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Energies, Volume 13, Issue 12 (June-2 2020) – 267 articles

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Cover Story (view full-size image) Cryogenic Liquid Injection in New Generation High Efficiency Energy Systems. View this [...] Read more.
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Open AccessArticle
An Improved SOM-Based Method for Multi-Robot Task Assignment and Cooperative Search in Unknown Dynamic Environments
Energies 2020, 13(12), 3296; https://doi.org/10.3390/en13123296 - 26 Jun 2020
Viewed by 322
Abstract
The methods of task assignment and path planning have been reported by many researchers, but they are mainly focused on environments with prior information. In unknown dynamic environments, in which the real-time acquisition of the location information of obstacles is required, an integrated [...] Read more.
The methods of task assignment and path planning have been reported by many researchers, but they are mainly focused on environments with prior information. In unknown dynamic environments, in which the real-time acquisition of the location information of obstacles is required, an integrated multi-robot dynamic task assignment and cooperative search method is proposed by combining an improved self-organizing map (SOM) neural network and the adaptive dynamic window approach (DWA). To avoid the robot oscillation and hovering issue that occurs with the SOM-based algorithm, an SOM neural network with a locking mechanism is developed to better realize task assignment. Then, in order to solve the obstacle avoidance problem and the speed jump problem, the weights of the winner of the SOM are updated by using an adaptive DWA. In addition, the proposed method can search dynamic multi-target in unknown dynamic environment, it can reassign tasks and re-plan searching paths in real time when the location of the targets and obstacle changes. The simulation results and comparative testing demonstrate the effectiveness and efficiency of the proposed method. Full article
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Open AccessArticle
Acoustic Emission Characteristics of Coal Samples under Different Stress Paths Corresponding to Different Mining Layouts
Energies 2020, 13(12), 3295; https://doi.org/10.3390/en13123295 - 26 Jun 2020
Viewed by 299
Abstract
Research on the mining-induced mechanical behavior and microcrack evolution of deep-mined coal has become increasingly important with the sharp increase in mining depth. For rock units in front of the working face, the microcrack evolution characteristics, structural characteristics, and stress state correspond well [...] Read more.
Research on the mining-induced mechanical behavior and microcrack evolution of deep-mined coal has become increasingly important with the sharp increase in mining depth. For rock units in front of the working face, the microcrack evolution characteristics, structural characteristics, and stress state correspond well to mining layouts and depths under deep mining. The acoustic emission (AE) characteristics of typical coal under deep mining were obtained by conducting laboratory experiments to simulate mining-induced behavior and utilizing AE techniques to capture the variation in AE temporal and spatial parameters in real time, which provide an important basis for studying the rupture mechanisms and mechanical behavior of deep-mined coal. The findings were as follows: (1) AE activity under deep mining was characterized by three stages, corresponding to crack initiation, crack stable propagation, and crack unstable propagation. As the three stages proceeded, the AE counting rate and AE energy rate presented stronger clustering characteristics, and the cumulative AE counting and cumulative AE energy exhibited a sharp increase by an order of magnitude. (2) The crack initiation and the main stages of crack propagation were determined by characteristic points of variation curves in the AE parameters over time. In the main crack propagation stage, the number of cumulative AE events and the cumulative AE counts were similar among the three mining conditions, while coal samples under coal pillar mining released the largest amount of AE energy. The amount of accumulated AE energy released by coal samples increased by one order of magnitude according to the sequence of protective coal-seam mining, top-coal caving mining, and nonpillar mining. (3) Fractal technology was applied to quantitatively analyze the AE spatial evolution process, showing that the fractal dimension of the AE location decreased as the peak stress increased, corresponding to protective seam mining, caving-coal mining, and nonpillar mining. The above results showed that the deformation and fracture characteristics of coal under deep mining followed a general law, but were affected by different mining conditions. The crack initiation and main rupture activity of coal occurred earlier under the conditions of protective seam mining, top-coal caving mining, and nonpillar mining, successively. Moreover, nonpillar mining induced the strongest and highest degree of unstable rupture of the coal body in front of the working face. Full article
(This article belongs to the Special Issue Green Coal Mining Techniques 2020)
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Open AccessArticle
Hydrophilic Coating of Copper Particle Monolayer Wicks for Enhanced Passive Water Transport
Energies 2020, 13(12), 3294; https://doi.org/10.3390/en13123294 - 26 Jun 2020
Viewed by 279
Abstract
Passive water transport through thin-surface wicks made of heat conducting material is important for developing thermal management devices such as heat pipes and spreaders. In this study, we demonstrated the hydrophilic coating of a Cu particle monolayer wick for enhanced water transport. We [...] Read more.
Passive water transport through thin-surface wicks made of heat conducting material is important for developing thermal management devices such as heat pipes and spreaders. In this study, we demonstrated the hydrophilic coating of a Cu particle monolayer wick for enhanced water transport. We fabricated a Cu particle monolayer using Cu powder with a nominal particle diameter of 100 μm and determined the particle size distribution using scanning electron microscopy (SEM). We observed a remarkable change in the water contact angle on the application of a hydrophilic coating, which demonstrated the enhanced passive water transport. The elemental mapping of Cu, O, and Si obtained by electron probe microanalysis confirmed the deposition of the SiO2-based coating material on each Cu particle. Although the Cu particles were only partially covered by SiO2, a remarkable enhancement in wettability was achieved. Finally, we conducted a rate-of-rise experiment to quantitatively characterize the water transport performance of the coated Cu particle monolayer. Thus, we propose hydrophilic coating as a simple and effective method to enhance passive water transport through Cu particle monolayer wicks. Full article
(This article belongs to the Section Energy Fundamentals and Conversion)
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Open AccessArticle
Voltage Control Methodologies in Active Distribution Networks
Energies 2020, 13(12), 3293; https://doi.org/10.3390/en13123293 - 26 Jun 2020
Viewed by 293
Abstract
Renewable Energy Sources are becoming widely spread, as they are sustainable and low-carbon emission. They are mostly penetrating the MV Distribution Networks as Distributed Generators, which has determined the evolution of the networks’ control and supervision systems, from almost a complete lack to [...] Read more.
