ALBERT

Description: Energies, Vol. 11, Pages 1622: Jatropha curcas, L. Pruning Residues for Energy: Characteristics of an Untapped By-Product Energies doi: 10.3390/en11071622 Authors: Luigi Pari Alessandro Suardi Leonardo Longo Monica Carnevale Francesco Gallucci Jatropha (Jatropha curcas, L.) is an energy crop mainly cultivated for the oil-seed, and the oil is usually used as bio-fuel. However, few studies have reported information about the utilization of the wood as a fuel for boiler heating systems. With 2500 jatropha trees per hectare, it is possible to produce about 3 t·ha−1·y−1 of woody biomass from pruning. In addition, jatropha trees are commonly cut down to a height of 45 cm once every 10 years, with a production of 80 t·ha−1 of dry matter of woody biomass. The use of this biomass has not yet been investigated. During the European project JatroMed, woody biomass from jatropha pruning was collected in Morocco. Chemical and physical characteristics of the wood were conducted according to UNI EN ISO standards. The following jatropha wood characteristics have been analyzed: Moisture and ash contents, the ash melting point, heating value, and concentrations of C, H, N, and S. This research focused on the evaluation of the potential use of jatropha pruning for energy production, and the results represent critical data that is useful for future studies and business potential.

Description: Energies, Vol. 11, Pages 1623: Gain Scheduled Torque Compensation of PMSG-Based Wind Turbine for Frequency Regulation in an Isolated Grid Energies doi: 10.3390/en11071623 Authors: Haixin Wang Junyou Yang Zhe Chen Weichun Ge Shiyan Hu Yiming Ma Yunlu Li Guanfeng Zhang Lijian Yang Frequency stability in an isolated grid can be easily impacted by sudden load or wind speed changes. Many frequency regulation techniques are utilized to solve this problem. However, there are only few studies designing torque compensation controllers based on power performances in different Speed Parts. It is a major challenge for a wind turbine generator (WTG) to achieve the satisfactory compensation performance in different Speed Parts. To tackle this challenge, this paper proposes a gain scheduled torque compensation strategy for permanent magnet synchronous generator (PMSG) based wind turbines. Our main idea is to improve the anti-disturbance ability for frequency regulation by compensating torque based on WTG speed Parts. To achieve higher power reserve in each Speed Part, an enhanced deloading method of WTG is proposed. We develop a new small-signal dynamic model through analyzing the steady-state performances of deloaded WTG in the whole range of wind speed. Subsequently, H∞ theory is leveraged in designing the gain scheduled torque compensation controller to effectively suppress frequency fluctuation. Moreover, since torque compensation brings about untimely power adjustment in over-rated wind speed condition, the conventional speed reference of pitch control system is improved. Our simulation and experimental results demonstrate that the proposed strategy can significantly improve frequency stability and smoothen power fluctuation resulting from wind speed variations. The minimum of frequency deviation with the proposed strategy is improved by up to 0.16 Hz at over-rated wind speed. Our technique can also improve anti-disturbance ability in frequency domain and achieve power balance.

Description: Energies, Vol. 11, Pages 1619: Study on the Propagation Characteristics of Partial Discharge in Switchgear Based on Near-Field to Far-Field Transformation Energies doi: 10.3390/en11071619 Authors: Yang Qi Yang Fan Bing Gao Yang Mengzhuo Ammad Jadoon Yu Peng Tian Jie Ultra-high frequency (UHF) electromagnetic (EM) signals generated by the partial discharge (PD) process of high-voltage equipment are now widely used in PD detection. The computation of EM propagation generated by a local discharge source using a uniformly hardwiring source can hardly reveal the discharge characteristics. In this paper, a method of near-field to far-field transformation is proposed to realize the study of the propagation characteristics of the PD signal. A short gap discharge model is established to get the near-field electromagnetics and the proposed method is validated by comparing the directly calculated results with the results of the near-field source. In the end, a model of switchgear is employed to study the propagation characteristics of the EM signal based on the proposed method. Via numerical calculation, the influence of the equipment in the switchgear on the propagation of the discharge EM is studied. It is found that the direction of the discharge source has a significant effect on the distribution of the electric field, which indicates that the discharge source cannot be simplified to a uniformly hardwiring source. In addition, it is also obtained that the amplitude of the electric field shows the same trend with the growth of the discharge channel, which gives a method for evaluating the development of the PD. Particularly, the near-field to far-field transformation can provide an effective method for studying the propagation of discharge EM waves in large-scale equipment.

Description: Energies, Vol. 11, Pages 1617: A Review and Analysis of Trends Related to Demand Response Energies doi: 10.3390/en11071617 Authors: Luis Alejandro Arias Edwin Rivas Francisco Santamaria Victor Hernandez This paper provides a review and analysis of trends related to demand response (DR). The authors have considered six different topics for the analysis of DR trends: Users, Network Services, Markets, Complementary Programs and Distributed Energy Resources (DER). A brief summary of the consulted articles is included and the behavior of the different DR trend-related topics is shown up to the year 2017 and their projections for 2020. As a result, the characterization of the main DR topics is obtained as well as its current and future trends. Based on the results of the study, it is concluded that the topic of complementary programs is a trendsetter for current trends and it is expected that there is a future change of focus towards the users and new services.

Description: Energies, Vol. 11, Pages 1629: Multiple Phase Change Material (PCM) Configuration for PCM-Based Heat Sinks—An Experimental Study Energies doi: 10.3390/en11071629 Authors: Idris Al Siyabi Sourav Khanna Tapas Mallick Senthilarasu Sundaram A small-scale phase change material (PCM)-based heat sink can regulate the temperature of electronics due to high latent-heat capacity. Three different heat sinks are examined to study the effects of PCM combination, arrangement of PCMs in multiple-PCM heat sink, PCM thickness, melting temperature and intensity of heat source on the thermal behavior of heat sink. Results are obtained for the temperature distribution across the heat sink and the PCM melting profile. It is concluded that (i) PCM combination RT50–RT55 increases the thermal regulation period and also reduces the heat sink temperature at the end of the operation, (ii) the RT58–RT47 arrangement slightly reduces the maximum temperature as compared to RT47–RT58, (iii) As PCM thickness increases from 30 mm to 60 mm, the thermal-regulation-period increases by 50 min, (iv) As the PCM melting temperature increases, the thermal-regulation-period and the heat sink temperature increase and (v) The thermal-regulation-period decreases as the power rating increases from 1 to 2 W.

Description: Energies, Vol. 11, Pages 1636: Optimal Deep Learning LSTM Model for Electric Load Forecasting using Feature Selection and Genetic Algorithm: Comparison with Machine Learning Approaches † Energies doi: 10.3390/en11071636 Authors: Salah Bouktif Ali Fiaz Ali Ouni Mohamed Adel Serhani Background: With the development of smart grids, accurate electric load forecasting has become increasingly important as it can help power companies in better load scheduling and reduce excessive electricity production. However, developing and selecting accurate time series models is a challenging task as this requires training several different models for selecting the best amongst them along with substantial feature engineering to derive informative features and finding optimal time lags, a commonly used input features for time series models. Methods: Our approach uses machine learning and a long short-term memory (LSTM)-based neural network with various configurations to construct forecasting models for short to medium term aggregate load forecasting. The research solves above mentioned problems by training several linear and non-linear machine learning algorithms and picking the best as baseline, choosing best features using wrapper and embedded feature selection methods and finally using genetic algorithm (GA) to find optimal time lags and number of layers for LSTM model predictive performance optimization. Results: Using France metropolitan’s electricity consumption data as a case study, obtained results show that LSTM based model has shown high accuracy then machine learning model that is optimized with hyperparameter tuning. Using the best features, optimal lags, layers and training various LSTM configurations further improved forecasting accuracy. Conclusions: A LSTM model using only optimally selected time lagged features captured all the characteristics of complex time series and showed decreased Mean Absolute Error (MAE) and Root Mean Square Error (RMSE) for medium to long range forecasting for a wider metropolitan area.

Description: Energies, Vol. 11, Pages 1630: Numerical Study on the Dynamic Behavior of a Francis Turbine Runner Model with a Crack Energies doi: 10.3390/en11071630 Authors: Ming Zhang David Valentin Carme Valero Mònica Egusquiza Weiqiang Zhao Crack appearance in the blade is the most common type of fatigue damage in Francis turbines. However, it is sometimes difficult to detect cracks in time using the current monitoring system, even when they are very large. To better monitor cracks, it is imperative to research the effect of a crack on the dynamic behavior of a Francis turbine. In this paper, the dynamic behavior of a Francis turbine runner model with a crack has been researched numerically. The intact numerical model was first validated by the experimental data available. Then, a crack was created at the intersection line between one blade and the crown. The change in dynamic behavior with increasing crack length has been investigated. Crack-induced vibration localization theory has been used to explain the dynamic behavior changes due to the crack. Modal analysis showed that the adopted theory could basically explain the modal behavior change due to the crack. The FFT results of the modal shapes and the localization factors (LF) has been used to explain the forced response changes due to the crack. Based on the above analysis, the challenge of crack monitoring has been analyzed. This research provides some references for more advanced monitoring technologies.

Description: Energies, Vol. 11, Pages 1632: Interleaved High Step-Up DC-DC Converter Based on Voltage Multiplier Cell and Voltage-Stacking Techniques for Renewable Energy Applications † Energies doi: 10.3390/en11071632 Authors: Shin-Ju Chen Sung-Pei Yang Chao-Ming Huang Huann-Ming Chou Meng-Jie Shen A novel interleaved high step-up DC-DC converter based on voltage multiplier cell and voltage-stacking techniques is proposed for the power conversion in renewable energy power systems. The circuit configuration incorporates an input-parallel output-series boost converter with coupled inductors, clamp circuits and a voltage multiplier cell stacking on the output side to extend the voltage gain. The converter achieves high voltage conversion ratio without working at extreme large duty ratio. The voltage stresses on the power switches are significantly lower than the output voltage. As a result, the low-voltage-rated metal-oxide-semiconductor field-effect transistors (MOSFETs) can be employed to reduce the conduction losses and higher conversion efficiency can be expected. The interleaved operation reduces the input current ripple. The leakage inductances of the coupled inductors act on mitigating the diode reverse recovery problem. The operating principle, steady-state analysis and design guidelines of the proposed converter are presented in detail. Finally, a 1-kW prototype with 28-V input and 380-V output voltages was implemented and tested. The experimental results are presented to validate the performance of the proposed converter.

Description: Energies, Vol. 11, Pages 1635: Game Theoretic Spectrum Allocation in Femtocell Networks for Smart Electric Distribution Grids Energies doi: 10.3390/en11071635 Authors: Ali Mohammadi Mohammad Javad Dehghani Elham Ghazizadeh Ever growing penetration of the behind-the-meter technologies is changing the electricity consumption profiles of end-users. Intelligent coordination of these emerging technologies through a robust communication infrastructure enables their seamless integration with electric utilities’ operation. In this context, an efficient and reliable communication infrastructure plays a pivotal role in enabling optimal integration of emerging resources. In this paper, we propose a game-theory based method to enhance efficiency of the underlying communication network. Specifically, we focus on Femtocell communication technology which is one the promising options for improving poor indoor communication coverage. The major drawback for using femtocell communication technology is cross-layer interference of femto users (FUs) and macro users (MUs) which adversely impact network performance. In this paper, we propose a novel approach for sharing spectrum in a cognitive radio system with FUs and MUs as primary and secondary users, respectively. The underlying problem is formulated as Stackelberg game that is joined with a convex optimization problem. In this study, MUs and FUs are assumed to be selfish, rational and motivated to achieve maximum utility function, while MUs are competing to obtain maximum bandwidth. Finally, we present a closed form solution for the proposed approach which obtains a unique Nash Equilibrium and prioritizes the access of MUs to femto-base stations. Simulation results provide proof of concept and verify the effectiveness of our mathematical modeling.

Description: Energies, Vol. 11, Pages 1640: Cycling Impact Assessment of Renewable Energy Generation in the Costs of Conventional Generators Energies doi: 10.3390/en11071640 Authors: Vincenzo Bassi Eduardo Pereira-Bonvallet Md Abu Abdullah Rodrigo Palma-Behnke This paper proposes a set of indicators to quantify the impact of conventional thermal generating unit cycling on its non-fuel variable costs (NFVC) due to generation mix changes in the system. A novel iterative cost adjustment framework is developed to evaluate the proposed indicators in order to assess the impacts of increasing installation of renewable resources on operation costs of the thermal units. The proposed framework allows private investors to estimate NFVC using a minimum level of information without a full knowledge of the system parameters. Additionally, the proposed framework is kept generic, which supports the NFVC adjustment for the conventional thermal units in a changing market environment. The impact of accelerated solar photovoltaic penetration on cycling and operational costs of existing thermal power plants in the Chilean power system is assessed using the indicators and methodology developed. The results suggest that natural gas driven peaking power plants are more susceptible to experiencing increased NFVC from solar photovoltaic growth than coal fired base load power plants.

Description: Energies, Vol. 11, Pages 1642: Analysis of the Operation of an Aerothermal Heat Pump in a Residential Building Using Building Information Modelling Energies doi: 10.3390/en11071642 Authors: Bárbara Torregrosa-Jaime Benjamín González Pedro J. Martínez Gaspar Payá-Ballester Heating, cooling and domestic hot water (DHW) are responsible for the largest share of energy use in residential buildings in Spain and play an important role in the implementation of nearly zero-energy buildings (NZEB). Building Information Modelling (BIM) is expected to promote more efficient buildings through evaluation of different design options. BIM can be used as a platform from which to gather information that can be conveyed to energy efficiency simulation tools. The objective of this paper was to implement the model of a reversible air-to-water heat pump in EnergyPlus 8.9. This model was employed to analyze the performance of an aerothermal heat pump system (B) in a residential building under different Spanish climates compared to a conventional Heating, Ventilating and Air Conditioning (HVAC) system (A). Significant primary energy savings were achieved with system B compared to system A. These energy savings were higher in climates with a significant heating demand such as Madrid (27.4%) and Burgos (33.6%), and in cities with a mild climate such as Barcelona (37%). The residential building studied in this work was classified as Class A according to the CO2 emissions scale when using the aerothermal heat pump system, and as Class B when using the conventional HVAC system.

Description: Energies, Vol. 11, Pages 1647: A Novel Electromagnetic Coupling Reactor Based Passive Power Filter with Dynamic Tunable Function Energies doi: 10.3390/en11071647 Authors: Yifei Wang Youxin Yuan Jing Chen Along with massive applications of power electronic equipment and non-linear loads, harmonic pollutions are becoming more serious than ever. This paper describes a novel electromagnetic coupling reactor based passive power filter with dynamic tuning to quickly eliminate harmony. Firstly, the structure and mathematic model of electromagnetic coupling reactor are presented; secondly, the employed parameters, including electromagnetic coupling reactance converter, primary winding coil, and secondary winding coil are designed based on a stable structure of data; then, the test plat of passive dynamic tunable filter is introduced, as well as the performance test. According to the experimental results, the newly designed electromagnetic coupling reactor can effectively eliminate the harmonies generated by the nonlinear load, greatly reducing the harm caused by harmonics on the grid. By fine-tuning the electromagnetic coupling reactor, the dissonance caused by the change of capacitance and other issues can be effectively solved. Finally, the detailed discussion of this paper is presented, and challenges and new future research are discussed.

