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Energies, Volume 12, Issue 22 (November-2 2019)

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Open AccessReview
A Review of Heat Pump Systems and Applications in Cold Climates: Evidence from Lithuania
Energies 2019, 12(22), 4331; https://doi.org/10.3390/en12224331 (registering DOI) - 13 Nov 2019
Abstract
Similar to other cold climate countries, space heating and domestic hot water (DHW) accounts form the largest share of household energy demand in Lithuania. Heat pump technology is considered to be one of the environmentally friendly solutions to increase energy efficiency and reduce [...] Read more.
Similar to other cold climate countries, space heating and domestic hot water (DHW) accounts form the largest share of household energy demand in Lithuania. Heat pump technology is considered to be one of the environmentally friendly solutions to increase energy efficiency and reduce the carbon footprint of buildings. Heat pumps have been finding their way into the Lithuanian market since 2002, and currently there are many good practice examples present in the country, especially in the residential and public sectors. Heat pump use is economically advantageous in the Baltic Region, and the market share of these systems is growing. Studies have reported seasonal performance factor (SPF) ranges within 1.8 and 5.6. The lower SPF values are typically attributable to air source heat pumps, whereas the higher efficiency is achieved by ground or water source heat pump applications. While the traditional heat pump techniques are well established in the region, there is a slow uptake of new technologies, such as solar-assisted heat pumps, absorption heat pumps and heat pumps integrated into foundations, tunnels or diaphragm walls. This paper provides a critical review of different heat pump technologies, using Lithuania as a cold climate case study to overview the market trends within the European context. Potential trends for the heat pump technology development in terms of application areas, cost-benefit predictions, as well as environmental aspects, are discussed. Full article
(This article belongs to the Special Issue Energy Efficiency in Buildings: Both New and Rehabilitated)
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Open AccessArticle
Validating Performance Models for Hybrid Power Plant Control Assessment
Energies 2019, 12(22), 4330; https://doi.org/10.3390/en12224330 (registering DOI) - 13 Nov 2019
Abstract
The need for simple, but accurate performance models of wind turbine generators (WTGs), photovoltaic (PV) plants, and battery energy storage systems (BESS) for various hybrid power plant (HPP) studies motivates the present work. Particularly, the development and verification stage of HPP controls requires [...] Read more.
The need for simple, but accurate performance models of wind turbine generators (WTGs), photovoltaic (PV) plants, and battery energy storage systems (BESS) for various hybrid power plant (HPP) studies motivates the present work. Particularly, the development and verification stage of HPP controls requires reduced-order models to minimize the complexity and computation effort of simulation platforms. In this paper, such models are proposed, and the most essential parts of the models are validated through field measurements. The models target power system integration studies involving active and reactive power, as well as frequency and voltage regulation where detailed models, as proposed in the standards, can be cumbersome. Field measurements of two Vestas WTGs, one 1-MW PV plant, and one 1-MW/1-MWh BESS are used for model validation. The results show that the WTG and PV performance models correctly estimate the power generation variability according to fluctuations in wind speed and solar irradiance. The BESS performance model provides satisfactory results related to grid-forming control performance and estimation of state-of-charge. The presented validation work enables using the proposed performance models for power system studies and HPP control design in all model-based design stages, that is, preliminary analysis, design, verification, and validation with a high level of confidence. Full article
(This article belongs to the Special Issue Modern Power System Dynamics, Stability and Control)
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Open AccessReview
Review on Power Performance and Efficiency of Wave Energy Converters
Energies 2019, 12(22), 4329; https://doi.org/10.3390/en12224329 (registering DOI) - 13 Nov 2019
Abstract
The level of awareness about ocean wave energy as a viable source of useful energy has been increasing recently. Different concepts and methods have been suggested by many researchers to harvest ocean wave energy. This paper reviews and compares the efficiencies and power [...] Read more.
The level of awareness about ocean wave energy as a viable source of useful energy has been increasing recently. Different concepts and methods have been suggested by many researchers to harvest ocean wave energy. This paper reviews and compares the efficiencies and power performance of different wave energy converters. The types of analyses used in deriving the reported efficiencies are identified, and the stage of the power conversion processes at which the efficiencies were determined is also identified. In order to find a common way to compare the efficiencies of different technologies, the hydrodynamic efficiency in relation to the characteristic width of the wave energy converters and the wave resource potential are chosen in this paper. The results show that the oscillating body systems have the highest ratio in terms of the efficiency per characteristic width, and overtopping devices have the lowest. In addition, with better understanding of the devices’ dynamics, the efficiencies of the newer oscillating water column and body systems would increase as the potential wave energy level increases, which shows that those newer designs could be suitable for more potential locations with large variations in wave energy potentials. At last, discussion about the cost of ocean wave energy is presented as well. Full article
(This article belongs to the Special Issue Wave Energy Potential, Behavior and Extraction 2020)
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Open AccessArticle
Neutral-Point Potential Balancing Control Strategy for Three-Level ANPC Converter Using SHEPWM Scheme
Energies 2019, 12(22), 4328; https://doi.org/10.3390/en12224328 (registering DOI) - 13 Nov 2019
Abstract
A selective harmonic elimination pulse width modulation (SHEPWM) control strategy is proposed to balance the neutral-point potential (NP) of a three-level active neutral-point-clamped (ANPC) converter. In this strategy, the chaotic ant colony algorithm (ACA) is adopted to solve the SHEPWM nonlinear equations, which [...] Read more.
A selective harmonic elimination pulse width modulation (SHEPWM) control strategy is proposed to balance the neutral-point potential (NP) of a three-level active neutral-point-clamped (ANPC) converter. In this strategy, the chaotic ant colony algorithm (ACA) is adopted to solve the SHEPWM nonlinear equations, which does not require presetting the initial values of solutions and can get multiple solutions in the same modulation index. The influence of different solutions corresponding to the SHEPWM switching states on the NP is different, namely, some make the NP increase and others make the NP decrease. Therefore, the NP balancing can be effectively controlled by choosing appropriate solutions in different fundamental periods. The simulation and experimental results further verify the feasibility and effectiveness of the control strategy. Full article
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Open AccessArticle
Assessment of Iron Oxide (III)–Therminol 66 Nanofluid as a Novel Working Fluid in a Convective Radiator Heating System for Buildings
Energies 2019, 12(22), 4327; https://doi.org/10.3390/en12224327 (registering DOI) - 13 Nov 2019
Abstract
This work investigates the use of iron oxide (III)–therminol 66 oil-based nanosuspensions in a convective heating system with potential heating applications in the buildings sector. In an experimental study, characteristics of nanofluids were measured, including heat capacity, thermal conductivity, and density. The influences [...] Read more.