Renewable Energy Sources are becoming widely spread, as they are sustainable and low-carbon emission. They are mostly penetrating the MV Distribution Networks as Distributed Generators, which has determined the evolution of the networks’ control and supervision systems, from almost a complete lack to becoming fully centralized. This paper proposes innovative voltage control architectures for the distribution networks, tailored for different development levels of the control and supervision systems encountered in real life: a Coordinated Control for networks with basic development, and an optimization-based Centralized Control for networks with fully articulated systems. The Centralized Control fits the requirements of the network: the challenging harmonization of the generator’s capability curves with the regulatory framework, and modelling of the discrete control of the On-Load Tap Changer transformer. A realistic network is used for tests and comparisons with the Local Strategy currently specified by regulations. The proposed Coordinated Control gives much better results with respect to the Local Strategy, in terms of loss minimization and voltage violations mitigation, and can be used for networks with poorly developed supervision and control systems, while Centralized Control proves the best solution, but can be applied only in fully supervised and controlled networks. Full article
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Open AccessArticle
Inertial Optimization Based Two-Step Methods for Solving Equilibrium Problems with Applications in Variational Inequality Problems and Growth Control Equilibrium Models
Energies 2020, 13(12), 3292; https://doi.org/10.3390/en13123292 - 26 Jun 2020
Viewed by 339
Abstract
This manuscript aims to incorporate an inertial scheme with Popov’s subgradient extragradient method to solve equilibrium problems that involve two different classes of bifunction. The novelty of our paper is that methods can also be used to solve problems in many fields, such [...] Read more.
This manuscript aims to incorporate an inertial scheme with Popov’s subgradient extragradient method to solve equilibrium problems that involve two different classes of bifunction. The novelty of our paper is that methods can also be used to solve problems in many fields, such as economics, mathematical finance, image reconstruction, transport, elasticity, networking, and optimization. We have established a weak convergence result based on the assumption of the pseudomonotone property and a certain Lipschitz-type cost bifunctional condition. The stepsize, in this case, depends upon on the Lipschitz-type constants and the extrapolation factor. The bifunction is strongly pseudomonotone in the second method, but stepsize does not depend on the strongly pseudomonotone and Lipschitz-type constants. In contrast, the first convergence result, we set up strong convergence with the use of a variable stepsize sequence, which is decreasing and non-summable. As the application, the variational inequality problems that involve pseudomonotone and strongly pseudomonotone operator are considered. Finally, two well-known Nash–Cournot equilibrium models for the numerical experiment are reviewed to examine our convergence results and show the competitive advantage of our suggested methods. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessArticle
Simulations and Tests of a KRET Aerospace Penetrator
Energies 2020, 13(12), 3291; https://doi.org/10.3390/en13123291 - 26 Jun 2020
Viewed by 267
Abstract
This manuscript presents the simulation tests of an aerospace penetrator conducted to check the device’s validity and functionality. For this work, the numerical model was created on the basis of engineering data, the laboratory model of the tube reluctance actuator was created on [...] Read more.
This manuscript presents the simulation tests of an aerospace penetrator conducted to check the device’s validity and functionality. For this work, the numerical model was created on the basis of engineering data, the laboratory model of the tube reluctance actuator was created on the basis of the numerical model, and a set of simulations were executed on the basis of both presented. Moreover, the mathematical model was supplemented by precise boundary conditions. The main goal was the analysis of the introduced device’s properties by comparing them to experimental values. Three different variants were taken into account to check the construction functionality and to study the most important parameters, e.g., the hammer displacement, hammer velocity, eddy currents and overall electromagnetic properties of the penetrator’s hammer displacement. The high-end values of the applied components were derived on the basis of the results and are presented in the summary of the concluded work. An overall method for checking the validity of the penetrator was proposed, which is cost and time effective. The model used was not limited in the representation of physical phenomena. Full article
(This article belongs to the Section Electric Vehicles)
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Open AccessArticle
SimBench—A Benchmark Dataset of Electric Power Systems to Compare Innovative Solutions Based on Power Flow Analysis
Energies 2020, 13(12), 3290; https://doi.org/10.3390/en13123290 - 26 Jun 2020
Viewed by 310
Abstract
Publicly accessible, elaborated grid datasets, i.e., benchmark grids, are well suited to publish and compare methods or study results. Similarly, developing innovative tools and algorithms in the fields of grid planning and grid operation is based on grid datasets. Therefore, a general methodology [...] Read more.
Publicly accessible, elaborated grid datasets, i.e., benchmark grids, are well suited to publish and compare methods or study results. Similarly, developing innovative tools and algorithms in the fields of grid planning and grid operation is based on grid datasets. Therefore, a general methodology to generate benchmark datasets and its voltage level dependent implementation is described in this paper. As a result, SimBench, a comprehensive dataset for the low, medium, high and extra-high voltage level, is presented. Besides grids that can be combined across several voltage levels, the dataset offers an added value by providing time series for a whole year as well as future scenarios. In this way, SimBench is applicable for many use cases and simplifies reproducing study results. As proof, different automated algorithms for grid planning are compared to show how to apply SimBench and make use of it as a simulation benchmark. Full article
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Open AccessArticle
Retrofit Methodology Based on Energy Simulation Modeling Applied for the Enhancement of a Historical Building in L’Aquila
Energies 2020, 13(12), 3289; https://doi.org/10.3390/en13123289 - 26 Jun 2020
Viewed by 252
Abstract
Energy loss has not been addressed effectively by policies introduced to encourage the preservation and enhancement of historical structures. Material and other constraints, together with safety standard improvements, do not always guarantee adequate levels of environmental performance. An optimization of retrofit measures to [...] Read more.