Description: Energies, Vol. 11, Pages 1684: An Efficient Regional Sensitivity Analysis Method Based on Failure Probability with Hybrid Uncertainty Energies doi: 10.3390/en11071684 Authors: Dawei Zhang Weilin Li Xiaohua Wu Tie Liu The application of reliability sensitivity analysis (RSA) to the high voltage direct current (HVDC) transmission systems is one of the hot topics in the future. A regional RSA method, the contribution to failure probability (CFP) plot, is investigated in this paper. This CFP plot contains both aleatory and epistemic uncertain variables modeled as random variables by probability theory and interval variables by evidence theory, respectively. A surrogate model of second-level limit state function needs to be established for each joint focal element (JFE), which is a time-consuming process. Additionally, an excessive number of Monte Carlo simulations (MCS) and optimizations may exceed the computing power of modern computers. In order to deal with the above problems and further decrease the computational cost, a more effective CFP calculation method under the framework of random-evidence hybrid reliability analysis is proposed. Three important improvements in the proposed method make the calculation of CFP more efficient and easy to implement. Firstly, an active learning kriging (ALK) based on the symbol prediction idea is employed to directly establish a surrogate model rather than a second-level limit state function with fewer function calls, which greatly simplifies construction of the model. Secondly, a random set-based Monte Carlo simulation (RS-MCS) is used to handle the issue of oversized optimization caused by too many JFEs. Thirdly, for further reducing the size of optimizations and improving the efficiency of the CFP calculation, a Karush-Kuhn-Tucker-based optimization (KKTO) method is recommended in the proposed method to solve the extreme value of performance function. A numerical example and an engineering example were studied to verify the accuracy, effectiveness and practicality of the proposed method. It can be seen from the results that regardless of whether it is modeling or computational efficiency, the proposed method is better than the original method.

Description: Energies, Vol. 11, Pages 1676: Optimal Scheduling of Integrated Energy Systems with Combined Heat and Power Generation, Photovoltaic and Energy Storage Considering Battery Lifetime Loss Energies doi: 10.3390/en11071676 Authors: Yongli Wang Haiyang Yu Mingyue Yong Yujing Huang Fuli Zhang Xiaohai Wang Integrated energy systems (IESs) are considered a trending solution for the energy crisis and environmental problems. However, the diversity of energy sources and the complexity of the IES have brought challenges to the economic operation of IESs. Aiming at achieving optimal scheduling of components, an IES operation optimization model including photovoltaic, combined heat and power generation system (CHP) and battery energy storage is developed in this paper. The goal of the optimization model is to minimize the operation cost under the system constraints. For the optimization process, an optimization principle is conducted, which achieves maximized utilization of photovoltaic by adjusting the controllable units such as energy storage and gas turbine, as well as taking into account the battery lifetime loss. In addition, an integrated energy system project is taken as a research case to validate the effectiveness of the model via the improved differential evolution algorithm (IDEA). The comparison between IDEA and a traditional differential evolution algorithm shows that IDEA could find the optimal solution faster, owing to the double variation differential strategy. The simulation results in three different battery states which show that the battery lifetime loss is an inevitable factor in the optimization model, and the optimized operation cost in 2016 drastically decreased compared with actual operation data.

Description: Energies, Vol. 11, Pages 1682: Designing a Reliable and Congested Multi-Modal Facility Location Problem for Biofuel Supply Chain Network Energies doi: 10.3390/en11071682 Authors: Sushil Poudel Mohammad Marufuzzaman Md Abdul Quddus Sudipta Chowdhury Linkan Bian Brian Smith This study presents a mathematical model that designs a reliable multi-modal transportation network for a biofuel supply chain system while site-dependent facility failure and congestion are taken into consideration. The proposed model locates the multi-modal facilities and biorefineries and determines the optimal production, storage, and routing plans in such a way that the overall system cost is minimized. We propose a hybrid Constraint generation-based Rolling horizon algorithm to solve this challenging NP-hard problem. The performance of this algorithm is tested in a example case study with numerical analysis showing that the hybrid algorithm can find near-optimal solutions to large-scale problem instances in a reasonable amount of time. Results indicate that the effect of congestion reduces the usage of multi-modal facilities in the biofuel supply chain network while bio-refineries and multi-modal facilities tend to move away from coastal areas when disruption probabilities are taken into consideration.

Description: Energies, Vol. 11, Pages 1677: Influence of the Design Parameters of a Fuel Thermal Management System on Its Thermal Endurance Energies doi: 10.3390/en11071677 Authors: Liping Pang Shuxin Li Meng Liu Rong A Aicheng Li Fanxin Meng The aerodynamic heating of a high-speed vehicle is destined to lead to a continuous fuselage temperature rise. However, its airborne thermal load rises exponentially. This will severely limit the thermal endurance of the high-speed vehicle and the working time of the electronic equipment. A jet-propelled high-speed vehicle usually uses fuel to generate thrust, so fuel thermal management technology has had much attention paid to it. During the vehicle design, its total amount of fuel should match its flight envelope. However, determining the amount of carried fuel is very difficult because it is affected by many factors. In order to analyze the relationship between the above influence factors and the flight envelope, a typical fuel thermal management system is set up for high-speed vehicles. Its dynamic characteristic equations are built correspondingly. A conception of thermal endurance is further presented to reveal the maximum flight time. Some flight conditions are used to analyze the influence of the main design parameters on the thermal endurance of high-speed vehicles. The results can help to design the parameters of fuel thermal management systems for high-speed vehicles.

Description: Energies, Vol. 11, Pages 1674: A Thermal Probability Density–Based Method to Detect the Internal Defects of Power Cable Joints Energies doi: 10.3390/en11071674 Authors: Li Zhang Xiyue LuoYang Yanjie Le Fan Yang Chun Gan Yinxian Zhang Internal defects inside power cable joints due to unqualified construction is the main issue of power cable failures, hence in this paper a method based on thermal probability density function to detect the internal defects of power cable joints is presented. First, the model to calculate the thermal distribution of power cable joints is set up and the thermal distribution is calculated. Then a thermal probability density (TPD)-based method that gives the statistics of isothermal points is presented. The TPD characteristics of normal power cable joints and those with internal defects, including insulation eccentricity and unqualified connection of conductors, are analyzed. The results indicate that TPD differs with the internal state of cable joints. Finally, experiments were conducted in which surface thermal distribution was measured by FLIR SC7000, and the corresponding TPDs are discussed.

Description: Energies, Vol. 11, Pages 1669: Detection Method for Soft Internal Short Circuit in Lithium-Ion Battery Pack by Extracting Open Circuit Voltage of Faulted Cell Energies doi: 10.3390/en11071669 Authors: Minhwan Seo Taedong Goh Minjun Park Sang Woo Kim Early detection of internal short circuit which is main cause of thermal runaway in a lithium-ion battery is necessary to ensure battery safety for users. As a promising fault index, internal short circuit resistance can directly represent degree of the fault because it describes self-discharge phenomenon caused by the internal short circuit clearly. However, when voltages of individual cells in a lithium-ion battery pack are not provided, the effect of internal short circuit in the battery pack is not readily observed in whole terminal voltage of the pack, leading to difficulty in estimating accurate internal short circuit resistance. In this paper, estimating the resistance with the whole terminal voltages and the load currents of the pack, a detection method for the soft internal short circuit in the pack is proposed. Open circuit voltage of a faulted cell in the pack is extracted to reflect the self-discharge phenomenon obviously; this process yields accurate estimates of the resistance. The proposed method is verified with various soft short conditions in both simulations and experiments. The error of estimated resistance does not exceed 31.2% in the experiment, thereby enabling the battery management system to detect the internal short circuit early.

Description: Energies, Vol. 11, Pages 1686: Differential Evolution-Based Load Frequency Robust Control for Micro-Grids with Energy Storage Systems Energies doi: 10.3390/en11071686 Authors: Hongyue Li Xihuai Wang Jianmei Xiao In this paper, the secondary load frequency controller of the power systems with renewable energies is investigated by taking into account internal parameter perturbations and stochastic disturbances induced by the integration of renewable energies, and the power unbalance caused between the supply side and demand side. For this, the μ-synthesis robust approach based on structure singular value is researched to design the load frequency controller. In the proposed control scheme, in order to improve the power system stability, an ultracapacitor is introduced to the system to rapidly respond to any power changes. Firstly, the load frequency control model with uncertainties is established, and then, the robust controller is designed based on μ-synthesis theory. Furthermore, a novel method using integrated system performance indexes is proposed to select the weighting function during controller design process, and solved by a differential evolution algorithm. Finally, the controller robust stability and robust performance are verified via the calculation results, and the system dynamic performance is tested via numerical simulation. The results show the proposed method greatly improved the load frequency stability of a micro-grid power system.

Description: Energies, Vol. 11, Pages 1679: Development of Honeycomb Methanation Catalyst and Its Application in Power to Gas Systems Energies doi: 10.3390/en11071679 Authors: Philipp Biegger Florian Kirchbacher Ana Roza Medved Martin Miltner Markus Lehner Michael Harasek Fluctuating energy sources require enhanced energy storage demand, in order to ensure safe energy supply. Power to gas offers a promising pathway for energy storage in existing natural gas infrastructure, if valid regulations are met. To improve interaction between energy supply and storage, a flexible power to gas process is necessary. An innovative multibed methanation concept, based on ceramic honeycomb catalysts combined with polyimide membrane gas upgrading, is presented in this study. Cordierite monoliths are coated with γ-Al2O3 and catalytically active nickel, and used in a two-stage methanation process at different operation conditions (p = 6–14 bar, GHSV = 3000–6000 h−1). To fulfill the requirements of the Austrian natural gas network, the product gas must achieve a CH4 content of ≥96 vol %. Hence, CH4 rich gas from methanation is fed to the subsequent gas upgrading unit, to separate remaining H2 and CO2. In the present study, two different membrane modules were investigated. The results of methanation and gas separation clearly indicate the high potential of the presented process. At preferred operation conditions, target concentration of 96 vol % CH4 can be achieved.

Description: Energies, Vol. 11, Pages 1672: Nuclear Energy Development in Bangladesh: A Study of Opportunities and Challenges Energies doi: 10.3390/en11071672 Authors: Ridoan Karim Mohammad Ershadul Karim Firdaus Muhammad-Sukki Siti Hawa Abu-Bakar Nurul Aini Bani Abu Bakar Munir Ahmed Imran Kabir Jorge Alfredo Ardila-Rey Abdullahi Abubakar Mas’ud It is a challenging task for Bangladesh to meet its increasing demand of energy while its economy is rapidly growing. Though prices of oil, coal, and fossil fuels around the world have been volatile, the price trend in Bangladesh demonstrates a persistent rise in the immediate past. This is further exacerbated by depleting reserves of natural gas. Cumulatively, these two effects heighten Bangladesh’s energy needs. Bangladesh presently mitigates her energy requirements predominantly through natural gas, supplemented by a handful of coal and furnace oil plants. Consequently, due to scarcity of natural gas, oil and coal resources, nuclear power surfaces as a palatable strategic option for Bangladesh’s future development agenda. However, a successful nuclear power program entails an extensive infrastructure. Just like the other nuclear energy-producing countries, Bangladesh also face challenges to safeguard the health and security of nuclear reactors, the proper management for nuclear waste treatment and the security concerns over the safe usage of nuclear materials. Additionally, the development of nuclear energy is also bewildered because of the complex nature of fission technology, lack of efficient human resources, and non-existence of proper legal instruments to guide safe nuclear power production. Moreover, the further challenges include the questions, for example: whether the nuclear power plant project is supported by the mass people or not? What are the strategies for nuclear waste disposal? Whether the recent initiatives for nuclear energy production is necessary or the country should more concern with renewable sources? This paper evaluates the nuclear energy development in Bangladesh. It operates under qualitative methodological framework and utilises secondary sources for analysis. We advance several recommendations in this paper to remedy the highlighted issues: (1) creating a comprehensive legal and regulatory system; (2) fortifying home-based technology of nuclear development and simultaneously localising of foreign-based technology; (3) reducing the cost of nuclear energy production; (4) fast-tracking the procedure of efficient development of nuclear technology; (5) accelerating the procedure of switching to more efficient reactor and nuclear fuel cycle, and (6) conducting effective profile-raising activities to generate greater awareness and spur common acceptance.

Description: Energies, Vol. 11, Pages 1689: Application of Dimensional Analysis to Project Laboratory Scale Numerical Modelling Prescribed Hydraulic Fracturing Results to Field Scales Energies doi: 10.3390/en11071689 Authors: Qingyuan He Jinhai Xu Joung Oh Chengguo Zhang Hydraulic fracturing has been applied in the cave mining industry with the purpose of re-creating an orebody rock mass condition that is suitable for caving. Prescribed hydraulic fracturing was proposed in a previous study as a supplement to the conventional hydraulic fracturing strategy. In this paper; dimensional analysis is used to project laboratory scale numerical modelling results to field scales in order to study the applicability of creating prescribed hydraulic fractures (PHFs) under field conditions. The results indicate that field scale PHFs are feasible if the stress shadows of the pre-located fractures are properly utilized. Water can be used to create the pre-located fractures that induce the local stress change in a low differential stress state; and the use of more proppants and a shorter pre-located fracture spacing lead to PHFs propagating more quickly towards the pre-located fractures. For field condition having high differential stresses, more viscous fluid must be used to create the pre-located fractures in order to enhance the stress shadows. In this case, a shorter pre-located fracture spacing does not necessarily result in the re-orientation of PHFs towards the pre-located fractures and may even lead to unsatisfactory pre-conditioning. A sufficiently high pre-located fracture net pressure to the differential stress ratio (close to 0.5) is the prerequisite for creating PHFs.

Description: Energies, Vol. 11, Pages 1990: Temperature Distribution of HBC Fuses with Asymmetric Electric Current Ratios Through Fuselinks Energies doi: 10.3390/en11081990 Authors: Adrian Plesca In many industrial applications high breaking capacity (HBC) fuses are used to protect electrical installations against overcurrents, especially in the power distribution network. At high rated current, HBC fuses have more parallel fuselinks mounted inside. The technological and mounting processes of the fuselinks inside the fuse results in an asymmetrical current distribution through the parallel fuselinks. In this article a model of a high breaking capacity fuse using two parallel fuselinks is proposed. The influence of electric current, cross-section of the notches, distance between notches and current imbalance through fuselinks on the maximum temperature rise of both fuselinks, has been investigated. Also, a 3D thermal model for the same HBC fuse has been developed. The temperature spread into the fuse and its elements has been obtained. In order to prove the validity of the mathematical and 3D model different tests have been considered. The experimental, simulation and computed results give similar values and it results that this model can also be used for fuses with many parallel fuselinks and it permits the design of new fuse elements with optimal thermal distribution.

Description: Energies, Vol. 11, Pages 1989: Solar Panel Supplier Selection for the Photovoltaic System Design by Using Fuzzy Multi-Criteria Decision Making (MCDM) Approaches Energies doi: 10.3390/en11081989 Authors: Tien-Chin Wang Su-Yuan Tsai The period of industrialization and modernization has increased energy demands around the world. As with other countries, the Taiwanese government is trying to increase the proportion of renewable energy, especially solar energy resources. Thus, there are many solar power plants built in Taiwan. One of the most important components of a solar power plant is the solar panel. The solar panel supplier selection process is a complex and multi-faceted decision that can reduce the cost of purchasing equipment and supply this equipment on time. In this research, we propose fuzzy MCDM approach that includes fuzzy analytical hierarchy process model (FAHP) and data envelopment analysis (DEA) for evaluation and selection of solar panel supplier for a photovoltaic system design in Taiwan. The main objective of this work is to design a fuzzy MCDM approach for solar panel supplier selection based on qualitative and quantitative factors. In the first step of this research, FAHP is applied to define the priority of suppliers. The AHP combined with fuzzy logic (FAHP) can be used to rank suppliers; however, the disadvantages of the FAHP model is that input data, expressed in linguistic terms, depends on experience of experts and the number of suppliers is practically limited, because of the number of pairwise comparison matrices. Thus, we applied several DEA models for ranking potential suppliers in the final stages. As the result, decision making unit 1 (DMU 1) is the optimal solar panel supplier for photovoltaic system design in Taiwan. The contribution of this research is a new fuzzy MCDM for supplier selection under fuzzy environment conditions. This paper also lies in the evolution of a new approach that is flexible and practical to the decision maker. It provides a useful guideline for solar panel supplier selection in many countries as well as a guideline for supplier selection in other industries.

