A Step-up mu-Power Converter for Solar Energy Harvesting Applications, using Hill Climbing Maximum Power Point Tracking

This paper presents a step-up micro-power converter for solar energy harvesting applications. The circuit uses a SC voltage tripler architecture, controlled by an MPPT circuit based on the Hill Climbing algorithm. This circuit was designed in a 0.13 mu m CMOS technology in order to work with an a-Si PV cell. The circuit has a local power supply voltage, created using a scaled down SC voltage tripler, controlled by the same MPPT circuit, to make the circuit robust to load and illumination variations. The SC circuits use a combination of PMOS and NMOS transistors to reduce the occupied area. A charge re-use scheme is used to compensate the large parasitic capacitors associated to the MOS transistors. The simulation results show that the circuit can deliver a power of 1266 mu W to the load using 1712 mu W of power from the PV cell, corresponding to an efficiency as high as 73.91%. The simulations also show that the circuit is capable of starting up with only 19% of the maximum illumination level.

The influence of microstructure on the maximum load and fracture energy of refractory castables

Refractory castables are composed of fractions of fine to fairly coarse particles. The fine fraction is constituted primarily of raw materials and calcium aluminate cement, which becomes hydrated, forming chemical bonds that stiffen the concrete during the curing process. The present study focused on an evaluation of several characteristics of two refractory castables with similar chemical compositions but containing aggregates of different sizes. The features evaluated were the maximum load, the fracture energy, and the ""relative crack-propagation work"" of the two castables heat-treated at 110, 650, 1100 and 1550 degrees C. The results enabled us to draw the following conclusions: the heat treatment temperature exerts a significant influence on the matrix/aggregate interaction, different microstructures form in the castables with temperature, and a relationship was noted between the maximum load and the fracture energy. (C) 2009 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Parameterized fast decoupled power flow methods for obtaining the maximum loading point of power systems. Part I. Mathematical modeling

The conventional Newton and fast decoupled power flow (FDPF) methods have been considered inadequate to obtain the maximum loading point of power systems due to ill-conditioning problems at and near this critical point. It is well known that the PV and Q-theta decoupling assumptions of the fast decoupled power flow formulation no longer hold in the vicinity of the critical point. Moreover, the Jacobian matrix of the Newton method becomes singular at this point. However, the maximum loading point can be efficiently computed through parameterization techniques of continuation methods. In this paper it is shown that by using either theta or V as a parameter, the new fast decoupled power flow versions (XB and BX) become adequate for the computation of the maximum loading point only with a few small modifications. The possible use of reactive power injection in a selected PV bus (Q(PV)) as continuation parameter (mu) for the computation of the maximum loading point is also shown. A trivial secant predictor, the modified zero-order polynomial which uses the current solution and a fixed increment in the parameter (V, theta, or mu) as an estimate for the next solution, is used in predictor step. These new versions are compared to each other with the purpose of pointing out their features, as well as the influence of reactive power and transformer tap limits. The results obtained with the new approach for the IEEE test systems (14, 30, 57 and 118 buses) are presented and discussed in the companion paper. The results show that the characteristics of the conventional method are enhanced and the region of convergence around the singular solution is enlarged. In addition, it is shown that parameters can be switched during the tracing process in order to efficiently determine all the PV curve points with few iterations. (C) 2003 Elsevier B.V. All rights reserved.

Parameterized fast decoupled power flow methods for obtaining the maximum loading point of power systems. Part II. Performance evaluation

The parameterized fast decoupled power flow (PFDPF), versions XB and BX, using either theta or V as a parameter have been proposed by the authors in Part I of this paper. The use of reactive power injection of a selected PVbus (Q(PV)) as the continuation parameter for the computation of the maximum loading point (MLP) was also investigated. In this paper, the proposed versions obtained only with small modifications of the conventional one are used for the computation of the MLP of IEEE test systems (14, 30, 57 and 118 buses). These new versions are compared to each other with the purpose of pointing out their features, as well as the influence of reactive power and transformer tap limits. The results obtained with the new approaches are presented and discussed. The results show that the characteristics of the conventional FDPF method are enhanced and the region of convergence around the singular solution is enlarged. In addition, it is shown that these versions can be switched during the tracing process in order to efficiently determine all the PV curve points with few iterations. A trivial secant predictor, the modified zero-order polynomial, which uses the current solution and a fixed increment in the parameter (V, theta, or mu) as an estimate for the next solution, is used for the predictor step. (C) 2003 Elsevier B.V. All rights reserved.

