Search Results (30)

Costa Rica is located along the narrow isthmus that connected South America to North America beginning in the mid-Cenozoic. The exchange of vertebrates between the two continents has received considerable study, but paleobotanical aspects are less known. The Pacific coast “ring of fire” volcanoes produced abundant hyaloclastic material that provided a source of silica for wood petrifaction, and the tropical forests contained diverse taxa. This combination resulted in the preservation of petrified wood at many sites in Costa Rica. Fossil wood ranges in age from Lower Miocene to Middle Pleistocene, but Miocene specimens are the most common. Our research involved the study of 54 specimens, with the goal of determining their mineral compositions and interpreting the fossilization processes. Data came from thin-section optical microscopy, SEM images, and X-ray diffraction. Two specimens were found to be mineralized with calcite, but most of the woods contained crystalline quartz and/or opal-CT. The preservation of anatomical detail is highly variable. Some specimens show evidence of decay or structural deformation that preceded mineralization, but other woods have well-preserved cell structures. This preliminary study demonstrates the abundance and botanical diversity of fossil wood in Costa Rica, hopefully opening a door into future studies that will consider the taxonomy and evolutionary aspects of the country’s fossil forests. Full article

Nowadays, the rise of DC loads along with distributed energy resources (DERs) and energy storage systems (ESSs) have led to a growing interest in using direct current (DC) microgrid systems. Conventional droop control methods face significant limitations when applied to parallel-connected distributed generation (DG) units, particularly in achieving balanced power sharing and minimizing voltage deviations. To overcome this issue, an enhanced power sharing control method is proposed in this paper to address load sharing in parallel-connected DG units based DC microgrids, considering unmatched line impedance and load variation. The enhanced control method aims to achieve balanced load power sharing and voltage control through the use of a Luenberger observer to estimate the Point of Common Coupling (PCC) bus voltage and accordingly estimate the voltage deviation. The proposed method compensates for the effects of unmatched line impedances and dynamic load variations, enabling accurate power sharing and precise DC bus voltage regulation. Various scenarios are studied to evaluate the performance of the proposed method under different operating conditions including system and load parameters variations. Finally, the performance of the proposed control method was validated through real-time simulation using OPAL-RT target, and compared with conventional droop control approaches. Full article

Optical microscopy and SEM imaging are powerful tools for evaluating the origins of quartz in fossil wood. Silicification is the most common mechanism for wood petrifaction, but this silica is commonly in cryptocrystalline or microcrystalline form (e.g., chalcedony or agate). Two essential requirements for the formation of macrocrystalline quartz is a concentration of dissolved silica low enough to allow the development of well-ordered lattices, and sufficient open space to allow euhedral or subhedral crystals to grow. Macrocrystalline quartz commonly occurs as a late-stage precipitate in open spaces that remained after initial mineralization had occurred. These spaces include vessels in angiosperm wood, and vugs and fractures in all types of wood. Exterior surfaces may also be suitable sites for quartz crystal growth. In some occurrences, crystalline quartz has directly encrusted or replaced wood cells. Diagenetic transformation of opal can produce cryptocrystalline or microcrystalline forms of quartz, but this process is not likely to produce macrocrystals. Full article

Robust control mechanisms are needed in microgrids to ensure voltage source inverters (VSIs) effectively integrate renewable energy sources such as solar photovoltaic (PV) systems into the power network. Current control approaches often have limitations regarding velocity, stability, and robustness. The paper details a newly developed method named Point of Common Coupling Direct Power Control (PCC-DPC) for renewable energy systems connected to the grid. PCC-DPC is used to instantly control voltage at the point of common coupling (PCC) inside the microgrid as opposed to other conventional techniques. This leads to a simplified controller design that does not require complex Park transformations and phase-locked loop (PLL) systems, and has a lower computational burden and less power fluctuation in a stable manner. Moreover, this research critically examines power quality phenomena through comparing PCC-DPC with a Vector Current Controller (VCC). Simulations performed on an Opal Re-al-Time simulator showed improved tracking performance and overall system efficiency due to the PCC-DPC approach over others. These results demonstrate that it can effectively be used as one of the most suitable methods for integrating renewable energy into electricity grids, which is reliable in regards to changes in power grid dynamics. Full article

