新型级联式分时复合高频DC/AC逆变器

通过数学建模详细分析了传统分时复合高频正弦波逆变器控制,当输入电压较低时,它难以胜任光伏逆变器的最大功率点跟踪。在此基础上提出了一种由降压型Current-Fed电路与分时复合脉冲调制逆变器级联组合而成的新型高频正弦波逆变器拓扑。与传统方案相比较,它有更高的功率密度和更好的控制特性。通过3.5kW的样机实验验证了控制方法的可行性。

单相分时复合并网逆变器新型间接电流控制方法

通过对分时复合并网逆变器进行小信号建模可知,在升压阶段,采用传统电网电流反馈控制方案将导致传递函数有一个转折频率较低的右半平面零点,系统难以补偿稳定。为此,提出一种基于瞬时功率守恒的新型"间接电流"控制方法,由分析可知,采用新的控制方案后的控制系统为最小相位系统;补偿后稳定裕度高。仿真和1 kW的样机实验证实了新型控制方法的有效性。

单相分时复合级联光伏逆变器的对比研究

本文通过对两种拓扑结构十分相似的单相分时复合级联光伏逆变器建模分析,揭示了这两种电路因滤波电路位置不同产生的本质区别。在"Boost"工作阶段,一个系统的小信号模型为典型四阶系统,而另一个为性能与二阶系统相似的特殊四阶系统。两个系统的补偿器设计也相差很多,一个系统需要设计包含两对共轭零点和一个积分器的补偿器,而另一个系统只需要一个简单的PID补偿器。理论分析和样机对比实验表明:特殊四阶系统拓扑在控制性能方面明显优于另一种拓扑。

两级式并网逆变器分时复合控制策略研究

针对光伏发电系统中的两级式并网逆变器,采用了一种新型的分时复合控制策略。首先详细分析了分时复合控制策略的基本工作原理,该方法可以使得前后两级电路交替进行高频开关工作,从而有利于减小损耗;在此基础上,对分时复合控制策略下的入网电流控制环路进行小信号建模,并给出了相应的控制环路参数设计,以保证具有良好的稳态和动态性能;最后搭建了一台1 k W实验样机并进行实验验证。实验结果表明所采用分时复合控制策略的可行性和有效性。

基于间接电流控制的分时复合并网逆变器

在此首先阐述了单相分时复合并网逆变器的工作原理,给出了其双极性控制的时序图,分析了该电路的闭环控制特性,然后提出了一种基于功率守恒原理的新型间接电流控制方式及其具体控制实现框图。所提出的控制方法有效克服了传统直接电网电流控制存在的闭环难以稳定的缺点。最后通过仿真及原理样机试验验证了该控制策略的有效性。

带有最大功率点跟踪算法的改进型分时复合控制策略

基于两级式光伏并网逆变器分时复合控制原理,提出一种带有最大功率点跟踪(maximum power point tracking,MPPT)算法的改进控制方案,并采用数字陷波器抑制光伏测二次电压脉动,为MPPT提供稳定的电压参考,同时给出输入侧解耦电容计算方法。在此基础上,采用损耗分析的方法,详细对比了改进型分时复合控制策略及传统控制策略下的系统损耗。最后搭建了两台实验样机进行实验验证,实验结果表明,所提出的改进分时复合控制策略的可行性,并得出分时复合策略较传统控制策略的优缺点。

Time-resolved photoluminescence of anatase/rutile TiO_2 phase junction revealing charge separation dynamics

Junctions are an important structure that allows charge separation in solar cells and photocatalysts. Here, we studied the charge transfer at an anatase/rutile TiO2 phase junction using time-resolved photoluminescence spectroscopy. Visible （-S00 nm） and near-infrared （NIR, -830 nm） emissions were monitored to give insight into the photoinduced charges of anatase and rutile in the junction, respectively, New fast photoluminescence decay components appeared in the visible emission of futile-phase dominated TiO2 and in the NIR emission of many mixed phase TiO2samples. The fast decays confirmed that the charge separation occurred at the phase junction. The visible emission intensity from the mixed phase TiO2 increased, revealing that charge transfer from rutile to anatase was the main pathway. The charge separation slowed the microsecond time scale photolumines- cence decay rate for charge carriers in both anatase and rutile. However, the millisecond decay of the charge carriers in anatase TiO2 was accelerated, while there was almost no change in the charge carrier dynamics of rutile TiO2. Thus, charge separation at the anatase/rutile phase junction caused an increase in the charge carrier concentration on a microsecond time scale, because of slower electron-hole recombination. The enhanced photocatalytic activity previously observed at ana- tase/rutile phase junctions is likely caused by the improved charge carrier dynamics we report here. These findings may contribute to the development of improved photocatalytic materials.

