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.     

Effective band gap of Si nanocrystals embedded in SiO2 matrix

Using a formulation of the Hartree-Fock formalism with the potential morphing method in the effective mass approximation, we calculate the effective band gap of Si nanocrystals embedded in SiO2 matrix without the existence of polysilane, as a function of their diameter in the size range 1-3.5 nm. Our results are in better agreement with the experimental data, in comparison with other existing theoretical data. For diameter smaller than 2 nm our results have the same tendency with the existing theoretical results, e.g., the discrepancy between theory and experiment seems to be essential.     

Infrastructure,automation and model-based operation strategy in a stand-alone hydrolytic solar-hydrogen production unit

An autonomous power system that exploits solar energy for the production of hydrogen through water electrolysis is fully assessed in terms of system implementation and optimal operating strategy. A 10 kW_p photovoltaic array supplies energy to a PEM electrolyzer rated at 6.9 kW_p. In order to maintain a smooth operation regardless of the inherent weather fluctuations, a 1000 Ah/24 V lead–acid accumulator stores energy excess and provides it when needed. The monitoring and control of the system is implemented through a Supervisory Control and Data Acquisition system (SCADA), while the interactions between electrical and chemical subsystems are addressed by a complete automation infrastructure. The mathematical models of each subsystem are validated based on real operational data and a model-based power management strategy is proposed and assessed through a parameter sensitivity analysis. Further on, an off-line optimization framework is evaluated regarding the optimal operation of the system in two diverse, but representative time periods. The optimal parameters are identified and their effect on hydrogen production and accumulator utilization is reported.     

A simple cost-effective sputtering-based method for micropatterning and materials microstructuring

Patterning of semiconductors results in the fabrication of micro- and nano-structures, which are desired in modern technologies. Such a patterning is usually realized with the help of e-beam-, high-energy ion-, X-ray- or laser-assisted techniques, which demand expensive equipments. In this work we present a simple cost-effective method realized via a radio-frequency driven magnetron-sputtering head in high vacuum. The target is a silicon wafer masked with metallic grids. If the grid is magnetic, e.g., nickel, it is attracted by the magnetic forces of the magnetron, otherwise, magnetic clamps are used. Soft sputtering conditions, i.e., 30-100 Watts are used and the result is a well-ordered micropatterning of the surface with nicely formed pits the size of which is entirely determined by the grid size and the depth by the sputtering power and time. The pits are monitored with the help of Optical and Atomic Force Microscopy. If the masked micropatterned silicon wafer is then used as a substrate, the pits may be partially filled by a material. As a first example we present square-like Co microstructures. The magnetic signal of these Co microstructures is recorded with the help of a computer-driven magneto-optic Kerr effect home-made magnetometer. This patterned material may be used in magnetic recording technology. More examples include the formation of Cu-microcolumns and Pt film microframeworks. For the latter ones, an etching process is applied to prepare porous silicon networks with photoluminescence, which may be used in optoelectronics.     

Structure and magnetic properties of hcp and fcc nanocrystalline thin Ni films and nanoparticles produced by radio frequency magnetron sputtering

We report on the growth of thin Ni films by radio frequency magnetron sputtering in Ar-plasma. The growth temperature was about 350 K and the films were deposited on various substrates such as glass, silicon, sapphire and alumina. The thickness of the thinnest films was estimated by the appearance of Kiessig fringes up to about 2theta = 8 degrees in the small-angle X-ray diffraction pattern, as expected for high-quality atomically-flat thin films. With the help of this, a quartz balance system was calibrated and used for measuring the thickness of thicker samples with an accuracy of better than 5%. Structural characterization via X-ray diffraction and high resolution transmission electron microscopy revealed an Ar-gas pressure window, where single phase hcp Ni films may be grown. The magnetic response of the Ni films was checked at room temperature via a newly established and fully automatic polar magneto-optic Kerr effect magnetometer. The hcp films show no magnetic response. Interestingly, the magnetic saturation field of fcc films deposited at low Ar pressure is comparable to the one of bulk Ni, while the one of fcc films deposited at high Ar pressures is decreased, revealing the presence of residual strain in the films. Finally, it is shown that it is possible to form films which contain magnetic Ni fcc nanoparticles in a non-magnetic hcp matrix, i.e., a system interesting for technological applications demanding a single Ni target for its production.     