Description: Energies, Vol. 11, Pages 1983: Fault Location Method for DC Distribution Systems Based on Parameter Identification Energies doi: 10.3390/en11081983 Authors: Yan Xu Jingyan Liu Weijia Jin Yuan Fu Hui Yang When a short circuit fault occurs on the DC side line, the fault current reaches the peak within a few milliseconds, and the voltage drops significantly. This phenomenon can cause overcurrent flowing through the DC line, semiconductor devices, and AC side, which is a major threat to the operation of the entire system. To solve this problem, this paper proposes a fault location scheme based on parameter identification. Firstly, the entire DC distribution system is regarded as a graph. The intersections of the distribution system lines are regarded as vertices. The current flow of each line is regarded as a directed edge. The network topology matrix is constructed and a fault type recognition algorithm is proposed based on graph theory. Secondly, the mathematical model of the pole-to-pole short-circuit fault and pole-to-ground short-circuit fault are analyzed with double-ended electrical quantities. Transform the fault location problem into a parameter identification problem, four parameters to be identified are extracted, and the fitness function is constructed separately for two kinds of fault cases. Thirdly, a genetic algorithm (GA) is adopted to identify the value of parameters. Considering the fault types, transition resistance and fault location, the Matlab/Simulink simulation platform is used to simulate 18 fault conditions. The simulation results show that the positioning error of the fault location method is less than 1%, which is not affected by the transition resistance and has strong robustness.

Description: Energies, Vol. 11, Pages 1936: Strengths and Weaknesses of Existing Building Green Retrofits: Case Study of a LEED EBOM Gold Project Energies doi: 10.3390/en11081936 Authors: Xiaonuan Sun Zhonghua Gou Yi Lu Yiqi Tao This study investigated the process of existing building green retrofits through examining a Leadership in Energy and Environmental Design for Existing Building: Operations and Maintenance (LEED EBOM) Gold project. The project demonstrated a standard green retrofit process for existing buildings, which includes energy auditing, building performance simulation, and measurement and verification. In this project, four energy conservation measures were applied to improve energy performance: light-emitting diode (LED) lighting, window films, green roofs, and chilled water plant upgrading and optimization. The expected energy saving was 30% after the retrofit; while the actual energy saving was 16%. The error of building performance simulation was one of uncertainties in this retrofit project. Occupancy conditions might be the main reason for this uncertainty. Strengths, weaknesses, opportunities and threats were identified and discussed for the green retrofit. The research results could be used to optimize the existing building retrofit process for better energy performance.

Description: Energies, Vol. 11, Pages 1939: A Practical Formulation for Ex-Ante Scheduling of Energy and Reserve in Renewable-Dominated Power Systems: Case Study of the Iberian Peninsula Energies doi: 10.3390/en11081939 Authors: Miguel Carrión Rafael Zárate-Miñano Ruth Domínguez Scheduling energy and reserve in power systems with a large number of intermittent units is a challenging problem. Traditionally, the reserve requirements are assigned after clearing the day-ahead energy market using ad hoc rules or solving computationally intense mathematical programming problems to co-optimize energy and reserve. While the former approach often leads to costly oversized reserve provisions, the computational time required by the latter makes it generally incompatible with the daily power system operational practices. This paper proposes an alternative deterministic formulation for computing the energy and reserve scheduling, considering the uncertainty of the demand and the intermittent power production in such a way that the resulting problem requires a lower number of constraints and variables than stochastic programming-based formulations. The performance of the proposed formulation has been compared with respect to two standard stochastic programming formulations in a small-size power system. Finally, a realistic case study based on the Iberian Peninsula power system has been solved and discussed.

Description: Energies, Vol. 11, Pages 1935: Impact of Shear Zone on Rockburst in the Deep Neelum-Jehlum Hydropower Tunnel: A Numerical Modeling Approach Energies doi: 10.3390/en11081935 Authors: Abdul Muntaqim Naji Hafeezur Rehman Muhammad Zaka Emad Hankyu Yoo Rockburst is a hazardous phenomenon in deep tunnels influenced by geological structural planes like faults, joints, and shear planes. Small-scale shear-plane-like structures have damaging impact on the boundaries of the tunnel, which act as barrier and accumulate high stresses. A shear plane combined with high stress conditions is very dangerous in deep excavations. Such a shear plane exposed in the side wall of the right headrace tunnel in the Neelum-Jehlum Hydropower Project. This project is constructed in the tectonically active Himalayas under high stress conditions. The influence of a shear zone on rockburst occurrence near the tunnel is studied. The FLAC3D explicit code simulated the shear zone in the right tunnel, revealing that the stresses are concentrated near the shear zone, while no such stress concentration is present in the left tunnel. The Rock mass got displaced near this shear zone. Modeling results confirm that the presence of shear zone in side wall of the right tunnel has a major influence on rockburst occurrence. A shear slip along this plane released huge amounts of accumulated energy, causing human fatalities and property damage. A comparison of numerical simulation with empirical rockburst criteria validates the actual conditions and help us to understand the phenomenon of stress concentration near the shear zone and its impact during deep tunneling.

Description: Energies, Vol. 11, Pages 1997: Partial Discharge Analysis in High-Frequency Transformer Based on High-Frequency Current Transducer Energies doi: 10.3390/en11081997 Authors: Jun Jiang Mingxin Zhao Chaohai Zhang Min Chen Haojun Liu Ricardo Albarracín High-frequency transformers are the core components of power electronic transformers (PET), whose insulation is deeply threatened by high voltage (HV) and high frequency (HF). The partial discharge (PD) test is an effective method to assess an electrical insulation system. A PD measurement platform applying different frequencies was set up in this manuscript. PD signals were acquired with a high-frequency current transducer (HFCT). For improving the signal-to-noise (SNR) ratio of PD pulses, empirical mode decomposition (EMD) was used to increase the SNR by 4 dB. PD characteristic parameters such as partial discharge inception voltage (PDIV) and PD phase, number, and magnitude were all analyzed as frequency dependent. High frequency led to high PDIV and a smaller discharge phase region. PD number and magnitude were first up and then down as the frequency increased. As a result, a suitable frequency for evaluating the insulation of high-frequency transformers is proposed at 8 kHz according to this work.

Description: Energies, Vol. 11, Pages 1993: A Fully Distributed Approach for Economic Dispatch Problem of Smart Grid Energies doi: 10.3390/en11081993 Authors: Bo Li Yudong Wang Jian Li Shengxian Cao The cooperative, reliable and responsive characteristics make smart grid more popular than traditional power grid. However, with the extensive employment of smart grid concepts, the traditional centralized control methods expose a lot of shortcomings, such as communication congestion, computing complexity in central management systems, and so on. The distributed control method with flexible characteristics can meet the timeliness and effectiveness of information management in smart grid and ensure the information collection timely and the power dispatch economically. This article presents a decentralized approach based on multi agent system (MAS) for solving data collection and economic dispatch problem of smart grid. First, considering the generators and loads are distributed on many nodes in the space, a flooding-based consensus algorithm is proposed to achieve generator and load information for each agent. Then, a suitable distributed algorithm called λ-consensus is used for solving the economic dispatch problem, eventually, all generators can automatically minimize the total cost in a collective sense. Simulation results in standard test cases are presented to demonstrate the effectiveness of the proposed control strategy.

Description: Energies, Vol. 11, Pages 2009: Design and Control of a Three-Coil Permanent Magnet Spherical Motor Energies doi: 10.3390/en11082009 Authors: Kleymilson N. Souza Ricardo S. T. Pontes Andressa P. Oliveira Guilherme A. Barreto The permanent magnet (PM) spherical motor has been subject to growing interest from the scientific community due to its potential for applications in distinct areas, particularly in robotics, prosthetics, satellite control, sensors or camera systems. Motivated by this movement, the current work presents all the steps for the efficient design and construction of a spherical motor model, using compound deposition technology with the aid of a 3D printer. Furthermore, we report comprehensive studies on the accuracy of the positioning system of the proposed motor using only three stator coils, which jointly act to move the rotor axis toward any point in the hemisphere. Unmodeled nonlinear phenomena, such as friction, impair accurate positioning of the motor actuator, but this is solved by means of a visual servo control system, which allows the user to collect input–output data to train an artificial neural network model. Details on the construction of the proposed motor are reported, in addition to the numerical results of the positioning control in tracking a desired trajectory.

Description: Energies, Vol. 11, Pages 2010: Reinforcement Learning Based Energy Management Algorithm for Smart Energy Buildings Energies doi: 10.3390/en11082010 Authors: Sunyong Kim Hyuk Lim A smart grid facilitates more effective energy management of an electrical grid system. Because both energy consumption and associated building operation costs are increasing rapidly around the world, the need for flexible and cost-effective management of the energy used by buildings in a smart grid environment is increasing. In this paper, we consider an energy management system for a smart energy building connected to an external grid (utility) as well as distributed energy resources including a renewable energy source, energy storage system, and vehicle-to-grid station. First, the energy management system is modeled using a Markov decision process that completely describes the state, action, transition probability, and rewards of the system. Subsequently, a reinforcement-learning-based energy management algorithm is proposed to reduce the operation energy costs of the target smart energy building under unknown future information. The results of numerical simulation based on the data measured in real environments show that the proposed energy management algorithm gradually reduces energy costs via learning processes compared to other random and non-learning-based algorithms.

Description: Energies, Vol. 11, Pages 2048: Optimal Component Sizing for Peak Shaving in Battery Energy Storage System for Industrial Applications Energies doi: 10.3390/en11082048 Authors: Rodrigo Martins Holger C. Hesse Johanna Jungbauer Thomas Vorbuchner Petr Musilek Recent attention to industrial peak shaving applications sparked an increased interest in battery energy storage. Batteries provide a fast and high power capability, making them an ideal solution for this task. This work proposes a general framework for sizing of battery energy storage system (BESS) in peak shaving applications. A cost-optimal sizing of the battery and power electronics is derived using linear programming based on local demand and billing scheme. A case study conducted with real-world industrial profiles shows the applicability of the approach as well as the return on investment dependence on the load profile. At the same time, the power flow optimization reveals the best storage operation patterns considering a trade-off between energy purchase, peak-power tariff, and battery aging. This underlines the need for a general mathematical optimization approach to efficiently tackle the challenge of peak shaving using an energy storage system. The case study also compares the applicability of yearly and monthly billing schemes, where the highest load of the year/month is the base for the price per kW. The results demonstrate that batteries in peak shaving applications can shorten the payback period when used for large industrial loads. They also show the impacts of peak shaving variation on the return of investment and battery aging of the system.

Description: Energies, Vol. 11, Pages 2033: Temperature Estimation of Stator Winding in Permanent Magnet Synchronous Motors Using d-Axis Current Injection Energies doi: 10.3390/en11082033 Authors: Bum-Su Jun Joon Sung Park Jun-Hyuk Choi Ki-Doek Lee Chung-Yuen Won This paper presents a stator winding temperature detection method for permanent magnet synchronous motors (PMSMs) using a motor parameter estimation method. PMSM performance is highly dependent on the motor parameters. However, the motor parameters vary with temperature. It is difficult to measure motor parameters using a voltage equation without additional sensors. Herein, a stator winding temperature estimation method based on a d-axis current injection method is proposed. The proposed estimation method can be used to obtain stator temperatures and to achieve reliable operation. The validity of the proposed method is verified through simulations and experimental results.

Description: Energies, Vol. 11, Pages 2036: Spatial Environmental Assessment Tool (SEAT): A Modeling Tool to Evaluate Potential Environmental Risks Associated with Wave Energy Converter Deployments Energies doi: 10.3390/en11082036 Authors: Craig Jones Grace Chang Kaustubha Raghukumar Samuel McWilliams Ann Dallman Jesse Roberts Wave energy converter (WEC) arrays deployed in coastal regions may create physical disturbances, potentially resulting in environmental stresses. Presently, limited information is available on the nature of these physical disturbance or the resultant effects. A quantitative Spatial Environmental Assessment Tool (SEAT) for evaluating the potential effects of wave energy converter (WEC) arrays on nearshore hydrodynamics and sediment transport is presented for the central Oregon coast (USA) through coupled numerical model simulations of an array of WECs. Derived climatological wave conditions were used as inputs to the model to allow for the calculation of risk metrics associated with various hydrodynamic and sediment transport variables such as maximum shear stress, bottom velocity, and change in bed elevation. The risk maps provided simple, quantitative, and spatially-resolved means of evaluating physical changes in the vicinity of a hypothetical WEC array in response to varying wave conditions. The near-field risk of sediment mobility was determined to be moderate in the lee of the densely spaced array, where the potential for increased sediment deposition could result in benthic habitat alteration. Modifications to the nearshore sediment deposition and erosion patterns were observed near headlands and topographic features, which could have implications for littoral sediment transport. The results illustrate the benefits of a risk evaluation tool for facilitating coastal resource management at early market marine renewable energy sites.

Description: Energies, Vol. 11, Pages 2037: Evaluation of SF6 Leakage from Gas Insulated Equipment on Electricity Networks in Great Britain Energies doi: 10.3390/en11082037 Authors: Phillip Widger Abderrahmane (Manu) Haddad This paper examines the data collected from the power industry over the last six years of actual reported emissions of sulphur hexafluoride (SF6) and the potential impact. The SF6 emissions have been collated from the 14 different regions in England, Scotland, and Wales (Great Britain) from the six distribution network operators. The emissions of SF6 due to the transmission network of Great Britain have also been collated from the three different transmission network operators. By collecting this SF6 emissions data from the power industry, in both the distribution and transmission networks, an overall view of the scale of SF6 emissions in Great Britain can be evaluated. Data from the power industry also shows the inventory of SF6 power equipment in use over the last six years in Great Britain and shows the calculated percentage leakage rate of all of this equipment. In this paper, these figures, as reported by the electrical power industry to the UK government, have been used to estimate the likely inventory of SF6 equipment in England, Scotland, and Wales by 2050 and the future emissions of SF6 that could be leaked into the atmosphere by this equipment.

Description: Energies, Vol. 11, Pages 2071: An Analytical Model for the Regeneration of Wind after Exiting a Wind Farm Energies doi: 10.3390/en11082071 Authors: Brian H. Fiedler The simplest model for an atmospheric boundary layer assumes a uniform steady wind over a certain depth, of order 1 km, with the forces of friction, pressure gradient and Coriolis in balance. A linear model is here employed for the adjustment of wind to this equilibrium, as the wake of a very wide wind farm. A length scale is predicted for the exponential adjustment to equilibrium. Calculation of this length scale is aided by knowledge of the angle for which the wind would normally cross the isobars in environmental conditions in the wake.

Description: Energies, Vol. 11, Pages 2066: A Novel Single-Terminal Fault Location Method for AC Transmission Lines in a MMC-HVDC-Based AC/DC Hybrid System Energies doi: 10.3390/en11082066 Authors: Shimin Xue Junchi Lu Chong Liu Yabing Sun Baibing Liu Cheng Gu Accurate and reliable fault location method for alternating current (AC) transmission lines is essential to the fault recovery. MMC-based converter brings exclusive non-linear characteristics to AC networks under single-phase-to-ground faults, thus influencing the performance of the fault location method. Fault characteristics are related to the control strategies of the converter. However, the existing fault location methods do not take the control strategies into account, with further study being required to solve this problem. The influence of the control strategies to the fault compound sequence network is analyzed in this paper first. Then, a unique boundary condition that the fault voltage and negative-sequence fault current merely meet the direct proportion linear relationship at the fault point, is derived. Based on these, a unary linear regression analysis is performed, and the fault can be located according to the minimum residual sum function principle. The effectiveness of the proposed method is verified by PSCAD/EMTDC simulation platform. A large number of simulation results are used to verify the advantages on sampling frequency, fault resistance, and fault distance. More importantly, it provides a higher ranging precision and has extensive applicability.

