Energies

16 pages, 2759 KiB

Open AccessReview

Guidelines for Determining the Initial Shape and Specifications of High-Speed AFPM Based on Recent Research

by Jae-Beom Kang, Ji-Young Lee and Ji-Heon Lee

Energies 2022, 15(16), 6099; https://doi.org/10.3390/en15166099 - 22 Aug 2022

Cited by 2 | Viewed by 1965

This paper presents guidelines for determining the initial shape and specifications of a high-speed axial-flux permanent-magnet (AFPM) machine in a hybrid-electric propulsion system in two steps based on previous studies and product review results related to high-speed AFPMs. In the first step, three [...] Read more.

This paper presents guidelines for determining the initial shape and specifications of a high-speed axial-flux permanent-magnet (AFPM) machine in a hybrid-electric propulsion system in two steps based on previous studies and product review results related to high-speed AFPMs. In the first step, three characteristics to be considered when designing AFPMs were classified as: electromagnetic, thermal, and mechanical. Then, the factors that should be considered in the design process to satisfy each characteristic were organized. In the second step, “the speed–output power” relationship was defined to predict the limits of applying AFPMs to high-speed applications, allowing an estimation of the limits of the speed range that can be used within the proposed output power. Full article

(This article belongs to the Section F: Electrical Engineering)

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24 pages, 3716 KiB

Open AccessArticle

Design, Greenhouse Emissions, and Environmental Payback of a Photovoltaic Solar Energy System

by Herwin Saito Schultz and Monica Carvalho

Energies 2022, 15(16), 6098; https://doi.org/10.3390/en15166098 - 22 Aug 2022

Cited by 11 | Viewed by 2220

Abstract

This study aims to design a 16.4 MW photovoltaic solar system located in the Brazilian Northeast and quantify the associated greenhouse gas emissions and environmental payback. The energy system was designed to minimize the Levelized Cost of Energy. The greenhouse gas emissions were [...] Read more.

This study aims to design a 16.4 MW photovoltaic solar system located in the Brazilian Northeast and quantify the associated greenhouse gas emissions and environmental payback. The energy system was designed to minimize the Levelized Cost of Energy. The greenhouse gas emissions were quantified with the Life Cycle Assessment methodology, expressing the environmental impact in terms of generated energy (kg CO₂-eq/kWh) and following ISO 14040 and 14044. The environmental payback considered the Brazilian electricity mix and degradation of the panels. The results indicated a system capable of producing 521,443 MWh in 25 years, with an emission factor of 0.044 kg CO₂-eq/kWh and environmental payback of five years and eight months. The emission factor is at least ten times lower than thermoelectric natural gas power plants. The solar panels were the main contributors to the greenhouse gas emissions, representing 90.59% of overall emissions. It is concluded that photovoltaic energy systems are crucial in the search for emissions mitigation, even in a country that presents a predominantly renewable electricity matrix, with demonstrated environmental benefits. Full article

(This article belongs to the Section B3: Carbon Emission and Utilization)

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21 pages, 5504 KiB

Open AccessArticle

There Is Plenty of Room inside a Bluff Body: A Hybrid Piezoelectric and Electromagnetic Wind Energy Harvester

by Mahmood Al-Riyami, Issam Bahadur and Hassen Ouakad

Energies 2022, 15(16), 6097; https://doi.org/10.3390/en15166097 - 22 Aug 2022

Cited by 6 | Viewed by 1529

Abstract

In this paper, a piezoelectric and electromagnetic hybrid wind energy harvester is proposed. The general design of the harvester comprises multiple cantilever piezoelectric energy harvesters (PEHs) and electromagnetic energy harvesters (EEHs) embedded inside the bluff body that is attached to the free end [...] Read more.

In this paper, a piezoelectric and electromagnetic hybrid wind energy harvester is proposed. The general design of the harvester comprises multiple cantilever piezoelectric energy harvesters (PEHs) and electromagnetic energy harvesters (EEHs) embedded inside the bluff body that is attached to the free end of PEHs. This research work investigates utilizing the room inside the bluff body to enclose harvesters to have a more compact and efficient harvesting system. A comprehensive coupled dynamic model of the harvester (HEH) is developed using Lagrange’s formulation. The electromechanical and electromagnetic coupling coefficient equations are derived. The coupled equations of motion are solved analytically and numerically with an exact agreement. A parametric analysis is conducted to study the effect of the design parameters on the overall performance of the harvester in terms of output power and bandwidth. The proposed design evidently presents itself as a promising concept in utilizing the room inside a bluff body. Full article

(This article belongs to the Section A3: Wind, Wave and Tidal Energy)

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20 pages, 24107 KiB

Open AccessArticle

Research on the Characteristics of the Solid–Liquid Two-Phase Flow Field of a Submersible Mixer Based on CFD-DEM

by Fei Tian, Erfeng Zhang, Chen Yang, Weidong Shi and Yonghua Chen

Energies 2022, 15(16), 6096; https://doi.org/10.3390/en15166096 - 22 Aug 2022

Cited by 4 | Viewed by 1417

Abstract

Submersible mixers are widely used in the sewage treatment process in various fields, such as agriculture and industry. They are mainly responsible for pushing flow and mixing activated sludge particles in a pool. Based on the CFD-DEM coupling method under the Euler–Lagrange framework, [...] Read more.

Submersible mixers are widely used in the sewage treatment process in various fields, such as agriculture and industry. They are mainly responsible for pushing flow and mixing activated sludge particles in a pool. Based on the CFD-DEM coupling method under the Euler–Lagrange framework, the solid–liquid two-phase flow of a submersible mixer was simulated in this paper, and the motion characteristics and distribution laws of particles in the pool were studied in axial, horizontal, and lateral directions, respectively. An evaluation method of distribution uniformity was proposed to analyze the velocity distribution of the flow field, the velocity distribution of particles, and the mixing uniformity of particles. The results show that the movement process of activated sludge particles in the pool can be roughly divided into three stages: the horizontal development stage, absorption–injection stage, and reflux-mixing stage, in which the reflux-mixing stage is the main stage for the uniform distribution of particles in the whole flow field. Particle accumulation occurs mainly in the dead zones of the flow field. Distribution of particles in the axial direction has the most homogeneous extent. Vortices can be generated near pool walls, causing accumulation of particles. This method can be a good guide for engineering practice. Full article

(This article belongs to the Special Issue Modelling and Computations of Fluid Mechanics for Energy System)

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18 pages, 5797 KiB

Open AccessArticle

A Novel Energy Efficient Threshold Based Algorithm for Wireless Body Sensor Network

by Suresh Kumar Arumugam, Amin Salih Mohammed, Kalpana Nagarajan, Kanagachidambaresan Ramasubramanian, S. B. Goyal, Chaman Verma, Traian Candin Mihaltan and Calin Ovidiu Safirescu

Energies 2022, 15(16), 6095; https://doi.org/10.3390/en15166095 - 22 Aug 2022

Cited by 12 | Viewed by 1434

Abstract

Wireless body sensor networks (WBSNs) monitor the changes within the human body by having continuous interactions within the nodes in the body network. Critical issues with these continuous interactions include the limited energy within the node and the nodes becoming isolated from the [...] Read more.

