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Energies, Volume 12, Issue 10 (May-2 2019)

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Cover Story (view full-size image) Deep borehole disposal relies on largely off-the-shelf technology to place long-lived nuclear waste [...] Read more.
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Open AccessReview
The Role of Off-Grid Houses in the Energy Transition with a Case Study in the Netherlands
Energies 2019, 12(10), 2033; https://doi.org/10.3390/en12102033
Received: 2 April 2019 / Revised: 16 May 2019 / Accepted: 23 May 2019 / Published: 27 May 2019
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Abstract
Off-grid houses can be considered an important concept to increase the access to electricity throughout the world. Although there are quite some initiatives in place to improve the access to electricity, the implementation rate of practical solutions is far below the UN Sustainable [...] Read more.
Off-grid houses can be considered an important concept to increase the access to electricity throughout the world. Although there are quite some initiatives in place to improve the access to electricity, the implementation rate of practical solutions is far below the UN Sustainable Development Goal 7: Energy (SDG 7) + for 2030. This situation is most apparent in Sub-Saharan Africa, where the current trends of electricity access calculated by the World Bank indicate that this region will not be able to achieve the SDG 7 target. Another worldwide trend which may help to increase electricity access is that currently, a lot of renewable energy generation is realized locally in houses (especially Solar Photovoltaics (PV)). This paper reviews the recent developments to increase the access to electricity in the world and the implementation of off-grid houses in different scenarios. The focus here is on the different efforts to create off-grid houses considering their challenges on a macro and micro level. Moreover, potential research directions for technologies in off-grid houses are presented in more detail. For this, a case description of a possible off-grid house in the Netherlands is presented together with some initial simulations results for this case using solar PV, the Sea-Salt battery, and a Glycerol Fuel Cell. The simulations use the DEMkit software and the analysis is performed using measured house load data for a period in winter and in summer. Full article
(This article belongs to the Special Issue Electricity for Energy Transition)
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Open AccessArticle
Roof Cutting Parameters Design for Gob-Side Entry in Deep Coal Mine: A Case Study
Energies 2019, 12(10), 2032; https://doi.org/10.3390/en12102032
Received: 6 May 2019 / Revised: 22 May 2019 / Accepted: 25 May 2019 / Published: 27 May 2019
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Abstract
Roof cutting is an effective technique for controlling the deformation and failure of the surrounding rock in deep gob-side entry. The determination of the roof cutting parameters has become a popular research subject. Initially, two mechanical models are established for the non-roof-cutting and [...] Read more.
Roof cutting is an effective technique for controlling the deformation and failure of the surrounding rock in deep gob-side entry. The determination of the roof cutting parameters has become a popular research subject. Initially, two mechanical models are established for the non-roof-cutting and roof-cutting of gob-side entry in deep mining conditions. On this basis, the necessity and significance of roof cutting is revealed by analysing the stress and displacement of roadside prop. The Universal Distinct Element Code numerical simulation model is established to determine the key roof-cutting parameters (cutting angle and cutting height) according to the on-site situation of No. 2415 headentry of the Suncun coal mine, China. The numerical simulation results show that with the cutting angle and height increase, the vertical stress and horizontal displacement of the coal wall first increase and then decrease, as in the case of the vertical stress and displacement of roadside prop. Therefore, the optimum roof cutting parameters are determined as a cutting angle of 70° and cutting height of 8 m. Finally, a field application was performed at the No. 2415 headentry of the Suncun coal mine. In situ investigations show that after 10 m lagged the working face, the stress and displacement of roadside prop are obviously reduced with the hanging roof smoothly cut down, and they are stable at 19 MPa and 145 mm at 32 m behind the working face, respectively. This indicates that the stability of the surrounding rock was effectively controlled. This research demonstrates that the key parameters determined through a numerical simulation satisfactorily meet the production requirements and provide a reference for ensuring safe production in deep mining conditions. Full article
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Open AccessFeature PaperArticle
Development of Hardware-in-the-Loop-Simulation Testbed for Pitch Control System Performance Test
Energies 2019, 12(10), 2031; https://doi.org/10.3390/en12102031
Received: 22 April 2019 / Revised: 22 May 2019 / Accepted: 23 May 2019 / Published: 27 May 2019
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Abstract
This paper deals with the development of a wind turbine pitch control system and the construction of a Hardware-in-the-Loop-Simulation (HILS) testbed for the performance test of the pitch control system. When the wind speed exceeds the rated wind speed, the wind turbine pitch [...] Read more.
This paper deals with the development of a wind turbine pitch control system and the construction of a Hardware-in-the-Loop-Simulation (HILS) testbed for the performance test of the pitch control system. When the wind speed exceeds the rated wind speed, the wind turbine pitch controller adjusts the blade pitch angles collectively to ensure that the rotor speed maintains the rated rotor speed. The pitch controller with the individual pitch control function can add individual pitch angles into the collective pitch angles to reduce the mechanical load applied to the blade periodically due to wind shear. Large wind turbines often experience mechanical loads caused by wind shear phenomena. To verify the performance of the pitch control system before applying it to an actual wind turbine, the pitch control system is tested on the HILS testbed, which acts like an actual wind turbine system. The testbed for evaluating the developed pitch control system consists of the pitch control system, a real-time unit for simulating the wind and the operations of the wind turbine, an operational computer with a human–machine interface, a load system for simulating the actual wind load applied to each blade, and a real pitch bearing. Through the several tests based on HILS test bed, how well the pitch controller performed the given roles for each area in the entire wind speed area from cut-in to cut-out wind speed can be shown. Full article
(This article belongs to the Special Issue Design, Fabrication and Performance of Wind Turbines)
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Open AccessArticle
Analysis of Research Topics and Scientific Collaborations in Energy Saving Using Bibliometric Techniques and Community Detection
Energies 2019, 12(10), 2030; https://doi.org/10.3390/en12102030
Received: 15 April 2019 / Revised: 21 May 2019 / Accepted: 24 May 2019 / Published: 27 May 2019
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Abstract
Concern about everything related to energy is increasingly latent in the world and therefore the use of energy saving concepts has been increasing over the past several years. The interest in the subject has allowed a conceptual evolution in the scientific community regarding [...] Read more.
