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Global Horizontal Irradiance Clear Sky Models: Implementation and Analysis

Reno, Matthew J.; Stein, Joshua

Clear sky models estimate the terrestrial solar radiation under a cloudless sky as a function of the solar elevation angle, site altitude, aerosol concentration, water vapor, and various atmospheric conditions. This report provides an overview of a number of global horizontal irradiance (GHI) clear sky models from very simple to complex. Validation of clear-sky models requires comparison of model results to measured irradiance during clear-sky periods. To facilitate validation, we present a new algorithm for automatically identifying clear-sky periods in a time series of GHI measurements. We evaluate the performance of selected clear-sky models using measured data from 30 different sites, totaling about 300 site-years of data. We analyze the variation of these errors across time and location. In terms of error averaged over all locations and times, we found that complex models that correctly account for all the atmospheric parameters are slightly more accurate than other models, but, primarily at low elevations, comparable accuracy can be obtained from some simpler models. However, simpler models often exhibit errors that vary with time of day and season, whereas the errors for complex models vary less over time.

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TYPE SAND Report YEAR 2012
DOIOSTI

Cyber Threat Metrics

Mateski, Mark E.; Trevino, Cassandra M.; Veitch, Cynthia K.; Michalski, John T.; Harris, James M.; Maruoka, Scott; Frye, Jason N.

Threats are generally much easier to list than to describe, and much easier to describe than to measure. As a result, many organizations list threats. Fewer describe them in useful terms, and still fewer measure them in meaningful ways. This is particularly true in the dynamic and nebulous domain of cyber threats - a domain that tends to resist easy measurement and, in some cases, appears to defy any measurement. We believe the problem is tractable. In this report we describe threat metrics and models for characterizing threats consistently and unambiguously. The purpose of this report is to support the Operational Threat Assessment (OTA) phase of risk and vulnerability assessment. To this end, we focus on the task of characterizing cyber threats using consistent threat metrics and models. In particular, we address threat metrics and models for describing malicious cyber threats to US FCEB agencies and systems.

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TYPE SAND Report YEAR 2012
DOIOSTI

850-nm VCSELs optimized for cryogenic data transmission

Proceedings of SPIE - The International Society for Optical Engineering

Serkland, Darwin K.; Geib, Kent M.; Peake, Gregory M.; Keeler, Gordon A.

We report on the development of 850-nm high-speed VCSELs optimized for low-power data transmission at cryogenic temperatures near 100 K. These VCSELs operate on the n=1 quantum well transition at cryogenic temperatures (near 100 K) and on the n=2 transition at room temperature (near 300 K) such that cryogenic cooling is not required for initial testing of the optical interconnects at room temperature. Relative to previous work at 950 nm, the shorter 850-nm wavelength of these VCSELs makes them compatible with high-speed receivers that employ GaAs photodiodes. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).

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TYPE Conference YEAR 2012
OSTIScopus

Comparison of open source visual analytics toolkits

Proceedings of SPIE - The International Society for Optical Engineering

Harger, John R.; Crossno, Patricia J.

We present the results of the first stage of a two-stage evaluation of open source visual analytics packages. This stage is a broad feature comparison over a range of open source toolkits. Although we had originally intended to restrict ourselves to comparing visual analytics toolkits, we quickly found that very few were available. So we expanded our study to include information visualization, graph analysis, and statistical packages. We examine three aspects of each toolkit: visualization functions, analysis capabilities, and development environments. With respect to development environments, we look at platforms, language bindings, multi-threading/parallelism, user interface frameworks, ease of installation, documentation, and whether the package is still being actively developed. © 2012 SPIE-IS&T.

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TYPE Conference YEAR 2012
OSTIScopus

Metal-organic framework materials as chemical sensors

Chemical Reviews

Kreno, Lauren E.; Leong-Hau, Kirsty; Farha, Omar K.; Allendorf, Mark; Van Duyne, Richard P.; Hupp, Joseph T.

A critical review of the literature on metal-organic frameworks (MOF) as chemical sensors is presented. Functional groups on the surface may nucleate MOF growth in a specific crystallographic direction, leading to preferentially oriented films. Lan and co-workers reported two fluorescent Zn-based MOFs capable of sensing nitro-containing molecules relevant to detection of explosives. Feng et al. expanded the concept of using MOF structure to tune luminescence by demonstrating that both dynamic structural changes and incorporation of extrinsic dopants within the MOF pores can be used to create intense new emission. Robinson and co-workers reported humidity detection over a very broad concentration range using SAWs coated with Cu-BTC. The MOF film was grown directly on the quartz of 96.5 MHz devices without an intervening SAM, using the layer-by-layer (LBL) growth method developed by Fischer et al.