Wireless body sensor networks (WBSNs) monitor the changes within the human body by having continuous interactions within the nodes in the body network. Critical issues with these continuous interactions include the limited energy within the node and the nodes becoming isolated from the network easily when it fails. Moreover, when the node’s burden increases because of the failure of other nodes, the energy utilization as well as the heat dissipated increases much more, causing damage to the network as well as human body. In this paper, we propose a threshold-based fail proof lifetime enhancement algorithm which schedules the nodes in an optimal way depending upon the available energy level. The proposed algorithm is experimented with a real time system setup and the proposed algorithm is compared with different routing mechanisms in terms of various network parameters. It is inferred that the proposed algorithm outperforms the existing routing mechanisms. Full article

(This article belongs to the Special Issue Energy Efficiency in Wireless Networks)

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18 pages, 2787 KiB

Open AccessArticle

Towards Data-Driven Models in the Prediction of Ship Performance (Speed—Power) in Actual Seas: A Comparative Study between Modern Approaches

by Kiriakos Alexiou, Efthimios G. Pariotis, Helen C. Leligou and Theodoros C. Zannis

Energies 2022, 15(16), 6094; https://doi.org/10.3390/en15166094 - 22 Aug 2022

Cited by 3 | Viewed by 1762

Abstract

In the extremely competitive environment of shipping, minimizing shipping cost is the key factor for the survival and growth of shipping companies. However, stricter rules and regulations that aim at the reduction of greenhouse gas emissions published by the International Maritime Organization, force [...] Read more.

In the extremely competitive environment of shipping, minimizing shipping cost is the key factor for the survival and growth of shipping companies. However, stricter rules and regulations that aim at the reduction of greenhouse gas emissions published by the International Maritime Organization, force shipping companies to increase the operational efficiency of their fleet. The prediction of a ship speed in actual seas with a given power by its engine is the most important performance indicator and thus makes it the “holy grail” in pursuing better efficiency. Traditionally, tank model tests and semi-empirical formulas were the preferred solution for the aforementioned prediction and are still widely applied. However, currently, with the increased computational power that is widely available, novel and more sophisticated methods taking into consideration computational fluid dynamics (CFD) and machine learning (ML) algorithms are emerging. In this paper, we briefly present the different approaches in the prediction of a ship’s speed but focus on ML methods comparing a representative number of the latest data-driven models used in papers, to provide guidelines, discover trends and identify the challenges to be faced by researchers. From this comparison, we can distinguish that artificial neural networks (ANN), being used in 73.3% of the reviewed papers, dominate as the algorithm of choice. Researchers mostly rely on physical laws governing the phenomena in the crucial part of data preprocessing tasks. Lastly, most researchers rely on data acquisition systems installed at ships in order to achieve usable results. Full article

(This article belongs to the Section K: State-of-the-Art Energy Related Technologies)

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19 pages, 7172 KiB

Open AccessArticle

A Study on a Casing Consisting of Three Flow Deflectors for Performance Improvement of Cross-Flow Wind Turbine

by Tadakazu Tanino, Ryo Yoshihara and Takeshi Miyaguni

Energies 2022, 15(16), 6093; https://doi.org/10.3390/en15166093 - 22 Aug 2022

Cited by 1 | Viewed by 1746

Abstract

We investigated the effective use of cross-flow wind turbines for small-scale wind power generation to increase the output power by using a casing, which is a kind of wind-collecting device, composed of three flow deflector plates having the shape of a circular-arc airfoil. [...] Read more.

We investigated the effective use of cross-flow wind turbines for small-scale wind power generation to increase the output power by using a casing, which is a kind of wind-collecting device, composed of three flow deflector plates having the shape of a circular-arc airfoil. Drag-type vertical-axis wind turbines have an undesirable part of about half of the swept area where the inflow of wind results in low output performance. To solve this problem, we devised a casing consisting of three flow deflector plates, two of which were to block the unwanted inflow of wind and the remaining flow deflector plate having an angle of attack with respect to the wind direction to increase the flow toward the rotor. In this study, output performance experiments using a wind tunnel and numerical fluid analysis were conducted on a cross-flow wind turbine with three flow deflector plates to evaluate the effectiveness of the casing on output performance improvement. As a result, it was confirmed that the casing could improve the output performance of the cross-flow wind turbine by approximately 60% at the maximum performance point and could also maintain the output performance about 50% higher compared to the bare cross-flow wind turbine without the casing within a deviation angle of ±10 degrees, even when the casing direction was inclined against the wind direction due to changes in wind direction. Full article

(This article belongs to the Special Issue Vertical-Axis Wind Turbine)

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15 pages, 12739 KiB

Open AccessArticle

Generalized Average Modeling of a Dual Active Bridge DC-DC Converter with Triple-Phase-Shift Modulation

by Michał Rolak, Maciej Twardy and Cezary Soból

Energies 2022, 15(16), 6092; https://doi.org/10.3390/en15166092 - 22 Aug 2022

Cited by 4 | Viewed by 2261

Abstract

This paper shows an elaboration of an equivalent electrical circuit of a Dual Active Bridge (DAB) and its application as a versatile tool for steady-states analysis in wide range of operating conditions. This work analyses the converter which is controlled with a coherently [...] Read more.

