ALBERT

Description: Energies, Vol. 10, Pages 2083: Core Abilities Evaluation Index System Exploration and Empirical Study on Distributed PV-Generation Projects Energies doi: 10.3390/en10122083 Authors: Lin He Chang-Ling Li Qing-Yun Nie Yan Men Hai Shao Jiang Zhu In line with the constraints of environmental problems and economic development, large-scale renewable-generation projects have been planned and constructed in recent years. In order to achieve sustainable power development and improve the power supply structure, China’s government has focused on distributed photovoltaic (PV) generation projects due to their advantages of clean emission and local consumption. However, their unstable output power still brings a series of problems concerning reliability, investment income, and available substitution proportion to traditional power, and so on. Therefore, it is imperative to understand the competitive development abilities of distributed PV generation projects and measure them effectively. First, through various investigation methods such as literature reviews, feasibility report analysis and expert interviews, the factors that influence the core abilities of distributed PV-generation projects were explored based on the micro-grid structure. Then, with the indexed exploration results, the factors were classified into 6 dimensions, i.e., investment and earning ability, production and operation ability, power-grid coordination ability, energy-conservation and emission-reduction ability, sustainable development ability, and society-serving ability. Meanwhile, an evaluation index system for core abilities of distributed PV-generation project was constructed using all quantitative indicators. Third, for examining the availability of the evaluation index system, combination weighting and techniques for order preference by similarity to an ideal solution (TOPSIS) methods were adopted to assess the practical distributed PV-generation projects. The case study results showed that installed capacity, local economy development, and grid-connected power quantity will influence the core abilities of distributed PV-generation project, obviously. The conclusions of the evaluation analysis on core abilities can provide useful references to operate and manage distributed PV-generation projects and promote their sustainable and health advantages. The proposed evaluation index system can also be used to assess power-generation projects in other types of energy, such as wind power and hydropower.

Description: Hydraulic manifolds are used to realize compact circuit layout, but may introduce a high pressure drop in the system. Their design is in fact oriented more toward achieving minimum size and weight than to reducing pressure losses. This work studies the pressure losses in hydraulic manifolds using different methods: Computational Fluid Dynamic (CFD) analysis; semi-empirical formulation derived from the scientific literature, when available; and experimental characterization. The purpose is to obtain the pressure losses when the channels’ connections within the manifold are not ascribable to the few classic cases studied in the literature, in particular for 90° bends (elbows) with expansion/contraction and offset intersection of channels. Moreover, since CFD analysis is used to predict pressure losses, general considerations of the manifold design may be outlined and this will help the design process in the optimization of flow passages. The main results obtained show how CFD analysis overestimates the experimental results; nevertheless, the numerical analysis represents the correct trends of the pressure losses.

Description: Traditional current transformers (CTs) suffer from DC and AC saturation and remanent magnetization in many industrial applications. Moreover, the drawbacks of traditional CTs, such as closed iron cores, bulky volume, and heavy weight, further limit the development of an intelligent power protection system. In order to compensate for these drawbacks, we proposed a novel current measurement method by using Hall sensors, which is called the Hall-effect current transformer (HCT). The existing commercial Hall sensors are electronic components, so the reliability of the HCT is normally worse than that of the traditional CT. Therefore, our study proposes a redundancy mechanism for the HCT to strengthen its reliability. With multiple sensor modules, the method has the ability to improve the accuracy of the HCT as well. Additionally, the proposed redundancy mechanism monitoring system provides a condition-based maintenance for the HCT. We verify our method with both simulations and an experimental test. The results demonstrate that the proposed HCT with a redundancy mechanism can almost achieve Class 0.2 for measuring CTs according to IEC Standard 60044-8.

Description: This paper proposes a model for strategic maintenance scheduling of offshore wind farms (SMSOWF) in a deregulated power system. The objective of the model is to plan the maintenance schedules in a way to maximize the profit of the offshore wind farm. In addition, some network constraints, such as transmission lines capacity, and wind farm constraints, such as labor working shift, wave height limit and wake effect, as well as unexpected outages, are included in deterministic and stochastic studies. Moreover, the proposedmodel provides theability to incorporate information from condition monitoring systems. SMSOWF is formulated through a bi-level formulation and then transformed into a single-level through Karush–Kuhn–Tucker conditions. The model is validated through a test system, and the results demonstrate applicability, advantages and challenges of harnessing the full potential of the model.

Description: Energy consumption in cellular networks is receiving significant attention from academia and the industry due to its significant potential economic and ecological influence. Energy efficiency and renewable energy are the main pillars of sustainability and environmental compatibility. Technological advancements and cost reduction for photovoltaics are making cellular base stations (BSs; a key source of energy consumption in cellular networks) powered by solar energy sources a long-term promising solution for the mobile cellular network industry. This paper addresses issues of deployment and operation of two solar-powered global system for mobile communications (GSM) BSs that are being deployed at present (GSM BS 2/2/2 and GSM BS 4/4/4). The study is based on the characteristics of South Korean solar radiation exposure. The optimum criteria as well as economic and technical feasibility for various BSs are analyzed using a hybrid optimization model for electric renewables. In addition, initial capital, replacement, operations, maintenance, and total net present costs for various solar-powered BSs are discussed. Furthermore, the economic feasibility of the proposed solar system is compared with conventional energy sources in urban and remote areas.

Description: The accurate peak power estimation of a battery pack is essential to the power-train control of electric vehicles (EVs). It helps to evaluate the maximum charge and discharge capability of the battery system, and thus to optimally control the power-train system to meet the requirement of acceleration, gradient climbing and regenerative braking while achieving a high energy efficiency. A novel online peak power estimation method for series-connected lithium-ion battery packs is proposed, which considers the influence of cell difference on the peak power of the battery packs. A new parameter identification algorithm based on adaptive ratio vectors is designed to online identify the parameters of each individual cell in a series-connected battery pack. The ratio vectors reflecting cell difference are deduced strictly based on the analysis of battery characteristics. Based on the online parameter identification, the peak power estimation considering cell difference is further developed. Some validation experiments in different battery aging conditions and with different current profiles have been implemented to verify the proposed method. The results indicate that the ratio vector-based identification algorithm can achieve the same accuracy as the repetitive RLS (recursive least squares) based identification while evidently reducing the computation cost, and the proposed peak power estimation method is more effective and reliable for series-connected battery packs due to the consideration of cell difference.

Description: Against the backdrop of increasingly serious global climate change and the development of the low-carbon economy, the coordination between energy consumption carbon emissions (ECCE) and regional population, resources, environment, economy and society has become an important subject. In this paper, the research focuses on the security early warning of ECCE in Hebei Province, China. First, an assessment index system of the security early warning of ECCE is constructed based on the pressure-state-response (P-S-R) model. Then, the variance method and linearity weighted method are used to calculate the security early warning index of ECCE. From the two dimensions of time series and spatial pattern, the security early warning conditions of ECCE are analyzed in depth. Finally, with the assessment analysis of the data from 2000 to 2014, the prediction of the security early warning of carbon emissions from 2015 to 2020 is given, using a back propagation neural network based on a kidney-inspired algorithm (KA-BPNN) model. The results indicate that: (1) from 2000 to 2014, the security comprehensive index of ECCE demonstrates a fluctuating upward trend in general and the trend of the alarm level is “Severe warning”–“Moderate warning”–“Slight warning”; (2) there is a big spatial difference in the security of ECCE, with relatively high-security alarm level in the north while it is relatively low in the other areas; (3) the security index shows the trend of continuing improvement from 2015 to 2020, however the security level will remain in the state of “Semi-secure” for a long time and the corresponding alarm is still in the state of “Slight warning”, reflecting that the situation is still not optimistic.

Description: The penetration of various types of renewable sources and on-site storage devices have recently focused attention towards DC power distribution in consumer grids to achieve the target of zero/positive energy buildings and communities. To achieve this target, the most important component is the DC consumer grid architecture which can integrate not only renewable sources and storage, but also enable the implementation in any conventional AC distribution network without any significant upgrade. To this end, a unique DC Transformer enabled DC microgrid architecture is presented in this paper. The architecture, called PCmRC (power controlling monitoring routing center) is proposed to manage distributed energy sources and storage at any stage and also directly interconnects the DC consumer grid with the conventional AC power grid. This paper also investigates detailed control algorithms of each component and the DC Transformer topology in addition to proposing four unique stages of grid operational modes to enhance the overall grid stability in any operational condition. The main objectives are to maximize the exploitation of renewable sources, to decrease reliance on fossil fuels, to boost the overall efficiency of the grid by reducing the power conversion losses and demand side management in all possible forms. The simulation platform is designed in MATLAB/Simulink. Simulation results of several types of case studies show the effectiveness of the proposed power distribution and management model.

Description: The main anthropogenic emissions (CO, CO2, NOx, SOx) produced by the processing (combustion) of wastes (coal filter cakes) were measured directly for the first time. The research considered the most widespread coal filter cakes: those of nonbaking, low-caking, coking, flame, and gas coals. These filter cakes are regarded as promising components for the technologies of coal-water slurry (CWS) and coal-water slurry containing petrochemicals (CWSP). According to our estimates, the annual increment of such wastes in the world is as high as 100 million tons. Consequently, the effective utilization of these wastes in the power industry is of high interest. The evaluation of hazardous emissions from the combustion of such wastes shows that filter cakes produce a similar amount of CO and CO2 as the initially-used coals but filter cakes are more cost-effective. We have established that CWS and CWSP technologies can be used to reduce NOx and SOx emissions. To reduce CO and CO2 emissions when burning filter cakes, we need to switch to low-temperature combustion. Lowering the combustion temperature of filter cakes from 850 °C down to 650 °C decreases the underburning insignificantly while decreasing CO and CO2 emissions by 30–40%.

Description: This paper discusses the modeling and analysis of three phase double stator slotted rotor permanent magnet generator (DSSR-PMG). The use of double stator topology through the double magnetic circuit helps to maximize the usage of flux linkage in the yoke structure of the single stator topology. The analytical computation is done using Permeance Analysis Method (PAM). Finite Element Analysis (FEA) is used for numerical verifications and to verify the design structure a prototype laboratory is performed. The analysis is done with various loading conditions to derive the electromagnetic torque, output power and efficiency for the proposed structure. The analytical, numerical and experimental results from the analysis are found to be in good agreement. The maximum power developed by this generator at rated speed of 2000 rpm is of 1 kW with the operational efficiency of 75%. A rectifier bridge circuit is used to make the generated voltage a storage capable constant voltage to make it suitable for mobile applications (such as Direct Current DC generator). The proposed generator structure is highly recommended for applications such as micro-hydro and small renewable plants.

Description: : In this paper, an effective method to achieve accurate and efficient torque control of an interior permanent magnet synchronous motor (IPMSM) in electric vehicles, based on low-resolution Hall-effect sensors, is proposed. The high-resolution rotor position is estimated by a proportional integral (PI) regulator using the deviation between actual output power and reference output power. This method can compensate for the Hall position sensor mounting error, and estimate rotor position continuously and accurately. The permanent magnetic flux linkage is also estimated based on a current PI controller. Other important parameters, such as the d-axis and q-axis inductances, stator resistance, and energy loss, are measured offline by experiments. The measured parameters are saved as lookup tables which cover the entire current operating range at different current levels. Based on these accurate parameters, a maximum torque per ampere (MTPA) control strategy, combined with the feedforward parameter iteration method, can be achieved for accurate and efficient torque control. The effectiveness of the proposed method is verified by both simulation and experimental results.

Description: The current load balance in the grid is managed mainly through peaking fossil-fuelled power plants that respond passively to the load changes. Intermittency, which comes from renewable energy sources, imposes additional requirements for even more flexible and faster responses from conventional power plants. A major challenge is to keep conventional generation running closest to the design condition with higher load factors and to avoid switching off periods if possible. Thermal energy storage (TES) integration into the power plant process cycle is considered as a possible solution for this issue. In this article, a technical feasibility study of TES integration into a 375-MW subcritical oil-fired conventional power plant is presented. Retrofitting is considered in order to avoid major changes in the power plant process cycle. The concept is tested based on the complete power plant model implemented in the ProTRAX software environment. Steam and water parameters are assessed for different TES integration scenarios as a function of the plant load level. The best candidate points for heat extraction in the TES charging and discharging processes are evaluated. The results demonstrate that the integration of TES with power plant cycle is feasible and provide a provisional guidance for the design of the TES system that will result in the minimal influence on the power plant cycle.

Description: Predicting the remaining useful life (RUL) of critical subassemblies can provide an advanced maintenance strategy for wind turbines installed in remote regions. This paper proposes a novel prognostic approach to predict the RUL of bearings in a wind turbine gearbox. An artificial neural network (NN) is used to train data-driven models and to predict short-term tendencies of feature series. By combining the predicted and training features, a polynomial curve reflecting the long-term degradation process of bearings is fitted. Through solving the intersection between the fitted curve and the pre-defined threshold, the RUL can be deduced. The presented approach is validated by an operating wind turbine with a faulty bearing in the gearbox.

Description: With the introduction of more non-linear loads, e.g., compact fluorescent lamps, electric vehicles, photovoltaics, etc., the need to determine the harmonic impact of the residential load is rising, illustrated by the many studies performed on their harmonic impact. Traditionally, these studies are performed for a single new device and single penetration level, neglecting the harmonic interaction between new types of devices, as well as giving little information at which moment in time possible problems may arise. A composite approach to access the impact of harmonic sources on the distribution network is therefore proposed. This method combines a bottom-up stochastic modeling of the residential load with harmonic measurement data and harmonic load-flows all based on a scenario analysis. The method is validated with measurement data and shows a good prediction of the current level of harmonics in a residential neighborhood for the current situation. To demonstrate the applicability of the proposed method, case studies are performed on the IEEE European Low Voltage Test Feeder. These case studies show a marked difference between applying individual device-based models and a composite modeling approach, demonstrating why the proposed approach is an adequate method for the determination of the impact of new devices on the harmonics.

Description: Energies, Vol. 10, Pages 1191: Pyrolysis Characteristics and Kinetics of Food Wastes Energies doi: 10.3390/en10081191 Authors: Jun-Ho Jo Seung-Soo Kim Jae-Wook Shin Ye-Eun Lee Yeong-Seok Yoo Pyrolysis is an environmental friendly alternative method compared with incineration, and the least time-consuming and smallest infrastructure footprint method compared with bio-chemical and thermo-chemical conversion. Baseline data for the pyrolysis of food waste was obtained in a kinetic study of the thermal decompositions by thermogravimetric analysis. To simulate the difference in the types of food waste, the study was done using model compounds, such as cereals, meat, vegetable, and mixed food waste; the pyrolysis commenced at 150 °C for most food waste and the process terminated at 450 °C to 500 °C. Between one and three peaks were observed on a differential thermogravimetry (DTG) graph, depending on the type of waste being pyrolyzed, reflecting the difference in the time required for pyrolysis of different components of food waste to take place. Depending on the composition of each food, one or four peaks were found, and the pyrolysis patterns of carbohydrate, protein, fat, and cellulose were found. Activation energies and frequency factors were calculated from the rates of conversion, using differential equation analyses. The activation energy increased from 10 kJ/mol to 50 kJ/mol as conversions increased from the 10% to 90%, regardless of the food waste type. The activation energy was measured as 50 kJ/mol, with a slight variation among the type of the food waste. Due to the activation energy being low, food waste consists of carbohydrates and other substances rather than cellulose, hemicellulose, and lignin.