Description: Energies, Vol. 11, Pages 2065: High Efficiency and Power Tracking Method for Wireless Charging System Based on Phase-Shift Control Energies doi: 10.3390/en11082065 Authors: Vladimir Kindl Martin Zavrel Pavel Drabek Tomas Kavalir The paper presents optimal operating point tracking algorithm for wireless charging system using identical coupling coils providing us to meet simultaneously high efficiency and high transmitted power under varied load and detuning conditions. The proposed method is suitable either for purely resistive load or battery load and it is based on phase-shift control between the primary and the secondary voltage. The paper also gives an intuitive mathematical description of the key control idea and demonstrates its operational abilities. The proposed algorithm is finally implemented into digital signal processor (DSP) and tested on 4 kW laboratory prototype of shielded wireless power transfer system.

Description: Energies, Vol. 11, Pages 2064: Streamer Inception from Ultra-Sharp Needles in Mineral Oil Based Nanofluids Energies doi: 10.3390/en11082064 Authors: Mauricio Aljure Marley Becerra Mattias E. Karlsson Positive and negative streamer inception voltages from ultra-sharp needle tips (with tip radii below 0.5 μm) are measured in TiO2, SiO2, Al2O3, ZnO and C60 nanofluids. The experiments are performed at several concentrations of nanoparticles dispersed in mineral oil. It is found that nanoparticles influence positive and negative streamers in different ways. TiO2, SiO2 and Al2O3 nanoparticles increase the positive streamer inception voltage only, whilst ZnO and C60 nanoparticles augment the streamer inception voltages in both polarities. Using these results, the main hypotheses explaining the improvement in the dielectric strength of the host oil due to the presence of nanoparticles are analyzed. It is found that the water adsorption hypothesis of nanoparticles is consistent with the increments in the reported positive streamer inception voltages. It is also shown that the hypothesis of nanoparticles reducing the electron velocity by hopping transport mechanisms fails to explain the results obtained for negative streamers. Finally, the hypothesis of nanoparticles attaching electrons according to their charging characteristics is found to be consistent with the results hereby presented on negative streamers.

Description: Energies, Vol. 11, Pages 2067: Power Quality: Scientific Collaboration Networks and Research Trends Energies doi: 10.3390/en11082067 Authors: Francisco G. Montoya Raul Baños Alfredo Alcayde Maria G. Montoya Francisco Manzano-Agugliaro Power quality is a research field related to the proper operation of devices and technological equipment in industry, service, and domestic activities. The level of power quality is determined by variations in voltage, frequency, and waveforms with respect to reference values. These variations correspond to different types of disturbances, including power fluctuations, interruptions, and transients. Several studies have been focused on analysing power quality issues. However, there is a lack of studies on the analysis of both the trending topics and the scientific collaboration network underlying the field of power quality. To address these aspects, an advanced model is used to retrieve data from publications related to power quality and analyse this information using a graph visualisation software and statistical tools. The results suggest that research interests are mainly focused on the analysis of power quality problems and mitigation techniques. Furthermore, they are observed important collaboration networks between researchers within and across countries.

Description: Energies, Vol. 11, Pages 2068: Sea-Water Desalination for Load Levelling of Gen-Sets in Small Off-Grid Islands Energies doi: 10.3390/en11082068 Authors: Alessandro Corsini Eileen Tortora This study deals with the energy/water nexus on small off-grid islands. Small islands share several characteristics that hinder the introduction of new plants, such as: Energy system balance when renewable sources are introduced; water shortages, usually addressed via shipping from the mainland; environmental and historical heritage values; and, scarce land availability. In these cases, it is mandatory to detect energy/water technology integration and management solutions respecting the peculiarities and boundaries of the sites. The present work proposes a desalination plant with a primary scope of load leveler and a secondary scope of water producer. The aim is to propose a simple and non-invasive solution for energy/water management in order to limit impacts on the local environment while improving the match between renewable energy and local generation by means of desalination. This study led to an integrated system composed of local diesel engine power plant, distributed roof-top photovoltaic plants and a desalination plant, managed by a dedicated control logic. Desalination from renewable energy and power adjustments of already active diesel engines are favored. The case study refers to Ponza island, in the Tyrrhenian Sea. This paper demonstrates the effective possibility of using a desalination plant with the double purpose of water production and load levelling, providing 98% of the local water demand while mitigating the renewable energy fluctuations effects on the gen-set. Moreover, the proposed system results in a 50% CO2eq emissions abatement over the current water supply carbon footprint.

Description: Energies, Vol. 11, Pages 2061: Identification of the Heat Equation Parameters for Estimation of a Bare Overhead Conductor’s Temperature by the Differential Evolution Algorithm Energies doi: 10.3390/en11082061 Authors: Mirza Sarajlić Jože Pihler Nermin Sarajlić Gorazd Štumberger This paper deals with the Differential Evolution (DE) based method for identification of the heat equation parameters applied for the estimation of a bare overhead conductor`s temperature. The parameters are determined in the optimization process using a dynamic model of the conductor; the measured environmental temperature, solar radiation and wind velocity; the current and temperature measured on the tested overhead conductor; and the DE, which is applied as the optimization tool. The main task of the DE is to minimise the difference between the measured and model-calculated conductor temperatures. The conductor model is relevant and suitable for the prediction of the conductor temperature, as the agreement between measured and model-calculated conductor temperatures is exceptional, where the deviation between mean and maximum measured and model-calculated conductor temperatures is less than 0.03 °C.

Description: Energies, Vol. 11, Pages 2059: Parameter Estimation of Electromechanical Oscillation Based on a Constrained EKF with C&I-PSO Energies doi: 10.3390/en11082059 Authors: Yonghui Sun Yi Wang Linquan Bai Yinlong Hu Denis Sidorov Daniil Panasetsky By combining together the extended Kalman filter with a newly developed C&I particle swarm optimization algorithm (C&I-PSO), a novel estimation method is proposed for parameter estimation of electromechanical oscillation, in which critical physical constraints on the parameters are taken into account. Based on the extended Kalman filtering algorithm, the constrained parameter estimation problem is formulated via the projection method. Then, by utilizing the penalty function method, the obtained constrained optimization problem could be converted into an equivalent unconstrained optimization problem; finally, the C&I-PSO algorithm is developed to address the unconstrained optimization problem. Therefore, the parameters of electromechanical oscillation with physical constraints can be successfully estimated and better performed. Finally, the effectiveness of the obtained results has been illustrated by several test systems.

Description: Energies, Vol. 11, Pages 2058: Thermophilic Anaerobic Digestion: Enhanced and Sustainable Methane Production from Co-Digestion of Food and Lignocellulosic Wastes Energies doi: 10.3390/en11082058 Authors: Aditi David Tanvi Govil Abhilash Kumar Tripathi Julie McGeary Kylie Farrar Rajesh Kumar Sani This article aims to study the codigestion of food waste (FW) and three different lignocellulosic wastes (LW) (Corn stover (CS), Prairie cordgrass (PCG), and Unbleached paper (UBP)) for thermophilic anaerobic digestion to overcome the limitations of digesting food waste alone (volatile fatty acids accumulation and low C:N ratio). Using an enriched thermophilic methanogenic consortium, all the food and lignocellulosic waste mixtures showed positive synergistic effects of codigestion. After 30 days of incubation at 60 °C (100 rpm), the highest methane yield of 305.45 L·kg−1 volatile solids (VS) was achieved with a combination of FW-PCG-CS followed by 279.31 L·kg−1 VS with a mixture of FW-PCG. The corresponding volatile solids reduction for these two co-digestion mixtures was 68% and 58%, respectively. This study demonstrated a reduced hydraulic retention time for methane production using FW and LW.

Description: Energies, Vol. 11, Pages 2060: An Electric Bus Power Consumption Model and Optimization of Charging Scheduling Concerning Multi-External Factors Energies doi: 10.3390/en11082060 Authors: Yajing Gao Shixiao Guo Jiafeng Ren Zheng Zhao Ali Ehsan Yanan Zheng With the large scale operation of electric buses (EBs), the arrangement of their charging optimization will have a significant impact on the operation and dispatch of EBs as well as the charging costs of EB companies. Thus, an accurate grasp of how external factors, such as the weather and policy, affect the electric consumption is of great importance. Especially in recent years, haze is becoming increasingly serious in some areas, which has a prominent impact on driving conditions and resident travel modes. Firstly, the grey relational analysis (GRA) method is used to analyze the various external factors that affect the power consumption of EBs, then a characteristic library of EBs concerning similar days is established. Then, the wavelet neural network (WNN) is used to train the power consumption factors together with power consumption data in the feature library, to establish the power consumption prediction model with multiple factors. In addition, the optimal charging model of EBs is put forward, and the reasonable charging time for the EB is used to achieve the minimum operating cost of the EB company. Finally, taking the electricity consumption data of EBs in Baoding and the data of relevant factors as an example, the power consumption prediction model and the charging optimization model of the EB are verified, which provides an important reference for the optimal charging of the EB, the trip arrangement of the EB, and the maximum profit of the electric public buses.

Description: Energies, Vol. 11, Pages 2062: An LQR-Based Controller Design for an LCL-Filtered Grid-Connected Inverter in Discrete-Time State-Space under Distorted Grid Environment Energies doi: 10.3390/en11082062 Authors: Thuy Vi Tran Seung-Jin Yoon Kyeong-Hwa Kim In order to alleviate the negative impacts of harmonically distorted grid conditions on inverters, this paper presents a linear quadratic regulator (LQR)-based current control design for an inductive-capacitive-inductive (LCL)-filtered grid-connected inverter. The proposed control scheme is constructed based on the internal model (IM) principle in which a full-state feedback controller is used for the purpose of stabilization and the integral terms as well as resonant terms are augmented into a control structure for the reference tracking and harmonic compensation, respectively. Additionally, the proposed scheme is implemented in the synchronous reference frame (SRF) to take advantage of the simultaneous compensation for both the negative and positive sequence harmonics by one resonant term. Since this leads to the decrease of necessary resonant terms by half, the computation effort of the controller can be reduced. With regard to the full-state feedback control approach for the LCL-filtered grid connected inverter, additional sensing devices are normally required to measure all of the system state variables. However, this causes a complexity in hardware and high implementation cost for measurement devices. To overcome this challenge, this paper presents a discrete-time current full-state observer that uses only the information from the control input, grid-side current sensor, and grid voltage sensor to estimate all of the system state variables with a high precision. Finally, an optimal linear quadratic control approach is introduced for the purpose of choosing optimal feedback gains, systematically, for both the controller and full-state observer. The simulation and experimental results are presented to prove the effectiveness and validity of the proposed control scheme.

Description: Energies, Vol. 11, Pages 2053: Productive Efficiency of Energy-Aware Data Centers Energies doi: 10.3390/en11082053 Authors: Damián Fernández-Cerero Alejandro Fernández-Montes Francisco Velasco Information technologies must be made aware of the sustainability of cost reduction. Data centers may reach energy consumption levels comparable to many industrial facilities and small-sized towns. Therefore, innovative and transparent energy policies should be applied to improve energy consumption and deliver the best performance. This paper compares, analyzes and evaluates various energy efficiency policies, which shut down underutilized machines, on an extensive set of data-center environments. Data envelopment analysis (DEA) is then conducted for the detection of the best energy efficiency policy and data-center characterization for each case. This analysis evaluates energy consumption and performance indicators for natural DEA and constant returns to scale (CRS). We identify the best energy policies and scheduling strategies for high and low data-center demands and for medium-sized and large data-centers; moreover, this work enables data-center managers to detect inefficiencies and to implement further corrective actions.

Description: Energies, Vol. 11, Pages 2063: The Efficiency of Long-Term Forecasting Model on Final Energy Consumption in Thailand’s Petroleum Industries Sector: Enriching the LT-ARIMAXS Model under a Sustainability Policy Energies doi: 10.3390/en11082063 Authors: Pruethsan Sutthichaimethee Kuskana Kubaha Presently, Thailand runs various sustainable development-based policies to boost the growth in economy, society, and environment. In this study, the economic and social growth was found to continuously increase and negatively deteriorate the environment at the same time due to a more massive final energy consumption in the petroleum industries sector than any other sectors. Therefore, it is necessary to establish national planning and it requires an effective forecasting model to support Thailand’s policy-making. This study aimed to construct a forecasting model for a final energy consumption prediction in Thailand’s petroleum industry sector for a longer-term (2018–2037) at a maximum efficiency from a certain class of methods. The Long Term-Autoregressive Integrated Moving Average with Exogeneous variables and Error Correction Mechanism model (LT-ARIMAXS model) (p, d, q, Xi, ECT(t−1)) was adapted from the autoregressive and moving average model incorporating influential variables together in both long-term relationships to produce the best model for prediction performance. All relevant variables in the model are stationary at Level I(0) or Level I(1). In terms of the extraneous variables, they consist of per capita GDP, population growth, oil price, energy intensity, urbanization rate, industrial structure, and net exports. The study found that the variables used are the causal factors and stationary at the first difference as well as co-integrated. With such features, it reflects that the variables are influential over the final energy consumption. The LT-ARIMAXS model (2,1,2) determined a proper period (t − i) through a white noise process with the Q test statistical method. It shows that the LT-ARIMAXS model (2,1,2) does not generate the issues of heteroskedasticity, multicollinearity, and autocorrelation. The performance of LT-ARIMAXS model (2,1,2) was tested based on the mean absolute percentage error (MAPE) and the root mean square error (RMSE). The LT-ARIMAXS model (2,1,2) can predict the final energy consumption based on the Sustainable Development Plan for the 20 years from 2018 to 2037. The results showed that the final energy consumption continues to increase steadily by 121,461 ktoe in 2037. Furthermore, the findings present that the growth rate (2037/2017) increases by 109.8%, which is not in line with Thailand’s reduction policy. In this study, the MAPE was valued at 0.97% and RMSE was valued at 2.12% when compared to the other old models. Therefore, the LT-ARIMAXS model (2,1,2) can be useful and appropriate for policy-making to achieve sustainability.

Description: Energies, Vol. 11, Pages 2087: Italian Experience on Electrical Storage Ageing for Primary Frequency Regulation Energies doi: 10.3390/en11082087 Authors: Roberto Benato Sebastian Dambone Sessa Maura Musio Francesco Palone Rosario Maria Polito The paper describes the results of different types of ageing tests performed by Terna (the Italian Transmission System Operator) applied to several electrochemical technologies, namely lithium-based and sodium-nickel chloride-based technologies. In particular, the tested lithium-based technologies exploit a graphite-based anode and the following cathode electrochemistries: lithium iron phosphate, lithium nickel cobalt aluminium, lithium nickel cobalt manganese, and lithium titanate. These tests have been performed in the storage labs located in Sardinia (Codrongianos) and Sicily (Ciminna). The aim of the storage labs is intended to give the electrical grid ancillary services, for example, primary frequency regulation, secondary frequency regulation, voltage regulation, synthetic rotational inertia provision, and many more. For the primary frequency regulation service, the ageing of the batteries is difficult to foresee as the ageing tests are not standardized. The authors proposed some novel cycle types, which showed that, in several cases, the frequency regulation cycle ages the batteries much more than the standard cycle. The standard cycle definition has been adopted in the paper to identify a battery cycle test that was carried out to uniformly compare and rank the different technologies. Moreover, sodium-nickel chloride batteries are unaffected by the types of cycle and have a negligible ageing. In addition, lithium manganese oxide and lithium titanate batteries show very good behaviour with a slight degradation of the dischargeable energy, irrespectively of the type of cycle. Inversely, lithium nickel cobalt aluminium technology shows a considerable ageing and a strong dependence on the cycle types. Even if the theoretical explanations of such aging behaviours need time to be understood and expounded, the authors are convinced that the scientific community should become aware of these experimental results.

