Minimizing Circulating Current in Parallel-Connected Photovoltaic Inverters

Parallel-connected photovoltaic inverters are required in large solar plants where it is not economically or technically reasonable to use a single inverter. Currently, parallel inverters require individual isolating transformers to cut the path for the circulating current. In this doctoral dissertation, the problem is approached by attempting to minimize the generated circulating current. The circulating current is a function of the generated common-mode voltages of the parallel inverters and can be minimized by synchronizing the inverters. The synchronization has previously been achieved by a communication link. However, in photovoltaic systems the inverters may be located far apart from each other. Thus, a control free of communication is desired. It is shown in this doctoral dissertation that the circulating current can also be obtained by a common-mode voltage measurement. A control method based on a short-time switching frequency transition is developed and tested with an actual photovoltaic environment of two parallel inverters connected to two 5 kW solar arrays. Controls based on the measurement of the circulating current and the common-mode voltage are generated and tested. A communication-free method of controlling the circulating current between parallelconnected inverters is developed and verified.

Design of a Traction Motor With Tooth-Coil Windings and Embedded Magnets

Traction motor design significantly differs from industrial machine design. The starting point is the load cycle instead of the steady-state rated operation point. The speed of the motor varies from zero to very high speeds. At low speeds, heavy overloading is used for starting, and the field-weakening region also plays an important role. Finding a suitable fieldweakening point is one of the important design targets. At the lowest speeds, a high torque output is desired, and all current reserves of the supplying converter unit are used to achieve the torque. In this paper, a 110-kW 2.5-p.u. starting torque and a maximum 2.5-p.u. speed permanent-magnet traction motor will be studied. The field-weakening point is altered by varying the number of winding turns of machine. One design is selected for prototyping. Theoretical results are verified by measurements.

Large scale solar power plant flat roof and carport installation in Finland

This work focuses on the 159.5 kW solar photovoltaic power plant project installed at the Lappeenranta University of Technology in 2013 as an example of what a solar plant project could be in Finland. The project consists of a two row carport and a flat roof installation on the roof of the university laboratories. The purpose of this project is not only its obvious energy savings potential but also to serve as research and teaching laboratory tool. By 2013, there were not many large scale solar power plants in Finland. For this reason, the installation and data experience from the solar power plant at LUT has brought valuable information for similar projects in northern countries. This work includes a first part for the design and acquisition of the project to continue explaining about the components and their installation. At the end, energy produced by this solar power plant is studied and calculated to find out some relevant economical results. For this, the radiation arriving to southern Finland, the losses of the system in cold weather and the impact of snow among other aspects are taken into account.

De adyto Melchisedeci

Invokaatio: I.N.J.

Deshidratación osmótica de la papaya chilena (Vasconcellea pubescens) e influencia de la temperatura y concentración de la solución sobre la cinética de transferencia de materia

El objetivo de este trabajo fue estudiar la cinética de deshidratación osmótica de la papaya chilena (Vasconcellea pubescens), utilizando dos variables experimentales: la temperatura (30, 40, 50 °C) y concentración (40, 50 y 60%) de la solución osmótica. Se determinó la variación de masa total, masa de agua y masa de sólidos solubles y se modeló con las ecuaciones difusionales, además en esta investigación se consideró y utilizó una modificación de la ecuación propuesta por Biswal-Bozorgmehr. Estos modelos fueron evaluados por medio del coeficiente de regresión lineal, suma de errores cuadrados, raíz media de los errores cuadrados y Chi-cuadrado. Al comparar los valores experimentales con los calculados, se demostró que el modelo Biswal-Bozorgmehr modificado obtuvo mejor ajuste sobre la variación de masa de agua y sólidos solubles. Mediante la ecuación de Arrhenius se analizó el efecto de la temperatura sobre los parámetros cinéticos (Kw y Kss) y difusividad efectiva de agua (Dwe) y sólidos solubles (Dsse). De acuerdo al diseño factorial, se observó que la temperatura no influyó sobre la difusividad efectiva de agua y de sólidos, pero sí la concentración de la solución osmótica. La mejor condición de salida de agua y ganancia de sólidos solubles durante la deshidratación osmótica de la papaya chilena fue a 30 °C con una concentración del 60%.