Energy loss has not been addressed effectively by policies introduced to encourage the preservation and enhancement of historical structures. Material and other constraints, together with safety standard improvements, do not always guarantee adequate levels of environmental performance. An optimization of retrofit measures to align with new uses, new standards of comfort, and energy saving are needed, as are studies based on new best practices for the enhancement of architectural heritage. This paper presents a method that uses dynamic models tared on non-destructive surveys, and based on compatible energy and structural interventions derived from preliminary analyses integrated into special design tools. Energy simulations were carried out using Design Builder (6.1.5.002, Designbuilder Software Ltd, Stroud, UK) software. The case study is a former hospital, S. Salvatore, in L’Aquila, an architecturally important building, severely damaged by an earthquake in 2009. The methodology presented in this research includes in-depth investigations coherently systematized into a multi-scenario output using simulation software. The results guarantee a high level of compatibility with restoration and seismic guidelines, and new building environmental performance requirements. Full article
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Open AccessArticle
Development of Future Compact and Eco-Friendly HVDC Gas-Insulated Systems: Shape Optimization of a DC Spacer Model and Novel Materials Investigation
Energies 2020, 13(12), 3288; https://doi.org/10.3390/en13123288 - 26 Jun 2020
Viewed by 253
Abstract
Testing and validating the electrical insulation performance of full-size compact high-voltage direct current (HVDC) gas-insulated systems, gas-insulated transmission lines (GIL) and gas-insulated switchgears (GIS) is very costly and take long time. Therefore, a reduced scale system was designed and constructed to study thoroughly [...] Read more.
Testing and validating the electrical insulation performance of full-size compact high-voltage direct current (HVDC) gas-insulated systems, gas-insulated transmission lines (GIL) and gas-insulated switchgears (GIS) is very costly and take long time. Therefore, a reduced scale system was designed and constructed to study thoroughly the spacer’s performance when subjected to higher electric fields under HVDC with different shapes, made of new advanced materials, and housed in new SF6-free gas environment. Since the stationary DC electric field distribution along the spacer is controlled by spacer material conductivity and strongly depends upon its shape, this, the first part of two articles, proposes in a first step based on electric field calculations with COMSOL Multiphysics software, an optimized shape of a spacer model using a standard high-voltage alternating current (HVAC) alumina-filled epoxy material. Then, two novel types of materials were introduced and investigated: (i) modified filled epoxy material with a lower temperature-dependent conductivity than that of the standard HVAC material, which is interpreted by a lower thermal activation energy; and (ii) nonlinear resistive field grading material with a low nonlinearity coefficient, with and without the presence of a temperature gradient which occurs under operating service load. The numerical results show that, despite that the DC optimized profile of the spacer made of standard HVAC, alumina-filled epoxy is very effective in relaxing the electric field magnitudes along the spacer under uniform temperature—its distribution is significantly affected by the presence of a high temperature gradient causing the maximum electric field shifts along the spacer surface towards the earthed flange. Under this condition, the modified filled epoxy material with a weaker temperature-dependent conductivity results in a significant reduction of the electric field enhancement, representing thus a relevant key solution for HVDC GIL/GIS applications. Nonlinear resistive field grading material is also effective but seems unnecessary. The optimized DC spacer models are being fabricated for tests verification with C4-Perfluoronitrile (C4-PFN, 3MTM NovecTM 4710)/CO2 and Trifluoroiodomethane (CF3I)/CO2 gas mixtures in the reduced scale gas-insulated test prototype. Full article
(This article belongs to the Section Advanced Energy Materials)
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Open AccessArticle
From Non-Modular to Modular Concept of Bidirectional Buck/Boost Converter for Microgrid Applications
Energies 2020, 13(12), 3287; https://doi.org/10.3390/en13123287 - 26 Jun 2020
Viewed by 243
Abstract
In this article, the practical comparison of the operational performance of the modular (or multiport) and non-modular bidirectional buck/boost (bi-BB) DC/DC converter is realized. The main contribution of the work is the evaluation of both concepts based on various aspects, considering the qualitative [...] Read more.
In this article, the practical comparison of the operational performance of the modular (or multiport) and non-modular bidirectional buck/boost (bi-BB) DC/DC converter is realized. The main contribution of the work is the evaluation of both concepts based on various aspects, considering the qualitative indicators of the systems relevant for microgrids. Here, we discuss efficiency, electrical properties, costs, and component values. At the same time, critical comparisons are provided for converters based on SiC and GaN technology (non-modular high-voltage SiC-based dual-interleaved converter and modular low-voltage GaN-based). The concepts are specific with their operating frequency, whereby for each solution, the switching frequency is different and directly influences relevant components. The efficiency, overall system volume, output voltage ripple, and input current ripple are compared mutually between both concepts with a dependency on power delivery. These factors, together with overall volume and costs, are very important considering modern converters for microgrid systems. The summary of pros and cons is realized for each of the proposed converters, whereby the evaluation criterion is reflected within the electrical properties targeting microgrid application. Full article
(This article belongs to the Special Issue Advanced DC-DC Power Converters and Switching Converters)
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Open AccessArticle
Predictive Adaptive Filter for Reducing Total Harmonics Distortion in PV Systems
Energies 2020, 13(12), 3286; https://doi.org/10.3390/en13123286 - 26 Jun 2020
Viewed by 304
Abstract
This paper presents a new method for reducing the total harmonic distortion (THD) of photovoltaic (PV) systems by using an adaptive filter based on a predictive model. Instead of reducing the produced THD at each stage of the PV system, a one-step process [...] Read more.
This paper presents a new method for reducing the total harmonic distortion (THD) of photovoltaic (PV) systems by using an adaptive filter based on a predictive model. Instead of reducing the produced THD at each stage of the PV system, a one-step process is implemented at the end stage. The connection topology of the adaptive filter is similar to normal active and passive filters. The main difference is its ability to adjust the filtering coefficients while others cannot. The proposed method is applied to a single-phase standalone PV system by adopting least mean square (LMS), normalized LMS (NLMS) and leaky LMS algorithms to verify the validity of the proposed method. Various values of filter length and step size are evaluated, and results indicate that the proposed method can reduce THD in the current signal of the PV system significantly by using all of the mentioned algorithms. Different step sizes and filter lengths directly influence the effectiveness of the THD reduction, with small step sizes and long filters being the most effective. Amongst the algorithms, NLMS reduces THD the most, and LMS reaches the peak current value the fastest. Full article
(This article belongs to the Section Smart Grids and Microgrids)
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Open AccessArticle
A Wind Energy Conversion System Based on a Generator with Modulated Magnetic Flux
Energies 2020, 13(12), 3285; https://doi.org/10.3390/en13123285 - 26 Jun 2020
Viewed by 323
Abstract
In this work, the concept of an energy conversion system for wind turbines based on the modified permanent magnet synchronous generator (PMSG) is presented. In the generator, a pair of three-phase windings is used, one of which is connected in a “star” and [...] Read more.
