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Energies, Volume 11, Issue 5 (May 2018)

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Cover Story (view full-size image) Lithium-ion battery manufacturing is a multi-step process involving synthesis, mixing, casting, [...] Read more.
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Open AccessFeature PaperArticle Development of Heat Transfer Surface Area Enhancements: A Test Facility for New Heat Exchanger Designs
Energies 2018, 11(5), 1322; https://doi.org/10.3390/en11051322
Received: 3 May 2018 / Revised: 15 May 2018 / Accepted: 19 May 2018 / Published: 22 May 2018
Cited by 1 | PDF Full-text (5007 KB) | HTML Full-text | XML Full-text
Abstract
Enhancing the heat transfer surface by usage of cellular metal structures, such as foams or wire structures, might allow enlarging the surface area, increasing the heat transfer coefficients, decreasing the material utilization, and enabling the flexibility of different geometrical dimensions. However their manufacturing
[...] Read more.
Enhancing the heat transfer surface by usage of cellular metal structures, such as foams or wire structures, might allow enlarging the surface area, increasing the heat transfer coefficients, decreasing the material utilization, and enabling the flexibility of different geometrical dimensions. However their manufacturing and assembling in a large heat exchanger for performance testing and optimizing can be costly. Therefore a test rig was constructed for experimental characterization of heat transfer surface area enhancements. Heat exchanger samples with dimensions in the centimeter range can be measured. The fluid flow and heat transfer features of a micro pin fin wire structure made from copper by soft-soldering were experimentally characterized under steady-state forced air convection. The results are compared to performance characteristics of louvered fins. Heat transfer coefficients of the pin fins are twice as high as for the louvered fins. The relative expanded uncertainty of the Nusselt number is ±7%. Full article
(This article belongs to the Section Energy Fundamentals and Conversion)
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Open AccessArticle Reconstruction of Hydraulic Fractures Using Passive Ultrasonic Travel-Time Tomography
Energies 2018, 11(5), 1321; https://doi.org/10.3390/en11051321
Received: 30 March 2018 / Revised: 13 May 2018 / Accepted: 14 May 2018 / Published: 22 May 2018
Cited by 1 | PDF Full-text (8245 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
The knowledge of hydraulic fracture morphology is significant for the analysis of fracture mechanisms. This paper utilizes passive Ultrasonic Travel-time Tomography (UTT) to characterize the hydraulic fracture. We constructed a velocity model based on X-ray computerized tomography (X-CT) images scanned on a real
[...] Read more.
The knowledge of hydraulic fracture morphology is significant for the analysis of fracture mechanisms. This paper utilizes passive Ultrasonic Travel-time Tomography (UTT) to characterize the hydraulic fracture. We constructed a velocity model based on X-ray computerized tomography (X-CT) images scanned on a real hydraulically fractured shale column. Then, ray-paths and travel times corresponding to the source-receiver configuration were calculated by curved ray-tracing schemes. Lastly, we performed tomographic inversions using total variation regularization (TVR). The simulation results showed that 3D passive UTT based on TVR is an accurate, efficient, and stable method to reconstruct the velocity structures with fractures, even in the case of sparse ray-coverage or high noise level. Meanwhile, we also verified that the passive UTT is a valid alternative to X-CT in depicting the hydraulic fracturing rock via a proper interpretation method. Full article
(This article belongs to the Special Issue Unconventional Natural Gas (UNG) Recoveries 2018)
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Open AccessArticle Conditional Maximum Likelihood of Three-Phase Phasor Estimation for μPMU in Active Distribution Networks
Energies 2018, 11(5), 1320; https://doi.org/10.3390/en11051320
Received: 17 April 2018 / Revised: 8 May 2018 / Accepted: 14 May 2018 / Published: 22 May 2018
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Abstract
Micro phasor measurement units (μPMU) installed in active distribution networks are very useful for improving observability by acquiring system real-time data. However, three-phase imbalance and harmonic power flows adversely impact the accuracy of synchronous measurements, which implies the importance of phasor estimation errors.
[...] Read more.
Micro phasor measurement units (μPMU) installed in active distribution networks are very useful for improving observability by acquiring system real-time data. However, three-phase imbalance and harmonic power flows adversely impact the accuracy of synchronous measurements, which implies the importance of phasor estimation errors. This paper proposes a new phasor estimation algorithm for μPMU in active distribution networks that uses a conditional maximum likelihood (CML) estimation method. Firstly, the signal model of three-phase, three-wire and four-wire imbalance systems is established. Then, the probability distributions of the magnitude and phase angles are derived from the geometric characteristics of the CML method by solving the geometric equation. Simulation results show that the proposed CML based method is effective for estimating phasor and impedance models of active distribution networks by using μPMU measurement data. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessArticle Breakdown Characteristics of Oil-Pressboard Insulation under AC-DC Combined Voltage and Its Mathematical Model
Energies 2018, 11(5), 1319; https://doi.org/10.3390/en11051319
Received: 3 May 2018 / Revised: 17 May 2018 / Accepted: 17 May 2018 / Published: 22 May 2018
Cited by 1 | PDF Full-text (4305 KB) | HTML Full-text | XML Full-text
Abstract
An AC-DC combined voltage is applied to the oil-pressboard insulation near the valve side during the operation of a converter transformer. To study the breakdown characteristics of an oil-pressboard insulation under such voltages, a typical plate electrode structure was employed in the laboratory
[...] Read more.
