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Energies, Volume 11, Issue 5 (May 2018)

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Cover Story (view full-size image) Lithium-ion battery manufacturing is a multi-step process involving synthesis, mixing, casting, [...] Read more.
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Open AccessFeature PaperArticle Development of Heat Transfer Surface Area Enhancements: A Test Facility for New Heat Exchanger Designs
Energies 2018, 11(5), 1322; https://doi.org/10.3390/en11051322
Received: 3 May 2018 / Revised: 15 May 2018 / Accepted: 19 May 2018 / Published: 22 May 2018
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Abstract
Enhancing the heat transfer surface by usage of cellular metal structures, such as foams or wire structures, might allow enlarging the surface area, increasing the heat transfer coefficients, decreasing the material utilization, and enabling the flexibility of different geometrical dimensions. However their manufacturing
[...] Read more.
Enhancing the heat transfer surface by usage of cellular metal structures, such as foams or wire structures, might allow enlarging the surface area, increasing the heat transfer coefficients, decreasing the material utilization, and enabling the flexibility of different geometrical dimensions. However their manufacturing and assembling in a large heat exchanger for performance testing and optimizing can be costly. Therefore a test rig was constructed for experimental characterization of heat transfer surface area enhancements. Heat exchanger samples with dimensions in the centimeter range can be measured. The fluid flow and heat transfer features of a micro pin fin wire structure made from copper by soft-soldering were experimentally characterized under steady-state forced air convection. The results are compared to performance characteristics of louvered fins. Heat transfer coefficients of the pin fins are twice as high as for the louvered fins. The relative expanded uncertainty of the Nusselt number is ±7%. Full article
(This article belongs to the Section Energy Fundamentals and Conversion)
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Open AccessArticle Reconstruction of Hydraulic Fractures Using Passive Ultrasonic Travel-Time Tomography
Energies 2018, 11(5), 1321; https://doi.org/10.3390/en11051321
Received: 30 March 2018 / Revised: 13 May 2018 / Accepted: 14 May 2018 / Published: 22 May 2018
Cited by 1 | PDF Full-text (8245 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
The knowledge of hydraulic fracture morphology is significant for the analysis of fracture mechanisms. This paper utilizes passive Ultrasonic Travel-time Tomography (UTT) to characterize the hydraulic fracture. We constructed a velocity model based on X-ray computerized tomography (X-CT) images scanned on a real
[...] Read more.
The knowledge of hydraulic fracture morphology is significant for the analysis of fracture mechanisms. This paper utilizes passive Ultrasonic Travel-time Tomography (UTT) to characterize the hydraulic fracture. We constructed a velocity model based on X-ray computerized tomography (X-CT) images scanned on a real hydraulically fractured shale column. Then, ray-paths and travel times corresponding to the source-receiver configuration were calculated by curved ray-tracing schemes. Lastly, we performed tomographic inversions using total variation regularization (TVR). The simulation results showed that 3D passive UTT based on TVR is an accurate, efficient, and stable method to reconstruct the velocity structures with fractures, even in the case of sparse ray-coverage or high noise level. Meanwhile, we also verified that the passive UTT is a valid alternative to X-CT in depicting the hydraulic fracturing rock via a proper interpretation method. Full article
(This article belongs to the Special Issue Unconventional Natural Gas (UNG) Recoveries 2018)
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Open AccessArticle Conditional Maximum Likelihood of Three-Phase Phasor Estimation for μPMU in Active Distribution Networks
Energies 2018, 11(5), 1320; https://doi.org/10.3390/en11051320
Received: 17 April 2018 / Revised: 8 May 2018 / Accepted: 14 May 2018 / Published: 22 May 2018
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Abstract
Micro phasor measurement units (μPMU) installed in active distribution networks are very useful for improving observability by acquiring system real-time data. However, three-phase imbalance and harmonic power flows adversely impact the accuracy of synchronous measurements, which implies the importance of phasor estimation errors.
[...] Read more.