This work investigates the use of iron oxide (III)–therminol 66 oil-based nanosuspensions in a convective heating system with potential heating applications in the buildings sector. In an experimental study, characteristics of nanofluids were measured, including heat capacity, thermal conductivity, and density. The influences of mass flow rate and concentration of nanofluid on various parameters were quantified, such as pressure loss, friction coefficient, and heat transfer rate. For a concentration of 0.3 wt.%, the heat transfer increased by 46.3% and the pressure drop increased by 37.5%. The latter is due to the higher friction and viscosity of the bulk of the nanofluid. Although the pressure drop is higher, the thermo-hydraulic efficiency still increased by 19%. As a result, iron oxide (III)–therminol 66 presented reasonable thermal performance, higher heat transfer coefficient, and a lower pressure drop value (19% better performance in comparison with water) for the air–liquid convective system. Results also showed that for nanosuspensions at 0.3 wt.%, the friction factor of the system increased by 10% in comparison with the performance of the system with water. Full article
(This article belongs to the Section Advanced Energy Materials)
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Open AccessArticle
Intelligent Energy Management Strategy for Automated Office Buildings
Energies 2019, 12(22), 4326; https://doi.org/10.3390/en12224326 (registering DOI) - 13 Nov 2019
Abstract
The increasing demand to reduce the high consumption of end-use energy in office buildings framed the objective of this work, which was to design an intelligent system management that could be utilized to minimize office buildings’ energy consumption from the national electricity grid. [...] Read more.
The increasing demand to reduce the high consumption of end-use energy in office buildings framed the objective of this work, which was to design an intelligent system management that could be utilized to minimize office buildings’ energy consumption from the national electricity grid. Heating, Ventilation and Air Conditioning (HVAC) and lighting are the two main consumers of electricity in office buildings. Advanced automation and control systems for buildings and their components have been developed by researchers to achieve low energy consumption in office buildings without considering integrating the load consumed and the Photovoltaic system (PV) input to the controller. This study investigated the use of PV to power the HVAC and lighting equipped with a suitable control strategy to improve energy saving within a building, especially in office buildings where there are reports of high misuse of electricity. The intelligent system was modelled using occupant activities, weather condition changes, load consumed and PV energy changes, as input to the control system of lighting and HVAC. The model was verified and tested using specialized simulation tools (Simulink®) and was subsequently used to investigate the impact of an integrated system on energy consumption, based on three scenarios. In addition, the direct impact on reduced energy cost was also analysed. The first scenario was tested in simulation of four offices building in a civil building in South Africa of a single occupant’s activities, weather conditions, temperature and the simulation resulted in savings of HVAC energy and lighting energy of 13% and 29%, respectively. In the second scenario, the four offices were tested in simulation due to the loads’ management plus temperature and occupancy and it resulted in a saving of 20% of HVAC energy and 29% of lighting electrical energy. The third scenario, which tested integrating PV energy (thus, the approach utilized) with the above-mentioned scenarios, resulted in, respectively, 64% and 73% of HVAC energy and lighting electrical energy saved. This saving was greater than that of the first two scenarios. The results of the system developed demonstrated that the loads’ control and the PV integration combined with the occupancy, weather and temperature control, could lead to a significant saving of energy within office buildings. Full article
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Open AccessArticle
Role of Shearing Dispersion and Stripping in Wax Deposition in Crude Oil Pipelines
Energies 2019, 12(22), 4325; https://doi.org/10.3390/en12224325 (registering DOI) - 13 Nov 2019
Abstract
Wax deposition during crude oil transmission can cause a series of negative effects and lead to problems associated with pipeline safety. A considerable number of previous works have investigated the wax deposition mechanism, inhibition technology, and remediation methods. However, studies on the shearing [...] Read more.
Wax deposition during crude oil transmission can cause a series of negative effects and lead to problems associated with pipeline safety. A considerable number of previous works have investigated the wax deposition mechanism, inhibition technology, and remediation methods. However, studies on the shearing mechanism of wax deposition have focused largely on the characterization of this phenomena. The role of the shearing mechanism on wax deposition has not been completely clarified. This mechanism can be divided into the shearing dispersion effect caused by radial migration of wax particles and the shearing stripping effect caused by hydrodynamic scouring. From the perspective of energy analysis, a novel wax deposition model was proposed that considered the flow parameters of waxy crude oil in pipelines instead of its rheological parameters. Considering the two effects of shearing dispersion and shearing stripping coexist, with either one of them being the dominant mechanism, a shearing dispersion flux model and a shearing stripping model were established. Furthermore, a quantitative method to distinguish between the roles of shearing dispersion and shearing stripping in wax deposition was developed. The results indicated that the shearing mechanism can contribute an average of approximately 10% and a maximum of nearly 30% to the wax deposition process. With an increase in the oil flow rate, the effect of the shearing mechanism on wax deposition is enhanced, and its contribution was demonstrated to be negative; shear stripping was observed to be the dominant mechanism. A critical flow rate was observed when the dominant effect changes. When the oil flow rate is lower than the critical flow rate, the shearing dispersion effect is the dominant effect; its contribution rate increases with an increase in the oil flow temperature. When the oil flow rate is higher than the critical flow rate, the shearing stripping effect is the dominant effect; its contribution rate increases with an increase in the oil flow temperature. This understanding can be used to design operational parameters of the actual crude oil pipelines and address the potential flow assurance problems. The results of this study are of great significance for understanding the wax deposition theory of crude oil and accelerating the development of petroleum industry pipelines. Full article
(This article belongs to the Special Issue Developments in Oil and Gas Engineering)
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Open AccessArticle
Gasification Characteristics and Kinetics of Unburned Pulverized Coal in Blast Furnaces
Energies 2019, 12(22), 4324; https://doi.org/10.3390/en12224324 (registering DOI) - 13 Nov 2019
Abstract
Pulverized coal injected into a blast furnace (BF) burns incompletely in a very limited amount of time. A considerable amount of unburned pulverized coal (UPC) escapes from the raceway to the coke layer. The unburned pulverized coal reacts with CO2 in the [...] Read more.
