The Multiple Microgrids (MMGs) concept has been identified as a promising solution for the management of large-scale power grids in order to maximize the use of widespread renewable energies sources. However, its deployment in realistic operation scenarios is still an open issue due to the presence of non-ideal and unreliable communication systems that allow each component within the power network to share information about its state. Indeed, due to technological constraints, multiple time-varying communication delays consistently appear during data acquisition and the transmission process and their effects must be considered in the control design phase. To this aim, this paper addresses the voltage regulation control problem for MMGs systems in the presence of time-varying communication delays. To solve this problem, we propose a novel hierarchical two-layer distributed control architecture that accounts for the presence of communication latencies in the information exchange. More spe... [more]

To predict the fire risk of spatter generated during shielded metal arc welding, the thermal characteristics of welding spatter were analyzed according to different welding times and electrical powers supplied to the electrode. An experimental apparatus for controlling the contact angle between the electrode and base metal as well as the feed rate was prepared. Moreover, the correlations among the volume, maximum diameter, scattering velocity, maximum number, and maximum temperature of the welding spatter were derived using welding power from 984−2067 W and welding times of 30 s, 50 s, and 70 s. It was found that the volume, maximum diameter, and maximum number of welding spatters increased proportionally as the welding time and electrical power increased, but the scattering velocity decreased as the particle diameter increased regardless of the welding time and electrical power. When the measured maximum temperature of the welding spatter was compared with an empirical formula, the ac... [more]

The electrification of vessels/ferries for green transformation requires onboard electrical energy storage as well as an energy supply network in the port area. In this context, a lot of efforts have been made in the last decade that have been reviewed in such a way that only a single aspect of the green transformation challenge is highlighted. Consequently, the objective of this research is to develop knowledge by examining the current state of affairs and provide, accordingly, abstract implementation guidelines for green transformation through vessel/ferry electrification. A comprehensive study on the electrification of vessels, in industry as well as in academia, is performed. Based on the data collected through a systematic study, a comparison of various pure electric and hybrid vessels in terms of certain performance attributes, such as battery capacity, passenger and cargo capacities, and size (length) of the vessel, is performed. Moreover, the distribution of vessels according t... [more]

The original water distribution characteristic plays an important role in the fracturing liquid retention in actual tight reservoirs. In this paper, an analytical model was proposed to characterize the water distribution and its effect on the spontaneous imbibition, based on the capillary tube model and fractal theory. Furthermore, the effect of the water film and the non-piston-like front related to the pore size are included in our model. The proposed model was successfully validated with the experimental results of core imbibition tests. Our work demonstrates that water distribution is influenced by displacement pressure and pore structure. For a small differential pressure, the porous media with richer large pores usually possesses a lower water saturation, and this difference will decrease with the increase of differential pressure. Moreover, compared with previous studies, the proposed imbibition model can not only distinguish the valid pores and invalid pores for imbibition but... [more]

Methanol is hygroscopic in a gaseous state and is a promising alternative fuel for internal combustion engines. It is understood that adding water can improve the antiknock performance for spark ignition engines, but this will also affect the flame speed and stability. In this work, laminar flame characteristics of methanol/water/air mixtures were experimentally investigated at a temperature range of 380−450 K, a pressure range of 1−4 bar, and water fractions (vaporous water molar fraction in the water−methanol fuel gas) of 0−40%. The results show that laminar burning velocity increases with temperature but decreases with pressure. The burning velocity decreases linearly with water fraction at a stoichiometric ratio. For rich mixtures and high pressures, the laminar flames tend to be more sensitive to stretch and, thus, more prone to being unstable. Increasing the water fraction can slightly increase the Markstein length. Increasing the initial pressure enhances the general flame insta... [more]

The wind power ramp event includes large fluctuations in wind power within a short period of time. To maintain grid stability, defining, identifying, and predicting the wind power ramp event is inevitable. Therefore, a comprehensive assessment method of wind power ramp events that combines the generalized information of the source, grid, and load sides is proposed. In this method, we put forward a channel self-selected multi-layer coefficient correction model (CSMCC) and wind power ramp threshold, according to the allowable value of a grid frequency change. Additionally, the availability of data-driven modeling methods is verified by performing autocorrelation analysis. Finally, the comprehensive evaluation method, which combines the back propagation (BP) neural network, supports the vector machine and CSMCC model is proved to be effective. This paper has a certain reference significance for basic research on large-scale wind power safety and efficient utilization.

