On-site measurement of limiting subcell in multijunction solar devices

It is well known that the response of any photovoltaic solar cell is dependent on the spectral characteristics of the incident radiation. This dependency is crucial in the output characteristics of a multijunction (MJ) cell where the spectral composition of the radiation determines the overall photocurrent produced, as either the top or the middle subcell will be limiting its response. The current mismatching between top and middle subcell is translated into energy losses, affecting the yield of the system. For research and commercial purposes it is interesting to measure accurately the incident solar radiation on a MJ cell, in terms of its spectral composition. This measurement will allows us to determine the photocurrent generated in each band of the multijunction device. Nowadays, the only way of measuring the photocurrent generated by each subcell is done with isotype cells or with spectroradiometers but there is no device capable of directly measuring each subcell photocurrent. In this paper it is described a device based on a commercial multijunction solar cell that is capable of measuring the direct irradiance for the top and middle bands thus it offers information of the limiting subcell (top or middle) in outdoors conditions.

Equipment and procedures for ON-SITE testing of PV plants and BIPV

Actual system performance of a PV system can differ from its expected behaviour.. This is the main reason why the performance of PV systems should be monitored, analyzed and, if needed, improved on. Some of the current testing procedures relating to the electrical behaviour of PV systems are appropriated for detecting electrical performance losses, but they are not well-suited to reveal hidden defects in the modules of PV plants and BIPV, which can lead to future losses. This paper reports on the tests and procedures used to evaluate the performance of PV systems, and especially on a novel procedure for quick on-site measurements and defect recognition caused by overheating in PV modules located in operating PV installations.

Effect of Initial On-Site Curing on 28-Day Cylinder Strength

The initial curing of concrete specimens for quality assurance is addressed in different ways in testing standards, which often specify requirements that are difficult to meet in practice unless very costly initial curing chambers are available. The failure to meet these requirements in many areas of the world does not appear to result in adverse consequences. This study analyzed six initial curing temperature schemes, all with cycles similar to natural conditions to avoid the simplifications inherent in constant temperature curing. Three strengths of concrete and two initial curing times (24 and 72 hours) were used in this study. The findings showed that initial curing time had no effect on 28-day strength. The 28-day strength also proved to be resilient to maximum and minimum initial curing temperatures outside the limits stated in the standards considered in this study

Capacitive load based on IGBTs for on-site characterization of PV arrays

This paper describes the practical design of a portable capacitive load based on insulated gate bipolar transistors (IGBTs), which is used to measure the I–V characteristics of PV arrays with short-circuit currents up to 80 A and open circuit voltages up to 800 V. Such measurement allows on-site characterization of PV arrays under real operating conditions and also provides information for the detection of potential array anomalies, such as broken cells or defective connections. The presented I–V load is easy to reproduce and low-cost, characteristics that are within the reach of small-scale organizations involved in PV electrification projects.

Optimal design of AC EMI filters with damping networks and effect on the systems power factor

The cutoff frequencies of an EMI filter are normally given by the noise attenuation requirements the filter has to fulfill. In order to select the component values of the filter elements, i.e. inductances and capacitances, an additional design criterium is needed. In this paper the effect of the EMI filter input and output impedances are considered. The input impedance influences the filters effect on the system displacement power factor and the output impedance plays a key role in the system stability. The effect of filter element values, the number of filter stages as well as additional damping networks are considered and a design procedure is provided. For this analysis a two-port description of the input filters employing ABCD-parameters is used.

On-site tests for the detection of potential induced degradation in modules

This paper presents different test alternatives which can be used on-site in a PV installation to detect potential induced degradation (PID) in modules. The testing procedures proposed are: thermal imaging; electroluminescence imaging; open circuit voltage measurements; operating voltage measurements; IV curve measurements; and dark IV curve measurements. Advantages and disadvantages of each test are reported.