Modern technologies, such as airborne laser scanning (ALS) and advanced data analysis algorithms, allow for the efficient and safe use of resources to protect infrastructure from potential threats. This publication presents a study to identify trees that may fall on highways. The study used free measurement data from airborne laser scanning and wind speed and direction data from the Institute of Meteorology and Water Management in Poland. Two methods were used to determine the crown tops of trees: PyCrown and OPALS. The effect of wind direction on potential hazards was then analyzed. The OPALS method showed the best performance in terms of detecting trees, with an accuracy of 74%. The analysis showed that the most common winds clustered between 260° and 290°. Potential threats, i.e., trees that could fall on the road, were selected. As a result of the analysis, OPALS detected between 140 and 577 trees, depending on the chosen strategy. The presented research shows that combining ALS technology with advanced algorithms and wind data can be an effective tool for identifying potential hazards associated with falling trees on highways. Full article

Reversible optical regulation has potential applications in optical anti-counterfeiting, storage, and catalysis. Compared to common power materials, the reverse opal structure has a larger specific surface area and an increased contact area for optical regulation, which is expected to achieve higher regulation rates. However, it is difficult to achieve reversible and repeatable regulation of the luminescent properties of photonic crystals, especially with the current research on the structural collapse of photonic crystals. In this work, WO₃: Yb³⁺, Er³⁺ inverse photonic crystals were prepared by the template approach, and reversible multi-mode optical modification was investigated. Upon heat treatment in a reducing atmosphere or air, the color of the photonic crystals can reversibly change from light yellow to dark green, accompanied by changes in absorption and upconversion of luminescence intensity. The stability and fatigue resistance of this reversible optical modification ability were explored through cyclic experiments, providing potential practical applications for photocatalysis, optical information storage, and electrochromism. Full article

This study proposes an adaptive high-frequency injection method (AHFI) aimed at mitigating common-mode voltage (CMV) on the AC side and alleviating current stress on power semiconductor devices within each arm of a medium-voltage motor propulsion system designed for modular multilevel converters. By adjusting the quantity of high-frequency components injected into each arm according to the fluctuation coefficient, the amplitude of injected high-frequency CMV and circulating currents can be reduced across medium to rated motor speeds. This approach enhances the start-up performance of medium-voltage motor drives while diminishing CMV effects on the motor side, resulting in decreased total harmonic distortion (THD) in the three-phase output waveforms. Furthermore, the effectiveness of the proposed AHFI method in SM voltage regulation and circulating current control under low-frequency operation is thoroughly analyzed. The validity of this method is established through comprehensive mathematical scrutiny and time–domain simulations performed using MATLAB/SIMULINK software (MATLAB version R2021b), along with real-time simulations conducted employing the real-time simulator OPAL/RT via hardware-in-the-loop simulation (HILS). Full article

This paper proposes an algorithm for determining the optimal capacitance by utilizing a mathematical model of a submodule (SM) capacitor in a modular multilevel converter (MMC) specifically for medium voltage motor drives (MVDs). By approximating the voltage fluctuation of the SM capacitor during low-frequency operation, it is feasible to ascertain the minimum capacitance required for the SM capacitor, ensuring that its voltage fluctuations remain within an acceptable limit that is predefined as a specified value. Moreover, the study considered the injection of both a high-frequency common-mode voltage (CMV) and a circulating current to alleviate the SM voltage fluctuation during the acceleration of motor drives. The effectiveness of the proposed method is validated through verification using time-domain simulation results obtained using the MATLAB/SIMULINK software and real-time simulation results acquired using the OPAL-RT simulator platform. Full article