A model developed at the University of Tomsk, Russia, for high latitudes (over 55° N) is proposed and applied to the analysis and observation of the solar resource in the state of Sonora in the northwest of Mexico. This model utilizes satellite data and geographical coordinates as inputs. The objective of this research work is to provide a low-cost and reliable alternative to field meteorological stations and also to obtain a wide illustration of the distribution of solar power in the state to visualize opportunities for sustainable energy production and reduce its carbon footprint. The model is compared against real-time data from meteorological stations and satellite data, using statistical methods to scrutinize its accuracy at local latitudes (26−32° N), where a satisfactory performance was observed. An annual geographical view of available solar radiation against maximum and minimum temperatures for all the state municipalities is provided to identify the photovoltaic electricity g... [more]

Accurate hysteresis models are necessary for modeling of magnetic components of devices such as transformers and motors. This study presents a hysteresis model with a convenient analytical form, based on hypergeometric functions with one free parameter, built upon a class of parameterized curves. The aim of this work is to explore suitability of the presented model for describing major and minor loops, as well as to demonstrate improved agreement between experimental and modeled hysteresis loops. The procedure for generating first order reversal curves is also discussed. The added parameter, introduced into the model, controls the shape of the model curve, especially near saturation. It can be adjusted to provide better agreement between measured and model curves. The model parameters are nonlinearly dependent; therefore, they are determined in a nonlinear curve fitting procedure. The choice of the initial approximation and a suitable set of constraints for the optimization procedure a... [more]

In this work we study long-term maximization of hydroelectric energy generation from complex multi-purpose reservoir systems, using the reservoir system of the Aliakmon River, Greece, as an application example. This system serves various purposes, like urban water supply, irrigation, hydroelectric energy production, cooling thermoelectric power plants and flood control, while preserving environmental flow. The system operator uses institutional rules for the annual scheduling of the outflows of the 2 largest reservoirs (Ilarion and Polyfyto) for additional safety and smooth distribution of energy production through the year. In this work, we focus on maximization of energy production. We have considered three different hydrological scenarios (dry, average and wet), both for the current and for anticipated future water demand. The multi-reservoir system’s operation was simulated and then optimized using a rather simple form of genetic algorithms, in order to maximize hydro energy produc... [more]

We conducted the Life Cycle Analysis (LCA) of energy production from biogas for maize and three types of wetland biomass: reed Phragmites australis, sedges Carex elata, and Carex gracilis, and “grassy vegetation” of wet meadows (WM). Biogas energy produced from maize reached over 90 GJ ha−1, which was more than four times higher than that gained from wetland biomass. However, an estimation of energy efficiency (EE) calculated as a ratio of energy input to the energy produced in a biogas plant showed that the wet fermentation (WF) of maize was similar to the values obtained for dry fermentation (DF) of sedge biomass (~0.30 GJ GJ−1). The greenhouse gases (GHG) emissions released during preparation of the feedstock and operation of the biogas plant were 150 g CO2 eq. kWhel.−1 for DF of sedges and 262 g CO2 eq. kWhel.−1 for WF of Phragmites. Compared to the prevailing coal-based power generation in Central Europe, anaerobic digestion (AD) of wetland biomass could contribute to a reduction... [more]

