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Energies, Volume 13, Issue 14 (July-2 2020) – 244 articles

Cover Story (view full-size image): Efficacious start-up is essential for SOFCs to produce the target power output. Ensuring sufficient electrical conductivity is part of this procedure and involves the reduction of the anode from NiO to Ni. In this text, we highlight the role of bipolar plate design in the reduction process and the power output implications if this is not accomplished.. View this paper.
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Open AccessArticle
Analysis of the Fire Propagation in a Sublevel Coal Mine
Energies 2020, 13(14), 3754; https://doi.org/10.3390/en13143754 - 21 Jul 2020
Cited by 4 | Viewed by 583
Abstract
A fire has been analyzed in a real underground coal mine, using a sublevel method, during an entire year. The study was focused on the collapsed area, reproducing a real mixture formed by coal, waste, and air gap. The analysis was done by [...] Read more.
A fire has been analyzed in a real underground coal mine, using a sublevel method, during an entire year. The study was focused on the collapsed area, reproducing a real mixture formed by coal, waste, and air gap. The analysis was done by means of an experimental analysis, a computational fuid dynamic model (CFD), and simulations using a mine ventilation software. Three scenarios were determined and studied regarding their influence on the evolution of the fire: (a) development of the fire without taking any action, (b) sealing off the affected areas, and (c) sealing and reducing the ventilation in the affected area and surrounding drifts. The study revealed the behavior of the fire in a real mine and the effectiveness of the main fire-fighting measures over time, verifying that none of the measures taken could eliminate the fire-induced in the collapsed area. Full article
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Open AccessArticle
Exergetic Evaluation of an Ethylene Refrigeration Cycle
Energies 2020, 13(14), 3753; https://doi.org/10.3390/en13143753 - 21 Jul 2020
Viewed by 552
Abstract
The production of light olefins by selective steam cracking is an energy-intensive process, and ethylene and propylene refrigeration cycles are key parts of it. The objective of this study was to identify opportunities for energy savings in an ethylene refrigeration cycle through an [...] Read more.
The production of light olefins by selective steam cracking is an energy-intensive process, and ethylene and propylene refrigeration cycles are key parts of it. The objective of this study was to identify opportunities for energy savings in an ethylene refrigeration cycle through an exergetic analysis. Two main causes of lower operational efficiency were identified: (1) Lower polytropic efficiency of the refrigerant compressor and (2) operating with the compressor mini-flow valve open to ensure reliability. The evaluation showed that the amount of irreversibilities generated by the cycle in operation is 22% higher than that predicted by the original design, which represents a 14% lower exergy efficiency. There is a potential savings of 0.20 MW in the cycle’s energy consumption with the implementation of the following improvements: recover refrigerant compressor efficiency by performing maintenance on the equipment and optimize the flow distribution between the recycle valve, the level control valve, and the temperature control valve. Full article
(This article belongs to the Section Thermal Management)
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Open AccessArticle
Assessment of Hybrid RANS/LES Models in Heat and Fluid Flows around Staggered Pin-Fin Arrays
Energies 2020, 13(14), 3752; https://doi.org/10.3390/en13143752 - 21 Jul 2020
Viewed by 597
Abstract
In the present work, the three-dimensional heat and fluid flows around staggered pin-fin arrays are predicted using two hybrid RANS/LES models (an improved delayed detached eddy simulation (IDDES) model and a stress-blended eddy simulation (SBES) model), and one transitional unsteady Reynolds averaged Navier-Stokes [...] Read more.
In the present work, the three-dimensional heat and fluid flows around staggered pin-fin arrays are predicted using two hybrid RANS/LES models (an improved delayed detached eddy simulation (IDDES) model and a stress-blended eddy simulation (SBES) model), and one transitional unsteady Reynolds averaged Navier-Stokes (URANS) model, called k-ω SSTLM. The periodic segment geometry with a total of nine pins is considered with a channel height of 2D and a distance of 2.5D between each pin. The corresponding Reynolds number based on the pin diameter and the maximum velocity between pins is 10,000. The two hybrid RANS/LES results show the superior prediction of the mean velocity profiles around the pins, pressure distributions on the pin wall, and Nusselt number distributions. However, the transitional model, k-ω SSTLM, shows large discrepancies except in front of the pins where the flow is not fully developed. The vortical structures are well resolved by the two hybrid RANS/LES models. The SBES model is particularly adept at capturing the 3-D vortex structures after the pins. The effects of the blending function switching between RANS and LES mode of the two hybrid RANS/LES models are also investigated. Full article
(This article belongs to the Special Issue Fluid Flow and Heat Transfer Ⅱ)
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Open AccessArticle
A Stochastic MPC Based Energy Management System for Simultaneous Participation in Continuous and Discrete Prosumer-to-Prosumer Energy Markets
Energies 2020, 13(14), 3751; https://doi.org/10.3390/en13143751 - 21 Jul 2020
Cited by 1 | Viewed by 564
Abstract
This article studies the exchange of self-produced renewable energy between prosumers (and with pure end consumers), through the discrete trading of energy packages and proposes a framework for optimizing this exchange. In order to mitigate the imbalances derived from discrepancies between production and [...] Read more.
This article studies the exchange of self-produced renewable energy between prosumers (and with pure end consumers), through the discrete trading of energy packages and proposes a framework for optimizing this exchange. In order to mitigate the imbalances derived from discrepancies between production and consumption and their respective forecasts, the simultaneous continuous trading of instantaneous power quotas is proposed, giving rise to a time-ahead market running in parallel with a real-time one. An energy management system (EMS) based on stochastic model predictive control (SMPC) simultaneously determines the optimal bidding strategies for both markets, as well as the optimal utilisation of any energy storage system (ESS). Simulations carried out for a heterogeneous group of agents show that those with SMPC-EMS achieve savings of between 3% and 15% in their energy operation economic result. The proposed structures allows the peer-to-peer (P2P) energy trading between end users without ESS and constitute a viable alternative to avoid deviation penalties in secondary regulation markets. Full article
(This article belongs to the Special Issue Model Predictive Control for Energy Management in Microgrids)
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Open AccessArticle
Automated Data Filtering Approach for ANN Modeling of Distributed Energy Systems: Exploring the Application of Machine Learning
Energies 2020, 13(14), 3750; https://doi.org/10.3390/en13143750 - 21 Jul 2020
Viewed by 670
Abstract
To realize the distributed generation and to make the partnership between the dispatchable units and variable renewable resources work efficiently, accurate and flexible monitoring needs to be implemented. Due to digital transformation in the energy industry, a large amount of data is and [...] Read more.
