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Energies, Volume 12, Issue 17 (September-1 2019)

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Cover Story (view full-size image) Rapid advancement of energy devices necessitates the development of new battery technologies. [...] Read more.
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Open AccessArticle
System Integrity Protection Scheme (SIPS) Development and an Optimal Bus-Splitting Scheme Supported by Phasor Measurement Units (PMUs)
Energies 2019, 12(17), 3404; https://doi.org/10.3390/en12173404 - 03 Sep 2019
Viewed by 307
Abstract
System integrity protection schemes (SIPS) are schemes that can, under potentially hazardous conditions, prevent a complete blackout of endangered parts of an electrical power system (EPS). The main objective of SIPS is to monitor the state of the power transmission network in real [...] Read more.
System integrity protection schemes (SIPS) are schemes that can, under potentially hazardous conditions, prevent a complete blackout of endangered parts of an electrical power system (EPS). The main objective of SIPS is to monitor the state of the power transmission network in real time and to react in emergency cases. This paper explores the use of phasor measurement unit (PMU) technology for the development of SIPS as a part of wide-area monitoring, protection, and control (WAMPAC) systems. A new SIPS development method is described using the experience from the real-time operation. The developed optimal bus-splitting scheme identifies potential actions that can eliminate or reduce power system overloads and protect the integrity of the power system. An optimal bus-splitting scheme based on a DC power flow model and PMU measurements is given as an example and is explained and tested on an IEEE 14 bus test system. Conducted simulations indicate that the described SIPS methodology supported by the PMU measurements can mitigate potential overloads of the observed network part. Full article
(This article belongs to the Special Issue Advanced Techniques for Electronic Power and Energy Systems)
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Open AccessArticle
Thermal State of the Blake Ridge Gas Hydrate Stability Zone (GHSZ)—Insights on Gas Hydrate Dynamics from a New Multi-Phase Numerical Model
Energies 2019, 12(17), 3403; https://doi.org/10.3390/en12173403 - 03 Sep 2019
Viewed by 288
Abstract
Marine sediments of the Blake Ridge province exhibit clearly defined geophysical indications for the presence of gas hydrates and a free gas phase. Despite being one of the world’s best-studied gas hydrate provinces and having been drilled during Ocean Drilling Program (ODP) Leg [...] Read more.
Marine sediments of the Blake Ridge province exhibit clearly defined geophysical indications for the presence of gas hydrates and a free gas phase. Despite being one of the world’s best-studied gas hydrate provinces and having been drilled during Ocean Drilling Program (ODP) Leg 164, discrepancies between previous model predictions and reported chemical profiles as well as hydrate concentrations result in uncertainty regarding methane sources and a possible co-existence between hydrates and free gas near the base of the gas hydrate stability zone (GHSZ). Here, by using a new multi-phase finite element (FE) numerical model, we investigate different scenarios of gas hydrate formation from both single and mixed methane sources (in-situ biogenic formation and a deep methane flux). Moreover, we explore the evolution of the GHSZ base for the past 10 Myr using reconstructed sedimentation rates and non-steady-state P-T solutions. We conclude that (1) the present-day base of the GHSZ predicted by our model is located at the depth of ~450 mbsf, thereby resolving a previously reported inconsistency between the location of the BSR at ODP Site 997 and the theoretical base of the GHSZ in the Blake Ridge region, (2) a single in-situ methane source results in a good fit between the simulated and measured geochemical profiles including the anaerobic oxidation of methane (AOM) zone, and (3) previously suggested 4 vol.%–7 vol.% gas hydrate concentrations would require a deep methane flux of ~170 mM (corresponds to the mass of methane flux of 1.6 × 10−11 kg s−1 m−2) in addition to methane generated in-situ by organic carbon (POC) degradation at the cost of deteriorating the fit between observed and modelled geochemical profiles. Full article
(This article belongs to the Special Issue Advances in Natural Gas Hydrates)
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Open AccessArticle
A Two-Stage Dispatch Mechanism for Virtual Power Plant Utilizing the CVaR Theory in the Electricity Spot Market
Energies 2019, 12(17), 3402; https://doi.org/10.3390/en12173402 - 03 Sep 2019
Viewed by 246
Abstract
The electricity spot market is now being implemented in China. Demand response, as a kind of flexible resource, is also being studied and explored for the constructed power market. Among the many demand response applications, the virtual power plant (VPP) as an aggregator [...] Read more.
The electricity spot market is now being implemented in China. Demand response, as a kind of flexible resource, is also being studied and explored for the constructed power market. Among the many demand response applications, the virtual power plant (VPP) as an aggregator of distributed energy resources (DERs), receives ever-increasing attention. However, the participation manner and related impacts of the VPP to the electricity spot market are still unknown within the current power market rules. Under this background, obeying the present trading rules of China’s electricity spot market, a two-stage dispatching model with optimized bidding and operating strategy in the day-ahead (DA) and real-time (RT) market for the VPP is proposed. In the designed model, the conditional risk value (CVaR) is adopted to address the risk encountered by the uncertainty of the electricity spot market price. The impact of the user-side over-deviated revenue mechanism (UORM) of the China spot market on the income of the VPP in the DA and RT market is also analyzed. For a full evaluation, different coefficients for the influence of DA and RT risk, UORM, and energy storage system (ESS) are tested to investigate their respective impacts on the revenue of the VPP. The simulation cases prove that the proposed method is helpful for the VPP to optimize DERs’ output in the electricity spot market according to its own risk preference. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessArticle
Identification of Noise, Vibration and Harshness Behavior of Wind Turbine Drivetrain under Different Operating Conditions
Energies 2019, 12(17), 3401; https://doi.org/10.3390/en12173401 - 03 Sep 2019
Viewed by 285
Abstract
Noise, vibration and harshness (NVH) problems are critical issues to be tackled for wind turbine drivetrains. Tracking the behavior of modal parameters of the machines’ fundamental modes during operation it is of high interest to validate complex simulation models. A powerful approach for [...] Read more.