Concern about everything related to energy is increasingly latent in the world and therefore the use of energy saving concepts has been increasing over the past several years. The interest in the subject has allowed a conceptual evolution in the scientific community regarding the understanding of the adequate use of energy. The objective of this work is to determine the contribution made by international institutions to the specialized publications in the area of energy-saving from 1939 to 2018, using Scopus Database API Interface. The methodology followed in this research was to perform a bibliometric analysis of the whole scientific production indexed in Scopus. The world’s scientific production has been analysed in the following domains: First the trend over time, types of publications and countries, second, the main subjects and keywords, third, main institutions and their main topics, and fourth, the main journals and proceedings that publish on this topic. Then, these data are presented using community detection algorithms and graph visualization software. With these techniques, it is possible to determine the main areas of research activity as well as to identify the structures of the collaboration network in the field of renewable energy. The results of the work show that the literature in this field have substantially increased during the last 10 years. Full article
(This article belongs to the Special Issue Energy Saving at Cities)
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Open AccessArticle
Public Access to Building Related Energy Data for Better Decision Making in Implementing Energy Efficiency Strategies: Legal Barriers and Technical Challenges
Energies 2019, 12(10), 2029; https://doi.org/10.3390/en12102029
Received: 22 April 2019 / Revised: 18 May 2019 / Accepted: 22 May 2019 / Published: 27 May 2019
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Abstract
The EU is committed to achieve ambitious energy efficiency and CO2 reduction targets. Regarding improving the energy performance of the building stock, detailed building related energy data made accessible by means of GIS are crucial. For this purpose, geo-referenced data from Energy [...] Read more.
The EU is committed to achieve ambitious energy efficiency and CO2 reduction targets. Regarding improving the energy performance of the building stock, detailed building related energy data made accessible by means of GIS are crucial. For this purpose, geo-referenced data from Energy Performance Certificates (EPC) according to the Energy Performance of Buildings Directive is of central importance, in order to facilitate decision-making about large-scale renovation projects and developing targeted products and services. The ENERFUND project developed a method and subsequently an internet based tool making use of EPC data from 13 EU member states as well as other open-source data (such as renovation costs), thus clearly demonstrating the feasibility and usefulness of this approach. However, also technical challenges and legal barriers were encountered, such as a lack of data and varying data specifications depending on the national transposition of the European Directive, and different interpretation of specific clauses of the General Data Protection Regulation depending on EU member states’ societal norms. Recommendations include the development of guidance notes to be issued by the responsible EU bodies to specify and harmonize data for mandatory public access, to ensure the effective implementation of energy efficiency and CO2 reduction policies in the EU’s building sector. Full article
(This article belongs to the Section Energy and Buildings)
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Open AccessArticle
Thermodynamic Selection of the Optimal Working Fluid for Organic Rankine Cycles
Energies 2019, 12(10), 2028; https://doi.org/10.3390/en12102028
Received: 6 May 2019 / Revised: 20 May 2019 / Accepted: 24 May 2019 / Published: 27 May 2019
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Abstract
A novel method proposed to choose the optimal working fluid—solely from the point of view of expansion route—for a given heat source and heat sink (characterized by a maximum and minimum temperature). The basis of this method is the novel classification of working [...] Read more.
A novel method proposed to choose the optimal working fluid—solely from the point of view of expansion route—for a given heat source and heat sink (characterized by a maximum and minimum temperature). The basis of this method is the novel classification of working fluids using the sequences of their characteristic points on temperature-entropy space. The most suitable existing working fluid can be selected, where an ideal adiabatic (isentropic) expansion step between a given upper and lower temperature is possible in a way, that the initial and final states are both saturated vapour states and the ideal (isentropic) expansion line runs in the superheated (dry) vapour region all along the expansion. Problems related to the presence of droplets or superheated dry steam in the final expansion state can be avoided or minimized by using the working fluid chosen with this method. Results obtained with real materials are compared with those gained with model (van der Waals) fluids; based on the results obtained with model fluids, erroneous experimental data-sets can be pinpointed. Since most of the known working fluids have optimal expansion routes at low temperatures, presently the method is most suitable to choose working fluids for cryogenic cycles, applied for example for heat recovery during LNG-regasification. Some of the materials, however, can be applied in ranges located at relatively higher temperatures, therefore the method can also be applied in some limited manner for the utilization of other low temperature heat sources (like geothermal or waste heat) as well. Full article
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Open AccessArticle
An Energy Potential Estimation Methodology and Novel Prototype Design for Building-Integrated Wind Turbines
Energies 2019, 12(10), 2027; https://doi.org/10.3390/en12102027
Received: 13 April 2019 / Revised: 11 May 2019 / Accepted: 23 May 2019 / Published: 27 May 2019
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Abstract
ROSEO-BIWT is a new Building-Integrated Wind Turbine (BIWT) intended for installation on the edge of buildings. It consists of a Savonius wind turbine and guiding vanes to accelerate the usual horizontal wind, together with the vertical upward air stream on the wall. This [...] Read more.
ROSEO-BIWT is a new Building-Integrated Wind Turbine (BIWT) intended for installation on the edge of buildings. It consists of a Savonius wind turbine and guiding vanes to accelerate the usual horizontal wind, together with the vertical upward air stream on the wall. This edge effect improves the performance of the wind turbine, and its architectural integration is also beneficial. The hypothetical performance and design configuration were studied for a university building in Eibar city using wind data from the ERA5 reanalysis (European Centre for Medium-Range Weather Forecasts’ reanalysis), an anemometer to calibrate the data, and the actual small-scale behavior in a wind tunnel. The data acquired by the anemometer show high correlations with the ERA5 data in the direction parallel to the valley, and the calibration is therefore valid. According to the results, a wind speed augmentation factor of three due to the edge effect and concentration vanes would lead to a increase in working hours at the rated power, resulting annually in more than 2000 h. Full article
(This article belongs to the Special Issue Alternative Energy Systems in Buildings)
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Open AccessArticle
Fundamental Investigation of the Effects of Modified Starch, Carboxymethylcellulose Sodium, and Xanthan Gum on Hydrate Formation under Different Driving Forces
Energies 2019, 12(10), 2026; https://doi.org/10.3390/en12102026
Received: 14 April 2019 / Revised: 17 May 2019 / Accepted: 24 May 2019 / Published: 27 May 2019
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Abstract
The development of a new drilling fluid system with hydrate inhibition is of great significance for drilling safety in gas hydrate-bearing sediments. Considering the importance of the selection of a suitable thickener for drilling fluid systems under weak and strong driving forces, the [...] Read more.