This paper shows an elaboration of an equivalent electrical circuit of a Dual Active Bridge (DAB) and its application as a versatile tool for steady-states analysis in wide range of operating conditions. This work analyses the converter which is controlled with a coherently defined Triple Phase-Shift (TPS) modulation which allows appropriate switching functions to be written, thus enabling the circuit’s state-space equations to be derived. Due to this approach, a Fourier series expansion may be easily applied to utilize Generalized Averaged Modeling (GAM)—a convenient method for modeling resonant and quasi-resonant power converters. Moreover, this paper shows the utilization of the GAM model higher harmonics’ complex magnitudes to calculate the steady-state power characteristics for bidirectional operation; additionally, a method for a particular state variable waveform signal reconstruction is presented. All the discussed model properites are validated with a 1.5 kW 100 kHz SiC-based DAB prototype. Full article

(This article belongs to the Special Issue New Topologies of Voltage Source Converters and Current Source Converters for Integration of Fast EV Charging Stations within Smart Grids)

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17 pages, 4437 KiB

Open AccessArticle

A Predicting Model for the Effective Thermal Conductivity of Anisotropic Open-Cell Foam

by Chao Zhang, Xiangzhuang Kong, Xian Wang, Yanxia Du and Guangming Xiao

Energies 2022, 15(16), 6091; https://doi.org/10.3390/en15166091 - 22 Aug 2022

Cited by 10 | Viewed by 1452

Abstract

The structural anisotropy of open-cell foam leads to the anisotropy of effective thermal conductivity (ETC). To quantitatively analyze the effect of structural anisotropy on the anisotropy of ETC, a new predicting model for the ETC of anisotropic open-cell foam was proposed based on [...] Read more.

The structural anisotropy of open-cell foam leads to the anisotropy of effective thermal conductivity (ETC). To quantitatively analyze the effect of structural anisotropy on the anisotropy of ETC, a new predicting model for the ETC of anisotropic open-cell foam was proposed based on an anisotropy tetrakaidecahedron cell (ATC). Feret diameters in three orthogonal directions obtained by morphological analysis of real foam structures were used to characterize the anisotropy of ATC. To validate our proposed anisotropic model, the ETCs of real foam structures in three orthogonal directions predicted by it were compared with the numerical results, for which the structures of numerical models are reconstructed by X-ray computed tomography (X-CT). Using the present anisotropic model, the influences of the thermal conductivity ratio (TCR) and porosity of the foams on the anisotropic ratios of ETCs are also investigated. Results show that there is good consistency between the ETCs obtained by the anisotropic model and the numerical method. The maximum relative errors between them are 2.84% and 13.57% when TCRs are 10 and 100, respectively. The present anisotropic model can not only predict the ETCs in different orthogonal directions but also quantitatively predict the anisotropy of ETC. The anisotropies of the ETCs decrease with porosity because the proportion of the foam skeleton decreases. However, the anisotropies of ETCs increase with TCR, and there exist asymptotic values in anisotropic ratios of ETCs as TCR approaches infinity and they are equal to the relative Feret diameters in different orthogonal directions. Full article

(This article belongs to the Special Issue Advanced Numerical Simulation Methods of Aerodynamics and Heat Transfer)

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13 pages, 3340 KiB

Open AccessArticle

Scalable Residential Building Geometry Characterisation Using Vehicle-Mounted Camera System

by Menglin Dai, Wil O. C. Ward, Hadi Arbabi, Danielle Densley Tingley and Martin Mayfield

Energies 2022, 15(16), 6090; https://doi.org/10.3390/en15166090 - 22 Aug 2022

Cited by 3 | Viewed by 1473

Abstract

Residential buildings are an important sector in the urban environment as they provide essential dwelling space, but they are also responsible for a significant share of final energy consumption. In addition, residential buildings that were built with outdated standards usually face difficulty meeting [...] Read more.

Residential buildings are an important sector in the urban environment as they provide essential dwelling space, but they are also responsible for a significant share of final energy consumption. In addition, residential buildings that were built with outdated standards usually face difficulty meeting current energy performance standards. The situation is especially common in Europe, as 35% of buildings were built over fifty years ago. Building retrofitting techniques provide a choice to improve building energy efficiency while maintaining the usable main structures, as opposed to demolition. The retrofit assessment requires the building stock information, including energy demand and material compositions. Therefore, understanding the building stock at scale becomes a critical demand. A significant piece of information is the building geometry, which is essential in building energy modelling and stock analysis. In this investigation, an approach has been developed to automatically measure building dimensions from remote sensing data. The approach is built on a combination of unsupervised machine learning algorithms, including K-means++, DBSCAN and RANSAC. This work is also the first attempt at using a vehicle-mounted data-capturing system to collect data as the input to characterise building geometry. The developed approach is tested on an automatically built and labelled point cloud model dataset of residential buildings and shows capability in acquiring comprehensive geometry information while keeping a high level of accuracy when processing an intact model. Full article

(This article belongs to the Special Issue Active Buildings: From Theory to Practice)

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20 pages, 1370 KiB

Open AccessArticle

Measures for Assessing the Effectiveness of Investments for Electricity and Heat Generation from the Hybrid Cooperation of a Photovoltaic Installation with a Heat Pump on the Example of a Household

by Mariusz Niekurzak, Wojciech Lewicki, Wojciech Drożdż and Paweł Miązek

Energies 2022, 15(16), 6089; https://doi.org/10.3390/en15166089 - 22 Aug 2022

Cited by 7 | Viewed by 1759

Abstract

In recent years, one of the key postulates in the European Union’s policy has become the development of renewable energy sources. In order to achieve the desired synergy effect, the idea of combining two selected sources of energy appeared. This article presents a [...] Read more.

In recent years, one of the key postulates in the European Union’s policy has become the development of renewable energy sources. In order to achieve the desired synergy effect, the idea of combining two selected sources of energy appeared. This article presents a technical and economic analysis of a hybrid connection of a ground source heat pump with a photovoltaic installation. Taking into account the heat demand of the building, a ground heat pump with a catalog nominal heating power of 25 kW was selected. This article presents the problem of the economic profitability of using a hybrid combination of a heat pump and photovoltaic panels in domestic hot water and central heating systems. The justification for the use of such heat sources in these installations is due to global trends and the gradual departure from conventional energy sources such as oil or gas boilers. This paper presents the economic and ecological results of using the pump heat connected together with photovoltaic panels. In the economic analysis, with the assumed installation costs related to the use of the considered heat pump and PV, two parameters commonly used in the investment analysis (static and dynamic) were used, namely, the simple payback period and the net present value of the investment. For the adopted assumptions, the usable area of the facility and the number of years of use were indicated, at which the investment in question is competitive with other alternative investment interest methods and will start to bring tangible benefits. The performed analysis also has measurable environmental benefits in the form of a reduction in carbon dioxide emissions at the level of 2893 kg/year into the atmosphere. The presented solution will help future investors understand the investment profitability mechanism for their households. Full article

(This article belongs to the Special Issue Advances in Organizations in Power Markets Applications for Industry 4.0)

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13 pages, 1337 KiB

Open AccessArticle

Performance Analysis of an Experimental Linear Encoder’s Reading Head under Different Mounting and Dynamic Conditions

by Donatas Gurauskis, Krzysztof Przystupa, Artūras Kilikevičius, Mikołaj Skowron, Matijošius Jonas, Joanna Michałowska and Kristina Kilikevičienė

Energies 2022, 15(16), 6088; https://doi.org/10.3390/en15166088 - 22 Aug 2022

Viewed by 2117

Abstract

The performance of an optical linear encoder is described and evaluated by certain parameters such as its resolution, accuracy and repeatability. The best encoder for a particular application, just like other sensors, is usually selected according these parameters. There are, however, many side [...] Read more.

