Publications

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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 99th 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.

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Photovoltaic systems are often priced in $/W{sub p}, where Wp refers to the DC power rating of the modules at Standard Test Conditions (1000 W/m{sup 2}, 25 C cell temperature) and $ refers to the installed cost of the system. However, the true value of the system is in the energy it will produce in kWhs, not the power rating. System energy production is a function of the system design and location, the mounting configuration, the power conversion system, and the module technology, as well as the solar resource. Even if all other variables are held constant, the annual energy yield (kWh/kW{sup p}) will vary among module technologies because of differences in response to low-light levels and temperature. Understanding energy yield is a key part of understanding system value. System performance models are used during project development to estimate the expected output of PV systems for a given design and location. Performance modeling is normally done by the system designer/system integrator. Often, an independent engineer will also model system output during a due diligence review of a project. A variety of system performance models are available. The most commonly used modeling tool for project development and due diligence in the United States is probably PVsyst, while those seeking a quick answer to expected energy production may use PVWatts. In this paper, we examine the variation in predicted energy output among modeling tools and users and compare that to measured output.

We propose and examine several statistical criteria for characterizing time series of solar irradiance. Time series of irradiance are used in analyses that seek to quantify the performance of photovoltaic (PV) power systems over time. Time series of irradiance are either measured or are simulated using models. Simulations of irradiance are often calibrated to or generated from statistics for observed irradiance and simulations are validated by comparing the simulation output to the observed irradiance. Criteria used in this comparison should derive from the context of the analyses in which the simulated irradiance is to be used. We examine three statistics that characterize time series and their use as criteria for comparing time series. We demonstrate these statistics using observed irradiance data recorded in August 2007 in Las Vegas, Nevada, and in June 2009 in Albuquerque, New Mexico.

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The Health and Safety Executive (HSE) has requested Sandia National Laboratories (SNL) to review the aircraft crash methodology for nuclear facilities that are being used in the United Kingdom (UK). The scope of the work included a review of one method utilized in the UK for assessing the potential for accidental airplane crashes into nuclear facilities (Task 1) and a comparison of the UK methodology against similar International Atomic Energy Agency (IAEA), United States (US) Department of Energy (DOE), and the US Nuclear Regulatory Commission (NRC) methods (Task 2). Based on the conclusions from Tasks 1 and 2, an additional Task 3 would provide an assessment of a site-specific crash frequency for the Dungeness B facility using one of the other methodologies. This report documents the results of Task 2. The comparison of the different methods was performed for the three primary contributors to aircraft crash risk at the Dungeness B site: airfield related crashes, crashes below airways, and background crashes. The methods and data specified in each methodology were compared for each of these risk contributors, differences in the methodologies were identified, and the importance of these differences was qualitatively and quantitatively assessed. The bases for each of the methods and the data used were considered in this assessment process. A comparison of the treatment of the consequences of the aircraft crashes was not included in this assessment because the frequency of crashes into critical structures is currently low based on the existing Dungeness B assessment. Although the comparison found substantial differences between the UK and the three alternative methodologies (IAEA, NRC, and DOE) this assessment concludes that use of any of these alternative methodologies would not change the conclusions reached for the Dungeness B site. Performance of Task 3 is thus not recommended.

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Rates of reactions can be expressed as dn/dt = kcf(n) where n is moles of reaction, k is a rate constant, c is a proportionality constant, and f(n) is a function of the properties of the sample. When the instrument time constant, ?, and k are sufficiently comparable that measured rates are significantly affected by instrument response, correction for instrument response must be done to obtain accurate reaction kinetics. Correction for instrument response has previously been done by truncating early data or by use of the Tian equation. Both methods can lead to significant errors. We describe a method for simultaneous determination of ?, k, and c by fitting equations describing the combined instrument response and rate law to rates observed as a function of time. The method was tested with data on the heat rate from acid-catalyzed hydrolysis of sucrose.

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