Noise, vibration and harshness (NVH) problems are critical issues to be tackled for wind turbine drivetrains. Tracking the behavior of modal parameters of the machines’ fundamental modes during operation it is of high interest to validate complex simulation models. A powerful approach for this purpose is represented by operational modal analysis (OMA). This paper describes the investigation of an automated technique for continuously tracking the modes of a rotating mechanical system running in normal operating conditions. The modal estimation procedure is based on an automatic version of the pLSCF (poly-reference Least-Square Complex Frequency-Domain) algorithm. The latter is coupled with a method that automatically tracks the modal parameters along different data sets. The use of OMA on a rotating component of the wind turbine creates the need to deal with harmonics in order to satisfy one of the assumptions of OMA. For this purpose, the use of a cepstrum editing procedure is analyzed and implemented. Modal estimates obtained from an automated analysis on stand still data and normal operating conditions data are compared, to test the added value of the cepstrum editing procedure and the robustness of the method when used on real data. To illustrate and validate the implemented methodology, data acquired during a long-term monitoring campaign of a wind turbine drivetrain are used. Full article
(This article belongs to the Special Issue Design, Fabrication and Performance of Wind Turbines)
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Open AccessArticle
Fault Characteristic and Low Voltage Ride-Through Requirements Applicability Analysis for a Permanent Magnet Synchronous Generator-Based Wind Farm
Energies 2019, 12(17), 3400; https://doi.org/10.3390/en12173400 - 03 Sep 2019
Viewed by 267
Abstract
As the penetration of wind energy is being dramatically increased, the impact of wind energy on the power system should be roundly studied, especially for the fault characteristics analysis and applicability analysis of low voltage ride-through (LVRT) requirements for a whole wind farm [...] Read more.
As the penetration of wind energy is being dramatically increased, the impact of wind energy on the power system should be roundly studied, especially for the fault characteristics analysis and applicability analysis of low voltage ride-through (LVRT) requirements for a whole wind farm (WF) and an individual wind turbine generator (WTG). This paper firstly describes a detailed modeling of a permanent magnet synchronous generator (PMSG)-based WF and analyzes the fault characteristics of the WF under various fault conditions. The validation of the fault characteristics analysis is carried out with the EMTP-RV generated data, with the consideration of different fault positions, fault types, and wind speeds. The relay protection and the related grid code are also taken into account. In addition, the applicability analysis of LVRT requirements for a WF and a WTG is also implemented, from the points of minimal grid-connection time and minimal dynamic reactive current support ability. The fault characteristic analysis of a PMSG-based WF could be helpful for developing new control or protection methods for a PMSG-based WF. Meanwhile, the applicability analysis of LVRT requirements could serve as a reference for WTG manufacturers, WF administrators, and grid operator. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessArticle
Stages in the Dynamics of Hydrate Formation and Consequences for Design of Experiments for Hydrate Formation in Sediments
Energies 2019, 12(17), 3399; https://doi.org/10.3390/en12173399 - 03 Sep 2019
Viewed by 260
Abstract
Natural gas hydrates in sediments can never reach thermodynamic equilibrium. Every section of any hydrate-filled reservoir is unique and resides in a stationary balance that depends on many factors. Fluxes of hydrocarbons from below support formation of new hydrate, and inflow of water [...] Read more.
Natural gas hydrates in sediments can never reach thermodynamic equilibrium. Every section of any hydrate-filled reservoir is unique and resides in a stationary balance that depends on many factors. Fluxes of hydrocarbons from below support formation of new hydrate, and inflow of water through fracture systems leads to hydrate dissociation. Mineral/fluid/hydrate interaction and geochemistry are some of the many other factors that determine local hydrate saturation in the pores. Even when using real sediments from coring it is impossible to reproduce in the laboratory a natural gas hydrate reservoir which has developed over geological time-scales. In this work we discuss the various stages of hydrate formation, with a focus on dynamic rate limiting processes which can lead to trapped pockets of gas and trapped liquid water inside hydrate. Heterogeneous hydrate nucleation on the interface between liquid water and the phase containing the hydrate former rapidly leads to mass transport limiting films of hydrate. These hydrate films can delay the onset of massive, and visible, hydrate growth by several hours. Heat transport in systems of liquid water and hydrate is orders of magnitude faster than mass transport. We demonstrate that a simple mass transport model is able to predict induction times for selective available experimental data for CO2 hydrate formation and CH4 hydrate formation. Another route to hydrate nucleation is towards mineral surfaces. CH4 cannot adsorb directly but can get trapped in water structures as a secondary adsorption. H2S has a significant dipole moment and can adsorb directly on mineral surfaces. The quadropole-moment in CO2 also plays a significant role in adsorption on minerals. Hydrate that nucleates toward minerals cannot stick to the mineral surfaces so the role of these nucleation sites is to produce hydrate cores for further growth elsewhere in the system. Various ways to overcome these obstacles and create realistic hydrate saturation in laboratory sediment are also discussed. Full article
(This article belongs to the Section Energy and Environment)
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Open AccessFeature PaperArticle
Aerodynamics of Monolithic Matrices for Supporting Solid Reactant or Catalyst
Energies 2019, 12(17), 3398; https://doi.org/10.3390/en12173398 - 03 Sep 2019
Viewed by 264
Abstract
Heterogeneous solid/fluid chemical reactions—as well as reactions dependent on solid catalysts—require spreading the active solid substance on the largest accessible area. The solution is a thin layer covering as much as possible convoluted surface of an inert support. This is nowadays the internal [...] Read more.