The performance of an optical linear encoder is described and evaluated by certain parameters such as its resolution, accuracy and repeatability. The best encoder for a particular application, just like other sensors, is usually selected according these parameters. There are, however, many side effects that have a direct influence on the optimal operation of an encoder. In order to understand how to minimize these harmful effects, a deeper knowledge of an encoder’s performance and a method for determining these factors are necessary. The main aspects of an encoder’s accuracy, resolution and repeatability are briefly reviewed in this paper. Discussed and developed in previous work, the experimental reading head for a Moiré effect-based optical linear encoder is used for the experimental analysis of the influence of different reading head designs on an encoder’s performance under various mounting inaccuracies and dynamic conditions. Full article

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10 pages, 277 KiB

Open AccessArticle

Special Cement Slurries for Strengthening Salt Rock Mass

by Stanisław Stryczek, Andrzej Gonet and Marcin Kremieniewski

Energies 2022, 15(16), 6087; https://doi.org/10.3390/en15166087 - 22 Aug 2022

Cited by 2 | Viewed by 1140

Abstract

Every year, the number of exploited mine workings necessary to seal the exploited mines increases in the world. As a result of experiments, technologies are developed that allow slurry to be pumped to fill free rock spaces or to liquidate rock mass discontinuities. [...] Read more.

Every year, the number of exploited mine workings necessary to seal the exploited mines increases in the world. As a result of experiments, technologies are developed that allow slurry to be pumped to fill free rock spaces or to liquidate rock mass discontinuities. The slurry preparation technologies can be divided into: subsurface and surface preparation and injection. Due to the pressure that forces the sealing slurry to move, the following can be distinguished: pressure technologies and technologies of gravity injection. The effectiveness of the work is determined by the correct selection of the technique and technology of the treatment and the selection of the optimal cement slurry recipe. The type of sealing liquid is especially important during works related to filling the exploited mine workings in salt mines. Therefore, this article presents the criteria for the selection of slurry recipes and their technological parameters, used for sealing and strengthening the salt rock mass. For this purpose, laboratory tests are carried out on various formulas of sealing slurries, prepared on the basis of full saturated brine and CEM I 32.5R Portland cement, ground granulated blast furnace slag, fly ash, and silt. The proposed concept for the selection of sealing slurry formulas has been positively verified during the performed works on sealing and strengthening the salt rock mass. Full article

23 pages, 4249 KiB

Open AccessArticle

Intelligent Optimization of Switched Reluctance Motor Using Genetic Aggregation Response Surface and Multi-Objective Genetic Algorithm for Improved Performance

by Chiweta Emmanuel Abunike, Ogbonnaya Inya Okoro and Sumeet S. Aphale

Energies 2022, 15(16), 6086; https://doi.org/10.3390/en15166086 - 22 Aug 2022

Cited by 4 | Viewed by 1910

Abstract

In this paper, a thorough framework for multiobjective design optimization of switched reluctance motor (SRM) is proposed. Selection of stator and rotor pole embrace coefficients is an essential step in the SRM design process since it influences torque output and torque ripple in [...] Read more.

In this paper, a thorough framework for multiobjective design optimization of switched reluctance motor (SRM) is proposed. Selection of stator and rotor pole embrace coefficients is an essential step in the SRM design process since it influences torque output and torque ripple in SRM. The problem of determining optimal pole embrace is formulated as a multi-objective optimization problem with the objective of optimizing average torque, efficiency and torque ripple, and response surface models were obtained based on the genetic aggregation method. The results obtained by genetic aggregation response surface (GARS) and the non-dominated genetic algorithm (NSGA-II) were validated with the finite element method (FEM) model of the initial SRM. The optimized model displayed better efficiency profile over a wide speed range. The initial and optimized models recorded maximum efficiencies of 85% and 94.05%, respectively, at 2000 rpm. The efficiency values of 93.97–94.05% were achieved for the three pareto optimal candidates. The findings indicate the viability of the suggested strategy and support the use of GARS and NSGA-II as useful methods for addressing SRM key challenges. Full article

(This article belongs to the Special Issue Modern Electric Power Delivery Systems)

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26 pages, 7687 KiB

Open AccessArticle

Cooling Modelling of an Electrically Heated Ceramic Heat Accumulator

by Dawid Taler, Jan Taler, Tomasz Sobota and Jarosław Tokarczyk

Energies 2022, 15(16), 6085; https://doi.org/10.3390/en15166085 - 22 Aug 2022

Cited by 2 | Viewed by 1428

Abstract

This paper presents a simple novel mathematical model of a heat accumulator with an arranged packing in the form of ceramic cylinders. The accumulator analysed in the paper can be heated with inexpensive electricity overnight or excess electricity from wind farms. It can [...] Read more.

This paper presents a simple novel mathematical model of a heat accumulator with an arranged packing in the form of ceramic cylinders. The accumulator analysed in the paper can be heated with inexpensive electricity overnight or excess electricity from wind farms. It can be used as a heat source in a hydronic heating system or for domestic hot water. The differential equations describing the transient temperature of the accumulator packing and flowing air were solved using the explicit Euler and Crank–Nicolson methods. The accuracy of both methods was assessed using exact analytical solutions and the superposition method for a uniform initial temperature and accounted for time changes in inlet air temperature. A numerical simulation of the accumulator cooled by flowing air was carried out. The correlation for the air-side Nusselt number was determined using the method of least squares based on experimental data. The calculated exit air temperature was compared with the measured data. The accumulator can operate as a heat source with dynamic discharge. The developed mathematical model of the accumulator can be used in a system to adjust the fan rotational speed so that the air temperature in the room is equal to the preset temperature. Full article

(This article belongs to the Special Issue Numerical Heat Transfer and Fluid Flow 2022)

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27 pages, 5363 KiB

Open AccessArticle

Long-Term Planning of Electrical Distribution Grids: How Load Uncertainty and Flexibility Affect the Investment Timing

by Marie-Cécile Alvarez-Herault, Jean-Pierre Dib, Oana Ionescu and Bertrand Raison

Energies 2022, 15(16), 6084; https://doi.org/10.3390/en15166084 - 22 Aug 2022

Cited by 3 | Viewed by 1782

Abstract

Due to the rise of smart grids, new players and services are emerging and can have an impact on the decision-making process in distribution networks, which, over the past decades, were only driven by linear demand growth with a low level of uncertainties. [...] Read more.

