Next Issue
Previous Issue

E-Mail Alert

Add your e-mail address to receive forthcoming issues of this journal:

Journal Browser

Journal Browser

Table of Contents

Energies, Volume 10, Issue 4 (April 2017)

  • Issues are regarded as officially published after their release is announced to the table of contents alert mailing list.
  • You may sign up for e-mail alerts to receive table of contents of newly released issues.
  • PDF is the official format for papers published in both, html and pdf forms. To view the papers in pdf format, click on the "PDF Full-text" link, and use the free Adobe Readerexternal link to open them.
Cover Story Optimized component behavior, for instance of check valves, turned out to be crucial for good [...] Read more.
View options order results:
result details:
Displaying articles 1-170
Export citation of selected articles as:

Research

Jump to: Review

Open AccessArticle Optimal Energy Management Strategy for a Plug-in Hybrid Electric Vehicle Based on Road Grade Information
Energies 2017, 10(4), 412; doi:10.3390/en10040412
Received: 9 January 2017 / Revised: 11 March 2017 / Accepted: 18 March 2017 / Published: 23 March 2017
Cited by 2 | PDF Full-text (7251 KB) | HTML Full-text | XML Full-text
Abstract
Energy management strategies (EMSs) are critical for the improvement of fuel economy of plug-in hybrid electric vehicles (PHEVs). However, conventional EMSs hardly consider the influence of uphill terrain on the fuel economy and battery life, leaving vehicles with insufficient battery power for continuous
[...] Read more.
Energy management strategies (EMSs) are critical for the improvement of fuel economy of plug-in hybrid electric vehicles (PHEVs). However, conventional EMSs hardly consider the influence of uphill terrain on the fuel economy and battery life, leaving vehicles with insufficient battery power for continuous uphill terrains. Hence, in this study, an optimal control strategy for a PHEV based on the road grade information is proposed. The target state of charge (SOC) is estimated based on the road grade information as well as the predicted driving cycle on uphill road obtained from the GPS/GIS system. Furthermore, the trajectory of the SOC is preplanned to ensure sufficient electricity for the uphill terrain in the charge depleting (CD) and charge sustaining (CS) modes. The genetic algorithm is applied to optimize the parameters of the control strategy to maintain the SOC of battery in the CD mode. The pre-charge mode is designed to charge the battery in the CS mode from a reasonable distance before the uphill terrain. Finally, the simulation model of the powertrain system for the PHEV is established using MATLAB/Simulink platform. The results show that the proposed control strategy based on road-grade information helps successfully achieve better fuel economy and longer battery life. Full article
(This article belongs to the Special Issue Advanced Energy Storage Technologies and Their Applications (AESA))
Figures

Figure 1

Open AccessArticle Small Scale Organic Rankine Cycle (ORC): A Techno-Economic Review
Energies 2017, 10(4), 413; doi:10.3390/en10040413
Received: 27 January 2017 / Revised: 16 March 2017 / Accepted: 17 March 2017 / Published: 23 March 2017
Cited by 5 | PDF Full-text (3389 KB) | HTML Full-text | XML Full-text
Abstract
The Organic Rankine Cycle (ORC) is widely considered as a promising technology to produce electrical power output from low-grade thermal sources. In the last decade, several power plants have been installed worldwide in the MW range. However, despite its market potential, the commercialization
[...] Read more.
The Organic Rankine Cycle (ORC) is widely considered as a promising technology to produce electrical power output from low-grade thermal sources. In the last decade, several power plants have been installed worldwide in the MW range. However, despite its market potential, the commercialization of ORC power plants in the kW range did not reach a high level of maturity, for several reasons. Firstly, the specific price is still too high to offer an attractive payback period, and secondly, potential costumers for small-scale ORCs are typically SMEs (Small-Medium Enterprises), generally less aware of the potential savings this technology could lead to. When it comes to small-scale plants, additional design issues arise that still limit the widespread availability of the technology. This review paper presents the state of the art of the technology, from a technical and economic perspective. Working fluid selection and expander design are illustrated in detail, as they represent the bottleneck of the ORC technology for small-scale power production. In addition, a European market analysis is presented, which constitutes a useful instrument to understand the future evolution of the technology. Full article
Figures

Figure 1

Open AccessArticle Characterization of the Diamond Wire Sawing Process for Monocrystalline Silicon by Raman Spectroscopy and SIREX Polarimetry
Energies 2017, 10(4), 414; doi:10.3390/en10040414
Received: 30 November 2016 / Revised: 16 March 2017 / Accepted: 18 March 2017 / Published: 23 March 2017
PDF Full-text (6654 KB) | HTML Full-text | XML Full-text
Abstract
A detailed approach to evaluate the sub-surface damage of diamond wire-sawn monocrystalline silicon wafers relating to the sawing process is presented. Residual stresses, the presence of amorphous silicon and microcracks are considered and related to diamond wire velocity and cutting ability. In particular,
[...] Read more.
A detailed approach to evaluate the sub-surface damage of diamond wire-sawn monocrystalline silicon wafers relating to the sawing process is presented. Residual stresses, the presence of amorphous silicon and microcracks are considered and related to diamond wire velocity and cutting ability. In particular, the degree of amorphization of the wafer surface is analyzed, as it may affect the etching performance (texturing) during solar cell manufacture. Raman spectroscopy and Scanning Infrared Stress Explorer (SIREX) measurements are used independently as non-destructive, contactless optical characterization methods to provide stress imaging with high spatial resolution. Raman mappings show that amorphous silicon layers can occur inhomogeneously across the surface of diamond wire-sawn wafers. The Raman and SIREX results reveal a connection between a higher fraction of the amorphous phase, a more inhomogeneous stress distribution and a lower peak maximum of the stress difference on wafers, depending on both the wire wear and the wire velocity. SIREX line scans of the in-plane difference of the principal stress components ∆σ taken across the sawing grooves show significant differences in magnitude and periodicity. Furthermore, the results are compared with the microcrack depth from the same investigation areas. The possibility to optimize the diamond wire sawing processes by analyzing the sub-surface stress of the wafers is offered by complementary use of both Raman and SIREX measurements. Full article
(This article belongs to the Special Issue Crystalline Silicon Solar Cells: Fundamentals and Technologies)
Figures

Figure 1

Open AccessArticle Image Recognition of Icing Thickness on Power Transmission Lines Based on a Least Squares Hough Transform
Energies 2017, 10(4), 415; doi:10.3390/en10040415
Received: 13 January 2017 / Revised: 12 March 2017 / Accepted: 18 March 2017 / Published: 23 March 2017
PDF Full-text (6284 KB) | HTML Full-text | XML Full-text
Abstract
In view of the shortcomings of current image detection methods for icing thickness on power transmission lines, an image measuring method for icing thickness based on remote online monitoring was proposed. In this method, a Canny operator is used to get the image
[...] Read more.
In view of the shortcomings of current image detection methods for icing thickness on power transmission lines, an image measuring method for icing thickness based on remote online monitoring was proposed. In this method, a Canny operator is used to get the image edge, in addition, a Hough transform and least squares are combined to solve the problems of traditional Hough transform in the parameter space whereby it is easily disturbed by the image background and noises, and eventually the edges of iced power transmission lines and un-iced power transmission lines are accurately detected in images which have low contrast, complex grayscale, and many noises. Furthermore, based on the imaging principle of the camera, a new geometric calculation model for icing thickness is established by using the radius of power transmission line as a reference, and automatic calculation of icing thickness is achieved. The results show that proposed image recognition method is rarely disturbed by noises and background, the image recognition results show good agreement with the real edges of iced power transmission lines and un-iced power transmission lines, and is simple and easy to program, which suggests that the method can be used for image recognition and calculation of icing thickness. Full article
Figures

Figure 1

Open AccessArticle Robust Operation of Energy Storage System with Uncertain Load Profiles
Energies 2017, 10(4), 416; doi:10.3390/en10040416
Received: 31 December 2016 / Revised: 2 March 2017 / Accepted: 18 March 2017 / Published: 23 March 2017
PDF Full-text (3012 KB) | HTML Full-text | XML Full-text
Abstract
In this paper, we propose novel techniques to reduce total cost and peak load of factories from a customer point of view. We control energy storage system (ESS) to minimize the total electricity bill under the Korea commercial and industrial (KCI) tariff, which
[...] Read more.
In this paper, we propose novel techniques to reduce total cost and peak load of factories from a customer point of view. We control energy storage system (ESS) to minimize the total electricity bill under the Korea commercial and industrial (KCI) tariff, which both considers peak load and time of use (ToU). Under the KCI tariff, the average peak load, which is the maximum among all average power consumptions measured every 15 min for the past 12 months, determines the monthly base cost, and thus peak load control is extremely critical. We aim to leverage ESS for both peak load reduction based on load prediction as well as energy arbitrage exploiting ToU. However, load prediction inevitably has uncertainty, which makes ESS operation challenging with KCI tariff. To tackle it, we apply robust optimization to minimize risk in a real environment. Our approach significantly reduces the peak load by 49.9% and the total cost by 10.8% compared to the case that does not consider load uncertainty. In doing this we also consider battery degradation cost and validate the practical use of the proposed techniques. Full article
(This article belongs to the Special Issue Industrial Energy Efficiency)
Figures

Open AccessArticle Analyzing the Impacts of System Parameters on MPC-Based Frequency Control for a Stand-Alone Microgrid
Energies 2017, 10(4), 417; doi:10.3390/en10040417
Received: 8 February 2017 / Revised: 17 March 2017 / Accepted: 20 March 2017 / Published: 23 March 2017
Cited by 2 | PDF Full-text (8990 KB) | HTML Full-text | XML Full-text
Abstract
Model predictive control (MPC) has been widely studied for regulating frequency in stand-alone microgrids (MGs), owing to the advantages of MPC such as fast response and robustness against the parameter uncertainties. Understanding the impacts of system parameters on the control performance of MPC
[...] Read more.
Model predictive control (MPC) has been widely studied for regulating frequency in stand-alone microgrids (MGs), owing to the advantages of MPC such as fast response and robustness against the parameter uncertainties. Understanding the impacts of system parameters on the control performance of MPC could be useful for the designing process of the controller to achieve better performance. This study analyzes the impact of system parameters on the control performance of MPC for frequency regulation in a stand-alone MG. The typical stand-alone MG, which consists of a diesel engine generator, an energy storage system (ESS), a wind turbine generator, and a load, is considered in this study. The diesel engine generator is in charge of primary frequency control whereas the ESS is responsible for secondary frequency control. The stand-alone MG is linearized to obtain the dynamic model that is used for designing MPC-based secondary frequency control. The robustness of MPC against the variation of system parameters is also analyzed in this study. A comparison study of MPC and proportional–integral (PI) control is presented. Simulation results show that MPC has a faster response time and lower overshoot compared to PI control. In addition, the robustness of MPC against the system uncertainties is stronger than conventional PI control. Full article
(This article belongs to the Special Issue Advanced Operation and Control of Smart Microgrids)
Figures

Figure 1

Open AccessArticle A High-Frequency Isolated Online Uninterruptible Power Supply (UPS) System with Small Battery Bank for Low Power Applications
Energies 2017, 10(4), 418; doi:10.3390/en10040418
Received: 22 December 2016 / Revised: 16 March 2017 / Accepted: 17 March 2017 / Published: 23 March 2017
PDF Full-text (5289 KB) | HTML Full-text | XML Full-text
Abstract
Uninterruptible power supplies (UPSs) are widely used to deliver reliable and high quality power to critical loads under all grid conditions. This paper proposes a high-frequency isolated online UPS system for low power applications. The proposed UPS consists of a single-stage AC-DC converter,
[...] Read more.
Uninterruptible power supplies (UPSs) are widely used to deliver reliable and high quality power to critical loads under all grid conditions. This paper proposes a high-frequency isolated online UPS system for low power applications. The proposed UPS consists of a single-stage AC-DC converter, boost DC-DC converter, and an inverter. The single-stage AC-DC converter provides galvanic isolation, input power factor correction, and continuous conduction of both input and output current. The low battery bank voltage is stepped up to high dc-link voltage by employing a high voltage gain boost converter, thus allows the reduction of battery bank to only 24 V parallel connected batteries. Operating batteries in parallel improves the battery performance and resolves the issues related to conventional battery banks that arrange the batteries in series combination. The inverter provides regulated output voltage to the load. A new cascaded slide mode (SM) and proportional-resonant (PR) control for the inverter has been proposed, which regulates the output voltage for both linear and non-linear loads. The controller shows excellent performance during load transients and step changes. Besides, the controller for boost converter and AC-DC converter is presented. Operating principle and experimental results of 1 kVA laboratory setup have been presented for the validation of proposed system. Full article
Figures

Figure 1

Open AccessArticle A Novel Multi-Objective Optimal Approach for Wind Power Interval Prediction
Energies 2017, 10(4), 419; doi:10.3390/en10040419
Received: 10 January 2017 / Revised: 14 March 2017 / Accepted: 20 March 2017 / Published: 23 March 2017
Cited by 1 | PDF Full-text (1576 KB) | HTML Full-text | XML Full-text
Abstract
Numerous studies on wind power forecasting show that random errors found in the prediction results cause uncertainty in wind power prediction and cannot be solved effectively using conventional point prediction methods. In contrast, interval prediction is gaining increasing attention as an effective approach
[...] Read more.
Numerous studies on wind power forecasting show that random errors found in the prediction results cause uncertainty in wind power prediction and cannot be solved effectively using conventional point prediction methods. In contrast, interval prediction is gaining increasing attention as an effective approach as it can describe the uncertainty of wind power. A wind power interval forecasting approach is proposed in this article. First, the original wind power series is decomposed into a series of subseries using variational mode decomposition (VMD); second, the prediction model is established through kernel extreme learning machine (KELM). Three indices are taken into account in a novel objective function, and the improved artificial bee colony algorithm (IABC) is used to search for the best wind power intervals. Finally, when compared with other competitive methods, the simulation results show that the proposed approach has much better performance. Full article
(This article belongs to the Special Issue Sustainable Energy Technologies)
Figures