Heterogeneous solid/fluid chemical reactions—as well as reactions dependent on solid catalysts—require spreading the active solid substance on the largest accessible area. The solution is a thin layer covering as much as possible convoluted surface of an inert support. This is nowadays the internal surface of narrow parallel passages. The supporting body is usually ceramic, its passages now mostly of square cross section. Reliable detailed knowledge of pressure drop across the set of passages has to be available, especially for flow control based on fluid property changes (e.g., with temperature or fluid composition). This paper presents results of laboratory measurements as well as numerical flowfield computations of the passage flows, with discovered universal law. Full article
(This article belongs to the Special Issue Fluid Mechanics and Thermodynamics: Theory, Methods and Applications)
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Open AccessArticle
Study on the Effect of Cable Group Laying Mode on Temperature Field Distribution and Cable Ampacity
Energies 2019, 12(17), 3397; https://doi.org/10.3390/en12173397 - 03 Sep 2019
Viewed by 214
Abstract
The reliability and service life of power cables is closely related to the cable ampacity and temperature rise. Therefore, studying the temperature field distribution and the cable ampacity is helpful to improve the construction guidelines of cable manufacturers. Taking a 8.7/15 kV YJV [...] Read more.
The reliability and service life of power cables is closely related to the cable ampacity and temperature rise. Therefore, studying the temperature field distribution and the cable ampacity is helpful to improve the construction guidelines of cable manufacturers. Taking a 8.7/15 kV YJV 1 × 400 XLPE three-loop power cable as the research object, cable temperature is calculated by IEC-60287 thermal circuit method and numerical simulation method, respectively. The results show that the numerical simulation method is more in line with the actual measured temperature, and the relative error is only 0.32% compared with the actual measured temperature. The temperature field and air velocity field of cluster cables with different laying methods are analyzed by finite element method. The corresponding cable ampacity are calculated by secant method. The results show that when the cable is laid at the bottom of the cable trench, the cable current is 420 A, which is 87.5% of the regular laying. Under irregular laying mode, the temperature of cable is higher than that of regular laying mode and the cable ampacity is lower than that of regular laying mode. At the same time, a multiparameter online monitoring system is developed to online monitor the temperature, water level and smoke concentration of the cable. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessArticle
Development of an SVR Model for the Fault Diagnosis of Large-Scale Doubly-Fed Wind Turbines Using SCADA Data
Energies 2019, 12(17), 3396; https://doi.org/10.3390/en12173396 - 03 Sep 2019
Viewed by 254
Abstract
Fault diagnosis and forecasting contribute significantly to the reduction of operating and maintenance associated costs, as well as to improve the resilience of wind turbine systems. Different from the existing fault diagnosis approaches using monitored vibration and acoustic data from the auxiliary equipment, [...] Read more.
Fault diagnosis and forecasting contribute significantly to the reduction of operating and maintenance associated costs, as well as to improve the resilience of wind turbine systems. Different from the existing fault diagnosis approaches using monitored vibration and acoustic data from the auxiliary equipment, this research presents a novel fault diagnosis and forecasting approach underpinned by a support vector regression model using data obtained by the supervisory control and data acquisition system (SCADA) of wind turbines (WT). To operate, the extraction of fault diagnosis features is conducted by measuring SCADA parameters. After that, confidence intervals are set up to guide the fault diagnosis implemented by the support vector regression (SVR) model. With the employment of confidence intervals as the performance indicators, an SVR-based fault detecting approach is then developed. Based on the WT SCADA data and the SVR model, a fault diagnosis strategy for large-scale doubly-fed wind turbine systems is investigated. A case study including a one-year monitoring SCADA data collected from a wind farm in Southern China is employed to validate the proposed methodology and demonstrate how it works. Results indicate that the proposed strategy can support the troubleshooting of wind turbine systems with high precision and effective response. Full article
(This article belongs to the Section Wind, Wave and Tidal Energy)
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Open AccessArticle
Investigations on EMI Mitigation Techniques: Intent to Reduce Grid-Tied PV Inverter Common Mode Current and Voltage
Energies 2019, 12(17), 3395; https://doi.org/10.3390/en12173395 - 03 Sep 2019
Viewed by 335
Abstract
Power inverters produce common mode voltage (CMV) and common mode current (CMC) which cause high-frequency electromagnetic interference (EMI) noise, leakage currents in electrical drives application and grid-connected systems, which consequently drops the efficiency of the system considerably. This CMV can be mitigated by [...] Read more.
Power inverters produce common mode voltage (CMV) and common mode current (CMC) which cause high-frequency electromagnetic interference (EMI) noise, leakage currents in electrical drives application and grid-connected systems, which consequently drops the efficiency of the system considerably. This CMV can be mitigated by designing suitable EMI filters and/or investigating the effects of different modulation strategies. In this paper, the effect of various modulation techniques over CMV and CMC are investigated for two-level and three-level inverters. It is observed that the modified third harmonic injection method reduced the CMV and CMC in the system by 60%. This modified pulse width modulation (PWM) technique is employed along with EMI chokes which results in reduced distortion of the system. Full article
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Open AccessArticle
Application of Response Surface Analysis to Evaluate the Effect of Concentrations of Ammonia and Propionic Acid on Acetate-Utilizing Methanogenesis
Energies 2019, 12(17), 3394; https://doi.org/10.3390/en12173394 - 03 Sep 2019
Viewed by 207
Abstract
Ammonia and propionate are known inhibitors of anaerobic methanogenesis at higher concentrations, and are likely to coexist in digesters treating high-strength wastewater. Therefore, this study was conducted to assess the effects of ammonia and propionate on acetate-utilizing methanogenesis when they coexist. Response surface [...] Read more.
