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Energies, Volume 12, Issue 12 (June-2 2019)

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Open AccessArticle
Predictive-Fixed Switching Current Control Strategy Applied to Six-Phase Induction Machine
Energies 2019, 12(12), 2294; https://doi.org/10.3390/en12122294 (registering DOI)
Received: 27 April 2019 / Revised: 20 May 2019 / Accepted: 30 May 2019 / Published: 15 June 2019
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Abstract
In applications such as multiphase motor drives, classical predictive control strategies are characterized by a variable switching frequency which adds high harmonic content and ripple in the stator currents. This paper proposes a model predictive current control adding a modulation stage based on [...] Read more.
In applications such as multiphase motor drives, classical predictive control strategies are characterized by a variable switching frequency which adds high harmonic content and ripple in the stator currents. This paper proposes a model predictive current control adding a modulation stage based on a switching pattern with the aim of generating a fixed switching frequency. Hence, the proposed controller takes into account the prediction of the two adjacent active vectors and null vector in the ( α - β ) frame defined by space vector modulation in order to reduce the (x-y) currents according to a defined cost function at each sampling period. Both simulation and experimental tests for a six-phase induction motor drive are provided and compared to the classical predictive control to validate the feasibility of the proposed control strategy. Full article
(This article belongs to the Special Issue Control of Multiphase Machines and Drives)
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Open AccessArticle
Sizing and Siting of Distributed Generators and Energy Storage in a Microgrid Considering Plug-in Electric Vehicles
Energies 2019, 12(12), 2293; https://doi.org/10.3390/en12122293 (registering DOI)
Received: 23 May 2019 / Revised: 13 June 2019 / Accepted: 13 June 2019 / Published: 15 June 2019
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Abstract
This paper presents a sizing and siting model for distributed generators (DGs) and energy storage systems (ESS) towards the design of a cost-efficient and reliable microgrid considering electric vehicles (EVs). The proposed model exploits the coordinated energy dispatching of DGs, ESS, and EVs, [...] Read more.
This paper presents a sizing and siting model for distributed generators (DGs) and energy storage systems (ESS) towards the design of a cost-efficient and reliable microgrid considering electric vehicles (EVs). The proposed model exploits the coordinated energy dispatching of DGs, ESS, and EVs, aiming at minimizing the overall planning and operating cost as well as meeting power supply reliability requirements. This issue is addressed in a two-stage framework. The upper stage determines the sizes and sites of candidate DGs and ESS, and the lower stage optimizes the microgrid’s economic power dispatch. Since the two-stage model contains both planning and operational variables, a two-stage iterative heuristic algorithm is designed. The effectiveness of the proposed approach is validated by case studies, and corresponding results demonstrate that the planning approach that considers coordinated management of an EV fleet and economic power dispatch of microgrid achieves better economics. In addition, the suggested approach can also better match distributed generation and power demands as well as securing microgrid power supply. Full article
(This article belongs to the Special Issue Planning and Operation of Distributed Energy Resources in Smart Grids)
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Open AccessArticle
Improvement of Combustion Process of Spark-Ignited Aviation Wankel Engine
Energies 2019, 12(12), 2292; https://doi.org/10.3390/en12122292 (registering DOI)
Received: 15 May 2019 / Revised: 8 June 2019 / Accepted: 13 June 2019 / Published: 15 June 2019
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Abstract
This paper deals with the creation of modern high-performance aircraft power units based on the Wankel rotary piston engine. One of the main problems of Wankel engines is high specific fuel consumption. This paper solves the problem of improving the efficiency of this [...] Read more.
This paper deals with the creation of modern high-performance aircraft power units based on the Wankel rotary piston engine. One of the main problems of Wankel engines is high specific fuel consumption. This paper solves the problem of improving the efficiency of this type of engine. The mathematical model of non-stationary processes of transfer of momentum, energy, mass, and the concentration of reacting substances in the estimated volume provides for the determination of local gas parameters in the entire computational region, which are presented as a sum of averaged and pulsation components. The k-ζ-f model is used as the turbulence model; the combustion is described by the coherent flame model (CFM) based on the concept of laminar flame propagation. As a result of the calculation, we obtained the values of temperature, pressure, and velocity of the working fluid in the working chamber cross-sections of a rotary–piston engine. Various options of the rotor recess shape are considered. Based on the data obtained, the rotor design was improved. The offered shape of the rotor recess has reduced emissions of both nitrogen oxides and carbon dioxide. Full article
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Open AccessArticle
Real-Time Energy Management of a Microgrid Using Deep Reinforcement Learning
Energies 2019, 12(12), 2291; https://doi.org/10.3390/en12122291 (registering DOI)
Received: 14 May 2019 / Revised: 11 June 2019 / Accepted: 12 June 2019 / Published: 15 June 2019
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Abstract
Driven by the recent advances and applications of smart-grid technologies, our electric power grid is undergoing radical modernization. Microgrid (MG) plays an important role in the course of modernization by providing a flexible way to integrate distributed renewable energy resources (RES) into the [...] Read more.
Driven by the recent advances and applications of smart-grid technologies, our electric power grid is undergoing radical modernization. Microgrid (MG) plays an important role in the course of modernization by providing a flexible way to integrate distributed renewable energy resources (RES) into the power grid. However, distributed RES, such as solar and wind, can be highly intermittent and stochastic. These uncertain resources combined with load demand result in random variations in both the supply and the demand sides, which make it difficult to effectively operate a MG. Focusing on this problem, this paper proposed a novel energy management approach for real-time scheduling of an MG considering the uncertainty of the load demand, renewable energy, and electricity price. Unlike the conventional model-based approaches requiring a predictor to estimate the uncertainty, the proposed solution is learning-based and does not require an explicit model of the uncertainty. Specifically, the MG energy management is modeled as a Markov Decision Process (MDP) with an objective of minimizing the daily operating cost. A deep reinforcement learning (DRL) approach is developed to solve the MDP. In the DRL approach, a deep feedforward neural network is designed to approximate the optimal action-value function, and the deep Q-network (DQN) algorithm is used to train the neural network. The proposed approach takes the state of the MG as inputs, and outputs directly the real-time generation schedules. Finally, using real power-grid data from the California Independent System Operator (CAISO), case studies are carried out to demonstrate the effectiveness of the proposed approach. Full article
(This article belongs to the Section Smart Grids and Microgrids)
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Open AccessArticle
Active Charge Equalizer of Li-Ion Battery Cells Using Double Energy Carriers
Energies 2019, 12(12), 2290; https://doi.org/10.3390/en12122290 (registering DOI)
Received: 1 May 2019 / Revised: 11 June 2019 / Accepted: 12 June 2019 / Published: 15 June 2019
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Abstract
In this work, a new active balancing circuit is proposed. This circuit consists of a cell-access network and an energy-transfer network. The cell-access network requires 2n + 6 switches, where n is the number of cells, and creates an energy-transfer path between [...] Read more.
