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Energies, Volume 11, Issue 6 (June 2018)

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Cover Story (view full-size image) The process of hydrocarbon generation is a function of kerogen content, type, and its maturity. [...] Read more.
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Open AccessArticle The Future of Fossil Fired Power Plants in Germany—A Lifetime Analysis
Energies 2018, 11(6), 1616; https://doi.org/10.3390/en11061616
Received: 27 April 2018 / Revised: 6 June 2018 / Accepted: 11 June 2018 / Published: 20 June 2018
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Abstract
In many German energy projections, the assumption of power plant lifetimes plays a central role, since it is often used in projections for the existing fleet of power plants or as a criterion for decommissioning in model-based investigations. The result of these analyses
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In many German energy projections, the assumption of power plant lifetimes plays a central role, since it is often used in projections for the existing fleet of power plants or as a criterion for decommissioning in model-based investigations. The result of these analyses is a power plant capacity retirement graph, which is then used to determine the replacement demand. Especially in the context of the German Energy transition (“Energiewende”) the amount and dynamics of replacement play an important role. Against this background, a large number of studies have been evaluated and fleet developments compared. Many studies refer to empirical values for the assumption of lifetimes without specifying them in greater detail. This approach was used to conduct an ex post lifetime analysis—accurate to each number of units—of German power plants that have been decommissioned since 1990. The analysis is conducted with the aid of a power plant database, which has been continuously updated for each individual unit since 1985. In addition to the power plants currently in operation, the database includes also includes power plant units that have been successively decommissioned over the past decades. The ex post analysis presents the first lifetime analysis for decommissioned German plants, which can serve as a basis for future power plant fleet projections. The analyses show that the lifetime of fossil-fired power plants has extended considerably. For example, whereas the real lifetimes of coal-fired power plants were in a range of 30 to 35 years in the 1990s, today they amount to 40 to 45 years on average. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessReview Thermal Performance through Heat Retention in Integrated Collector-Storage Solar Water Heaters: A Review
Energies 2018, 11(6), 1615; https://doi.org/10.3390/en11061615
Received: 16 May 2018 / Revised: 7 June 2018 / Accepted: 19 June 2018 / Published: 20 June 2018
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Abstract
Solar thermal systems are a long-standing technology that is receiving increased attention, in terms of research and development, due to ambitious climate change targets and the need for renewable energy solutions. Integrated collector-storage solar water heaters (ICSSWHs) are a potential contributing solution and
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Solar thermal systems are a long-standing technology that is receiving increased attention, in terms of research and development, due to ambitious climate change targets and the need for renewable energy solutions. Integrated collector-storage solar water heaters (ICSSWHs) are a potential contributing solution and numerous studies have focussed on the optimisation of their thermal performance and efficiency. A major drawback of these systems is the heavy heat losses experienced during non-collection periods. To combat this, various heat retention strategies have been proposed and evaluated, including baffles plates, additional insulation, multiple glazing layers, selective coatings, and phase change materials. This paper aims to bring together these studies through a systematic review of the existing literature surrounding the performance of ICSSWH systems, focusing on heat retention. This review provides a comprehensive and up-to-date point of reference on relevant research and developments for researchers in this field. Full article
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Open AccessArticle SnSb Alloy Blended with Hard Carbon as Anode for Na-Ion Batteries
Energies 2018, 11(6), 1614; https://doi.org/10.3390/en11061614
Received: 15 May 2018 / Revised: 9 June 2018 / Accepted: 15 June 2018 / Published: 20 June 2018
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Abstract
SnSb binary alloys blended with reduced graphene oxide (SnSb/RGO) or mixtures of SnSb/RGO with hard carbon (SnSb/RGO+HC) were tested as anode materials for sodium-ion batteries. The presence of hard carbon in the SnSb/RGO+HC blend improves its cycle efficiency and rate performance substantially. The