Due to the rise of smart grids, new players and services are emerging and can have an impact on the decision-making process in distribution networks, which, over the past decades, were only driven by linear demand growth with a low level of uncertainties. Nowadays, the evolution of distribution networks and investment decisions (conductors and transformers) can no longer be based solely on deterministic assumptions of load evolution since there is a high level of uncertainties related to the development of electrical loads such as electric vehicles. In this paper, we focus on the uncertainty of the peak power, key elements for triggering an investment, and the flexibility to choose between different topologies of electric networks. To solve this problem, we apply a real option approach and provide an analytical model with closed-form solutions that allows a full treatment of the dynamic aspects of the decision to reconsider the topology of the network. Moreover, through a comparative statics analysis, we infer the sensitivity of the option value to modify the network with respect to the volatility of the peak power, the associated investment cost or other types of costs of power losses, the growth rate, or the discount rate. Full article

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21 pages, 1061 KiB

Open AccessArticle

Validated Analytical Modeling of Eccentricity and Dynamic Displacement in Diesel Engines with Flexible Crankshaft

by Salah A. M. Elmoselhy, Waleed F. Faris and Hesham A. Rakha

Energies 2022, 15(16), 6083; https://doi.org/10.3390/en15166083 - 22 Aug 2022

Viewed by 1436

Abstract

In spite of the fact that the flexibility of the crankshaft of diesel engines exhibits notable nonlinearities, analytical modeling of such nonlinearities is not yet realized. The present study thus analytically models the effect of eccentricity on flexible crankshaft and piston secondary motion. [...] Read more.

In spite of the fact that the flexibility of the crankshaft of diesel engines exhibits notable nonlinearities, analytical modeling of such nonlinearities is not yet realized. The present study thus analytically models the effect of eccentricity on flexible crankshaft and piston secondary motion. The eccentricity of the crankshaft is modeled as the summation of the hydrodynamic eccentricity and the dynamic mass eccentricity of the crankshaft. The study also models the absolute value of the vibrational dynamic displacement of the center of the crankshaft. The paper proves that such dynamic displacement of the center of the crankshaft is sensitive to the changes in its independent variables. It was found that the most influential parameters on the dynamic displacement of the center of the crankshaft due to vibration are the natural frequency and the eccentricity of the crankshaft. The modeling of the dynamic displacement in a flexible crankshaft was validated using a case study based on the eccentricity of the crankshaft showing a relative error of 4%, which is less than the relative error in the CMEM and GT-Power. Furthermore, the analytical modeling of the dynamic displacement in the flexible crankshaft was validated using another case study based on fatigue analysis of the crankshaft showing a relative error of 9%, which is less than that the relative error in Newman’s model of diesel engine fuel consumption and Lansky’s model of diesel engine cylinders. The paper also presents a proposed approach of fatigue failure analysis for vehicular dynamic components and presents a proposed nanostructure of crankshafts for improving such fatigue performance. The developed models would help develop efficient diesel engines and help prolong their service life. Full article

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18 pages, 4897 KiB

Open AccessArticle

Heat Transfer Enhancement through Thermodynamical Activity of H₂O/Clay Nanofluid Flow over an Infinite Upright Plate with Caputo Fractional-Order Derivative

by J. Kayalvizhi, A. G. Vijaya Kumar, Hakan F. Öztop, Ndolane Sene and Nidal H. Abu-Hamdeh

Energies 2022, 15(16), 6082; https://doi.org/10.3390/en15166082 - 22 Aug 2022

Cited by 5 | Viewed by 1181

Abstract

This paper presents a modelling of nanofluid flow using Caputo fractional derivatives through conservative equations of mass and momentum, and provides an exact solution on un-steady convective flow over a vertical plate with the mass diffusion effect, in association with an energy equation. [...] Read more.

This paper presents a modelling of nanofluid flow using Caputo fractional derivatives through conservative equations of mass and momentum, and provides an exact solution on un-steady convective flow over a vertical plate with the mass diffusion effect, in association with an energy equation. H₂O is the base liquid with clay nanoparticles floating in it in a uniform way. Boussinessq’s approach is used in the momentum equation for pressure gradient. The non-dimensional fluid temperature, species concentration and fluid transport are derived together with Jacob Fourier sine and Laplace transform techniques in terms of exponential decay function, and the inverse is computed further in terms of the Mittag-Leffler function. The impact of various physical quantities is interpreted with the fractional order of the Caputo derivatives. The obtained temperature, transport and species concentration profiles show behaviors for 0 <

α

< 1, where

α

is the fractional parameter. The rate of heat and mass transfer coefficients for the significance of physical quantities of interest are also obtained and presented through graphs. The impact of the nanoparticle volume fraction on the flow field is observed. At larger values of the fractional parameter, the velocity, temperature, and concentration distributions grow more quickly. In addition to that, it is found the concentration profiles behave in the opposite way for the volume fraction of nanofluids. Full article

(This article belongs to the Special Issue Heat Transfer and Mathematical Modeling of Multi-Phase Flow through Porous Media)

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15 pages, 1611 KiB

Open AccessArticle

Probabilistic Peak Demand Estimation Using Members of the Clayton Generalized Gamma Copula Family

by Moshe Kelner, Zinoviy Landsman and Udi E. Makov

Energies 2022, 15(16), 6081; https://doi.org/10.3390/en15166081 - 22 Aug 2022

Viewed by 1247

Abstract

Climate change impacts many aspects of life and requires innovative thinking on various issues. The electricity sector is affected in several ways, including changes in the production components and consumption patterns. One of the most important issues for Independent System Operators, a state-controlled [...] Read more.

