Energies

21 pages, 3776 KiB

Open AccessArticle

Robust Speed Controller Design Using H_infinity Theory for High-Performance Sensorless Induction Motor Drives

by Ahmed A. Zaki Diab, Abou-Hashema M. El-Sayed, Hossam Hefnawy Abbas and Montaser Abd El Sattar

Energies 2019, 12(5), 961; https://doi.org/10.3390/en12050961 - 12 Mar 2019

Cited by 9 | Viewed by 4349

In this paper, a robust speed control scheme for high dynamic performance sensorless induction motor drives based on the H_infinity (H_∞) theory has been presented and analyzed. The proposed controller is robust against system parameter variations and achieves good dynamic performance. [...] Read more.

In this paper, a robust speed control scheme for high dynamic performance sensorless induction motor drives based on the H_infinity (H_∞) theory has been presented and analyzed. The proposed controller is robust against system parameter variations and achieves good dynamic performance. In addition, it rejects disturbances well and can minimize system noise. The H_∞ controller design has a standard form that emphasizes the selection of the weighting functions that achieve the robustness and performance goals of motor drives in a wide range of operating conditions. Moreover, for eliminating the speed encoder—which increases the cost and decreases the overall system reliability—a motor speed estimation using a Model Reference Adaptive System (MRAS) is included. The estimated speed of the motor is used as a control signal in a sensor-free field-oriented control mechanism for induction motor drives. To explore the effectiveness of the suggested robust control scheme, the performance of the control scheme with the proposed controllers at different operating conditions such as a sudden change of the speed command/load torque disturbance is compared with that when using a classical controller. Experimental and simulation results demonstrate that the presented control scheme with the H_∞ controller and MRAS speed estimator has a reasonable estimated motor speed accuracy and a good dynamic performance. Full article

(This article belongs to the Special Issue Advances in Rotating Electric Machines)

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18 pages, 969 KiB

Open AccessArticle

A Transformer Fault Diagnosis Model Based on Chemical Reaction Optimization and Twin Support Vector Machine

by Fang Yuan, Jiang Guo, Zhihuai Xiao, Bing Zeng, Wenqiang Zhu and Sixu Huang

Energies 2019, 12(5), 960; https://doi.org/10.3390/en12050960 - 12 Mar 2019

Cited by 32 | Viewed by 4293

Abstract

The condition monitoring and fault diagnosis of power transformers plays a significant role in the safe, stable and reliable operation of the whole power system. Dissolved gas analysis (DGA) methods are widely used for fault diagnosis, however, their accuracy is limited by the [...] Read more.

The condition monitoring and fault diagnosis of power transformers plays a significant role in the safe, stable and reliable operation of the whole power system. Dissolved gas analysis (DGA) methods are widely used for fault diagnosis, however, their accuracy is limited by the selection of DGA features and the performance of fault diagnosis models, for example, the classical support vector machine (SVM), is easily affected by unbalanced training samples. This paper presents a transformer fault diagnosis model based on chemical reaction optimization and a twin support vector machine. Twin support vector machines (TWSVMs) are used as classifiers for solving problems involving unbalanced and insufficient samples. Restricted Boltzmann machines (RBMs) are used for data preprocessing to ensure the effective identification of feature parameters and improve the efficiency and accuracy of fault diagnosis. The chemical reaction optimization (CRO) algorithm is used to optimize TWSVM parameters to select the optimal training parameters. The cross-validation (CV) method is used to ensure the reliability and generalization ability of the diagnostic model. Finally, the validity of the model is verified using real fault samples and random testing. Full article

(This article belongs to the Section A1: Smart Grids and Microgrids)

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19 pages, 6417 KiB

Open AccessArticle

Experimental and Numerical Analysis of the Effect of Vortex Generator Height on Vortex Characteristics and Airfoil Aerodynamic Performance

by Xinkai Li, Ke Yang and Xiaodong Wang

Energies 2019, 12(5), 959; https://doi.org/10.3390/en12050959 - 12 Mar 2019

Cited by 41 | Viewed by 6542

Abstract

To explore the effect of the height of vortex generators (VGs) on the control effect of boundary-layer flow, the vortex characteristics of a plate and the aerodynamic characteristics of an airfoil for VGs were studied by both wind tunnel experiments and numerical methods. [...] Read more.

To explore the effect of the height of vortex generators (VGs) on the control effect of boundary-layer flow, the vortex characteristics of a plate and the aerodynamic characteristics of an airfoil for VGs were studied by both wind tunnel experiments and numerical methods. Firstly, the ratio of VG height (H) to boundary layer thickness (δ) was studied on a flat plate boundary layer; the values of H are 0.1δ, 0.2δ, 0.5δ, 1.0δ, 1.5δ, and 2.0δ. Results show that the concentrated vortex intensity and VG height present a logarithmic relationship, and vortex intensity is proportional to the average kinetic energy of the fluid in the height range of the VG. Secondly, the effects of height on the aerodynamic performance of airfoils were studied in a wind tunnel using three VGs with H = 0.66δ, 1.0δ, and 1.33δ. The stall angle of the airfoil with and without VGs is 18° and 8°, respectively, so the VGs increase the stall angle by 10°. The maximum lift coefficient of the airfoil with VGs increases by 48.7% compared with the airfoil without VGs, and the drag coefficient of the airfoil with VGs is 84.9% lower than that of the airfoil without VGs at an angle of attack of 18°. The maximum lift–drag ratio of the airfoil with VGs is lower than that of the airfoil without VGs, so the VGs do not affect the maximum lift–drag ratio of the airfoil. However, a VG does increase the angle of attack of the best lift–drag ratio. Full article

(This article belongs to the Special Issue Recent Advances in Aerodynamics of Wind Turbines)

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15 pages, 2287 KiB

Open AccessArticle

An Optimization Study on Soot-Blowing of Air Preheaters in Coal-Fired Power Plant Boilers

by Yuanhao Shi, Jie Wen, Fangshu Cui and Jingcheng Wang

Energies 2019, 12(5), 958; https://doi.org/10.3390/en12050958 - 12 Mar 2019

Cited by 12 | Viewed by 5323

Abstract

This paper presents a comprehensive approach for optimization of soot-blowing of air preheaters in a coal-fired power plant boiler. In the method, modeling of the cleanliness factor is firstly proposed to monitor the ash deposition status of the air preheaters. Then, the statistical [...] Read more.