Identification and characterization of microbial contaminants isolated from stored aviation fuels by DNA sequencing and restriction fragment length analysis of a PCR-amplified region of the 16S rRNA gene

Microorganisms contaminating aircraft fuel tanks containing jet propellant-8 (JP-8) were isolated and identified by 16S rRNA gene sequencing. Thirty-three samples from six geographically separated airport bases were collected. Two bacterial genera were identified by DNA sequencing including the following isolates: Agrobacterium tumefaciens and Staphylococcus epidermidis. Populations of the above species recovered on culture media ranged from 10 to 10³ CFU/ml with Staphylococcus and Agrobacterium being identified in samples taken from more than one airport base. From the bacterial communities isolated and identified in this study, only Staphylococcus was previously reported to thrive in aviation fuel tanks. Changes in aviation fuel composition and/or the ability of certain microbes to grow under certain environmental conditions may account for the isolation of previously undocumented bacterial species in aviation fuel tanks. Restriction fragment length analysis of the PCR-product proved a more reliable, simpler and quicker method to distinguish S. epidermidis and A. tumefaciens than conventional DNA sequencing.     

Influence of electrolyte co-additives on the performance of dye-sensitized solar cells

The presence of specific chemical additives in the redox electrolyte results in an efficient increase of the photovoltaic performance of dye-sensitized solar cells (DSCs). The most effective additives are 4-tert-butylpyridine (TBP), N-methylbenzimidazole (NMBI) and guanidinium thiocyanate (GuNCS) that are adsorbed onto the photoelectrode/electrolyte interface, thus shifting the semiconductor's conduction band edge and preventing recombination with triiodides. In a comparative work, we investigated in detail the action of TBP and NMBI additives in ionic liquid-based redox electrolytes with varying iodine concentrations, in order to extract the optimum additive/I₂ ratio for each system. Different optimum additive/I₂ ratios were determined for TBP and NMBI, despite the fact that both generally work in a similar way. Further addition of GuNCS in the optimized electrolytic media causes significant synergistic effects, the action of GuNCS being strongly influenced by the nature of the corresponding co-additive. Under the best operation conditions, power conversion efficiencies as high as 8% were obtained.     

Analytical Solutions and Simulation Approaches for Double Permeability Aquifers

In this paper we develop analytical solutions for 1D unsteady flow in an infinite double permeability aquifer. Two asymptotic cases are considered: the near-hydraulic-equilibrium-state and the far-hydraulic-equilibrium-state. It is demonstrated that for both cases the equation for the piezometric head can be expressed in leading order, as the classical solution of the simple permeability medium, to which correction terms are added. On the basis of these findings the accuracy of the simulations using conventional approaches (i.e. MODFLOW, MT3D codes) is discussed, while decision tools for the choice of the adequate double continuum model are also provided.     

ApoA1 Deficiency Reshapes the Phenotypic and Molecular Characteristics of Bone Marrow Adipocytes in Mice

In the present study, we studied the effect of apolipoprotein A-1 (APOA1) on the spatial and molecular characteristics of bone marrow adipocytes, using well-characterized ApoA1 knockout mice. APOA1 is a central regulator of high-density lipoprotein cholesterol (HDL-C) metabolism, and thus HDL; our recent work showed that deficiency of APOA1 increases bone marrow adiposity in mice. We found that ApoA1 deficient mice have greatly elevated adipocytes within their bone marrow compared to wild type counterparts. Morphologically, the increased adipocytes were similar to white adipocytes, and displayed proximal tibial-end localization. Marrow adipocytes from wild type mice were significantly fewer and did not display a bone-end distribution pattern. The mRNA levels of the brown/beige adipocyte-specific markers Ucp1, Dio2, Pat2, and Pgc1a; and the expression of leptin were greatly reduced in the ApoA1 knock-out in comparison to the wild-type mice. In the knock-out mice, adiponectin was remarkably elevated. In keeping with the close ties of hematopoietic stem cells and marrow adipocytes, using flow cytometry we found that the elevated adiposity in the ApoA1 knockout mice is associated with a significant reduction in the compartments of hematopoietic stem cells and common myeloid, but not of the common lymphoid, progenitors. Moreover, the ‘beiging’-related marker osteopontin and the angiogenic factor VEGF were also reduced in the ApoA1 knock-out mice, further supporting the notion that APOA1—and most probably HDL-C—regulate bone marrow microenvironment, favoring beige/brown adipocyte characteristics.