Albuquerque, NM, United States
Albuquerque, NM, United States

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Peters D.W.,Sandia National Laboratories | Boye R.R.,Sandia National Laboratories | Wendt J.R.,Sandia National Laboratories | Kellogg R.A.,Sandia National Laboratories | And 3 more authors.
Optics Letters | Year: 2010

We demonstrate a two-dimensional (2D) polarization-independent resonant subwavelength grating (RSG) in a filter array. RSGs, also called guided mode resonant filters, are traditionally one-dimensional gratings; however, this leads to TE and TM resonances at different wavelengths and with different spectral shape. A 2D grating can remove the polarization dependence at normal incidence, while maintaining the desirable RSG properties of high reflectivity, narrow passband, and low sidebands without ripple. We designed and fabricated 2D gratings with near-identical responses for both polarizations at normal incidence in the telecommunication band. Ninety percent reflectivity is achieved at the resonant wavelengths. © 2010 Optical Society of America.

Tidwell V.C.,Sandia National Laboratories | Macknick J.,National Renewable Energy Laboratory | Zemlick K.,Sandia Staffing Alliance | Sanchez J.,Colorado School of Mines | Woldeyesus T.,National Renewable Energy Laboratory
Applied Energy | Year: 2014

Drought poses important risks to thermoelectric power production in the United States because of the significant water use in this sector. Here a scoping level analysis is performed to identify the technical tradeoffs and initial cost estimates for retrofitting existing thermoelectric generation to achieve zero freshwater withdrawal and thus reduce drought related vulnerabilities. Specifically, conversion of existing plants to dry cooling or a wet cooling system utilizing non-potable water is considered. The least cost alternative is determined for each of the 1178 freshwater using power plants in the United States. The projected increase in levelized cost of electricity ranges roughly from $0.20 to $20/MWh with a median value of $3.53/MWh. With a wholesale price of electricity running about $35/MWh, many retrofits could be accomplished at levels that would add less than 10% to current power plant generation expenses. Such retrofits would alleviate power plant vulnerabilities to thermal discharge limits in times of drought (particularly in the East) and would save 3.2Mm3/d of freshwater consumption in watersheds with limited water availability (principally in the West). The estimated impact of retrofits on wastewater and brackish water supply is minimal requiring only a fraction of the available resource. Total parasitic energy requirements to achieve zero freshwater withdrawal are estimated at 140million MWh or roughly 4.5% of the total production from the retrofitted plants. © 2014 Elsevier Ltd.

Jordan T.S.,Sandia National Laboratories | Scott S.,Purdue University | Leonhardt D.,Sandia National Laboratories | Custer J.O.,Sandia Staffing Alliance | And 3 more authors.
IEEE Transactions on Electron Devices | Year: 2014

This paper investigates and models the dc behavior of thin-film-based switching devices. The devices are based on sputtered vanadium dioxide thin films that transition from 200 k} at room temperature to 390}/ at temperatures above 68°C, with the transition occurring over a narrow temperature range. The device resistance is characterized over temperature and under current-and voltage-sourced electrical bias. The finite-element model predicts the device's nonuniform switching behavior. Electrothermally heated devices show the same transition ratio and switching behavior as externally heated devices suggesting a purely electrothermal switching mechanism. © 2014 IEEE.

Ho C.K.,Sandia National Laboratories | Khalsa S.S.,Sandia Staffing Alliance | Kolb G.J.,Sandia National Laboratories
Solar Energy | Year: 2011

Probabilistic modeling of concentrating solar power technologies provides important information regarding uncertainties and sensitivities not available from deterministic models. Benefits of using probabilistic models include quantification of uncertainties inherent in the system and characterization of their impact on system performance and economics. This paper presents the tools necessary to conduct probabilistic modeling of concentrating solar technologies. The probabilistic method begins with the identification of uncertain variables and the assignment of appropriate distributions for those variables. Those parameters are then sampled using a stratified method (Latin Hypercube Sampling) to ensure complete and representative sampling from each distribution. Models of performance, reliability, and/or cost are then simulated multiple times using the sampled set of parameters. The results yield a cumulative distribution function that can be analyzed to quantify the probability of achieving a particular metric (e.g., net energy output or levelized energy cost) and to rank the importance of the uncertain input parameters. © 2010 Elsevier Ltd.