Pulverized coal injected into a blast furnace (BF) burns incompletely in a very limited amount of time. A considerable amount of unburned pulverized coal (UPC) escapes from the raceway to the coke layer. The unburned pulverized coal reacts with CO2 in the coke layer, and this has a very significant impact on the operation of the BF. The gasification reaction characteristics of the UPC with CO2 were assessed by thermogravimetric analysis. The microstructure and specific surface area of the pulverized coal and UPC were characterized by scanning electron microscopy (SEM), and a specific surface area testing apparatus together with the Brunauer-Emmett-Teller (BET) method, respectively. The results showed that Qingding UPC requires a higher temperature to complete the gasification reaction. At the same heating rate, the Tm (maximum reaction rate temperature) of the Shenhua UPC is much lower and the reaction rate is larger than those of the Qingding UPC. An increased heating rate is beneficial for the gasification reaction of the two UPCs. The kinetics analysis results showed that the optimal mechanism function models for the Shenhua and Qingding UPCs are chemical reaction models. The apparent activation energies of gasification of the Shenhua UPC and Qingding UPC under different conditions were 269.89–223.41 KJ/mol and 266.70–251.54 KJ/mol, respectively. Full article
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Open AccessArticle
Modeling of Light Pipes for the Optimal Disposition in Buildings
Energies 2019, 12(22), 4323; https://doi.org/10.3390/en12224323 (registering DOI) - 13 Nov 2019
Abstract
A light pipe is an excellent solution to transport and distribute daylight into environments without or with little lighting, guaranteeing comfort inside the rooms. As stated in the literature, the evaluation of the performances of light pipes presents numerous complexities, making the work [...] Read more.
A light pipe is an excellent solution to transport and distribute daylight into environments without or with little lighting, guaranteeing comfort inside the rooms. As stated in the literature, the evaluation of the performances of light pipes presents numerous complexities, making the work very difficult for technicians and designers. This study is aimed to present a methodology that is able to identify the potential of light pipes using indices such as daylight autonomy (DA), continuous daylight autonomy (DAc), and useful daylight illuminance (UDI). This paper presents an analysis of daylight obtained by several configurations of simple models of light pipes installed into a 5 × 5 m plant area room. All simulations are carried out in a DAYSIM environment, which allows calculating the annual availability of daylight based on a RADIANCE raytracer backward. Several daylight conditions were analyzed for different light pipe configurations, considering different pipe lengths and a variable number of light pipes. The light pipes are tested also in the horizontal position, for different orientations. The results of all the combinations were compared with the performances of a window with dimensions equal to 1/8 of the internal surface, which was in accordance with the minimum value to be guaranteed by the Italian Regulation (D.M. 5 July 1975 n. 190) for different orientations. The results indicated a difference in daylight distribution, showing a strong correlation between the percentage levels of DA and DAc with the length and number of pipes, during different periods of the year. The simulated model is strongly influenced by the aspect ratio (R = diameter/length). The results show that the illuminance levels decrease drastically, increasing the length. Full article
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Open AccessArticle
Design and Performance Comparison of Methanol Production Processes with Carbon Dioxide Utilization
Energies 2019, 12(22), 4322; https://doi.org/10.3390/en12224322 (registering DOI) - 13 Nov 2019
Abstract
Carbon dioxide recycling is one of the possible contributions to CO2 mitigation and provides an opportunity to use a low-cost carbon source. Methanol is a commodity chemical that serves as an important basic chemical and energy feedstock with growing demand. For each [...] Read more.
Carbon dioxide recycling is one of the possible contributions to CO2 mitigation and provides an opportunity to use a low-cost carbon source. Methanol is a commodity chemical that serves as an important basic chemical and energy feedstock with growing demand. For each of the four types of industrial methanol production processes from natural gas (methane), i.e., steam reforming (SR), autothermal reforming (ATR), combined reforming (CR), and two-step reforming (TSR), CO2 utilization cases of (A) no utilization, (B) as reforming step feedstock, and (C) as methanol synthesis step feedstock were designed based on common industrial operation conditions and analyzed for energy consumption, exergy loss (EXloss), net CO2 reduction (NCR) and internal rate of return (IRR). The utilization of CO2 can reduce energy consumption. The processes with the lowest and the highest EXloss are SR and ATR, respectively. All SR processes give negative NCR. All the B-type processes are positive in NCR except B-SR. The highest NCR is obtained from the B-ATR process with a value of 0.23 kg CO2/kg methanol. All the processes are profitable with positive IRR results and the highest IRR of 41% can be obtained from B-ATR. The utilization of CO2 in the industrial methanol process can realize substantial carbon reduction and is beneficial to process economics. Full article
(This article belongs to the Special Issue Carbon Dioxide (CO2) Utilization)
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Open AccessReview
Hardware Approach to Mitigate the Effects of Module Mismatch in a Grid-connected Photovoltaic System: A Review
Energies 2019, 12(22), 4321; https://doi.org/10.3390/en12224321 (registering DOI) - 13 Nov 2019
Abstract
This study reviews the hardware approach to mitigate the effects of module mismatch in a grid-connected photovoltaic (PV) system. Unlike software solutions, i.e. the maximum power tracking algorithm, hardware techniques are well suited to enhance energy yield because of their inherent ability to [...] Read more.
This study reviews the hardware approach to mitigate the effects of module mismatch in a grid-connected photovoltaic (PV) system. Unlike software solutions, i.e. the maximum power tracking algorithm, hardware techniques are well suited to enhance energy yield because of their inherent ability to extract energy from the mismatched module. Despite the extra cost of the additional circuitry, hardware techniques have recently gained popularity because of their long-term financial benefits. Notwithstanding the growing interest in this topic, review papers that provide updates on the technological developments of the three main hardware solutions, namely micro inverter, DC power optimizer, and energy recovery circuits, are lacking. This is in contrast to software solutions, which have had a considerable number of reputable reviews. Thus, a comprehensive review paper is appropriate at this juncture to provide up-to-date information on the latest topologies, highlight their merits/drawbacks, and evaluate their comparative performance. Full article
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Open AccessArticle
Optimal Dispatch of Aggregated HVAC Units for Demand Response: An Industry 4.0 Approach
Energies 2019, 12(22), 4320; https://doi.org/10.3390/en12224320 (registering DOI) - 13 Nov 2019
Abstract
Demand response (DR) involves economic incentives aimed at balancing energy demand during critical demand periods. In doing so DR offers the potential to assist with grid balancing, integrate renewable energy generation and improve energy network security. Buildings account for roughly 40% of global [...] Read more.