To realize the distributed generation and to make the partnership between the dispatchable units and variable renewable resources work efficiently, accurate and flexible monitoring needs to be implemented. Due to digital transformation in the energy industry, a large amount of data is and will be captured every day, but the inability to process them in real time challenges the conventional monitoring and maintenance practices. Access to automated and reliable data-filtering tools seems to be crucial for the monitoring of many distributed generation units, avoiding false warnings and improving the reliability. This study aims to evaluate a machine-learning-based methodology for autodetecting outliers from real data, exploring an interdisciplinary solution to replace the conventional manual approach that was very time-consuming and error-prone. The raw data used in this study was collected from experiments on a 100-kW micro gas turbine test rig in Norway. The proposed method uses Density-Based Spatial Clustering of Applications with Noise (DBSCAN) to detect and filter out the outliers. The filtered datasets are used to develop artificial neural networks (ANNs) as a baseline to predict the normal performance of the system for monitoring applications. Results show that the filtering method presented is reliable and fast, minimizing time and resources for data processing. It was also shown that the proposed method has the potential to enhance the performance of the predictive models and ANN-based monitoring. Full article
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Open AccessArticle
Least Squares Method for Identification of IGBT Thermal Impedance Networks Using Direct Temperature Measurements
Energies 2020, 13(14), 3749; https://doi.org/10.3390/en13143749 - 21 Jul 2020
Cited by 1 | Viewed by 526
Abstract
State-of-the-art methods for determining thermal impedance networks for IGBT (Insulated Gate Bipolar Transistor) modules usually involves the establishment of the relationship between the measured transistor or diode voltage and temperature under homogenous temperature distribution across the IGBT module. The junction temperature is recomputed [...] Read more.
State-of-the-art methods for determining thermal impedance networks for IGBT (Insulated Gate Bipolar Transistor) modules usually involves the establishment of the relationship between the measured transistor or diode voltage and temperature under homogenous temperature distribution across the IGBT module. The junction temperature is recomputed from the established voltage–temperature relationship and used in determining the thermal impedance network. This method requires accurate measurement of voltage drop across the transistors and diodes under specific designed heating and cooling profiles. Validation of the junction temperature is usually done using infrared camera or sensors placed close to the transistors or diodes (in some cases and open IGBT module) so that the measured temperature is as close to the junction as possible. In this paper, we propose an alternative method for determining the IGBT thermal impedance network using the principles of least squares. This method uses measured temperatures for defined heating and cooling cycles under different cooling conditions to determine the thermal impedance network. The results from the proposed method are compared with those obtained using state-of-the-art methods. Full article
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Open AccessArticle
Pre-Selection of the Optimal Sitting of Phase-Shifting Transformers Based on an Optimization Problem Solved within a Coordinated Cross-Border Congestion Management Process
Energies 2020, 13(14), 3748; https://doi.org/10.3390/en13143748 - 21 Jul 2020
Cited by 1 | Viewed by 605
Abstract
The current European policy roadmap aims at forcing the TSOs to coordinate remedial actions used for relieving the congestions in the synchronous power system. In this paper, an optimization problem for coordinated congestion management is described and its results obtained for a real [...] Read more.
The current European policy roadmap aims at forcing the TSOs to coordinate remedial actions used for relieving the congestions in the synchronous power system. In this paper, an optimization problem for coordinated congestion management is described and its results obtained for a real European use cases created in the H2020 EU-SysFlex project are presented. First of all, these results prove the feasibility of a central optimization problem for the coordination of the cross-border congestion management process. Next, the formulated optimization problem is used to tackle the issue of planning the investments in phase-shifting transformers (PSTs), for the purpose of increasing the efficiency/decreasing the cost of congestion management. Finally, this paper introduces two optimization-based indicators for pre-selecting the investment sites, which may be used to support the decision makers aiming at decreasing the costs of coordinated congestion management. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Step-Up Series Resonant DC–DC Converter with Bidirectional-Switch-Based Boost Rectifier for Wide Input Voltage Range Photovoltaic Applications
Energies 2020, 13(14), 3747; https://doi.org/10.3390/en13143747 - 21 Jul 2020
Cited by 2 | Viewed by 650
Abstract
This paper proposes a high gain DC–DC converter based on the series resonant converter (SRC) for photovoltaic (PV) applications. This study considers low power applications, where the resonant inductance is usually relatively small to reduce the cost of the converter realization, which results [...] Read more.
