Optimization of YIG/Bi stacks for spin-to-charge conversion and influence of aging

We show that an optimized growth of magnetic layer/non-magnetic layer stacks allows for the improvement of the spin-to-charge conversion efficiency. From the analysis of the voltage signal generated in spin pumping experiments due to the inverse spin Hall effect (ISHE) on Y3Fe5O12 (YIG)/Bi stacks, we have determined values for the spin Hall angle and the spin-diffusion length in Bi of 0.0068(8) and 17.8(9) nm, respectively. Based on these results, we have also studied the influence of aging on the spin-to-charge conversion efficiency by performing spin pumping experiments on YIG/Bi stacks after exposing the samples to ambient conditions for several days and up to 150 days. We have found that in YIG/Bi samples with Bi thicknesses around or below the spin-diffusion length, the ISHE voltage signal is still above 80% of its initial value after 100 days

Novel utility-scale photovoltaic plant electroluminescence maintenance technique by means of bidirectional power inverter controller

Producción CientíficaNowadays, photovoltaic (PV) silicon plants dominate the growth in renewable energies generation. Utility-scale photovoltaic plants (USPVPs) have increased exponentially in size and power in the last decade and, therefore, it is crucial to develop optimum maintenance techniques. One of the most promising maintenance techniques is the study of electroluminescence (EL) images as a complement of infrared thermography (IRT) analysis. However, its high cost has prevented its use regularly up to date. This paper proposes a maintenance methodology to perform on-site EL inspections as efficiently as possible. First, current USPVP characteristics and the requirements to apply EL on them are studied. Next, an increase over the automation level by means of adding automatic elements in the current PV plant design is studied. The new elements and their configuration are explained, and a control strategy for applying this technique on large photovoltaic plants is developed. With the aim of getting on-site EL images on a real plant, a PV inverter has been developed to validate the proposed methodology on a small-scale solar plant. Both the electrical parameters measured during the tests and the images taken have been analysed. Finally, the implementation cost of the solution has been calculated and optimised. The results conclude the technical viability to perform on-site EL inspections on PV plants without the need to measure and analyse the panel defects out of the PV installation.Ministerio de Industria, Economía y Competitividad (grant number RTC-2017-6712-3)Junta de Castilla y León (grant VA283P18

Low-Cost Three-Quadrant Single Solar Cell I-V Tracer

An I-V curve measurement technique is one of the most important techniques available for characterising photovoltaic cells. Measuring an accurate I-V curve at the single-cell level is a challenging task because of the low voltages and high currents implied, requiring the management of very low impedances. In this paper, the authors propose a low-cost device for I-V curve measurements of single (or small amounts) of cells in a series based on the charge transfer between two capacitors of equal capacitance. Our measurement strategy allows us to trace the usual first quadrant curve (the normal working region of solar cells) as well as the second and fourth quadrants of the I-V curve, which are quite important for research purposes. A prototype was built to demonstrate the feasibility and successful measurements of the three-quadrant I-V curve, obtained for more than 20 different cells. To use the device in a laboratory, without depending on the solar irradiation, a modular platform was 3D-printed, integrating a board with infrared LEDs as irradiating devices, and housing (to place the solar cell under test). The result is a useful low-cost setup for three-quadrant I-V curve tracing that works as expected.Proyecto de Investigación PID2020-113533RB-C33 (MCIN

A Resonant Ring Topology Approach to Power Line Communication Systems within Photovoltaic Plants

Within this study, single-cable propagation facilitated by PV strings’ wiring characteristics is considered for an adapted design of PLC electronics. We propose to close the communications signal path, resulting in a ring topology where a resonance condition could be implemented. A PLC topology using the resulting circular closed-loop path of a PV series string as its physical communication support is designed and leveraged for practical use. When the path length or the number of transceivers is changed, the resonance properties that come with the circular path as the physical support are affected but are shown to be preserved with the application of automatic adjustable tuning. This automatic tuning guarantees that the resonance improves propagation parameters and reverts the system to its optimal values at the chosen carrier frequency.This study was supported by the Universidad of Valladolid with the predoctoral contracts of 2020 cofunded by Santander Bank. This study was supported by the Universidad of Valladolid with ERASMUS+ KA-107. Partial funding for open access charge: Universidad de Málag

Nondestructive characterization of solar PV cells defects by means of electroluminescence, infrared thermography, I–V curves and visual tests: Experimental study and comparison

Photovoltaic (PV) modules are the core of every PV system, representing the power generation and their operation will affect the overall plant performance. It is one of the elements within a PV site with the higher failure appearance, being essential their proper operation to produce reliable, efficient and safety energy. Quantitative analysis and characterization of manufacturing, soldering and breaking PV defects is performed by a combination of electroluminescence (EL), infrared thermography (IRT), electrical current voltage (I-V) curves and visual inspection. Equivalent-circuit model characterization and microscope inspection are also performed as additional techniques when they contribute to the defects characterization. A 60-cells polycrystalline module has been ad hoc manufactured for this research, with different defective and non-defective cells. All cells are accessible from the backside of the module and the module includes similar kinds of defects in the same bypass string. This paper characterizes different defects of PV modules to control, mitigate or eliminate their influence and being able to do a quality assessment of a whole PV module, relating the individual cells performance with the combination of defective and non-defective cells within the module strings, with the objective of determining their interaction and mismatch effects, apart from their discrete performance.Ministerio de Ciencia e Innovación (Proyecto de Investigación ENE2017-89561-C4-3-R)Ministerio de Ciencia e Innovación (Proyecto de Investigación RTC-2017-6712-3

