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Energies, Volume 12, Issue 20 (October-2 2019)

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Open AccessArticle
Characterization of a Practical-Based Ohmic Series Resistance Model under Life-Cycle Changes for a Lithium-Ion Battery
Energies 2019, 12(20), 3888; https://doi.org/10.3390/en12203888 (registering DOI) - 14 Oct 2019
Abstract
Understanding battery characteristic behaviors is indispensable in designing and managing large-scale battery-based energy storage systems in high-power applications. This paper presents a practical-based characterization method to model the ohmic series resistance of lithium-ion batteries under life-cycle consideration. Aging cells were prepared in a [...] Read more.
Understanding battery characteristic behaviors is indispensable in designing and managing large-scale battery-based energy storage systems in high-power applications. This paper presents a practical-based characterization method to model the ohmic series resistance of lithium-ion batteries under life-cycle consideration. Aging cells were prepared in a controlled environment, and the testing information was automatically characterized using a developed computer-based battery test system. An experimental methodology based on the cycling of pulse tests is applied for modeling the ohmic series resistance. Several aspects of the testing parameters during the cycling operations, such as the characteristic changes of the ohmic series resistance, amplitudes of the periodic test current, cell capacity, state of charge, and the rate of change of the resistance increment, are also investigated and analyzed so as to fulfill the resistance model. The accuracy of the proposed model is verified by comparing the testing information, showing a satisfactory result. Full article
(This article belongs to the Special Issue Smart Power & Internet Energy Systems )
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Open AccessArticle
Mitigation Method of Slot Harmonic Cogging Torque Considering Unevenly Magnetized Permanent Magnets in PMSM
Energies 2019, 12(20), 3887; https://doi.org/10.3390/en12203887 (registering DOI) - 14 Oct 2019
Abstract
This paper presents a mitigation method of slot harmonic cogging torque considering unevenly magnetized magnets in a permanent magnet synchronous motor. In previous studies, it has been confirmed that non-uniformly magnetized permanent magnets cause an unexpected increase of cogging torque because of additional [...] Read more.
This paper presents a mitigation method of slot harmonic cogging torque considering unevenly magnetized magnets in a permanent magnet synchronous motor. In previous studies, it has been confirmed that non-uniformly magnetized permanent magnets cause an unexpected increase of cogging torque because of additional slot harmonic components. However, these studies did not offer a countermeasure against it. First, in this study, the relationship between the residual magnetic flux density of the permanent magnet and the cogging torque is derived from the basic form of the Maxwell stress tensor equation. Second, the principle of the slot harmonic cogging torque generation is explained qualitatively, and the mitigation method of the slot harmonic component is proposed. Finally, the proposed method is verified with the finite element analysis and experimental results. Full article
(This article belongs to the Special Issue Advances in Rotating Electric Machines)
Open AccessArticle
Adaptive-Gain Second-Order Sliding Mode Direct Power Control for Wind-Turbine-Driven DFIG under Balanced and Unbalanced Grid Voltage
Energies 2019, 12(20), 3886; https://doi.org/10.3390/en12203886 (registering DOI) - 14 Oct 2019
Abstract
In a wind turbine system, a doubly-fed induction generator (DFIG), with nonlinear and high-dimensional dynamics, is generally subjected to unbalanced grid voltage and unknown uncertainty. This paper proposes a novel adaptive-gain second-order sliding mode direct power control (AGSOSM-DPC) strategy for a wind-turbine-driven DFIG, [...] Read more.
In a wind turbine system, a doubly-fed induction generator (DFIG), with nonlinear and high-dimensional dynamics, is generally subjected to unbalanced grid voltage and unknown uncertainty. This paper proposes a novel adaptive-gain second-order sliding mode direct power control (AGSOSM-DPC) strategy for a wind-turbine-driven DFIG, valid for both balanced and unbalanced grid voltage. The AGSOSM-DPC control scheme is presented in detail to restrain rotor voltage chattering and deal with the scenario of unknown uncertainty upper bound. Stator current harmonics and electromagnetic torque ripples can be simultaneously restrained without phase-locked loop (PLL) and phase sequence decomposition using new active power expression. Adaptive control gains are deduced based on the Lyapunov stability method. Comparative simulations under three DPC schemes are executed on a 2-MW DFIG under both balanced and unbalanced grid voltage. The proposed strategy achieved active and reactive power regulation under a two-phase stationary reference frame for both balanced and unbalanced grid voltage. An uncertainty upper bound is not needed in advance, and the sliding mode control chattering is greatly restrained. The simulation results verify the effectiveness, robustness, and superiority of the AGSOSM-DPC strategy. Full article
(This article belongs to the Special Issue Sliding Mode Control of Power Converters in Renewable Energy Systems)
Open AccessArticle
Boosting Hydrogen Production from Formic Acid over Pd Catalysts by Deposition of N-Containing Precursors on the Carbon Support
Energies 2019, 12(20), 3885; https://doi.org/10.3390/en12203885 (registering DOI) - 14 Oct 2019
Abstract
Formic acid is a promising liquid organic hydrogen carrier (LOHC) since it has relatively high hydrogen content (4.4 wt%), low inflammability, low toxicity and can be obtained from biomass or from CO2. The aim of the present research was the creation [...] Read more.