In this work, a new active balancing circuit is proposed. This circuit consists of a cell-access network and an energy-transfer network. The cell-access network requires 2n + 6 switches, where n is the number of cells, and creates an energy-transfer path between unbalanced cells and the energy-transfer network. The energy-transfer network has double energy carriers and simultaneously implements cell-to-pack and pack-to-cell balancing operations without overlapping. As a result, a high power rate and fast balancing operation can be achieved by using two energy carriers in a single balancing circuit. The prototype of a proposed balancing circuit was built for six cells and then tested under various conditions; all cells in the state of charge (SOC) region of 70% to 80% were equalized after 93 min, and one charging/discharging period in the SOC region of 10% to 90% was increased by 8.58% compared to the non-balancing operation. These results show that the proposed circuit is a good way to balance charges among batteries in a battery pack. Full article
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Open AccessArticle
Analysis and Control of Fault Ride-Through Capability Improvement for Wind Turbine Based on a Permanent Magnet Synchronous Generator Using an Interval Type-2 Fuzzy Logic System
Energies 2019, 12(12), 2289; https://doi.org/10.3390/en12122289 (registering DOI)
Received: 24 April 2019 / Revised: 22 May 2019 / Accepted: 11 June 2019 / Published: 15 June 2019
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Abstract
Recently, wind energy conversion systems in renewable energy sources have attracted attention due to their effective application. Wind turbine systems have a complex structure; however, traditional control systems are inadequate in answering the demands of complex systems. Therefore, expert control systems are applied [...] Read more.
Recently, wind energy conversion systems in renewable energy sources have attracted attention due to their effective application. Wind turbine systems have a complex structure; however, traditional control systems are inadequate in answering the demands of complex systems. Therefore, expert control systems are applied to wind turbines, such as type-1 and interval type-2 fuzzy logic control (IT-2 FLC) systems. An IT-2 FLC system is used to solve the complexity of the wind turbine system and increases the efficiency of the wind turbine. This paper proposes a new control approach using the IT-2 FLC method applied to a wind turbine based on a permanent magnet synchronous generator (PMSG) to improve the transient stability during grid faults. An IT-2 FLC was designed to enhance the fault ride-through performance of a wind turbine and was implemented to control the machine side converter and grid side converter of a wind turbine. The proposed algorithm performance of a wind turbine based on a PMSG was investigated for different types of grid fault. The analysis results verify that the interval type-2 fuzzy logic control system is robustly utilized under different operational conditions. Full article
(This article belongs to the Special Issue Fuel Cell Renewable Hybrid Power Systems)
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Open AccessArticle
Thermal Analysis of Busbars from a High Current Power Supply System
Energies 2019, 12(12), 2288; https://doi.org/10.3390/en12122288 (registering DOI)
Received: 8 May 2019 / Revised: 3 June 2019 / Accepted: 13 June 2019 / Published: 14 June 2019
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Abstract
Copper busbar technology is widely used with the aim to achieve electrical connections with power distribution systems because of their flexibility and compactness. The thermal analysis takes into account the heat conduction and convection of a copper busbar system used to supply a [...] Read more.
Copper busbar technology is widely used with the aim to achieve electrical connections with power distribution systems because of their flexibility and compactness. The thermal analysis takes into account the heat conduction and convection of a copper busbar system used to supply a test bench with high currents in order to check the electro-thermal behaviour of power circuit breakers during overload and short circuit conditions. This paper proposes a mathematical model for busbars used within a high current power supply. The obtained thermal model can be used to analyse the thermal behaviour of busbars in steady-state conditions at different values of the electric current, cross-section and length of the busbar. Also, the mathematical model allows to calculate the temperature distribution along the busbar at different values of the contact resistances at junction points with other conductors. There is a good correlation between calculated, simulated and experimental results. Full article
Open AccessFeature PaperArticle
Proposal of a Predictive Mixed Experimental- Numerical Approach for Assessing the Performance of Farm Tractor Engines Fuelled with Diesel- Biodiesel-Bioethanol Blends
Energies 2019, 12(12), 2287; https://doi.org/10.3390/en12122287 (registering DOI)
Received: 25 April 2019 / Revised: 4 June 2019 / Accepted: 10 June 2019 / Published: 14 June 2019
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Abstract
The effect of biofuel blends on the engine performance and emissions of agricultural machines can be extremely complex to predict even if the properties and the effects of the pure substances in the blends can be sourced from the literature. Indeed, on the [...] Read more.