An AC-DC combined voltage is applied to the oil-pressboard insulation near the valve side during the operation of a converter transformer. To study the breakdown characteristics of an oil-pressboard insulation under such voltages, a typical plate electrode structure was employed in the laboratory to conduct a breakdown test on the oil-pressboard insulation. The electrical field distribution and the DC contents of the transformer oil and the pressboard in composite insulation under the AC-DC combined voltage were simulated by their dielectric parameters. The breakdown strength of the transformer oil decreases with the increase in the DC content of the applied voltage, whereas that of the pressboard increases. For the oil-pressboard insulation, the breakdown voltage increases first and then decreases. The electric field strength decreases in the transformer oil with the increase in the DC content, whereas it increases in the pressboard. And the DC contents of the transformer and the pressboard in composite insulation were different from that of the applied voltage. Finally, based on the above results, a mathematical model was proposed to describe the breakdown characteristics of the oil-pressboard insulation under the AC-DC combined voltage; the theoretical and experimental results were in good agreement. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessArticle Regulation of Voltage and Frequency in Solid Oxide Fuel Cell-Based Autonomous Microgrids Using the Whales Optimisation Algorithm
Energies 2018, 11(5), 1318; https://doi.org/10.3390/en11051318
Received: 18 April 2018 / Revised: 10 May 2018 / Accepted: 18 May 2018 / Published: 22 May 2018
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Abstract
This study explores the Whales Optimization Algorithm (WOA)-based PI controller for regulating the voltage and frequency of an inverter-based autonomous microgrid (MG). The MG comprises two 50 kW DGs (solid oxide fuel cells, SOFCs) interfaced using a power electronics-based voltage source inverter (VSI)
[...] Read more.
This study explores the Whales Optimization Algorithm (WOA)-based PI controller for regulating the voltage and frequency of an inverter-based autonomous microgrid (MG). The MG comprises two 50 kW DGs (solid oxide fuel cells, SOFCs) interfaced using a power electronics-based voltage source inverter (VSI) with a 120-kV conventional grid. Four PI controller schemes for the MG are implemented: (i) stationary PI controller with fixed gain values (Kp and Ki), (ii) PSO tuned PI controller, (iii) GWO tuned PI controller, and (iv) WOA tuned PI controller. The performance of these controllers is evaluated by monitoring the system voltage and frequency during the transition of MG operation mode and changes in the load. The MATLAB/SIMULINK tool is utilised to design the proposed model of grid-tied MG alongside the MATLAB m-file to apply an optimisation technique. The simulation results show that the WOA-based PI controller which optimises the control parameters, achieve 62.7% and 59% better results for voltage and frequency regulation, respectively. The eigenvalue analysis is also provided to check the stability of the proposed controller. Furthermore, the proposed system also satisfies the limits specified in IEEE-1547-2003 for voltage and frequency. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessEditorial Smart Green Applications: From Renewable Energy Management to Intelligent Transportation Systems
Energies 2018, 11(5), 1317; https://doi.org/10.3390/en11051317
Received: 4 May 2018 / Revised: 10 May 2018 / Accepted: 10 May 2018 / Published: 22 May 2018
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Open AccessArticle Power System Restoration Planning Strategy Based on Optimal Energizing Time of Sectionalizing Islands
Energies 2018, 11(5), 1316; https://doi.org/10.3390/en11051316
Received: 14 April 2018 / Revised: 11 May 2018 / Accepted: 14 May 2018 / Published: 22 May 2018
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Abstract
Common power system restoration planning strategy is based on a ‘build up’ approach, where a blackout system is sectionalized among several islands for parallel restoration prior to resynchronization. In order to speed up the resynchronization of the islands, each island must have similar
[...] Read more.
Common power system restoration planning strategy is based on a ‘build up’ approach, where a blackout system is sectionalized among several islands for parallel restoration prior to resynchronization. In order to speed up the resynchronization of the islands, each island must have similar energizing times. However, there is a huge number of possible combinations of islands that can be formed. Thus, this paper proposes a method to determine optimal islands that have similar energizing times. The method involves identifying transmission lines that should not be connected to form the islands. The proposed method is based on the combination of heuristic and discrete optimization methods. The heuristic technique is proposed to find initial solution that is close to the optimal solution. This solution will guide the optimization technique, which is the discrete Artificial Bee Colony optimization method, to find the optimum solution. The proposed method also considers restoration constraints including black start generator availability, load-generation balance, and the maintenance of acceptable voltage magnitude within each island. The proposed method is validated via simulation using IEEE 39, 118-bus and 89-bus European systems. The advantage of the proposed method in terms of restoration time is demonstrated through a comparison with other literature. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessArticle A Comprehensive Approach for Modelling Horizontal Diffuse Radiation, Direct Normal Irradiance and Total Tilted Solar Radiation Based on Global Radiation under Danish Climate Conditions
Energies 2018, 11(5), 1315; https://doi.org/10.3390/en11051315
Received: 17 April 2018 / Revised: 3 May 2018 / Accepted: 5 May 2018 / Published: 22 May 2018
Cited by 1 | PDF Full-text (4770 KB) | HTML Full-text | XML Full-text
Abstract
A novel combined solar heating plant with flat plate collectors (FPC) and parabolic trough collectors (PTC) was constructed and put into operation in Taars, 30 km north of Aalborg, Denmark in August 2015. To assess the thermal performance of the solar heating plant,
[...] Read more.
A novel combined solar heating plant with flat plate collectors (FPC) and parabolic trough collectors (PTC) was constructed and put into operation in Taars, 30 km north of Aalborg, Denmark in August 2015. To assess the thermal performance of the solar heating plant, global radiation, direct normal irradiance (DNI) and total radiation on the tilted collector plane of the flat plate collector field were measured. To determine the accuracy of the measurements, the calculated solar radiations, including horizontal diffuse radiation, DNI and total tilted solar radiation with seven empirical models, were compared each month based on an hourly time step. In addition, the split of measured global radiation into diffuse and beam radiation based on a model developed by DTU (Technical University of Denmark) and the Reduced Reindl correlation model was investigated. A new method of combining empirical models, only based on measured global radiation, was proposed for estimating hourly total radiation on tilted surfaces. The results showed that the DTU model could be used to calculate diffuse radiation on the horizontal surface, and that the anisotropic models (Perez I and Perez II) were the most accurate for calculation of total radiation on tilted collector surfaces based only on global radiation under Danish climate conditions. The proposed method was used to determine reliable horizontal diffuse radiation, DNI and total tilted radiation with only the measurement of global radiation. Only a small difference compared to measured data, was found. The proposed method was cost-effective and needed fewer measurements to obtain reliable DNI and total radiation on the tilted plane. This method may be extended to other Nordic areas that have similar weather. Full article
(This article belongs to the Special Issue Solar Thermal Energy Utilization Technologies in Buildings)
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Open AccessArticle Active-Reactive Additional Damping Control of a Doubly-Fed Induction Generator Based on Active Disturbance Rejection Control
Energies 2018, 11(5), 1314; https://doi.org/10.3390/en11051314
Received: 21 March 2018 / Revised: 21 April 2018 / Accepted: 17 May 2018 / Published: 21 May 2018
Cited by 1 | PDF Full-text (3997 KB) | HTML Full-text | XML Full-text
Abstract
Large-scale wind power interfacing to the power grid has an impact on the stability of the power system. However, with an additional damping controller of the wind generator, new ways for improving system damping and suppressing the low frequency oscillation (LFO) of power
[...] Read more.