Ammonia and propionate are known inhibitors of anaerobic methanogenesis at higher concentrations, and are likely to coexist in digesters treating high-strength wastewater. Therefore, this study was conducted to assess the effects of ammonia and propionate on acetate-utilizing methanogenesis when they coexist. Response surface analysis with face-centered central composite design was used to explore the total ammonia nitrogen (TAN) level of 2–5 g/L and the propionate level of 2–8 g/L in acetate-fed batch incubation. Two models were successfully derived to estimate the lag period and the methane yield in response to the concentrations of the two chemicals. The lag period was affected by both inhibitors, with clues obtained of a synergistic effect at a higher concentration rage ([TAN] > 3.5 g/L and [propionate] > 5 g/L). The methane yield was also affected by the two inhibitors; between the two, it was more significantly dependent on the TAN concentration than on the propionate concentration. Real-time PCR showed that Methanosarcinaceae was the major methanogen group in this system. The results of this study improve our understanding of the inhibition of biogas reactors. Full article
(This article belongs to the Special Issue Anaerobic Digestion for the Production of Energy and Chemicals)
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Open AccessReview
Building Energy Retrofit Measures in Hot-Summer–Cold-Winter Climates: A Case Study in Shanghai
Energies 2019, 12(17), 3393; https://doi.org/10.3390/en12173393 - 03 Sep 2019
Viewed by 207
Abstract
Building retrofit measures provide a significant means of mitigating the effect of climate change on buildings by enhancing building energy performance at a beneficial cost-effectiveness. An insight into the applicable building retrofit measures within a climate zone will guide the optimisation framework to [...] Read more.
Building retrofit measures provide a significant means of mitigating the effect of climate change on buildings by enhancing building energy performance at a beneficial cost-effectiveness. An insight into the applicable building retrofit measures within a climate zone will guide the optimisation framework to attaining sustainability in architecture and the built environment. This article presents a brief overview of recent studies on retrofit measures and its application on a variety of buildings in hot-summer–cold-winter climates, with emphasis on Shanghai. Findings show that the major retrofit measures include improvement in the building envelope, heating, ventilation and cooling (HVAC) and lighting, supported by photovoltaic (PV) systems, accordingly. Furthermore, the study identifies key elements and plausible challenges for the evaluation of building retrofit measures in this region. In this regard, financial barriers and lack of standards and regulatory support are the main challenges identified. These insights provide a systematic approach to guide building researchers, practitioners and decision-makers in the design and development of existing and new retrofit measures for the future of rapidly growing cities with a broad climate variation scope. Full article
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Open AccessArticle
Condition Monitoring of Bearing Faults Using the Stator Current and Shrinkage Methods
Energies 2019, 12(17), 3392; https://doi.org/10.3390/en12173392 - 03 Sep 2019
Viewed by 234
Abstract
Condition monitoring of bearings is an open issue. The use of the stator current to monitor induction motors has been validated as a very advantageous and practical way to detect several types of faults. Nevertheless, for bearing faults, the use of vibrations or [...] Read more.
Condition monitoring of bearings is an open issue. The use of the stator current to monitor induction motors has been validated as a very advantageous and practical way to detect several types of faults. Nevertheless, for bearing faults, the use of vibrations or sound generally offers better results in the accuracy of the detection, although with some disadvantages related to the sensors used for monitoring. To improve the performance of bearing monitoring, it is proposed to take advantage of more information available in the current spectra, beyond the usually employed, incorporating the amplitude of a significant number of sidebands around the first eleven harmonics, growing exponentially the number of fault signatures. This is especially interesting for inverter-fed motors. But, in turn, this leads to the problem of overfitting when applying a classifier to perform the fault diagnosis. To overcome this problem, and still exploit all the useful information available in the spectra, it is proposed to use shrinkage methods, which have been lately proposed in machine learning to solve the overfitting issue when the problem has many more variables than examples to classify. A case study with a motor is shown to prove the validity of the proposal. Full article
(This article belongs to the Special Issue Fault Diagnosis in Electric Motors)
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Open AccessArticle
Electrospun Core-Shell Nanofiber as Separator for Lithium-Ion Batteries with High Performance and Improved Safety
Energies 2019, 12(17), 3391; https://doi.org/10.3390/en12173391 - 03 Sep 2019
Viewed by 294
Abstract
Though the energy density of lithium-ion batteries continues to increase, safety issues related to the internal short circuit and the resulting combustion of highly flammable electrolytes impede the further development of lithium-ion batteries. It has been well-accepted that a thermal stable separator is [...] Read more.
Though the energy density of lithium-ion batteries continues to increase, safety issues related to the internal short circuit and the resulting combustion of highly flammable electrolytes impede the further development of lithium-ion batteries. It has been well-accepted that a thermal stable separator is important to postpone the entire battery short circuit and thermal runaway. Traditional methods to improve the thermal stability of separators include surface modification and/or developing alternate material systems for separators, which may affect the battery performance negatively. Herein, a thermostable and shrink-free separator with little compromise in battery performance was prepared by coaxial electrospinning and tested. The separator consisted of core-shell fiber networks where poly (vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) layer served as shell and polyacrylonitrile (PAN) as the core. This core-shell fiber network exhibited little or even no shrinking/melting at elevated temperature over 250 °C. Meanwhile, it showed excellent electrolyte wettability and could take large amounts of liquid electrolyte, three times more than that of conventional Celgard 2400 separator. In addition, the half-cell using LiNi1/3Co1/3Mn1/3O2 as cathode and the aforementioned electrospun core-shell fiber network as separator demonstrated superior electrochemical behavior, stably cycling for 200 cycles at 1 C with a reversible capacity of 130 mA·h·g−1 and little capacity decay. Full article
(This article belongs to the Special Issue Advanced Nanomaterials for Li- and Na-Ion Batteries)
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Open AccessReview
Overview of Recent Advancements in the Microbial Fuel Cell from Fundamentals to Applications: Design, Major Elements, and Scalability
Energies 2019, 12(17), 3390; https://doi.org/10.3390/en12173390 - 03 Sep 2019
Viewed by 244
Abstract
Microbial fuel cell (MFC) technology offers an alternative means for producing energy from waste products. In this review, several characteristics of MFC technology that make it revolutionary will be highlighted. First, a brief history presents how bioelectrochemical systems have advanced, ultimately describing the [...] Read more.
