604 research outputs found
Power management of islanded Self-Excited Induction Generator reinforced by energy storage systems
Self-Excited Induction Generators (SEIGs), e.g., Small-Scale Embedded wind generation, are increasingly used in electricity distribution networks. The operational stability of stand-alone SEIG is constrained by the local load conditions: stability can be achieved by maintaining the load’s active and reactive power at optimal values. Changes in power demand are dependent on customers’ requirements, and any deviation from the pre-calculated optimum setting will affect a machine’s operating voltage and frequency. This paper presents an investigation of the operation of the SEIG in islanding mode of operation under different load conditions, with the aid of batteries as an energy storage source. In this research a current-controlled voltage-source converter is proposed to regulate the power exchange between a direct current (DC) energy storage source and an alternating current (AC) grid, the converter’s controller is driven by any variation between machine capability and load demand. In order to prolong the system stability when the battery reaches its operation constraints, it is recommended that an ancillary generator and a dummy local load be embedded in the system. The results show the robustness and operability of the proposed system in the islanding mode of the SEIG under different load conditions
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Human β-Cell Proliferation and Intracellular Signaling: Driving in the Dark Without a Road Map
A major goal in diabetes research is to find ways to enhance the mass and function of insulin secreting β-cells in the endocrine pancreas to prevent and/or delay the onset or even reverse overt diabetes. In this Perspectives in Diabetes article, we highlight the contrast between the relatively large body of information that is available in regard to signaling pathways, proteins, and mechanisms that together provide a road map for efforts to regenerate β-cells in rodents versus the scant information in human β-cells. To reverse the state of ignorance regarding human β-cell signaling, we suggest a series of questions for consideration by the scientific community to construct a human β-cell proliferation road map. The hope is that the knowledge from the new studies will allow the community to move faster towards developing therapeutic approaches to enhance human β-cell mass in the long-term goal of preventing and/or curing type 1 and type 2 diabetes
Spin dynamics in a structurally ordered non-Fermi liquid compound: YbRh_2Si_2
Muon spin relaxation (muSR) experiments have been carried out at low
temperatures in the non-Fermi-liquid heavy-fermion compound YbRh_2Si_2. The
longitudinal-field muSR relaxation function is exponential, indicative that the
dynamic spin fluctuations are homogeneous. The relaxation rate 1/T_1 varies
with applied field as H^{-y}, y = 1.0 \pm 0.1, which implies a scaling law of
the form \chi''(\omega) \propto \omega^{-y} f(\omega/T), \lim_{x\to0} f(x) = x
for the dynamic spin susceptibility.Comment: 5 pages, 2 figures. To be published in proceedings of musr2002
(Physica B
Towards a Classification of the Effects of Disorder on Materials Properties
Many 'interesting; correlated electron materials exhibit an unusual
sensitivity of measured properties to external perturbations, and in particular
to imperfections in the sample being measured. It is argued that in addition to
its inconvenience, this sensitivity may indicated potentially useful
properties. A partial classification of causes of such sensitivity is given.Comment: Solid State Communications, in press (Proceedings of the June 2002
Williamsburg conference on Muon Spin Rotation
Killing spinor initial data sets
A 3+1 decomposition of the twistor and valence-2 Killing spinor equation is
made using the space spinor formalism. Conditions on initial data sets for the
Einstein vacuum equations are given so that their developments contain
solutions to the twistor and/or Killing equations. These lead to the notions of
twistor and Killing spinor initial data. These notions are used to obtain a
characterisation of initial data sets whose development are of Petrov type N or
D.Comment: 31 pages, submitted to J. Geom. Phy
Disorder, inhomogeneity and spin dynamics in f-electron non-Fermi liquid systems
Muon spin rotation and relaxation (SR) experiments have yielded evidence
that structural disorder is an important factor in many f-electron-based
