147 research outputs found
From Megawatts to Kilowatts: A Review of Small Wind Turbine Applications, Lessons From The US to Brazil
Increased use of fossil fuels has contributed to global warming due to greenhouse gas emissions, which has led countries to implement policies that favor the gradual replacement of their use with renewable energy sources. Wind expansion in Brazil is a success story, but its adherence to distributed generation is still a big challenge. In this context, the authors of this paper argue that the development of robust and viable distributed power grids will also depend in the future on improving small wind generation as an important alternative to the diversity of decentralized power grids. In this study, the authors present an overview of the small-sized Aeolic (or wind) energy market in Brazil, with the objective to support the debate regarding its expansion. Promoting the small wind market in Brazil is still a big challenge, but lessons can be learned from the United States. In this context, the article uses the United States learning curve, analyzing barriers that were found, as well as public policies implemented to overcome them. The lessons learned in the American market may guide public policies aimed at fostering this technology in Brazil. If technological improvements, certification and introduction of financial incentives were implemented in Brazil, the small wind industry chain could grow substantially, building a trajectory to promote the low carbon economy
Delayed Self-Synchronization in Homoclinic Chaos
The chaotic spike train of a homoclinic dynamical system is self-synchronized
by re-inserting a small fraction of the delayed output. Due to the sensitive
nature of the homoclinic chaos to external perturbations, stabilization of very
long periodic orbits is possible. On these orbits, the dynamics appears chaotic
over a finite time, but then it repeats with a recurrence time that is slightly
longer than the delay time. The effect, called delayed self-synchronization
(DSS), displays analogies with neurodynamic events which occur in the build-up
of long term memories.Comment: Submitted to Phys. Rev. Lett., 13 pages, 7 figure
Localized f electrons in CexLa1-xRhIn5: dHvA Measurements
Measurements of the de Haas-van Alphen effect in CexLa1-xRhIn5 reveal that
the Ce 4f electrons remain localized for all x, with the mass enhancement and
progressive loss of one spin from the de Haas-van Alphen signal resulting from
spin fluctuation effects. This behavior may be typical of antiferromagnetic
heavy fermion compounds, inspite of the fact that the 4f electron localization
in CeRhIn5 is driven, in part, by a spin-density wave instability.Comment: 4 pages, 4 figures, submitted to PR
Force and Motion Generation of Molecular Motors: A Generic Description
We review the properties of biological motor proteins which move along linear
filaments that are polar and periodic. The physics of the operation of such
motors can be described by simple stochastic models which are coupled to a
chemical reaction. We analyze the essential features of force and motion
generation and discuss the general properties of single motors in the framework
of two-state models. Systems which contain large numbers of motors such as
muscles and flagella motivate the study of many interacting motors within the
framework of simple models. In this case, collective effects can lead to new
types of behaviors such as dynamic instabilities of the steady states and
oscillatory motion.Comment: 29 pages, 9 figure
Upper critical field for underdoped high-T_c superconductors. Pseudogap and stripe--phase
We investigate the upper critical field in a stripe--phase and in the
presence of a phenomenological pseudogap. Our results indicate that the
formation of stripes affects the Landau orbits and results in an enhancement of
. On the other hand, phenomenologically introduced pseudogap leads to a
reduction of the upper critical field. This effect is of particular importance
when the magnitude of the gap is of the order of the superconducting transition
temperature. We have found that a suppression of the upper critical field takes
place also for the gap that originates from the charge--density waves.Comment: 7 pages, 5 figure
Topological doping and the stability of stripe phases
We analyze the properties of a general Ginzburg-Landau free energy with
competing order parameters, long-range interactions, and global constraints
(e.g., a fixed value of a total ``charge'') to address the physics of stripe
phases in underdoped high-Tc and related materials. For a local free energy
limited to quadratic terms of the gradient expansion, only uniform or
phase-separated configurations are thermodynamically stable. ``Stripe'' or
other non-uniform phases can be stabilized by long-range forces, but can only
have non-topological (in-phase) domain walls where the components of the
antiferromagnetic order parameter never change sign, and the periods of charge
and spin density waves coincide. The antiphase domain walls observed
experimentally require physics on an intermediate lengthscale, and they are
absent from a model that involves only long-distance physics. Dense stripe
phases can be stable even in the absence of long-range forces, but domain walls
always attract at large distances, i.e., there is a ubiquitous tendency to
phase separation at small doping. The implications for the phase diagram of
underdoped cuprates are discussed.Comment: 18 two-column pages, 2 figures, revtex+eps
Dispersion of Ordered Stripe Phases in the Cuprates
A phase separation model is presented for the stripe phase of the cuprates,
which allows the doping dependence of the photoemission spectra to be
calculated. The idealized limit of a well-ordered array of magnetic and charged
stripes is analyzed, including effects of long-range Coulomb repulsion.
Remarkably, down to the limit of two-cell wide stripes, the dispersion can be
interpreted as essentially a superposition of the two end-phase dispersions,
with superposed minigaps associated with the lattice periodicity. The largest
minigap falls near the Fermi level; it can be enhanced by proximity to a (bulk)
Van Hove singularity. The calculated spectra are dominated by two features --
this charge stripe minigap plus the magnetic stripe Hubbard gap. There is a
strong correlation between these two features and the experimental
photoemission results of a two-peak dispersion in LaSrCuO, and
the peak-dip-hump spectra in BiSrCaCuO. The
differences are suggestive of the role of increasing stripe fluctuations. The
1/8 anomaly is associated with a quantum critical point, here expressed as a
percolation-like crossover. A model is proposed for the limiting minority
magnetic phase as an isolated two-leg ladder.Comment: 24 pages, 26 PS figure
Formation of Large-Scale Semi-Organized Structures in Turbulent Convection
A new mean-field theory of turbulent convection is developed. This theory
predicts the convective wind instability in a shear-free turbulent convection
which causes formation of large-scale semi-organized fluid motions in the form
of cells or rolls. Spatial characteristics of these motions, such as the
minimum size of the growing perturbations and the size of perturbations with
the maximum growth rate, are determined. This study predicts also the existence
of the convective shear instability in a sheared turbulent convection which
results in generation of convective shear waves with a nonzero hydrodynamic
helicity. Increase of shear promotes excitation of the convective shear
instability. Applications of the obtained results to the atmospheric turbulent
convection and the laboratory experiments on turbulent convection are
discussed. This theory can be applied also for the describing a mesogranular
turbulent convection in astrophysics.Comment: 16 pages, 10 figures, REVTEX4, PHYSICAL REVIEW E, v. 67, in press
(2003
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