Ho C.K.,Sandia National Laboratories | Christian J.M.,Sandia Staffing Alliance
ASME 2013 7th Int. Conf. on Energy Sustainability Collocated with the ASME 2013 Heat Transfer Summer Conf. and the ASME 2013 11th Int. Conf. on Fuel Cell Science, Engineering and Technology, ES 2013 | Year: 2013

This paper presents simulations and designs of a prototype falling particle solar receiver with air recirculation as a means of mitigating heat loss and impacts of external wind on particle flow. The flow and dispersion of different sizes of ceramic proppant (CARBO HSP) particles (1 mm, 100 ?m, and 10 ?m) were simulated in recirculating air flows without heating effects. Particles on the order of 0.1 - 1 mm yielded desirable simulated flow patterns when falling through the cavity receiver with an air-injection velocity of 3 m/s. Simulations of smaller particles on the order of 10 microns yielded unstable flow patterns that may lead to large losses of particles through the aperture. A prototype cavity receiver with air recirculation was designed and fabricated to validate the unheated simulations. The blower nozzles and suction plenum were engineered to yield the most uniform flow pattern along the entire width of the aperture. Observed and simulated air velocity distributions around the aperture and particle flow patterns using CARBO HSP particles with an average diameter of ~0.7 mm were found to be qualitatively similar. Copyright © 2013 by ASME.

Griffin P.J.,Sandia National Laboratories | Peters C.D.,Sandia Staffing Alliance
Journal of the Korean Physical Society | Year: 2011

Improved nuclear physics modelling codes and computational methods have made significant advances in recent years and have enabled the cross section modelling community to automate the parameter variation and generate model-based cross section evaluations that include covariance matrices. Current dosimetry community standards do not permit the use of purely model-based cross sections for many applications, e.g., for spectrum adjustment in support of pressure vessel surveillance activities. This paper examines the implications of using model-based cross section libraries for typical dosimetry applications.

Klem J.F.,Sandia National Laboratories | Kim J.K.,Sandia National Laboratories | Cich M.J.,Sandia National Laboratories | Hawkins S.D.,Sandia National Laboratories | And 2 more authors.
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2010

We have fabricated mid-wave infrared photodetectors containing InAsSb absorber regions and AlAsSb barriers in n-barrier-n (nBn) and n-barrier-p (nBp) configurations, and characterized them by current-voltage, photocurrent, and capacitance-voltage measurements in the 100-200 K temperature range. Efficient collection of photocurrent in the nBn structure requires application of a small reverse bias resulting in a minimum dark current, while the nBp devices have high responsivity at zero bias. When biasing both types of devices for equal dark currents, the nBn structure exhibits a differential resistance significantly higher than the nBp, although the nBp device may be biased for arbitrarily low dark current at the expense of much lower dynamic resistance. Capacitance-voltage measurements allow determination of the electron concentration in the unintentionally-doped absorber material, and demonstrate the existence of an electron accumulation layer at the absorber/barrier interface in the nBn device. Numerical simulations of idealized nBn devices demonstrate that photocurrent collection is possible under conditions of minimal absorber region depletion, thereby strongly suppressing depletion region Shockley-Read-Hall generation. © 2010 Copyright SPIE - The International Society for Optical Engineering.

Smith E.J.,Sandia Staffing Alliance | Ho C.K.,Sandia National Laboratories
Energy Procedia | Year: 2013