Microbial fuel cell (MFC) technology offers an alternative means for producing energy from waste products. In this review, several characteristics of MFC technology that make it revolutionary will be highlighted. First, a brief history presents how bioelectrochemical systems have advanced, ultimately describing the development of microbial fuel cells. Second, the focus is shifted to the attributes that enable MFCs to work efficiently. Next, follows the design of various MFC systems in use including their components and how they are assembled, along with an explanation of how they work. Finally, microbial fuel cell designs and types of main configurations used are presented along with the scalability of the technology for proper application. The present review shows importance of design and elements to reduce energy loss for scaling up the MFC system including the type of electrode, shape of the single reactor, electrical connection method, stack direction, and modulation. These aspects precede making economically applicable large-scale MFCs (over 1 m3 scale) a reality. Full article
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Open AccessArticle
Optimal Sizing and Operation of Electric and Thermal Storage in a Net Zero Multi Energy System
Energies 2019, 12(17), 3389; https://doi.org/10.3390/en12173389 - 03 Sep 2019
Viewed by 291
Abstract
In this this paper, the optimal sizing of electric and thermal storage is applied to the novel definition of a net zero multi energy system (NZEMS). A NZMES is based on producing electricity exclusively from renewable energy sources (RES) and converting it into [...] Read more.
In this this paper, the optimal sizing of electric and thermal storage is applied to the novel definition of a net zero multi energy system (NZEMS). A NZMES is based on producing electricity exclusively from renewable energy sources (RES) and converting it into other energy forms to satisfy multiple energy needs of a community. Due to the intermittent nature of RES, storage resources are needed to increase the self-sufficiency of the system. Possible storage sizing choices are examined considering, on an annual basis, the solution of a predictive control problem aimed at optimizing daily operation. For each day of the year, a predictive control problem is formulated and solved, aimed at minimizing operating costs. Electric, thermal, and (electric) transportation daily curves and expected RES production are assessed by means of a model that includes environmental parameters. Test results, based on the energy model of a small rural village, show expected technical-economic performance of different planning solutions, highlighting how the renewable energy mix influences the choice of both thermal and electric storage, and how self-sufficiency can affect the overall cost of energy. Full article
(This article belongs to the Special Issue Optimization of Multicarrier Energy Systems)
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Open AccessArticle
Backbone—An Adaptable Energy Systems Modelling Framework
Energies 2019, 12(17), 3388; https://doi.org/10.3390/en12173388 - 02 Sep 2019
Viewed by 294
Abstract
Backbone represents a highly adaptable energy systems modelling framework, which can be utilised to create models for studying the design and operation of energy systems, both from investment planning and scheduling perspectives. It includes a wide range of features and constraints, such as [...] Read more.
Backbone represents a highly adaptable energy systems modelling framework, which can be utilised to create models for studying the design and operation of energy systems, both from investment planning and scheduling perspectives. It includes a wide range of features and constraints, such as stochastic parameters, multiple reserve products, energy storage units, controlled and uncontrolled energy transfers, and, most significantly, multiple energy sectors. The formulation is based on mixed-integer programming and takes into account unit commitment decisions for power plants and other energy conversion facilities. Both high-level large-scale systems and fully detailed smaller-scale systems can be appropriately modelled. The framework has been implemented as the open-source Backbone modelling tool using General Algebraic Modeling System (GAMS). An application of the framework is demonstrated using a power system example, and Backbone is shown to produce results comparable to a commercial tool. However, the adaptability of Backbone further enables the creation and solution of energy systems models relatively easily for many different purposes and thus it improves on the available methodologies. Full article
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Open AccessFeature PaperArticle
Power Oscillation Damping from Offshore Wind Farms Connected to HVDC via Diode Rectifiers
Energies 2019, 12(17), 3387; https://doi.org/10.3390/en12173387 - 02 Sep 2019
Viewed by 373
Abstract
Diode rectifiers (DRs) have elicited increasing interest from both industry and academia as a feasible alternative for connecting offshore wind farms (OWFs) to HVDC networks. However, before such technology is deployed, more studies are needed to assess the actual capabilities of DR-connected OWFs [...] Read more.
Diode rectifiers (DRs) have elicited increasing interest from both industry and academia as a feasible alternative for connecting offshore wind farms (OWFs) to HVDC networks. However, before such technology is deployed, more studies are needed to assess the actual capabilities of DR-connected OWFs to contribute to the secure operation of the networks linked to them. This study assessed the capability of such an OWF to provide support to an onshore AC network by means of (active) power oscillation damping (POD). A semi-aggregated OWF representation was considered in order to examine the dynamics of each grid-forming wind turbine (WT) within a string when providing POD, while achieving reasonable simulation times. Simulation results corroborate that such an OWF can provide POD by means of OWF active power controls similar to those developed for OWFs connected to HVDC via voltage source converters, while its grid-forming WTs share the reactive power consumption/production and keep the offshore voltage frequency and magnitude within their normal operating ranges. Open-loop test results show that such capability can, however, be restricted at operating points corresponding to the lowest and highest values of active power output. Full article
(This article belongs to the Special Issue Control and Protection of HVDC-Connected Offshore Wind Power Plants)
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Open AccessArticle
Study on Riblet Drag Reduction Considering the Effect of Sweep Angle
Energies 2019, 12(17), 3386; https://doi.org/10.3390/en12173386 - 02 Sep 2019
Viewed by 244
Abstract
This study computationally evaluates the riblet drag reduction effect considering the effect of sweep angle. An implicit large eddy simulation is performed on a channel flow and an infinite swept wing. First, three different inclined angles between the riblets and the flow direction [...] Read more.