Description: Energies, Vol. 10, Pages 1185: Off-Design Performances of Subcritical and Supercritical Organic Rankine Cycles in Geothermal Power Systems under an Optimal Control Strategy Energies doi: 10.3390/en10081185 Authors: Tieyu Gao Changwei Liu The conditions of heat source and heat sink in a geothermal ORC system may frequently vary due to variations in geological conditions, ambient temperature and actual operation. In this study, an off-design performance prediction model for geothermal ORC systems is developed according to special designs of critical components, and an optimal control strategy which regards the turbine guide vane angle, the refrigerant pump rotational speed and the cooling water mass flow rate as control variables is proposed to maximize the net power output. Off-design performances of both subcritical and supercritical ORCs are analyzed. The results indicate that, under the optimal control strategy, the net power output of both ORCs increase with greater geothermal water mass flow rate, higher geothermal water inlet temperature and lower cooling water inlet temperature, which is mainly due to a greater working fluid mass flow rate, higher turbine inlet pressure and lower condensing pressure, respectively. The net power output of supercritical ORC is always greater than that of subcritical ORC within the range of this study, but the difference tends to decrease when supercritical ORC activates the geothermal water reinjection temperature restriction.

Description: Energies, Vol. 10, Pages 1189: Theoretical and Experimental Investigation of Switching Ripple in the DC-Link Voltage of Single-Phase H-Bridge PWM Inverters Energies doi: 10.3390/en10081189 Authors: Marija Vujacic Manel Hammami Milan Srndovic Gabriele Grandi Direct current (DC)-link voltage ripple analysis is essential for determining harmonic noise and for DC-link capacitor design and selection in single-phase pulse-width modulation (PWM) inverters. This paper provides an extensive theoretical analysis of DC-link voltage ripple for full-bridge (H-bridge) inverters, with simulation and experimental verifications, considering a DC source impedance (non-ideal DC voltage source). The DC voltage ripple amplitude is theoretically estimated as a function of the output current, both amplitude and phase angle, and the modulation index. It consists of a switching frequency component and a double-fundamental frequency component (i.e., 100 Hz), thereby both components are considered in the analysis. In particular, the peak-to-peak distribution, maximum amplitude, and root mean square (RMS) values of the voltage switching ripple over the fundamental period are obtained. Based on the DC voltage requirements, simple and effective guidelines for designing DC-link capacitors are obtained.

Description: Energies, Vol. 10, Pages 1207: Enhanced Production of Bioethanol by Fermentation of Autohydrolyzed and C4mimOAc-Treated Sugarcane Bagasse Employing Various Yeast Strains Energies doi: 10.3390/en10081207 Authors: Muzna Hashmi Aamer Shah Abdul Hameed Arthur Ragauskas This study examines the fermentation of autohydrolyzed and 1-n-butyl-3-methylimidazolium acetate (C4mimOAc) pretreated sugarcane bagasse, using four different yeast strains to determine the efficiency of bioethanol production. Three strains of Saccharomyces cerevisiae (S. cerevisiae) and one of Scheffersomyces stipitis (S. stipitis) were employed in this study. It was observed that the sugarcane bagasse autohydrolyzed at 205 °C for 6 min with subsequent enzymatic hydrolysis exhibited the maximum ethanol yield of 70.92 ± 0.09 mg/g-substrate when S. cerevisiae MZ-4 was used. However, a slightly higher ethanol yield of 78.78 ± 0.94 mg/g-substrate was obtained from C4mimOAc pretreated bagasse employing S. cerevisiae MZ-4. The study showed that the newly isolated MZ-4 strain exhibited better ethanol yield as compared to commercially available yeast strains S. cerevisiae Uvaferm-43, S. cerevisiae Lalvin EC-1118, and S. stipitis.

Description: Energies, Vol. 10, Pages 1211: Performance Evaluation of a Hydrogen-Based Clean Energy Hub with Electrolyzers as a Self-Regulating Demand Response Management Mechanism Energies doi: 10.3390/en10081211 Authors: Weiliang Wang Dan Wang Hongjie Jia Guixiong He Qing’e Hu Pang-Chieh Sui Menghua Fan Energy management of hybrid resources has become a critical issue in integrated energy system analysis. In this study, as a self-regulating demand response (DR) management mechanism, deferrable electrolyzers are used as a main controlled resource in a hydrogen-based clean energy hub (CEH), which includes a traditional generation plant (TGP), a low-carbon generation plant (LGP), and wind energy. Based on the hysteresis control model for aggregated electrolyzers, a comfort-constrained optimal energy state regulation (OESR) control strategy is implemented to model the deregulation feature of aggregated electrolyzers. The electrolyzers’ population can be integrated as a controlled efficient power plant (EPP) to provide the virtual spinning reserve for CEH. As a flexible and self-regulating participant, the electrolyzer-based EPP is integrated into the hybrid resource constrained optimization model; this reduces the total cost of CEH and carbon emissions and improves the integration of wind energy. Combined with TGP, LGP, and wind energy, the simulation results show that the deployment of aggregated electrolyzers on both the supply and demand sides of the CEH contributes to significant amounts of low-carbon hydrogen. The simulation also illustrates that the DR control strategy has a positive effect on active power and reserve re-dispatch.

Description: Energies, Vol. 10, Pages 1216: Recent Advances in the Quest for a New Insulation Gas with a Low Impact on the Environment to Replace Sulfur Hexafluoride (SF6) Gas in High-Voltage Power Network Applications Energies doi: 10.3390/en10081216 Authors: Abderrahmane Beroual Abderrahmane (Manu) Haddad The growing environmental challenge of electrical energy systems has prompted a substantial increase in renewable energy generation. Such generation systems allow for significant reduction of CO2 emissions compared with a traditional fossil fuel plant. Furthermore, several improvements in power systems network configuration and operation combined with new technologies have enabled reduction of losses and energy demand, thus contributing to reduction of CO2 emissions. Another environmental threat identified in electrical networks is the leaking of insulating sulfur hexafluoride (SF6) gas used in electrical gas insulated substations (GIS) and equipment. Because of its Global Warming Potential (GWP) of nearly 24,000 and its long life in the atmosphere (over 3000 years), SF6 gas was recognized as a greenhouse gas at the 1997 COP3; since then its use and emissions in the atmosphere have been regulated by international treaties. It is expected that as soon as an alternative insulating gas is found, SF6 use in high-voltage (HV) equipment will be banned. This paper presents an overview of the key research advances made in recent years in the quest to find eco-friendly gases to replace SF6. The review reports the main properties of candidate gases that are being investigated; in particular, natural gases (dry air, N2 or CO2) and polyfluorinated gases especially Trifluoroiodomethane (CF3I), Perfluorinated Ketones, Octafluorotetra-hydrofuran, Hydrofluoroolefin (HFOs), and Fluoronitriles are presented and their strengths and weaknesses are discussed with an emphasis on their dielectric properties (especially their dielectric strength), GWP, and boiling point with respect to the minimum operating temperature for HV power network applications.

Description: Energies, Vol. 10, Pages 1229: Wind Energy Potential of Gaza Using Small Wind Turbines: A Feasibility Study Energies doi: 10.3390/en10081229 Authors: Mohamed Elnaggar Ezzaldeen Edwan Matthias Ritter In this paper, we conduct a feasibility study of the wind energy potential in Gaza, which suffers from a severe shortage of energy supplies. Our calculated energy harvested from the wind is based on data for a typical meteorological year, which are fed into a small wind turbine of 5 kW power rating installable on the roof of residential buildings. The expected annual energy output at a height of 10 m amounts to 2695 kWh, but it can be increased by 35–125% at higher altitudes between 20 m and 70 m. The results also depict the great potential of wind energy to complement other renewable resources such as solar energy: the harvested energy of a wind system constitutes to up to 84% of the annual output of an equivalent power rating photovoltaic system and even outperforms the solar energy in the winter months. We also show that one wind turbine and one comparable photovoltaic system together could provide enough energy for 3.7 households. Hence, a combination of wind and solar energy could stabilize the decentralized energy production in Gaza. This is very important in a region where people seek to reach energy self-sufficient buildings due to the severe electricity shortage in the local grid.

Description: Energies, Vol. 10, Pages 1242: Modelling and Control of Parallel-Connected Transformerless Inverters for Large Photovoltaic Farms Energies doi: 10.3390/en10081242 Authors: Marian Liberos Raúl González-Medina Gabriel Garcerá Emilio Figueres This paper presents a control structure for transformerless photovoltaic inverters connected in parallel to manage photovoltaic fields in the MW range. Large photovoltaic farms are usually divided into several photovoltaic fields, each one of them managed by a centralized high power inverter. The current tendency to build up centralized inverters in the MW range is the use of several transformerless inverters connected in parallel, a topology that provokes the appearance of significant zero-sequence circulating currents among inverters. To eliminate this inconvenience, this paper proposes a control structure that avoids the appearance of circulating currents by controlling the zero-sequence component of the inverters. A second contribution of the paper is the development of a model of n parallel-connected inverters. To validate the concept, the proposed control structure has been applied to a photovoltaic field of 2 MW managed by four 500 kW photovoltaic inverters connected in parallel.

Description: Energies, Vol. 10, Pages 1246: Effects of Loading Rate on Gas Seepage and Temperature in Coal and Its Potential for Coal-Gas Disaster Early-Warning Energies doi: 10.3390/en10091246 Authors: Chong Zhang Xiaofei Liu Guang Xu Xiaoran Wang The seepage velocity and temperature externally manifest the changing structure, gas desorption and energy release that occurs in coal containing gas failure under loading. By using the system of coal containing gas failure under loading, this paper studies the law of seepage velocity and temperature under different loading rates and at 1.0 MPa confining pressure and 0.5 MPa gas pressure, and combined the on-site results of gas pressure and temperature. The results show that the stress directly affects the seepage velocity and temperature of coal containing gas, and the pressure and content of gas have the most sensitivity to mining stress. Although the temperature is not sensitive to mining stress, it has great correlation with mining stress. Seepage velocity has the characteristic of critically slowing down under loading. This is demonstrated by the variance increasing before the main failure of the samples. Therefore, the variance of seepage velocity with time and temperature can provide an early warning for coal containing gas failing and gas disasters in a coal mine.

Description: Energies, Vol. 10, Pages 1258: An Intelligent Hybrid Heuristic Scheme for Smart Metering based Demand Side Management in Smart Homes Energies doi: 10.3390/en10091258 Authors: Awais Manzoor Nadeem Javaid Ibrar Ullah Wadood Abdul Ahmad Almogren Atif Alamri Smart grid is an emerging technology which is considered to be an ultimate solution to meet the increasing power demand challenges. Modern communication technologies have enabled the successful implementation of smart grid (SG), which aims at provision of demand side management mechanisms (DSM), such as demand response (DR). In this paper, we propose a hybrid technique named as teacher learning genetic optimization (TLGO) by combining genetic algorithm (GA) with teacher learning based optimization (TLBO) algorithm for residential load scheduling, assuming that electric prices are announced on a day-ahead basis. User discomfort is one of the key aspects which must be addressed along with cost minimization. The major focus of this work is to minimize consumer electricity bill at minimum user discomfort. Load scheduling is formulated as an optimization problem and an optimal schedule is achieved by solving the minimization problem. We also investigated the effect of power-flexible appliances on consumers’ bill. Furthermore, a relationship among power consumption, cost and user discomfort is also demonstrated by feasible region. Simulation results validate that our proposed technique performs better in terms of cost reduction and user discomfort minimization, and is able to obtain the desired trade-off between consumer electricity bill and user discomfort.

Description: Energies, Vol. 10, Pages 1262: Layout Optimisation of Wave Energy Converter Arrays Energies doi: 10.3390/en10091262 Authors: Pau Ruiz Vincenzo Nava Mathew Topper Pablo Minguela Francesco Ferri Jens Kofoed This paper proposes an optimisation strategy for the layout design of wave energy converter (WEC) arrays. Optimal layouts are sought so as to maximise the absorbed power given a minimum q-factor, the minimum distance between WECs, and an area of deployment. To guarantee an efficient optimisation, a four-parameter layout description is proposed. Three different optimisation algorithms are further compared in terms of performance and computational cost. These are the covariance matrix adaptation evolution strategy (CMA), a genetic algorithm (GA) and the glowworm swarm optimisation (GSO) algorithm. The results show slightly higher performances for the latter two algorithms; however, the first turns out to be significantly less computationally demanding.

Description: Energies, Vol. 10, Pages 1257: A Three-Part Electricity Price Mechanism for Photovoltaic-Battery Energy Storage Power Plants Considering the Power Quality and Ancillary Service Energies doi: 10.3390/en10091257 Authors: Yajing Gao Fushen Xue Wenhai Yang Yanping Sun Yongjian Sun Haifeng Liang Peng Li To solve the problem of solar abandoning, which is accompanied by the rapid development of photovoltaic (PV) power generation, a demonstration of a photovoltaic-battery energy storage system (PV-BESS) power plant has been constructed in Qinghai province in China. However, it is difficult for the PV-BESS power plant to survive and develop with the current electricity price mechanism and subsidy policy. In this paper, a three-part electricity price mechanism is proposed based on a deep analysis of the construction and operation costs and economic income. The on-grid electricity price is divided into three parts: the capacity price, graded electricity price, and ancillary service price. First, to ensure that the investment of the PV-BESS power plant would achieve the industry benchmark income, the capacity price and benchmark electricity price are calculated using the discounted cash flow method. Then, the graded electricity price is calculated according to the grade of the quality of grid-connected power. Finally, the ancillary service price is calculated based on the graded electricity price and ancillary service compensation. The case studies verify the validity of the three-part electricity price mechanism. The verification shows that the three-part electricity price mechanism can help PV-BESS power plants to obtain good economic returns, which can promote the development of PV-BESS power plants.

Description: Energies, Vol. 10, Pages 1256: Towards a Residential Air-Conditioner Usage Model for Australia Energies doi: 10.3390/en10091256 Authors: Mark Goldsworthy Realistic models of occupant behaviour in relation to air-conditioner (a/c) use are fundamentally important for developing accurate building energy simulation tools. In Australia and elsewhere, such simulation tools are inextricably bound both in legislation and in the design of new technology, electricity infrastructure and regulatory schemes. An increasing number of studies in the literature confirm just how important occupants are in determining overall energy consumption, but obtaining the data on which to build behaviour models is a non-trivial task. Here data is presented on air-conditioner usage derived from three different types of case study analyses. These are: (i) use of aggregate energy consumption data coupled with weather, demographic and building statistics across Australia to estimate key predictors of energy use at the aggregate level; (ii) use of survey data to determine characteristic a/c switch on/off behaviours and usage frequencies; and (iii) use of detailed household level sub-circuit monitoring from 140 households to determine a/c switch on/off probabilities and their dependence on different building and occupant parameters. These case studies are used to assess the difficulties associated with translation of different forms of individual, aggregate and survey based information into a/c behaviour simulation models. Finally a method of linking the data gathering methodologies with the model development is suggested. This method would combine whole-of-house “smart”-meter data measurements with linked targeted occupant surveying.