Formic acid is a promising liquid organic hydrogen carrier (LOHC) since it has relatively high hydrogen content (4.4 wt%), low inflammability, low toxicity and can be obtained from biomass or from CO2. The aim of the present research was the creation of efficient 1 wt% Pd catalysts supported on mesoporous graphitic carbon (Sibunit) for the hydrogen production from gas-phase formic acid. For this purpose, the carbon support was modified by pyrolysis of deposited precursors containing pyridinic nitrogen such as melamine (Mel), 2,2′-bipyridine (Bpy) or 1,10-phenanthroline (Phen) at 673 K. The following activity trend of the catalysts Pd/Mel/C > Pd/C ~ Pd/Bpy/C > Pd/Phen/C was obtained. The activity of the Pd/Mel/C catalyst was by a factor of 4 higher than the activity of the Pd/C catalyst at about 373 K and the apparent activation energy was significantly lower than those for the other catalysts (32 vs. 42–46 kJ/mol). The high activity of the melamine-based samples was explained by a high dispersion of Pd nanoparticles (~2 nm, HRTEM) and their strong electron-deficient character (XPS) provided by interaction of Pd with pyridinic nitrogen species of the support. The presented results can be used for the development of supported Pd catalysts for hydrogen production from different liquid organic hydrogen carriers. Full article
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Open AccessArticle
Present Geothermal Characteristics and Influencing Factors in the Xiong’an New Area, North China
Energies 2019, 12(20), 3884; https://doi.org/10.3390/en12203884 (registering DOI) - 14 Oct 2019
Abstract
The present geothermal characteristics and influencing factors are analyzed to conduct geothermal resource exploration in the Xiong’an New Area. Thermal conductivity data for 100 rock samples are obtained from different wells and a sedimentary strata thermal conductivity column is proposed. From these data, [...] Read more.
The present geothermal characteristics and influencing factors are analyzed to conduct geothermal resource exploration in the Xiong’an New Area. Thermal conductivity data for 100 rock samples are obtained from different wells and a sedimentary strata thermal conductivity column is proposed. From these data, heat flow distribution in the area is mapped using equilibrium temperature logs obtained for 32 wells. The heat flow in this area is found to be 53.3–106.5 mW·m−2 (average: 73 mW·m−2). The uplift heat value in Niutuozhen and Rongcheng uplift is 106.5 and 90 mW·m−2, respectively. The sag heat flow is relatively low and the Baxian sag’s heat flow value is 48.9–61.6 mW·m−2. Thermal conductivity differences among Cenozoic caprock, Proterozoic carbonate reservoirs, and basement rock mainly affects the geothermal distribution. The low and high thermal conductivities of the caprock and thermal reservoir as well as basement, respectively, cause heat flow redistribution in the surface during conduction. Groundwater rises to geothermal reservoirs through heat-controlling faults, causing convective heat transfer and increasing the geothermal reservoir temperature; therefore, high-temperature groundwater accumulates in the shallow uplift areas. The caprock’s thin uplift area exhibits a high geothermal background due to water convergence. Understanding the geothermal characteristics and influencing factors is necessary for understanding the distribution law and factors influencing geothermal resources and guiding geothermal exploration and development in the Xiong’an New Area. Full article
(This article belongs to the Section Geo-Energy)
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Open AccessArticle
Feasibility Analysis and Performance Evaluation and Optimization of a DXSAHP Water Heater Based on the Thermal Capacity of the System: A Case Study
Energies 2019, 12(20), 3883; https://doi.org/10.3390/en12203883 (registering DOI) - 14 Oct 2019
Abstract
The present work conducts an evaluation of the feasibility and the overall performance and consequent optimization of a direct expansion solar assisted heat pump (DXSAHP) employed for domestic water heating. For the study conducted R134a, R404A, R407C and R410A working fluids were evaluated [...] Read more.
The present work conducts an evaluation of the feasibility and the overall performance and consequent optimization of a direct expansion solar assisted heat pump (DXSAHP) employed for domestic water heating. For the study conducted R134a, R404A, R407C and R410A working fluids were evaluated as well as the use of four, six and eight flat-plate solar collectors and a worktime ranging from 1 to 6 h. The case study is based in Mexico City with a 300 L container and a hot water outlet temperature of 51 °C. The paper introduces a new evaluation criterion based on the thermal capacity and all the evaluations conducted throughout this research revolve around this performance metric. The results show that, the system would require at least 4 h of operation to achieve the outlet temperature. Additionally, it was found that the R410A refrigerant has the best heat transfer properties; with an average condensation heat rate of 6.31 kW, followed by the R407C with 5.72 kW, the R404A with 5.42 kW and the R134a with 5.18 kW. Diversely, the R134a refrigerant requires 0.402 kW of compression work, 62% less than the R410A, which requires 1.06 kW. Consequently, R134a delivers the highest COP, which ranges from 7 to 14, followed by the R407C and R404A refrigerants, which present a similar behaviour between them, with COP ranging from 5 to 9 and 4 to 8, respectively, and finally the R410A, achieving the lowest COP, ranging from 3.5 to 6.5. Moreover, it was found that the R134a presents a higher dispersion regarding the energy exchange rate, which reveals that it is the fluid most susceptible to external factors, such as the weather. Contrarily, the remaining refrigerants present a more consistent performance. Finally, the optimization revealed that the R407C refrigerant is the most suitable given that it requires 20% less compression work than the R404A. This provides the heat pump system with a steadier behaviour, a COP ranging from 7 to 8, 30% higher than R410A, a worktime decrease of 1.5 h and heat transfer area of 5.5 flat-plate solar collectors, equivalent to a 31% reduction, both compared to R134a. Full article
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Open AccessArticle
Application of Dipole Array Acoustic Logging in the Evaluation of Shale Gas Reservoirs
Energies 2019, 12(20), 3882; https://doi.org/10.3390/en12203882 (registering DOI) - 14 Oct 2019
Abstract
In order to effectively evaluate shale gas reservoirs with low porosity, extra-low permeability, and no natural productivity, dipole array acoustic logging, which can provide various types of information including P-wave slowness (DTC) and S-wave slowness (DTS), is widely used. As the dipole array [...] Read more.
