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Volume 13, March-2

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Energies, Volume 13, Issue 7 (April-1 2020) – 183 articles

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Open AccessArticle
Heat Transport Analysis in Rectangular Shields Using the Laplace and Poisson Equations
Energies 2020, 13(7), 1714; https://doi.org/10.3390/en13071714 (registering DOI) - 03 Apr 2020
Abstract
In the design of a building envelope, there is the issue of heat flow through the partitions. In the heat flow process, we distinguish steady and dynamic states in which heat fluxes need to be obtained as part of building physics calculations. This [...] Read more.
In the design of a building envelope, there is the issue of heat flow through the partitions. In the heat flow process, we distinguish steady and dynamic states in which heat fluxes need to be obtained as part of building physics calculations. This article describes the issue of determining the size of those heat fluxes. The search for the temperature field in a two-dimensional problem is common in building physics and heat exchange in general. Both numerical and analytical methods can be used to obtain a solution. Two methods were dealt with, the first of which was used to obtain the solution in the steady state and the other in the transient. In the steady state a method of initial functions, the basics of which were given by W.Z. Vlasov and A.Y. Lur’e was adopted. Originally MIF was used for analysis of the loads of a flat elastic medium. Since then it was used for solving concrete beams, plates and composite materials problems. Polynomial half-reverse solutions are used in the theory of a continuous medium. Here solutions were obtained by the direct method. As a result, polynomial forms of the considered temperature field were obtained. A Cartesian coordinate system and rectangular shape of the plate were assumed. The problem is governed by the Laplace equation in the steady state and Poisson in the transient state. Boundary conditions in the form of temperature (τ(x), t(y)) or/and flux (p(x), q(y)) can be provided. In the steady state the solution T(x, y) was assumed in the form of an infinite power series developed in relation to the variable y with coefficients Cn depending on x. The assumed solution was substituted into the Fourier equation and after expanding into the Taylor series the boundary condition for y = 0 and y = h was taken into account. From this condition the coefficient Cn can be calculated and, therefore, a closed solution for the temperature field in the plate. Full article
(This article belongs to the Special Issue Recent Developments in Building Physics)
Open AccessArticle
Machine Performance and Hog Fuel Quality Evaluation in Olive Tree Pruning Harvesting Conducted Using a Towed Shredder on Flat and Hilly Fields
Energies 2020, 13(7), 1713; https://doi.org/10.3390/en13071713 (registering DOI) - 03 Apr 2020
Abstract
Pruning residues from olive groves represent an important biomass source. Until now, the management of pruning residue has generally represented a disposal problem rather than an opportunity for additional revenue. The main problem is the lack of a well-organized pruning biomass supply chain. [...] Read more.
Pruning residues from olive groves represent an important biomass source. Until now, the management of pruning residue has generally represented a disposal problem rather than an opportunity for additional revenue. The main problem is the lack of a well-organized pruning biomass supply chain. In particular, harvesting is a key stage that influences the product quality, the type of logistics chain, and the economic sustainability of the pruning supply chain. The aim of the present paper was the evaluation of the machine performance of the Facma Comby TR200 towed shredder. The harvesting tests took place in Agios Konstantinos, Fthiotida, Central Greece. Two different experimental fields were used for the evaluation of this harvesting system; these fields were characterized by different slopes to check the convenience of using such a towed shredder on both hilly slopes and flat terrains. Analysis was conducted focusing on both the work productivity and costs. Moreover, an evaluation of the obtained hog fuel quality was performed. The Facma Comby TR200 showed good work performances on both flat (2.60 tdm·h−1) and hilly (2.74 tdm·h−1) land, even if a consistent influence of the pruning biomass yield on the work performances was reported. The biomass quality could be consistently improved by modifying the pick-up systems to avoid the collection of inert materials (soil and rocks). In fact, the analysis showed a high ash content in the comminuted material (4% dry basis). Finally, the economic aspects of this study’s results were in line with those reported in the literature. The applied harvesting system showed a cost equal to 29.88 and 16.59 €·tfm−1 on flat and hilly land, respectively. Full article
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Open AccessArticle
Future Renewable Fuel Mixes in Transport in Germany under RED II and Climate Protection Targets
Energies 2020, 13(7), 1712; https://doi.org/10.3390/en13071712 (registering DOI) - 03 Apr 2020
Abstract
With the Renewable Energy Directive 2018/2001 (RED II), adopted in December 2018, the EU is continuing the political framework for the use of renewable energy sources in the transport sector for the period from 2021 to 2030. At the same time, the German [...] Read more.
With the Renewable Energy Directive 2018/2001 (RED II), adopted in December 2018, the EU is continuing the political framework for the use of renewable energy sources in the transport sector for the period from 2021 to 2030. At the same time, the German federal government has set a target of reducing greenhouse gas (GHG) emissions in the transport sector by at least 40% to 42% by 2030 compared to the 1990 GHG level. To investigate the possible effects of the European and national requirements on the German GHG quota, cost-optimal fuel mixes were modelled to achieve the GHG targets of 26 fuel options in each of the nine different scenarios. The results show clear differences between the scenarios that implement the RED II targets (including 14% renewables in transport by 2030) and those that implement the climate protection target (40%–42% GHG reduction compared to 1990 by 2030). If only the minimum requirements of RED II are met, the German climate protection target is clearly missed without further measures. In order to achieve the climate protection target, a significant reduction in the final energy consumption in transport is required, as well as a very high GHG quota of 34.5%, meaning a high proportion of renewables of ca. 40% and using almost all the fuel options considered. Full article
(This article belongs to the Special Issue Prospects of Biomass-Based Biofuels)
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Open AccessArticle
Design Method of Dual Active Bridge Converters for Photovoltaic Systems with High Voltage Gain
Energies 2020, 13(7), 1711; https://doi.org/10.3390/en13071711 (registering DOI) - 03 Apr 2020
Abstract
In this paper, a design method for a photovoltaic system based on a dual active bridge converter and a photovoltaic module is proposed. The method is supported by analytical results and theoretical predictions, which are confirmed with circuital simulations. The analytical development, the [...] Read more.