Large-scale wind power interfacing to the power grid has an impact on the stability of the power system. However, with an additional damping controller of the wind generator, new ways for improving system damping and suppressing the low frequency oscillation (LFO) of power systems can be put forward. In this paper, an active-reactive power additional damping controller based on active disturbance rejection control (ADRC) is proposed. In order to improve the precision of the controller, the theory of data driven control is adopted, using the numerical algorithms for subspace state space system identification (N4SID) to obtain the two order model of the ADRC controlled object. Based on the identification model, the ADRC additional damping controller is designed. Taking a 2-area 4-machine system containing the doubly fed induction generator (DFIG) wind farm as an example, it is verified that the active-reactive additional damping controller designed in this paper performs well in suppressing negative-damping LFO and forced power oscillation. When the operation state of the power system changes, it can still restrain the LFO effectively, showing stronger robustness and better effectiveness compared to the traditional proportional–integral–derivative (PID) additional damping controller. Full article
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Open AccessArticle Islanding Detection Method Based on Injecting Perturbation Signal and Rate of Change of Output Power in DC Grid-Connected Photovoltaic System
Energies 2018, 11(5), 1313; https://doi.org/10.3390/en11051313
Received: 19 April 2018 / Revised: 11 May 2018 / Accepted: 18 May 2018 / Published: 21 May 2018
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Abstract
The emergence of Distributed Generation (DG) in the electric system has brought about the appearance of the islanding phenomenon. In AC networks, there are a lot of Islanding Detection Methods (IDMs) have been studied. However, not too much IDMs in DC networks have
[...] Read more.
The emergence of Distributed Generation (DG) in the electric system has brought about the appearance of the islanding phenomenon. In AC networks, there are a lot of Islanding Detection Methods (IDMs) have been studied. However, not too much IDMs in DC networks have been published because of the absence of frequency and reactive power. The hybrid IDM based on injected perturbation signal and rate of change of power output is proposed. This IDM can detect islanding condition not only in the worst case (the power of load and PV are equal) but also in another case (the power of load is greater than the power of PV). It can be applicable to both single and multi-PV operation scenarios. Besides, the effectiveness of the proposed method is verified by simulation in Matlab/Simulink. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessArticle Explorative Multidimensional Analysis for Energy Efficiency: DataViz versus Clustering Algorithms
Energies 2018, 11(5), 1312; https://doi.org/10.3390/en11051312
Received: 10 April 2018 / Revised: 8 May 2018 / Accepted: 10 May 2018 / Published: 21 May 2018
Cited by 1 | PDF Full-text (3687 KB) | HTML Full-text | XML Full-text
Abstract
We propose a simple tool to help the energy management of a large building stock defining clusters of buildings with the same function, setting alert thresholds for each cluster, and easily recognizing outliers. The objective is to enable a building management system to
[...] Read more.
We propose a simple tool to help the energy management of a large building stock defining clusters of buildings with the same function, setting alert thresholds for each cluster, and easily recognizing outliers. The objective is to enable a building management system to be used for detection of abnormal energy use. We start reviewing energy performance indicators, and how they feed into data visualization (DataViz) tools for a large building stock, especially for university campuses. After a brief presentation of the University of Turin’s building stock which represents our case study, we perform an explorative analysis based on the Multidimensional Detective approach by Inselberg, using the Scatter Plot Matrix and the Parallel Coordinates methods. The k-means clustering algorithm is then applied on the same dataset to test the hypotheses made during the explorative analysis. Our results show that DataViz techniques provide quick and user-friendly solutions for the energy management of a large stock of buildings. In particular, they help identifying clusters of buildings and outliers and setting alert thresholds for various Energy Efficiency Indices. Full article
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Open AccessArticle Improved Synchronous Machine Rotor Design for the Easy Assembly of Excitation Coils Based on Surrogate Optimization
Energies 2018, 11(5), 1311; https://doi.org/10.3390/en11051311
Received: 10 April 2018 / Revised: 4 May 2018 / Accepted: 16 May 2018 / Published: 21 May 2018
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Abstract
This paper introduces a new rotor design for the easy insertion and removal of rotor windings. The shape of the rotor is optimized based on a surrogate method in order to achieve low power loss under the maximum power output. The synchronous machine
[...] Read more.
This paper introduces a new rotor design for the easy insertion and removal of rotor windings. The shape of the rotor is optimized based on a surrogate method in order to achieve low power loss under the maximum power output. The synchronous machine with the new rotor is evaluated in 2-D finite element software and validated by experiments. This rotor shows great potential for reducing the maintenance and repair costs of synchronous machines, making it particularly suited for low-cost mass production markets including gen-sets, steam turbines, wind power generators, and hybrid electric vehicles. Full article
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Open AccessArticle Research on the Robustness of the Constant Speed Control of Hydraulic Energy Storage Generation
Energies 2018, 11(5), 1310; https://doi.org/10.3390/en11051310
Received: 24 April 2018 / Revised: 17 May 2018 / Accepted: 17 May 2018 / Published: 21 May 2018
PDF Full-text (2842 KB) | HTML Full-text | XML Full-text
Abstract
Energy storage plays a major role in solving the fluctuation and intermittence problem of wind and the effective use of wind power. The application of the hydraulic accumulator is the most efficient and convenient way to store wind energy in hydraulic wind turbines.
[...] Read more.
