143 research outputs found
New insights on the empirical predictability of spectral indicators for PV performance
Accurate produced PV energy estimation is critical to business decisions under long-term investments
in PV on a utility scale. PV energy yield is affected by different sites' specific conditions. The variability
of the spectral distribution after temperature and irradiation is a site condition that impacts energy yield
estimates. Evaluating the impact of the spectral irradiance distribution on the PV performance generally
requires accurate information about the PV device's spectral response and the site’s measured spectra.
Detailed spectral and device information may not always be available. This study analyzes the
interrelations between device-dependent and device-independent energetic spectral indicators with
spectral data from nine sites with different climates and latitudes, aiming to relax the requirement for
detailed device and spectral information. First, an apparent correlation of each site's spectral
distributions' yearly Average Photon Energy with the corresponding latitude is observed. As the
commonly applied device-dependent spectral indicator, it can be observed that the monthly mismatch
factors of all nine sites exhibit a global linear relationship with the monthly average photon energies.
This linear relationship with measured spectral data provides a predictive character for each PV device
technology by allowing the estimation of the annual spectral impact from the annual Average Photon
Energy, potentially for any site. This work also analyzes the validity of the Spectral Average Useful
Fraction and the Spectral Enhancement Factor as alternative device-dependent spectral indicators.
These require average spectra and, thus, would reduce the calculation complexity for spectral indicators.
Finally, the proposed method was validated qualitatively using synthetic spectral data from the National
Solar Radiation Database. The trends of the scatter plot between the synthetic Spectral Mismatch Factor
and the Average Photon Energy that follow the experimental linear regression give an idea of the
proposed method's functionality, despite the synthetic data's uncertainties.La estimación precisa de la energía fotovoltaica producida es fundamental para las decisiones
comerciales en el marco de inversiones a largo plazo en proyectos fotovoltaicos. El rendimiento
de la energía fotovoltaica se ve afectado por las condiciones locales epecíficas. La variabilidad
de la distribución espectral después de la temperatura y la irradiación es una condición del sitio
que afecta las estimaciones de rendimiento energético. Uno de los desafíos al evaluar el
impacto de la variabilidad espectral es reducir la complejidad del cálculo. Esto implica realizar
una estimación precisa y rápida del impacto espectral con la mínima información requerida a
priori. Con este fin, la presente tesis busca analizar las interrelaciones entre los indicadores
espectrales energéticos dependientes e independientes del dispositivo fotovoltaico con datos
espectrales de varios climas y latitudes en todo el mundo. Debido al enfoque reduccionista que
proveen los indicadores espectrales, analizamos la dependencia de la distribución espectral
representada por la energía fotónica promedio con la latitud en diferentes escalas de tiempo
mensuales y anuales. Al analizar los indicadores espectrales dependientes del dispositivo, se
destaca el Spectral mismatch factor, que exhibe una relación lineal global con el la energía
fotónica promedio en una escala mensual. El análisis exhaustivo de esta relación con los datos
espectrales medidos también proporciona un carácter predictivo al permitir el cálculo del
impacto espectral anual a partir de la energía fotónica promedio anual, y por lo tanto tal relación
lineal propone ser un modelo empírico para el cálculo directo y sencillo del impacto espectral
anual. Adicionalmente, analizamos la validez de manera global de dos indicadores espectrales
dependientes del dispositivo, el Spectral Enhancement Factor y el Spectral Average Useful
Fraction para los sitios seleccionados. Con ello se busca ofrecer un catálogo multiclimático
integrado de las interrelaciones de indicadores espectrales en escala de tiempo anual y mensual.
Finalmente, se realizó una validación cualitativa del método propuesto utilizando datos
espectrales sintéticos de la National Solar Radiation Database. Las tendencias de los Spectral
mismatch factor y Average Photon Energy anuales basados en data espectral sintética siguen a
la regresión lineal experimental y por ende, dan una idea de la funcionalidad del método
propuesto, pese a las incertidumbres propias de la data espectral sintética
Synchronization of multihop wireless sensor networks at the application layer
Time synchronization is a key issue in wireless
sensor networks; timestamping collected
data, tasks scheduling, and efficient communications
are just some applications. From all the
existing techniques to achieve synchronization,
those based on precisely time-stamping sync
messages are the most accurate. However, working
with standard protocols such as Bluetooth or
ZigBee usually prevents the user from accessing
lower layers and consequently reduces accuracy.