Automatic fault detection on BIPV systems without solar irradiation data

BIPV systems are small PV generation units spread out over the territory, and whose characteristics are very diverse. This makes difficult a cost-effective procedure for monitoring, fault detection, performance analyses, operation and maintenance. As a result, many problems affecting BIPV systems go undetected. In order to carry out effective automatic fault detection procedures, we need a performance indicator that is reliable and that can be applied on many PV systems at a very low cost. The existing approaches for analyzing the performance of PV systems are often based on the Performance Ratio (PR), whose accuracy depends on good solar irradiation data, which in turn can be very difficult to obtain or cost-prohibitive for the BIPV owner. We present an alternative fault detection procedure based on a performance indicator that can be constructed on the sole basis of the energy production data measured at the BIPV systems. This procedure does not require the input of operating conditions data, such as solar irradiation, air temperature, or wind speed. The performance indicator, called Performance to Peers (P2P), is constructed from spatial and temporal correlations between the energy output of neighboring and similar PV systems. This method was developed from the analysis of the energy production data of approximately 10,000 BIPV systems located in Europe. The results of our procedure are illustrated on the hourly, daily and monthly data monitored during one year at one BIPV system located in the South of Belgium. Our results confirm that it is possible to carry out automatic fault detection procedures without solar irradiation data. P2P proves to be more stable than PR most of the time, and thus constitutes a more reliable performance indicator for fault detection procedures. We also discuss the main limitations of this novel methodology, and we suggest several future lines of research that seem promising to improve on these procedures.

Automatic expert system based on images for accuracy crop row detection in maize fields

This paper proposes an automatic expert system for accuracy crop row detection in maize fields based on images acquired from a vision system. Different applications in maize, particularly those based on site specific treatments, require the identification of the crop rows. The vision system is designed with a defined geometry and installed onboard a mobile agricultural vehicle, i.e. submitted to vibrations, gyros or uncontrolled movements. Crop rows can be estimated by applying geometrical parameters under image perspective projection. Because of the above undesired effects, most often, the estimation results inaccurate as compared to the real crop rows. The proposed expert system exploits the human knowledge which is mapped into two modules based on image processing techniques. The first one is intended for separating green plants (crops and weeds) from the rest (soil, stones and others). The second one is based on the system geometry where the expected crop lines are mapped onto the image and then a correction is applied through the well-tested and robust Theil–Sen estimator in order to adjust them to the real ones. Its performance is favorably compared against the classical Pearson product–moment correlation coefficient.

Evaluación de la profundidad de la fisuración superficial descendente en pavimentos asfálticos mediante técnicas de ultrasonidos. Validación teórico-práctica y modelos

