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Energies, Volume 12, Issue 1 (January-1 2019)

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Cover Story (view full-size image) Introduction to Figure: Saturation fluid type and level of saturation in oil/gas reservoir rocks, [...] Read more.
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Open AccessArticle Distributed Settlement of Frequency Regulation Based on a Battery Energy Storage System
Energies 2019, 12(1), 199; https://doi.org/10.3390/en12010199
Received: 26 October 2018 / Revised: 28 December 2018 / Accepted: 29 December 2018 / Published: 8 January 2019
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Abstract
Battery energy storage systems (BESS) have wide applicability for frequency regulation services in power systems, owing to their fast response and flexibility. In this paper, a distributed method for frequency regulation based on the BESS is proposed, where the method includes two layers.
[...] Read more.
Battery energy storage systems (BESS) have wide applicability for frequency regulation services in power systems, owing to their fast response and flexibility. In this paper, a distributed method for frequency regulation based on the BESS is proposed, where the method includes two layers. The upper layer is a communication network composed of agents, which is used to transmit and process information, whilst the bottom layer comprises the power system with the BESS, which provides a frequency regulation service for the system. Furthermore, a set of fully distributed control laws for the BESS are derived from the proposed distributed method, where economic power dispatch and frequency recovery are simultaneously achieved. Finally, simulations were conducted to evaluate the effectiveness of the proposed method. The results show that the system frequency regulation and economic power dispatch are achieved after considering the limits of the battery state of charge and communication delays. Full article
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Open AccessArticle Analytical and Numerical Investigation of Fe3O4–Water Nanofluid Flow over a Moveable Plane in a Parallel Stream with High Suction
Energies 2019, 12(1), 198; https://doi.org/10.3390/en12010198
Received: 5 December 2018 / Revised: 4 January 2019 / Accepted: 7 January 2019 / Published: 8 January 2019
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Abstract
In the current framework, a model is constituted to explore the impacts of high suction and partial slip on Fe3O4–water nanoliquid flow over a porous moveable surface in a parallel free stream. The mechanisms of heat transfer are also
[...] Read more.
In the current framework, a model is constituted to explore the impacts of high suction and partial slip on Fe3O4–water nanoliquid flow over a porous moveable surface in a parallel free stream. The mechanisms of heat transfer are also modeled in the existence of Newtonian heating effect. The obtaining PDEs are transformed into a non-linear ODE system employing appropriate boundary conditions to diverse physical parameters. The governing ODE system is solved using a singular perturbation technique that results in an analytical asymptotic solution as a function of the physical parameters. The obtained solution allows us to carry out an analytical parametric study to investigate the impact of the physical parameters on the nonlinear attitude of the system. The precision of the proposed method is verified by comparisons between the numerical and analytical results. The results confirm that the proposed technique yields a good approximation to the solution as well as the solution calculation has no CPU time-consuming or round off error. Numerical solutions are computed and clarified in graphs for the model embedded parameters. Moreover, profiles of the skin friction coefficient and the heat transfer rate are also portrayed and deliberated. The data manifests that both solid volume fraction and slip impact significantly alter the flow profiles. Moreover, an upward trend in temperature is anticipated for enhancing Newtonian heating strength. Additionally, it was found that both the nanofluid velocity and temperature distributions are decelerated when the solid volume fraction and suction parameters increase. Furthermore, a rise in slip parameter causes an increment in velocity profiles, and a rise in Biot number causes an increment in the temperature profiles. Full article
(This article belongs to the Section Energy Fundamentals and Conversion)
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Open AccessEditorial Acknowledgement to Reviewers of Energies in 2018
Energies 2019, 12(1), 197; https://doi.org/10.3390/en12010197
Published: 8 January 2019
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Abstract
Rigorous peer-review is the corner-stone of high-quality academic publishing[...] Full article
Open AccessArticle Prediction of China’s Energy Consumption Based on Robust Principal Component Analysis and PSO-LSSVM Optimized by the Tabu Search Algorithm
Energies 2019, 12(1), 196; https://doi.org/10.3390/en12010196
Received: 3 December 2018 / Revised: 25 December 2018 / Accepted: 28 December 2018 / Published: 8 January 2019
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Abstract
China’s energy consumption issues are closely associated with global climate issues, and the scale of energy consumption, peak energy consumption, and consumption investment are all the focus of national attention. In order to forecast the amount of energy consumption of China accurately, this
[...] Read more.
China’s energy consumption issues are closely associated with global climate issues, and the scale of energy consumption, peak energy consumption, and consumption investment are all the focus of national attention. In order to forecast the amount of energy consumption of China accurately, this article selected GDP, population, industrial structure and energy consumption structure, energy intensity, total imports and exports, fixed asset investment, energy efficiency, urbanization, the level of consumption, and fixed investment in the energy industry as a preliminary set of factors; Secondly, we corrected the traditional principal component analysis (PCA) algorithm from the perspective of eliminating “bad points” and then judged a “bad spot” sample based on signal reconstruction ideas. Based on the above content, we put forward a robust principal component analysis (RPCA) algorithm and chose the first five principal components as main factors affecting energy consumption, including: GDP, population, industrial structure and energy consumption structure, urbanization; Then, we applied the Tabu search (TS) algorithm to the least square to support vector machine (LSSVM) optimized by the particle swarm optimization (PSO) algorithm to forecast China’s energy consumption. We collected data from 1996 to 2010 as a training set and from 2010 to 2016 as the test set. For easy comparison, the sample data was input into the LSSVM algorithm and the PSO-LSSVM algorithm at the same time. We used statistical indicators including goodness of fit determination coefficient (R2), the root means square error (RMSE), and the mean radial error (MRE) to compare the training results of the three forecasting models, which demonstrated that the proposed TS-PSO-LSSVM forecasting model had higher prediction accuracy, generalization ability, and higher training speed. Finally, the TS-PSO-LSSVM forecasting model was applied to forecast the energy consumption of China from 2017 to 2030. According to predictions, we found that China shows a gradual increase in energy consumption trends from 2017 to 2030 and will breakthrough 6000 million tons in 2030. However, the growth rate is gradually tightening and China’s energy consumption economy will transfer to a state of diminishing returns around 2026, which guides China to put more emphasis on the field of energy investment. Full article
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Open AccessArticle A Two-Step Framework for Energy Local Area Network Scheduling Problem with Electric Vehicles Based on Global–Local Optimization Method
Energies 2019, 12(1), 195; https://doi.org/10.3390/en12010195
Received: 27 November 2018 / Revised: 23 December 2018 / Accepted: 1 January 2019 / Published: 8 January 2019
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Abstract
To reduce the fluctuation of renewable energy (RE) supply and improve the economic efficiency of the power grid, the energy local area network (ELAN), which is a subnetwork of the energy internet (EI), plays an important role in specific regions. Electric vehicles (EVs),
[...] Read more.
