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Energies, Volume 12, Issue 23 (December-1 2019)

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Cover Story (view full-size image) Industrial food waste can be a valuable resource for anaerobic co-digestion due to its homogeneity [...] Read more.
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Open AccessArticle
Characteristics and Control of Mining Induced Fractures above Longwall Mines Using Backfilling
Energies 2019, 12(23), 4604; https://doi.org/10.3390/en12234604 - 03 Dec 2019
Abstract
Water conservation in mining is the key to solving the conflict between coal resource exploitation and ecological environment protection, especially in arid and semi-arid mining areas. Continuous excavation and continuous backfilling (CECB) in longwall mining is an important method to realize water conservation [...] Read more.
Water conservation in mining is the key to solving the conflict between coal resource exploitation and ecological environment protection, especially in arid and semi-arid mining areas. Continuous excavation and continuous backfilling (CECB) in longwall mining is an important method to realize water conservation mining. Considering the different boundary conditions of the main roof stress in different mining phases, the mechanical models of clamped–clamped beam, continuous beam, and elastic foundation beam among filling body, main roof, and strata are established. Furthermore, the spatio-temporal evolution mechanisms of mining-induced fractures (MIF) are studied. It is found that there is a hyperbolic function relationship between MIF and the mining roadway (MR) filling percentages. Based on mining the XV coal seam under CECB in the Wangtaipu Coal Mine, the distribution patterns of MIF are studied. It is concluded that the distribution pattern is an isosceles trapezoid with the moving angle of overlying strata as the bottom angle, and the upper and lower boundary of MIF as the two parallel sides. Based on the influence coefficient of MR filling percentages on MIF, the curve of the MIF height is divided into three ranges, which include the stability control range, the critical range, and the lost control range. The controlling effects of MR filling percentages are studied, and the calculation expression of the MIF height in the stability control range is given. In engineering practice, 90% MR filling percentage is used for CECB. The MIF height is about 3.0 times of mining height, and the main roof beam is not broken. The water-resisting property of aquiclude III is not destroyed, thus, the mining does not adversely impact the water. The results provide theories and practices for controlling MIF under CECB in the conditions of extremely close distance aquifers. Full article
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Open AccessReview
Transmission-Scale Battery Energy Storage Systems: A Systematic Literature Review
Energies 2019, 12(23), 4603; https://doi.org/10.3390/en12234603 - 03 Dec 2019
Abstract
When the transmission capacity of an electrical system is insufficient to adequately serve customer demand, the transmission system is said to be experiencing congestion. More transmission lines can be built to increase capacity. However, transmission congestion typically only occurs during periods of peak [...] Read more.
When the transmission capacity of an electrical system is insufficient to adequately serve customer demand, the transmission system is said to be experiencing congestion. More transmission lines can be built to increase capacity. However, transmission congestion typically only occurs during periods of peak demand, which occur just a few times per year; capitol-intensive investments in new transmission capacity address problems that occur infrequently. Alternative solutions to alleviated transmission congestion have been devised, including generation curtailment, demand response programs, and various remedial action schema. Though not currently a common solution, battery energy storage systems can also provide transmission congestion relief. Technological and market trends indicate the growing production capacity of battery energy storage systems and decreasing prices, which indicate the technology may soon become a viable option for providing congestion relief. Batteries can provide multiple ancillary services, and so can concurrently provide value through multiple revenue streams. In this manuscript, the authors present a systematic review of literature, technology, regulations, and projects related to the use of battery energy storage systems to provide transmission congestion relief. Full article
Open AccessArticle
New Curved Reflectors for Significantly Enhanced Solar Power Generation in Four Seasons
Energies 2019, 12(23), 4602; https://doi.org/10.3390/en12234602 - 03 Dec 2019
Abstract
A new curved-type reflector for solar power generation is proposed. By adopting the curved-type reflector between consecutive solar panel arrays, all incoming sunlight can be utilized and thus, the generated power is significantly increased. Furthermore, the proposed curved-type reflector can be generally used [...] Read more.
A new curved-type reflector for solar power generation is proposed. By adopting the curved-type reflector between consecutive solar panel arrays, all incoming sunlight can be utilized and thus, the generated power is significantly increased. Furthermore, the proposed curved-type reflector can be generally used in four seasons regardless of the altitude or angle of the installation environment. The optimum design rule for the curved-reflector, comparing to a plane-type reflector, is completely developed in this paper. A new solar cell configuration best fit for the proposed curved-reflector is also provided. Experimental results showed that the curved-type reflector improves the spatial average solar power by 61% compared to no reflector case, which is even 11% higher than the plane-type reflector. Reflectors, especially curved-type reflectors, are found to be one of promising solutions for highly efficient solar power generation. Full article
(This article belongs to the Section Solar Energy and Photovoltaic Systems)
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Open AccessArticle
PV Module-Level CHB Inverter with Integrated Battery Energy Storage System
Energies 2019, 12(23), 4601; https://doi.org/10.3390/en12234601 - 03 Dec 2019
Abstract
In this paper, a photovoltaic (PV) module-level Cascaded H-Bridge (CHB) inverter with an integrated Battery Energy Storage System (BESS) is proposed. The advantages and drawbacks of the CHB circuit architecture in distributed PV generation systems are highlighted. The main benefits are related to [...] Read more.