Figure 1

Open AccessArticle Improved Separation and Collection of Charge Carriers in Micro-Pyramidal-Structured Silicon/PEDOT:PSS Hybrid Solar Cells
Energies 2017, 10(4), 420; doi:10.3390/en10040420
Received: 5 February 2017 / Revised: 14 March 2017 / Accepted: 21 March 2017 / Published: 23 March 2017
PDF Full-text (2534 KB) | HTML Full-text | XML Full-text
Abstract
Silicon (Si)/organic polymer hybrid solar cells have great potential for becoming cost-effective and efficient energy-harvesting devices. We report herein on the effects of polymer coverage and the rear electrode on the device performance of Si/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hybrid solar cells with micro-pyramidal structures. These
[...] Read more.
Silicon (Si)/organic polymer hybrid solar cells have great potential for becoming cost-effective and efficient energy-harvesting devices. We report herein on the effects of polymer coverage and the rear electrode on the device performance of Si/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hybrid solar cells with micro-pyramidal structures. These hybrid solar cells provided adequate generation of charge carriers owing to the suppression of reflectance to below 13%. Additionally, the separation of the photogenerated charge carriers at the micro-pyramidal-structured Si/PEDOT:PSS interface regions and their collection at the electrodes were dramatically improved by tuning the adhesion areas of the PEDOT:PSS layer and the rear electrode materials, thereby attaining a power conversion efficiency of 8.25%. These findings suggest that it is important to control the PEDOT:PSS coverage and to optimize the rear electrode materials in order to achieve highly efficient separation of the charge carriers and their effective collection in micro-textured hybrid solar cells. Full article
Figures

Figure 1

Open AccessArticle A 3-D Coupled Magneto-Fluid-Thermal Analysis of a 220 kV Three-Phase Three-Limb Transformer under DC Bias
Energies 2017, 10(4), 422; doi:10.3390/en10040422
Received: 11 January 2017 / Revised: 28 February 2017 / Accepted: 20 March 2017 / Published: 23 March 2017
Cited by 1 | PDF Full-text (3608 KB) | HTML Full-text | XML Full-text
Abstract
This paper takes a typical 220 kV three-phase three-limb oil-immersed transformer as an example, this paper building transient field-circuit coupled model and 3D coupled magneto -fluid-thermal model. Considering a nonlinear B–H curve, the magneto model uses the field-circuit coupled finite element method (FEM)
[...] Read more.
This paper takes a typical 220 kV three-phase three-limb oil-immersed transformer as an example, this paper building transient field-circuit coupled model and 3D coupled magneto -fluid-thermal model. Considering a nonlinear B–H curve, the magneto model uses the field-circuit coupled finite element method (FEM) to calculate the magnetic flux distribution of the core and the current distribution of the windings when the transformer is at a rated current and under direct current (DC) bias. Taking the electric power losses of the core and windings as a heat source, the temperature inside the transformer and the velocity of the transformer oil are analyzed by the finite volume method (FVM) in a fluid-thermal field. In order to improve the accuracy of the calculation results, the influence of temperature on the electrical resistivity of the windings and the physical parameter of the transformer oil are taken into account in the paper. Meanwhile, the convective heat transfer coefficient of the FVM model boundary is determined by its temperature. By iterative computations, the model is updated according to the thermal field calculation result until the maximum difference in hot spot temperature between the two adjacent steps is less than 0.01 K. The result calculated by the coupling method agrees well with the empirical equation result according to IEC 60076-7. Full article
Figures

Figure 1

Open AccessArticle Coordinated Control and Fault Protection Investigation of a Renewable Energy Integration Facility with Solar PVs and a Micro-Turbine
Energies 2017, 10(4), 423; doi:10.3390/en10040423
Received: 16 February 2017 / Revised: 21 March 2017 / Accepted: 22 March 2017 / Published: 23 March 2017
Cited by 1 | PDF Full-text (2815 KB) | HTML Full-text | XML Full-text
Abstract
In this paper, a renewable energy integration facility (REIF) with photovoltaic (PV) distributed generation resources for micro-grid applications is studied. In grid-tied operation, the PV system together with the grid supply the power to the local loads while the surplus energy is fed
[...] Read more.
In this paper, a renewable energy integration facility (REIF) with photovoltaic (PV) distributed generation resources for micro-grid applications is studied. In grid-tied operation, the PV system together with the grid supply the power to the local loads while the surplus energy is fed back to the grid. In stand-alone mode, a gas micro-turbine is operated as a master to establish the common AC bus voltage to which the PV inverters can synchronize The experimental results demonstrate the stable operation of the REIF under various generation and load conditions. The power quality can meet the IEEE Standard 1547. Furthermore, the responses of the REIF under different fault conditions are investigated. Relevant protection mechanisms are then developed, providing insights into the fault protection for the future grid. Full article
Figures

Figure 1

Open AccessArticle Scale-Dependent Light Scattering Analysis of Textured Structures on LED Light Extraction Enhancement Using Hybrid Full-Wave Finite-Difference Time-Domain and Ray-Tracing Methods
Energies 2017, 10(4), 424; doi:10.3390/en10040424
Received: 7 February 2017 / Revised: 14 March 2017 / Accepted: 20 March 2017 / Published: 23 March 2017
PDF Full-text (7505 KB) | HTML Full-text | XML Full-text
Abstract
A multiscale model that enables quantitative understanding and prediction of the size effect on the scattering properties of micro- and nanostructures is crucial for the design of light-emitting diode (LED) surface textures optimized for high light extraction efficiency (LEE). In this paper, a
[...] Read more.
A multiscale model that enables quantitative understanding and prediction of the size effect on the scattering properties of micro- and nanostructures is crucial for the design of light-emitting diode (LED) surface textures optimized for high light extraction efficiency (LEE). In this paper, a hybrid process for combining full-wave finite-difference time-domain simulation and a ray-tracing technique based on a bidirectional scattering distribution function model is proposed. We apply this method to study the influence of different pattern sizes of a patterned sapphire substrate on GaN-based LED light extraction from the micro-scale to the nano-scale. The results show that near-wavelength–scale patterns with strong diffraction are not expected to enhance the LEE. By contrast, micro-scale patterns with optical diffusion behavior have the highest LEE at a specific aspect ratio, and subwavelength-scale patterns that have antireflection properties show a marked enhancement of the LEE for a wide range of aspect ratios. Full article
(This article belongs to the Special Issue Solid State Lighting)
Figures

Figure 1

Open AccessArticle The EU Electricity Security Decision-Analytic Framework: Status and Perspective Developments
Energies 2017, 10(4), 425; doi:10.3390/en10040425
Received: 12 January 2017 / Revised: 9 March 2017 / Accepted: 18 March 2017 / Published: 23 March 2017
PDF Full-text (5207 KB) | HTML Full-text | XML Full-text
Abstract
Electricity security, generally referring to a system’s capability to provide electricity to its users, is a multi-faceted problem attracting mounting attention from policy makers and scientists around the world. Electricity security encompasses largely different properties based upon the time/geographical scales of the factors
[...] Read more.
Electricity security, generally referring to a system’s capability to provide electricity to its users, is a multi-faceted problem attracting mounting attention from policy makers and scientists around the world. Electricity security encompasses largely different properties based upon the time/geographical scales of the factors affecting electricity delivery; it is challenged by threats surfacing in spheres far beyond the physical one; it involves a myriad of stakeholders spanning manifold disciplines and with considerably different expectations from the electricity commodity or services; it can be studied as a complicated techno-economic problem or as a complex socio-economic problem. All the above reasons, in a framework of changing European Union (EU) and global energy scenarios, render electricity security ever more challenging to assess and critical to safeguard. Against this background, this work presents recommendations to bring science and policy making closer towards evaluating and handling EU electricity security. More in detail, this is done by:
  • Characterising electricity security via features at the cross-roads of policy and science.
  • Reviewing the electricity security modelling and assessment approaches across sectors.
  • Proposing elements for a novel electricity security decision-analytic framework for the EU.
  • Contextualising the proposed framework in EU’s Energy Union grid design initiatives.
Full article
Figures

Figure 1

Open AccessArticle Pareto-Efficient Capacity Planning for Residential Photovoltaic Generation and Energy Storage with Demand-Side Load Management
Energies 2017, 10(4), 426; doi:10.3390/en10040426
Received: 6 February 2017 / Revised: 13 March 2017 / Accepted: 17 March 2017 / Published: 23 March 2017
Cited by 4 | PDF Full-text (1011 KB) | HTML Full-text | XML Full-text
Abstract
Optimal sizing of residential photovoltaic (PV) generation and energy storage (ES) systems is a timely issue since government polices aggressively promote installing renewable energy sources in many countries, and small-sized PV and ES systems have been recently developed for easy use in residential
[...] Read more.
Optimal sizing of residential photovoltaic (PV) generation and energy storage (ES) systems is a timely issue since government polices aggressively promote installing renewable energy sources in many countries, and small-sized PV and ES systems have been recently developed for easy use in residential areas. We in this paper investigate the problem of finding the optimal capacities of PV and ES systems in the context of home load management in smart grids. Unlike existing studies on optimal sizing of PV and ES that have been treated as a part of designing hybrid energy systems or polygeneration systems that are stand-alone or connected to the grid with a fixed energy price, our model explicitly considers the varying electricity price that is a result of individual load management of the customers in the market. The problem we have is formulated by a D-day capacity planning problem, the goal of which is to minimize the overall expense paid by each customer for the planning period. The overall expense is the sum of expenses to buy electricity and to install PV and ES during D days. Since each customer wants to minimize his/her own monetary expense, their objectives look conflicting, and we first regard the problem as a multi-objective optimization problem. Additionally, we secondly formulate the problem as a D-day noncooperative game between customers, which can be solved in a distributed manner and, thus, is better fit to the pricing practice in smart grids. In order to have a converging result of the best-response game, we use the so-called proximal point algorithm. With numerical investigation, we find Pareto-efficient trajectories of the problem, and the converged game-theoretic solution is shown to be mostly worse than the Pareto-efficient solutions. Full article
Figures

Figure 1

Open AccessArticle Modeling and Analysis of a DC Electrical System and Controllers for Implementation of a Grid-Interactive Building
Energies 2017, 10(4), 427; doi:10.3390/en10040427
Received: 8 February 2017 / Revised: 11 March 2017 / Accepted: 22 March 2017 / Published: 23 March 2017
Cited by 1 | PDF Full-text (4538 KB) | HTML Full-text | XML Full-text
Abstract
As the penetration of photovoltaic (PV) systems on building rooftops increases, the accumulated effect of the rooftop PV power outputs on electric network operation is no longer negligible. Energy storage resources (ESRs) have been used to smooth PV power outputs, particularly when building
[...] Read more.
As the penetration of photovoltaic (PV) systems on building rooftops increases, the accumulated effect of the rooftop PV power outputs on electric network operation is no longer negligible. Energy storage resources (ESRs) have been used to smooth PV power outputs, particularly when building load becomes low. In commercial buildings, the batteries of plug-in electric vehicles (PEVs) can be regarded as distributed ESRs. This paper proposes a DC electrical system in a commercial building that enables PEVs to compensate for rooftop PV power fluctuation and participate in tracking signals for grid frequency regulation (GFR). The proposed building system and associated controllers are modeled considering steady-state and dynamic operations of the PV system and PEV batteries. Simulation case studies are conducted to demonstrate the performance of the proposed building system under various conditions, determined by such factors as the maximum voltage, minimum state-of-charge, and desired charging end-time of PEVs batteries. Full article
(This article belongs to the Special Issue DC Systems)
Figures

Figure 1

Open AccessArticle Predictive Direct Flux Control—A New Control Method of Voltage Source Inverters in Distributed Generation Applications
Energies 2017, 10(4), 428; doi:10.3390/en10040428
Received: 16 February 2017 / Revised: 21 March 2017 / Accepted: 21 March 2017 / Published: 23 March 2017
PDF Full-text (2715 KB) | HTML Full-text | XML Full-text
Abstract
Voltage source inverters (VSIs) have been widely utilized in electric drives and distributed generations (DGs), where electromagnetic torque, currents and voltages are usually the control objectives. The inverter flux, defined as the integral of the inverter voltage, however, is seldom studied. Although a
[...] Read more.
Voltage source inverters (VSIs) have been widely utilized in electric drives and distributed generations (DGs), where electromagnetic torque, currents and voltages are usually the control objectives. The inverter flux, defined as the integral of the inverter voltage, however, is seldom studied. Although a conventional flux control approach has been developed, it presents major drawbacks of large flux ripples, leading to distorted inverter output currents and large power ripples. This paper proposes a new control strategy of VSIs by controlling the inverter flux. To improve the system’s steady-state and transient performance, a predictive control scheme is adopted. The flux amplitude and flux angle can be well regulated by choosing the optimum inverter control action according to formulated selection criteria. Hence, the inverter flux can be controlled to have a specified magnitude and a specified position relative to the grid flux with less ripples. This results in a satisfactory line current performance with a fast transient response. The proposed predictive direct flux control (PDFC) method is tested in a 3 MW high-power grid-connected VSI system in the MATLAB/Simulink environment, and the results demonstrate its effectiveness. Full article
Figures

Figure 1

Open AccessArticle A Solar Energy Solution for Sustainable Third Generation Mobile Networks
Energies 2017, 10(4), 429; doi:10.3390/en10040429
Received: 14 February 2017 / Revised: 20 March 2017 / Accepted: 21 March 2017 / Published: 24 March 2017
Cited by 2 | PDF Full-text (2279 KB) | HTML Full-text | XML Full-text
Abstract
The energy consumption of cellular networks has become increasingly important to cellular network operators, due to the significant economic and ecological influence of these networks in the future. The development of alternative energy technologies has resulted in the consideration of a solar powered
[...] Read more.
The energy consumption of cellular networks has become increasingly important to cellular network operators, due to the significant economic and ecological influence of these networks in the future. The development of alternative energy technologies has resulted in the consideration of a solar powered base station (BS) as a long-term solution for the mobile cellular network industry, to reduce the operational expenditures and CO2 footprints of cellular networks. This study addresses the deployment and operational issues of a solar powered universal mobile telecommunications system (UMTS; a third generation mobile cellular system) BS (i.e., Node B) that is currently deployed (i.e., UMTS Node B 2/2/2 and UMTS Node B 4/4/4). In addition, this study employs a hybrid optimization model for an electric renewable software simulator developed by the American National Renewable Energy Laboratory. Four key aspects are discussed in this study: optimal solar system architecture, energy production, the cash flow of the solar powered UMTS Node B project, and the economic feasibility of a solar powered system compared with traditional sources. Simulation results show that the proposed solution ensures 100% energy autonomy and long-term energy balance for the UMTS Node B, with cost effectiveness. Full article
Figures