To reduce the fluctuation of renewable energy (RE) supply and improve the economic efficiency of the power grid, the energy local area network (ELAN), which is a subnetwork of the energy internet (EI), plays an important role in specific regions. Electric vehicles (EVs), as virtual energy storage (VES) in ELANs, are helpful to decrease the fluctuations of RE supply. However, how to use EVs in ELANs is a complex issue, considering the uncertainties of EVs’ charging demand, the forecast data errors of RE sources, etc. In this paper, a typical ELAN structure is established, taking into account RE sources, load response system, and a distributed energy storage (DES) system including EVs. A two-step optimization framework for ELAN scheduling problem is proposed. A global optimization model based on forecast data is built to maximize the income of ELAN, and an online local optimization model is introduced to minimize the correction cost utilizing prior knowledge. Finally, the proposed two-step optimization framework is applied to a series of real-world ELAN scheduling problems. The results show that DES system with EVs can reduce the volatility of RE supply evidently, and the proposed method is able to maximize the income of the ELAN efficiently. Full article
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Open AccessArticle Application of Spectral Kurtosis to Characterize Amplitude Variability in Power Systems’ Harmonics
Energies 2019, 12(1), 194; https://doi.org/10.3390/en12010194
Received: 30 November 2018 / Revised: 31 December 2018 / Accepted: 2 January 2019 / Published: 8 January 2019
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Abstract
The highly-changing concept of Power Quality (PQ) needs to be continuously reformulated due to the new schemas of the power grid or Smart Grid (SG). In general, the spectral content is characterized by their averaged or extreme values. However, new PQ events may
[...] Read more.
The highly-changing concept of Power Quality (PQ) needs to be continuously reformulated due to the new schemas of the power grid or Smart Grid (SG). In general, the spectral content is characterized by their averaged or extreme values. However, new PQ events may consist of large variations in amplitude that occur in a short time or small variations in amplitude that take place continuously. Thus, the former second-order techniques are not suitable to monitor the dynamics of the power spectrum. In this work, a strategy based on Spectral Kurtosis (SK) is introduced to detect frequency components with a constant amplitude trend, which accounts for amplitude values’ dispersion related to the mean value of that spectral component. SK has been proven to measure frequency components that follow a constant amplitude trend. Two practical real-life cases have been considered: electric current time-series from an arc furnace and the power grid voltage supply. Both cases confirm that the more concentrated the amplitude values are around the mean value, the lower the SK values are. All this confirms SK as an effective tool for evaluating frequency components with a constant amplitude trend, being able to provide information beyond maximum variation around the mean value and giving a progressive index of value dispersion around the mean amplitude value, for each frequency component. Full article
(This article belongs to the Special Issue Analysis for Power Quality Monitoring)
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Open AccessArticle Sample Entropy Based Net Load Tracing Dispatch of New Energy Power System
Energies 2019, 12(1), 193; https://doi.org/10.3390/en12010193
Received: 18 November 2018 / Revised: 25 December 2018 / Accepted: 7 January 2019 / Published: 8 January 2019
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Abstract
The high-proportion of renewable energies is gradually becoming one of the main power supply sources and bringing strong uncertainties to the power grid. In this paper, a sample entropy (SampEn) based net load tracing dispatch strategy with a specific thermal generating mode is
[...] Read more.
The high-proportion of renewable energies is gradually becoming one of the main power supply sources and bringing strong uncertainties to the power grid. In this paper, a sample entropy (SampEn) based net load tracing dispatch strategy with a specific thermal generating mode is proposed. In this strategy, renewable energies are fully and preferentially consumed by electric loads, turned to net loads, to maximize the utilization of renewable energies. SampEn theory is utilized to evaluate the complexity of net load time series, based on which, the traditional power generators trace the complexity of the net load flexibly. According to the SampEn, a specific generating model of thermal generators is determined and the cooperation between thermal generators and pumped storage is realized, aiming at reducing the ramp power of thermal generators and increasing the throughput of pumped storage. The experiment simulation is developed on the 10-unit test system. Results show that the ramping power of the thermal generators are reduced 43% and 13% in the two cases together with the throughput of pumped storage is increased 44% and 27% on the premise that the economy of the system is maintained and renewable energies are fully consumed. Therefore, the efficiency and reasonability of the proposed dispatch strategy are confirmed. Full article
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Open AccessArticle Flow Separation Control in a Curved Diffuser with Rigid Traveling Wave Wall and Its Mechanism
Energies 2019, 12(1), 192; https://doi.org/10.3390/en12010192
Received: 2 December 2018 / Revised: 3 January 2019 / Accepted: 4 January 2019 / Published: 8 January 2019
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Traveling wave wall is a useful method to suppress flow separation. However, the interaction between the traveling wave wall and unsteady separation flow is complex, which causes difficulty in discovering the corresponding mechanism. To reveal the mechanism of traveling wave wall control, numerical
[...] Read more.