Demand response (DR) involves economic incentives aimed at balancing energy demand during critical demand periods. In doing so DR offers the potential to assist with grid balancing, integrate renewable energy generation and improve energy network security. Buildings account for roughly 40% of global energy consumption. Therefore, the potential for DR using building stock offers a largely untapped resource. Heating, ventilation and air conditioning (HVAC) systems provide one of the largest possible sources for DR in buildings. However, coordinating the real-time aggregated response of multiple HVAC units across large numbers of buildings and stakeholders poses a challenging problem. Leveraging upon the concepts of Industry 4.0, this paper presents a large-scale decentralized discrete optimization framework to address this problem. Specifically, the paper first focuses upon the real-time dispatch problem for individual HVAC units in the presence of a tertiary DR program. The dispatch problem is formulated as a non-linear constrained predictive control problem, and an efficient dynamic programming (DP) algorithm with fixed memory and computation time overheads is developed for its efficient solution in real-time on individual HVAC units. Subsequently, in order to coordinate dispatch among multiple HVAC units in parallel by a DR aggregator, a flexible and efficient allocation/reallocation DP algorithm is developed to extract the cost-optimal solution and generate dispatch instructions for individual units. Accurate baselining at individual unit and aggregated levels for post-settlement is considered as an integrated component of the presented algorithms. A number of calibrated simulation studies and practical experimental tests are described to verify and illustrate the performance of the proposed schemes. The results illustrate that the distributed optimization algorithm enables a scalable, flexible solution helping to deliver the provision of aggregated tertiary DR for HVAC systems for both aggregators and individual customers. The paper concludes with a discussion of future work. Full article
(This article belongs to the Special Issue Future Smart Grid Systems)
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Open AccessArticle
Reduction of DC Current Ripples by Virtual Space Vector Modulation for Three-Phase AC–DC Matrix Converters
Energies 2019, 12(22), 4319; https://doi.org/10.3390/en12224319 (registering DOI) - 13 Nov 2019
Abstract
In this paper, the virtual space vector modulation for the AC–DC (alternating current–direct current) matrix converters is proposed to reduce the DC current ripples in the whole modulation index range. In the proposed method, each virtual vector is synthesized by the two nearest [...] Read more.
In this paper, the virtual space vector modulation for the AC–DC (alternating current–direct current) matrix converters is proposed to reduce the DC current ripples in the whole modulation index range. In the proposed method, each virtual vector is synthesized by the two nearest original active vectors. To synthesize the current reference vector, two virtual vectors and one zero vector are used in every switching period. The main principle of the proposed method is to reduce the dwelling period of the largest active current vector in each sector. In addition, the optimized switching patterns are proposed to further reduce the DC current ripples at both high- and low-power operation. Finally, simulation and experimental results are illustrated to validate the effectiveness of the proposed strategy. Full article
(This article belongs to the Special Issue Power Electronics Applications in Renewable Energy Systems)
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Open AccessArticle
Distributed Peer-to-Peer Electricity Trading Considering Network Loss in a Distribution System
Energies 2019, 12(22), 4318; https://doi.org/10.3390/en12224318 (registering DOI) - 13 Nov 2019
Abstract
In this paper, a distributed peer-to-peer (P2P) electricity trading model was proposed to study economic interactions between load aggregators (LAs) and microgrid operators (MGOs) considering network losses in a distribution system. In this model, the economic interactions among market participants were formulated as [...] Read more.
In this paper, a distributed peer-to-peer (P2P) electricity trading model was proposed to study economic interactions between load aggregators (LAs) and microgrid operators (MGOs) considering network losses in a distribution system. In this model, the economic interactions among market participants were formulated as a Nash bargaining game, where LAs and MGOs can bargain with each other on the trading volume of electricity and payment. To achieve the Nash bargaining solution, the game was divided into two sub-problems: social welfare maximization and payment bargaining. Then, the alternating direction method of multipliers was used to solve the two sub-problems with limited information exchange. Finally, we tested the proposed model on a 12 × 12 km2 distribution system, and the results verify its effectiveness. Full article
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Open AccessArticle
Demand Price Elasticity of Residential Electricity Consumers with Zonal Tariff Settlement Based on Their Load Profiles
Energies 2019, 12(22), 4317; https://doi.org/10.3390/en12224317 (registering DOI) - 13 Nov 2019
Abstract
The concept of price elasticity of demand has been widely used for the assessment of the consumers’ behavior in the electricity market. As the residential consumers represent a significant percentage of the total load, price elasticity of their demand may be used to [...] Read more.
The concept of price elasticity of demand has been widely used for the assessment of the consumers’ behavior in the electricity market. As the residential consumers represent a significant percentage of the total load, price elasticity of their demand may be used to design desirable demand side response programs in order to manage peak load in a power system. The method presented in this study proposes an alternative approach towards price elasticity determination for zonal tariff users, based on comparisons of load profiles of consumers settled according to flat and time-of-use electricity tariffs. A detailed explanation of the proposed method is presented, followed by a case-study of price elasticity determination for residential electricity consumers in Poland. The forecasted values of price elasticity of demand for the Polish households using time-of-use (TOU) tariff vary between −1.7 and −2.3, depending on the consumers’ annual electricity consumption. Moreover, an efficiency study of residential zonal tariff is performed to assess the operation of currently applicable electricity tariffs. Presented analysis is based on load profiles published by Distribution System Operators and statistical data, but the method can be applied to the real-life measurements from the smart metering systems as well when such systems are accessible for residential consumers. Full article
(This article belongs to the Section Energy Economics and Policy)
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Open AccessArticle
A Novel Photovoltaic Array Outlier Cleaning Algorithm Based on Sliding Standard Deviation Mutation
Energies 2019, 12(22), 4316; https://doi.org/10.3390/en12224316 (registering DOI) - 13 Nov 2019
Abstract
There is a large number of outliers in the operation data of photovoltaic (PV) array, which is caused by array abnormalities and faults, communication issues, sensor failure, and array shutdown during PV power plant operation. The outlier will reduce the accuracy of PV [...] Read more.
There is a large number of outliers in the operation data of photovoltaic (PV) array, which is caused by array abnormalities and faults, communication issues, sensor failure, and array shutdown during PV power plant operation. The outlier will reduce the accuracy of PV system performance analysis and modeling, and make it difficult for fault diagnosis of PV power plant. The conventional data cleaning method is affected by the outlier data distribution. In order to solve the above problems, this paper presents a method for identifying PV array outliers based on sliding standard deviation mutation. Considering the PV array output characteristics under actual environmental conditions, the distribution of array outliers is analyzed. Then, an outlier identification method is established based on sliding standard deviation calculation. This method can identify outliers by analyzing the degree of dispersion of the operational data. The verification part is illustrated by case study and algorithm comparison. In the case study, multiple sets of actual operating data of different inverters are cleaned, which is selected from a large grid-connected power station. The cleaning results illustrate the availability of the algorithm. Then, the comparison against the quantile-algorithm-based outlier identification method explains the effectiveness of the proposed algorithm. Full article
(This article belongs to the Special Issue Photovoltaic Modules)
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Open AccessArticle
Effect of Mixed-Flow Fans with a Newly Shaped Diffuser on Heat Stress of Dairy Cows Based on CFD
Energies 2019, 12(22), 4315; https://doi.org/10.3390/en12224315 (registering DOI) - 12 Nov 2019
Abstract
Mixed-flow fans (MFF) are widely used to reduce the heat stress in dairy cows in summer. Our research team developed MFFs with a newly shaped diffuser with the length of 250 mm and the circumferential angle of 150°, which have better performance in [...] Read more.