The effect of biofuel blends on the engine performance and emissions of agricultural machines can be extremely complex to predict even if the properties and the effects of the pure substances in the blends can be sourced from the literature. Indeed, on the one hand, internal combustion engines (ICEs) have a high intrinsic operational complexity; on the other hand, biofuels show antithetic effects on engine performance and present positive or negative interactions that are difficult to determine a priori. This study applies the Response Surface Methodology (RSM), a numerical method typically applied in other disciplines (e.g., industrial engineering) and for other purposes (e.g., set-up of production machines), to analyse a large set of experimental data regarding the mechanical and environmental performances of an ICE used to power a farm tractor. The aim is twofold: i) to demonstrate the effectiveness of RSM in quantitatively assessing the effects of biofuels on a complex system like an ICE; ii) to supply easy-to-use correlations for the users to predict the effect of biofuel blends on performance and emissions of tractor engines. The methodology showed good prediction capabilities and yielded interesting outcomes. The effects of biofuel blends and physical fuel parameters were adopted to study the engine performance. Among all possible parameters depending on the fuel mixture, the viscosity of a fuel blend demonstrated a high statistical significance on some system responses directly related to the engine mechanical performances. This parameter can constitute an interesting indirect estimator of the mechanical performances of an engine fuelled with such blend, while it showed poor accuracy in predicting the emissions of the ICE (NOx, CO concentration and opacity of the exhaust gases) due to a higher influence of the chemical composition of the fuel blend on these parameters; rather, the blend composition showed a much higher accuracy in the assessment of the mechanical performance of the ICE. Full article
(This article belongs to the Special Issue Internal Combustion Engines 2018)
Open AccessArticle
Climate Classification for the Use of Solar Thermal Systems in East Asia
Energies 2019, 12(12), 2286; https://doi.org/10.3390/en12122286 (registering DOI)
Received: 25 April 2019 / Revised: 10 June 2019 / Accepted: 12 June 2019 / Published: 14 June 2019
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Abstract
To examine the effects of solar heat utilization, it is necessary to classify the solar-use climate in East Asia based on climatic factors that affect load reduction by solar heating systems. In this study, annual load simulations are performed on weather data in [...] Read more.
To examine the effects of solar heat utilization, it is necessary to classify the solar-use climate in East Asia based on climatic factors that affect load reduction by solar heating systems. In this study, annual load simulations are performed on weather data in 842 sites in Japan to analyze the climatic factors that impact the load reduction effect of the solar heating system. The analysis results were based on three climatic factors that strongly affect the heating load of a building: heating degree day (HDD18-18), daily solar global horizontal irradiance, and solar-heat available days. Furthermore, to distinguish the climate for the use of solar heat in East Asia, climate classification was done according to three climatic factors using the weather data of 1176 sites in East Asia. Consequently, the climate of solar thermal utilization in East Asia can be classified into 88 areas. Based on the regional classification for solar heat utilization proposed in this study, it will be possible to examine the effect of load reduction and applicability according to the use of solar heat in each region in the future. Full article
Open AccessArticle
Performance Analysis of Single-Phase Electrical Machine for Military Applications
Energies 2019, 12(12), 2285; https://doi.org/10.3390/en12122285 (registering DOI)
Received: 8 May 2019 / Revised: 20 May 2019 / Accepted: 27 May 2019 / Published: 14 June 2019
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Abstract
A permanent magnet assisted synchronous reluctance generator (PMA-SynRG) and an induction generator (IG) were compared for portable generator applications. PMA-SynRG with two rotor configurations, namely rotors with ferrite magnet and NdFeB, were designed. Furthermore, a design strategy for both PMA-SynRG and IG is [...] Read more.
A permanent magnet assisted synchronous reluctance generator (PMA-SynRG) and an induction generator (IG) were compared for portable generator applications. PMA-SynRG with two rotor configurations, namely rotors with ferrite magnet and NdFeB, were designed. Furthermore, a design strategy for both PMA-SynRG and IG is presented with their geometrical dimensions. The machine was designed and results were analyzed using finite element analysis. Results such as flux density, open circuit and full load voltages, torque in generating mode, weight comparison and detailed cost analysis were investigated. In addition, thermal analysis for various ambient conditions (−40 °C, +30 °C, +65 °C) was evaluated for both PMA-SynRG and IG. Furthermore, acoustic versus frequency plot and acoustic pressure level were investigated for both the generators. Finally, the results confirmed that the machine with a higher power-to-weight ratio was the right choice for military applications. Full article
(This article belongs to the Special Issue Electrical Machine Design)
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Open AccessArticle
Hybrid Model for Determining Dual String Gas Lift Split Factor in Oil Producers
Energies 2019, 12(12), 2284; https://doi.org/10.3390/en12122284 (registering DOI)
Received: 22 March 2019 / Revised: 15 April 2019 / Accepted: 15 April 2019 / Published: 14 June 2019
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Abstract
Upstream oil production using dual string completion, i.e., two tubing inside a well casing, is common due to its cost advantage. High pressure gas is employed to lift the oil to the surface when there is insufficient reservoir energy to overcome the liquids [...] Read more.
Upstream oil production using dual string completion, i.e., two tubing inside a well casing, is common due to its cost advantage. High pressure gas is employed to lift the oil to the surface when there is insufficient reservoir energy to overcome the liquids static head in the tubing. However, gas lifting for this type of completion can be complicated. This is due to the operating condition where total gas is injected into the common annulus and then allowed to be distributed among the two strings without any surface control. High uncertainties often result from the methods used to determine the split factor—the ratio between the gas lift rate to one string over the total gas injected. A hybrid model which combined three platforms: the Visual Basics for Application programme, PROSPER (a nodal analysis tool) and Excel spreadsheet, is proposed for the estimation of the split factor. The model takes into consideration two important parameters, i.e., the lift gas pressure gradient along the annulus and the multiphase pressure drop inside the tubing to estimate the gas lift rate to the individual string and subsequently the split factor. The proposed model is able to predict the split factor to within 2% to 7% accuracy from the field measured data. Accurate knowledge of the amount of gas injected into each string leads to a more efficient use of lift gas, improving the energy efficiency of the oil productions facilities and contributing toward the sustainability of fossil fuel. Full article
(This article belongs to the Special Issue Sustainability of Fossil Fuels)
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Open AccessArticle
Study on Selecting the Optimal Algorithm and the Effective Methodology to ANN-Based Short-Term Load Forecasting Model for the Southern Power Company in Vietnam
Energies 2019, 12(12), 2283; https://doi.org/10.3390/en12122283 (registering DOI)
Received: 11 March 2019 / Revised: 27 May 2019 / Accepted: 3 June 2019 / Published: 14 June 2019
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Abstract
Recently, power companies apply optimal algorithms for short-term load forecasting, especially the daily load. However, in Vietnam, the load forecasting of the power system has not focused on this solution. Optimal algorithms and can help experts improve forecasting results including accuracy and the [...] Read more.