Na3Ni2BiO6 with Honeycomb structure suffers from poor cycle stability when applied as cathode material for sodium-ion batteries. Herein, the strategy to improve the stability is to substitute Ni and Bi with inactive Ti. Monoclinic Na3Ni2-xBi1-yTix+yO6 powders with different Ti content were successfully synthesized via sol gel method, and 0.3 mol of Ti was determined as a maximum concentration to obtain a phase-pure compound. A solid-solution in the system of O3-NaNi0.5Ti0.5O2 and O3-Na3Ni2BiO6 is obtained when this critical concentration is not exceeded. The capacity of the first desodiation process at 0.1 C of Na3Ni2BiO6 (~93 mAh g−1) decreases with the increasing Ti concentration to ~77 mAh g−1 for Na3Ni2Bi0.9Ti0.1O6 and to ~82 mAh g−1 for Na3Ni0.9Bi0.8Ti0.3O6, respectively. After 100 cycles at 1 C, a better electrochemical kinetics is obtained for the Ti-containing structures, where a fast diffusion effect of Na+-ions is more pronounced. As a result of in operando synchrotron radiat... [more]

Renewable energy sources will represent the only alternative to limit fossil fuel usage and pollution. For this reason, photovoltaic (PV) power plants represent one of the main systems adopted to produce clean energy. Monitoring the state of health of a system is fundamental. However, these techniques are time demanding, cause stops to the energy generation, and often require laboratory instrumentation, thus being not cost-effective for frequent inspections. Moreover, PV plants are often located in inaccessible places, making any intervention dangerous. In this paper, we propose solAIr, an artificial intelligence system based on deep learning for anomaly cells detection in photovoltaic images obtained from unmanned aerial vehicles equipped with a thermal infrared sensor. The proposed anomaly cells detection system is based on the mask region-based convolutional neural network (Mask R-CNN) architecture, adopted because it simultaneously performs object detection and instance segmentatio... [more]

Energy from hydrogen is an appropriate technological choice in the context of sustainable development. The opportunities offered by the use of energy from hydrogen also represent a significant challenge for mobile technologies and daily life. Nevertheless, despite a significant amount of research and information regarding the benefits of hydrogen energy, it creates considerable controversy in many countries. Globally, there is a lack of understanding about the production process of hydrogen energy and the benefits it provides, which leads to concerns regarding the consistency of its use. In this study, an original questionnaire was used as a research tool to determine the opinions of inhabitants of countries in which hydrogen energy is underutilized and where the infrastructure for hydrogen energy is underdeveloped. Respondents presented their attitude to ecology, and indicated their knowledge regarding the operation of hydrogen energy and the use of hydrogen fuel. The results indicate... [more]

The microscopic alga Coccomyxa subglobosa, collected from the Głowoniowa Nyża Cave (Tatra Mountains, Poland), is a source of fatty acids (FAs) that could be used for biodiesel production. FAs from subaerial algae have unlimited availability because of the ubiquity of algae in nature. Algal culture was carried out under laboratory conditions and algal biomass was measured during growth phase, resulting in 5 g of dry weight (32% oil). The fatty acid methyl ester (FAME) profile was analyzed by means of gas chromatography−mass spectrometry (GC−MS). The presence of lipids and chloroplasts in C. subglobosa was demonstrated using GC−MS and confocal laser microscopy. Naturally occurring FAMEs contained C12−C24 compounds, and methyl palmitate (28.5%) and methyl stearate (45%) were the predominant lipid species. Aerophytic algae could be an important component of biodiesel production, as they are omnipresent and environmentally friendly, contain more methyl esters than seaweed, and can be easily... [more]

