Optimal planning of energy storage system in active distribution system based on fuzzy multi-objective bi-level optimization

A fuzzy multi-objective bi-level optimization problem is proposed to model the planning of energy storage system(ESS) in active distribution systems(ADS). The proposed model enables us to take into account how optimal operation strategy of ESS in the lower level can affect and be affected by the optimal allocation of ESS in the upper level. The power characteristic model of micro-grid(MG)and typical daily scenarios are established to take full consideration of time-variable nature of renewable energy generations(REGs) and load demand while easing the burden of computation. To solve the bi-level mixed integer problem, a multi-subgroup hierarchical chaos hybrid algorithm is introduced based on differential evolution(DE) and particle swarm optimization(PSO). The modified IEEE-33 bus benchmark distribution system is utilized to investigate the availability and effectiveness of the proposed model and the hybrid algorithm. Results indicate that the planningmodel gives an adequate consideration to the optimal operation and different roles of ESS, and has the advantages of objectiveness and reasonableness.

CFD simulation of smooth and T-abrupt exits in circulating fluidized bed risers

Gas-solid flow in circulating fluidized bed （CFB） risers depends not only on operating conditions but also on exit configurations. Few studies investigated the effects of exit configurations on flow structure using computational fluid dynamics （CFD）. This paper provides a 2D two-fluid model to simulate a cold bench-scale square cross-section riser with smooth and T-abrupt exits. The drag force between the gas and solid phases plays an important role in CFD. Since the drag force model based on homogeneous two- phase flow, such as the Wen-Yu correlation, could not capture the heterogeneous structures in gas-solid flow, the structure-dependent energy-minimization multi-scale （EMMS） drag force model （Wang, Ge, ＆ Li, 2008）, applicable for Geldart B particles （sand）, was integrated into the two-fluid model. The calculated axial solids hold-up profiles were respectively exponential curve for smooth exit and C-shaped curve for T-abrupt exit, both consistent with experimental data. This study once again proves the key role of drag force in CFD simulation and also shows the validity of CFD simulation （two-fluid model） to describe exit effects on ~as-solid flow in CFB risers.

Power flow control scheme for multiport power electronics transformers

Development of power electronics transformers(PETs)has made dc distribution and hybrid ac/dc systems a competitive solution for future network expansion.This study focuses on the functions of a multiport PET interfacing medium-voltage and low-voltage networks.A comprehensive power flow control scheme based on a globalised multiport transmission model of the PET and droop control is derived.This allows for a coordinated energy exchange between the ports of the PET,which enables autonomous operation.Simulations of the proposed approach in a hybrid ac/dc distribution network with different levels of distributed generation and loads verify the effectiveness of the method.

3D full-loop simulation and experimental verification of gas-solid flow hydrodynamics in a dense circulating fluidized bed

Because of their advantages of high efficiency and low cost, numerical research methods for large-scale circulating fluidized bed （CFB） apparatus are gaining ever more importance. This article presents a numer- ical study of gas-solid flow dynamics using the Eulerian granular multiphase model with a drag coefficient correction based on the energy-minimization multi-scale （EMMS） model. A three-dimensional, full-loop, time-dependent simulation of the hydrodynamics of a dense CFB apparatus is performed. The process parameters （e.g., operating and initial conditions） are provided in accordance with the real experiment to enhance the accuracy of the simulation. The axial profiles of the averaged solid volume fractions and the solids flux at the outlet of the cyclone are in reasonable agreement with experimental data, thereby verifying the applicability of the mathematical and physical models. As a result, the streamline in the riser and standpipe as well as the solids distribution contours at the cross sections is analyzed. Computational fluid dynamics （CFD） serves as a basis for CFB modeling to help resolve certain issues long in dispute but difficult to address experimentally. The results of this study provide the basis of a general approach to describing dynamic simulations of gas-solid flows.