La fisuración iniciada en la superficie de los pavimentos asfálticos constituye uno de los más frecuentes e importantes modos de deterioro que tienen lugar en los firmes bituminosos, como han demostrado los estudios teóricos y experimentales llevados a cabo en la última década. Sin embargo, este mecanismo de fallo no ha sido considerado por los métodos tradicionales de diseño de estos firmes. El concepto de firmes de larga duración se fundamenta en un adecuado seguimiento del proceso de avance en profundidad de estos deterioros y la intervención en el momento más apropiado para conseguir mantenerlos confinados como fisuras de profundidad parcial en la capa superficial más fácilmente accesible y reparable, de manera que pueda prolongarse la durabilidad y funcionalidad del firme y reducir los costes generalizados de su ciclo de vida. Por lo tanto, para la selección de la estrategia óptima de conservación de los firmes resulta esencial disponer de metodologías que posibiliten la identificación precisa in situ de la fisuración descendente, su seguimiento y control, y que además permitan una determinación fiable y con alto rendimiento de su profundidad y extensión. En esta Tesis Doctoral se presentan los resultados obtenidos mediante la investigación sistemática de laboratorio e in situ llevada a cabo para la obtención de datos sobre fisuración descendente en firmes asfálticos y para el estudio de procedimientos de evaluación de la profundidad de este tipo de fisuras empleando técnicas de ultrasonidos. Dichos resultados han permitido comprobar que la metodología no destructiva propuesta, de rápida ejecución, bajo coste y sencilla implementación (principalmente empleada hasta el momento en estructuras metálicas y de hormigón, debido a las dificultades que introduce la naturaleza viscoelástica de los materiales bituminosos) puede ser aplicada con suficiente fiabilidad y repetibilidad sobre firmes asfálticos. Las medidas resultan asimismo independientes del espesor total del firme. Además, permite resolver algunos de los inconvenientes frecuentes que presentan otros métodos de diagnóstico de las fisuras de pavimentos, tales como la extracción de testigos (sistema destructivo, de alto coste y prolongados tiempos de interrupción del tráfico) o algunas otras técnicas no destructivas como las basadas en medidas de deflexiones o el georradar, las cuales no resultan suficientemente precisas para la investigación de fisuras superficiales. Para ello se han realizado varias campañas de ensayos sobre probetas de laboratorio en las que se han estudiado diferentes condiciones empíricas como, por ejemplo, distintos tipos de mezclas bituminosas en caliente (AC, SMA y PA), espesores de firme y adherencias entre capas, temperaturas, texturas superficiales, materiales de relleno y agua en el interior de las grietas, posición de los sensores y un amplio rango de posibles profundidades de fisura. Los métodos empleados se basan en la realización de varias medidas de velocidad o de tiempo de transmisión del pulso ultrasónico sobre una única cara o superficie accesible del material, de manera que resulte posible obtener un coeficiente de transmisión de la señal (mediciones relativas o autocompensadas). Las mediciones se han realizado a bajas frecuencias de excitación mediante dos equipos de ultrasonidos diferentes dotados, en un caso, de transductores de contacto puntual seco (DPC) y siendo en el otro instrumento de contacto plano a través de un material especialmente seleccionado para el acoplamiento (CPC). Ello ha permitido superar algunos de los tradicionales inconvenientes que presenta el uso de los transductores convencionales y no precisar preparación previa de las superficies. La técnica de autocalibración empleada elimina los errores sistemáticos y la necesidad de una calibración local previa, demostrando el potencial de esta tecnología. Los resultados experimentales han sido comparados con modelos teóricos simplificados que simulan la propagación de las ondas ultrasónicas en estos materiales bituminosos fisurados, los cuales han sido deducidos previamente mediante un planteamiento analítico y han permitido la correcta interpretación de dichos datos empíricos. Posteriormente, estos modelos se han calibrado mediante los resultados de laboratorio, proporcionándose sus expresiones matemáticas generalizadas y gráficas para su uso rutinario en las aplicaciones prácticas. Mediante los ensayos con ultrasonidos efectuados en campañas llevadas a cabo in situ, acompañados de la extracción de testigos del firme, se han podido evaluar los modelos propuestos. El máximo error relativo promedio en la estimación de la profundidad de las fisuras al aplicar dichos modelos no ha superado el 13%, con un nivel de confianza del 95%, en el conjunto de todos los ensayos