This study provides a demonstration of an eight-parameter heliostat tracking-error correction method applied to heliostats at the National Solar Thermal Test Facility at Sandia National Laboratories. This method models the non-random physical error sources that arise from imperfect mount fabrication and installation processes. Previous work demonstrated this method for a single heliostat over a one-month period [1]. This study extends the previous work by expanding the number of heliostats modeled and tested to over 200. The duration over which errors were collected and modeled spanned 7 months. Smaller subsets of heliostats were observed and tested over an even longer, 10 month period. Error data was obtained by bringing heliostat beams (one by one) to a surveyed target on the Solar Tower and recording the measured elevation and azimuthal offsets. Beam position was determined by the centroid of the intensity image. Using automated sequencing, we were able to limit the time to process a heliostat to ∼32 seconds. This allowed us to process the entire 200 heliostat field in just 2 full days (weather permitting). These 2-day collections were performed about once every 2-3 months, to cover the full annual range of motion. All error data were automatically collected, time tagged and synchronized, in real-time. The error data were then converted, by a post-processor, to updated model parameters. The updated model data were subsequently uploaded to the heliostat field for model performance testing. In this way, almost 50, 000 error observations were collected for 200 heliostats during the course of the study. In addition, 19, 000 model performance observations were also recorded. Initial results indicate this method was able to halve the RMS average pointing error from 1.5 mrad to 0.78 mrad across the field of 200 heliostats. © 2013 The Authors.

Pepin M.,Sandia Staffing Alliance
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2015

The location of point scatterers in Synthetic Aperture Radar (SAR) data is exploited in several modern analyzes including persistent scatter tracking, terrain deformation, and object identification. The changes in scatterers over time (pulse-to-pulse including vibration and movement, or pass-to-pass including direct follow on, time of day, and season), can be used to draw more information about the data collection. Multiple pass and multiple antenna SAR scenarios have extended these analyzes to location in three dimensions. Either multiple passes at different elevation angles may be.own or an antenna array with an elevation baseline performs a single pass. Parametric spectral estimation in each dimension allows sub-pixel localization of point scatterers in some cases additionally exploiting the multiple samples in each cross dimension. The accuracy of parametric estimation is increased when several azimuth passes or elevations (snapshots) are summed to mitigate measurement noise. Inherent range curvature across the aperture however limits the accuracy in the range dimension to that attained from a single pulse. Unlike the stationary case where radar returns may be averaged the movement necessary to create the synthetic aperture is only approximately (to pixel level accuracy) removed to form SAR images. In parametric estimation increased accuracy is attained when two dimensions are used to jointly estimate locations. This paper involves jointly estimating azimuth and elevation to attain increased accuracy 3D location estimates. In this way the full 2D array of azimuth and elevation samples is used to obtain the maximum possible accuracy. In addition the independent dimension collection geometry requires choosing which dimension azimuth or elevation attains the highest accuracy while joint estimation increases accuracy in both dimensions. When maximum parametric estimation accuracy in azimuth is selected the standard interferometric SAR scenario results. When maximum estimation accuracy in elevation is selected the multiple baseline interferometric SAR scenario results. Use of a 2D parametric estimation method attains the best accuracy possible in both dimensions. When in some scenarios particularly the orbital case where the azimuth dimension is only approximately linear the full accuracy increase of linear joint azimuth and elevation is not fully attained. Images and point cloud estimates are shown for several linear and orbital SAR scenarios. Images provide a visual representation of the data while the quantitative point cloud data is a direct input for the multiple analyzes listed earlier. © 2015 SPIE.

Ho C.K.,Sandia National Laboratories | Khalsa S.S.,Sandia Staffing Alliance
ASME 2011 5th International Conference on Energy Sustainability, ES 2011 | Year: 2011

A new method is described to determine irradiance distributions on receivers and targets from heliostats or other collectors for concentrating solar power applications. The method uses a CCD camera, and, unlike previous beam characterization systems, it does not require additional sensors, calorimeters, or flux gauges on the receiver or target. In addition, spillage can exist (the beam does not need to be contained within the target). The only additional information required besides the digital images recorded from the CCD camera is the direct normal irradiance and the reflectivity of the receiver. Methods are described to calculate either an average reflectivity or a reflectivity distribution for the receiver using the CCD camera. The novel feature of this new PHLUX method is the use of recorded images of the sun to scale both the magnitude of each pixel value and the subtended angle of each pixel. A test was performed to evaluate the PHLUX method using a heliostat beam on the central receiver tower at the National Solar Thermal Test Facility in Albuquerque, NM. Results showed that the PHLUX method was capable of producing an accurate flux map of the heliostat beam with a relative error in the peak flux of 2%. Copyright © 2011 by ASME.

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