In this paper, a photovoltaic (PV) module-level Cascaded H-Bridge (CHB) inverter with an integrated Battery Energy Storage System (BESS) is proposed. The advantages and drawbacks of the CHB circuit architecture in distributed PV generation systems are highlighted. The main benefits are related to the higher granularity of the PV power control, which mitigates mismatch effects, thus increasing the power harvesting. Nevertheless, heavy unbalanced configurations due to the intermittent nature of PV sources need to be properly addressed. In order to smooth the PV fluctuations, a Battery Energy Storage System is used to provide both an energy buffer and coordination of power supply and demand to obtain a flat profile of the output power. In particular, by exploiting the inherent modularity of the conversion circuit, a distributed storage system is also implemented by splitting the battery into smaller units each of which represents the backup module of a single power cell of the PV CHB. The proposed design and control strategy allows overcoming the operating limits of PV CHB inverter. Simulation results, carried out on a single-phase nineteen-level PV CHB inverter, evidence the effectiveness of the proposed design and control approach to minimize the adverse impact of deep mismatch conditions, thus enabling continuous power output by compensating PV power fluctuations. Full article
(This article belongs to the Special Issue PV Systems: from Small- to Large-Scale)
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Open AccessArticle
Microgeneration of Electricity Using a Solar Photovoltaic System in Ireland
Energies 2019, 12(23), 4600; https://doi.org/10.3390/en12234600 - 03 Dec 2019
Abstract
Microgeneration of electricity using solar photovoltaic (PV) systems is a sustainable form of renewable energy, however uptake in Ireland remains very low. The aim of this study is to assess the potential of the community-based roof top solar PV microgeneration system to supply [...] Read more.
Microgeneration of electricity using solar photovoltaic (PV) systems is a sustainable form of renewable energy, however uptake in Ireland remains very low. The aim of this study is to assess the potential of the community-based roof top solar PV microgeneration system to supply electricity to the grid, and to explore a crowd funding mechanism for community ownership of microgeneration projects. A modelled microgeneration project was developed: the electricity load profiles of 68 residential units were estimated; a community-based roof top solar PV system was designed; an electricity network model, based on a real network supplying a town and its surrounding areas, was created; and power flow analysis on the electrical network for system peak and minimum loads was carried out. The embodied energy, energy payback time, GHG payback time, carbon credits and financial cost relating to the proposed solar PV system were calculated. Different crowdfunding models were assessed. Results show the deployment of community solar PV system projects have significant potential to reduce the peak demand, smooth the load profile, assist in the voltage regulation and reduce electrical losses and deliver cost savings to distribution system operator and the consumer. Full article
(This article belongs to the Special Issue Power Electronics Applications in Renewable Energy Systems)
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Open AccessReview
A Review of Energy Assessment Methodology for Water Supply Systems
Energies 2019, 12(23), 4599; https://doi.org/10.3390/en12234599 - 03 Dec 2019
Abstract
Energy assessment is one of the most important parts of water utility performance evaluation. In recent years, many methods for the evaluation of energy transformation in water distribution systems have been presented in the literature. The methods vary in terms of the scope [...] Read more.
Energy assessment is one of the most important parts of water utility performance evaluation. In recent years, many methods for the evaluation of energy transformation in water distribution systems have been presented in the literature. The methods vary in terms of the scope and aim of assessment. The main objective of this paper was to give a review and comparative analysis of the methods for assessing the energy efficiency of water supply systems, which are described in the scientific literature and technical standards. Because the energy assessment of water supply systems is performed for different purposes, it is not possible to indicate one universal method. The main aim of the research was to present and analyze the methods currently in use. The review of different performance assessment methodologies provided information on issues related to the classification of methods, energy balancing, determination of reference values for performance assessment, the use of supporting computer tools, and the importance of data reliability. Gaps, challenges, and possibilities for future research were also described. Full article
(This article belongs to the Section Energy and Environment)
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Open AccessArticle
Multi-Period Newsvendor Problem for the Management of Battery Energy Storage Systems in Support of Distributed Generation
Energies 2019, 12(23), 4598; https://doi.org/10.3390/en12234598 - 03 Dec 2019
Abstract
Stakeholders’ interests on renewable and clean energy sources experienced a huge increase in the last decades, thanks to the remarkable benefits on climate, economic, and social issues. The integration of flexible energy storage systems represents a great chance for a further increased penetration [...] Read more.
Stakeholders’ interests on renewable and clean energy sources experienced a huge increase in the last decades, thanks to the remarkable benefits on climate, economic, and social issues. The integration of flexible energy storage systems represents a great chance for a further increased penetration as they support the mitigation of renewables’ main drawbacks (i.e., stochastic behavior) and guarantee the balance between energy supply and demand enabling non-simultaneous production and consumption. The increased focus on distributed generation and storage was also of interest to the research community which investigated both the economic and technical performances of the integrated systems. The operations management branch addressed this topic, since storage devices present many similarities with traditional inventory management applied to regular commodities. At a user level, the relation between energy production and storage can be studied by analogy with inventory models. Specifically, this study presents a multi-period newsvendor model for the management and optimal sizing of a battery energy storage system installed to increase the self-consumption rate by allowing loads shifting. This work aims to extend the traditional inventory management applying its concepts to energy systems operations in order to minimize the total energy cost. A numerical study is provided to show the behavior of the model. Full article
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Open AccessTechnical Note
An SNA-DEA Prioritization Framework to Identify Critical Nodes of Gas Networks: The Case of the US Interstate Gas Infrastructure
Energies 2019, 12(23), 4597; https://doi.org/10.3390/en12234597 - 03 Dec 2019
Abstract
This paper presents a framework to identify critical nodes of a gas pipeline network. This framework calculates a set of metrics typical of the social network analysis considering the topological characteristics of the network. Such metrics are utilized as inputs and outputs of [...] Read more.