This paper presents a comprehensive approach for optimization of soot-blowing of air preheaters in a coal-fired power plant boiler. In the method, modeling of the cleanliness factor is firstly proposed to monitor the ash deposition status of the air preheaters. Then, the statistical fitting of the ash fouling status is subsequently obtained to analyze the ash fouling dynamics and assessment of optimized soot-blowing strategies. Soot-blowing strategies are finally developed to optimize the steam consumption and heat transfer efficiency. Our methods can achieve the fouling monitoring and soot-blowing optimization of air preheater (APH) by using the existing monitoring data, not requiring additional special instruments and complex computing systems. The methodology is validated with the actual operating data of a 300 MW coal-fired power plant boiler. The results show the effectiveness of the proposed method. It can be used for the soot-blowing optimization in most coal-fired power plant boiler with air preheaters. Full article

(This article belongs to the Section I: Energy Fundamentals and Conversion)

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25 pages, 4845 KiB

Open AccessArticle

Building Variable Productivity Ratios for Improving Large Scale Spatially Explicit Pruning Biomass Assessments

by Daniel García-Galindo, Arkadiusz Dyjakon and Fernando Cay Villa-Ceballos

Energies 2019, 12(5), 957; https://doi.org/10.3390/en12050957 - 12 Mar 2019

Cited by 7 | Viewed by 3196

Abstract

Biomass assessments of agro–residues performed at large geographical scales generally base calculations on single constant pruning productivity ratios (RSRs). Reliability of biomass assessments shall be improved if RSRs respond to prevailing regional crop growing conditions. The present paper describes the methodology applied to [...] Read more.

Biomass assessments of agro–residues performed at large geographical scales generally base calculations on single constant pruning productivity ratios (RSRs). Reliability of biomass assessments shall be improved if RSRs respond to prevailing regional crop growing conditions. The present paper describes the methodology applied to create geographically varying pruning RSR ratios–tons of dry matter per hectare—for five crop groups: vineyard, olive, fruit species, citrus and dry fruits. A newly created database containing 230 records–from seven EU28 countries—is submitted to statistical analysis. Results reveal that agro-climatic conditions are able to explain a not negligible share of the pruning productivity as dependent variable. Subsequent regression analysis provides two equations—for vineyard and citrus—achieving a reasonable good fitting (R² 0.18 and 0.42 respectively) between RSR and the agroclimatic variables. Analysis of olive, fruit species and dry fruits scatter and whisker plots were useful for zoning and inducing ramp functions. A Geographical Information System (GIS) was utilised to apply the functions to the agroclimatic raster coverages in order to obtain RSR raster grids. Zonal statistic procedures applied by European regional units (NUTs0, NUTs2, NUTs3) provide a specific crop RSR ratio per administrative unit as a principal output of the present work. Full article

(This article belongs to the Section L: Energy Sources)

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25 pages, 6489 KiB

Open AccessArticle

HyFlow—A Hybrid Load Flow-Modelling Framework to Evaluate the Effects of Energy Storage and Sector Coupling on the Electrical Load Flows

by Benjamin Böckl, Matthias Greiml, Lukas Leitner, Patrick Pichler, Lukas Kriechbaum and Thomas Kienberger

Energies 2019, 12(5), 956; https://doi.org/10.3390/en12050956 - 12 Mar 2019

Cited by 25 | Viewed by 4891

Abstract

HyFlow is a grid-based multi-energy system (MES) modelling framework. It aims to model the status quo of current energy systems, future scenarios with a high share of fluctuating energy sources or additional consumers like electric vehicles, and to compare solution strategies if certain [...] Read more.

HyFlow is a grid-based multi-energy system (MES) modelling framework. It aims to model the status quo of current energy systems, future scenarios with a high share of fluctuating energy sources or additional consumers like electric vehicles, and to compare solution strategies if certain parts of the infrastructure are congested. In order to evaluate the congestion limits and the feasibility and suitability of solution strategies (e.g., energy storage, sector coupling technologies, demand response (DR)), load flow calculations of all three main grid-bound energy carriers are implemented in one single modelling framework. In addition to the implemented load flow models, it allows the interaction of these grids with the use of hybrid elements. This measure enables a proper assessment of future scenarios, not only for the infrastructure of one energy carrier, but for the overall energy system. The calculation workflow of HyFlow, including the implemented load flow calculations, as well as the implementation of the flexibility options, is described in detail in the methodology section. To demonstrate the wide range of applicability of HyFlow with different spatial ranges, two case studies referring to current research problems are presented: a city and a region surrounding the mentioned city. The calculations for the mentioned case studies are performed for three levels. A “status quo” level, a “high-stress” level with added fluctuating energy sources and consumers, and an “improvement” level, where flexibility options are introduced to the system. The effect of the flexibility options on future energy grids is, therefore, analyzed and evaluated. A wide variety of evaluation criteria can be selected. For example, the maximum load of certain power lines, the self-sufficiency of the overall system, the total transport losses or the total energy consumption. Full article

(This article belongs to the Special Issue Model Coupling and Energy Systems)

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17 pages, 1521 KiB

Open AccessEditor’s ChoiceArticle

A Techno-Economic Analysis of Vehicle-to-Building: Battery Degradation and Efficiency Analysis in the Context of Coordinated Electric Vehicle Charging

by Stefan Englberger, Holger Hesse, Daniel Kucevic and Andreas Jossen

Energies 2019, 12(5), 955; https://doi.org/10.3390/en12050955 - 12 Mar 2019

Cited by 27 | Viewed by 10768

Abstract

In the context of the increased acceptance and usage of electric vehicles (EVs), vehicle-to-building (V2B) has proven to be a new and promising use case. Although this topic is already being discussed in literature, there is still a lack of experience on how [...] Read more.

In the context of the increased acceptance and usage of electric vehicles (EVs), vehicle-to-building (V2B) has proven to be a new and promising use case. Although this topic is already being discussed in literature, there is still a lack of experience on how such a system, of allowing bidirectional power flows between an EV and building, will work in a residential environment. The challenge is to optimize the interplay of electrical load, photovoltaic (PV) generation, EV, and optionally a home energy storage system (HES). In total, fourteen different scenarios are explored for a German household. A two-step approach is used, which combines a computationally efficient linear optimizer with a detailed modelling of the non-linear effects on the battery. The change in battery degradation, storage system efficiency, and operating expenses (OPEX) as a result of different, unidirectional and bidirectional, EV charging schemes is examined for both an EV battery and a HES. The simulations show that optimizing unidirectional charging can improve the OPEX by 15%. The addition of V2B leads to a further 11% cost reduction, however, this corresponds with a 12% decrease in EV battery lifetime. Techno-economic analysis reveals that the V2B charging solution with no HES leads to strong self-consumption improvements (EUR 1381 savings over ten years), whereas, this charging scheme would not be justified for a residential prosumer with a HES (only EUR 160 savings). Full article

(This article belongs to the Special Issue Energy Storage and Management for Electric Vehicles)

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Graphical abstract

20 pages, 8923 KiB

Open AccessArticle

Unsteadiness of Tip Leakage Flow in the Detached-Eddy Simulation on a Transonic Rotor with Vortex Breakdown Phenomenon

by Xiangyu Su, Xiaodong Ren, Xuesong Li and Chunwei Gu

Energies 2019, 12(5), 954; https://doi.org/10.3390/en12050954 - 12 Mar 2019

Cited by 20 | Viewed by 4021

Abstract

Tip leakage vortex (TLV) in a transonic compressor rotor was investigated numerically using detached-eddy simulation (DES) method at different working conditions. Strong unsteadiness was found at the tip region, causing a considerable fluctuation in total pressure distribution and flow angle distribution above 80% [...] Read more.

