Next Issue
Volume 13, July-2
Previous Issue
Volume 13, June-2

Table of Contents

Energies, Volume 13, Issue 13 (July-1 2020) – 214 articles

  • Issues are regarded as officially published after their release is announced to the table of contents alert mailing list.
  • You may sign up for e-mail alerts to receive table of contents of newly released issues.
  • PDF is the official format for papers published in both, html and pdf forms. To view the papers in pdf format, click on the "PDF Full-text" link, and use the free Adobe Readerexternal link to open them.
Cover Story (view full-size image) Lab testing of integrated lighting system retrofits found energy savings ranging from 20% (daylight [...] Read more.
Order results
Result details
Select all
Export citation of selected articles as:
Open AccessFeature PaperArticle
Optimal Sizing of Fuel Cell Hybrid Power Sources with Reliability Consideration
Energies 2020, 13(13), 3510; https://doi.org/10.3390/en13133510 - 07 Jul 2020
Viewed by 273
Abstract
This paper addresses the issue of optimal sizing reliability applied to a fuel cell/battery hybrid system. This specific problem raises the global problem of strong coupling between hardware and control parameters. To tackle this matter, the proposed methodology uses nested optimization loops. Furthermore, [...] Read more.
This paper addresses the issue of optimal sizing reliability applied to a fuel cell/battery hybrid system. This specific problem raises the global problem of strong coupling between hardware and control parameters. To tackle this matter, the proposed methodology uses nested optimization loops. Furthermore, to increase the optimal design relevance, a reliability assessment of the optimal sizing set is introduced. This new paradigm enables showing the early impact of the reliability criteria on design choices regarding energetic performance index. It leads to a smart design methodology permitting to avoid complexity and save computing time. It considerably helps design engineers set up the best hybridization rate and enables practicing tradeoffs, including reliability aspects in the early design stages. Full article
Show Figures

Graphical abstract

Open AccessArticle
Hydrogen Storage Properties of Mg-Ni Alloys Processed by Fast Forging
Energies 2020, 13(13), 3509; https://doi.org/10.3390/en13133509 - 07 Jul 2020
Viewed by 234
Abstract
Fast forging of compacts made up of Mg and Ni powders is shown to be an effective method to induce severe plastic deformation with improved H2 sorption properties. Here, using such processed samples, a comprehensive analysis of the sorption properties reveals that [...] Read more.
Fast forging of compacts made up of Mg and Ni powders is shown to be an effective method to induce severe plastic deformation with improved H2 sorption properties. Here, using such processed samples, a comprehensive analysis of the sorption properties reveals that the first hydrogenation sequence significantly depends on the forging temperature, through different microstructures. More in detail, no phase transformation occurs upon cold forging, while solid-state reaction leads to the formation of the Mg2Ni intermetallic compound upon forging above 400 °C. Forging below the brittle-to-ductile transition (225–250 °C) leads to faster H2 uptake upon first absorption owing to a more textured fiber along the c-axis and internal strains which promote hydrogen diffusion through the bulk material. Desorption kinetics remain slower with low-temperature forging, despite Ni recombining to form Mg2Ni during the first desorption. After several cycles, a two-step behavior is observed with a fast absorption step occurring up to about 3 wt.%. Despite this limited uptake performance, the forging process can be considered as a straightforward, safe, and cost-efficient process to produce large amounts of Mg-based alloys for hydrogen storage. In particular, such severe plastic deformation processes can be considered as reliable substitutes for ball-milling, which is highly efficient but energy- and time-consuming. Full article
(This article belongs to the Special Issue Hydrogen Storage Properties of Materials)
Show Figures

Graphical abstract

Open AccessFeature PaperArticle
Economic Management Based on Hybrid MPC for Microgrids: A Brazilian Energy Market Solution
Energies 2020, 13(13), 3508; https://doi.org/10.3390/en13133508 - 07 Jul 2020
Viewed by 242
Abstract
This paper proposes a microgrid central controller (MGCC) solution to the energy management problem of a renewable energy-based microgrid (MG). This MG is a case study from the Brazilian energy market context and, thus, has some operational particularities and rules to be obeyed. [...] Read more.
This paper proposes a microgrid central controller (MGCC) solution to the energy management problem of a renewable energy-based microgrid (MG). This MG is a case study from the Brazilian energy market context and, thus, has some operational particularities and rules to be obeyed. The MGCC development was based on a hybrid model predictive control (HMPC) strategy using the mixed logical dynamic (MLD) approach to deal with logical constraints within the HMPC structure, which results in a mixed integer programming (MIP) problem. The development of the solution is done through economic and dynamic modeling of the MG components; furthermore, it also takes into account the energy compensation rules of the Brazilian energy market and the white energy tariff. These conditions are specified through a set of MLD constraints. The effectiveness and performance of the proposed solution are evaluated through high-fidelity numerical simulation. Full article
(This article belongs to the Special Issue Model Predictive Control for Energy Management in Microgrids)
Show Figures

Graphical abstract

Open AccessArticle
Extending the Application of the Smart Readiness Indicator—A Methodology for the Quantitative Assessment of the Load Shifting Potential of Smart Districts
Energies 2020, 13(13), 3507; https://doi.org/10.3390/en13133507 - 07 Jul 2020
Viewed by 213
Abstract
In 2018, the revised Energy Performance of Buildings Directive (EPBD) included for the first time the application of a smart readiness indicator (SRI). Based on the fact that load shifting in and across buildings plays an increasingly important role to improve efficiency and [...] Read more.
In 2018, the revised Energy Performance of Buildings Directive (EPBD) included for the first time the application of a smart readiness indicator (SRI). Based on the fact that load shifting in and across buildings plays an increasingly important role to improve efficiency and alleviate the integration of renewable energy systems, the SRI is also aimed at providing an indication of how well buildings can interact with the energy grids. With the clustering of buildings into larger entities, synergies related to the integration of renewable energy and load shifting can be efficiently exploited. However, current proposals for the SRI focus mainly on qualitative appraisals of the smartness of buildings and do not include the wider context of the districts. Quantitative approaches that can be easily applied at an early planning stage are still mostly missing. To optimize infrastructure decisions on a larger scale, a quantifiable perspective beyond the building level is necessary to evaluate and leverage the larger load shifting capacities. This article builds on a previously published methodology for smart buildings with the aim to provide a numerical model-based approach on the assessment of whole districts based on their overall energy storage capacity, load shifting potential and their ability to actively interact with the energy grids. It also delivers the equivalent CO2 savings potential compared to a non-interactive system. The methodology is applied to theoretical use cases for validation. The results highlight that the proposed quantitative model can provide a meaningful and objective assessment of the load shifting potentials of smart districts. Full article
(This article belongs to the Special Issue Evaluation of Energy Efficiency and Flexibility in Smart Buildings)
Show Figures