Figure 1

Open AccessArticle Financial Appraisal of Small Hydro-Power Considering the Cradle-to-Grave Environmental Cost: A Case from Greece
Energies 2017, 10(4), 430; doi:10.3390/en10040430
Received: 20 November 2016 / Revised: 15 March 2017 / Accepted: 20 March 2017 / Published: 28 March 2017
PDF Full-text (2485 KB) | HTML Full-text | XML Full-text
Abstract
In the last decades increasing attention to environmental issues has come to the fore due to the looming issue of climate change. The growing demand for energy, coupled with the increasing greenhouse gas (GHG) emissions, have forced the study and development of energy
[...] Read more.
In the last decades increasing attention to environmental issues has come to the fore due to the looming issue of climate change. The growing demand for energy, coupled with the increasing greenhouse gas (GHG) emissions, have forced the study and development of energy plants that use renewable energy sources (RES), as electricity generation is one of the major contributors to anthropogenic emissions. Small hydropower plants are of particular interest as their potential is assumed to be high. The aim of this study is to provide a comprehensive assessment of the environmental impacts of small hydropower plants (SHPs) using Life Cycle Assessment (LCA) methodology. The main parameter set for our simplified LCA model was the weight of the components used to construct and operate the plant: concrete, aggregates and steel. Through LCA, air pollutant externalities were associated with the life cycle of SHPs. This was accomplished by applying the NEEDS framework. The results are given per impact type (human health, loss of biodiversity, crop yield, material damage and climate change). The spearhead of the study is that the environmental cost must be a parameter of the total investment cost, which may affect the indexes of the financial evaluation of the project. Full article
Figures

Figure 1

Open AccessArticle Comparison of Impulse Wave and Sweep Frequency Response Analysis Methods for Diagnosis of Transformer Winding Faults
Energies 2017, 10(4), 431; doi:10.3390/en10040431
Received: 27 January 2017 / Revised: 13 March 2017 / Accepted: 20 March 2017 / Published: 28 March 2017
Cited by 1 | PDF Full-text (5569 KB) | HTML Full-text | XML Full-text
Abstract
Monitoring of winding faults is the most important item used to determine the maintenance status of a transformer. Commonly used methods for winding-fault diagnosis require the transformer to exit operation before testing and an external exciting signal, whether the transformer is malfunctioning or
[...] Read more.
Monitoring of winding faults is the most important item used to determine the maintenance status of a transformer. Commonly used methods for winding-fault diagnosis require the transformer to exit operation before testing and an external exciting signal, whether the transformer is malfunctioning or not. However, if an overvoltage signal can be regarded as a broadband excitation source for fault diagnosis, then the interference caused by signal injection can be eliminated without the need for additional pulse or impulse signals. In this paper, a tapped transformer is designed and test platforms are built to compare winding diagnoses using the impulse wave and sweep frequency response analysis methods by recording voltage responses on both the high- and low-voltage sides and calculating the respective transfer functions. Based on comparison of statistical indicators, it is found that the sensitivities of both methods are similar for detecting conditions of winding-ground and winding-interlayer short circuits. It is concluded that it is feasible to use a transient overvoltage monitoring system for winding-fault diagnosis. Full article
(This article belongs to the Special Issue Electric Power Systems Research 2017)
Figures

Figure 1

Open AccessArticle Multi-Scale Parameter Identification of Lithium-Ion Battery Electric Models Using a PSO-LM Algorithm
Energies 2017, 10(4), 432; doi:10.3390/en10040432
Received: 1 February 2017 / Revised: 17 March 2017 / Accepted: 20 March 2017 / Published: 27 March 2017
PDF Full-text (2990 KB) | HTML Full-text | XML Full-text
Abstract
This paper proposes a multi-scale parameter identification algorithm for the lithium-ion battery (LIB) electric model by using a combination of particle swarm optimization (PSO) and Levenberg-Marquardt (LM) algorithms. Two-dimensional Poisson equations with unknown parameters are used to describe the potential and current density
[...] Read more.
This paper proposes a multi-scale parameter identification algorithm for the lithium-ion battery (LIB) electric model by using a combination of particle swarm optimization (PSO) and Levenberg-Marquardt (LM) algorithms. Two-dimensional Poisson equations with unknown parameters are used to describe the potential and current density distribution (PDD) of the positive and negative electrodes in the LIB electric model. The model parameters are difficult to determine in the simulation due to the nonlinear complexity of the model. In the proposed identification algorithm, PSO is used for the coarse-scale parameter identification and the LM algorithm is applied for the fine-scale parameter identification. The experiment results show that the multi-scale identification not only improves the convergence rate and effectively escapes from the stagnation of PSO, but also overcomes the local minimum entrapment drawback of the LM algorithm. The terminal voltage curves from the PDD model with the identified parameter values are in good agreement with those from the experiments at different discharge/charge rates. Full article
Figures

Figure 1

Open AccessArticle Stability Analysis of DC Distribution Systems with Droop-Based Charge Sharing on Energy Storage Devices
Energies 2017, 10(4), 433; doi:10.3390/en10040433
Received: 30 September 2016 / Revised: 11 January 2017 / Accepted: 16 March 2017 / Published: 27 March 2017
Cited by 3 | PDF Full-text (2726 KB) | HTML Full-text | XML Full-text
Abstract
Direct current (DC) distribution systems and DC microgrids are becoming a reliable and efficient alternative energy system, compatible with the DC nature of most of the distributed energy resources (DERs), storage devices and loads. The challenging problem of redesigning an autonomous DC-grid system
[...] Read more.
Direct current (DC) distribution systems and DC microgrids are becoming a reliable and efficient alternative energy system, compatible with the DC nature of most of the distributed energy resources (DERs), storage devices and loads. The challenging problem of redesigning an autonomous DC-grid system in view of using energy storage devices to balance the power produced and absorbed, by applying simple decentralized controllers on the electronic power interfaces, is investigated in this paper. To this end, a complete nonlinear DC-grid model has been deployed that includes different DC-DERs, two controlled parallel battery branches, and different varying DC loads. Since many loads in modern distribution systems are connected through power converters, both constant power loads and simple resistive loads are considered in parallel. Within this system, suitable cascaded controllers on the DC/DC power converter interfaces to the battery branches are proposed, in a manner that ensures stability and charge sharing between the two branches at the desired ratio. To achieve this task, inner-loop current controllers are combined with outer-loop voltage, droop-based controllers. The proportional-integral (PI) inner-loop current controllers include damping terms and are fully independent from the system parameters. The controller scheme is incorporated into the system model and a globally valid nonlinear stability analysis is conducted; this differs from small-signal linear methods that are valid only for specific systems, usually via eigenvalue investigations. In the present study, under the virtual cost of applying advanced Lyapunov techniques on the entire nonlinear system, a rigorous analysis is formulated to prove stability and convergence to the desired operation, regardless of the particular system characteristics. The theoretical results are evaluated by detailed simulations, with the system performance being very satisfactory. Full article
(This article belongs to the Special Issue Microgrids 2016)
Figures

Figure 1

Open AccessArticle A High-Efficiency Isolated-Type Three-Port Bidirectional DC/DC Converter for Photovoltaic Systems
Energies 2017, 10(4), 434; doi:10.3390/en10040434
Received: 3 February 2017 / Revised: 17 March 2017 / Accepted: 22 March 2017 / Published: 25 March 2017
PDF Full-text (6528 KB) | HTML Full-text | XML Full-text
Abstract
This paper proposes a novel high-efficiency isolated three-port bidirectional DC/DC device for photovoltaic (PV) systems. The device contains a high step-up converter for PV modules to supply power to the DC bus, and a bidirectional charge/discharge control circuit for the battery with an
[...] Read more.
This paper proposes a novel high-efficiency isolated three-port bidirectional DC/DC device for photovoltaic (PV) systems. The device contains a high step-up converter for PV modules to supply power to the DC bus, and a bidirectional charge/discharge control circuit for the battery with an improved boost-flyback converter. When the PV modules supply sufficient energy, their output can be stepped up and energy supply to the DC bus and charging of the battery can be achieved simultaneously. However, when the energy supplied is insufficient, the battery provides energy to the DC bus. When the proposed converter is operated in the step-down mode, the DC-blocking capacitor on the high-voltage side is used to reduce the voltage on the transformer and achieve high step-down performance. Moreover, to improve the overall efficiency of the system, the energy stored in the leakage inductance is recycled and supplied to the DC-blocking capacitor during operation in the step-up mode. Finally, to verify the feasibility and practicability of the proposed devices, a 500 W three-port bidirectional DC/DC devices was implemented. The highest efficiencies achieved for operation in different modes were as follows: high step-up mode for the PV modules, 95.2%; battery step-up mode, 94.2%; and step-down mode, 97.6%. Full article
Figures

Figure 1

Open AccessArticle GeroMAG: In-House Prototype of an Innovative Sealed, Compact and Non-Shaft-Driven Gerotor Pump with Magnetically-Driving Outer Rotor
Energies 2017, 10(4), 435; doi:10.3390/en10040435
Received: 26 February 2017 / Accepted: 22 March 2017 / Published: 26 March 2017
Cited by 2 | PDF Full-text (5733 KB) | HTML Full-text | XML Full-text
Abstract
The technology of gerotor pumps is progressing towards cutting-edge applications in emerging sectors, which are more demanding for pump performance. Moreover, recent environmental standards are heading towards leakage-free and noiseless hydraulic systems. Hence, in order to respond to these demands, this study, which
[...] Read more.
The technology of gerotor pumps is progressing towards cutting-edge applications in emerging sectors, which are more demanding for pump performance. Moreover, recent environmental standards are heading towards leakage-free and noiseless hydraulic systems. Hence, in order to respond to these demands, this study, which will be referred to as the GeroMAG concept, aims to make a leap from the standard gerotor pump technology: a sealed, compact, non-shaft-driven gerotor pump with a magnetically-driving outer rotor. The GeroMAG pump is conceived as a variable-flow pump to accomplish a standard volumetric flow rate at low rotational speed with satisfactory volumetric efficiency. By following the authors’ methodology based on a catalogue of best-practice rules, a custom trochoidal gear set is designed. Then, two main technological challenges are encountered: how to generate the rotational movement of the driving outer rotor and how to produce the guide of rotation of the gear set once there is no drive shaft. To confront them, a quiet magnet brushless motor powers the driving outer rotor through pole pieces placed in its external sideway and the rotational movement is guided by the inner edgewise pads carved on it. Subsequently, GeroMAG pump architecture, prototype, housing, methodology, materials and manufacture will be presented. As a principal conclusion, the GeroMAG proof of concept and pump prototype are feasible, which is corroborated by experimental results and performance indexes. Full article
(This article belongs to the Special Issue Energy Efficiency and Controllability of Fluid Power Systems)
Figures

Figure 1

Open AccessArticle Optimizing the Energy-Efficient Metro Train Timetable and Control Strategy in Off-Peak Hours with Uncertain Passenger Demands
Energies 2017, 10(4), 436; doi:10.3390/en10040436
Received: 6 January 2017 / Revised: 19 February 2017 / Accepted: 15 March 2017 / Published: 29 March 2017
PDF Full-text (4454 KB) | HTML Full-text | XML Full-text
Abstract
How to reduce the energy consumption of metro trains by optimizing both the timetable and control strategy is a major focus. Due to the complexity and difficulty of the combinatorial operation problem, the commonly-used method to optimize the train operation problem is based
[...] Read more.
How to reduce the energy consumption of metro trains by optimizing both the timetable and control strategy is a major focus. Due to the complexity and difficulty of the combinatorial operation problem, the commonly-used method to optimize the train operation problem is based on an unchanged dwelling time for all trains at a specific station. Here, we develop a simulation-based method to design an energy-efficient train control strategy under the optimized timetable constraints, which assign the dwelling time margin to the running time. This time margin is caused by dynamically uncertain passenger demands in off-peak hours. Firstly, we formulate a dwelling time calculation model to minimize the passenger boarding and alighting time. Secondly, we design an optimal train control strategy with fixed time and develop a time-based model to describe mass-belt train movement. Finally, based on this simulation module, we present numerical examples based on the real-world operation data from the Beijing metro Line 2, in which the energy consumption of one train can be reduced by 21.9%. These results support the usefulness of the proposed approach. Full article
Figures

Figure 1

Open AccessArticle Design Methodology of a Power Split Type Plug-In Hybrid Electric Vehicle Considering Drivetrain Losses
Energies 2017, 10(4), 437; doi:10.3390/en10040437
Received: 11 January 2017 / Revised: 13 March 2017 / Accepted: 23 March 2017 / Published: 25 March 2017
Cited by 2 | PDF Full-text (11711 KB) | HTML Full-text | XML Full-text
Abstract
This paper proposes a design methodology for a power split type plug-in hybrid electric vehicle (PHEV) by considering drivetrain losses. Selecting the input split type PHEV with a single planetary gear as the reference topology, the locations of the engine, motor and generators
[...] Read more.
This paper proposes a design methodology for a power split type plug-in hybrid electric vehicle (PHEV) by considering drivetrain losses. Selecting the input split type PHEV with a single planetary gear as the reference topology, the locations of the engine, motor and generators (MGs), on the speed lever were determined by using the mechanical point considering the system efficiency. Based on the reference topology, feasible candidates were selected by considering the operation conditions of the engine, MG1, and a redundant element. To evaluate the fuel economy of the selected candidates, the loss models of the power electronic system and drivetrain components were obtained from the mathematical governing equation and the experimental results. Based on the component loss model, a comparative analysis was performed using a dynamic programming approach under the presence or absence of the drivetrain losses. It was found that the selection of the operating mode and the operation time of each mode vary since the drivetrain loss affects the system efficiency. In addition, even if the additional modes provide the flexibility of selecting the operating mode that results in a higher system efficiency for the given driving condition, additional drivetrain elements for realizing the modes can deteriorate the fuel economy due to their various losses. Full article
Figures

Open AccessArticle Combustion Characteristics and Slagging during Co-Combustion of Rice Husk and Sewage Sludge Blends
Energies 2017, 10(4), 438; doi:10.3390/en10040438
Received: 10 February 2017 / Revised: 21 March 2017 / Accepted: 21 March 2017 / Published: 30 March 2017
PDF Full-text (4494 KB) | HTML Full-text | XML Full-text
Abstract
In this work, the thermal behavior of rice husk, sewage sludge, and their blends during combustion processes was investigated by means of thermogravimetric analysis (TGA), and the slagging characteristics were studied through X-ray fluorescence (XRF) and melting temperature. The effects of the proportion
[...] Read more.
In this work, the thermal behavior of rice husk, sewage sludge, and their blends during combustion processes was investigated by means of thermogravimetric analysis (TGA), and the slagging characteristics were studied through X-ray fluorescence (XRF) and melting temperature. The effects of the proportion of rice husk and sewage sludge blends on the combustion process, ignition and burnout characteristics were also studied. The blends had rice husk percentages of 30, 50, 70 and 100%. The results indicate that there are four main stages of the material burning processes: dehydration, volatile oxidation, and decomposition/oxidation. The reactivity of the blends improved with increasing amounts of rice husk and the results suggest synergistic interactions between rice husk and sewage sludge during the co-combustion process. All co-combustion ashes showed a lower slagging potential owing to their high amorphous SiO2 content. On the basis of combustion properties and slagging characteristics of ash, the ratio of sewage sludge in the blends should not exceed 30%. Full article
Figures