Mixed-flow fans (MFF) are widely used to reduce the heat stress in dairy cows in summer. Our research team developed MFFs with a newly shaped diffuser with the length of 250 mm and the circumferential angle of 150°, which have better performance in terms of maximum flow flux and energy efficiency. However, how the elevation angle of the diffuser influences the performance of MFFs and how the optimal fan perform in the field experiment has not been studied yet. In this paper, the diffuser was optimized by CFD (Computational Fluid Dynamics) simulation of the fan and a laboratory prototype test. An orthogonal test showed no interaction among length, circumferential angle, and elevation angle. The diffuser with an elevation angle of 10° performed better than that with an elevation angle of 0°, showing increased jet lengths, flow flux, and energy efficiency by 0.5 m, 0.69%, and 1.39%, respectively, and attaining greater axial wind speeds and better non-uniformity coefficients at the dairy cattle height. Then, through on-site controlled trials, we found that the 10°/150°/250 mm diffusers increased the overall average wind speeds by 9.4% with respect to the MFFs without a diffuser. MFFs with the newly shaped diffuser were used for field tests, and their effectiveness in alleviating heat stress in dairy cows was evaluated by testing environmental parameters and dairy cows’ physiological indicators. Although the temperature–humidity indexes (THIs) in the experimental barn with the optimized fan at different times were lower than those in the controlled barn, the environmental conditions corresponded to moderate heat stress. However, this was not consistent with cow's respiratory rate and rectal temperature. Finally, on the basis of the CFD simulation of a dairy cow barn, the equivalent temperature of cattle (ETIC), which takes into account the effect of air velocity, showed that the environment caused moderate heat stress only at 13:00, but not at other times of the day. This shows that ETIC is more accurate to evaluate heat stress. Full article
(This article belongs to the Special Issue Large-Eddy Simulations of Turbulent Flows)
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Open AccessArticle
Assessing Financial and Flexibility Incentives for Integrating Wind Energy in the Grid Via Agent-Based Modeling
Energies 2019, 12(22), 4314; https://doi.org/10.3390/en12224314 (registering DOI) - 12 Nov 2019
Abstract
This article provides an agent-based model of a hypothetical standalone electricity network to identify how the feed-in tariffs and the installed capacity of wind power, calculated in percentage of total system demand, affect the electricity consumption from renewables. It includes the mechanism of [...] Read more.
This article provides an agent-based model of a hypothetical standalone electricity network to identify how the feed-in tariffs and the installed capacity of wind power, calculated in percentage of total system demand, affect the electricity consumption from renewables. It includes the mechanism of electricity pricing on the Day Ahead Market (DAM) and the Imbalance Market (IM). The extra production volumes of Electricity from Renewable Energy Sources (RES-E) and the flexibility of electrical consumption of industries is provided as reserves on the IM. Five thousand simulations were run by using the agent-based model to gather data that were then fit in linear regression models. This helped to quantify the effect of feed-in tariffs and installed capacity of wind power on the consumption from renewable energy and market prices. The consumption from renewable sources, expressed as percentage of total system consumption, increased by 8.17% for every 10% increase in installed capacity of wind power. The sharpest increase in renewable energy consumption is observed when a feed-in tariff of 0.04 €/kWh is provided to the wind farm owners, resulting in an average increase of 9.1% and 5.1% in the consumption from renewable sources while the maximum installed capacity of wind power is 35% and 100%, respectively. The regression model for the annualized DAM prices showed an increase by 0.01 €cents/kWh in the DAM prices for every 10% increase in the installed wind power capacity. With every increase of 0.01 €/kWh in the value of feed-in tariffs, the mean DAM price is lowered as compared to the previous value of the feed-in tariff. DAM prices only decrease with increasing installed wind capacity when a feed-in tariff of 0.04 €/kWh is provided. This is observed because all wind power being traded on DAM at a very cheap price. Hence, no volume of electricity is being stored for availability on IM. The regression models for predicting IM prices show that, with every 10% increase in installed capacity of wind power, the annualized IM price decreases by 0.031 and 0.34 €cents/kWh, when installed capacity of wind power is between 0 and 25%, and between 25 and 100%, respectively. The models also showed that, until the maximum installed capacity of wind power is less than 25%, the IM prices increase when the value of feed-in tariff is 0.01 and 0.04 €/kWh, but decrease for a feed-in tariff of 0.02 and 0.03 €/kWh. When installed capacity of wind power is between 25 and 100%, increasing feed-in tariffs to the value of 0.03 €/kWh result in lowering the mean IM price. However, at 0.04 €/kWh, the mean IM price is higher, showing the effect of no storage reserves being available on IM and more expensive reserves being engaged on the IM. The study concludes that the effect of increasing installed capacity of wind power is more significant on increasing consumption of renewable energy and decreasing the DAM and IM prices than the effect of feed-in tariffs. However, the effect of increasing values of both factors on the profit of RES-E producers with storage facilities is not positive, pointing to the need for customized rules and incentives to encourage their market participation and investment in storage facilities. Full article
(This article belongs to the Special Issue 100% Renewable Energy Transition: Pathways and Implementation)
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Open AccessArticle
A Study on the Performance of a Cascade Heat Pump for Generating Hot Water
Energies 2019, 12(22), 4313; https://doi.org/10.3390/en12224313 (registering DOI) - 12 Nov 2019
Abstract
The use of cascade heat pumps for hot water generation has gained much attention in recent times. The big question that has attracted much research interest is how to enhance the performance and energy saving potential of these cascade heat pumps. This study [...] Read more.