Figure 1

Open AccessArticle
Frequency Stability Evaluation in Low Inertia Systems Utilizing Smart Hierarchical Controllers
Energies 2020, 13(13), 3506; https://doi.org/10.3390/en13133506 - 07 Jul 2020
Viewed by 228
Abstract
The high penetration of the Renewable Energy Sources and other emerging technologies likely to be installed in future power grids will pose new operational challenges to grid operators. One of the main issues expected to affect the operation of the power grid is [...] Read more.
The high penetration of the Renewable Energy Sources and other emerging technologies likely to be installed in future power grids will pose new operational challenges to grid operators. One of the main issues expected to affect the operation of the power grid is the impact of inverter-based technologies to the power system inertia and, hence, to system stability. Consequently, the main challenge of the future grid is the evaluation of the frequency stability in the presence of inverter-based systems and how the aforementioned technology can support frequency stability without the help of the rotating masses of the traditional power grid systems. To assess the above problem, this paper proposes a methodology to evaluate the frequency stability in a projection of the real distribution grid in Cyprus with the time horizon to be the year 2030. The power grid under investigation is evaluated with and without the presence of smart hierarchical controllers for providing support to the power system under disturbance conditions. The advanced controllers were applied to manage the available power resource in a fast and effective manner to maintain frequency within nominal levels. The controllers have been implemented in two hierarchical levels revealing useful responses for managing low-inertia networks. The first is set to act locally within a preselected area and the second level effectively supporting the different areas for optimal operation. After undertaking a significant number of simulations for time-series of one year, it was concluded from the results that the local control approach manages to minimize the frequency excursion effectively and influence all related attributes including the rate of change of frequency (RoCoF), frequency nadir and frequency zenith. Full article
(This article belongs to the Special Issue Smart Photovoltaic Energy Systems for a Sustainable Future)
Show Figures

Graphical abstract

Open AccessArticle
Analysis of the Influence of Diesel Spray Injection on the Ignition and Soot Formation in Multiple Injection Strategy
Energies 2020, 13(13), 3505; https://doi.org/10.3390/en13133505 - 07 Jul 2020
Viewed by 226
Abstract
Multiple injection strategies have increased their capabilities along with the evolution of injection system technologies up to the point that nowadays it is possible to inject eight different pulses, permitting to improve the engine performance, and consequently, emissions. The present work was realized [...] Read more.
Multiple injection strategies have increased their capabilities along with the evolution of injection system technologies up to the point that nowadays it is possible to inject eight different pulses, permitting to improve the engine performance, and consequently, emissions. The present work was realized for two simplified strategies: a pilot-main and a main-post, in order to analyze the influence of an auxiliary pulse on the main and otherwise, in reactive conditions for two pilot/post quantities and four hydraulic dwell times. The study was carried out by employing two optical techniques: diffused back-illumination with flame bandpass chemiluminescence for measuring soot, represented by soot-maps distribution, and single-pass schlieren for ignition delay (ID). Furthermore, a novel methodology for decoupling the start of combustion (SOC) of the second pulse was developed and successfully validated. From the ignition delay results, it was found for all test points that the pilot injection enhanced conditions, which promote a faster ignition of the main pulse, also at higher chamber temperatures, all conditions presented a separate combustion event for each injection. In emission terms, soot increased in the pilot-main strategies compared to its single injection case, as well as, in conditions that promote faster-premixed combustion. Full article
Show Figures

Graphical abstract

Open AccessArticle
Investigative Coupled Thermo-Hydro-Mechanical Modelling Approach for Geothermal Heat Extraction through Multistage Hydraulic Fracturing from Hot Geothermal Sedimentary Systems
Energies 2020, 13(13), 3504; https://doi.org/10.3390/en13133504 - 07 Jul 2020
Viewed by 195
Abstract
The meaningful utilization of artificially created multiple fractures in tight formations is associated with the performance behavior of such flow channels, especially in the case of thermal energy extraction from sedimentary geothermal system. In this study, an innovative idea is presented to develop [...] Read more.
The meaningful utilization of artificially created multiple fractures in tight formations is associated with the performance behavior of such flow channels, especially in the case of thermal energy extraction from sedimentary geothermal system. In this study, an innovative idea is presented to develop a numerical model for geothermal energy production based on concrete physical performance of an artificially created tensile multi-fracture system in a simplified manner. The state-of-the-art software FLAC3Dplus-TOUGH2MP-TMVOC are integrated to develop a coupled thermo-hydro-mechanical (THM) fictive model for constructing a multi-fracture scheme and estimating heat extraction performance. By incorporating the actual fracture width of newly created subsequent fracture under the effect of stress shadow, cubic law is implemented for fluid flow and geothermal energy production. The results depict that fracture spacing plays a vital role in the energy contribution through multiple fractures. Afterwards, a field case study to design huge multiple hydraulic fractures was performed in the geothermal well GB X1 in North Germany. The attenuation of fracture propagation becomes more significant when massive multiple fracturing operation is performed especially in the case of lower fracture spacing. The fictive model results will be extended to study the geothermal utilization of the North German basin through massive multiple fractures in our future work. Full article
(This article belongs to the Section Geo-Energy)
Show Figures