Energy efficiency and moisture diffusivity of apple slices during convective drying

Abstract The present study aimed at investigating the influences of drying air temperature and flow rate on energy parameters and dehydration behaviour of apple slices. For this purpose, apple slices were dried in a convective dryer at air temperatures of 50, 60 and 70 °C, and air velocities of 1, 1.5 and 2 m s–1. Dehydration rate increased as the air temperature and flow rate increased from 50 to 70 °C and 1 to 2 m s–1, respectively. The effective moisture diffusivity was determined to be in the range of 6.75×10–10-1.28×10–9 m2 s–1. Results of data analysis showed that the maximum energy consumption (23.94 kW h) belonged to 50 °C and 2 m s–1 and the minimum (13.89 kW h) belonged to 70 °C and 1 m s–1 treatment. Energy efficiency values were in the range of 2.87-9.11%. Moreover, the results indicated that any increment in the air temperature increases thermal and drying efficiencies while any increment in the air flow rate decreases both of them.

Mikro-ORC-energianmuuntimen suorituskyvyn todentaminen ja kaupallistaminen

Tässä työssä esitellään yleisesti ORC-prosessi, sen toimintaperiaate ja käyttökohteet. Työn tavoitteena oli todentaa diesel-moottorin savukaasujen lämpöenergian sähköenergiaksi muuntavan mikro-ORC-energianmuuntimen suorituskyky. Suorituskyky pyrittiin toteamaan laskemalla laboratoriomittauksista saadusta datasta koelaitoksen sähköntuotannon hyötysuhde ja vertaamalla sitä mallinnuksessa laskettuun hyötysuhteeseen. Esitys käytännöstä suorituskyvyn todentamiseen kuuluu työn sisältöön. Koelaitoksen suorituskykyä ei pystytty toteamaan turbogeneraattoriin liittyvien ongelmien vuoksi. Tarkasteltavaksi tähän työhön jäi koelaitoksen suorituskykyyn olennaisesti liittyvien laitoskomponenttien toiminta niille tyypillisten mittausdatasta laskettujen tunnuslukujen kautta. Koelaitoksella käytettyjen lämmönsiirrinten todettiin olevan kykeneviä siirtämään tarpeeksi lämpöenergiaa 130 kW jarruteholla toimivan diesel-moottorin savukaasujen lämmöstä sähköenergian tuotantoon. Laitoksen kaupallistamista tarkasteltiin asiakkaan ja valmistajan näkökulmasta. Tarkasteluun sisältyi katsaus kaupalliseen versioon kuuluvista ominaisuuksista, alihankinnasta ja säädöksistä, jotka laitoksen on täytettävä markkinoille päästäkseen.

Maalämpöpumpun ja aurinkosähköjärjestelmän yhteiskäyttö pientalon lämpimän käyttöveden tuotannossa

Tässä kandidaatintyössä tutkitaan maalämpöpumpun ja aurinkosähköjärjestelmän yhteis-käyttöä pientalon lämpimän käyttöveden tuotannossa. Tarkoituksena on tuottaa päivittäin tarvittava lämmin käyttövesi maalämpöpumpulla, jonka tarvitsema sähkö tuotetaan aurin-kosähköjärjestelmän avulla keskimäärin kello 12–15. Aurinkosähköjärjestelmän tuotto simuloidaan kuvitteelliselle Lappeenrannassa sijaitsevalle omakotitalolle Homer-ohjelmistolla. Maalämpöpumpun koko pidetään vakiona ja vertai-lussa on 4, 5 ja 6 kW:n kokoiset aurinkosähköjärjestelmät. Yli 5 kW:n aurinkosähköjärjestelmällä saadaan katettua talon peruskuorman lisäksi, myös maalämpöpumpun tarvitsema teho kyseisenä ajanjaksona. 4 kW:n aurinkosähköjärjestel-mällä ja maalämpöpumpulla saadaan tuotettua päivässä riittävästi energiaa neljän henkilön tarvitsemaan käyttöveteen, mutta tällöin maalämpöpumppua täytyy käyttää pidempi ajan-jakso, jos lämmitykseen halutaan käyttää vain tuotettua aurinkosähköä.

