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Energies, Volume 11, Issue 8 (August 2018)

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Cover Story (view full-size image) The assessment of the outdoor air needs to define a domain which accurately represents the natural [...] Read more.
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Open AccessReview A Review of the Design and Control of Free-Piston Linear Generator
Energies 2018, 11(8), 2179; https://doi.org/10.3390/en11082179
Received: 16 July 2018 / Revised: 12 August 2018 / Accepted: 14 August 2018 / Published: 20 August 2018
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Abstract
The Free-piston linear generator (FPLG) is a novel energy converter which can generate electrical energy and is regarded as a potential technology for solving the restriction of the short driving range of electric vehicles. Getting rid of the crank and flywheel mechanism, FPLG
[...] Read more.
The Free-piston linear generator (FPLG) is a novel energy converter which can generate electrical energy and is regarded as a potential technology for solving the restriction of the short driving range of electric vehicles. Getting rid of the crank and flywheel mechanism, FPLG obtains some advantages of a variable compression ratio, compact size, and highly-efficient power generation. Linear electric machine (LEM) design and piston motion control are two key technologies of FPLG. However, they are currently the main obstacles to the favorable performance of FPLG. LEM being used to drive the piston motion or generate electric energy is an integrated design including a motor/generator. Various types of LEMs are investigated, and suitable application scenarios based on advantages and disadvantages are discussed. The FPLG’s controller is used to ensure stable operation and highly-efficient output. However, cycle-to-cycle variations of the combustion process and motor/generator switching make it difficult to improve the performance of the piston motion control. Comments on the advantages and disadvantages of different piston motion control methods are also given in this paper. Full article
(This article belongs to the Special Issue Power Electronics for Energy Storage)
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Open AccessArticle Investment Determinants in Self-Consumption Facilities: Characterization and Qualitative Analysis in Spain
Energies 2018, 11(8), 2178; https://doi.org/10.3390/en11082178
Received: 11 July 2018 / Revised: 6 August 2018 / Accepted: 9 August 2018 / Published: 20 August 2018
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Abstract
Self-consumption energy facilities are presented as viable and sustainable solutions in the energy transition scenario in which many countries are immersed. However, they rely on dispersed and private investments in the territory. Given the uneven growth in the number of self-consumption facilities in
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Self-consumption energy facilities are presented as viable and sustainable solutions in the energy transition scenario in which many countries are immersed. However, they rely on dispersed and private investments in the territory. Given the uneven growth in the number of self-consumption facilities in Europe, the main objective of this study is to identify and measure the investment determinants in self-consumption facilities. To this end, the main influential incentives and barriers are identified through the aggregate analysis of the regulatory framework for self-consumption in several European countries, and the empirical characterization of Spanish facilities as a multiple case study, to define the common features of the investments made. The technical, economic, and financial characterization of real self-consumption facilities in climatic zones of southern Europe is a significant contribution of the present work. There are few samples of this type in the studies published to date, which have mainly been prepared from case studies or statistical data without identifying particular facilities. Cost-related variables have been identified as the most important variables in private investment decisions, and potential influential factors on these variables that could be regulated have been pointed out as relevant. It is also worth highlighting the elaboration of an analytical framework based on this conceptual approach, which has been proven to be useful to depict regulatory scenarios and to compare the positioning for the development of self-consumption systems in different countries. A model that transfers the influence of the determining factors to the deployment of self-consumption under specific regulatory scenarios has been developed and applied to the case of Spain. As a general reflection, to increase the adoption of this kind of technology and encourage consumers to make private investments, policies for renewable energy must consider self-consumption and microgeneration as the main axis, by increasing the availability of energy when necessary. For instance, the promotion of energy storage from these kinds of facilities could receive priority treatment, as well as rewarding the electricity surplus in the interests of security of supply in a period of energy transition towards a new, more sustainable model. Incentive schemes, aids to compensate for the additional costs resulting from the battery storage or easing restrictions in terms of contracted power would foreseeably increase the rates of adoption of the technology, favoring its faster development in terms of research and development and product innovation. Full article
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Open AccessArticle Research on a Plug-In Hybrid Electric Bus Energy Management Strategy Considering Drivability
Energies 2018, 11(8), 2177; https://doi.org/10.3390/en11082177
Received: 25 July 2018 / Revised: 10 August 2018 / Accepted: 17 August 2018 / Published: 20 August 2018
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Abstract
Plug-in hybrid electric buses (PHEBs) is some of the most promising products to address air pollution and the energy crisis. Considering the switching between different working modes often bring about sudden changes of the torque and the speed of different power sources, which
[...] Read more.
Plug-in hybrid electric buses (PHEBs) is some of the most promising products to address air pollution and the energy crisis. Considering the switching between different working modes often bring about sudden changes of the torque and the speed of different power sources, which may lead to the instability of the power output and affect the driving performance and ride comfort, it is of great significance to develop a real-time optimal energy management strategy for PHEBs to achieve the optimization of fuel economy and drivability. In this study, the proposed strategy includes an offline part and an online part. In the offline part, firstly, the energy conversion coefficient s(t) is optimized by linear weight particle swarm optimization algorithm (LinWPSO), then, the optimization results of s(t) are converted into a 2-dimensional look-up table. Secondly, combined with three typical driving cycle conditions, the gear-shifting correction and mode switching boundary parameters that affect the drivability of the vehicle are extracted by dynamic programming (DP) algorithm. In the online part, combined with the s(t), the gear-shifting correction and mode switching boundary parameters which are obtained through offline optimization, the real-time energy management strategy is proposed to solve the trade-off problem between minimizing the fuel consumption and improving the drivability and riding comfort. Finally, the proposed strategy is verified with simulation, the results show that the proposed strategy can guarantee the engine and the electric motor (EM) work in the high-efficiency area with optimal energy distribution while keeping drivability in the variation of driving circle. The overall performance is improved by 18.54% compared with the rule-based control strategy. The proposed strategy may provide theoretical support for the optimal control of PHEB. Full article
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Open AccessArticle Online Speed Estimation Using Artificial Neural Network for Speed Sensorless Direct Torque Control of Induction Motor based on Constant V/F Control Technique
Energies 2018, 11(8), 2176; https://doi.org/10.3390/en11082176
Received: 9 August 2018 / Revised: 15 August 2018 / Accepted: 19 August 2018 / Published: 20 August 2018
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Abstract
This paper presents the speed estimator for speed sensorless direct torque control of a three-phase induction motor based on constant voltage per frequency (V/F) control technique, using artificial neural network (ANN). The estimated stator current equation is derived and rearranged consistent with the
[...] Read more.