The use of cascade heat pumps for hot water generation has gained much attention in recent times. The big question that has attracted much research interest is how to enhance the performance and energy saving potential of these cascade heat pumps. This study therefore proposed a new cycle to enhance performance of the cascade heat pump by adopting an auxiliary heat exchanger (AHX) in desuperheater, heater and parallel positions at the low stage (LS) side. The new cascade cycle with AHX in desuperheater position was found to have better performance than that with AHX at heater and parallel positions. Compared to the conventional cycle, heating capacity and coefficient of performance (COP) of the new cascade cycle with AHX in desuperheater position increased up to 7.4% and 14.9% respectively. Full article
Open AccessArticle
Macroeconomic Effects of EU Energy Efficiency Regulations on Household Dishwashers, Washing Machines and Washer Dryers
Energies 2019, 12(22), 4312; https://doi.org/10.3390/en12224312 (registering DOI) - 12 Nov 2019
Abstract
Testing the relationship between economic performance and energy consumption is of utmost importance in nearly all countries. Taking the European Union as scope, this paper analyses the impacts of energy efficiency legislation on a selection of household appliances. In particular, it analyses the [...] Read more.
Testing the relationship between economic performance and energy consumption is of utmost importance in nearly all countries. Taking the European Union as scope, this paper analyses the impacts of energy efficiency legislation on a selection of household appliances. In particular, it analyses the employment and value added impacts of the stricter energy efficiency requirements for dishwashers, washing machines, and washer dryers. To do so, this paper combines a bottom-up stock model with a macro-econometric dynamic general equilibrium model (FIDELIO) to quantify the direct and indirect value added and employment impacts in the European Union. The analysis shows that stricter energy efficiency requirements on household dishwashers, washing machines, and washer dryers have a net negative macroeconomic impact on value added (roughly 0.01 % of the total European Union value added) and a slightly net positive impact on employment. In fact, the regulations cause a shift in the composition of the household consumption basket that seems to favor labor-intensive industries. Full article
(This article belongs to the Special Issue Advances in Low Carbon Technologies and Transition)
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Open AccessArticle
A Novel Framework for Cost Optimization of Renewable Energy Installations: A Case Study of Nigeria
Energies 2019, 12(22), 4311; https://doi.org/10.3390/en12224311 (registering DOI) - 12 Nov 2019
Abstract
The merits of utilizing renewable energy sources (RESs) in electricity generation, especially in the developing countries, are to improve the electricity access, economic development and energy sustainability. Nigeria is a developing country with an acute electricity problem. The country is blessed with rich [...] Read more.
The merits of utilizing renewable energy sources (RESs) in electricity generation, especially in the developing countries, are to improve the electricity access, economic development and energy sustainability. Nigeria is a developing country with an acute electricity problem. The country is blessed with rich renewable energy (RE) resources. However, most of these resources are yet to be exploited. A look at the energy sector in Nigeria suggests that for the country to be energy sufficient, it must embrace domestic RESs in its energy mix. However, RE technologies are capital intensive. Hence, by using Nigeria as a motivation, this study aims at developing a general framework that can be used for any country or region in determining the optimal total capacities of RESs to be installed in different locations, to increase the use of RE in a cost-effective manner. The designed optimization problem aims to minimize the total cost of installing RE technologies while satisfying some predetermined constraints that include demand and supply, RE potential, area and system reserve requirement. To this end, three different scenarios, namely prospective off-grid, on-grid, and all-off-grid are introduced. The first scenario aims at finding the optimal cost of installing RESs in order to improve electricity access at each off-grid installation location (a distribution company (DISCO) location with electricity access level below 50%). The second scenario optimizes cost of installing RESs for on-grid installation locations (DISCO locations (DLs) with electricity access level of 50% and above). The third scenario finds the optimal cost of installing off-grid RESs for all the DLs. Linear optimization technique is used to solve the problem. The results show that the total installation costs of the first and second scenarios (which means having off-grid installations for some DLs and having on-grid installations for other DLs) are $97.46 billion and $114.03 billion respectively, with a total cost of $211.49 billion. On the other hand, the result of third installation scenario (only off-grid installations for all DLs), is found to be $244.33 billion. These results reveal that the combination of off-grid and on-grid installations (first and second scenarios) has the minimum installation cost, for the case study of Nigeria. As the framework presented in this study is used to provide the minimum RE related total installation cost and related RE planning in Nigeria, it can also be useful for other countries or regions considering RE planning. Full article
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Open AccessArticle
Optimal Power Flow for Transmission Power Networks Using a Novel Metaheuristic Algorithm
Energies 2019, 12(22), 4310; https://doi.org/10.3390/en12224310 (registering DOI) - 12 Nov 2019
Abstract
In the paper, a modified coyote optimization algorithm (MCOA) is proposed for finding highly effective solutions for the optimal power flow (OPF) problem. In the OPF problem, total active power losses in all transmission lines and total electric generation cost of all available [...] Read more.
In the paper, a modified coyote optimization algorithm (MCOA) is proposed for finding highly effective solutions for the optimal power flow (OPF) problem. In the OPF problem, total active power losses in all transmission lines and total electric generation cost of all available thermal units are considered to be reduced as much as possible meanwhile all constraints of transmission power systems such as generation and voltage limits of generators, generation limits of capacitors, secondary voltage limits of transformers, and limit of transmission lines are required to be exactly satisfied. MCOA is an improved version of the original coyote optimization algorithm (OCOA) with two modifications in two new solution generation techniques and one modification in the solution exchange technique. As compared to OCOA, the proposed MCOA has high contributions as follows: (i) finding more promising optimal solutions with a faster manner, (ii) shortening computation steps, and (iii) reaching higher success rate. Three IEEE transmission power networks are used for comparing MCOA with OCOA and other existing conventional methods, improved versions of these conventional methods, and hybrid methods. About the constraint handling ability, the success rate of MCOA is, respectively, 100%, 96%, and 52% meanwhile those of OCOA is, respectively, 88%, 74%, and 16%. About the obtained solutions, the improvement level of MCOA over OCOA can be up to 30.21% whereas the improvement level over other existing methods is up to 43.88%. Furthermore, these two methods are also executed for determining the best location of a photovoltaic system (PVS) with rated power of 2.0 MW in an IEEE 30-bus system. As a result, MCOA can reduce fuel cost and power loss by 0.5% and 24.36%. Therefore, MCOA can be recommended to be a powerful method for optimal power flow study on transmission power networks with considering the presence of renewable energies. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessArticle
A Study on Utility of Retrofit that Minimizes the Replacement of Heat-Source System in Large Offices
Energies 2019, 12(22), 4309; https://doi.org/10.3390/en12224309 (registering DOI) - 12 Nov 2019
Abstract
In a general building retrofit process, the reinforcement of insulation performance or air-tightness in walls and windows are conducted to reduce the maximum cooling and heating load of buildings. A heat source consists of heat-source equipment and water-pipe systems, which are replaced with [...] Read more.