In this paper, a design method for a photovoltaic system based on a dual active bridge converter and a photovoltaic module is proposed. The method is supported by analytical results and theoretical predictions, which are confirmed with circuital simulations. The analytical development, the theoretical predictions, and the validation through circuital simulations, are the main contributions of the paper. The dual active bridge converter is selected due to its high efficiency, high input and output voltages range, and high voltage-conversion ratio, which enables the interface of low-voltage photovoltaic modules with a high-voltage dc bus, such as the input of a micro-inverter. To propose the design method, the circuital analysis of the dual active bridge converter is performed to describe the general waveforms derived from the circuit behavior. Then, the analysis of the dual active bridge converter, interacting with a photovoltaic module driven by a maximum power point tracking algorithm, is used to establish the mathematical expressions for the leakage inductor current, the photovoltaic current, and the range of operation for the phase shift. The design method also provides analytical equations for both the high-frequency transformer equivalent leakage inductor and the photovoltaic side capacitor. The design method is validated through detailed circuital simulations of the whole photovoltaic system, which confirm that the maximum power of the photovoltaic module can be extracted with a correct design of the dual active bridge converter. Also, the theoretical restrictions of the photovoltaic system, such as the photovoltaic voltage and power ripples, are fulfilled with errors lower than 2% with respect to the circuital simulations. Finally, the simulation results also demonstrate that the maximum power point for different environmental conditions is reached, optimizing the phase shift factor with a maximum power point tracking algorithm. Full article
(This article belongs to the Special Issue Energy Harvesting Systems: Analysis, Design and Optimization)
Open AccessArticle
AC Volume Breakdown and Surface Flashover of a 4% NovecTM 4710/96% CO2 Gas Mixture Compared to CO2 in Highly Nonhomogeneous Fields
Energies 2020, 13(7), 1710; https://doi.org/10.3390/en13071710 (registering DOI) - 03 Apr 2020
Abstract
AC pre-discharge currents, breakdown, and flashover voltage measurements are reported in a 10 mm needle-plane arrangement in a 4% NovecTM 4710/96% CO2 gas mixture and compared with CO2 for pressures up to 8.8 bar abs. Flashover measurements were performed on [...] Read more.
AC pre-discharge currents, breakdown, and flashover voltage measurements are reported in a 10 mm needle-plane arrangement in a 4% NovecTM 4710/96% CO2 gas mixture and compared with CO2 for pressures up to 8.8 bar abs. Flashover measurements were performed on different solid dielectrics (Al2O3_filled epoxy resins, PTFE (Polytetrafluoroethylene) and PE (polyethylene)) for different roughness surface finishes. The effect of fixed conducting needles at various positions on electrical strength is reported. A strong nonlinear behaviour as function of gas pressure was observed for all the studied parameters (gas, needle position, solid insulator, insulator roughness). The non-linear behaviour is attributed to the inception and quenching of glow corona, in the interval between inception and breakdown or flashover voltages. It is found that a 4% concentration of NovecTM 4710 in CO2 has a breakdown/flashover voltage ≈ 1.14 higher than CO2. The glow corona-induced stabilization effect is seen for pressures between 2 and 5 bar abs for all the studied parameters. The peak flashover voltage and its associated pressure of the different insulators are strongly dependent on surface roughness. At 8.8 bar abs, the flashover voltage level obtained with various materials was ordered as follows: PTFE > PE-UHMW > Epoxy > HDPE(High-density polyethylene). Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessArticle
Development and Performance Assessment of Prefabricated Insulation Elements for Deep Energy Renovation of Apartment Buildings
Energies 2020, 13(7), 1709; https://doi.org/10.3390/en13071709 (registering DOI) - 03 Apr 2020
Abstract
A need for the refurbishment and renewal of the existing building stock has been in focus for many decades, principally because of excessive global energy consumption and pollution. This paper presents a methodology and the results of analysis of choices of realizable sets [...] Read more.
A need for the refurbishment and renewal of the existing building stock has been in focus for many decades, principally because of excessive global energy consumption and pollution. This paper presents a methodology and the results of analysis of choices of realizable sets of timber frame prefabricated insulation elements for major renovation of apartment buildings. Numerous combinations of elements with different characteristics were analyzed by applying measurements, interviews, and building performance simulation software, and thereupon their performance, installation eligibility, and concurrent cost levels were compared. Mineral wool board with a special wind barrier facing was found to be the best material as a wind barrier from the perspective of hygrothermal performance. An air and vapor barrier should have sufficient vapor permeability to allow dry-out of constructional moisture. It is possible to renovate apartment buildings to meet the nZEB energy performance requirements and their moisture safety can be guaranteed without paying high relative difference cost. Calculations showed that the global cost was lower for solutions with some mold growth risk. Great care is needed when decreasing costs without simultaneous hygrothermal analyses. The facade cladding was found to have the highest influence on the initial cost of the prefabricated insulation element. Full article
(This article belongs to the Special Issue Energy Performance of Buildings)
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Open AccessArticle
Co-Combustion of Municipal Sewage Sludge and Biomass in a Grate Fired Boiler for Phosphorus Recovery in Bottom Ash
Energies 2020, 13(7), 1708; https://doi.org/10.3390/en13071708 (registering DOI) - 03 Apr 2020
Abstract
Phosphorus has been identified as a critical element by the European Union and recycling efforts are increasingly common. An important phosphorus-containing waste stream for recycling is municipal sewage sludge (MSS), which is used directly as fertilizer to farmland. However, it contains pollutants such [...] Read more.
Phosphorus has been identified as a critical element by the European Union and recycling efforts are increasingly common. An important phosphorus-containing waste stream for recycling is municipal sewage sludge (MSS), which is used directly as fertilizer to farmland. However, it contains pollutants such as heavy metals, pharmaceutical residues, polychlorinated bi-phenyls (PCBs) and nano-plastics. The interest in combustion of MSS is continuously growing, as it both reduces the volume as well as destroys the organic materials and could separate certain heavy metals from the produced ashes. This results in ashes with a potential for either direct use as fertilizer or as a suitable feedstock for upgrading processes. The aim of this study was to investigate co-combustion of MSS and biomass to create a phosphorus-rich bottom ash with a low heavy metal content. A laboratory-scale fixed-bed reactor in addition to an 8 MWth grate-boiler was used for the experimental work. The concentration of phosphorus and selected heavy metals in the bottom ashes were compared to European Union regulation on fertilizers, ash application to Swedish forests and Swedish regulations on sewage sludge application to farmland. Element concentrations were determined by ICP-AES complemented by analysis of spatial distribution with SEM-EDS and XRD analysis to determine crystalline compounds. The results show that most of the phosphorus was retained in the bottom ash, corresponding to 9–16 wt.% P2O5, while the concentration of cadmium, mercury, lead and zinc was below the limits of the regulations. However, copper, chromium and nickel concentrations exceeded these standards. Full article
(This article belongs to the Special Issue Materials Recycling and Energy Use of Waste)
Open AccessArticle
Revision of Threshold Luminance Levels in Tunnels Aiming to Minimize Energy Consumption at No Cost: Methodology and Case Studies
Energies 2020, 13(7), 1707; https://doi.org/10.3390/en13071707 (registering DOI) - 03 Apr 2020
Abstract
Because of the absence of lighting calculation tools at the initial stage of tunnel design, the lighting systems are usually over-dimensioned, leading to over illumination and increased energy consumption. For this reason, a fine-tuning method for switching lighting stages according to the traffic [...] Read more.