This paper presents the speed estimator for speed sensorless direct torque control of a three-phase induction motor based on constant voltage per frequency (V/F) control technique, using artificial neural network (ANN). The estimated stator current equation is derived and rearranged consistent with the control algorithm and ANN structure. For the speed estimation, a weight in ANN, which relates to the speed, is adjusted by using Widrow–Hoff learning rule to minimize the sum of squared errors between the measured stator current and the estimated stator current from ANN output. The consequence of using this method leads to the ability of online speed estimation and simple ANN structure. The simulation and experimental results in high- and low-speed regions have confirmed the validity of the proposed speed estimation method. Full article
(This article belongs to the Special Issue Selected Papers from IEEE ICKII 2018...)
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Open AccessArticle The Effects of Dynamic Pricing of Electric Power on Consumer Behavior: A Propensity Score Analysis for Empirical Study on Nushima Island, Japan
Energies 2018, 11(8), 2175; https://doi.org/10.3390/en11082175
Received: 20 July 2018 / Revised: 17 August 2018 / Accepted: 18 August 2018 / Published: 20 August 2018
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Abstract
This study aimed to investigate the change of consumer behavior in electric power consumption after the application of dynamic pricing via real-time feedback. Afield experiment of dynamic pricing was carried out on Nushima Island, which is located in Hyogo Prefecture in central Japan.
[...] Read more.
This study aimed to investigate the change of consumer behavior in electric power consumption after the application of dynamic pricing via real-time feedback. Afield experiment of dynamic pricing was carried out on Nushima Island, which is located in Hyogo Prefecture in central Japan. The panel data of hourly electric power consumption among 50 households (including 22 control households and 28 treated households) were collected from a baseline survey (14 days before the dynamic pricing experiment was conducted) and during the 14-day experimental period. Propensity score analysis with local linear matching was employed to analyze the average treatment effects of dynamic pricing on consumer behavior. The results report that dynamic pricing plays a crucial role in reducing consumers’ electric power consumption—by 9.6% compared to the pre-experimental period. Full article
(This article belongs to the Special Issue Demand Response in Electricity Markets)
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Open AccessArticle An Overview of Energy Scenarios, Storage Systems and the Infrastructure for Vehicle-to-Grid Technology
Energies 2018, 11(8), 2174; https://doi.org/10.3390/en11082174
Received: 21 June 2018 / Revised: 7 August 2018 / Accepted: 13 August 2018 / Published: 20 August 2018
Cited by 2 | PDF Full-text (1173 KB) | HTML Full-text | XML Full-text
Abstract
The increase in the emission of greenhouse gases (GHG) is one of the most important problems in the world. Decreasing GHG emissions will be a big challenge in the future. The transportation sector uses a significant part of petroleum production in the world,
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The increase in the emission of greenhouse gases (GHG) is one of the most important problems in the world. Decreasing GHG emissions will be a big challenge in the future. The transportation sector uses a significant part of petroleum production in the world, and this leads to an increase in the emission of GHG. The result of this issue is that the population of the world befouls the environment by the transportation system automatically. Electric Vehicles (EV) have the potential to solve a big part of GHG emission and energy efficiency issues such as the stability and reliability of energy. Therefore, the EV and grid relation is limited to the Vehicle-to-Grid (V2G) or Grid-to-Vehicle (G2V) function. Consequently, the grid has temporary energy storage in EVs’ batteries and electricity in exchange for fossil energy in vehicles. The energy actors and their research teams have determined some targets for 2050; hence, they hope to decrease the world temperature by 6 °C, or at least by 2 °C in the normal condition. Fulfilment of these scenarios requires suitable grid infrastructure, but in most countries, the grid does not have a suitable background to apply in those scenarios. In this paper, some problems regarding energy scenarios, energy storage systems, grid infrastructure and communication systems in the supply and demand side of the grid are reviewed. Full article
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Open AccessArticle Combustion Characteristics and NOx Emission through a Swirling Burner with Adjustable Flaring Angle
Energies 2018, 11(8), 2173; https://doi.org/10.3390/en11082173
Received: 1 July 2018 / Revised: 13 August 2018 / Accepted: 18 August 2018 / Published: 20 August 2018
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Abstract
A swirling burner with a variable inner secondary air (ISA) flaring angle β is proposed and a laboratory scale opposed-firing furnace is built. Temperature distribution and NOx emission are designedly measured. The combustion characteristics affected by variable β are experimentally evaluated from
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A swirling burner with a variable inner secondary air (ISA) flaring angle β is proposed and a laboratory scale opposed-firing furnace is built. Temperature distribution and NOx emission are designedly measured. The combustion characteristics affected by variable β are experimentally evaluated from ignition and burnout data. Meanwhile, NOx reduction by the variable β is analyzed through emissions measurements. Different inner/outer primary coal-air concentration ratios γ, thermal loads and coal types are considered in this study. Results indicate that β variation provides a new approach to promote ignition and burnout, as well as NOx emission reduction under conditions of fuel rich/lean combustion and load variation. The recommended β of a swirling burner under different conditions is not always constant. The optimal βopt of the swirling burner under all conditions for different burning performance are summarized in the form of curves, which could provide reference for exquisite combustion adjustment. Full article
(This article belongs to the Special Issue Cleaner Combustion)
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Open AccessArticle Comparison of the Stator Step Skewed Structures for Cogging Force Reduction of Linear Flux Switching Permanent Magnet Machines
Energies 2018, 11(8), 2172; https://doi.org/10.3390/en11082172
Received: 27 July 2018 / Revised: 15 August 2018 / Accepted: 18 August 2018 / Published: 20 August 2018
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Abstract
Linear flux switching permanent magnetic (LFSPM) machines, with the armature windings and magnets both on the mover in addition to a robust stator, are a good choice for long stoke applications, however, a large cogging force is also inevitable due to the double
[...] Read more.