Traveling wave wall is a useful method to suppress flow separation. However, the interaction between the traveling wave wall and unsteady separation flow is complex, which causes difficulty in discovering the corresponding mechanism. To reveal the mechanism of traveling wave wall control, numerical simulation of a separated curved diffuser using rigid traveling wave wall flow control is performed, which shows some unique characteristics. Then, a nonlinear simplified model is used to explain this phenomenon in flow control in consideration of nonlinear dynamics and order of degree. Flow field data from the numerical simulation are further analyzed using fast Fourier transform analysis, linear stability theory of free shear layers, and the nonlinear simplified model to reveal the control mechanism of traveling wave wall. Full article
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Open AccessArticle Research on the Comprehensive Performance of Hygroscopic Materials in an Office Building Based on EnergyPlus
Energies 2019, 12(1), 191; https://doi.org/10.3390/en12010191
Received: 5 November 2018 / Revised: 1 January 2019 / Accepted: 2 January 2019 / Published: 8 January 2019
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Abstract
This paper presents the research status of hygroscopic materials, points out the weak links as targets for major breakthroughs, and introduces humidifying mechanisms and their categories. In this paper, we simulated a single-monomer Shenyang office building with different envelopes of inner-surface hygroscopic materials
[...] Read more.
This paper presents the research status of hygroscopic materials, points out the weak links as targets for major breakthroughs, and introduces humidifying mechanisms and their categories. In this paper, we simulated a single-monomer Shenyang office building with different envelopes of inner-surface hygroscopic materials for indoor humidity conditions, energy consumption, and economy, which are three aspects of energy consumption analysis in EnergyPlus software. To obtain the best moisture buffering performance from hygroscopic materials, we also simulated different cases including the laying area, ventilation strategy, thickness, and initial moisture content of different hygroscopic materials. The humidity fluctuation, with changes in the style of hygroscopic materials and usage conditions, of a room in a building can be analyzed by numerical simulation. This allows the determination of the best moisture buffering performance of the building structure. The results show that hygroscopic materials have great advantages in three energy saving aspects of building assessment. Hygroscopic materials can regulate indoor air humidity and reduce energy consumption. In addition, the entire life-cycle cost can be minimized. Lower rates of air exchange and larger usable areas can help enhance the level of performance of hygroscopic materials. The thickness and initial moisture content of hygroscopic materials have little impact on the moisture buffering value. This study strived to provide a theoretical basis and technical guidance for the production and installation of hygroscopic materials. It also promoted the passive materials market and the building’s energy savings. The best moisture buffering performance, evaluated at room level in this paper, can be obtained through real-world environmental simulation. Full article
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Open AccessReview Microorganism Assisted Synthesized Nanoparticles for Catalytic Applications
Energies 2019, 12(1), 190; https://doi.org/10.3390/en12010190
Received: 28 November 2018 / Revised: 21 December 2018 / Accepted: 4 January 2019 / Published: 8 January 2019
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Abstract
Metal and metalloid nanoparticles (NPs) have attracted substantial attention from research communities over the past few decades. Traditional methodologies for NP fabrication have also been intensely explored. However, drawbacks such as the use of toxic agents and the high energy consumption involved in
[...] Read more.
Metal and metalloid nanoparticles (NPs) have attracted substantial attention from research communities over the past few decades. Traditional methodologies for NP fabrication have also been intensely explored. However, drawbacks such as the use of toxic agents and the high energy consumption involved in chemical and physical processes hinder their further application in various fields. It is well known that some bacteria are capable of binding and concentrating dissolved metal and metalloid ions, thereby detoxifying their environments. Bioinspired fabrication of NPs is environmentally friendly and inexpensive and requires only low energy consumption. Some biosynthesized NPs are usually used as heterogeneous catalysts in environmental remediation and show higher catalytic efficiency because of their enhanced biocompatibility, stability and large specific surface areas. Therefore, bacteria used as nanofactories can provide a novel approach for removing metal or metalloid ions and fabricating materials with unique properties. Even though a wide range of NPs have been biosynthesized, and their synthetic mechanisms have been proposed, some of these mechanisms are not known in detail. This review focuses on the synthesis and catalytic applications of NPs obtained using bacteria. The known mechanisms of bioreduction and prospects in the design of NPs for catalytic applications are also discussed. Full article
(This article belongs to the Special Issue Nanotechnology for Energy Materials)
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Open AccessArticle Medium-Term Hydropower Scheduling with Variable Head under Inflow, Energyand Reserve Capacity Price Uncertainty
Energies 2019, 12(1), 189; https://doi.org/10.3390/en12010189
Received: 11 November 2018 / Revised: 30 December 2018 / Accepted: 2 January 2019 / Published: 8 January 2019
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We propose a model for medium-term hydropower scheduling (MTHS) with variable head and uncertainty in inflow, reserve capacity, and energy price. With an increase of intermittent energy sources in the generation mix, it is expected that a flexible hydropower producer can obtain added
[...] Read more.
We propose a model for medium-term hydropower scheduling (MTHS) with variable head and uncertainty in inflow, reserve capacity, and energy price. With an increase of intermittent energy sources in the generation mix, it is expected that a flexible hydropower producer can obtain added profits by participating in markets other than just the energy market. To capture this added potential, the hydropower system should be modeled with a higher level of detail. In this context, we apply an algorithm based on stochastic dual dynamic programming (SDDP) to solve the nonconvex MTHS problem and show that the use of strengthened Benders (SB) cuts to represent the expected future profit (EFP) function provides accurate scheduling results for slightly nonconvex problems. A method to visualize the EFP function in a dynamic programming setting is provided, serving as a useful tool for a priori inspection of the EFP shape and its nonconvexity. Full article
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Open AccessArticle Data-Driven Evaluation of Residential HVAC System Efficiency Using Energy and Environmental Data
Energies 2019, 12(1), 188; https://doi.org/10.3390/en12010188
Received: 2 December 2018 / Revised: 31 December 2018 / Accepted: 4 January 2019 / Published: 8 January 2019
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Abstract
In the U.S., the heating, ventilation, and air conditioning (HVAC) system is generally the largest electricity-consuming end-use in a residential building. However, homeowners are less likely to have their HVAC system serviced regularly, thus inefficiencies in operation are also more likely to occur.