A high efficiency microturbine concept

There is a growing trend towards decentralized electricity and heat production throughout the world. Reciprocating engines and gas turbines have an essential role in the global decentralized energy markets and any improvement in their electrical efficiency has a significant impact from the environmental and economic viewpoints. This paper introduces an inter-cooled and recuperated two-shaft microturbine at 500 kW electric output range. The microturbine is optimized for a realistic combination of the turbine inlet temperature, the recuperation rate and the pressure ratio. The new microturbine design aims to achieve significantly increased performance within the range of microturbines and even competing with the efficiencies achieved in large industrial gas turbines. The simulated electrical efficiency is 45%. Improving the efficiency of combined heat and power (CHP) systems will significantly decrease the emissions and operating costs of decentralized heat and electricity production. Cost-effective, compact and environmentally friendly micro-and small-scale CHP turbine systems with high electrical efficiency will have an opportunity to successfully compete against reciprocating engines, which today are used in heat and power generation all over the world and manufactured in large production series. This paper presents a small-scale gas turbine process, capable of competing with reciprocating engine in terms of electrical efficiency.

Evaluation of the Efficiency of Line-Start Permanent-Magnet Machines as a Function of the Operating Temperature

The standard squirrel-cage induction machine has nearly reached its maximum efficiency. In order to further increase the energy efficiency of electrical machines, the use of permanent magnets in combination with the robust design and the line start capability of the induction machine is extensively investigated. Many experimental designs have been suggested in literature, but recently, these line-start permanent-magnet machines (LSPMMs) have become off-the-shelf products available in a power range up to 7.5 kW. The permanent magnet flux density is a function of the operating temperature. Consequently, the temperature will affect almost every electrical quantity of the machine, including current, torque, and efficiency. In this paper, the efficiency of an off-the-shelf 4-kW three-phase LSPMM is evaluated as a function of the temperature by both finite-element modeling and by practical measurements. In order to obtain stator, rotor, and permanent magnet temperatures, lumped thermal modeling is used.

Le Petit journal illustré

1933/09/03 (Numéro 2228).

L'Écho de Paris

1890/06/24 (Numéro 2228).

Programa de Responsabilidad Social Empresarial

Servicios registrales

What motivates registered nurses to participate in continuing education activities?

The study was undertaken to identify what motivates registered nurses to participate in continuing education activities. The primary questions were whether basic nursing education, employment status, clinical area, and position, as well as readiness for selfdirected learning influenced Canadian nurses' motivational orientations when deciding to participate in continuing education activities. Other individual differences (e.g., age) were also examined. The sample included 142 registered nurses employed at an urban community hospital. Three instruments were used for data collection: the Education Participation Scale, the Self-Directed Learning Readiness Scale, and a nursing survey consisting of demographic questions. Basic nursing education and employment status did not effect motivational orientation or self-directed learning readiness. Clinical area and level of position significantly influenced nurses' decisions to participate in continuing education activities. Motivational orientation had a significant relationship with selfdirected learning readiness. Implications for practice as a result of this study involves program planning and delivery. The identification of the motivational orientations of participants may assist in the development and delivery of continuing education programs that are beneficial, relevant, and address the identified learning needs of participants. Implications for future research also exist in relation to studying different groups of nurses, for example, registered nursing assistants, and investigating related issues, for example, what are the deterrents to participation in continuing education?

Satisfacción del paciente con la atención de enfermería a su egreso de la unidad de urgencias

Tesis (Maestría en Ciencias de Enfermería con Énfasis en Administración de Servicios) UANL