This paper analyzes the impact of policy changes on the Romanian renewable energy producers. Attracted by a generous subsidy scheme, foreign and domestic investors flocked to the market. Consequently, the sector witnessed remarkable progress, especially in the wind power category. Romania fast approached the national target set by the European Union concerning the share of the country’s energy consumption from renewable sources. However, frequent changes in the support scheme and in the regulations issued by public authorities led to chaos. The aim of the paper was to emphasize the evolution of renewable energy policy in Romania, to investigate the incentives and their effects, and to critically assess the impact of the changes on renewable energy producers. It highlights, by means of an exploratory study and several interviews with executives of renewable energy companies, the challenges and shortcomings of policymaking. The main finding was that the revision of the subsidy scheme and... [more]

Frequency response analysis is widely used to diagnose transformer winding deformation faults due to its high sensitivity, strong anti-interference capability, and equipment portability, but the results of frequency response analysis can be affected by insulation aging and moisture in the transformer, leading to errors in the diagnosis of winding deformation faults. Currently, there is no effective method to prevent such errors. This paper focuses on optimizing the criterion for diagnosing winding deformations when insulation aging and moisture are present. First, the winding frequency response curves of oil-paper insulation were determined by combining insulation aging and moisture tests of the oil-paper insulation with frequency response simulations of the transformer winding. Next, the winding deformation criterion predicting the likelihood and extent of errors diagnosing transformer winding deformations due to the insulation aging and moisture content is discussed. Finally, the cor... [more]

This paper proposes a particle filter (PF)-based electricity load prediction method to improve the accuracy of the microgrid day-ahead scheduling. While most of the existing prediction methods assume electricity loads follow normal distributions, we consider it is a nonlinear and non-Gaussian process which is closer to the reality. To handle the nonlinear and non-Gaussian characteristics of electricity load profile, the PF-based method is implemented to improve the prediction accuracy. These load predictions are used to provide the microgrid day-ahead scheduling. The impact of load prediction error on the scheduling decision is analyzed based on actual data. Comparison results on a distribution system show that the estimation precision of electricity load based on the PF method is the highest among several conventional intelligent methods such as the Elman neural network (ENN) and support vector machine (SVM). Furthermore, the impact of the different parameter settings are analyzed for... [more]

The assessment of regional bioenergy potentials from different types of natural land cover is an integral part of simulation tools that aim to assess local renewable energy systems. This work introduces a new workflow, which evaluates regional bioenergy potentials and its impact on water demand based on geographical information system (GIS)-based land use data, satellite maps on local crop types and soil types, and conversion factors from biomass to bioenergy. The actual annual biomass yield of crops is assessed through an automated process considering the factors of local climate, crop type, soil, and irrigation. The crop biomass yields are validated with historic statistical data, with deviation less than 7% in most cases. Additionally, the resulting bioenergy potentials yield between 10.7 and 12.0 GWh/ha compared with 13.3 GWh/ha from other studies. The potential contribution from bioenergy on the energy demand were investigated in the two case studies, representing the agricultural... [more]

Metallic interconnects represent the main component of a solid oxide fuel cell (SOFC) stack in terms of weight and volume. They are typically made of ferritic stainless steel (FSS) coated on the air side. At the stack operating conditions, the interconnect is exposed to a dual atmosphere: air at the cathode side; fuel (a hydrogen-rich mixture) at the anode side. The stacks considered in this study were field operated in reformed natural gas for 5000, 9000 and 20,000 h respectively. The analyzed interconnects are made from CROFER22APU and coated on the air side with Co-Mn base spinel. One interconnect has been studied for each stack by sampling and preparing cross section the inlet and outlet positions. The samples were characterized by SEM-EDXS in order to investigate the evolution of the interconnect at the air side. The interaction between the metal substrate and the coating is investigated highlighting the formation of chromia based thermal grown oxide (at the FSS/coating interface)... [more]