This paper proposes a high gain DC–DC converter based on the series resonant converter (SRC) for photovoltaic (PV) applications. This study considers low power applications, where the resonant inductance is usually relatively small to reduce the cost of the converter realization, which results in low-quality factor values. On the other hand, these SRCs can be controlled at a fixed switching frequency. The proposed topology utilizes a bidirectional switch (AC switch) to regulate the input voltage in a wide range. This study shows that the existing topology with a bidirectional switch has a limited input voltage regulation range. To avoid this issue, the resonant tank is rearranged in the proposed converter to the resonance capacitor before the bidirectional switch. By this rearrangement, the dependence of the DC voltage gain on the duty cycle is changed, so the proposed converter requires a smaller duty cycle than that of the existing counterpart at the same gain. Theoretical analysis shows that the input voltage regulation range is extended to the region of high DC voltage gain values at the maximum input current. Contrary to the existing counterpart, the proposed converter can be realized with a wide range of the resonant inductance values without compromising the input voltage regulation range. Nevertheless, the proposed converter maintains advantages of the SRC, such as zero voltage switching (ZVS) turn-on of the primary-side semiconductor switches. In addition, the output-side diodes are turned off at zero current. The proposed converter is analyzed and compared with the existing counterpart theoretically and experimentally. A 300 W experimental prototype is used to validate the theoretical analysis of the proposed converter. The peak efficiency of the converter is 96.5%. Full article
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Open AccessArticle
Comparison of the Use of Energy Storages and Energy Curtailment as an Addition to the Allocation of Renewable Energy in the Distribution System in Order to Minimize Development Costs
Energies 2020, 13(14), 3746; https://doi.org/10.3390/en13143746 - 21 Jul 2020
Cited by 6 | Viewed by 582
Abstract
This paper presents a comparison of the efficiency of energy storage and energy curtailment as an addition to the allocation of renewable energy in the distribution system in order to minimize development costs using a Mixed Integer-Linear Programming (MILP). Energy sources and energy [...] Read more.
This paper presents a comparison of the efficiency of energy storage and energy curtailment as an addition to the allocation of renewable energy in the distribution system in order to minimize development costs using a Mixed Integer-Linear Programming (MILP). Energy sources and energy storages are selected, sized and allocated under operational circumstances such as grid congestions and weather conditions. Loads and power units are modeled by daily consumption and generation profiles respectively, to reflect the intermittent character of renewable generation and consumption of energy. The optimization is carried out for a one-year time horizon using twenty-four representative days. The method is verified on three main simulation scenarios and three sub-scenarios for each of them, allowing for the comparison of the efficiency of each used tool. The main scenarios differ in their share of energy from renewable energy sources (RES) in total consumption. In the sub-scenarios, different tools (RES sizing and allocation, energy storages (ES) sizing and allocation and energy curtailment) are used. The results of this research confirm that energy curtailment is a more efficient additional tool for RES sizing and allocation than energy storages. This method can find practical application for Distribution System Operators in elaborating grid development strategies. Full article
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Open AccessArticle
Effects of Inflow Shear on Wake Characteristics of Wind-Turbines over Flat Terrain
Energies 2020, 13(14), 3745; https://doi.org/10.3390/en13143745 - 21 Jul 2020
Cited by 1 | Viewed by 634
Abstract
The scope of the present study was to understand the wake characteristics of wind-turbines under various inflow shears. First, in order to verify the prediction accuracy of the in-house large-eddy simulation (LES) solver, called RIAM-COMPACT, based on a Cartesian staggered grid, we conducted [...] Read more.
The scope of the present study was to understand the wake characteristics of wind-turbines under various inflow shears. First, in order to verify the prediction accuracy of the in-house large-eddy simulation (LES) solver, called RIAM-COMPACT, based on a Cartesian staggered grid, we conducted a wind-tunnel experiment using a wind-turbine scale model and compared the numerical and experimental results. The total number of grid points in the computational domain was about 235 million. Parallel computation based on a hybrid LES/actuator line (AL) model approach was performed with a new SX-Aurora TSUBASA vector supercomputer. The comparison between wind-tunnel experiment and high-resolution LES results showed that the AL model implemented in the in-house LES solver in this study could accurately reproduce both performances of the wind-turbine scale model and flow characteristics in the wake region. Next, with the LES solver developed in-house, flow past the entire wind-turbine, including the nacelle and the tower, was simulated for a tip-speed ratio (TSR) of 4, the optimal TSR. Three types of inflow shear, N = 4, N = 10, and uniform flow, were set at the inflow boundary. In these calculations, the calculation domain in the streamwise direction was very long, 30.0 D (D being the wind-turbine rotor diameter) from the center of the wind-turbine hub. Long-term integration of t = 0 to 400 R/Uin was performed. Various turbulence statistics were calculated at t = 200 to 400 R/Uin. Here, R is the wind-turbine rotor radius, and Uin is the wind speed at the hub-center height. On the basis of the obtained results, we numerically investigated the effects of inflow shear on the wake characteristics of wind-turbines over a flat terrain. Focusing on the center of the wind-turbine hub, all results showed almost the same behavior regardless of the difference in the three types of inflow shear. Full article
(This article belongs to the Section Wind, Wave and Tidal Energy)
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Open AccessArticle
Influence of Control and Structure Parameters on the Starting Performance of a 12/8 Pole Switched Reluctance Motor
Energies 2020, 13(14), 3744; https://doi.org/10.3390/en13143744 - 21 Jul 2020
Viewed by 413
Abstract
To investigate the influence of control and structure parameters on the starting performance of a switched reluctance motor, a 12/8 pole switched reluctance motor is analyzed in this paper. The novel field-circuit coupled finite element method of switched reluctance motor is proposed in [...] Read more.
To investigate the influence of control and structure parameters on the starting performance of a switched reluctance motor, a 12/8 pole switched reluctance motor is analyzed in this paper. The novel field-circuit coupled finite element method of switched reluctance motor is proposed in the paper. The influence of the controller on the switched reluctance motor is considered. The influence of rotor initial position angle, starting mode, starting current, and structure parameters on the starting performance of the switched reluctance motor is studied using the field-circuit coupled finite element method. The starting performance of the switched reluctance motor is obtained under the different control and structure parameters. The alternating starting mode of single- and two-phase winding can improve the starting torque of switched reluctance motor (SRM). As the stator pole arc coefficient increases, the starting torque of SRM increases. The appropriate reduction of the air gap length can improve the starting torque of SRM. Experimental results of the prototype are compared with the calculation results, which verifies the reliability of the calculation method and accuracy of the calculation results. Full article
(This article belongs to the Section Electric Vehicles)
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Open AccessArticle
Aggregated Conducted Electromagnetic Interference Generated by DC/DC Converters with Deterministic and Random Modulation
Energies 2020, 13(14), 3698; https://doi.org/10.3390/en13143698 - 21 Jul 2020
Cited by 1 | Viewed by 617
Abstract
The assessment of electromagnetic compatibility (EMC) is important for both technical and legal reasons. This manuscript addresses specific issues that should be taken into account for proper EMC assessment of energy systems that use power electronic interfaces. The standardized EMC measuring techniques have [...] Read more.