Tip leakage vortex (TLV) in a transonic compressor rotor was investigated numerically using detached-eddy simulation (DES) method at different working conditions. Strong unsteadiness was found at the tip region, causing a considerable fluctuation in total pressure distribution and flow angle distribution above 80% span. The unsteadiness at near choke point and peak efficiency point is not obvious. DES method can resolve more detailed flow patterns than RANS (Reynolds-averaged Navier–Stokes) results, and detailed structures of the tip leakage flow were captured. A spiral-type breakdown structure of the TLV was successfully observed at the near stall point when the TLV passed through the bow shock. The breakdown of TLV contributed to the unsteadiness and the blockage effect at the tip region. Full article

(This article belongs to the Special Issue Fluid Flow and Heat Transfer)

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18 pages, 4479 KiB

Open AccessFeature PaperArticle

A Vibration Signal-Based Method for Fault Identification and Classification in Hydraulic Axial Piston Pumps

by Paolo Casoli, Mirko Pastori, Fabio Scolari and Massimo Rundo

Energies 2019, 12(5), 953; https://doi.org/10.3390/en12050953 - 12 Mar 2019

Cited by 46 | Viewed by 5682

Abstract

In recent years, the interest of industry towards condition-based maintenance, substituting traditional time-based maintenance, is growing. Indeed, condition-based maintenance can increase the system uptime with a consequent economic advantage. In this paper, a solution to detect the health state of a variable displacement [...] Read more.

In recent years, the interest of industry towards condition-based maintenance, substituting traditional time-based maintenance, is growing. Indeed, condition-based maintenance can increase the system uptime with a consequent economic advantage. In this paper, a solution to detect the health state of a variable displacement axial-piston pump based on vibration signals is proposed. The pump was tested on the test bench in different operating points, both in healthy and faulty conditions, the latter obtained by assembling damaged components in the pump. The vibration signals were acquired and exploited to extract features for fault identification. After the extraction, the obtained features were reduced to decrease the computational effort and used to train different types of classifiers. The classification algorithm that presents the greater accuracy with reduced features was identified. The analysis has also showed that using the time sampling raw signal, a satisfying accuracy could be obtained, which will permit onboard implementation. Results have shown the capability of the algorithm to identify which fault occurred in the system (fault identification) for each working condition. In future works, the classification algorithm will be implemented onboard to validate its effectiveness for the online identification of the typical incipient faults in axial-piston pumps. Full article

(This article belongs to the Special Issue Energy Efficiency and Controllability of Fluid Power Systems 2018)

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23 pages, 3008 KiB

Open AccessArticle

Optimal Sizing of Cascade Hydropower and Distributed Photovoltaic Included Virtual Power Plant Considering Investments and Complementary Benefits in Electricity Markets

by Jichun Liu, Jianhua Li, Yue Xiang, Xin Zhang and Wanxiao Jiang

Energies 2019, 12(5), 952; https://doi.org/10.3390/en12050952 - 12 Mar 2019

Cited by 8 | Viewed by 3617

Abstract

Due to an imminent fossil energy crisis and environmental pollution, renewable energy, such as photovoltaics, has been vigorously developing. However, the output of photovoltaic energy has strong volatility and intermittency. Thus, the photovoltaic generation system cannot constantly meet the load demand. To address [...] Read more.

Due to an imminent fossil energy crisis and environmental pollution, renewable energy, such as photovoltaics, has been vigorously developing. However, the output of photovoltaic energy has strong volatility and intermittency. Thus, the photovoltaic generation system cannot constantly meet the load demand. To address this problem, a virtual power plant with hydro-photovoltaic-thermal generation is proposed in this paper. This virtual power plant utilizes the complementary characteristics of the output of the power sources to ensure a smooth and stable total output curve, and the power supply quality of the virtual power plant is improved. Further, the nonlinear operating cost model of the virtual power plant, with output changing over time, is established on the weighted output of hydro, photovoltaic, and thermal power; then, the corresponding marginal cost model of the virtual power plant is obtained. In the electricity market, three typical mid- to long-term electricity decomposition methods based on average, tracking load and spot price are constructed, and the spot price is predicted by the auto regressive moving average model (ARIMA) model, while the relationship between the spot price and the marginal cost of the virtual power plant is obtained; the marginal cost could also be adjusted based on the ARIMA model. Based on above factors, the sizing model of the virtual power plant is established, considering investment and complementary benefits. Finally, a case study is undertaken, where the sizing scheme for the increasing local load in the typical scenarios of the planning year and the corresponding annual rate of return are obtained. Sensitivity analysis of the influence for the above factors on the sizing of the virtual power plant is carried out. The optimal ratio of mid- to long-term electricity and its decomposition methods, as well as the capacity of the virtual power plant and the sizing ratio of hydropower, photovoltaic, and thermal power are obtained. Full article

(This article belongs to the Section A1: Smart Grids and Microgrids)

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39 pages, 27902 KiB

Open AccessArticle

Studies of Energy Consumption by a City Bus Powered by a Hybrid Energy Storage System in Variable Road Conditions

by Andrzej Łebkowski

Energies 2019, 12(5), 951; https://doi.org/10.3390/en12050951 - 12 Mar 2019

Cited by 46 | Viewed by 8409

Abstract

This article analyzes various configurations of Hybrid Energy Storage Systems consisting of batteries only, combinations of batteries and supercapacitors, and supercapacitors only. For the presented configurations, mathematical models that were used in research in terms of energy consumption and carbon dioxide emissions were [...] Read more.

This article analyzes various configurations of Hybrid Energy Storage Systems consisting of batteries only, combinations of batteries and supercapacitors, and supercapacitors only. For the presented configurations, mathematical models that were used in research in terms of energy consumption and carbon dioxide emissions were developed, employing a 12-m city bus as a test bed. The tests were carried out using standard test cycles for heavy vehicles as well as routes developed on the basis of actual road conditions. The obtained test results confirmed that the lowest energy consumption is characterized by the system supplied exclusively by batteries (855 Wh/km), followed by a hybrid system of a large battery with a small supercapacitor (941 Wh/km), a hybrid system with a large supercapacitor and a small battery pack (1087 Wh/km), and finally a system with a supercapacitor only (1091 Wh/km). In comparison with the conventional diesel power system (3967 Wh/km), the CO₂ emission reductions ranged from 27% to 43%, depending on the source of electrical energy. Full article

(This article belongs to the Section D: Energy Storage and Application)

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22 pages, 7354 KiB

Open AccessArticle

Forecasting the Carbon Price Using Extreme-Point Symmetric Mode Decomposition and Extreme Learning Machine Optimized by the Grey Wolf Optimizer Algorithm

by Jianguo Zhou, Xuejing Huo, Xiaolei Xu and Yushuo Li

Energies 2019, 12(5), 950; https://doi.org/10.3390/en12050950 - 12 Mar 2019

Cited by 42 | Viewed by 3820

Abstract

Due to the nonlinear and non-stationary characteristics of the carbon price, it is difficult to predict the carbon price accurately. This paper proposes a new novel hybrid model for carbon price prediction. The proposed model consists of an extreme-point symmetric mode decomposition, an [...] Read more.