This paper presents a framework to identify critical nodes of a gas pipeline network. This framework calculates a set of metrics typical of the social network analysis considering the topological characteristics of the network. Such metrics are utilized as inputs and outputs of a (Data Envelopment Analysis) DEA model to generate a cross-efficiency index that identifies the most important nodes in the network. The framework was implemented to assess the US interstate gas network between 2013 and 2017 from both the demand and supply-side perspectives. Results emerging from the US gas network case suggest that different analysis perspectives should necessarily be considered to have a more in-depth and comprehensive view of the network capacity and performance. Full article
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Open AccessArticle
An Acoustic Source Model for Applications in Low Mach Number Turbulent Flows, Such as a Large-Scale Wind Turbine Blade
Energies 2019, 12(23), 4596; https://doi.org/10.3390/en12234596 - 03 Dec 2019
Abstract
Aerodynamic noise from wind turbine blades is one of the major hindrances for the widespread use of large-scale wind turbines generating green energy. In order to more accurately guide wind turbine blade manufacturers to optimize the blade geometry for aerodynamic noise reduction, an [...] Read more.
Aerodynamic noise from wind turbine blades is one of the major hindrances for the widespread use of large-scale wind turbines generating green energy. In order to more accurately guide wind turbine blade manufacturers to optimize the blade geometry for aerodynamic noise reduction, an acoustic model that not only understands the relation between the behavior of the sound source and the sound generation, but also accounts for the compressibility effect, was derived by rearranging the continuity and Navier–Stokes equations as a wave equation with a lump of source terms, including the material derivative and square of the velocity divergence. Our acoustic model was applied to low Mach number, weakly compressible turbulent flows around NACA0012 airfoil. For the computation of flow fields, a large-eddy simulation (LES) with the dynamic Smagorinsky subgrid scale (SGS) model and the cubic interpolated pseudo particle (CIP)-combined unified numerical procedure method were conducted. The reproduced turbulent flow around NACA0012 airfoil was in good agreement with the experimental data. For the estimation of acoustic fields, our acoustic model and classical sound source models, such as Lighthill and Powell, were performed using our LES database. The investigation suggested that the derived material derivative of the velocity divergence plays a dominant role as sound source. The distribution of the sources in our acoustic model was consistent with that of the classical sound source models. The sound pressure level (SPL) predicted based on the above-mentioned LES and our newly derived acoustic model was in reasonable agreement with the experimental data. The influence of the increase of Mach number on the acoustic field was investigated. Our acoustic source model was verified to be capable of treating the influence of Mach numbers on the acoustic field. Full article
(This article belongs to the Special Issue Numerical Simulation of Wind Turbine)
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Open AccessArticle
3D Dynamic Simulation of Heat Conduction through a Building Corner Using a BEM Model in the Frequency Domain
Energies 2019, 12(23), 4595; https://doi.org/10.3390/en12234595 - 03 Dec 2019
Abstract
This paper sets out a three-dimensional (3D) boundary element method (BEM) formulation in the frequency domain to simulate heat transfer through a point thermal bridge (PTB) at a corner in a building envelope. The main purpose was to quantify the dynamic effect of [...] Read more.
This paper sets out a three-dimensional (3D) boundary element method (BEM) formulation in the frequency domain to simulate heat transfer through a point thermal bridge (PTB) at a corner in a building envelope. The main purpose was to quantify the dynamic effect of a geometrical PTB in terms of distribution of temperatures and heat fluxes, which is useful for evaluating moisture condensation risk. The numerical model is first validated experimentally using a hot box to measure the dynamic heat behavior of a 3D timber building corner. The proposed model is then used to study the dynamic thermal bridging effect in the vicinity of a 3D concrete corner. Given the importance of the risk of condensation, this study looks at the influence of an insulating material and its position on the temperature and heat flux distribution through the PTB under steady state and dynamic conditions. Full article
(This article belongs to the Section Energy and Buildings)
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Open AccessArticle
Kinetic Study of Corn and Sugarcane Waste Oxidative Pyrolysis
Energies 2019, 12(23), 4594; https://doi.org/10.3390/en12234594 - 03 Dec 2019
Abstract
The behavior of the kinetics and pyrolysis of the corn and sugarcane waste (cob corn, husk corn, and bagasse) produced in Colombia have been evaluated in a thermobalance as a step toward their valorization, in order to recover this type of waste. For [...] Read more.
The behavior of the kinetics and pyrolysis of the corn and sugarcane waste (cob corn, husk corn, and bagasse) produced in Colombia have been evaluated in a thermobalance as a step toward their valorization, in order to recover this type of waste. For this, a kinetic model has been developed consisting of a multicomponent mechanism that seeks to describe the formation of volatile materials. This model evaluates the decomposition of hemicellulose, cellulose, and lignin from three parallel and independent reactions network. It also involves the production of other products such as fixed coal and ashes. The evaluation of the model from the kinetic parameters and the thermogravimetric curves were compared with other types of waste, in the same way the chemical characteristics of the studied waste were determined. Although the wastes of this study are completely different, it was found that the degradation behavior of the residues of this study are very similar to other lignocellulosic materials, which demonstrates again that the pyrolysis valorization is possible. Full article
(This article belongs to the Section Bio-Energy)
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Open AccessReview
Hydrogen Fuel Cell Technology for the Sustainable Future of Stationary Applications
Energies 2019, 12(23), 4593; https://doi.org/10.3390/en12234593 - 03 Dec 2019
Abstract
The climate changes that are becoming visible today are a challenge for the global research community. The stationary applications sector is one of the most important energy consumers. Harnessing the potential of renewable energy worldwide is currently being considered to find alternatives for [...] Read more.