Climate change impacts many aspects of life and requires innovative thinking on various issues. The electricity sector is affected in several ways, including changes in the production components and consumption patterns. One of the most important issues for Independent System Operators, a state-controlled organization responsible for ensuring the reliability, availability, and quality of electricity delivery in the country, is the response to climate change. This is reflected in the appropriate design of production units to cope with the increase in demand due to extreme heat and cold events and the development of models aimed at predicting the probability of such events. In our work, we address this challenge by proposing a novel probability model for peak demand as a function of wet temperature (henceforth simply temperature), which is a weighting of temperature and humidity. We study the relationship between peak demand and temperature using a new Archimedean copula family, shown to be effective for this purpose. This family, the Clayton generalized Gamma, is a multi-parameter copula function that comprises several members. Two new measures of fit, an economic measure and a conditional coverage measure, were introduced to select the most appropriate family member based on the empirical data of daily peak demand and minimum temperature in the winter. The Clayton Gamma copula showed the lowest cost measure and the best conditional coverage and was, therefore, proven to be the most appropriate member of the family. Full article

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2 pages, 174 KiB

Open AccessEditorial

Advances and Challenges in Solar PV Systems’ Performance

by Jesús Polo

Energies 2022, 15(16), 6080; https://doi.org/10.3390/en15166080 - 22 Aug 2022

Cited by 1 | Viewed by 1078

Abstract

The increase in the dissemination of renewable energies constitutes a new energy model that can be used to face the global challenges derived from the reduction in fossil fuels to limit the effects of global warming [...] Full article

11 pages, 2896 KiB

Open AccessArticle

Numerical Simulation of Transient Combustion and the Acoustic Environment of Obstacle Vortex-Driven Flow

by Afaque Ahmed Bhutto, Khanji Harijan, Mukkarum Hussain, Syed Feroz Shah and Laveet Kumar

Energies 2022, 15(16), 6079; https://doi.org/10.3390/en15166079 - 22 Aug 2022

Cited by 8 | Viewed by 1265

Abstract

Solid fuel combustion in a chamber does not necessarily occur at a constant rate and may show fluctuations due to variables such as varying burning rates, chamber pressure, and residual combustion. These variables can cause the fuel to burn disproportionately. The acoustic environment [...] Read more.

Solid fuel combustion in a chamber does not necessarily occur at a constant rate and may show fluctuations due to variables such as varying burning rates, chamber pressure, and residual combustion. These variables can cause the fuel to burn disproportionately. The acoustic environment of obstacle vortex-driven flow due to transient combustion with pressure oscillations in a solid fuel chamber is numerically investigated in the present study. Solid fuel combustion is considered transient, and flow characteristics of the present problem are governed by large eddies shed from an obstacle. Since unsteady Reynolds-averaged Navier-Stokes (URANS) simulations are not appropriate to compute the present flow phenomenon, therefore, a detached eddy simulation (DES) is performed to precisely predict the flow behavior. Simulation of steady-state combustion is carried out to validate the numerical results with available experimental data from the literature. The simulation of transient combustion shows that if the combustion frequency is close to the chamber’s modal frequency of the chamber, its amplitude increases greatly and creates an acute acoustic environment. This will result in fuel savings. The amplitude of pressure oscillation up to 18% and 5% of mean pressure are evident at the first and second mode of forced oscillation frequencies respectively. Interestingly, it is also found that pressure oscillation always occurs at inlet mass flux disturbance frequency and not between the disturbance and natural frequency of the chamber. As a result, it is evident that the combustion process or chamber configuration could be modified to ensure that both frequencies are far away enough to interact and create both a harsh acoustic environment and sufficient fuel to burn disproportionately. Full article

(This article belongs to the Special Issue Advanced Studies in Clean and Green Energy Technologies)

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15 pages, 2164 KiB

Open AccessArticle

Assessment Criteria of Changes in Health Index Values over Time—A Transformer Population Study

by Patryk Bohatyrewicz and Szymon Banaszak

Energies 2022, 15(16), 6078; https://doi.org/10.3390/en15166078 - 22 Aug 2022

Cited by 2 | Viewed by 1446

Abstract

The current use of health index algorithms is mainly limited to single assessments of the unit’s condition or the device comparison. The paper focuses on the changes in the health index values between the consecutive analyses. The algorithm used for this purpose was [...] Read more.

The current use of health index algorithms is mainly limited to single assessments of the unit’s condition or the device comparison. The paper focuses on the changes in the health index values between the consecutive analyses. The algorithm used for this purpose was previously developed by the authors. The test group included 359 complete oil evaluation results from 86 power transformers monitored over several years. For each outcome, the influence of the sub-components of the main score was calculated. Additional health index increase simulations were performed based on the IEC 60599 standard guidelines. The highest increases and decreases in the total score were listed and analyzed to determine the main factors behind the changes. The study has shown that the changes in dissolved gases concentrations have a much more significant influence on the health index values than the changes in physicochemical properties of the oil and furfural content. Based on the magnitude of the observed changes and the simulation outcomes, the authors have proposed two assessment thresholds—the 50th percentile health index increase within a population as an alarm zone, and the 90th or 95th percentile increase as a pre-failure zone. Full article

(This article belongs to the Special Issue State-of-the-Art Solutions and Innovations for Testing and Condition Monitoring of Power Transformers)

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16 pages, 6045 KiB

Open AccessArticle

Constant Power Factor Model of DFIG-Based Wind Turbine for Steady State Load Flow Studies

by Rudy Gianto

Energies 2022, 15(16), 6077; https://doi.org/10.3390/en15166077 - 22 Aug 2022

Cited by 4 | Viewed by 1595

Abstract

DFIG (Doubly Fed Induction Generator)-based WPP (Wind Power Plant) is the most popular type of wind-driven electric power generation configuration. The main reason for its popularity is that the DFIG system can capture wind energy more optimally than other WPP configurations. Due to [...] Read more.