Due to the nonlinear and non-stationary characteristics of the carbon price, it is difficult to predict the carbon price accurately. This paper proposes a new novel hybrid model for carbon price prediction. The proposed model consists of an extreme-point symmetric mode decomposition, an extreme learning machine, and a grey wolf optimizer algorithm. Firstly, the extreme-point symmetric mode decomposition is employed to decompose the carbon price into several intrinsic mode functions and one residue. Then, the partial autocorrelation function is utilized to determine the input variables of the intrinsic mode functions, and the residue of the extreme learning machine. In the end, the grey wolf optimizer algorithm is applied to optimize the extreme learning machine, to forecast the carbon price. To illustrate the superiority of the proposed model, the Hubei, Beijing, Shanghai, and Guangdong carbon price series are selected for the predictions. The empirical results confirm that the proposed model is superior to the other benchmark methods. Consequently, the proposed model can be employed as an effective method for carbon price series analysis and forecasting. Full article

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22 pages, 1993 KiB

Open AccessArticle

Analyzing National and Local Pathways to Carbon-Neutrality from Technology, Emissions, and Resilience Perspectives—Case of Finland

by Sannamari Pilpola, Vahid Arabzadeh, Jani Mikkola and Peter D. Lund

Energies 2019, 12(5), 949; https://doi.org/10.3390/en12050949 - 12 Mar 2019

Cited by 62 | Viewed by 6464

Abstract

The Paris Climate Accord calls for urgent CO₂ reductions. Here we investigate low and zero carbon pathways based on clean electricity and sector coupling. Effects from different spatialities are considered through city and national cases (Helsinki and Finland). The methodology employs techno-economic [...] Read more.

The Paris Climate Accord calls for urgent CO₂ reductions. Here we investigate low and zero carbon pathways based on clean electricity and sector coupling. Effects from different spatialities are considered through city and national cases (Helsinki and Finland). The methodology employs techno-economic energy system optimization, including resilience aspects. In the Finnish case, wind, nuclear, and biomass coupled to power-to-heat and other flexibility measures could provide a cost-effective carbon-neutral pathway (annual costs −18%), but nuclear and wind are, to some extent, exclusionary. A (near) carbon-neutral energy system seems possible even without nuclear (−94% CO₂). Zero-carbon energy production benefits from a stronger link to the broader electricity market albeit flexibility measures. On the city level, wind would not easily replace local combined heat and power (CHP), but may increase electricity export. In the Helsinki case, a business-as-usual approach could halve emissions and annual costs, while in a comprehensive zero-emission approach, the operating costs (OPEX) could decrease by 87%. Generally, electrification of heat production could be effective to reduce CO₂. Low or zero carbon solutions have a positive impact on resilience, but in the heating sector this is more problematic, e.g., power outage and adequacy of supply during peak demand will require more attention when planning future carbon-free energy systems. Full article

(This article belongs to the Special Issue Sustainable Energy Technologies for Eco Cities and Environment—Papers from the 17th International Conference on Sustainable Energy Technologies (SET2018))

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21 pages, 8298 KiB

Open AccessArticle

An Estimation Model with Generalization Characteristics for the Internal Impedance of the Rechargeable Batteries by Means of Dual ANN Model

by Minella Bezha, Ryo Gondo and Naoto Nagaoka

Energies 2019, 12(5), 948; https://doi.org/10.3390/en12050948 - 12 Mar 2019

Cited by 6 | Viewed by 3569

Abstract

An estimation method of equivalent circuit parameters for rechargeable batteries that follows Artificial Neural Network (ANN) logic is proposed in this paper. The capability of the nonlinear analysis of the ANN is suitable for estimating the parameters that are nonlinearly involved in the [...] Read more.

An estimation method of equivalent circuit parameters for rechargeable batteries that follows Artificial Neural Network (ANN) logic is proposed in this paper. The capability of the nonlinear analysis of the ANN is suitable for estimating the parameters that are nonlinearly involved in the complex circuit equation. The parameters have to be obtained from the complex internal impedances, which are measured in a wide frequency range. The accuracy is improved by dividing this wide range into a low-frequency and a high-frequency region. These regions are strongly related to the capacity fade and the maximum chargeable/dischargeable current, respectively. The improved method will determine the optimal frequency region for three different rechargeable batteries, which are composed of Li-Ion, Pb and Ni-MH. The accuracy of the proposed method is confirmed by a comparison with the measured results obtained using a conventional frequency domain method. For obtaining the real-time diagnostics of the battery, an improved dual ANN system, which employs unequal sampling, is proposed to obtain the circuit parameters. The deterioration of a battery can be detected from the estimated parameters, which can help in further investigations that aim to develop diagnostic models for the embedded circuit in industrial applications. Full article

(This article belongs to the Special Issue Advances in Electrical Power Engineering—Select papers from 53rd International Universities Power Engineering Conference (UPEC2018))

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19 pages, 2727 KiB

Open AccessArticle

Monitoring the Bioactive Compounds Status in Olea europaea According to Collecting Period and Drying Conditions

by Amani Taamalli, Jesus Lozano Sánchez, Haifa Jebabli, Najla Trabelsi, Leila Abaza, Antonio Segura Carretero, Jae Youl Cho and David Arráez Román

Energies 2019, 12(5), 947; https://doi.org/10.3390/en12050947 - 12 Mar 2019

Cited by 19 | Viewed by 3602

Abstract

Polyphenols and triterpenoids in olive have relevant importance both in the physiology of the plant and the nutritional and biological value of its products. Olive leaf extracts are of special interest for their numerous health-promoting properties. The present research is investigating the occurrence [...] Read more.

Polyphenols and triterpenoids in olive have relevant importance both in the physiology of the plant and the nutritional and biological value of its products. Olive leaf extracts are of special interest for their numerous health-promoting properties. The present research is investigating the occurrence of phytochemicals in supercritical fluid extracts from leaves with regard to collection time and drying temperature. The phytochemical profiles of the olive leaf extracts were determined by reversed-phase high-performance liquid chromatography (HPLC) coupled to electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) detection. The main extracted phytochemicals were phenols and terpenoids. A significant variation in the amounts of the different components was observed as a function of the different drying temperature and collecting time (p < 0.05). Among samples, the maximal contents of polyphenols and secoiridoid derivatives were found in the extracts from olive leaves collected in November and dried at 120 °C, whereas triterpenoids showed the highest content in fresh leaves collected in August. Full article

(This article belongs to the Special Issue Biorefinery Based on Olive Biomass)

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19 pages, 2495 KiB

Open AccessReview

Prediction of Electric Vehicle Range: A Comprehensive Review of Current Issues and Challenges

by Bogdan Ovidiu Varga, Arsen Sagoian and Florin Mariasiu

Energies 2019, 12(5), 946; https://doi.org/10.3390/en12050946 - 12 Mar 2019

Cited by 136 | Viewed by 17348

Abstract

Electric vehicles (EV) are the immediate solution to drastically reducing pollutant emissions from the transport sector. There is a continuing increase in the number of EVs in use, but their widespread and massive acceptance by automotive consumers is related to the performance they [...] Read more.