[...] Read more.
In the U.S., the heating, ventilation, and air conditioning (HVAC) system is generally the largest electricity-consuming end-use in a residential building. However, homeowners are less likely to have their HVAC system serviced regularly, thus inefficiencies in operation are also more likely to occur. To address this challenge, this research works towards a non-intrusive data-driven assessment method using building assessors’ data, HVAC electricity demand data, and outdoor environmental data. Building assessors’ data is first used to estimate the HVAC system size, then estimate the electricity demand curve of the HVAC system. A comparison of the proposed electricity demand curve development method demonstrates strong agreement with physics-based HVAC model results. An HVAC efficiency rating is then proposed, which compares the model-predicted and actual performance data to define whether an HVAC system is operating as expected. As a case study, detailed data for 39 occupied, conditioned residential buildings in Austin, Texas, was used demonstrating the identification of the presence of potential HVAC inefficiencies. The results prove beneficial for utilities to help target residential HVAC systems in need of service or energy efficiency upgrades, as well as for homeowners as a continuous assessment tool for HVAC performance. Full article
(This article belongs to the Special Issue Energy Efficiency in Buildings: Both New and Rehabilitated)
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Open AccessArticle Implementation of a Microgrid Scheme Using a MVDC Connection between Gapado Island and Marado Island in South Korea
Energies 2019, 12(1), 187; https://doi.org/10.3390/en12010187
Received: 31 October 2018 / Revised: 3 January 2019 / Accepted: 3 January 2019 / Published: 8 January 2019
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In this paper, we propose a microgrid (MG) implementation method through Medium-Voltage Direct Current (MVDC) connection between Gapado Island and Marado Island in Korea. MVDC is a facility that can be efficiently applied between small power generation complexes. The structure of power generation
[...] Read more.
In this paper, we propose a microgrid (MG) implementation method through Medium-Voltage Direct Current (MVDC) connection between Gapado Island and Marado Island in Korea. MVDC is a facility that can be efficiently applied between small power generation complexes. The structure of power generation facilities is mainly supplied by diesel generators, while solar and wind power generators supply additional power. An Energy Storage System (ESS) is also used to reduce the output fluctuations of wind and solar power generation. Since power systems in such areas are low-voltage and low-power distribution systems, problems can arise in terms of power management due to power generators with variable output characteristics such as solar power and wind power generators. In addition, when a major power source such as a diesel generator is dropped, the power system collapses. However, these problems can be solved by interchanging the power between the micro-grids through the connection of MVDCs. With the MVDC connected, we verify the impact of the power system on Marado Island and Gapado Island due to the input and opening of solar, wind and diesel generators. The proposed configuration uses the PSCAD/EMTDC simulation program. Full article
(This article belongs to the Special Issue Power Electronics for Energy Storage)
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Open AccessArticle Analysis of the Performance of Various PV Module Technologies in Peru
Energies 2019, 12(1), 186; https://doi.org/10.3390/en12010186
Received: 13 December 2018 / Revised: 2 January 2019 / Accepted: 2 January 2019 / Published: 8 January 2019
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A knowledge gap exists about the actual behavior of PV grid-connected systems (PVGCS) using various PV technologies in Peru. This paper presents the results of an over three-year-long performance evaluation of a 3.3-kWp monocrystalline silicon (sc-Si) PVGCS located in Arequipa, a 3.3-kWp sc-Si
[...] Read more.
A knowledge gap exists about the actual behavior of PV grid-connected systems (PVGCS) using various PV technologies in Peru. This paper presents the results of an over three-year-long performance evaluation of a 3.3-kWp monocrystalline silicon (sc-Si) PVGCS located in Arequipa, a 3.3-kWp sc-Si PVGCS located in Tacna, and a 3-kWp policrystalline (mc-Si) PVGCS located in Lima. An assessment of the performance of a 3.5-kWp amorphous silicon/crystalline silicon hetero-junction (a-Si/µc-Si) PVGCS during over one and a half years of being in Lima is also presented. The annual final yields obtained lie within 1770–1992 kWh/kW, 1505–1540 kWh/kW, and 736–833 kWh/kW for Arequipa, Tacna, and Lima, respectively, while the annual PV array energy yield achieved by a-Si/µc-Si is 1338 kWh/kW. The annual performance ratio stays in the vicinity of 0.83 for sc-Si in Arequipa and Tacna while this parameter ranges from 0.70 to 0.77 for mc-Si in Lima. An outstanding DC annual performance ratio of 0.97 is found for a-Si/µc-Si in the latter site. The use of sc-Si and presumably, mc-Si PV modules in desert climates, such as that of Arequipa and Tacna, is encouraged. However, sc-Si and presumably, mc-Si-technologies experience remarkable temperature and low irradiance losses in Lima. By contrast, a-Si/µc-Si PV modules perform much better in the latter site thanks to being less influenced by both temperature and low light levels. Full article
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Open AccessReview The Role of Photovoltaics (PV) in the Present and Future Situation of Suriname
Energies 2019, 12(1), 185; https://doi.org/10.3390/en12010185
Received: 21 November 2018 / Revised: 21 December 2018 / Accepted: 30 December 2018 / Published: 8 January 2019
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The aim of this paper is to give an overview of the energy sector and the current status of photovoltaic (PV) systems in Suriname and to investigate which role PV systems can play in this country’s future energy transition. At this moment, 64%
[...] Read more.
