxygen in the cathode catalyst layer was increased from 0.2 to 0.3. It is also expected for the flowing of oxygen to flush away any water formation, so the water formation was limited. Due to the water mass balance taking place in the model, limiting liquid water formation increased the dissolved water inside of the membrane. The electrochemical surface area was increased to account for the higher surface area of the Pt catalyst in our proprietary MEA design.These modifications showed an increase in performance for the tubular cell when compared with the standard planar design as seen in Figure 2. Limiting the liquid water formation and selecting a geometry with a more uniform pressure distribution resulted in a cell with higher power density. The removal of the GDL and increase of air flow allowed for a higher oxygen concentration at the cathode catalyst interface. With oxygen no longer being a limiting reactant, the cell was able to perform at a higher power output. Figure 1

In-process monitoring and empirical modeling of the tool wear in turning of aluminum alloys using thermoelectric signals

Tool wear is an important criterion for economic machining. It does not only determine the tool life, but also affects the surface layer properties of metallic components and thus their performance. In addition to the machining parameters, there are numerous other influencing factors like batch variations, workpiece inhomogeneities, process vibrations, or the formation of built-up edges that can impact the wear progression. Consequently, it is difficult to predict the tool wear analytically. Therefore, the determination of the tool condition during machining is of great importance. The tool-workpiece thermocouple enables the in-process measurement of a thermoelectric voltage and current during machining generated by the Seebeck effect at the interface between two electrically conductive materials. Thus, the thermoelectric signals are sensitive for the changing contact interface between the tool and the specimen. To allow for an in-process characterization of the tool wear, finding an empirical relationship between the flank wear land width of cemented carbide indexable inserts and the thermoelectric signals in turning is aspired. To vary the flank wear land width in a reproducible manner, tools with a flank face chamfer resulting in a clearance angle of 0° and five different flank face land widths (80 µm – 280 µm) are used. These geometrical properties are measured with a 3D laser scanning microscope. Afterwards, cylindrical specimens of the aluminum alloy EN AW-2017 are machined by turning applying the modified tools and cutting speeds in the range of 300 m/min to 550 m/min. The depth of cut (0.4 mm) and the feed (0.04 mm) are kept constant. Besides the tool-workpiece thermocouple, the components of the resultant force are measured by a dynamometer. The geometrical properties of the machined surfaces are characterized using a stylus measurement instrument.The experimental investigations show an increase of the temperature and the components of the resultant forces with increasing flank wear land width, which impacts the geometrical properties of the surface. An empirical regression model derived from the in-process measurement results depicts the relationship between the thermoelectric signals, the force components, and the changing area of the tool which is in contact with the specimen. Consequently, the combined in-process measurement of the process forces and thermoelectric signals allows for an in-process determination of the tool wear progression and therefore enables the prevention of a strong impairment of the surface-layer properties by a timely tool change or a controlled adjustment of the machining parameters.

Lateral Vibrations in Deep Hole Drilling Due to Land Width Variation

In drilling processes, self-excited vibrations have a negative influence on the achievable drilling qualities and tool life. Low-frequency radial vibrations are characterised by oscillating movements of the tool tip and the generation of lobed holes in drilling. This study focuses on the participation and influence of the land widths of twisted drilling tools on these vibrations. Therefore, drilling tools with different land widths and a length-to-diameter ratio of 12 are used, and different parameters, such as feed rate, cutting speed, coolant pressure, pre-hole diameter, and chuck selection, are varied in a drill-out process. The results show that the land widths have a statistically significant influence on the formation of lateral vibrations and the generation of lobed holes.

Experimental Studies of Effect of Land Width in PEM Fuel Cells with Serpentine Flow Field and Carbon Cloth

Flow field plays an important role in the performance of proton exchange membrane (PEM) fuel cells, such as transporting reactants and removing water products. Therefore, the performance of a PEM fuel cell can be improved by optimizing the flow field dimensions and designs. In this work, single serpentine flow fields with four different land widths are used in PEM fuel cells to study the effects of the land width. The gas diffusion layers are made of carbon cloth. Since different land widths may be most suitable for different reactant flow rates, three different inlet flow rates are studied for all the flow fields with four different land widths. The effects of land width and inlet flow rate on fuel cell performance are studied based on the polarization curves and power densities. Without considering the pumping power, the cell performance always increases with the decrease in the land width and the increase in the inlet flow rates. However, when taking into consideration the pumping power, the net power density reaches the maximum at different combinations of land widths and reactant flow rates at different cell potentials.