Energy storage plays a major role in solving the fluctuation and intermittence problem of wind and the effective use of wind power. The application of the hydraulic accumulator is the most efficient and convenient way to store wind energy in hydraulic wind turbines. A hydraulic energy storage generation system (HESGS) can transform hydraulic energy stored in the hydraulic accumulator into stable and constant electrical energy by controlling the variable motor, regardless of wind changes. The aim of the present study is to design a constant speed control method for the variable motor in the HESGS and investigate the influence of the controller’s main parameters on the resistance of the HESGS to external load power disturbances. Mathematical equations of all components in this system are introduced and an entire system simulation model is built. A double closed-loop control method of the variable motor is presented within this paper, which keeps the motor speed constant for the fixed frequency of electrical power generated by the HESGS. Ultimately, a series of simulations with different proportional gains and integral gains under the environment of changeless load power step are conducted. At the same time, comparison analyses of the experiment and simulation under variable load power step are performed. The results verify the correctness and the usability of the simulation model, and also indicate that the proposed control method is robust to the disturbances of changing load power. Full article
(This article belongs to the Section Energy Storage and Application)
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Open AccessArticle Methods for Advanced Wind Turbine Condition Monitoring and Early Diagnosis: A Literature Review
Energies 2018, 11(5), 1309; https://doi.org/10.3390/en11051309
Received: 23 April 2018 / Revised: 10 May 2018 / Accepted: 12 May 2018 / Published: 21 May 2018
Cited by 3 | PDF Full-text (1747 KB) | HTML Full-text | XML Full-text
Abstract
Condition monitoring and early fault diagnosis for wind turbines have become essential industry practice as they help improve wind farm reliability, overall performance and productivity. If not detected and rectified at early stages, some faults can be catastrophic with significant loss or revenue
[...] Read more.
Condition monitoring and early fault diagnosis for wind turbines have become essential industry practice as they help improve wind farm reliability, overall performance and productivity. If not detected and rectified at early stages, some faults can be catastrophic with significant loss or revenue along with interruption to the business relying mainly on wind energy. The failure of Wind turbine results in system downtime and repairing or replacement expenses that significantly reduce the annual income. Such failures call for more systematized operation and maintenance schemes to ensure the reliability of wind energy conversion systems. Condition monitoring and fault diagnosis systems of wind turbine play an important role in reducing maintenance and operational costs and increase system reliability. This paper is aimed at providing the reader with the overall feature for wind turbine condition monitoring and fault diagnosis which includes various potential fault types and locations along with the signals to be analyzed with different signal processing methods. Full article
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Open AccessArticle Hybrid Coupled Multifracture and Multicontinuum Models for Shale Gas Simulation by Use of Semi-Analytical Approach
Energies 2018, 11(5), 1308; https://doi.org/10.3390/en11051308
Received: 26 April 2018 / Revised: 5 May 2018 / Accepted: 14 May 2018 / Published: 21 May 2018
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Abstract
Combining the advantages of multicontinuum and multifracture representations provides an easy-to-use tool to adequately capture the characteristic of the multiscaled fracture system in shale gas reservoir. A hybrid model is established on the basis of simplified conceptual productivity assumption, where the matrix volume
[...] Read more.
Combining the advantages of multicontinuum and multifracture representations provides an easy-to-use tool to adequately capture the characteristic of the multiscaled fracture system in shale gas reservoir. A hybrid model is established on the basis of simplified conceptual productivity assumption, where the matrix volume is divided into two sub-domains (triple-porosity model and dual-depletion flowing model) and the fracture volume is represented by discrete finite conductivity fracture. In addition, the mechanisms of instant desorption, viscous flow and dual-depletion in matrix are taken into account. The rate transient responses are then obtained by use of semi-analytical approach. Based on the model, type curves are plotted and verified by comparing with alternative reliable methods. Different flow regimes in shale gas reservoirs can be identified and detected. The Generalized Likelihood Uncertainty Estimation methodology, based on probabilistic aggregation theory, is employed to integrating those two productivity models together such that the production can be predicted more accurately. A field example is applied to validate the applicability of this new model. Finally, it is concluded that the proposed model can predict the rate and cumulative rate more easily and practically. Full article
(This article belongs to the Section Energy Sources)
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Open AccessArticle ACEnet: Approximate Thinning-Based Judicious Network Control for Energy-Efficient Ultra-Dense Networks
Energies 2018, 11(5), 1307; https://doi.org/10.3390/en11051307
Received: 14 April 2018 / Revised: 14 May 2018 / Accepted: 17 May 2018 / Published: 21 May 2018
PDF Full-text (1037 KB) | HTML Full-text | XML Full-text
Abstract
This study considers a ultra-dense network (UDN) in which the enormous number of base stations (BSs) are densely deployed to support the massive amount of data traffic generated by many mobile devices. In this paper, we propose an approximate thinning-based judicious network control
[...] Read more.
This study considers a ultra-dense network (UDN) in which the enormous number of base stations (BSs) are densely deployed to support the massive amount of data traffic generated by many mobile devices. In this paper, we propose an approximate thinning-based judicious network control algorithm for energy-efficient UDNs (ACEnet) to improve the area throughput while diminishing the network energy consumption. The main idea of the proposed ACEnet algorithm is to judiciously adjust the modes of the BSs according to active-user density based on the thinning operation in stochastic geometry framework. The stochastic geometry framework is exploited to analyze the performance of the proposed algorithm, which includes the signal-to-interference-plus-noise ratio (SINR), average achievable rate of users, area throughput, and energy efficiency. Through intensive simulations, it shows that the proposed algorithm outperforms conventional algorithms. We also demonstrate that the analytical results are well matched with the simulation results. Full article
(This article belongs to the Section Energy Storage and Application)
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Open AccessArticle Risk Assessment and Reduction for an Innovative Subsurface Well Completion System
Energies 2018, 11(5), 1306; https://doi.org/10.3390/en11051306
Received: 4 April 2018 / Revised: 3 May 2018 / Accepted: 14 May 2018 / Published: 20 May 2018
PDF Full-text (5714 KB) | HTML Full-text | XML Full-text
Abstract
In recent years, many oil and gas fields have been discovered in ultra-deep sea (UDS). Some of these fields are evaluated to have no commercial value if existing oil field development approaches are used, especially while the oil prices remain low. A new
[...] Read more.