The climate changes that are becoming visible today are a challenge for the global research community. The stationary applications sector is one of the most important energy consumers. Harnessing the potential of renewable energy worldwide is currently being considered to find alternatives for obtaining energy by using technologies that offer maximum efficiency and minimum pollution. In this context, new energy generation technologies are needed to both generate low carbon emissions, as well as identifying, planning and implementing the directions for harnessing the potential of renewable energy sources. Hydrogen fuel cell technology represents one of the alternative solutions for future clean energy systems. This article reviews the specific characteristics of hydrogen energy, which recommends it as a clean energy to power stationary applications. The aim of review was to provide an overview of the sustainability elements and the potential of using hydrogen as an alternative energy source for stationary applications, and for identifying the possibilities of increasing the share of hydrogen energy in stationary applications, respectively. As a study method was applied a SWOT analysis, following which a series of strategies that could be adopted in order to increase the degree of use of hydrogen energy as an alternative to the classical energy for stationary applications were recommended. The SWOT analysis conducted in the present study highlights that the implementation of the hydrogen economy depends decisively on the following main factors: legislative framework, energy decision makers, information and interest from the end beneficiaries, potential investors, and existence of specialists in this field. Full article
(This article belongs to the Special Issue Fuel Cell Renewable Hybrid Power Systems)
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Open AccessArticle
Sensitivity Analysis of Rock Electrical Influencing Factors of Natural Gas Hydrate Reservoir in Permafrost Region of Qilian Mountain, China
Energies 2019, 12(23), 4592; https://doi.org/10.3390/en12234592 - 03 Dec 2019
Abstract
It has been found that the relatively low abundance of gas hydrate in the Muli area of the Qilian Mountain causes gas hydrate reservoirs to have low-resistivity characteristics similar to those of low-resistivity oil and gas reservoirs. Therefore, it has great significance to [...] Read more.
It has been found that the relatively low abundance of gas hydrate in the Muli area of the Qilian Mountain causes gas hydrate reservoirs to have low-resistivity characteristics similar to those of low-resistivity oil and gas reservoirs. Therefore, it has great significance to research the main controlling factors affecting the electrical properties, and then come up a new logging identification and evaluation model for low-resistivity gas hydrate reservoirs. In this investigation, the rock samples of sandstone from gas hydrate reservoirs were scanned by CT and combined with gas hydrate distribution characteristics. The three-dimensional digital rocks with different hydrate saturation were constructed using the diffusion limited aggregation (DLA) model, and the resistivity was simulated via the finite element method. After sorting out the influencing factors of electrical characteristics, the sensitivity of the factors affecting electrical properties was evaluated using orthogonal analysis, using variance analysis and trend analysis to quantitatively evaluate the influencing factors of rock electrical sensitivity, so as to distinguish the main and secondary factors affecting rock electrical sensitivity. The results show that the sensitivity of rock electrical properties to the six influencing factors from strong to weak are: formation water salinity, water film thickness, shale content, conductive mineral content, micropores, and average coordination number. Full article
(This article belongs to the Special Issue Advances in Natural Gas Hydrates)
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Open AccessArticle
Model Predictive Control-Based Coordinated Control Algorithm with a Hybrid Energy Storage System to Smooth Wind Power Fluctuations
Energies 2019, 12(23), 4591; https://doi.org/10.3390/en12234591 - 03 Dec 2019
Abstract
Stochastically fluctuating wind power has an escalating impact on the stability of power grid operations. To smooth out short- and long-term fluctuations, this paper presents a coordinated control algorithm using model predictive control (MPC) to manage a hybrid energy storage system (HESS) consisting [...] Read more.
Stochastically fluctuating wind power has an escalating impact on the stability of power grid operations. To smooth out short- and long-term fluctuations, this paper presents a coordinated control algorithm using model predictive control (MPC) to manage a hybrid energy storage system (HESS) consisting of ultra-capacitor (UC) and lithium-ion battery (LB) banks. In the HESS-computing period, the algorithm minimizes HESS operating costs in the subsequent prediction horizon by optimizing the time constant of a flexible first-delay filter (FDF) to obtain the UC power output. In the LB-computing period, the algorithm keeps the optimal time constant of the FDF from the previous period to directly obtain the power output of the UC bank to minimize the power output of the LB bank in the next prediction horizon. A relaxation technique is deployed when the problem is unsolvable. Thus, the fluctuation mitigation requirements are fulfilled with a large probability even in extreme conditions. A state-of-charge (SOC) feedback control strategy is proposed to regulate the SOC of the HESS within its proper range. Case studies and quantitative comparisons demonstrate that the proposed MPC-based algorithm uses a lower power rating and storage capacity than other conventional algorithms to satisfy one-minute and 30-min fluctuation mitigation requirements (FMR). Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessArticle
Analysis of the Nexus of CO2 Emissions, Economic Growth, Land under Cereal Crops and Agriculture Value-Added in Pakistan Using an ARDL Approach
Energies 2019, 12(23), 4590; https://doi.org/10.3390/en12234590 - 02 Dec 2019
Abstract
The present study attempts to explore the correlation between carbon dioxide emissions (CO2 e), gross domestic product (GDP), land under cereal crops (LCC) and agriculture value-added (AVA) in Pakistan. The study exploits time-series data from 1961 to 2014 and further applies descriptive [...] Read more.
The present study attempts to explore the correlation between carbon dioxide emissions (CO2 e), gross domestic product (GDP), land under cereal crops (LCC) and agriculture value-added (AVA) in Pakistan. The study exploits time-series data from 1961 to 2014 and further applies descriptive statistical analysis, unit root test, Johansen co-integration test, autoregressive distributed lag (ARDL) model and pairwise Granger causality test. The study employes augmented Dickey–Fuller (ADF) and Phillips–Perron (PP) tests to check the stationarity of the variables. The results of the analysis reveal that there is both short- and long-run association between agricultural production, economic growth and carbon dioxide emissions in the country. The long-run results estimate that there is a positive and insignificant association between carbon dioxide emissions, land under cereal crops, and agriculture value-added. The results of the short-run analysis point out that there is a negative and statistically insignificant association between carbon dioxide emissions and gross domestic product. It is very important for the Government of Pakistan’s policymakers to build up agricultural policies, strategies and planning in order to reduce carbon dioxide emissions. Consequently, the country should promote environmentally friendly agricultural practices in order to strengthen its efforts to achieve sustainable agriculture. Full article
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Open AccessArticle
Modeling and Simulation Performance Evaluation of a Proposed Calorimeter for Testing a Heat Pump System
Energies 2019, 12(23), 4589; https://doi.org/10.3390/en12234589 - 02 Dec 2019
Abstract
The energy consumption for heating and cooling in the building sector accounts for more than one-third of total energy used worldwide. In view of that, it is important to develop energy efficient cooling and heating systems in order to conserve energy in buildings [...] Read more.