In order to effectively evaluate shale gas reservoirs with low porosity, extra-low permeability, and no natural productivity, dipole array acoustic logging, which can provide various types of information including P-wave slowness (DTC) and S-wave slowness (DTS), is widely used. As the dipole array acoustic logging tool has a larger investigation depth and is suitable for complex borehole environments, such as those with a high wellbore temperature, high drilling fluid column pressure, or irregular borehole wall, it has been mainly applied to the evaluation of lithology, gas potential, fractures, and stimulation potential in shale gas reservoirs. The findings from a case study of the Sichuan Basin in China reveal that the acoustic slowness, S-P wave slowness ratio (RMSC), and S-wave anisotropy of the dipole array acoustic logging can be used to qualitatively identify reservoir lithology, gas potential, and fractures. Using the relationship between DTC and the total porosity of shale gas reservoirs, and combined with the compensated neutron (CNL) and shale content (Vsh) of the reservoir, a mathematical model for accurately calculating the total porosity of the shale gas reservoir can be established. By using the relationship between the RMSC and gas saturation in shale gas reservoirs and tied with density log (DEN), a mathematical model of gas saturation can be established, and the determination of gas saturation by the non-resistivity method can be achieved, delivering a solution to the issue that the electric model is not applicable under low resistivity conditions. The DTS, DTC, and DEN of shale can be used to calculate rock mechanic parameters such as the Poisson’s ratio (POIS) and Young’s modulus (YMOD), which can be used to evaluate the shale stimulation potential. Full article
(This article belongs to the Section Geo-Energy)
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Open AccessArticle
Experimental Study on Water Shutoff Technology Using In-Situ Ion Precipitation for Gas Reservoirs
Energies 2019, 12(20), 3881; https://doi.org/10.3390/en12203881 (registering DOI) - 14 Oct 2019
Abstract
Once a gas well begins to produce water, gas production will be seriously affected. If no effective measures are taken, the gas well will be shut down. Although some methods can be adopted to limit the production of unwanted water in gas reservoirs, [...] Read more.
Once a gas well begins to produce water, gas production will be seriously affected. If no effective measures are taken, the gas well will be shut down. Although some methods can be adopted to limit the production of unwanted water in gas reservoirs, they do not radically solve the problem of excessive water production, which may cause gas shutoff and dramatically increase the cost of dealing with disposing of the unwanted water. In this study, water shutoff technology with in-situ ion precipitation was tested for a gas well in southwest China, with results demonstrated through experiments of ion precipitation distribution and core displacement. The results of the ion precipitation distribution experiments show that it can be artificially controlled to produce ion precipitation blocking the water layer. The distribution of ion precipitation depends on influencing factors including injection flow rate, injection height, and ion concentration, which is generally hill-shaped. Dynamic displacement experiments through two types of cores (matrix core and fracture core) show that during the process of injecting gas-field water into the core, the ion precipitation caused by the in-situ reaction constantly blocked the seepage channel, resulting in a significant decrease in the injection rate. After injection of the gas-field water, the permeability of the core decreased, the starting pressure gradient increased, and the effect of water shutoff was remarkable. Given the experimental results, the in-situ ion precipitation water-plugging technology for gas reservoirs can directly solve the problem of water production in gas wells in the water layer because it can block the water layer by using formation water itself. This technology has promise for use in southwest China and can provide technical guidance for other gas reservoirs. Full article
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Open AccessArticle
Biogas and Methane Potential of Pre-Thermally Disintegrated Bio-Waste
Energies 2019, 12(20), 3880; https://doi.org/10.3390/en12203880 (registering DOI) - 14 Oct 2019
Abstract
One of the environmental solutions employed in order to achieve circular economy goals is methane fermentation—a technology that is beneficial both for the stabilization and reduction of organic waste and for alternative energy generation. The article presents the results of research aimed at [...] Read more.
One of the environmental solutions employed in order to achieve circular economy goals is methane fermentation—a technology that is beneficial both for the stabilization and reduction of organic waste and for alternative energy generation. The article presents the results of research aimed at determining the biogas and methane potential of bio-waste which has been pre-thermally disintegrated, and determining the influence of variable process parameters of disintegration on the kinetics of fermentation. A first-order kinetic model was used to describe the fermentation as well as two mathematical models: logistic and Gompertz. It has been found that process parameters such as time (0.5, 1 and 2 h) and temperature (between 55 to 175 °C) have a significant effect on the solubilization efficiency of the bio-waste. The methane fermentation of thermally disintegrated bio-waste showed that the highest biogas potential is characterized by samples treated, respectively, for 0.5 h at 155 °C and for 2 h at 175 °C. The best match for the experimental data of biogas production from disintegrated substrates was demonstrated for the Gompertz model. Full article
(This article belongs to the Section Bio-Energy)
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Open AccessArticle
Impact of Primary Frequency Control of Offshore HVDC Grids on Interarea Modes of Power Systems
Energies 2019, 12(20), 3879; https://doi.org/10.3390/en12203879 (registering DOI) - 14 Oct 2019
Abstract
Offshore high-voltage DC (HVDC) grids are developing as a technically reliable and economical solution to transfer more offshore wind power to onshore power systems. It is also foreseen that the offshore HVDC grids pave the way for offshore wind participation in power systems’ [...] Read more.
Offshore high-voltage DC (HVDC) grids are developing as a technically reliable and economical solution to transfer more offshore wind power to onshore power systems. It is also foreseen that the offshore HVDC grids pave the way for offshore wind participation in power systems’ balancing process through frequency support. The primary frequency control mechanism in an HVDC grid can be either centralized using communication links between HVDC terminals or decentralized by the simultaneous use of DC voltage and frequency droop controls. This paper investigates the impact of both types of primary frequency control of offshore HVDC grids on onshore power system dynamics. Parametric presentation of power systems’ electro-mechanical dynamics and HVDC controls is developed to analytically prove that the primary frequency control can improve the damping of interarea modes of onshore power systems. The key findings of the paper include showing that the simultaneous use of frequency and DC voltage droop controls on onshore converters results in an autonomous share of damping torque between onshore power systems even without any participation of offshore wind farms in the frequency control. It is also found that the resulting damping from the frequency control of offshore HVDC is not always reliable as it can be nullified by the power limits of HVDC converters or wind farms. Therefore, using power oscillation damping control in parallel with frequency control is suggested. The analytical findings are verified by simulations on a three-terminal offshore HVDC grid. Full article
(This article belongs to the Special Issue Control and Protection of HVDC-Connected Offshore Wind Power Plants)
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Open AccessEditorial
Special Issue “Nanogrids, Microgrids, and the Internet of Things (IoT): Towards the Digital Energy Network”
Energies 2019, 12(20), 3878; https://doi.org/10.3390/en12203878 - 14 Oct 2019
Abstract
I started hearing a lot about energy digitization a little over a year ago, talking to my colleagues during conferences and meetings [...] Full article
Open AccessArticle
Experimental and Numerical Analysis of a Seawall’s Effect on Wind Turbine Performance
Energies 2019, 12(20), 3877; https://doi.org/10.3390/en12203877 - 14 Oct 2019
Abstract
For the purposes of this study, a wind tunnel experiment and a numerical analysis during ebb and high tides were conducted to determine the positive and negative effects of wind flow influenced by a seawall structure on the performance of wind turbines installed [...] Read more.