Micro phasor measurement units (μPMU) installed in active distribution networks are very useful for improving observability by acquiring system real-time data. However, three-phase imbalance and harmonic power flows adversely impact the accuracy of synchronous measurements, which implies the importance of phasor estimation errors. This paper proposes a new phasor estimation algorithm for μPMU in active distribution networks that uses a conditional maximum likelihood (CML) estimation method. Firstly, the signal model of three-phase, three-wire and four-wire imbalance systems is established. Then, the probability distributions of the magnitude and phase angles are derived from the geometric characteristics of the CML method by solving the geometric equation. Simulation results show that the proposed CML based method is effective for estimating phasor and impedance models of active distribution networks by using μPMU measurement data. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessArticle Breakdown Characteristics of Oil-Pressboard Insulation under AC-DC Combined Voltage and Its Mathematical Model
Energies 2018, 11(5), 1319; https://doi.org/10.3390/en11051319
Received: 3 May 2018 / Revised: 17 May 2018 / Accepted: 17 May 2018 / Published: 22 May 2018
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Abstract
An AC-DC combined voltage is applied to the oil-pressboard insulation near the valve side during the operation of a converter transformer. To study the breakdown characteristics of an oil-pressboard insulation under such voltages, a typical plate electrode structure was employed in the laboratory
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An AC-DC combined voltage is applied to the oil-pressboard insulation near the valve side during the operation of a converter transformer. To study the breakdown characteristics of an oil-pressboard insulation under such voltages, a typical plate electrode structure was employed in the laboratory to conduct a breakdown test on the oil-pressboard insulation. The electrical field distribution and the DC contents of the transformer oil and the pressboard in composite insulation under the AC-DC combined voltage were simulated by their dielectric parameters. The breakdown strength of the transformer oil decreases with the increase in the DC content of the applied voltage, whereas that of the pressboard increases. For the oil-pressboard insulation, the breakdown voltage increases first and then decreases. The electric field strength decreases in the transformer oil with the increase in the DC content, whereas it increases in the pressboard. And the DC contents of the transformer and the pressboard in composite insulation were different from that of the applied voltage. Finally, based on the above results, a mathematical model was proposed to describe the breakdown characteristics of the oil-pressboard insulation under the AC-DC combined voltage; the theoretical and experimental results were in good agreement. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessArticle Regulation of Voltage and Frequency in Solid Oxide Fuel Cell-Based Autonomous Microgrids Using the Whales Optimisation Algorithm
Energies 2018, 11(5), 1318; https://doi.org/10.3390/en11051318
Received: 18 April 2018 / Revised: 10 May 2018 / Accepted: 18 May 2018 / Published: 22 May 2018
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Abstract
This study explores the Whales Optimization Algorithm (WOA)-based PI controller for regulating the voltage and frequency of an inverter-based autonomous microgrid (MG). The MG comprises two 50 kW DGs (solid oxide fuel cells, SOFCs) interfaced using a power electronics-based voltage source inverter (VSI)
[...] Read more.
This study explores the Whales Optimization Algorithm (WOA)-based PI controller for regulating the voltage and frequency of an inverter-based autonomous microgrid (MG). The MG comprises two 50 kW DGs (solid oxide fuel cells, SOFCs) interfaced using a power electronics-based voltage source inverter (VSI) with a 120-kV conventional grid. Four PI controller schemes for the MG are implemented: (i) stationary PI controller with fixed gain values (Kp and Ki), (ii) PSO tuned PI controller, (iii) GWO tuned PI controller, and (iv) WOA tuned PI controller. The performance of these controllers is evaluated by monitoring the system voltage and frequency during the transition of MG operation mode and changes in the load. The MATLAB/SIMULINK tool is utilised to design the proposed model of grid-tied MG alongside the MATLAB m-file to apply an optimisation technique. The simulation results show that the WOA-based PI controller which optimises the control parameters, achieve 62.7% and 59% better results for voltage and frequency regulation, respectively. The eigenvalue analysis is also provided to check the stability of the proposed controller. Furthermore, the proposed system also satisfies the limits specified in IEEE-1547-2003 for voltage and frequency. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessEditorial Smart Green Applications: From Renewable Energy Management to Intelligent Transportation Systems
Energies 2018, 11(5), 1317; https://doi.org/10.3390/en11051317
Received: 4 May 2018 / Revised: 10 May 2018 / Accepted: 10 May 2018 / Published: 22 May 2018
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Open AccessArticle Power System Restoration Planning Strategy Based on Optimal Energizing Time of Sectionalizing Islands
Energies 2018, 11(5), 1316; https://doi.org/10.3390/en11051316
Received: 14 April 2018 / Revised: 11 May 2018 / Accepted: 14 May 2018 / Published: 22 May 2018
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Abstract
Common power system restoration planning strategy is based on a ‘build up’ approach, where a blackout system is sectionalized among several islands for parallel restoration prior to resynchronization. In order to speed up the resynchronization of the islands, each island must have similar