In this work, the concept of an energy conversion system for wind turbines based on the modified permanent magnet synchronous generator (PMSG) is presented. In the generator, a pair of three-phase windings is used, one of which is connected in a “star” and the second in a “delta” configuration. At the outputs of both windings, two six-pulse uncontrolled (diode) rectifiers are included. These rectifiers are mutually coupled by a specially designed pulse transformer, whose primary winding is powered by the power electronics converter—the so-called “current modulator”—which, in this case, operates as a magnetic flux modulator, in the generator. The modulator provides a quasi-sinusoidal magnetomotive force (mmf) in the stator of the machine. The whole system is connected to the power grid via a dedicated voltage source inverter (VSI) converter. The main objective of the elaborated solution is to provide high efficiency conversion of mechanical (wind) energy into electricity by means of a relatively simple electrical system. Full article
(This article belongs to the Section Wind, Wave and Tidal Energy)
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Open AccessArticle
Hydrothermal Desorption of Cs with Oxalic Acid from Hydrobiotite and Wastewater Treatment by Chemical Precipitation
Energies 2020, 13(12), 3284; https://doi.org/10.3390/en13123284 - 25 Jun 2020
Viewed by 381
Abstract
A hydrobiotite (HBT) clay contains more cesium (Cs)-specific adsorption sites than illitic clay, and the capacity of frayed edge sites can increase as the weathering of micaceous minerals proceeds. Thus, Cs can be selectively adsorbed to HBT clay. In this study, we investigated [...] Read more.
A hydrobiotite (HBT) clay contains more cesium (Cs)-specific adsorption sites than illitic clay, and the capacity of frayed edge sites can increase as the weathering of micaceous minerals proceeds. Thus, Cs can be selectively adsorbed to HBT clay. In this study, we investigated the removal efficiency of non-radioactive (133Cs) and radioactive (137Cs) Cs from HBT, using oxalic acid. We found the minimum optimal concentration of 0.15 M oxalic acid removed more than 90% of Cs. Subsequently, cations and Cs ions were removed using Ca(OH)2 and sodium tetraphenylborate (NaTPB) to treat the washing wastewater generated at the optimum concentration of the desorbent (0.15 M oxalic acid). In order to remove cations and heavy metal ions in the waste solution, Ca(OH)2 was treated at a mass ratio of 0.025 g/mL and pH 9–10 to derive optimal conditions. As a final step, to remove Cs, NaTPB was treated with a mass ratio of 2 mg/mL and reduced to below 0.1 mg/L Cs to find the optimal dose. The novelty of this study is that the amount of radioactive waste can be drastically reduced by removing the non-radioactive cations and heavy metals separately in the first step and removing the remaining radioactive Cs in the second step. Full article
(This article belongs to the Special Issue Wastewater Treatment and Resource Recovery)
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Open AccessArticle
Hydrokinetic Power Conversion Using Vortex-Induced Oscillation with Cubic Restoring Force
Energies 2020, 13(12), 3283; https://doi.org/10.3390/en13123283 - 25 Jun 2020
Viewed by 315
Abstract
A cubic-spring restoring function with high-deformation stiffening is introduced to passively improve the harnessed marine hydrokinetic power by using flow-induced oscillations/vibrations (FIO/V) of a cylinder. In these FIO/V experiments, a smooth, rigid, single-cylinder on elastic end-supports is tested at Reynolds numbers ranging from [...] Read more.
A cubic-spring restoring function with high-deformation stiffening is introduced to passively improve the harnessed marine hydrokinetic power by using flow-induced oscillations/vibrations (FIO/V) of a cylinder. In these FIO/V experiments, a smooth, rigid, single-cylinder on elastic end-supports is tested at Reynolds numbers ranging from 24,000 < Re < 120,000. The parameters of the tested current energy converter (CEC) are cubic stiffness and linear damping. Using the second generation of digital virtual spring-damping (Vck) controller developed by the Marine Renewable Energy Laboratory (MRELab), the cubic modeling of the oscillator stiffness is tested. Experimental results show the influence of the parameter variation on the amplitude, frequency, energy conversion, energy efficiency, and power of the converter. All experiments are conducted in the low turbulence-free surface water (LTFSW) channel of the MRELab of the University of Michigan. The main conclusions are: (1) The nonlinearity in the cubic oscillator is an effective way to extend the vortex-induced vibration (VIV) upper branch, which results in higher harnessing power and efficiency compared to the linear stiffness cylinder converter. (2) Compared to the linear converter, the overall power increase is substantial. The nonlinear power optimum, occurring at the end of the VIV upper branch, is 63% higher than its linear counterpart. (3) The cubic stiffness converter with low harnessing damping achieves consistently good performance in all the VIV regions because of the hardening restoring force, especially at higher flow velocity. Full article
(This article belongs to the Special Issue Hydrokinetic Energy Conversion: Technology, Research, and Outlook)
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Open AccessArticle
Predictions of Rock Temperature Evolution at the Lahendong Geothermal Field by Coupled Numerical Model with Discrete Fracture Model Scheme
Energies 2020, 13(12), 3282; https://doi.org/10.3390/en13123282 - 25 Jun 2020
Viewed by 341
Abstract
The comprehensive exploitation of geothermal fields has an impact on the productivity of the reservoirs. To realize sustainable steam production, changes in the rock temperature need to be predicted and controlled. A coupled thermo-hydro-mechanical (THM) model employing COMSOL Multiphysics was proposed to study [...] Read more.
The comprehensive exploitation of geothermal fields has an impact on the productivity of the reservoirs. To realize sustainable steam production, changes in the rock temperature need to be predicted and controlled. A coupled thermo-hydro-mechanical (THM) model employing COMSOL Multiphysics was proposed to study the characteristics of heat transfer, fluid flow, and solid deformation at the Lahendong geothermal field, in North Sulawesi, Indonesia. The numerical results were compared with analytical and measured data in order to validate the numerical simulation. Based on the results, the predicted temperatures of the production wells showed significant decrease with the production time. In addition, a reduction in the reservoir temperature leads to lower specific gross electrical power within the production well, which should significantly reduce the sustainability of the power plant. Full article
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Open AccessArticle
Analysis of the Main Coal Mining Restructuring Policy Objectives in the Light of Polish Mining Companies’ Ability to Change
Energies 2020, 13(12), 3281; https://doi.org/10.3390/en13123281 - 25 Jun 2020
Viewed by 309
Abstract
This article presents the results from research performed on the implementation of restructuring policies of Polish mining enterprises. The study sought to verify whether the changes planned in Poland since the 1990s have been successfully introduced. The main objective of the restructuring of [...] Read more.