The aim of this paper is to give an overview of the energy sector and the current status of photovoltaic (PV) systems in Suriname and to investigate which role PV systems can play in this country’s future energy transition. At this moment, 64% of the power is available from diesel/heavy fuel oil (HFO) gensets while 36% is available from renewables namely hydroelectric power systems and PV systems. Suriname has renewable energy (RE) targets for 2017 and 2022 which already have been achieved by this 36%. However, the RE target of 2027 of 47% must be achieved yet. As there is abundant irradiance available, on an average 1792 kWh/m2/year and because several PV systems have already been successfully implemented, PV can play an important role in the energy transition of Suriname. In order to achieve the 2027 target with only PV systems, an additional 110 MWp of installed PV capacity will be required. Governmental and non-governmental institutes have planned PV projects. If these will be executed in the future than annually 0.8 TWh electricity will be produced by PV systems. In order to meet the electricity demand of 2027 fully, 2.2 TWh PV electricity will be required which implies that more PV systems must be implemented in Suriname besides the already scheduled ones. Full article
(This article belongs to the Section Sustainable Energy)
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Open AccessArticle Modeling the Total Energy Consumption of Mobile Network Services and Applications
Energies 2019, 12(1), 184; https://doi.org/10.3390/en12010184
Received: 28 November 2018 / Revised: 26 December 2018 / Accepted: 3 January 2019 / Published: 7 January 2019
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Reducing the energy consumption of Internet services requires knowledge about the specific traffic and energy consumption characteristics, as well as the associated end-to-end topology and the energy consumption of each network segment. Here, we propose a shift from segment-specific to service-specific end-to-end energy-efficiency
[...] Read more.
Reducing the energy consumption of Internet services requires knowledge about the specific traffic and energy consumption characteristics, as well as the associated end-to-end topology and the energy consumption of each network segment. Here, we propose a shift from segment-specific to service-specific end-to-end energy-efficiency modeling to align engineering with activity-based accounting principles. We use the model to assess a range of the most popular instant messaging and video play applications to emerging augmented reality and virtual reality applications. We demonstrate how measurements can be conducted and used in service-specific end-to-end energy consumption assessments. Since the energy consumption is dependent on user behavior, we then conduct a sensitivity analysis on different usage patterns and identify the root causes of service-specific energy consumption. Our main findings show that smartphones are the main energy consumers for web browsing and instant messaging applications, whereas the LTE wireless network is the main consumer for heavy data applications such as video play, video chat and virtual reality applications. By using small cell offloading and mobile edge caching, our results show that the energy consumption of popular and emerging applications could potentially be reduced by over 80%. Full article
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Open AccessFeature PaperArticle State of Charge Estimation for Lithium-Bismuth Liquid Metal Batteries
Energies 2019, 12(1), 183; https://doi.org/10.3390/en12010183
Received: 13 December 2018 / Revised: 1 January 2019 / Accepted: 2 January 2019 / Published: 7 January 2019
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Abstract
Lithium-bismuth liquid metal batteries have much potential for stationary energy storage applications, with characteristics such as a large capacity, high energy density, low cost, long life-span and an ability for high current charge and discharge. However, there are no publications on battery management
[...] Read more.
Lithium-bismuth liquid metal batteries have much potential for stationary energy storage applications, with characteristics such as a large capacity, high energy density, low cost, long life-span and an ability for high current charge and discharge. However, there are no publications on battery management systems or state-of-charge (SoC) estimation methods, designed specifically for these devices. In this paper, we introduce the properties of lithium-bismuth liquid metal batteries. In analyzing the difficulties of traditional SoC estimation techniques for these devices, we establish an equivalent circuit network model of a battery and evaluate three SoC estimation algorithms (the extended Kalman filter, the unscented Kalman filter and the particle filter), using constant current discharge, pulse discharge and hybrid pulse (containing charging and discharging processes) profiles. The results of experiments performed using the equivalent circuit battery model show that the unscented Kalman filter gives the most robust and accurate performance, with the least convergence time and an acceptable computation time, especially in hybrid pulse current tests. The time spent on one estimation with the three algorithms are 0.26 ms, 0.5 ms and 1.5 ms. Full article
(This article belongs to the Special Issue Battery Storage Technology for a Sustainable Future)
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Open AccessEditorial Special Issue “Offshore Renewable Energy: Ocean Waves, Tides and Offshore Wind”
Energies 2019, 12(1), 182; https://doi.org/10.3390/en12010182
Received: 18 December 2018 / Revised: 3 January 2019 / Accepted: 4 January 2019 / Published: 7 January 2019
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Offshore renewable energy includes several forms of energy extraction from oceans and seas, and the most common and successful offshore technologies developed so far are based on wind, wave and tides. [...] Full article
(This article belongs to the Special Issue Offshore Renewable Energy: Ocean Waves, Tides and Offshore Wind)
Open AccessArticle Optimal Economic Dispatch in Microgrids with Renewable Energy Sources
Energies 2019, 12(1), 181; https://doi.org/10.3390/en12010181
Received: 23 November 2018 / Revised: 16 December 2018 / Accepted: 25 December 2018 / Published: 7 January 2019
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Due to the opening of the energy market and agreements for the reduction of pollution emissions, the use of microgrids attracts more attention in the scientific community, but the management of the distribution of electricity has new challenges. This paper considers different distributed
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Due to the opening of the energy market and agreements for the reduction of pollution emissions, the use of microgrids attracts more attention in the scientific community, but the management of the distribution of electricity has new challenges. This paper considers different distributed generation systems as a main part to design a microgrid and the resources management is defined in a period through proposed dynamic economic dispatch approach. The inputs are obtained by the model predictive control algorithm considering variations of both pattern of consumption and generation systems capacity, including conventional and renewable energy sources. Furthermore, the proposed approach considers a benefits program to customers involving a demand restriction and the costs of regeneration of the pollutants produced by conventional generation systems. The dispatch strategy through a mathematical programming approach seeks to reduce to the minimum the fuel cost of conventional generators, the energy transactions, the regeneration of polluted emissions and, finally, includes the benefit in electricity demand reduction satisfying all restrictions through mathematical programming strategy. The model is implemented in LINGO 17.0 software (Lindo Systems, 1415 North Dayton Street, Chicago, IL, USA). The results exhibit the proposed approach effectiveness through a study case under different considerations. Full article