The assessment of electromagnetic compatibility (EMC) is important for both technical and legal reasons. This manuscript addresses specific issues that should be taken into account for proper EMC assessment of energy systems that use power electronic interfaces. The standardized EMC measuring techniques have been used in a laboratory setup consisting in two identical DC/DC converters with deterministic and random modulations. Measuring difficulties caused by the low frequency envelopes, resulting from frequency beating accompanying aggregation of harmonic components of similar frequencies, were indicated as a phenomenon that might lead to significant problems during the EMC assessment using currently binding standards. The experimental results describing deterministic and random modulated converters might be useful for practitioners implementing power interfaces in microgrids and power systems as well as for researchers involved in EMC assurance of power systems consisting in multiple power electronic interfaces. Full article
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Analysis and Design of Self-Oscillating Resonant Converters with Loss-Free Resistor Characteristics
Energies 2020, 13(14), 3743; https://doi.org/10.3390/en13143743 - 20 Jul 2020
Viewed by 464
Abstract
A general approach for the analysis and design of self-oscillating resonant converters is presented in this paper, for a particular class of circuits in which the change of input voltage polarity is caused by the zero-crossings of the input inductor current. The key [...] Read more.
A general approach for the analysis and design of self-oscillating resonant converters is presented in this paper, for a particular class of circuits in which the change of input voltage polarity is caused by the zero-crossings of the input inductor current. The key features of the method are an analytical description in the time-domain of a spiral that eventually converges into an ellipse, and a frequency–domain analysis that explains the behavior of the ellipse as a limit cycle. On a theoretical basis, this class of circuits behaves as loss-free resistors (LFR) because in steady-state the input inductor current is in phase with the first harmonic of the input voltage. The proposed analytical procedure predicts accurately the amplitude and frequency of the limit cycle and justifies the stability of its generation. This accuracy is reflected in the close agreement between the theoretical expressions and the corresponding simulated and measured waveforms. Third and fourth order resonant converters are designed following simple guidelines derived from the theoretical analysis. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessArticle
Thermoelectric Generation in Hybrid Electric Vehicles
Energies 2020, 13(14), 3742; https://doi.org/10.3390/en13143742 - 20 Jul 2020
Viewed by 702
Abstract
Improving the efficiency of an internal combustion engine (ICE) leads to the reduction of fuel consumption, which improves the performance of a hybrid vehicle. Waste heat recovery (WHR) systems offer options to improve the efficiency of an ICE. This is due to the [...] Read more.
Improving the efficiency of an internal combustion engine (ICE) leads to the reduction of fuel consumption, which improves the performance of a hybrid vehicle. Waste heat recovery (WHR) systems offer options to improve the efficiency of an ICE. This is due to the ICE releasing approximately one third of the combustion energy as waste heat into the atmosphere. This paper focuses on one such upcoming system by analysing the efficiency of a thermoelectric generator (TEG) used as a waste heat recovery system in a hybrid electric vehicle (HEV). It summarises how the efficiency of the TEG can be improved by considering parameters such as the size of module, materials used, and the number of modules needed for the TEG system. The results obtained are then compared with other types of WHR system such as the Organic Rankine Cycle (ORC) and turbocompounding (T/C) implemented on the same type of engine. The research is based on a 1.8 L Toyota Prius-type engine. The TEG model simulated in this research can generate a maximum power of 1015 W at an engine speed of 5200 RPM. The overall system efficiency of TEG implemented on the HEV model is 6% with the average engine speed operating at 2000 RPM. Full article
(This article belongs to the Section Advanced Energy Materials)
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Open AccessArticle
Rapid Determination of Wood and Rice Husk Pellets’ Proximate Analysis and Heating Value
Energies 2020, 13(14), 3741; https://doi.org/10.3390/en13143741 - 20 Jul 2020
Cited by 1 | Viewed by 574
Abstract
Biomass pellets are a potential renewable and clean energy source. With the advantages of perfect combustion performance and easy storage and transport, biomass pellets have gradually replaced fossil fuels and become widely used. Rapid and accurate determination of biomass pellets’ quality is critical [...] Read more.
Biomass pellets are a potential renewable and clean energy source. With the advantages of perfect combustion performance and easy storage and transport, biomass pellets have gradually replaced fossil fuels and become widely used. Rapid and accurate determination of biomass pellets’ quality is critical to efficient energy use. Laser-induced breakdown spectroscopy (LIBS) combined with chemometric methods were utilized. The gross calorific value (CV) and ash content (Ash), volatile matter (VM) and fixed carbon (FC) were firstly measured and analyzed. LIBS spectra and their corresponding elements of biomass pellet samples were analyzed. Three quantitative analysis models for quality indexes including partial least-squares regression (PLSR), least squares-support vector machines (LS-SVM), extreme learning machines (ELM) were further built. All models performed well, especially the LS-SVM model which obtained the best determination results, with all R2 values over 0.95. Concurrently, the modeling performance of ash was slightly better than that of the other three quality indexes, which further confirmed the feasibility of using relevant elements to predict biomass quality indexes. The overall results indicated that LIBS coupled with suitable chemometrics could be an alternative promising method to determine quality indexes of biomass pellets and further improve energy utilization by using biomass materials with better quality. Full article
(This article belongs to the Special Issue Advanced Technologies for Biomass)
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Open AccessArticle
General Formula for SHE Problem Solution
Energies 2020, 13(14), 3740; https://doi.org/10.3390/en13143740 - 20 Jul 2020
Cited by 2 | Viewed by 524
Abstract
This paper considers cascaded H-bridges multilevel inverters with 2n dc sources, n integer, n>0 and proposes a new general formula to compute those 2n switching angles capable of eliminating n+1 harmonics and their respective multiples from the [...] Read more.