realizados. La comprobación in situ de los modelos ha permitido establecer los criterios y las necesarias recomendaciones para su utilización sobre firmes en servicio. La experiencia obtenida posibilita la integración de esta metodología entre las técnicas de auscultación para la gestión de su conservación. Abstract Surface-initiated cracking of asphalt pavements constitutes one of the most frequent and important types of distress that occur in flexible bituminous pavements, as clearly has been demonstrated in the technical and experimental studies done over the past decade. However, this failure mechanism has not been taken into consideration for traditional methods of flexible pavement design. The concept of long-lasting pavements is based on adequate monitoring of the depth and extent of these deteriorations and on intervention at the most appropriate moment so as to contain them in the surface layer in the form of easily-accessible and repairable partial-depth topdown cracks, thereby prolonging the durability and serviceability of the pavement and reducing the overall cost of its life cycle. Therefore, to select the optimal maintenance strategy for perpetual pavements, it becomes essential to have access to methodologies that enable precise on-site identification, monitoring and control of top-down propagated cracks and that also permit a reliable, high-performance determination of the extent and depth of cracking. This PhD Thesis presents the results of systematic laboratory and in situ research carried out to obtain information about top-down cracking in asphalt pavements and to study methods of depth evaluation of this type of cracking using ultrasonic techniques. These results have demonstrated that the proposed non-destructive methodology –cost-effective, fast and easy-to-implement– (mainly used to date for concrete and metal structures, due to the difficulties caused by the viscoelastic nature of bituminous materials) can be applied with sufficient reliability and repeatability to asphalt pavements. Measurements are also independent of the asphalt thickness. Furthermore, it resolves some of the common inconveniences presented by other methods used to evaluate pavement cracking, such as core extraction (a destructive and expensive procedure that requires prolonged traffic interruptions) and other non-destructive techniques, such as those based on deflection measurements or ground-penetrating radar, which are not sufficiently precise to measure surface cracks. To obtain these results, extensive tests were performed on laboratory specimens. Different empirical conditions were studied, such as various types of hot bituminous mixtures (AC, SMA and PA), differing thicknesses of asphalt and adhesions between layers, varied temperatures, surface textures, filling materials and water within the crack, different sensor positions, as well as an ample range of possible crack depths. The methods employed in the study are based on a series of measurements of ultrasonic pulse velocities or transmission times over a single accessible side or surface of the material that make it possible to obtain a signal transmission coefficient (relative or auto-calibrated readings). Measurements were taken at low frequencies by two short-pulse ultrasonic devices: one equipped with dry point contact transducers (DPC) and the other with flat contact transducers that require a specially-selected coupling material (CPC). In this way, some of the traditional inconveniences presented by the use of conventional transducers were overcome and a prior preparation of the surfaces was not required. The auto-compensating technique eliminated systematic errors and the need for previous local calibration, demonstrating the potential for this technology. The experimental results have been compared with simplified theoretical models that simulate ultrasonic wave propagation in cracked bituminous materials, which had been previously deduced using an analytical approach and have permitted the correct interpretation of the aforementioned empirical results. These models were subsequently calibrated using the laboratory results, providing generalized mathematical expressions and graphics for routine use in practical applications. Through a series of on-site ultrasound test campaigns, accompanied by asphalt core extraction, it was possible to evaluate the proposed models, with differences between predicted crack depths and those measured in situ lower than 13% (with a confidence level of 95%). Thereby, the criteria and the necessary recommendations for their implementation on in-service asphalt pavements have been established. The experience obtained through this study makes it possible to integrate this methodology into the evaluation techniques for pavement management systems.

