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Journal Energies

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Published: 24 March 2020
by MDPI
Energies, Volume 13; doi:10.3390/en13061524

Abstract:
A novel type of multi-degree-of-freedom (multi-DOF) deflecting-type permanent-magnet synchronous wind generator (PMSWG) is constructed to improve the reliability and utilization of wind energy. The basic working principle of the multi-DOF deflecting-type permanent-magnet synchronous generator (PMSG) is introduced, and its structural size is experimentally and theoretically determined. Subsequently, the multi-DOF deflecting-type PMSG was used to operate a complete wind turbine. A prototype and three-dimensional (3D) model of the wind turbine is simulated, allowing one to analyze the aerodynamics of the turbine and power generation performance. The electromagnetic field analysis is performed via analytical methods, followed by a 3D finite element and torque analyses. Furthermore, the wind turbine power generation characteristics curves are obtained through simulation software. Finally, transient analysis of post deflection is demonstrated. The before and after deflection values of the generator voltage, current, flux linkage, and induced voltage are compared and analyzed, relying on simulations and experiments. Additionally, the wind tunnel experiment is used to compare voltage variation with wind direction. The comparison reveals that the wind generator phase voltage remains maximized with wind direction variation. The results confirm that the proposed PMSWG has excellent performance and future research potential.
Published: 24 March 2020
by MDPI
Energies, Volume 13; doi:10.3390/en13061525

Abstract:
This paper advances a practical tool for production forecasting, using a 2-segment Decline Curve Analysis (DCA) method, based on an analytical flow-cell model for multi-stage fractured shale wells. The flow-cell model uses a type well and can forecast the production rate and estimated ultimate recovery (EUR) of newly planned wells, accounting for changes in completion design (fracture spacing, height, half-length), total well length, and well spacing. The basic equations for the flow-cell model have been derived in two earlier papers, the first one dedicated to well forecasts with fracture down-spacing, the second one to well performance forecasts when inter-well spacing changes (and for wells drilled at different times, to account for parent-child well interaction). The present paper provides a practical workflow, introduces correction parameters to account for acreage quality and fracture treatment quality. Further adjustments to the flow-cell model based 2-segment DCA method are made after history matching field data and numerical reservoir simulations, which indicate that terminal decline is not exponential (b = 0) but hyperbolic (with 0 < b< 1). The timing for the onset of boundary dominated flow was also better constrained, using inputs from a reservoir simulator. The new 2-segment DCA method is applied to real field data from the Eagle Ford Formation. Among the major insights of our analyses are: (1) fracture down-spacing does not increase the long-term EUR, and (2) fracture down-spacing of real wells does not result in the rate increases predicted by either the flow-cell model based 2-segment DCA (or its matching reservoir simulations) with the assumed perfect fractures in the down-spaced well models. Our conclusion is that real wells with down-spaced fracture clusters, involving up to 5000 perforations, are unlikely to develop successful hydraulic fractures from each cluster. The fracture treatment quality factor (TQF) or failure rate (1-TQF) can be estimated by comparing the actual well performance with the well forecast based on the ideal well model (albeit flow-cell model or reservoir model, both history-matched on the type curve).
Published: 24 March 2020
by MDPI
Energies, Volume 13; doi:10.3390/en13061527

Abstract:
In recent years, different off-the-shelf solutions for the rapid control prototyping of power electronics converters have been commercialised. The main benefits of those systems are based on a fast and easy-to-use environment due to high-level programming. However, most of those systems are very expensive and are closed software and hardware solutions. In this context, this paper presents the design and implementation of a control platform targeting at the segment in between expensive off-the-shelf control platforms and low-cost controllers. The control platform is based on the Launchpad TMS320F28379D from Texas Instruments, and it is equipped with an expansion board that provide analogue-to-digital measurements, switching signals and hardware protections. The performance of the control platform is experimentally tested on a 20 kVA power converter.
Published: 24 March 2020
by MDPI
Energies, Volume 13; doi:10.3390/en13061526

Abstract:
The energy recovered with regenerative braking system can greatly improve energy efficiency of range-extended electric vehicle (R-EEV). Nevertheless, maximizing braking energy recovery while maintaining braking performance remains a challenging issue, and it is also difficult to reduce the adverse effects of regenerative current on battery capacity loss rate (Qloss,%) to extend its service life. To solve this problem, a revised regenerative braking control strategy (RRBCS) with the rate and shape of regenerative braking current considerations is proposed. Firstly, the initial regenerative braking control strategy (IRBCS) is researched in this paper. Then, the battery capacity loss model is established by using battery capacity test results. Eventually, RRBCS is obtained based on IRBCS to optimize and modify the allocation logic of braking work-point. The simulation results show that compared with IRBCS, the regenerative braking energy is slightly reduced by 16.6% and Qloss,% is reduced by 79.2%. It means that the RRBCS can reduce Qloss,% at the expense of small braking energy recovery loss. As expected, RRBCS has a positive effect on prolonging the battery service life while ensuring braking safety while maximizing recovery energy. This result can be used to develop regenerative braking control system to improve comprehensive performance levels.
Published: 23 March 2020
by MDPI
Energies, Volume 13; doi:10.3390/en13061515