This paper considers cascaded H-bridges multilevel inverters with 2 n dc sources, n integer, n > 0 and proposes a new general formula to compute those 2 n switching angles capable of eliminating n + 1 harmonics and their respective multiples from the output voltage waveform. The proposed procedure uses only scalar products and avoids linear systems, therefore it has a low computational cost. Computed angles do not depend on modulation index, moreover, voltage sources vary linearly. A mathematical proof is given to validate the formula. Three-phase implementations eliminate or mitigate a significant amount of low order harmonics, thus resulting in very low total harmonic distortion. The proposed formula has been experimentally validated using a single-phase nine-level cascaded H-bridge inverter prototype, resulting in a Total Harmonic Distortion (THD) of 5.59%; the first not-mitigated harmonic is the 17th. Full article
(This article belongs to the Special Issue Multilevel Power Converters Control and Modulation Techniques)
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Open AccessArticle
Constant Output-Voltage Design for Bi-Directional Wireless Power Transfer System with Multiple Stages
Energies 2020, 13(14), 3739; https://doi.org/10.3390/en13143739 - 20 Jul 2020
Viewed by 492
Abstract
For the application of wireless power transfer (WPT) technology in a robot (like the snake robot), the power is supposed to be transferred to each device across multiple robot joints. This paper proposes a multi-stage bi-directional WPT (MB-WPT) system that not only provides [...] Read more.
For the application of wireless power transfer (WPT) technology in a robot (like the snake robot), the power is supposed to be transferred to each device across multiple robot joints. This paper proposes a multi-stage bi-directional WPT (MB-WPT) system that not only provides power to multiple loads but also increases the power transfer distance. Besides, the last stage can charge for the preceding stages by reverse power transfer. The constant output voltage can be achieved whether the power is transmitted in a forward or reverse direction, and different output voltages for each stage can be achieved to satisfy the respective voltage requirement through the parameter design method. The validity of the theoretical analysis and the feasibility of the system are verified by experiments. Full article
(This article belongs to the Section Smart Grids and Microgrids)
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Open AccessArticle
Histidine Decorated Nanoparticles of CdS for Highly Efficient H2 Production via Water Splitting
Energies 2020, 13(14), 3738; https://doi.org/10.3390/en13143738 - 20 Jul 2020
Viewed by 509
Abstract
Pure cadmium sulfide and histidine decorated cadmium sulfide nanocomposites are prepared by the hydrothermal or solvothermal method. Scanning electron microscopy (SEM) analysis shows that the particle sizes of pure cadmium sulfide (pu/CdS) and histidine decorated cadmium sulfide prepared by the hydrothermal method (hi/CdS) [...] Read more.
Pure cadmium sulfide and histidine decorated cadmium sulfide nanocomposites are prepared by the hydrothermal or solvothermal method. Scanning electron microscopy (SEM) analysis shows that the particle sizes of pure cadmium sulfide (pu/CdS) and histidine decorated cadmium sulfide prepared by the hydrothermal method (hi/CdS) range from 0.75 to 3.0 μm. However, when a solvothermal method is used, the particle size of histidine decorated cadmium sulfide (so/CdS) ranges from 50 to 300 nm. X-ray diffraction (XRD) patterns show that all samples (pu/CdS, hi/CdS and so/CdS) have a hexagonal wurtzite crystal structure but so/CdS has a poor crystallinity compared to the others. The as-prepared samples are applied to photocatalytic hydrogen production via water splitting and the results show that the highest H2 evolution rate for pu/CdS and hi/CdS are 1250 and 1950 μmol·g−1·h−1, respectively. On the other hand, the so/CdS sample has a rate of 6020 μmol·g−1·h−1, which is about five times higher than that of the pu/CdS sample. The increased specific surface area of so/CdS nanoparticles and effective charge separation by histidine molecules are attributed to the improved H2 evolution. Full article
(This article belongs to the Special Issue Nanotechnology for Solar Energy Conversion)
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Open AccessArticle
Method of Averaging the Effective Thermal Conductivity Based on Thermal Response Tests of Borehole Heat Exchangers
Energies 2020, 13(14), 3737; https://doi.org/10.3390/en13143737 - 20 Jul 2020
Cited by 2 | Viewed by 473
Abstract
This work concerns borehole heat exchangers and their testing using apparatus for thermal response tests. In the theoretical part of the article, an equation was derived from the known equation of heat flow, on which the interpretation of the thermal response test was [...] Read more.
This work concerns borehole heat exchangers and their testing using apparatus for thermal response tests. In the theoretical part of the article, an equation was derived from the known equation of heat flow, on which the interpretation of the thermal response test was based. The practical part presents the results of several measurements taken in the AGH Laboratory of Geoenergetics. They were aimed at examining the potential heat exchange capacity between the heat carrier and rock mass. Measurement results in the form of graphs are shown in relation to the examined, briefly described wells. Result analysis made it possible to draw conclusions regarding the interpretation of the thermal response test. The method of averaging the measurement results was subjected to further study. The measuring apparatus recorded data at a frequency of one second, however such accuracy was too large to be analyzed efficiently. Therefore, an average of every 1 min, every 10 min, and every 60 min was proposed. The conclusions stemming from the differences in the values of effective thermal conductivity in the borehole heat exchanger, resulting from different data averaging, were described. In the case of three borehole heat exchangers, ground properties were identical. The effective thermal conductivity λeff was shown to depend on various borehole heat exchanger (BHE) designs, heat carrier flow geometry, and grout parameters. It is important to consider the position of the pipes relative to each other. As shown in the charts, the best (the highest) effective thermal conductivity λeff occurred in BHE-1 with a coaxial construction. At the same time, this value was closest to the theoretical value of thermal conductivity of rocks λ, determined on the basis of literature. The standard deviation and the coefficient of variation confirmed that the effective thermal conductivity λeff, calculated for different time intervals, showed little variation in value. The values of effective thermal conductivity λeff for each time interval for the same borehole exchanger were similar in value. The lowest values of effective thermal conductivity λeff most often appeared for analysis with averaging every 60 min, and the highest—for analysis with averaging every 1 min. For safety reasons, when designing (number of BHEs), safer values should be taken for analysis, i.e., lower, averaging every 60 min. Full article
(This article belongs to the Special Issue Thermal Response Tests for Shallow Geothermal Systems)
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Numerical Analysis of Thermal Behaviour of DC Fuse
Energies 2020, 13(14), 3736; https://doi.org/10.3390/en13143736 - 20 Jul 2020
Cited by 1 | Viewed by 432
Abstract
One of the very well-known protections for electrical apparatus against overloads or short circuits is the fuse. It can be used to protect both AC or DC electrical installations and it has also proven its effectiveness in the protection of different loads. This [...] Read more.
