535 research outputs found
An Electrical Grid with Discrete Energy Levels
Minimizing both power fluctuations and energy waste in an electrical grid is
a central challenge to energy policy. Any discrepancy between power production
and loads may lead to inefficiencies and instability in the system. Right now,
the electrical grid is an analog system that reacts to power demands. While
sensors and predictive models - based on historical data - have been
implemented to make the grid more immune to instabilities, they have difficulty
addressing fluctuations in real time. The question that we ask is: what happens
if the fluctuating grid delivers discrete energy units to the nodes instead of
continuous power levels? This immediately implies that the grid needs to be
supported by local, short-term energy storage units (STESU) that maintain such
energy system. We assume that the energy delivered and the node's energy
consumption are random with some limits. A model is constructed and analyzed
within the framework of a randomly perturbed Markovian chain. Under reasonable
assumptions, we show that this grid maintains stability over long periods of
time while subjected to randomly fluctuating energy conditions.Comment: 13 pages, 7 figure
Raman spectroscopy of active-carbon electrodes when Au colloids are placed at the electrolyte/electrode interface
In the past, when Au nanoparticles (AuNPs) were placed just above (but not
included in) active carbon (A-C) electrodes, a 10-fold amplification of
gravitational specific capacitance were demonstrated, despite the small
mass-ratio between the AuNPs and A-C; a ratio of 1:5000. We use surface enhance
Raman spectroscopy (SERS) in further studying this system. Supercapacitors are
volumetric elements while SERS is a surface interrogating method, whose signal
could be impacted by many factors, including local colloid's preferential
points (hot spots). Here, we use the ratio between the D- and G-lines of the
A-C electrode, ID/IG, as a marker to eliminate the surface inconsistencies. At
some concentration levels of AuNPs, the A-C Raman signature shows a clear
preference towards the 1600 cm-1 vibration mode (G-line). Following that point,
the cell exhibits a large specific capacitance.Comment: 13 pages; 10 figure
Numerical calculations of the phase diagram of cubic blue phases in cholesteric liquid crystals
We study the static properties of cubic blue phases by numerically minimising
the three-dimensional, Landau-de Gennes free energy for a cholesteric liquid
crystal close to the isotropic-cholesteric phase transition. Thus we are able
to refine the powerful but approximate, semi-analytic frameworks that have been
used previously. We obtain the equilibrium phase diagram and discuss it in
relation to previous results. We find that the value of the chirality above
which blue phases appear is shifted by 20% (towards experimentally more
accessible regions) with respect to previous estimates. We also find that the
region of stability of the O5 structure -- which has not been observed
experimentally -- shrinks, while that of BP I (O8-) increases thus giving the
correct order of appearance of blue phases at small chirality. We also study
the approach to equilibrium starting from the infinite chirality solutions and
we find that in some cases the disclination network has to assemble during the
equilibration. In these situations disclinations are formed via the merging of
isolated aligned defects.Comment: 16 pages, 5 figures. Accepted for publication in Phys. Rev.
Ancient stars beyond the Local Group: RR Lyrae variables and Blue Horizontal Branch stars in Sculptor Group Dwarf Galaxies
We have used Hubble Space Telescope ACS images to generate color-magnitude
diagrams that reach below the magnitude of the horizontal branch in the
Sculptor Group dwarf galaxies ESO294-010 and ESO410-005. In both diagrams blue
horizontal branch stars are unambiguously present, a signature of the existence
of an ancient stellar population whose age is comparable to that of the
Galactic halo globular clusters. The result is reinforced by the discovery of
numerous RR Lyrae variables in both galaxies. The occurrence of these stars in
the first direct confirmation of the existence of ancient stellar populations
beyond the Local Group and indicates that star formation can occur at the
earliest epochs even in low density environments.Comment: 11 pages, 2 figures, accepted for publication in Astrophysical
Journal Letter
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