In a general building retrofit process, the reinforcement of insulation performance or air-tightness in walls and windows are conducted to reduce the maximum cooling and heating load of buildings. A heat source consists of heat-source equipment and water-pipe systems, which are replaced with high-efficient specification materials. Most of them are simply replaced with the same capacity as the previous heating equipment. This study aims to investigate matters required for decision making in a retrofit plan, such as conducting an investigation of the maximum load reduction in buildings obtained by the retrofit and the capacity of heat-source equipment that reflects the reduction, and the re-use or replacement of the water-pipe system, etc. in advance. This study verified that when the capacity of heat-source equipment was reduced, the pipe diameter of the water-pipe system was also decreased, but if existing pipes were re-used, the transportation power of the pump was reduced due to the reduction in flow velocity. The changes in maximum cooling and heating load through retrofit were quantitatively verified compared to that of the initial design of the building based on previous study results, and flow rates of cool and hot water were determined by re-calculating the capacity of the heat-source equipment. Using the results, the water-pipe system was re-designed, and the annual transportation power of the pump was calculated through simulations. The calculation results verified that the transportation power decreased by up to approximately 10% when oversized pipes were re-used from the existing water-pipe system. Additionally, when the capacity of the heat-source equipment was decreased, reasonable measures considering remodeling, construction duration, and cost were derived. Full article
(This article belongs to the Section Energy and Buildings)
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Open AccessArticle
Huff-n-Puff Experimental Studies of CO2 with Heavy Oil
Energies 2019, 12(22), 4308; https://doi.org/10.3390/en12224308 (registering DOI) - 12 Nov 2019
Abstract
This work is devoted to CO2 Huff-n-Puff studies on heavy oil. Oil recovery for heavy oil reservoirs is sufficiently small in comparison with conventional reservoirs, and, due to the physical limitation of oil flow through porous media, a strong need for better [...] Read more.
This work is devoted to CO2 Huff-n-Puff studies on heavy oil. Oil recovery for heavy oil reservoirs is sufficiently small in comparison with conventional reservoirs, and, due to the physical limitation of oil flow through porous media, a strong need for better understanding of tertiary recovery mechanisms of heavy oil exists. Notwithstanding that the idea of Huff-n-Puff gas injection technology for enhanced oil recovery has existed for dozens of years, there is still no any precise methodology for evaluating the applicability and efficiency of this technology in heavy oil reservoirs. Oil recovery factor is a question of vital importance for heavy oil reservoirs. In this work, we repeated Huff-n-Puff tests more than three times at five distinct pressure points to evaluate the applicability and efficiency of CO2 Huff-n-Puff injection to the heavy oil reservoirs. Additionally, the most critical factor that affects oil recovery in gas injection operation is the condition of miscibility. Experimental data allowed to distinguish the mixing zone of the light fractions of studied heavy oil samples. The experimental results showed that the pressure increase in the Huff-n-Puff injection process does not affect the oil recovery when the injection pressure stays between miscibility pressure of light components of oil and minimum miscibility pressure. It was detected that permeability decreases after Huff-n-Puff CO2 tests. Full article
(This article belongs to the Special Issue CO2 EOR and CO2 Storage in Oil Reservoirs)
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Open AccessArticle
Development of Virtual Air Flow Sensor Using In-Situ Damper Performance Curve in VAV Terminal Unit
Energies 2019, 12(22), 4307; https://doi.org/10.3390/en12224307 (registering DOI) - 12 Nov 2019
Abstract
In this study, we developed a virtual air flow sensor using an in-situ damper performance curve to secure the stability of control of the variable air volume (VAV) terminal unit, and also established the in-situ measurement procedure. The minimum air flow rate of [...] Read more.
In this study, we developed a virtual air flow sensor using an in-situ damper performance curve to secure the stability of control of the variable air volume (VAV) terminal unit, and also established the in-situ measurement procedure. The minimum air flow rate of the VAV terminal unit was related to the energy consumption, it was important to determine the minimum air flow rate suitable for the situation of each room in terms of energy saving. However, it was difficult to set the minimum air flow rate low setpoint due to the low accuracy of the air flow sensor of the VAV terminal unit. This paper suggested a virtual air flow sensing method using an in-situ damper performance curve in the VAV terminal unit. The input factors of the virtual sensor were developed on the basis of the supply fan speed and damper opening ratio, which could be easily obtained from the existing control system. In addition, the in-situ measurement procedure of the virtual air flow sensor was developed by dividing the procedure into five steps. Finally, reliability of the virtual air flow sensor was evaluated through uncertainty analysis of input variables and relative error analysis, in comparison with the conventional air flow rate measurement method. The developed virtual air flow sensor was found to have an uncertainty of up to 8.8%, and it was also found that the closer to the maximum the values of the input variables, the lower the uncertainty. In addition, verification of relative error with respect to the measured values by the hot-wire anemometer was conducted by varying operation conditions to a total of 12 cases, and as a result, relative error was found to be up to 5.6%. In addition, the results of long-term experiment showed that relative error was within about 9.5%, and thus, the feasibility for field application and control was confirmed. Full article
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Open AccessArticle
Preliminary Study on the Use of Biodiesel Obtained from Waste Vegetable Oils for Blending with Hydrotreated Kerosene Fossil Fuel Using Calcium Oxide (CaO) from Natural Waste Materials as Heterogeneous Catalyst
Energies 2019, 12(22), 4306; https://doi.org/10.3390/en12224306 (registering DOI) - 12 Nov 2019
Abstract
In this experimental work, calcium from natural seafood wastes was used as a heterogeneous catalyst separately or in a blend of “shell mix” for producing biodiesel. Several chemical reaction runs were conducted at varied reaction times ranging from 30 min to 8 h, [...] Read more.
In this experimental work, calcium from natural seafood wastes was used as a heterogeneous catalyst separately or in a blend of “shell mix” for producing biodiesel. Several chemical reaction runs were conducted at varied reaction times ranging from 30 min to 8 h, at 60 °C, with a mass content of 5% (Wcat./Woil) and a methanol/oil molar ratio of 12. After the purification process, the biodiesel with fatty acid methyl ester (FAME) weight content measured was higher than 99%, which indicated that it was a pure biodiesel. This work also showed that the inorganic solid waste shell mixture used as the heterogeneous catalyst can be reused three times and the reused mixture still resulted in a FAME content higher than 99%. After 40 different transesterification reactions were performed using liquid (waste cooking oils) and solid (calcium seafood shells) wastes for producing biodiesel, under the specific conditions stated above, we found a successful, innovative, and promising way to produce biodiesel. In addition, blends prepared with jet fuel A1 and biodiesel were recorded with no invalid results after certain tests, at 25 °C. In this case, except for the 10% blend, the added biodiesel had no significant effect on the viscosity (fluidity) of the biojet fuel. Full article
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Open AccessArticle
The Influence of Parasitic Components on LLC Resonant Converter
Energies 2019, 12(22), 4305; https://doi.org/10.3390/en12224305 (registering DOI) - 12 Nov 2019
Abstract
In recent years, the LLC resonant converter has been widely used in DC–DC conversion applications. However, the parasitic components of the LLC resonant converter have a significant impact in practical applications, such as influence on the conduction loss and the soft-switching of power [...] Read more.