In recent years, many oil and gas fields have been discovered in ultra-deep sea (UDS). Some of these fields are evaluated to have no commercial value if existing oil field development approaches are used, especially while the oil prices remain low. A new alternative field development solution, termed as Subsurface Well Completion (SWC) system, is proposed with the aim to produce oil and gas in a cost-effective manner in UDS. This system primarily consists of four parts: a tethered subsurface platform, the rigid riser, SWC equipment and flexible jumper. Obviously, central to the evaluation and application of the new SWC technology is the inherent risk relative to acceptance level. In particular, an uncontrolled release of hydrocarbons to sea, which may lead to catastrophical consequences involving personnel risk, environmental damage and economic losses, is a main contributor to the total risk and of great concern to the offshore petroleum industry. As for the new SWC system, any failure will not be a direct source of risk for the personnel on the surface installation due to its offset feature. In this context, this paper proposes a quantitative risk assessment (QRA) framework to assess such uncontrolled releases to sea with regard to the SWC system for an oil field in the production phase based on the new Subsurface Tension Leg Production (STLP) facility. According to the QRA results presented in this paper, the identified scenarios representing uncontrolled releases to sea are subsea wellhead leaks, rigid riser leaks, subsurface wellhead leaks, releases from X-mas tree and flexible jumper leaks. Among these scenarios, subsea wellhead is found to be the high-risk area. Compared with the established risk acceptance criteria (RAC), the environmental risk levels for the subsea wellhead’s leak lie within the As Low As Reasonably Practicable (ALARP) region while other risks are all below ALARP limits, which means that there is a need for improved consideration of the existing design with regard to the subsea wellhead area, and the corresponding risk reduction measures are proposed. Furthermore, the sources and effects of uncertainties are reviewed and sensitivity studies are carried out to illustrate the effect of some of the important assumptions in the risk model. It can be found that some assumptions made are conservative or optimistic while others are unknown. However, the final QRA results can be regarded as somewhat conservative. This paper concludes that the new SWC technology has a distinct advantage with respect to the leakage duration time in UDS, and thus mitigates the environmental and commercial impacts to a large extent. Besides, relaxed design requirements for the X-mas tree and flexible jumper can be accepted. It is also concluded that there are no serious and major commercial losses for all the identified accidental release scenarios, which is of great importance and attractiveness to oil producers. Full article
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Open AccessArticle Comparative Analysis of Bearing Current in Wind Turbine Generators
Energies 2018, 11(5), 1305; https://doi.org/10.3390/en11051305
Received: 3 April 2018 / Revised: 16 May 2018 / Accepted: 16 May 2018 / Published: 20 May 2018
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Abstract
Bearing current problems frequently appear in wind turbine systems, which cause wind turbines the break down and result in very large losses. This paper investigates and compares bearing current problems in three kinds of wind turbine generators, namely doubly-fed induction generator (DFIG), direct-drive
[...] Read more.
Bearing current problems frequently appear in wind turbine systems, which cause wind turbines the break down and result in very large losses. This paper investigates and compares bearing current problems in three kinds of wind turbine generators, namely doubly-fed induction generator (DFIG), direct-drive permanent magnet synchronous generator (PMSG), and semi-direct-drive PMSG turbines. Common mode voltage (CMV) of converters is introduced firstly. Then stray capacitances of three kinds of generators are calculated and compared through the finite element method. The bearing current equivalent circuits are proposed and simulations of the bearing current are carried out. It is verified that the bearing currents of DFIGs are more serious than the two kinds of PMSG, while common mode current (CMC) of the direct-drive PMSG is much greater than the other two types of wind turbine generators. Full article
(This article belongs to the Special Issue Modeling of Wind Turbines and Wind Farms)
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Open AccessArticle Experimental Validation of Peer-to-Peer Distributed Voltage Control System
Energies 2018, 11(5), 1304; https://doi.org/10.3390/en11051304
Received: 18 April 2018 / Revised: 8 May 2018 / Accepted: 16 May 2018 / Published: 20 May 2018
Cited by 1 | PDF Full-text (3123 KB) | HTML Full-text | XML Full-text
Abstract
This paper presents experimental validation of a distributed optimization-based voltage control system. The dual-decomposition method is used in this paper to solve the voltage optimization problem in a fully distributed way. Device-to-device communication is implemented to enable peer-to-peer data exchange between agents of
[...] Read more.