The development of a new drilling fluid system with hydrate inhibition is of great significance for drilling safety in gas hydrate-bearing sediments. Considering the importance of the selection of a suitable thickener for drilling fluid systems under weak and strong driving forces, the hydrate inhibition of 0.1–0.5 wt% modified starch (MS), carboxymethylcellulose sodium (CMC), and xanthan gum (XG) aqueous solutions was studied. The applicability of these three thickeners were investigated through hydrate formation experiments, mesostructure observations, water activity tests, bubble retention observations, and rheological property tests. The results show that (1) under weak driving force, 0.3 wt% or higher concentration CMC and 0.3 wt% XG can almost completely inhibit hydrate formation due to the interactions between relatively small amounts of free water and CH4 molecules. Furthermore, the hydrate inhibition of higher XG concentrations was decreased due to their strong foam stability, leading to good contact between free water and CH4 molecules. Meanwhile, the hydrate inhibition of MS was weaker when compared with that of CMC and XG at the same concentrations. (2) Under strong driving force, the existence of the three 0.1–0.5 wt% thickeners could only slow down the hydrate formation rate, and hydrate inhibition due to XG was slightly better than that of the other two. This result implies that the effects of the different mesostructures on hydrate formation were severely weakened. Finally, (3) the tackifying effect of CMC was found to be stronger than that of XG and MS, and the rheological properties of the CMC solution were shown to be relatively weak compared to those of the XG and MS solutions; the CMC solution showed a more significant increase in viscosity with decreasing temperature, which is related to the differences in the mesostructures. Therefore, when the driving force of hydrate formation is relatively low, CMC is a good choice for the drilling fluid system when there is no requirement for cooling, while XG is more applicable for a system that needs cooling. In the case of a stronger driving force, XG is the optimal choice irrespective of whether the drilling fluid system needs cooling or not. Full article
(This article belongs to the Section Energy Sources)
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Open AccessArticle
City-Level Features of Energy Footprints and Carbon Dioxide Emissions in Sichuan Province of China
Energies 2019, 12(10), 2025; https://doi.org/10.3390/en12102025
Received: 26 April 2019 / Revised: 12 May 2019 / Accepted: 23 May 2019 / Published: 27 May 2019
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Abstract
The sustainable development of the western region of China has always been essential to the national development strategy. The Western region has undertaken an industrial transfer from the Eastern and Central regions. Therefore, the CO2 emission intensity in the western region is [...] Read more.
The sustainable development of the western region of China has always been essential to the national development strategy. The Western region has undertaken an industrial transfer from the Eastern and Central regions. Therefore, the CO2 emission intensity in the western region is higher than those of the Eastern and Central regions of China, and consequently its low-carbon development pathway has an important impact for China as a whole. Sichuan Province is not only the province with the highest CO2 emissions, but also the most economically developed province in Western China in 2018. In order to promote low carbon development in the western region, it is important to understand the features of emissions in Sichuan Province and to formulate effective energy strategies accordingly. This paper uses the IPCC regional emission accounting method to calculate the carbon emissions of 15 cities in Sichuan province, and to comply with the city-level emission accounts. The results show that the total carbon emissions of Sichuan province over the past 10 years was 3258.32 mt and reached a peak in 2012. The smelting and pressing of ferrous metals, coal mining and dressing were the leading sectors that contributed to the emissions, accounting for 17.86% and 15.82%, respectively. Raw coal, cleaned coal, and coke were the most significant contributors to CO2 emissions, accounting for 43.73%, 9.55%, and 6.60%, respectively. Following the above results, the Sichuan provincial government can formulate differentiated energy structure policies according to different energy consumption structures and carbon emission levels in the 15 cities. By controlling the level of total emissions and regulating larger industrial emitters in Sichuan province, some useful information could be provided as an essential reference for low-carbon development in Western China, and contribute to the promotion of emissions mitigation from a more holistic perspective. Full article
(This article belongs to the Special Issue Carbon Footprinting and Life Cycle Assessment)
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Open AccessReview
Energy Management Strategies for Hybrid Construction Machinery: Evolution, Classification, Comparison and Future Trends
Energies 2019, 12(10), 2024; https://doi.org/10.3390/en12102024
Received: 6 May 2019 / Revised: 19 May 2019 / Accepted: 22 May 2019 / Published: 27 May 2019
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Abstract
Hybrid Construction Machinery (HCM), known as an effective and crucial solution for the issues of environment pollution and energy shortage, has aroused increasing attention from manufacturers and researchers. A suitable energy management strategy is the vital technology to determine the energy saving and [...] Read more.
Hybrid Construction Machinery (HCM), known as an effective and crucial solution for the issues of environment pollution and energy shortage, has aroused increasing attention from manufacturers and researchers. A suitable energy management strategy is the vital technology to determine the energy saving and emission reduction performance of HCM. In the present paper, the difference between construction machinery and automobiles is first analyzed from the perspective of configuration, and the energy-based HCM configuration classification method is introduced and analyzed. Second, the development of HCM energy management strategy is reviewed along with relevant references, and the HCM energy management strategies are classified and summarized. In the meantime, the characteristics of each strategy are compared and analyzed, and the application of HCM energy management strategy is analyzed based on the relevant research results. Lastly, the state, challenges facing and the trend of HCM energy management strategy are analyzed on the levels of theory, manufacturer and market. According to the analysis, though progress has been achieved in energy management technology of HCM driven by market and policy, many challenges and problems remain in the electrification and intellectualization of HCM and the testing, application and improvement of the strategy. The contribution of this paper can be identified in three points: First, it can be referenced to solve relevant engineering problems. Second, it lays the foundation for the proposal of new ideas. Third, it highlights the state-of-the-art trends and avoids what has already been done. Full article
(This article belongs to the Section Energy Storage and Application)
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Open AccessArticle
Coupled Motion Characteristics of Offshore Wind Turbines during the Integrated Transportation Process
Energies 2019, 12(10), 2023; https://doi.org/10.3390/en12102023
Received: 4 April 2019 / Revised: 16 May 2019 / Accepted: 21 May 2019 / Published: 27 May 2019
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Abstract
The offshore wind turbine (OWT) supported by bucket foundations can be installed in the integrated transportation process by a dedicated vessel. During the integrated transportation process, the wind turbine is considered as a coupling system with the transport ship, which is easily influenced [...] Read more.