Figure 1

Open AccessArticle Battery Pack Grouping and Capacity Improvement for Electric Vehicles Based on a Genetic Algorithm
Energies 2017, 10(4), 439; doi:10.3390/en10040439
Received: 27 February 2017 / Revised: 20 March 2017 / Accepted: 22 March 2017 / Published: 31 March 2017
PDF Full-text (2708 KB) | HTML Full-text | XML Full-text
Abstract
This paper proposes an optimal grouping method for battery packs of electric vehicles (EVs). Based on modeling the vehicle powertrain, analyzing the battery degradation performance and setting up the driving cycle of an EV, a genetic algorithm (GA) is applied to optimize the
[...] Read more.
This paper proposes an optimal grouping method for battery packs of electric vehicles (EVs). Based on modeling the vehicle powertrain, analyzing the battery degradation performance and setting up the driving cycle of an EV, a genetic algorithm (GA) is applied to optimize the battery grouping topology with the objective of minimizing the total cost of ownership (TCO). The battery capacity and the serial and parallel amounts of the pack can thus be determined considering the influence of battery degradation. The results show that the optimized pack grouping can be solved by GA within around 9 min. Compared with the results of maximum discharge efficiency within a fixed lifetime, the proposed method can not only achieve a higher discharge efficiency, but also reduce the TCO by 2.29%. To enlarge the applications of the proposed method, the sensitivity to driving conditions is also analyzed to further prove the feasibility of the proposed method. Full article
Figures

Figure 1

Open AccessFeature PaperArticle Numerical Analysis of an Organic Rankine Cycle with Adjustable Working Fluid Composition, a Volumetric Expander and a Recuperator
Energies 2017, 10(4), 440; doi:10.3390/en10040440
Received: 31 January 2017 / Revised: 10 March 2017 / Accepted: 22 March 2017 / Published: 27 March 2017
Cited by 1 | PDF Full-text (3626 KB) | HTML Full-text | XML Full-text
Abstract
Conventional Organic Rankine Cycles (ORCs) using ambient air as their coolant cannot fully utilize the greater temperature differential available to them during the colder months. However, changing the working fluid composition so its boiling temperature matches the ambient temperature as it changes has
[...] Read more.
Conventional Organic Rankine Cycles (ORCs) using ambient air as their coolant cannot fully utilize the greater temperature differential available to them during the colder months. However, changing the working fluid composition so its boiling temperature matches the ambient temperature as it changes has been shown to have potential to increase year-round electricity generation. Previous research has assumed that the cycle pressure ratio is able to vary without a major loss in the isentropic efficiency of the turbine. This paper investigates if small scale ORC systems that normally use positive-displacement expanders with fixed expansion ratios could also benefit from this new concept. A numerical model was firstly established, based on which a comprehensive analysis was then conducted. The results showed that it can be applied to systems with positive-displacement expanders and improve their year-round electricity generation. However, such an improvement is less than that of the systems using turbine expanders with variable expansion ratios. Furthermore, such an improvement relies on heat recovery via the recuperator. This is because expanders with a fixed expansion ratio have a relatively constant pressure ratio between their inlet and outlet. The increase of pressure ratio between the evaporator and condenser by tuning the condensing temperature to match colder ambient condition in winter cannot be utilised by such expanders. However, with the recuperator in place, the higher discharging temperature of the expander could increase the heat recovery and consequently reduce the heat input at the evaporator, increasing the thermal efficiency and the specific power. The higher the amount of heat energy transferred in the recuperator, the higher the efficiency improvement. Full article
Figures

Figure 1

Open AccessArticle Data-Driven Predictive Torque Coordination Control during Mode Transition Process of Hybrid Electric Vehicles
Energies 2017, 10(4), 441; doi:10.3390/en10040441
Received: 16 November 2016 / Revised: 20 March 2017 / Accepted: 22 March 2017 / Published: 1 April 2017
Cited by 2 | PDF Full-text (2188 KB) | HTML Full-text | XML Full-text
Abstract
Torque coordination control significantly affects the mode transition quality during the mode transition dynamic process of hybrid electric vehicles (HEV). Most of the existing torque coordination control methods are based on the mechanism model, whose control effect heavily depends on the modeling accuracy
[...] Read more.
Torque coordination control significantly affects the mode transition quality during the mode transition dynamic process of hybrid electric vehicles (HEV). Most of the existing torque coordination control methods are based on the mechanism model, whose control effect heavily depends on the modeling accuracy of the HEV powertrain. However, the powertrain structure is so complex, that it is difficult to establish its precise mechanism model. In this paper, a torque coordination control strategy using the data-driven predictive control (DDPC) technique is proposed to overcome the shortcomings of mechanism model-based control methods for a clutch-enabled HEV. The proposed control strategy is only based on the measured input-output data in the HEV powertrain, and no mechanism model is needed. The conflicting control requirements of comfortability and economy are included in the cost function. The actual physical constraints of actuators are also explicitly taken into account in the solving process of the data-driven predictive controller. The co-simulation results in Cruise and Simulink validate the effectiveness of the proposed control strategy and demonstrate that the DDPC method can achieve less vehicle jerk, faster mode transition and smaller clutch frictional losses compared with the traditional model predictive control (MPC) method. Full article
(This article belongs to the Special Issue Advanced Energy Storage Technologies and Their Applications (AESA))
Figures

Figure 1

Open AccessArticle Research on Optimal Planning of Access Location and Access Capacity of Large-Scale Integrated Wind Power Plants
Energies 2017, 10(4), 442; doi:10.3390/en10040442
Received: 13 December 2016 / Revised: 18 March 2017 / Accepted: 21 March 2017 / Published: 1 April 2017
PDF Full-text (888 KB) | HTML Full-text | XML Full-text
Abstract
This paper proposes a multi-objective optimal planning model of access location and access capacity for large-scale integrated wind power generation considering the mutual restriction between the planning of large-scale wind power plants and the planning of power system network. In this model, the
[...] Read more.
This paper proposes a multi-objective optimal planning model of access location and access capacity for large-scale integrated wind power generation considering the mutual restriction between the planning of large-scale wind power plants and the planning of power system network. In this model, the power flow equilibrium degree, investment costs and active network loss are taken as the optimization goals. The improved differential evolution (IDE) algorithm is applied to calculate the Pareto optimal solution set of wind power’s access planning. With the solution results described by the Pareto pattern, all the alternative solutions are then ranked based on the entropy weight method and the final compromised solution is selected by the method of technique for order preference by similarity to ideal (TOPSIS). And the proposed optimal planning model is tested based on a practical planning need of large-scale integrated wind power generation in an actual power grid of China in 2020. The simulation results show that applied with the proposed optimization model and matching algorithm, the planning scheme of large-scale wind power’s access location and access capacity under complex and practical power system circumstances has been successfully optimized. Full article
Figures

Figure 1

Open AccessArticle Wind Power Potentials in Cameroon and Nigeria: Lessons from South Africa
Energies 2017, 10(4), 443; doi:10.3390/en10040443
Received: 23 February 2017 / Revised: 19 March 2017 / Accepted: 21 March 2017 / Published: 27 March 2017
PDF Full-text (1934 KB) | HTML Full-text | XML Full-text
Abstract
Wind energy has seen a tremendous growth for electricity generation worldwide and reached 456 GW by the end of June 2016. According to the World Wind Energy Association, global wind power will reach 500 GW by the end of 2016. Africa is a
[...] Read more.
Wind energy has seen a tremendous growth for electricity generation worldwide and reached 456 GW by the end of June 2016. According to the World Wind Energy Association, global wind power will reach 500 GW by the end of 2016. Africa is a continent that possesses huge under-utilized wind potentials. Some African countries, e.g., Morocco, Egypt, Tunisia and South Africa, have already adopted wind as an alternative power generation source in their energy mix. Among these countries, South Africa has invested heavily in wind energy with operational wind farms supplying up to 26,000 GWh annually to the national grid. However, two African countries, i.e., Cameroon and Nigeria, have vast potentials, but currently are lagging behind in wind energy development. For Nigeria, there is slow implementation of renewable energy policy, with no visible operational wind farms; while Cameroon does not have any policy plan for wind power. These issues are severely hindering both direct foreign and local investments into the electricity sector. Cameroon and Nigeria have huge wind energy potentials with similar climatic conditions and can benefit greatly from the huge success recorded in South Africa in terms of policy implementation, research, development and technical considerations. Therefore, this paper reviews the wind energy potentials, policies and future renewable energy road-maps in Cameroon and Nigeria and identifies their strength and weakness, as well as providing necessary actions for future improvement that South Africa has already adopted. Full article
(This article belongs to the Special Issue Sustainable Energy Technologies)
Figures

Open AccessArticle A Dual Half-Bridge Converter with Adaptive Energy Storage to Achieve ZVS over Full Range of Operation Conditions
Energies 2017, 10(4), 444; doi:10.3390/en10040444
Received: 1 December 2016 / Revised: 7 March 2017 / Accepted: 22 March 2017 / Published: 28 March 2017
Cited by 1 | PDF Full-text (3494 KB) | HTML Full-text | XML Full-text
Abstract
The phase-shifted full-bridge (PSFB) converter is widely employed in high-power applications. However, circulating current, duty-cycle loss, secondary voltage oscillation, and narrow zero-voltage-switching (ZVS) range are the main drawbacks of the conventional PSFB converter. This paper proposes a novel full-bridge converter to improve the
[...] Read more.
The phase-shifted full-bridge (PSFB) converter is widely employed in high-power applications. However, circulating current, duty-cycle loss, secondary voltage oscillation, and narrow zero-voltage-switching (ZVS) range are the main drawbacks of the conventional PSFB converter. This paper proposes a novel full-bridge converter to improve the performance of the conventional PSFB converter. The proposed converter contains two paralleled half-bridge inverters and an auxiliary inductor on the primary side. The rectifier stage is composed of six diodes connected with the form of full-bridge rectification. This structure allows the stored energy for ZVS operation to change adaptively with duty-cycle. The power can be transferred from the primary side to the secondary side during the whole period. Therefore, the requirement of output filter inductance is reduced and the circulating current is removed. The proposed converter is a good candidate for high power, high voltage and variable input voltage applications. The operation principle and performance are verified on a laboratory prototype. Full article
(This article belongs to the collection Smart Grid)
Figures

Figure 1

Open AccessArticle Performance of an Energy Efficient Low Power Stepper Converter
Energies 2017, 10(4), 445; doi:10.3390/en10040445
Received: 21 February 2017 / Revised: 17 March 2017 / Accepted: 22 March 2017 / Published: 28 March 2017
PDF Full-text (4901 KB) | HTML Full-text | XML Full-text
Abstract
This paper presents the development of an energy efficient low power stepper converter. A prototype with a hydraulic output power of ≈600 W was designed, manufactured, investigated and improved. The converter consists of a hydraulic cylinder piston unit controlled by a fast switching
[...] Read more.
This paper presents the development of an energy efficient low power stepper converter. A prototype with a hydraulic output power of ≈600 W was designed, manufactured, investigated and improved. The converter consists of a hydraulic cylinder piston unit controlled by a fast switching valve to displace a defined fluid quantum by the limited forward stroke of the piston in its cylinder. The displaced fluid generates a precise, incremental motion of a load cylinder which should be controlled. Energy saving is achieved by storing the pressure surplus intermediately in the kinetic energy of the piston to displace a part of the fluid quantum without hydraulic energy from the supply line. Energy recuperation can be done in a similar way. Simulations and experiments showed two main efficiency improvement measures of the first converter prototype. The weak points were the commercially available check valves and the used guidance system for the pistons. The second part of the paper reports about the development of a fast check valve and of a combined hydrostatic hydrodynamic bearing system based on the elastic deformation of plastics. The theoretical and experimental results show a significant improvement of the energy efficiency, the potential of this drive technology and further improvement potential. Expressed in terms of numbers an energy efficiency increase compared to a resistance control up to 30% and a maximum recuperation energy efficiency over 60% were measured. Full article
(This article belongs to the Special Issue Energy Efficiency and Controllability of Fluid Power Systems)
Figures

Figure 1

Open AccessArticle An Isolated Three-Port Bidirectional DC-DC Converter with Enlarged ZVS Region for HESS Applications in DC Microgrids
Energies 2017, 10(4), 446; doi:10.3390/en10040446
Received: 15 February 2017 / Revised: 20 March 2017 / Accepted: 24 March 2017 / Published: 1 April 2017
Cited by 1 | PDF Full-text (18700 KB) | HTML Full-text | XML Full-text
Abstract
In this paper, a two-stage three-port isolated bidirectional DC-DC converter (BDC) for hybrid energy storage system (HESS) applications in DC microgrids is proposed. It has an enlarged zero-voltage-switching (ZVS) region and reduced power circulation loss. A front-end three-phase interleaved BDC is introduced to
[...] Read more.
In this paper, a two-stage three-port isolated bidirectional DC-DC converter (BDC) for hybrid energy storage system (HESS) applications in DC microgrids is proposed. It has an enlarged zero-voltage-switching (ZVS) region and reduced power circulation loss. A front-end three-phase interleaved BDC is introduced to the supercapacitor (SC) channel to compensate voltage variations of SC. Consequently, wide ZVS range and reduced circulation power loss for SC and DC bus ports are achieved under large-scale fluctuating SC voltage. Furthermore, a novel modified pulse-width-modulation (PWM) and phase-shift (PHS) hybrid control method with two phase-shift angles is proposed for BA port. And it contributes to an increasing number of switches operating in ZVS mode with varying battery (BA) voltage. Phase shift control with fixed driving frequency is applied to manage power flow. The ZVS range as well as the current stress of resonant tanks under varying port voltages is analyzed in detail. Finally, a 1 kW prototype with peak efficiency of 94.9% is built, and the theoretical analysis and control method are verified by experiments. Full article
Figures