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TYPE Journal Article YEAR 2012
ScopusOSTI

Effects of thermodynamic ensembles and mineral surfaces on interfacial water structure

Physical Chemistry Chemical Physics

Zeitler, Todd Z.; Greathouse, Jeffery A.; Cygan, Randall T.

While performing molecular dynamics simulations of water or aqueous solutions in a slab geometry, such as at mineral surfaces, it is important to match bulk water density in the diffuse region of the model system with that expected for the appropriate experimental conditions. Typically, a slab geometry represents parallel surfaces with a variable region of confined water (this region can range in size from a few Ångstroms to many tens of Ångstroms). While constant-pressure simulations usually result in appropriate density values in the bulk diffuse region removed from either surface, constant-volume simulations have also been widely used, sometimes without careful consideration of the method for determining water content. Simulations using two thermodynamic ensembles as well as two methods for calculating the water-accessible volume have been investigated for two distinct silicate surfaces - hydrophilic cristobalite (111) and hydrophobic pyrophyllite (001). In cases where NPT simulations are not feasible, a simple geometry-based treatment of the accessible volume can be sufficient to replicate bulk water density far from the surface. However, the use of the Connolly method can be more appropriate in cases where a surface is less well-defined. Specific water-surface interactions (e.g., hydrophobic repulsion) also play a role in determining water content in a confined water simulation. While reported here for planar surfaces, these results can be extended to an interface with any solvent, or to other types of surfaces and geometries. © the Owner Societies 2012.

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TYPE Journal Article YEAR 2012
OSTIScopus

Effects of thermodynamic ensembles and mineral surfaces on interfacial water structure

Physical Chemistry Chemical Physics

Zeitler, Todd Z.; Greathouse, Jeffery A.; Cygan, Randall T.

While performing molecular dynamics simulations of water or aqueous solutions in a slab geometry, such as at mineral surfaces, it is important to match bulk water density in the diffuse region of the model system with that expected for the appropriate experimental conditions. Typically, a slab geometry represents parallel surfaces with a variable region of confined water (this region can range in size from a few Ångstroms to many tens of Ångstroms). While constant-pressure simulations usually result in appropriate density values in the bulk diffuse region removed from either surface, constant-volume simulations have also been widely used, sometimes without careful consideration of the method for determining water content. Simulations using two thermodynamic ensembles as well as two methods for calculating the water-accessible volume have been investigated for two distinct silicate surfaces - hydrophilic cristobalite (111) and hydrophobic pyrophyllite (001). In cases where NPT simulations are not feasible, a simple geometry-based treatment of the accessible volume can be sufficient to replicate bulk water density far from the surface. However, the use of the Connolly method can be more appropriate in cases where a surface is less well-defined. Specific water-surface interactions (e.g., hydrophobic repulsion) also play a role in determining water content in a confined water simulation. While reported here for planar surfaces, these results can be extended to an interface with any solvent, or to other types of surfaces and geometries. © the Owner Societies 2012.

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TYPE Conference YEAR 2012
OSTIScopus

Effect of chain stiffness on nanoparticle segregation in polymer/nanoparticle blends near a substrate

Macromolecular Theory and Simulations

Padmanabhan, Venkat; Frischknecht, Amalie L.; MacKay, Michael E.

Nanoparticles in a flexible polymer melt film often segregate to the substrate due to attractive depletion interactions between the nanoparticles and the substrate. Here, molecular dynamics simulations are performed to study the effect of chain stiffness on this segregation. The nanoparticles are modeled as spheres and the polymers as semi-flexible bead-spring chains. Both purely repulsive and attractive forces are considered, while assuming non-selective interactions among all species. The nanoparticles are found to be well-dispersed in the system having repulsive forces only and aggregate into clusters in the completely attractive system. For the repulsive system, adding chain stiffness substantially decreases the nanoparticles' segregation, and hence their concentration, at the substrate. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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TYPE Journal Article YEAR 2012
ScopusOSTI

Critical conditions for ferric chloride-induced flocculation of freshwater algae

Biotechnology and Bioengineering

Hughes, Lindsey; Brady, Patrick V.; Hewson, John C.; Grillet, Anne M.; Hankins, Matthew G.; Pohl, Phillip I.