DFIG (Doubly Fed Induction Generator)-based WPP (Wind Power Plant) is the most popular type of wind-driven electric power generation configuration. The main reason for its popularity is that the DFIG system can capture wind energy more optimally than other WPP configurations. Due to the increasing penetration of WPP in power systems, the need to study its impact on power system performance is becoming increasingly important. To enable such a study to be conducted properly, the first and probably the most crucial step is to model all system components (including the WPP). This paper proposes a new steady state model of DFIG-based WPP for load flow analysis. The proposed model is derived based on DFIG power formulas (i.e., DFIG stator and rotor power formulas). The model in the present work is simple and can easily be incorporated into load flow analysis. Representation of the DFIG in both sub-synchronous and super-synchronous conditions can be carried out by using a single mathematical model. Furthermore, since DFIG can be operated at a constant power factor (i.e., unity, leading, or lagging power factor), this important feature is also considered in the proposed model development. Full article

(This article belongs to the Special Issue Power System Simulation and Modeling)

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21 pages, 4658 KiB

Open AccessReview

Main Trends and Research Directions in Hydrogen Generation Using Low Temperature Electrolysis: A Systematic Literature Review

by Cristina Hora, Florin Ciprian Dan, Nicolae Rancov, Gabriela Elena Badea and Calin Secui

Energies 2022, 15(16), 6076; https://doi.org/10.3390/en15166076 - 22 Aug 2022

Cited by 8 | Viewed by 2183

Abstract

Hydrogen (H₂) is the most abundant element in the universe and it is also a neutral energy carrier, meaning the environmental effects of using it are strictly related to the effects of creating the means of producing of that amount of [...] Read more.

Hydrogen (H₂) is the most abundant element in the universe and it is also a neutral energy carrier, meaning the environmental effects of using it are strictly related to the effects of creating the means of producing of that amount of Hydrogen. So far, the H₂ generation by water electrolysis research field did not manage to break the efficiency barrier in order to consider H₂ production as a technology that sustains financially its self-development. However, given the complexity of this technology and the overall environmental impacts, an up-to-date research and development status review is critical. Thus, this study aims to identify the main trends, achievements and research directions of the H₂ generation using pure and alkaline water electrolysis, providing a review of the state of the art in the specific literature. Methods: In order to deliver this, a Systematic Literature Review was carried out, using PRISMA methodology, highlighting the research trends and results in peer review publish articles over more than two years (2020–2022). Findings: This review identifies niches and actual status of the H₂ generation by water and alkaline water electrolysis and points out, in numbers, the boundaries of the 2020–2022 timeline research. Full article

(This article belongs to the Collection Hydrogen Energy Reviews)

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13 pages, 3665 KiB

Open AccessArticle

The Design and Optimization of Ground-Side Coils for Dynamic Wireless Power Transfer Considering Coupling Variations

by Wenbo Wang, Junjun Deng, Zhenpo Wang and Shuo Wang

Energies 2022, 15(16), 6075; https://doi.org/10.3390/en15166075 - 22 Aug 2022

Viewed by 1182

Abstract

Dynamic wireless power transfer (DWPT) has attracted widespread attention for its charging flexibility; short-segmented DWPT systems are more suitable for EV charging scenarios because of their higher charging efficiency and lower electromagnetic radiation, compared to long-track DWPT systems. For short-segmented DWPT systems, the [...] Read more.

Dynamic wireless power transfer (DWPT) has attracted widespread attention for its charging flexibility; short-segmented DWPT systems are more suitable for EV charging scenarios because of their higher charging efficiency and lower electromagnetic radiation, compared to long-track DWPT systems. For short-segmented DWPT systems, the structural design of the ground-side coil affects the coupling characteristics of the system, while simultaneously the electric vehicle driving speed and coil arrangement also cause coupling variations, and this will inevitably have an impact on the system’s performance. Therefore, this paper demonstrates the coupler design of a short-segmented system for electric vehicles, focusing on the optimization of ground-side coil. The coupling variations causing by driving speed of EV and coil arrangement are taken into account. Considering the tradeoffs and restrictions, a multi-objective optimization process of coils in DWPT systems is proposed based on the Pareto optimizing method, with three objectives: transfer power, high efficiency and low cost. A reasonable optimal solution is selected from the Pareto front to verify the optimizing method through a constructed prototype. Full article

(This article belongs to the Special Issue High-Power and Efficient Wireless Charging for Electrified Transportation)

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15 pages, 4407 KiB

Open AccessArticle

A Rapid Solver for the Prediction of Flow-Field of High-Speed Vehicle Moving in a Tube

by Mohammed Abdulla and Khalid A. Juhany

Energies 2022, 15(16), 6074; https://doi.org/10.3390/en15166074 - 22 Aug 2022

Cited by 3 | Viewed by 1304

Abstract

High-speed vehicles traveling in a tube with pressures similar to those experienced by aircraft at their maximum altitude are presented. Although the concept resembles Hyperloop, the pressure level investigated here is much higher and safer than that suggested by Hyperloop, and, therefore, the [...] Read more.

High-speed vehicles traveling in a tube with pressures similar to those experienced by aircraft at their maximum altitude are presented. Although the concept resembles Hyperloop, the pressure level investigated here is much higher and safer than that suggested by Hyperloop, and, therefore, the system design is markedly different. Calculating a vehicle’s aerodynamic performance in the initial design stages requires low-budget computational tools to enable iterative design processes. This study presents an algorithm for rapid flow-field prediction based on a one-dimensional Reimann solution, including viscosity and heat transfer effects. The flow-field is divided into near- and far-fields, where the near-field represents the solution directly around the vehicle. The far-field demonstrates the impact of the vehicle’s motion on the vehicle’s flow-field upstream and downstream. Two-dimensional URANS models are compared to the current numerical scheme. The developed algorithm analyzes the flow-field and the propagation of pressure waves along the tube to simulate the vehicle’s movement. The one-dimensional model shows the robustness and predictability of the near and far flow-fields. The results from the developed scheme provide good agreement, with less than a few percent deviations, compared to CFD simulations but with significantly lower computational resources. Full article

(This article belongs to the Special Issue Hyperloop and Associated Technologies)

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15 pages, 798 KiB

Open AccessArticle

A Method for Grading the Hidden Dangers of Urban Gas Polyethylene Pipelines Based on Improved PLC Methods

by Yunlong Wang, Zhiting Liu, Xinru Huang, Haizhou Lv, Yun Wu and Kai Zhou

Energies 2022, 15(16), 6073; https://doi.org/10.3390/en15166073 - 22 Aug 2022

Cited by 2 | Viewed by 1325

Abstract

The classification of hidden dangers in urban gas pipelines plays a vital role in the smooth operation of urban gas pipelines and in solving the problem of hidden safety dangers in urban gas pipelines. In recent years, the number and proportion of polyethylene [...] Read more.