The article presents issues related to the possibility of using electric and hybrid systems to drive Small Waterplane Area Twin Hull (SWATH) vessels. Ships of this type have significantly less sway and heave compared to monohull crafts and catamarans. Thanks to the synergistic combination of the hydrodynamic properties of SWATH hull and electric drive systems, they can be an interesting proposition for use in transport of passengers and offshore wind farms service crews. The paper presents comparative test results of an electric drive system powered by Hybrid Energy Storage System, which are a combination of systems consisting of batteries (BAT), hydrogen fuel cells (FC) and diesel generators (D). For the presented configurations of propulsion systems, mathematical models taking into account the hydrodynamic resistance of the hull of the vessel have been developed and implemented in the Modelica simulation environment. The tests carried out for various configurations of the drive syste... [more]

In order to achieve long-distance measurement, a bridge differential inductance detection circuit is employed; on this basis, an automatic zero adjustment technique for sensors using an integral−proportional-integral controller is proposed in this work to achieve consistent product production and efficient installation and debugging, and the mathematical model of the bridge differential inductance detection circuit is established to effectively design the controller parameters. Furthermore, an implementation method for an inductive proximity sensor with an attenuation coefficient of 1 is also proposed based on the bridge differential inductance detection circuit by querying the proximity distance table in the field-programmable gate array (FPGA) to detect multiple target metal objects at the same inductive distance. Simulation and experimental results show that the proposed method is correct and effective.

The increasing deployment of wind power is reducing inertia in power systems. High-voltage direct current (HVDC) technology can help to improve the stability of AC areas in which a frequency response is required. Moreover, multi-terminal DC (MTDC) networks can be optimized to distribute active power to several AC areas by droop control setting schemes that adjust converter control parameters. To this end, in this paper, particle swarm optimization (PSO) is used to improve the primary frequency response in AC areas considering several grid limitations and constraints. The frequency control uses an optimization process that minimizes the frequency nadir and the settling time in the primary frequency response. Secondly, another layer is proposed for the redistribution of active power among several AC areas, if required, without reserving wind power capacity. This method takes advantage of the MTDC topology and considers the grid code limitations at the same time. Two scenarios are defined... [more]

Thermostatically controlled loads (TCLs) can effectively support network operation through their intrinsic flexibility and play a pivotal role in delivering cost effective decarbonization. This paper proposes a scalable distributed solution for the operation of large populations of TCLs providing frequency response and performing energy arbitrage. Each TCL is described as a price-responsive rational agent that participates in an integrated energy/frequency response market and schedules its operation in order to minimize its energy costs and maximize the revenues from frequency response provision. A mean field game formulation is used to implement a compact description of the interactions between typical power system characteristics and TCLs flexibility properties. In order to accommodate the heterogeneity of the thermostatic loads into the mean field equations, the whole population of TCLs is clustered into smaller subsets of devices with similar properties, using k-means clustering te... [more]

Photovoltaic-thermal panels are hybrid systems that combine the two types of conventional solar energy technologies (photovoltaic and thermal panels) and simultaneously generate both thermal and electrical energy in a micro-cogeneration system. Like any co-generation system, there is an optimal balance that can be achieved between the thermal and electrical energy produced. For this reason, it is important to establish the relationship and inter-connection between the two. Limited research is available on the cogeneration interaction in a PVT system, so the novelty of this article lies in the consideration of the entire energy system connected to the PVT panel, including the storage tank and the consumer demand curve, and the investigation of the thermal parametric variation. This study analyses the impact of the variation of some thermal parameters of a domestic hot water tank on the electrical efficiency of a photovoltaic-thermal panel. A model of a system of photovoltaic-thermal pan... [more]

This paper investigates the feasibility of using the three-dimensional omnidirectional inductive channel for power transfer and as a power line communication (PLC) for ground-based vehicle, electric air vehicle, or space applications. The simulation results were performed by the advanced design system software using lumped equivalent circuit model. The power transfer efficiency was determined based on multiport scattering (S)-parameters numerical simulation results while the theoretical channel capacity was calculated based on Matlab software as a function of the coupling coefficient considering an additive white Gaussian noise. Furthermore, the magnetic field distribution was evaluated as function of the misalignment angle θ between the receiver and the three orthogonal transmitters coils.