For the purposes of this study, a wind tunnel experiment and a numerical analysis during ebb and high tides were conducted to determine the positive and negative effects of wind flow influenced by a seawall structure on the performance of wind turbines installed along a coastal seawall. The comparison of the wind flow field between a wind tunnel experiment performed with a 1/100 scale model and a computational fluid dynamics (CFD) analysis confirmed that the MP k-turbulence model estimated flow separation on the leeside of the seawall the most accurately. The CFD analysis verified that wind speed-up occurred due to the virtual hill effect caused by the seawall’s windward slope and the recirculation zone of its rear face, which created a positive effect by mitigating wind shear while increasing the mean wind speed in the wind turbine’s rotor plane. In contrast, the turbulence effect of flow separation on the seawall’s leeside was limited to the area below the wind turbine rotor, and had no negative effect. The use of the CFD verified with the comparison with the wind tunnel experiment was extended to the full-scale seawall, and the results of the analysis based on the wind turbine Supervisory Control and Data Acquisition (SCADA) data of a wind farm confirmed that the seawall effect was equivalent to a 1.5% increase in power generation as a result of a mitigation of the wind profile. Full article
(This article belongs to the Special Issue Computational Fluid Dynamics (CFD) 2018)
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Open AccessArticle
High-Resolution Household Load Profiling and Evaluation of Rooftop PV Systems in Selected Houses in Qatar
Energies 2019, 12(20), 3876; https://doi.org/10.3390/en12203876 - 14 Oct 2019
Abstract
Even though Qatar’s per capita electricity consumption is one of the highest in the world, little is currently known about behind-the-meter power consumption. The residential sector is the largest consumer of electricity, accounting for approximately 59% of the overall consumption of electricity. As [...] Read more.
Even though Qatar’s per capita electricity consumption is one of the highest in the world, little is currently known about behind-the-meter power consumption. The residential sector is the largest consumer of electricity, accounting for approximately 59% of the overall consumption of electricity. As energy subsidies lead to budget deficits and overconsumption of carbon resources, there is a pressing need to examine the residential load profile to better understand consumption patterns and uncover potential solutions for more efficient usage. Residential load profiles are typically influenced by seasonal and socio-economic factors. Furthermore, household load profiles can be used to examine the viability of rooftop photovoltaic (PV) systems. In this study, a total of 10 houses in Qatar were chosen, and their power demand was monitored for over a year using smart energy monitors. This empirical research was conducted to achieve the following goals: (1) creation of the first high-resolution residential load profiles in Qatar and in the Gulf region; (2) analyses of the acquired load profiles and the determining factors that affect energy consumption; and (3) calculation of self-consumption values, analysis of the viability of household rooftop PV systems, and discussing potential use-cases for energy storage systems. Investigation of this topic is particularly important for Qatar as the country is adopting a sizable portion of PV systems (5% by 2021) and promotes sustainable energy options as a part of a national development strategy. Results show that there are significant differences between per-household and per-capita consumption due to factors such as electricity subsidies, household income and size, and air-conditioner type. Moreover, due to high electricity consumption, distributed energy storage units for bill management applications have limited applicability with current pricing tariffs. To the best of authors’ knowledge, this is the first study conducted in Qatar and in the Gulf region where a growing amount of interest is given to measure and improve building energy performance. Full article
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Open AccessArticle
Segmental Track Analysis in Dynamic Wireless Power Transfer
Energies 2019, 12(20), 3875; https://doi.org/10.3390/en12203875 - 14 Oct 2019
Abstract
Electric vehicles have gained more and more attention because of the serious oil crisis and environmental problems. However, the disadvantages of the electric vehicle, such as short driving range, high battery cost, and inconvenient charging, are hindering its market development and expansion. The [...] Read more.
Electric vehicles have gained more and more attention because of the serious oil crisis and environmental problems. However, the disadvantages of the electric vehicle, such as short driving range, high battery cost, and inconvenient charging, are hindering its market development and expansion. The realization of on-road wireless power transfer technology can effectively solve the problems of short driving range, prevent the battery from being completely discharged to prolong its service life, and reduce requirement of on-board battery. In this paper, the charging mode and the compensation topology of wireless power transfer technology are discussed and then the equivalent circuit model of segmental wireless power transfer system is built. We carried out some magnetic field simulation to analyze how the track shape and length influence coupling coefficient, which is later verified by experiments. Full article
(This article belongs to the collection Electric and Hybrid Vehicles Collection)
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Open AccessArticle
Application of Synthetic Ester for Drying Distribution Transformer Insulation—The Influence of Cellulose Thickness on Drying Efficiency
Energies 2019, 12(20), 3874; https://doi.org/10.3390/en12203874 - 13 Oct 2019
Viewed by 162
Abstract
A high level of insulation moisture in a transformer increases the breakdown probability and forces a reduction of its load. Therefore, there is a need to dry the transformer insulation. For technical reasons, there are some restrictions on the use of common drying [...] Read more.