Recently, power companies apply optimal algorithms for short-term load forecasting, especially the daily load. However, in Vietnam, the load forecasting of the power system has not focused on this solution. Optimal algorithms and can help experts improve forecasting results including accuracy and the time required for forecasting. To achieve both goals, the combinations of different algorithms are still being studied. This article describes research using a new combination of two optimal algorithms: Genetic Algorithm (GA) and Particle Swarm Optimization (PSO). This combination limits the weakness of the convergence speed of GA as well as the weakness of PSO that it easily falls into local optima (thereby reducing accuracy). This new hybrid algorithm was applied to the Southern Power Corporation’s (SPC—a large Power company in Vietnam) daily load forecasting. The results show the algorithm’s potential to provide a solution. The most accurate result was for the forecasting of a normal working day with an average error of 1.15% while the largest error was 3.74% and the smallest was 0.02%. For holidays and weekends, the average error always approximated the allowable limit of 3%. On the other hand, some poor results also provide an opportunity to re-check the real data provided by SPC. Full article
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Open AccessArticle
Investigation of the Relationship between Bacteria Growth and Lipid Production Cultivating of Microalgae Chlorella Vulgaris in Seafood Wastewater
Energies 2019, 12(12), 2282; https://doi.org/10.3390/en12122282 (registering DOI)
Received: 21 April 2019 / Revised: 4 May 2019 / Accepted: 10 May 2019 / Published: 14 June 2019
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Abstract
Algae biorefinery is gaining much attention for the sustainable production of value-added products (e.g., biofuels, protein supplements etc.) globally. The current study aimed to investigate the relationship between lipid production and bacteria growth by an initial microalgae Chlorella vulgaris density culture in seafood [...] Read more.
Algae biorefinery is gaining much attention for the sustainable production of value-added products (e.g., biofuels, protein supplements etc.) globally. The current study aimed to investigate the relationship between lipid production and bacteria growth by an initial microalgae Chlorella vulgaris density culture in seafood wastewater effluent (SWE). According to our results, the initial C. vulgaris concentration in SWE influenced lipid accumulation. The concentration ranged from 25–35 mg·L−1 which corresponds to SWE’s chemical oxygen demand concentration of 365.67 ± 3.45 mg·L−1. A higher microalgae growth rate and lipid content of 32.15 ± 1.45% was successfully attained. A higher lipid content, approximately double, was observed when compared to the control (16.8 ± 0.5%). Moreover, this study demonstrates that bacteria inhibited microalgae growth as the initial cell density stepped over 35 mg·L−1, which also affected lipid accumulation. This study shows an optimal lipid accumulation attained at moderate Chlorella vulgaris density culture in SWE. Hence, wastewater treatment incorporating microalgae culture could be greatly developed in the future to achieve a greener environment. Full article
(This article belongs to the Special Issue Algal Biotechnology)
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Open AccessArticle
Wind Power Economic Feasibility under Uncertainty and the Application of ANN in Sensitivity Analysis
Energies 2019, 12(12), 2281; https://doi.org/10.3390/en12122281 (registering DOI)
Received: 23 April 2019 / Revised: 16 May 2019 / Accepted: 21 May 2019 / Published: 14 June 2019
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Abstract
Wind power has grown popular in past recent years due to environmental issues and the search for alternative energy sources. Thus, the viability for wind power generation projects must be studied in order to attend to the environmental concerns and still be attractive [...] Read more.
Wind power has grown popular in past recent years due to environmental issues and the search for alternative energy sources. Thus, the viability for wind power generation projects must be studied in order to attend to the environmental concerns and still be attractive and profitable. Therefore, this article aims to perform a sensitive analysis in order to identify the variables that influence most in the viability of a wind power investment for small size companies in the Brazilian northeast. For this, a stochastic analysis of viability through Monte Carlo Simulation (MCS) will be made and afterwards, Artificial Neural Networks (ANN) models will be applied for the most relevant variables identification. Through the sensitivity, it appears that the most relevant factors in the analysis are the speed of wind, energy tariff and the investment amount. Thus, the viability of the investment is straightly tied to the region where the wind turbine is installed, and the government incentives may allow decreasing in the investment amount for wind power. Based on this, incentives programs for the production of clean energy include cheaper purchase of wind turbines, lower taxing and financing rates, can make wind power more profitable and attractive. Full article
(This article belongs to the Section Energy Economics and Policy)
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Open AccessArticle
An Investigation of the Effect of Fuel Supply Parameters on Combustion Process of the Heavy-Duty Dual-Fuel Diesel Ignited Gas Engine
Energies 2019, 12(12), 2280; https://doi.org/10.3390/en12122280 (registering DOI)
Received: 13 May 2019 / Revised: 3 June 2019 / Accepted: 10 June 2019 / Published: 14 June 2019
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Abstract
Modern research in the area of internal combustion engines is focused on researching and investigating the technologies that will improve fuel efficiency and decrease emissions. Application of dual-fuel engines is considered as a potential solution to these problems. In the dual-fuel engine, a [...] Read more.