A receiver-to-receiver schema improves timestamping
performance because it eliminates the
largest non-deterministic error at the sender’s
side: the medium access time. Nevertheless, utilization
of existing methods in multihop networks
is not feasible since the amount of extra
traffic required is excessive. In this article, we
present a method that allows accurate synchronization
of large multihop networks, working at
the application layer while keeping the message
exchange to a minimum. Through an extensive
experimental study, we evaluate the protocol’s
performance and discuss the factors that influence
synchronization accuracy the most.Ministerio de Ciencia y Tecnología TIN2006-15617-C0
Comparison and evaluation of measured and simulated high-frequency capacitance-voltage curves of MOS structures for different interface passivation parameters
Semiconductor-insulator interfaces play an important role in the performance of
many different electronic and optoelectronic devices such as transistors, LEDs, lasers
and solar cells. Particularly, the recombination of photo-generated charge carriers
at interfaces in crystalline silicon solar cells causes a dramatic efficiency reduction.
Therefore, during the fabrication process, the crystalline silicon must be subjected
to prior superficial passivation; typically through an insulating layer such as SiO2,
SiNx or AlOx. The function of this passivating layer is to reduce electrical recombination
losses in interfacial defect states originating from dangling bonds. The
associated passivation parameters are, on the one hand, stable charges within the
insulating layer (Qox) that by repelling a certain type of charge carrier from the
crystalline silicon surface, reduces its recombination effectiveness (Field Effect Passivation).
On the other hand, the density of surface defect states or the interface
trap density (Dit), which is reduced by the passivation layer (Chemical Passivation).
These passivation parameters (Qox and Dit) turn out to be relevant when evaluating
the effectiveness of a new material with passivating properties, as well as relevant for
different theoretical models that allow simulations of the spectral response and/or
efficiency in solar cells under different passivation conditions. One of the techniques
widely used for studying the interfacial passivation properties of semiconductor electronic
devices is the extraction of these interfacial passivation parameters through
of capacitance-voltage (C-V) measurements on metal-oxide-semiconductor (MOS)
or metal-insulator-semiconductor (MIS) systems.
In the present work, a simulation tool for High-Frequency C-V curves based on simulated
Qox and the Dit was developed using Python. As a first step, the simulation
was developed for an ideal MOS system, i.e. for Qox = 0 and Dit = 0. A verification
of the resulting, simulated band-bending was reached through a band diagram
simulator (The Multi-Dielectric Band-Diagram program). As a second step, the program was subjected to an evaluation and validation through
experimental data. This data comprises measurements of C-V and their respective
extracted parameters for a sample of silicon dioxide thermally grown on crystalline
silicon wafer (SiO2/c-Si). Using three different models for the Dit distribution within
the band gap energy: Gaussian model, U-shape model, and a constant value, approximations
of the corresponding experimental C-V curve were obtained. It was
evident that the C-V curve simulated from the Dit based on the model with Gaussian
distributions for the defect centers and exponentials for the band tails resulted
in the best approximation of the experimental C-V curve. It should be noted that
the other two models were adjusted based on the value of the Dit near to midgap
energy, where the recombination probability and rate are the highest. In this way,
the constant model of the Dit at the midgap presented the largest deviation in the
simulated C-V curve among the used models.
An implicit fitting method of the Dit through the experimental C-V curve fitting
is proposed. For this, the U-shape model is used because it only depends on three
parameters. The average values of the fitted and the experimentally extracted Dit
are compared.
The parameter D0
it, which defines the value at midgap in the U-shape model could
be interpreted as an average estimation of the Dit energetic range values around the
midgap where recombinations are most significant. Therefore, this parameter could
determine a representative value of the Dit.