Description: Energies, Vol. 10, Pages 1259: Analysis of Vibrations in Interior Permanent Magnet Synchronous Motors Considering Air-Gap Deformation Energies doi: 10.3390/en10091259 Authors: Yi Li Feng Chai Zaixin Song Zongyang Li This paper studies the non-uniform air-gap caused by stator and rotor deformations, together with its effects on the spatial and temporal spectrum of the radial magnetic force density in an interior permanent magnet synchronous motor (IPMSM). According to the mathematical model of the deformed air-gap length, the superposition method is adopted to derive the air-gap permeance. Then, the formulas of the magnetic flux field and radial force density of the IPMSM considering air-gap deformation are obtained. Considering the stator oval deformation and the rotor centrifugal distortion in the electromagnetic finite element models (FEMs), the finite element analysis (FEA) and experiments of the investigated IPMSM are carried out to verify the results obtained by the theoretical analysis at different operations. Finally, the mathematical correlation between air-gap deformation and electromagnetic vibration is obtained. The result is helpful in solving problems of mutual influence between electromagnetic and mechanical characteristics during the optimization design of IPMSM.

Description: Energies, Vol. 10, Pages 1261: Models for Flow Rate Simulation in Gear Pumps: A Review Energies doi: 10.3390/en10091261 Authors: Massimo Rundo Gear pumps represent the majority of the fixed displacement machines used for flow generation in fluid power systems. In this context, the paper presents a review of the different methodologies used in the last years for the simulation of the flow rates generated by gerotor, external gear and crescent pumps. As far as the lumped parameter models are concerned, different ways of selecting the control volumes into which the pump is split are analyzed and the main governing equations are presented. The principles and the applications of distributed models from 1D to 3D are reported. A specific section is dedicated to the methods for the evaluation of the necessary geometric quantities: analytic, numerical and Computer-Aided Design (CAD)-based. The more recent studies taking into account the influence on leakages of the interactions between the fluid and the mechanical parts are explained. Finally the models for the simulation of the fluid aeration are described. The review brings to evidence the increasing effort for improving the simulation models used for the design and the optimization of the gear machines.

Description: Energies, Vol. 10, Pages 1260: Game-Based Generation Scheduling Optimization for Power Plants Considering Long-Distance Consumption of Wind-Solar-Thermal Hybrid Systems Energies doi: 10.3390/en10091260 Authors: Tiejiang Yuan Tingting Ma Yiqian Sun Ning Chen Bingtuan Gao With the increasing penetration of renewable energy in power systems, fluctuation of renewable energy power plants has great influence on stability of the system, and renewable power curtailment is also becoming more and more serious due to the insufficient consumptive ability of local power grid. In order to maximize the utilization of renewable energy, this paper focuses on the generation scheduling optimization for a wind-solar-thermal hybrid system considering that the produced energy will be transmitted over a long distance to satisfy the demands of the receiving end system through ultra-high voltage (UHV) transmission lines. Accordingly, a bilevel optimization based on a non-cooperative game method is proposed to maximize the profit of power plants in the hybrid system. Users in the receiving end system are at the lower level of the bilevel programming, and power plants in the transmitting end system are at the upper level. Competitive behavior among power plants is formulated as a non-cooperative game and the profit of power plant is scheduled by adjusting generation and bidding strategies in both day-ahead markets and intraday markets. In addition, generation cost, wheeling cost, and carbon emissions are all considered in the non-cooperative game model. Moreover, a distributed algorithm is presented to obtain the generalized Nash equilibrium solution, which realizes the optimization in terms of maximizing profit. Finally, several simulations are implemented and analyzed to verify the effectiveness of the proposed optimization method.

Description: Energies, Vol. 10, Pages 1277: The Effect of Imbalanced Carrier Transport on the Efficiency Droop in GaInN-Based Blue and Green Light-Emitting Diodes Energies doi: 10.3390/en10091277 Authors: Jun Hyuk Park Jaehee Cho E. Fred Schubert Jong Kyu Kim The effect of strongly-imbalanced carrier concentration and mobility on efficiency droop is studied by comparing the onset voltage of high injection, the onset current density of the droop, and the magnitude of the droop, as well as their temperature dependence, of GaInN-based blue and green light-emitting diodes (LEDs). An n-to-p asymmetry factor is defined as σn/σp, and was found to be 17.1 for blue LEDs and 50.1 for green LEDs. Green LEDs, when compared to blue LEDs, were shown to enter the high-injection regime at a lower voltage, which is attributed to their less favorable p-type transport characteristics. Green LEDs, with lower hole concentration and mobility, have a lower onset current density of the efficiency droop and a higher magnitude of the efficiency droop when compared to blue LEDs. The experimental results are in quantitative agreement with the imbalanced carrier transport causing the efficiency droop, thus providing guidance for alleviating the phenomenon of efficiency droop.

Description: Energies, Vol. 10, Pages 1282: Development of a Transmission and Distribution Integrated Monitoring and Analysis System for High Distributed Generation Penetration Energies doi: 10.3390/en10091282 Authors: Jaewan Suh Sungchul Hwang Gilsoo Jang To date, power system analysis has been performed separately for transmission and distribution systems. Due to the small influence of distribution systems on transmission systems, separate analyses have had no accuracy problems in existing power systems. However, as the amount of distributed generation (DG) in distribution systems increases, neighboring distribution systems and even transmission systems can be affected by the distributed generation. Therefore, a power system operator needs a new system to analyze the power system, one that considers the mutual interactions between the transmission and distribution systems. This paper presents with applications and case studies a transmission and distribution integrated monitoring and analysis system for high DG penetration. The integrated system analyzes the mutual interaction between the transmission and distribution systems due to DG. The preliminary evaluation of the DG connections is automated in this system, using real time online data. Case studies with practical data show the need and effectiveness of transmission and distribution integrated monitoring and analysis for real power systems with high DG penetration.

Description: Energies, Vol. 10, Pages 1296: One-Step Self-Assembly Synthesis α-Fe2O3 with Carbon-Coated Nanoparticles for Stabilized and Enhanced Supercapacitors Electrode Energies doi: 10.3390/en10091296 Authors: Yizhi Yan Haolin Tang Fan Wu Rui Wang Mu Pan A cocoon-like α-Fe2O3 nanocomposite with a novel carbon-coated structure was synthesized via a simple one-step hydrothermal self-assembly method and employed as supercapacitor electrode material. It was observed from electrochemical measurements that the obtained α-Fe2O3@C electrode showed a good specific capacitance (406.9 Fg−1 at 0.5 Ag−1) and excellent cycling stability, with 90.7% specific capacitance retained after 2000 cycles at high current density of 10 Ag−1. These impressive results, presented here, demonstrated that α-Fe2O3@C could be a promising alternative material for application in high energy density storage.

Description: Energies, Vol. 10, Pages 1300: Overview of AC Microgrid Controls with Inverter-Interfaced Generations Energies doi: 10.3390/en10091300 Authors: Md Hossain Hemanshu Pota Walid Issa Md Hossain Distributed generation (DG) is one of the key components of the emerging microgrid concept that enables renewable energy integration in a distribution network. In DG unit operation, inverters play a vital role in interfacing energy sources with the grid utility. An effective interfacing can successfully be accomplished by operating inverters with effective control techniques. This paper reviews and categorises different control methods (voltage and primary) for improving microgrid power quality, stability and power sharing approaches. In addition, the specific characteristics of microgrids are summarised to distinguish from distribution network control. Moreover, various control approaches including inner-loop controls and primary controls are compared according to their relative advantages and disadvantages. Finally, future research trends for microgrid control are discussed pointing out the research opportunities. This review paper will be a good basis for researchers working in microgrids and for industry to implement the ongoing research improvement in real systems.

Description: Energies, Vol. 10, Pages 1302: A Simulation-Based Optimization Method for Hybrid Frequency Regulation System Configuration Energies doi: 10.3390/en10091302 Authors: Jie Song Xin Pan Chao Lu Hanchen Xu Frequency regulation is essential for the stability of a power grid with high load fluctuation and integration of new energies. Constrained by the large ramping, a generator alone is not capable of conducting load frequency controls effectively and economically. In this paper, an energy storage system (ESS) is introduced to coordinate with generators in automatic generation control (AGC), where ESS and the generator respectively deal with high-frequency load fluctuation and low-portion. We develop a system configuration framework for such a hybrid system, including the operation strategy and capacity optimization. Due to the complexity of the hybrid system, the operation process is captured by a simulation model which considers practical constraints as well as remaining energy management of ESS. Taking advantage of the gradient-based approximation algorithm, we are then able to optimize the capacity of a hybrid system. According to the numerical experiments with real historical AGC data, the hybrid system is shown to perform well in cost reduction and to achieve the regulation tasks.

Description: Energies, Vol. 10, Pages 1301: Modeling and Simulating Long-Timescale Cascading Faults in Power Systems Caused by Line-Galloping Events Energies doi: 10.3390/en10091301 Authors: Lizheng Chen Hengxu Zhang Changgang Li Huadong Sun With the increasing occurrence of extreme weather events, the short circuit and line-breaking faults in transmission lines caused by line galloping have been threatening the security operation of power systems. These faults are also hard to be simulated with current simulation tools. A numerical simulation approach of power systems is presented to simulate the clustered, cascading faults of long-timescale caused by line-galloping events. A simulation framework is constructed in which large numbers of fault scenarios are simulated to reflect the randomness of line galloping. The interaction mechanism between power system operation states and line galloping processes is revealed and simulated by the solution of differences of timescales and parameters. Based on Power System Simulator/Engineering (PSS/E), an extended software package for line galloping simulation is developed with Python, which extends the functionalities of the PSS/E in power system simulation. An example is given to demonstrate the feasibility of the proposed simulation method.

Description: Energies, Vol. 10, Pages 1310: Investigation on Slot–Pole Combinations of a PM Vernier Motor with Fractional-Slot Concentrated Winding Configurations Energies doi: 10.3390/en10091310 Authors: Byungtaek Kim This paper presents a new method to find available slot–pole combination of a permanent magnet (PM) vernier motor with fractional-slot concentrated winding (FSCW) configurations instead of the conventional design rule. To this aim, for a common structure of PM vernier machines with FSCW, the air gap flux densities including modulation flux due to vernier effects are investigated from the magnetic view points and then a general condition to effectively use the modulation flux is derived. Under the obtained condition, the specific design condition for slot–pole combinations are established for the most popular FSCW configurations with the coil spans of ±2π/3. Using the established condition, all available vernier structures including those which could not be found by the previous rule are obtained, and the back-electromotive force (EMF) of each structure is analytically estimated to check the vernier effects. During these procedures, it is also revealed that some of general FSCW-PM motors possess vernier effects and thus can be classified into vernier motors. To verify the proposed ideas, the characteristics of back EMF are analyzed through finite element (FE)-simulations for various models, and compared with their analytical calculation results. Finally, the characteristics of torque regarding to slot–pole combinations of the vernier motors are discussed.

Description: Energies, Vol. 10, Pages 1328: On the Influence of Operational and Control Parameters in Thermal Response Testing of Borehole Heat Exchangers Energies doi: 10.3390/en10091328 Authors: Borja Badenes Miguel Mateo Pla Lenin Lemus-Zúñiga Begoña Sáiz Mauleón Javier Urchueguía Thermal response test (TRT) is a common procedure for characterization of ground and borehole thermal properties needed for the design of a shallow geothermal heat pump system. In order to investigate and to develop more accurate and robust procedures for TRT control, modelling, and evaluation in semi-permeable soils with large water content, a pilot borehole heat exchanger was built in the main campus of the Universitat Politècnica de València. The present work shows the results of the experiments performed at the site, analysing the improvements that have been introduced both in the control of the heat injected during TRTs and in the methods to infer the ground thermal parameter. Three models are compared: two based on the infinite-line source theory and one based on the finite-line source scheme. The models were tested under two possible configurations of the equipment, i.e., with and without strict control of injected heat. Our results show the importance of heat injection control for a robust parameter assessment and the existence of additional heat transfer processes that the used models cannot completely characterize and that are related to the presence of significant groundwater flow at the site. In addition, our experience with the current installation and the knowledge about its strengths and weaknesses have allowed us to design a new and more complete test-site to help in the analysis and validation of new ground heat exchanger geometries.

Description: Energies, Vol. 10, Pages 1334: Multi-Objective Optimization of the Hydrodynamic Performance of the Second Stage of a Multi-Phase Pump Energies doi: 10.3390/en10091334 Authors: Jun-Won Suh Jin-Woo Kim Young-Seok Choi Jin-Hyuk Kim Won-Gu Joo Kyoung-Yong Lee Most multi-phase pumps used in crude oil production have been developed to satisfy certain pressure specifications. In the design of these pumps, the flow characteristics of the posterior stage are different from those of the prior stage. For this reason, the design of the second stage needs to be supplemented. To optimize performance in this stage, multi-objective optimization to simultaneously increase pressure and efficiency is reported in this article. Flow analyses of the single and multiple phases of the multi-phase pump were conducted by solving three-dimensional steady Reynolds-averaged Navier–Stokes equations. For the numerical optimization, two design variables related to the blade inlet angle were selected. The impeller and the diffuser blades were optimized using a systematic optimization technique combined with a central composite method and a hybrid multi-objective evolutionary algorithm coupled with a surrogate model. The selected optimal model yielded better hydrodynamic performance than the base model, and reasons for this are investigated through internal flow field analysis.

Description: Energies, Vol. 10, Pages 1330: Experimental Validation of the Simulation Model of a DOAS Equipped with a Desiccant Wheel and a Vapor Compression Refrigeration System Energies doi: 10.3390/en10091330 Authors: Pedro Martínez Carlos Llorca José Pla Pedro Martínez A dedicated outdoor air system (DOAS) can be designed to supply 100% of the outside air and meet the latent load of the room with dry air. The objectives of this study were to develop a model of a DOAS equipped with a desiccant wheel and a vapor-compression refrigeration system, build a prototype, validate the model with experimental data, and gain knowledge about the system operation. The test facility was designed with the desiccant wheel downstream of the cooling coil to take advantage of the operating principles of cooling coils and desiccants. A model of the DOAS was developed in the TRNSYS environment. The root mean standard error (RMSE) was used for model validation by comparing the measured air and refrigerant properties with the corresponding calculated values. The results obtained with the developed model showed that the DOAS was able to maintain an indoor humidity ratio depending on outdoor conditions. Laboratory tests were also used to investigate the effect of changes in the regeneration air temperature and the process airflow rate on the process air humidity ratio at the outlet of the wheel. The results are consistent with the technical literature.