Modern research in the area of internal combustion engines is focused on researching and investigating the technologies that will improve fuel efficiency and decrease emissions. Application of dual-fuel engines is considered as a potential solution to these problems. In the dual-fuel engine, a natural gas-air mixture is ignited by a small amount of the diesel fuel directly injected into a combustion chamber. Pilot fuel injection parameters can strongly effect the combustion process. The aim of this paper is to investigate the effect of such fuel-supply parameters as pilot fuel mass, pilot fuel injection pressure, pilot fuel injection timing and excess air ratio on the combustion process. Investigation is based on the data obtained during bench tests conducted with the use of measurement equipment. The dependences of engine characteristics from the fuel supply parameters under review were obtained based on the results of the experimental study. Optimal values for every investigated fuel-supply parameter were chosen based on the obtained results. Over the course of the investigation, the coefficient for heat release rate according to the Vibe equation was calculated for each operating point. Full article
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Open AccessArticle
Performance Analysis of a Small-Scale ORC Trigeneration System Powered by the Combustion of Olive Pomace
Energies 2019, 12(12), 2279; https://doi.org/10.3390/en12122279
Received: 24 April 2019 / Revised: 6 June 2019 / Accepted: 7 June 2019 / Published: 14 June 2019
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Abstract
The utilisation of low- and medium-temperature energy allows to reduce the energy shortage and environmental pollution problems because low-grade energy is plentiful in nature and renewable as well. In the past two decades, thanks to its feasibility and reliability, the organic Rankine cycle [...] Read more.
The utilisation of low- and medium-temperature energy allows to reduce the energy shortage and environmental pollution problems because low-grade energy is plentiful in nature and renewable as well. In the past two decades, thanks to its feasibility and reliability, the organic Rankine cycle (ORC) has received great attention. The present work is focused on a small-scale (7.5 kW nominal electric power) combined cooling, heating and power ORC system powered by the combustion of olive pomace obtained as a by-product in the olive oil production process from an olive farm situated in the central part of Italy. The analysis of the employment of this energy system is based on experimental data and Aspen Plus simulation, including biomass and combustion tests, biomass availability and energy production analysis, Combined Cooling Heat and Power (CCHP) system sizing and assessment. Different low environmental impact working fluids and various operative process parameters were investigated. Olive pomace has been demonstrated to be suitable for the energy application and, in this case, to be able to satisfy the energy consumption of the same olive farm with the option of responding to further energy users. Global electrical efficiency varied from 12.7% to 19.4%, depending on the organic fluid used and the working pressure at the steam generator. Full article
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Open AccessArticle
Analysis and Mitigation of Stray Capacitance Effects in Resistive High-Voltage Dividers
Energies 2019, 12(12), 2278; https://doi.org/10.3390/en12122278
Received: 15 May 2019 / Revised: 6 June 2019 / Accepted: 10 June 2019 / Published: 14 June 2019
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Abstract
This work analyzes the effects of the parasitic or stray distributed capacitance to ground in high-voltage environments and assesses the effectiveness of different corrective actions to minimize such effects. To this end, the stray capacitance of a 130 kV RMS high-voltage resistive divider [...] Read more.
This work analyzes the effects of the parasitic or stray distributed capacitance to ground in high-voltage environments and assesses the effectiveness of different corrective actions to minimize such effects. To this end, the stray capacitance of a 130 kV RMS high-voltage resistive divider is studied because it can severely influence the behavior of such devices when operating under alternating current or transient conditions. The stray capacitance is calculated by means of three-dimensional finite element analysis (FEA) simulations. Different laboratory experiments under direct current (DC) and alternating current (AC) supply are conducted to corroborate the theoretical findings, and different possibilities to mitigate stray capacitance effects are analyzed and discussed. The effects of the capacitance are important in applications, such as large electrical machines including transformers, motors, and generators or in high-voltage applications involving voltage dividers, conductors or insulator strings, among others. The paper also proves the usefulness of FEA simulations in predicting the stray capacitance, since they can deal with a wide range of configurations and allow determining the effectiveness of different corrective configurations. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessArticle
Model-Based Pre-Ignition Diagnostics in a Race Car Application
Energies 2019, 12(12), 2277; https://doi.org/10.3390/en12122277
Received: 12 May 2019 / Revised: 7 June 2019 / Accepted: 12 June 2019 / Published: 14 June 2019
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Abstract
Since 2014, Formula 1 engines have been turbocharged spark-ignited engines. In this scenario, the maximum engine power available in full-load conditions can be achieved only by optimizing combustion phasing within the cycle, i.e., by advancing the center of combustion until the limit established [...] Read more.
Since 2014, Formula 1 engines have been turbocharged spark-ignited engines. In this scenario, the maximum engine power available in full-load conditions can be achieved only by optimizing combustion phasing within the cycle, i.e., by advancing the center of combustion until the limit established by the occurrence of abnormal combustion. High in-cylinder pressure peaks and the possible occurrence of knocking combustion significantly increase the heat transfer to the walls and might generate hot spots inside the combustion chamber. This work presents a methodology suitable to properly diagnose and control the occurrence of pre-ignition events that emanate from hot spots. The methodology is based on a control-oriented model of the ignition delay, which is compared to the actual ignition delay calculated from the real-time processing of the in-cylinder pressure trace. When the measured ignition delay becomes significantly smaller than that modeled, it means that ignition has been activated by a hot spot instead of the spark plug. In this case, the presented approach, implemented in the electronic control unit (ECU) that manages the whole hybrid power unit, detects a pre-ignition event and corrects the injection pattern to avoid the occurrence of further abnormal combustion. Full article
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Open AccessArticle
A 3D Numerical Study of Supersonic Steam Dumping Process of the Pressurizer Relief Tank
Energies 2019, 12(12), 2276; https://doi.org/10.3390/en12122276
Received: 5 May 2019 / Revised: 11 June 2019 / Accepted: 12 June 2019 / Published: 14 June 2019
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Abstract
Simulating the steam dumping process of a pressurized relief tank is a challenging engineering problem, due to the massive computing resource requirements and its complex physical models. This study gave a comprehensive 3D numerical study for the transient dumping process from the PRT [...] Read more.