Linear flux switching permanent magnetic (LFSPM) machines, with the armature windings and magnets both on the mover in addition to a robust stator, are a good choice for long stoke applications, however, a large cogging force is also inevitable due to the double salient structure, and will worsen the system performance. Skewing methods are always employed for the rotary machines to reduce the cogging torque, and the rotor step-skewed method is a low-cost approximation of regular skewing. The step skewed method can also be applied to the linear machines, namely, the stator step skewed. In this paper, three stator step skewed structures, which are a three-step skewed stator, a two-step skewed stator and an improved two-step skewed stator, are employed for the cogging force reduction of two types of LFSPM machines. The three structures are analyzed and compared with emphasize on the influence of the skewed displacement on the cogging force and the average thrust force. Based on finite element analysis (FEA), proper skewed displacements are selected according to maximum difference between the reduction ratio of the cogging force and the decrease ratio of the average thrust force, then, the corresponding results are compared, and finally, valuable conclusions are drawn according to the comparison. The comparison presented in this paper will be useful to the cogging force reduction of LFSPM machines in general. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessArticle Low Cost Position Controller for Exhaust Gas Recirculation Valve System
Energies 2018, 11(8), 2171; https://doi.org/10.3390/en11082171
Received: 30 July 2018 / Revised: 17 August 2018 / Accepted: 17 August 2018 / Published: 20 August 2018
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Abstract
This paper proposes a position control method for a low-cost exhaust gas recirculation (EGR) valve system for automotive applications. Generally, position control systems used in automotive applications have many restrictions, such as cost and space. The mechanical structure of the actuator causes high
[...] Read more.
This paper proposes a position control method for a low-cost exhaust gas recirculation (EGR) valve system for automotive applications. Generally, position control systems used in automotive applications have many restrictions, such as cost and space. The mechanical structure of the actuator causes high friction and large differences between static friction and coulomb friction. When this large friction difference occurs, the position control vibrates when the controller uses a conventional linear controller such as the P or PI controller. In this paper, we introduce an inexpensive position control method that can be applied under the high-difference-friction mechanical systems. The proposed method is verified through the use of experiments by comparing it with the results obtained when using a conventional control system. Full article
(This article belongs to the Special Issue Intelligent Control in Energy Systems)
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Open AccessArticle Magnetotelluric Imaging of the Zhangzhou Basin Geothermal Zone, Southeastern China
Energies 2018, 11(8), 2170; https://doi.org/10.3390/en11082170
Received: 25 May 2018 / Revised: 30 July 2018 / Accepted: 13 August 2018 / Published: 20 August 2018
Cited by 1 | PDF Full-text (4112 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
The geothermal zone of southeast China, which is one of the country’s known geothermal zones, contains significant natural geothermal resources. To understand the formation of geothermal resources, a magnetotelluric (MT) investigation with a site spacing of 1–2 km was carried out around the
[...] Read more.
The geothermal zone of southeast China, which is one of the country’s known geothermal zones, contains significant natural geothermal resources. To understand the formation of geothermal resources, a magnetotelluric (MT) investigation with a site spacing of 1–2 km was carried out around the Zhangzhou Basin. The recorded MT data were processed by robust time series and remote reference processing techniques. The data analysis results revealed that two-dimensional (2-D) modeling can be used to approximately determine the electrical structure. The joint inversions of TE and TM modes have been performed after distortion decomposition. In the inversion models, a low resistivity cap of 200–800 m thickness was observed, which represented the blanketing sediments composed of Quaternary and volcanic rocks of the late Jurassic period. The presence of high resistivity above a depth of 20 km indicates the granites are widely developed in the upper and middle crust. MT measurements have revealed some deep-seated high conductive zones, which were inferred to be partially melting at depth of 8–17 km, which is likely to be reason behind the formation of higher-temperature hot springs. The results also show that there is a shallower Moho, which indicates that the heat from the upper mantle may have a big contribution to the surface heat flow. Fractures-controlled meteoric fluid circulation is the most likely explanation for the hot springs. Full article
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Open AccessArticle Maintenance Factor Identification in Outdoor Lighting Installations Using Simulation and Optimization Techniques
Energies 2018, 11(8), 2169; https://doi.org/10.3390/en11082169
Received: 20 July 2018 / Revised: 8 August 2018 / Accepted: 14 August 2018 / Published: 20 August 2018
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Abstract
This document addresses the development of a novel methodology to identify the actual maintenance factor of the luminaires of an outdoor lighting installation in order to assess their lighting performance. The method is based on the combined use of Radiance, a free and
[...] Read more.
This document addresses the development of a novel methodology to identify the actual maintenance factor of the luminaires of an outdoor lighting installation in order to assess their lighting performance. The method is based on the combined use of Radiance, a free and open-source tool, for the modeling and simulation of lighting scenes, and GenOpt, a generic optimization program, for the calibration of the model. The application of this methodology allows the quantification of the deterioration of the road lighting system and the identification of luminaires that show irregularities in their operation. Values lower than 9% for the error confirm that this research can contribute to the management of street lighting by assessing real road conditions. Full article
(This article belongs to the Special Issue Optimization Methods Applied to Power Systems)
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Open AccessFeature PaperArticle Autoignition Behavior of an Ethanol-Methylcellulose Gel Droplet in a Hot Environment
Energies 2018, 11(8), 2168; https://doi.org/10.3390/en11082168
Received: 27 July 2018 / Revised: 17 August 2018 / Accepted: 17 August 2018 / Published: 19 August 2018
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Abstract
Autoignition of an ethanol-based gel droplet was experimentally investigated by adding 10 wt % of methylcellulose as gellant to liquid ethanol. Experimental studies of the ignition behavior of the gel droplet were found to be quite rare. The initial droplet diameter was 1.17
[...] Read more.