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SnSb binary alloys blended with reduced graphene oxide (SnSb/RGO) or mixtures of SnSb/RGO with hard carbon (SnSb/RGO+HC) were tested as anode materials for sodium-ion batteries. The presence of hard carbon in the SnSb/RGO+HC blend improves its cycle efficiency and rate performance substantially. The synergy between the SnSb/RGO and the hard carbon phase is explained by the buffer action of the hard carbon, preventing SnSb interparticle agglomeration during the repeated recharge cycles. The feasibility of SnSb alloy anode for sodium-ion batteries was confirmed in full cell configuration vs. Na3V2(PO4)2F3 cathode. Full article
(This article belongs to the Special Issue Electrochemical Energy Conversion and Storage Technologies)
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Open AccessArticle Fully-distributed Load Frequency Control Strategy in an Islanded Microgrid Considering Plug-In Electric Vehicles
Energies 2018, 11(6), 1613; https://doi.org/10.3390/en11061613
Received: 22 May 2018 / Revised: 13 June 2018 / Accepted: 19 June 2018 / Published: 20 June 2018
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Abstract
With large-scale integration of electric vehicles, this paper investigates the load frequency control problem in an islanded microgrid with plug-in electric vehicles (PEVs), which can be regarded as mobile battery energy storages to provide a valuable contribution to frequency regulation. A novel fully-distributed
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With large-scale integration of electric vehicles, this paper investigates the load frequency control problem in an islanded microgrid with plug-in electric vehicles (PEVs), which can be regarded as mobile battery energy storages to provide a valuable contribution to frequency regulation. A novel fully-distributed control strategy is proposed to achieve fast frequency regulation of islanded microgrids and effective coordination control of distributed energy sources. Firstly, distributed control based on an improved linear active disturbance rejection algorithm is realized through a multi-agent system, and it greatly enhances the anti-disturbance capability of the microgrid. Then, in order to guarantee the effectiveness of PEVs in frequency regulation, PEVs are controlled following the controllable power rate (CPR) calculated from the consensus-based multi-agent system. Furthermore, the system control construction in this paper is well designed to avoid the negative effects caused by system communication time delay. Finally, numerical simulations under different disturbances are carried out to demonstrate the effectiveness of the proposed control strategy in comparison with other previous control strategies. Full article
(This article belongs to the Special Issue Electric Power Systems Research 2018)
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Open AccessArticle Research on the Pressure Ratio Characteristics of a Swash Plate-Rotating Hydraulic Transformer
Energies 2018, 11(6), 1612; https://doi.org/10.3390/en11061612
Received: 16 May 2018 / Revised: 8 June 2018 / Accepted: 8 June 2018 / Published: 20 June 2018
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Abstract
This paper presents a theoretical model and its experimental validation for the pressure ratio of a swash plate-rotating hydraulic transformer. The structure and principle of the new type of transformer are described. The swash plate-rotating type can reduce the throttling loss caused by
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This paper presents a theoretical model and its experimental validation for the pressure ratio of a swash plate-rotating hydraulic transformer. The structure and principle of the new type of transformer are described. The swash plate-rotating type can reduce the throttling loss caused by the valve plate in traditional hydraulic transformers. The theoretical model of the pressure ratio was derived based on the displacements functioning as the pump and motor in the transformer, accounting for the friction losses. A specific experimental setup including the prototype was established to validate the principle and pressure ratio of the machine. The results show that the transformer has a wider pressure range. The increase in pressure at port A and the rotating speed of the cylinder can reduce the pressure ratio slightly due to the torque loss. The present work indicates the useful potential of the swash plate-rotating hydraulic transformer. Full article
(This article belongs to the Special Issue Energy Efficiency and Controllability of Fluid Power Systems 2018)
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Open AccessArticle 3-Leg Inverter Control for 2-Phase Outer Rotor Coreless Torque Actuator in Hybrid Multi-D.O.F System
Energies 2018, 11(6), 1611; https://doi.org/10.3390/en11061611
Received: 31 March 2018 / Revised: 6 June 2018 / Accepted: 19 June 2018 / Published: 20 June 2018
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Abstract
Since an existing 3-phase inner rotor torque actuator (TA) has severe torque ripples, it is not appropriate for a gimbal system that requires precise position control. Therefore, a coreless TA is considered to eliminate the core causing torque ripples. In order to compensate