Simulating the steam dumping process of a pressurized relief tank is a challenging engineering problem, due to the massive computing resource requirements and its complex physical models. This study gave a comprehensive 3D numerical study for the transient dumping process from the PRT (Pressurizer Relief Tank) to the room containing the tank. The physical model, geometry design and meshing strategy, along with the numerical techniques, have been described in detail. Through parallel simulations based on the open source CFD toolbox OpenFOAM, numerical results for the temperature, pressure, and the velocity distribution are presented. The results show that the maximum velocity throughout the whole domain is 967 m/s over Mach 2 and the maximum pressure on the roof of the room is 2.8 atm. It could provide the guidance information for the safety design of the reactor coolant system. Additionally, comparison cases between OpenFOAM and CFX are tested, and it turns out that OpenFOAM could produce comparable accuracy with commercial CFD software and scale to much more computing cores in parallel simulations. Full article
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Open AccessArticle
Primal-Dual Learning Based Risk-Averse Optimal Integrated Allocation of Hybrid Energy Generation Plants under Uncertainty
Energies 2019, 12(12), 2275; https://doi.org/10.3390/en12122275
Received: 13 May 2019 / Revised: 3 June 2019 / Accepted: 4 June 2019 / Published: 13 June 2019
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Abstract
A groundswell of opinion in utilizing environmentally friendly energy technologies has been put forth worldwide. In this paper, we consider an energy generation plant distribution and allocation problem under uncertainty to get the utmost out of available developments, as well as to control [...] Read more.
A groundswell of opinion in utilizing environmentally friendly energy technologies has been put forth worldwide. In this paper, we consider an energy generation plant distribution and allocation problem under uncertainty to get the utmost out of available developments, as well as to control costs and greenhouse emissions. Different clean and traditional energy technologies are considered in this paper. In particular, we present a risk-averse stochastic mixed-integer linear programming (MILP) model to minimize the total expected costs and control the risk of CO2 emissions exceeding a certain budget. We employ the conditional value-at-risk (CVaR) model to represent risk preference and risk constraint of emissions. We prove that our risk-averse model can be equivalent to the traditional risk-neutral model under certain conditions. Moreover, we suggest that the risk-averse model can provide solutions generating less CO2 than traditional models. To handle the computational difficulty in uncertain scenarios, we propose a Lagrange primal-dual learning algorithm to solve the model. We show that the algorithm allows the probability distribution of uncertainty to be unknown, and that desirable approximation can be achieved by utilizing historical data. Finally, an experiment is presented to demonstrate the performance of our method. The risk-averse model encourages the expansion of clean energy plants over traditional models for the reduction CO2 emissions. Full article
Open AccessArticle
Noninferior Solution Grey Wolf Optimizer with an Independent Local Search Mechanism for Solving Economic Load Dispatch Problems
Energies 2019, 12(12), 2274; https://doi.org/10.3390/en12122274
Received: 5 May 2019 / Revised: 6 June 2019 / Accepted: 10 June 2019 / Published: 13 June 2019
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Abstract
The economic load dispatch (ELD) problem is a complex optimization problem in power systems. The main task for this optimization problem is to minimize the total fuel cost of generators while also meeting the conditional constraints of valve-point loading effects, prohibited operating zones, [...] Read more.
The economic load dispatch (ELD) problem is a complex optimization problem in power systems. The main task for this optimization problem is to minimize the total fuel cost of generators while also meeting the conditional constraints of valve-point loading effects, prohibited operating zones, and nonsmooth cost functions. In this paper, a novel grey wolf optimization (GWO), abbreviated as NGWO, is proposed to solve the ELD problem by introducing an independent local search strategy and a noninferior solution neighborhood independent local search technique to the original GWO algorithm to achieve the best problem solution. A local search strategy is added to the standard GWO algorithm in the NGWO, which is called GWOI, to search the local neighborhood of the global optimal point in depth and to guarantee a better candidate. In addition, a noninferior solution neighborhood independent local search method is introduced into the GWOI algorithm to find a better solution in the noninferior solution neighborhood and ensure the high probability of jumping out of the local optimum. The feasibility of the proposed NGWO method is verified on five different power systems, and it is compared with other selected methods in terms of the solution quality, convergence rate, and robustness. The compared experimental results indicate that the proposed NGWO method can efficiently solve ELD problems with higher-quality solutions. Full article
Open AccessArticle
Feedback Linearization and Reaching Law Based Sliding Mode Control Design for Nonlinear Hydraulic Turbine Governing System
Energies 2019, 12(12), 2273; https://doi.org/10.3390/en12122273
Received: 8 May 2019 / Revised: 8 June 2019 / Accepted: 12 June 2019 / Published: 13 June 2019
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Abstract
Hydropower as renewable energy has continually expanded at a relatively high rate in the last decade. This expansion calls for more accurate scheme design in hydraulic turbine governing system (HTGS) to ensure its high efficiency. Sliding mode control (SMC) as a robust control [...] Read more.
Hydropower as renewable energy has continually expanded at a relatively high rate in the last decade. This expansion calls for more accurate scheme design in hydraulic turbine governing system (HTGS) to ensure its high efficiency. Sliding mode control (SMC) as a robust control method which is insensitive to system uncertainties and disturbances raises interest in the application in HTGS. However, the feature of highly coupled state variables reflects the nonlinear essence of HTGS and SMC studies on the related mathematical model under certain fluctuations are not satisfied. In this regard, a novel SMC design with proportional-integral-derivative manifold is firstly applied to a nonlinear HTGS with a complex conduit system. In dealing with certain fluctuations in speed and load around the rated working condition, the proposed SMC is capable of driving the system to the desired state with smooth and light responses in aspects of the key state variables. The exponential reaching law and introduced boundary layer fasten the speed of converging time and suppress chattering. A necessary integral of sliding parameter added to manifold successfully reduces the latency caused by the anti-regulation feature of HTGS. Three operating scenarios are simulated compared with the PSO-PID method, and results imply that the proposed SMC method equips with accurate trajectory tracking ability and smooth responses. Finally, the strong robustness against system uncertainties is tested. Full article
(This article belongs to the Special Issue Modern Power System Dynamics, Stability and Control)
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Open AccessArticle
Real-Time Analysis of Time-Critical Messages in IEC 61850 Electrical Substation Communication Systems
Energies 2019, 12(12), 2272; https://doi.org/10.3390/en12122272
Received: 22 April 2019 / Revised: 28 May 2019 / Accepted: 8 June 2019 / Published: 13 June 2019
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Abstract
IEC 61850 is a standard for the design and operation of electrical Substation Automation Systems (SAS) that defines how data may be transferred among Intelligent Electronic Devices (IEDs). The defined data models can be mapped into application protocols, such as SV or GOOSE, [...] Read more.