This study computationally evaluates the riblet drag reduction effect considering the effect of sweep angle. An implicit large eddy simulation is performed on a channel flow and an infinite swept wing. First, three different inclined angles between the riblets and the flow direction are tested in the channel flow. The results show that with increases in the inclined angle, the friction drag decreases, while the pressure drag increases approximately quadratically. The riblets with a 30° inclined angle increase the total drag of the channel flow. Then, an infinite wing with a 30° swept angle with and without riblets is studied. The riblets demonstrate satisfactory drag reduction efficiency because the cross flow over most parts of the wing is mild. The lift and friction drag follow the relation of the cosine law of a swept wing. Moreover, the cross flow and the turbulence fluctuation are suppressed by the riblets. Full article
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Open AccessArticle
Supply System for Three-Level Inverters Using Multi-Pulse Rectifiers with Coupled Reactors
Energies 2019, 12(17), 3385; https://doi.org/10.3390/en12173385 - 02 Sep 2019
Viewed by 264
Abstract
The paper presents a proposal of the multi-pulse AC/DC converter for providing supply for multilevel inverters and considerably reducing the line current distortion factor. It can be applied as a suitable source of DC voltage and is convenient to supply such inverters that [...] Read more.
The paper presents a proposal of the multi-pulse AC/DC converter for providing supply for multilevel inverters and considerably reducing the line current distortion factor. It can be applied as a suitable source of DC voltage and is convenient to supply such inverters that need three or more voltage terminals to supply all sections of the inverter. The converter in question works as a multi-pulse rectifier supplying a three-level NPC (Neutral Point Clamped) inverter. The load introduced by the inverter is simulated as an impedance depended on a selected inverter state. All possible states are determined by using a set of three digits: 0, 1, 2. The sourcing converter is constructed from diode bridge rectifiers connected in parallel as well as the system of magnetically coupled reactors. The reactors are used to shift subsequent line voltages so that they create a multiphase voltage system supplying component rectifiers. The article presents two alternative rectifiers: A 12-pulse diode rectifier using one magnetically coupled reactor unit 3CRλ, and a 24-pulse one using three 3CRλ units. Simulation and experimental tests have been executed and selected results included. Conclusion and discussion are added. Full article
(This article belongs to the Section Energy Storage and Application)
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Open AccessArticle
Optimal Dispatching of Offshore Microgrid Considering Probability Prediction of Tidal Current Speed
Energies 2019, 12(17), 3384; https://doi.org/10.3390/en12173384 - 02 Sep 2019
Viewed by 232
Abstract
Oceans contain rich tidal current energy, which can provide sufficient power for offshore microgrids. However, the uncertainty of tidal flow may endanger the operational reliability of an offshore microgrid. In this paper, a probabilistic prediction model of tidal current is established based on [...] Read more.
Oceans contain rich tidal current energy, which can provide sufficient power for offshore microgrids. However, the uncertainty of tidal flow may endanger the operational reliability of an offshore microgrid. In this paper, a probabilistic prediction model of tidal current is established based on support vector quantile regression to reduce the influence of uncertainty. Firstly, the penalty factors and kernel parameters of the proposed prediction model was optimized by the dragonfly algorithm to predict the tidal speed of any time of a day in different quantiles. Secondly, combining the above result with the kernel density to predict the probability density function of the tidal current speed, which is to improve the accuracy of prediction in the absence of information. Thirdly, an optimal generation dispatching strategy with tidal current generators is proposed to minimize the fuel consumption of offshore microgrids. Finally, a case study based on the offshore oil and gas platform in Bohai shows that the mean absolute percent error of the proposed model is 2.8142%, which is better than support vector quantile regression model and support vector regression model optimized by the genetic algorithm. Full article
(This article belongs to the Section Wind, Wave and Tidal Energy)
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Open AccessArticle
A Nonlinear-Model-Based Observer for a State-of-Charge Estimation of a Lithium-Ion Battery in Electric Vehicles
Energies 2019, 12(17), 3383; https://doi.org/10.3390/en12173383 - 02 Sep 2019
Viewed by 224
Abstract
This paper presents a nonlinear-model-based observer for the state of charge estimation of a lithium-ion battery cell that always exhibits a nonlinear relationship between the state of charge and the open-circuit voltage. The proposed nonlinear model for the battery cell and its observer [...] Read more.
This paper presents a nonlinear-model-based observer for the state of charge estimation of a lithium-ion battery cell that always exhibits a nonlinear relationship between the state of charge and the open-circuit voltage. The proposed nonlinear model for the battery cell and its observer can estimate the state of charge without the linearization technique commonly adopted by previous studies. The proposed method has the following advantages: (1) The observability condition of the proposed nonlinear-model-based observer is derived regardless of the shape of the open circuit voltage curve, and (2) because the terminal voltage is contained in the state vector, the proposed model and its observer are insensitive to sensor noise. A series of experiments using an INR 18650 25R battery cell are performed, and it is shown that the proposed method produces convincing results for the state of charge estimation compared to conventional SOC estimation methods. Full article
(This article belongs to the Special Issue Energy Storage and Management for Electric Vehicles)
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Open AccessArticle
Facile Electrodeposition of Poly(3,4-ethylenedioxythiophene) on Poly(vinyl alcohol) Nanofibers as the Positive Electrode for High-Performance Asymmetric Supercapacitor
Energies 2019, 12(17), 3382; https://doi.org/10.3390/en12173382 - 02 Sep 2019
Viewed by 219
Abstract
Poly(vinyl alcohol)/poly(3,4-ethylenedioxythiophene) (PVA/PEDOT) nanofibers were synthesized as a positive electrode for high-performance asymmetric supercapacitor (ASC). PVA/PEDOT nanofibers were prepared through electrospinning and electrodeposition meanwhile reduced graphene oxide (rGO) was obtained by electrochemical reduction. The PVA/PEDOT nanofibers demonstrated cauliflower-like morphology showing that PEDOT was [...] Read more.