Finally, the developed program allows an in-depth analysis of the passivation parameters
from which the surface passivation is evaluated.La interfaz entre un semiconductor y un aislante juega un papel importante en el
desempeño de diferentes dispositivos electrónicos y optoelectrónicos, tales como transistores,
LEDs, láseres y celdas solares. Una de las técnicas ampliamente empleadas
en el estudio de las propiedades interfaciales de dispositivos electrónicos semiconductores
es la extracción de parámetros interfaciales por medio del modelo de un sistema
Metal-óxido o aislante-semiconductor (MOS o MIS) sobre medidas de Capacitancia
en función del voltaje (C-V). Uno de estos dispositivos, en el cual se encuentra una
fuerte aplicabilidad debido al interés de investigación son las celdas solares de alta
eficiencia basadas en silicio cristalino, las cuales, en la mayoría de los casos deben ser
sometidas a una previa pasivacion superficial del material absorbente (comunmente
silicio cristalino), por medio de una capa pasivadora, aislante (como el SiO2, SiNx o
AlOx). La función de esta capa pasivadora es la de reducir las pérdidas eléctricas
por recombinación en defectos interfaciales. Los parámetros asociados son por un
lado, cargas estables dentro la capa aislante (Qox) que al repeler un cierto tipo de
la superficie del silicio cristalino, reduce su efectividad de recombinación (pasivación por efecto de campo) y por lo lado, la reducción de la densidad de estados superficiales
Dit del semiconductor (pasivación química). Estos parámetros de pasivación
(Qox y Dit) resultan ser relevantes al momento de evaluar la efectividad de un nuevo
material con propiedades pasivadoras, así como también son relevantes para los distintos
modelos teóricos que permiten hacer simulaciones de la respuesta espectral
y/o de la eficiencia en celdas solares bajo distintas condiciones. Es conocido que
como primera aproximación la representación de una Dit por medio de un único
estado resulta ser un buen punto de partida para estos modelos así como también
una forma práctica de comparación de la pasivación química para distintas capas
pasivadoras.
En el presente trabajo se desarrolló, mediante el lenguaje Python, una herramienta
de simulación de curvas C-V medidas a alta frecuencia en base a Qox y Dit simulados. Inicialmente la simulación es desarrollada para un sistema MOS ideal para diferentes
conjuntos de ecuaciones, una solución exacta y otra aproximada del modelo usado,
encontrándose una buena estimación de ambas curvas simuladas. En un primer
instante un parámetro principal (band-bending potential) del programa, a partir del
cual se construyen las curvas C-V, es validado con un simulador de diagrama de
bandas (Multi-Dielectric Band-Diagram) obteniéndose un buen ajuste para el bandbending
potential.
Como segunda medida el programa fue sometido a una evaluación y validación
por medio de datos experimentales. Estos datos comprenden medidas de C-V y
sus respectivos parámetros extraídos para una muestra de óxido de silicio crecido
térmicamente sobre obleas de silicio cristalino (SiO2/c-Si). Usando tres diferentes
modelos, modelo gausiano, modelo U-shape y de valor constante, para simular la
Dit. A partir de estos modelos se obtuvieron aproximaciones de la curva C-V experimental.
Además comparando los distintos modelos se evidenció que el modelo
gausiano es el más aproximado. Cabe señalar que los otros dos modelos se ajustaron
en base al valor de la Dit en la mitad del ancho de banda (valor energético dónde
más efectiva es la recombinación) el cual es conocido como midgap. De este modo, el
modelo constante de Dit en el midgap presenta el mayor error entre los tres modelos
usados.
Un método de ajuste implícito de la Dit a través del ajuste de la curva C-V experimental
es planteado. Para ello el model U-shape es usado debido a que solo depende
de tres parámetros. Los valores promedios de la Dit ajustada y experimentalmente
extraída son comparados, obteniéndose una aproximación hasta la segunda cifra
significativa.
Se da una supuesta interpretación de uno de los parámetros asociados a este modelo
U-shape, el valor constante que define el midgap y sus alrededores, como el promedio
de los valores centrales de la Dit experimental, cuya recombinación es significativa
respecto al valor en el midgap. Por lo tanto este parámetro hallado podría determinar
una Dit representativa a la hora de comparar diferentes curvas de Dit.