Description: Energies, Vol. 10, Pages 1337: Flame Front Propagation in an Optical GDI Engine under Stoichiometric and Lean Burn Conditions Energies doi: 10.3390/en10091337 Authors: Santiago Martinez Adrian Irimescu Simona Merola Pedro Lacava Pedro Curto-Riso Lean fueling of spark ignited (SI) engines is a valid method for increasing efficiency and reducing nitric oxide (NOx) emissions. Gasoline direct injection (GDI) allows better fuel economy with respect to the port-fuel injection configuration, through greater flexibility to load changes, reduced tendency to abnormal combustion, and reduction of pumping and heat losses. During homogenous charge operation with lean mixtures, flame development is prolonged and incomplete combustion can even occur, causing a decrease in stability and engine efficiency. On the other hand, charge stratification results in fuel impingement on the combustion chamber walls and high particle emissions. Therefore, lean operation requires a fundamentally new understanding of in-cylinder processes for developing the next generation of direct-injection (DI) SI engines. In this paper, combustion was investigated in an optically accessible DISI single cylinder research engine fueled with gasoline. Stoichiometric and lean operations were studied in detail through a combined thermodynamic and optical approach. The engine was operated at a fixed rotational speed (1000 rpm), with a wide open throttle, and at the start of the injection during the intake stroke. The excess air ratio was raised from 1 to values close to the flammability limit, and spark timing was adopted according to the maximum brake torque setting for each case. Cycle resolved digital imaging and spectroscopy were applied; the optical data were correlated to in-cylinder pressure traces and exhaust gas emission measurements. Flame front propagation speed, flame morphology parameters, and centroid motion were evaluated through image processing. Chemical kinetics were characterized based on spectroscopy data. Lean burn operation demonstrated increased flame distortion and center movement from the location of the spark plug compared to the stoichiometric case; engine stability decreased as the lean flammability limit was approached.

Description: Energies, Vol. 10, Pages 1356: A Combined Approach Effectively Enhancing Traffic Performance for HSR Protocol in Smart Grids Energies doi: 10.3390/en10091356 Authors: Nguyen Tien Jong Rhee Sang Park In this paper, we propose a very effectively filtering approach (EFA) to enhance network traffic performance for high-availability seamless redundancy (HSR) protocol in smart grids. The EFA combines a novel filtering technique for QuadBox rings (FQR) with two existing filtering techniques, including quick removing (QR) and port locking (PL), to effectively reduce redundant unicast traffic within HSR networks. The EFA filters unicast traffic for both unused terminal rings by using the PL technique and unused QuadBox rings based on the newly-proposed FQR technique. In addition, by using the QR technique, the EFA prevents the unicast frames from being duplicated and circulated in rings; the EFA thus significantly reduces redundant unicast traffic in HSR networks compared with the standard HSR protocol and existing traffic filtering techniques. The EFA also reduces control overhead compared with the filtering HSR traffic (FHT) technique. In this study, the performance of EFA was analyzed, evaluated, and compared to that of the standard HSR protocol and existing techniques, and various simulations were conducted to validate the performance analysis. The analytical and simulation results showed that for the sample networks, the proposed EFA reduced network unicast traffic by 80% compared with the standard HSR protocol and by 26–62% compared with existing techniques. The proposed EFA also reduced control overhead by up to 90% compared with the FHT, thus decreasing control overhead, freeing up network bandwidth, and improving network traffic performance.

Description: Energies, Vol. 10, Pages 1357: Modeling and Analysis of the Common Mode Voltage in a Cascaded H-Bridge Electronic Power Transformer Energies doi: 10.3390/en10091357 Authors: Yun Yang Chengxiong Mao Dan Wang Jie Tian Ming Yang Electronic power transformers (EPTs) have been identified as emerging intelligent electronic devices in the future smart grid, e.g., the Energy Internet, especially in the application of renewable energy conversion and management. Considering that the EPT is directly connected to the medium-voltage grid, e.g., a10 kV distribution system, and its cascaded H-bridges structure, the common mode voltage (CMV) issue will be more complex and severe. The CMV will threaten the insulation of the entire EPT device and even produce common mode current. This paper investigates the generated mechanism and characteristics of the CMV in a cascaded H-bridge EPT (CHB-EPT) under both balanced and fault grid conditions. First, the CHB-EPT system is introduced. Then, a three-phase simplified circuit model of the high-voltage side of the EPT system is presented. Combined with a unipolar modulation strategy and carrier phase shifting technology by rigorous mathematical analysis and derivation, the EPT internal CMV and its characteristics are obtained. Moreover, the influence of the sinusoidal pulse width modulation dead time is considered and discussed based on analytical calculation. Finally, the simulation results are provided to verify the validity of the aforementioned model and the analysis results. The proposed theoretical analysis method is also suitable for other similar cascaded converters and can provide a useful theoretical guide for structural design and power density optimization.

Description: Energies, Vol. 10, Pages 1362: Enumerative Optimization Procedure for the Gear Train Optimization Problem of a Two-Speed Dedicated Electric Transmission Energies doi: 10.3390/en10091362 Authors: Xiangyang Xu Zhifeng Chen Yanjing Liu Peng Dong Yanfang Liu Gear train optimization problems (GTOPs) can be very difficult. This paper proposes an enumerative optimization procedure (EOP) for the GTOP of a two-speed dedicated electric transmission (2DET) for electric vehicles (EVs). The EOP combines enumeration with the Min-Max Principle of Optimality (MMPO). First, the requirements of the EV and the requirements of manufacture and operation were checked in a dedicated order to obtain the feasible region of the GTOP. Then, the MMPO was implemented within the feasible region to reveal the global optimum in terms of the performance of the EV, the load capacity of the gears and the size of the gear train (GT). Results demonstrated that the EOP was effective in determining the feasible region and simultaneously and globally optimizing multiple criteria for the GTOP. The idea of combining enumeration with optimization, as the EOP presents, may be helpful to solve other GTOPs and provide global optima that are immediately practical and applicable.

Description: Energies, Vol. 10, Pages 1363: Gas Turbine Engine Identification Based on a Bank of Self-Tuning Wiener Models Using Fast Kernel Extreme Learning Machine Energies doi: 10.3390/en10091363 Authors: Feng Lu Yu Ye Jinquan Huang In order to simultaneously obtain global optimal model structure and coefficients, this paper proposes a novel Wiener model to identify the dynamic and static behavior of a gas turbine engine. An improved kernel extreme learning machine is presented to build up a bank of self-tuning block-oriented Wiener models; the time constant values of linear dynamic element in Wiener model are designed to tune engine operating conditions. Reduced-dimension matrix inversion incorporated with the fast leave one out cross validation strategy is utilized to decrease computational time for the selection of engine model feature parameters. An optimization algorithm is no longer needed compared to the former method. The contribution of this study is that a more convenient and appropriate methodology is developed to describe aircraft engine thermodynamic behavior during its static and dynamic operations. The methodology is evaluated in terms of computational efforts, dynamic and static estimation accuracy through a case study involving data that are generated by general aircraft engine simulation. The results confirm our viewpoints in this paper.

Description: Energies, Vol. 10, Pages 1361: Development of an Integrated Thermal Energy Storage and Free-Piston Stirling Generator for a Concentrating Solar Power System Energies doi: 10.3390/en10091361 Authors: Songgang Qiu Laura Solomon Garrett Rinker Incorporating thermal energy storage (TES) into a concentrating solar power (CSP) system extends the power production hours, eliminating intermittency and reducing the Levelized Cost of the Energy (LCOE). The designed TES system was integrated with a 3 kW free-piston Stirling convertor. A NaF–NaCl eutectic salt was chosen as the phase change material (PCM) with a melting temperature of 680 °C. This eutectic salt has an energy density that is 5 to 10 times that of a typical molten salt PCM. In order to overcome the drawbacks of the material having a low thermal conductivity, heat pipes were embedded into the PCM to enhance the heat transfer rate within the system. Since the dish collector tracks the sun over the course of the day, two operational extremes were tested on the system; horizontal (zero solar elevation at sunrise/sunset) and vertical (solar noon). Although the system’s performance was below the expectations due to improperly sized wicks in the secondary heat pipes, the results indicated that the Stirling engine was able to produce 1.3 kWh of electricity by extracting latent heat energy from the PCM; thus, the concept of the design was validated.

Description: Energies, Vol. 10, Pages 1355: Impedance-Based Stability Analysis in Grid Interconnection Impact Study Owing to the Increased Adoption of Converter-Interfaced Generators Energies doi: 10.3390/en10091355 Authors: Youngho Cho Kyeon Hur Yong Kang Eduard Muljadi This study investigates the emerging harmonic stability concerns to be addressed by grid planners in generation interconnection studies, owing to the increased adoption of renewable energy resources connected to the grid via power electronic converters. The wideband and high-frequency electromagnetic transient (EMT) characteristics of these converter-interfaced generators (CIGs) and their interaction with the grid impedance are not accurately captured in the typical dynamic studies conducted by grid planners. This paper thus identifies the desired components to be studied and subsequently develops a practical process for integrating a new CIG into a grid with the existing CIGs. The steps of this process are as follows: the impedance equation of a CIG using its control dynamics and an interface filter to the grid, for example, an LCL filter (inductor-capacitor-inductor type), is developed; an equivalent impedance model including the existing CIGs nearby and the grid observed from the point of common coupling are derived; the system stability for credible operating scenarios is assessed. Detailed EMT simulations validate the accuracy of the impedance models and stability assessment for various connection scenarios. By complementing the conventional EMT simulation studies, the proposed analytical approach enables grid planners to identify critical design parameters for seamlessly integrating a new CIG and ensuring the reliability of the grid.

Description: Energies, Vol. 10, Pages 1358: A Data Analysis Technique to Estimate the Thermal Characteristics of a House Energies doi: 10.3390/en10091358 Authors: Seyed Tabatabaei Wim van der Ham Michel C. A. Klein Jan Treur Almost one third of the energy is used in the residential sector, and space heating is the largest part of energy consumption in our houses. Knowledge about the thermal characteristics of a house can increase the awareness of homeowners about the options to save energy, for example by showing that there is room for improvement of the insulation level. However, calculating the exact value of these characteristics is not possible without precise thermal experiments. In this paper, we propose a method to automatically estimate two of the most important thermal characteristics of a house, i.e., the loss rate and the heat capacity, based on collected data about the temperature and gas usage. The method is evaluated with a data set that has been collected in a real-life case study. Although a ground truth is lacking, the analyses show that there is evidence that this method could provide a feasible way to estimate those values from the thermostat data. More detailed data about the houses in which the data was collected is required to draw stronger conclusions. We conclude that the proposed method is a promising way to add energy saving advice to smart thermostats.

Description: Energies, Vol. 10, Pages 1360: Biodiesel Production from Bombacopsis glabra Oil by Methyl Transesterification Method Energies doi: 10.3390/en10091360 Authors: Francisca da Silva Araújo Antonio do Nascimento Cavalcante Maria das Dores Sousa Carla de Moura Mariana Chaves Sabria Aued-Pimentel Miriam Fernandes Caruso Luimar Tozetto Soane Kaline Morais Chaves The objective of this work was to produce methyl biodiesel from Bombacopis glabra (B. glabra) oil degummed with H3PO4. The methyl biodiesel was prepared in an alkaline medium, and characterized by physico-chemical parameters, thin-layer chrmatograghy (TLC), gas chromatograph (GC), (Nuclear magnetic resonance of hydrogen (H-NMR), thermogravimetry and infrared analysis. The physico-chemical parameters of biodiesel were in accordance with the limits established by National Agency of Petroleum, Natural Gas and Biofuels (ANP) Resolution 45/2014, except oxidation stability, where it was corrected with the addition of antioxidants such as TBHQ and BHT.

Description: Energies, Vol. 10, Pages 1365: A Torque Error Compensation Algorithm for Surface Mounted Permanent Magnet Synchronous Machines with Respect to Magnet Temperature Variations Energies doi: 10.3390/en10091365 Authors: Chang-Seok Park Jae Suk Lee This paper presents a torque error compensation algorithm for a surface mounted permanent magnet synchronous machine (SPMSM) through real time permanent magnet (PM) flux linkage estimation at various temperature conditions from medium to rated speed. As known, the PM flux linkage in SPMSMs varies with the thermal conditions. Since a maximum torque per ampere look up table, a control method used for copper loss minimization, is developed based on estimated PM flux linkage, variation of PM flux linkage results in undesired torque development of SPMSM drives. In this paper, PM flux linkage is estimated through a stator flux linkage observer and the torque error is compensated in real time using the estimated PM flux linkage. In this paper, the proposed torque error compensation algorithm is verified in simulation and experiment.

Description: Energies, Vol. 10, Pages 1374: Improved Reactive Current Detection Method of SVG Energies doi: 10.3390/en10091374 Authors: Xueliang Wei Guorong Zhu Jianghua Lu Wenjing Li and Erjie Qi The static VAR generator (SVG) is an important device in flexible AC transmission systems (FACTS) for the development of smart grids. Based on the basis principle of SVG and instantaneous reactive power theory, the conventional ip–iq and dq methods have a certain error when the three phase voltages are unbalanced. In this paper, the current detection algorithm is improved in cases of three-phase power asymmetry by using the fundamental positive-sequence reactive current instead of the voltage as the input of phase locked loop (PLL). So the problems caused by unbalanced three-phase voltages could be avoided. In addition, a moving average filter is designed to improve the performance of the detection accuracy and dynamic response. Experiments verify the correctness and effectiveness of the proposed scheme.

Description: Energies, Vol. 10, Pages 1368: Using Thermostats for Indoor Climate Control in Office Buildings: The Effect on Thermal Comfort Energies doi: 10.3390/en10091368 Authors: Georgios Kontes Georgios Giannakis Philip Horn Simone Steiger Dimitrios Rovas Thermostats are widely used in temperature regulation of indoor spaces and have a direct impact on energy use and occupant thermal comfort. Existing guidelines make recommendations for properly selecting set points to reduce energy use, but there is little or no information regarding the actual achieved thermal comfort of the occupants. While dry-bulb air temperature measured at the thermostat location is sometimes a good proxy, there is less understanding of whether thermal comfort targets are actually met. In this direction, we have defined an experimental simulation protocol involving two office buildings; the buildings have contrasting geometrical and construction characteristics, as well as different building services systems for meeting heating and cooling demands. A parametric analysis is performed for combinations of controlled variables and boundary conditions. In all cases, occupant thermal comfort is estimated using the Fanger index, as defined in ISO 7730. The results of the parametric study suggest that simple bounds on the dry-bulb air temperature are not sufficient to ensure comfort, and in many cases, more detailed considerations taking into account building characteristics, as well as the types of building heating and cooling services are required. The implication is that the calculation or estimation of detailed comfort indices, or even the use of personalised comfort models, is key towards a more human-centric approach to building design and operation.