Poly(vinyl alcohol)/poly(3,4-ethylenedioxythiophene) (PVA/PEDOT) nanofibers were synthesized as a positive electrode for high-performance asymmetric supercapacitor (ASC). PVA/PEDOT nanofibers were prepared through electrospinning and electrodeposition meanwhile reduced graphene oxide (rGO) was obtained by electrochemical reduction. The PVA/PEDOT nanofibers demonstrated cauliflower-like morphology showing that PEDOT was uniformly coated on the smooth cross-linking structure of PVA nanofibers. In addition, the ASC showed a remarkable energy output efficiency by delivering specific energy of 21.45 Wh·kg−1 at a specific power of 335.50 W·kg−1 with good cyclability performance (83% capacitance retained) after 5000 CV cycles. The outstanding supercapacitive performance is contributed from the synergistic effects of both PVA/PEDOT//rGO, which gives promising materials for designing high-performance supercapacitor applications. Full article
(This article belongs to the Section Energy Storage and Application)
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Open AccessArticle
Multiscale Apparent Permeability Model of Shale Nanopores Based on Fractal Theory
Energies 2019, 12(17), 3381; https://doi.org/10.3390/en12173381 - 02 Sep 2019
Viewed by 222
Abstract
Based on fractal geometry theory, the Hagen–Poiseuille law, and the Langmuir adsorption law, this paper established a mathematical model of gas flow in nano-pores of shale, and deduced a new shale apparent permeability model. This model considers such flow mechanisms as pore size [...] Read more.
Based on fractal geometry theory, the Hagen–Poiseuille law, and the Langmuir adsorption law, this paper established a mathematical model of gas flow in nano-pores of shale, and deduced a new shale apparent permeability model. This model considers such flow mechanisms as pore size distribution, tortuosity, slippage effect, Knudsen diffusion, and surface extension of shale matrix. This model is closely related to the pore structure and size parameters of shale, and can better reflect the distribution characteristics of nano-pores in shale. The correctness of the model is verified by comparison with the classical experimental data. Finally, the influences of pressure, temperature, integral shape dimension of pore surface and tortuous fractal dimension on apparent permeability, slip flow, Knudsen diffusion and surface diffusion of shale gas transport mechanism on shale gas transport capacity are analyzed, and gas transport behaviors and rules in multi-scale shale pores are revealed. The proposed model is conducive to a more profound and clear understanding of the flow mechanism of shale gas nanopores. Full article
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Open AccessArticle
Resilience-Oriented Optimal Operation Strategy of Active Distribution Network
Energies 2019, 12(17), 3380; https://doi.org/10.3390/en12173380 - 02 Sep 2019
Viewed by 214
Abstract
The ability to withstand extreme disasters has a profound impact on the distribution network operation. This paper proposes a novel optimal operation strategy for an active distribution network to enhance system resilience.. The objectives in the proposed optimal strategy include, the resilience, operation [...] Read more.
The ability to withstand extreme disasters has a profound impact on the distribution network operation. This paper proposes a novel optimal operation strategy for an active distribution network to enhance system resilience.. The objectives in the proposed optimal strategy include, the resilience, operation cost, and its pollutant emissions. According to the existence of uncontrollable distributed energy resources in the active distribution network, the problem which takes the uncertainty most into account, is this multi-objective optimization problem. Thus, it can be treated as a min-max dual robust optimization problem. Benders decomposition is employed to decouple the problem, then non-dominated sorting genetic algorithm II is applied to search the multi-objective optimal solution which has an extremely low CPU time. The modified standard IEEE 34-node system, with different distributed energy resources types, is employed, as a studied case, to demonstrate the effectiveness of the proposed optimal operation strategy. The simulation results illustrate that, compared to other economic-oriented robust optimal operation models, the proposed strategy can enhance system resilience without a significant increase in the operation cost and pollutant emissions. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessArticle
The Impact of Jumps and Leverage in Forecasting the Co-Volatility of Oil and Gold Futures
Energies 2019, 12(17), 3379; https://doi.org/10.3390/en12173379 - 02 Sep 2019
Viewed by 203
Abstract
This paper investigates the impact of jumps in forecasting co-volatility in the presence of leverage effects for daily crude oil and gold futures. We use a modified version of the jump-robust covariance estimator of Koike (2016), such that the estimated matrix is positive [...] Read more.
This paper investigates the impact of jumps in forecasting co-volatility in the presence of leverage effects for daily crude oil and gold futures. We use a modified version of the jump-robust covariance estimator of Koike (2016), such that the estimated matrix is positive definite. Using this approach, we can disentangle the estimates of the integrated co-volatility matrix and jump variations from the quadratic covariation matrix. Empirical results show that more than 80% of the co-volatility of the two futures contains jump variations and that they have significant impacts on future co-volatility but that the impact is negligible in forecasting weekly and monthly horizons. Full article
(This article belongs to the Special Issue Multivariate Modelling of Fossil Fuel and Carbon Emission Prices)
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Open AccessArticle
Modeling and Validation of an Electrohydraulic Power Take-Off System for a Portable Wave Energy Convertor with Compressed Energy Storage
Energies 2019, 12(17), 3378; https://doi.org/10.3390/en12173378 - 02 Sep 2019
Viewed by 248
Abstract
Small-scale, portable generation of electricity from ocean waves provides a versatile solution to power the ocean sensors network, in addition to the traditional large-scale wave energy conversion facilities. However, one issue of small-scale wave energy convertor (WEC) is the low capturable power density, [...] Read more.