A Model for Proactivity in Mobile, Context-aware Recommender Systems

A proactive recommender system pushes recommendations to the user when the current situation seems appropriate, without explicit user request. This is conceivable in mobile scenarios such as restaurant or gas station recommendations. In this paper, we present a model for proactivity in mobile recommender systems. The model relies on domain-dependent context modeling in several categories. The recommendation process is divided into two phases to first analyze the current situation and then examine the suitability of particular items. We have implemented a prototype gas station recommender and conducted a survey for evaluation. Results showed good correlation of the output of our system with the assessment of users regarding the question when to generate recommendations.

Engineering an Ontology for Autonomous Systems - The OASys Ontology

This paper describes the development of an ontology for autonomous systems, as the initial stage of a research programe on autonomous systems’ engineering within a model-based control approach. The ontology aims at providing a unified conceptual framework for the autonomous systems’ stakeholders, from developers to software engineers. The modular ontology contains both generic and domain-specific concepts for autonomous systems description and engineering. The ontology serves as the basis in a methodology to obtain the autonomous system’s conceptual models. The objective is to obtain and to use these models as main input for the autonomous system’s model-based control system.

DBpedia Spotlight: Shedding Light on the Web of Documents

Interlinking text documents with Linked Open Data enables the Web of Data to be used as background knowledge within document-oriented applications such as search and faceted browsing. As a step towards interconnecting the Web of Documents with the Web of Data, we developed DBpedia Spotlight, a system for automatically annotating text documents with DBpedia URIs. DBpedia Spotlight allows users to configure the annotations to their specific needs through the DBpedia Ontology and quality measures such as prominence, topical pertinence, contextual ambiguity and disambiguation confidence. We compare our approach with the state of the art in disambiguation, and evaluate our results in light of three baselines and six publicly available annotation systems, demonstrating the competitiveness of our system. DBpedia Spotlight is shared as open source and deployed as a Web Service freely available for public use.

Dynamic Effects On Culverts for High Speed Trains

Culverts are very common in recent railway lines. Wild life corridors and drainage conducts often fall in this category of partially buried structures. Their dynamic behavior has received far less attention than other structures such as bridges but its large number makes that study an interesting challenge from the point of view of safety and savings. In this paper a complete study of a culvert, including on-site measurements as well as numerical modelling, will be presented. The structure belongs to the high speed railway line linking Segovia and Valladolid, in Spain. The line was opened to traffic in 2004. Its dimensions (3x3m) are the most frequent along the line. Other factors such as reduced overburden (0.6m) and an almost right angle with the track axis make it an interesting example to extract generalized conclusions. On site measurements have been performed in the structure recording the dynamic response at selected points of the structure during the passage of high speed trains at speeds ranging between 200 and 300km/h. The measurements by themselves provide a good insight into the main features of the dynamic behaviour of the structure. A 3D finite element model of the structure, representing its key features was also studied as it allows further understanding of the dynamic response to the train loads . In the paper the discrepancies between predicted and measured vibration levels will be analyzed and some advices on numerical modelling will be proposed

An empirical mixed model to quantify climate influence on the growth of Pinus halepensis Mill. stands in South-Eastern Spain

The influence of climate on forest stand composition, development and growth is undeniable. Many studies have tried to quantify the effect of climatic variables on forest growth and yield. These works become especially important because there is a need to predict the effects of climate change on the development of forest ecosystems. One of the ways of facing this problem is the inclusion of climatic variables into the classic empirical growth models. The work has a double objective: (i) to identify the indicators which best describe the effect of climate on Pinus halepensis growth and (ii) to quantify such effect in several scenarios of rainfall decrease which are likely to occur in the Mediterranean area. A growth mixed model for P. halepensis including climatic variables is presented in this work. Growth estimates are based on data from the Spanish National Forest Inventory (SNFI). The best results are obtained for the indices including rainfall, or rainfall and temperature together, with annual precipitation, precipitation effectiveness, Emberger?s index or free bioclimatic intensity standing out among them. The final model includes Emberger?s index, free bioclimatic intensity and interactions between competition and climate indices. The results obtained show that a rainfall decrease about 5% leads to a decrease in volume growth of 5.5?7.5% depending on site quality.

Wireless Optical Communications for Intra-Spacecraft Networks Based on OCDMA with Random Optical Codes

In recent years, spacial agencies have shown a growing interest in optical wireless as an alternative to wired and radio-frequency communications. The use of these techniques for intra-spacecraft communications reduces the effect of take-off acceleration and vibrations on the systems by avoiding the need for rugged connectors and provides a significant mass reduction. Diffuse transmission also eases the design process as terminals can be placed almost anywhere without a tight planification to ensure the proper system behaviour. Previous studies have compared the performance of radio-frequency and infrared optical communications. In an intra-satellite environment optical techniques help reduce EMI related problems, and their main disadvantages - multipath dispersion and the need for line-of-sight - can be neglected due to the reduced cavity size. Channel studies demonstrate that the effect of the channel can be neglected in small environments if data bandwidth is lower than some hundreds of MHz.