The offshore wind turbine (OWT) supported by bucket foundations can be installed in the integrated transportation process by a dedicated vessel. During the integrated transportation process, the wind turbine is considered as a coupling system with the transport ship, which is easily influenced by waves and storms. In view of the motion response and influential factors, the heave and rock stiffness of the entire floating system was proposed, and then the analytical dynamic motion model of the coupling system was established based on the movement mechanism of the traditional floating body in the wave in this paper. Subsequently, the rationality of the proposed motion model was verified based on the field observation data, with the maximum deviation of the motion responses less than 14%. Further, the influence on the heave and pitch motion of the coupling system considering different factors (vessel speed, wave height, wind speed and wave angle) and the factor sensitivity were discussed by the novel analytical model. It is explained that the heave and pitch motion responses rise with the increase of the wave height and wave angle. Simultaneously, the responses decrease as the vessel speed increases considering sailing along the waves. On the contrary, the responses show an obvious increasing trend with the increase of vessel speed in the case of the top wave sailing. In addition, it is also illustrated that the wave height has the greatest influence on the heave and pitch motion responses, followed by the vessel speed. The wave angle has the lowest sensitivity when the heave and pitch motion are far away from its harmonic resonance region. Full article
(This article belongs to the Special Issue Recent Advances in Offshore Wind Technology)
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Open AccessArticle
Volumetric Measurements of Methane-Coal Adsorption and Desorption Isotherms—Effects of Equations of State and Implication for Initial Gas Reserves
Energies 2019, 12(10), 2022; https://doi.org/10.3390/en12102022
Received: 10 May 2019 / Revised: 22 May 2019 / Accepted: 23 May 2019 / Published: 27 May 2019
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Abstract
This study presents the effects of equations of state (EOSs) on methane adsorption capacity, sorption hysteresis and initial gas reserves of a medium volatile bituminous coal. The sorption experiments were performed, at temperatures of 25 °C and 40 °C and up to 7MPa [...] Read more.
This study presents the effects of equations of state (EOSs) on methane adsorption capacity, sorption hysteresis and initial gas reserves of a medium volatile bituminous coal. The sorption experiments were performed, at temperatures of 25 °C and 40 °C and up to 7MPa pressure, using a high-pressure volumetric analyzer (HPVA-II). The measured isotherms were parameterized with the modified (three-parameter) Langmuir model. Gas compressibility factors were calculated using six popular equations of state and the results were compared with those obtained using gas compressibility factors from NIST-Refprop® (which implies McCarty and Arp’s EOS for Z-factor of helium and Setzmann and Wagner’s EOS for that of methane). Significant variations were observed in the resulting isotherms and associated model parameters with EOS. Negligible hysteresis was observed with NIST-refprop at both experimental temperatures, with the desorption isotherm being slightly lower than the adsorption isotherm at 25 °C. Compared to NIST-refprop, it was observed that equations of state that gave lower values of Z-factor for methane resulted in “positive hysteresis”, (one in which the desorption isotherm is above the corresponding adsorption curve) and the more negatively deviated the Z-factors are, the bigger the observed hysteresis loop. Conversely, equations of state that gave positively deviated Z-factors of methane relatively produced “negative hysteresis” loops where the desorption isotherms are lower than the corresponding adsorption isotherms. Adsorbed gas accounted for over 90% of the calculated original gas in place (OGIP) and the larger the Langmuir volume, the larger the proportion of OGIP that was adsorbed. Full article
(This article belongs to the Special Issue Development of Unconventional Reservoirs)
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Open AccessArticle
A Four-Winding Inductive Filtering Transformer to Enhance Power Quality in a High-Voltage Distribution Network Supplying Nonlinear Loads
Energies 2019, 12(10), 2021; https://doi.org/10.3390/en12102021
Received: 26 April 2019 / Revised: 22 May 2019 / Accepted: 24 May 2019 / Published: 27 May 2019
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Abstract
This paper solves the problem of reactive power and harmonics compensation in a high-voltage (HV) distribution network supplying nonlinear loads. An inductive filtering (IF) approach where passive filters connect to the filtering winding of a four-winding inductive filtering transformer (FW-IFT) is presented to [...] Read more.
This paper solves the problem of reactive power and harmonics compensation in a high-voltage (HV) distribution network supplying nonlinear loads. An inductive filtering (IF) approach where passive filters connect to the filtering winding of a four-winding inductive filtering transformer (FW-IFT) is presented to enhance the power quality of the public grid. This method can not only greatly suppress harmonic currents of the medium and/or low-voltage (LV) side, but also prevent them from flowing into the public grid. The new main circuit topology, where the FW-IFT has specific filtering winding by adopting the ampere-turn balance of the transformer, is presented. On the basis of the structure of the FW-IFT, the magnetic potential balanced equation and inductive filtering technology, its equivalent circuit and mathematical model are established, and the filtering performances are analyzed in detail. Simulation and experimental results rated at SN-10/0.38 of the FW-IFT are presented to prove the efficacy of the comprehensive enhancement of power quality on the grid side. Full article
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Open AccessArticle
Measured Performance of a Mixed-Use Commercial-Building Ground Source Heat Pump System in Sweden
Energies 2019, 12(10), 2020; https://doi.org/10.3390/en12102020
Received: 3 May 2019 / Revised: 20 May 2019 / Accepted: 20 May 2019 / Published: 27 May 2019
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Abstract
When the new student center at Stockholm University in Sweden was completed in the fall of 2013 it was thoroughly instrumented. The 6300 m2 four-story building with offices, a restaurant, study lounges, and meeting rooms was designed to be energy efficient with [...] Read more.
When the new student center at Stockholm University in Sweden was completed in the fall of 2013 it was thoroughly instrumented. The 6300 m2 four-story building with offices, a restaurant, study lounges, and meeting rooms was designed to be energy efficient with a planned total energy use of 25 kWh/m2/year. Space heating and hot water are provided by a ground source heat pump (GSHP) system consisting of five 40 kW off-the-shelf water-to-water heat pumps connected to 20 boreholes in hard rock, drilled to a depth of 200 m. Space cooling is provided by direct cooling from the boreholes. This paper uses measured performance data from Studenthuset to calculate the actual thermal performance of the GSHP system during one of its early years of operation. Monthly system coefficients-of-performance and coefficients-of-performance for both heating and cooling operation are presented. In the first months of operation, several problems were corrected, leading to improved performance. This paper provides long-term measured system performance data from a recently installed GSHP system, shows how the various system components affect the performance, presents an uncertainty analysis, and describes how some unanticipated consequences of the design may be ameliorated. Seasonal performance factors (SPF) are evaluated based on the SEPEMO (“SEasonal PErformance factor and MOnitoring for heat pump systems”) boundary schema. For heating (“H”), SPFs of 3.7 ± 0.2 and 2.7 ± 0.13 were obtained for boundaries H2 and H3, respectively. For cooling (“C”), a C2 SPF of 27 ± 5 was obtained. Results are compared to measured performance data from 55 GSHP systems serving commercial buildings that are reported in the literature. Full article
(This article belongs to the Special Issue Modelling and Monitoring of Geothermal Heating and Cooling Systems)
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Open AccessArticle
A Vine-Copula Based Voltage State Assessment with Wind Power Integration
Energies 2019, 12(10), 2019; https://doi.org/10.3390/en12102019
Received: 11 February 2019 / Revised: 21 May 2019 / Accepted: 23 May 2019 / Published: 27 May 2019
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Abstract
With the increasing rate of wind power installed capacity, voltage state assessment with large-scale wind power integration is of great significance. In this paper, a vine-copula based voltage state assessment method with large-scale wind power integration is proposed. Firstly, the nonparametric kernel density [...] Read more.