Autoignition of an ethanol-based gel droplet was experimentally investigated by adding 10 wt % of methylcellulose as gellant to liquid ethanol. Experimental studies of the ignition behavior of the gel droplet were found to be quite rare. The initial droplet diameter was 1.17 ± 0.23 mm. The gel droplet was suspended on a K-type thermocouple and its evaporation, ignition and combustion characteristics were evaluated and compared with pure ethanol at an ambient temperature of 600, 700, and 800 °C under atmospheric pressure conditions. The gel droplet exhibited swelling and vapor jetting phenomena. Before ignition, a linear decrease in droplet diameter followed by a sudden increase was repeatedly observed, which was caused by evaporation and swelling processes, respectively. Major droplet swelling was detected just before the onset of ignition at all temperatures. But no further swelling was detected after ignition. For the gel droplet, the ignition delay accounted for 93% of the droplet lifetime at 600 °C, and 88% at 700 °C, but only 31% at 800 °C. Its average burning rate was also evaluated for all temperatures. At 800 °C, the gellant layer no longer exerts any influence on the combustion of the gel droplet. Full article
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Open AccessArticle Optimal Energy Management within a Microgrid: A Comparative Study
Energies 2018, 11(8), 2167; https://doi.org/10.3390/en11082167
Received: 21 June 2018 / Revised: 19 July 2018 / Accepted: 16 August 2018 / Published: 19 August 2018
Cited by 2 | PDF Full-text (3795 KB) | HTML Full-text | XML Full-text
Abstract
In this work, we focus on optimal energy management within the context of the tertiary control of a microgrid operating in grid-connected mode. Specifically, the optimal energy management problem is solved in a unified way by using the optimal power flow (OPF) and
[...] Read more.
In this work, we focus on optimal energy management within the context of the tertiary control of a microgrid operating in grid-connected mode. Specifically, the optimal energy management problem is solved in a unified way by using the optimal power flow (OPF) and day-ahead concepts. The elements considered in the microgrid are a photovoltaic panel, a wind turbine, electric vehicles, a storage system, and a point of common coupling with the main grid. The aim of this paper consists of optimizing the economic energy dispatch within the microgrid considering known predictions of electricity demand, solar radiation, and wind speed for a given period of time. The OPF is solved using three different algorithms provided by the optimization toolbox of MATLAB® (R2015a, MathWorks®, Natick, MA, USA): the interior point method (IP), a hybrid genetic algorithm with interior point (GA-IP), and a hybrid direct search with interior point (patternsearch-IP). The efficiency and effectiveness of the algorithms to optimize the energy dispatch within the microgrid are verified and analyzed through a case study, where real climatological data of solar irradiance, wind speed in Almería city, photovoltaic system data, and room load from a bioclimatic building were considered. Full article
(This article belongs to the Special Issue Microgrids-2018)
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Open AccessArticle Lighting Control Including Daylight and Energy Efficiency Improvements Analysis
Energies 2018, 11(8), 2166; https://doi.org/10.3390/en11082166
Received: 26 July 2018 / Revised: 13 August 2018 / Accepted: 16 August 2018 / Published: 19 August 2018
Cited by 1 | PDF Full-text (6643 KB) | HTML Full-text | XML Full-text
Abstract
Energy used for lighting is one of the major components of total energy consumption in buildings. Nowadays, buildings have a great potential to reduce their energy consumption, but to achieve this purpose additional efforts are indispensable. In this study, the need for energy
[...] Read more.
Energy used for lighting is one of the major components of total energy consumption in buildings. Nowadays, buildings have a great potential to reduce their energy consumption, but to achieve this purpose additional efforts are indispensable. In this study, the need for energy savings evaluation before the implementation of lighting control algorithms for a specified building is highlighted. Therefore, experimental tests have been carried out in a university building with laboratories and other rooms, equipped with KNX building automation system. A dimmable control strategy has been investigated, dependent on daylight illuminance. Moreover, a relationship between external and internal daylight illuminance levels has been evaluated as well. Based on the experimental results, the authors proposed a method for the rough estimation of electrical energy savings. Since, according to the EN 15232 standard, Building Automation and Control Systems (BACS) play an important role in buildings’ energy efficiency improvements, the BACS efficiency factors from this standard have been used to verify the experimental results presented in the paper. The potential to reduce energy consumption from lighting in non-residential buildings by 28% for offices and 24% for educational buildings has been confirmed, but its dependence on specific building parameters has been discussed as well. Full article
(This article belongs to the Special Issue Automation Control and Energy Efficiency in Complex Systems)
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Open AccessArticle Internet of Energy Approach for Sustainable Use of Electric Vehicles as Energy Storage of Prosumer Buildings
Energies 2018, 11(8), 2165; https://doi.org/10.3390/en11082165
Received: 31 July 2018 / Revised: 15 August 2018 / Accepted: 17 August 2018 / Published: 19 August 2018
Cited by 2 | PDF Full-text (2448 KB) | HTML Full-text | XML Full-text
Abstract
Vehicle-to-building (V2B) technology permits bypassing the power grid in order to supply power to a building from electric vehicle (EV) batteries in the parking lot. This paper investigates the hypothesis stating that the increasing number of EVs on our roads can be also
[...] Read more.
Vehicle-to-building (V2B) technology permits bypassing the power grid in order to supply power to a building from electric vehicle (EV) batteries in the parking lot. This paper investigates the hypothesis stating that the increasing number of EVs on our roads can be also beneficial for making buildings sustainably greener on account of using V2B technology in conjunction with local photovoltaic (PV) generation. It is assumed that there is no local battery storage other than EVs and that the EV batteries are fully available for driving, so that the EVs batteries must be at the intended state of charge at the departure time announced by the EV driver. Our goal is to exploit the potential of the EV batteries capacity as much as possible in order to permit a large area of solar panels, so that even on sunny days all PV power can be used to supply the building needs or the EV charging at the parking lot. A system architecture and collaboration protocols that account for uncertainties in EV behaviour are proposed. The proposed approach is proven in simulation covering one year period for three locations in different climatic regions of the US, resulting in the electricity bill reductions of 15.8%, 9.1% and 4.9% for California, New Jersey and Alaska, respectively. These results are compared to state-of-the-art research in combining V2B with PV or wind power generation. It is concluded that the achieved electricity bill reductions are superior to the state-of-the-art, because previous work is based on problem formulations that exploit only a part of the potential EV battery capacity. Full article
(This article belongs to the collection Smart Grid)
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Open AccessFeature PaperArticle On Energy Management Control of a PV-Diesel-ESS Based Microgrid in a Stand-Alone Context
Energies 2018, 11(8), 2164; https://doi.org/10.3390/en11082164
Received: 22 July 2018 / Revised: 14 August 2018 / Accepted: 16 August 2018 / Published: 18 August 2018
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Abstract
This paper deals with the energy management control of a PV-Diesel-ESS-based microgrid in a stand-alone context. In terms of control, an Isolated Mode Control (IMC) strategy based on a resonant regulator is proposed. In Parallel Mode Control (PMC) conditions, the diesel generator (DG)
[...] Read more.