Comparative analysis of current and voltage-controlled photovoltaic Maximum Power Point tracking

Maximum Power Point tracking (MPPT) in photovoltaic (PV) systems may be achieved by controlling either the voltage or current of the PV device. There is no consensus in the technical literature about how is the best choice. This paper provides a comparative analysis performance among current and voltage control using two different MPPT strategies: the perturb and observe (P&O) and the incremental conductance techniques. © 2011 IEEE.

Main maximum power point tracking strategies intended for photovoltaics

This paper presents evaluations among the most usual MPPT techniques, doing meaningful comparisons with respect to the amount of energy extracted from the photovoltaic panel (PV) (Tracking Factor - TF) in relation to the available power, PV voltage ripple, dynamic response and use of sensors. Using MatLab/Simulink® and DSpace platforms, a digitally controlled boost DC-DC converter was implemented and connected to an Agilent Solar Array E4350B simulator in order to verify the analytical procedures. The main experimental results are presented and a contribution in the implementation of the IC algorithm is performed and called IC based on PI. Moreover, the dynamic response and the tracking factor are also evaluated using a Friendly User Interface, which is capable of online program power curves and compute the TF. Finally, a typical daily insulation is used in order to verify the experimental results for the main PV MPPT methods. © 2011 IEEE.

Six-element circuit for maximum power point tracking in photovoltaic-motor systems with variable-frequency drives

A low-cost circuit was developed for stable and efficient maximum power point (MPP) tracking in autonomous photo voltaic-motor systems with variable-frequency drives (VFDs). The circuit is made of two resistors, two capacitors, and two Zener diodes. Its input is the photovoltaic (PV) array voltage and its output feeds the proportional-integral-derivative (PID) controller usually integrated into, the drive. The steady-state frequency-voltage oscillations induced by the circuit were treated in a simplified mathematical model, which was validated by widely characterizing a PV-powered centrifugal pump. General procedures for circuit and controller tuning were recommended based on model equations. The tracking circuit presented here is widely applicable to PV-motor system with VFDs, offering an. efficient open-access technology of unique simplicity. Copyright (C) 2010 John Wiley & Sons, Ltd.

Variable Step Maximum Power Point Tracking for Photovoltaic Modules with Embedded Temperature Sensing

Thesis (Master's)--University of Washington, 2016-06

Locating the maximum power point for thermoelectric generators for constant heat operation

Thermoelectric generators (TEGs) are solid-state devices that can be used for the direct conversion between heat and electricity. These devices are an attractive option for generating clean energy from heat. There are two modes of operation for TEGs; constant heat and constant temperature. It is a well-known fact that for constant temperature operation, TEGs have a maximum power point lying at half the open circuit voltage of the TEG, for a particular temperature. This work aimed to investigate the position of the maximum power point for Bismuth Telluride TEGs working under constant heat conditions i.e. the heat supply to the TEG is fixed however the temperature across the TEG can vary depending upon its operating conditions. It was found that for constant heat operation, the maximum power point for a TEG is greater than half the open circuit voltage of the TEG.

Mode III interlaminar fracture of carbon/epoxy laminates using the edge crack torsion (ECT) test

The mode III interlaminar fracture of carbon/epoxy laminates was evaluated with the edge crack torsion (ECT) test. Three-dimensional finite element analyses were performed in order to select two specimen geometries and an experimental data reduction scheme. Test results showed considerable non-linearity before the maximum load point and a significant R-curve effect. These features prevented an accurate definition of the initiation point. Nevertheless, analyses of non-linearity zones showed two likely initiation points corresponding to GIIIc values between 850 and 1100 J/m2 for both specimen geometries. Although any of these values is realistic, the range is too broad, thus showing the limitations of the ECT test and the need for further research.

Aplicação do método de Newton desacoplado para o fluxo de carga continuado

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Técnicas de parametrização para o fluxo de carga continuado desenvolvidas a partir da análise das trajetórias de soluções do fluxo de carga