Because of the absence of lighting calculation tools at the initial stage of tunnel design, the lighting systems are usually over-dimensioned, leading to over illumination and increased energy consumption. For this reason, a fine-tuning method for switching lighting stages according to the traffic weighted L20 luminance is proposed at no additional cost. The method was applied in a real –case scenario, where L20 luminance of the access zone at eleven (11) existing tunnels was calculated. The traffic weighted method of CR14380 was used in order to calculate the actual luminance levels for the entrance zone. The new transition zone, which decreases luminance curves, was produced and compared with the existing ones. Thus, a new switching control was proposed and programed for the Supervisory Control and Data Acquisition (SCADA) system of the tunnel. The signals of the corresponding eleven L20 meters for a period of eight days were used and the corresponding annual energy consumptions were calculated using the proposed switching program for each tunnel. The results were compared with a number of scenarios in which the existing lighting system was retrofitted with Lighting Emitting Diodes (LED) luminaires. In these scenarios, the new luminaire arrangement was based not only on the existing luminance demand value for the threshold zone, but also on the newly proposed one with two different control techniques (continuous dimming and 10% step dimming). The fine-tuning method for switching resulted in energy savings between 11% and 54% depending on the tunnel when the scenario of the existing installation at no extra cost was used. Energy savings, when LED luminaires were installed, varied between 57% (for the scenario with existing luminance demand value for the threshold zone and 10% step dimming) and 85% (for the scenario with the new calculated luminance demand and continuous dimming). Full article
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Open AccessArticle
A Bi-Layer Multi-Objective Techno-Economical Optimization Model for Optimal Integration of Distributed Energy Resources into Smart/Micro Grids
Energies 2020, 13(7), 1706; https://doi.org/10.3390/en13071706 (registering DOI) - 03 Apr 2020
Abstract
The energy management system is executed in microgrids for optimal integration of distributed energy resources (DERs) into the power distribution grids. To this end, various strategies have been more focused on cost reduction, whereas effectively both economic and technical indices/factors have to be [...] Read more.
The energy management system is executed in microgrids for optimal integration of distributed energy resources (DERs) into the power distribution grids. To this end, various strategies have been more focused on cost reduction, whereas effectively both economic and technical indices/factors have to be considered simultaneously. Therefore, in this paper, a two-layer optimization model is proposed to minimize the operation costs, voltage fluctuations, and power losses of smart microgrids. In the outer-layer, the size and capacity of DERs including renewable energy sources (RES), electric vehicles (EV) charging stations and energy storage systems (ESS), are obtained simultaneously. The inner-layer corresponds to the scheduled operation of EVs and ESSs using an integrated coordination model (ICM). The ICM is a fuzzy interface that has been adopted to address the multi-objectivity of the cost function developed based on hourly demand response, state of charges of EVs and ESS, and electricity price. Demand response is implemented in the ICM to investigate the effect of time-of-use electricity prices on optimal energy management. To solve the optimization problem and load-flow equations, hybrid genetic algorithm (GA)-particle swarm optimization (PSO) and backward-forward sweep algorithms are deployed, respectively. One-day simulation results confirm that the proposed model can reduce the power loss, voltage fluctuations and electricity supply cost by 51%, 40.77%, and 55.21%, respectively, which can considerably improve power system stability and energy efficiency. Full article
(This article belongs to the Special Issue Computational Intelligence Applications in Smart Grid Optimization)
Open AccessArticle
Vehicle-To-Grid for Peak Shaving to Unlock the Integration of Distributed Heat Pumps in a Swedish Neighborhood
Energies 2020, 13(7), 1705; https://doi.org/10.3390/en13071705 (registering DOI) - 03 Apr 2020
Abstract
The city of Stockholm is close to hitting the capacity limits of its power grid. As an additional challenge, electricity has been identified as a key resource to help the city to meet its environmental targets. This has pushed citizens to prefer power-based [...] Read more.
The city of Stockholm is close to hitting the capacity limits of its power grid. As an additional challenge, electricity has been identified as a key resource to help the city to meet its environmental targets. This has pushed citizens to prefer power-based technologies, like heat pumps and electric vehicles, thus endangering the stability of the grid. The focus of this paper is on the district of Hammarby Sjöstad. Here, plans are set to switch from district heating to heat pumps. A previous study verified that this choice will cause overloadings on the electricity distribution grid. The present paper tackles this problem by proposing a new energy storage option. By considering the increasing share of electric vehicles, the potential of using the electricity stored in their batteries to support the grid is explored through technical performance simulations. The objective was to enable a bi-directional flow and use the electric vehicles’ (EVs)’ discharging to shave the peak demand caused by the heat pumps. It was found that this solution can eliminate overloadings up to 50%, with a 100% EV penetration. To overcome the mismatch between the availability of EVs and the overloadings’ occurrence, the minimum state of charge for discharging should be lower than 70%. Full article
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Open AccessArticle
Analysis and Design of Three-Phase Buck Rectifier Employing UPS to Supply High Reliable DC Power
Energies 2020, 13(7), 1704; https://doi.org/10.3390/en13071704 (registering DOI) - 03 Apr 2020
Abstract
In the DC distribution system, to step down the DC voltage level from the AC grid voltage, the conventional topologies require multiple power conversion stages and bulky line-frequency transformers, which degrade their power density and cost-effectiveness. In addition, the conventional topologies suffer from [...] Read more.