One of the very well-known protections for electrical apparatus against overloads or short circuits is the fuse. It can be used to protect both AC or DC electrical installations and it has also proven its effectiveness in the protection of different loads. This paper describes a three-dimensional model of a DC fuse with two different types of fuselink notches: circular and rhombic. The obtained 3D thermal model can be used to investigate the thermal behaviour of DC fuses in both steady-state and transient conditions at different values of overloads or short circuits. With the aim to validate the proposed 3D thermal model, a series of experimental tests have been achieved. The thermal simulated values are in good concordance with the experimental results (a relative error less than ±6% has been obtained between experimental and simulation data). Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessFeature PaperArticle
Effects of Injector Spray Angle on Performance of an Opposed-Piston Free-Piston Engine
Energies 2020, 13(14), 3735; https://doi.org/10.3390/en13143735 - 20 Jul 2020
Cited by 1 | Viewed by 456
Abstract
A free-piston engine is a novel internal combustion engine which has the advantages of a variable compression ratio and multi-fuel adaptability. This paper focuses on numerical simulation for combustion process and spray angle optimization of an opposed-piston free-piston engine. The working principle and [...] Read more.
A free-piston engine is a novel internal combustion engine which has the advantages of a variable compression ratio and multi-fuel adaptability. This paper focuses on numerical simulation for combustion process and spray angle optimization of an opposed-piston free-piston engine. The working principle and spray-guided central combustor structure of the engine are discussed. A three-dimensional computational fluid dynamic model with moving mesh is presented based on the tested piston motion of the prototype. Calculation conditions, spray models, and combustion models were set-up according to the same prototype. The effects of spray angle on fuel evaporation rate, mixture distribution, heat release rate, in-cylinder pressure, in-cylinder temperature, and emissions were simulated and analyzed in detail. The research results indicate that the performance of the engine was very sensitive to the spray angle. The combustion efficiency and the indicated thermal efficiencies of 97.5% and 39.7% were obtained as the spray angle reached 40°. Full article
(This article belongs to the Section Energy Storage and Application)
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Open AccessArticle
1D Simulation and Experimental Analysis on the Effects of the Injection Parameters in Methane–Diesel Dual-Fuel Combustion
Energies 2020, 13(14), 3734; https://doi.org/10.3390/en13143734 - 20 Jul 2020
Cited by 3 | Viewed by 758
Abstract
Notwithstanding the policies that move towards electrified powertrains, the transportation sector mainly employs internal combustion engines as the primary propulsion system. In this regard, for medium- to heavy-duty applications, as well as for on- and off-road applications, diesel engines are preferred because of [...] Read more.
Notwithstanding the policies that move towards electrified powertrains, the transportation sector mainly employs internal combustion engines as the primary propulsion system. In this regard, for medium- to heavy-duty applications, as well as for on- and off-road applications, diesel engines are preferred because of the better efficiency, lower CO2, and greater robustness compared to spark-ignition engines. Due to its use at a large scale, the internal combustion engines as a source of energy depletion and pollutant emissions must further improved. In this sense, the adoption of alternative combustion concepts using cleaner fuels than diesel (e.g., natural gas, ethanol and methanol) presents a viable solution for improving the efficiency and emissions of the future powertrains. Particularly, the methane–diesel dual-fuel concept represents a possible solution for compression ignition engines because the use of the low-carbon methane fuel, a main constituent of natural gas, as primary fuel significantly reduces the CO2 emissions compared to conventional liquid fuels. Nonetheless, other issues concerning higher total hydrocarbon (THC) and CO emissions, mainly at low load conditions, are found. To minimize this issue, this research paper evaluates, through a new and alternative approach, the effects of different engine control parameters, such as rail pressure, pilot quantity, start of injection and premixed ratio in terms of efficiency and emissions, and compared to the conventional diesel combustion mode. Indeed, for a deeper understanding of the results, a 1-Dimensional spray model is used to model the air-fuel mixing phenomenon in response to the variations of the calibration parameters that condition the subsequent dual-fuel combustion evolution. Specific variation settings, in terms of premixed ratio, injection pressure, pilot quantity and combustion phasing are proposed for further efficiency improvements. Full article
(This article belongs to the Special Issue Research and Development in Single-Cylinder Engines)
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Open AccessArticle
Comparative Study of CFD and LedaFlow Models for Riser-Induced Slug Flow
Energies 2020, 13(14), 3733; https://doi.org/10.3390/en13143733 - 20 Jul 2020
Viewed by 499
Abstract
The goal of this study is to compare mainstream Computational Fluid Dynamics (CFD) with the widely used 1D transient model LedaFlow in their ability to predict riser induced slug flow and to determine if it is relevant for the offshore oil and gas [...] Read more.