Electric vehicles (EV) are the immediate solution to drastically reducing pollutant emissions from the transport sector. There is a continuing increase in the number of EVs in use, but their widespread and massive acceptance by automotive consumers is related to the performance they can deliver. The most important feature here (a hot topic at present in EV research) is related to the possibility of providing a more accurate prediction of range. Range prediction is a complex problem because it depends on a lot of influence factors (internal, external, constant, variables) and the present paper aims to investigate the effect of these factors on the range of EVs. The results and aspects of current worldwide research on this theme are presented through the analysis of the main classes of influence factors: Vehicle design, the driver and the environment. Further, the weight and effect of each potential factor which influences EV range was analyzed by presenting current issues. An exhaustive and comprehensive analysis has made it possible to identify future research and development directions in the EV research field, resulting in massive future and immediate EV penetration in the automotive market. Full article

(This article belongs to the Collection Electric and Hybrid Vehicles Collection)

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15 pages, 4256 KiB

Open AccessArticle

Coordinated Control for Operating Characteristics Improvement of UHVDC Transmission Systems under Hierarchical Connection Scheme with STATCOM

by Atiq Ur Rehman, Chunyi Guo and Chengyong Zhao

Energies 2019, 12(5), 945; https://doi.org/10.3390/en12050945 - 12 Mar 2019

Cited by 7 | Viewed by 3288

Abstract

Ultra-high voltage direct current (UHVDC) systems under hierarchical connection schemes (HCSs) linked to AC grids with different voltage levels (500 and 1000 kV) have been a great concern for power utilities to transfer bulk power. They have some operating issues like cascaded commutation [...] Read more.

Ultra-high voltage direct current (UHVDC) systems under hierarchical connection schemes (HCSs) linked to AC grids with different voltage levels (500 and 1000 kV) have been a great concern for power utilities to transfer bulk power. They have some operating issues like cascaded commutation failures and longer fault recovery time under certain fault conditions. Since STATCOM has the ability to effectively regulate AC busbar voltages, thus it is considered in this paper to improve the operating characteristics of UHVDC-HCS systems. To further improve the operating characteristics, a coordinated control between an UHVDC-HCS system and STATCOM is presented. To validate the effectiveness of coordinated control, the comparison between different control modes such as reactive power control (Q-control) and voltage control (V-control) in the outer loop control of STATCOM are conducted in detail. Various indices like commutation failure immunity index (CFII) and commutation failure probability index (CFPI) are also comprehensively evaluated in order to investigate robustness of the adopted coordinated control. An UHVDC-HCS system with multiple STATCOMs on the inverter side (500 kV bus) is developed in PSCAD/EMTDC. The impact of coordinated control on commutation failure phenomena and fault recovery time during single and three phase AC faults is analyzed. The analysis shows that coordinated control with V-control mode of STATCOM exhibits better performance in enhancing the operating characteristics of UHVDC-HCS system by improving the CFII, effectively reducing the CFPI and fault recovery time under various AC faults. Full article

(This article belongs to the Section F: Electrical Engineering)

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20 pages, 3811 KiB

Open AccessArticle

A Simplified Methodology for Existing Tertiary Buildings’ Cooling Energy Need Estimation at District Level: A Feasibility Study of a District Cooling System in Marrakech

by Saeid Charani Shandiz, Alice Denarie, Gabriele Cassetti, Marco Calderoni, Antoine Frein and Mario Motta

Energies 2019, 12(5), 944; https://doi.org/10.3390/en12050944 - 12 Mar 2019

Cited by 7 | Viewed by 4797

Abstract

In district energy systems planning, the calculation of energy needs is a crucial step in making the investment profitable. Although several computational approaches exist for estimating the thermal energy need of individual buildings, this is challenging at the district level due to the [...] Read more.

In district energy systems planning, the calculation of energy needs is a crucial step in making the investment profitable. Although several computational approaches exist for estimating the thermal energy need of individual buildings, this is challenging at the district level due to the amount of data needed, the diversity of building types, and the uncertainty of connections. The aim of this paper is to present a simplified measurement-based methodology for estimating the cooling energy needs at the district level, which can be employed in the preliminary sizing and design of a district cooling network. The methodology proposed is suitable for tertiary buildings and is based on building electricity bills as historical data to calculate the yearly cooling demand. Then, the developed method is applied to a real case study: the feasibility analysis of a sustainable district cooling network for a hotel district in the city of Marrakech. The designed system foresees a 23-MW_cold district cooling network that is 4 km long, supplying 26 GWh of cooling to the tourist area. The results show that the proposed methodology for cooling demand estimation is coherent with the other existing methods in the literature. Full article

(This article belongs to the Special Issue 4th International Conference on Smart Energy Systems and 4th Generation District Heating)

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19 pages, 5680 KiB

Open AccessArticle

Sliding Mode-Based Robust Control for Piezoelectric Actuators with Inverse Dynamics Estimation

by Ander Chouza, Oscar Barambones, Isidro Calvo and Javier Velasco

Energies 2019, 12(5), 943; https://doi.org/10.3390/en12050943 - 12 Mar 2019

Cited by 21 | Viewed by 3829

Abstract

This paper presents an improved control approach to be used for piezoelectric actuators. The proposed approach is based on sliding mode control with estimation perturbation (SMCPE) techniques. Also, a proportional–integral–derivative (PID)-type sliding surface is proposed for position tracking. The proposed approach has been [...] Read more.

This paper presents an improved control approach to be used for piezoelectric actuators. The proposed approach is based on sliding mode control with estimation perturbation (SMCPE) techniques. Also, a proportional–integral–derivative (PID)-type sliding surface is proposed for position tracking. The proposed approach has been studied and implemented in a commercial actuator. A model for the system is introduced, which includes the Bouc–Wen (BW) model to represent the hysteresis, and it is identified by means of the System Identification Toolbox in Matlab/Simulink. Experimental data show that the proposed controller has a better performance when compared to a proportional-integral (PI) controller or a conventional SMCPE in motion tracking. Furthermore, a sub-micrometer accuracy tracking can be obtained while compensating for the hysteresis effect. Full article

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31 pages, 6858 KiB

Open AccessArticle

Uncertainty Evaluation of Safe Mud Weight Window Utilizing the Reliability Assessment Method

by Tianshou Ma, Tao Tang, Ping Chen and Chunhe Yang

Energies 2019, 12(5), 942; https://doi.org/10.3390/en12050942 - 12 Mar 2019

Cited by 25 | Viewed by 4613

Abstract

Due to the uncertainty of formation properties and improper wellbore stability analysis methods, the input parameters are often uncertain and the required mud weight to prevent wellbore collapse is too large, which might cause an incorrect result. However, the uncertainty evaluation of input [...] Read more.