In the DC distribution system, to step down the DC voltage level from the AC grid voltage, the conventional topologies require multiple power conversion stages and bulky line-frequency transformers, which degrade their power density and cost-effectiveness. In addition, the conventional topologies suffer from a shoot-through problem resulting in their low system reliability. In this paper, to overcome the above issues, systematic design approaches of a three-phase buck rectifier with an uninterruptible power supply (UPS) and a protection algorithm are proposed to obtain the high reliability of the DC distribution system, which can deal with fault conditions and can regulate the output voltage level. It only requires a single stage of the three-phase buck rectifier. Also, a thyristor switch is added without any commutation circuits to cut off the output from the fault circuit. The shoot-through faults do not occur in the buck rectifier, leading to high reliability. A dual-active-bridge (DAB) DC-DC converter is applied as the UPS to supply the electric power from the battery when the buck rectifier is shut down under the fault conditions. Finally, the protection algorithm is proposed to detect the fault conditions and to regulate the output voltage level. Full article
(This article belongs to the Section Electrical Power and Energy System)
Open AccessArticle
Economic Dispatch of Renewable Generators and BESS in DC Microgrids Using Second-Order Cone Optimization
Energies 2020, 13(7), 1703; https://doi.org/10.3390/en13071703 (registering DOI) - 03 Apr 2020
Abstract
A convex mathematical model based on second-order cone programming (SOCP) for the optimal operation in direct current microgrids (DCMGs) with high-level penetration of renewable energies and battery energy storage systems (BESSs) is developed in this paper. The SOCP formulation allows converting the non-convex [...] Read more.
A convex mathematical model based on second-order cone programming (SOCP) for the optimal operation in direct current microgrids (DCMGs) with high-level penetration of renewable energies and battery energy storage systems (BESSs) is developed in this paper. The SOCP formulation allows converting the non-convex model of economic dispatch into a convex approach that guarantees the global optimum and has an easy implementation in specialized software, i.e., CVX. This conversion is accomplished by performing a mathematical relaxation to ensure the global optimum in DCMG. The SOCP model includes changeable energy purchase prices in the DCMG operation, which makes it in a suitable formulation to be implemented in real-time operation. An energy short-term forecasting model based on a receding horizon control (RHC) plus an artificial neural network (ANN) is used to forecast primary sources of renewable energy for periods of 0.5h. The proposed mathematical approach is compared to the non-convex model and semidefinite programming (SDP) in three simulation scenarios to validate its accuracy and efficiency. Full article
(This article belongs to the Special Issue Solar and Wind Power and Energy Forecasting)
Open AccessArticle
Thermal Comfort Aspects of Solar Gains during the Heating Season
Energies 2020, 13(7), 1702; https://doi.org/10.3390/en13071702 (registering DOI) - 03 Apr 2020
Abstract
Glazing plays a key role in the energy balance of buildings. The aim of this paper is to enlighten the thermal discomfort caused by large glazed areas in the heating season and to point out a possible solution that can provide proper thermal [...] Read more.
Glazing plays a key role in the energy balance of buildings. The aim of this paper is to enlighten the thermal discomfort caused by large glazed areas in the heating season and to point out a possible solution that can provide proper thermal comfort with low energy use. It is unusual to discuss the negative effects of solar gains on thermal comfort during the heating season. However, there are cases when glazing may lead to unforeseen indoor thermal discomfort conditions. Laboratory and on site measurements were performed in order to assess thermal discomfort caused by direct and diffuse radiation. It was shown that the WBGT (Wet Bulb Globe Temperature) index may exceed even 30 °C in the winter season in a room having large glazed area oriented to east. Laboratory tests performed in climate chamber have shown that the high PMV values cannot be reduced below 1.0, increasing the air change rate in the room. Using opaque drapes, the WBGT index was reduced by 2 °C, but the daylighting decreased substantially. It was demonstrated that by using advanced personalized ventilation systems, the appropriate thermal comfort can be provided avoiding the reduction of daylighting. Full article
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Open AccessArticle
A PV-Powered TE Cooling System with Heat Recovery: Energy Balance and Environmental Impact Indicators
Energies 2020, 13(7), 1701; https://doi.org/10.3390/en13071701 (registering DOI) - 03 Apr 2020
Abstract
Over the past decades, clean and renewable energy has become a subject of great interest to both science and industry in response to the pollution caused by conventional energy sources. Its useful form should always meet the requirements of high performance and low [...] Read more.
Over the past decades, clean and renewable energy has become a subject of great interest to both science and industry in response to the pollution caused by conventional energy sources. Its useful form should always meet the requirements of high performance and low environmental impact, while remaining within the scope of the expected functionality. The purpose of study presented in this paper was to determine the operational characteristics for a recently developed photovoltaic (PV)-powered thermoelectric (TE) cooling system with heat recovery. The characteristics of operation of the tested system were determined within the use of a specially developed measurement system. The conducted experimental research allowed describing the conditions of power supply for TE module using PV system, calculate the coefficient of performance (COP) for the whole TE cooling system with heat recovery and calculate the environmental impact indicators based on the material and energy balance used for life cycle assessment (LCA). Full article
(This article belongs to the Special Issue Life Cycle Assessment and Carbon Footprint in Energy Systems)
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Open AccessArticle
Socio-Economic Effect on ICT-Based Persuasive Interventions Towards Energy Efficiency in Tertiary Buildings
Energies 2020, 13(7), 1700; https://doi.org/10.3390/en13071700 (registering DOI) - 03 Apr 2020
Abstract
Occupants of tertiary environments rarely care about their energy consumption. This fact is even more accentuated in cases of buildings of public use. Such unawareness has been identified by many scholars as one of the main untapped opportunities with high energy saving potential [...] Read more.
