Publications

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Many popular models for photovoltaic system performance (e.g., [1], [2]) employ a single diode model (e.g., [3]) to compute the IV curve for a module or string of modules for given irradiance and temperature conditions. Most commonly (e.g., [4]), parameters are determined using only current and voltage at short circuit, open circuit and maximum power from a single IV curve at standard test conditions, along with reported temperature coefficients. In contrast, module testing frequently records IV curves at a wide range of irradiance and temperature conditions, such as those specified in IEC 61853-1 [5], which, when available, should also be used to parameterize the performance model. We propose a parameter estimation method that makes use of the full range of available IV curves, and demonstrate the accuracy of the resulting performance model. © 2013 IEEE.

Variability of photovoltaic (PV) power output is a potential concern to utilities because it can lead to voltage changes on the distribution system and have other adverse impacts on power quality unless additional equipment is added or operational practices are changed to mitigate these effects. This paper develops and evaluates a simple yet novel approach for quantifying irradiance variability over various timescales. The approach involves comparison between measured irradiance and a reference, clear sky irradiance, determined from a model. Conceptually, the "Variability Index" is the ratio of the "length" of the measured irradiance plotted against time divided by the "length" of the reference clear sky irradiance signal. Adjustments are proposed that correct for different measurement intervals. By evaluating the variability index at several sites, we show how annual and monthly distributions of this metric can help to classify sites and periods of time when variability is significant. Copyright © (2012) by American Solar Energy Society.

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Extensive work has been carried out by the U.S. Department of Energy (DOE) in the development of a proposed geologic repository at Yucca Mountain (YM), Nevada, for the disposal of high-level radioactive waste. As part of this development, a detailed performance assessment (PA) for the YM repository was completed in 2008 and supported a license application by the DOE to the U.S. Nuclear Regulatory Commission (NRC) for the construction of the YM repository. The following aspects of the 2008 YM PA are described in this presentation: (i) conceptual structure and computational organization, (ii) uncertainty and sensitivity analysis techniques in use, (iii) uncertainty and sensitivity analysis for physical processes, and (iv) uncertainty and sensitivity analysis for expected dose to the reasonably maximally exposed individual (RMEI) specified the NRC's regulations for the YM repository. © 2011 Elsevier Ltd.

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One year of power output was simulated at one-minute intervals for each of fourteen hypothetical utility-scale photovoltaic power plants and for the aggregate power output from a large number of distribution-connected photovoltaic systems. For utility-scale plants, the simulation first constructs one-year time series of global horizontal irradiance at one-minute intervals at each plant location, and a performance model translates irradiance and weather information to AC output power. Distribution-connected photovoltaic systems comprise a variety of system configurations: residential-scale rooftop systems at various tilts; commercial-scale flat-roof mounted systems; and commercial-scale ground-mounted tracked systems. For distribution-connected PV systems, the simulation estimates the time series of spatially-averaged irradiance for the region containing the systems, and the performance model is employed to estimate power aggregate power from all systems. The simulation results are validated by comparing statistics for the time series of irradiance with statistics for measured irradiance within the region.

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Sandia National Laboratories has developed a modeling approach to simulate time-synchronized, 1-minute power output from large PV plants in locations where only hourly irradiance measurements are available via satellite sources. The approach uses 1-min irradiance measurements from analogue sites in a similar geographic area. PV output datasets generated for 2007 in southern Nevada are being used for a Solar PV Grid Integration Study to estimate the integration costs associated with various utility-scale PV generation levels. Plant designs considered include both fixed-tilt thin-film, and singleaxis- tracked polycrystalline Si systems ranging in size from 5 to 300 MWAC. Simulated power output profiles at 1-min intervals were generated for five scenarios (149.5 MW, 222 WM, 292 MW, 492 MW, and 892 MW) each comprising as many as 10 geographically separated PV plants. Copyright© (2011) by the American Solar Energy Society.

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This report describes in-depth analysis of photovoltaic (PV) output variability in a high-penetration residential PV installation in the Pal Town neighborhood of Ota City, Japan. Pal Town is a unique test bed of high-penetration PV deployment. A total of 553 homes (approximately 80% of the neighborhood) have grid-connected PV totaling over 2 MW, and all are on a common distribution line. Power output at each house and irradiance at several locations were measured once per second in 2006 and 2007. Analysis of the Ota City data allowed for detailed characterization of distributed PV output variability and a better understanding of how variability scales spatially and temporally. For a highly variable test day, extreme power ramp rates (defined as the 99^th percentile) were found to initially decrease with an increase in the number of houses at all timescales, but the reduction became negligible after a certain number of houses. Wavelet analysis resolved the variability reduction due to geographic diversity at various timescales, and the effect of geographic smoothing was found to be much more significant at shorter timescales.

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Published results of performance assessments for deep geologic disposal of high-level radioactive waste and spent nuclear fuel provide insight into those aspects of the waste form that are potentially important to the long-term performance of a repository system. Alternative waste forms, such as might result from new technologies for processing spent fuel and advances in nuclear reactor design, have the potential to affect the long-term performance of a geologic repository. This paper reviews relevant results of existing performance assessments for a range of disposal concepts and provides observations about how hypothetical modifications to waste characteristics (e.g., changes in radionuclide inventory, thermal loading, and durability of waste forms) might impact results of the performance assessment models. Disposal concepts considered include geologic repositories in both saturated and unsaturated environments. Specifically, we consider four recent performance assessments as representative of a range of disposal concepts. We examine the extent to which results of these performance assessments are affected by (i) thermal loading of the waste proposed for disposal; (ii) mechanical and chemical lifetime of the waste form; and (iii) radionuclide content of the waste. We find that peak subsurface temperature generally is a constraint that can be met through engineering solutions and that processing of wastes to reduce thermal power may enable more efficient use of repositories rather than improved repository performance. We observe that the rate of radionuclide release is often limited by geologic or chemical processes other than waste form degradation. Thus, the effects on repository performance of extending waste-form lifetime may be relatively small unless the waste form lifetime becomes sufficiently long relative to the period of repository performance. Finally, we find that changes to radionuclide content of waste (e.g., by separation or transmutation processes) do not in general correspond to proportional effects on repository performance. Rather, the effect of changes to radionuclide content depends on the relative mobility of various radionuclides through the repository system, and consequently on repository geology and geochemistry.

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Design and operation of the electric power grid (EPG) relies heavily on computational models. High-fidelity, full-order models are used to study transient phenomena on only a small part of the network. Reduced-order dynamic and power flow models are used when analysis involving thousands of nodes are required due to the computational demands when simulating large numbers of nodes. The level of complexity of the future EPG will dramatically increase due to large-scale deployment of variable renewable generation, active load and distributed generation resources, adaptive protection and control systems, and price-responsive demand. High-fidelity modeling of this future grid will require significant advances in coupled, multi-scale tools and their use on high performance computing (HPC) platforms. This LDRD report demonstrates SNL's capability to apply HPC resources to these 3 tasks: (1) High-fidelity, large-scale modeling of power system dynamics; (2) Statistical assessment of grid security via Monte-Carlo simulations of cyber attacks; and (3) Development of models to predict variability of solar resources at locations where little or no ground-based measurements are available.