Due to the uncertainty of formation properties and improper wellbore stability analysis methods, the input parameters are often uncertain and the required mud weight to prevent wellbore collapse is too large, which might cause an incorrect result. However, the uncertainty evaluation of input parameters and their influence on safe mud weight window (SMWW) is seldom investigated. Therefore, the present paper aims to propose an uncertain evaluation method to evaluate the uncertainty of SMWW. The reliability assessment theory was introduced, and the uncertain SMWW model was proposed by involving the tolerable breakout, the Mogi-Coulomb (MG-C) criterion and the reliability assessment theory. The influence of uncertain parameters on wellbore collapse, wellbore fracture and SMWW were systematically simulated and investigated by utilizing Monte Carlo simulation. Finally, the field observation of well SC-101X was reported and discussed. The results indicated that the MG-C criterion and tolerable breakout is recommended for wellbore stability analysis. The higher the coefficient of variance is, the higher the level of uncertainty is, the larger the impact on SMWW will be, and the higher the risk of well kick, wellbore collapse and fracture will be. The uncertainty of basic parameters has a very significant impact on SMWW, and it cannot be ignored. For well SC-101X, the SMWW predicted by analytical solution is 0.9921–1.6020 g/cm³, compared to the SMWW estimated by the reliability assessment method, the reliability assessment method tends to give a very narrow SMWW of 1.0756–1.0935 g/cm³ and its probability is only 80%, and the field observation for well kick and wellbore fracture verified the analysis results. For narrow SMWW formation drilling, some kinds of advanced technology, such as the underbalanced drilling (UBD), managed pressure drilling (MPD), micro-flow drilling (MFD) and wider the SMWW, can be utilized to maintain drilling safety. Full article

(This article belongs to the Special Issue New Stimulation Methods for Recovery of Energy and Minerals from Ultra-low-permeability Rock Formations)

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19 pages, 2767 KiB

Open AccessFeature PaperArticle

Is It Possible to Supply Norwegian Apartment Blocks with 4th Generation District Heating?

by Øystein Rønneseth, Nina Holck Sandberg and Igor Sartori

Energies 2019, 12(5), 941; https://doi.org/10.3390/en12050941 - 12 Mar 2019

Cited by 10 | Viewed by 4156

Abstract

Direct electricity is widely used for heating purposes in Norway, leading to significant strain on the electricity grid during the heating season. Conversion to 4th generation district heating (4GDH) is an effective method for reducing the need for large investments in the electricity [...] Read more.

Direct electricity is widely used for heating purposes in Norway, leading to significant strain on the electricity grid during the heating season. Conversion to 4th generation district heating (4GDH) is an effective method for reducing the need for large investments in the electricity grid, while simultaneously improving the energy efficiency of district heating systems. This article evaluates the possibility of reducing the supply temperature in existing Norwegian apartment blocks by improving the thermal envelope and reducing the temperature levels for the heating system. The analysis is based on simulations in IDA ICE (IDA Indoor Climate and Energy) focusing on whether the reduced supply temperature guarantees thermal comfort in the building, considering the coldest room with a heating setpoint of 22 °C. Based on a recommended minimum acceptable indoor temperature of 19 °C from the Norwegian building regulations (TEK), it should be possible to lower the radiator supply temperature from 80 to 60 °C for apartment blocks newer than 1971. For older buildings, an “intermediate” renovation is necessary to maintain temperatures above 19 °C, however, a “standard” renovation is recommended to ensure thermal comfort and improve the energy efficiency of the building stock. Full article

(This article belongs to the Special Issue 4th International Conference on Smart Energy Systems and 4th Generation District Heating)

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24 pages, 7737 KiB

Open AccessArticle

Development of a Computational System to Improve Wind Farm Layout, Part I: Model Validation and Near Wake Analysis

by Rafael V. Rodrigues and Corinne Lengsfeld

Energies 2019, 12(5), 940; https://doi.org/10.3390/en12050940 - 12 Mar 2019

Cited by 17 | Viewed by 5983

Abstract

The first part of this work describes the validation of a wind turbine farm Computational Fluid Dynamics (CFD) simulation using literature velocity wake data from the MEXICO (Model Experiments in Controlled Conditions) experiment. The work is intended to establish a computational framework from [...] Read more.

The first part of this work describes the validation of a wind turbine farm Computational Fluid Dynamics (CFD) simulation using literature velocity wake data from the MEXICO (Model Experiments in Controlled Conditions) experiment. The work is intended to establish a computational framework from which to investigate wind farm layout, seeking to validate the simulation and identify parameters influencing the wake. A CFD model was designed to mimic the MEXICO rotor experimental conditions and simulate new operating conditions with regards to tip speed ratio and pitch angle. The validation showed that the computational results qualitatively agree with the experimental data. Considering the designed tip speed ratio (TSR) of 6.6, the deficit of velocity in the wake remains at rate of approximately 15% of the free-stream velocity per rotor diameter regardless of the free-stream velocity applied. Moreover, analysis of a radial traverse right behind the rotor showed an increase of 20% in the velocity deficit as the TSR varied from TSR = 6 to TSR = 10, corresponding to an increase ratio of approximately 5% m·s⁻¹ per dimensionless unit of TSR. We conclude that the near wake characteristics of a wind turbine are strongly influenced by the TSR and the pitch angle. Full article

(This article belongs to the Special Issue Modeling of Wind Turbines and Wind Farms)

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12 pages, 15198 KiB

Open AccessArticle

Numerical Analysis of the Influence of Design Parameters on the Efficiency of an OWC Axial Impulse Turbine for Wave Energy Conversion

by Yongyao Luo, Alexandre Presas and Zhengwei Wang

Energies 2019, 12(5), 939; https://doi.org/10.3390/en12050939 - 12 Mar 2019

Cited by 12 | Viewed by 4333

Abstract

Oscillating water column (OWC) axial impulse turbines permit the conversion of wave energy into electrical power. Unlike other hydropower units with a mature and well established technology, such turbines have been recently developed, there are still few prototypes operating and therefore there is [...] Read more.

Oscillating water column (OWC) axial impulse turbines permit the conversion of wave energy into electrical power. Unlike other hydropower units with a mature and well established technology, such turbines have been recently developed, there are still few prototypes operating and therefore there is a large space for optimizing its design. Many recent studies focus on the improvement of the efficiency and transient characteristics by means of experimentation and also simulation techniques. In the present paper we use a 3D numerical simulation model (computational fluid dynamics model with ANSYS-Fluent 18) to analyze the influence of different geometrical parameters on the efficiency of the turbine, which have been less discussed yet. A reference configuration case has been used to validate our simulation model by comparing it with previous experimental results. Then, parametric variations in the guide vane number and type, gaps between the rotating and stationary part and hub to tip ratio have been introduced in the model to discuss the influence of these effects. It is found that some of these parameters have an important influence on the efficiency of the turbine and therefore, the results presented in this paper can help to optimize future designs of OWC impulse turbines. Full article

(This article belongs to the Section A3: Wind, Wave and Tidal Energy)

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22 pages, 1015 KiB

Open AccessArticle

Quantifying the Benefits of a Solar Home System-Based DC Microgrid for Rural Electrification

by Nishant Narayan, Ali Chamseddine, Victor Vega-Garita, Zian Qin, Jelena Popovic-Gerber, Pavol Bauer and Miroslav Zeman

Energies 2019, 12(5), 938; https://doi.org/10.3390/en12050938 - 11 Mar 2019

Cited by 28 | Viewed by 4008

Abstract

Off-grid solar home systems (SHSs) currently constitute a major source of providing basic electricity needs in un(der)-electrified regions of the world, with around 73 million households having benefited from off-grid solar solutions by 2017. However, in and of itself, state-of-the-art SHSs can only [...] Read more.