Occupants of tertiary environments rarely care about their energy consumption. This fact is even more accentuated in cases of buildings of public use. Such unawareness has been identified by many scholars as one of the main untapped opportunities with high energy saving potential in terms of cost-effectiveness. Towards that direction, there have been numerous studies exploring energy-related behaviour and the impact that our daily actions have on energy efficiency, demand response and flexibility of power systems. Nevertheless, there are still certain aspects that remain controversial and unidentified, especially in terms of socio-economic characteristics of the occupants with regards to bespoke tailored motivational and awareness-based campaigns. The presented work introduces a two-step survey, publicly available through Zenodo repository that covers social, economic, behavioural and demographic factors. The survey analysis aims to fully depict the drivers that affect occupant energy-related behaviour at tertiary buildings and the barriers which may hinder green actions. Moreover, the survey reports evidence on respondents’ self-assessment of fifteen known principles of persuasion intended to motivate them to behave pro-environmentally. The outcomes from the self-assessment help to shed light on understanding which of the Persuasive Principles may work better to nudge different user profiles towards doing greener actions at workplace. This study was conducted in four EU countries, six different cities and seven buildings, reaching more than three-hundred-and-fifty people. Specifically, a questionnaire was delivered before (PRE) and after (POST) a recommendation-based intervention towards pro-environmental behaviour through Information and Communication Technologies (ICT). The findings from the PRE-pilot stage were used to refine the POST-pilot survey (e.g., we removed some questions that did not add value to one or several research questions or dismissed the assessment of Persuasive Principles (PPs) which were of low value to respondents in the pre-pilot survey). Both surveys validate “Cause and Effect”, “Conditioning” and “Self-monitoring” as the top PPs for affecting energy-related behaviour in a workplace context. Among other results, the descriptive and prescriptive analysis reveals the association effects of specific barriers, pro-environmental intentions and confidence in technology on forming new pro-environmental behaviour. The results of this study intend to set the foundations for future interventions based on persuasion through ICT to reduce unnecessary energy consumption. Among all types of tertiary buildings, we emphasise on the validity of the results provided for buildings of public use. Full article
(This article belongs to the Special Issue End-Users’ Perspectives on Energy Policy and Technology)
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Open AccessArticle
Co-Gasification of Crude Glycerol/Animal Fat Mixtures
Energies 2020, 13(7), 1699; https://doi.org/10.3390/en13071699 (registering DOI) - 03 Apr 2020
Abstract
The aim of this work was to assess the technical viability of glycerol/fat co-gasification. The gasification performance was studied in a downflow fixed bed reactor using activated alumina particles as bed material and steam as oxidizing agent. The effect of gasification temperature, from [...] Read more.
The aim of this work was to assess the technical viability of glycerol/fat co-gasification. The gasification performance was studied in a downflow fixed bed reactor using activated alumina particles as bed material and steam as oxidizing agent. The effect of gasification temperature, from 800 to 950 °C was studied with a feed mixture with 10% (w/w) of animal fat. The influence of fat incorporation on the feedstock in the overall gasification process was also performed, using 3% (w/w) and 5% (w/w) of fat in feed mixtures. Samples of dry gas from the gasifier were collected and analyzed by gas chromatography in order to determine the CO, CO2, CH4, and H2 content. The best results were obtained using the highest tested temperature, 950 °C, and using 3% (w/w) of animal fat in the feed mixture. The overall results revealed that the co-gasification of glycerol/animal fat mixtures seems to be a feasible technical option. Full article
(This article belongs to the Special Issue Challenges Towards Sustainable Energy)
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Open AccessArticle
Thermal Effects of Natural Gas and Syngas Co-Firing System on Heat Treatment Process in the Preheating Furnace
Energies 2020, 13(7), 1698; https://doi.org/10.3390/en13071698 (registering DOI) - 03 Apr 2020
Abstract
Preheating furnaces, which are commonly used in many production sectors (e.g., iron and steel), are simultaneously one of the most energy-intensive devices used in the industry. Partial replacement of natural gas with biomass-derived synthesis gas as a fuel used for heating would be [...] Read more.
Preheating furnaces, which are commonly used in many production sectors (e.g., iron and steel), are simultaneously one of the most energy-intensive devices used in the industry. Partial replacement of natural gas with biomass-derived synthesis gas as a fuel used for heating would be an important step towards limiting industrial CO2 emissions. The time dependent computational fluid dynamics (CFD) model of an exemplary furnace was created to evaluate whether it is possible to obtain 40% of energy from syngas combustion without deterioration of thermal parameters of the treated load. As an outcome, a promising method to organize co-firing in the furnace was indicated. The obtained results show that the co-firing method (up to 40% thermal natural gas replacement with syngas), assuming low air-to-fuel equivalence ratio (λNG = 2.0) and even distribution of power among the furnace corners, lead to satisfactory efficiency of the heat treatment process—the heat transferred to the load exceeds 95% of the heat delivered to the load in the reference case), while carbon dioxide emission is reduced from 285.5 to 171.3 kg CO2/h. This study showed that it is feasible (from the heat transfer point of view) to decrease the environmental impact of the process industries by the use of renewable fuels. Full article
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Open AccessFeature PaperArticle
Real-Time Active-Reactive Optimal Power Flow with Flexible Operation of Battery Storage Systems
Energies 2020, 13(7), 1697; https://doi.org/10.3390/en13071697 (registering DOI) - 03 Apr 2020
Abstract
In this paper, a multi-phase multi-time-scale real-time dynamic active-reactive optimal power flow (RT-DAR-OPF) framework is developed to optimally deal with spontaneous changes in wind power in distribution networks (DNs) with battery storage systems (BSSs). The most challenging issue hereby is that a large-scale [...] Read more.
In this paper, a multi-phase multi-time-scale real-time dynamic active-reactive optimal power flow (RT-DAR-OPF) framework is developed to optimally deal with spontaneous changes in wind power in distribution networks (DNs) with battery storage systems (BSSs). The most challenging issue hereby is that a large-scale ‘dynamic’ (i.e., with differential/difference equations rather than only algebraic equations) mixed-integer nonlinear programming (MINLP) problem has to be solved in real time. Moreover, considering the active-reactive power capabilities of BSSs with flexible operation strategies, as well as minimizing the expended life costs of BSSs further increases the complexity of the problem. To solve this problem, in the first phase, we implement simultaneous optimization of a huge number of mixed-integer decision variables to compute optimal operations of BSSs on a day-to-day basis. In the second phase, based on the forecasted wind power values for short prediction horizons, wind power scenarios are generated to describe uncertain wind power with non-Gaussian distribution. Then, MINLP AR-OPF problems corresponding to the scenarios are solved and reconciled in advance of each prediction horizon. In the third phase, based on the measured actual values of wind power, one of the solutions is selected, modified, and realized to the network for very short intervals. The applicability of the proposed RT-DAR-OPF is demonstrated using a medium-voltage DN. Full article
(This article belongs to the Special Issue Optimal Design and Operation of Sustainable Energy Systems)
Open AccessArticle
Fault Ride Through Capability Improvement of DFIG Based Wind Farm Using Nonlinear Controller Based Bridge-Type Flux Coupling Non-Superconducting Fault Current Limiter
Energies 2020, 13(7), 1696; https://doi.org/10.3390/en13071696 (registering DOI) - 03 Apr 2020
Abstract
High penetration of Doubly Fed Induction Generator (DFIG) into existing power grid can attribute complex issues as they are very sensitive to the grid faults. In addition, Fault Ride Through (FRT) is one of the main requirements of the grid code for integrating [...] Read more.