With the increasing rate of wind power installed capacity, voltage state assessment with large-scale wind power integration is of great significance. In this paper, a vine-copula based voltage state assessment method with large-scale wind power integration is proposed. Firstly, the nonparametric kernel density estimation is used to fit the wind speed distribution, and vine-copula is used to construct the wind speed joint distribution model of multiple regions. In order to obtain voltage distribution characteristics, probabilistic load flow based on the semi-invariant method and wind speed independent transformation based on the Rosenblatt transformation are described. On this basis, a voltage state assessment index is established for the more comprehensive evaluation of voltage characteristics, and a voltage state assessment procedure is proposed. Taking actual wind speed as an example, the case study of the IEEE 24-node power system and the east Inner Mongolia power system for voltage state assessment with large-scale wind power integration are studied. The simulation results verify the effectiveness of the proposed voltage state assessment method. Full article
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Open AccessArticle
Energy Consumption and Trade Openness Nexus in Egypt: Asymmetry Analysis
Energies 2019, 12(10), 2018; https://doi.org/10.3390/en12102018
Received: 23 April 2019 / Revised: 9 May 2019 / Accepted: 22 May 2019 / Published: 27 May 2019
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Abstract
Trade openness may support the economic growth of any country but its environmental effects due to increasing energy consumption cannot be ignored. This research hypothesizes the asymmetrical effects of both economic growth and trade openness on the energy consumption of Egypt from 1971–2014. [...] Read more.
Trade openness may support the economic growth of any country but its environmental effects due to increasing energy consumption cannot be ignored. This research hypothesizes the asymmetrical effects of both economic growth and trade openness on the energy consumption of Egypt from 1971–2014. Our estimates suggest that both economic growth and trade openness have asymmetrical effects on the energy consumption in both long and short runs because magnitude of the effects are found unequal. Both increasing and decreasing economic growth have positive effects on the energy consumption in the long and short runs except an insignificant effect of decreasing economic growth in the short run. Increasing and decreasing trade openness have also positive effects on the energy consumption in the long and short runs except an insignificant effect of decreasing trade openness in the long run. The increasing energy consumption, as results of increasing economic growth and/or trade openness, may have environmental consequence. Therefore, we recommend the Egyptian government to diversify the energy consumption from fossil fuel sources. Full article
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Open AccessArticle
Accurate Expressions of Mutual Inductance and Their Calculation of Archimedean Spiral Coils
Energies 2019, 12(10), 2017; https://doi.org/10.3390/en12102017
Received: 25 April 2019 / Revised: 20 May 2019 / Accepted: 23 May 2019 / Published: 26 May 2019
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Abstract
Considering the helicity of Archimedean spiral coils, this paper proposes accurate expressions of mutual inductance and their numerical calculation methods, which can be applied in the wireless power transmission field, etc. Accurate expressions of mutual inductance are deduced respectively for two coils that [...] Read more.
Considering the helicity of Archimedean spiral coils, this paper proposes accurate expressions of mutual inductance and their numerical calculation methods, which can be applied in the wireless power transmission field, etc. Accurate expressions of mutual inductance are deduced respectively for two coils that are coaxial, laterally misaligned, or non-parallel, and numerical calculations are performed using Gaussian integration as well. In the case of coaxial coils, the calculation results are verified by the 3D finite element method (3D FEM) and compared with the results gained by the traditional method that approximates two spiral coils to two clusters of series-connected circular coils ignoring helicity. The comparison of the three methods shows that results achieved by the proposed expression are close to that of 3D FEM, while there is increasing error with the screw pitches of the coils when using the traditional circular coil approximation method. The influence of relative position on the mutual inductance of the two coils is also studied and it is further explained through magnetic field distribution. Finally, the validity of the proposed expressions of mutual inductance is verified by experimental results. Full article
(This article belongs to the Special Issue Electricity for Energy Transition)
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Open AccessArticle
Improving Microgrid Frequency Regulation Based on the Virtual Inertia Concept while Considering Communication System Delay
Energies 2019, 12(10), 2016; https://doi.org/10.3390/en12102016
Received: 6 April 2019 / Revised: 21 May 2019 / Accepted: 22 May 2019 / Published: 26 May 2019
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Abstract
Frequency stability is an important issue for the operation of islanded microgrids. Since the upstream grid does not support the islanded microgrids, the power control and frequency regulation encounter serious problems. By increasing the penetration of the renewable energy sources in microgrids, optimizing [...] Read more.
Frequency stability is an important issue for the operation of islanded microgrids. Since the upstream grid does not support the islanded microgrids, the power control and frequency regulation encounter serious problems. By increasing the penetration of the renewable energy sources in microgrids, optimizing the parameters of the load frequency controller plays a great role in frequency stability, which is currently being investigated by researchers. The status of loads and generation sources are received by the control center of a microgrid via a communication system and the control center can regulate the output power of renewable energy sources and/or power storage devices. An inherent delay in the communication system or other parts like sensors sampling rates may lead microgrids to have unstable operation states. Reducing the delay in the communication system, as one of the main delay origins, can play an important role in improving fluctuation mitigation, which on the other hand increases the cost of communication system operation. In addition, application of ultra-capacitor banks, as a virtual inertial tool, can be considered as an effective solution to damp frequency oscillations. However, when the ultra-capacitor size is increased, the virtual inertia also increases, which in turn increases the costs. Therefore, it is essential to use a suitable optimization algorithm to determine the optimum parameters. In this paper, the communication system delay and ultra-capacitor size along with the parameters of the secondary controller are obtained by using a Non-dominated Sorting Genetic Algorithm II (NSGA-II) algorithm as well as by considering the costs. To cover frequency oscillations and the cost of microgrid operation, two fitness functions are defined. The frequency oscillations of the case study are investigated considering the stochastic behavior of the load and the output of the renewable energy sources. Full article
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Open AccessArticle
Effect of Actual Gas Turbine Operating Conditions on Mist/Steam Cooling Performance in a Ribbed Passage
Energies 2019, 12(10), 2015; https://doi.org/10.3390/en12102015
Received: 17 March 2019 / Revised: 8 May 2019 / Accepted: 22 May 2019 / Published: 26 May 2019
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Abstract
This study numerically examines the effect of actual gas turbine operating conditions on heat transfer characteristics in a ribbed passage with mist/steam cooling. A 60° ribbed passage with aspect ratio of 1/1 was investigated at Reynolds number of 300,000, and steam cooling was [...] Read more.