Pós-graduação em Engenharia Elétrica - FEIS

Estudio de la rotura en barras de acero : aspectos experimentales y numéricos

El acero es, junto con el hormigón, el material más ampliamente empleado en la construcción de obra civil y de edificación. Además de su elevada resistencia, su carácter dúctil resulta un aspecto de particular interés desde el punto de vista de la seguridad estructural, ya que permite redistribuir esfuerzos a elementos adyacentes y, por tanto, almacenar una mayor energía antes del colapso final de la estructura. No obstante, a pesar de su extendida utilización, todavía existen aspectos relacionados con su comportamiento en rotura que necesitan una mayor clarificación y que permitirían un mejor aprovechamiento de sus propiedades. Cuando un elemento de acero es ensayado a tracción y alcanza la carga máxima, sufre la aparición de un cuello de estricción que plantea dificultades para conocer el comportamiento del material desde dicho instante hasta la rotura. La norma ISO 6892-1, que define el método a emplear en un ensayo de tracción con materiales metálicos, establece procedimientos para determinar los parámetros relacionados con este tramo último de la curva F − E. No obstante, la definición de dichos parámetros resulta controvertida, ya que éstos presentan una baja reproducibilidad y una baja repetibilidad que resultan difíciles de explicar. En esta Tesis se busca profundizar en el conocimiento del último tramo de la curva F − E de los aceros de construcción. Para ello se ha realizado una amplia campaña experimental sobre dos aceros representativos en el campo de la construcción civil: el alambrón de partida empleado en la fabricación de alambres de pretensado y un acero empleado como refuerzo en hormigón armado. Los dos materiales analizados presentan formas de rotura diferentes: mientras el primero de ellos presenta una superficie de rotura plana con una región oscura claramente apreciable en su interior, el segundo rompe según la clásica superficie en forma de copa y cono. La rotura en forma de copa y cono ha sido ampliamente estudiada en el pasado y existen modelos de rotura que han logrado reproducirla con éxito, en especial el modelo de Gurson- Tvergaard-Needleman (GTN). En cuanto a la rotura exhibida por el primer material, en principio nada impide abordar su reproducción numérica con un modelo GTN, sin embargo, las diferencias observadas entre ambos materiales en los ensayos experimentales permiten pensar en otro criterio de rotura. En la presente Tesis se realiza una amplia campaña experimental con probetas cilíndricas fabricadas con dos aceros representativos de los empleados en construcción con comportamientos en rotura diferentes. Por un lado se analiza el alambrón de partida empleado en la fabricación de alambres de pretensado, cuyo frente de rotura es plano y perpendicular a la dirección de aplicación de la carga con una región oscura en su interior. Por otro lado, se estudian barras de acero empleadas como armadura pasiva tipo B 500 SD, cuyo frente de rotura presenta la clásica superficie en forma de copa y cono. Estos trabajos experimentales han permitido distinguir dos comportamientos en rotura claramente diferenciados entre ambos materiales y, en el caso del primer material, se ha identificado un comportamiento asemejable al exhibido por materiales frágiles. En este trabajo se plantea la hipótesis de que el primer material, cuya rotura provoca un frente de rotura plano y perpendicular a la dirección de aplicación de la carga, rompe de manera cuasifrágil como consecuencia de un proceso de decohesión, de manera que la región oscura que se observa en el centro del frente de rotura se asemeja a una entalla circular perpendicular a la dirección de aplicación de la carga. Para la reproducción numérica de la rotura exhibida por el primer material, se plantea un criterio de rotura basado en un modelo cohesivo que, como aspecto novedoso, se hace depender de la triaxialidad de tensiones, parámetro determinante en el fallo de este tipo de materiales. Este tipo de modelos presenta varias ventajas respecto a los modelos GTN habitualmente empleados. Mientras los modelos GTN precisan de numerosos parámetros para su calibración, los modelos cohesivos precisan fundamentalmente de dos parámetros para definir su curva de ablandamiento: la tensión de decohesión ft y la energía de fractura GF . Además, los parámetros de los modelos GTN no son medibles de manera experimental, mientras que GF sí lo es. En cuanto a ft, aunque no existe un método para su determinación experimental, sí resulta un parámetro más fácilmente interpretable que los empleados por los modelos GTN, que utilizan valores como el porcentaje de huecos presentes en el material para iniciar el fenómeno de coalescencia o el porcentaje de poros que provoca una pérdida total de la capacidad resistente. Para implementar este criterio de rotura se ha desarrollado un elemento de intercara cohesivo dependiente de la triaxialidad de tensiones. Se han reproducido con éxito los ensayos de tracción llevados a cabo en la campaña experimental empleando dicho elemento de intercara. Además, en estos modelos la rotura se produce fenomenológicamente de la misma manera observada en los ensayos experimentales: produciéndose una decohesión circular en torno al eje de la probeta. En definitiva, los trabajos desarrollados en esta Tesis, tanto experimentales como numéricos, contribuyen a clarificar el comportamiento de los aceros de construcción en el último tramo de la curva F − E y los mecanismos desencadenantes de la rotura final del material, aspecto que puede contribuir a un mejor aprovechamiento de las propiedades de estos aceros en el futuro y a mejorar la seguridad de las estructuras construidas con ellos. Steel is, together with concrete, the most widely used material in civil engineering works. Not only its high strength, but also its ductility is of special interest from the point of view of the structural safety, since it enables