Off-grid solar home systems (SHSs) currently constitute a major source of providing basic electricity needs in un(der)-electrified regions of the world, with around 73 million households having benefited from off-grid solar solutions by 2017. However, in and of itself, state-of-the-art SHSs can only provide electricity access with adequate power supply availability up to tier 2, and to some extent, tier 3 levels of the Multi-tier Framework (MTF) for measuring household electricity access. When considering system metrics of loss of load probability (LLP) and battery size, meeting the electricity needs of tiers 4 and 5 is untenable through SHSs alone. Alternatively, a bottom-up microgrid composed of interconnected SHSs is proposed. Such an approach can enable the so-called climb up the rural electrification ladder. The impact of the microgrid size on the system metrics like LLP and energy deficit is evaluated. Finally, it is found that the interconnected SHS-based microgrid can provide more than 40% and 30% gains in battery sizing for the same LLP level as compared to the standalone SHSs sizes for tiers 4 and 5 of the MTF, respectively, thus quantifying the definite gains of an SHS-based microgrid over standalone SHSs. This study paves the way for visualizing SHS-based rural DC microgrids that can not only enable electricity access to the higher tiers of the MTF with lower battery storage needs but also make use of existing SHS infrastructure, thus enabling a technologically easy climb up the rural electrification ladder. Full article

(This article belongs to the Section A1: Smart Grids and Microgrids)

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15 pages, 6735 KiB

Open AccessArticle

Research on Distribution and Shielding of Spatial Magnetic Field of a DC Air Core Smoothing Reactor

by Jiameng Yang, Wenfang Zhang, Liang Zou, Yali Wang, Youliang Sun and Yingchun Feng

Energies 2019, 12(5), 937; https://doi.org/10.3390/en12050937 - 11 Mar 2019

Cited by 5 | Viewed by 4046

Abstract

With the rapid development of ultra-high-voltage direct current (UHVDC) transmission, air core smoothing reactors have become the main source of electromagnetic contamination in converter substations. The actual magnetic field distribution was obtained by measuring the magnetic induction intensity of the polar busbar smoothing [...] Read more.

With the rapid development of ultra-high-voltage direct current (UHVDC) transmission, air core smoothing reactors have become the main source of electromagnetic contamination in converter substations. The actual magnetic field distribution was obtained by measuring the magnetic induction intensity of the polar busbar smoothing reactor under full load operation condition of the Jiaodong ±660 kV converter substation. A method combined with the measured data to eliminate the influence of the geomagnetic field is proposed. The magnetic field distribution model of the smoothing reactor is established and the rationality and validity of the model for magnetic field distribution is verified. Some magnetic shielding measures are proposed and their effectiveness is verified by simulation and small-scale experiments. Full article

(This article belongs to the Section F: Electrical Engineering)

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15 pages, 2074 KiB

Open AccessArticle

Fuzzy Rule-Based and Particle Swarm Optimisation MPPT Techniques for a Fuel Cell Stack

by Doudou N. Luta and Atanda K. Raji

Energies 2019, 12(5), 936; https://doi.org/10.3390/en12050936 - 11 Mar 2019

Cited by 53 | Viewed by 4402

Abstract

The negative environmental impact and the rapidly declining reserve of fossil fuel-based energy sources for electricity generation is a big challenge to finding sustainable alternatives. This scenario is complicated by the ever-increasing world population growth demanding a higher standard of living. A fuel [...] Read more.

The negative environmental impact and the rapidly declining reserve of fossil fuel-based energy sources for electricity generation is a big challenge to finding sustainable alternatives. This scenario is complicated by the ever-increasing world population growth demanding a higher standard of living. A fuel cell system is able to generate electricity and water with higher energy efficiency while producing near-zero emissions. A common fuel cell stack displays a nonlinear power characteristic as a result of internal limitations and operating parameters such as temperature, hydrogen and oxygen partial pressures and humidity levels, leading to a reduced overall system performance. It is therefore important to extract as much power as possible from the stack, thus hindering excessive fuel use. This study considers and compares two Maximum Power Point Tracking (MPPT) approaches; one based on the Mamdani Fuzzy Inference System and the other on the Particle Swarm Optimisation (PSO) algorithm to maintain the output power of a fuel cell stack extremely close to its maximum. To ensure that, the power converter interfaced to the fuel cell unit must be able to continuously self-modify its parameters, hence changing its voltage and current depending upon the Maximum Power Point position. While various methods exist for Maximum Power Point tracker design, this paper analyses the response characteristics of a Mamdani Fuzzy Inference Engine and the Particle Swarm Optimisation technique. The investigation was conducted on a 53 kW Proton Exchange Membrane Fuel Cell interfaced to a DC-to-DC boost converter supplying 1.2 kV from a 625 V input DC voltage. The modelling was accomplished using a Matlab/Simulink environment. The results showed that the MPPT controller based on the PSO algorithm presented better tracking efficiency as compared to the Mamdani controller. Furthermore, the rise time of the PSO controller was slightly shorter than the Mamdani controller and the overshoot of the PSO controller was 2% lower than that of the Mamdani controller. Full article

(This article belongs to the Special Issue Selected Papers from 5th International Conference on Power and Energy Systems Engineering (CPESE 2018))

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18 pages, 2649 KiB

Open AccessFeature PaperArticle

Optimal Operation Strategies into Deregulated Markets for 50 MW_e Parabolic Trough Solar Thermal Power Plants with Thermal Storage

by Jorge M. Llamas, David Bullejos and Manuel Ruiz de Adana

Energies 2019, 12(5), 935; https://doi.org/10.3390/en12050935 - 11 Mar 2019

Cited by 13 | Viewed by 3091

Abstract

The evolution of electric generation systems, according to relevant legislation, allows for the parallel evolution of the installed power capacity of renewable resources with the development of technologies for renewable resources, therefore optimizing the choice of energy mix from renewable resources by prioritizing [...] Read more.