This paper deals with the energy management control of a PV-Diesel-ESS-based microgrid in a stand-alone context. In terms of control, an Isolated Mode Control (IMC) strategy based on a resonant regulator is proposed. In Parallel Mode Control (PMC) conditions, the diesel generator (DG) is controlled to operate at its nominal power. In this context, a supervisory algorithm optimizing the power flow between the microgrid’s various components ensures switching between the two modes for different possible scenarios. To prove the effectiveness of the proposed control strategies, the energy management control (EMC) is tested first using a standard state of charge (SOC) profile emulating the microgrid different states. Then real data are used to simulate the load and solar radiations. An experimental validation on a reduced scale test bench is carried out to prove the feasibility and the effectiveness of the proposed energy management control strategies. Full article
(This article belongs to the Section Sustainable Energy)
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Open AccessArticle Gated Recurrent Unit Network-Based Short-Term Photovoltaic Forecasting
Energies 2018, 11(8), 2163; https://doi.org/10.3390/en11082163
Received: 19 July 2018 / Revised: 8 August 2018 / Accepted: 15 August 2018 / Published: 18 August 2018
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Abstract
Photovoltaic power has great volatility and intermittency due to environmental factors. Forecasting photovoltaic power is of great significance to ensure the safe and economical operation of distribution network. This paper proposes a novel approach to forecast short-term photovoltaic power based on a gated
[...] Read more.
Photovoltaic power has great volatility and intermittency due to environmental factors. Forecasting photovoltaic power is of great significance to ensure the safe and economical operation of distribution network. This paper proposes a novel approach to forecast short-term photovoltaic power based on a gated recurrent unit (GRU) network. Firstly, the Pearson coefficient is used to extract the main features that affect photovoltaic power output at the next moment, and qualitatively analyze the relationship between the historical photovoltaic power and the future photovoltaic power output. Secondly, the K-means method is utilized to divide training sets into several groups based on the similarities of each feature, and then GRU network training is applied to each group. The output of each GRU network is averaged to obtain the photovoltaic power output at the next moment. The case study shows that the proposed approach can effectively consider the influence of features and historical photovoltaic power on the future photovoltaic power output, and has higher accuracy than the traditional methods. Full article
(This article belongs to the Special Issue Solar and Wind Energy Forecasting)
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Open AccessArticle The Impact of Pulse Charging Parameters on the Life Cycle of Lithium-Ion Polymer Batteries
Energies 2018, 11(8), 2162; https://doi.org/10.3390/en11082162
Received: 2 August 2018 / Revised: 14 August 2018 / Accepted: 15 August 2018 / Published: 18 August 2018
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Abstract
The pulse charging algorithm is seen as a promising battery charging technique to satisfy the needs of electronic device consumers to have fast charging and increased battery charge and energy efficiencies. However, to get the benefits of pulse charging, the pulse charge current
[...] Read more.
The pulse charging algorithm is seen as a promising battery charging technique to satisfy the needs of electronic device consumers to have fast charging and increased battery charge and energy efficiencies. However, to get the benefits of pulse charging, the pulse charge current parameters have to be chosen carefully to ensure optimal battery performance and also extend the life cycle of the battery. The impact of pulse charge current factors on the life cycle and battery characteristics are seldom investigated. This paper seeks to evaluate the impact of pulse charge current factors, such as frequency and duty cycle, on the life cycle and impedance parameters of lithium-ion polymer batteries (LiPo) while using a design of experiments approach, Taguchi orthogonal arrays. The results are compared with the benchmark constant current-constant voltage (CC-CV) charging algorithm and it is observed that by using a pulse charger at optimal parameters, the cycle life of a LiPo battery can be increased by as much as 100 cycles. It is also determined that the duty cycle of the pulse charge current has the most impact on the cycle life of the battery. The battery impedance characteristics were also examined by using non-destructive techniques, such as electrochemical impedance spectroscopy, and it was determined that the ambient temperature at which the battery was charged had the most effect on the battery impedance parameters. Full article
(This article belongs to the Section Energy Storage and Application)
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Open AccessArticle A Power and Data Decoupled Transmission Method for Wireless Power Transfer Systems via a Shared Inductive Link
Energies 2018, 11(8), 2161; https://doi.org/10.3390/en11082161
Received: 17 July 2018 / Revised: 11 August 2018 / Accepted: 11 August 2018 / Published: 18 August 2018
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Abstract
Wireless Power Transfer (WPT) technology is gaining global popularity. However, in some applications, data transmission is also required to monitor the load states. This paper presents an alternative wireless power and data transmission method via the shared inductive link. With the method, the
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Wireless Power Transfer (WPT) technology is gaining global popularity. However, in some applications, data transmission is also required to monitor the load states. This paper presents an alternative wireless power and data transmission method via the shared inductive link. With the method, the system presents three characteristics: (1) controllability and stability of the output voltage; (2) miniaturization in volume of the system; (3) decoupled transmission of power and data. The output voltage control is realized by a non-inductive hysteresis control method. In particular, data is transferred when the power transmission is blocked (i.e., the hysteresis switch is off). The interference between power and data transmission is very small. The signal to noise ratio (SNR) performance which is relevant to the interference from power transfer to data transfer and data transfer capacity, is studied and optimized. Both simulation and experimental results have verified the proposed method. Full article
(This article belongs to the Special Issue Wireless Power Transfer 2018)
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Open AccessArticle Real-World Fuel Consumption, Fuel Cost and Exhaust Emissions of Different Bus Powertrain Technologies
Energies 2018, 11(8), 2160; https://doi.org/10.3390/en11082160
Received: 17 July 2018 / Revised: 11 August 2018 / Accepted: 15 August 2018 / Published: 18 August 2018
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Abstract
Air quality in urban areas is strongly influenced by exhaust emitted by the public transport fleet. The aim of this study was to analyze benefits in the fuel consumption, fuel costs and exhaust emissions when replacing baseline diesel fueled EURO III city buses