This study numerically examines the effect of actual gas turbine operating conditions on heat transfer characteristics in a ribbed passage with mist/steam cooling. A 60° ribbed passage with aspect ratio of 1/1 was investigated at Reynolds number of 300,000, and steam cooling was used to provide a contrast. Three main factors were considered: coolant temperature, operating pressure, and wall heat flux density. The heat transfer enhancement mechanism of mist/steam cooling was explored, and the results showed that the heat transfer performance of mist/steam cooling was superior to steam cooling. When the coolant temperature varied from 300 to 500 °C, the average Nusselt number of mist/steam cooling decreased by 26.6%, and the heat transfer enhancement ratio dropped from 15% to 10%. As operating pressure increased, the heat transfer performance factor of mist/steam firstly increased and then decreased. At an operating pressure of 1.5 MPa, the heat transfer achieved its optimal performance, and the heat transfer enhancement ratio achieved its maximum value of 15.9%. Larger wall heat flux density provided less heat transfer enhancement. When the heat flux density increased from 100,000 to 300,000 W·m−2, the average Nusselt number of mist/steam cooling decreased by 13.8%, while the heat transfer enhancement ratio decreased from 25.3% to 12.6%. Full article
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Open AccessArticle
Use of Buckwheat Straw to Produce Ethyl Alcohol Using Ionic Liquids
Energies 2019, 12(10), 2014; https://doi.org/10.3390/en12102014
Received: 8 March 2019 / Revised: 18 April 2019 / Accepted: 10 May 2019 / Published: 26 May 2019
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Abstract
Background: Common buckwheat (Fagopyrum esculentum Moench) is an annual spring-emerging crop that is classified among the dicotyledons, due to the manner of its cultivation, use, and chemical composition of seeds. The use of buckwheat straw for energy purposes—for example, for the production [...] Read more.
Background: Common buckwheat (Fagopyrum esculentum Moench) is an annual spring-emerging crop that is classified among the dicotyledons, due to the manner of its cultivation, use, and chemical composition of seeds. The use of buckwheat straw for energy purposes—for example, for the production of second generation bioethanol—might enable its wider application and increase the cost-effectiveness of tillage. Methods: In this study, we examined the usability of buckwheat straw for the production of bioethanol. We pretreated the raw material with ionic liquids and subsequently performed enzymatic hydrolysis and alcoholic fermentation. The obtained chemometric data were analyzed using the Partial Least Squares (PLS) regression model. PLS regression in combination with spectral analysis within the near-infrared (NIR) spectrum allowed for the rapid determination of the amount of cellulose in the raw material and also provided information on the changes taking place in its structure. Results: We obtained good results for the combination of 1-ethyl-3-methylimidazolium acetate as the ionic liquid and Cellic CTec2 as the enzymatic preparation for the pretreatment of buckwheat straw. The highest concentration of glucose following 72 h of enzymatic hydrolysis was found to be around 5.5 g/dm3. The highest concentration of ethanol (3.31 g/dm3) was obtained with the combination of 1-butyl-3-methylimidazolium acetate for the pretreatment and cellulase from Trichoderma reesei for enzymatic hydrolysis. Conclusions: In summary, the efficiency of the fermentation process is strictly associated with the pool of available fermenting sugars, and it depends on the type of ionic liquid used during the pretreatment and on the enzymatic preparation. It is possible to obtain bioethanol from buckwheat straw using ionic liquid for pretreatment of the raw material prior to the enzymatic hydrolysis and alcoholic fermentation of the material. Full article
(This article belongs to the Special Issue Biomass and Biofuels 2019)
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Open AccessArticle
Unsteady Flow Numerical Simulations on Internal Energy Dissipation for a Low-Head Centrifugal Pump at Part-Load Operating Conditions
Energies 2019, 12(10), 2013; https://doi.org/10.3390/en12102013
Received: 4 May 2019 / Revised: 21 May 2019 / Accepted: 23 May 2019 / Published: 26 May 2019
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Abstract
Dredge pumps are usually operated at part-load conditions, in which the low-solidity centrifugal impeller could experience large internal energy dissipation, related to flow separation and vortices. In this study, SST k-ω and SAS-SST turbulence models were used, in steady and unsteady [...] Read more.
Dredge pumps are usually operated at part-load conditions, in which the low-solidity centrifugal impeller could experience large internal energy dissipation, related to flow separation and vortices. In this study, SST k-ω and SAS-SST turbulence models were used, in steady and unsteady simulations, for a low-head centrifugal pump with a three-bladed impeller. The main focus of the present work was to investigate the internal energy dissipation in rotating an impeller at part-load operating conditions, related to flow separation and stall. The unsteady nature of these operating conditions was investigated. Performance experiments and transient wall pressure measurements were conducted for validation. A methodology for internal energy dissipation analysis has been proposed; and the unsteady pressure fluctuations were analyzed in the rotating impeller. The internal power losses in the volute and the impeller were mostly found in the centrifugal pump. The rotating stall phenomenon occurred with flow separation and detachment at the part-load operating condition, leading to a dissipation of the internal energy in the impeller. The rotating impeller experienced pressure fluctuations with low frequencies, at part-load operating conditions, while in the design operating condition only experienced rotating frequency. Full article
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Open AccessArticle
Co-Optimized Analysis and Design of Electric and Natural Gas Infrastructures
Energies 2019, 12(10), 2012; https://doi.org/10.3390/en12102012
Received: 22 March 2019 / Revised: 7 May 2019 / Accepted: 8 May 2019 / Published: 26 May 2019
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Abstract
This paper proposes and implements a long-term deterministic capacity expansion model for the co-optimization of electric and natural gas infrastructures. It determines the required investments in generation units, transmission lines and pipelines for meeting future demands, while representing electricity and natural gas flows [...] Read more.