This article presents the results from research performed on the implementation of restructuring policies of Polish mining enterprises. The study sought to verify whether the changes planned in Poland since the 1990s have been successfully introduced. The main objective of the restructuring of the Polish mining industry was to transform it into a profitable, cost-effective sector, which would be able to survive in a competitive market. The study also allowed the determination of a ratio giving a clear and synthetic outcome—called the indicator of the ability to change (WZZ). This indicator included criteria for assessing the efficiency of a company during the process of change, the fundamental errors committed during the change process, and the reasons for a low level of ability to change. The WZZ indicator achieved only an average level of 30%. The main errors committed during the change process are the mining enterprises’ lack of experience in change management and a lack of executive conviction about the need not only for proper management of the process of change, but also for change in general. The problem is also a lack of adequate knowledge, competence in the field of change management, and communication with employees. Low level of ability to change is caused by the inflexible organizational structure, the organizational culture, a lack of motivation, and the limited flexibility of the human factor. Full article
(This article belongs to the Section Energy Economics and Policy)
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Open AccessArticle
Anti-Thermal Shock Binding of Liquid-State Food Waste to Non-Wood Pellets
Energies 2020, 13(12), 3280; https://doi.org/10.3390/en13123280 - 25 Jun 2020
Viewed by 439
Abstract
The development and implementation of strategies to assist safe and effective transport and storage of pellets in containers and indoor facilities without heating systems are challenging. This study primarily aimed to reshape the organic fraction of municipal solid waste into a liquid-state binder [...] Read more.
The development and implementation of strategies to assist safe and effective transport and storage of pellets in containers and indoor facilities without heating systems are challenging. This study primarily aimed to reshape the organic fraction of municipal solid waste into a liquid-state binder in order to develop freezing–defrosting-proof non-wood pellets. The introduction of the standard solution of food waste into the process of pelleting consisted of stirring it together with the residual biomass from distillation of cellulosic bioethanol or alternatively spraying very fine droplets on the layer of the starting material before it entered the pilot-scale automatic machine at 200 MPa and 125 °C. The addition by spraying of carbohydrate-rich supplement boiled for five minutes caused the pellets to show increases in apparent density (1250.8500 kg·m−3), durability (99.7665%), and hydrophobicity (93.9785%), and consistently prevented them from suffering severe mechanical fracture by thermal shock. The fractal dimension of breakpoints, cracks, and delamination on the finished surface for these products was the smallest at 1.7500–1.7505. Sprayed pellets would fall into the strictest grid of products for residential heat-and-power units, even after freezing and defrosting. The conclusion is therefore that spraying can spectacularly ensure the reliability of liquid-state food waste as an anti-thermal shock binder for non-wood pellets. Full article
(This article belongs to the Special Issue Sustainable Waste-to-Energy Systems)
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Open AccessFeature PaperArticle
A State-Space Model of an Inverter-Based Microgrid for Multivariable Feedback Control Analysis and Design
Energies 2020, 13(12), 3279; https://doi.org/10.3390/en13123279 - 25 Jun 2020
Viewed by 610
Abstract
In this work, a synchronous model for grid-connected and islanded microgrids is presented. The grid-connected model is based on the premise that the reference frame is synchronized with the AC bus. The quadrature component of the AC bus voltage can be cancelled, which [...] Read more.
In this work, a synchronous model for grid-connected and islanded microgrids is presented. The grid-connected model is based on the premise that the reference frame is synchronized with the AC bus. The quadrature component of the AC bus voltage can be cancelled, which allows to express output power as a linear equation for nominal values in the AC bus amplitude voltage. The model for the islanded microgrid is developed by integrating all the inverter dynamics using a state-space model for the load currents. This model is presented in a comprehensive way such that it could be scalable to any number of inverter-based generators using inductor–capacitor–inductor (LCL) output filters. The use of these models allows designers to assess microgrid stability and robustness using modern control methods such as eigenvalue analysis and singular value diagrams. Both models were tested and validated in an experimental setup to demonstrate their accuracy in describing microgrid dynamics. In addition, three scenarios are presented: non-controlled model, Linear-Quadratic Integrator (LQI) power control, and Power-Voltage (PQ/Vdq) droop–boost controller. Experimental results demonstrate the effectiveness of the control strategies and the accuracy of the models to describe microgrid dynamics. Full article
(This article belongs to the Special Issue Control Strategies for Power Conversion Systems)
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Open AccessArticle
Performance Enhancement of Nitrogen Dual Expander and Single Mixed Refrigerant LNG Processes Using Jaya Optimization Approach
Energies 2020, 13(12), 3278; https://doi.org/10.3390/en13123278 - 25 Jun 2020
Viewed by 284
Abstract
The nitrogen (N2) expander and single mixed refrigerant (SMR) liquefaction processes are recognized as the most favorable options to produce liquefied natural gas (LNG) at small-scale and offshore sites. These processes have a simple and compact design that make them efficient [...] Read more.