Abstract:
This paper presents a model combining the LCA (Life Cycle Assessment) of fossil fuel extraction with its quality parameters and related CO2, SO2 and dust emissions at the stage of the combustion process. The model which was developed aims to identify the environmental impact of the processes of electricity production from selected energy carriers over their whole life cycle. The model takes into account the full LCA of fossil fuel extraction (of both hard and brown coal), its enrichment and fuel production as well as the environmental impact associated with emissions introduced into the air at the stage of electricity generation based on the fuels evaluated. Such an approach allows one to determine the fuel quality parameters that affect the environmental impact of energy production based on an LCA of mining and assigns the degree of environmental impact involved in particular production processes. Overall, the results obtained based on the proposed model permit the identification and prioritisation of the individual processes in the electricity generation life cycle which contribute the highest share in the general environmental impact indicator, having taken into account the modelling of the quality of the fuels used (calorific value, ash and sulphur content).
Published: 23 March 2020
by MDPI
Energies, Volume 13; doi:10.3390/en13061518

Abstract:
Charging and discharge currents measured in low-density polyethylene (LDPE) and LDPE/Al2O3 nanocomposite are analyzed. The experiments were conducted at temperatures of 40–80 °C utilizing a consecutive charging–discharging procedure, with the charging step at electric fields varying between 20 and 60 kV/mm. A quasi-steady state of the charging currents was earlier observed for the nanofilled specimens and it was attributed to the enhanced trapping process at polymer–nanofiller interfaces. An anomalous behavior of the discharge currents was found at elevated temperatures for both the studied materials and its occurrence at lower temperatures in the nanofilled LDPE was due to the presence of deeply trapped charges at polymer–nanofiller interfaces. The field dependence of the quasi-steady charging currents is examined by testing for different conduction mechanisms. It is shown that the space-charge-limited process is dominant and the average trap site separation is estimated at less than 2 nm for the pristine LDPE and it is at about 5–7 nm for the LDPE/Al2O3 nanocomposite. Also, location of the trapping sites in the band gap structure of the nanofilled material is altered, which substantially weakens electrical transport as compared to the unfilled counterpart.
Published: 23 March 2020
by MDPI
Energies, Volume 13; doi:10.3390/en13061519

Abstract:
The shape of the temperature vs. specific entropy diagram of a working fluid is very important to understanding the behavior of fluid during the expansion phase of the organic Rankine cycle or similar processes. Traditional wet-dry-isentropic classifications of these materials are not sufficient; several materials remain unclassified or misclassified, while materials listed in the same class might show crucial differences. A novel classification, based on the characteristic points of the T–s diagrams was introduced recently, listing eight different classes. In this paper, we present a map of these classes for a model material, namely, the van der Waals fluid in reduced temperature (i.e., reduced molecular degree of freedom) space; the latter quantity is related to the molar isochoric specific heat. Although van der Waals fluid cannot be used to predict material properties quantitatively, the model gives a very good and proper qualitative description. Using this map, some peculiarities related to T–s diagrams of working fluids can be understood.
Published: 23 March 2020
by MDPI
Energies, Volume 13; doi:10.3390/en13061517

Abstract:
Environmental and economic needs drive the increased penetration of intermittent renewable energy in electricity grids, enhancing uncertainty in the prediction of market conditions and network constraints. Thereafter, the importance of energy systems with flexible dispatch is reinforced, ensuring energy storage as an essential asset for these systems to be able to balance production and demand. In order to do so, such systems should participate in wholesale energy markets, enabling competition among all players, including conventional power plants. Consequently, an effective dispatch schedule considering market and resource uncertainties is crucial. In this context, an innovative dispatch optimization strategy for schedule planning of renewable systems with storage is presented. Based on an optimization algorithm combined with a machine-learning approach, the proposed method develops a financial optimal schedule with the incorporation of uncertainty information. Simulations performed with a concentrated solar power plant model following the proposed optimization strategy demonstrate promising financial improvement with a dynamic and intuitive dispatch planning method (up to 4% of improvement in comparison to an approach that does not consider uncertainties), emphasizing the importance of uncertainty treatment on the enhanced quality of renewable systems scheduling.
Published: 23 March 2020
by MDPI
Energies, Volume 13; doi:10.3390/en13061521

Abstract:
The impact of environmental regulations implemented in the power industry that affect the consumption of solid fuels is of key importance to coal-based power generation systems, such as that in Poland. In this context, the main purpose of the paper was to determine the future demand for hard coal and brown coal in the Polish power sector by 2050 with reference to the environmental regulations implemented in the power sector. To achieve these goals, a mathematical model was developed using the linear programming approach, which reflected the key relationships between the hard and brown coal mining sector and the power sector in the context of the environmental regulations discussed. The environmental regulations selected had a great influence on the future demand for hard and brown coal in the power generation sector. The scope of this influence depended on particular regulations. The prices of CO2 emission allowances and stricter emissions standards stemming from the Industrial Emissions Directive and the BAT (Best Available Techniques) conclusions had the largest influence on the reduction of hard coal demand. In the case of brown coal, no new power generating units would be deployed; hence, brown coal consumption would drop practically to zero in 2050 under all the scenarios considered.
Published: 23 March 2020
by MDPI
Energies, Volume 13; doi:10.3390/en13061520

Abstract:
This paper proposes a local control strategy applied in the soft open point (SOP) to suppress voltage fluctuation when adding a renewable energy source into the system. The mathematic model of the grid connected to SOP is established based on the characteristics of a low-voltage distribution network. Combined with the mathematic model and local voltage information, the local control strategy is proposed to optimize the active and reactive power distribution and consume the minimum apparent power of the converter. The local control strategy can effectively suppress the voltage fluctuation caused by renewable energy access, which was testified by MATLAB/Simulink simulation. In addition, the local control strategy can deduce the communication resource and increase the response speed compared to global optimization. This paper is meaningful for renewable energy source distribution and voltage balance in low-voltage distribution systems.
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