High penetration of Doubly Fed Induction Generator (DFIG) into existing power grid can attribute complex issues as they are very sensitive to the grid faults. In addition, Fault Ride Through (FRT) is one of the main requirements of the grid code for integrating Wind Farms (WFs) into the power grid. In this work, to enhance the FRT capability of the DFIG based WFs, a Bridge-Type Flux Coupling Non-Superconducting Fault Current Limiter (BFC-NSFCL) is proposed. The effectiveness of the proposed BFC-NSFCL is evaluated through performance comparison with that of the Bridge-Type Fault Current Limiter (BFCL) and Series Dynamic Braking Resistor (SDBR). Moreover, a dynamic nonlinear controller is also proposed for controlling the operation of the BFC-NSFCL. Extensive simulations are carried out in the MATLAB/SIMULINK environment for both symmetrical and unsymmetrical temporary as well as permanent faults. Based on the simulation results and different numerical analysis, it is found that the proposed nonlinear controller based BFC-NSFCL is very effective in enhancing the FRT capability of the WF. Also, the BFC-NSFCL outperforms the conventional BFCL and SDBR by maintaining a near-seamless performance during various grid fault situations. Full article
Open AccessArticle
Data-Driven Modeling of Fuel Consumption for Turboprop-Powered Civil Airliners
Energies 2020, 13(7), 1695; https://doi.org/10.3390/en13071695 (registering DOI) - 03 Apr 2020
Abstract
Next to empirical correlations for the specific range, fuel flow rate, and specific fuel consumption, a response surface model for estimates of the fuel consumption in early design stages is presented and validated. The response-surface’s coefficients are themselves predicted from empirical correlations based [...] Read more.
Next to empirical correlations for the specific range, fuel flow rate, and specific fuel consumption, a response surface model for estimates of the fuel consumption in early design stages is presented and validated. The response-surface’s coefficients are themselves predicted from empirical correlations based solely on the operating empty weight. The model and correlations are all derived from fuel consumption data of nine current civil turbo-propeller aircraft and are validated on a separate set. The model can accurately predict fuel weights of new designs for any combination of payload and range within the current range of efficiency of the propulsion. The accuracy of the model makes it suited for preliminary and conceptual design of near-in-kind turbo-propeller aircraft. The model can shorten the design cycle by delivering fast and accurate fuel weight estimates from the first design iteration once the operating empty weight is known. Since it is based solely on the operating empty weight and it is accurate, the model is a sound variant to the Breguet range equation in order to make accurate fuel weight estimates. Full article
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Open AccessArticle
Fault Tolerant Boost Converter with Multiple Serial Inputs and Output Voltage Regulation for Vehicle-to-Aid Services
Energies 2020, 13(7), 1694; https://doi.org/10.3390/en13071694 (registering DOI) - 03 Apr 2020
Abstract
The operation of electric vehicles (EV) is currently being segmented into a scenario of smart grids, including vehicle-to-grid (V2G), vehicle-to-home (V2H), vehicle-to-building (V2B), and vehicle-to-load (V2L), among others. Energy-providing services from EVs for medical/health assistance (human, animal, agronomist, environmental, etc.), including emergency services [...] Read more.
The operation of electric vehicles (EV) is currently being segmented into a scenario of smart grids, including vehicle-to-grid (V2G), vehicle-to-home (V2H), vehicle-to-building (V2B), and vehicle-to-load (V2L), among others. Energy-providing services from EVs for medical/health assistance (human, animal, agronomist, environmental, etc.), including emergency services (patrols, fire trucks, etc.), are named/classified in this article as vehicle-to-aid (V2A), since it is expected that they will require special characteristics. For instance, an EV for V2A services must supply regulated voltage by a power electronic converter, even during possible failures, including short-circuits and damages on its components. In this paper, a new configuration of boost converter is proposed, with unlimited serial inputs ( n ), and important properties of fault tolerance, even if the power sources are not isolated; this includes robustness against component failures, variations in the parameters and design errors. Analytic, numerical, and experimental results that validate the operation of the proposed configuration against failures and parameter variation are presented. A numerical comparison with series-connected boost converters is also presented, showing best closed loop performance (PI) with n fewer diodes and n fewer capacitors. Full article
(This article belongs to the Special Issue Power Electronics and Power Quality 2019)
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Open AccessArticle
Acid-Catalyzed Wet Torrefaction for Enhancing the Heating Value of Barley Straw
Energies 2020, 13(7), 1693; https://doi.org/10.3390/en13071693 (registering DOI) - 03 Apr 2020
Abstract
In the present study, the possibility of improving the higher heating value (HHV) of lignocellulosic biomass, especially barley straw, was examined. The research deals with the treatment of barley straw by acid-catalyzed wet torrefaction (ACWT), also called acid hydrolysis, in a batch reactor [...] Read more.
In the present study, the possibility of improving the higher heating value (HHV) of lignocellulosic biomass, especially barley straw, was examined. The research deals with the treatment of barley straw by acid-catalyzed wet torrefaction (ACWT), also called acid hydrolysis, in a batch reactor (autoclave) Parr 4553 3.75 L. In this case, two different simulation approaches were applied: (i) combined severity factor (CSF) and (ii) response surface methodology (RSM) based on Box–Behnken design of experiments (DoE). Sulfuric acid (SA) concentration, temperature and time were the ACWT parameters examined herein. An oxygen bomb calorimeter was used for the HHV measurement. The findings indicated that the composition changes of the straw due to ACWT had a significant effect on the HHV of the pretreated material. In this study, treatment conditions were 10–35 mM SA, 160–200 °C and an isothermal reaction time 0–40 min (preheating period not included in these values). In conclusion, there was a significant increase in the HHV up to 24.3 MJ/kg for the ACWT barley straw, compared to 17.5 MJ/kg for the untreated straw, at optimal conditions of 200 °C for 25 min (isothermal period) and 35 mM SA. This resulted in a 1.39 enhancement factor (EF) and 68% energy yield (EY). Full article
(This article belongs to the Special Issue Solid Fuels Technology and Applications.)