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Common power system restoration planning strategy is based on a ‘build up’ approach, where a blackout system is sectionalized among several islands for parallel restoration prior to resynchronization. In order to speed up the resynchronization of the islands, each island must have similar energizing times. However, there is a huge number of possible combinations of islands that can be formed. Thus, this paper proposes a method to determine optimal islands that have similar energizing times. The method involves identifying transmission lines that should not be connected to form the islands. The proposed method is based on the combination of heuristic and discrete optimization methods. The heuristic technique is proposed to find initial solution that is close to the optimal solution. This solution will guide the optimization technique, which is the discrete Artificial Bee Colony optimization method, to find the optimum solution. The proposed method also considers restoration constraints including black start generator availability, load-generation balance, and the maintenance of acceptable voltage magnitude within each island. The proposed method is validated via simulation using IEEE 39, 118-bus and 89-bus European systems. The advantage of the proposed method in terms of restoration time is demonstrated through a comparison with other literature. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessArticle A Comprehensive Approach for Modelling Horizontal Diffuse Radiation, Direct Normal Irradiance and Total Tilted Solar Radiation Based on Global Radiation under Danish Climate Conditions
Energies 2018, 11(5), 1315; https://doi.org/10.3390/en11051315
Received: 17 April 2018 / Revised: 3 May 2018 / Accepted: 5 May 2018 / Published: 22 May 2018
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Abstract
A novel combined solar heating plant with flat plate collectors (FPC) and parabolic trough collectors (PTC) was constructed and put into operation in Taars, 30 km north of Aalborg, Denmark in August 2015. To assess the thermal performance of the solar heating plant,
[...] Read more.
A novel combined solar heating plant with flat plate collectors (FPC) and parabolic trough collectors (PTC) was constructed and put into operation in Taars, 30 km north of Aalborg, Denmark in August 2015. To assess the thermal performance of the solar heating plant, global radiation, direct normal irradiance (DNI) and total radiation on the tilted collector plane of the flat plate collector field were measured. To determine the accuracy of the measurements, the calculated solar radiations, including horizontal diffuse radiation, DNI and total tilted solar radiation with seven empirical models, were compared each month based on an hourly time step. In addition, the split of measured global radiation into diffuse and beam radiation based on a model developed by DTU (Technical University of Denmark) and the Reduced Reindl correlation model was investigated. A new method of combining empirical models, only based on measured global radiation, was proposed for estimating hourly total radiation on tilted surfaces. The results showed that the DTU model could be used to calculate diffuse radiation on the horizontal surface, and that the anisotropic models (Perez I and Perez II) were the most accurate for calculation of total radiation on tilted collector surfaces based only on global radiation under Danish climate conditions. The proposed method was used to determine reliable horizontal diffuse radiation, DNI and total tilted radiation with only the measurement of global radiation. Only a small difference compared to measured data, was found. The proposed method was cost-effective and needed fewer measurements to obtain reliable DNI and total radiation on the tilted plane. This method may be extended to other Nordic areas that have similar weather. Full article
(This article belongs to the Special Issue Solar Thermal Energy Utilization Technologies in Buildings)
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Open AccessArticle Active-Reactive Additional Damping Control of a Doubly-Fed Induction Generator Based on Active Disturbance Rejection Control
Energies 2018, 11(5), 1314; https://doi.org/10.3390/en11051314
Received: 21 March 2018 / Revised: 21 April 2018 / Accepted: 17 May 2018 / Published: 21 May 2018
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Abstract
Large-scale wind power interfacing to the power grid has an impact on the stability of the power system. However, with an additional damping controller of the wind generator, new ways for improving system damping and suppressing the low frequency oscillation (LFO) of power
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Large-scale wind power interfacing to the power grid has an impact on the stability of the power system. However, with an additional damping controller of the wind generator, new ways for improving system damping and suppressing the low frequency oscillation (LFO) of power systems can be put forward. In this paper, an active-reactive power additional damping controller based on active disturbance rejection control (ADRC) is proposed. In order to improve the precision of the controller, the theory of data driven control is adopted, using the numerical algorithms for subspace state space system identification (N4SID) to obtain the two order model of the ADRC controlled object. Based on the identification model, the ADRC additional damping controller is designed. Taking a 2-area 4-machine system containing the doubly fed induction generator (DFIG) wind farm as an example, it is verified that the active-reactive additional damping controller designed in this paper performs well in suppressing negative-damping LFO and forced power oscillation. When the operation state of the power system changes, it can still restrain the LFO effectively, showing stronger robustness and better effectiveness compared to the traditional proportional–integral–derivative (PID) additional damping controller. Full article
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Open AccessArticle Islanding Detection Method Based on Injecting Perturbation Signal and Rate of Change of Output Power in DC Grid-Connected Photovoltaic System
Energies 2018, 11(5), 1313; https://doi.org/10.3390/en11051313
Received: 19 April 2018 / Revised: 11 May 2018 / Accepted: 18 May 2018 / Published: 21 May 2018
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Abstract
The emergence of Distributed Generation (DG) in the electric system has brought about the appearance of the islanding phenomenon. In AC networks, there are a lot of Islanding Detection Methods (IDMs) have been studied. However, not too much IDMs in DC networks have
[...] Read more.