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Since an existing 3-phase inner rotor torque actuator (TA) has severe torque ripples, it is not appropriate for a gimbal system that requires precise position control. Therefore, a coreless TA is considered to eliminate the core causing torque ripples. In order to compensate for several problems (e.g., problems of production structures and output degradation) when a coreless type is used, the final 2-phase outer rotor is proposed for the low vibration and high power TA in the gimbal system. To control the 2-phase TA applied to such the gimbal system, special inverter control methods, such as bi-directional drive for tilting control and control for output torque improvement, are required. The 2-phase 3-leg inverter is free of DC capacitor voltage unbalance compared to the 2-leg inverter, and is economical because it uses less power switches than the 4-leg inverter. Therefore, the 2-phase 3-leg inverter is applied to drive the 2-phase outer rotor coreless TA of a hybrid gimbal system, and it is verified through simulation. Full article
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Open AccessArticle Electro-Thermal Modeling of Metal-Oxide Arrester under Power Frequency Applied Voltages
Energies 2018, 11(6), 1610; https://doi.org/10.3390/en11061610
Received: 30 May 2018 / Revised: 11 June 2018 / Accepted: 14 June 2018 / Published: 20 June 2018
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Abstract
Metal-oxide arresters (MOAs) are used to absorb the electrical energy resulting from overvoltages in power systems. However, temperature rises caused by the absorbed energy can lead to the electrothermal failure of MOAs. Therefore, it is necessary to analyze the electric and thermal characteristics
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Metal-oxide arresters (MOAs) are used to absorb the electrical energy resulting from overvoltages in power systems. However, temperature rises caused by the absorbed energy can lead to the electrothermal failure of MOAs. Therefore, it is necessary to analyze the electric and thermal characteristics of MOAs. In this paper, in order to study the electric and thermal characteristics of MOAs under power frequency voltage, an improved electrothermal model of an MOA is presented. The proposed electrothermal model can be divided into an electric model and a thermal model. In the electric model, based on the conventional MOA electric circuit, the effect of temperature on the voltage–current (V–I) characteristics of an MOA has been obtained. Using temperature and applied voltage as input data, the current flows through the MOA can be calculated using the artificial neural network (ANN) method. In the thermal model, the thermal circuit of a MOA has been built. The varistor power loss obtained from the electric model is used as input data, and the temperature of the zinc oxide varistors can be calculated. Therefore, compared with the existing MOA models, the interaction of leakage current and temperature can be considered in the proposed model. Finally, experimental validations have been done, and the electrothermal characteristics of an MOA have been studied by simulation and experimental methods. The electrothermal model proposed in this paper can assist with the prediction of the electric and thermal characteristics of MOAs. Full article
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Open AccessArticle Wind Farm Blockage and the Consequences of Neglecting Its Impact on Energy Production
Energies 2018, 11(6), 1609; https://doi.org/10.3390/en11061609
Received: 22 March 2018 / Revised: 14 June 2018 / Accepted: 15 June 2018 / Published: 20 June 2018
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Abstract
Measurements taken before and after the commissioning of three wind farms reveal that the wind speeds just upstream of each wind farm decrease relative to locations farther away after the turbines are turned on. At a distance of two rotor diameters upstream, the
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Measurements taken before and after the commissioning of three wind farms reveal that the wind speeds just upstream of each wind farm decrease relative to locations farther away after the turbines are turned on. At a distance of two rotor diameters upstream, the average derived relative slowdown is 3.4%; at seven to ten rotor diameters upstream, the average slowdown is 1.9%. Reynolds-Averaged Navier-Stokes (RANS) simulations point to wind-farm-scale blockage as the primary cause of these slowdowns. Blockage effects also cause front row turbines to produce less energy than they each would operating in isolation. Wind energy prediction procedures in use today ignore this effect, resulting in an overprediction bias that pervades the entire wind farm. Full article
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Open AccessArticle A Fully Coupled Numerical Model for Microwave Heating Enhanced Shale Gas Recovery
Energies 2018, 11(6), 1608; https://doi.org/10.3390/en11061608
Received: 18 May 2018 / Revised: 7 June 2018 / Accepted: 12 June 2018 / Published: 20 June 2018
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Abstract