The classification of hidden dangers in urban gas pipelines plays a vital role in the smooth operation of urban gas pipelines and in solving the problem of hidden safety dangers in urban gas pipelines. In recent years, the number and proportion of polyethylene (PE) pipelines in urban gas pipelines are increasing day by day, but the current classification of hidden dangers in urban gas pipelines is still based on steel pipelines, and the classification method is highly subjective. Therefore, this paper proposes an improved PLC method that integrates the use of a risk matrix and compensation coefficient to solve the problem of grading the hidden dangers of PE pipelines of urban gas. The improved PLC method is based on the failure database of urban gas PE pipelines to obtain the vulnerability and severity of consequences when determining the initial level of hidden dangers, and the compensation coefficient is modified according to regional vulnerability, ease of rectification, condition around the pipeline, positioning technology, leak detection technology, and emergency ability, which can effectively reduce the subjectivity of hidden danger classification. Using the improved PLC method to classify urban gas pipelines for hidden dangers can provide pipeline operating companies with a basis for decision making in the process of hidden danger disposal and effectively reduce pipeline safety risks. Full article

(This article belongs to the Special Issue Advanced Regulations and Strategies for Energy Management and Investment)

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16 pages, 9769 KiB

Open AccessFeature PaperArticle

Improving Transient Stability of a Synchronous Generator Using UIPC with a Unified Control Scheme

by Saeed Ghafouri, Mohammad Ali Hajiahmadi, Mehdi Firouzi, Gevork B. Gharehpetian, Saleh Mobayen and Paweł Skruch

Energies 2022, 15(16), 6072; https://doi.org/10.3390/en15166072 - 22 Aug 2022

Cited by 1 | Viewed by 1630

Abstract

In this paper, the Unified Interphase Power Controller (UIPC) is utilized to protect the synchronous generator in case of faults occurring in the transmission system. The UIPC not only maintains the generator’s stability by keeping its load angle within safe operational limits but [...] Read more.

In this paper, the Unified Interphase Power Controller (UIPC) is utilized to protect the synchronous generator in case of faults occurring in the transmission system. The UIPC not only maintains the generator’s stability by keeping its load angle within safe operational limits but also prevents high-amplitude currents from flowing through the stator windings. This also allows for more loading on the generator without compromising the system’s stability. Moreover, utilization of the UIPC improves the LVRT capability of the generator by injecting reactive power at the faulted location. Additionally, a novel unified control scheme is proposed for the UIPC that enhances its performance by omitting the necessity of fault detection algorithms. To evaluate the performance of the proposed controller and the efficacy of the UIPC in protecting the synchronous generator under the faults, simulations have been conducted in a MATLAB/Simulink environment. A test grid was developed comprising a synchronous generator, transmission line model, UIPC, and an infinite grid representing the Point of Common Coupling (PCC), and three fault scenarios have been implemented in the transmission system. The comparative analysis of simulation results demonstrates the capability and efficacy of UIPC in isolating the synchronous generator from the faulted location, which in turn not only enhances transient stability of the generator, but also protects generator windings from detrimental faults currents. Moreover, according to the results, UIPC also contributes to recovering the voltage dip of the fault location via injecting reactive power. Full article

(This article belongs to the Special Issue Advances in Power System Analysis and Control)

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19 pages, 6280 KiB

Open AccessArticle

Effects of Curing Pressure on the Long-Term Strength Retrogression of Oil Well Cement Cured under 200 °C

by Hongtao Liu, Jiankun Qin, Bo Zhou, Zhongfei Liu, Zhongtao Yuan, Zhi Zhang, Zhengqing Ai, Xueyu Pang and Xiaolin Liu

Energies 2022, 15(16), 6071; https://doi.org/10.3390/en15166071 - 21 Aug 2022

Cited by 11 | Viewed by 1685

Abstract

The influences of curing pressure on the physical and mechanical property development of oil well cement during long-term curing were studied. Five silica-enriched cement slurries designed without and with reinforcement materials (latex fiber and nano-graphene) were autoclaved at 200 °C under two different [...] Read more.

The influences of curing pressure on the physical and mechanical property development of oil well cement during long-term curing were studied. Five silica-enriched cement slurries designed without and with reinforcement materials (latex fiber and nano-graphene) were autoclaved at 200 °C under two different pressures. The low pressure (50 MPa) curing was conducted for 2, 60, 90 and 180 days; the high pressure (150 MPa) curing was conducted for 2 and 360 days. The physical and mechanical properties of set cement were characterized by compressive strength, Young’s modulus, and water/gas permeability; the mineral composition and microstructure were determined by X-ray diffraction, mercury intrusion porosimetry, thermogravimetry and scanning electron microscope. Test results showed that high pressure (150 MPa) curing led to a more compact microstructure, which reduced the rate of strength retrogression in the long term. Samples with reinforcement materials, especially the latex fiber, showed higher compressive strength, Young’s modulus and lower permeability during long-term curing at both pressures. Full article

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19 pages, 1608 KiB

Open AccessArticle

Tail Dependency and Risk Spillover between Oil Market and Chinese Sectoral Stock Markets—An Assessment of the 2013 Refined Oil Pricing Reform

by Jiliang Sheng, Juchao Li and Jun Yang

Energies 2022, 15(16), 6070; https://doi.org/10.3390/en15166070 - 21 Aug 2022

Cited by 3 | Viewed by 1365

Abstract

The Chinese refined oil pricing reform in 2013 has brought its refined oil price to be more aligned with the international oil price, helping to mitigate prior distorted pricing mechanisms. Its impact on the correlation, tail risks, and spillover effects between the international [...] Read more.

The Chinese refined oil pricing reform in 2013 has brought its refined oil price to be more aligned with the international oil price, helping to mitigate prior distorted pricing mechanisms. Its impact on the correlation, tail risks, and spillover effects between the international crude oil market and Chinese sectoral stock markets warrants empirical assessments. Time-varying copula models and conditional VaR (CoVaR) are employed to examine the correlation between the international oil market and Chinese sectoral stock indexes before and after the 2013 pricing reform, as well as the tail risk and spillover effects of the extreme and moderate oil markets. The results show that: (1) the correlation between the oil market and all 11 Chinese stock sectors is positive both before and after the reform, but the correlation is weaker after the reform than before; (2) The downside tail risk of the extreme and moderate oil markets to most Chinese stock market sectors, and the upside tail risk of the moderate oil market to most stock sectors are lower after the reform; (3) Tail risk spillover effects of extreme oil market on all sectors exist before and after the reform; (4) The upside tail risk spillover effects of moderate oil market exist in most sectors before the reform, but they almost all disappear after the reform. The downside risk spillover effects of the moderate oil market do not exist before or after the reform. The findings provide valuable references for portfolio management and future policy update. Full article

(This article belongs to the Section C: Energy Economics and Policy)

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