Description: Energies, Vol. 10, Pages 1375: Spectrometer-Based Line-of-Sight Temperature Measurements during Alkali-Pulverized Coal Combustion in a Power Station Boiler Energies doi: 10.3390/en10091375 Authors: Weijie Yan Yunqi Ya Feng Du Hao Shao Peitao Zhao A portable spectrometer system that simultaneously measures the temperature, emissivity, and radiation intensity of an alkali metal was used in a 1000 MW coal-fired power plant boiler furnace. A calibrated fiber-optic spectrometer system was applied to obtain the radiation intensity of the flame. A simple method based on polynomial fitting was used to separate the continuous baseline from the measured flame spectra that contained both continuous and discontinuous bands. Nine synthetic spectra that included the baseline, noise, and three simulated discontinuous bands based on a Gaussian function were created to test the accuracy of the separation method. The accuracy of the estimated continuous baseline was evaluated by the goodness-of-fit coefficient quality metric. The results indicated good spectral matching for the selected profiles. The soot emissivity model by Hottel and Broughton was employed to calculate temperature and emissivity. The influence of discontinuous emission spectra on the temperature and emissivity calculations was evaluated. The results showed that the maximum difference of the measurement points of the calculated temperature was only 6 K and that the relative difference in emissivity among the measurement points was less than 5%. In addition, a comparison between the actual intensity of the alkali metal and the calculated temperature indicated that the change in the radiation intensity of the alkali metal followed the trend of the calculated temperature. This study serves as a preliminary investigation for measuring gas-phase alkali metal concentrations in a furnace.

Description: Energies, Vol. 10, Pages 1371: Hanging Wall Pressure Relief Mechanism of Horizontal Section Top-Coal Caving Face and Its Application—A Case Study of the Urumqi Coalfield, China Energies doi: 10.3390/en10091371 Authors: Jinshuai Guo Liqiang Ma Ye Wang Fangtian Wang Abundant steeply-dipping thick coal seams (SDTCS) have been found in Xinjiang, China, and they are mined largely by the horizontal section top-coal caving (HSTCC) method. The hanging wall of the HSTCC face is nearly vertical and does not fracture easily after the underlying coal is extracted. As a result, stress tends to concentrate in the hanging wall of the lower-section working face (LSWF) and then induce dynamic disasters. In this study, a mechanical model of a HSTCC face’s hanging wall in steeply-dipping seams was constructed to study the characteristics of hanging wall deformation. The mechanism of hanging wall pressure relief by deep-hole blasting (DHB) was analyzed and the effectiveness of DHB was investigated by simulation using the LS-DYNA software. Based on these studies, parameters relevant to pressure relief by DHB were determined and then DHB was applied to the 4301 working face in the Jiangou coal mine. The results show that the average pressure of measured at the 4301 working face decreased about 34% from those at the 4501 face where the hanging wall was not blasted. Accidents related to dynamic rock pressure, such as support crushing and large-scale rib fall, did not occur at the 4301 working face throughout the mining process. Additionally, in order to constrain the surface “V”-shaped collapsed grooves induced by repeated mining of HSTCC faces and prevent the subsequent failure of the surrounding rock on the sides of the collapsed grooves, loess was used to fill in the grooves to provide constraint and dynamic control on the surrounding rock. The two complementary technologies proposed in this study provide a guide on how to control hanging wall of SDTCS in similar conditions.

Description: Energies, Vol. 10, Pages 1370: Biomass Production of Three Biofuel Energy Plants’ Use of a New Carbon Resource by Carbonic Anhydrase in Simulated Karst Soils: Mechanism and Capacity Energies doi: 10.3390/en10091370 Authors: Rui Wang Yanyou Wu Deke Xing Hongtao Hang Xiaolin Xie Xiuqun Yang Kaiyan Zhang Sen Rao To determine whether the bicarbonate in karst limestone soil could be used as a new carbon resource for biomass production by the catalysis of carbonic anhydrase (CA), a simulative karst drought stress experiment was designed and performed. Three plants used for biofuel energy, Orychophragmus violaceus L. (Ov), Brassica juncea L. (Bj), and Euphorbia lathyris L. (El), were grown under simulated karst drought stress. In response to drought stress, the photosynthesis of the three energy plants was inhibited, but their CA activity increased. The hypothesis was confirmed by plant physiological and stable isotope techniques. The obtained results showed that plant biomass was produced with atmospheric CO2 as well as bicarbonate under drought stress. Bicarbonate use was proportional to the CA activity of the plants. With high CA activity over a long period, El had the highest proportional bicarbonate use compared to Ov and Bj, reaching 26.95%. Additionally, a new method is proposed for the screening of plants grown for energy in karst habitats.

Description: Energies, Vol. 10, Pages 1373: New Design of a CNG-H2-AIR Mixer for Internal Combustion Engines: An Experimental and Numerical Study Energies doi: 10.3390/en10091373 Authors: Hussein A. Mahmood Nor Mariah. Adam B. B. Sahari S. U. Masuri Several studies have aimed to convert diesel engines to dual- or tri-fuel engines to improve their fuel economy and reduce the emissions from diesel engine, however, most of these studies do not consider enhancing the homogeneity of fuel mixtures inside the engine and accurately controlling the air fuel ratio. In this study, a new air-fuel mixer was designed, manufactured and tested. The proposed air-gaseous fuel mixer design was conceived to be suitable for mixing air with compressed natural gas (CNG) and a blend of hydrogen and compressed natural gas (HCNG) that gives homogenous mixtures with high uniformity index and also to be easily connected with an Electronic Control Unit (ECU) for controlling accurately the air-gaseous fuel ratio for different engine speeds. For optimizing the homogeneity inside the new mixer, fourteen different mixer models were created to investigate the effects of diameter, location, and the number of holes inside the mixer on the homogeneity and distribution of the mixtures. Computational fluid dynamics analysis software was used to check the flow behavior, distribution and homogeneity of mixtures inside the new mixer models. The simulation results revealed that the best uniformity index (UI) values are obtained in model 7 where the UI values are 0.939 and 0.937, respectively, for an air fuel ratio for a blend of hydrogen and compressed natural gas (AFRHCNG) = 51.31 and the air fuel ratio for compressed natural gas (AFRCNG) = 34.15. According to the numerical and experimental results for the new mixer (model 7) under different engine speeds (1000–4000) and air-CNG ratio of 34.15, a meaningful agreement is reached between the experimental and numerical values for AFRCNG (coefficient of determination (R2) = 0.96 and coefficient of variation (CoV) = 0.001494).

Description: Energies, Vol. 10, Pages 1387: Proportional-Type Performance Recovery DC-Link Voltage Tracking Algorithm for Permanent Magnet Synchronous Generators Energies doi: 10.3390/en10091387 Authors: Seok-Kyoon Kim This study proposes a disturbance observer-based proportional-type DC-link voltage tracking algorithm for permanent magnet synchronous generators (PMSGs). The proposed technique feedbacks the only proportional term of the tracking errors, and it contains the nominal static and dynamic feed-forward compensators coming from the first-order disturbance observers. It is rigorously proved that the proposed method ensures the performance recovery and offset-free properties without the use of the integrators of the tracking errors. A wind power generation system has been simulated to verify the efficacy of the proposed method using the PSIM (PowerSIM) software with the DLL (Dynamic Link Library) block.

Description: Energies, Vol. 10, Pages 1381: Use of Rod Compactors for High Voltage Overhead Power Lines Magnetic Field Mitigation Energies doi: 10.3390/en10091381 Authors: Fabio Bignucolo Massimiliano Coppo Andrea Savio Roberto Turri In the last decades, strengthening the high voltage transmission system through the installation of new overhead power lines has become critical, especially in highly developed areas. Present laws concerning the human exposure to electric and magnetic fields introduce constraints to be considered in both new line construction and existing systems. In the paper, a technique for passive magnetic field mitigation in areas close to overhead power lines is introduced, fully modelled and discussed through a parametric analysis. The investigated solution, which basically consists in approaching line conductors along the span making use of rod insulators, is applicable on both existing and under-design overhead lines as an alternative to other mitigating actions. Making use of a 3-dimensional representation, the procedure computes both positions of phase conductors and forces acting on insulators, towers, conductors and compactors, with the aim of evaluating the additional mechanical stress introduced by the compactors. Finally, a real case study is reported to demonstrate and quantify the benefits in terms of ground magnetic field reduction achievable by applying the proposed solution, in comparison to a traditional configuration. Furthermore, using compactors to passively reduce the magnetic field is simple to be applied, minimally invasive and quite inexpensive as regards to alternative mitigating actions.

Description: Energies, Vol. 10, Pages 1384: A Novel Sensorless Control Strategy for Brushless Direct Current Motor Based on the Estimation of Line Back Electro-Motive Force Energies doi: 10.3390/en10091384 Authors: Chengde Tong Mingqiao Wang Baige Zhao Zuosheng Yin Ping Zheng In this paper, a novel sensorless control strategy based on the estimation of line back electro-motive force (BEMF) is proposed. According to the phase relationship between the ideal commutation points of the brushless direct current motor (BLDCM) and the zero-crossing points (ZCPs) of the line BEMF, the calculation formula of line BEMF is simplified properly and the commutation rule for different positions of the rotor is presented. The estimation error of line BEMF caused by the freewheeling current of silent phase is analyzed, and the solution is given. With the phase shift of the low-pass filter considered, a compensation method using “60°-α” and “120°-α” is studied in this paper to eliminate the error. Finally, the simulation and experimental results show that the rotor-position-detection error is reduced effectively and the motor driven by the accurate commutation signal can work well at low and high speed.

Description: Energies, Vol. 10, Pages 1383: The Impact of Spectral Composition of White LEDs on Spinach (Spinacia oleracea) Growth and Development Energies doi: 10.3390/en10091383 Authors: Chiara Burattini Benedetta Mattoni Fabio Bisegna Light-emitting diodes (LED) are a promising light source for the cultivation of edible vegetables in greenhouses. The spectral radiation of the light sources has an impact on plants physiological parameters, as well as on morphological features. In this study the growth of spinach plants has been carried out in experimental boxes under two white LED treatments having different correlate color temperature (CCT): the cold lighting (CL) corresponded to 6500 K, while the warm lighting (WL) to 3000 K. The work was aimed to investigate the influence of the two light spectra on plant development and comparing the results. Results showed that the different lighting treatments impact differently on plant development and on growth parameters.

Description: Energies, Vol. 10, Pages 1386: A Modular AC-DC Power Converter with Zero Voltage Transition for Electric Vehicles Energies doi: 10.3390/en10091386 Authors: Jazmin Ramirez-Hernandez Ismael Araujo-Vargas Marco Rivera A study of the fundamental of operation of a three-phase AC-DC power converter that uses Zero-Voltage Transition (ZVT) together with Space Vector Pulse Width Modulation (SVPWM) is presented. The converter is basically an active rectifier divided into two converters: a matrix converter and an H bridge, which transfer energy through a high-frequency transformer, resulting in a modular AC-DC wireless converter appropriate for Plug-in Electric Vehicles (PEVs). The principle of operation of this converter considers high power quality, output regulation and low semiconductor power loss. The circuit operation, idealized waveforms and modulation strategy are explained together with simulation results of a 5 kW design.

Description: Energies, Vol. 10, Pages 1396: Potential for Energy Production from Farm Wastes Using Anaerobic Digestion in the UK: An Economic Comparison of Different Size Plants Energies doi: 10.3390/en10091396 Authors: Gabriel Oreggioni Baboo Gowreesunker Savvas Tassou Giuseppe Bianchi Matthew Reilly Marie Kirby Trisha Toop Mike Theodorou Anaerobic digestion (AD) plants enable renewable fuel, heat, and electricity production, with their efficiency and capital cost strongly dependent on their installed capacity. In this work, the technical and economic feasibility of different scale AD combined heat and power (CHP) plants was analyzed. Process configurations involving the use of waste produced in different farms as feedstock for a centralized AD plant were assessed too. The results show that the levelized cost of electricity are lower for large-scale plants due to the use of more efficient conversion devices and their lower capital cost per unit of electricity produced. The levelized cost of electricity was estimated to be 4.3 p/kWhe for AD plants processing the waste of 125 dairy cow sized herds compared to 1.9 p/kWhe for AD plants processing waste of 1000 dairy cow sized herds. The techno-economic feasibility of the installation of CO2 capture units in centralized AD-CHP plants was also undertaken. The conducted research demonstrated that negative CO2 emission AD power generation plants could be economically viable with currently paid feed-in tariffs in the UK.

Description: Energies, Vol. 10, Pages 1400: A Study of Two Multi-Element Resonant DC-DC Topologies with Loss Distribution Analyses Energies doi: 10.3390/en10091400 Authors: Yifeng Wang Liang Yang Fuqiang Han Shijie Tu Weiya Zhang In this paper, two multi-element resonant DC-DC converters are analyzed in detail. Since their resonant tanks have multiple resonant components, the converters display different resonant characteristics within different operating frequency ranges. Through appropriate design, both of the two proposed converters successfully lower the conversion losses and, meanwhile, broaden the voltage gain ranges as well: one converter is able to take full usage of the third order harmonic to deliver the active power, and thus the effective utilization rate of the resonant current is elevated; while the another minimizes the entire switching losses for power switching devices by restricting the input impedance angle of the resonant tank. Besides, the loss distribution is analyzed for the purpose of guiding the component design. In the end, two 500 W prototypes are fabricated to test the theoretical analyses. The results demonstrate that the two proposed converters can achieve wide voltage gain with the small frequency deviation, which noticeably contributes to highly efficient conversion. Their peak efficiencies are measured as 95.4% and 95.3%, respectively.

Description: Energies, Vol. 10, Pages 1397: Energy Flexibility Management Based on Predictive Dispatch Model of Domestic Energy Management System Energies doi: 10.3390/en10091397 Authors: Amin Gazafroudi Francisco Prieto-Castrillo Tiago Pinto Javier Prieto Juan Corchado Javier Bajo This paper proposes a predictive dispatch model to manage energy flexibility in the domestic energy system. Electric Vehicles (EV), batteries and shiftable loads are devices that provide energy flexibility in the proposed system. The proposed energy management problem consists of two stages: day-ahead and real time. A hybrid method is defined for the first time in this paper to model the uncertainty of the PV power generation based on its power prediction. In the day-ahead stage, the uncertainty is modeled by interval bands. On the other hand, the uncertainty of PV power generation is modeled through a stochastic scenario-based method in the real-time stage. The performance of the proposed hybrid Interval-Stochastic (InterStoch) method is compared with the Modified Stochastic Predicted Band (MSPB) method. Moreover, the impacts of energy flexibility and the demand response program on the expected profit and transacted electrical energy of the system are assessed in the case study presented in this paper.