Long-Term Statistics of Extreme Tsunami Height at Crescent City

Historically, Crescent City is one of the most vulnerable communities impacted by tsunamis along the west coast of the United States, largely attributed to its offshore geography. Trans-ocean tsunamis usually produce large wave runup at Crescent Harbor resulting in catastrophic damages, property loss and human death. How to determine the return values of tsunami height using relatively short-term observation data is of great significance to assess the tsunami hazards and improve engineering design along the coast of Crescent City. In the present study, the extreme tsunami heights observed along the coast of Crescent City from 1938 to 2015 are fitted using six different probabilistic distributions, namely, the Gumbel distribution, the Weibull distribution, the maximum entropy distribution, the lognormal distribution, the generalized extreme value distribution and the generalized Pareto distribution. The maximum likelihood method is applied to estimate the parameters of all above distributions. Both Kolmogorov-Smirnov test and root mean square error method are utilized for goodness-of-fit test and the better fitting distribution is selected. Assuming that the occurrence frequency of tsunami in each year follows the Poisson distribution, the Poisson compound extreme value distribution can be used to fit the annual maximum tsunami amplitude, and then the point and interval estimations of return tsunami heights are calculated for structural design. The results show that the Poisson compound extreme value distribution fits tsunami heights very well and is suitable to determine the return tsunami heights for coastal disaster prevention.

Plasmonic-enhanced two-photon fluorescence with single gold nanoshell

Single gold nanoshell with mutilpolar plasmon resonances is proposed to enhance two-photon fluorescence efficiently.The single emitter single nanoshell configuration is studied systematically by employing the finite-difference time-domain method.The emitter located inside or outside the nanoshell at various positions leads to a significantly different enhancement effect.The fluorescent emitter placed outside the nanoshell can achieve large fluorescence intensity given that both the position and orientation of the emission dipole are optimally controlled.In contrast,for the case of the emitter placed inside the nanoshell,it can experience substantial two-photon fluorescence enhancement without strict requirements upon the position and dipole orientations.Metallic nanoshell encapsulating many fluorescent emitters should be a promising nanocomposite configuration for bright two-photon fluorescence label.The results provide a comprehensive understanding about the plasmonic-enhanced two-photon fluorescence behaviors,and the nanocomposite configuration has great potential for optical detecting,imaging and sensing in biological applications.

Enhanced Fabry-Perot resonance in GaAs nanowires through local field enhancement and surface passivation

We report substantial improvements in the photoluminescence （PL） efficiency and Fabry-Perot （FP） resonance of individual GaAs nanowires through surface passivation and local field enhancement, enabling FP peaks to be observed even at room temperature. For bare GaAs nanowires, strong FP resonance peaks can be observed at 4 K, but not at room temperature. However, depositing the nanowires on gold substrates leads to substantial enhancement in the PL intensity （5X） and 3.7X to infinite enhancement of FP peaks. Finite-difference time-domain （FDTD） simulations show that the gold substrate enhances the PL spectra predominately through enhanced absorption （11X） rather than enhanced emission （1.3X）, predicting a total PL enhancement of 14X in the absence of non-radiative recombination. Despite the increased intensity of the FP peaks, lower Q factors are observed due to losses associated with the underlying gold substrate. As a means of reducing the non-radiative recombination in these nanowires, the surface states in the nanowires can be passivated by either an ionic liquid （1-ethyl-3-methylimidazolium bis（trifluoromethylsulfonyl）imide （EMIM TFSI）） or an A1GaAs surface layer to achieve up to 12X enhancement of the photoluminescence intensity and observation of FP peaks at room temperature without a gold substrate.