This paper proposes and implements a long-term deterministic capacity expansion model for the co-optimization of electric and natural gas infrastructures. It determines the required investments in generation units, transmission lines and pipelines for meeting future demands, while representing electricity and natural gas flows using DC Power Flow and Weymouth equations, respectively. A Mixed Integer Nonlinear Programming (MINLP) problem is developed, from which a linearized version is derived. A 26 node integrated gas-electric system for the Eastern Region of the United States is used to demonstrate the model’s capabilities. Results show that the model provides an accurate operational representation of the integrated system, and, therefore, enhances the expansion planning process. Full article
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Open AccessArticle
Mixing Characteristics of Binary Mixture with Biomass in a Gas-Solid Rectangular Fluidized Bed
Energies 2019, 12(10), 2011; https://doi.org/10.3390/en12102011
Received: 18 April 2019 / Revised: 17 May 2019 / Accepted: 24 May 2019 / Published: 26 May 2019
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Abstract
Aiming to better understand the biomass pyrolysis and gasification processes, a detailed experimental study of the mixing characteristics is conducted in a fluidized bed with binary mixtures. Rapeseed is used as biomass, and silica sand or resin as inert material. The effect of [...] Read more.
Aiming to better understand the biomass pyrolysis and gasification processes, a detailed experimental study of the mixing characteristics is conducted in a fluidized bed with binary mixtures. Rapeseed is used as biomass, and silica sand or resin as inert material. The effect of mixture composition, initial packing manner, and superficial gas velocity on the concentration distribution is investigated in a rectangular fluidized bed by means of photography and sampling methods. The results show that the mixture composition plays an important role in the axial solids profile of binary mixtures. The mixing behavior of binary mixture is dominated by the bubble movement. The axial distribution of binary mixtures becomes uniform with increasing superficial gas velocity, whilst no obvious effect of initial packing manner is observed in this study. Full article
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Open AccessArticle
A Robust Feedback Path Tracking Control Algorithm for an Indoor Carrier Robot Considering Energy Optimization
Energies 2019, 12(10), 2010; https://doi.org/10.3390/en12102010
Received: 25 March 2019 / Revised: 13 May 2019 / Accepted: 22 May 2019 / Published: 26 May 2019
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Abstract
This work develops an indoor carrier robot for people with disabilities, where the precise tracking of designated route is crucial. The parameter uncertainties and disturbances of the robot impose significant challenges for tracking. The present paper first investigates the dynamic of mechanical structure [...] Read more.
This work develops an indoor carrier robot for people with disabilities, where the precise tracking of designated route is crucial. The parameter uncertainties and disturbances of the robot impose significant challenges for tracking. The present paper first investigates the dynamic of mechanical structure and modeling of actuator motors and constructs a new dynamic model by considering all main parameter uncertainties and disturbances. A novel robust feedback tracking controller considering both the optimization of path tracking and the minimization of the power consumption energy is proposed. It is proved that the tracking errors e and e ˙ satisfy a H performance indicator while the energy consumption is minimum. A simulation example was performed and the results show that this novel algorithm can effectively reduce the tracking error from 0.2 m to 0.006 m while guaranteeing the minimum energy consumption. Furthermore, the effectiveness of the proposed method was validated by experiment compared with the non-robust one. Full article
(This article belongs to the Special Issue Robotics, Micronanosensor and Smart Devices for Control)
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Open AccessArticle
Research on Dynamic Evolution Simulation and Sustainability Evaluation Model of China’s Power Supply and Demand System
Energies 2019, 12(10), 2009; https://doi.org/10.3390/en12102009
Received: 15 April 2019 / Revised: 17 May 2019 / Accepted: 22 May 2019 / Published: 25 May 2019
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Abstract
Power supply and demand systems are important support systems for industrial production and residents’ lives. They have multiple influencing factors, and complex mechanisms of interaction exist among these factors. In view of the present sustainability problems faced by China’s power supply and demand [...] Read more.
Power supply and demand systems are important support systems for industrial production and residents’ lives. They have multiple influencing factors, and complex mechanisms of interaction exist among these factors. In view of the present sustainability problems faced by China’s power supply and demand system, this research adopts a system dynamics (SD) model to simulate the evolution of China’s power supply and demand system, and analyzes the interaction mechanism of various elements of the system. Based on this, an innovative index system for the evaluation of the sustainability of power supply and demand systems is proposed based on the four elements of total amount, structure, technology and environment. Furthermore, by integrating Principal Component Analysis (PCA) and State Space (SS) method, a PCA-SS evaluation model is constructed to explore the development bottleneck of China’s power supply and demand system. The results show that there is still a large gap between the actual sustainability and the ideal range, and that the sustainability of structural and environmental layers needs further improvement. This research expands the knowledge system regarding the evaluation of the sustainability of power supply and demand systems and provides a theoretical reference for the optimization of China’s power supply and demand system. Full article
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Open AccessArticle
Research Insights and Knowledge Headways for Developing Remote, Off-Grid Microgrids in Developing Countries
Energies 2019, 12(10), 2008; https://doi.org/10.3390/en12102008
Received: 1 May 2019 / Revised: 16 May 2019 / Accepted: 22 May 2019 / Published: 25 May 2019
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Abstract
Recent reports from international energy agencies indicate that more than a billion of the population in the world is deprived of basic electricity provisions, confined mainly to the remote communities of developing nations. Microgrids are promoted as a potential technology for electricity provisions [...] Read more.
Recent reports from international energy agencies indicate that more than a billion of the population in the world is deprived of basic electricity provisions, confined mainly to the remote communities of developing nations. Microgrids are promoted as a potential technology for electricity provisions to off-grid rural communities, but have failed to reach their value proposition in the context of rural electrification access. In view of the rampant rural electrification issues, the objective of this paper is to furnish an understanding of, and advance the knowledge into, methods to facilitate the design and development of microgrid systems for remote communities in developing countries. The methodology involves an integrative review process of an annotated bibliography to summarise past empirical or theoretical literature. As such, this research is based on evaluation attributes, and identifies the challenges and barriers for remote microgrids through an analysis of 19 case studies. The paper concludes by proposing key aspects that need to be considered for developing a framework to improve the sustainability of electricity provisions for off-grid rural communities in developing countries. Full article
(This article belongs to the Special Issue Hybrid Renewable Energy Systems in Remote Sites 2019)
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Open AccessArticle
Multi-Input Ćuk-Derived Buck-Boost Voltage Source Inverter for Photovoltaic Systems in Microgrid Applications
Energies 2019, 12(10), 2007; https://doi.org/10.3390/en12102007
Received: 26 March 2019 / Revised: 21 May 2019 / Accepted: 22 May 2019 / Published: 25 May 2019
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Abstract
This paper presents a multi-input Ćuk-derived Buck-Boost voltage source inverter (CBBVSI) for Photovoltaic (PV) systems. The proposed topology consists of a single-stage DC-AC inverter that combines both DC-DC and DC-AC stages. The DC-DC stage is used for stepping-up the voltage from the PV [...] Read more.