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Air quality in urban areas is strongly influenced by exhaust emitted by the public transport fleet. The aim of this study was to analyze benefits in the fuel consumption, fuel costs and exhaust emissions when replacing baseline diesel fueled EURO III city buses by the compressed natural gas (CNG)-fueled EURO V buses and by hydraulic series hybrid diesel-fueled EURO V buses. Real-world measurements were performed on the regular bus route to access realistic energy consumption and exhaust emissions. Instantaneous gaseous emission (CO2, CO, NOx and THC) were measured together with the instantaneous PM10 mass emission. Innovativeness of the presented approach thus arises from the systematic comparison of different powertrain technologies under real-world drive cycles and measuring time traces of not only gaseous but also of PM10 mass emissions. Furthermore, lumped cycle averaged emissions are interpreted and explained by typical powertrain performance parameters and exhaust emission time traces. Cumulative results indicate that application of the CNG fueled buses does not necessary reduce CO2 emissions compared to diesel-fueled buses whereas reduction in fuel costs is evident. Additionally, it is shown that hybrid operation of the hydraulic series hybrid diesel-fueled bus resulted in higher fuel consumption due to poorly optimized hybrid topology and control strategy. Furthermore, analyses of the time traces point out inadequate lambda control of CNG-fueled buses and nucleation mode-based particle number emissions during deceleration. Full article
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Open AccessArticle Power Transfer Efficiency Analysis for Omnidirectional Wireless Power Transfer System Using Three-Phase-Shifted Drive
Energies 2018, 11(8), 2159; https://doi.org/10.3390/en11082159
Received: 25 July 2018 / Revised: 9 August 2018 / Accepted: 11 August 2018 / Published: 18 August 2018
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Abstract
In order to implement the omnidirectional wireless power transfer (WPT), a novel three-phase-shifted drive omnidirectional WPT system is proposed. This system is comprised of three independent phase-adjusted excitation sources, three orthogonal transmitting coils, and one planar receiving coil. Based on the mutual coupling
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In order to implement the omnidirectional wireless power transfer (WPT), a novel three-phase-shifted drive omnidirectional WPT system is proposed. This system is comprised of three independent phase-adjusted excitation sources, three orthogonal transmitting coils, and one planar receiving coil. Based on the mutual coupling theory, the power transfer efficiency is derived and the corresponding control mechanism for maximizing this efficiency is presented. This control mechanism only depends on the currents’ root-mean-square (RMS) values of the three transmitting coils and simple calculations after each location and/or posture change of the receiving coil, which provides the real-time possibility to design an omnidirectional WPT system comparing with the other omnidirectional systems. In aid of computer emulation technique, the efficiency characteristic versus the omnidirectional location and posture of the receiving coil is analyzed, and the analytical results verify the validity of the control mechanism. Lastly, a hardware prototype has been set up, and its omnidirectional power transmission capacity has been successfully verified. The experimental results show that the wireless power is omnidirectional and it can be effectively transmitted to a load even though its receiving coil moves and/or rotates in a 3-D energy region. Full article
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Open AccessFeature PaperArticle Migration and Transformation of Vanadium and Nickel in High Sulfur Petroleum Coke during Gasification Processes
Energies 2018, 11(8), 2158; https://doi.org/10.3390/en11082158
Received: 26 July 2018 / Revised: 12 August 2018 / Accepted: 14 August 2018 / Published: 18 August 2018
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Abstract
The volatilization characteristics and occurrence forms of V and Ni in petroleum coke (petcoke) were investigated during steam (H2O) and carbon dioxide (CO2) gasification on a fixed bed reactor at 800–1100 °C. The Tessier sequential chemical extraction procedure was
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The volatilization characteristics and occurrence forms of V and Ni in petroleum coke (petcoke) were investigated during steam (H2O) and carbon dioxide (CO2) gasification on a fixed bed reactor at 800–1100 °C. The Tessier sequential chemical extraction procedure was employed to determine the different forms of V and Ni. The results showed their volatilities were not dependent on the gasification atmosphere, but rather relied mainly on the reaction temperature. The CO2 atmosphere accelerated the conversion of organic-bound nickel to residual form at low temperature and promoted Fe-Mn oxides formation at high temperature. However, the H2O atmosphere was conducive to form vanadium bound to Fe-Mn oxides and promoted the decomposition of residual forms. In addition, the thermodynamic equilibrium calculations showed the volatilization of Ni mainly released Ni3S2 between 800–1100 °C. The H2O atmosphere was favorable to generate the more stable NixSy compound, thereby suppressing the volatilization of Ni, while the presence of CO2 led to an increase in residual V and decrease of Fe-Mn oxides. The V and Ni mainly caused erosion problems under the CO2 atmosphere while the fouling and slagging obviously increased under the H2O atmosphere with impacts gradually weakened with the increase of temperature. Full article
(This article belongs to the Special Issue Solid Waste Gasification)
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Open AccessArticle Experimental Study on the Performance of Controllers for the Hydrogen Gas Production Demanded by an Internal Combustion Engine
Energies 2018, 11(8), 2157; https://doi.org/10.3390/en11082157
Received: 16 July 2018 / Revised: 7 August 2018 / Accepted: 9 August 2018 / Published: 18 August 2018
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Abstract
This work presents the design and application of two control techniques—a model predictive control (MPC) and a proportional integral derivative control (PID), both in combination with a multilayer perceptron neural network—to produce hydrogen gas on-demand, in order to use it as an additive
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This work presents the design and application of two control techniques—a model predictive control (MPC) and a proportional integral derivative control (PID), both in combination with a multilayer perceptron neural network—to produce hydrogen gas on-demand, in order to use it as an additive in a spark ignition internal combustion engine. For the design of the controllers, a control-oriented model, identified with the Hammerstein technique, was used. For the implementation of both controllers, only 1% of the overall air entering through the throttle valve reacted with hydrogen gas, allowing maintenance of the hydrogen–air stoichiometric ratio at 34.3 and the air–gasoline ratio at 14.6. Experimental results showed that the average settling time of the MPC controller was 1 s faster than the settling time of the PID controller. Additionally, MPC presented better reference tracking, error rates and standard deviation of 1.03 × 10 7 and 1.06 × 10 14 , and had a greater insensitivity to measurement noise, resulting in greater robustness to disturbances. Finally, with the use of hydrogen as an additive to gasoline, there was an improvement in thermal and combustion efficiency of 4% and 0.6%, respectively, and an increase in power of 545 W, translating into a reduction of fossil fuel use. Full article
(This article belongs to the Section Sustainable Energy)
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Open AccessArticle Analysis and Design of a Compound-Structure Permanent-Magnet Motor for Hybrid Electric Vehicles