Flow field plates with trap-shape channels to enhance power density of polymer electrolyte membrane fuel cells

Power density of proton exchange membrane fuel cells (PEMFCs) can be enhanced by optimal design of the shape and dimensions of bipolar plate channels. By introducing a novel, trap-shape channel design, the present work investigates the impact of having smaller land widths at different cross-sections of each bipolar plate on the PEMFC performance. The possible capabilities of this trap-form channel are investigated through a three-dimensional non-isothermal model in ANSYS Fluent.It is found that having narrower lands at a number of plate cross-sections enhances the current density per unit active area. However, there are critical (optimum) values for both length and number of these traps where the PEMFC performance is maximized. The results show that channels with two 8-mm-long traps provide considerably higher current densities. These two-trap channels also improve the distribution of oxygen and water over the cathode catalyst, which consequently results in less cathodic overpotential across the cell. As a result, the proposed trap-shape channel significantly enhances the power density while being as simple and inexpensive as the conventional/classical straight (no-trap) channels for fabrication. This new, trap shape of channels can therefore be considered one of the promising channel designs for the next generation of PEMFCs.

A Designing and Generating Method for Grinding Relief Surfaces of a Dual-Cone Double-Enveloping Hourglass Worm Gear Hob

Land widths and relief angles of a dual-cone double-enveloping hourglass worm gear hob are important factors that influence the life and the hobbing performance of the hob. Both of them are obtained by generating relief surfaces of the hob. Due to the reason that all teeth of this type of hob have different profiles with each other, all of the relief surfaces are difficult to generate for keeping all cutting teeth with uniformed relief angles and uniformed land widths. For the purpose that land widths and relief angles could be machined precisely, this paper puts forward a designing and generating method for grinding the relief surfaces. The relief surfaces are ground using the same double-cone grinding wheel as grinding the helical surfaces of the worm. Based on the theory of gearing, the mathematical model for grinding relief surfaces is built. Motion parameters when grinding the different points of the land edges on different teeth of the hob are solved. A generating simulation is built by putting those motion parameters into a four-axis hourglass worm-grinding machine. The results of the simulation show that the relief surfaces can be ground continuously and the land widths and the relief angles meet the requirements.

Experimental Performance Characterization of Polymer Electrolyte Fuel Cells with Varying Channel Dimensions

Proper flow field design is an active research topic in the pursuit of improving performance for Polymer Electrolyte Membrane Fuel Cells (PEMFC). Important concerns remain as PEMFCs come to mass market including increasing power density, and decreasing system cost. Increasing the power density of fuel cells reduces the size and cost of a system for a given output power. Understanding the performance benefits and drawbacks of various gas distributor channel dimensions is relevant for designing the next generation of PEMFCs. Two important flow field designs are parallel and interdigitated flow fields. In contrast to parallel flow fields, interdigitated flow fields cause convective transport between channels by forcing flow through the gas diffusion layer via pressure differences, also known as cross flow. Cross flow reduces the diffusion length between reactants and the catalyst layer, reducing performance losses, and helping to remove water from land areas. However, cross flow raises pumping losses. This study experimentally examines the cathode bipolar plate channel/land width and channel depth while taking into account pumping losses. This has been experimentally examined for parallel flow fields, but has only been computationally explored for interdigitated flow fields, such as in Lin and Nguyen (A Two-Dimensional Two-Phase Model of a PEM Fuel Cell, Journal of the Electrochemical Society, 2006). This study seeks to understand how electrochemical and overall performance is affected by channel dimension in interdigitated flow fields, relative to parallel flow fields, from an experimental perspective. Six bipolar plates with different combinations of channel/land width x channel depth were tested: 1 x 1 mm, 1 x 0.5 mm, 1 x 0.25 mm, 0.5 x 0.5 mm, 0.5 x 0.25 mm, and 0.25 x 0.25 mm. Channel and land widths were equal for the experiment, to keep a constant channel to land ratio. The bipolar plates are 20 cm long and 1.5 cm wide, for 30 cm2 cell area. The bipolar plates are aluminum coated in rhodium for corrosion protection. The bipolar plates fit into a cell superstructure that can use valves to switch from a parallel to interdigitated flow field in situ. Both parallel and interdigitated flow fields were tested, as well as two stoichiometries - high stoichiometry at 2.0 anode / 4.0 cathode and low stoichiometry of 1.5 anode / 2.0 cathode. Each combination of cathode bipolar plate, flow field and stoichiometry was tested three times in random order. The polarization curve and associated pressure drop through the cell were measured for each bipolar plate combination. Based on these, the raw and net power density, and limiting current density of each condition are determined. The electrochemical performance results will be examined with respect to individual channel size, and overall bipolar plate permeability. The results will also be examined using relevant diffusive and convective length criteria. Initial testing for this study has been completed and analysis is underway. In the analysis, it was discovered that the 1x0.25 mm plates suffered from GDL intrusion, and so they were removed. Some preliminary results have been obtained, such as the peak raw and net power density results in Figure 1. The peak raw power density and peak net power density, after accounting for pumping power losses, are plotted against the hydraulic diameter (HD) of the channels. The results show an optimal HD for net power density to be approximately 0.4 mm. There are still gains in performance for smaller channels, but these gains are overwhelmed by the pumping losses due to increased pressure to drive flow through the cell. The parallel flow fields experienced a smaller net power decrease than the interdigitated at low HD, as they incur a reduced pressure increase, due to the lack of cross flow. Major remaining analysis includes determining the performance gains due to increased pressure in the cell, ohmic losses and the relative permeability of the bipolar plates. By subtracting off these gains (and losses) the actual electrochemical performance of the cells can be examined, as well as the net system performance. This will clarify what sort of diffusion and flow benefits are available at lower channel dimensions. Once this study is completed, it will provide a guide for PEM bipolar plate designers on how the size & shape of channels, and the permeability of bipolar plates affect the performance of resulting fuel cell. It will also provide critical information on which design parameters are most important for each flow field. This can be used to design bipolar plates to fit a wide range of channel dimension or sizes in the future. Figure 1