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Open AccessArticle
Study on Optimization of Active Control Schemes for Considering Transient Processes in the Case of Pipeline Leakage
Energies 2020, 13(7), 1692; https://doi.org/10.3390/en13071692 (registering DOI) - 03 Apr 2020
Abstract
Pipeline leakage of crude oil, refined oil or other petroleum derivatives can cause serious damage to the environment, soil, and more importantly, pose a serious threat to personal safety. The losses can be minimized to a degree by active control. Therefore, timely and [...] Read more.
Pipeline leakage of crude oil, refined oil or other petroleum derivatives can cause serious damage to the environment, soil, and more importantly, pose a serious threat to personal safety. The losses can be minimized to a degree by active control. Therefore, timely and effective control measures should be taken to minimize the leak volume whenever a pipeline leaks. However, the complexity of pipeline hydraulic systems makes it difficult to optimize control schemes for pipeline hydraulic devices under leak conditions, and existing studies rarely consider complex transient processes. This paper aims to establish a mixed integer linear programming model considering transient processes, hydraulic constraints, equipment constraints and flow constraints, and develop a detailed control scheme of the devices by the branch and bound algorithm. Moreover, it is the objective of the model to figure out the most optimal control plan to minimize the leakage. Experiments on a real-world liquid pipeline have proved the practicability and high reliability of the model. Full article
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Open AccessArticle
Control Design and Experimental Validation of a HB-NPC as a Shunt Active Power Filter
Energies 2020, 13(7), 1691; https://doi.org/10.3390/en13071691 (registering DOI) - 03 Apr 2020
Abstract
This work presents the design of a control law based on the average model of a shunt active power filter considering an H-bridge neutral point clamped topology and its experimental validation. Therefore, the proposed controller is formed by three control loops, namely current [...] Read more.
This work presents the design of a control law based on the average model of a shunt active power filter considering an H-bridge neutral point clamped topology and its experimental validation. Therefore, the proposed controller is formed by three control loops, namely current (inner), regulation (outer), and balance control loops. The current loop aims to compensate both the displacement power factor and the harmonic distortion produced by nonlinear loads connected to the point of common coupling. To deal with harmonic current distortion, the current loop involves an adaptive mechanism based on a bank of resonant filters tuned at odd harmonics of the fundamental grid frequency. The regulation and balance loops are aimed to maintain the voltage of the capacitors forming the DC-link at a desired constant level. For this, proportional-integral controllers are designed. The design of all three loops considers the average model of the system. The performance of the proposed multi-loop control law is evaluated through numerical results and real-time experimental implementation, both considering a 2 kW academic benchmark with a constant switching frequency of 7 kHz. In order to provide harmonic distortion, a nonlinear load based on an uncontrolled diode bridge rectifier is considered. Additionally, step-load changes from 0.5 kW to 1 kW are considered for the nonlinear load. As a result, a suitable current tracking, voltage regulation, and balance are observed despite parametric uncertainties, load variations, and harmonic distortion. As a consequence, in steady state, simulation results indicate that the compensated grid current THD is 1.75%; meanwhile, the nonlinear load current THD is 52.5%. Experimental results indicate that the compensated grid current THD is 2.32%; meanwhile, the nonlinear load current THD is 53.8%. Full article
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Open AccessArticle
Nanostructure Effect on Methane Adsorption Capacity of Shale with Type III Kerogen
Energies 2020, 13(7), 1690; https://doi.org/10.3390/en13071690 - 03 Apr 2020
Abstract
This study focuses on the nanostructure of shale samples with type III kerogen and its effect on methane adsorption capacity. The composition, pore size distribution, and methane adsorption capacities of 12 shale samples were analyzed by using the high-pressure mercury injection experiment, low-temperature [...] Read more.
This study focuses on the nanostructure of shale samples with type III kerogen and its effect on methane adsorption capacity. The composition, pore size distribution, and methane adsorption capacities of 12 shale samples were analyzed by using the high-pressure mercury injection experiment, low-temperature N2/CO2 adsorption experiments, and the isothermal methane adsorption experiment. The results show that the total organic carbon (TOC) content of the 12 shale samples ranges from 0.70% to ~35.84%. In shales with type III kerogen, clay minerals and organic matter tend to be deposited simultaneously. When the TOC content is higher than 10%, the clay minerals in these shale samples contribute more than 70% of the total inorganic matter. The CO2 adsorption experimental results show that micropores in shales with type III kerogen are mainly formed in organic matter. However, mesopores and macropores are significantly affected by the contents of clay minerals and quartz. The methane isothermal capacity experimental results show that the Langmuir volume, indicating the maximum methane adsorption capacity, of all the shale samples is between 0.78 cm3/g and 9.26 cm3/g. Moreover, methane is mainly adsorbed in micropores and developed in organic matter, whereas the influence of mesopores and macropores on the methane adsorption capacity of shale with type III kerogen is small. At different stages, the influencing factors of methane adsorption capacity are different. When the TOC content is <1.4% or >4.5%, the methane adsorption capacity is positively correlated with the TOC content. When the TOC content is in the range of 1.4–4.5%, clay minerals have obviously positive effects on the methane adsorption capacity. Full article
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Open AccessArticle
A Building Energy Management System Based on an Equivalent Electric Circuit Model
Energies 2020, 13(7), 1689; https://doi.org/10.3390/en13071689 - 03 Apr 2020
Abstract
In recent decades, many EU and national regulations have been issued in order to increase the energy efficiency in different sectors and, consequently, to reduce environmental pollution. In the building sector, energy efficiency interventions are usually based on the use of innovative insulated [...] Read more.