The emergence of Distributed Generation (DG) in the electric system has brought about the appearance of the islanding phenomenon. In AC networks, there are a lot of Islanding Detection Methods (IDMs) have been studied. However, not too much IDMs in DC networks have been published because of the absence of frequency and reactive power. The hybrid IDM based on injected perturbation signal and rate of change of power output is proposed. This IDM can detect islanding condition not only in the worst case (the power of load and PV are equal) but also in another case (the power of load is greater than the power of PV). It can be applicable to both single and multi-PV operation scenarios. Besides, the effectiveness of the proposed method is verified by simulation in Matlab/Simulink. Full article
(This article belongs to the Section Electrical Power and Energy System)
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Open AccessArticle Explorative Multidimensional Analysis for Energy Efficiency: DataViz versus Clustering Algorithms
Energies 2018, 11(5), 1312; https://doi.org/10.3390/en11051312
Received: 10 April 2018 / Revised: 8 May 2018 / Accepted: 10 May 2018 / Published: 21 May 2018
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Abstract
We propose a simple tool to help the energy management of a large building stock defining clusters of buildings with the same function, setting alert thresholds for each cluster, and easily recognizing outliers. The objective is to enable a building management system to
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We propose a simple tool to help the energy management of a large building stock defining clusters of buildings with the same function, setting alert thresholds for each cluster, and easily recognizing outliers. The objective is to enable a building management system to be used for detection of abnormal energy use. We start reviewing energy performance indicators, and how they feed into data visualization (DataViz) tools for a large building stock, especially for university campuses. After a brief presentation of the University of Turin’s building stock which represents our case study, we perform an explorative analysis based on the Multidimensional Detective approach by Inselberg, using the Scatter Plot Matrix and the Parallel Coordinates methods. The k-means clustering algorithm is then applied on the same dataset to test the hypotheses made during the explorative analysis. Our results show that DataViz techniques provide quick and user-friendly solutions for the energy management of a large stock of buildings. In particular, they help identifying clusters of buildings and outliers and setting alert thresholds for various Energy Efficiency Indices. Full article
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Open AccessArticle Improved Synchronous Machine Rotor Design for the Easy Assembly of Excitation Coils Based on Surrogate Optimization
Energies 2018, 11(5), 1311; https://doi.org/10.3390/en11051311
Received: 10 April 2018 / Revised: 4 May 2018 / Accepted: 16 May 2018 / Published: 21 May 2018
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Abstract
This paper introduces a new rotor design for the easy insertion and removal of rotor windings. The shape of the rotor is optimized based on a surrogate method in order to achieve low power loss under the maximum power output. The synchronous machine
[...] Read more.
This paper introduces a new rotor design for the easy insertion and removal of rotor windings. The shape of the rotor is optimized based on a surrogate method in order to achieve low power loss under the maximum power output. The synchronous machine with the new rotor is evaluated in 2-D finite element software and validated by experiments. This rotor shows great potential for reducing the maintenance and repair costs of synchronous machines, making it particularly suited for low-cost mass production markets including gen-sets, steam turbines, wind power generators, and hybrid electric vehicles. Full article
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Open AccessArticle Research on the Robustness of the Constant Speed Control of Hydraulic Energy Storage Generation
Energies 2018, 11(5), 1310; https://doi.org/10.3390/en11051310
Received: 24 April 2018 / Revised: 17 May 2018 / Accepted: 17 May 2018 / Published: 21 May 2018
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Abstract
Energy storage plays a major role in solving the fluctuation and intermittence problem of wind and the effective use of wind power. The application of the hydraulic accumulator is the most efficient and convenient way to store wind energy in hydraulic wind turbines.
