Large-scale unit commitment under uncertainty: an updated literature survey

The Unit Commitment problem in energy management aims at finding the optimal production schedule of a set of generation units, while meeting various system-wide constraints. It has always been a large-scale, non-convex, difficult problem, especially in view of the fact that, due to operational requirements, it has to be solved in an unreasonably small time for its size. Recently, growing renewable energy shares have strongly increased the level of uncertainty in the system, making the (ideal) Unit Commitment model a large-scale, non-convex and uncertain (stochastic, robust, chance-constrained) program. We provide a survey of the literature on methods for the Uncertain Unit Commitment problem, in all its variants. We start with a review of the main contributions on solution methods for the deterministic versions of the problem, focussing on those based on mathematical programming techniques that are more relevant for the uncertain versions of the problem. We then present and categorize the approaches to the latter, while providing entry points to the relevant literature on optimization under uncertainty. This is an updated version of the paper "Large-scale Unit Commitment under uncertainty: a literature survey" that appeared in 4OR 13(2), 115--171 (2015); this version has over 170 more citations, most of which appeared in the last three years, proving how fast the literature on uncertain Unit Commitment evolves, and therefore the interest in this subject

Niobium addition effect in molds at last cooling step on EN-GJL250 gray cast iron: Microstructural changes and electrochemical behavior

The purpose of this study was to determine the impact of niobium addition as an inoculation element on the microstructure and electrochemical properties of EN-FGL250 gray cast iron. Niobium additions are in a powder form and have a 0.5 mm particle size at dfferent proportions of 1wt.% and 3wt.%. The addition was done during casting of the metal in the mold at the last cooling step of the melt cast iron. These additions have a significant impact on the phenomenon of solidification as the metal powder deposited in the sand molds creates new centers of germination and absorbs a lot of heat. The cooling rate directly affects the microstructure and electrochemical behavior. This is confirmed by SEM observations and electrochemical tests. Furthermore, the addition of niobium transforms the microstructure of gray cast iron from cellular structure into totally dendritic structure. As a consequence, the niobium addition affected the shape and size of graphite, thus considerably reducing the corrosion current density by increasing the polarization resistance Rp

Hemispheric asymmetries in the ionosphere response observed during the high-speed solar wind streams of the 24-28 August 2010

International audienceThis paper presents the geomagnetic and ionospheric responses to a high speed solar wind stream (HSS) impacting the magnetosphere on 24 August 2010. We focus our study on the interhemispheric conjugated behavior. The solar wind speed remained very high during 5 days from 24 to 28 August 2010. By using magnetometer and ground-based GPS data from various approximately conjugated magnetic observatories and GPS stations, we studied the hemispheric asymmetries in the magnetic signature, Vertical Total Electron Content (VTEC) and scintillation activity during this HSS event. Geomagnetic activity reveals larger disturbances in amplitude in the Northern Hemisphere (NH) than in the southern Hemisphere (SH), and stronger asymmetries at higher latitudes, than at lower latitudes, between the conjugate observatories. VTEC variations reveal large increases in amplitude in the NH; while these effects are less pronounced in the SH. We investigate also the GPS scintillation activities occurring in the conjugated polar regions under HSSs conditions. At auroral latitudes, our results show a good correlation between the rate of VTEC index (ROTI) and auroral Al index, with more intense phase fluctuations in the NH

Wavelet based multiscale analysis of geophysical downhole measurements: Application to a clayey siliclastic sequence

International audienceIn order to reveal the structural properties at small scale within a geological unit apparently homogeneous at large scale, we explore the stochastic components of petrophysical measurements recorded in a borehole drilled through the Cretaceous siliclastic units of the South-eastern Basin of France. First, we analyse the instantaneous fluctuations of gamma ray log measurements searching for scaling laws in its power spectrum. Power spectra indicate a scaling behaviour with average exponent beta a values mostly falling in the range

Transient variations of vertical total electron content at low latitude during the period 2013–2017