Energies 2018, 11(8), 2156; https://doi.org/10.3390/en11082156
Received: 19 July 2018 / Revised: 5 August 2018 / Accepted: 9 August 2018 / Published: 17 August 2018
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Abstract
On the basis of the excellent driving force demand of hybrid electric vehicles (HEVs), this paper studies the torque property of the compound-structure permanent-magnet motor (CSPM motor) used for HEVs, which is influenced by magnetic field oversaturation and variable nonlinear parameters. Firstly, the
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On the basis of the excellent driving force demand of hybrid electric vehicles (HEVs), this paper studies the torque property of the compound-structure permanent-magnet motor (CSPM motor) used for HEVs, which is influenced by magnetic field oversaturation and variable nonlinear parameters. Firstly, the system configuration of HEVs based on CSPM motor and its working mode are introduced. Next, the state equation of CSPM motor in three-phase stationary coordinate system is proposed in order to investigate its torque performance; then, the factors affecting the output torque are gained. Finite element method (FEM)-based electromagnetic parameters analysis and design is carried out, to raise the output torque and reduce the torque ripple of CSPM motor. Besides, optimized design parameters are used to establish the FEM model, and the simulation results of electromagnetic performances for the CSPM motor before and after optimization are given to verify the rationality of optimization. Full article
(This article belongs to the Special Issue Demand Response in Electricity Markets)
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Open AccessArticle Impact of Demand-Side Management on the Reliability of Generation Systems
Energies 2018, 11(8), 2155; https://doi.org/10.3390/en11082155
Received: 1 August 2018 / Revised: 13 August 2018 / Accepted: 15 August 2018 / Published: 17 August 2018
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Abstract
The load shifting strategy is a form of demand side management program suitable for increasing the reliability of power supply in an electrical network. It functions by clipping the load demand that is above an operator-defined level, at which time is known as
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The load shifting strategy is a form of demand side management program suitable for increasing the reliability of power supply in an electrical network. It functions by clipping the load demand that is above an operator-defined level, at which time is known as peak period, and replaces it at off-peak periods. The load shifting strategy is conventionally performed using the preventive load shifting (PLS) program. In this paper, the corrective load shifting (CLS) program is proven as the better alternative. PLS is implemented when power systems experience contingencies that jeopardise the reliability of the power supply, whereas CLS is implemented only when the inadequacy of the power supply is encountered. The disadvantages of the PLS approach are twofold. First, the clipped energy cannot be totally recovered when it is more than the unused capacity of the off-peak period. The unused capacity is the maximum amount of extra load that can be filled before exceeding the operator-defined level. Second, the PLS approach performs load curtailment without discrimination. This means that load clipping is performed as long as the load is above the operator-defined level even if the power supply is adequate. The CLS program has none of these disadvantages because it is implemented only when there is power supply inadequacy, during which the amount of load clipping is mostly much smaller than the unused capacity of the off-peak period. The performance of the CLS was compared with the PLS by considering chronological load model, duty cycle and the probability of start-up failure for peaking and cycling generators, planned maintenance of the generators and load forecast uncertainty. A newly proposed expected-energy-not-recovered (EENR) index and the well-known expected-energy-not-supplied (EENS) were used to evaluate the performance of proposed CLS. Due to the chronological factor and huge combinations of power system states, the sequential Monte Carlo was employed in this study. The results from this paper show that the proposed CLS yields lower EENS and EENR than PLS and is, therefore, a more robust strategy to be implemented. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessArticle Modeling and Parameter Design of Voltage-Controlled Inverters Based on Discrete Control
Energies 2018, 11(8), 2154; https://doi.org/10.3390/en11082154
Received: 31 July 2018 / Revised: 11 August 2018 / Accepted: 13 August 2018 / Published: 17 August 2018
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Abstract
Grid-connected inverters are widely used to interface renewable energy and energy storage resources into the grid. Voltage-controlled inverters have attracted more and more attention due to their grid-friendly characteristics. The mathematical models of the voltage and current loops are developed in this paper,
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Grid-connected inverters are widely used to interface renewable energy and energy storage resources into the grid. Voltage-controlled inverters have attracted more and more attention due to their grid-friendly characteristics. The mathematical models of the voltage and current loops are developed in this paper, considering especially the discrete control delay caused by calculation and modulation. In order to suppress the resonance peak in the current loop, the frequency characteristics of the current loop are analyzed in detail. The optimum design flow of the current controller and voltage controller parameters are presented based on numerical analysis, and the stability, dynamic performance and the resonance peak suppression in voltage loop are also considered. Finally, the validity of the mathematical model and the effectiveness of the controller parameters design method are verified by simulation and experimental results. Full article
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Open AccessArticle Nitric Acid Pretreatment of Jerusalem Artichoke Stalks for Enzymatic Saccharification and Bioethanol Production
Energies 2018, 11(8), 2153; https://doi.org/10.3390/en11082153
Received: 8 June 2018 / Revised: 9 August 2018 / Accepted: 13 August 2018 / Published: 17 August 2018
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Abstract
This paper evaluated the effectiveness of nitric acid pretreatment on the hydrolysis and subsequent fermentation of Jerusalem artichoke stalks (JAS). Jerusalem artichoke is considered a potential candidate for producing bioethanol due to its low soil and climate requirements, and high biomass yield. However,
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This paper evaluated the effectiveness of nitric acid pretreatment on the hydrolysis and subsequent fermentation of Jerusalem artichoke stalks (JAS). Jerusalem artichoke is considered a potential candidate for producing bioethanol due to its low soil and climate requirements, and high biomass yield. However, its stalks have a complexed lignocellulosic structure, so appropriate pretreatment is necessary prior to enzymatic hydrolysis, to enhance the amount of sugar that can be obtained. Nitric acid is a promising catalyst for the pretreatment of lignocellulosic biomass due to the high efficiency with which it removes hemicelluloses. Nitric acid was found to be the most effective catalyst of JAS biomass. A higher concentration of glucose and ethanol was achieved after hydrolysis and fermentation of 5% (w/v) HNO3-pretreated JAS, leading to 38.5 g/L of glucose after saccharification, which corresponds to 89% of theoretical enzymatic hydrolysis yield, and 9.5 g/L of ethanol. However, after fermentation there was still a significant amount of glucose in the medium. In comparison to more commonly used acids (H2SO4 and HCl) and alkalis (NaOH and KOH), glucose yield (% of theoretical yield) was approximately 47–74% higher with HNO3. The fermentation of 5% nitric-acid pretreated hydrolysates with the absence of solid residues, led to an increase in ethanol yield by almost 30%, reaching 77–82% of theoretical yield. Full article
