Multilevel converters for single-phase grid connected photovoltaic systems: an overview

Multilevel voltage source inverters offer several advantages compared to their conventional counterparts. By synthesising the AC output terminal voltage from several levels of DC voltages, staircase waveforms can be produced, which approach the sinusoidal waveform with low harmonic distortion, thus reducing filter requirements. The need of several sources on the DC side of the converter makes multilevel technology attractive for photovoltaic applications. This paper provides an overview on different multilevel topologies and investigates their suitability for single-phase grid connected photovoltaic systems. Several transformerless photovoltaic systems incorporating multilevel converters are compared regarding issues such as component count and stress, system power rating and the influence of the photovoltaic array earth capacitance.     

Electrode and electrolyte materials for electrochemical capacitors

Among different electric energy storage technologies electrochemical capacitors are used for energy storage applications when high power delivery or uptake is needed. Their energy and power densities, durability and efficiency are influenced by electrode and electrolyte materials however due to a high cost/performance ratio; their widespread use in energy storage systems has not been attained yet.Thanks to their properties such as high surface area, controllable pore size, low electrical resistance, good polarizability and inertness; activated carbons derived from polymeric precursors are the most used electrode materials in electrochemical capacitors at present. Other electrode materials such as shaped nano-carbons or metal oxides are also investigated as electrode materials in electrochemical capacitors, but only as useful research tools.Most commercially used electrochemical capacitors employ organic electrolytes when offering concomitant high energy and high power densities. The use of aqueous based electrolytes in electrochemical capacitor applications is mainly limited to research purposes as a result of their narrow operating voltage. Recent studies on room temperature ionic liquids to be employed as electrolyte for electrochemical capacitor applications are focused on fine tuning their physical and transport properties in order to bring the energy density of the device closer to that of batteries without compromising the power densities.In this paper a performance analysis, recent progress and the direction of future developments of various types of materials used in the fabrication of electrodes for electrochemical capacitors are presented. The influence of different types of electrolytes on the performance of electrochemical capacitors such as their output voltage and energy/power densities is also discussed.     

Roles of the Na+/H+ Exchanger Isoform 1 and Urokinase in Prostate Cancer Cell Migration and Invasion

Prostate cancer is a leading cause of cancer-associated deaths in men over 60 years of age. Most patients are killed by tumor metastasis. Recent evidence has implicated a role of the tumor microenvironment and urokinase plasminogen activator (uPA) in cancer cell migration, invasion, and metastasis. Here, we examine the role of the Na⁺/H⁺ exchanger isoform 1 (NHE1) and uPA in DU 145 prostate cancer cell migration and colony formation. Knockout of NHE1 reduced cell migration. The effects of a series of novel NHE1/uPA hexamethylene-amiloride-based inhibitors with varying efficacy towards NHE1 and uPA were examined on prostate cancer cells. Inhibition of NHE1—alone, or with inhibitors combining NHE1 or uPA inhibition—generally did not prevent prostate cancer cell migration. However, uPA inhibition—but not NHE1 inhibition—prevented anchorage-dependent colony formation. Application of inhibitors at concentrations that only saturate uPA inhibition decreased tumor invasion in vivo. The results suggest that while knockout of NHE1 affects cell migration, these effects are not due to NHE1-dependent proton translocation. Additionally, while neither NHE1 nor uPA activity was critical in cell migration, only uPA activity appeared to be critical in anchorage-dependent colony formation of DU 145 prostate cancer cells and invasion in vivo.     

Video bridging for multipoint video conferencing

With the growth of video conferencing, the need for multipoint conferences is also growing. In a multipoint videoconference over a wide-area network, the users are connected to a multipoint control unit (MCU) in a central office. A video bridge in the MCU combines the input video streams into a suitable format for presentation. In this article, we discuss various approaches of video bridging for continuous presence multipoint video conferencing. We also discuss the video quality issues and describe some multipoint video bridge prototypes we have developed. © 1997 John Wiley & Sons, Inc. Int J Imaging Syst Technol, 8, 427–431, 1997     

Revisit the crystallization mechanism of vectra,™ a liquid crystal polymer

We have studied isothermal transition kinetics and crystallization mechanisms of the Hoechst Celanese liquid crystal polymer Vectra™ A‐950 using a differential scanning calorimetry. This material is a random copolymer with a composition made of 73 mol %/27 mol % of HBA (1,4‐dihydroxybenzoic acid)/HNA (2,6‐dihydroxynaphthoic acid). When comparing our results to previous work on 75/25 HBA/HNA, we found some similarities as well as dissimilarities. In the case of similarities, both polymers show two types of transition processes in the low‐temperature region (below 495 K). One is a fast process, which can be regarded as liquid crystal transition, and is characterized by a heat of fusion that does not vary significantly with annealing time. The other transition is a slow process related to crystal perfection and shows increases in the heat of transition and the transition temperature with increasing annealing time. However, the apparently slight difference in polymer composition also leads to a few surprising results. For example, previous work on 75/25 HBA/HNA two transition peaks (slow and fast processes) were observed after annealing it at 505 K, while in the current work only one transition peak in the case of 73/27 HBA/HNA was observed. In addition, based on the relationship between heat of fusion and annealing time, the LCP made from 73/27 HBA/HNA may perfect faster (in the time scale we study); hence, its posttreatment processes may be accomplished more efficiently than that of 75/25 HBA/HNA. This result also suggests that the former may have a better sequence or a better matching of molecular dimensions for crystallization than that of the latter. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1139–1150, 1999     

金属空间 自然生活

20世纪80年代初,纽约建筑师威廉格莱克曼在平凡无奇的小镇Sagaponak构建了一栋全不锈钢规格的房子。25年后,一位纽约房地产开发商和他的妻子发现了它。通过他们的改造,新家将室内与室外连接在一起,就像一个开放的流动帐篷让他们置身在大自然之中。     

Influence of Zinc on Distiller's Yeast: Cellular Accumulation of Zinc and Impact on Spirit Congeners

Accumulation of zinc by a whisky distilling yeast strain of Saccharomyces cerevisiae was studied during fermentation of malt wort and synthetic defined medium. Zinc uptake by yeast cells was very rapid in malt wort, as zinc (0.32 μg/mL) was completely removed from the fermentation medium within one hour. The type of fermentable carbohydrate had an impact on the kinetics of zinc accumulation, with maltose most effective at enhancing metal uptake at zinc concentrations above 3.2 μg/mL. Enriching yeast cells with zinc by “preconditioning” impacted on the production of flavour congeners in the distillates produced from fermented cultures. Such distillates were characterised by an altered flavour and aroma profile. In particular, the production of some higher alcohols increased when yeast cells were preconditioned with zinc. This phenomenon is yeast strain related. Industrial fermentation processes, including brewing and distilling, may benefit from optimization of zinc bioavailability in yeast cultures resulting in more efficient fermentations and improved product quality.     

升级或替换?--谈亚洲数字电视的天馈线系统

要使射频广播系统的架构能够支持数字电视系统,需要设些什么?对整个网络进行进个站点的深入研究是必需的,RFS公司无线技术小组的Mike Dallimore这样认为。本文介绍了RFS公司通过对各种因素的分析研究得出的将现有系统介级为数字电视系统的最经济方案。