Description: Energies, Vol. 10, Pages 1394: Liquid vs. Gas Phase CO2 Photoreduction Process: Which Is the Effect of the Reaction Medium? Energies doi: 10.3390/en10091394 Authors: Alberto Olivo Elena Ghedini Michela Signoretto Matteo Compagnoni Ilenia Rossetti The use of carbon dioxide, the most concerning environmental issue of the 21st century, as a feedstock for fuels productions still represents an innovative, yet challenging, task for the scientific community. CO2 photoreduction processes have the potential to transform this hazardous pollutant into important products for the energy industry (e.g., methane and methanol) employing a photocatalyst and light as the only energy input. In order to design an effective process, the high sustainability of this reaction should be matched with the perfect reaction conditions to allow the reactant, photocatalyst, and light source to come together: therefore, the choice of reaction conditions, and in particular its medium, is a crucial issue that needs to be investigated. Throughout this paper, a careful study of carbon dioxide photoreduction in liquid and vapour phases are reported, focusing on their effect on catalyst performances in terms of light harvesting, productivity, and selectivity. Different from most papers in the literature, catalytic tests were performed under extremely low light irradiance, in order to minimise the primary energy input, highlighting that this experimental variable has a great effect on the reaction pathway and, thus, product distribution.

Description: Energies, Vol. 10, Pages 1389: Elimination of Chattering by Control Strategy Based on the Multiphase Sliding Model Control for Efficient Power Conversion in a DC-DC Circuit Energies doi: 10.3390/en10091389 Authors: Woonki Na Pengyuan Chen Jonghoon Kim In this paper, a multiphase sliding mode (MPSM) control method with a master–slave structure is proposed and analyzed in order to suppress ripples at input terminals or output terminals in DC/DC converter applications. With the proposed MPSM, the switching frequency of a multiphase DC/DC converter does not need to be operating such a high switching frequency as adopted in a conventional sliding mode control-based DC/DC converter. As a result, switching power losses could be reduced in this converter. Another advantage of this proposed method is that the master–slave multiphase slide control loop could figure out a proper phase shift between each switching phase of a DC/DC converter instead of precalculated phase shift. The proposed concepts are proven by the PSIM computer simulations and the feasibility of the proposed concepts is validated through the test experiments.

Description: Energies, Vol. 10, Pages 1403: Backflow Power Optimization Control for Dual Active Bridge DC-DC Converters Energies doi: 10.3390/en10091403 Authors: Fei Xiong Junyong Wu Liangliang Hao Zicheng Liu This paper proposes optimized control methods for global minimum backflow power based on a triple-phase-shift (TPS) control strategy. Three global optimized methods are derived to minimize the backflow power on the primary side, on the secondary side and on both sides, respectively. Backflow power transmission is just a portion of non-active power transmission in a dual active bridge (DAB) converter. Non-active power transmission time is proposed in this paper, which unifies zero power transmission and backflow power transmission. Based on the proposed index, an optimized control method is derived to achieve both the maximum effective power transmission time and minimum current stress of DAB at the same time. A comparative analysis is performed to show the limitations of the minimum backflow power optimization method. Finally, a prototype is built to verify the effectiveness of our theoretical analysis and the proposed control methods by experimental results.

Description: Energies, Vol. 10, Pages 1405: Optimization Model of an Efficient Collaborative Power Dispatching System for Carbon Emissions Trading in China Energies doi: 10.3390/en10091405 Authors: Qinliang Tan Yihong Ding Yimei Zhang In this paper, a collaborative power dispatching system (CPDS) was developed to maximize the profit of a regional biomass power system consisting of an independent power grid. A power generating, dispatching and carbon emissions trading system (CETS) could be engaged in joint strategic planning and operational execution. The principal of CPDS is interactive planning of generating units in power generation and carbon emissions trading. An efficient carbon emissions trading plan for a CPDS would lead to optimized power generation levels under available power production capacities and carbon emissions. In a case study, four generator policies are proposed by considering basic CETSs to comparatively analyze the function of each generator in the CPDS. Results of four scenarios are compared, showing that biomass energy could replace thermal units to a certain extent, the carbon emissions and coal consumption of the CPDS would lie at a lower level, and a pumped storage unit could adjust the load fluctuations. The results of a carbon trading analysis show that the CETS has no significant impact on the CPDS, but along with the increase in trading price or the decrease in the free quota, the economic interests of power plants will be reduced accordingly. This may lead to carrying out low-carbon projects and reducing carbon emissions. Therefore, it is imperative to reduce carbon emissions by replacing power units with high energy consumption, and improve the consumption capacity of renewable energy.

Description: Energies, Vol. 10, Pages 1412: Stability of DC Distribution Systems: An Algebraic Derivation Energies doi: 10.3390/en10091412 Authors: Nils van der Blij Laura Ramirez-Elizondo Matthijs Spaan Pavol Bauer Instability caused by low inertia and constant power loads is a major challenge of DC distribution grids. Previous research uses oversimplified models or does not provide general rules for stability. Therefore, a comprehensive approach to analyze the stability of DC distribution systems is desired. This paper presents a method to algebraically analyze the stability of any DC distribution system through the eigenvalues of its state-space matrices. Furthermore, using this method, requirements are found for the stability of three example systems. Additionally, a sensitivity analysis is performed, which considers the effect of several system parameters on the stability and disputes some overgeneralized conclusions of previous research. Moreover, various simulations are performed to illustrate the dynamic behavior of a stable and an unstable DC distribution system.

Description: Energies, Vol. 10, Pages 1422: A Hybrid Wind Speed Forecasting System Based on a ‘Decomposition and Ensemble’ Strategy and Fuzzy Time Series Energies doi: 10.3390/en10091422 Authors: Hufang Yang Zaiping Jiang Haiyan Lu Accurate and stable wind speed forecasting is of critical importance in the wind power industry and has measurable influence on power-system management and the stability of market economics. However, most traditional wind speed forecasting models require a large amount of historical data and face restrictions due to assumptions, such as normality postulates. Additionally, any data volatility leads to increased forecasting instability. Therefore, in this paper, a hybrid forecasting system, which combines the ‘decomposition and ensemble’ strategy and fuzzy time series forecasting algorithm, is proposed that comprises two modules—data pre-processing and forecasting. Moreover, the statistical model, artificial neural network, and Support Vector Regression model are employed to compare with the proposed hybrid system, which is proven to be very effective in forecasting wind speed data affected by noise and instability. The results of these comparisons demonstrate that the hybrid forecasting system can improve the forecasting accuracy and stability significantly, and supervised discretization methods outperform the unsupervised methods for fuzzy time series in most cases.

Description: Energies, Vol. 10, Pages 1421: State-Of-The-Art in Microgrid-Integrated Distributed Energy Storage Sizing Energies doi: 10.3390/en10091421 Authors: Ibrahim Alsaidan Abdulaziz Alanazi Wenzhong Gao Hongyu Wu Amin Khodaei Distributed energy storage (DES) plays an important role in microgrid operation and control, as it can potentially improve local reliability and resilience, reduce operation cost, and mitigate challenges caused by high penetration renewable generation. However, to ensure an acceptable economic and technical performance, DES must be optimally sized and placed. This paper reviews the existing DES sizing methods for microgrid applications and presents a generic sizing method that enables microgrid planners to efficiently determine the optimal DES size, technology, and location. The proposed method takes into consideration the impact of DES operation on its lifetime to enhance the obtained results accuracy and practicality. The presented model can be used for both grid-tied (considering both grid-connected and islanded modes) and isolated microgrids.

Description: Energies, Vol. 10, Pages 1423: Olive Mill Wastewater: From a Pollutant to Green Fuels, Agricultural Water Source and Bio-Fertilizer—Part 1. The Drying Kinetics Energies doi: 10.3390/en10091423 Authors: Mejdi Jeguirim Patrick Dutournié Antonis A. Zorpas Lionel Limousy Olive Mill Wastewater (OMWW) treatment is considered to be one of the main challenges that Mediterranean countries face. Although several procedures and technologies are mentioned in the literature, these techniques have several disadvantages or have been limited to laboratory pilot validation without posterior industrial projection. Recently, an advanced environmental friendly strategy for the recovery of OMWW was established involving the impregnation of OMWW on dry biomasses, drying of these impregnated samples, and finally green fuels and biochar production. This established strategy revealed that the drying step is crucial for the success of the entire recovery process. Hence, two impregnated samples were prepared through OMWW impregnation on sawdust (IS) and olive mill solid waste (ISW). The drying kinetics of OMWW and impregnated samples (IS and ISW) were examined in a convective dryer (air velocity range from 0.7–1.3 m/s and the temperature from 40–60 °C). The experimental results indicated that the drying of the impregnated samples occurred twice as fast as for the OMWW sample. Such behavior was attributed to the remaining thin layer of oil on the OMWW surface Furthermore, the Henderson and Pabis model showed the suitable fit of the drying curves with a determination coefficient R2 above 0.97. The drying rates were extracted from the mathematical models and the drying process was analyzed. The coefficient of effective diffusivity varied between 2.8 and 11.7 × 10−10 m2/s. In addition, the activation energy values ranged between 28.7 and 44.9 kJ/mol. These values were in the same range as those obtained during the drying of other agrifood byproducts. The final results could be very helpful to engineers aiming to improve and optimize the OMWW drying process.

Description: Energies, Vol. 10, Pages 1424: Electric Arc Furnace Modeling with Artificial Neural Networks and Arc Length with Variable Voltage Gradient Energies doi: 10.3390/en10091424 Authors: Raul Garcia-Segura Javier Vázquez Castillo Fernando Martell-Chavez Omar Longoria-Gandara Jaime Ortegón Aguilar Electric arc furnaces (EAFs) contribute to almost one third of the global steel production. Arc furnaces use a large amount of electrical energy to process scrap or reduced iron and are relevant to study because small improvements in their efficiency account for significant energy savings. Optimal controllers need to be designed and proposed to enhance both process performance and energy consumption. Due to the random and chaotic nature of the electric arcs, neural networks and other soft computing techniques have been used for modeling EAFs. This study proposes a methodology for modeling EAFs that considers the time varying arc length as a relevant input parameter to the arc furnace model. Based on actual voltages and current measurements taken from an arc furnace, it was possible to estimate an arc length suitable for modeling the arc furnace using neural networks. The obtained results show that the model reproduces not only the stable arc conditions but also the unstable arc conditions, which are difficult to identify in a real heat process. The presented model can be applied for the development and testing of control systems to improve furnace energy efficiency and productivity.

Description: Energies, Vol. 10, Pages 1426: Hierarchical Distributed Motion Control for Multiple Linear Switched Reluctance Machines Energies doi: 10.3390/en10091426 Authors: Bo Zhang Jianping Yuan Jianjun Luo Xiaoyu Wu Li Qiu J.F. Pan This paper investigates a distributed, coordinated motion control network based on multiple direct-drive, linear switched reluctance machines (LSRMs). A hierarchical, two-level synchronization control strategy is proposed for the four LSRMs based motion control network. The high-level, reference signals agreement algorithm is first employed to correct the asynchronous behaviors of the position commands. Then, the low-level tracking synchronization method is applied for the collaborative position control of the four LSRMs. The proposed two-level, fault-tolerant control strategy eliminates the asynchrony of the reference signals and it also guarantees the coordinated tracking control performance of the four LSRMs. Experimental results demonstrate that effective coordinated tracking control can be ensured, based on the successful agreement of reference signals and an absolute tracking error falling within 2 mm can be achieved.

Description: Energies, Vol. 10, Pages 1427: Design and Numerical Simulations of a Flow Induced Vibration Energy Converter for Underwater Mooring Platforms Energies doi: 10.3390/en10091427 Authors: Wenlong Tian Zhaoyong Mao Fuliang Zhao Limited battery energy restricts the duration of the underwater operation of underwater mooring platforms (UMPs). In this paper, a flow-induced vibration energy converter (FIVEC) is designed to produce power for the UMPs and extend their operational time. The FIVEC is equipped with a thin plate to capture the kinetic energy in the vortices shed from the surface of the UMP. A magnetic coupling (MC) is applied for the non-contacting transmission of the plate torque to the generators so that the friction loss can be minimized. In order to quantify and evaluate the performance of the FIVEC, two-dimensional computational fluid dynamics (CFD) simulations are performed. Simulations are based on the Reynolds Averaged Navier-Stokes (RANS) equations and the shear stress transport (SST) k-ω turbulent model is utilized. The CFD method is firstly validated using existing experimental data. Then the influences of plate length and system damping on the performance of the FIVEC are evaluated. The results show that the device has a maximum averaged power coefficient of 0.0520 (13.86 W) in the considered situations. The results also demonstrate the feasibility of this energy converter plan.

Description: Energies, Vol. 10, Pages 1440: Application of Dynamic Non-Linear Programming Technique to Non-Convex Short-Term Hydrothermal Scheduling Problem Energies doi: 10.3390/en10091440 Authors: Omid Hoseynpour Behnam Mohammadi-ivatloo Morteza Nazari-Heris Somayeh Asadi Short-term hydro-thermal scheduling aims to obtain optimal generation scheduling of hydro and thermal units for a one-day or a one-week scheduling time horizon. The main goal of the problem is to minimize total operational cost considering a series of equality and inequality constraints. The problem is considered as a non-linear and complex problem involving the valve-point loading effect of conventional thermal units, the water transport delay between connected reservoirs, and transmission loss with a set of equality and inequality constraints such as power balance, water dynamic balance, water discharge, initial and end reservoir storage volume, reservoir volume limits and the operation limits of hydro and thermal plants. A solution methodology to the short-term hydro-thermal scheduling problem with continuous and non-smooth/non-convex cost function is introduced in this research applying dynamic non-linear programming. In this study, the proposed approach is applied to two test systems with different characteristics. The simulation results obtained in this paper are compared with those reported in recent research studies, which show the effectiveness of the presented technique in terms of total operational cost. In addition, the obtained results ensure the capability of the proposed optimization procedure for solving short-term hydro-thermal scheduling problem with transmission losses and valve-point effects.

Description: Energies, Vol. 10, Pages 1441: Generic Type 3 Wind Turbine Model Based on IEC 61400-27-1: Parameter Analysis and Transient Response under Voltage Dips Energies doi: 10.3390/en10091441 Authors: Alberto Lorenzo-Bonache Andrés Honrubia-Escribano Francisco Jiménez-Buendía Ángel Molina-García Emilio Gómez-Lázaro This paper analyzes the response under voltage dips of a Type 3 wind turbine topology based on IEC 61400-27-1. The evolution of both active power and rotational speed is discussed in detail when some of the most relevant control parameters, included in the mechanical, active power and pitch control models, are modified. Extensive results are also included to explore the influence of these parameters on the model dynamic response. This work thus provides an extensive analysis of the generic Type 3 wind turbine model and provides an estimation of parameters not previously discussed in the specific literature. Indeed, the International Standard IEC 61400-27-1, recently published in February 2015, defines these generic dynamic simulation models for wind turbines, but does not provide values for the parameters to simulate the response of these models. Thus, there is a pressing need to establish correlations between IEC generic models and specific wind turbine manufacturer models to estimate suitable parameters for simulation purposes. Extensive results and simulations are also included in the paper.