The goal of this study is to compare mainstream Computational Fluid Dynamics (CFD) with the widely used 1D transient model LedaFlow in their ability to predict riser induced slug flow and to determine if it is relevant for the offshore oil and gas industry to consider making the switch from LedaFlow to CFD. Presently, the industry use relatively simple 1D-models, such as LedaFlow, to predict flow patterns in pipelines. The reduction in cost of computational power in recent years have made it relevant to compare the performance of these codes with high fidelity CFD simulations. A laboratory test facility was used to obtain data for pressure and mass flow rates for the two-phase flow of air and water. A benchmark case of slug flow served for evaluation of the numerical models. A 3D unsteady CFD simulation was performed based on Reynolds-Averaged Navier-Stokes (RANS) formulation and the Volume of Fluid (VOF) model using the open-source CFD code OpenFOAM. Unsteady simulations using the commercial 1D LedaFlow solver were performed using the same boundary conditions and fluid properties as the CFD simulation. Both the CFD and LedaFlow model underpredicted the experimentally determined slug frequency by 22% and 16% respectively. Both models predicted a classical blowout, in which the riser is completely evacuated of water, while only a partial evacuation of the riser was observed experimentally. The CFD model had a runtime of 57 h while the LedaFlow model had a runtime of 13 min. It can be concluded that the prediction capabilities of the CFD and LedaFlow models are similar for riser-induced slug flow while the CFD model is much more computational intensive. Full article
(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020) Printed Edition available
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Open AccessArticle
Influence of a Thermal Pad on Selected Parameters of Power LEDs
Energies 2020, 13(14), 3732; https://doi.org/10.3390/en13143732 - 20 Jul 2020
Viewed by 441
Abstract
This paper is devoted to the analysis of the influence of thermal pads on electric, optical, and thermal parameters of power LEDs. Measurements of parameters, such as thermal resistance, optical efficiency, and optical power, were performed for selected types of power LEDs operating [...] Read more.
This paper is devoted to the analysis of the influence of thermal pads on electric, optical, and thermal parameters of power LEDs. Measurements of parameters, such as thermal resistance, optical efficiency, and optical power, were performed for selected types of power LEDs operating with a thermal pad and without it at different values of the diode forward current and temperature of the cold plate. First, the measurement set-up used in the paper is described in detail. Then, the measurement results obtained for both considered manners of power LED assembly are compared. Some characteristics that illustrate the influence of forward current and temperature of the cold plate on electric, thermal, and optical properties of the tested devices are presented and discussed. It is shown that the use of the thermal pad makes it possible to achieve more advantageous values of operating parameters of the considered semiconductor devices at lower values of their junction temperature, which guarantees an increase in their lifetime. Full article
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Open AccessArticle
Simulation and Thermo-Energy Analysis of Building Types in the Dominican Republic to Evaluate and Introduce Energy Efficiency in the Envelope
Energies 2020, 13(14), 3731; https://doi.org/10.3390/en13143731 - 20 Jul 2020
Viewed by 501
Abstract
The improvement of the energy performance in buildings is key for sustainable development, even more so in the case of the Dominican Republic (DR), which is committed to this goal but which has neither regulation nor specific social behavior in this field. The [...] Read more.
The improvement of the energy performance in buildings is key for sustainable development, even more so in the case of the Dominican Republic (DR), which is committed to this goal but which has neither regulation nor specific social behavior in this field. The main goal of this work is double; on one hand it is aimed at providing useable information for those who have the responsibly of making regulation norms and on the other, it is desirable to give an essential, technically proven and handy tool to those involved in the construction sector in improving the envelopes of buildings and to introduce good practices into the management of the energy systems of buildings. A case study of eight administrative buildings located in different climatic zones of the DR was carried out. A simulation tool was used for the study, and one of the buildings was monitored to verify the simulation work. Those factors that affect the development of the buildings in relation to thermo-energy consumption have been detailed. The large-scale heat gains resulting from the common glazing used by the tertiary sector in the Dominican Republic (including office buildings, hospitals and shops among others) illustrate the need for economically viable solutions in this sector. As a conclusion, it has been proved that the incidental thermal load of buildings could be reduced by up to 40%, thus in turn reducing the costs associated with the electricity needed to maintain the users’ desired thermal comfort level, as their influence in this sector is significant. Full article
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Open AccessReview
Advanced Distribution Measurement Technologies and Data Applications for Smart Grids: A Review
Energies 2020, 13(14), 3730; https://doi.org/10.3390/en13143730 - 20 Jul 2020
Cited by 2 | Viewed by 944
Abstract
The integration of advanced measuring technologies in distribution systems allows distribution system operators to have better observability of dynamic and transient events. In this work, the applications of distribution grid measurement technologies are explored in detail. The main contributions of this review are: [...] Read more.
The integration of advanced measuring technologies in distribution systems allows distribution system operators to have better observability of dynamic and transient events. In this work, the applications of distribution grid measurement technologies are explored in detail. The main contributions of this review are: (a) a comparison of eight advanced measurement devices for distribution networks, based on their technical characteristics, including reporting periods, measuring data, precision, and sample rate; (b) a review of the most recent applications of micro-Phasor Measurement Units, Smart Meters, and Power Quality Monitoring devices used in distribution systems, considering different novel methods applied for data analysis; and (c) an input-output table that relates measured quantities from micro-Phasor Measurement Units and Smart Meters needed for each specific application found in this extensive review. This paper aims to serve as an important guide for researches and engineers studying smart grids. Full article
(This article belongs to the Special Issue Smart Distribution Grid Technologies and Applications)
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Open AccessArticle
Techno-Economic Assessment of Three Modes of Large-Scale Crop Residue Utilization Projects in China
Energies 2020, 13(14), 3729; https://doi.org/10.3390/en13143729 - 20 Jul 2020
Cited by 4 | Viewed by 941
Abstract
In China, the non-exploitation of bioenergy poses major problems and challenges. To solve bioenergy problems, considerable efforts have been made to expedite the construction of large-scale crop residue utilization projects. In this study, three principal supported modes of large-scale crop residue utilization projects [...] Read more.