Figure 1

Open AccessArticle Hybridising Human Judgment, AHP, Grey Theory, and Fuzzy Expert Systems for Candidate Well Selection in Fractured Reservoirs
Energies 2017, 10(4), 447; doi:10.3390/en10040447
Received: 9 January 2017 / Revised: 23 February 2017 / Accepted: 24 March 2017 / Published: 1 April 2017
Cited by 3 | PDF Full-text (3494 KB) | HTML Full-text | XML Full-text
Abstract
The selection of appropriate wells for hydraulic fracturing is one of the most important decisions faced by oilfield engineers. It has significant implications for the future development of an oilfield in terms of its productivity and economics. In this study, we developed a
[...] Read more.
The selection of appropriate wells for hydraulic fracturing is one of the most important decisions faced by oilfield engineers. It has significant implications for the future development of an oilfield in terms of its productivity and economics. In this study, we developed a fuzzy model for well selection that combines the major objective criteria with the subjective judgments of decision makers. This was done by fusing the analytic hierarchy process (AHP) method, grey theory and an advanced version of fuzzy logic theory (FLT). The AHP component was used to identify the relevant criteria involved in selecting wells for hydraulic fracturing. Grey theory was used to determine the relative importance of those criteria. Then a fuzzy expert system was applied to fuzzily process the aggregated inputs using a Type-2 fuzzy logic system. This undertakes approximate reasoning and generates recommendations for candidate wells. These techniques and technologies were hybridized by using an intercommunication job-sharing method that integrates human judgment. The proposed method was tested on data from an oilfield in Western China and finally the most appropriate candidate wells for hydraulic fracturing were ranked in order of their projected output after fracturing. Full article
(This article belongs to the Special Issue Oil and Gas Engineering)
Figures

Figure 1

Open AccessArticle Active, Reactive and Harmonic Control for Distributed Energy Micro-Storage Systems in Smart Communities Homes
Energies 2017, 10(4), 448; doi:10.3390/en10040448
Received: 8 February 2017 / Revised: 20 March 2017 / Accepted: 24 March 2017 / Published: 1 April 2017
Cited by 2 | PDF Full-text (1615 KB) | HTML Full-text | XML Full-text
Abstract
This paper aims to provide control strategies for distributed micro-storage energy systems at the residential level to contribute to smart grid goals. A simulation model of an energy storage system (ESS) charger has been implemented to test these proposed control strategies. The smart
[...] Read more.
This paper aims to provide control strategies for distributed micro-storage energy systems at the residential level to contribute to smart grid goals. A simulation model of an energy storage system (ESS) charger has been implemented to test these proposed control strategies. The smart community energy management system (SCEMS), acting as an aggregator of resources in the community according to the expected demand and production, sends to each individual home the active and reactive power set-points. Besides, in case the ESS has available capacity, once the SCEMS requirements are satisfied, it is used to absorb the harmonic current components demanded by the household circuitry. It allows a local improvement in the power quality of the demanded current, and thus contributes to the global power quality consumption of the community. Simulation results showing the operation of a local ESS at a home in a Smart Community are presented to validate the proposed control strategies. Full article
Figures

Figure 1

Open AccessArticle Enhancement of Power System Stability Using a Novel Power System Stabilizer with Large Critical Gain
Energies 2017, 10(4), 449; doi:10.3390/en10040449
Received: 2 February 2017 / Revised: 15 March 2017 / Accepted: 23 March 2017 / Published: 1 April 2017
Cited by 1 | PDF Full-text (4663 KB) | HTML Full-text | XML Full-text
Abstract
Power system stabilizers (PSSs) are widely used for suppressing low frequency oscillations in practical power systems. However, according to the requirement of the “guide for setting test of power system stabilizer” of China, PSS gain is limited to no more than 1/3 of
[...] Read more.
Power system stabilizers (PSSs) are widely used for suppressing low frequency oscillations in practical power systems. However, according to the requirement of the “guide for setting test of power system stabilizer” of China, PSS gain is limited to no more than 1/3 of its critical gain. As a result, PSSs may not provide enough damping to the inter-area mode oscillations. Through analyzing the Heffron-Phillips (H-P) model of the generator with PSS, it is found that exciter mode will become unstable when PSS exceeds its critical gain. This exciter mode is formed by the natural characteristic of the exciter-PSS loop. To address this problem, a novel PSS with a parallel component added to the conventional PSS is proposed to improve its critical gain. Therefore, large gain can be chosen for the proposed PSS to meet the critical gain requirements of the guide and provide enough damping to the inter-area modes simultaneously. Simulation results on the Ximeng coal power station of China verify the effectiveness of the proposed PSS. Full article
(This article belongs to the Section Electrical Power and Energy System)
Figures

Figure 1

Open AccessArticle Aggregators’ Optimal Bidding Strategy in Sequential Day-Ahead and Intraday Electricity Spot Markets
Energies 2017, 10(4), 450; doi:10.3390/en10040450
Received: 2 February 2017 / Revised: 21 March 2017 / Accepted: 22 March 2017 / Published: 1 April 2017
PDF Full-text (3449 KB) | HTML Full-text | XML Full-text
Abstract
This paper proposes a probabilistic optimization method that produces optimal bidding curves to be submitted by an aggregator to the day-ahead electricity market and the intraday market, considering the flexible demand of his customers (based in time dependent resources such as batteries and
[...] Read more.
This paper proposes a probabilistic optimization method that produces optimal bidding curves to be submitted by an aggregator to the day-ahead electricity market and the intraday market, considering the flexible demand of his customers (based in time dependent resources such as batteries and shiftable demand) and taking into account the possible imbalance costs as well as the uncertainty of forecasts (market prices, demand, and renewable energy sources (RES) generation). The optimization strategy aims to minimize the total cost of the traded energy over a whole day, taking into account the intertemporal constraints. The proposed formulation leads to the solution of different linear optimization problems, following the natural temporal sequence of electricity spot markets. Intertemporal constraints regarding time dependent resources are fulfilled through a scheduling process performed after the day-ahead market clearing. Each of the different problems is of moderate dimension and requires short computation times. The benefits of the proposed strategy are assessed comparing the payments done by an aggregator over a sample period of one year following different deterministic and probabilistic strategies. Results show that probabilistic strategy reports better benefits for aggregators participating in power markets. Full article
(This article belongs to the Special Issue Distributed Energy Resources Management)
Figures

Figure 1

Open AccessArticle Power-to-Steel: Reducing CO2 through the Integration of Renewable Energy and Hydrogen into the German Steel Industry
Energies 2017, 10(4), 451; doi:10.3390/en10040451
Received: 13 February 2017 / Revised: 16 March 2017 / Accepted: 17 March 2017 / Published: 1 April 2017
Cited by 7 | PDF Full-text (2207 KB) | HTML Full-text | XML Full-text
Abstract
This paper analyses some possible means by which renewable power could be integrated into the steel manufacturing process, with techniques such as blast furnace gas recirculation (BF-GR), furnaces that utilize carbon capture, a higher share of electrical arc furnaces (EAFs) and the use
[...] Read more.
This paper analyses some possible means by which renewable power could be integrated into the steel manufacturing process, with techniques such as blast furnace gas recirculation (BF-GR), furnaces that utilize carbon capture, a higher share of electrical arc furnaces (EAFs) and the use of direct reduced iron with hydrogen as reduction agent (H-DR). It is demonstrated that these processes could lead to less dependence on—and ultimately complete independence from—coal. This opens the possibility of providing the steel industry with power and heat by coupling to renewable power generation (sector coupling). In this context, it is shown using the example of Germany that with these technologies, reductions of 47–95% of CO2 emissions against 1990 levels and 27–95% of primary energy demand against 2008 can be achieved through the integration of 12–274 TWh of renewable electrical power into the steel industry. Thereby, a substantial contribution to reducing CO2 emissions and fuel demand could be made (although it would fall short of realizing the German government’s target of a 50% reduction in power consumption by 2050). Full article
(This article belongs to the Section Electrical Power and Energy System)
Figures

Figure 1

Open AccessFeature PaperArticle A Comprehensive Study on the Avalanche Breakdown Robustness of Silicon Carbide Power MOSFETs
Energies 2017, 10(4), 452; doi:10.3390/en10040452
Received: 13 January 2017 / Revised: 20 March 2017 / Accepted: 21 March 2017 / Published: 1 April 2017
PDF Full-text (16000 KB) | HTML Full-text | XML Full-text
Abstract
This paper presents an in-depth investigation into the avalanche breakdown robustness of commercial state-of-the-art silicon carbide (SiC) power MOSFETs comprising of functional as well as structural characterization and the corresponding underlying physical mechanisms responsible for device failure. One aspect of robustness for power
[...] Read more.
This paper presents an in-depth investigation into the avalanche breakdown robustness of commercial state-of-the-art silicon carbide (SiC) power MOSFETs comprising of functional as well as structural characterization and the corresponding underlying physical mechanisms responsible for device failure. One aspect of robustness for power MOSFETs is determined by its ability to withstand energy during avalanche breakdown. Avalanche energy (EAV) is an important figure of merit for all applications requiring load dumping and/or to benefit from snubber-less converter design. 2D TCAD electro-thermal simulations were performed to get important insight into the failure mechanism of SiC power MOSFETs during avalanche breakdown. Full article
(This article belongs to the Special Issue Semiconductor Power Devices)
Figures

Figure 1

Open AccessArticle Energy Management with Support of PV Partial Shading Modelling in Micro Grid Environments
Energies 2017, 10(4), 453; doi:10.3390/en10040453
Received: 19 February 2017 / Revised: 17 March 2017 / Accepted: 24 March 2017 / Published: 1 April 2017
Cited by 1 | PDF Full-text (2274 KB) | HTML Full-text | XML Full-text
Abstract
Although photovoltaic power plants are suitable local energy sources in Micro Grid environments, when large plants are involved, partial shading and inaccurate modelling of the plant can affect both the design of the Micro Grid as well as the energy management process that
[...] Read more.
Although photovoltaic power plants are suitable local energy sources in Micro Grid environments, when large plants are involved, partial shading and inaccurate modelling of the plant can affect both the design of the Micro Grid as well as the energy management process that allows for lowering the overall Micro Grid demand towards the main grid. To investigate the issue, a Photovoltaic Plant simulation model, based on a real life power plant, and an energy management system, based on a real life Micro Grid environment, have been integrated to evaluate the performance of a Micro Grid under partial shading conditions. Using a baseline energy production model as a reference, the energy demand of the Micro Grid has been computed in sunny and partial shading conditions. The experiments reveal that an estimation based on a simplified PV model can exceed by 65% the actual production. With regards to Micro Grid design, on sunny days, the expected costs, based on a simplified PV model, can be 5.5% lower than the cost based on the double inverter model. In single cloud scenarios, the underrating can reach 28.3%. With regard to the management process, if the energy yield is estimated by means of a simplified PV model, the actual cost can be from 17.1% to 21.5% higher than the theoretical cost expected at design time. Full article
Figures

Figure 1

Open AccessArticle The Performance of Polymer Flooding in Heterogeneous Type II Reservoirs—An Experimental and Field Investigation
Energies 2017, 10(4), 454; doi:10.3390/en10040454
Received: 12 January 2017 / Revised: 22 February 2017 / Accepted: 27 March 2017 / Published: 1 April 2017
Cited by 2 | PDF Full-text (6536 KB) | HTML Full-text | XML Full-text
Abstract
The polymer flooding process has already been applied to the medium permeability type II reservoirs of the Daqing Oilfield (China) to enhance oil recovery. However, this process faces a number of challenges, such as the flooding efficiency, high injection pressure, formation blockage and
[...] Read more.
The polymer flooding process has already been applied to the medium permeability type II reservoirs of the Daqing Oilfield (China) to enhance oil recovery. However, this process faces a number of challenges, such as the flooding efficiency, high injection pressure, formation blockage and damage, unbalanced absorption ratio, and economical justification. In this study, single-phase and two-phase flow experiments are performed to investigate polymer injection adaptability with natural cores of type II reservoirs. The enhanced oil recovery (EOR) effects of the polymer are studied by physical simulation experiments, and the results of application in an actual field are also presented. The results indicate that the flow characteristics and injection capability are dominated by the reservoir permeability in polymer flooding. Moreover, the adsorption of polymer molecules and the injection pressure gradient, which reflect formation damage, are affected more significantly by the concentration than by the molecular weight in type II reservoirs. Using the matching relationship, the injection-production process is stable, and additional oil recoveries of 10%–15% can be obtained in heterogeneous type II reservoirs with a high water saturation. This work is significant in that it further accelerates the application of polymer flooding EOR in medium permeability heterogeneous oilfields with high water saturation. Full article
(This article belongs to the Special Issue Oil and Gas Engineering)
Figures

Figure 1

Open AccessArticle A New Method for Distribution Network Reconfiguration Analysis under Different Load Demands
Energies 2017, 10(4), 455; doi:10.3390/en10040455
Received: 23 December 2016 / Revised: 21 March 2017 / Accepted: 23 March 2017 / Published: 1 April 2017
Cited by 2 | PDF Full-text (1943 KB) | HTML Full-text | XML Full-text
Abstract
The strategies of distribution network reconfiguration are applicable for minimizing power loss and saving electrical energy in the distribution system. Network reconfiguration is usually represented by constant load demand so ignoring the variability of load demand causes uncertainty and misleading results in the
[...] Read more.
The strategies of distribution network reconfiguration are applicable for minimizing power loss and saving electrical energy in the distribution system. Network reconfiguration is usually represented by constant load demand so ignoring the variability of load demand causes uncertainty and misleading results in the minimization of power loss. This paper consists of two parts: first, the reconfiguration was accomplished using an optimization framework based on constant load to find sets of optimal switches. The minimization of active power loss was taken as an objective function while bus voltage, branch current and system radiality were taken as system constraints. The study was applied to a 33-bus test distribution network, which is exceedingly used as test examples for solving reconfiguration problems. Second, lists of the configurations set obtained from the first part, as well as other different optimization methods proposed earlier under constant load demand were taken as test switches. Additionally, the network in the presence of distributed generators was taken to analyze the reconfiguration under an active network. Two types of load demands; the variable load and voltage-dependent load, are proposed to represent the practical load demands. This paper presents a new method for good analysis as it defines the effect of loading levels and loading patterns on a distribution system performance for passive and active networks. The proposed approach tries to find the actual power loss under different characteristics of loads. Therefore, the probable benefit of this approach is the contribution to providing more flexibility for electrical utilities in terms of distribution system operation, while also opening new prospects in the automation of smart distribution systems. Full article
Figures