Description: Energies, Vol. 11, Pages 2086: Ventilation System Influence on Hydrogen Explosion Hazards in Industrial Lead-Acid Battery Rooms Energies doi: 10.3390/en11082086 Authors: Dorota Brzezińska When charging most types of industrial lead-acid batteries, hydrogen gas is emitted. A large number of batteries, especially in relatively small areas/enclosures, and in the absence of an adequate ventilation system, may create an explosion hazard. This paper describes full scale tests, which demonstrate conditions that can occur in a battery room in the event of a ventilation system breakdown. Over the course of the tests, full scale hydrogen emission experiments were performed to study emission time and flammable cloud formation according to the assumed emission velocity. On this basis, the characteristics of dispersion of hydrogen in the battery room were obtained. The CFD model Fire Dynamic Simulator created by National Institute of Standards and Technology (NIST) was used for confirmation that the lack of ventilation in a battery room can be the cause of an explosive atmosphere developing, and leading to, a potential huge explosive hazard. It was demonstrated that different ventilation systems provide battery rooms with varying efficiencies of hydrogen removal. The most effective type appeared to be natural ventilation, which proved more effective than mechanical means.

Description: Energies, Vol. 11, Pages 2085: Efficient and Privacy-Preserving Data Aggregation and Dynamic Billing in Smart Grid Metering Networks Energies doi: 10.3390/en11082085 Authors: An Braeken Pardeep Kumar Andrew Martin The smart grid enables convenient data collection between smart meters and operation centers via data concentrators. However, it presents security and privacy issues for the customer. For instance, a malicious data concentrator cannot only use consumption data for malicious purposes but also can reveal life patterns of the customers. Recently, several methods in different groups (e.g., secure data aggregation, etc.) have been proposed to collect the consumption usage in a privacy-preserving manner. Nevertheless, most of the schemes either introduce computational complexities in data aggregation or fail to support privacy-preserving billing against the internal adversaries (e.g., malicious data concentrators). In this paper, we propose an efficient and privacy-preserving data aggregation scheme that supports dynamic billing and provides security against internal adversaries in the smart grid. The proposed scheme actively includes the customer in the registration process, leading to end-to-end secure data aggregation, together with accurate and dynamic billing offering privacy protection. Compared with the related work, the scheme provides a balanced trade-off between security and efficacy (i.e., low communication and computation overhead while providing robust security).

Description: Energies, Vol. 11, Pages 2084: A New Gob-Side Entry Layout Method for Two-Entry Longwall Systems Energies doi: 10.3390/en11082084 Authors: Rui Wu Qingyuan He Joung Oh Zecheng Li Chengguo Zhang The gob-side entry layout is popular at two-entry longwall mine sites in China for the benefit of improving the coal recovery rate. Currently, two methods have been widely used to develop gob-side entries, including gob-side entry retaining and gob-side entry driving. Gob-side entry retaining maximizes the recovery rate by pillarless mining but increases the difficulty in gob-side entry support. Also, this method has limited applications in hard roof conditions. The gob-side entry driving mine site uses the rib pillar to separate the gob entry and the gob area of the previous panel, which leads to additional coal losses. The waste is more intolerable in large-cutting-height panels and longwall top coal caving panels as the Chinese government limits the minimum recovery rate of longwall panels using these mining methods. In this paper, a new gob-side entry layout method, termed gob-side pre-backfill driving, is established to overcome the shortcomings of the existing methods. The new method eliminates rib pillar losses and enhances gob-side entry stability. The feasibility of gob-side pre-backfill driving is studied by numerical modelling and a field trial at Changcun Mine in China. The results indicate that gob-side pre-backfill driving is an alternative for gob-side entry development. This method is practical and also has the potential to bring significant economic benefits to the mining industry.

Description: Energies, Vol. 11, Pages 2083: A 120 W Class-E Power Module with an Adaptive Power Combiner for a 6.78 MHz Wireless Power Transfer System Energies doi: 10.3390/en11082083 Authors: Ui-Gyu Choi Jong-Ryul Yang In this article, a highly efficient power module is presented with two class-E power amplifiers and an adaptive power combiner for transmitting output powers >100 W at 6.78 MHz in a wireless power transfer system. The losses caused by the combiners and interstage matching circuits or mismatching between the amplifier, and the combiners can significantly reduce the overall efficiency of the power module. To achieve an efficient combination of the output amplifier signals, the adaptive power combiner is proposed based on the consideration of the optimum load impedance characteristics of the power amplifiers. The input impedance of the combiner is designed using series capacitors and resistors between the two input ports of the combiner and the two output signals of the class-E amplifiers at the optimum load condition. The output performances of the proposed module can decrease based on the component mismatch between the two power amplifiers. The proposed power module was implemented on an FR4 PCB, with a 15 mm metal heat sink, and demonstrated an output power of 123.3 W, a power-added efficiency of 85.7%, and a power gain of 25.6 dB at 6.78 MHz. The second harmonic suppression of the module was 37 dBc.

Description: Energies, Vol. 11, Pages 2080: Empirical Comparison of Neural Network and Auto-Regressive Models in Short-Term Load Forecasting Energies doi: 10.3390/en11082080 Authors: Miguel López Carlos Sans Sergio Valero Carolina Senabre Artificial Intelligence (AI) has been widely used in Short-Term Load Forecasting (STLF) in the last 20 years and it has partly displaced older time-series and statistical methods to a second row. However, the STLF problem is very particular and specific to each case and, while there are many papers about AI applications, there is little research determining which features of an STLF system is better suited for a specific data set. In many occasions both classical and modern methods coexist, providing combined forecasts that outperform the individual ones. This paper presents a thorough empirical comparison between Neural Networks (NN) and Autoregressive (AR) models as forecasting engines. The objective of this paper is to determine the circumstances under which each model shows a better performance. It analyzes one of the models currently in use at the National Transport System Operator in Spain, Red Eléctrica de España (REE), which combines both techniques. The parameters that are tested are the availability of historical data, the treatment of exogenous variables, the training frequency and the configuration of the model. The performance of each model is measured as RMSE over a one-year period and analyzed under several factors like special days or extreme temperatures. The AR model has 0.13% lower error than the NN under ideal conditions. However, the NN model performs more accurately under certain stress situations.

Description: Energies, Vol. 11, Pages 2082: Numerical Investigation of the Effects of Steam Mole Fraction and the Inlet Velocity of Reforming Reactants on an Industrial-Scale Steam Methane Reformer Energies doi: 10.3390/en11082082 Authors: Chun-Lang Yeh Steam methane reforming (SMR) is the most common commercial method of industrial hydrogen production. Control of the catalyst tube temperature is a fundamental demand of the reformer design because the tube temperature must be maintained within a range that the catalysts have high activity and the tube has minor damage. In this paper, the transport and chemical reaction in an industrial-scale steam methane reformer are simulated using computational fluid dynamics (CFD). Two factors influencing the reformer temperature, hydrogen yield and stress distribution are discussed: (1) the mole fraction of steam (YH2O) and (2) the inlet velocity of the reforming reactants. The purpose of this paper is to get a better understanding of the flow and thermal development in a reformer and thus, to make it possible to improve the performance and lifetime of a steam reformer. It is found that the lowest temperature at the reforming tube surface occurs when YH2O is 0.5. Hydrogen yield has the highest value when YH2O is 0.5. The wall shear stress at the reforming tube surface is higher at a higher YH2O. The surface temperature of a reforming tube increases with the inlet velocity of the reforming reactants. Finally, the wall shear stress at the reforming tube surface increases with the inlet velocity of the reforming reactants.

Description: Energies, Vol. 11, Pages 2079: A DC Short-Circuit Fault Ride Through Strategy of MMC-HVDC Based on the Cascaded Star Converter Energies doi: 10.3390/en11082079 Authors: Yingjie Wang Bo Yang Huifang Zuo Haiyuan Liu Haohao Yan A modular multilevel converter based high voltage direct current (MMC-HVDC) with DC fault self-clearing is adopted to deal with the DC short-circuit fault. However, the constant power load characteristic of the sub-modules causes capacitor voltages to diverge and the converter to go out of hot standby. To address this problem, a novel DC short-circuit fault ride through strategy is proposed. According to the polarities of grid voltages, the working or blockage of the upper and lower bridge arms is chosen according to six sections to obtain a cascaded star converter. The capacitor voltages of MMC sub-modules are maintained and balanced through the control similar to the cascaded star converter. Moreover, in order not to change zero crossing, a cluster balancing control method by scaling the amplitudes of the modulated waves is proposed to balance the capacitor voltages between phase clusters. The strategy also achieves the DC Bus line-to-line equipotential and no fault current generated. With the switches of two modes (normal operation and fault ride through operation) after the fault is cleared, the power transfer of MMC-HVDC can be recovered quickly. Finally, the effectiveness of the proposed fault ride through strategy is demonstrated on the 21-level MMC-HVDC simulation model in PSCAD/EMTDC.

Description: Energies, Vol. 11, Pages 2081: Design and Prototyping Medium-Frequency Transformers Featuring a Nanocrystalline Core for DC–DC Converters Energies doi: 10.3390/en11082081 Authors: Dante Ruiz-Robles Vicente Venegas-Rebollar Adolfo Anaya-Ruiz Edgar L. Moreno-Goytia Juan R. Rodríguez-Rodríguez Medium frequency transformers (MFTs) are a key component of DC–DC dual active bridge (DAB)-type converters. These technologies are becoming a quintessential part of renewable energy solutions, such as photovoltaic systems and wind energy power plants, as well as in modern power grid interfaces functioning as solid-state transformers in smart-grid environments. The weight and physical dimensions of an MFT are key data for the design of these devices. The size of an MFT is reduced by increasing its operating frequency. This reduction implicates higher power density through the transformer windings, as well as other design requirements distinct to those used for conventional 60/50 Hz transformers; therefore, new MFT design procedures are needed. This paper introduces a novel methodology for designing MFTs, using nanocrystalline cores, and tests it using an MFT–DAB lab prototype. Different to other MFT design procedures, this new design approach uses a modified version of the area-product technique, which consists of smartly modifying the core losses computation, and includes nanocrystalline cores. The core losses computation is supported by a full analysis of the dispersion inductance. For purposes of validation, a model MFT connected to a DAB converter is simulated in Matlab-Simulink (The MathWorks, v2014a, Mexico City, Mexico). In addition, a MFT–DAB lab prototype (1 kVA at 5 kHz) is implemented to experimentally probe further the validity of the methodology just proposed. These results demonstrate that the analytic calculations results match those obtained from simulations and lab experiments. In all cases, the efficiency of the MFT is greater than 99%.

Description: Energies, Vol. 11, Pages 2095: Site Selection of Hybrid Offshore Wind and Wave Energy Systems in Greece Incorporating Environmental Impact Assessment Energies doi: 10.3390/en11082095 Authors: Eva Loukogeorgaki Dimitra G. Vagiona Margarita Vasileiou This paper presents a methodological framework for evaluating marine areas in Greece for the purpose of identifying the most adequate sites for Hybrid Offshore Wind and Wave Energy Systems (HOWiWaES), with special focus on the HOWiWaES’ environmental impact assessment evaluation. Nine evaluation criteria that reflect various environmental, economic, technical and socio-political aspects are considered, including Wind Velocity (WV), Wave Energy Potential (WEP), Water Depth (WD), Distance from Shore (DS), Connection to Local Electrical Grid (CLEG), Population Served (PS), Shipping Density (SD), Distance from Ports (DP) and Environmental Performance Value (EPV). Analytical Hierarchy Process (AHP) is performed to hierarchically rank twelve predefined siting alternatives. Questionnaires are used to collect information on pairwise comparisons of the evaluation criteria from a group of stakeholders/experts. Geographic Information Systems (GIS) are used as a metric tool for pairwise comparisons of each siting alternative with respect to the first eight evaluation criteria, while the last criterion is assessed through the development of an innovative environmental impact assessment tool. The results indicate that WV, WEP and EPV present the evaluation criteria with the highest relative significance, while PS, DP and SD correspond to less influencing criteria. The proposed methodology can be easily applied to other countries worldwide for supporting socially accepted siting of HOWiWaES.

Description: Energies, Vol. 11, Pages 2123: Controlled Mechanical Ventilation in Buildings: A Comparison between Energy Use and Primary Energy among Twenty Different Devices Energies doi: 10.3390/en11082123 Authors: Lamberto Tronchin Kristian Fabbri Chiara Bertolli Indoor air quality (IAQ) of buildings is a problem that affects both comfort for occupants and the energy consumption of the structure. Controlled mechanical ventilation systems (CMVs) make it possible to control the air exchange rate. When using CMV systems, it is interesting to investigate the relationship between the useful thermal energy requirements for ventilation and the energy consumption of these systems. This paper addresses whether there is a correlation between these two parameters. The methodology used in this work involves the application of equations of technical Italian regulations UNI/TS 11300 applied to a case study. The case study is represented by a 54 m3 room, which is assumed to have three CMV systems installed (extraction, insertion, insertion and extraction) for twenty different devices available on the market. Afterwards, simulations of useful thermal energy requirements QH,ve and primary energy EP,V were performed according to the electrical power of each fan W and the ventilation flow. The results show that the two values are not linearly correlated: it is not possible to clearly associate the operating cost for CMV systems according to building requirements. The study also shows that CMV systems are particularly efficient for high-performance buildings, where there is no leakage that can be ascribed to windows infiltrations.

Description: Energies, Vol. 11, Pages 2122: Design of a Ventilation System Coupled with a Horizontal Air-Ground Heat Exchanger (HAGHE) for a Residential Building in a Warm Climate Energies doi: 10.3390/en11082122 Authors: Cristina Baglivo Delia D’Agostino Paolo Maria Congedo Energy consumption in new buildings can be reduced at the design stage. This study optimizes the ventilation system design of a new residential building located in a warm climate (Southern Italy). Different system options of horizontal air-ground heat exchangers (HAGHEs), also called earth-to-air heat exchangers (EAHX), have been considered to search for the optimal configuration. The thermal behaviour of the obtained configurations has been modelled by the dynamic simulation software TRNSYS 17. The pipe numbers, the air flow rate, and the soil thermal conductivity are among the simulated building components. For each of them, different design options have been analysed to study how each parameter impacts the building thermal behaviour in winter and summer. The operative air temperature (TOP) has been evaluated inside the building prototype to investigate the indoor comfort. The paper demonstrates that HAGHEs permit to assure a suitable indoor climatization if the building envelope is optimized for a warm area. These conditions require high values of heat storage capacity to keep under control the internal temperature fluctuations, especially in summer. The paper confirms the importance of geothermal systems and design optimization to increase energy savings.

Description: Energies, Vol. 11, Pages 2119: Regional Energy, CO2, and Economic and Air Quality Index Performances in China: A Meta-Frontier Approach Energies doi: 10.3390/en11082119 Authors: Ying Li Yung-Ho Chiu Liang Chun Lu Rapid economic development has resulted in a significant increase in energy consumption and pollution such as carbon dioxide (CO2), particulate matter (PM2.5), particulate matter 10 (PM10), SO2, and NO2 emissions, which can cause cardiovascular and respiratory diseases. Therefore, to ensure a sustainable future, it is essential to improve economic efficiency and reduce emissions. Using a Meta-frontier Non-radial Directional Distance Function model, this study took energy consumption, the labor force, and fixed asset investments as the inputs, Gross domestic product (GDP) as the desirable output, and CO2 and the Air Quality Index (AQI) scores as the undesirable outputs to assess energy efficiency and air pollutant index efficiency scores in China from 2013–2016 and to identify the areas in which improvements was necessary. It was found that there was a large gap between the western and eastern cities in China. A comparison of the CO2 and AQI in 31 Chinese cities showed a significant difference in the CO2 emissions and AQI efficiency scores, with the lower scoring cities being mainly concentrated in China’s western region. It was therefore concluded that China needs to pay greater attention to the differences in the economic levels, stages of social development, and energy structures in the western cities when developing appropriately focused improvement plans.