[...] Read more.
Energy storage plays a major role in solving the fluctuation and intermittence problem of wind and the effective use of wind power. The application of the hydraulic accumulator is the most efficient and convenient way to store wind energy in hydraulic wind turbines. A hydraulic energy storage generation system (HESGS) can transform hydraulic energy stored in the hydraulic accumulator into stable and constant electrical energy by controlling the variable motor, regardless of wind changes. The aim of the present study is to design a constant speed control method for the variable motor in the HESGS and investigate the influence of the controller’s main parameters on the resistance of the HESGS to external load power disturbances. Mathematical equations of all components in this system are introduced and an entire system simulation model is built. A double closed-loop control method of the variable motor is presented within this paper, which keeps the motor speed constant for the fixed frequency of electrical power generated by the HESGS. Ultimately, a series of simulations with different proportional gains and integral gains under the environment of changeless load power step are conducted. At the same time, comparison analyses of the experiment and simulation under variable load power step are performed. The results verify the correctness and the usability of the simulation model, and also indicate that the proposed control method is robust to the disturbances of changing load power. Full article
(This article belongs to the Section Energy Storage and Application)
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Open AccessArticle Methods for Advanced Wind Turbine Condition Monitoring and Early Diagnosis: A Literature Review
Energies 2018, 11(5), 1309; https://doi.org/10.3390/en11051309
Received: 23 April 2018 / Revised: 10 May 2018 / Accepted: 12 May 2018 / Published: 21 May 2018
Cited by 1 | PDF Full-text (1747 KB) | HTML Full-text | XML Full-text
Abstract
Condition monitoring and early fault diagnosis for wind turbines have become essential industry practice as they help improve wind farm reliability, overall performance and productivity. If not detected and rectified at early stages, some faults can be catastrophic with significant loss or revenue
[...] Read more.
Condition monitoring and early fault diagnosis for wind turbines have become essential industry practice as they help improve wind farm reliability, overall performance and productivity. If not detected and rectified at early stages, some faults can be catastrophic with significant loss or revenue along with interruption to the business relying mainly on wind energy. The failure of Wind turbine results in system downtime and repairing or replacement expenses that significantly reduce the annual income. Such failures call for more systematized operation and maintenance schemes to ensure the reliability of wind energy conversion systems. Condition monitoring and fault diagnosis systems of wind turbine play an important role in reducing maintenance and operational costs and increase system reliability. This paper is aimed at providing the reader with the overall feature for wind turbine condition monitoring and fault diagnosis which includes various potential fault types and locations along with the signals to be analyzed with different signal processing methods. Full article
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Open AccessArticle Hybrid Coupled Multifracture and Multicontinuum Models for Shale Gas Simulation by Use of Semi-Analytical Approach
Energies 2018, 11(5), 1308; https://doi.org/10.3390/en11051308
Received: 26 April 2018 / Revised: 5 May 2018 / Accepted: 14 May 2018 / Published: 21 May 2018
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Abstract
Combining the advantages of multicontinuum and multifracture representations provides an easy-to-use tool to adequately capture the characteristic of the multiscaled fracture system in shale gas reservoir. A hybrid model is established on the basis of simplified conceptual productivity assumption, where the matrix volume
[...] Read more.
Combining the advantages of multicontinuum and multifracture representations provides an easy-to-use tool to adequately capture the characteristic of the multiscaled fracture system in shale gas reservoir. A hybrid model is established on the basis of simplified conceptual productivity assumption, where the matrix volume is divided into two sub-domains (triple-porosity model and dual-depletion flowing model) and the fracture volume is represented by discrete finite conductivity fracture. In addition, the mechanisms of instant desorption, viscous flow and dual-depletion in matrix are taken into account. The rate transient responses are then obtained by use of semi-analytical approach. Based on the model, type curves are plotted and verified by comparing with alternative reliable methods. Different flow regimes in shale gas reservoirs can be identified and detected. The Generalized Likelihood Uncertainty Estimation methodology, based on probabilistic aggregation theory, is employed to integrating those two productivity models together such that the production can be predicted more accurately. A field example is applied to validate the applicability of this new model. Finally, it is concluded that the proposed model can predict the rate and cumulative rate more easily and practically. Full article
(This article belongs to the Section Energy Sources)
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