Finalmente el programa desarrollado podría permitir un análisis profundo de los
parámetros de pasivación a partir de los cuales la pasivación superficial es evaluada.Tesi
Processing, microstructure and mechanical characterization of dispersion strengthened Cu-1%Y
Dispersion strengthened Cu-1%Y (wt%) has been produced by mechanical alloying and subsequent consolidation by hot isostatic pressing (HIP). Samples of this alloy have been submitted to an equal channel angular pressing (ECAP) process and the effects on the microstructure and mechanical properties analyzed. The characteristics of the microstructure, such as the size distributions of both, the grains and Y-rich particles dispersed in the Cu matrix, have been studied by high resolution electron scanning microscopy and electron backscatter diffraction. The as-HIP alloy exhibits a quasi-bimodal distribution with an average diameter of 17 ± 14 μm. The ECAP treatment refines the average grain size to 1.3 ± 0.9 μm besides changing the size distribution of the Y-rich particles, which shifted from average size from 94 ± 9 nm to 55 ± 8 nm after ECAP. The mechanical characteristics have been investigated by means of microhardness measurements, and stress-strain tests in the temperature range 293 ― 573 K. The ECAP deformation resulted in an increase of the mechanical strength and a decrease in ductility. It is found that the Voce law can satisfactorily describe the plastic and hardening rate behavior of these alloys. The strain hardening rate plots as a function of flow stress for the samples tested at 293 ≤ T ≤ 773 K exhibited a two-stage behavior, comprising a transient stage at low stresses followed by the characteristic linear dependence for the stage III of hardening in f.c.c. metals
Microstructural and mechanical characterization of Cu-0.8wt%Y
Proceeding of the 28th Symposium On Fusion Technology (SOFT-28), September 29th - October 3rd, 2014, San Sebastián, Spain.Dispersion strengthened Cu-0.8 wt.%Y has been produced by a powder metallurgy route and subsequent consolidation by hot isostatic pressing at 1123 K and 172 MPa. A fully dense alloy has been obtained that exhibits a microstructure characterized by equiaxed grains with sizes ranging from 0.5 to 50 μm. Yttrium-rich particles with an average size of 0.92 μm have been observed inside the grains and decorating the grain boundaries. As expected, the tensile tests carried out from room temperature to 773 K have revealed that both the YS and the UTS decrease with increasing temperature. This alloy exhibits better tensile properties and microhardness than OFHC Cu. This improvement is attributed to the presence of the Y-rich particles.This research has been supported by Ministerio de Economía y
Competitividad of Spain (ENE2012-39787-C06-05). We also thank
the Comunidad de Madrid for the financial support, through the
program TECHNOFUSION (S2009/ENE-1679). Additional subvention from EURATOM/CIEMAT association through contract EFDA
(WP12-MAT-HHFMAM-02) has also been received
High-heat flux Cu-0.8Y alloys investigated by positron annihilation spectroscopy
This work studies the thermal stability of the microstructure and the evolution of the defects of two high-heat flux Cu-0.8 wt%Y alloys fabricated following two alternative powder metallurgy routes. One batch was produced by direct hot isostatic pressing (HIP) consolidation of Cu-0.8 wt%Y pre-alloyed atomized powders while an additional ball milling processing step was introduced before HIP sintering for the second alloy. The stability and recovery characteristics of the vacancy type defects in these alloys in the as-produced state and after processing by severe equal channel angular pressing to achieve a refine microstructure have been investigated by positron lifetime and coincidence Doppler broadening measurements in samples subjected to isochronal annealing from room temperature to 900 °C. Microhardness measurements and electron transmission microscopy analysis have also been performed to support the results obtained from the positron annihilation spectroscopy analysis techniques. The recovery curves of the positron lifetime and S-W plots show a recovery stage in agreement with the recovery stage V for Cu. However, a full recovery is not accomplished, and a stage that reverts the previous recovery takes place after annealing above ~600 °C, that leads to the formation of very stable defects at temperatures up to 900 °C, identified as vacancy aggregates and nanocavities. The characteristic shape of the coincidence Doppler broadening indicates that the dispersed Y-O particles in the Cu matrix appear to be responsible for stabilizing the vacancy aggregates and nanocavities for temperatures above 600-700 °C.This research was supported by the Spanish Ministerio de Economia y Competitividad (MINECO) in the form of Project ENE2015-70300-C3-2-R and by the Regional Government of Madrid (Spain) through TECHNOFUSIÓN(III)CM (S2018/EMT-4437), and Comunidad de Madrid (Spain) - multiannual agreement with UC3M ("Excelencia para el Profesorado Universitario" EPUC3M14) - Fifth regional research plan 2016-2020; and also Spanish Ministerio de Ciencia e Innovación through project DAMAINSOL [grant number RTI2018-101020-B-I00]