A high level of insulation moisture in a transformer increases the breakdown probability and forces a reduction of its load. Therefore, there is a need to dry the transformer insulation. For technical reasons, there are some restrictions on the use of common drying methods for cellulose insulation available on the market. The aim of the research was to check the possibility of using synthetic ester for effective drying of cellulose materials of various thickness and an evaluation of the drying dynamics. The replacement of mineral oil with a synthetic ester caused a reduction of moisture in paper and thin pressboard by one percentage point. It was possible in the case of drying these materials for seven days at a temperature of 70 °C. The effects of drying were much smaller in the case of thicker cellulose samples. This paper also shows the complex problem of simultaneously drying materials of different thicknesses. Drying thin paper and thick pressboard at the same time significantly slows down the drying process of the pressboard. Presented results will be used to develop a procedure for drying the transformer insulation system using a synthetic ester. Full article
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Open AccessArticle
Modelling and Analysis of Energy Harvesting in Internet of Things (IoT): Characterization of a Thermal Energy Harvesting Circuit for IoT based Applications with LTC3108
Energies 2019, 12(20), 3873; https://doi.org/10.3390/en12203873 - 13 Oct 2019
Viewed by 214
Abstract
This paper presents a simulation-based study for characterizing and analyzing the performance of a commercially available thermoelectric cooler (TEC) as a generator for harvesting heat energy along with a commercial-off-the-shelf (COTS) power management integrated circuit (PMIC); LTC3108. In this model, the transformation of [...] Read more.
This paper presents a simulation-based study for characterizing and analyzing the performance of a commercially available thermoelectric cooler (TEC) as a generator for harvesting heat energy along with a commercial-off-the-shelf (COTS) power management integrated circuit (PMIC); LTC3108. In this model, the transformation of heat was considered in terms of an electrical circuit simulation perspective, where temperature experienced by TEC on both cold and hot sides was incorporated with voltage supply as Vth and Vtc in the circuit. When it comes to modeling a system in a simulation program with an integrated circuit emphasis (SPICE) like environment, the selection of thermoelectric generator (TEG) and extraction methods are not straightforward as well as the lack of information from manufacturer’s datasheets can limit the grip over the analysis parameters of the module. Therefore, it is mandatory to create a prototype before implementing it over a physical system for energy harvesting circuit (EHC) optimization. The major goal was to establish the basis for devising the thermal energy scavenging based Internet of Things (IoT) system with two configurations of voltage settings for the same TEG model. This study measured the data in terms of current, voltage, series of resistive loads and various temperature gradients for generating the required power. These generated power levels from EHC prototype were able to sustain the available IoT component’s power requirement, hence it could be considered for the implementation of IoT based applications. Full article
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Open AccessArticle
Active and Reactive Power Control of a PV Generator for Grid Code Compliance
Energies 2019, 12(20), 3872; https://doi.org/10.3390/en12203872 - 12 Oct 2019
Viewed by 161
Abstract
As new grid codes have been created to permit the integration of large scale photovoltaic power plants into the transmission system, the enhancement of the local control of the photovoltaic (PV) generators is necessary. Thus, the objective of this paper is to present [...] Read more.
As new grid codes have been created to permit the integration of large scale photovoltaic power plants into the transmission system, the enhancement of the local control of the photovoltaic (PV) generators is necessary. Thus, the objective of this paper is to present a local controller of active and reactive power to comply the new requirements asked by the transmission system operators despite the variation of ambient conditions without using extra devices. For this purpose, the control considers the instantaneous capability curves of the PV generator which vary due to the change of solar irradiance, temperature, dc voltage and modulation index. To validate the control, the PV generator is modeled in DIgSILENT PowerFactory ® and tested under different ambient conditions. The results show that the control developed can modify the active and reactive power delivered to the desired value at different solar irradiance and temperature. Full article
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Open AccessArticle
Control Method of Parallel Inverters with Self-Synchronizing Characteristics in Distributed Microgrid
Energies 2019, 12(20), 3871; https://doi.org/10.3390/en12203871 - 12 Oct 2019
Viewed by 143
Abstract
The centralized control mode is no longer applicable for microgrid operation due to the high penetration rate of distributed energy, which is responsible for the widespread interest in the use of the distributed microgrid. Focusing on the issues of power coupling and uncontrollable [...] Read more.
The centralized control mode is no longer applicable for microgrid operation due to the high penetration rate of distributed energy, which is responsible for the widespread interest in the use of the distributed microgrid. Focusing on the issues of power coupling and uncontrollable droop coefficient at the terminal of the connecting line between the micro-source and AC bus, which is rarely considered, this paper proposes an improved virtual synchronous generator (VSG) control strategy based on local data considering precise control of the droop coefficient and realizing the power decoupling and the expected droop characteristics. Then, combined with the virtual rotor characteristic matching method, the reasonable active and reactive power sharing of the parallel microgrid inverters are realized in terms of static and dynamic performance without additional improvement of reactive power control. Finally, the effectiveness and feasibility of the proposed method are verified based on the MATLAB/Simulink simulation platform. The combination of the proved strategy and matching principle endows inverters with self-synchronization characteristics, forming the self-synchronizing voltage sources, which gives the distributed microgrid a higher self-stability, autonomy and robustness to ensure the stable operation of the microgrid. Full article
(This article belongs to the Special Issue Advanced Control in Microgrid Systems)
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Open AccessArticle
Global Transportation Demand Development with Impacts on the Energy Demand and Greenhouse Gas Emissions in a Climate-Constrained World
Energies 2019, 12(20), 3870; https://doi.org/10.3390/en12203870 - 12 Oct 2019
Viewed by 233
Abstract
The pivotal target of the Paris Agreement is to keep temperature rise well below 2 °C above the pre-industrial level and pursue efforts to limit temperature rise to 1.5 °C. To meet this target, all energy-consuming sectors, including the transport sector, need to [...] Read more.