Description: Energies, Vol. 11, Pages 2118: Auto-Adaptive Filtering-Based Energy Management Strategy for Fuel Cell Hybrid Electric Vehicles Energies doi: 10.3390/en11082118 Authors: Jamila Snoussi Seifeddine Ben Elghali Mohamed Benbouzid Mohamed Faouzi Mimouni The global need to solve pollution problems has conducted automotive engineers to promote the development and the use of electric vehicle technologies. This paper focuses on the fuel cell hybrid electric vehicle which uses a proton exchange membrane fuel cell as a main source associated to hybrid storage device: lithium ion battery and ultracapacitors. A common interest in such technology is to spread out the energy flow between its different sources in order to satisfy the power demand for any requested mission. However, the challenging task stills the optimization of this split to reduce hydrogen consumption and respect, at the same time, the system limitations such as admissible limits of storage system capacities and battery current variation. An adaptive filtering-based energy management strategy is proposed in this paper to ensure an optimum distribution of the energy between the sources taking into account dynamic and energetic constraints of each device. For more performance, a fuzzy logic system is used to adapt the frequency of separation with the system state evolution. A sliding mode control is applied to control electric characteristics (voltage and currents) in the considered hybrid power supply. Simulation results, obtained under MATLAB®/SimPowerSystems® for four driving cycles are presented. The proposed strategy achieved good performances by respecting the ultracapacitors state of charge while preserving the battery lifetime under various driving missions.

Description: Energies, Vol. 11, Pages 2117: Synthesis of the ZnO@ZnS Nanorod for Lithium-Ion Batteries Energies doi: 10.3390/en11082117 Authors: Haipeng Li Jiayi Wang Yan Zhao Taizhe Tan The ZnO@ZnS nanorod is synthesized by solvothermal method as an anode material for lithium ion batteries. ZnS is deposited on ZnO and assembles in nanorod geometry successfully. The nanosized rod structure supports ion diffusion by substantially reducing the ion channel. The close-linking of ZnS and ZnO improves the synergetic effect. ZnS is in the middle of the ZnO core and the external environment, which would greatly relieve the volume change of the ZnO core during the Li+ intercalation/de-intercalation processes; therefore, the ZnO@ZnS nanorod is helpful in maintaining excellent cycle stability. The ZnO@ZnS nanorod shows a high discharge capacity of 513.4 mAh g−1 at a current density of 200 mA g−1 after 100 cycles, while a reversible capacity of 385.6 mAh g−1 is achieved at 1000 mA g−1.

Description: Energies, Vol. 11, Pages 2115: Analysis of Propagation Delay for Multi-Terminal High Voltage Direct Current Networks Interconnecting the Large-Scale Off-Shore Renewable Energy Energies doi: 10.3390/en11082115 Authors: Muhammad Haroon Nadeem Xiaodong Zheng Nengling Tai Mehr Gul Sohaib Tahir Voltage-source-converter-based multi-terminal high voltage direct current (MTDC) networks are extensively recognized as a viable solution for meeting the increasing demand of electrical energy and escalating penetration of renewable energy sources. DC faults are major limitations to the development of MTDC networks. The analysis of variable constraints has become mandatory in order to develop a reliable protection scheme. This paper contributes in assessing the propagation delay with the analytical approximation in MTDC networks. The propagation delay is analyzed in the time domain by taking only the forward traveling wave into account and considering the initial voltage step of magnitude at the fault position. Numerous simulations were carried out for different parameters and arrangements in Power System Computer Aided Design (PSCAD) to explore the proposed expressions. The results accurately depicted the time development of fault current. The results obtained from the real-time digital simulator (RTDS) confirmed that the proposed approach is capable of evaluating propagation delay in MTDC networks. Moreover, the influence of fault resistance is also taken into account for investigating its effect on the system parameters.

Description: Energies, Vol. 11, Pages 2114: Application of Markov Model to Estimate Individual Condition Parameters for Transformers Energies doi: 10.3390/en11082114 Authors: Amran Mohd Selva Norhafiz Azis Muhammad Sharil Yahaya Mohd Zainal Abidin Ab Kadir Jasronita Jasni Young Zaidey Yang Ghazali Mohd Aizam Talib This paper presents a study to estimate individual condition parameters of the transformer population based on Markov Model (MM). The condition parameters under study were hydrogen (H2), methane (CH4), acetylene (C2H2), ethylene (C2H4), ethane (C2H6), carbon monoxide (CO), carbon dioxide (CO2), dielectric breakdown voltage, interfacial tension, colour, acidity, water content, and 2-furfuraldehyde (2-FAL). First, the individual condition parameter of the transformer population was ranked and sorted based on recommended limits as per IEEE Std. C57. 104-2008 and IEEE Std. C57.106-2015. Next, the mean for each of the condition parameters was computed and the transition probabilities for each condition parameters were obtained based on non-linear optimization technique. Next, the future states probability distribution was computed based on the MM prediction model. Chi-square test and percentage of absolute error analysis were carried out to find the goodness-of-fit between predicted and computed condition parameters. It is found that estimation for majority of the individual condition parameter of the transformer population can be carried out by MM. The Chi-square test reveals that apart from CH4 and C2H4, the condition parameters are outside the rejection region that indicates agreement between predicted and computed values. It is also observed that the lowest and highest percentages of differences between predicted and computed values of all the condition parameters are 81.46% and 98.52%, respectively.

Description: Energies, Vol. 11, Pages 2113: Thermally Induced Mechanical Stress in the Stator Windings of Electrical Machines Energies doi: 10.3390/en11082113 Authors: Bishal Silwal Peter Sergeant The lifetime of an electrical machine mainly depends on the thermal overloading. Modern day applications of electrical machines on one hand require compact machines with high power density, while on the other hand force electrical machines to undergo frequent temperature cycling. Until recently, in the case of electrical machines, the main factor related to the degradation of the winding insulation was thought to be the thermal oxidization of the insulation materials. It has now been revealed that thermal overloading can also induce mechanical stress in the windings of electrical machines, which over time could lead to fatigue and degradation. In this paper, a comprehensive study of the thermally induced mechanical stress in the windings of an electrical machine is presented. The study is performed using combined thermo-mechanical models. The numerical results are validated by experiments on a segmented stator winding set-up.

Description: Energies, Vol. 11, Pages 2112: A Bottom-Up Model for Household Load Profile Based on the Consumption Behavior of Residents Energies doi: 10.3390/en11082112 Authors: Bingtuan Gao Xiaofeng Liu Zhenyu Zhu The forecasting of the load profile of the domestic sector is an area of increased concern for the power grid as it appears in many applications, such as grid operations, demand side management, energy trading, and so forth. Accordingly, a bottom-up forecasting framework is presented in this paper based upon bottom level data about the electricity consumption of household appliances. In the proposed framework, a load profile for group households is obtained with a similar day extraction module, household behavior analysis module, and household behavior prediction module. Concretely, similar day extraction module is the core of the prediction and is employed to extract similar historical days by considering the external environmental and household internal influence factors on energy consumption. The household behavior analysis module is used to analyse and formulate the consumption behavior probability of appliances according to the statistical characteristics of appliances’ switch state in historical similar days. Based on the former two modules, household behavior prediction module is responsible for the load profile of group households. Finally, a case study based on the measured data in a practical residential community is performed to illustrate the feasibility and effectiveness of the proposed bottom-up household load forecasting approach.

Description: Energies, Vol. 11, Pages 2151: On the Accuracy of Three-Dimensional Actuator Disc Approach in Modelling a Large-Scale Tidal Turbine in a Simple Channel Energies doi: 10.3390/en11082151 Authors: Anas Rahman Vengatesan Venugopal Jerome Thiebot To date, only a few studies have examined the execution of the actuator disc approximation for a full-size turbine. Small-scale models have fewer constraints than large-scale models because the range of time-scale and length-scale is narrower. Hence, this article presents the methodology in implementing the actuator disc approach via the Reynolds-Averaged Navier-Stokes (RANS) momentum source term for a 20-m diameter turbine in an idealised channel. A structured grid, which varied from 0.5 m to 4 m across rotor diameter and width was used at the turbine location to allow for better representation of the disc. The model was tuned to match known coefficient of thrust and operational profiles for a set of validation cases based on published experimental data. Predictions of velocity deficit and turbulent intensity became almost independent of the grid density beyond 11 diameters downstream of the disc. However, in several instances the finer meshes showed larger errors than coarser meshes when compared to the measurements data. This observation was attributed to the way nodes were distributed across the disc swept area. The results demonstrate that the accuracy of the actuator disc was highly influenced by the vertical resolutions, as well as the grid density of the disc enclosure.

Description: Energies, Vol. 11, Pages 2154: Modeling and Parameter Design of Voltage-Controlled Inverters Based on Discrete Control Energies doi: 10.3390/en11082154 Authors: Ningbo Dong Huan Yang Junfei Han Rongxiang Zhao Grid-connected inverters are widely used to interface renewable energy and energy storage resources into the grid. Voltage-controlled inverters have attracted more and more attention due to their grid-friendly characteristics. The mathematical models of the voltage and current loops are developed in this paper, considering especially the discrete control delay caused by calculation and modulation. In order to suppress the resonance peak in the current loop, the frequency characteristics of the current loop are analyzed in detail. The optimum design flow of the current controller and voltage controller parameters are presented based on numerical analysis, and the stability, dynamic performance and the resonance peak suppression in voltage loop are also considered. Finally, the validity of the mathematical model and the effectiveness of the controller parameters design method are verified by simulation and experimental results.

Description: Energies, Vol. 11, Pages 2148: Reconfiguration of a Multilevel Inverter with Trapezoidal Pulse Width Modulation Energies doi: 10.3390/en11082148 Authors: Nataraj Prabaharan V. Arun Padmanaban Sanjeevikumar Lucian Mihet-Popa Frede Blaabjerg This paper presents different multi-carrier unipolar trapezoidal pulse width modulation strategies for a reduced switch asymmetrical multilevel inverter. The different strategies are phase disposition, alternative phase opposition and disposition, and carrier overlapping and variable frequency that involve triangular waves as carriers with a unipolar trapezoidal wave as a reference. The reduced switch, asymmetrical multilevel inverter operation was examined for generating the seven-level output voltage using Matlab/Simulink 2009b and the results were verified with a real-time laboratory-based experimental setup using a field-programmable gate array. Different parameter analyses, such as total harmonic distortion, fundamental root mean square voltage, and distortion factor, were analyzed with different modulation indices to investigate the performance of the selected topology. Unipolar trapezoidal pulse width modulation provides a higher root mean square voltage value.

Description: Energies, Vol. 11, Pages 2146: Combustion of Flax Shives, Beech Wood, Pure Woody Pseudo-Components and Their Chars: A Thermal and Kinetic Study Energies doi: 10.3390/en11082146 Authors: Nourelhouda Boukaous Lokmane Abdelouahed Mustapha Chikhi Abdeslam-Hassen Meniai Chetna Mohabeer Taouk Bechara Thermogravimetric analysis was employed to investigate the combustion characteristics of flax shives, beech wood, hemicellulose, cellulose, lignin, and their chars. The chars were prepared from raw materials in a fixed-bed reactor at 850 °C. In this study, the thermal behavior based on characteristic temperatures (ignition, maximum, and final temperatures), burnout time and maximum rate was investigated. The kinetic parameters for the combustion of different materials were determined based on the Coats-Redfern approach. The results of our study revealed that the combustion of pure pseudo-components behaved differently from that of biomass. Indeed, principal component analysis showed that the thermal behavior of both biomasses was generally similar to that of pure hemicellulose. However, pure cellulose and lignin showed different behaviors compared to flax shives, beech wood, and hemicellulose. Hemicellulose and cellulose chars had almost the same behaviors, while being different from biomass and lignin chars. Despite the difference between flax shives and beech wood, they showed almost the same thermal characteristics and apparent activation energies. Also, the combustion of the hemicellulose and cellulose chars showed that they have almost the same structure. Their overall thermal and kinetic behavior remained between that of biomass and lignin.

Description: Energies, Vol. 11, Pages 2144: A Novel Stability Improvement Strategy for a Multi-Inverter System in a Weak Grid Utilizing Dual-Mode Control Energies doi: 10.3390/en11082144 Authors: Ming Li Xing Zhang Wei Zhao Due to the increasing penetration of distributed generations (DGS) and non-negligible grid impedance, the instability problem of the multi-inverter system operating in current source mode (CSM) is becoming serious. In this paper, a closed-loop transfer function model of such a multi-inverter system is established, by which it is concluded that output current resonance will occur with the increase in the grid impedance. In order to address this problem, this paper presents a novel dual-mode control scheme of multiple inverters: one inverter operating in CSM will be alternated into voltage source mode (VSM) if the grid impedance is high. It is theoretically proved that the coupling between the inverters and the resonance in the output current can be suppressed effectively with the proposed scheme. Finally, the validity of the proposed theory is demonstrated by extensive simulations and experiments.

Description: Energies, Vol. 11, Pages 2143: PLL-Less Three-Phase Four-Wire SAPF with STF-dq0 Technique for Harmonics Mitigation under Distorted Supply Voltage and Unbalanced Load Conditions Energies doi: 10.3390/en11082143 Authors: Yap Hoon Mohd Amran Mohd Radzi This paper presents a non-iterative technique that generates reference current to manage operation of a three-phase four-wire shunt active power filter which employs a three-leg split capacitor voltage source inverter (VSI) topology. The proposed technique integrates together a self-tuning-filter (STF) and direct-quadrature-zero (dq0) principle (referred here as STF-dq0), allowing the controlled shunt active power filter (SAPF) to perform effectively under distorted source voltages and unbalanced load conditions. Unlike the previous technique developed based on the standard dq0 principle, the proposed technique does not require any service from a phase-locked loop (PLL) where two STFs are applied to separate harmonic and fundamental elements for the purpose of generating synchronization phases and reference current, respectively. Simulation work which includes connection of the SAPF circuits, design of control techniques and all the necessary assessments are conducted in MATLAB-Simulink platform. Performance achieved by the SAPF while utilizing the proposed technique is thoroughly investigated and benchmarked with that demonstrated by the SAPF while using the standard dq0 technique, to evaluate the inherent advantages. Exhaustive simulation results are provided and thoroughly discussed to support design concept, effectiveness, and benefits of the proposed technique.

Description: Energies, Vol. 11, Pages 2149: Fault Detection for Gas Turbine Hot Components Based on a Convolutional Neural Network Energies doi: 10.3390/en11082149 Authors: Jiao Liu Jinfu Liu Daren Yu Myeongsu Kang Weizhong Yan Zhongqi Wang Michael G. Pecht Gas turbine hot component failures often cause catastrophic consequences. Fault detection can improve the availability and economy of hot components. The exhaust gas temperature (EGT) profile is usually used to monitor the performance of the hot components. The EGT profile is uniform when the hot component is healthy, whereas hot component faults lead to large temperature differences between different EGT values. The EGT profile swirl under different operating and ambient conditions also cause temperature differences. Therefore, the influence of EGT profile swirl on EGT values must be eliminated. To improve the detection sensitivity, this paper develops a fault detection method for hot components based on a convolutional neural network (CNN). This paper demonstrates that a CNN can extract the information between adjacent EGT values and consider the impact of the EGT profile swirl. This paper reveals, in principle, that a CNN is a viable solution for dealing with fault detection for hot components. Based on the distribution characteristics of EGT thermocouples, the circular padding method is developed in the CNN. The sensitivity of the developed method is verified by real-world data. Moreover, the developed method is visualized in detail. The visualization results reveal that the CNN effectively considers the influence of the EGT profile swirl.