Formation heat treatment (FHT) can be achieved by converting electromagnetic energy into heat energy (that is microwave heating or MWH). Experimental evidence shows that such FHT can significantly enhance oil and gas recovery. As relatively few research studies have been reported on microwave
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Formation heat treatment (FHT) can be achieved by converting electromagnetic energy into heat energy (that is microwave heating or MWH). Experimental evidence shows that such FHT can significantly enhance oil and gas recovery. As relatively few research studies have been reported on microwave heating enhanced shale gas recovery (MWH-EGR), a fully coupled electromagnetic-thermo-hydro-mechanical (ETHM) model is developed for the MWH-EGR in the present study. In the ETHM model, a thermal-induced gas adsorption model is firstly proposed for shale gas adsorption and fitted by experimental data. This thermal-induced adsorption model considers the increase of matrix pore space due to the desorption of the adsorbed phase. Further, a thermal-induced fracture model in shale matrix is established and fitted by experimental data. Finally, this ETHM model is applied to a fractured shale gas reservoir to simulate gas production. Numerical results indicated that the thermal-induced fracturing and gas desorption make predominant contributions to the evolution of matrix porosity. The MWH can increase cumulative gas production by 44.9% after 31.7 years through promoting gas desorption and matrix diffusion. These outcomes can provide effective insights into shale gas recovery enhancement by microwave assistance. Full article
(This article belongs to the Section Energy Sources)
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Open AccessArticle The Effect of a Vertical Electric Field on the Surface Flashover Characteristics of a Bushing Model
Energies 2018, 11(6), 1607; https://doi.org/10.3390/en11061607
Received: 27 May 2018 / Revised: 13 June 2018 / Accepted: 15 June 2018 / Published: 20 June 2018
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Abstract
High-voltage bushings play a crucial role in energy conveyance. Their specialized electric structure makes the bushing more vulnerable to surface discharge. However, the influence of a vertical electric field on the surface flashover of bushing structures remains unclear. To investigate this mechanism, four
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High-voltage bushings play a crucial role in energy conveyance. Their specialized electric structure makes the bushing more vulnerable to surface discharge. However, the influence of a vertical electric field on the surface flashover of bushing structures remains unclear. To investigate this mechanism, four simplified bushing samples were built and the influence of pollution, leakage length, and the electric field component vertical to the dielectric surface on flashover properties of the bushing samples were tested. It was found that the surface pollution level was the decisive factor that influenced flashover voltage. When the leakage length and form factor were the same, the pollution flashover of the bushing structure was lower than that of the post structure. It was also found that increasing the leakage length was not very effective in improving the flashover voltage of bushings when the equivalent salt deposit density (ESDD) was high. No obvious correlation was found between pollution flashover voltage and electric field stress. Furthermore, the uneven wetting flashover performance of the bushings was tested. Under this condition, the flashover voltage decreased with an increase of the electric field component vertical to the dielectric surface. In addition, the electric field distribution of the samples was calculated and the results were in accordance with the experimental results. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessArticle Reactive Power Planning for Regional Power Grids Based on Active and Reactive Power Adjustments of DGs
Energies 2018, 11(6), 1606; https://doi.org/10.3390/en11061606
Received: 19 May 2018 / Revised: 9 June 2018 / Accepted: 12 June 2018 / Published: 20 June 2018
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Abstract
To deal with extreme overvoltage scenarios with small probabilities in regional power grids, the traditional reactive power planning model requires a huge VAR compensator investment. Obviously, such a decision that makes a large investment to cope with a small probability event is not
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To deal with extreme overvoltage scenarios with small probabilities in regional power grids, the traditional reactive power planning model requires a huge VAR compensator investment. Obviously, such a decision that makes a large investment to cope with a small probability event is not economic. Therefore, based on the scenario analysis of power outputs of distributed generations and load consumption, a novel reactive power planning model considering the active and reactive power adjustments of distributed generations is proposed to derive the optimal allocation of VAR compensators and ensure bus voltages within an acceptable range under extreme overvoltage scenarios. The objective of the proposed reactive power planning model is to minimize the VAR compensator investment cost and active power adjustment cost of distributed generations. Moreover, since the proposed reactive power planning model is formulated as a mixed-integer nonlinear programming problem, a primal-dual interior point method-based particle swarm optimization algorithm is developed to effectively solve the proposed model. Simulation results were conducted with the modified IEEE 30-bus system to verify the effectiveness of the proposed reactive power planning model. Full article