Microscopic analysis of fossils from the Lightning Ridge district of northwestern New South Wales, Australia, shows that opal has been typically deposited in variable cavities left by the degradation of the original organic material. Fine-grained, clay-rich sediments have preserved the external morphology, and opalization has produced detailed casts with different modes of preservation of internal details. Plant remains include cones, cone scales, fruiting bodies, and seeds, but the most common specimens are twigs, stems, and wood fragments. These specimens commonly contain angular inclusions that represent small tissue fragments produced by the degradation of the original wood. Inclusions commonly have a “hollow box” structure where the organic material has decomposed after the initial opal filling of the mold. These spaces commonly contain traces of the cellular architecture, in the form of wood fiber textures imprinted on the cavity wall, degraded cellular material, and silicified tracheids. Opal casts of mollusk shells and crustacean bioliths preserve the shape but no calcium carbonate residue. Likewise, opal casts of vertebrate remains (bones, teeth, osteoderms) lack preservation of the original bioapatite. These compositions are evidence that burial in fine clays and silts, isolated from the effects of water and oxygen, caused protracted delays between the timing of burial, decomposition, and the development of vacuities in the claystones that became sites for opal precipitation. The length of time required for the dissolution of cellulosic/ligninitic plant remains, calcium carbonate items, and calcium phosphates in bones and teeth cannot be quantified, but evidence from opal-bearing formations worldwide reveals that these processes can be very slow. The timing of opalization can be inferred from previous studies that concluded that Cenozoic tectonism produced faults and fissures that allowed horizontal and lateral movement of silica-bearing groundwater. Comparisons of Australian opal-AG with opal from international localities suggest that opalization was a Neogene phenomenon. The transformation of Opal-AG → Opal-CT is well-documented for the diagenesis of siliceous biogenic sediments and siliceous sinter from geothermal areas. Likewise, precious and common opal from the late Miocene Virgin Valley Formation in northern Nevada, USA, shows the rapidity of the Opal-AG → Opal-CT transformation. Taken together, we consider this evidence to indicate a Neogene age for Lightning Ridge opalization and by inference for the opalization of the extensive opal deposits of the Great Artesian Basin in Australia. New paleontology discoveries include a surprising level of cellular detail in plant fossils, the preservation of individual tracheids as opal casts, evidence of opalized plant pith or vascular tissue (non-gymnosperm), and the first report of Early Cretaceous coprolites from New South Wales, Australia. Full article

Renewable penetration, particularly the increasing deployment of PV by residential customers, organizations, and utilities, is leading to the rapid evolution of the power grid. However, the power system’s architectural changes affect the quality of supply and give rise to power quality issues such as harmonics, fluctuations, disturbances, etc., at the point of common coupling (PCC). Therefore, in this work, a power network was modeled to study the impact of PV systems on PCC. At first, a detailed review is presented for on-grid PV systems with different inverter topologies, control techniques, sources of harmonic generation, and their mitigation strategies. After that, several use cases considering various sources of harmonics in a network with on-grid PV are modeled and simulated using MATLAB/Simulink. In-depth research was performed in this work to examine the many variables that affect harmonics, such as solar radiation levels, controller tuning, and load changes. Results with a real-time simulation platform (OPAL-RT) are presented in this paper for several use cases. Lastly, comprehensive discussions are presented from the acquired offline and real-time simulation results. Full article

Ocimum basilicum L. is the most common Ocimum species, and it is used as an ornamental plant and in food condiments. This unique study examined the chemical composition and biological activities of six extracts from five basil cultivars, including their antimicrobial, antidiabetic, antilipidemic, neuroprotective, and anticollagenase activity. Moreover, their toxicological effects were studied using the zebrafish Danio rerio. Volatile components were determined using HS-SPME and GC-MS, while total polyphenols were detected using HPLC and the spectrophotometric Folin–Ciocalteu method. Spectrophotometric assays (DPPH, ABTS, ORAC, FRAP) were performed to determine antioxidant activity, collagenase inhibition, acetylcholinesterase inhibition, and pancreatic lipase inhibition. Antimicrobial activity was determined using the broth microdilution test. The study found that the biological activities of different basil cultivars varied depending on the proportion of active compounds, as determined by chemical analyses. All six basil extracts significantly inhibited α-amylase, while Purple basil extract most significantly inhibited the activity of collagenase, acetylcholinesterase, and pancreatic lipase. Purple basil and Dark Opal basil I extracts exhibited the highest antimicrobial activity, while the Dark Opal basil II extract had the most significant antioxidant potential. The findings in this study suggest that ethanolic basil extracts have the potential to be used as dietary drugs and implemented in antiaging products. This study is unique in its aims to compare the chemical composition and biological activities of basil cultivars from Croatia and to evaluate potential toxicological effects through embryotoxicity tests on zebrafish Danio rerio embryos, and it reports the first evidence of anticollagenase, antidiabetic, and antilipidemic activities for these cultivars. Full article