In China, the non-exploitation of bioenergy poses major problems and challenges. To solve bioenergy problems, considerable efforts have been made to expedite the construction of large-scale crop residue utilization projects. In this study, three principal supported modes of large-scale crop residue utilization projects were taken as empirical cases in Hubei province bioenergy planning. In terms of the overall benefit and sustainable development, a third-grade evaluation index system was established. The analysis was carried out using the analytical hierarchy process, principal component projection, and grey relational analysis. The conclusion indicates that according to the evaluation values, the sequence from best to worst was crop residue biogas project, crop residue briquette fuel project, and crop residue gasification project. Nevertheless, there was no remarkable difference in the overall evaluation values. The biogas project had certain advantages in terms of the production cost, soil improvement, and expenditure saving, whereas the gasification project was comparatively insufficient in environmental efficiency, product benefit, by-product disposal, and technical rationality. According to actual evaluation results, the unilateral determination approach of the single weight index can be seen as being overcome through the unified adaptation of the evaluation methods. The research results can serve as a reference for making investment decisions to build large-scale crop residue utilization projects. Full article
(This article belongs to the Section Bio-Energy)
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Open AccessArticle
Structural, Textural, and Catalytic Properties of Ni-CexZr1−xO2 Catalysts for Methane Dry Reforming Prepared by Continuous Synthesis in Supercritical Isopropanol
Energies 2020, 13(14), 3728; https://doi.org/10.3390/en13143728 - 20 Jul 2020
Viewed by 448
Abstract
A series of 5%Ni-CexZr1−xO2 (x = 0.3, 0.5, 0.7) catalysts has been prepared via one-pot solvothermal continuous synthesis in supercritical isopropanol and incipient wetness impregnation of CexZr1−xO2 obtained by the same route. The [...] Read more.
A series of 5%Ni-CexZr1−xO2 (x = 0.3, 0.5, 0.7) catalysts has been prepared via one-pot solvothermal continuous synthesis in supercritical isopropanol and incipient wetness impregnation of CexZr1−xO2 obtained by the same route. The textural, structural, red-ox, and catalytic properties in methane dry reforming (MDR) of Ni-modified Ce-Zr oxides synthesized by two routes have been compared. It was shown by XRD, TEM, and Raman spectroscopy that the method of Ni introduction does not affect the phase composition of the catalysts, but determines the dispersion of NiO. Despite a high dispersion of NiO and near-uniform distribution of Ni within Ce-Zr particles observed for the one-pot catalysts, they have shown a lower activity and stability in MDR as compared with impregnated ones. This is a result of a low Ni concentration in the surface layer due to segregation of Ce and decoration of nickel nanoparticles with support species. Full article
(This article belongs to the Special Issue Hydrogen and Syngas Generation)
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Open AccessArticle
A Three-Level DC-Link Quasi-Switch Boost T-Type Inverter with Voltage Stress Reduction
Energies 2020, 13(14), 3727; https://doi.org/10.3390/en13143727 - 20 Jul 2020
Viewed by 506
Abstract
In recent years, the three-level T-Type inverter has been considered the best choice for many low and medium power applications. Nevertheless, this topology is known as a buck converter. Therefore, in this paper, a new topology incorporating the dc-link type quasi-switched boost network [...] Read more.
In recent years, the three-level T-Type inverter has been considered the best choice for many low and medium power applications. Nevertheless, this topology is known as a buck converter. Therefore, in this paper, a new topology incorporating the dc-link type quasi-switched boost network with the traditional three-level T-type inverter is proposed to overcome the limit of traditional three-level T-Type inverter. The space vector pulse width modulation scheme is considered to control this topology, which provides some benefits such as enhancing modulation index and reducing the magnitude of common-mode voltage. For this scheme, the zero, medium, and large vectors are utilized to generate the output voltage. The shoot-through state which is adopted by turning on all power switches of inverter leg is inserted into zero vector to boost the dc-link voltage. As a result, there is no distortion at the output waveform. The control signal of intermediate network power switches is also detailed to improve the boost factor and voltage gain. As a result, the voltage stress on power devices like capacitors, diodes, and switches is decreased significantly. To demonstrate the outstanding of proposed structure and its control strategy, some comparisons between the proposed method and other ones are performed. Simulation and experimental prototype results are conducted to verify the accuracy of the theory and effectiveness of the inverter. Full article
(This article belongs to the Special Issue Impedance Source Converters: Topologies, Control, and Applications)
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Open AccessArticle
Dehydration Leads to Hydrocarbon Gas Formation in Thermal Degradation of Gas-Phase Polyalcohols
Energies 2020, 13(14), 3726; https://doi.org/10.3390/en13143726 - 20 Jul 2020
Viewed by 420
Abstract
To understand the molecular mechanisms of hydrocarbon gas formation in biomass gasification, gasification of simple polyalcohols (glycerol, propylene glycol, and ethylene glycol) were studied at 400, 600, and 800 °C (residence times: 0.9–1.4 s) from the viewpoint of dehydration reactions that form aldehydes [...] Read more.
To understand the molecular mechanisms of hydrocarbon gas formation in biomass gasification, gasification of simple polyalcohols (glycerol, propylene glycol, and ethylene glycol) were studied at 400, 600, and 800 °C (residence times: 0.9–1.4 s) from the viewpoint of dehydration reactions that form aldehydes with various substituents as intermediates to produce hydrocarbon gases. The results were also compared with those of glyceraldehyde and dihydroxyacetone, which are reported to produce syngas (H2 and CO) selectively. All polyalcohols became reactive at 600 °C to form condensable products in 15.7–24.7% yields (C-based), corresponding to 33.9–38.4% based on the amounts of reacted polyalcohols. These condensable products, mostly aldehydes, act as gas-forming intermediates, because the polyalcohols were completely gasified at 800 °C (hydrocarbon gas contents: 20.3–35.3%, C-based). Yields of the intermediates bearing alkyl groups at 600 °C were proportionally correlated to the yields of hydrocarbon gases at 800 °C, suggesting that the alkyl groups are further converted into hydrocarbon gases via the fragmentation of acyl radicals. Dehydration reactions were suggested to occur in both heterolytic and radical mechanisms by theoretical calculations. Glyceraldehyde tended to fragment directly into CO and H2, instead of forming a dehydration intermediate. These results are informative for controlling the product gas composition in biomass gasification. Full article
(This article belongs to the Special Issue Biomass Conversion Technologies)
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