Description: Energies, Vol. 10, Pages 1444: Optimal Analytical Solution for a Capacitive Wireless Power Transfer System with One Transmitter and Two Receivers Energies doi: 10.3390/en10091444 Authors: Ben Minnaert Nobby Stevens Wireless power transfer from one transmitter to multiple receivers through inductive coupling is slowly entering the market. However, for certain applications, capacitive wireless power transfer (CWPT) using electric coupling might be preferable. In this work, we determine closed-form expressions for a CWPT system with one transmitter and two receivers. We determine the optimal solution for two design requirements: (i) maximum power transfer, and (ii) maximum system efficiency. We derive the optimal loads and provide the analytical expressions for the efficiency and power. We show that the optimal load conductances for the maximum power configuration are always larger than for the maximum efficiency configuration. Furthermore, it is demonstrated that if the receivers are coupled, this can be compensated for by introducing susceptances that have the same value for both configurations. Finally, we numerically verify our results. We illustrate the similarities to the inductive wireless power transfer (IWPT) solution and find that the same, but dual, expressions apply.

Description: Energies, Vol. 10, Pages 1443: Control Strategies for Improving Energy Efficiency and Reliability in Autonomous Microgrids with Communication Constraints Energies doi: 10.3390/en10091443 Authors: Francisco Portelinha Júnior Antonio Carlos Zambroni de Souza Miguel Castilla Denisson Queiroz Oliveira Paulo Ribeiro Microgrids are a feasible path to deploy smart grids, an intelligent and highly automated power system. Their operation demands a dedicated communication infrastructure to manage, control and monitor the intermittent sources of energy and loads. Therefore, smart devices will be connected to support the growth of grid smartness increasing the dependency on communication networks, which consumes a high amount of power. In an energy-limited scenario, one of the main issues is to enhance the power supply time. Therefore, this paper proposes a hybrid methodology for microgrid energy management, integrated with a communication infrastructure to improve and to optimize islanded microgrid operation at maximum energy efficiency. The hybrid methodology applies some control management rules, such as intentional load shedding, priority load management, and communication energy saving. These energy saving rules establish a trade-off between increasing microgrid energy availability and communication system reliability. To achieve a compromised solution, a continuous time Markov chain model describes the impact of energy saving policies into system reliability. The proposed methodology is simulated and tested with the help of the modified IEEE 34 node test-system.

Description: Energies, Vol. 10, Pages 1195: A New Method for Evaluating Moisture Content and Aging Degree of Transformer Oil-Paper Insulation Based on Frequency Domain Spectroscopy Energies doi: 10.3390/en10081195 Authors: Guoqiang Xia Guangning Wu Bo Gao Haojie Yin Feibao Yang The condition of oil-paper insulation is closely related to the life expectancy of a transformer. The accurate results of oil-paper have not been obtained due to the impact of influencing factors. Therefore, in order to improve the evaluation accuracy of oil-paper insulation, in this paper, oil-paper samples which were prepared with different aging and moisture content were analyzed by frequency domain spectroscopy (FDS). Results show that when the moisture content is less than 2%, the range of 101~103 Hz is mainly affected by moisture and aging has little effect. However, with the increase of moisture content, the effect of aging degree on this band became increasingly prominent. Sm, which represents the integral value from 10−1 to 10−3 Hz, and SDP, which represents the integral value from 101 to 103 Hz, were extracted as characteristic parameters of moisture content and aging degree respectively. Compensation factors γ which represents the influence ratio of moisture on SDP and φ which represents the influence ratio of aging on Sm were introduced to compensate for the influence of moisture content and aging degree on characteristics respectively. Then, a new method was proposed to evaluate the condition of oil-paper based on compensation factors. Through this method, the influence in characteristics can be eliminated by the obtained actual compensation factors, thus distinguishing the internal influence between moisture content and aging degree on FDS. Finally, this method was verified by field test.

Description: Energies, Vol. 10, Pages 1192: Bioenergy Potential and Utilization Costs for the Supply of Forest Woody Biomass for Energetic Use at a Regional Scale in Mexico Energies doi: 10.3390/en10081192 Authors: Ulises Flores Hernández Dirk Jaeger Jorge Islas Samperio While considering constraints in regard to sustainability, this paper reviews the development of a methodology to assess the introduction of bioenergy supply chains in Mexico based on forest woody biomass. Three research modules include analyses of forest biomass utilization residues that originated from: (1) harvesting activities, (2) non-extracted stands and (3) sawmills. A regional case study focusing on tree species of commercial importance (pine, oak and fir) in the 10 provinces with the highest timber production located in the north and central-south part of the country, is analyzed. After applying the methodology, the theoretical potential of available woody biomass for energetic use amounts to 6,357,482 m3. When applying the sustainability constraints, the technically feasible supply of forest woody biomass for energetic use sums up to 5,798,722 m3, which relate to a technical energy potential of 45.96 PJ for 2013. Moreover, a biomass energy flow chart showing energy values for each analyzed source and species is presented. Monte Carlo simulations were carried out for each cost involved in utilizing the resulting available woody biomass for energetic use. In the absence of national studies which include forest operations and bioenergy transformation to calculate the sustainable energy potential, the developed methodology adds innovation for assessing woody biomass availability.

Description: Energies, Vol. 10, Pages 1198: Theoretical Study of the Effects of Spark Timing on the Performance and Emissions of a Light-Duty Spark Ignited Engine Running under Either Gasoline or Ethanol or Butanol Fuel Operating Modes Energies doi: 10.3390/en10081198 Authors: Roussos Papagiannakis Dimitrios Rakopoulos Constantine Rakopoulos Much research is ongoing to find suitable alternate fuels in order to reduce the exhaust emission levels without deteriorating the basic performance characteristics of conventional spark-ignited (SI) engines. One of the methods to achieve the above problem is the use of alcohols as full supplement fuels to normal gasoline. At the same time, many related research studies have shown that the use of alcohols has a negative impact on some basic engine performance characteristics, e.g., brake power output, etc. On the other hand, spark timing is one of the critical engine operating parameters that significantly influences the combustion mechanism inside the combustion chamber of a SI engine. Therefore, the primary objective of the present work is to investigate the effect of spark timing on the performance and emissions characteristics of a conventional, four-stroke, SI engine running under three different fuel operating modes, viz. with conventional gasoline or ethanol or butanol. The specific investigation is conducted by using an in-house, comprehensive, two-zone phenomenological model. The predictive ability of the model is tested against pertinent experimental data and it is found that the computed results are in good agreement with the respective experimental ones. For all test cases examined herein, the results concern basic engine performance characteristics, i.e., cylinder pressure, power output, specific fuel consumption etc., as well as NO and CO emissions. The main objectives of the work were to record and evaluate the impact that spark timing has on the performance characteristics and emitted pollutants of a conventional SI engine, operating under either conventional gasoline or ethanol or butanol fuel operating modes. Moreover, it deals with the determination of an optimum combination between the type of fuel used and the spark timing, so that probable undesirable effects on engine performance characteristics would be avoided. By comparing this investigation results, it is revealed that the use of alcohols as a full substitute fuel of gasoline accompanied with an appropriate alteration of the spark timing, could be a promising solution to improving both the efficiency and environmental behavior of a light-duty, spark-ignited (SI) engine, without causing any harmful problems to the engine operational lifetime. The conclusions from the study may prove valuable for the application of this technological solution to existing conventional SI engines.

Description: Energies, Vol. 10, Pages 1203: DG Placement in Loop Distribution Network with New Voltage Stability Index and Loss Minimization Condition Based Planning Approach under Load Growth Energies doi: 10.3390/en10081203 Authors: Syed Kazmi Dong Shin This paper presents a new planning approach based on voltage stability index (VSI) together with improved loss minimization (LM) formulations. The method has employed for application of distributed generation (DG) unit placement (location and size) in a loop (configured) test distribution network (LDN). Initially, VSI relationship for equivalent loop model has employed to find out potential locations for DG placement in LDN. Later, loss minimization formulations and loss minimization conditions (LMC) have been derived on the basis of an equivalent electrical model of LDN, for single and two DGs operating at various power factors, respectively. The proposed approach is comprised of two variants and has demonstrated on the 69-bus test distribution network. The first planning variant as a single case has applied for DG allocation (location, size, number) in LDN under normal load. Similarly, the second planning variant has demonstrated with three cases (six scenarios per case), evaluated under normal load and impact of load growth (across five years), respectively. The proposed approach has analyzed in terms of various performance indicators and results obtained have compared and found in close agreement with existed works in literature. Simulation results verify the validity of the proposed planning approach and establish that LDN performs better than radial distribution network from the perspective of load growth.

Description: Energies, Vol. 10, Pages 1201: Efficiency Evaluation and Policy Analysis of Industrial Wastewater Control in China Energies doi: 10.3390/en10081201 Authors: Weixin Yang Lingguang Li With increasing emissions of industrial wastewater and poor control measures, environmental pollution has become a serious issue haunting China’s economic development. Meanwhile, the current pollution management policy system in China is mainly under the supervision of the central government and executed by local governments. Under the current economic growth model of China, the industrial sector remains the dominant segment of our economy, which makes the Total Factor Efficiency (TFE) evaluation and policy analysis of industrial wastewater control decisive factors concerning China’s future economic growth and sustainable development. Based on existing studies of China and abroad, and with the help of a Data Envelope Analysis (DEA) model, this paper used 39 industrial sectors and their input-output data from 2003 to 2014 of China as Decision Making Units to calculate the TFE of wastewater control in different industrial sectors of China. Moreover, we have designed and adopted our own MATLAB programming for optimization solutions of multi-variable constrained nonlinear functions in order to obtain a more accurate estimation of the TFE of wastewater control. Based on our calculation results, this paper further explained the difference in TFE and policy implications across typical industries in China, and offered policy recommendations accordingly.

Description: Energies, Vol. 10, Pages 1202: Development of an Integrated, Personalized Comfort Methodology for Office Buildings Energies doi: 10.3390/en10081202 Authors: Panagiota Antoniadou Agis Papadopoulos Despite the significant progress observed over the last decades, the European building stock still consumes significant amounts of energy (39% of the total final energy), whilst it does not always provide the conditions required for occupants’ well-being and thermal comfort sensation. In order to achieve the goal of nearly or even zero energy buildings, a deep refurbishment of the building stock is imperative. As the literature indicates, a firm evaluation of indoor conditions is essential, while having at the epicenter the occupants’ comfort perception, with emphasis on their individual characteristics. In this respect, a methodological framework is developed and a preliminary implementation is performed. The main goal of the methodological approach is the consideration of both the classical comfort parameters along with the occupants’ socioeconomic and personalized characteristics that affect their perception and can differentiate their needs even under the same conditions. Among other important findings this preliminary implementation achieved some very promising results, highlighting that occupants’ individual characteristics such as recycling and exercising can affect the occupants’ comfort perception.

Description: Energies, Vol. 10, Pages 1214: Effect of Summer Ventilation on the Thermal Performance and Energy Efficiency of Buildings Utilizing Phase Change Materials Energies doi: 10.3390/en10081214 Authors: Yi Zhang Hongzhi Cui Waiching Tang Guochen Sang Hong Wu To analyze the effect of summer ventilation on the thermal performance and energy efficiency of buildings utilizing phase change materials (PCMs), this paper simulated the indoor temperature variation and energy saving performance of buildings constructed with PCM under different ventilation conditions from June to September. With EnergyPlus and degree-day method, 48 ventilation schemes, including eight ventilation periods (3 h per period) and six ventilation quantities (0.5 ac/h to 3 ac/h), were modeled and simulated in five cities located in different climate regions in China. According to the results, it is believed that the simultaneous use of PCM and ventilation can significantly improve the indoor thermal comfort and offer a good energy saving performance in summer. Considering the economic benefits, different optimal ventilation schemes (including ventilation periods and ventilation quantities) were suggested for different climate regions.

Description: Energies, Vol. 10, Pages 1217: A Comprehensive Study of Key Electric Vehicle (EV) Components, Technologies, Challenges, Impacts, and Future Direction of Development Energies doi: 10.3390/en10081217 Authors: Fuad Un-Noor Sanjeevikumar Padmanaban Lucian Mihet-Popa Mohammad Mollah Eklas Hossain Electric vehicles (EV), including Battery Electric Vehicle (BEV), Hybrid Electric Vehicle (HEV), Plug-in Hybrid Electric Vehicle (PHEV), Fuel Cell Electric Vehicle (FCEV), are becoming more commonplace in the transportation sector in recent times. As the present trend suggests, this mode of transport is likely to replace internal combustion engine (ICE) vehicles in the near future. Each of the main EV components has a number of technologies that are currently in use or can become prominent in the future. EVs can cause significant impacts on the environment, power system, and other related sectors. The present power system could face huge instabilities with enough EV penetration, but with proper management and coordination, EVs can be turned into a major contributor to the successful implementation of the smart grid concept. There are possibilities of immense environmental benefits as well, as the EVs can extensively reduce the greenhouse gas emissions produced by the transportation sector. However, there are some major obstacles for EVs to overcome before totally replacing ICE vehicles. This paper is focused on reviewing all the useful data available on EV configurations, battery energy sources, electrical machines, charging techniques, optimization techniques, impacts, trends, and possible directions of future developments. Its objective is to provide an overall picture of the current EV technology and ways of future development to assist in future researches in this sector.

Description: Energies, Vol. 10, Pages 1224: Best Practices for Recovering Rural Abandoned Towers through the Installation of Small-Scale Biogas Plants Energies doi: 10.3390/en10081224 Authors: Mattia Manni Valentina Coccia Gianluca Cavalaglio Andrea Nicolini Alessandro Petrozzi The massive and continuous development of renewable energy systems is making it possible to achieve the European goals regarding environment and sustainability. On the other hand, it leads to the progression of significant problems such as low renewable energy density (i), social acceptability (ii), and non-programmability of renewable energy sources (iii). The rural architecture, which is largely present in the countryside of central Italy, is generally equipped with several annexes such as dovecotes (i), grain stores (ii), and tobacco drying kilns (iii). Nowadays, those towers appear in decay because of the decline of agricultural activities, although they are classed as Environmental and Historical Heritage sites. The present work aims to propose a methodology for improving the energy grid in the countryside, while reusing abandoned buildings by modifying their function and maintaining their aspect as much as possible. The proposed workflow was applied to a rural silo, which has fallen into disuse, in Sant’Apollinare (Marsciano, Perugia) by converting it into a mini-biogas plant. The function of the annex which was chosen as the case study changes from agricultural use to energy production: it becomes an on-site renewable energy-based electric grid that can produce clean energy from agricultural and forestry residues. The project turns out to be sustainable not only in terms of energy and the environment, but also from an economic point of view as a result of the recent regulations and incentives for renewable energy production.