The energy consumption for heating and cooling in the building sector accounts for more than one-third of total energy used worldwide. In view of that, it is important to develop energy efficient cooling and heating systems in order to conserve energy in buildings as well as reduce greenhouse gas emissions. In both commercial and residential buildings, the heat pump has been adopted as an energy efficient technology for space heating and cooling purposes as compared to conventional air conditioning systems. However, heat pumps undergo standard testing, rating, and certification procedures to ascertain their system performance. Essentially, the calorimeter for testing heat pumps has two test chambers to serve as a heat source and heat sink to control and maintain the test conditions required to simulate the heat pump indoor and outdoor units, simultaneously. In air-to-air heat pump units, the conventional calorimeter controls the air temperature and humidity conditions in each test chamber with separate air handling units consisting of a refrigerator, heater, humidifier, and supply fan, which results in high energy consumption. In this study, using dynamic modeling and simulation, a new calorimeter for controlling air conditions in each test chamber is proposed. The performance analysis based on simulation results showed that the newly proposed calorimeter predicted at least 43% energy savings with the use of a heat recovery unit and small refrigerator capacity as compared to the conventional calorimeter that utilized a large refrigerator capacity for all the weather conditions and load capacities that we investigated. Full article
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Open AccessArticle
Exergetic, Energetic, and Quality Performance Evaluation of Paddy Drying in a Novel Industrial Multi-Field Synergistic Dryer
Energies 2019, 12(23), 4588; https://doi.org/10.3390/en12234588 - 02 Dec 2019
Abstract
The present work proposes a novel industrial multi-field synergistic dryer with a drying capacity of 3.45 t/h. The energy, exergy, and quality aspects of the drying process were studied. An energy–exergy methodology was employed to estimate the energetic and exergetic performance, heat loss [...] Read more.
The present work proposes a novel industrial multi-field synergistic dryer with a drying capacity of 3.45 t/h. The energy, exergy, and quality aspects of the drying process were studied. An energy–exergy methodology was employed to estimate the energetic and exergetic performance, heat loss characteristics and heat recovery behavior of the dryer. Additionally, the quality of the dried paddy seeds was evaluated by its crackle ratio, generation potential, and generation rate. The results showed that the overall energy and exergy efficiency ranged from 13.26% to 56.63% and 39.03% to 60.23%, respectively. The improvement potential rates of the whole system varied from the lowest 8.49 kW to the highest 15.83 kW and respectively accounted for 15.81–29.48% of the total exergy input, indicating that the performance of the dryer is acceptable. The total recovered radiant energy and radiant exergy recover rate were respectively ascertained to be 237.64 MJ and 0.26 kW. As for the quality aspect, the generation potential and generation rate of the dried paddy seeds respectively ranged from 75% to 90% and 69% to 88% while the crackle ratio of the paddy seeds was 1%, which indicated that the quality performance of the dried seed is of economic viability. Full article
(This article belongs to the Section Energy Fundamentals and Conversion)
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Open AccessArticle
Fault Ride-Through Characteristics of Small Wind Turbines
Energies 2019, 12(23), 4587; https://doi.org/10.3390/en12234587 - 02 Dec 2019
Abstract
There is significant potential for an increase in the use of kilowatt-class small wind turbines (SWTs) in Japan due to reduced limitations with respect to installation, despite their high cost. At this stage, the Japanese grid code has not been considered sufficiently with [...] Read more.
There is significant potential for an increase in the use of kilowatt-class small wind turbines (SWTs) in Japan due to reduced limitations with respect to installation, despite their high cost. At this stage, the Japanese grid code has not been considered sufficiently with respect to grid-connected SWTs, and the addition of fault ride-through (FRT) requirements for SWTs has been requested. Moreover, the FRT of SWTs is challenging to achieve owing to the low inertia constants when compared with those of large-scale wind turbines, which result in significant acceleration of the rotor speed and an increase in the input voltage of the power conditioning system (PCS) during FRT operation. In this study, FRT field tests were conducted on SWT systems against a voltage dip with a duration of ~ 1 s, and it was confirmed that the SWT systems satisfied the FRT requirements for photovoltaic (PV) systems connected to low-voltage distribution lines in Japan. The behaviors of the rotational speed of the SWTs and the PCS input voltage in an FRT operation were then analyzed, and it was noted that the increase in the PCS input voltage with the overspeed of the turbine can reach the upper limit and make the PCS cease operation, which indicates failure of the FRT. The overvoltage, therefore, requires restriction using a method such as pitch control, furling, and electrical and/or mechanical brakes. Full article
(This article belongs to the Special Issue Electrical Engineering for Sustainable and Renewable Energy)
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Open AccessArticle
A New Method to Determine the Annual Energy Output of Liquid-Based Solar Collectors
Energies 2019, 12(23), 4586; https://doi.org/10.3390/en12234586 - 02 Dec 2019
Abstract
One of the most effective ways to reduce greenhouse gas emissions is the widespread use of solar panels. A new approach to estimating the annual energy output ESC of liquid-based solar thermal collectors is presented in this paper. The main parameters of [...] Read more.