Methodology based on FBG sensors for monitoring of FRP-strengthened beams
Advanced composite materials are increasingly used in the strengthening of reinforced concrete (RC) structures. The use of externally bonded strips made of fibre-reinforced plastics (FRP) as strengthening method has gained widespread acceptance in recent years since it has many advantages over the traditional techniques. However, unfortunately, this strengthening method is often associated with a brittle and sudden failure caused by some form of FRP bond failure, originated at the termination of the FRP material or at intermediate areas in the vicinity of flexural cracks in the RC beam. Up to date, little effort in the early prediction of the debonding in its initial instants even though this effect is not noticeable by simple visual observation. An early detection of this phenomenon might help in taking actions to prevent future catastrophes. Fibre-optic Bragg grating (FBG) sensors are able to measure strains locally with high resolution and accuracy. Furthermore, as their physical size is extremely small compared with other strain measuring components, it enables to be embedded at the concrete-FRP interface for determining the strain distribution without influencing the mechanical properties of the host materials. This paper shows the development of a debonding identification methodology based on strains experimentally measured. For, it a simplified model is implemented to simulate the behaviour of FRP-strengthened reinforced concrete beams. This model is taken as a basis to. develop an model updating procedure able to detect minor debonding at the concrete-FRP interface from experimental strains obtained by using FBG sensors embedded at the interfac
Identification of intermediate debonding damage in FRP-plated RC beams based on multi-objective particle swarm optimization without updated baseline model
Fiber reinforced polymer composites (FRP) have found widespread usage in the repair and strengthening of concrete structures. FRP composites exhibit high strength-to-weight ratio, corrosion resistance, and are convenient to use in repair applications. Externally bonded FRP flexural strengthening of concrete beams is the most extended application of this technique. A common cause of failure in such members is associated with intermediate crack-induced debonding (IC debonding) of the FRP substrate from the concrete in an abrupt manner. Continuous monitoring of the concrete?FRP interface is essential to pre- vent IC debonding. Objective condition assessment and performance evaluation are challenging activities since they require some type of monitoring to track the response over a period of time. In this paper, a multi-objective model updating method integrated in the context of structural health monitoring is demonstrated as promising technology for the safety and reliability of this kind of strengthening technique. The proposed method, solved by a multi-objective extension of the particle swarm optimization method, is based on strain measurements under controlled loading. The use of permanently installed fiber Bragg grating (FBG) sensors embedded into the FRP-concrete interface or bonded onto the FRP strip together with the proposed methodology results in an automated method able to operate in an unsupervised mode
Thermal conductivity and diffusivity of Cu-Y alloys produced by different powder metallurgy routes
Full density Cu-1%Y and Cu-0.8%Y alloys have been produced by different powder metallurgy routes and subsequent hot isostatic pressing. Some of the alloys have been subjected to equal channel angular pressing (ECAP) via BC route up to 8 passes. ECAP deformation homogenizes and refines the microstructure up to attaining a sub-micron grain structure. Thermal properties have been characterized by the laser flash method in the temperature range 373–773 K. The ECAP process, irrespective of the production route, enhanced the thermal conductivity to values similar to those for CuCrZr (ITER grade). The linear thermal expansion coefficient was temperature independent for all materials
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