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Open AccessArticle Numerical Simulation Three-Dimensional Nonlinear Seepage in a Pumped-Storage Power Station: Case Study
Energies 2019, 12(1), 180; https://doi.org/10.3390/en12010180
Received: 9 December 2018 / Revised: 30 December 2018 / Accepted: 3 January 2019 / Published: 7 January 2019
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Abstract
Due to high water pressure in the concrete reinforced hydraulic tunnels, surrounding rocks are confronted with nonlinear seepage problem in the pumped storage power station. In this study, to conduct nonlinear seepage numerical simulation, a nonlinear seepage numerical model combining the Forchheimer nonlinear
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Due to high water pressure in the concrete reinforced hydraulic tunnels, surrounding rocks are confronted with nonlinear seepage problem in the pumped storage power station. In this study, to conduct nonlinear seepage numerical simulation, a nonlinear seepage numerical model combining the Forchheimer nonlinear flow theory, the discrete variational inequality formulation of Signorini’s type and an adaptive penalized Heaviside function is established. This numerical seepage model is employed to the seepage analysis of the hydraulic tunnel surrounding rocks in the Yangjiang pumped-storage power station, which is the highest water pressure tunnel under construction in China. Moreover, the permeability of the surrounding rocks under high water pressure is determined by high pressure packer test and its approximate analytical model. It is shown that the flow in the surrounding rocks is particularly prone to become nonlinear as a result of the high flow velocities and hydraulic gradients in the nearby of the seepage-control measures and the high permeability fault. The nonlinear flow theory generates smaller flow rate than the Darcy flow theory. With the increase of nonlinear flow, this observation would become more remarkable. Full article
(This article belongs to the Special Issue Electrical Energy Production in the Water Sector)
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Open AccessArticle Inverse-System Decoupling Control of DC/DC Converters
Energies 2019, 12(1), 179; https://doi.org/10.3390/en12010179
Received: 18 November 2018 / Revised: 21 December 2018 / Accepted: 30 December 2018 / Published: 7 January 2019
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Abstract
Existing large-signal control schemes for DC/DC converters formulate control strategies based primarily on nonlinear control theory, and the associated design and implementation are relatively complex. In this work, a decomposition modeling and inverse-system decoupling control method is proposed for DC/DC converters that operate
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Existing large-signal control schemes for DC/DC converters formulate control strategies based primarily on nonlinear control theory, and the associated design and implementation are relatively complex. In this work, a decomposition modeling and inverse-system decoupling control method is proposed for DC/DC converters that operate under large-signal disturbances. First, a large-signal circuit-averaged model for DC/DC converters is established. The proposed control system has a double closed-loop control structure composed of a voltage loop and a current loop. Then, the voltage-loop and current-loop controlled subsystems are decoupled and compensated to first-order integral elements using the inverse system method. Several linear feedback controllers are designed for first-order integral systems under various optimization criteria using the optimal control theory. Simulation and experiment were performed on buck–boost converters with resistive and constant power loads. The results show that under the control of the proposed controller, all systems exhibited excellent dynamic and steady-state performance. The proposed method allows the disturbance control of the DC/DC converter, the dynamic behavior control of the voltage loop, and the current loop to become independent processes. The local controller design follows the classical linear control design method and is a simple and effective large-signal control strategy. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessArticle Combustion of Miscanthus: Composition of the Ash by Particle Size
Energies 2019, 12(1), 178; https://doi.org/10.3390/en12010178
Received: 19 November 2018 / Revised: 26 December 2018 / Accepted: 1 January 2019 / Published: 7 January 2019
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Abstract
Miscanthus is an energy crop considered to show potential for a substantial contribution to sustainable energy production. In miscanthus combustion, 2.0% to 3.5% of the mass of the fuel remains as ash. This ash is less contaminated by heavy metals than ash from
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Miscanthus is an energy crop considered to show potential for a substantial contribution to sustainable energy production. In miscanthus combustion, 2.0% to 3.5% of the mass of the fuel remains as ash. This ash is less contaminated by heavy metals than ash from wood combustion. The concentrations are well below the typical limit concentrations for use as a soil conditioner on agricultural land and forests. The potassium concentration in the investigated miscanthus ash of 14.1% K2O was significantly higher than the typical concentration of potassium in ashes from wood combustion (3% to 7% K2O). However, in comparison to wood ashes, only very little enrichment of potassium in the fine size fractions of miscanthus ash was found. For most of the other elements, the enrichment in the fine size fractions was also low. Therefore, the production of a potassium-rich material by classification for the production of potassium fertilizer is not feasible. The absence of such an enrichment can be explained on the one hand by the significantly lower combustion temperature in the miscanthus combustion plant and, on the other hand, by the higher molar ratio of K to Cl and the low ratio of K to Si. Thus, the most sensible utilization of miscanthus ash is its direct recycling to the soil such as where the miscanthus plants are grown. Full article
(This article belongs to the Section Sustainable Energy)
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Open AccessArticle Optimized Dimensioning and Operation Automation for a Solar-Combi System for Indoor Space Heating. A Case Study for a School Building in Crete
Energies 2019, 12(1), 177; https://doi.org/10.3390/en12010177
Received: 30 November 2018 / Revised: 2 January 2019 / Accepted: 3 January 2019 / Published: 7 January 2019
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Abstract
This article investigates the introduction of hybrid power plants for thermal energy production for the indoor space heating loads coverage. The plant consists of flat plate solar collectors with selective coating, water tanks as thermal energy storage and a biomass heater. A new