The evolution of electric generation systems, according to relevant legislation, allows for the parallel evolution of the installed power capacity of renewable resources with the development of technologies for renewable resources, therefore optimizing the choice of energy mix from renewable resources by prioritizing the implementation of concentrating solar thermal plants. Thanks to their great potential, parabolic trough solar thermal power plants have become the most widely spread type of electricity generation by renewable solar energy. Nonetheless, the operation of the plant is not unique; it must be adapted to the parameters of solar radiation and market behavior for each specific location. This work focuses on the search for the optimal strategies of operation by a mathematical model of a 50 MW_e parabolic trough thermal power plant with thermal storage. The analysis of the different ways of operation throughout a whole year, including model verification via a currently operating plant, provides meaningful insights into the electricity generated. Focused to work under non-regulated electricity markets to adjust this type of technology to the European directives, the presented model of optimization allows for the adaptation of the curve of generation to the network demands and market prices, rising the profitability of the power plant. Thus, related to solar resources and market price, the economic benefit derived from the electricity production improves between 5.17% and 7.79%. Full article

(This article belongs to the Section F: Electrical Engineering)

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19 pages, 17680 KiB

Open AccessArticle

Key Parameters of Roof Cutting of Gob-Side Entry Retaining in a Deep Inclined Thick Coal Seam with Hard Roof

by Jinzhu Hu, Manchao He, Jiong Wang, Zimin Ma, Yajun Wang and Xingyu Zhang

Energies 2019, 12(5), 934; https://doi.org/10.3390/en12050934 - 11 Mar 2019

Cited by 46 | Viewed by 3749

Abstract

Gob-side entry retaining by roof cutting (GERRC) employed in a deep inclined thick coal seam (DITCS) can not only increase economic benefits and coal recovery, but also optimize surrounding rock structure. In accordance with the principles of GERRC, the technology of GERRC in [...] Read more.

Gob-side entry retaining by roof cutting (GERRC) employed in a deep inclined thick coal seam (DITCS) can not only increase economic benefits and coal recovery, but also optimize surrounding rock structure. In accordance with the principles of GERRC, the technology of GERRC in DITCS is introduced and a roof-cutting mechanical model of GERRC is proposed to determine the key parameters of the depth and angle of RC. The results show that the greater the RC angle, the easier the caving of the goaf roof, but the length of cantilever beam increases. The depth of RC should account for the dip angle of the coal seam when the angle is above 20°. Increasing the coal seam dip angle could reduce the volume of rock falling of the goaf roof, but increase the filling height of the upper gangue to slide down. According to numerical model analysis of the stress and displacement of surrounding rock at different depths and angles of RC, when the depth of RC increased from 9 m to 13 m, the distance between the stress concentration zone and the coal side is increased. When the angle of RC increased from 0° to 20°, the value of roof separation is decreased. GERRC was applied in a DITCS with 11 m depth and 20° RC angle, and the field-measured data verified the conclusions of the numerical model. Full article

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15 pages, 3359 KiB

Open AccessArticle

An Investigation of a Root Zone Heating System and Its Effects on the Morphology of Winter-Grown Green Peppers

by Muhammad Ameen, Zhuo Zhang, Xiaochan Wang, Muhammad Yaseen, Muhammad Umair, Rana Shahzad Noor, Wei Lu, Khurram Yousaf, Fahim Ullah and Muhammad Sohail Memon

Energies 2019, 12(5), 933; https://doi.org/10.3390/en12050933 - 11 Mar 2019

Cited by 7 | Viewed by 5100

Abstract

The winter season in Nanjing is from December to February, with extremely low temperature and high humidity due to seasonal snowfall. During these extreme cold climatic conditions, plants have to survive severe heat stress conditions, even if they are being kept in greenhouses. [...] Read more.

The winter season in Nanjing is from December to February, with extremely low temperature and high humidity due to seasonal snowfall. During these extreme cold climatic conditions, plants have to survive severe heat stress conditions, even if they are being kept in greenhouses. The objective of this study was to investigate a heating system that can provide heat directly to the root zone instead of heating the entire greenhouse, which is a viable option to reduce energy consumption. Root zone heating could be an effective alternative for the sustainable development of plants during the winter. A novel type of root zone heating system was applied to evaluate the energy consumption during different greenhouse ambient temperature conditions, the effects of root zone heating systems on pepper plant morphology, and heat transfer rates to plant canopy in the greenhouse. The temperature treatments in root zone heating system were T-15, T-20, T-25, T-30, and a control treatment (TC) at 15 °C, 20 °C, 25 °C, and 30 °C, respectively, while TC received no heat. A simulation study was carried out to validate the root zone temperature. The results of the current investigation revealed that energy consumption has an inverse relationship to the ambient temperature of the greenhouse, while temperature gradients to the plant canopy observed from the lower to the upper part of the plant and the upper canopy experienced less temperature fluctuation as compared to the lower part of the plant. The results also showed that treatment T-20 had the maximum in terms of the leaf dry weight, stem diameter, and the number of leaves, while T-25 showed the maximum root dry weight and stem dry weight; T-30 and T-15 had minimum dry weights of plant segments among all treatments. Control treatment (TC) showed a minimum dry mass of plant. The root zone heating with optimal root zone temperature was found to be a viable and adaptable option as this leads to improved energy consumption patterns for the sustainable growth and development of plants in greenhouses during extremely low temperatures. Full article

(This article belongs to the Special Issue Agriculture and Energy 2019)

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16 pages, 9214 KiB

Open AccessArticle

Investigation of the Effect of Natural Fractures on Multiple Shale-Gas Well Performance Using Non-Intrusive EDFM Technology

by Wei Yu, Xiaohu Hu, Malin Liu and Weihong Wang

Energies 2019, 12(5), 932; https://doi.org/10.3390/en12050932 - 10 Mar 2019

Cited by 11 | Viewed by 4204

Abstract

The influence of complex natural fractures on multiple shale-gas well performance with varying well spacing is poorly understood. It is difficult to apply the traditional local grid refinement with structured or unstructured gridding techniques to accurately and efficiently handle complex natural fractures. In [...] Read more.

The influence of complex natural fractures on multiple shale-gas well performance with varying well spacing is poorly understood. It is difficult to apply the traditional local grid refinement with structured or unstructured gridding techniques to accurately and efficiently handle complex natural fractures. In this study, we introduced a powerful non-intrusive embedded discrete fracture model (EDFM) technology to overcome the limitations of exiting methods. Through this unique technology, complex fracture configurations can be easily and explicitly embedded into structured matrix blocks. We set up a field-scale two-phase reservoir model to history match field production data and predict long-term recovery from Marcellus. The effective fracture properties were determined thorough history matching. In addition, we extended the single-well model to include two horizontal wells with and without including natural fractures. The effects of different numbers of natural fractures on two-well performance with varying well spacing of 200 m, 300 m, and 400 m were examined. The simulation results illustrate that gas productivity almost linearly increases with the number of two-set natural fractures. Furthermore, the difference of well performance between different well spacing increases with an increase in natural fracture density. A larger well spacing is preferred for economically developing the shale-gas reservoirs with a larger natural fracture density. The findings of this study provide key insights into understanding the effect of natural fractures on well performance and well spacing optimization. Full article

(This article belongs to the Special Issue Improved Reservoir Models and Production Forecasting Techniques for Multi-Stage Fractured Hydrocarbon Wells)

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Energies, Volume 12, Issue 5 (March-1 2019) – 194 articles

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