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Open AccessArticle Short-Term Forecasting for Energy Consumption through Stacking Heterogeneous Ensemble Learning Model
Energies 2018, 11(6), 1605; https://doi.org/10.3390/en11061605
Received: 20 May 2018 / Revised: 8 June 2018 / Accepted: 13 June 2018 / Published: 19 June 2018
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Abstract
In the real-life, time-series data comprise a complicated pattern, hence it may be challenging to increase prediction accuracy rates by using machine learning and conventional statistical methods as single learners. This research outlines and investigates the Stacking Multi-Learning Ensemble (SMLE) model for time
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In the real-life, time-series data comprise a complicated pattern, hence it may be challenging to increase prediction accuracy rates by using machine learning and conventional statistical methods as single learners. This research outlines and investigates the Stacking Multi-Learning Ensemble (SMLE) model for time series prediction problem over various horizons with a focus on the forecasts accuracy, directions hit-rate, and the average growth rate of total oil demand. This investigation presents a flexible ensemble framework in light of blend heterogeneous models for demonstrating and forecasting nonlinear time series. The proposed SMLE model combines support vector regression (SVR), backpropagation neural network (BPNN), and linear regression (LR) learners, the ensemble architecture consists of four phases: generation, pruning, integration, and ensemble prediction task. We have conducted an empirical study to evaluate and compare the performance of SMLE using Global Oil Consumption (GOC). Thus, the assessment of the proposed model was conducted at single and multistep horizon prediction using unique benchmark techniques. The final results reveal that the proposed SMLE model outperforms all the other benchmark methods listed in this study at various levels such as error rate, similarity, and directional accuracy by 0.74%, 0.020%, and 91.24%, respectively. Therefore, this study demonstrates that the ensemble model is an extremely encouraging methodology for complex time series forecasting. Full article
(This article belongs to the Special Issue Short-Term Load Forecasting by Artificial Intelligent Technologies)
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Open AccessArticle Improvement in Harmonic Compensation of a Smart Charger with a Constant DC-Capacitor Voltage-Control-Based Strategy for Electric Vehicles in Single-Phase Three-Wire Distribution Feeders
Energies 2018, 11(6), 1604; https://doi.org/10.3390/en11061604
Received: 19 April 2018 / Revised: 10 June 2018 / Accepted: 14 June 2018 / Published: 19 June 2018
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Abstract
This paper presents an improvement in harmonic compensation performance of a previously proposed smart charger (SC) with a constant dc-capacitor voltage-control (CDCVC) strategy for electric vehicles (EVs) in single-phase three-wire distribution feeders (SPTWDFs). A controller for 3rd harmonic currents in d-q
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This paper presents an improvement in harmonic compensation performance of a previously proposed smart charger (SC) with a constant dc-capacitor voltage-control (CDCVC) strategy for electric vehicles (EVs) in single-phase three-wire distribution feeders (SPTWDFs). A controller for 3rd harmonic currents in d-q coordinates is added to the previously proposed SC. This addition improves harmonic compensation performance of the source currents. We briefly introduce harmonic current compensation using the previously proposed CDCVC-based algorithm for the SC. Then, the basic principles of the proposed controller for the 3rd harmonic currents in d-q coordinates are discussed in detail. It is shown that synchronization of the current controllers for both the fundamental and 3rd harmonic components is required. The switching frequency of a three-leg pulse-width modulated rectifier with a bidirectional dc–dc converter, which performs the SC, is determined considering the synchronization of the current controllers. Simulation and experimental results demonstrate that balanced and sinusoidal source currents with a unity power factor are achieved during both battery charging and discharging operations in EVs, improving the harmonic compensation performance of the previously proposed SC. Experimental results also demonstrate that the total harmonic distortion values of source currents are improved by 8.4% and 3.6% with the proposed controller for 3rd harmonic currents, when the SC is discharging, for example. Full article