This paper presents a multi-input Ćuk-derived Buck-Boost voltage source inverter (CBBVSI) for Photovoltaic (PV) systems. The proposed topology consists of a single-stage DC-AC inverter that combines both DC-DC and DC-AC stages. The DC-DC stage is used for stepping-up the voltage from the PV generator. Simultaneously, the DC-AC stage is used for interfacing the PV source with the AC grid. The topology allows three sources to utilize the antiparallel diodes for each inverter leg for transferring the energy. The proposed system exhibits several features such as a reduction of the number of components compared to typical two-stage structures, and Split-Source Inverter (SSI), and Z-Source Inverter (ZSI) topologies. Moreover, the power of each PV source can be harvested either simultaneously or separately since independent Maximum Power Point Tracking (MPPT) is performed. The system was simulated using MATLAB/SIMULINK software and a 1 kW laboratory prototype was implemented to verify the operation of the proposed CBBVSI. The numerical simulations are presented together with the experimental results, showing a good agreement. Full article
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Open AccessArticle
Quantitative Evaluation of the “Non-Enclosed” Microseismic Array: A Case Study in a Deeply Buried Twin-Tube Tunnel
Energies 2019, 12(10), 2006; https://doi.org/10.3390/en12102006
Received: 6 May 2019 / Revised: 20 May 2019 / Accepted: 23 May 2019 / Published: 25 May 2019
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Abstract
The high-stress hazards of underground engineering have stimulated the exploration of microseismic monitoring and early warning methods. To achieve a good monitoring effect, the monitoring object is usually enclosed by a microseismic array (sensor array) (e.g., slope engineering, etc.). However, some characteristics of [...] Read more.
The high-stress hazards of underground engineering have stimulated the exploration of microseismic monitoring and early warning methods. To achieve a good monitoring effect, the monitoring object is usually enclosed by a microseismic array (sensor array) (e.g., slope engineering, etc.). However, some characteristics of a buried tunnel, including “linear”, “deep-buried”, and “long”, make it difficult to deploy a reasonable microseismic array, which leads to the microseismic array being non-enclosed for the monitoring object. Application of the non-enclosed microseismic array yields decreases the accuracy of the source location. To solve the problem wisely, this paper deals with the feasibility of non-enclosed microseismic arrays (axial-extended, lateral-extended, and twin-tube arrays) by introducing a quantitative method. To this end, an optimized microseismic array with the best source location accuracy for a twin-tube expressway tunnel is proposed. The obtained results reveal that the non-enclosed microseismic arrays, which are unavoidable in expressway tunnel engineering, do not introduce errors but reduce the ability to resist them. Further, the twin-tube array achieves a better source location accuracy than the axial and lateral-extended arrays. In the application of the source location based on the particle swarm optimization (PSO) algorithm to the twin-tube array, microseismic events, which cluster in the rockburst section, are wholly gathered, and the maximum error is reduced by about 30–50 m, indicating its greater feasibility with respect to the single-tube array. Full article
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Open AccessArticle
Recognition and Classification of Incipient Cable Failures Based on Variational Mode Decomposition and a Convolutional Neural Network
Energies 2019, 12(10), 2005; https://doi.org/10.3390/en12102005
Received: 16 April 2019 / Revised: 17 May 2019 / Accepted: 20 May 2019 / Published: 25 May 2019
Cited by 1 | Viewed by 421 | PDF Full-text (3489 KB) | HTML Full-text | XML Full-text
Abstract
To avoid power supply hazards caused by cable failures, this paper presents an approach of incipient cable failure recognition and classification based on variational mode decomposition (VMD) and a convolutional neural network (CNN). By using VMD, the original current signal is decomposed into [...] Read more.
To avoid power supply hazards caused by cable failures, this paper presents an approach of incipient cable failure recognition and classification based on variational mode decomposition (VMD) and a convolutional neural network (CNN). By using VMD, the original current signal is decomposed into seven modes with different center frequencies. Then, 42 features are extracted for the seven modes and used to construct a feature vector as input of the CNN to classify incipient cable failure through deep learning. Compared with using the original signals directly as the CNN input, the proposed approach is more efficient and robust. Experiments on different classifiers, namely, the decision tree (DT), K-nearest neighbor (KNN), BP neural network (BP) and support vector machine (SVM), and show that the CNN outperforms the other classifiers in terms of accuracy. Full article
(This article belongs to the Special Issue Applications of Computational Intelligence to Power Systems)
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Open AccessArticle
A Study on the Effect of Closed-Loop Wind Farm Control on Power and Tower Load in Derating the TSO Command Condition
Energies 2019, 12(10), 2004; https://doi.org/10.3390/en12102004
Received: 15 April 2019 / Revised: 20 May 2019 / Accepted: 20 May 2019 / Published: 25 May 2019
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Abstract
This study was conducted to analyze the impact of surrounding environmental changes on the feedback gain and performance of a closed-loop wind farm controller that reduces the error between total power output of wind farm and power command of transmission system operator. To [...] Read more.
This study was conducted to analyze the impact of surrounding environmental changes on the feedback gain and performance of a closed-loop wind farm controller that reduces the error between total power output of wind farm and power command of transmission system operator. To analyze the impact of environment changes on wind farm controller feedback gain, the feedback gain was manually changed from 0 to 0.9 with a 0.1 interval. In this study, wind speed and wind direction changes were considered as environment changes; it was found by simulation code that the wind farm controller gain is in inverse proportion to wake recovery rate. In other words, the feedback gain should be higher if the distance between upstream and downstream wind turbine is not sufficient to wake recovery. Furthermore, the feedback gain should be lower when the upstream wind turbine generates a relatively weak wake by operating above the rated wind speed. The wind farm simulation was performed using reference 5 MW wind turbines from the National Renewable Energy Laboratory (NREL), which are numerically modeled for each element so that wind farm power output and tower load can be calculated according to the variation of the power command by using a modified wake model with improved accuracy. All the simulations performed in this study were carried out to review the power output accuracy of wind farms, but only if the transmission system operator’s power command was lower than the available power of wind farm. In this study, the gain of the wind farm controller was applied differently depending on the wind speed and direction to consider benefits in terms of power and tower load, especially if the wake effect of the upstream wind turbine was rapidly transferred to the downstream wind turbine. Ultimately, a simple, but more effective, power distribution method was proposed for distributing power commands to wind turbines that constitute wind farms and the study indicated the need for controller gain adjustment based on surrounding environmental changes. Full article
(This article belongs to the Section Wind, Wave and Tidal Energy)
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