One of the most effective ways to reduce greenhouse gas emissions is the widespread use of solar panels. A new approach to estimating the annual energy output ESC of liquid-based solar thermal collectors is presented in this paper. The main parameters of the surrounding environment that affect the energy yield from solar water heating (SWH) systems are total solar radiation ES and outdoor air temperature θO. Here, two functions are developed that allow the specification of the ESC value based on these two previously determined factors. The article presents the subsequent stages that led to the development of this method. In the first step, the computer model of a solar domestic hot water (SDHW) system was built in the EnergyPlus environment. An object of the modeling procedure was the real solar installation located at Bialystok University of Technology. The database containing the annual energy output of the solar collectors, total solar radiation, and outdoor air temperature was developed based on the results of simulations made for 24 Polish cities. Then, the multiple regression method (implemented in Statistica v.13.1) was used to correlate these variables. Two contour graphs were also developed to determine the energy yield from solar collectors. Full article
(This article belongs to the Special Issue Innovations-Sustainability-Modernity-Openness in Energy Research 2019)
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Open AccessArticle
Photovoltaic-Driven SiC MOSFET Circuit Breaker with Latching and Current Limiting Capability
Energies 2019, 12(23), 4585; https://doi.org/10.3390/en12234585 - 02 Dec 2019
Abstract
This paper introduces a Solid State Circuit Breaker with Latching and Current Limiting capabilities for DC distribution systems. The proposed circuit uses very few electronic parts and it is fully analog. A SiC N-MOSFET driven by a photovoltaic driver and a maximum current [...] Read more.
This paper introduces a Solid State Circuit Breaker with Latching and Current Limiting capabilities for DC distribution systems. The proposed circuit uses very few electronic parts and it is fully analog. A SiC N-MOSFET driven by a photovoltaic driver and a maximum current detector circuit are the core elements of the system. This work details circuit operation under different conditions and includes experimental validation at 1 kVdc. Wide versatility, highly configurable, and very fast response, less than 1 µs in the case of short-circuit, are the most remarkable outcomes. Full article
(This article belongs to the Special Issue Micro Grid Protection)
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Open AccessArticle
Study on Operating Strategy of Electric–Gas Combined System Considering the Improvement of Dispatchability
Energies 2019, 12(23), 4584; https://doi.org/10.3390/en12234584 - 02 Dec 2019
Abstract
The uncertainty of distributed energy (DG) and load in the electric–gas combined system (EGCS) requires EGCS to have higher dispatching capacity. A novel strategy is introduced in this paper to operate EGCS considering dispatchability evaluation indexes in order to improve the dispatchability of [...] Read more.
The uncertainty of distributed energy (DG) and load in the electric–gas combined system (EGCS) requires EGCS to have higher dispatching capacity. A novel strategy is introduced in this paper to operate EGCS considering dispatchability evaluation indexes in order to improve the dispatchability of EGCS. Firstly, the paper describes the physical architecture of EGCS and its main devices. Based on the typical structure of EGCS, the main coupling modes between the two networks are analyzed and summarized, and a power flow model of deep coupling EGCS is established. Then, it proposes a unified quantitative modeling method of dispatchability, and qualitatively analyzes the dispatchability capability of different types of resources in the system through the definition, connotation, and multi-dimensional attributes of EGCS dispatchability. In order to characterize the strength of the overall dispatchability of EGCS, two evaluation indexes, upward/downward dispatchability margin, are proposed. The case study validates the applicability of the proposed dispatchability indexes through simulation. The uncertainties existing in various sources, namely networks and loads of EGCS, the output power of wind farms, and photovoltaic plants, are analyzed emphatically through actual data of a certain area. The EGCS economic dispatching model is established by considering the DG output prediction errors, introducing the expected penalty term of insufficient dispatchability into the objective function, and calculating the dispatchability margin through the simulation model to quantitatively analyze the dispatchability capability of the system. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessFeature PaperArticle
Experimental and Numerical Analysis of the Effect of Vortex Generator Installation Angle on Flow Separation Control
Energies 2019, 12(23), 4583; https://doi.org/10.3390/en12234583 - 02 Dec 2019
Abstract
In order to explore the effect of the installation angle of vortex generator (VG) on boundary-layer flow control, the vortex characteristics of plate VG and their effect on the aerodynamic characteristics of an airfoil was studied numerically and using wind tunnel experiments. The [...] Read more.
In order to explore the effect of the installation angle of vortex generator (VG) on boundary-layer flow control, the vortex characteristics of plate VG and their effect on the aerodynamic characteristics of an airfoil was studied numerically and using wind tunnel experiments. The effects of five VG installation angles (β) of 10°, 15°, 20°, 25°, and 30° on the characteristics of vortices were studied. The results show that the strength of vortices on the leeward side of VG increases with an increased installation angle until, eventually, the vortex core breaks down. During the downstream development of the VG leading-edge separation vortices, these vortices deviate in the radial direction. The larger the installation angle, the larger this deviation distance in the radial direction becomes. The effects of installation angle on the aerodynamic performance of airfoils were studied in a wind tunnel using the same five VG installation angles. The results show that VG can delay flow separation on the airfoil suction surface, thereby increasing lift and reducing drag. The stall angle of the airfoil with VG was increased by 10°. When the installation angle of the VG was 20°, the maximum lift coefficient of airfoil increased by 48.77%. For an airfoil angle of attack (AoA) of 18°, the drag of the airfoil decreased by 88%, and the lift-drag ratio increased by 1146.04%. Considering the best overall distribution of lift-drag ratio, the positive effect of the VG was found to be when β = 20° and the worst VG effectiveness was observed at β = 30°. Full article
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Open AccessArticle
Experimental Investigation of a 10 MW Prototype Kaplan Turbine during Start-Up Operation
Energies 2019, 12(23), 4582; https://doi.org/10.3390/en12234582 - 02 Dec 2019
Abstract
An increase in the start/stop cycles of hydraulic turbines due to the penetration of intermittent renewable energy sources is important. Hydraulic instabilities that occur in hydraulic turbines during start/stops may cause structural issues in the turbine components. High-stress fluctuations on the runner blades [...] Read more.