Figure 1

Open AccessArticle Methodology Applied to the Evaluation of Natural Ventilation in Residential Building Retrofits: A Case Study
Energies 2017, 10(4), 456; doi:10.3390/en10040456
Received: 22 December 2016 / Revised: 11 March 2017 / Accepted: 18 March 2017 / Published: 1 April 2017
Cited by 2 | PDF Full-text (16905 KB) | HTML Full-text | XML Full-text
Abstract
The primary objective of this paper is to present the use of a steady model that is able to qualify and quantify available natural ventilation flows applied to the energy retrofitting of urban residential districts. In terms of air quality, natural ventilation presents
[...] Read more.
The primary objective of this paper is to present the use of a steady model that is able to qualify and quantify available natural ventilation flows applied to the energy retrofitting of urban residential districts. In terms of air quality, natural ventilation presents more efficient solutions compared to active systems. This method combines numeric simulations, through the utilization of Ansys Fluent R15.0® and Engineering Equation Solver EES®, with on-site pressurization tests. Testing consists of the application of the seasonal pressure gradient on the building’s envelope and the calculation of the ventilation flows in three climatic representative conditions (summer, winter, and annual average). Through the implementation of this methodology to existing buildings it is possible to evaluate the influence of the built environment, as well as key parameters (relative height of the dwelling, number of vertical ventilation ducts, and airtightness of windows) of available natural ventilation. Full article
(This article belongs to the Special Issue Energy Conservation in Infrastructures 2016)
Figures

Figure 1

Open AccessArticle A Cubature Particle Filter Algorithm to Estimate the State of the Charge of Lithium-Ion Batteries Based on a Second-Order Equivalent Circuit Model
Energies 2017, 10(4), 457; doi:10.3390/en10040457
Received: 13 January 2017 / Revised: 21 March 2017 / Accepted: 23 March 2017 / Published: 1 April 2017
Cited by 3 | PDF Full-text (11257 KB) | HTML Full-text | XML Full-text
Abstract
The state of charge (SOC) is the residual capacity of a battery. The SOC value indicates the mileage endurance, and an accurate SOC value is required to ensure the safe use of the battery to prevent over- and over-discharging. However, unlike
[...] Read more.
The state of charge (SOC) is the residual capacity of a battery. The SOC value indicates the mileage endurance, and an accurate SOC value is required to ensure the safe use of the battery to prevent over- and over-discharging. However, unlike size and weight, battery power is not easily determined. As a consequence, we can only estimate the SOC value based on the external characteristics of the battery. In this paper, a cubature particle filter (CPF) based on the cubature Kalman filter (CKF) and the particle filter (PF) is presented for accurate and reliable SOC estimation. The CPF algorithm combines the CKF and PF algorithms to generate a suggested density function for the PF algorithm based on the CKF. The second-order resistor-capacitor (RC) equivalent circuit model was used to approximate the dynamic performance of the battery, and the model parameters were identified by fitting. A dynamic stress test (DST) was used to separately estimate the accuracy and robustness of the CKF and the CPF algorithms. The experimental results show that the CPF algorithm exhibited better accuracy and robustness than the CKF algorithm. Full article
Figures

Figure 1

Open AccessArticle Combustion and Emission Characteristics of Coconut-Based Biodiesel in a Liquid Fuel Burner
Energies 2017, 10(4), 458; doi:10.3390/en10040458
Received: 29 November 2016 / Revised: 16 February 2017 / Accepted: 17 February 2017 / Published: 1 April 2017
PDF Full-text (2821 KB) | HTML Full-text | XML Full-text
Abstract
This paper presents an investigation on the combustion performance of different Coconut Methyl Ester (CME) biodiesel blends with Conventional Diesel Fuel (CDF) under B5 (5% CME, 95% CDF), B15 (15% CME, 85% CDF), and B25 (25% CME, 75% CDF) conditions. The performances of
[...] Read more.
This paper presents an investigation on the combustion performance of different Coconut Methyl Ester (CME) biodiesel blends with Conventional Diesel Fuel (CDF) under B5 (5% CME, 95% CDF), B15 (15% CME, 85% CDF), and B25 (25% CME, 75% CDF) conditions. The performances of these fuels were evaluated based on the temperature profiles of the combustor wall and emission concentration of Oxides of Nitrogen (NOx), Sulphur Dioxide (SO2), and Carbon Monoxide (CO). The fuel properties of the CME biodiesel blends were measured and compared with CDF. All tested fuels were combusted using an open-ended combustion chamber at three different equivalence ratios, i.e., lean fuel to air mixture (Ф = 0.8), stoichiometry (Ф = 1.0), and rich fuel to air mixture (Ф =1.2), using a standard solid spray fuel nozzle. The results indicated that CME biodiesel blends combust at a lower temperature and produce less emission in comparison with CDF for all equivalence ratios. Moreover, the increase of CME content in biodiesel blends reduced the temperature of the combustor wall and the emission concentration. Results also proved that the utilization of biodiesel is beneficial to various industrial applications, especially in the transportation sector due to it being environmentally friendly, and serves as an alternative to petroleum diesel fuel. Full article
Figures

Figure 1

Open AccessArticle A Novel Remaining Useful Life Prediction Approach for Superbuck Converter Circuits Based on Modified Grey Wolf Optimizer-Support Vector Regression
Energies 2017, 10(4), 459; doi:10.3390/en10040459
Received: 9 January 2017 / Revised: 19 March 2017 / Accepted: 20 March 2017 / Published: 2 April 2017
Cited by 1 | PDF Full-text (4541 KB) | HTML Full-text | XML Full-text
Abstract
The reliability of power packs is very important for the performance of electronic equipment and ensuring the reliability of power electronic circuits is especially vital for equipment security. An alteration in the converter component parameter can lead to the decline of the power
[...] Read more.
The reliability of power packs is very important for the performance of electronic equipment and ensuring the reliability of power electronic circuits is especially vital for equipment security. An alteration in the converter component parameter can lead to the decline of the power supply quality. In order to effectively prevent failure and estimate the remaining useful life (RUL) of superbuck converters, a circuit failure prognostics framework is proposed in this paper. We employ the average value and ripple value of circuit output voltage as a feature set to calculate the Mahalanobis distance (MD) in order to reflect the health status of the circuit. Time varying MD sets form the circuit state time series. According to the working condition time series that have been obtained, we can predict the later situation with support vector regression (SVR). SVR has been improved by a modified grey wolf optimizer (MGWO) algorithm before estimating the RUL. This is the first attempt to apply the modified version of the grey wolf optimizer (GWO) to circuit prognostics and system health management (PHM). Subsequently, benchmark functions have been used to validate the performance of the MGWO. Finally, the simulation results of comparative experiments demonstrate that MGWO-SVR can predict the RUL of circuits with smaller error and higher prediction precision. Full article
(This article belongs to the Special Issue DC Systems)
Figures

Figure 1

Open AccessArticle A Method for Energy and Resource Assessment of Waves in Finite Water Depths
Energies 2017, 10(4), 460; doi:10.3390/en10040460
Received: 21 December 2016 / Revised: 24 March 2017 / Accepted: 28 March 2017 / Published: 2 April 2017
Cited by 1 | PDF Full-text (7859 KB) | HTML Full-text | XML Full-text
Abstract
This paper presents a new method for improving the assessment of energy and resources of waves in the cases of finite water depths in which the historical and some ongoing sea wave measurements are simply given in forms of scatter diagrams or the
[...] Read more.
This paper presents a new method for improving the assessment of energy and resources of waves in the cases of finite water depths in which the historical and some ongoing sea wave measurements are simply given in forms of scatter diagrams or the forms of (significant) wave heights and the relevant statistical wave periods, whilst the detailed spectrum information has been discarded, thus no longer available for the purpose of analysis. As a result of such simplified wave data, the assessment for embracing the effects of water depths on wave energy and resources becomes either difficult or inaccurate. In many practical cases, the effects of water depths are simply ignored because the formulas for deep-water waves are frequently employed. This simplification may cause large energy under-estimations for the sea waves in finite water depths. To improve the wave energy assessment for such much-simplified wave data, an approximate method is proposed for approximating the effect of water depth in this research, for which the wave energy period or the calculated peak period can be taken as the reference period for implementing the approximation. The examples for both theoretical and measured spectra show that the proposed method can significantly reduce the errors on wave energy assessment due to the approximations and inclusions of the effects of finite water depths. Full article
(This article belongs to the Special Issue Numerical Modelling of Wave and Tidal Energy)
Figures

Figure 1

Open AccessArticle A Multiple Data Fusion Approach to Wheel Slip Control for Decentralized Electric Vehicles
Energies 2017, 10(4), 461; doi:10.3390/en10040461
Received: 29 January 2017 / Revised: 24 March 2017 / Accepted: 27 March 2017 / Published: 2 April 2017
Cited by 1 | PDF Full-text (8091 KB) | HTML Full-text | XML Full-text
Abstract
Currently, active safety control methods for cars, i.e., the antilock braking system (ABS), the traction control system (TCS), and electronic stability control (ESC), govern the wheel slip control based on the wheel slip ratio, which relies on the information from non-driven wheels. However,
[...] Read more.
Currently, active safety control methods for cars, i.e., the antilock braking system (ABS), the traction control system (TCS), and electronic stability control (ESC), govern the wheel slip control based on the wheel slip ratio, which relies on the information from non-driven wheels. However, these methods are not applicable in the cases without non-driven wheels, e.g., a four-wheel decentralized electric vehicle. Therefore, this paper proposes a new wheel slip control approach based on a novel data fusion method to ensure good traction performance in any driving condition. Firstly, with the proposed data fusion algorithm, the acceleration estimator makes use of the data measured by the sensor installed near the vehicle center of mass (CM) to calculate the reference acceleration of each wheel center. Then, the wheel slip is constrained by controlling the acceleration deviation between the actual wheel and the reference wheel center. By comparison with non-control and model following control (MFC) cases in double lane change tests, the simulation results demonstrate that the proposed control method has significant anti-slip effectiveness and stabilizing control performance. Full article
(This article belongs to the Special Issue Advances in Electric Vehicles and Plug-in Hybrid Vehicles 2017)
Figures

Figure 1

Open AccessArticle Energy Demand Modeling Methodology of Key State Transitions of Turning Processes
Energies 2017, 10(4), 462; doi:10.3390/en10040462
Received: 5 January 2017 / Revised: 5 March 2017 / Accepted: 24 March 2017 / Published: 2 April 2017
PDF Full-text (5332 KB) | HTML Full-text | XML Full-text
Abstract
Energy demand modeling of machining processes is the foundation of energy optimization. Energy demand of machining state transition is integral to the energy requirements of the machining process. However, research focus on energy modeling of state transition is scarce. To fill this gap,
[...] Read more.
Energy demand modeling of machining processes is the foundation of energy optimization. Energy demand of machining state transition is integral to the energy requirements of the machining process. However, research focus on energy modeling of state transition is scarce. To fill this gap, an energy demand modeling methodology of key state transitions of the turning process is proposed. The establishment of an energy demand model of state transition could improve the accuracy of the energy model of the machining process, which also provides an accurate model and reliable data for energy optimization of the machining process. Finally, case studies were conducted on a CK6153i CNC lathe, the results demonstrating that predictive accuracy with the proposed method is generally above 90% for the state transition cases. Full article
(This article belongs to the Special Issue Energy Efficient Manufacturing)
Figures

Figure 1

Open AccessArticle Power Loss Analysis for Wind Power Grid Integration Based on Weibull Distribution
Energies 2017, 10(4), 463; doi:10.3390/en10040463
Received: 3 January 2017 / Revised: 28 March 2017 / Accepted: 29 March 2017 / Published: 2 April 2017
PDF Full-text (3115 KB) | HTML Full-text | XML Full-text
Abstract
The growth of electrical demand increases the need of renewable energy sources, such as wind energy, to meet that need. Electrical power losses are an important factor when wind farm location and size are selected. The capitalized cost of constant power losses during
[...] Read more.
The growth of electrical demand increases the need of renewable energy sources, such as wind energy, to meet that need. Electrical power losses are an important factor when wind farm location and size are selected. The capitalized cost of constant power losses during the life of a wind farm will continue to high levels. During the operation period, a method to determine if the losses meet the requirements of the design is significantly needed. This article presents a Simulink simulation of wind farm integration into the grid; the aim is to achieve a better understanding of wind variation impact on grid losses. The real power losses are set as a function of the annual variation, considering a Weibull distribution. An analytical method has been used to select the size and placement of a wind farm, taking into account active power loss reduction. It proposes a fast linear model estimation to find the optimal capacity of a wind farm based on DC power flow and graph theory. The results show that the analytical approach is capable of predicting the optimal size and location of wind turbines. Furthermore, it revealed that the annual variation of wind speed could have a strong effect on real power loss calculations. In addition to helping to improve utility efficiency, the proposed method can develop specific designs to speeding up integration of wind farms into grids. Full article
(This article belongs to the collection Wind Turbines)
Figures

Figure 1

Open AccessArticle The Role of Logistics in Practical Levelized Cost of Energy Reduction Implementation and Government Sponsored Cost Reduction Studies: Day and Night in Offshore Wind Operations and Maintenance Logistics
Energies 2017, 10(4), 464; doi:10.3390/en10040464
Received: 11 February 2017 / Revised: 24 March 2017 / Accepted: 28 March 2017 / Published: 2 April 2017
PDF Full-text (2298 KB) | HTML Full-text | XML Full-text
Abstract
This paper reveals that logistics make up at least 17% of annual operational expenditure costs for offshore wind farms. Annual operational expenditure is found to vary by a factor of 9.5, making its share of levelized cost of energy for offshore wind range
[...] Read more.
This paper reveals that logistics make up at least 17% of annual operational expenditure costs for offshore wind farms. Annual operational expenditure is found to vary by a factor of 9.5, making its share of levelized cost of energy for offshore wind range from 13% to 57%. These are key findings of a 20-month research project targeting cost reduction initiatives for offshore wind systems. The findings reveal that cost-out measures are difficult to implement due to cultural differences. Implementation efforts are rendered by personnel located offshore in a harsh sea environment which is in stark contrast to the shore-based office personnel who develop studies directing cost reduction efforts. This paper details the company motivation to join industry-wide cost reduction initiatives. A business case for offshore wind operations and maintenance logistics yielding 1% savings in levelized cost of energy is included on how to expand working hours from daytime to also work at night. Full article
(This article belongs to the Section Energy Sources)
Figures