The pivotal target of the Paris Agreement is to keep temperature rise well below 2 °C above the pre-industrial level and pursue efforts to limit temperature rise to 1.5 °C. To meet this target, all energy-consuming sectors, including the transport sector, need to be restructured. The transport sector accounted for 19% of the global final energy demand in 2015, of which the vast majority was supplied by fossil fuels (around 31,080 TWh). Fossil-fuel consumption leads to greenhouse gas emissions, which accounted for about 8260 MtCO2eq from the transport sector in 2015. This paper examines the transportation demand that can be expected and how alternative transportation technologies along with new sustainable energy sources can impact the energy demand and emissions trend in the transport sector until 2050. Battery-electric vehicles and fuel-cell electric vehicles are the two most promising technologies for the future on roads. Electric ships and airplanes for shorter distances and hydrogen-based synthetic fuels for longer distances may appear around 2030 onwards to reduce the emissions from the marine and aviation transport modes. The rail mode will remain the least energy-demanding, compared to other transport modes. An ambitious scenario for achieving zero greenhouse gas emissions by 2050 is applied, also demonstrating the very high relevance of direct and indirect electrification of the transport sector. Fossil-fuel demand can be reduced to zero by 2050; however, the electricity demand is projected to rise from 125 TWhel in 2015 to about 51,610 TWhel in 2050, substantially driven by indirect electricity demand for the production of synthetic fuels. While the transportation demand roughly triples from 2015 to 2050, substantial efficiency gains enable an almost stable final energy demand for the transport sector, as a consequence of broad electrification. The overall well-to-wheel efficiency in the transport sector increases from 26% in 2015 to 39% in 2050, resulting in a respective reduction of overall losses from primary energy to mechanical energy in vehicles. Power-to-fuels needed mainly for marine and aviation transport is not a significant burden for overall transport sector efficiency. The primary energy base of the transport sector switches in the next decades from fossil resources to renewable electricity, driven by higher efficiency and sustainability. Full article
(This article belongs to the Special Issue 100% Renewable Energy Transition: Pathways and Implementation)
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Open AccessArticle
Merit Order Effect Modeling: The Case of the Hellenic Electricity Market
Energies 2019, 12(20), 3869; https://doi.org/10.3390/en12203869 - 12 Oct 2019
Viewed by 130
Abstract
A simple effective model is proposed for the day-ahead electricity market. The model considers the main factors which govern the process, predicts the seasonal and daily variation of electricity demand, renewable production, system marginal price, and merit order effect. The accuracy of the [...] Read more.
A simple effective model is proposed for the day-ahead electricity market. The model considers the main factors which govern the process, predicts the seasonal and daily variation of electricity demand, renewable production, system marginal price, and merit order effect. The accuracy of the model is increased by fitting to historic data of the Hellenic electricity market. During the period between October 2016 and December 2018, the Hellenic electricity market calculated explicitly the merit order effect using an innovative mechanism to directly charge the electricity suppliers (retailers). On the basis of the proposed model and the market recorded data, the effect of the renewable penetration on the wholesale Hellenic electricity prices is revealed. The model is further used to analyze the market future behavior when basic factors (electricity demand, conventional power, and renewable penetration) are known or estimated. The effect of merit order effect on the Hellenic legislation is discussed and the appropriate measures adopted by the Hellenic authorities are analyzed and evaluated. Full article
(This article belongs to the Special Issue Market Design for a High-Renewables Electricity System)
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Open AccessArticle
Design Considerations of Switched Flux Memory Machine with Partitioned Stators
Energies 2019, 12(20), 3868; https://doi.org/10.3390/en12203868 - 12 Oct 2019
Viewed by 139
Abstract
This paper presents general design considerations of a partitioned stator switched flux hybrid magnet memory machine (PS-SF-HMMM). The armature windings and permanent magnets (PMs) are placed on two separate stators, respectively, in the PS-SF-HMMM, and thus both high torque density and wide flux [...] Read more.
This paper presents general design considerations of a partitioned stator switched flux hybrid magnet memory machine (PS-SF-HMMM). The armature windings and permanent magnets (PMs) are placed on two separate stators, respectively, in the PS-SF-HMMM, and thus both high torque density and wide flux regulation capability can be obtained. The topology and working principle of the machine are introduced briefly first, and then different magnet arrangements and stator/rotor pole combinations are investigated. In addition, various design parameters are optimized based on finite element (FE) methods. Finally, a prototype is fabricated to experimentally validate the FE results. Full article
(This article belongs to the Special Issue Design, Control, and Optimization of Flux Switching Machine)
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Open AccessArticle
Energy and Efficiency Evaluation of Feedback Branch Design in Thermoacoustic Stirling-Like Engines
Energies 2019, 12(20), 3867; https://doi.org/10.3390/en12203867 - 12 Oct 2019
Viewed by 126
Abstract
Stirling-like thermoacoustic generators are external combustion engines that provide useful acoustic power in the absence of moving parts with high reliability and respect for the environment. The study of these systems involves a great complexity since the parameters that describe them, besides being [...] Read more.
Stirling-like thermoacoustic generators are external combustion engines that provide useful acoustic power in the absence of moving parts with high reliability and respect for the environment. The study of these systems involves a great complexity since the parameters that describe them, besides being numerous, present a high degree of coupling between them. This implies a great difficulty in characterizing the effects of any parametric variation on the performance of these devices. Due to the huge amount of data to analyze, the experiments and simulations required to address the problem involve high investments in time and resources, sometimes unaffordable. This article presents, how a sensitivity analysis applying the response surface methodology can be applied to optimize the feedback branch of a thermoacoustic Stirling-like engine. The proposed study is made by evaluating the comparative relevance of seven design variables. The dimensional reduction process identifies three significant factors: the frequency of operation, the internal diameter of compliance, and the inertance. Subsequently, the Response Surface Methodology is applied to assess the interaction effects of these three design parameters on the efficiency of the thermoacoustic engine, and an improvement of 6% has been achieved. The enhanced values given by the response surface methodology are validated using the DeltaEC software. Full article
(This article belongs to the Special Issue Internal Combustion Engine Waste Heat Recovery)
Open AccessArticle
Solid Particles in Natural Gas from a Transportation Network: A Chemical Composition Study
Energies 2019, 12(20), 3866; https://doi.org/10.3390/en12203866 - 12 Oct 2019
Viewed by 117
Abstract
This paper aims to provide the elemental composition of particles found in natural gas. Particle sampling is performed on cellulose filters obtained from an industrial gas storage facility, and the qualitative particle composition is determined by scanning electron microscopy and energy dispersive X-ray [...] Read more.