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This article investigates the introduction of hybrid power plants for thermal energy production for the indoor space heating loads coverage. The plant consists of flat plate solar collectors with selective coating, water tanks as thermal energy storage and a biomass heater. A new operation algorithm is applied, maximizing the exploitation of the available thermal energy storage capacity and, eventually, the thermal power production from the solar collectors. An automation system is also designed and proposed for the realization of the newly introduced algorithm. The solar-combi system is computationally simulated, using annual time series of average hourly steps. A dimensioning optimization process is proposed, using as criterion the minimization of the thermal energy production levelized cost. The overall approach is validated on a school building with 1000 m2 of covered area, located in the hinterland of the island of Crete. It is seen that, given the high available solar radiation in the specific area, the proposed solar-combi system can guarantee the 100% annual heating load coverage of the examined building, with an annual contribution from the solar collectors higher than 45%. The annually average thermal power production levelized cost is calculated at 0.15 €/kWhth. Full article
(This article belongs to the Special Issue Alternative Sources of Energy Modeling and Automation)
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Open AccessArticle An Analytical Solution of the Pseudosteady State Productivity Index for the Fracture Geometry Optimization of Fractured Wells
Energies 2019, 12(1), 176; https://doi.org/10.3390/en12010176
Received: 13 November 2018 / Revised: 16 December 2018 / Accepted: 30 December 2018 / Published: 6 January 2019
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Abstract
The pseudosteady state productivity index is very important for evaluating the production from oil and gas wells. It is usually used as an objective function for the optimization of fractured wells. However, there is no analytical solution for it, especially when the proppant
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The pseudosteady state productivity index is very important for evaluating the production from oil and gas wells. It is usually used as an objective function for the optimization of fractured wells. However, there is no analytical solution for it, especially when the proppant number of the fractured well is greater than 0.1. This paper extends the established fitting solution for proppant numbers less than 0.1 by introducing an explicit expression of the shape factor. It also proposes a new asymptotic solution based on the trilinear-flow model for proppant numbers greater than 0.1. The two solutions are combined to evaluate the pseudosteady state productivity index. The evaluation results are verified by the numerical method. The new solution can be directly used for fracture geometry optimization. The optimization results are consistent with those given by the unified fracture design (UFD) method. Using the analytical solution for the pseudosteady state productivity index, optimization results can be obtained for rectangular drainage areas with arbitrary aspect ratios without requiring any interpolation or extrapolation. Moreover, the new solution provides more rigorous optimization results than the UFD method, especially for fractured horizontal wells. Full article
(This article belongs to the Section Energy Sources)
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Open AccessArticle Torrefaction as a Valorization Method Used Prior to the Gasification of Sewage Sludge
Energies 2019, 12(1), 175; https://doi.org/10.3390/en12010175
Received: 30 November 2018 / Revised: 29 December 2018 / Accepted: 2 January 2019 / Published: 6 January 2019
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Abstract
The gasification and torrefaction of sewage sludge have the potential to make the thermal utilization of sewage sludge fully sustainable, thus limiting the use of expensive fossil fuels in the process. This includes sustainability in terms of electricity consumption. Although a great deal
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The gasification and torrefaction of sewage sludge have the potential to make the thermal utilization of sewage sludge fully sustainable, thus limiting the use of expensive fossil fuels in the process. This includes sustainability in terms of electricity consumption. Although a great deal of work has been performed so far regarding the gasification of sewage sludge and some investigations have been performed in the area of its torrefaction, there is still a gap in terms of the influence of the torrefaction of the sewage sludge on its subsequent gasification. This study presents the results from the torrefaction tests, performed on a pilot scale reactor, as well as two consecutive steam gasification tests, performed in an allothermal fixed bed gasifier, in order to determine if torrefaction can be deemed as a primary method of the reduction of tar content for the producer gas, from the aforementioned gasification process. A comparative analysis is performed based on the results obtained during both tests, with special emphasis on the concentrations of condensable compounds (tars). The obtained results show that the torrefaction of sewage sludge, performed prior to gasification, can indeed have a positive influence on the gas quality. This is beneficial especially in terms of the content of heavy tars with melting points above 40 °C. Full article
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Open AccessFeature PaperArticle Determination of Optimal Location and Sizing of Solar Photovoltaic Distribution Generation Units in Radial Distribution Systems
Energies 2019, 12(1), 174; https://doi.org/10.3390/en12010174
Received: 7 December 2018 / Revised: 28 December 2018 / Accepted: 3 January 2019 / Published: 6 January 2019
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Abstract
This paper presents an effective biogeography-based optimization (BBO) for optimal location and sizing of solar photovoltaic distributed generation (PVDG) units to reduce power losses while maintaining voltage profile and voltage harmonic distortion at the limits. This applied algorithm was motivated by biogeography, that
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This paper presents an effective biogeography-based optimization (BBO) for optimal location and sizing of solar photovoltaic distributed generation (PVDG) units to reduce power losses while maintaining voltage profile and voltage harmonic distortion at the limits. This applied algorithm was motivated by biogeography, that the study of the distribution of biological species through time and space. This technique is able to expand the searching space and retain good solution group at each generation. Therefore, the applied method can significantly improve performance. The effectiveness of the applied algorithm is validated by testing it on IEEE 33-bus and IEEE 69-bus radial distribution systems. The obtained results are compared with the genetic algorithm (GA), the particle swarm optimization algorithm (PSO) and the artificial bee colony algorithm (ABC). As a result, the applied algorithm offers better solution quality and accuracy with faster convergence. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessFeature PaperArticle Adaptive Air-Fuel Ratio Regulation for Port-Injected Spark-Ignited Engines Based on a Generalized Predictive Control Method
Energies 2019, 12(1), 173; https://doi.org/10.3390/en12010173
Received: 8 December 2018 / Revised: 1 January 2019 / Accepted: 3 January 2019 / Published: 6 January 2019
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Abstract
The accurate air-fuel ratio (AFR) control is crucial for the exhaust emission reduction based on the three-way catalytic converter in the spark ignition (SI) engine. The difficulties in transient cylinder air mass flow measurement, the existing fuel mass wall-wetting phenomenon, and the unfixed