Figure 1

Open AccessReview
Design of Nanomaterials for Hydrogen Storage
Energies 2020, 13(13), 3503; https://doi.org/10.3390/en13133503 - 07 Jul 2020
Viewed by 236
Abstract
The interaction of hydrogen with solids and the mechanisms of hydride formation experience significant changes in nanomaterials due to a number of structural features. This review aims at illustrating the design principles that have recently inspired the development of new nanomaterials for hydrogen [...] Read more.
The interaction of hydrogen with solids and the mechanisms of hydride formation experience significant changes in nanomaterials due to a number of structural features. This review aims at illustrating the design principles that have recently inspired the development of new nanomaterials for hydrogen storage. After a general discussion about the influence of nanomaterials’ microstructure on their hydrogen sorption properties, several scientific cases and hot topics are illustrated surveying various classes of materials. These include bulk-like nanomaterials processed by mechanochemical routes, thin films and multilayers, nano-objects with composite architectures such as core–shell or composite nanoparticles, and nanoparticles on porous or graphene-like supports. Finally, selected examples of recent in situ studies of metal–hydride transformation mechanisms using microscopy and spectroscopy techniques are highlighted. Full article
(This article belongs to the Special Issue Design of Materials for Solid State Hydrogen Storage)
Show Figures

Graphical abstract

Open AccessArticle
Numerical Study on the Gravity Effect on Heat Transfer of Supercritical CO2 in a Vertical Tube
Energies 2020, 13(13), 3502; https://doi.org/10.3390/en13133502 - 07 Jul 2020
Viewed by 214
Abstract
The effects of gravity on the heat transfer performance of supercritical CO2 flowing within a vertical tube with a diameter of 4.75 mm are numerically studied in this paper. The main objectives are to comprehensively investigate the action of gravity and buoyancy [...] Read more.
The effects of gravity on the heat transfer performance of supercritical CO2 flowing within a vertical tube with a diameter of 4.75 mm are numerically studied in this paper. The main objectives are to comprehensively investigate the action of gravity and buoyancy on the supercritical heat transfer. An effective numerical method, which employs a modified Shear Stress Transfer k-ω model (SST k-ω), is applied at various gravity conditions. It is found that, for both upward and downward flows, the heat transfer of supercritical CO2 is improved with increased gravity magnitude. The effect of gravity on heat transfer are more pronounced under a low mass flux condition than that under a high mass flux condition and it is closely related to the variations of thermal properties. For the upward flow, the increased gravity magnitude accelerates the near wall fluid and creates a classic “M-shaped” radial velocity distribution. For the downward flow, the increased gravity magnitude decelerates the near wall fluid and creates a parabola-like radial velocity distribution. On one hand, the turbulent kinetic energies of both the upward and downward flows are enhanced as the gravity magnitude increases, which benefits heat transfer dominated by turbulent eddy diffusion. On the other hand, high-density fluid with high thermal conductivity occupies the near wall region as the gravity magnitude increases, which benefits heat transfer dominated by molecular diffusion. The results might provide some instructive advice to improve the design and operation safety of heat exchanger at various gravity conditions. Full article
Show Figures

Figure 1

Open AccessArticle
Enabling Methodologies for Predictive Power System Resilience Analysis in the Presence of Extreme Wind Gusts
Energies 2020, 13(13), 3501; https://doi.org/10.3390/en13133501 - 07 Jul 2020
Viewed by 215
Abstract
Modern power system operation should comply with strictly reliability and security constraints, which aim at guarantee the correct system operation also in the presence of severe internal and external disturbances. Amongst the possible phenomena perturbing correct system operation, the predictive assessment of the [...] Read more.
Modern power system operation should comply with strictly reliability and security constraints, which aim at guarantee the correct system operation also in the presence of severe internal and external disturbances. Amongst the possible phenomena perturbing correct system operation, the predictive assessment of the impacts induced by extreme weather events has been considered as one of the most critical issues to address, since they can induce multiple, and large-scale system contingencies. In this context, the development of new computing paradigms for resilience analysis has been recognized as a very promising research direction. To address this issue, this paper proposes two methodologies, which are based on Time Varying Markov Chain and Dynamic Bayesian Network, for assessing the system resilience against extreme wind gusts. The main difference between the proposed methodologies and the traditional solution techniques is the improved capability in modelling the occurrence of multiple component faults and repairing, which cannot be neglected in the presence of extreme events, as experienced worldwide by several Transmission System Operators. Several cases studies and benchmark comparisons are presented and discussed in order to demonstrate the effectiveness of the proposed methods in the task of assessing the power system resilience in realistic operation scenarios. Full article
(This article belongs to the Special Issue Data Mining in Smart Grids)
Show Figures

Figure 1

Open AccessArticle
Smart Energy Management of Residential Microgrid System by a Novel Hybrid MGWOSCACSA Algorithm
Energies 2020, 13(13), 3500; https://doi.org/10.3390/en13133500 - 07 Jul 2020
Viewed by 222
Abstract
Optimal scheduling of distributed energy resources (DERs) of a low-voltage utility-connected microgrid system is studied in this paper. DERs include both dispatchable fossil-fueled generators and non-dispatchable renewable energy resources. Various real constraints associated with adjustable loads, charging/discharging limitations of battery, and the start-up/shut-down [...] Read more.
Optimal scheduling of distributed energy resources (DERs) of a low-voltage utility-connected microgrid system is studied in this paper. DERs include both dispatchable fossil-fueled generators and non-dispatchable renewable energy resources. Various real constraints associated with adjustable loads, charging/discharging limitations of battery, and the start-up/shut-down time of the dispatchable DERs are considered during the scheduling process. Adjustable loads are assumed to the residential loads which either operates throughout the day or for a particular period during the day. The impact of these loads on the generation cost of the microgrid system is studied. A novel hybrid approach considers the grey wolf optimizer (GWO), sine cosine algorithm (SCA), and crow search algorithm (CSA) to minimize the overall generation cost of the microgrid system. It has been found that the generation costs rise 50% when the residential loads were included along with the fixed loads. Active participation of the utility incurred 9–17% savings in the system generation cost compared to the cases when the microgrid was operating in islanded mode. Finally, statistical analysis has been employed to validate the proposed hybrid Modified Grey Wolf Optimization-Sine Cosine Algorithm-Crow Search Algorithm (MGWOSCACSA) over other algorithms used. Full article
(This article belongs to the Special Issue Future Maintenance Management in Renewable Energies)
Show Figures

