Liquid water saturation profiles were determined using high resolution neutron radiography for commercially available fuel cell materials and hardware. Temperature, pressure, and relative humidity ...(concentration) gradients were imposed on the cell to determine individual influences on water content for each gradient. The asymmetric anode/cathode channel/land architecture used in this work results in significant water accumulation in the anode diffusion media with saturation values of up to ∼50%. Anode water content was found to change substantially with imposed pressure or concentration gradient, whereas the cathode saturation profile remained relatively consistent, indicating the channel/land ratio and thickness have a determinant role in diffusion media retention. The data generated in this work has been made publicly available through www.pemfcdata.org, and should be useful for computational modelers seeking validation data.
•High resolution neutron radiography used to determine through-plane water content.•Test conditions chosen to push limitations of multi-phase models.•Online database provides rich validation data for multiphase models.•Flow field geometry significantly influences diffusion media saturation levels.•Anode was more sensitive to water content with asymmetric flow field.
A proton exchange membrane fuel cell (PEMFC) must maintain a balance between the hydration level required for efficient proton transfer and excess liquid water that can impede the flow of gases to ...the electrodes where the reactions take place. Therefore, it is critically important to understand the two-phase flow of liquid water combined with either the hydrogen (anode) or air (cathode) streams. In this paper, we describe the design of an in situ test apparatus that enables investigation of two-phase channel flow within PEMFCs, including the flow of water from the porous gas diffusion layer (GDL) into the channel gas flows; the flow of water within the bipolar plate channels themselves; and the dynamics of flow through multiple channels connected to common manifolds which maintain a uniform pressure differential across all possible flow paths. These two-phase flow effects have been studied at relatively low operating temperatures under steady-state conditions and during transient air purging sequences.
► Spatiotemporal measurements of lithium through the electrode thickness were quantified with high resolution neutron imaging. ► A nonuniform lithium distribution was observed early in the first ...intercalation cycle but relaxed as the electrode filled with lithium. ► Through-plane transport resistance in the bulk of the graphite composite electrode was measured. ► The distribution of lost capacity associated with trapped lithium was quantified and linked to regions with low intercalation rates.
Lithium intercalation into graphite electrodes is widely studied, but few direct in situ diagnostic methods exist. Such diagnostic methods are desired to probe the influence of factors such as charge rate, electrode structure and solid electrolyte interphase layer transport resistance as they relate to lithium-ion battery performance and durability. In this work, we present a continuous measurement of through-plane lithium distributions in a composite graphite/lithium metal electrochemical cell. Capacity change in a thick graphite electrode was measured during several charge/discharge cycles with high resolution (14μm) neutron imaging. A custom test fixture and a method for quantifying lithium are described. The measured lithium distribution within the graphite electrode is given as a function of state of charge. Bulk transport resistance is considered by comparing intercalation rates through the thickness of the electrode near the separator and current collector. The residual lithium content associated with irreversible capacity loss that results from cycling is also measured.
► An experimental methodology to consider water retention vs. proton resistance. ► Thermal resistance of the GDL significantly influences liquid water accumulation. ► A small amount of water in the ...GDL is required to minimize proton resistance. ► Beyond this low GDL water saturation level, proton resistance remains constant.
Proton transport resistance in the polymer electrolyte of proton exchange membrane fuel cells (PEMFCs) is a significant contributor to ohmic overpotential. Conductivity is largely controlled by water uptake, which requires a delicate water balance to avoid an increase in mass transport overpotential due to diffusion resistance in porous components. The present work explores this relationship with distributed liquid water and high frequency resistance (HFR) measurements. The HFR response to local changes in liquid water content is characterized for two gas diffusion layers (GDLs) with different thermal resistivities. From these experiments, it is shown that liquid water must be present in the gas diffusion layer (GDL) to minimize the HFR. Furthermore, the impact of varied saturation levels induced by GDL thermal gradients on drying transients is considered with both in-plane and through-plane neutron imaging.
Anton 2 Shaw, David E.; Grossman, J. P.; Bank, Joseph A. ...
Proceedings of the International Conference for High Performance Computing, Networking, Storage and Analysis,
11/2014
Conference Proceeding
Anton 2 is a second-generation special-purpose supercomputer for molecular dynamics simulations that achieves significant gains in performance, programmability, and capacity compared to its ...predecessor, Anton 1. The architecture of Anton 2 is tailored for fine-grained event-driven operation, which improves performance by increasing the overlap of computation with communication, and also allows a wider range of algorithms to run efficiently, enabling many new software-based optimizations. A 512-node Anton 2 machine, currently in operation, is up to ten times faster than Anton 1 with the same number of nodes, greatly expanding the reach of all-atom biomolecular simulations. Anton 2 is the first platform to achieve simulation rates of multiple microseconds of physical time per day for systems with millions of atoms. Demonstrating strong scaling, the machine simulates a standard 23,558-atom benchmark system at a rate of 85 μs/day---180 times faster than any commodity hardware platform or general-purpose supercomputer.
Anton, a special-purpose machine for molecular dynamics simulation Shaw, David E.; Deneroff, Martin M.; Dror, Ron O. ...
International Symposium on Computer Architecture: Proceedings of the 34th annual international symposium on Computer architecture; 09-13 June 2007,
06/2007
Conference Proceeding
Odprti dostop
The ability to perform long, accurate molecular dynamics (MD) simulations involving proteins and other biological macro-molecules could in principle provide answers to some of the most important ...currently outstanding questions in the fields of biology, chemistry and medicine. A wide range of biologically interesting phenomena, however, occur over time scales on the order of a millisecond--about three orders of magnitude beyond the duration of the longest current MD simulations.
In this paper, we describe a massively parallel machine called Anton, which should be capable of executing millisecond-scale classical MD simulations of such biomolecular systems. The machine, which is scheduled for completion by the end of 2008, is based on 512 identical MD-specific ASICs that interact in a tightly coupled manner using a specialized high-speed communication network. Anton has been designed to use both novel parallel algorithms and special-purpose logic to dramatically accelerate those calculations that dominate the time required for a typical MD simulation. The remainder of the simulation algorithm is executed by a programmable portion of each chip that achieves a substantial degree of parallelism while preserving the flexibility necessary to accommodate anticipated advances in physical models and simulation methods.
* Understand biomolecular systems through their motions *Numerical integration of Newton's laws of motion - Model atoms as point masses - Compute forces on every atom based on current positions - ...Update atom velocities and positions in discrete time steps of a few femtoseconds * Force computation described by a model: the force field
The ability to perform long, accurate molecular dynamics (MD) simulations involving proteins and other biological macro-molecules could in principle provide answers to some of the most important ...currently outstanding questions in the fields of biology, chemistry and medicine. A wide range of biologically interesting phenomena, however, occur over time scales on the order of a millisecond--about three orders of magnitude beyond the duration of the longest current MD simulations.
In this paper, we describe a massively parallel machine called Anton, which should be capable of executing millisecond-scale classical MD simulations of such biomolecular systems. The machine, which is scheduled for completion by the end of 2008, is based on 512 identical MD-specific ASICs that interact in a tightly coupled manner using a specialized high-speed communication network. Anton has been designed to use both novel parallel algorithms and special-purpose logic to dramatically accelerate those calculations that dominate the time required for a typical MD simulation. The remainder of the simulation algorithm is executed by a programmable portion of each chip that achieves a substantial degree of parallelism while preserving the flexibility necessary to accommodate anticipated advances in physical models and simulation methods.
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