This paper presents experimental validation of a distributed optimization-based voltage control system. The dual-decomposition method is used in this paper to solve the voltage optimization problem in a fully distributed way. Device-to-device communication is implemented to enable peer-to-peer data exchange between agents of the proposed voltage control system. The paper presents the design, development and hardware setup of a laboratory-based testbed used to validate the performance of the proposed dual-decomposition-based peer-to-peer voltage control. The architecture of the setup consists of four layers: microgrid, control, communication, and monitoring. The key question motivating this research was whether distributed voltage control systems are a technically effective alternative to centralized ones. The results discussed in this paper show that distributed voltage control systems can indeed provide satisfactory regulation of the voltage profiles. Full article
(This article belongs to the Special Issue Methods and Concepts for Designing and Validating Smart Grid Systems)
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Open AccessArticle A New Perspective on Improving Hospital Energy Administration Based on Recurrence Interval Analysis
Energies 2018, 11(5), 1303; https://doi.org/10.3390/en11051303
Received: 14 April 2018 / Revised: 6 May 2018 / Accepted: 16 May 2018 / Published: 20 May 2018
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Abstract
Based on 15-min high-frequency power load data from a Chinese hospital, by adopting recurrence interval analysis, an attempt is made to provide a new perspective for improving hospital energy administration in electrical efficiency and safety. Initially, the definition of extreme fluctuation of the
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Based on 15-min high-frequency power load data from a Chinese hospital, by adopting recurrence interval analysis, an attempt is made to provide a new perspective for improving hospital energy administration in electrical efficiency and safety. Initially, the definition of extreme fluctuation of the power load, as well as the recurrence interval, is given. Next, the stretched exponential distribution function is provided, which fits quite well with the probability density distribution of recurrence intervals. Then, tests on recurrence intervals, including scaling behavior and short-term and long-term memory effect are conducted. At last, a risk estimation method of VaR is proposed for hospital energy administrator to forecast risk probability. Results clearly indicate that the recurrence interval analysis (RIA) method works well on forecasting extreme power load fluctuation in hospital. However, there is no evidence to support the existence of the long-term memory effect of recurrence intervals, which means that hospital energy management plans have to be continuously fixed and updated with time. Some relevant applicant suggestions are provided for the energy administrator at the end of this paper. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessFeature PaperArticle Plasma-Assisted Biomass Gasification with Focus on Carbon Conversion and Reaction Kinetics Compared to Thermal Gasification
Energies 2018, 11(5), 1302; https://doi.org/10.3390/en11051302
Received: 5 April 2018 / Revised: 15 May 2018 / Accepted: 15 May 2018 / Published: 20 May 2018
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Abstract
Compared to conventional allothermal gasification of solid fuels (e.g., biomass, charcoal, lignite, etc.), plasma-assisted gasification offers an efficient method for applying energy to the gasification process to increase the flexibility of operation conditions and to increase the reaction kinetics. In particular, non-thermal plasmas
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Compared to conventional allothermal gasification of solid fuels (e.g., biomass, charcoal, lignite, etc.), plasma-assisted gasification offers an efficient method for applying energy to the gasification process to increase the flexibility of operation conditions and to increase the reaction kinetics. In particular, non-thermal plasmas (NTP) are promising, in which thermal equilibrium is not reached and electrons have a substantially higher mean energy than gas molecules. Thus, it is generally assumed that in NTP the supplied energy is utilized more efficiently for generating free radicals initiating gasification reactions than thermal plasma processes. In order to investigate this hypothesis, we compared purely thermal to non-thermal plasma-assisted gasification of biomass in steam in a drop tube reactor at atmospheric pressure. The NTP was provided by means of gliding arcs between two electrodes aligned in the inlet steam flow with an electric power of about 1 kW. Reaction yields and rates were evaluated using measured gas temperatures by the optical technique. The first experimental results show that the non-thermal plasma not only promotes the carbon conversion of the fuel particles, but also accelerates the reaction kinetics. The carbon conversion is increased by nearly 10% using wood powder as the fuel. With charcoal powder, more than 3% are converted into syngas. Full article
(This article belongs to the Special Issue Electric Fields in Energy & Process Engineering)
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Open AccessArticle Pretreatment of Corn Stover Using Organosolv with Hydrogen Peroxide for Effective Enzymatic Saccharification
Energies 2018, 11(5), 1301; https://doi.org/10.3390/en11051301
Received: 21 April 2018 / Revised: 17 May 2018 / Accepted: 18 May 2018 / Published: 20 May 2018
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Abstract
In this study, the chemical pretreatment method using ethanol organosolv with hydrogen peroxide was investigated to improve enzymatic saccharification of corn stover. The pretreatment method using ethanol with hydrogen peroxide in a flow-through reaction was proposed to lower the reaction severity such as
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In this study, the chemical pretreatment method using ethanol organosolv with hydrogen peroxide was investigated to improve enzymatic saccharification of corn stover. The pretreatment method using ethanol with hydrogen peroxide in a flow-through reaction was proposed to lower the reaction severity such as the pretreatment temperature. With the same reaction time, the pretreatment process using organosolv (30 wt.% ethanol) containing 1 wt.% hydrogen peroxide at 150 °C resulted in a similar conversion yield as the result of the alkali pretreatment method using 15 wt.% aqueous ammonia at 170 °C. When corn stover was pretreated with 30 wt.% ethanol solution containing 5 wt.% hydrogen peroxide, a glucose conversion yield of 69.7 wt.% and glucose production of 23.8 g were achieved. Full article
(This article belongs to the Section Sustainable Energy)
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Open AccessArticle Wind Turbine Power Curve Upgrades
Energies 2018, 11(5), 1300; https://doi.org/10.3390/en11051300
Received: 20 April 2018 / Revised: 11 May 2018 / Accepted: 16 May 2018 / Published: 19 May 2018
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Abstract
Full-scale wind turbine is a mature technology and therefore several retrofitting techniques have recently been spreading in the industry to further improve the efficiency of wind kinetic energy conversion. This kind of interventions is costly and, furthermore, the energy improvement is commonly estimated