An increase in the start/stop cycles of hydraulic turbines due to the penetration of intermittent renewable energy sources is important. Hydraulic instabilities that occur in hydraulic turbines during start/stops may cause structural issues in the turbine components. High-stress fluctuations on the runner blades are expected during start-ups due to the unsteady pressure loading on the runner blades. This paper presents experiments performed on a 10 MW prototype Kaplan turbine at the Porjus Hydropower Center during a start-up cycle. Synchronized unsteady pressure and strain measurements on a runner blade and axial, bending (in two directions) and torsion strain measurements on the shaft were performed. In addition, the general parameters of the turbine (e.g., rotational speed, guide vane opening and runner blade angle) were acquired. Low-frequency fluctuations (0–15 Hz) were observed in the pressure data on the runner blade after opening the guide vanes from the completely closed position. A higher strain value was observed on the strain gauges installed on the runner blade near the hub (200–500 μ m / m ) compared to the ones near the shroud at the leading and trailing edge. The strain fluctuation level on the shaft decreased after loading the generator by further opening the guide vanes. Higher fluctuations were observed in the torsion strain compared to axial and bending strain. In addition, the torsion strain peak-to-peak value reached 12 times its corresponding value at 61% guide vane opening. Full article
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Open AccessArticle
Analyzing Oil Price Shocks and Exchange Rates Movements in Korea using Markov Regime-Switching Models
Energies 2019, 12(23), 4581; https://doi.org/10.3390/en12234581 - 01 Dec 2019
Abstract
Korea imports all of its crude oil, and is the world's fifth largest oil importing country. We analyze the effects of oil prices, interest rates, consumer price indexes (CPIs), and industrial production indexes (IPIs) on the regime shift behavior of the Korean exchange [...] Read more.
Korea imports all of its crude oil, and is the world's fifth largest oil importing country. We analyze the effects of oil prices, interest rates, consumer price indexes (CPIs), and industrial production indexes (IPIs) on the regime shift behavior of the Korean exchange rates against the USA from January 1991 to March 2019. We use the Markov regime switching model (MRSM) to detect the regime shift behavior of the movements of Korean exchange rates. In order to select the optimal MRSM, we fit a total of 30 models considering four explanatory variables. The selected model based on Akaike information criteria (AIC) and maximum log likelihood (MLL) includes the log-differentials of oil prices, the log-differentials of CPIs compared to those of the US, and its own auto-regressive terms. Based on the selected MRSM model, throughout all markets, we find evidence to support the existence of two distinct regimes: a stable regime with low-volatility, and an unstable regime with high-volatility. The regime with high-volatility includes the Asian financial crisis of 1997 and the global financial crisis of 2008–2009 in the Korean exchange rates market. In the regime with low-volatility, the Korean exchange rates are not significantly influenced by any of the explanatory variables, except for its own auto-regressive terms. In the regime with high-volatility, the Korean exchange rates are significantly influenced by the CPIs and oil prices. The transition probability from the regime with low-volatility to the regime with high-volatility is about ten times that of the opposite case. Full article
(This article belongs to the Special Issue Energy Markets and Economics Ⅱ)
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Open AccessArticle
Comprehensive Evaluation of the New Energy Power Generation Development at the Regional Level: An Empirical Analysis from China
Energies 2019, 12(23), 4580; https://doi.org/10.3390/en12234580 - 01 Dec 2019
Abstract
In order to build an environment-friendly society and realize the coordinated allocation and effective utilization of resources and finally achieve China’s energy supply security, it is imperative to vigorously develop new energy sources. This study establishes a four-level new energy power generation (NEPG) [...] Read more.
In order to build an environment-friendly society and realize the coordinated allocation and effective utilization of resources and finally achieve China’s energy supply security, it is imperative to vigorously develop new energy sources. This study establishes a four-level new energy power generation (NEPG) development index system from multiple dimensions. Taking the installed capacity and generating capacity of China’s NEPG in 2016 and 2017 as samples, we used the improved entropy method, to analyze the development of different types of NEPG among 31 provinces from three aspects: absolute value, relative value, and incremental value. Finally, we comprehensively evaluated the NEPG development in each province. The empirical analysis shows that the spatial distribution of NEPG development in China is uneven, the growth rate is different, the development gap is obvious, and the development efficiency is quite different. Full article
(This article belongs to the Section Sustainable Energy)
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Open AccessArticle
Cooling Technologies for High Power Density Electrical Machines for Aviation Applications
Energies 2019, 12(23), 4579; https://doi.org/10.3390/en12234579 - 01 Dec 2019
Abstract
This paper is aimed at giving an overview of possible cooling technologies for electrical machines and their assessment for aviation applications, e.g., fan or propeller drives. The most important demand for aircraft is the minimization of the drive system weight comprising electrical machine, [...] Read more.
