ALBERT

Description: This paper pays attention to magnetic flux linkage optimization of a direct-driven surface-mounted permanent magnet synchronous generator (D-SPMSG). A new compact representation of the D-SPMSG nonlinear dynamic differential equations to reduce system parameters is established. Furthermore, the nonlinear dynamic characteristics of new D-SPMSG equations in the process of varying magnetic flux linkage are considered, which are illustrated by Lyapunov exponent spectrums, phase orbits, Poincaré maps, time waveforms and bifurcation diagrams, and the magnetic flux linkage stable region of D-SPMSG is acquired concurrently. Based on the above modeling and analyses, a novel method of magnetic flux linkage optimization is presented. In addition, a 2 MW D-SPMSG 2D/3D model is designed by ANSYS software according to the practical design requirements. Finally, five cases of D-SPMSG models with different magnetic flux linkages are simulated by using the finite element analysis (FEA) method. The nephograms of magnetic flux density are agreement with theoretical analysis, which both confirm the correctness and effectiveness of the proposed approach.

Description: Energies, Vol. 11, Pages 1556: Petrographic Controls on Pore and Fissure Characteristics of Coals from the Southern Junggar Coalfield, Northwest China Energies doi: 10.3390/en11061556 Authors: Sandong Zhou Dameng Liu Yidong Cai Zuleima Karpyn Yanbin Yao The productive potential of coalbed methane projects is controlled by pore and fissure characteristics, which are intrinsically related to coal petrology. This work attempts to identify the influence of petrographic factors on the development of pore and fissure systems in the southern Junggar Coalfield, Northwest China. Here, Middle Jurassic coal (lignite and subbituminous) petrology in coal seam No. 45 of the southern Junggar Coalfield (SJC) is studied with respect to the characteristics of pore and fissure structure with the aid of optical microscopes, scanning electron microscopy, mercury intrusion porosimetry, and nuclear magnetic resonance analysis. Maceral analysis shows coals at the SJC are dominated by vitrinite (38–87 vol %), with moderate quantities of inertinite (1–28 vol %) and liptinite (0.5–30 vol %). Decomposition of plants occurs under slightly oxic–anoxic conditions, with good tissue retention. Four types of coal facies are classified using petrographic indices, comprising (1) lower delta plain marsh, (2) lower delta plain fen, (3) upper delta plain wet forest swamp; and (4) piedmont plain moor. Pores and fissures are generally observed in telinite, collotelinite, fusinite, and semifusinite in SJC coals, indicating that the generation of pores and fissures is strongly influenced by coal macerals. Pore and fissure structures of coals in coal facies (1) appear weakly connected, whereas those in coal facies (2) reveal good connectivity. Coals in coal facies (3) and (4) show moderate connectivity between pore and fissure structure. Therefore, pore and fissure structures are significantly controlled by coal facies. This work provides practical recommendations and implementation methods for petrological studies in future coalbed methane exploration/development in the SJC. This study also serves to predict the physical properties of pores and fissures and interpret the control mechanism of coalbed methane production using coal petrology.

Description: Energies, Vol. 11, Pages 1817: A Real Options Analysis for Renewable Energy Investment Decisions under China Carbon Trading Market Energies doi: 10.3390/en11071817 Authors: Yanbin Li Min Wu Zhen Li Under the carbon trading mechanism, renewable energy projects can gain additional benefits through Chinese Certified Emission Reduction transactions. Due to the uncertainty of carbon trading system, carbon prices will fluctuate randomly, which will affect the investment timing of renewable energy projects. Thus, the value of the option will be generated. Therefore, renewable energy power generation project investment has the right of option. However, the traditional investment decision-making method can no longer meet the requirements of renewable energy investment in the current stage. In this paper, a real option model considering carbon price fluctuation is proposed as a tool for renewable energy investment. Considering optimal investment timing and carbon price, the model introduces a carbon price fluctuation as part of the optimization, studies the flexibility of enterprises’ delayed investment under the fluctuation of carbon price. A case study is carried out to verify the effectiveness of the proposed real option model by selecting a wind farm in North China. The model is expected to help investors to assess the volatility and risk of renewable energy projects more accurately, and help investors to make a complete plan for the project investment, thus promoting the efficient allocation of resources in the energy industry.

Description: Superconducting magnetic energy storage (SMES) systems, in which the proportional-integral (PI) method is usually used to control the SMESs, have been used in microgrids for improving the control performance. However, the robustness of PI-based SMES controllers may be unsatisfactory due to the high nonlinearity and coupling of the SMES system. In this study, the energy shaping passivity (ESP)-based control strategy, which is a novel nonlinear control based on the methodology of interconnection and damping assignment (IDA), is proposed for robustness improvement of SMES systems. A step-by-step design of the ESP-based method considering the robustness of SMES systems is presented. A comparative analysis of the performance between ESP-based and PI control strategies is shown. Simulation and experimental results prove that the ESP-based strategy achieves the stronger robustness toward the system parameter uncertainties than the conventional PI control. Besides, the use of ESP-based control method can reduce the eddy current losses of a SMES system due to the significant reduction of 2nd and 3rd harmonics of superconducting coil DC current.

Description: Battery energy storage systems (BESSs) with proportional-integral (PI) control methods have been widely studied in microgrids (MGs). However, the performance of PI control methods might be unsatisfactory for BESSs due to the nonlinear characteristics of the system. To overcome this problem, an energy-based (EB) control method is applied to control the converter of a BESS in this study. The EB method is a robust nonlinear control method based on passivity theory with good performance in both transient and steady states. The detailed design process of the EB method in the BESS by adopting an interconnection and damping assignment (IDA) strategy is described. The design process comprises three steps: the construction of the port-controlled Hamiltonian model, the determination of the equilibrium point and the solution of the undetermined matrix. In addition, integral action is combined to eliminate the steady state error generated by the model mismatch. To establish the correctness and validity of the proposed method, we implement several case simulation studies based on a test MG system and compare the control performance of the EB and PI methods carefully. The case simulation results demonstrate that the EB method has better tracking and anti-disturbance performance compared with the classic PI method. Moreover, the proposed EB method shows stronger robustness to the uncertainty of system parameters.

Description: Energies, Vol. 11, Pages 2453: An Analysis of the Multi-Criteria Decision-Making Problem for Distributed Energy Systems Energies doi: 10.3390/en11092453 Authors: Yanbin Li Shuangshuang Shao Feng Zhang Choosing a distributed energy system (DES) is a multi-criteria decision-making problem. Decision-makers should not only consider the cost of the system, but also consider the energy efficiency and environmental protection of the system. In order to help decision-makers choose the best DES, this paper designs seven different DESs based on specific examples, using five criteria: investment cost, operation cost, primary energy consumption, primary energy utilization, and yearly CO2 emission. Additionally, three methods of super-efficiency Data Envelopment Analysis (DEA), Technique for Order Preference by Similarity to an Ideal Solution (TOPSIS), and Complex Proportional Assessment (COPRAS) are used to evaluate the system priority and analyze the sensitivity under different decision-making scenarios. The results show that when decision-makers only consider cost factors, traditional systems are the best choice. However, renewable energy systems are the best choice when decision-makers consider energy efficiency and environmental protection rather than cost. Among them, the photovoltaic storage system is the best system in many decision-making scenarios, because of its comprehensive advantages in cost, energy efficiency, and environmental benefit. Simultaneously, the system’s prioritization of different decision-making methods is different. In this paper, according to the Spearman correlation index test, the results achieved from TOPSIS and COPRAS are relevant and feasible.