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Full-scale wind turbine is a mature technology and therefore several retrofitting techniques have recently been spreading in the industry to further improve the efficiency of wind kinetic energy conversion. This kind of interventions is costly and, furthermore, the energy improvement is commonly estimated under the hypothesis of ideal wind conditions, but real ones can be very different because of wake interactions and/or wind shear induced by the terrain. A precise quantification of the energy gained in real environment is therefore precious. Wind turbines are subjected to non-stationary conditions and therefore it makes little sense to compare energy production before and after an upgrade: the post-upgrade production should rather be compared to a model of the pre-upgrade production under the same conditions. Since the energy improvement is typically of the order of few percents, a very precise model of wind turbine power output is needed and therefore it should be data-driven. Furthermore, the formulation of the model is heavily affected by the features of the available data set and by the nature of the problem. The objective of this work is the discussion of some wind turbine power curve upgrades on the grounds of operational data analysis. The selected test cases are: improved start-up through pitch angle adjustment near the cut-in, aerodynamic blade retrofitting by means of vortex generators and passive flow control devices, and extension of the power curve through a soft cut-out strategy for very high wind speed. The criticality of each test case is discussed and appropriate data-driven models are formulated. These are employed to estimate the energy improvement from each of the upgrades under investigation. The general outcome of this work is a catalog of generalizable methods for studying wind turbine power curve upgrades. In particular, from the study of the selected test cases, it arises that complex wind conditions might affect wind turbine operation such that the production improvement is non-negligibly different from what can be estimated under the hypothesis of ideal wind conditions. A complex wind flow might actually impact on the efficiency of vortex generators and the soft cut-out strategies at high wind speeds. The general lesson is therefore that it is very important to estimate wind turbine upgrades on real environments through operational data. Full article
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Open AccessArticle Decomposing Air Pollutant Emissions in Asia: Determinants and Projections
Energies 2018, 11(5), 1299; https://doi.org/10.3390/en11051299
Received: 26 April 2018 / Revised: 17 May 2018 / Accepted: 17 May 2018 / Published: 19 May 2018
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Abstract
High levels of air pollution pose an urgent social and public health challenge in many Asian regions. This study evaluates the role of key factors that determined the changes in emission levels in China, India and Japan over the past 25 years. While
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High levels of air pollution pose an urgent social and public health challenge in many Asian regions. This study evaluates the role of key factors that determined the changes in emission levels in China, India and Japan over the past 25 years. While emissions of air pollutants have been declining in Japan since the 1990s, China and India have experienced a rapid growth in pollution levels in recent years. Around 2005, control measures for sulfur emissions started to deliver expected reductions in China, followed by cuts in nitrogen oxides ten years later. Despite recent policy interventions, growing emission trends in India persist. A decomposition analysis of emission-driving factors indicates that emission levels would have been at least two-times higher without the improvements in energy intensity and efficiency, combined with end-of-pipe measures. Due to the continuous reliance on fossil fuels, the abatement effect of a cleaner fuel mix was in most cases significantly smaller than other factors. A reassessment of emission projections developed in the past suggests a decisive impact of energy and environmental policies. It is expected that targeted legislative instruments will play a dominant role in achieving future air-quality goals in Asia. Full article
(This article belongs to the Special Issue Energy and Environment)
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Open AccessArticle The Impact of Injector Hole Diameter on Spray Behaviour for Butanol-Diesel Blends
Energies 2018, 11(5), 1298; https://doi.org/10.3390/en11051298
Received: 6 April 2018 / Revised: 9 May 2018 / Accepted: 10 May 2018 / Published: 19 May 2018
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Abstract
Optimising the combustion process in compression ignition (CI) engines is of interest in current research as a potential means to reduce fuel consumption and emission levels. Combustion optimisation can be achieved as a result of understanding the relationship between spraying technique and combustion
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Optimising the combustion process in compression ignition (CI) engines is of interest in current research as a potential means to reduce fuel consumption and emission levels. Combustion optimisation can be achieved as a result of understanding the relationship between spraying technique and combustion characteristics. Understanding macroscopic characteristics of spray is an important step in predicting combustion behaviour. This study investigates the impact of injector hole diameter on macroscopic spray characteristics (spray penetration, spray cone angle, and spray volume) of butanol-diesel blends. In the current study, a Bosch (0.18 mm diameter) and a Delphi (0.198 mm) injector were used. Spray tests were carried out in a constant volume vessel (CVV) under different injection conditions. The test blends were injected using a solenoid injector with a common rail injection system and images captured using a high-speed camera. The experimental results showed that the spray penetration (S) was increased with larger hole diameter. Spray penetration of a 20% butanol-80% diesel blend was slightly further than that of neat diesel. Spray penetration of all test fuels was increased as a result of increased injection pressure (IP), while spray cone angle (θ) was slightly widened due to the increase in either hole diameter or injection pressure. Spray volume of all test fuels was increased as a result of increased hole diameter or injection pressure. Thus, an efficient diesel engine performance can be achieved as a result of controlling injection characteristics, especially when using a promising additive like butanol blended with diesel. Full article
(This article belongs to the Section Energy Sources)
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Open AccessArticle An Overall Bubble Diameter Model for the Flow Boiling and Numerical Analysis through Global Information Searching
Energies 2018, 11(5), 1297; https://doi.org/10.3390/en11051297
Received: 17 April 2018 / Revised: 8 May 2018 / Accepted: 14 May 2018 / Published: 19 May 2018
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Abstract
Bubble diameter is important for the three-dimensional nucleate boiling two-phase flow simulation. However, the bubble diameter is rarely considered as a variable because it is difficult to estimate. In this paper, a novel bubble diameter model is proposed for the boiling flow. The
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Bubble diameter is important for the three-dimensional nucleate boiling two-phase flow simulation. However, the bubble diameter is rarely considered as a variable because it is difficult to estimate. In this paper, a novel bubble diameter model is proposed for the boiling flow. The heat transfer growth, the breakup or coalescence of collision and the liquid impacting separation are considered as the factors affecting the bubble diameter, and three bubble diameter sub-models are developed to calculate the overall bubble diameter based on a departure diameter. In the model, the heat transfer growth is calculated after estimating the bubble location. The collision variation is deduced from the interfacial area concentration equation. The liquid impacting variation is calculated through phase interaction force and gas viscous force. The proposed model is conducted through global information searching in the flow boiling simulation and validated using the void fraction and temperature data from the literature. The effects of the heat transfer growth, the collision, and the separating volume on the bubble diameter are discussed. Full article
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Open AccessArticle Parameterized Disturbance Observer Based Controller to Reduce Cyclic Loads of Wind Turbine
Energies 2018, 11(5), 1296; https://doi.org/10.3390/en11051296
Received: 30 March 2018 / Revised: 2 May 2018 / Accepted: 8 May 2018 / Published: 19 May 2018
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Abstract
This paper is concerned with bump-less transfer of parameterized disturbance observer based controller with individual pitch control strategy to reduce cyclic loads of wind turbine in full load operation. Cyclic loads are generated due to wind shear and tower shadow effects. Multivariable disturbance