In recent decades, many EU and national regulations have been issued in order to increase the energy efficiency in different sectors and, consequently, to reduce environmental pollution. In the building sector, energy efficiency interventions are usually based on the use of innovative insulated materials and on the installation of cogeneration and tri-generation units, as well as solar technologies. New and retrofitted buildings are more and more commonly being called “smart buildings”, since they are characterized by the installation of electric and thermal power generation units, energy storage systems, and flexible loads; the presence of such technologies determines the necessity of installing Building Energy Management Systems (BEMSs), which are used to optimally manage their operation. The present paper proposes a BEMS for a smart building, equipped with plants based on renewables (photovoltaics, solar thermal panels, and geothermal heat pump), where the heating and cooling demand are satisfied by a Heating, Ventilation and Air Conditioning System (HVAC) fed by a geothermal heat pump. The developed BEMS is composed of two different modules: an optimization tool used to optimally manage the HVAC plant, in order to guarantee a desired level of comfort inside rooms, and a simulation tool, based on an equivalent electric circuit model and used to evaluate the thermal dynamic behavior of the building. The paper describes the two modules and shows the main results of the validation phase that has been conducted on a real test-case represented by the Smart Energy Building (SEB) located at the Savona Campus of the University of Genoa, Italy. Full article
(This article belongs to the Special Issue Optimal Control of Hybrid Systems and Renewable Energies)
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Open AccessArticle
Air Terminal Devices Developed for Personal Ventilation Systems
Energies 2020, 13(7), 1688; https://doi.org/10.3390/en13071688 - 03 Apr 2020
Abstract
Using the personal ventilation systems may improve the thermal comfort sensation. At the University of Debrecen, a personal ventilation system was developed named ALTAIR. This paper presents the results of mean air velocity, turbulence grade, and draught measurements related to newly developed air [...] Read more.
Using the personal ventilation systems may improve the thermal comfort sensation. At the University of Debrecen, a personal ventilation system was developed named ALTAIR. This paper presents the results of mean air velocity, turbulence grade, and draught measurements related to newly developed air terminal devices which are connected to the ALTAIR personal ventilation system. In order to define the measurement points it was essential to test the new air terminal devices (ATDs) in front of a black wall and smoke puffs. A series of measurements were carried out with isothermal air flow, mean air velocity, turbulence grade, and draught around the occupant head region in order to improve the thermal comfort sensation. Five different ATDs were analyzed. Full article
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Open AccessArticle
An Interval Forecasting Model Based on Phase Space Reconstruction and Weighted Least Squares Support Vector Machine for Time Series of Dissolved Gas Content in Transformer Oil
Energies 2020, 13(7), 1687; https://doi.org/10.3390/en13071687 - 03 Apr 2020
Abstract
Transformer state forecasting and fault forecasting are important for the stable operation of power equipment and the normal operation of power systems. Forecasting of the dissolved gas content in oil is widely conducted for transformer faults, but its accuracy is affected by data [...] Read more.
Transformer state forecasting and fault forecasting are important for the stable operation of power equipment and the normal operation of power systems. Forecasting of the dissolved gas content in oil is widely conducted for transformer faults, but its accuracy is affected by data scale and data characteristics. Based on phase space reconstruction (PSR) and weighted least squares support vector machine (WLSSVM), a forecasting model of time series of dissolved gas content in transformer oil is proposed in this paper. The phase spaces of time series of the dissolved gas content sequence are reconstructed by chaos theory, and the delay time and dimension are obtained by the C-C method. The WLSSVM model is used to forecast time series of dissolved gas content, the chemical reaction optimization (CRO) algorithm is used to optimize training parameters, the bootstrap method is used to build forecasting intervals. Finally, the accuracy and generalization ability of the forecasting model are verified by the analysis of actual case and the comparison of different models. Full article
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Open AccessArticle
A Novel Hybrid Model for Cu–Al2O3/H2O Nanofluid Flow and Heat Transfer in Convergent/Divergent Channels
Energies 2020, 13(7), 1686; https://doi.org/10.3390/en13071686 (registering DOI) - 03 Apr 2020
Abstract
In the present study, our aim is to present a novel model for the flow of hybrid nanofluids in oblique channels. Copper and aluminum oxide have been used to obtain a novel hybrid nanofluid. The equations that govern the flow of hybrid nanofluids [...] Read more.
In the present study, our aim is to present a novel model for the flow of hybrid nanofluids in oblique channels. Copper and aluminum oxide have been used to obtain a novel hybrid nanofluid. The equations that govern the flow of hybrid nanofluids have been transformed to a set of nonlinear equations with the implementation of self-similar variables. The resulting system is treated numerically by using coupled shooting and Runge–Kutta (R-K) scheme. The behavior of velocity and temperature is examined by altering the flow parameters. The cases for narrowing (convergent) and opening (divergent) channels are discussed, and the influence of various parameters on Nusselt number is also presented. To indicate the reliability of the study, a comparison is made that confirms the accuracy of the study presented. Full article
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Open AccessArticle
Research on the Roof Advanced Breaking Position and Influences of Large Mining Height Working Face in Shallow Coal Seam
Energies 2020, 13(7), 1685; https://doi.org/10.3390/en13071685 - 03 Apr 2020
Abstract
The large mining height (LMH) in shallow coal seam has been widely applied in the Shenfu coalfield, China. The dynamic load is obvious, and the rib spalling is serious when the LMH working face concerns roof weighting. The advanced breaking position of the [...] Read more.
The large mining height (LMH) in shallow coal seam has been widely applied in the Shenfu coalfield, China. The dynamic load is obvious, and the rib spalling is serious when the LMH working face concerns roof weighting. The advanced breaking position of the roof affects the strength of the ground pressure when the roof is broken. Firstly, based on a large number of actual measurements and physical simulation experiments, the rock formation in the fall zone, where the articulated structure cannot be articulated between the coal seam and the main roof, is called the equivalent immediate roof (EIR). When the mining height increases, the thickness of the EIR increases non-linearly. Next, based on the theory of “elastic foundation beam”, a mechanical model for the advanced breaking of the roof is established in shallow coal seam, and the calculation equation for the advanced breaking position of the roof is given; then, designed and carry out boreholes of the no. 22201 working face in the Zhangjiamao Coal Mine. The theoretical calculation of key strata results (5.6–6.9 m) are in the range of field measurement results (5–8 m). According to the field measurement results, the roof movement of the LMH working face is ahead of the roof weighting. Finally, we define the thickness of EIR and the mining height ratio as the immediate mining ratio ki, which affects the degree of filling of the goaf and determines the structural form of the main roof. When the ki is small, the goaf is fully filled; when the ki is large, the goaf is fully filled. Under the same conditions, different filling rate conditions will form different roof structures. Results of this research can be helpful to control roof weighting and provide early warning of possible safety problems related to the LMH working face in shallow coal seam. Full article
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