Description: Energies, Vol. 11, Pages 2145: Flow Simulation of Artificially Induced Microfractures Using Digital Rock and Lattice Boltzmann Methods Energies doi: 10.3390/en11082145 Authors: Yongfei Yang Zhihui Liu Jun Yao Lei Zhang Jingsheng Ma S. Hossein Hejazi Linda Luquot Toussaint Dono Ngarta Microfractures have great significance in the study of reservoir development because they are an effective reserving space and main contributor to permeability in a large amount of reservoirs. Usually, microfractures are divided into natural microfractures and induced microfractures. Artificially induced rough microfractures are our research objects, the existence of which will affect the fluid-flow system (expand the production radius of production wells), and act as a flow path for the leakage of fluids injected to the wells, and even facilitate depletion in tight reservoirs. Therefore, the characteristic of the flow in artificially induced fractures is of great significance. The Lattice Boltzmann Method (LBM) was used to calculate the equivalent permeability of artificially induced three-dimensional (3D) fractures. The 3D box fractal dimensions and porosity of artificially induced fractures in Berea sandstone were calculated based on the fractal theory and image-segmentation method, respectively. The geometrical parameters (surface roughness, minimum fracture aperture, and mean fracture aperture), were also calculated on the base of digital cores of fractures. According to the results, the permeability lies between 0.071–3.759 (dimensionless LB units) in artificially induced fractures. The wide range of permeability indicates that artificially induced fractures have complex structures and connectivity. It was also found that 3D fractal dimensions of artificially induced fractures in Berea sandstone are between 2.247 and 2.367, which shows that the artificially induced fractures have the characteristics of self-similarity. Finally, the following relations were studied: (a) exponentially increasing permeability with increasing 3D box fractal dimension, (b) linearly increasing permeability with increasing square of mean fracture aperture, (c) indistinct relationship between permeability and surface roughness, and (d) linearly increasing 3D box fractal dimension with increasing porosity.

Description: Energies, Vol. 11, Pages 2147: Feasibility of a 100 MW Installed Capacity Wind Farm for Different Climatic Conditions Energies doi: 10.3390/en11082147 Authors: M. Mujahid Rafique Shafiqur Rehman Md. Mahbub Alam Luai M. Alhems Wind power is the world’s fastest-growing energy source. More power can be generated from wind energy through the use of new wind machine designs and techniques. The objective of the present work is to encourage people and governments to develop wind energy-based power plants to achieve sustainable energy infrastructures, especially in developing countries. In this paper, a feasibility study of a 100 MW grid-connected wind farm is conducted for five different cities of Saudi Arabia (KSA). The results indicate that the proposed power plant is feasible both technically and economically. All sites are found to be within the profitable range with Dhahran being the most feasible site among the others for the installation of the wind farm. A sensitivity analysis has also been carried out to find out the effects of different incentives on the payback period of the project.

Description: Energies, Vol. 11, Pages 2212: A Hybrid Electric Vehicle Dynamic Optimization Energy Management Strategy Based on a Compound-Structured Permanent-Magnet Motor Energies doi: 10.3390/en11092212 Authors: Qiwei Xu Yunqi Mao Meng Zhao Shumei Cui A dynamic optimization energy management strategy called Hybrid Electric Vehicle Based on Compound Structured Permanent-Magnet Motor (CSPM-HEV) is investigated in this paper. CSPM-HEV has obvious advantages in power density, heat dissipation efficiency, torque performance and energy transmission efficiency. This paper describes the topology and working principle of the CSPM-HEV, and analyzes its operating mode and corresponding energy flow laws. On this basis, the relationship about the power loss of the vehicle, the CSPM transmission ratio iCSPM and the CSPM-HEV power distribution coefficient f1 were derived. According to the optimal combination of (iCSPM, f1), the engine power and speed which minimize the power loss of the vehicle, were calculated, thus realizing the instantaneous optimal control of the vehicle. In addition, in order to improve the instantaneously optimized control processing speed, a neural network controller was established. The drive axle demand power, speed and battery State of Charge (SOC), were taken as input variables. Then, the engine power and speed were taken as output variables. The simulation results show that the average speed of the instantaneous optimization strategy after BP neural network optimization is increased by 98.1%, the control effect is significant, and it has high application value.

Description: Energies, Vol. 11, Pages 2210: Performance Analysis on the Optimum Control of a Calorimeter with a Heat Recovery Unit for a Heat Pump Energies doi: 10.3390/en11092210 Authors: Kofi Owura Amoabeng Jong Min Choi Heat pumps are used in many applications, both in households and industries, for space air conditioning and hot water provision. The calorimeter is the equipment used in testing the heat pump system to obtain performance data. In the conventional testing mode and under standard conditions, the calorimeter utilizes a lot of energy through refrigeration and heating systems. In this study, a newly developed calorimeter with a heat recovery unit was used to test the performance of a water-to-water heat pump system. The aim was to minimize the rate of energy used in the conventional calorimeter. Two heat recovery control methods were adopted. In the control (1), the heat recovery unit was used to control the inlet water temperature setpoint for the heat pump indoor heat exchanger, whereas in control (2), the heat recovery unit was used to control the inlet water temperature setpoint for the heat pump outdoor heat exchanger. Tests were executed by varying the operating mode and test conditions. For the heating operating mode, the inlet water setpoint temperatures for the indoor and outdoor heat pump heat exchangers were 40 °C and 5 °C, respectively, whereas for the cooling mode, the inlet water setpoint temperatures for the outdoor and indoor heat pump heat exchangers were 25 °C and 12 °C, respectively. The analyses of the experimental results revealed that the energy saving of the calorimeter with heat recovery was about 71% in cooling mode and 73% in heating mode compared to the conventional calorimeter. Also, the energy consumption of the proposed calorimeter was analyzed based on the control methods. In heating mode, the calorimeter performance was enhanced when the control (2) strategy was used because the energy saving was about 8 to 13% compared to control (1). However, in the cooling mode test, it was the control (1) method that resulted in energy savings of about 6.4 to 21% compared to the control (2) method.

Description: Energies, Vol. 11, Pages 2200: Theoretical Study and Experimental Validation of a Hydrostatic Transmission Control for a City Bus Hybrid Driveline with Kinetic Energy Storage Energies doi: 10.3390/en11092200 Authors: Venelin Jivkov Vutko Draganov A city bus with hybrid drive system was studied for its performance. The driveline under consideration consists of two alternative energy sources—an internal combustion engine (ICE) and kinetic energy storage (KES)—a hydrostatic transmission (HST), a drive axle and corresponding gears. A generalized law for HST control is obtained that satisfies kinematic and torque requirements for the alternative energy sources and the different modes of operation of the bus. A test stand was developed for validation of the chosen control strategy and for the energy flow simulations through the HST. The estimated maximum energy recovery potential is around 20–25%.

Description: Energies, Vol. 11, Pages 2201: Studying the Effectiveness of Polyacrylamide (PAM) Application in Hydrocarbon Reservoirs at Different Operational Conditions Energies doi: 10.3390/en11092201 Authors: Kingsley Godwin Uranta Sina Rezaei-Gomari Paul Russell Faik Hamad The water-soluble polymer PAM (polyacrylamide) is used in enhanced oil recovery (EOR) operations. It is pumped into water injection wells to increase the viscosity of the injected water and in turn to direct more oil towards production wells. This EOR process is proven to be sensitive to operational well conditions such as hydrocarbon reservoir temperature, as well as the salinity of the injected water and/or formation water. These operational conditions lead to technical challenges ranging from the solubility of PAM in injection water to the behaviour of PAM inside the reservoir. To gain a clear picture of the functionality of PAM in EOR applications, this report characterizes its behaviour of in terms of degree of hydrolysis and changes in solution viscosity determined using Perkin Elmer spectrum 100 Fourier transform infrared-Attenuated total reflection (FTIR)-ATR and nuclear magnetic resonance spectroscopy (1H NMR) and a Fann model 35 Couette and Cole Parmer rotational viscometer, respectively. Different shear rates were investigated to determine the effect of shear on PAM gel stability. Experiments were performed for PAM mixed with formation brine at 50, 70, and 90 °C for ageing times of up to 30 days. The results indicate that the degree of hydrolysis achieved after 30 days is much higher in saline solutions than in pure water, and that this effect is more pronounced at higher temperatures. For example, after 30 days at 50 °C, the hydrolysis level was observed to be 53%, rising to 65% at 70 °C and 75% at 90 °C in PAM mixed with brines. Similar trends were observed with viscosity, where lower viscosity was observed for samples at higher temperatures and salinities. It is thus reasonable to conclude that the degree of hydrolysis causes changes in the viscosity of the polymer gel, leading to a decline in its performance as it ages.

Description: Energies, Vol. 11, Pages 2199: Optimized Sizing and Scheduling of Hybrid Energy Storage Systems for High-Speed Railway Traction Substations Energies doi: 10.3390/en11092199 Authors: Yuanli Liu Minwu Chen Shaofeng Lu Yinyu Chen Qunzhan Li The integration of hybrid energy storage systems (HESS) in alternating current (AC) electrified railway systems is attracting widespread interest. However, little attention has been paid to the interaction of optimal size and daily dispatch of HESS within the entire project period. Therefore, a novel bi-level model of railway traction substation energy management (RTSEM) system is developed, which includes a slave level of diurnal HESS dispatch and a master level of HESS sizing. The slave level is formulated as a mixed integer linear programming (MILP) model by coordinating HESS, traction load, regenerative braking energy and renewable energy. As for the master level model, comprehensive cost study within the project period is conducted, with batteries degradation and replacement cost taken into account. Grey wolf optimization technique with embedded CPLEX solver is utilized to solve this RTSEM problem. The proposed model is tested with a real high-speed railway line case in China. The simulation results of several cases with different system elements are presented, and the sensitivity analyses of several parameters are also performed. The obtained results reveal that it shows significant economic-saving potentials with the integration of HESS and renewable energy.

Description: Energies, Vol. 11, Pages 2195: Optimal Scheduling of Power System Incorporating the Flexibility of Thermal Units Energies doi: 10.3390/en11092195 Authors: Tong Guo Yajing Gao Xiaojie Zhou Yonggang Li Jiaomin Liu Due to the randomness, volatility and intermittent nature of wind power, power systems with significant wind penetration face serious “curtailment” problems. The flexibility of a power system is an important factor that affects the large-scale consumption of wind power. Based on this fact, this paper takes into account the economics and flexibility of the system, and proposes an optimal scheduling method that takes the flexibility of each thermal power unit into account. Firstly, a comprehensive evaluation index system of thermal power unit flexibility is designed by an analytic hierarchy process and entropy method. The system covers the technical indexes and economic characteristics of thermal power units and is able to quantitatively evaluate the different types of thermal power units in the system. Secondly, a multi-objective optimization scheduling model involving the overall flexibility of the unit and the total power generation cost is established. Finally, the correctness and effectiveness of the proposed indicators and models are verified by a case study.

Description: Energies, Vol. 11, Pages 2192: Maximum Power Point Tracking for a Point Absorber Device with a Tubular Linear Switched Reluctance Generator Energies doi: 10.3390/en11092192 Authors: Rui Mendes Maria do Rosário Calado Sílvio Mariano This paper addresses the control of a Tubular Linear Switched Reluctance Generator (TLSRG) with application in a point absorber device. A maximum power point tracking (MPPT) strategy is proposed to maximize the power extraction from ocean waves. The generator is characterized by average maximum force of 120 kN and a maximum velocity of 1.3 m/s. The proposed MPPT is achieved by changing the generator damping load according to the excitation force induced by a regular wave. A hysteresis controller is applied to regulate the phase current intensity which allows the control of the linear force provided by the generator. The conversion system direct current (DC) bus voltage is adjusted by an isolated DC/DC converter with a proportional integral controller to define the appropriate duty-cycle.

Description: Energies, Vol. 11, Pages 2193: Integrated Energy Planning with a High Share of Variable Renewable Energy Sources for a Caribbean Island Energies doi: 10.3390/en11092193 Authors: Dominik Franjo Dominković Greg Stark Bri-Mathias Hodge Allan Schrøder Pedersen Although it can be complex to integrate variable renewable energy sources such as wind power and photovoltaics into an energy system, the potential benefits are large, as it can help reduce fuel imports, balance the trade, and mitigate the negative impacts in terms of climate change. In order to try to integrate a very large share of variable renewable energy sources into the energy system, an integrated energy planning approach was used, including ice storage in the cooling sector, a smart charging option in the transport sector, and an excess capacity of reverse osmosis technology that was utilised in order to provide flexibility to the energy system. A unit commitment and economic dispatch tool (PLEXOS) was used, and the model was run with both 5 min and 1 h time resolutions. The case study was carried out for a typical Caribbean island nation, based on data derived from measured data from Aruba. The results showed that 78.1% of the final electricity demand in 2020 was met by variable renewable energy sources, having 1.0% of curtailed energy in the energy system. The total economic cost of the modelled energy system was similar to the current energy system, dominated by the fossil fuel imports. The results are relevant for many populated islands and island nations.

Description: Energies, Vol. 11, Pages 2217: The Impact of Substituting Production Technologies on the Economic Demand Response Potential in Industrial Processes Energies doi: 10.3390/en11092217 Authors: Michael Schoepf Martin Weibelzahl Lisa Nowka Given the low carbon transformation of our energy systems, demand response has the potential to increase the adaptability of electricity demand to a volatile electricity supply. In this article, we investigate the demand response potential for the case where substituting technologies are available in an energy-intensive industrial production process. The available production technologies may not only differ in their technical characteristics, but also vary by the necessary input materials. We present a generic linear optimization model for such a production process and apply it to a real-world example in the paper industry. The results show that the question of which substituting technologies are used in an optimal production schedule to which degree, is highly influenced by the combination of current input parameters such as prices. In direct consequence, the corresponding demand response potential is not a fixed number. From an operational perspective, this input dependency implies that the price relation of raw input materials used in substituting technologies can be a crucial driving force for the ability and willingness of industrial enterprises to provide demand response. In addition, from a strategic perspective, long-run investments in demand response potentials may rely on expected price development of major input factors.

Description: Energies, Vol. 11, Pages 2213: An Assessment of Near-to-Mid-Term Economic Impacts and Energy Transitions under “2 °C” and “1.5 °C” Scenarios for India Energies doi: 10.3390/en11092213 Authors: Shivika Mittal Jing-Yu Liu Shinichiro Fujimori Priyadarshi Ramprasad Shukla The goal of limiting global temperature rise to “well below” 2 °C has been reaffirmed in the Paris Agreement on climate change at the 21st Conference of the Parties (COP21). Almost all countries submitted their decarbonization targets in their Intended Nationally Determined Contributions (INDC) to the United Nations Framework Convention on Climate Change (UNFCCC) and India did as well. India’s nationally determined contribution (NDC) aims to reduce greenhouse gas (GHG) emissions intensity of national GDP in 2030 by 33–35% compared to 2005. This paper analyzes how India’s NDC commitments compare with emission trajectories consistent with well below 2 °C and 1.5 °C global temperature stabilization goals. A top-down computable general equilibrium model is used for the analysis. Our analysis shows that there are significant emission gaps between NDC and global climate stabilization targets in 2030. The energy system requires significant changes, mostly relying on renewable energy and carbon capture and storage (CCS) technology. The mitigation costs would increase if India delays its abatement efforts and is locked into NDC pathways till 2030. India’s GHG emissions would peak 10 years earlier under 1.5 °C global temperature stabilization compared to the 2 °C goal. The results imply that India would need financial and technological support from developed countries to achieve emissions reductions aligned with the global long-term goal.

Description: Energies, Vol. 11, Pages 2219: An Energy-Balanced Geographic Routing Algorithm for Mobile Ad Hoc Networks Energies doi: 10.3390/en11092219 Authors: Dong Yang Hongxing Xia Erfei Xu Dongliang Jing Hailin Zhang To mitigate the frequent link breakage and node death caused by node mobility and energy constraints in mobile ad-hoc networks, we propose an energy-balanced routing algorithm for energy and mobility greedy perimeter stateless routing (EM-GPSR) based on geographical location. In the proposed algorithm, the forward region is divided into four sub-regions. Then, according to the remaining lifetime of each node and the distance between the source node and the destination node, we select the next-hop node in the candidate sub-regions. Since the energy consumption rate of the node is taken into account, the next-hop selection favors the nodes with longer remaining lifetimes. Simulation results show that compared with conventional greedy perimeter stateless routing (GPSR) and speed up-greedy perimeter stateless routing (SU-GPSR) routing algorithms, the proposed algorithm can lead to a lower end-to-end delay, longer service time, and higher transmission efficiency for the network.