This paper aims to provide the elemental composition of particles found in natural gas. Particle sampling is performed on cellulose filters obtained from an industrial gas storage facility, and the qualitative particle composition is determined by scanning electron microscopy and energy dispersive X-ray spectroscopy. Our results establish that natural gas may contain solid particles, with sizes ranging from less than 1 μm to more than 50 μm. The observed particles are composed of numerous elements, such as aluminum (Al), silica (Si), sulphur (S), chloride (Cl), chromium (Cr), zinc (Zn), sodium (Na), manganese (Mg), calcium (Ca), iron (Fe), titanium (Ti), nickel (Ni), vanadium (V), potassium (K), copper (Cu), manganese (Mn), silver (Ag), cobalt (Co), iodine (I), and barium (Ba), with relative occurrences ranging from 1 to 85%. Moreover, metallic elements enable the formation of larger particles as a result of the agglomeration of smaller particles. Full article
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Open AccessArticle
Behaviour of Distribution Grids with the Highest PV Share Using the Volt/Var Control Chain Strategy
Energies 2019, 12(20), 3865; https://doi.org/10.3390/en12203865 - 12 Oct 2019
Viewed by 105
Abstract
The large-scale integration of rooftop PVs stalls due to the voltage limit violations they provoke, the uncontrolled reactive power flow in the superordinate grids and the information and communications technology (ICT) related challenges that arise in solving the voltage limit violation problem. This [...] Read more.
The large-scale integration of rooftop PVs stalls due to the voltage limit violations they provoke, the uncontrolled reactive power flow in the superordinate grids and the information and communications technology (ICT) related challenges that arise in solving the voltage limit violation problem. This paper attempts to solve these issues using the LINK-based holistic architecture, which takes into account the behaviour of the entire power system, including customer plants. It focuses on the analysis of the behaviour of distribution grids with the highest PV share, leading to the determination of the structure of the Volt/var control chain. The voltage limit violations in low voltage grid and the ICT challenge are solved by using concentrated reactive devices at the end of low voltage feeders. Q-Autarkic customer plants relieve grids from the load-related reactive power. The optimal arrangement of the compensation devices is determined by a series of simulations. They are conducted in a common model of medium and low voltage grids. Results show that the best performance is achieved by placing compensation devices at the secondary side of the supplying transformer. The Volt/var control chain consists of two Volt/var secondary controls; one at medium voltage level (which also controls the TSO-DSO reactive power exchange), the other at the customer plant level. Full article
(This article belongs to the Special Issue Integration of PV in Distribution Networks)
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Open AccessArticle
CO2 Efficiency Break Points for Processes Associated to Wood and Coal Transport and Heating
Energies 2019, 12(20), 3864; https://doi.org/10.3390/en12203864 - 12 Oct 2019
Viewed by 122
Abstract
This paper aims to deal with CO2 emissions in energy production process in an original way, based on calculations of total specific CO2 emissions, depending on the type of fuel and the transport distance. This paper has ambition to set a [...] Read more.
This paper aims to deal with CO2 emissions in energy production process in an original way, based on calculations of total specific CO2 emissions, depending on the type of fuel and the transport distance. This paper has ambition to set a break point from where it is not worthwhile to use wood as an energy carrier as the alternative to coal. The reason for our study is the social urgency of selected problem. For example, in the area of public sector decision-making, wood heating is promoted regardless of the availability within the reasonable distance. From the current state of the research, it is also clear that none of the studies compare coal and biomass fuel transportation from the point of view of CO2 production. For this purpose, an original methodology has been proposed. It is based on a modified life cycle assessment (LCA), supplemented with a system of equations. The proposed methodology has a generalizable nature, and therefore, it can be applied to different regions. However, calculation inputs and modelling are based on specific site data. Based on the presented numerical analysis, the key finding is the break point for associated processes at a distance of 1779.64 km, since when that it is better to burn brown coal than wood in terms of total CO2 emissions. We can conclude that, in some cases, it is more efficient to use coal instead of wood as fuel in terms of CO2 emissions, particularly in regard to transport distance and type of transport. Full article
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Open AccessArticle
Triggering Optimal Control of Air Conditioning Systems by Event-Driven Mechanism: Comparing Direct and Indirect Approaches
Energies 2019, 12(20), 3863; https://doi.org/10.3390/en12203863 - 12 Oct 2019
Viewed by 117
Abstract
Real-time optimal control of air conditioning (AC) is important, and should respond to the condition changes for an energy efficient operation. The traditional optimal control triggering mechanism is based on the “time clock” (called time-driven), and has certain drawbacks (e.g., delayed or unnecessary [...] Read more.
Real-time optimal control of air conditioning (AC) is important, and should respond to the condition changes for an energy efficient operation. The traditional optimal control triggering mechanism is based on the “time clock” (called time-driven), and has certain drawbacks (e.g., delayed or unnecessary actions). Thus, an event-driven optimal control (EDOC) was proposed. In previous studies, the part-load ratio (PLR) of chiller plants was used as events to trigger optimal control actions. However, PLR is an indirect indicator of operation efficiency, which could misrepresent the system coefficient of performance (SCOP). This study thus proposes to directly monitor the SCOP deviations from the desired SCOP values. Two events are defined based on transient and cumulative SCOP deviations, which are systematically investigated in terms of energy performance and robustness. The PLR-based and SCOP-based EDOC are compared, in which energy saving and optimal control triggering time are analyzed. Results suggest that SCOP-based EDOC has better energy performance compared with PLR-based EDOC, but the frequent event triggering might happen due to the parameter uncertainty. For actual applications, the SCOP-based EDOC can be recommended when the ideal SCOP model is available with the properly-handled uncertainty. Nevertheless, the PLR-based EDOC could still be a more practical option to replace the traditional TDOC considering its acceptable energy performance and better robustness. Full article
(This article belongs to the Special Issue Evaluation of Energy Efficiency and Flexibility in Smart Buildings)
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Open AccessArticle
Smart Battery Pack for Electric Vehicles Based on Active Balancing with Wireless Communication Feedback
Energies 2019, 12(20), 3862; https://doi.org/10.3390/en12203862 - 12 Oct 2019
Viewed by 102
Abstract
In this paper, the concept of smart battery pack is introduced. The smart battery pack is based on wireless feedback from individual battery cells and is capable to be applied to electric vehicle applications. The proposed solution increases the usable capacity and prolongs [...] Read more.