Small-scale, portable generation of electricity from ocean waves provides a versatile solution to power the ocean sensors network, in addition to the traditional large-scale wave energy conversion facilities. However, one issue of small-scale wave energy convertor (WEC) is the low capturable power density, challenging the design of the efficient power take-off (PTO) system. To tackle this challenge, in this paper, an electrohydraulic PTO system with compressed energy storage was proposed to boost output power of a portable WEC. Lumped-parameter kinematics and dynamics of the four-bar mechanism, the fluid dynamics of the digital fluid power circuit, and the mechanical and volumetric power losses were modeled and experimentally validated. Initial test results of the 0.64 m2 footprint prototype showed that the inclusion of storage improved the averaged electric power output over 40 times compared to the traditional architecture, and the proposed device can deliver up to 122 W at peaks. Full article
(This article belongs to the Special Issue Wave Energy Conversion)
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Open AccessArticle
Improving the Performance of a Dual Kalman Filter for the Identification of PEM Fuel Cells in Impedance Spectroscopy Experiments
Energies 2019, 12(17), 3377; https://doi.org/10.3390/en12173377 - 02 Sep 2019
Viewed by 272
Abstract
In this paper, the Dual Kalman Filter (DKF) is used for the parametric identification of an RC model of a Polymer Electrolyte Membrane Fuel Cell (FC) stack. The identification is performed for diagnostic purposes, starting from time-domain voltage and current signals in the [...] Read more.
In this paper, the Dual Kalman Filter (DKF) is used for the parametric identification of an RC model of a Polymer Electrolyte Membrane Fuel Cell (FC) stack. The identification is performed for diagnostic purposes, starting from time-domain voltage and current signals in the framework of Electrochemical Impedance Spectroscopy (EIS) tests. Here, the sinusoidal input of the tests makes the identification of DKF parameters challenging. The paper analyzes the filter performance and proposes a possible approach to address the filter tuning to let it work with FC operating either in normal conditions or in the presence of drying and flooding fault conditions, or in fuel starvation mode. The analysis is mainly performed in a simulated environment, where the Fouquet model is used to simulate the FC. Some criteria to tune the filter are derived from the analysis and used also with experimental data produced by some EIS tests, to achieve the best estimate in constrained conditions. The results show that the DKF can be turned into a valuable tool to identify the model parameters even with signals developed for other scopes. The identification results envisage the possibility of assisting the model-based FC diagnosis by means of a very simple tool that can run on a low-cost embedded device. Indeed, the simplicity of the filter approach and a lightweight implementation allow the deployment of the algorithm in embedded solutions. Full article
(This article belongs to the Special Issue Fuel Cell Systems Design and Control)
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Open AccessArticle
Optimization of Energy and Resource Efficiency in a Multistage Drying Process of Phosphate Pellets
Energies 2019, 12(17), 3376; https://doi.org/10.3390/en12173376 - 02 Sep 2019
Viewed by 267
Abstract
The roasting of phosphate pellets is an energy-intensive process the optimization of which can give rise to considerable efficiency improvements. To this purpose, the mathematical modeling and the computer-aided simulation of a multi-stage process of phosphate pellets roasting have been developed. This process [...] Read more.
The roasting of phosphate pellets is an energy-intensive process the optimization of which can give rise to considerable efficiency improvements. To this purpose, the mathematical modeling and the computer-aided simulation of a multi-stage process of phosphate pellets roasting have been developed. This process includes drying, dissociation reaction of carbonates and a sintering process in a moving, dense, multilayer mass of phosphate pellets in a special horizontal grate apparatus. A theoretical model for the physical-chemical processes of a mass of phosphate pellets moving in a multistage process and subject to external operation variables (i.e., temperatures and flowrates of the drying gas at different positions of the conveyor length) has been developed. By letting these control variables vary over suitable ranges limited by technical constraints, it is possible to construct an algorithm that minimizes the overall energy consumption subject to the attainment of the required quality indicators of the pellets. Therefore, the set of constraints includes bounds on both the operational variables and quality indicators of the final product. A dynamic programming algorithm subject to suitable penalty functions for avoiding constraints violation has been used. The results obtained show that the scientifically based operation described in this paper achieves the goal of attaining considerable energy savings while assuring the quality of the finished pellets. Furthermore, the results highlight the convenience of extending the general strategy developed in this article to other chemical energy engineering processes. Full article
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Open AccessArticle
A Modified One-Cycle Control for Vienna Rectifiers with Functionality of Input Power Factor Regulation and Input Current Distortion Mitigation
Energies 2019, 12(17), 3375; https://doi.org/10.3390/en12173375 - 02 Sep 2019
Viewed by 272
Abstract
In this paper, aiming at incorporating reactive power compensation functionality into the Vienna rectifiers, a modified one-cycle-control (MOCC) strategy is proposed by which the three-phase Vienna rectifier can be regulated in leading, lagging or unity power factors with near-sinusoidal input current waveform. First, [...] Read more.
In this paper, aiming at incorporating reactive power compensation functionality into the Vienna rectifiers, a modified one-cycle-control (MOCC) strategy is proposed by which the three-phase Vienna rectifier can be regulated in leading, lagging or unity power factors with near-sinusoidal input current waveform. First, a brief review of the working principle of the conventional OCC (COCC) strategy is conducted. Then, the MOCC strategy with the functionality of input current phase-shift control is discussed in detail. To mitigate input current distortion caused by the current phase-shift, a method whereby the signal of one phase current which is flowing in an uncontrollable region is injected into the other two phases’ current command signals is further presented. The constraints to the implementation of the MOCC scheme and the reactive power compensation capacity of the rectifier under MOCC control are analyzed as well. The proposed MOCC strategy is as easy to implement as the COCC strategy. Moreover, the MOCC strategy also preserves all other advantages of the COCC strategy, such as no phase-locked loop, no frame transformation and constant switching frequency. Finally, the theoretical analysis of the proposed MOCC strategy is fully verified by simulation and experimental results from a 1 kV·A three-phase Vienna rectifier prototype. Full article
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