The nitrogen (N2) expander and single mixed refrigerant (SMR) liquefaction processes are recognized as the most favorable options to produce liquefied natural gas (LNG) at small-scale and offshore sites. These processes have a simple and compact design that make them efficient with respect to their capital costs. Nevertheless, huge operating costs, mainly due to their lower energy efficiency, remains an ongoing issue. Utilization of design variables having non-optimal values is the primary cause for the lower energy efficiency; which, in turn, leads to exergy destruction (i.e., entropy generation), and ultimately the overall energy consumption is increased. The optimal execution of the design variables of LNG processes can be obtained through effective design optimization. However, the complex and highly non-linear interactions between design variables (refrigerant flowrates and operating pressures) and objective function (overall energy consumption) make the design optimization a difficult and challenging task. In this context, this study examines a new optimization algorithm, named “Jaya”, to reduce the operating costs of nitrogen dual expander and SMR LNG processes. The Jaya approach is an algorithm-specific parameter-less optimization methodology. It was found that by using the Jaya algorithm, the energy efficiency of the SMR process and nitrogen dual expander natural gas (NG) liquefaction process can be enhanced up to 14.3% and 11.6%, respectively, as compared to their respective base cases. Using the Jaya approach, significant improved results were observed even compared to other previously used optimization approaches for design optimization. Results of conventional exergy analysis revealed that the exergy destruction of SMR and N2 dual expander process can be reduced by 17.4% and 14%, respectively. Moreover, economic analysis identified the 13.3% and 11.6% relative operating costs savings for SMR and N2 dual expander LNG processes, respectively. Full article
(This article belongs to the Special Issue Exergy Analysis and Optimization of Energy Systems and Processes)
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Open AccessArticle
Ethanolic Media Effect on the Susceptibility to Stress Corrosion Cracking in an X-70 Microalloyed Steel with Different Aging Treatments
Energies 2020, 13(12), 3277; https://doi.org/10.3390/en13123277 - 25 Jun 2020
Viewed by 254
Abstract
The objective of this research was to evaluate the stress corrosion cracking (SCC) of X-70 micro-alloyed steel in contact with bio-ethanol and E-50 gasohol. Environmental factors, including water (1%, 3% and 5%) and NaCl (10 mg/L and 32 mg/L), as well as two [...] Read more.
The objective of this research was to evaluate the stress corrosion cracking (SCC) of X-70 micro-alloyed steel in contact with bio-ethanol and E-50 gasohol. Environmental factors, including water (1%, 3% and 5%) and NaCl (10 mg/L and 32 mg/L), as well as two aging treatments were studied. Experimental values were obtained by the Slow Strain Rate Test (SSRT) technique, X-ray fluorescence (XRF), and tensile test according to the information reported in the literature. The results of the SSRT for the SCC determination showed that this steel in these conditions did not show evidence of SCC, which was attributed to the formation of an oxide (Fe2O3) not soluble in ethanol. The oxide layer acts as a protector preventing the formation of pitting, one of the main causes of cracks initiation in SCC. Full article
(This article belongs to the Section Thermal Management)
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Open AccessArticle
Structural Design, Analysis, and Testing of a 10 kW Fabric-Covered Wind Turbine Blade
Energies 2020, 13(12), 3276; https://doi.org/10.3390/en13123276 - 24 Jun 2020
Viewed by 375
Abstract
Reducing the weight of a wind turbine blade is a major issue. Wind turbines have become larger in size to increase power generating efficiency. The blade has also grown in length to take more wind energy. A fabric-based wind turbine blade, introduced by [...] Read more.
Reducing the weight of a wind turbine blade is a major issue. Wind turbines have become larger in size to increase power generating efficiency. The blade has also grown in length to take more wind energy. A fabric-based wind turbine blade, introduced by General Electric Co., reduced the blade weight. In this study, a small fabric-covered blade for a 10 kW wind turbine was developed to verify structural ability. The blade was designed on the cross-section using variational asymptotic beam sectional analysis (VABS), structural analysis was carried out using MSC.Nastran for the design loads. A modal analysis was performed to compare the modal frequency and mode shapes. Static structural testing and modal testing were fulfilled. The analysis results were compared with the testing results. The fabric-covered structure was confirmed to reduce the blade mass with sufficient strength. Full article
(This article belongs to the Special Issue Innovation in Wind Turbine Blade Design and Aeroelasticity)
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Open AccessArticle
Effects of the Circuit Arrangement on the Thermal Performance of Double U-Tube Ground Heat Exchangers
Energies 2020, 13(12), 3275; https://doi.org/10.3390/en13123275 - 24 Jun 2020
Viewed by 326
Abstract
Given that the issue of variations in geometrical parameters of the borehole heat exchanger (BHE) revolves around the phenomenon of thermal resistance, a thorough understanding of these parameters is beneficial in enhancing thermal performance of BHEs. The present study seeks to identify relative [...] Read more.
Given that the issue of variations in geometrical parameters of the borehole heat exchanger (BHE) revolves around the phenomenon of thermal resistance, a thorough understanding of these parameters is beneficial in enhancing thermal performance of BHEs. The present study seeks to identify relative changes in the thermal performance of double U-tube BHEs triggered by alterations in circuit arrangements, as well as the shank spacing and the borehole length. The thermal performance of double U-tube BHEs with different configurations is comprehensively analyzed through a 3D transient numerical code developed by means of the finite element method. The sensitivity of each circuit configuration in terms of the thermal performance to variations of the borehole length and shank spacing is investigated. The impact of the thermal interference between flowing legs, namely thermal short-circuiting, on parameters affecting the borehole thermal resistance is addressed. Furthermore, the energy exchange characteristics for different circuit configurations are quantified by introducing the thermal effectiveness coefficient. The results indicate that the borehole length is more influential than shank spacing in increasing the discrepancy between thermal performances of different circuit configurations. It is shown that deviation of the averaged-over-the-depth mean fluid temperature from the arithmetic mean of the inlet and outlet temperatures is more critical for lower shank spacings and higher borehole lengths. Full article
(This article belongs to the Special Issue Computational Geothermal Energy Applications)
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Open AccessArticle
Application of Second-Law Analysis for the Environmental Control Unit at High Ambient Temperature
Energies 2020, 13(12), 3274; https://doi.org/10.3390/en13123274 - 24 Jun 2020
Viewed by 358
Abstract
This paper assesses the application of the second-law of thermodynamics in a military Environmental Control Unit (ECU) to evaluate the exergy destruction (or irreversibility) in each component when operating at high ambient temperature. Experimental testings were conducted on three ECUs, 1.5 (5.3 kW), [...] Read more.