International audienceWe use GPS networks to measure the vertical Total Electron Content (VTEC) variations at low latitude, in three longitude sectors: America, Europe-Africa and Asia, collected during the period 2013-2017. This period corresponds to the increasing phase of the solar cycle 24 (SC#24) observed around 2013-2014 as well as the decreasing phase around 2014-2017. Our results discussed a morphological analysis of regular variations in ionization during different phases of solar activity: daytime variations, seasonal and semiannual variations and variations based on the solar cycle 24 in three longitude sectors. In all longitude sectors, the highest VTEC values are displayed during the two months of the spring, located after sunrise and before sunset. The lowest values are found during the summer and winter seasons. We found that the winter anomaly and the presence of equinoxial peaks are the most pronounced effects in VTECs in the increasing and decreasing phase of the SC#24. A strong asymmetry is detected between equinoxial peaks and the location of peaks occurring in March/April and October/November at maximum in the solar flux variations during the increase phase. We show that the daily VTEC maximum values were registered between 14:00 and 16:00 LT and the minimum values between 4:00 and 6:00 LT. Double ionization peak in the morning and evening is observed in VTEC annual variations, due to the proximity of the equatorial fountain stations. From the statistical analysis part, we observed practically the same distribution of the different classes of VTEC (two peaks, bell-shaped and plateau-shaped) variations in the three sectors of longitude. These observations indicate longitudinal variation in the presence of the winter anomaly in the Equatorial Ionized Anomaly (EIA) region. Additionally, we can note a longitudinal variation of the spring-autumn VTEC asymmetry in the EIA region during the five years 2013-2017. We observe also that the occurrence of nocturnal peak recorded around 19 local time (LT) shows the same characteristics as the vertical drift E x B (B: magnetic field is perpendicular to E: electric field.) with respect to solar cycle, season and longitude. Three essential characteristics we noted: 1) the occurrence of the nocturnal peak generally follows the solar cycle. 2) The occurrence of the nocturnal peak is generally stronger at the equinoxes than at the solstices. 3) The occurrence of the nocturnal peak is stronger in the Europe-Africa and America sectors than in the Asia sector. As a result, nocturnal peak occurrence is well related to the PRE at the origin of the GNSS signal scintillations

Local regularity analysis of strata heterogeneities from sonic logs

Borehole logs provide geological information about the rocks crossed by the wells. Several properties of rocks can be interpreted in terms of lithology, type and quantity of the fluid filling the pores and fractures. <br><br> Here, the logs are assumed to be nonhomogeneous Brownian motions (nhBms) which are generalized fractional Brownian motions (fBms) indexed by depth-dependent Hurst parameters H(z). Three techniques, the local wavelet approach (LWA), the average-local wavelet approach (ALWA), and Peltier Algorithm (PA), are suggested to estimate the Hurst functions (or the regularity profiles) from the logs. <br><br> First, two synthetic sonic logs with different parameters, shaped by the successive random additions (SRA) algorithm, are used to demonstrate the potential of the proposed methods. The obtained Hurst functions are close to the theoretical Hurst functions. Besides, the transitions between the modeled layers are marked by Hurst values discontinuities. It is also shown that PA leads to the best Hurst value estimations. <br><br> Second, we investigate the multifractional property of sonic logs data recorded at two scientific deep boreholes: the pilot hole VB and the ultra deep main hole HB, drilled for the German Continental Deep Drilling Program (KTB). All the regularity profiles independently obtained for the logs provide a clear correlation with lithology, and from each regularity profile, we derive a similar segmentation in terms of lithological units. The lithological discontinuities (strata' bounds and faults contacts) are located at the local extrema of the Hurst functions. Moreover, the regularity profiles are compared with the KTB estimated porosity logs, showing a significant relation between the local extrema of the Hurst functions and the fluid-filled fractures. The Hurst function may then constitute a tool to characterize underground heterogeneities

Sand Dune Characterization for Preparing Metallurgical Grade Silicon

Dune sand, from Biskra, Algeria which is in the Great Algerian huge desert attracts tourists from all over the world. Dune sand composes around 80% of the total area of the desert. To date only a few studies have been conducted on this sand. The purpose of the present study is to shed light on the textural and physico-chemical characteristics of this sand using several characterization methods, and to understand the potential for using for photovoltaic applications. The chemical composition of Biskra sand indicated a high purity quartz with 97.6% silica, and a presence of others oxides. Granular analysis enabled the determination of the size of the sand grains which was found to be fine to medium. On the other hand, a Scanning Electron Microscopy and optical observations were carried out, the micrographs observations indicated the presence of different shapes of sand grain, some rounded whereas others are angular edged or elongated. X-ray diffraction indicates the highly crystalline nature of the Biskra sand