Graphical abstract

Open AccessArticle
Improving Fuel Economy and Engine Performance through Gasoline Fuel Octane Rating
Energies 2020, 13(13), 3499; https://doi.org/10.3390/en13133499 - 07 Jul 2020
Viewed by 205
Abstract
The octane number is a measure of the resistance of gasoline fuels to auto-ignition. Therefore, high octane numbers reduce the engine knocking risk, leading to higher compression threshold and, consequently, higher engine efficiencies. This allows higher compression ratios to be considered during the [...] Read more.
The octane number is a measure of the resistance of gasoline fuels to auto-ignition. Therefore, high octane numbers reduce the engine knocking risk, leading to higher compression threshold and, consequently, higher engine efficiencies. This allows higher compression ratios to be considered during the engine design stage. Current spark-ignited (SI) engines use knock sensors to protect the engine from knocking, usually adapting the operation parameters (boost pressure, spark timing, lambda). Moreover, some engines can move the settings towards optimized parameters if knock is not detected, leading to higher performance and fuel economy. In this work, three gasolines with different octane ratings (95, 98 and 100 RON (research octane number)) were fueled in a high-performance vehicle. Tests were performed in a chassis dyno at controlled ambient conditions, including a driving sequence composed of full-load accelerations and two steady-state modes. Vehicle power significantly increased with the octane rating of the fuel, thus decreasing the time needed for acceleration. Moreover, the specific fuel consumption decreased as the octane rating increased, proving that the fuel can take an active part in reducing greenhouse gas emissions. The boost pressure, which increased with the octane number, was identified as the main factor, whereas the ignition advance was the second relevant factor. Full article
(This article belongs to the Special Issue New Trends on the Combustion Processes in Spark Ignition Engines)
Show Figures

Figure 1

Open AccessArticle
Absorption of Cu(II) and Zn(II) from Aqueous Solutions onto Biochars Derived from Apple Tree Branches
Energies 2020, 13(13), 3498; https://doi.org/10.3390/en13133498 - 07 Jul 2020
Viewed by 191
Abstract
The aim of this study was to investigate the adsorption of Cu(II) and Zn(II) onto apple tree branches biochar (BC) produced at 300, 400, 500 and 600 °C (BC300, BC400, BC500, and BC600), respectively. The effect of adsorbent dosage, pH value, contact time, [...] Read more.
The aim of this study was to investigate the adsorption of Cu(II) and Zn(II) onto apple tree branches biochar (BC) produced at 300, 400, 500 and 600 °C (BC300, BC400, BC500, and BC600), respectively. The effect of adsorbent dosage, pH value, contact time, initial concentration of Cu(II) or Zn(II), and temperature on the adsorption process were investigated. The result showed that 5 g BC·L−1 was the optimal dosage to remove Cu(II) and Zn(II) from wastewater and the maximum adsorption efficiency was achieved at a pH of 5.0 for all the BCs when the initial concentration of Cu(II) and Zn(II) were 64 and 65 mg L−1, respectively. Adsorption kinetics and isotherm experiments showed that the pseudo-second order equation and the Langmuir isotherm could best describe the adsorption process, indicating that the adsorption of Cu(II) and Zn(II) onto BCs were monolayer processes and chemisorption was the rate limiting step. The values of ΔG0 for the absorption of Cu(II) and Zn(II) on all BCs were negative, while the values of ΔH0 were positive, suggesting that the absorption was a spontaneous endothermic process. The mechanisms of BC adsorption of metal ions adsorption include surface precipitation, ion exchange, and minor contribution by cation-π interaction. BC500 had highest Cu(II) and Zn(II) adsorption capacity under various conditions (except at pH 2.0). The maximum adsorption capacities of Cu(II) and Zn(II) on BC500 were 11.41 and 10.22 mg·g−1, respectively. Therefore, BC derived from apple tree branches produced at 500 °C can be used as an adsorbent to remove Cu(II) and Zn(II) from wastewater. Full article
(This article belongs to the Special Issue Wastewater Treatment and Resource Recovery)
Show Figures

Figure 1

Open AccessArticle
Buildings Energy Efficiency Analysis and Classification Using Various Machine Learning Technique Classifiers
Energies 2020, 13(13), 3497; https://doi.org/10.3390/en13133497 - 07 Jul 2020
Viewed by 247
Abstract
Energy efficiency is a major concern to achieve sustainability in modern society. Smart cities sustainability depends on the availability of energy-efficient infrastructures and services. Buildings compose most of the city, and they are responsible for most of the energy consumption and emissions to [...] Read more.
Energy efficiency is a major concern to achieve sustainability in modern society. Smart cities sustainability depends on the availability of energy-efficient infrastructures and services. Buildings compose most of the city, and they are responsible for most of the energy consumption and emissions to the atmosphere (40%). Smart cities need smart buildings to achieve sustainability goals. Building’s thermal modeling is essential to face the energy efficiency race. In this paper, we show how ICT and data science technologies and techniques can be applied to evaluate the energy efficiency of buildings. In concrete, we apply machine learning techniques to classify buildings based on their energy efficiency. Particularly, our focus is on single-family buildings in residential areas. Along this paper, we demonstrate the capabilities of machine learning techniques to classify buildings depending on their energy efficiency. Moreover, we analyze and compare the performance of different classifiers. Furthermore, we introduce new parameters which have some impact on the buildings thermal modeling, especially those concerning the environment where the building is located. We also make an insight on ICT and remark the growing relevance in data acquisition and monitoring of relevant parameters by using wireless sensor networks. It is worthy to remark the need for an appropriate and reliable dataset to achieve the best results. Moreover, we demonstrate that reliable classification is feasible with a few featured parameters. Full article
Show Figures