Figure 1

Open AccessArticle A Time-Efficient Approach for Modelling and Simulation of Aggregated Multiple Photovoltaic Microinverters
Energies 2017, 10(4), 465; doi:10.3390/en10040465
Received: 14 February 2017 / Revised: 26 March 2017 / Accepted: 29 March 2017 / Published: 31 March 2017
PDF Full-text (4577 KB) | HTML Full-text | XML Full-text
Abstract
This paper presents a time-efficient modeling and simulation strategy for aggregated microinverters in large-scale photovoltaic systems. As photovoltaic microinverter systems are typically comprised of multiple power electronic converters, a suitable modeling and simulation strategy that can be used for rapid prototyping is required.
[...] Read more.
This paper presents a time-efficient modeling and simulation strategy for aggregated microinverters in large-scale photovoltaic systems. As photovoltaic microinverter systems are typically comprised of multiple power electronic converters, a suitable modeling and simulation strategy that can be used for rapid prototyping is required. Dynamic models incorporating switching action may induce significant computational burdens and long simulation durations. This paper introduces a single-matrix-form approach using the average model of a basic microinverter with two power stages consisting of a dc-dc and dc-ac converter. The proposed methodology using a common or intermediate source between two average models of cascaded converters to find the overall average model is introduced and is applicable to many other converter topologies and combinations. It provides better flexibility and simplicity when investigating various power topologies in system-level studies of microinverter and other power electronic systems. A 200 W prototype microinverter is tested to verify the proposed average and dynamic models. Furthermore, MATLAB/Simulink (2010a, Mathworks, Natick, MA, USA) is used to show the improved simulation speed and maintained accuracy of the multiple microinverter configurations when the derived average model is compared to a dynamic switching simulation model. Full article
(This article belongs to the Special Issue Power Electronics and Power Quality)
Figures

Figure 1

Open AccessArticle Fatigue Reliability Analysis of Wind Turbine Cast Components
Energies 2017, 10(4), 466; doi:10.3390/en10040466
Received: 14 December 2016 / Revised: 23 February 2017 / Accepted: 28 March 2017 / Published: 2 April 2017
PDF Full-text (1714 KB) | HTML Full-text | XML Full-text
Abstract
The fatigue life of wind turbine cast components, such as the main shaft in a drivetrain, is generally determined by defects from the casting process. These defects may reduce the fatigue life and they are generally distributed randomly in components. The foundries, cutting
[...] Read more.
The fatigue life of wind turbine cast components, such as the main shaft in a drivetrain, is generally determined by defects from the casting process. These defects may reduce the fatigue life and they are generally distributed randomly in components. The foundries, cutting facilities and test facilities can affect the verification of properties by testing. Hence, it is important to have a tool to identify which foundry, cutting and/or test facility produces components which, based on the relevant uncertainties, have the largest expected fatigue life or, alternatively, have the largest reliability to be used for decision-making if additional cost considerations are added. In this paper, a statistical approach is presented based on statistical hypothesis testing and analysis of covariance (ANCOVA) which can be applied to compare different groups (manufacturers, suppliers, test facilities, etc.) and to quantify the relevant uncertainties using available fatigue tests. Illustrative results are presented as obtained by statistical analysis of a large set of fatigue data for casted test components typically used for wind turbines. Furthermore, the SN curves (fatigue life curves based on applied stress) for fatigue assessment are estimated based on the statistical analyses and by introduction of physical, model and statistical uncertainties used for the illustration of reliability assessment. Full article
(This article belongs to the collection Wind Turbines)
Figures

Figure 1

Open AccessArticle Experimental Investigations of Physical and Chemical Properties for Microalgae HTL Bio-Crude Using a Large Batch Reactor
Energies 2017, 10(4), 467; doi:10.3390/en10040467
Received: 21 December 2016 / Revised: 10 March 2017 / Accepted: 28 March 2017 / Published: 5 April 2017
Cited by 1 | PDF Full-text (1126 KB) | HTML Full-text | XML Full-text
Abstract
As a biofuel feedstock, microalgae has good scalability and potential to supply a significant proportion of world energy compared to most types of biofuel feedstock. Hydrothermal liquefaction (HTL) is well-suited to wet biomass (such as microalgae) as it greatly reduces the energy requirements
[...] Read more.
As a biofuel feedstock, microalgae has good scalability and potential to supply a significant proportion of world energy compared to most types of biofuel feedstock. Hydrothermal liquefaction (HTL) is well-suited to wet biomass (such as microalgae) as it greatly reduces the energy requirements associated with dewatering and drying. This article presents experimental analyses of chemical and physical properties of bio-crude oil produced via HTL using a high growth-rate microalga Scenedesmus sp. in a large batch reactor. The overarching goal was to investigate the suitability of microalgae HTL bio-crude produced in a large batch reactor for direct application in marine diesel engines. To this end we characterized the chemical and physical properties of the bio-crudes produced. HTL literature mostly reports work using very small batch reactors which are preferred by researchers, so there are few experimental and parametric measurements for bio-crude physical properties, such as viscosity and density. In the course of this study, a difference between traditionally calculated values and measured values was noted. In the parametric study, the bio-crude viscosity was significantly closer to regular diesel and biodiesel standards than transesterified (FAME) microalgae biodiesel. Under optimised conditions, HTL bio-crude’s high density (0.97–1.04 kg·L−1) and its high viscosity (70.77–73.89 mm2·s−1) had enough similarity to marine heavy fuels. although the measured higher heating value, HHV, was lower (29.8 MJ·kg−1). The reaction temperature was explored in the range 280–350 °C and bio-crude oil yield and HHV reached their maxima at the highest temperature. Slurry concentration was explored between 15% and 30% at this temperature and the best HHV, O:C, and N:C were found to occur at 25%. Two solvents (dichloromethane and n-hexane) were used to recover the bio-crude oil, affecting the yield and chemical composition of the bio-crude. Full article
(This article belongs to the collection Bioenergy and Biofuel)
Figures

Figure 1

Open AccessArticle Determining the Minimal Power Capacity of Energy Storage to Accommodate Renewable Generation
Energies 2017, 10(4), 468; doi:10.3390/en10040468
Received: 15 February 2017 / Revised: 23 March 2017 / Accepted: 24 March 2017 / Published: 2 April 2017
Cited by 2 | PDF Full-text (593 KB) | HTML Full-text | XML Full-text
Abstract
The increasing penetration of renewable generation increases the need for flexibility to accommodate for growing uncertainties. The level of flexibility is measured by the available power that can be provided by flexible resources, such as dispatachable generators, in a certain time period under
[...] Read more.
The increasing penetration of renewable generation increases the need for flexibility to accommodate for growing uncertainties. The level of flexibility is measured by the available power that can be provided by flexible resources, such as dispatachable generators, in a certain time period under the constraint of transmission capacity. In addition to conventional flexible resources, energy storage is also expected as a supplementary flexible resource for variability accommodation. To aid the cost-effective planning of energy storage in power grids with intensive renewable generation, this study proposed an approach to determine the minimal requirement of power capacity and the appropriate location for the energy storage. In the proposed approach, the variation of renewable generation is limited within uncertainty sets, then a linear model is proposed for dispatchable generators and candidate energy storage to accommodate the variation in renewable generation under the power balance and transmission network constraints. The target of the proposed approach is to minimize the total power capacity of candidate energy storage facilities when the availability of existing flexible resources is maximized. After that, the robust linear optimization method is employed to convert and solve the proposed model with uncertainties. Case studies are carried out in a modified Garver 6-bus system and the Liaoning provincial power system in China. Simulation results well demonstrate the proposed optimization can provide the optimal location of energy storage with small power capacities. The minimal power capacity of allocated energy storage obtained from the proposed approach only accounts for 1/30 of the capacity of the particular transmission line that is required for network expansion. Besides being adopted for energy storage planning, the proposed approach can also be a potential tool for identifying the sufficiency of flexibility when a priority is given to renewable generation. Full article
(This article belongs to the Section Energy Storage and Application)
Figures

Figure 1

Open AccessArticle Thermal Properties of Biochars Derived from Waste Biomass Generated by Agricultural and Forestry Sectors
Energies 2017, 10(4), 469; doi:10.3390/en10040469
Received: 18 December 2016 / Revised: 20 March 2017 / Accepted: 24 March 2017 / Published: 2 April 2017
Cited by 3 | PDF Full-text (3289 KB) | HTML Full-text | XML Full-text
Abstract
Waste residues produced by agricultural and forestry industries can generate energy and are regarded as a promising source of sustainable fuels. Pyrolysis, where waste biomass is heated under low-oxygen conditions, has recently attracted attention as a means to add value to these residues.
[...] Read more.
Waste residues produced by agricultural and forestry industries can generate energy and are regarded as a promising source of sustainable fuels. Pyrolysis, where waste biomass is heated under low-oxygen conditions, has recently attracted attention as a means to add value to these residues. The material is carbonized and yields a solid product known as biochar. In this study, eight types of biomass were evaluated for their suitability as raw material to produce biochar. Material was pyrolyzed at either 350 °C or 500 °C and changes in ash content, volatile solids, fixed carbon, higher heating value (HHV) and yield were assessed. For pyrolysis at 350 °C, significant correlations (p < 0.01) between the biochars’ ash and fixed carbon content and their HHVs were observed. Masson pine wood and Chinese fir wood biochars pyrolyzed at 350 °C and the bamboo sawdust biochar pyrolyzed at 500 °C were suitable for direct use in fuel applications, as reflected by their higher HHVs, higher energy density, greater fixed carbon and lower ash contents. Rice straw was a poor substrate as the resultant biochar contained less than 60% fixed carbon and a relatively low HHV. Of the suitable residues, carbonization via pyrolysis is a promising technology to add value to pecan shells and Miscanthus. Full article
(This article belongs to the Special Issue Biomass Chars: Elaboration, Characterization and Applications)
Figures

Figure 1

Open AccessArticle Impedance Decoupling in DC Distributed Systems to Maintain Stability and Dynamic Performance
Energies 2017, 10(4), 470; doi:10.3390/en10040470
Received: 10 February 2017 / Revised: 25 March 2017 / Accepted: 28 March 2017 / Published: 2 April 2017
Cited by 1 | PDF Full-text (2445 KB) | HTML Full-text | XML Full-text
Abstract
DC distributed systems are highly reliable and efficient means of delivering DC power or adopting renewable energy resources. However, DC distributed systems are prone to instability and dynamic performance degradation due to the negative incremental input impedance of DC-DC converts. In this paper,
[...] Read more.
DC distributed systems are highly reliable and efficient means of delivering DC power or adopting renewable energy resources. However, DC distributed systems are prone to instability and dynamic performance degradation due to the negative incremental input impedance of DC-DC converts. In this paper, we propose a generic method to eliminate the impact of the negative input impedance on DC systems by shaping the source output impedance such that its bode-plot is restricted in the area that is contained below the product of the source’s duty ratio and its characteristic impedance. The performance deterioration originates whenever the output impedance of the source exceeds, in magnitude, the input impedance of the load converter due to deficiency in stability margins. Hence, confining the impedance in the proposed region helps decouple the interaction between the converters and preserve their own dynamic performances. The proposed method was proven by analytical analysis, time-based simulation, and practical experiments. All of their outcomes were in agreement, proving the effectiveness of the proposed method in preserving the dynamic performance of distributed systems. Full article
(This article belongs to the Special Issue Distribution Power Systems and Power Quality)
Figures

Figure 1

Open AccessArticle Effects of Lean Zones on Steam-Assisted Gravity Drainage Performance
Energies 2017, 10(4), 471; doi:10.3390/en10040471
Received: 8 February 2017 / Revised: 21 March 2017 / Accepted: 29 March 2017 / Published: 3 April 2017
PDF Full-text (6705 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
A thorough understanding of the effects of lean zones and the improvement of steam-assisted gravity drainage (SAGD) operations with such heterogeneities is critically important for reducing the disadvantages of lean zones. The numerical model shows: (1) SAGD is most influenced by the single-layer
[...] Read more.
A thorough understanding of the effects of lean zones and the improvement of steam-assisted gravity drainage (SAGD) operations with such heterogeneities is critically important for reducing the disadvantages of lean zones. The numerical model shows: (1) SAGD is most influenced by the single-layer lean zone with the above-injector (AI) location; with the decrease of interval distance and increase of thickness and water saturation in lean zones, the detrimental effect of single-layer lean zones on SAGD performance increases; (2) with the increase of period and decrease of connate and initial water saturations in lean zones, the detrimental effect of multiple-layer lean zones on SAGD performance increases; (3) reducing the injection pressure properly improves SAGD performance in leaky oil sands. The field-scale study indicates: (1) well pair 1 is most affected by lean zones in the studied pad due to the widest distribution of lean zones above its injector, and a hybrid cyclic steam stimulation (CSS)/SAGD method is proposed to overcome the practical problem of a low injection pressure in this area; (2) simulation results prove that the hybrid CSS/SAGD method is better than the conventional SAGD method in leaky oil sands. Full article
(This article belongs to the Section Energy Sources)
Figures

Figure 1

Open AccessArticle System Identification of a Heaving Point Absorber: Design of Experiment and Device Modeling
Energies 2017, 10(4), 472; doi:10.3390/en10040472
Received: 2 January 2017 / Revised: 3 March 2017 / Accepted: 15 March 2017 / Published: 3 April 2017
PDF Full-text (2240 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Empirically based modeling is an essential aspect of design for a wave energy converter. Empirically based models are used in structural, mechanical and control design processes, as well as for performance prediction. Both the design of experiments and methods used in system identification
[...] Read more.
Empirically based modeling is an essential aspect of design for a wave energy converter. Empirically based models are used in structural, mechanical and control design processes, as well as for performance prediction. Both the design of experiments and methods used in system identification have a strong impact on the quality of the resulting model. This study considers the system identification and model validation process based on data collected from a wave tank test of a model-scale wave energy converter. Experimental design and data processing techniques based on general system identification procedures are discussed and compared with the practices often followed for wave tank testing. The general system identification processes are shown to have a number of advantages, including an increased signal-to-noise ratio, reduced experimental time and higher frequency resolution. The experimental wave tank data is used to produce multiple models using different formulations to represent the dynamics of the wave energy converter. These models are validated and their performance is compared against one another. While most models of wave energy converters use a formulation with surface elevation as an input, this study shows that a model using a hull pressure measurement to incorporate the wave excitation phenomenon has better accuracy. Full article
(This article belongs to the Special Issue Marine Energy)
Figures