Description: Energies, Vol. 10, Pages 1223: Sensitivity Analysis of Heavy Fuel Oil Spray and Combustion under Low-Speed Marine Engine-Like Conditions Energies doi: 10.3390/en10081223 Authors: Lei Zhou Aifang Shao Haiqiao Wei Xi Chen On account of their high power, thermal efficiency, good reliability, safety, and durability, low-speed two-stroke marine diesel engines are used as the main drive devices for large fuel and cargo ships. Most marine engines use heavy fuel oil (HFO) as the primary fuel, however, the physical and chemical characteristics of HFO are not clear because of its complex thermophysical properties. The present study was conducted to investigate the effects of fuel properties on the spray and combustion characteristics under two-stroke marine engine-like conditions via a sensitivity analysis. The sensitivity analysis of fuel properties for non-reacting and reacting simulations are conducted by comparing two fuels having different physical properties, such as fuel density, dynamic viscosity, critical temperature, and surface tension. The performances of the fuels are comprehensively studied under different ambient pressures, ambient temperatures, fuel temperatures, and swirl flow conditions. From the results of non-reacting simulations of HFO and diesel fuel properties in a constant volume combustion chamber, it can be found that the increase of the ambient pressure promotes fuel evaporation, resulting in a reduction in the steady liquid penetration of both diesel and HFO; however, the difference in the vapor penetrations of HFO and diesel reduces. Increasing the swirl flow significantly influences the atomization of both HFO and diesel, especially the liquid distribution of diesel. It is also found that the ambient temperature and fuel temperature have the negative effects on Sauter mean diameter (SMD) distribution. For low-speed marine engines, the combustion performance of HFO is not sensitive to activation energy in a certain range of activation energy. At higher engine speed, the difference in the effects of different activation energies on the in-cylinder pressure increases. The swirl flow in the cylinder can significantly promote fuel evaporation and reduce soot production.

Description: Energies, Vol. 10, Pages 1222: Quantifying Cathode Water Transport via Anode Relative Humidity Measurements in a Polymer Electrolyte Membrane Fuel Cell Energies doi: 10.3390/en10081222 Authors: Logan Battrell Aubree Trunkle Erica Eggleton Lifeng Zhang Ryan Anderson A relative humidity (RH) measurement based on pressure drop analysis is presented as a diagnostic tool to experimentally quantify the amount of excess water on the cathode side of a polymer electrolyte membrane fuel cell (PEMFC). Ex-situ pressure drop calibration curves collected at fixed RH values, used with a set of well-defined equations for the anode pressure drop, allows for an estimate of in-situ relative humidity values. During the in-situ test, a dry anode inlet stream at increasing flow rates is used to create an evaporative gradient to drive water from the cathode to the anode. This combination of techniques thus quantitatively determines the changing net cell water flux. Knowing the cathodic water production rate, the net water flux to the anode can explain the influence of liquid and vapor transport as a function of GDL selection. Experimentally obtained quantified values for the water removal rate for a variety of cathode gas diffusion layer (GDL) setups are presented, which were chosen to experimentally vary a range of water management abilities, from high to low performance. The results show that more water is transported to the anode when a GDL with poor water management capabilities is used, due to the higher levels of initial saturation occurring on the cathode. At sufficiently high concentration gradients, the anode removes more water than is produced by the reaction, allowing for the quantification of excess water saturating the cathode. The protocol is broadly accessible and applicable as a quantitative diagnostic tool of water management in PEMFCs.

Description: Energies, Vol. 10, Pages 1225: Hydrogen Storage Capacity of Tetrahydrofuran and Tetra-N-Butylammonium Bromide Hydrates Under Favorable Thermodynamic Conditions Energies doi: 10.3390/en10081225 Authors: Joshua T. Weissman Stephen M. Masutani An experimental study was conducted to evaluate the feasibility of employing binary hydrates as a medium for H2 storage. Two reagents, tetrahydrofuran (THF) and tetra-n-butylammonium bromide (TBAB), which had been reported previously to have potential to form binary hydrates with H2 under favorable conditions (i.e., low pressures and high temperatures), were investigated using differential scanning calorimetry and Raman spectroscopy. A scale-up facility was employed to quantify the hydrogen storage capacity of THF binary hydrate. Gas chromatography (GC) and pressure drop analyses indicated that the weight percentages of H2 in hydrate were less than 0.1%. The major conclusions of this investigation were: (1) H2 can be stored in binary hydrates at relatively modest pressures and temperatures which are probably feasible for transportation applications; and (2) the storage capacity of H2 in binary hydrate formed from aqueous solutions of THF over a concentration range extending from 2.78 to 8.34 mol % and at temperatures above 263 K and pressures below 11 MPa was <0.1 wt %.

Description: Energies, Vol. 10, Pages 1218: Valuation of Real Options in Crude Oil Production Energies doi: 10.3390/en10081218 Authors: Luis Mª Abadie José M. Chamorro Oil producers are going through a hard period. They have a number of real options at their disposal. This paper addresses the valuation of two of them: the option to delay investment and the option to abandon a producing field. A prerequisite for this is to determine the value of a producing well. For this purpose we draw on a stochastic model of oil price with three risk factors: spot price, long-term price, and spot price volatility. This model is estimated with spot and futures West Texas Intermediate (WTI) oil prices. The numerical estimates of the underlying parameters allow calculate the value of a producing well over a fixed time horizon. We delineate the optimal boundary that separates the investment region from the wait region in the spot price/unit cost space. We similarly draw the boundary governing the optimal exercise of the option to abandon and the one governing the active/inactive production decision when there is no such option.

Description: Energies, Vol. 10, Pages 1163: A Methodology for Determining Permissible Operating Region of Power Systems According to Conditions of Static Stability Limit Energies doi: 10.3390/en10081163 Authors: Van Ngo Dinh Le Kim Le Van Pham Alberto Berizzi For power systems with long-distance ultra-high-voltage (UHV) transmission lines, power transmission limits are often determined by static stability limits. Therefore, the assessment of stability and finding solutions to improve the stability reserve are essential for the operation of the system. This article presents an analytical approach to construct limit characteristics according to static stability conditions on a power plane. Based on the approach proposed, a program is developed and tested on a system with long-distance UHV transmission lines, showing a good performance.

Description: Energies, Vol. 10, Pages 1220: The UK Solar Farm Fleet: A Challenge for the National Grid? † Energies doi: 10.3390/en10081220 Authors: Diane Palmer Elena Koubli Tom Betts Ralph Gottschalg Currently, in the UK, it is widely believed that supply from renewable energy sources is capable of reaching proportions too great for the transmission system. This research investigates this topic objectively by offering an understanding of year-to-year and area-to-area variability of PV (photovoltaic) performance, measured in terms of specific yield (kWh/kWp). The dataset is created using publicly available data that gives an indication of impact on the grid. The daily and seasonal variance is determined, demonstrating a surprisingly good energy yield in April (second only to August). The geographic divergence of generation from large scale solar systems is studied for various sized regions. Generation is compared to demand. Timing of output is analyzed and probability of achieving peak output ascertained. Output and demand are not well matched, as regards location. Nevertheless, the existing grid infrastructure is shown to have sufficient capacity to handle electricity flow from large scale PV. Full nameplate capacity is never reached by the examples studied. Although little information is available about oversizing of array-to-inverter ratios, this is considered unlikely to be a major contributor to grid instability. It is determined that output from UK solar farms currently presents scant danger to grid stability.

Description: Energies, Vol. 10, Pages 1228: Optimization of Drilling Layouts Based on Controlled Presplitting Blasting through Strata for Gas Drainage in Coal Roadway Strips Energies doi: 10.3390/en10081228 Authors: Zhicheng Xie Dongming Zhang Zhenlong Song Minghui Li Chao Liu Dongling Sun The controlled presplitting blasting technique is widely used in mining engineering to improve the permeability and gas extraction efficiency of coal seams. One of the key factors is the appropriate arrangement of the blasting and drainage holes, which can help improve the gas drainage quantity. To optimize the drilling layout to enhance gas-drainage efficiency, a series of controlled presplitting blasting tests were conducted at the Pingdingshan No. 8 coal mine. Based on the analysis of variations in stress and longitudinal-wave velocity of the coal in different blasting ranges, the results show that the stress on the coal at a distance of 1 m from the blasting hole decreased significantly after blasting; thus, the coal exhibited negligible bearing capacity and the longitudinal-wave velocity decreased by 56%. However, the coal exhibited particular bearing capacity at a distance of 3 m away from the blasting hole, and the longitudinal-wave velocity decreased by 35%. The stress and longitudinal-wave velocity at a distance of 5 m from the blasting hole were unaffected by the blasting. The blasting integrity rate of coal kv was defined to characterize the effect of blasting on the coal-seam fracture. The effective cracking and effective influence radii of blasting under these working conditions were predicted to be in the ranges 3.3–3.4 m and 7.2–7.3 m, respectively. According to the test results, the borehole layout was optimized in the field testes for gas drainage in coal roadway strips, and the amounts of pure gas extracted after blasting were thus increased by 1.54–2.24 times the amount before blasting.

Description: Energies, Vol. 10, Pages 1233: A Metric-Based Validation Process to Assess the Realism of Synthetic Power Grids Energies doi: 10.3390/en10081233 Authors: Adam Birchfield Eran Schweitzer Mir Athari Ti Xu Thomas Overbye Anna Scaglione Zhifang Wang Public power system test cases that are of high quality benefit the power systems research community with expanded resources for testing, demonstrating, and cross-validating new innovations. Building synthetic grid models for this purpose is a relatively new problem, for which a challenge is to show that created cases are sufficiently realistic. This paper puts forth a validation process based on a set of metrics observed from actual power system cases. These metrics follow the structure, proportions, and parameters of key power system elements, which can be used in assessing and validating the quality of synthetic power grids. Though wide diversity exists in the characteristics of power systems, the paper focuses on an initial set of common quantitative metrics to capture the distribution of typical values from real power systems. The process is applied to two new public test cases, which are shown to meet the criteria specified in the metrics of this paper.

Description: Energies, Vol. 10, Pages 1237: Extended Kalman Filter-Based State of Charge and State of Power Estimation Algorithm for Unmanned Aerial Vehicle Li-Po Battery Packs Energies doi: 10.3390/en10081237 Authors: Sunghun Jung Heon Jeong Customer requirements for unmanned aerial vehicles (UAVs) with long flight times are increasing exponentially in the personal, commercial, and military use areas. Due to their limited payload, large numbers of on-board battery packs cannot be used and this is the main reason behind the need for battery management software (BMS) packages with state of charge (SOC) estimation functions to increase the flight time. At the same time, as the UAV application range has extended widely, the size of UAVs has increased and heavy-duty UAVs are slowly appearing. As a result, the system operating power of the UAVs has been increased tremendously and their safe system power operation has become an issue. This is the main reason for the need of BMS having state of power (SOP) estimation functions. In this work a 6 S Li-Po battery pack is simulated with two ladder equivalent circuit models (ECMs) considering an impedance effect whose parameters are found using hybrid pulse power characterization (HPPC) current patterns with parameter determination using the table-based linear interpolation (TBLI) method. Two state estimation methods, including the current integration method and the extended Kalman filter (EKF) method are developed and the estimation accuracies of SOC and SOP are compared. Results show that the most accurate SOC estimation turns out to be 0.1477% (indoor test with HPPC), 0.1324% (outdoor test with 0 kg payload), and 0.2021% (outdoor test with 10 kg payload). Also, the most accurate SOP estimation error turns out to be 1.2% (indoor test with HPPC), 3.6% (outdoor test with 0 kg payload), and 4.2% (outdoor test with 10 kg payload).

Description: Energies, Vol. 10, Pages 1243: Feasibility Study of a Hybrid Building High Voltage Alternating Current and Domestic Hot Water System Combining a Photovoltaic‐Thermal System and a Ground Source Heat Pump Energies doi: 10.3390/en10081243 Authors: Yong-Dae Jeong Min Gyung Yu Yujin Nam Renewable energy systems have received a lot of attention as sustainable technology in building sector. However, the efficiency of the renewable energy systems depends on the surrounding conditions, and it could gradually decrease by excessive and long-term operation. As a solution, a hybrid system can increase the reliability of energy production and decrease investment costs through by reducing the system capacity. The hybrid system operates at the ideal performance, but the design and operation method for hybrid system have not been established. In this paper, the performance of the hybrid system combined with photovoltaic/thermal (PVT) system and ground source heat pump (GSHP) system was analyzed using TRNSYS 17 and feasibility was assessed. The energy consumption and performance efficiency of hybrid system were calculated according to operating modes. Furthermore, seasonal performance factor (SPF) of hybrid system was compared with that of conventional GSHP system. System performance was analyzed in various conditions such as the usage of storage tank heating and set temperature for solar heating. As a result, the average SPF of the developed system increased about 55.3% compared with the GSHP system.

Description: Energies, Vol. 10, Pages 1250: A Novel Method for Idle-Stop-Start Control of Micro Hybrid Construction Equipment—Part B: A Real-Time Comparative Study Energies doi: 10.3390/en10091250 Authors: Truong Dinh James Marco Hui Niu David Greenwood Lee Harper David Corrochano Micro hybrid propulsion (MHP) technologies have emerged as promising solutions for minimisation of fuel consumption and pollutant emissions of off-highway construction machines (OHCMs). Their performance and economic feasibility strongly depend on the way they utilize the idle-stop-start control (ISSC) concept. The ISSC design process and performance evaluation are particularly challenging due to the peculiar structures and dynamics of OHCMs compared to other vehicles and, therefore, require significant development time and efforts. This paper is the second of a two-part study focusing on prediction-based idle-start-stop control (PISSC) for micro hybrid OHCMs. In part A, the powertrain model and the procedure to design the PISSC system have been presented. The PISSC-based engine control performance has been investigated through numerical simulations with the designed model. In this Part B, a hardware-in-the-loop (HIL) test platform is established in HIL Control Laboratory for the rapid validation of the proposed technique in terms of the fuel/pollutant emission saving in real-time. First, the powertrain architecture and PISSC algorithm presented in Part A are briefly reviewed. Second, the process to build the HIL test platform is clearly stated. Third, experiments and analysis are carried out for a number of comparative studies to validate the superiority and practical applicability of the PISSC approach.

Description: Energies, Vol. 10, Pages 1252: Proposal for a Simplified CFD Procedure for Obtaining Patterns of the Age of Air in Outdoor Spaces for the Natural Ventilation of Buildings Energies doi: 10.3390/en10091252 Authors: Miguel Padilla-Marcos Alberto Meiss Jesús Feijó-Muñoz Built urban environments modify the air quality of the natural ventilation processes in buildings. The experimental assessment of outdoor air change (OACH) processes is complex due to the air volumes involved. The quality of the OACH can be analysed through the ventilation efficiency concept, which has been extensively studied in order to characterize the ventilation of indoor spaces. Ventilation efficiency through the age-of-the-air concept assesses the air-change capacity of a certain space. A procedure formulation for obtaining an adequate control domain (CD) to evaluate outdoor air change efficiency (OACE) is proposed. A methodology in two phases is presented: first, an evaluation stage, in which the outdoor air behaviour patterns are studied; then, a second phase where an ideal control domain (ICD) is obtained in order to comparatively assess several cases in similar urban environments. The outcomes determine a proposed ICD for evaluating the design of efficiently ventilated buildings in open urban built environments. The methodology of validation proposed simplifies the assessment of the building shape and its impact on the quality of the OACH by the OACE. The suggested ICD has dimensions of L = 5H; D = 3H; and T = 2H. ICD enables the representation of the air behaviour and fulfils the imposed requirements to evaluate efficiency with a mean accuracy lower than 0.6%.