non-Fermi-liquid (NFL) systems. Disorder-driven mechanisms for NFL behaviour
are suggested by the observed broad and strongly temperature-dependent SR
(and NMR) linewidths in several NFL compounds and alloys. Local disorder-driven
theories (Kondo disorder, Griffiths-McCoy singularity) are, however, not
capable of describing the time-field scaling seen in muon spin relaxation
experiments, which suggest cooperative and critical spin fluctuations rather
than a distribution of local fluctuation rates. A strong empirical correlation
is established between electronic disorder and slow spin fluctuations in NFL
materialsComment: 24 pages, 15 figures, submitted to J. Phys.: Condens. Matte
Order and nFl Behavior in UCu4Pd
We have studied the role of disorder in the non-Fermi liquid system UCu4Pd
using annealing as a control parameter. Measurement of the lattice parameter
indicates that this procedure increases the crystallographic order by
rearranging the Pd atoms from the 16e to the 4c sites. We find that the low
temperature properties depend strongly on annealing. Whereas the non-Fermi
liquid behavior in the specific heat can be observed over a larger temperature
range after annealing, the clear non-Fermi liquid behavior of the resistivity
of the unannealed sample below 10 K disappears. We come to the conclusion that
this argues against the Kondo disorder model as an explanation for the
non-Fermi liquid properties of both as-prepared and annealed UCu4Pd
Multilateralismo e unilateralismo na polÃtica mundial: América Latina frente à ordem mundial em transição
Specific Heat of URuSi in Fields to 42 T: Clues to the 'Hidden Order'
The large C observed at 17.5 K in URuSi is inconsistent
with the small, 0.04 moment measured for the antiferromagnetism
observed starting (perhaps coincidentally) at the same temperature. We report
measurements of this specific heat transition, thought to be due to some
'hidden order', in magnetic fields between 24 and 42 T, i. e. through the field
region where three metamagnetic transtions are known to occur at 35.8, 37.3,
and 39.4 T. The response of C in single crystal URuSi to
magnetic field, which includes a change to C being possibly associated
with a first order phase transition for high fields, is analyzed to shed
further light on the possible explanations of this unknown ordering process. At
fields above 35 T, a new high field phase comes into being; the connection
between this high field phase revealed by the specific heat and earlier
magnetization data is discussed
Characterization of white spot lesions formed on human enamel under microcosm biofilm for different experimental periods
The initial characteristics of white spot lesion (WSLs), such as the degree of integrated mineral loss (ΔZ), depth and pattern of mineral distribution, have an impact on further demineralization and remineralization. However, these lesion parameters have not been evaluated in WSLs produced from microcosm biofilms. Objective: This study characterized artificial white spot lesions produced on human enamel under microcosm biofilm for different experimental periods. Methodology: In total, 100 human enamel specimens (4x4mm) were assigned to 5 distinct groups (n=20/group) differing according to the period of biofilm formation (2, 4, 6, 8 or 10 days). Microcosm biofilm was produced on the specimens from a mixture of human and McBain saliva at the first 8h. Enamel samples were then exposed to McBain saliva containing 0.2% sucrose. WSLs formed were characterized by quantitative light-induced fluorescence (QLF) and transverse microradiography (TMR). Data were analyzed by ANOVA/Tukey or Kruskal-Wallis/Dunn tests (p<0.05). Results: A clear time-response pattern was observed for both analyses, but TMR was able to better discriminate among the lesions. Regarding QLF analysis, median (95%CI; %) changes in fluorescence ∆Z were -7.74(-7.74:-6.45)a, -8.52(-8.75:-8.00)ab, -9.17(-10.00:-8.71)bc, -9.58(-10.53:-8.99)bc and -10.01(-11.44:-9.72)c for 2, 4, 6, 8, and 10 days, respectively. For TMR, median (95%CI; vol%.µm) ∆Z were 1410(1299-1479)a, 2420(2327-2604)ab, 2775(2573-2899)bc, 3305(3192-3406)cd and 4330(3972-4465)d, whereas mean (SD; µm) lesion depth were 53.7(12.3)a, 71.4(12.0)a, 103.8(24.8)b, 130.5(27.2)bc, 167.2(39.3)c for 2, 4, 6, 8 and 10 days, respectively. Conclusion: The progression of WSLs formed on human enamel under microcosm biofilm can be characterized over 2-10 days, both by QLF and TMR analyses, although the latter provides better discrimination among the lesions
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