The effects of algae concentration, ferric chloride dose, and pH on the flocculation efficiency of the freshwater algae Chlorella zofingiensis can be understood by considering the nature of the electrostatic charges on the algae and precipitate surfaces. Two critical conditions are identified which, when met, result in flocculation efficiencies in excess of 90% for freshwater algae. First, a minimum concentration of ferric chloride is required to overcome the electrostatic stabilization of the algae and promote bridging of algae cells by hydroxide precipitates. At low algae concentrations, the minimum amount of ferric chloride required increases linearly with algae concentration, characteristic of flocculation primarily through electrostatic bridging by hydroxide precipitates. At higher algae concentrations, the minimum required concentration of ferric chloride for flocculation is independent of algae concentration, suggesting a change in the primary flocculation mechanism from bridging to sweep flocculation. Second, the algae must have a negative surface charge. Experiments and surface complexation modeling show that the surface charge of C. zofingiensis is negative above a pH of 4.0±0.3 which agrees well with the minimum pH required for effective flocculation. These critical flocculation criteria can be extended to other freshwater algae to design effective flocculation systems. © 2011 Wiley Periodicals, Inc.

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TYPE Journal Article YEAR 2012
OSTIScopus

Development of a hydrogen catalytic heater for heating metal hydride hydrogen storage systems

International Journal of Hydrogen Energy

Johnson, Terry A.; Kanouff, Michael P.

This paper describes the design, fabrication and performance evaluation of a high efficiency, compact heater that uses the catalytic oxidation of hydrogen to provide heat to a hydrogen storage system. The heater was designed to transfer up to 30 kW of heat from the catalytic reaction to the hydrogen storage system via a recirculating heat transfer fluid. The catalytic heater consists of three main parts: 1) the reactor, 2) the gas heat recuperator, and 3) oil and gas flow distribution manifolds. The reactor and recuperator are integrated, compact, finned-plate heat exchangers to maximize heat transfer efficiency and minimize mass and volume. Detailed, three-dimensional, multi-physics computational models were used to design and optimize the system. At full power the heater was able to catalytically combust a 10% hydrogen/air mixture flowing at over 80 cubic feet per minute and transfer 30 kW of heat to a 30 gallon per minute flow of oil over a temperature range from 100 °C to 220 °C. The total efficiency of the catalytic heater, defined as the heat transferred to the oil divided by the inlet hydrogen chemical energy, was determined to exceed the design goal of 80% for oil temperatures from 60 °C to 165 °C. © 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

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TYPE Journal Article YEAR 2012
ScopusOSTI

Fibrous filter efficiency and pressure drop in the viscous-inertial transition flow regime

Aerosol Science and Technology

Hubbard, Joshua A.; Brockmann, John E.; Dellinger, Jennifer; Lucero, Daniel A.; Sanchez, Andres L.; Servantes, Brandon L.

Fibrous filter pressure drop and aerosol collection efficiency were measured at low air pressures (0.2-0.8 atm) and high face velocities (5-19 m/s) to give fiber Reynolds numbers lying in the viscous-inertial transition flow regime (1-15). In this regime, contemporary filtration theory based on Kuwabara's viscous flow through an ensemble of fibers underpredicts single fiber impaction by several orders of magnitude. Streamline curvature increases substantially as air stream inertial forces become significant. Dimensionless pressure drop measurements followed the viscous-inertial theory of Robinson and Franklin (1972) rather than Darcy's linear pressure-velocity relationship. Sodium chloride and iron nano-agglomerate aerosols were tested to provide a comparison between particles of dissimilar densities and shape factors. Total filter efficiency collapsed when plotted against the particle Stokes number and fiber Reynolds number. Efficiencies were then modeled with an impactor type equation where the cutpoint Stokes number and a steepness parameter described data well in the sharply increasing portion of the curve (20%-80% efficiency). A minimum in collection efficiency was observed at small Stokes numbers and attributed to interception and diffusive effects. The cutpoint Stokes number was a linearly decreasing function of fiber Reynolds number. Single fiber efficiencies were calculated from total filter efficiencies and compared to contemporary viscous flow impaction theory (Stechkina et al. 1969), and numerical simulations of single fiber efficiencies from the literature. Existing theories underpredicted measured single fiber efficiencies, although comparison is problematic. The assumption of uniform flow conditions for each successive layer of fibers is questionable; thus, the common exponential relationship between single fiber efficiency and total filter efficiency may not be appropriate in this regime. Copyright © American Association for Aerosol Research.

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TYPE Conference YEAR 2012
ScopusOSTI
Results 65001–65200 of 99,299
Results 65001–65200 of 99,299