微电网中光伏并网发电系统的建模仿真

微电网技术的发展能有效地解决分布式发电的接入问题,有利于新能源的开发.太阳能作为最具有利用价值的清洁能源之一,并网光伏发电系统的应用前景越来越广阔.通过利用Mat-lab\Simulink软件,开发出了适用于工程计算的光伏电池模型,基于扰动观察法建立了最大功率跟踪模型,结合空间矢量调制(SVPWM)算法设计出了电压外环-电流内环双环控制结构的逆变器并网控制策略,在此基础上搭建了完整的光伏并网发电系统运行与控制模型.仿真结果表明建立的模型能较好的反映三相光伏并网系统运行特性,为微电网的进一步研究提供了基础.     

云计算环境下基于密文策略的权重属性加密方案

在综合分析现有基于密文策略属性加密方案的基础上,针对现有基于密文策略属性加密方案较少考虑属性权重的现状,将权重的概念引入到基于密文策略的属性加密方案中,提出了基于密文策略的权重属性加密方案,证明了在选择权重属性集模型下安全性。并将此方案引入到云计算环境下,给出了系统级的基于密文策略的权重属性加密方案。分析表明,基于密文策略权重属性的加密方案相比现有方案更适用于云计算环境。     

Stochastic formulation of particle kinetics in wall-bounded two-phase flows

This paper presents a generalized framework of stochastic modeling for particle kinetics in wall-bounded flow. We modified a reflected Brownian motion process and straightforwardly obtained a Kramers equation for particle probability density function (PDF). After the wall effects were accounted for as a drift from zero in the mean displacement and suppression in the diffusivity of a particle, an analytical solution was worked out for PDF. Three distinguishable mechanisms were identified to affect the profile of particle probability distribution: external forces, turbophoresis effect, and wall-drift effect. The proposed formulation covers the Huang et al. (2009) model of a wall that produces electrostatic repulsion force and van der Waals force, as well as Monte-Carlo solutions for the Peter and Barenbrug (2002) model under a variety of relaxation times. Moreover, it successfully reproduces the two patterns of particle concentration profiles observed in experiments of sediment-laden open-channel flows. The strength of the wall-drift effect was found to be connected with the interaction frequency between particle and wall. Further exploration of the relationship among flow turbulence, particle inertia, and particle concentration is worthwhile.     

A miniature liquid helium temperature JT cryocooler for space application

Along with the development of space science and technology,miniature liquid helium temperature long life cryocooler is a focus subject in cryogenic study.Since it is the precondition of space detection researches,institutions of space in many countries do the research on it.In this article,we designed a compound cooling system.A three-stage high frequency thermal coupled pulse tube cryocooler was used to precool a Joule-Thomson(JT)cryocooler.This system has no moving parts at low temperatures and is hence suitable for space operation.Liquid helium temperature was successfully achieved in both open loop and closed cycle experiments.In the closed cycle experiment,when 473 W electric power was inputted,the cooling system reached a no-load temperature of 4.4 K,and a cooling capacity of 11.6 mW was provided at 4.54 K.It is the first miniature liquid helium temperature JT cryocooler in China and the research achievement paves a way for the space application of ultra-long wave infrared detection and THz technologies.     

An index for deformation controllability of small-volume materials

Mechanical tests on small-volume materials show that in addition to the usual attributes of strength and ductility,the controllability of deformation would be crucial for the purpose of precise plastic shaping.In our present work,a"mechanical controllability index"(MCI)has been proposed to assess the controllability of mechanical deformation quantitatively.The index allows quantitative evaluation of the relative fraction of the controllable plastic strain out of the total strain.MCI=0 means completely uncontrollable plastic deformation,MCI=∞means perfectly controllable plastic shaping.The application of the index is demonstrated here by comparing two example cases:0.273 to 0.429 for single crystal Al nanopillars that exhibit obvious strain bursts,versus 3.17 to 4.2 for polycrystalline Al nanopillars of similar size for which the stress-strain curve is smoother.     

离网型风光互补发电系统多模态能量控制与管理

离网型风光互补发电系统是一种合理配置新能源的独立电源系统,由于自然界中风能、太阳能出现的不可预测性,以及负载、储能状态的随机性,提出一种基于风光互补发电系统多模态能量流的分析法,研究了各模态及模态间转化特性,给出了四种典型状态下风光互补发电系统的能量控制及管理。采用闭环电压控制使风力发电通道和光伏发电通道输出电压恒定,为用电设备和储能元件提供安全可靠的电能。通过MATLAB/Simulink软件对离网型风光互补发电系统多模态能量控制进行了仿真,结果表明离网型风光互补发电系统工作可靠,验证了多模态分析法的有效性。     

Upeonversion Emission and Paramagnctism of Colloid Ba2ErF7 and Ba2ErFT：Yb3＋ Nanoerystals Synthesized with Solvothermal Method

Ultrasmall near-monodisperse Ba2ErF7 nanocrystals with average crystal size 9.6 nm were synthesized with solvothermal method. The X-ray diffraction （XRD） and transmission electron microscopy （TEM） assays reveal that the as-synthesized Ba2ErF7 nanocrystals are of the cubic structure with the cell parameter of 5.943 A, instead of the reported orthorhombic and tetragonal structure. Two emission bands originated from 2Hwj4H3/2 → 4F5/2 and 4F9/2 ----＋ 4115,2 of Er3＋ can be observed under a 980 nm laser excitation. The magnetic mass susceptibility of the as-synthesized BazErF7 nanocrystals reaches 4.293 × 10-5 emu g-1 Oe-1.     

基于组合优化BP神经网络的模拟电路故障诊断

电子系统的可靠性已成为影响系统正常运行的关键,因此电路故障的诊断越来越受到重视。基于BP神经网络的诊断方法是目前实现模拟电路故障诊断的有效方法之一。文章针对已有BP神经网络模拟电路故障诊断技术的不足,提出了一种组合优化的诊断方案。该方案采用遗传算法优化确定BP神经网络的初始权值,以规避BP神经网络易陷入局部极小值的不足,然后应用L-M方法在这个局部解空间里对BP神经网络进行精调,搜索出最优解或者近似最优解。该方案发挥了BP神经网络的广泛映射能力和遗传算法的全局搜索能力,加快了网络的学习速度,综合提高了整个学习过程中的逼近能力和泛化能力,有效提升了基于BP神经网络模拟电路故障诊断的性能。     

The influence of oleic acid on the hydration and mechanical properties of portland cement

To investigate the effects of OA on the portland cement using oleic acid （OA） as grinding aid and the effects of OA on the portland cement, we characterized the different perfermances of the cement, discussed the characteristics of properties. The results reveal that the OA can reduce the water requirement of normal consistency. With the content of OA added, setting time was extended. The OA can significantly improve the rheological properties of cement-based materials, while the compressive strength decreases.     

Effect of dy on structure and magnetic properties of CoPt alloys

The effects of Dy on the microstructure and magnetic properties of Dy _x Co_50?x Pt₅₀ alloys were investigated. The XRD results indicate that all the alloys homogenized at 1000 °C contain only a single A1 (fcc) phase, while the alloys annealed at 675 °C consist of a hard-magnetic face-centered-tetragonal (fct) phase and a magnetically soft face-centered-cube (fcc) phase. Maximum values for the coercivity H _c and remanence ratio m _r were achieved in Dy_0.4Co_49.6Pt₅₀ alloys annealed at 675 °C for 80 min. For the series of Dy _x Co_50?x Pt₅₀ alloys annealed at 675 °C for 60 min, H _c decreases monotonically with increasing Dy concentration, but m _r is first enhanced and then weakened.