In recent years, the LLC resonant converter has been widely used in DC–DC conversion applications. However, the parasitic components of the LLC resonant converter have a significant impact in practical applications, such as influence on the conduction loss and the soft-switching of power devices, the voltage oscillation across rectifier diodes, the unregulated output voltage at light load condition and so on. It is hard to analyze the higher-order circuits by the conventional analysis methods. Focusing on the operational principle of the LLC converter with parasitic components, the differential equation model is presented and solved by the numerical method in this paper. The simulation results verify the correctness of the theoretical analysis. The causes of two different frequency oscillations and the voltage spike are clarified. The design considerations and a specific example of the LLC converter are given. The experimental results are consistent with the simulation results, and the soft-switching of primary-side switches can be achieved in the prototype. Full article
(This article belongs to the Section Energy Fundamentals and Conversion)
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Open AccessArticle
Impact of Heating Control Strategy and Occupant Behavior on the Energy Consumption in a Building with Natural Ventilation in Poland
Energies 2019, 12(22), 4304; https://doi.org/10.3390/en12224304 - 12 Nov 2019
Abstract
This study aims to provide an experimental assessment of energy consumption in an existing public building in Poland, in order to analyze the impact of occupant behavior on that consumption. The building is naturally ventilated and the occupants have the freedom to change [...] Read more.
This study aims to provide an experimental assessment of energy consumption in an existing public building in Poland, in order to analyze the impact of occupant behavior on that consumption. The building is naturally ventilated and the occupants have the freedom to change the temperature set point and open or close the windows. The energy consumption is calculated and the calculation results are compared with the experimental data. An analysis of occupants’ behavior has revealed that they choose temperature set points in a wide range recognized as thermal comfort, and window opening is accidental and difficult to predict. The implemented heating control algorithms take into account the strong influence of individual occupant preferences on the feeling of comfort. The energy consumption assessment has revealed that the lowering of temperature set point by 1 °C results in an energy saving of about 5%. Comparisons of energy consumption with heating control and without any controls showed that the potential for energy reduction due to heating control reached approximately 10%. The use of windows control, which allows to turn off the heating after opening the window and its impact on energy savings have been discussed as well. Full article
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Open AccessArticle
Analysis and Disposal of Typical Breakdown Failure for Resin Impregnated Paper Bushing in the Valve Side of HVDC Converter Transformer
Energies 2019, 12(22), 4303; https://doi.org/10.3390/en12224303 - 12 Nov 2019
Abstract
This paper presents analysis, diagnosis and disposal with a typical internal breakdown failure of the resin impregnated paper (RIP) valve side bushing in high voltage direct current (HVDC) converter transformer. Based on the analysis of fault current characteristics at the time of the [...] Read more.
This paper presents analysis, diagnosis and disposal with a typical internal breakdown failure of the resin impregnated paper (RIP) valve side bushing in high voltage direct current (HVDC) converter transformer. Based on the analysis of fault current characteristics at the time of the RIP valve side bushing failure, and field test results of insulation parameters, a method of diagnosing typical breakdown failures of valve side bushings is proposed. Through disassembly inspection of the internal overheating and arcing traces on the failure bushing, the root cause of this typical breakdown failure is found, which is upper axial flashover along the RIP condenser/SF6 interface caused by the abnormal contact of two current-carrying conductive tubes. Temperature distribution inside the bushing with an abnormal contact resistance between the copper conductive tube and aluminum conductive tube under different load current is simulated by using the finite element method. An special device is also developed for repairing defective bushing on-site, and 75 bushings with conductive contact defects have been repaired on the premise of not pushing converter transformers away from the valve hall and even without pulling out defective bushings. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessArticle
Insight into the Pore Characteristics of a Saudi Arabian Tight Gas Sand Reservoir
Energies 2019, 12(22), 4302; https://doi.org/10.3390/en12224302 - 12 Nov 2019
Abstract
The petrophysical characterization of tight gas sands can be affected by clay minerals, gas adsorption, microfractures, and the presence of high-density minerals. In this study, we conducted various petrophysical, petrographic, and high-resolution image analyses on Saudi Arabian tight sand in order to understand [...] Read more.
The petrophysical characterization of tight gas sands can be affected by clay minerals, gas adsorption, microfractures, and the presence of high-density minerals. In this study, we conducted various petrophysical, petrographic, and high-resolution image analyses on Saudi Arabian tight sand in order to understand how a complex pore system responds to measurement tools. About 140 plug samples extracted from six wells were subjected to routine core analyses including cleaning, drying, and porosity–permeability measurements. The porosity–permeability data was used to identify hydraulic flow units (HFU). In order to probe the factors contributing to the heterogeneity of this tight sand, 12 subsamples representing the different HFUs were selected for petrographic study and high-resolution image analysis using SEM, quantitative evaluation of minerals by scanning electron microscope (QEMSCAN), and micro-computed tomography (µCT). Nuclear magnetic resonance (NMR) and electrical resistivity measurements were also conducted on 56 subsamples representing various lithofacies. NMR porosity showed good agreement with other porosity measurements. The agreement was remarkable in specific lithofacies with porosity ranging from 0.1% to 7%. Above this range, significant scatters were seen between the porosity methods. QEMSCAN results revealed that samples with <7% porosity contain a higher proportion of clay than those with porosity >7%, which are either microfractured or contain partially dissolved labile minerals. The NMR T2 profiles also showed that samples with porosity <7% are dominated by micropores while samples with porosity >7% are dominated by macropores. Analysis of the µCT images revealed that pore throat sizes may be responsible for the poor correlation between NMR porosity and other porosity methods. NMR permeability values estimated using the Shlumberger Doll Research (SDR) method are fairly correlated with helium permeability (with an R2 of 0.6). Electrical resistivity measurements showed that the different rock types fall on the same slope of the formation factors versus porosity, with a cementation factor of 1.5. Full article
(This article belongs to the Special Issue Development of Unconventional Reservoirs)
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