Sources of Current Density Distribution in the Land-Channel Direction of a PEMFC

A proton exchange membrane fuel cell (PEMFC) was segmented to measure local current density, electrochemical surface area, and high frequency resistance (HFR) distribution in the land-channel direction at resolution of 350 μm. An in-house catalyst coated membrane of 3 mm × 3 mm active area was prepared to represent a small area in a larger scale cell with 1 mm land and channel widths. This design was employed to measure current density and HFR distribution at 60°C with several different operating conditions. Local electrical resistance was also measured separately so that local protonic resistances can be discerned from local HFR. To analyze the effect of the land-channel geometry a method was developed to quantify the sources of current distribution, such as distributions of oxygen concentration at the electrode, oxygen transport resistance, cathode catalyst layer resistance, and membrane water content. Current density distribution is strongly correlated with the distribution of membrane water content and electrode resistance in dry condition, and oxygen concentration distribution in wet condition, while in moderate condition both oxygen concentration and water content in membrane are critical to the local current density distribution. The results imply the limitation of uniform condition assumption used in a differential cell study.

Is FSI Dependent on Land Availability and Densities? A Comparative Review of FSI in Indian Cities

In the present era of rapid urbanization, the prime concern is how to regulate development in our cities considering the present growth dynamics. FSI, (Floor space index-the ratio of built-up space on a plot to the area of the plot) is one such regulation predominant in the development control norms, which primarily dictates the overall urban form. In this article, the author investigates the current FSI norms in codes, guidelines and development controls across cities that are classified as large cities. Author takes the secondary data of six cities to find out how FSI is varying in cities with respect to the available land and densities through an empirical analysis. Sample study area building byelaws are taken to make a comparative study on FSI. The result shows FSI in study area are not based on densities or available land but road widths and setbacks and in other cities it varies based on nature of the building activity. Finally, it is found that the FSI is independent of the land availability and density in India and there is no standard model for assigning FSI. To increase or decrease the FSI in a city, shall depend upon some key parameters and an investigation of those parameters shall indicate the FSI control in the cities. Keywords: Floor space index, cities, development controls, density, land.

Effect of channel length on interdigitated flow-field PEMFC performance: A computational and experimental study

This study examines the effect of increased channel length on the distribution of flow gases and cell performance in an interdigitated flow-field PEMFC. A numerical model was used to simulate the pressure distribution and gas flow in 5 cm and 25 cm channel length interdigitated flow fields that have eight channels with fixed channel and land widths of 1 mm. The results show the distribution of flow under land areas (cross flow) is subject to maldistribution in the longer cell while the shorter cell produces relatively homogenous cross flow along its length. An experimental test cell was designed to run with the same channel lengths, under similar conditions to those modeled. Inlet pressure data was recorded to account for parasitic pump losses, used to calculate net system power curves. Results show that a shorter channel interdigitated flow field may produce both higher maximum power and limiting current densities compared with longer cells.

In-situ measurements of GDL effective permeability and under-land cross-flow in a PEM fuel cell

When reactant gases flow along a channel in serpentine flow field of a proton exchange membrane (PEM) fuel cell, there is a pressure difference between the adjacent channels and it produces an under-land cross-flow (or under-rib convection) from the higher pressure side to the lower pressure side through the gas diffusion layer (GDL). A unique experimental setup is developed for in-situ measurement of this cross-flow and the GDL effective permeability at the cathode side of a PEM fuel cell under dry and realistic humidified gas conditions. The non-Darcy effect, defined as a function of the Forchheimer number is studied and compared for both 1 mm and 2 mm land widths and both dry and humidified air conditions. Finally, a dimensional analysis is performed and the non-dimensional cross-flowrate is shown to increases linearly with the increase of the non-dimensional pressure difference.