(This article belongs to the Special Issue Power Electronics for Energy Storage)
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Open AccessArticle Investigation of Discharge Coefficients for Single Element Lean Direct Injection Modules
Energies 2018, 11(6), 1603; https://doi.org/10.3390/en11061603
Received: 11 May 2018 / Revised: 5 June 2018 / Accepted: 12 June 2018 / Published: 19 June 2018
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Abstract
Lean direct injection (LDI) combustion has a high potential as a low pollution combustion method for gas turbines. The present research aims to further investigate the discharge coefficient of an LDI module, axial swirler and convergent outlet under non-reaction and reaction conditions by
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Lean direct injection (LDI) combustion has a high potential as a low pollution combustion method for gas turbines. The present research aims to further investigate the discharge coefficient of an LDI module, axial swirler and convergent outlet under non-reaction and reaction conditions by theoretical, numerical and experimental methods. The functional relationship between the discharge coefficient of the LDI module, axial swirler and convergent outlet was established, and the effect of swirl angle (30°, 32°, 34°, 36°, 38°, 40°) and vane number (11, 12, 13, 14, 15, 16) on discharge coefficient was studied, and finally the differences in effective flow area of LDI combustor under different inlet conditions were analyzed. The results indicate that the flow separation on the suction side increases as the swirl angle increases, which leads to a decrease of the discharge coefficient of the axial swirler, however the discharge coefficient of the convergent outlet remains unchanged first and then decreases. As the vane number increases, the flow separation on the suction side decreases and the flow friction loss increases, so that the discharge coefficient of the axial swirler and convergent outlet will first increase with the increase of vane number and then decrease with further increases. The effective flow area of combustor changes as the conditions change, but it is approximately equal under high power conditions and normal temperature and pressure conditions. Full article
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Open AccessArticle Strategic Behavior of Retailers for Risk Reduction and Profit Increment via Distributed Generators and Demand Response Programs
Energies 2018, 11(6), 1602; https://doi.org/10.3390/en11061602
Received: 9 May 2018 / Revised: 5 June 2018 / Accepted: 8 June 2018 / Published: 19 June 2018
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Abstract
Following restructuring of power industry, electricity supply to end-use customers has undergone fundamental changes. In the restructured power system, some of the responsibilities of the vertically integrated distribution companies have been assigned to network managers and retailers. Under the new situation, retailers are
[...] Read more.
Following restructuring of power industry, electricity supply to end-use customers has undergone fundamental changes. In the restructured power system, some of the responsibilities of the vertically integrated distribution companies have been assigned to network managers and retailers. Under the new situation, retailers are in charge of providing electrical energy to electricity consumers who have already signed contract with them. Retailers usually provide the required energy at a variable price, from wholesale electricity markets, forward contracts with energy producers, or distributed energy generators, and sell it at a fixed retail price to its clients. Different strategies are implemented by retailers to reduce the potential financial losses and risks associated with the uncertain nature of wholesale spot electricity market prices and electrical load of the consumers. In this paper, the strategic behavior of retailers in implementing forward contracts, distributed energy sources, and demand-response programs with the aim of increasing their profit and reducing their risk, while keeping their retail prices as low as possible, is investigated. For this purpose, risk management problem of the retailer companies collaborating with wholesale electricity markets, is modeled through bi-level programming approach and a comprehensive framework for retail electricity pricing, considering customers’ constraints, is provided in this paper. In the first level of the proposed bi-level optimization problem, the retailer maximizes its expected profit for a given risk level of profit variability, while in the second level, the customers minimize their consumption costs. The proposed programming problem is modeled as Mixed Integer programming (MIP) problem and can be efficiently solved using available commercial solvers. The simulation results on a test case approve the effectiveness of the proposed demand-response program based on dynamic pricing approach on reducing the retailer’s risk and increasing its profit. Full article
(This article belongs to the Section Electrical Power and Energy System)
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