This paper assesses the application of the second-law of thermodynamics in a military Environmental Control Unit (ECU) to evaluate the exergy destruction (or irreversibility) in each component when operating at high ambient temperature. Experimental testings were conducted on three ECUs, 1.5 (5.3 kW), 3 (10.6 kW), and 5 (17.6 kW) tons of refrigeration (RT), to assess the potential contribution of each component to enhance the overall energy efficiency of the system, and to determine the feasibility of the thermodynamic model presented herein. The analysis provided for extreme high ambient conditions up to 51.7 °C (125 °F). The results yielded that the highest irreversibility was associated with compressors (32.4% to 42.5%). This is followed by the heat exchanges (19.6% to 32.9%) in the case of 1.5-RT and 3-RT units, whereas for the 5-RT unit, the highest irreversibility was associated with the evaporator followed by the one of the compressors. In the 3-RT ECU, the condenser’s second-law efficiency enhanced due to an additional fan, yet the working refrigerant increased the irreversibility in the expansion device. The second-law analysis recognized the components with the highest exergy destruction and identified the direction to enhance the exergetic efficiency of any ECU operating at high-temperature climate. Full article
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Open AccessArticle
Coordinated Flexibility Scheduling for Urban Integrated Heat and Power Systems by Considering the Temperature Dynamics of Heating Network
Energies 2020, 13(12), 3273; https://doi.org/10.3390/en13123273 - 24 Jun 2020
Viewed by 326
Abstract
The coordinated heat-electricity dispatch of the urban integrated energy system (UIES) helps to improve the system flexibility, thereby overcoming the adverse effects caused by the random fluctuations of renewable energy (RE) and promoting the penetration of RE. Among them, the dynamic characteristics of [...] Read more.
The coordinated heat-electricity dispatch of the urban integrated energy system (UIES) helps to improve the system flexibility, thereby overcoming the adverse effects caused by the random fluctuations of renewable energy (RE) and promoting the penetration of RE. Among them, the dynamic characteristics of the urban heat network (UHN) are important features that need to be considered for the operating scheduling of the UIES. This paper aims to establish a flexibility scheduling model for UIES based on the dynamic characteristics of the UHN. First, the typical structure and key equipment model of the urban integrated heat and power system (UIHPS) with the dynamic characteristics of the UHN is proposed. Then, the definition and model of the UIHPS flexibility and the assessment index of the flexibility are developed. Moreover, a flexibility scheduling model for a UIHPS that considers the dynamic characteristics of a UHN is established. Finally, the validity of the proposed model is validated by case studies, and the applicability of flexibility scheduling and the effect of heat load (HL) are analyzed. Full article
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Open AccessArticle
Assessing the Alkyl Chain Effect of Ammonium Hydroxides Ionic Liquids on the Kinetics of Pure Methane and Carbon Dioxide Hydrates
Energies 2020, 13(12), 3272; https://doi.org/10.3390/en13123272 - 24 Jun 2020
Viewed by 442
Abstract
In this study, four ammonium hydroxide ionic liquids (AHILs) with varying alkyl chains were evaluated for their kinetic hydrate inhibition (KHI) impact on pure carbon dioxide (CO2) and methane (CH4) gas hydrate systems. The constant cooling technique was used [...] Read more.
In this study, four ammonium hydroxide ionic liquids (AHILs) with varying alkyl chains were evaluated for their kinetic hydrate inhibition (KHI) impact on pure carbon dioxide (CO2) and methane (CH4) gas hydrate systems. The constant cooling technique was used to determine the induction time, the initial rate of hydrate formation, and the amount of gas uptake for CH4-AHILs and CO2-AHILs systems at 8.0 and 3.50 MPa, respectively, at 1 wt.% aqueous AHILs solutions. In addition, the effect of hydrate formation sub-cooling temperature on the performance of the AHILs was conducted at experimental temperatures 274.0 and 277.0 K. The tested AHILs kinetically inhibited both CH4 and CO2 hydrates at the studied sub-cooling temperatures by delaying the hydrate induction time and reducing the initial rate of hydrate formation and gas uptake. The hydrate inhibition performance of AHILs increases with increasing alkyl chain length, due to the better surface adsorption on the hydrate crystal surface with alkyl chain length enhancement. TPrAOH efficiently inhibited the induction time of both CH4 and CO2 hydrate with an average inhibition percentage of 50% and 84%, respectively. Tetramethylammonium Hydroxide (TMAOH) and Tetrabutylammonium Hydroxide (TBAOH) best reduced CH4 and CO2 total uptake on average, with TMAOH and Tetraethylammonium Hydroxide (TEAOH) suitably reducing the average initial rate of CH4 and CO2 hydrate formation, respectively. The findings in this study could provide a roadmap for the potential use of AHILs as KHI inhibitors, especially in offshore environs. Full article
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Open AccessArticle
On-Line Partial Discharge Monitoring System for Power Transformers Based on the Simultaneous Detection of High Frequency, Ultra-High Frequency, and Acoustic Emission Signals
Energies 2020, 13(12), 3271; https://doi.org/10.3390/en13123271 - 24 Jun 2020
Viewed by 339
Abstract
The article presents a novel on-line partial discharge (PD) monitoring system for power transformers, whose functioning is based on the simultaneous use of three unconventional methods of PD detection: high-frequency (HF), ultra-high frequency (UHF), and acoustic emission (AE). It is the first monitoring [...] Read more.
The article presents a novel on-line partial discharge (PD) monitoring system for power transformers, whose functioning is based on the simultaneous use of three unconventional methods of PD detection: high-frequency (HF), ultra-high frequency (UHF), and acoustic emission (AE). It is the first monitoring system equipped in an active dielectric window (ADW), which is a combined ultrasonic and electromagnetic PD sensor. The article discusses in detail the process of designing and building individual modules of hardware and software layers of the system, wherein the most attention was paid to the PD sensors, i.e., meandered planar inverted-F antenna (MPIFA), high-frequency current transformer (HFCT), and active dielectric window with ultrasonic transducer, which were optimized for detection of PDs occurring in oil-paper insulation. The prototype of the hybrid monitoring system was first checked on a 330 MVA large power transformer during the induced voltage test with partial discharge measurement (IVPD). Next, it was installed on a 31.5 MVA substation power transformer and integrated according to the standard IEC 61850 with SCADA (Supervisory Control and Data Acquisition) system registering voltage, active power, and oil temperature of the monitored unit. The obtained results showed high sensitivity of the manufactured PD sensors as well as the advantages of the simultaneous use of three techniques of PD detection and the possibility of discharge parameter correlation with other power transformer parameters. Full article
(This article belongs to the Special Issue Condition Monitoring and Diagnosis of Electrical Machines)
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Open AccessArticle
Advanced Control for Hydrogen Pyrolysis Installations
Energies 2020, 13(12), 3270; https://doi.org/10.3390/en13123270 - 24 Jun 2020
Viewed by 284
Abstract
Today, hydrogen production plays an important part in the industry due to the increasing use of hydrogen in significant domains, such as chemistry, transportation, or energy. In this paper, we aim to design a numerical control solution based on the thermodynamic analysis of [...] Read more.