Figure 1

Open AccessArticle A Novel High Controllable Voltage Gain Push-Pull Topology for Wireless Power Transfer System
Energies 2017, 10(4), 474; doi:10.3390/en10040474
Received: 2 February 2017 / Revised: 27 March 2017 / Accepted: 29 March 2017 / Published: 1 April 2017
Cited by 1 | PDF Full-text (4026 KB) | HTML Full-text | XML Full-text
Abstract
Wireless Power Transfer (WPT) is commonly used to transmit power from a transmitting coil to various movable power devices. In the WPT system, due to a resonant tank inherent characteristic, the system cannot achieve a high output voltage gain. This paper proposes a
[...] Read more.
Wireless Power Transfer (WPT) is commonly used to transmit power from a transmitting coil to various movable power devices. In the WPT system, due to a resonant tank inherent characteristic, the system cannot achieve a high output voltage gain. This paper proposes a novel current-fed push–pull circuit to realize high output voltage gain by adding a bi-directional switch between the resonant network and inverter. To obtain a high voltage gain, this paper proposes energy storage and energy injection mode to realize an energy boost function. A duty cycle control method for mode switching is also proposed. The proposed method allows the converter to operate with a variable voltage gain over a wide range with high efficiency. Experimental validation shows that the system gain of a proposed circuit can achieve a variable gain from 2 to 7 of which the converter can be two times higher than the classical system with the same condition. Full article
(This article belongs to the Section Electrical Power and Energy System)
Figures

Open AccessArticle Risk-Based Dynamic Security Assessment for Power System Operation and Operational Planning
Energies 2017, 10(4), 475; doi:10.3390/en10040475
Received: 23 January 2017 / Revised: 9 March 2017 / Accepted: 28 March 2017 / Published: 1 April 2017
Cited by 1 | PDF Full-text (3079 KB) | HTML Full-text | XML Full-text
Abstract
Assessment of dynamic stability in a modern power system (PS) is becoming a stringent requirement both in operational planning and in on-line operation, due to the increasingly complex dynamics of a PS. Further, growing uncertainties in forecast state and in the response to
[...] Read more.
Assessment of dynamic stability in a modern power system (PS) is becoming a stringent requirement both in operational planning and in on-line operation, due to the increasingly complex dynamics of a PS. Further, growing uncertainties in forecast state and in the response to disturbances suggests the adoption of risk-based approaches in Dynamic Security Assessment (DSA). The present paper describes a probabilistic risk-based DSA, which provides instability risk indicators by combining an innovative probabilistic hazard/vulnerability analysis with the assessment of contingency impacts via time domain simulation. The tool implementing the method can be applied to both current and forecast PS states, the latter characterized in terms of renewable and load forecast uncertainties, providing valuable results for operation and operational planning contexts. Some results from a real PS model are discussed. Full article
(This article belongs to the Special Issue Advances in Power System Operations and Planning)
Figures

Figure 1

Open AccessArticle A Novel Caving Model of Overburden Strata Movement Induced by Coal Mining
Energies 2017, 10(4), 476; doi:10.3390/en10040476
Received: 20 December 2016 / Revised: 28 March 2017 / Accepted: 29 March 2017 / Published: 1 April 2017
Cited by 1 | PDF Full-text (1443 KB) | HTML Full-text | XML Full-text
Abstract
The broken pattern of the overburden strata induced by mining has a non-ignorable effect on overlying strata movement, failure, and safety in mining production. To study the caving pattern of overlying strata and determine the calculation method of fracture pathway parameters due to
[...] Read more.
The broken pattern of the overburden strata induced by mining has a non-ignorable effect on overlying strata movement, failure, and safety in mining production. To study the caving pattern of overlying strata and determine the calculation method of fracture pathway parameters due to roof caving induced by coal mining, the trapezoidal broken models were developed to explain and prevent water leakage, and even water inrush, during the mining process. By incorporating the variation of the volume expansion coefficient, a connection among the parameters of the fracture pathways and fracture angles, face width, and mining height could be established, which shows that the larger the degree of the fracture angle is, the smaller the value of the volume expansion coefficient and face width is with a relatively larger mining height. This relationship was also used to determine the eventual evolution configuration of the trapezoidal broken model. The presented approaches may help us to better understand the movement of overburden strata and provide an idea to help settle conflicts related to fracture space calculations induced by coal mining. Full article
Figures

Figure 1

Open AccessArticle Measurement of Line-to-Ground Capacitance in Distribution Network Considering Magnetizing Impedance’s Frequency Characteristic
Energies 2017, 10(4), 477; doi:10.3390/en10040477
Received: 1 February 2017 / Revised: 21 March 2017 / Accepted: 27 March 2017 / Published: 3 April 2017
Cited by 1 | PDF Full-text (2443 KB) | HTML Full-text | XML Full-text
Abstract
Signal injection method (SIM) is widely applied to the insulation parameters’ measurement in distribution network for its convenience and safety. It can be divided into two kinds of patterns: injecting a specific frequency signal or several frequencies’ groups, and scanning frequency in a
[...] Read more.
Signal injection method (SIM) is widely applied to the insulation parameters’ measurement in distribution network for its convenience and safety. It can be divided into two kinds of patterns: injecting a specific frequency signal or several frequencies’ groups, and scanning frequency in a scheduled frequency scope. In order to avoid the disadvantages in related researches, improved signal injection method (ISIM), in which the frequency characteristic of the transformer magnetizing impedance is taken into consideration, is proposed. In addition, optimization for signal injection position has been accomplished, and the corresponding three calculation methods of line-to-ground capacitance has been derived. Calculations are carried out through the vector information (vector calculation method), the amplitude information (amplitude calculation method), the phase information (phase calculation method) of voltage and current in signal injecting port, respectively. The line-to-ground capacitance is represented by lumped parameter capacitances in high-voltage simulation test. Eight different sinusoidal signals are injected into zero-sequence circuit, and then line-to-ground capacitance is calculated with the above-mentioned vector calculation method based on the voltage and the current data of the injecting port. The results obtained by the vector calculation method show that ISIM has a wider application frequency range compared with signal injection method with rated parameters (RSIM) and SIM. The RSIM is calculated with the rated transformer parameters of magnetizing impedance, and the SIM based on the ideal transformer model, and the relative errors of calculation results of ISIM are smaller than that for other methods in general. The six groups of two-frequency set are chosen in a specific scope which is recommended by vector calculation results. Based on ISIM, the line-to-ground capacitance calculations through the amplitude calculation method and phase calculation method are compared, and then its application frequency range, which can work as a guidance for line-to-ground capacitance measurement, is concluded. Full article
(This article belongs to the Special Issue Electric Power Systems Research 2017)
Figures

Figure 1

Open AccessArticle Numerical Simulations of a VAWT in the Wake of a Moving Car
Energies 2017, 10(4), 478; doi:10.3390/en10040478
Received: 23 January 2017 / Revised: 7 March 2017 / Accepted: 28 March 2017 / Published: 3 April 2017
PDF Full-text (5970 KB) | HTML Full-text | XML Full-text
Abstract
Wind energy generated from the wake of moving cars has a large energy potential that has not yet been utilized. In this study, a vertical axis wind turbine (VAWT) was used to recover energy from the wakes of moving cars. The turbine was
[...] Read more.
Wind energy generated from the wake of moving cars has a large energy potential that has not yet been utilized. In this study, a vertical axis wind turbine (VAWT) was used to recover energy from the wakes of moving cars. The turbine was designed to be planted by the side of the car lane and driven by the wake produced by the car. Transient computational fluid dynamics (CFD) simulations were performed to evaluate the performance of the VAWT. The influence of two main factors on the performance of the VAWT, the velocity of the car and the gap between the car and the rotor, were studied. The simulations confirmed the feasibility of this plan, and in the tested cases, the VAWT was able to generate a maximum energy output of 100.49 J from the wake of a car. The results also showed that the performance of the VAWT decreased with the velocity of the car, and the increased gap between the car and the VAWT. Full article
(This article belongs to the Section Energy Fundamentals and Conversion)
Figures

Figure 1

Open AccessArticle Analysis of the Average Annual Consumption of Water in the Hospitals of Extremadura (Spain)
Energies 2017, 10(4), 479; doi:10.3390/en10040479
Received: 9 March 2017 / Revised: 27 March 2017 / Accepted: 29 March 2017 / Published: 3 April 2017
PDF Full-text (2678 KB) | HTML Full-text | XML Full-text
Abstract
The aim of this paper is to quantity the annual average consumption of water in 13 public hospitals in Extremadura (Spain). An analytical study in order to reduce water demand was conducted from 2010 to 2014 in the above-mentioned hospitals. The study concluded
[...] Read more.
The aim of this paper is to quantity the annual average consumption of water in 13 public hospitals in Extremadura (Spain). An analytical study in order to reduce water demand was conducted from 2010 to 2014 in the above-mentioned hospitals. The study concluded that, in order to determine the average annual water consumption, a fixed ratio is not the appropriate tool. A parametric type related to the built surface area and/or number of beds by hospital should be used instead. The average annual consumption of cold water for human consumption (CWHC) was 262.82 m3 (102.10) per bed and 1.65 m3 (0.46) per built surface area. The mean annual consumption of domestic hot water (DHW) was 92.96 m3 (35.72) per bed and 0.59 m3 (0.18) per built surface area. The ratio between DHW and CWHC was 35.62% (5.53). The time period of greatest demand was between 12 p.m. and 6 p.m. Considering B as the number of hospital beds and S its built surface area, to calculate the average annual cold water for human consumption in a hospital, the equation 165B + 12,100 (m3) or 1.568S + 2400 (m3) should be used. Similarly, in terms of hot domestic water, the average annual consumption in m3 corresponds to either 53.65B + 5170 (m3) or 0.53S + 1400 (m3). Full article
(This article belongs to the Special Issue Energy and Water, Current and Future Crisis)
Figures

Figure 1

Open AccessArticle Automatic J–A Model Parameter Tuning Algorithm for High Accuracy Inrush Current Simulation
Energies 2017, 10(4), 480; doi:10.3390/en10040480
Received: 5 February 2017 / Revised: 18 March 2017 / Accepted: 23 March 2017 / Published: 4 April 2017
PDF Full-text (807 KB) | HTML Full-text | XML Full-text
Abstract
Inrush current simulation plays an important role in many tasks of the power system, such as power transformer protection. However, the accuracy of the inrush current simulation can hardly be ensured. In this paper, a Jiles–Atherton (J–A) theory based model is proposed to
[...] Read more.
Inrush current simulation plays an important role in many tasks of the power system, such as power transformer protection. However, the accuracy of the inrush current simulation can hardly be ensured. In this paper, a Jiles–Atherton (J–A) theory based model is proposed to simulate the inrush current of power transformers. The characteristics of the inrush current curve are analyzed and results show that the entire inrush current curve can be well featured by the crest value of the first two cycles. With comprehensive consideration of both of the features of the inrush current curve and the J–A parameters, an automatic J–A parameter estimation algorithm is proposed. The proposed algorithm can obtain more reasonable J–A parameters, which improve the accuracy of simulation. Experimental results have verified the efficiency of the proposed algorithm. Full article
(This article belongs to the Section Electrical Power and Energy System)
Figures

Figure 1

Open AccessArticle Co-Production Performance Evaluation of a Novel Solar Combi System for Simultaneous Pure Water and Hot Water Supply in Urban Households of UAE
Energies 2017, 10(4), 481; doi:10.3390/en10040481
Received: 26 October 2016 / Revised: 26 March 2017 / Accepted: 28 March 2017 / Published: 4 April 2017
Cited by 1 | PDF Full-text (9016 KB) | HTML Full-text | XML Full-text
Abstract
Water is the most desirable and sparse resource in Gulf cooperation council (GCC) region. Utilization of point-of-use (POU) water treatment devices has been gaining huge market recently due to increase in knowledge of urban population on health related issues over contaminants in decentralized
[...] Read more.
Water is the most desirable and sparse resource in Gulf cooperation council (GCC) region. Utilization of point-of-use (POU) water treatment devices has been gaining huge market recently due to increase in knowledge of urban population on health related issues over contaminants in decentralized water distribution networks. However, there is no foolproof way of knowing whether the treated water is free of contaminants harmful for drinking and hence reliance on certified bottled water has increased worldwide. The bottling process right from treatment to delivery is highly unsustainable due to huge energy demand along the supply chain. As a step towards sustainability, we investigated various ways of coupling of membrane distillation (MD) process with solar domestic heaters for co-production of domestic heat and pure water. Performance dynamics of various integration techniques have been evaluated and appropriate configuration has been identified for real scale application. A solar combi MD (SCMD) system is experimentally tested for single household application for production 20 L/day of pure water and 250 L/day of hot water simultaneously without any auxiliary heating device. The efficiency of co-production system is compared with individual operation of solar heaters and solar membrane distillation. Full article
Figures

Figure 1

Open AccessArticle A Highly Relevant Method for Incorporation of Shunt Connected FACTS Device into Multi-Machine Power System to Dampen Electromechanical Oscillations
Energies 2017, 10(4), 482; doi:10.3390/en10040482
Received: 18 January 2017 / Revised: 26 March 2017 / Accepted: 29 March 2017 / Published: 4 April 2017
Cited by 1 | PDF Full-text (3493 KB) | HTML Full-text | XML Full-text
Abstract
A number of techniques have been proposed to dampen the power system oscillations in the electric power systems. Flexible alternating current transmission system (FACTS) devices are becoming one of them. Among the FACTS family, the static synchronous compensator (STATCOM), a shunt connected FACTS
[...] Read more.
A number of techniques have been proposed to dampen the power system oscillations in the electric power systems. Flexible alternating current transmission system (FACTS) devices are becoming one of them. Among the FACTS family, the static synchronous compensator (STATCOM), a shunt connected FACTS device, has been widely used to provide smooth and rapid steady state, limit transient voltage, and improve the power system stability and performance by absorbing or injecting reactive power. However, the influence ability depends on its placement, control signal, and place of receiving-signal in the network. In order to satisfy these issues, this paper proposes a method for optimal setting and signal position of the STATCOM into the multi-machine power systems with the aim for damping the electromechanical oscillations. This method is developed from the energy approach based on Gramian matrices considering multiple tasks on the Lyapunov equation, in which the observability Gramian matrix is used to seek an optimal location for STATCOM placement. The another is the controllability one used to determine the best local input signal placement that is chosen as a feedback signal for the power oscillation damping (POD) of STATCOM. In addition, the Krylov-based model reduction method is introduced to shorten the calculation time. The proposed method has been verified on the IEEE 24-bus system by analyzing the small-signal stability to search several feasible placements, and then the transient stability is analyzed to compare and determine an optimal placement through testing various cases. The obtained result is also compared with other optimal method. Full article
(This article belongs to the Special Issue Electric Power Systems Research 2017)
Figures

Figure 1