Graphical abstract

Open AccessArticle
Detection of Partial Demagnetization Faults in Five-Phase Permanent Magnet Assisted Synchronous Reluctance Machines
Energies 2020, 13(13), 3496; https://doi.org/10.3390/en13133496 - 06 Jul 2020
Viewed by 279
Abstract
This paper analyzes partial demagnetization faults in a five-phase permanent magnet assisted synchronous reluctance motor (fPMa-SynRM) incorporating ferrite permanent magnets (PMs). These faults are relevant because of the application of field weakening, or due to high operating temperatures or short circuit currents, the [...] Read more.
This paper analyzes partial demagnetization faults in a five-phase permanent magnet assisted synchronous reluctance motor (fPMa-SynRM) incorporating ferrite permanent magnets (PMs). These faults are relevant because of the application of field weakening, or due to high operating temperatures or short circuit currents, the PMs can become irreversibly demagnetized, thus affecting the performance and safe operation of the machine. This paper proposes fault indicators to detect such fault modes with low demagnetization levels between 5.0% to 16.7% relative demagnetization. Four partial demagnetization fault detection methods are tested, which are based on the analysis of the harmonic content of the electromotive force (EMF) under no load conditions, the harmonic content of the line currents, the harmonic content of the zero-sequence voltage component (ZSVC) and the analysis of the power factor (PF). This work also compares the sensitivity and performance of the proposed detection methods. According to the fault indicators proposed in this paper, the results show that the analysis of the EMF, ZSVC and PF are the most sensitive detection methods. Experimental results are presented to validate finite element analysis (FEA) simulations. Full article
(This article belongs to the Special Issue Electrical Machine Design 2020)
Show Figures

Graphical abstract

Open AccessArticle
Location and Sizing of Micro-Grids to Improve Continuity of Supply in Radial Distribution Networks
Energies 2020, 13(13), 3495; https://doi.org/10.3390/en13133495 - 06 Jul 2020
Viewed by 281
Abstract
The steady decline in the prices of distributed energy resources (DERs), such as distributed renewable generation and storage systems, together with more sophisticated monitoring and control strategies allow power distribution companies to enhance the performance of the distribution network, for instance improving voltage [...] Read more.
The steady decline in the prices of distributed energy resources (DERs), such as distributed renewable generation and storage systems, together with more sophisticated monitoring and control strategies allow power distribution companies to enhance the performance of the distribution network, for instance improving voltage control, congestion management, or reliability. The latter will be the subject of this paper. This paper addresses the improvement of continuity of supply in radial distribution grids in rural areas, where traditional reinforcements cannot be carried out because they are located in secluded areas or in naturally protected zones, where the permits to build new lines are difficult to obtain. When a contingency occurs in such a feeder, protection systems isolate it, and all downstream users suffer an interruption until the service is restored. This paper proposes a novel methodology to determine the optimal location and size of micro-grid systems (MGs) used to reduce non-served energy, considering reliability and investment costs. The proposed model additionally determines the most suitable combination of DER technologies. The resulting set of MGs would be used to supply consumers located in the isolated area while the upstream fault is being repaired. The proposed methodology is validated through its application to a case study of an actual rural feeder which suffers from reliability issues due to the difficulties in obtaining the necessary permissions to undertake conventional grid reinforcements. Full article
Show Figures

Figure 1

Open AccessArticle
Distributed Machine Learning on Dynamic Power System Data Features to Improve Resiliency for the Purpose of Self-Healing
Energies 2020, 13(13), 3494; https://doi.org/10.3390/en13133494 - 06 Jul 2020
Viewed by 307
Abstract
Numerous online methods for post-fault restoration have been tested on different types of systems. Modern power systems are usually operated at design limits and therefore more prone to post-fault instability. However, traditional online methods often struggle to accurately identify events from time series [...] Read more.
Numerous online methods for post-fault restoration have been tested on different types of systems. Modern power systems are usually operated at design limits and therefore more prone to post-fault instability. However, traditional online methods often struggle to accurately identify events from time series data, as pattern-recognition in a stochastic post-fault dynamic scenario requires fast and accurate fault identification in order to safely restore the system. One of the most prominent methods of pattern-recognition is machine learning. However, machine learning alone is neither sufficient nor accurate enough for making decisions with time series data. This article analyses the application of feature selection to assist a machine learning algorithm to make better decisions in order to restore a multi-machine network which has become islanded due to faults. Within an islanded multi-machine system the number of attributes significantly increases, which makes application of machine learning algorithms even more erroneous. This article contributes by proposing a distributed offline-online architecture. The proposal explores the potential of introducing relevant features from a reduced time series data set, in order to accurately identify dynamic events occurring in different islands simultaneously. The identification of events helps the decision making process more accurate. Full article
(This article belongs to the Special Issue AI Applications to Power Systems)
Show Figures

Graphical abstract

Open AccessArticle
Very Short-Term Power Forecasting of High Concentrator Photovoltaic Power Facility by Implementing Artificial Neural Network
Energies 2020, 13(13), 3493; https://doi.org/10.3390/en13133493 - 06 Jul 2020
Viewed by 304
Abstract
Concentrator photovoltaic (CPV) is used to obtain cheaper and more stable renewable energy. Methods which predict the energy production of a power system under specific circumstances are highly important to reach the goal of using this system as a part of a bigger [...] Read more.
Concentrator photovoltaic (CPV) is used to obtain cheaper and more stable renewable energy. Methods which predict the energy production of a power system under specific circumstances are highly important to reach the goal of using this system as a part of a bigger one or of making it integrated with the grid. In this paper, the development of a model to predict the energy of a High CPV (HCPV) system using an Artificial Neural Network (ANN) is described. This system is located at the University of Rabat. The performed experiments show a quick prediction with encouraging results for a very short-term prediction horizon, considering the small amount of data available. These conclusions are based on the processes of obtaining the ANN models and detailed discussion of the results, which have been validated using real data. Full article
Show Figures

Graphical abstract

Open AccessArticle
A Study of Heat Exchange Processes within the Channels of Disk Pulse Devices
Energies 2020, 13(13), 3492; https://doi.org/10.3390/en13133492 - 06 Jul 2020
Viewed by 246
Abstract
The effect of basic parameters of the channels of disk pulse devices on the heat exchange efficiency was studied both analytically and experimentally, especially in terms of pulse acting on the heat carrier. A methodology to determine the main parameters, namely the pressure [...] Read more.
The effect of basic parameters of the channels of disk pulse devices on the heat exchange efficiency was studied both analytically and experimentally, especially in terms of pulse acting on the heat carrier. A methodology to determine the main parameters, namely the pressure and the temperature of the heat carrier as well as the pulse effect on the fluid, was proposed. The mathematical models of the effect of the structural and technological parameters of the channels in the disk pulse device on the heat exchange efficiency were developed. The models’ adequacy was proved based on a series of experimental studies involving devices with one-stage and multistage systems of pulsed heat carrier processing. This enabled the development, testing, and implementation of practical construction designs of pulse disk heat generators for decentralized heating of commercial and domestic buildings with one-stage and multistage systems of pulsed heat carrier processing. Taking into account the results of the mathematical modeling, the developed method of multistage pulse action was proved experimentally and implemented in regard to the structural design of a working chamber of the disk pulse heat generator. An efficient geometry of the working chamber of the disk pulse heat generator was specified for its further integration into the system of decentralized heat supply. One of the developed heat generators with the multistage pulse action on the heat carrier was integrated into the heating system of a greenhouse complex with a 0.86–0.9 efficiency coefficient. Full article
Show Figures