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This paper is concerned with bump-less transfer of parameterized disturbance observer based controller with individual pitch control strategy to reduce cyclic loads of wind turbine in full load operation. Cyclic loads are generated due to wind shear and tower shadow effects. Multivariable disturbance observer based linear controllers are designed with objective to reduce output power fluctuation, tower oscillation and drive-train torsion using optimal control theory. Linear parameterized controllers are designed by using a smooth scheduling mechanism between the controllers. The proposed parameterized controller with individual pitch was tested on nonlinear Fatigue, Aerodynamics, Structures, and Turbulence (FAST) code model of National Renewable Energy Laboratory (NREL)’s 5 MW wind turbine. The closed-loop system performance was assessed by comparing the simulation results of proposed controller with a fixed gain and parameterized controller with collective pitch for full load operation of wind turbine. Simulations are performed with step wind to see the behavior of the system with wind shear and tower shadow effects. Then, turbulent wind is applied to see the smooth transition of the controllers. It can be concluded from the results that the proposed parameterized control shows smooth transition from one controller to another controller. Moreover, 3p and 6p harmonics are well mitigated as compared to fixed gain DOBC and parameterized DOBC with collective pitch. Full article
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Open AccessArticle Wind–PV-Based Hybrid DC Microgrid (DCMG) Development: An Experimental Investigation and Comparative Economic Analysis
Energies 2018, 11(5), 1295; https://doi.org/10.3390/en11051295
Received: 10 April 2018 / Revised: 30 April 2018 / Accepted: 10 May 2018 / Published: 18 May 2018
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Abstract
The cyclical nature and high investment costs of the wind and photovoltaic renewable energy sources are the two critical issues seeking attention for the use of such systems in backup or isolated applications. This paper aims to present the experimental and economic analysis
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The cyclical nature and high investment costs of the wind and photovoltaic renewable energy sources are the two critical issues seeking attention for the use of such systems in backup or isolated applications. This paper aims to present the experimental and economic analysis of a wind–photovoltaic-based hybrid direct current microgrid (DCMG) system for backup power and off-grid isolated power generation system for emergency purposes. The two distributed generating units comprising photovoltaic panels and wind generator were designed and developed for the experimental study. A lead-acid battery is also added as an energy storage system to enhance the system supply. The electric load of this system comprise of 42 DC light emitting diode (LED) lamps of 12 Watt each and a 25 Watt DC fan. The charge controller provides the control and protection features for the designed system. The complete system design and fabrication of this system have been undertaken at Mehran University of Engineering & Technology (MUET, Jamshoro, Pakistan). The compatibility of the designed system has been analysed by comparing the Levelized Cost of Energy (LCOE) with a conventional gasoline generator system of the same capacity. The capital, running and lifetime costs of DCMG are found to be 1.29, 0.15 and 0.29 times those of the gasoline generator, respectively. Moreover, it is found that per unit cost of gasoline generator is $0.3 (i.e., PKR 31.4) which is almost 3.4 times higher than that of the hybrid DCMG system. The performance and cost evaluation of the designed system indicate its broad potential to be adopted for commercialisation to meet backup power and off-grid power requirements. This study concludes that proposed DCMG system is a not only low cost, but also a pollution-free alternative option compared to the existing traditional small gasoline generator system. Full article
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Open AccessArticle Hybrid Electric Powertrain with Fuel Cells for a Series Vehicle
Energies 2018, 11(5), 1294; https://doi.org/10.3390/en11051294
Received: 18 April 2018 / Revised: 11 May 2018 / Accepted: 15 May 2018 / Published: 18 May 2018
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Abstract
Recent environmental and climate change issues make it imperative to persistently approach research into the development of technologies designed to ensure the sustainability of global mobility. At the European Union level, the transport sector is responsible for approximately 28% of greenhouse gas emissions,
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Recent environmental and climate change issues make it imperative to persistently approach research into the development of technologies designed to ensure the sustainability of global mobility. At the European Union level, the transport sector is responsible for approximately 28% of greenhouse gas emissions, and 84% of them are associated with road transport. One of the most effective ways to enhance the de-carbonization process of the transport sector is through the promotion of electric propulsion, which involves overcoming barriers related to reduced driving autonomy and the long time required to recharge the batteries. This paper develops and implements a method meant to increase the autonomy and reduce the battery charging time of an electric car to comparable levels of an internal combustion engine vehicle. By doing so, the cost of such vehicles is the only remaining significant barrier in the way of a mass spread of electric propulsion. The chosen method is to hybridize the electric powertrain by using an additional source of fuel; hydrogen gas stored in pressurized cylinders is converted, in situ, into electrical energy by means of a proton exchange membrane fuel cell. The power generated on board can then be used, under the command of a dedicated management system, for battery charging, leading to an increase in the vehicle’s autonomy. Modeling and simulation results served to easily adjust the size of the fuel cell hybrid electric powertrain. After optimization, an actual fuel cell was built and implemented on a vehicle that used the body of a Jeep Wrangler, from which the thermal engine, associated subassemblies, and gearbox were removed. Once completed, the vehicle was tested in traffic conditions and its functional performance was established. Full article
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Open AccessArticle Experimental Investigation of the Loading Strategy of an Automotive Diesel Engine under Transient Operation Conditions
Energies 2018, 11(5), 1293; https://doi.org/10.3390/en11051293
Received: 13 March 2018 / Revised: 4 May 2018 / Accepted: 10 May 2018 / Published: 18 May 2018
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Abstract
Targeting the performance optimization of an automotive diesel engine under transient operation conditions, in this research, the effect of several non-linear loading strategies on diesel performance have been experimentally analyzed using a heavy-duty turbocharged diesel engine running under transient conditions based on the
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Targeting the performance optimization of an automotive diesel engine under transient operation conditions, in this research, the effect of several non-linear loading strategies on diesel performance have been experimentally analyzed using a heavy-duty turbocharged diesel engine running under transient conditions based on the constant 1650 r/min speed, the load is increased from 10% to 100% in a 5 s transition time The results show that the larger the early loading rate and change point load, the better the dynamic torque response. The peak values of smoke and CO and the transient average of brake specific fuel consumption (BSFC), soot and CO can be decreased by increasing the early loading rate by the loading strategies with the appropriate change point load during transient operation. However, combustion deteriorates under the loading process with an overlarge change point load, causing emissions to increase, and the larger the early loading rate, the worse the worsening. Based on the trade-off consisting of torque dynamic response, transient total and transient average of the BSFC and brake specific emissions, peak values of smoke and CO emissions, it is concluded that the loading strategy with the early loading rate is the 50% load per second and the change point load in the 25% load is the most suitable in these strategies. Full article
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