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The accurate air-fuel ratio (AFR) control is crucial for the exhaust emission reduction based on the three-way catalytic converter in the spark ignition (SI) engine. The difficulties in transient cylinder air mass flow measurement, the existing fuel mass wall-wetting phenomenon, and the unfixed AFR path dynamic variations make the design of the AFR controller a challenging task. In this paper, an adaptive AFR regulation controller is designed using the feedforward and feedback control scheme based on the dynamical modelling of the AFR path. The generalized predictive control method is proposed to solve the problems of inherent nonlinearities, time delays, parameter variations, and uncertainties in the AFR closed loop. The simulation analysis is investigated for the effectiveness of noise suppression, online prediction, and self-correction on the SI engine system. Moreover, the experimental verification shows an acceptable performance of the designed controller and the potential usage of the generalized predictive control in AFR regulation application. Full article
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Open AccessArticle An LCC-P Compensated Wireless Power Transfer System with a Constant Current Output and Reduced Receiver Size
Energies 2019, 12(1), 172; https://doi.org/10.3390/en12010172
Received: 14 November 2018 / Revised: 28 December 2018 / Accepted: 4 January 2019 / Published: 6 January 2019
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Abstract
Wireless Power Transfer (WPT) for autonomous underwater vehicles (AUVs) has been a research focus in recent years. This paper studies the inductor-capacitor-capacitor and parallel (LCC-P) compensation topology to achieve a compact receiver for AUVs. Unlike the series-series (SS) compensation topology, the LCC-P topology
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Wireless Power Transfer (WPT) for autonomous underwater vehicles (AUVs) has been a research focus in recent years. This paper studies the inductor-capacitor-capacitor and parallel (LCC-P) compensation topology to achieve a compact receiver for AUVs. Unlike the series-series (SS) compensation topology, the LCC-P topology retains the advantages of the double-sided LCC topology and has a more compact receiver than the double-sided LCC topology with fewer elements used on the receiver side. The analytical model of such a WPT system is established to analyze the output power and transfer efficiency. The LCC-P topology has a higher efficiency compared to the SS topology due to the smaller conduction loss of the inverter. Moreover, a method of eliminating the DC filter inductor L0 is proposed to further decrease the size and weight of the receiver. The amplitude of the withstanding voltage on the receiver compensation capacitor without L0 is approximately decreased by 40% compared to that with L0. Both cases of with and without L0 have a constant current output and the peak efficiency without L0 is about 94%, which is 1% lower than that with L0. A prototype was built and the experimental results verified the theoretical analysis. Full article
(This article belongs to the Special Issue Wireless Power for Electric Vehicles)
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Open AccessArticle Integration Capability Evaluation of Wind and Photovoltaic Generation in Power Systems Based on Temporal and Spatial Correlations
Energies 2019, 12(1), 171; https://doi.org/10.3390/en12010171
Received: 5 December 2018 / Revised: 28 December 2018 / Accepted: 31 December 2018 / Published: 5 January 2019
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Abstract
With the rapid growth of renewable energy generation, it has become essential to give a comprehensive evaluation of renewable energy integration capability in power systems to reduce renewable generation curtailment. Existing research has not considered the correlations between wind power and photovoltaic (PV)
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With the rapid growth of renewable energy generation, it has become essential to give a comprehensive evaluation of renewable energy integration capability in power systems to reduce renewable generation curtailment. Existing research has not considered the correlations between wind power and photovoltaic (PV) power. In this paper, temporal and spatial correlations among different renewable generations are utilized to evaluate the integration capability of power systems based on the copula model. Firstly, the temporal and spatial correlation between wind and PV power generation is analyzed. Secondly, the temporal and spatial distribution model of both wind and PV power generation output is formulated based on the copula model. Thirdly, aggregated generation output scenarios of wind and PV power are generated. Fourthly, wind and PV power scenarios are utilized in an optimal power flow calculation model of power systems. Lastly, the integration capacity of wind power and PV power is shown to be able to be evaluated by satisfying the reliability of power system operation. Simulation results of a modified IEEE RTS-24 bus system indicate that the integration capability of renewable energy generation in power systems can be comprehensively evaluated based on the temporal and spatial correlations of renewable energy generation. Full article
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Open AccessArticle Current Control of a Six-Phase Induction Machine Drive Based on Discrete-Time Sliding Mode with Time Delay Estimation
Energies 2019, 12(1), 170; https://doi.org/10.3390/en12010170
Received: 22 November 2018 / Revised: 27 December 2018 / Accepted: 29 December 2018 / Published: 5 January 2019
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Abstract
This paper proposes a robust nonlinear current controller that deals with the problem of the stator current control of a six-phase induction motor drive. The current control is performed by using a state-space representation of the system, explicitly considering the unmeasurable states, uncertainties
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This paper proposes a robust nonlinear current controller that deals with the problem of the stator current control of a six-phase induction motor drive. The current control is performed by using a state-space representation of the system, explicitly considering the unmeasurable states, uncertainties and external disturbances. To estimate these latter effectively, a time delay estimation technique is used. The proposed control architecture consists of inner and outer loops. The inner current control loop is based on a robust discrete-time sliding mode controller combined with a time delay estimation method. As said before, the objective of the time delay estimation is to reconstruct the unmeasurable states and uncertainties, while the sliding mode aims is to suppress the estimation error, to ensure robustness and finite-time convergence of the stator currents to their desired references. The outer loop is based on a proportional-integral controller to control the speed. The stability of the current closed-loop system is proven by establishing sufficient conditions on the switching gains. Experimental work has been conducted to verify the performance and the effectiveness of the proposed robust control scheme for a six-phase induction motor drive. The results obtained have shown that the proposed method allows good performances in terms of current tracking, in their corresponding planes. Full article
(This article belongs to the Special Issue Control of Multiphase Machines and Drives)
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