IEC 61850 is a standard for the design and operation of electrical Substation Automation Systems (SAS) that defines how data may be transferred among Intelligent Electronic Devices (IEDs). The defined data models can be mapped into application protocols, such as SV or GOOSE, which may run upon high speed Ethernet LANs bridged by IEEE 802.1Q compliant switches. The communication system must cope with the timing requirements associated to protective relaying strategies. Given the time constrained nature of SAS applications, a thorough analysis of its timing behavior is required. In this paper, we propose an analytical model for the timing assessment of SV and GOOSE message exchanges in an IEC 61850 process bus. The proposed model allows the communication timing assessment, by analyzing the response time of each message stream of the SAS. This feature is an advantage for the expansion of the SAS, as it allows the evaluation at design time of the maximum number of IEDs that can be supported by the underlying communication system. The results from the proposed analytical model were validated for a typical IEC 61850 communication scenario, both through simulation and through an experimental assessment with IEC 61850 compliant equipment. Full article
(This article belongs to the Special Issue Real-time Communications for Smart Grids and Industry)
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Open AccessArticle
Classification of Renewable Sources of Electricity in the Context of Sustainable Development of the New EU Member States
Energies 2019, 12(12), 2271; https://doi.org/10.3390/en12122271
Received: 12 May 2019 / Revised: 28 May 2019 / Accepted: 10 June 2019 / Published: 13 June 2019
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Abstract
Climate change and awareness of the need to care for the environment have resulted in a global increase in the interest in renewable energy sources. The European Union (EU) is active in this respect and requires Member States to fulfill specific plans in [...] Read more.
Climate change and awareness of the need to care for the environment have resulted in a global increase in the interest in renewable energy sources. The European Union (EU) is active in this respect and requires Member States to fulfill specific plans in the transformation of their energy systems. We employed hierarchical cluster analysis in an attempt to distinguish those countries among the new EU Member States that increased their electrical capacity from renewable energy sources to the greatest extent while paying attention to their energy intensity. The analyses were conducted in two scenarios for both 2004 and 2016. The first scenario assumed an analysis of all known renewable energy sources, whereas in the second scenario, only renewable energy sources from wind and solar power plants were included. The division of analyses into these two variants showed the importance of the differences in the energy assessment of individual countries, depending on classification of renewable energy sources. We identified groups of countries where electrical capacity from renewable energy sources increased the most. Conducting analyses using two variants allowed distinguishing countries that based most of their renewable energy on modern renewable energy sources, such as solar and wind power plants. The inclusion of gross domestic product in the analyses allowed us to identify countries with the worst energy efficiency value. Full article
(This article belongs to the Special Issue Market Design for a High-Renewables Electricity System)
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Open AccessArticle
An Optimal Scheduling Method for Multi-Energy Hub Systems Using Game Theory
Energies 2019, 12(12), 2270; https://doi.org/10.3390/en12122270
Received: 22 May 2019 / Revised: 6 June 2019 / Accepted: 11 June 2019 / Published: 13 June 2019
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Abstract
The optimal scheduling of multi-energy hub systems plays an important role in the safety, stability, and economic operation of the system. However, due to the strong uncertainty of renewable energy access, serious coupling, and the interaction among energy hubs of multi-energy hub systems, [...] Read more.
The optimal scheduling of multi-energy hub systems plays an important role in the safety, stability, and economic operation of the system. However, due to the strong uncertainty of renewable energy access, serious coupling, and the interaction among energy hubs of multi-energy hub systems, it is difficult for the traditional optimal scheduling method to solve these problems. Therefore, game theory was used to solve the optimal scheduling problem of multi-energy hub systems. According to the internal connection mode and energy conversion relationship of energy hubs, along with the competitive and cooperative relationship between multi-energy hubs, the game theoretic optimal scheduling model of the multi-energy hub system was established. Then, two cases and 50 groups of wind speed series were used to test the robustness of the proposed method. Simulation results show that the total power injection is −16,805.8, 104.1847, and −865.561 and the natural gas injection is 46,046.81, 27,727.65, and 63,039.54 in spring/autumn, summer, and winter, respectively, which is consistent with the characteristics of the four seasons. Furthermore, the optimal scheduling method using game theory has a strong robustness in multi-energy hub systems. Full article
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Open AccessArticle
Numerical Simulation of Bubble-Liquid Two-Phase Turbulent Flows in Shallow Bioreactor
Energies 2019, 12(12), 2269; https://doi.org/10.3390/en12122269
Received: 5 May 2019 / Revised: 9 June 2019 / Accepted: 10 June 2019 / Published: 13 June 2019
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Abstract
An improved second-order moment bubble-liquid two-phase turbulent model is developed to predict the hydrodynamic characteristics of the shallow bioreactor using two height-to-diameter ratios of H/D = 1.4 and H/D = 2.9. The two-phase hydrodynamic parameters, the bubble normal and shear stress, the bubble [...] Read more.
An improved second-order moment bubble-liquid two-phase turbulent model is developed to predict the hydrodynamic characteristics of the shallow bioreactor using two height-to-diameter ratios of H/D = 1.4 and H/D = 2.9. The two-phase hydrodynamic parameters, the bubble normal and shear stress, the bubble energy dissipation rate, the bubble turbulent kinetic energy, etc. were numerically simulated. These parameters increased along with flow direction and constituted a threat to cells living at far distance away from the gas jetting inlet regions, rather than a finding of higher cell damage at near the jetting inlet region, as reported by Babosa et al. 2003. A new correlation named the turbulent energy production of bubble-liquid two-phase flow was proposed to successfully verify this experimental observation. A smaller H/D ratio makes more contributions to the generation of lower turbulent energy productions, which are in favor of the alleviation of cell damage. The extremely long and narrow shape of the bioreactor is deteriorative for cell living. Full article
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Open AccessArticle
Series-Series/Series Compensated Inductive Power Transmission System with Symmetrical Half-Bridge Resonant Converter: Design, Analysis, and Experimental Assessment
Energies 2019, 12(12), 2268; https://doi.org/10.3390/en12122268
Received: 24 April 2019 / Revised: 3 June 2019 / Accepted: 8 June 2019 / Published: 13 June 2019
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Abstract
In order to compensate the large leakage inductance and improve the power transmission capacity, capacitors are widely used in inductive power transfer (IPT) systems, which results in high voltage or current stresses in the resonant tanks and limits higher volt-ampere (VA) rating of [...] Read more.