Figure 1

Open AccessArticle
Comparison of Control Strategies to Realize Synthetic Inertia in Converters
Energies 2020, 13(13), 3491; https://doi.org/10.3390/en13133491 - 06 Jul 2020
Viewed by 280
Abstract
The increasing amount of renewable energy sources in the electrical energy system leads to an increasing number of converter-based generators connected to the electrical power grid. Other than conventional power plants that are often connected to the grid via synchronous generators, converter-based generators [...] Read more.
The increasing amount of renewable energy sources in the electrical energy system leads to an increasing number of converter-based generators connected to the electrical power grid. Other than conventional power plants that are often connected to the grid via synchronous generators, converter-based generators do not provide mechanical inertia intrinsically. Therefore, ensuring frequency stability in the electrical power grid might become even more difficult in the future. With the concept of synthetic inertia, the converter-based generators partially imitate the behavior of conventional generators. By implementing such a concept in converters, they are capable of contributing to frequency stability as well. This paper compares two strategies to realize synthetic inertia by modeling converter-based generators in MATLAB/SIMULINK and simulating their behavior in a small Microgrid. The results prove that any kind of realization of synthetic inertia helps to improve frequency stability. Each of the two investigated strategies may have their scope of application in a future electrical energy system. Full article
(This article belongs to the Special Issue Advanced Control Applications for Electrical Energy Systems)
Show Figures

Figure 1

Open AccessArticle
A Review of the Power Converter Interfaces for Switched Reluctance Machines
Energies 2020, 13(13), 3490; https://doi.org/10.3390/en13133490 - 06 Jul 2020
Viewed by 230
Abstract
The use of power electronic converters is essential for the operation of Switched Reluctance Machines (SRMs). Many topologies and structures have been developed over the last years considering several specific applications for this kind of machine, improving the control strategies, performance range, fault-tolerant [...] Read more.
The use of power electronic converters is essential for the operation of Switched Reluctance Machines (SRMs). Many topologies and structures have been developed over the last years considering several specific applications for this kind of machine, improving the control strategies, performance range, fault-tolerant operation, among other aspects. Thus, due to the great importance of power electronic converters in such applications, this paper is focused on a detailed review of main structures and topologies for SRM drives. The proposed study is not limited to the classic two-level power converters topologies dedicated to the SRMs; it also presents a review about recent approaches, such as multilevel topologies and based on impedance source network. Moreover, this review is also focused on a new class of topologies associated to these machines, namely the ones with fault-tolerant capability. This new category of topologies has been a topic of research in recent years, being currently considered an area of great interest for future research work. An analysis, taking into consideration the main features of each structure and topology, was addressed in this review. A classification and comparison of the several structures and topologies for each kind of converter, considering modularity, boost capability, number of necessary switches and phases, integration in the machine design, control complexity, available voltage levels and fault-tolerant capability to different failure modes, is also presented. In this way, this review also includes a description of the presented solutions taking into consideration the reliability of the SRM drive. Full article
(This article belongs to the Section Smart Grids and Microgrids)
Show Figures

Figure 1

Open AccessArticle
The Economic and Geographical Aspects of the Status of Small-Scale Photovoltaic Systems in Hungary—A Case Study
Energies 2020, 13(13), 3489; https://doi.org/10.3390/en13133489 - 06 Jul 2020
Viewed by 251
Abstract
The use of solar energy is an obvious choice; the energy of the sun is not only indispensable for most processes in nature but it is also a clean, abundant, sustainable, and—most importantly—universally available resource. Although the further spread of photovoltaic systems, which [...] Read more.
The use of solar energy is an obvious choice; the energy of the sun is not only indispensable for most processes in nature but it is also a clean, abundant, sustainable, and—most importantly—universally available resource. Although the further spread of photovoltaic systems, which make use of this source of energy, is expected in the future all around the world, no comprehensive investigation has been conducted into the current situation of the small-scale photovoltaic power plants in Hungary, where this type of photovoltaic system is the most popular. By means of a case study, whose novelty lies in its focus on small-scale power plants and their complex examination, including economic and geographic indicators, this paper analyzes their status in Hungary. The study endeavors to establish the reasons for the popularity of this type of power plant and to identify some typical geographical locations with well-illustrated photovoltaic density. Residential, as well as business prosumers, were examined with the aim of learning more about the density of the small-scale photovoltaic systems and their geographical locations. Another goal was to calculate the average size of small-scale photovoltaic power plants and to gain more understanding of their economic aspects. The outcomes of this research include maps displaying the density of the small-scale photovoltaic power plants in Hungary and the results of the economic calculations for such investments. Full article
(This article belongs to the Special Issue Smart Photovoltaic Energy Systems for a Sustainable Future)
Show Figures