In this paper, the concept of smart battery pack is introduced. The smart battery pack is based on wireless feedback from individual battery cells and is capable to be applied to electric vehicle applications. The proposed solution increases the usable capacity and prolongs the life cycle of the batteries by directly integrating the battery management system in the battery pack. The battery cells are connected through half-bridge chopper circuits, which allow either the insertion or the bypass of a single cell depending on the current states of charge. This consequently leads to the balancing of the whole pack during both the typical charging and discharging time of an electric vehicle and enables the fault-tolerant operation of the pack. A wireless feedback for implementing the balancing method is proposed. This solution reduces the need for cabling and simplifies the assembling of the battery pack, making also possible a direct off-board diagnosis. The paper validates the proposed smart battery pack and the wireless feedback through simulations and experimental results by adopting a battery cell emulator. Full article
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Open AccessArticle
A Formulation of the Thrust Coefficient for Representing Finite-Sized Farms of Tidal Energy Converters
Energies 2019, 12(20), 3861; https://doi.org/10.3390/en12203861 - 12 Oct 2019
Viewed by 84
Abstract
Tidal energy converter (TEC) arrays in tidal channels generate complex flow phenomena due to interactions with the local environment and among devices. Models with different resolutions are thus employed to study flows past TEC farms, which consider multiple spatial and temporal scales. Simulations [...] Read more.
Tidal energy converter (TEC) arrays in tidal channels generate complex flow phenomena due to interactions with the local environment and among devices. Models with different resolutions are thus employed to study flows past TEC farms, which consider multiple spatial and temporal scales. Simulations over tidal cycles use mesoscale ocean circulation models, incorporating a thrust coefficient to model the momentum sink that represents the effects of the array. In this work, we propose an expression for a thrust coefficient to represent finite-sized farms of TEC turbines at larger scales, C t F a r m , which depends on the spatial organization of the devices. We use a coherent-structure resolving turbulence model coupled with the actuator disk approach to simulate staggered turbine configurations in more detail, varying the separation among devices and the ratios between the channel depths and hub heights. Based on these simulations, we calculate the resultant force for various subsets of devices within the farm, and their corresponding effective thrust coefficient, C t F a r m . We conclude that the thrust coefficient depends solely on the lateral separation of the devices, S y , for farms with only two rows. For farms with more than two rows, the streamwise distance, S x , must be considered as well. With the proposed expression, it is possible to calculate efficiently the effects of finite-sized TEC farms and incorporate a momentum sink into ocean circulation models, without assuming a constant coefficient derived from an infinite farm approximation. Full article
(This article belongs to the Special Issue Fluid Dynamics in Marine and Hydrokinetic Energy System)
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Open AccessArticle
Reducing the Energy and Steam Consumption of SAGD Through Cyclic Solvent Co-Injection
Energies 2019, 12(20), 3860; https://doi.org/10.3390/en12203860 - 12 Oct 2019
Viewed by 90
Abstract
The amount of oil that is contained in the Canadian oil sands represent the third largest oil accumulation in the world. Approximately half of the daily oil production from the oil sands comes from mining processes and the other half is produced mostly [...] Read more.
The amount of oil that is contained in the Canadian oil sands represent the third largest oil accumulation in the world. Approximately half of the daily oil production from the oil sands comes from mining processes and the other half is produced mostly using steam assisted gravity drainage (SAGD). This method is effective at reducing the viscosity of the oil. However, the generation of steam requires a significant amount of energy. Thus, there is an ongoing effort to reduce the energy needed to produce oil from the oil sands. In this article the intermittent injection of a solvent, along with steam, is investigated as a means of reducing the amount of energy required to extract oil from the Canadian oil sands. A simulation-based study examined the effect of the type of solvent, the cycles’ duration, the solvent concentration and the number of cycles. The simulations covered a time span of 10 years during which several different solvents (methane, ethane, propane, butane, pentane, hexane, and CO2) were injected under varying injection schedules. The solvents that were investigated are compounds that are likely to be readily available at a heavy oil production site. The solvent injection periods ranged from six to 24 months in length. The results reveal that SAGD combined with intermittent injection of hexane resulted in the most significant improvement to the cumulative oil production and in the cumulative energy-oil ratio (cEOR). Compared to SAGD without solvent injection, the cumulative oil production was increased by 45% and the cEOR was reduced by 23%. It was also seen that the performance of the proposed process is highly dependent on the resulting physical properties of the solvent-bitumen mixture. Finally, a simplified economic analysis also identified SAGD with intermittent hexane injection as the scheme that resulted in the highest net present value. Compared to SAGD without solvent injection, the intermittent injection of hexane led to an 85% increase in the net present value. Full article
(This article belongs to the Section Energy Sources)
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Open AccessArticle
Analysis and Design of High-Efficiency Bidirectional GaN-Based CLLC Resonant Converter
Energies 2019, 12(20), 3859; https://doi.org/10.3390/en12203859 - 12 Oct 2019
Viewed by 86
Abstract
A bidirectional CLLC resonant converter (CLLC-BRC) based on GaN transistors is analyzed and designed in this paper. Similar resonant topologies are listed and commented on, with the CLLC topology showing competitiveness in bidirectional energy transmission. The analysis of the aforementioned converter has been [...] Read more.
A bidirectional CLLC resonant converter (CLLC-BRC) based on GaN transistors is analyzed and designed in this paper. Similar resonant topologies are listed and commented on, with the CLLC topology showing competitiveness in bidirectional energy transmission. The analysis of the aforementioned converter has been provided, including the reveal of resonant frequencies of the CLLC topology and an improved zero-voltage switching (ZVS) condition with operation principles of the reverse mode and relevant parasitic parameters taken into account. The design methodology of the aforementioned converter based on pulse frequency modulation (PFM) is further discussed in detail. A prototype with a rated power of 400 W and a maximal operating frequency that is larger than 0.5 MHz was built to verify the proposed design methodology. The highest conversion efficiency of the prototype was 97.02% in the forward mode, and it was 95.96% in the reverse mode. Full article
(This article belongs to the Special Issue Advanced in Resonant Converter and Dual Active Bridge Converter)
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