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Abstract In recent decades, increasing attention has been paid on accurate modeling of circulating fluidized bed (CFB) risers to provide valuable guidance to design, optimization, and operation of reactors. Turbulence model plays an important role in the accurate prediction of complex gas-solid flows. Recently developed Wray–Agarwal (WA) model is a one-equation turbulence model with the advantages of high computational efficiency and competitive accuracy with two-equation models. In this paper for the first time, the Eulerian–Eulerian approach coupled with different turbulence models including WA model, standard κ–ɛ model, and shear stress transport (SST) κ–ω model is employed to simulate two-phase flows of gas phase and solid phase in two CFB risers, in order to assess accuracy and efficiency of WA model compared to other well-known two-equation models. Predicted gas-solid flow dynamic characteristics including the gas-solid volume fraction distributions in radial and axial directions, pressure profiles, and solid mass flux distributions are compared with data obtained from an experiment in detail. The results demonstrate that the WA model is very promising for accurate and efficient simulation of gas-solid multiphase flows.

This paper initially proposes a real options model for the investment of the Power-to-gas (P2G) plant based on uncertain operating cost which mainly refers to the price of electricity. Through the analysis of the uncertainty parameters affecting the operation of the P2G project, the mathematical model expressing the relations between the parameters of P2G operation cost, electricity price, sunk cost, and other parameters are established. The Brownian motion is utilized to describe the operation cost, based on which, the option value and the project value models of P2G are derived in detail. According to these two models, the optimal investment timing of the P2G device and the corresponding optimal investment capacity can be determined. The above models are verified by numerical simulation. In addition, the influence of the change of parameters on the investment timing and investment capacity in the real options model is studied. The results show that the volatility of electricity price has a greater impact on the option value of P2G project than that of other parameters. When there is a high operating cost uncertainty, waiting is a better option, and the investment can be performed when the operating cost falls to the cost with reference to the optimal investment timing.

A hybrid energy storage system (HESS) consists of two or more types of energy storage components and the power electronics circuit to connect them. Therefore, the real-time capacity of this system highly depends on the state of the system and cannot be simply evaluated with traditional battery models. To tackle this challenge, an equivalent state of charge (ESOC) which reflects the remaining capacity of a HESS unit in a specific operation mode, is proposed in this paper. Furthermore, the proposed ESOC is applied to the control of the distributed HESS which contains several units with their own local targets. To optimally distribute the overall power target among these units, a sparse communication network-based hierarchical control framework is proposed. This framework considers the distributed control and optimal power distribution in the HESS from two aspects - the power output capability and the ESOC balance. Based on the primary droop control, the total power is allocated according to the maximum output capacity of each unit, and the secondary control is used to adjust the power from the perspective of ESOC balance. Therefore, each energy storage unit can be controlled to meet the local power demand of the microgrid. Simulation results based on MATLAB/Simulink verify the effectiveness of the application of the proposed equivalent SOC.

Abstract This paper presents a novel seamless transfer strategy for microgrids (MGs) that enables both grid-connected and islanding modes, with no need of forced controller switching between different controllers and with improved performance in both reference tracking and disturbance attenuation. A unified control structure based on droop technique is utilized to generate the voltage and frequency references, accommodating all operation modes. Alternatively, the dynamical feedforward with reference model control and disturbance observer are introduced into the cascaded voltage-current control, which not only estimates and compensates the influences of power fluctuation as well as model uncertainties timely, but also provides the perfect reference tracking capability. Then the detailed analysis on the deduced mathematical model of inverter is carried out, and the superiority of the proposed strategy is demonstrated in tracking performance, disturbance attenuation and robustness. Simulation and experimental results show that the harmful transients during the operation mode transition can be effectively eliminated with the overall dynamic performance improved so that the effectiveness of the proposed control scheme is verified.

Global food insecurity is becoming more severe under the threat of rising global carbon dioxide concentrations, increasing population, and shrinking farmlands and their degeneration. We acquired the ISI Web of Science platform for over 31 years (1988–2018) to review the research on how climate change impacts global food security, and then performed cluster analysis and research hotspot analysis with VosViewer software. We found there were two drawbacks that exist in the current research. Firstly, current field research data were defective because they were collected from various facilities and were hard to integrate. The other drawback is the representativeness of field research site selection as most studies were carried out in developed countries and very few in developing countries. Therefore, more attention should be paid to developing countries, especially some African and Asian countries. At the same time, new modified mathematical models should be utilized to process and integrate the data from various facilities and regions. Finally, we suggested that governments and organizations across the world should be united to wrestle with the impact of climate change on food security.

Very limited information is available on the drought tolerance of European beech and silver fir in mixed cultivation, both for mature forests and natural regeneration. Particularly, little information is available regarding the significance on memory effects of drought exposure. Therefore, drought memory was analyzed in seedlings of these species grown in mixed cultivation in the present study. The results showed that previous-year drought hardening mediated enhanced biomass accumulation of beech leaves and root in the subsequent year, but did not impact fir growth. Total carbon (C) content was decreased by drought hardening in both the leaves and roots of beech and previous-year needles and roots of fir, in beech probably as a consequence of increased growth. Previous-year drought hardening had no significant effect on relative water contents, total nitrogen (N), or soluble protein contents in leaves and roots of beech and fir, but resulted in decreased total amino acid contents of beech leaves and fir needles. It further reduced structural N in current-year fir needles and decreased C/N ratios in roots of both beech and silver fir seedlings. Generally, the number of interspecific neighbors had no considerable effect on biomass or total C or N contents, as well as N partitioning in leaves and roots of beech and fir seedlings. The present study highlights that drought hardening induces memory effects in European beech and silver fir seedlings in their mixture in the subsequent year of growth, but these memory effects are stronger in beech than in fir.

Abstract A unified optimal power flow (OPF) model for AC/DC grids integrated with natural gas systems is proposed for the real-time scheduling of power systems. Herein, the primary physical couplings underlying this coordinated system are modeled and investigated. In addition, the uncertainties of gas loads are considered when studying the role of gas supply for gas-fired units in power system operations. The nonlinear gas system constraints are converted to the second-order cone forms that allow for the use of the Benders decomposition techniques and the interior-point method to obtain the optimal solution. The numerical results of the modified IEEE 118-bus test system that integrates the Belgium 20-node natural gas system demonstrate the effectiveness of the proposed model. The effects of gas demand uncertainties on the optimal schedule of thermal generators are investigated as well.