Figure 1

Open AccessArticle
Electromagnetic Vibration Analysis and Slot–Pole Structural Optimization for a Novel Integrated Permanent Magnet In-Wheel Motor
Energies 2020, 13(13), 3488; https://doi.org/10.3390/en13133488 - 06 Jul 2020
Viewed by 219
Abstract
This paper presents a novel integrated permanent magnet (PM) in-wheel motor (IWM) driving system for electric vehicles (EVs), in order to overcome the disadvantages of electromagnetic vibration and cogging torque in the topology scheme, on the basis of maintaining high output torque. Firstly, [...] Read more.
This paper presents a novel integrated permanent magnet (PM) in-wheel motor (IWM) driving system for electric vehicles (EVs), in order to overcome the disadvantages of electromagnetic vibration and cogging torque in the topology scheme, on the basis of maintaining high output torque. Firstly, the transient magnetic field of the integrated PM motor is analyzed using the improved analytical subdomain model and finite element (FE) model. The harmonic component of magnetic force density (MFD) is obtained with no-load condition. Furthermore, the vertical dynamic model for the dynamic vibration absorber is established to investigate the influence of the magnetic force harmonic on the vibration response of the stator and rotor. On this basis, the multi-objective optimization design of the pole–slot structure parameters is carried out by using the adaptive weighted particle swarm optimization (AWPSO) algorithm. Finally, the optimization results are compared and verified by FE analysis. The investigation shows that the unbalanced magnetic force and cogging torque is significantly reduced by the adjustment of the pole-arc coefficient, PM thickness, stator slot width and slot opening width. Full article
(This article belongs to the Section Electric Vehicles)
Show Figures

Figure 1

Open AccessArticle
Lithium-Rich Cobalt-Free Manganese-Based Layered Cathode Materials for Li-Ion Batteries: Suppressing the Voltage Fading
Energies 2020, 13(13), 3487; https://doi.org/10.3390/en13133487 - 06 Jul 2020
Viewed by 271
Abstract
Lithium-rich layered oxides are recognized as promising materials for Li-ion batteries, owing to higher capacity than the currently available commercialized cathode, for their lower cost. However, their voltage decay and cycling instability during the charge/discharge process are problems that need to be solved [...] Read more.
Lithium-rich layered oxides are recognized as promising materials for Li-ion batteries, owing to higher capacity than the currently available commercialized cathode, for their lower cost. However, their voltage decay and cycling instability during the charge/discharge process are problems that need to be solved before their practical application can be envisioned. These problems are mainly associated with a phase transition of the surface layer from the layered structure to the spinel structure. In this paper, we report the AlF3-coating of the Li-rich Co-free layered Li1.2Ni0.2Mn0.6O2 (LLNMO) oxide as an effective strategy to solve these problems. The samples were synthesized via the hydrothermal route that insures a very good crystallization in the layered structure, probed by XRD, energy-dispersive X-ray (EDX) spectroscopy, and Raman spectroscopy. The hydrothermally synthesized samples before and after AlF3 coating are well crystallized in the layered structure with particle sizes of about 180 nm (crystallites of ~65 nm), with high porosity (pore size 5 nm) determined by Brunauer–Emmett–Teller (BET) specific surface area method. Subsequent improvements in discharge capacity are obtained with a ~5-nm thick coating layer. AlF3-coated Li1.2Ni0.2Mn0.6O2 delivers a capacity of 248 mAh g−1 stable over the 100 cycles, and it exhibits a voltage fading rate of 1.40 mV per cycle. According to the analysis from galvanostatic charge-discharge and electrochemical impedance spectroscopy, the electrochemical performance enhancement is discussed and compared with literature data. Post-mortem analysis confirms that the AlF3 coating is a very efficient surface modification to improve the stability of the layered phase of the Li-rich material, at the origin of the significant improvement of the electrochemical properties. Full article
(This article belongs to the Section Advanced Energy Materials)
Show Figures

Figure 1

Open AccessArticle
An Electric Vehicle Charge Scheduling Approach Suited to Local and Supplying Distribution Transformers
Energies 2020, 13(13), 3486; https://doi.org/10.3390/en13133486 - 06 Jul 2020
Viewed by 231
Abstract
Distribution networks with high electric vehicle (EV) penetration levels can experience transformer overloading and voltage instability issues. A charge scheduling approach is proposed to mitigate against these issues that suits smart home settings in residential areas. It comprises measurement systems located at distribution [...] Read more.
Distribution networks with high electric vehicle (EV) penetration levels can experience transformer overloading and voltage instability issues. A charge scheduling approach is proposed to mitigate against these issues that suits smart home settings in residential areas. It comprises measurement systems located at distribution transformers that communicate directly with fuzzy logic controller (FLC) systems embedded within EV supply equipment (EVSE). This realizes a reduction in data processing requirements compared to more centralized control approaches, which is advantageous for distribution networks with large numbers of transformers and EV scheduling requests. A case study employing the proposed approach is presented. Realistic driver behavior patterns, EV types, and multivariate probabilistic modeling were used to estimate EV charging demands, daily travel mileage, and plug-in times. A Monte Carlo simulation approach was developed to obtain EV charging loads. The effectiveness of mitigation in terms of reducing distribution transformer peak load levels and losses, as well as improving voltage stability is demonstrated for a distribution network in Jakarta, Indonesia. Full article
Show Figures

Figure 1

Open AccessArticle
Developing a Mathematical Model for Wind Power Plant Siting and Sizing in Distribution Networks
Energies 2020, 13(13), 3485; https://doi.org/10.3390/en13133485 - 06 Jul 2020
Viewed by 239
Abstract
Wind Power Plants (WPPs) are generally located in remote areas with weak distribution connections. Hence, the value of Short Circuit Capacity (SCC), WPP size and the short circuit impedance angle ratio (X/R) are all very critical in the voltage stability of a distribution [...] Read more.
Wind Power Plants (WPPs) are generally located in remote areas with weak distribution connections. Hence, the value of Short Circuit Capacity (SCC), WPP size and the short circuit impedance angle ratio (X/R) are all very critical in the voltage stability of a distribution system connected WPP. This paper presents a new voltage stability model based on the mathematical relations between voltage, the level of wind power penetration, SCC and X/R at a given Point of Common Coupling (PCC) of a distribution network connected WPP. The proposed model introduces six equations based on the SCC and X/R values seen from a particular PCC point. The equations were developed for two common types of Wind Turbine Generators (WTGs), including: the Induction Generator (IG) and the Double Fed Induction Generator (DFIG). Taking advantage of the proposed equations, design engineers can predict how the steady-state PCC voltage will behave in response to different penetrations of IG- and DFIG-based WPPs. In addition, the proposed equations enable computing the maximum size of the WPP, ensuring grid code requirements at the given PCC without the need to carry out complex and time-consuming computational tasks or modelling of the system, which is a significant advantage over existing WPP sizing approaches. Full article
(This article belongs to the Section Electrical Power and Energy System)
Show Figures