Description: Energies, Vol. 11, Pages 2236: Adaptive Consensus Algorithm for Distributed Heat-Electricity Energy Management of an Islanded Microgrid Energies doi: 10.3390/en11092236 Authors: Xiaofeng Dong Xiaoshun Zhang Tong Jiang This paper proposes a novel adaptive consensus algorithm (ACA) for distributed heat-electricity energy management (HEEM) of an islanded microgrid. In order to simultaneously satisfy the heat-electricity energy balance constraints, ACA is implemented with a switch between unified consensus and independent consensus according to the dynamic energy mismatches. The feasible operation region of a combined heat and power (CHP) unit is decomposed into eight searching sub-regions, thus its electricity and heat energy outputs can simultaneously match the incremental cost consensus requirement and the heat-electricity energy balance constraints. Case studies are thoroughly carried out to verify the performance of ACA for distributed HEEM of an islanded microgrid.

Description: Energies, Vol. 11, Pages 2256: Quota Allocation for Carbon Emissions in China′s Electric Power Industry Based Upon the Fairness Principle Energies doi: 10.3390/en11092256 Authors: Ming Meng Lixue Wang Qu Chen As an essential measure to mitigate the CO2 emissions, China is constructing a nationwide carbon emission trading (CET) market. The electric power industry is the first sector that will be introduced into this market, but the quota allocation scheme, as the key foundation of market transactions, is still undetermined. This research employed the gross domestic product (GDP), energy consumption, and electric generation data of 30 provinces from 2001 to 2015, a hybrid trend forecasting model, and a three-indicator allocation model to measure the provincial quota allocation for carbon emissions in China′s electric power sector. The conclusions drawn from the empirical analysis can be summarized as follows: (1) The carbon emission peak in China′s electric power sector will appear in 2027, and peak emissions will be 3.63 billion tons, which will surpass the total carbon emissions of the European Union (EU) and approximately equal to 2/3 of the United States of America (USA). (2) The developed provinces that are supported by traditional industries should take more responsibility for carbon mitigation. (3) Nine provinces are expected to be the buyers in the CET market. These provinces are mostly located in eastern China, and account for approximately 63.65% of China′s carbon emissions generated by the electric power sector. (4) The long-distance electric power transmission shifts the carbon emissions and then has an impact on the quotas allocation for carbon emissions. (5) The development and effective utilization of clean power generation will play a positive role for carbon mitigation in China′s electric sector.

Description: Energies, Vol. 11, Pages 2251: Alternating Current Discharge Characteristics and Simulation Analysis of Rod-Plane Short Air Gaps under Salt Fog Conditions Energies doi: 10.3390/en11092251 Authors: Yunpeng Liu Shuo Jiang Zheng Zhong Jianghai Geng Fangcheng Lv In this paper, smog meteorological conditions in the natural environment is simulated by the salt fog method. The study of the alternating current (AC) discharge characteristics of rod-plane short air gaps in salt fog environments has important guiding significance for how to strengthen the external insulation strength of ultra-high voltage (UHV) transmission lines and electrical equipment in smog environments. The rod-plane short air gap is selected as the model to simulate the extremely uneven electric field. The AC discharge test is carried out in the salt fog environment with different conductivity, and the finite element method (FEM) is used to simulate the distribution of electric field in air gap under salt fog environment conditions. The results show that under clean fog conditions the AC discharge voltage in the air gap increased by 15.1% to 35.5% compared to that under dry conditions. With the increased conductivity of salt fog, the AC discharge voltage in air gap decreased by 4.1% to 9.2% compared to that under clean fog conditions, and the reduction is within 10%. The distortion of the electric field and the adsorption of free electrons in the gap by droplets lead to the decrease of the electric field intensity in the air gap. With the increase of the conductivity, the electric field intensity in the air gap increases slightly. Meanwhile, the influence of salt fog and its conductivity on the AC discharge voltage of rod-plane short air gap is examined, becoming saturated with the increase of the gap distance and the conductivity of salt fog.

Description: Energies, Vol. 11, Pages 2260: A Study of the Production and Combustion Characteristics of Pyrolytic Oil from Sewage Sludge Using the Taguchi Method Energies doi: 10.3390/en11092260 Authors: Guan-Bang Chen Jia-Wen Li Hsien-Tsung Lin Fang-Hsien Wu Yei-Chin Chao Sewage sludge is a common form of municipal solid waste, and can be utilized as a renewable energy source. This study examines the effects of different key operational parameters on sewage sludge pyrolysis process for pyrolytic oil production using the Taguchi method. The digested sewage sludge was provided by the urban wastewater treatment plant of Tainan, Taiwan. The experimental results indicate that the maximum pyrolytic oil yield, 10.19% (18.4% on dry ash free (daf) basis) by weight achieved, is obtained under the operation conditions of 450 °C pyrolytic temperature, residence time of 60 min, 10 °C/min heating rate, and 700 mL/min nitrogen flow rate. According to the experimental results, the order of sensitivity of the parameters that affect the yield of sludge pyrolytic oil is the nitrogen flow rate, pyrolytic temperature, heating rate and residence time. The pyrolysis and oxidation reactions of sludge pyrolytic oil are also investigated using thermogravimetric analysis. The combustion performance parameters, such as the ignition temperature, burnout temperature, flammability index and combustion characteristics index are calculated and compared with those of heavy fuel oil. For the blend of sludge pyrolytic oil with heavy fuel oil, a synergistic effect occurs and the results show that sludge pyrolytic oil significantly enhances the ignition and combustion of heavy fuel oil.

Description: Energies, Vol. 11, Pages 2258: Analysis of the Effects of Blade Installation Angle and Blade Number on Radial-Inflow Turbine Stator Flow Performance Energies doi: 10.3390/en11092258 Authors: Peng Li Zhonghe Han Xiaoqiang Jia Zhongkai Mei Xu Han Organic Rankine cycle (ORC) is a reliable technology to recover low-grade heat sources. The radial-inflow turbine is a critical component, which has a significant influence on the overall efficiency of ORC system. This study investigates the effects of the blade installation angle and blade number on the flow performance of radial-inflow turbine stator. R245fa and toluene were selected as the working fluids in the low and high temperature range, respectively. Two-dimensional stator blades model for the two working fluids were established, and numerical simulation was conducted through Computational Fluid Dynamics (CFD) software. The results show that for low temperature working fluid R245fa, when the installation angle is 32° and blade number is 22, the distribution of static pressure along the stator blade has no obvious pressure fluctuation, and the flow loss is least. Meanwhile, the stator blade obtained the optimal performance. For high temperature working fluid toluene, when the installation angle is 28° and blade number is 32, the average outlet temperature is the lowest, while the average outlet velocity is the largest. The flow state is well and smooth, and the remarkable flow separation and shock wave are not present. Moreover, the stator blade for R245fa has a larger chord length, cascade inlet diameter, and cascade outside diameter but a lower blade number compared to toluene.

Description: Energies, Vol. 11, Pages 2262: A Cost-Effective Decentralized Control for AC-Stacked Photovoltaic Inverters Energies doi: 10.3390/en11092262 Authors: Hua Han Chao Luo Xiaochao Hou Mei Su Wenbin Yuan Zhangjie Liu Josep M. Guerrero For an AC-stacked photovoltaic (PV) inverter system with N cascaded inverters, existing control methods require at least N communication links to acquire the grid synchronization signal. In this paper, a novel decentralized control is proposed. For N inverters, only one inverter nearest the point of common coupling (PCC) needs a communication link to acquire the grid voltage phase and all other N − 1 inverters use only local measured information to achieved fully decentralized local control. Specifically, one inverter with a communication link utilizes the grid voltage phase and adopts current control mode to achieve a required power factor (PF). All other inverters need only local information without communication links and adopt voltage control mode to achieve maximum power point tracking (MPPT) and self-synchronization with grid voltage. Compared with existing methods, the communication link and complexity is greatly reduced, thus improved reliability and reduced communication costs are achieved. The effectiveness of the proposed control is verified by simulation tests.

Description: Energies, Vol. 11, Pages 2261: On the Application of Proper Orthogonal Decomposition (POD) for In-Cylinder Flow Analysis Energies doi: 10.3390/en11092261 Authors: Mohammed El-Adawy Morgan R. Heikal A. Rashid A. Aziz Ibrahim Khalil Adam Mhadi A. Ismael Mohammed E. Babiker Masri B. Baharom Firmansyah Ezrann Zharif Zainal Abidin Proper orthogonal decomposition (POD) is a coherent structure identification technique based on either measured or computed data sets. Recently, POD has been adopted for the analysis of the in-cylinder flows inside internal combustion engines. In this study, stereoscopic particle image velocimetry (Stereo-PIV) measurements were carried out at the central vertical tumble plane inside an engine cylinder to acquire the velocity vector fields for the in-cylinder flow under different experimental conditions. Afterwards, the POD analysis were performed firstly on synthetic velocity vector fields with known characteristics in order to extract some fundamental properties of the POD technique. These data were used to reveal how the physical properties of coherent structures were captured and distributed among the POD modes, in addition to illustrate the difference between subtracting and non-subtracting the ensemble average prior to conducting POD on datasets. Moreover, two case studies for the in-cylinder flow at different valve lifts and different pressure differences across the air intake valves were presented and discussed as the effect of both valve lifts and pressure difference have not been investigated before using phase-invariant POD analysis. The results demonstrated that for repeatable flow pattern, only the first mode was sufficient to reconstruct the physical properties of the flow. Furthermore, POD analysis confirmed the negligible effect of pressure difference and subsequently the effect of engine speed on flow structures.

Description: Energies, Vol. 11, Pages 2290: Digital Control of an Interleaving Operated Buck-Boost Synchronous Converter Used in a Low-Cost Testing System for an Automotive Powertrain Energies doi: 10.3390/en11092290 Authors: Miran Rodič Miro Milanovič Mitja Truntič Based on the standardization in the automotive industry, systems require extensive testing, which represents significant costs regarding personnel and equipment. The testing systems must be built in such a way that a bidirectional power flow is possible between the power source and the tested system. Additionally, applied testing systems have to possess high disturbance immunity. Classical current programmed control performed using an analogue approach suffers from low disturbance rejection during switching operation. The digital control of DC–DC converter can solve this problem with the use of digital integration in a measurement chain. The integrals of values are obtained by using a Voltage Control Oscillator (VCO) and appropriate counters. Digital control of an interleaving operated bidirectional buck-boost synchronous converter can be applied in the testing system for automotive powertrains. The voltage and current measurements with the application of an integral-measurement principle act as low-pass filters, which remove the disturbances from the measured values. The digital implementation of a compensation ramp (current mode control) and method for choice of control parameters are described. All the tasks for measurements, as well as current and voltage control, were implemented within the FPGA (Field Programmable Gate Array). The presented converter can operate as a close to ideal voltage or current source, and satisfies the requirements of testing electric motor drive-trains with bidirectional DC–AC converters that are applied in automotive applications. The proposed system was verified by simulation

Description: Energies, Vol. 11, Pages 2289: Historical Evolution of the Wave Resource and Energy Production off the Chilean Coast over the 20th Century Energies doi: 10.3390/en11092289 Authors: Alain Ulazia Markel Penalba Arkaitz Rabanal Gabriel Ibarra-Berastegi John Ringwood Jon Sáenz The wave energy resource in the Chilean coast shows particularly profitable characteristics for wave energy production, with relatively high mean wave power and low inter-annual resource variability. This combination is as interesting as unusual, since high energetic locations are usually also highly variable, such as the west coast of Ireland. Long-term wave resource variations are also an important aspect when designing wave energy converters (WECs), which are often neglected in resource assessment. The present paper studies the long-term resource variability of the Chilean coast, dividing the 20th century into five do-decades and analysing the variations between the different do-decades. To that end, the ERA20C reanalysis of the European Centre for Medium-Range Weather Forecasts is calibrated versus the ERA-Interim reanalysis and validated against buoy measurements collected in different points of the Chilean coast. Historical resource variations off the Chilean coast are compared to resource variations off the west coast in Ireland, showing a significantly more consistent wave resource. In addition, the impact of historical wave resource variations on a realistic WEC, similar to the Corpower device, is studied, comparing the results to those obtained off the west coast of Ireland. The annual power production off the Chilean coast is demonstrated to be remarkably more regular over the 20th century, with variations of just 1% between the different do-decades.

Description: Energies, Vol. 11, Pages 2288: Dynamic Optimization of Combined Cooling, Heating, and Power Systems with Energy Storage Units Energies doi: 10.3390/en11092288 Authors: Jiyuan Kuang Chenghui Zhang Fan Li Bo Sun In this paper, a combined cooling, heating, and power (CCHP) system with thermal storage tanks is introduced. Considering the plants’ off-design performance, an efficient methodology is introduced to determine the most economical operation schedule. The complex CCHP system’s state transition equation is extracted by selecting the stored cooling and heating energy as the discretized state variables. Referring to the concept of variable cost and constant cost, repeated computations are saved in phase operating cost calculations. Therefore, the most economical operation schedule is obtained by employing a dynamic solving framework in an extremely short time. The simulation results indicated that the optimized operating cost is reduced by 40.8% compared to the traditional energy supply system.

Description: Energies, Vol. 11, Pages 2284: A Modified Firefly Algorithm with Rapid Response Maximum Power Point Tracking for Photovoltaic Systems under Partial Shading Conditions Energies doi: 10.3390/en11092284 Authors: Yu-Pei Huang Cheng-En Ye Xiang Chen A rapid response optimization technique for photovoltaic maximum power point tracking (MPPT) under partial shading conditions (PSCs) is proposed in this study. To improve the solar MPPT tracking speed for rapidly-changing environmental conditions and to prevent the conventional firefly algorithm (FA) from becoming trapped at the local peaks and oscillations during the search process, a novel fusion algorithm, named the modified firefly algorithm (MFA), is proposed. The MFA integrates and modifies the processes of two algorithms, namely the firefly algorithm with neighborhood attraction (NaFA) and simplified firefly algorithm (SFA). A modified attraction process for the NaFA is used in the first iteration to avoid trapping at local maximum power points (LMPPs). In addition, in order to improve the convergence speed, the attractiveness factor of the attraction process is designed to be related to the power and position difference of the fireflies. Furthermore, the number of fireflies is designed to decrease in proportion with the iterations in the modified SFA process. Results from both the simulations and evaluations verify that the proposed algorithm offers rapid response with high accuracy and efficiency when encountering PSCs. In addition, the MFA can avoid becoming trapped at LMPPs and ease the oscillations during the search process. Consequently, the proposed method could be considered to be one of the most promising substitutes for existing approaches. In addition, the proposed method is adaptable to different types of solar panels and different system formats with specifically designed parameters.

Description: Energies, Vol. 11, Pages 2273: Lyapunov-Based Large Signal Stability Assessment for VSG Controlled Inverter-Interfaced Distributed Generators Energies doi: 10.3390/en11092273 Authors: Meiyi Li Wentao Huang Nengling Tai Moduo Yu Inverter-interfaced distributed generators (IIDGs) have been widely applied due to their control flexibility. The stability problems of IIDGs under large signal disturbances, such as large load variations and feeder faults, will cause serious impacts on the system. The virtual synchronous generator (VSG) control is an effective scheme for IIDGs to increase transient stability. However, the existing linearized stability models of IIDGs are limited to small disturbances. Hence, this paper proposes a Lyapunov approach based on non-linearized models to assess the large signal stability of VSG-IIDG. The electrostatic machine model is introduced to establish the equivalent nonlinear model. On the basis of Popov’s theory, a Lyapunov function is derived to calculate the transient stability domain. The stability mechanism is revealed by depicting the stability domain using the locus of the angle and the angular frequency. Large signal stability of the VSG-IIDG is quantified according to the boundary of the stability domain. Effects and sensitivity analysis of the key parameters including the cable impedance, the load power, and the virtual inertia on the stability of the VSG-IIDG are also presented. The simulations are performed in PSCAD/EMTDC and the results demonstrate the proposed large signal stability assessment method.