In order to compensate the large leakage inductance and improve the power transmission capacity, capacitors are widely used in inductive power transfer (IPT) systems, which results in high voltage or current stresses in the resonant tanks and limits higher volt-ampere (VA) rating of the transfer power, especially in medium and low frequency applications. This paper presents a symmetrical half-bridge resonant converter (SHRC) for series-series/series compensated IPT systems with detailed analysis and design. It operates at a relatively low frequency of 12.5 kHz, suitable for IGBT applications. The theoretical analysis shows that, compared with full-bridge resonant converter (FRC) for IPT, the symmetrical half-bridge resonant converter achieves a higher efficiency. Simulation and a prototype of 1500 W power output were built to verify the theoretical analysis. The experimental results show that the power loss of SHRC is 39.7 W while that of FRC is 79.4 W, which is consistent with the theoretical analysis. The global efficiency of the IPT based on the proposed converter is 91.6%. Full article
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Open AccessArticle
A Comparative Analysis of House Owners in Need of Energy Efficiency Measures but with Different Intentions
Energies 2019, 12(12), 2267; https://doi.org/10.3390/en12122267
Received: 1 May 2019 / Revised: 2 June 2019 / Accepted: 8 June 2019 / Published: 13 June 2019
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Abstract
Existing private homes in Germany and throughout Europe often are in need of energy efficient refurbishment measures (EERMs). However, these EERMs are not realized on the required level in order to achieve environment-related political targets. Therefore we investigate, based on an online survey [...] Read more.
Existing private homes in Germany and throughout Europe often are in need of energy efficient refurbishment measures (EERMs). However, these EERMs are not realized on the required level in order to achieve environment-related political targets. Therefore we investigate, based on an online survey of 1085 German owner-occupiers, the factors that differentiate two groups of single- and two-family house owners in need of EERM. Using an extended version of the Theory of Planned Behavior as a research framework, the performed logistic regression analysis shows that e.g., behavioral beliefs are significant factors for differentiating “Future-Refurbishers” from “Non-Refurbishers”. Based on our results we suggest an enhancement of practice-orientated initiatives, e.g., refurbishment workshops or best-practice presentations. By presenting the aesthetic appearance of refurbished buildings or providing knowledge, other owner-occupiers could be motivated to engage in EERM. In addition to funding programs, initiatives like this can be used to increase the general energy efficiency of buildings and specifically of those in cities and urban districts, where a high share of the mentioned houses is located and greenhouse gas emissions are caused to a great extent. Full article
(This article belongs to the Special Issue Energy Efficient Cities of Today and Tomorrow)
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Open AccessArticle
Secure Load Frequency Control of Smart Grids under Deception Attack: A Piecewise Delay Approach
Energies 2019, 12(12), 2266; https://doi.org/10.3390/en12122266
Received: 14 May 2019 / Revised: 5 June 2019 / Accepted: 11 June 2019 / Published: 13 June 2019
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Abstract
The problem of secure load frequency control of smart grids is investigated in this paper. The networked data transmission within the smart grid is corrupted by stochastic deception attacks. First, a unified Load frequency control model is constructed to account for both network-induced [...] Read more.
The problem of secure load frequency control of smart grids is investigated in this paper. The networked data transmission within the smart grid is corrupted by stochastic deception attacks. First, a unified Load frequency control model is constructed to account for both network-induced effects and deception attacks. Second, with the Lyapunov functional method, a piecewise delay analysis is conducted to study the stability of the established model, which is of less conservativeness. Third, based on the stability analysis, a controller design method is provided in terms of linear matrix inequalities. Finally, a case study is carried out to demonstrate the derived results. Full article
(This article belongs to the Special Issue Advanced Control in Microgrid Systems)
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Open AccessArticle
Cost–Benefit Analysis of Rooftop PV Systems on Utilities and Ratepayers in Thailand
Energies 2019, 12(12), 2265; https://doi.org/10.3390/en12122265
Received: 6 May 2019 / Revised: 27 May 2019 / Accepted: 5 June 2019 / Published: 13 June 2019
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Abstract
Driven by falling photovoltaic (PV) installation costs and potential support policies, rooftop PV is expected to expand rapidly in Thailand. As a result, the relevant stakeholders, especially utilities, have concerns about the net economic impacts of high PV adoption. Using a cost–benefit analysis, [...] Read more.
Driven by falling photovoltaic (PV) installation costs and potential support policies, rooftop PV is expected to expand rapidly in Thailand. As a result, the relevant stakeholders, especially utilities, have concerns about the net economic impacts of high PV adoption. Using a cost–benefit analysis, this study quantifies the net economic impacts of rooftop PV systems on three utilities and on ratepayers in Thailand by applying nine different PV adoption scenarios with various buyback rates and annual percentages of PV cost reduction. Under Thailand’s current electricity tariff structure, Thai utilities are well-protected and able to pass all costs due to PV onto the ratepayers in terms of changes in retail rates. We find that when PV adoption is low, the net economic impacts on both the utilities and retail rates are small and the impacts on each utility depend on its specific characteristics. On the other hand, when PV adoption ranges from 9–14% in energy basis, five-year retail rate impacts become noticeable and are between 6% and 11% as compared to the projected retail rates in 2036 depending on the PV adoption level. Thus, it is necessary for Thailand to make tradeoffs among the stakeholders and maximize the benefits of rooftop PV adoption. Full article
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