Figure 1

Open AccessArticle
Exergo-Economic and Environmental Analysis of a Solar Integrated Thermo-Electric Storage
Energies 2020, 13(13), 3484; https://doi.org/10.3390/en13133484 - 06 Jul 2020
Viewed by 225
Abstract
Renewable energies are often subject to stochastic resources and daily cycles. Energy storage systems are consequently applied to provide a solution for the mismatch between power production possibility and its utilization period. In this study, a solar integrated thermo-electric energy storage (S-TEES) is [...] Read more.
Renewable energies are often subject to stochastic resources and daily cycles. Energy storage systems are consequently applied to provide a solution for the mismatch between power production possibility and its utilization period. In this study, a solar integrated thermo-electric energy storage (S-TEES) is analyzed both from an economic and environmental point of view. The analyzed power plant with energy storage includes three main cycles, a supercritical CO2 power cycle, a heat pump and a refrigeration cycle, indirectly connected by sensible heat storages. The hot reservoir is pressurized water at 120/160 °C, while the cold reservoir is a mixture of water and ethylene glycol, maintained at −10/−20 °C. Additionally, the power cycle’s evaporator section rests on a solar-heated intermediate temperature (95/40 °C) heat reservoir. Exergo-economic and exergo-environmental analyses are performed to identify the most critical components of the system and to obtain the levelized cost of electricity (LCOE), as well as the environmental indicators of the system. Both economic and environmental analyses revealed that solar energy converting devices are burdened with the highest impact indicators. According to the results of exergo-economic analysis, it turned out that average annual LCOE of S-TEES can be more than two times higher than the regular electricity prices. However, the true features of the S-TEES system should be only fully assessed if the economic results are balanced with environmental analysis. Life cycle assessment (LCA) revealed that the proposed S-TEES system has about two times lower environmental impact than referential hydrogen storage systems compared in the study. Full article
(This article belongs to the Section Energy Storage and Application)
Show Figures

Figure 1

Open AccessArticle
Review on Multi-Objective Control Strategies for Distributed Generation on Inverter-Based Microgrids
Energies 2020, 13(13), 3483; https://doi.org/10.3390/en13133483 - 06 Jul 2020
Viewed by 230
Abstract
Microgrids have emerged as a solution to address new challenges in power systems with the integration of distributed energy resources (DER). Inverter-based microgrids (IBMG) need to implement proper control systems to avoid stability and reliability issues. Thus, several researchers have introduced multi-objective control [...] Read more.
Microgrids have emerged as a solution to address new challenges in power systems with the integration of distributed energy resources (DER). Inverter-based microgrids (IBMG) need to implement proper control systems to avoid stability and reliability issues. Thus, several researchers have introduced multi-objective control strategies for distributed generation on IBMG. This paper presents a review of the different approaches that have been proposed by several authors of multi-objective control. This work describes the main features of the inverter as a key component of microgrids. Details related to accomplishing efficient generation from a control systems’ view have been observed. This study addresses the potential of multi-objective control to overcome conflicting objectives with balanced results. Finally, this paper shows future trends in control objectives and discussion of the different multi-objective approaches. Full article
Show Figures

Figure 1

Open AccessArticle
Capacitive Wireless Power Transfer with Multiple Transmitters: Efficiency Optimization
Energies 2020, 13(13), 3482; https://doi.org/10.3390/en13133482 - 06 Jul 2020
Viewed by 231
Abstract
Wireless power transfer with multiple transmitters can have several advantages, including more robustness against misalignment and extending the mobility and range of the receiver(s). In this work, the efficiency maximization problem is analytically solved for a capacitive wireless power transfer system with multiple [...] Read more.
Wireless power transfer with multiple transmitters can have several advantages, including more robustness against misalignment and extending the mobility and range of the receiver(s). In this work, the efficiency maximization problem is analytically solved for a capacitive wireless power transfer system with multiple coupled transmitters and a single receiver. It is found that the system efficiency can be increased by adding more transmitters. Moreover, it is proven that the cross-coupling between the transmitters can be eliminated by adding shunt susceptances at the input ports. Optimal values for the input currents and receiver load are determined to achieve maximum efficiency. As well the optimal load, the optimal input currents and the maximum efficiency are independent on the cross-coupling. By impedance-matching the internal conductances of the generators, the maximum-efficiency solution also becomes the one that provides the maximum output power. Finally, by expressing each transmitter–receiver link with its kQ-product, the maximum system efficiency can be calculated. The analytical results are verified by circuital simulation. Full article
(This article belongs to the Special Issue Modelling of Wireless Power Transfer)
Show Figures

Graphical abstract

Open AccessArticle
Estimation of Bearing Fault Severity in Line-Connected and Inverter-Fed Three-Phase Induction Motors
Energies 2020, 13(13), 3481; https://doi.org/10.3390/en13133481 - 06 Jul 2020
Viewed by 236
Abstract
This paper addresses a comprehensive evaluation of a bearing fault evolution and its consequent prediction concerning the remaining useful life. The proper prediction of bearing faults in their early stage is a crucial factor for predictive maintenance and mainly for the production management [...] Read more.
This paper addresses a comprehensive evaluation of a bearing fault evolution and its consequent prediction concerning the remaining useful life. The proper prediction of bearing faults in their early stage is a crucial factor for predictive maintenance and mainly for the production management schedule. The detection and estimation of the progressive evolution of a bearing fault are performed by monitoring the amplitude of the current signals at the time domain. Data gathered from line-fed and inverter-fed three-phase induction motors were used to validate the proposed approach. To assess classification accuracy and fault estimation, the models described in this paper are investigated by using Artificial Neural Networks models. The paper also provides process flowcharts and classification tables to present the prognostic models used to estimate the remaining useful life of a defective bearing. Experimental results confirmed the method robustness and provide an accurate diagnosis regardless of the bearing fault stage, motor speed, load level, and type of supply. Full article
(This article belongs to the Special Issue Incipient Fault Detection and Diagnosis, Fault-Tolerant Control)
Show Figures

Figure 1

Previous Issue
Next Issue
Back to TopTop