This paper presents a new Remote Hyperspectral Imaging System (RHIS) embedded on an Unmanned Aquatic Drone (UAD) for plastic detection and identification in coastal and freshwater environments. This original system, namely the Remotely Operated Vehicle of the University of Littoral Côte d’Opale (ROV-ULCO), works in a near-field of view, where the distance between the hyperspectral camera and the water surface is about 45 cm. In this paper, the new ROV-ULCO system with all its components is firstly presented. Then, a hyperspectral image database of plastic litter acquired with this system is described. This database contains hyperspectral data cubes of different plastic types and polymers corresponding to the most-common plastic litter items found in aquatic environments. An in situ spectral analysis was conducted from this benchmark database to characterize the hyperspectral reflectance of these items in order to identify the absorption feature wavelengths for each type of plastic. Finally, the ability of our original system RHIS to automatically recognize different types of plastic litter was assessed by applying different supervised machine learning methods on a set of representative image patches of marine litter. The obtained results highlighted the plastic litter classification capability with an overall accuracy close to 90%. This paper showed that the newly presented RHIS coupled with the UAD is a promising approach to identify plastic waste in aquatic environments. Full article

For applications of nanolattices in low-temperature nanoelectronics, the inter-unit space can be filled with superconducting metallic alloys. However, superconductivity under nanoconfinement is expected to be strongly affected by size-effects and other factors. We studied the magnetic properties and structure of the Ga-Sn eutectic alloy within regular nanopores of an opal template, to understand the specifics of the alloy superconductivity. Two superconducting transitions were observed, in contrast to the bulk alloy. The transitions were ascribed to the segregates with the structures of tetragonal tin and a particular gallium polymorph. The superconducting-phase diagram was constructed, which demonstrated crossovers from the positive- to the common negative-curvature of the upper critical-field lines. Hysteresis was found between the susceptibilities obtained at cooling and warming in the applied magnetic field. Full article

Recently, because of the increase in the number of connections to Distributed Generation (DG), the problem of lowering voltage stability in the distribution system has become an issue. Reactive power compensators, such as Static Synchronous Compensators (STATCOM), may be used to solve the problem of voltage stability degradation. However, because of the complexity of the distribution system, it is very difficult to select the installation location for STATCOM. Furthermore, when installed in the wrong location, economical efficiency and availability problems may occur. This paper proposes a Virtual STATCOM Configuration and Control method that would operate like a single STATCOM based on multiple DGs connected to the system. The proposed Virtual STATCOM has the merit of being economical by using existing facilities without adding new power facilities, and it solves the problem of the difficulty of selecting the installation location because of the complexity of the distribution system. In addition, while the conventional STATCOM uses an independent control method in consideration of the power quality of the access point, the Virtual STATCOM performs the Point of Common Coupling (PCC) power quality compensation using the integrated control of multiple DGs connected to the system. In the proposed method, the Virtual STATCOM integrated control algorithm is configured by adopting linear programming, and the compensation is performed while considering the distance between DG and PCC, the inverter’s rated capacity, and the power generation. The performance of the Virtual STATCOM power quality compensation was verified using MATLAB/SIMULINK and Real Time Simulator (OPAL-RT). Full article

Silicified wood occurs abundantly in Middle Miocene flows and sedimentary interbeds of the Columbia River Basalt Group (CRBG) in central Washington State, USA. These fossil localities are well-dated based on radiometric ages determined for the host lava. Paleoenvironments include wood transported by lahars (Ginkgo Petrified Forest State Park), fluvial and palludal environments (Saddle Mountain and Yakima Canyon fossil localities), and standing forests engulfed by advancing lava (Yakima Ridge fossil forest). At all of these localities, the mineralogy of fossil wood is diverse, with silica minerals that include opal-A, opal-CT, chalcedony, and macrocrystalline quartz. Some specimens are composed of only a single form of silica; more commonly, specimens contain multiple phases. Opal-A and Opal-CT often coexist. Some woods are mineralized only with chalcedony; however, chalcedony and macrocrystalline quartz are common as minor constituents in opal wood. In these specimens, crystalline silica filling fractures, rot pockets, and cell lumen may occur. These occurrences are evidence that silicification occurred as a sequential process, where changes in the geochemical environment or anatomical structures affected the precipitation of silica. Fossilization typically began with precipitation of amorphous silica within cell walls, leaving cell lumen and conductive vessels open. Diagenetic transformation of opal-A to opal-CT in fossil wood has long been a widely accepted hypothesis; however, in opaline CRBG specimens, the two silica polymorphs usually appear to have formed independently, e.g., woods in which cell walls are mineralized with opal-A but in which lumen contain opal-CT. Similarly, opal-CT has been inferred to sometimes transform to chalcedony; however, in CRBG, these mixed assemblages commonly resulted from multiple mineralization episodes. Full article