Impact of land width and channel span on fuel cell performance

The impact of land width and channel span on fuel cell performance was tested on 50 cm 2 flow fields. Combinations of land widths and channel spans of 0.25, 0.5 and 1.0 mm were evaluated. Polarization curves and high-frequency resistance were measured for several humidity levels for each of the flow fields. For performance at high current density, the land width was found to be the dominant factor. Reductions to the smallest size tested (0.25 mm) continued to showed improvement over wider lands. Adequate land fraction (preferably about 50%) needs to be provided to minimize contact resistance.

고분자전해질형연료전지의 가스 채널 최적화를 위한 수치적 연구 (II) - 가스 채널 치수가 온도와 액체포화 분포에 미치는 영향성 -

Using the multi-dimensional, multi-phase, nonisothermal Polymer Electrolyte Fuel Cell (PEFC) model presented in Part I, the effects of land/channel flow-field on temperature and liquid saturation distributions inside PEFCs are investigated in Part II. The focus is placed on exploring the coupled water transport and heat transfer phenomena within the nonisothermal and two-phase zone existing in the diffusion media (DM) of PEFCs. Numerical simulations are performed varying the land and channel widths and simulation results reveal that the water profile and temperature rise inside PEFCs are considerably altered by changing the land and channel widths, which indicates that oxygen supply and heat removal from the channel to the land regions and liquid water removal from the land toward the gas channels are key factors in determining the water and temperature distributions inside PEFCs. In addition, the adverse liquid saturation gradient along the thru-plane direction is predicted near the land regions by the numerical model, which is due to the vapor-phase diffusion driven by the temperature gradient in the nonisothermal two-phase DM where water evaporates at the hotter catalyst layer, diffuses as a vapor form and then condenses on the cooler land region. Therefore, the vapor phase diffusion exacerbates DM flooding near the land region, while it alleviates DM flooding near the gas channel.

Nanoimprint mold fabrication and duplication for embedded servo and discrete track recording media

A master mold for nanoimprint lithography was fabricated for discrete track recording (DTR) media using electron beam lithography and conventional etching techniques. The DTR pattern, containing 167 tracks of 120 nm pitch (60 nm land and groove widths) and embedded servo information, was automatically generated using an in-house developed program and was optimized for faster electron beam writing on an x-y stage. A daughter mold was duplicated from the master mold by nanoimprinting, using UV-curable resist and an intermediate polymer stamp technique. Scanning electron microscope images showed that the daughter mold was accurately and completely reproduced from the master mold.

Finish Hobbing of Medium Hardness Gears with Cermet-Tipped Hobs.

The cermet-tipped finishing hob shows good results in economical and productive finishing of tooth surfaces. As the cermet finishing hob has poor grindability, it takes a long time to manufacture the cermet hobs and it is difficult to keep the hob tooth profile accurate. In this paper, small lands of zero relief angle are made on the cutting edges of the cermet finishing hob in order to manufacture a highly accurate hob. By finish hobbing tests with the cermet hobs having various land widths, the hob wear and the tooth surface roughness vs the width of lands are investigated. The hob with the small land width of about 0.1mm is superior in wear resistance and improvement of the tooth surface roughness to those with land width greater than 0.2mm.

Load Capacity and Stiffness Considerations for Hydrostatic Journal Bearings

A theoretical solution, using finite element method, to predict performance characteristics of a capillary compensated hydrostatic oil journal bearing has been presented. Load capacity curves for two load directions of a typical bearing with four pockets are given for various design parameters and eccentricity ratios, and the four fluid film stiffness coefficients are computed. The effect of the feeding system on bearing load capacity and stiffness is discussed. Without using the flow approximations of Raimondi and Boyd [1] and Kher and Cowley [2], which are essentially valid for small eccentricity ratios and small circumferential land widths, the bearing flow has been computed both as the sum of nodal flows on the external boundary and as the inflow to the pockets.

Load capacity and flow characteristics of a hydrostatically lubricated four-pocket journal bearing by finite element method

The performance of a capillary compensated hydrostatic journal bearing is analyzed by finite element method. Non-dimensional load carrying capacity, flow, and pocket pressures are presented for various bearing design parameters, and eccentricity ratios. Load capacity varies significantly with changes in these parameters. Bearing flow for any eccentricity is obtained by summing up the boundary nodal flows and it is not necessary to use approximate expressions [1,2] for the axial and circumferential flows which are valid only for small eccentricity ratios and small circumferential land widths.