stress distribution with adjacent elements and, therefore, more energy can be stored before reaching the structural failure. However, despite of being extensively used, there are still some aspects related to its fracture behaviour that need to be clarified and that will allow for a better use of its properties. When a steel item is tested under tension and reaches the maximum load point, necking process begins, which makes difficult to define the material behaviour from that moment onward. The ISO standard 6892-1, which defines the tensile testing method for metallic materials, describes the procedures to obtain some parameters related to this last section of the F − E curve. Nevertheless, these parameters have proved to be controversial, since they have low reproducibility and repeatibility rates that are difficult to explain. This Thesis tries to deepen the knowledge of the last section of the F − E curve for construction steels. An extensive experimental campaign has been carried out with two representative steels used in civil engineering works: a steel rod used for manufacturing prestressing steel wires, before the cold-drawing process is applied, and steel bars used in reinforced concrete structures. Both materials have different fracture surfaces: while the first of them shows a flat fracture surface, perpendicular to the loading direction with a dark region in the centre of it, the second one shows the classical cup-cone fracture surface. The cup-cone fracture surface has been deeply studied in the past and different numerical models have been able to reproduce it with success, with a special mention to the Gurson-Tvergaard-Needleman model (GTN). Regarding the failure surface shown by the first material, in principle it can be numerically reproduced by a GTN model, but the differences observed between both materials in the experimental campaign suggest thinking of a different failure criterium. In the present Thesis, an extensive experimental campaign has been carried out using cylindrical specimens made of two representative construction steels with different fracture behaviours. On one hand, the initial eutectoid steel rod used for manufacturing prestressing steel wires is analysed, which presents a flat fracture surface, perpendicular to the loading direction, and with a dark region in the centre of it. On the other hand, B 500 SD steel bars, typically used in reinforced concrete structures and with the typical cup-cone fracture surface, are studied. These experimental works have allowed distinguishing two clearly different fracture behaviours between both materials and, in the case of the first one, a fragile-like behaviour has been identified. For the first material, which shows a flat fracture surface perpendicular to the loading direction, the following hypothesis is proposed in this study: a quasi-brittle fracture is developed as a consequence of a decohesion process, with the dark region acting as a circular crack perpendicular to the loading direction. To reproduce numerically the fracture behaviour shown by the first material, a failure criterium based on a cohesive model is proposed in this Thesis. As an innovative contribution, this failure criterium depends on the stress triaxiality state of the material, which is a key parameter when studying fracture in this kind of materials. This type of models have some advantages when compared to the widely used GTN models. While GTN models need a high number of parameters to be defined, cohesive models need basically two parameters to define the softening curve: the decohesion stress ft and the fracture energy GF . In addition to this, GTN models parameters cannot be measured experimentally, while GF is indeed. Regarding ft, although no experimental procedure is defined for its obtention, it has an easier interpretation than the parameters used by the GTN models like, for instance, the void volume needed for the coalescence process to start or the void volume that leads to a total loss of the bearing capacity. In order to implement this failure criterium, a triaxiality-dependent cohesive interface element has been developed. The experimental results obtained in the experimental campaign have been successfully reproduced by using this interface element. Furthermore, in these models the failure mechanism is developed in the same way as observed experimentally: with a circular decohesive process taking place around the longitudinal axis of the specimen. In summary, the works developed in this Thesis, both experimental and numerical, contribute to clarify the behaviour of construction steels in the last section of the F − E curve and the mechanisms responsible for the eventual material failure, an aspect that can lead to a better use of the properties of these steels in the future and a safety improvement in the structures built with them.

A DC-DC Step-Up mu-Power Converter for Energy Harvesting Applications, Using Maximum Power PointTracking, Based on Fractional Open Circuit Voltage

A DC-DC step-up micro power converter for solar energy harvesting applications is presented. The circuit is based on a switched-capacitorvoltage tripler architecture with MOSFET capacitors, which results in an, area approximately eight times smaller than using MiM capacitors for the 0.131mu m CMOS technology. In order to compensate for the loss of efficiency, due to the larger parasitic capacitances, a charge reutilization scheme is employed. The circuit is self-clocked, using a phase controller designed specifically to work with an amorphous silicon solar cell, in order to obtain themaximum available power from the cell. This will be done by tracking its maximum power point (MPPT) using the fractional open circuit voltage method. Electrical simulations of the circuit, together with an equivalent electrical model of an amorphous silicon solar cell, show that the circuit can deliver apower of 1132 mu W to the load, corresponding to a maximum efficiency of 66.81%.