Today, hydrogen production plays an important part in the industry due to the increasing use of hydrogen in significant domains, such as chemistry, transportation, or energy. In this paper, we aim to design a numerical control solution based on the thermodynamic analysis of the pyrolysis reactions for hydrogen production and to present novel research developments that highlight industrial applications. Beginning with the evaluation of the technological aspects for the pyrolysis chemical process, the paper studies the thermodynamic evaluation of the system equilibrium for the pyrolysis reactions set, to recommend an appropriate automatic control solution for hydrogen pyrolysis installations. The numerical control architecture is organized on two levels, a control level dedicated to key technological parameters, and a supervisory decision level for optimizing the conversion performances of the pyrolysis process. The data employed for modelling, identification, control, and optimization tasks, were obtained from an experimental platform. The scientific results can be implemented on dedicated equipment, to achieve an optimal exploitation of the industrial pyrolysis process. Full article
(This article belongs to the Special Issue Advanced Control Design and Fault Diagnosis)
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Open AccessArticle
Enhancing Heating Performance of Low-Temperature Air Source Heat Pumps Using Compressor Casing Thermal Storage
Energies 2020, 13(12), 3269; https://doi.org/10.3390/en13123269 - 24 Jun 2020
Viewed by 260
Abstract
Air source heat pumps (ASHPs) are widely recognized as energy-saving and environmentally friendly heating and air-conditioning equipment with broad applications. However, when conventional ASHPs are operated at a low ambient temperature, they suffer from problems such as high discharge temperature and low heating [...] Read more.
Air source heat pumps (ASHPs) are widely recognized as energy-saving and environmentally friendly heating and air-conditioning equipment with broad applications. However, when conventional ASHPs are operated at a low ambient temperature, they suffer from problems such as high discharge temperature and low heating efficiency. To address these problems, this study designed a new type of dual evaporator combined with a compressor casing thermal storage heat pump system (DE-CCTS) on the basis of a low-temperature air source heat pump water heater with enhanced vapor injection (EVI). The proposed DE-CCTS used thermal storage phase change material (PCM), which was filled in the secondary evaporator (the thermal storage heat exchanger), to recover the waste heat of the compressor casing. Unlike that in the original system under different ambient temperatures, the suction temperature increased by 0.1–1 °C, the discharge temperature decreased by 0.1–0.5 °C, and the coefficient of performance (COP) of DE-CCTS increased by 0.85–4.72% under the proposed system. These effects were especially evident at low temperatures. Full article
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Open AccessArticle
Enhanced Intelligent Energy Management System for a Renewable Energy-Based AC Microgrid
Energies 2020, 13(12), 3268; https://doi.org/10.3390/en13123268 - 24 Jun 2020
Viewed by 355
Abstract
This paper proposes an enhanced energy management system (EEMS) for a residential AC microgrid. The renewable energy-based AC microgrid with hybrid energy storage is broken down into three distinct parts: a photovoltaic (PV) array as a green energy source, a battery (BT) and [...] Read more.
This paper proposes an enhanced energy management system (EEMS) for a residential AC microgrid. The renewable energy-based AC microgrid with hybrid energy storage is broken down into three distinct parts: a photovoltaic (PV) array as a green energy source, a battery (BT) and a supercapacitor (SC) as a hybrid energy storage system (HESS), and apartments and electric vehicles, given that the system is for residential areas. The developed EEMS ensures the optimal use of the PV arrays’ production, aiming to decrease electricity bills while reducing fast power changes in the battery, which increases the reliability of the system, since the battery undergoes fewer charging/discharging cycles. The proposed EEMS is a hybrid control strategy, which is composed of two stages: a state machine (SM) control to ensure the optimal operation of the battery, and an operating mode (OM) for the best operation of the SC. The obtained results show that the EEMS successfully involves SC during fast load and PV generation changes by decreasing the number of BT charging/discharging cycles, which significantly increases the system’s life span. Moreover, power loss is decreased during passing clouds phases by decreasing the power error between the extracted power by the sources and the required equivalent; the improvement in efficiency reaches 9.5%. Full article
(This article belongs to the Special Issue Microgrids 2020)
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Open AccessReview
Advanced Laboratory Testing Methods Using Real-Time Simulation and Hardware-in-the-Loop Techniques: A Survey of Smart Grid International Research Facility Network Activities
Energies 2020, 13(12), 3267; https://doi.org/10.3390/en13123267 - 24 Jun 2020
Viewed by 415
Abstract
The integration of smart grid technologies in interconnected power system networks presents multiple challenges for the power industry and the scientific community. To address these challenges, researchers are creating new methods for the validation of: control, interoperability, reliability of Internet of Things systems, [...] Read more.
The integration of smart grid technologies in interconnected power system networks presents multiple challenges for the power industry and the scientific community. To address these challenges, researchers are creating new methods for the validation of: control, interoperability, reliability of Internet of Things systems, distributed energy resources, modern power equipment for applications covering power system stability, operation, control, and cybersecurity. Novel methods for laboratory testing of electrical power systems incorporate novel simulation techniques spanning real-time simulation, Power Hardware-in-the-Loop, Controller Hardware-in-the-Loop, Power System-in-the-Loop, and co-simulation technologies. These methods directly support the acceleration of electrical systems and power electronics component research by validating technological solutions in high-fidelity environments. In this paper, members of the Survey of Smart Grid International Research Facility Network task on Advanced Laboratory Testing Methods present a review of methods, test procedures, studies, and experiences employing advanced laboratory techniques for validation of range of research and development prototypes and novel power system solutions. Full article
(This article belongs to the Special Issue Advancements in Real-Time Simulation of Power and Energy Systems)
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