(This article belongs to the Special Issue Biofuel and Bioenergy Technology)
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Open AccessArticle Model-Based Fault Detection of Inverter-Based Microgrids and a Mathematical Framework to Analyze and Avoid Nuisance Tripping and Blinding Scenarios
Energies 2018, 11(8), 2152; https://doi.org/10.3390/en11082152
Received: 1 August 2018 / Revised: 13 August 2018 / Accepted: 15 August 2018 / Published: 17 August 2018
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Abstract
Traditional protection methods such as over-current or under-voltage methods are unreliable in inverter-based microgrid applications. This is primarily due to low fault current levels because of power electronic interfaces to the distributed energy resources (DER), and IEEE1547 low-voltage-ride-through (LVRT) requirements for renewables in
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Traditional protection methods such as over-current or under-voltage methods are unreliable in inverter-based microgrid applications. This is primarily due to low fault current levels because of power electronic interfaces to the distributed energy resources (DER), and IEEE1547 low-voltage-ride-through (LVRT) requirements for renewables in microgrids. However, when faults occur in a microgrid feeder, system changes occur which manipulate the internal circuit structure altering the system dynamic relationships. This observation establishes the basis for a proposed, novel, model-based, communication-free fault detection technique for inverter-based microgrids. The method can detect faults regardless of the fault current level and the microgrid mode of operation. The approach utilizes fewer measurements to avoid the use of a communication system. Protecting the microgrid without communication channels could lead to blinding (circuit breakers not tripping for faults) or nuisance tripping (tripping incorrectly). However, these events can be avoided with proper system design, specifically with appropriately sized system impedance. Thus, a major contribution of this article is the development of a mathematical framework to analyze and avoid blinding and nuisance tripping scenarios by quantifying the bounds of the proposed fault detection technique. As part of this analysis, the impedance based constraints for microgrid system feeders are included. The performance of the proposed technique is demonstrated in the MATLAB/SIMULINK (MathWorks, Natick, MA, USA) simulation environment on a representative microgrid architecture showing that the proposed technique can detect faults for a wide range of load impedances and fault impedances. Full article
(This article belongs to the Special Issue Microgrids-2018)
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Open AccessArticle On the Accuracy of Three-Dimensional Actuator Disc Approach in Modelling a Large-Scale Tidal Turbine in a Simple Channel
Energies 2018, 11(8), 2151; https://doi.org/10.3390/en11082151
Received: 17 July 2018 / Revised: 10 August 2018 / Accepted: 10 August 2018 / Published: 17 August 2018
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Abstract
To date, only a few studies have examined the execution of the actuator disc approximation for a full-size turbine. Small-scale models have fewer constraints than large-scale models because the range of time-scale and length-scale is narrower. Hence, this article presents the methodology in
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To date, only a few studies have examined the execution of the actuator disc approximation for a full-size turbine. Small-scale models have fewer constraints than large-scale models because the range of time-scale and length-scale is narrower. Hence, this article presents the methodology in implementing the actuator disc approach via the Reynolds-Averaged Navier-Stokes (RANS) momentum source term for a 20-m diameter turbine in an idealised channel. A structured grid, which varied from 0.5 m to 4 m across rotor diameter and width was used at the turbine location to allow for better representation of the disc. The model was tuned to match known coefficient of thrust and operational profiles for a set of validation cases based on published experimental data. Predictions of velocity deficit and turbulent intensity became almost independent of the grid density beyond 11 diameters downstream of the disc. However, in several instances the finer meshes showed larger errors than coarser meshes when compared to the measurements data. This observation was attributed to the way nodes were distributed across the disc swept area. The results demonstrate that the accuracy of the actuator disc was highly influenced by the vertical resolutions, as well as the grid density of the disc enclosure. Full article
(This article belongs to the Special Issue Offshore Renewable Energy: Ocean Waves, Tides and Offshore Wind)
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Open AccessArticle An Improved Multi-Timescale Coordinated Control Strategy for Stand-Alone Microgrid with Hybrid Energy Storage System
Energies 2018, 11(8), 2150; https://doi.org/10.3390/en11082150
Received: 29 July 2018 / Revised: 10 August 2018 / Accepted: 15 August 2018 / Published: 17 August 2018
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Abstract
A scientific and effective coordinated control strategy is crucial to the safe and economic operation of a microgrid (MG). With the continuous improvement of the renewable energy source (RES) penetration rate in MG, the randomness and intermittency of its output lead to the
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A scientific and effective coordinated control strategy is crucial to the safe and economic operation of a microgrid (MG). With the continuous improvement of the renewable energy source (RES) penetration rate in MG, the randomness and intermittency of its output lead to the increasing regulation pressure of the conventional controllable units, the increase of the operating risk of MG and the difficulty in improving the operational economy. To solve the mentioned problems and take advantage of hybrid energy storage system (HESS), this study proposes a multi-time scale coordinated control scheme of “day-ahead optimization (DAO) + intraday rolling (IDR) + quasi-real-time correction (QRTC) + real-time coordinated control (RTCC).” Considering the shortcomings of existing low prediction accuracy of distributed RES and loads, the soft constraints such as unit commitment scheduling errors and load switching scheduling errors are introduced in the intraday rolling model, allowing the correction of day-ahead unit commitment and load switching schedule. In the quasi-real-time coordinated control, an integrated criterion is introduced to decide the adjustment priority of the distributed generations. In the real-time coordinated control, the HESS adopts an improved first order low pass filtering algorithm to adaptively compensate the second-level unbalanced power. Compared with the traditional coordinated control strategy, the proposed improved model has the advantages of good robustness and fast solving speed and provides some guidance for the intelligent solution for stable and economic operation of stand-alone MG with HESS. Full article
(This article belongs to the Special Issue Power Quality in Microgrids Based on Distributed Generators)
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