This paper is aimed at giving an overview of possible cooling technologies for electrical machines and their assessment for aviation applications, e.g., fan or propeller drives. The most important demand for aircraft is the minimization of the drive system weight comprising electrical machine, power electronics, and the cooling system. The potential of aluminum winding an overview about several cooling technologies with the Rankine or Brayton cycle or utilizing the phase change of the cooling fluid is given. As an alternative approach, the cooling structure inside the machine is studied. A very interesting potential was discovered with direct slot cooling (DSC) removing the heat where it is produced and, thus, simplifying the cooling system effort and its weight. Since it is one of the most promising approaches, this cooling method is studied in depth. Furthermore, it can also be combined with one of the cooling technologies discussed above. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessFeature PaperArticle
Threshold Pore Pressure Gradients in Water-Bearing Tight Sandstone Gas Reservoirs
Energies 2019, 12(23), 4578; https://doi.org/10.3390/en12234578 - 01 Dec 2019
Abstract
Tight gas reservoirs commonly occur in clastic formations having a complex pore structure and a high water saturation, which results in a threshold pressure gradient (TPG) for gas seepage. The micropore characteristics of a tight sandstone gas reservoir (Tuha oilfield, Xinjiang, China) were [...] Read more.
Tight gas reservoirs commonly occur in clastic formations having a complex pore structure and a high water saturation, which results in a threshold pressure gradient (TPG) for gas seepage. The micropore characteristics of a tight sandstone gas reservoir (Tuha oilfield, Xinjiang, China) were studied, based on X-ray diffraction, scanning electron microscopy and high pressure mercury testing. The TPG of gas in cores of the tight gas reservoir was investigated under various water saturation conditions, paying special attention to core permeability and water saturation impact on the TPG. A mathematical TPG model applied a multiple linear regression method to evaluate the influence of core permeability and water saturation. The results show that the tight sandstone gas reservoir has a high content of clay minerals, and especially a large proportion of illite–smectite mixed layers. The pore diameter is distributed below 1 micron, comprising mesopores and micropores. With a decrease of reservoir permeability, the number of micropores increases sharply. Saturated water tight cores show an obvious non-linear seepage characteristic, and the TPG of gas increases with a decrease of core permeability or an increase of water saturation. The TPG model has a high prediction accuracy and shows that permeability has a greater impact on TPG at high water saturation, while water saturation has a greater impact on TPG at low permeability. Full article
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Open AccessFeature PaperArticle
Data-Driven Distributionally Robust Stochastic Control of Energy Storage for Wind Power Ramp Management Using the Wasserstein Metric
Energies 2019, 12(23), 4577; https://doi.org/10.3390/en12234577 - 01 Dec 2019
Abstract
The integration of wind energy into the power grid is challenging because of its variability, which causes high ramp events that may threaten the reliability and efficiency of power systems. In this paper, we propose a novel distributionally robust solution to wind power [...] Read more.
The integration of wind energy into the power grid is challenging because of its variability, which causes high ramp events that may threaten the reliability and efficiency of power systems. In this paper, we propose a novel distributionally robust solution to wind power ramp management using energy storage. The proposed storage operation strategy minimizes the expected ramp penalty under the worst-case wind power ramp distribution in the Wasserstein ambiguity set, a statistical ball centered at an empirical distribution obtained from historical data. Thus, the resulting distributionally robust control policy presents a robust ramp management performance even when the future wind power ramp distribution deviates from the empirical distribution, unlike the standard stochastic optimal control method. For a tractable numerical solution, a duality-based dynamic programming algorithm is designed with a piecewise linear approximation of the optimal value function. The performance and utility of the proposed method are demonstrated and analyzed through case studies using the wind power data in the Bonneville Power Administration area for the year 2018. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessArticle
Variable Renewable Energy and Market Design: New Products and a Real-World Study
Energies 2019, 12(23), 4576; https://doi.org/10.3390/en12234576 - 30 Nov 2019
Abstract
Most existing energy markets (EMs) were not designed to take into account an active participation of variable renewable energy (VRE). This situation results typically in imbalances and substantial costs in balancing markets. Such costs are reflected both in the energy and the VRE [...] Read more.
Most existing energy markets (EMs) were not designed to take into account an active participation of variable renewable energy (VRE). This situation results typically in imbalances and substantial costs in balancing markets. Such costs are reflected both in the energy and the VRE parts of the consumer tariffs. Both appropriate market products and new elements of market design may largely facilitate the large-scale integration of VRE in EMs. Accordingly, this article presents a new bilateral energy contract and introduces two new marketplaces that can contribute to reduce the imbalances resulting from VRE producers. It also presents a study conducted with the help of an agent-based tool, called MATREM. The results indicate a significant decrease in the imbalances and the associated costs. Full article
(This article belongs to the Special Issue Multi-Agent Energy Systems Simulation)
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Open AccessArticle
Flow Angularity Investigations in an Automotive Slotted Wall Wind Tunnel
Energies 2019, 12(23), 4575; https://doi.org/10.3390/en12234575 - 30 Nov 2019
Abstract
The Volvo Cars aerodynamic wind tunnel has had a vortical flow angularity pattern in the test section since its original commissioning in 1986. The vortical flow nature persisted after an upgrade in 2006, when the fan was replaced and a moving ground system [...] Read more.
The Volvo Cars aerodynamic wind tunnel has had a vortical flow angularity pattern in the test section since its original commissioning in 1986. The vortical flow nature persisted after an upgrade in 2006, when the fan was replaced and a moving ground system was introduced. It has been hypothesized that the cause for this flow angularity pattern was leakages around the heat exchanger installed in the settling chamber. The present paper tests this hypothesis by measuring the flow angularity in the test section before and after sealing the leakages. The findings show that the leakage path around the heat exchanger does not influence the flow angularity, and that the current pattern is different compared to the commissioning after the upgrade. This prompted an investigation of the influence from the turbulence screens, which were changed after the upgrade commissioning. These investigations indicate that the probable cause of the vortical flow angularity pattern is residual swirl from the fan. Force measurements on a reference car with and without extra induced flow angularity show that the flow angles measured in the tunnel for regular operation are most likely small enough to not have a significant effect on the measured aerodynamic forces. Full article
(This article belongs to the Special Issue Recent Advances in Vehicle Aerodynamics)
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