Design, implementation and validation of advanced measurements and communications within photovoltaic plants

Design and validation of a low cost integrated electronic system for the diagnosis and maintenance improvement of photovoltaic solar plants. The system is able to trace I-V curves for each solar module individually without disconnecting them from the plant, without interrupting the electric generation and with a minimum impact over the energy production. Moreover the I-V data obtained are evacuated from the module under test through a novel PLC communications system using the same power wires installed in the plant as physical support.The main novelty of this system is the implementation of a single cable ring shaped topology pushed to work in resonance compatible with the physical topology of the power cabling in the solar plant.Diseño y validación de un sistema electrónico integrado de bajo coste para la mejora de las labores de mantenimiento y diagnóstico de una planta solar fotovoltaica. El sistema es capaz de realizar trazas I-V de módulos fotovoltaicos individuales sin desconectarlos de la planta, sin interrumpir la generación eléctrica y con un impacto mínimo en la producción de energía. Además, los datos obtenidos de estas trazas I-V se evacúan del módulo analizado mediante un novedoso sistema de comunicaciones PLC (Power Line Communications) que utiliza los mismos cables de potencia ya instalados en la planta como soporte físico. La principal novedad de este sistema es la utilización de una topología de un solo cable en forma de anillo que se lleva a trabajar en resonancia, consecuente con la topología física del cableado de potencia de la planta.Escuela de DoctoradoDoctorado en Ingeniería Industria

Low-Cost Three-Quadrant Single Solar Cell I-V Tracer

An I-V curve measurement technique is one of the most important techniques available for characterising photovoltaic cells. Measuring an accurate I-V curve at the single-cell level is a challenging task because of the low voltages and high currents implied, requiring the management of very low impedances. In this paper, the authors propose a low-cost device for I-V curve measurements of single (or small amounts) of cells in a series based on the charge transfer between two capacitors of equal capacitance. Our measurement strategy allows us to trace the usual first quadrant curve (the normal working region of solar cells) as well as the second and fourth quadrants of the I-V curve, which are quite important for research purposes. A prototype was built to demonstrate the feasibility and successful measurements of the three-quadrant I-V curve, obtained for more than 20 different cells. To use the device in a laboratory, without depending on the solar irradiation, a modular platform was 3D-printed, integrating a board with infrared LEDs as irradiating devices, and housing (to place the solar cell under test). The result is a useful low-cost setup for three-quadrant I-V curve tracing that works as expected

Optimized estimator of the output power of PV cells using EL images and I–V curves

Producción CientíficaIn this work, a method to predict the output power of Photovoltaic (PV) cells using their Electroluminescence (EL) images is presented. The data used includes Electroluminescence Images and the value of the Max Power Point computed from the Current–Voltage Curve of the cells. The method is used as follows: Firstly, the images are preprocessed to improve their quality. After that, a comparison between different Machine Learning methods from Traditional ones, such as Random Forest or Gradient Boosting, to Deep Learning methods, such as Recurrent Neural Networks or Convolutional Neural Networks is performed. Another significant contribution of this paper is that it analyzes the problem of unbalanced data, trying to solve it using Synthetic Images created by a Generative Adversarial Network. Our results show that the best model is the Gradient-Boosting based method using a pre-trained Resnet50 as a feature extraction method with a Mean Absolute Error (MAE) of 0.0341 and a Mean Squared Error (MSE) of 0.00211. The results also shows how the models trained with the unbalanced dataset are capable of obtaining results similar to the models trained with the balanced dataset.Ministerio de Ciencia e Innovación de España (PID2020-113533RB-C33

A Review of I–V Tracers for Photovoltaic Modules: Topologies and Challenges

Current–voltage (I–V) curve tracers are used for measuring voltage and current in photovoltaic (PV) modules. I–V curves allow identifying certain faults in the photovoltaic module, as well as quantifying the power performance of the device. I–V curve tracers are present in different topologies and configurations, by means of rheostats, capacitive loads, electronic loads, transistors, or by means of DC–DC converters. This article focuses on presenting all these configurations. The paper shows the electrical parameters to which the electronic elements of the equipment are exposed using LTSpice, facilitating the appropriate topology selection. Additionally, a comparison has been included between the different I–V tracers’ topologies, analyzing their advantages and disadvantages, considering different factors such as their flexibility, modularity, cost, precision, speed or rating, as well as the characteristics of the different DC–DC converters

Online Distributed Measurement of Dark I-V Curves in Photovoltaic Plants

The inspection techniques for defects in photovoltaic modules are diverse. Among them, the inspection with measurements using current–voltage (I-V) curves is one of the most outstanding. I-V curves, which can be carried under illumination or in dark conditions, are widely used to detect certain defects in photovoltaic modules. In a traditional way, these measurements are carried out by disconnecting the photovoltaic module from the string inside the photovoltaic plant. In this work, the researchers propose a methodology to perform online dark I-V curves of modules in photovoltaic plants without the need of disconnecting them from the string. For this, a combination of electronic boards in the photovoltaic modules and a bidirectional inverter are employed. The results are highly promising, and this methodology could be widely used in upcoming photovoltaic plants