From irradiance to output power fluctuations: the pv plant as a low pass filter

The power generated by large grid-connected photovoltaic (PV) plants depends greatly on the solar irradiance. This paper studies the effects of the solar irradiance variability analyzing experimental 1-s data collected throughout a year at six PV plants, totaling 18 MWp. Each PV plant was modeled as a first order filter function based on an analysis in the frequency domain of the irradiance data and the output power signals. An empiric expression which relates the filter parameters and the PV plant size has been proposed. This simple model has been successfully validated precisely determining the daily maximum output power fluctuation from incident irradiance measurements.

Domestic hot water consumption vs. solar thermal energy storage: the optimum size of the storage tank

Many efforts have been made in order to adequate the production of a solar thermal collector field to the consumption of domestic hot water of the inhabitants of a building. In that sense, much has been achieved in different domains: research agencies, government policies and manufacturers. However, most of the design rules of the solar plants are based on steady state models, whereas solar irradiance, consumption and thermal accumulation are inherently transient processes. As a result of this lack of physical accuracy, thermal storage tanks are sometimes left to be as large as the designer decides without any aforementioned precise recommendation. This can be a problem if solar thermal systems are meant to be implemented in nowadays buildings, where there is a shortage of space. In addition to that, an excessive storage volume could not result more efficient in many residential applications, but costly, extreme in space consumption and in some cases too heavy. A proprietary transient simulation program has been developed and validated with a detailed measurement campaign in an experimental facility. In situ environmental data have been obtained through a whole year of operation. They have been gathered at intervals of 10 min for a solar plant of 50 m2 with a storage tank of 3 m3, including the equipment for domestic hot water production of a typical apartment building. This program has been used to obtain the design and dimensioning criteria of DHW solar plants under daily transient conditions throughout a year and more specifically the size of the storage tank for a multi storey apartment building. Comparison of the simulation results with the current Spanish regulation applicable, “Código Técnico de la Edificación” (CTE 2006), offers fruitful details and establishes solar facilities dimensioning criteria.

Predicción espacio-temporal de la irradiancia solar global a corto plazo en España mediante geoestadística y redes neuronales artificiales

El enriquecimiento del conocimiento sobre la Irradiancia Solar (IS) a nivel de superficie terrestre, así como su predicción, cobran gran interés para las Energías Renovables (ER) - Energía Solar (ES)-, y para distintas aplicaciones industriales o ecológicas. En el ámbito de las ER, el uso óptimo de la ES implica contar con datos de la IS en superficie que ayuden tanto, en la selección de emplazamientos para instalaciones de ES, como en su etapa de diseño (dimensionar la producción) y, finalmente, en su explotación. En este último caso, la observación y la predicción es útil para el mercado energético, la planificación y gestión de la energía (generadoras y operadoras del sistema eléctrico), especialmente en los nuevos contextos de las redes inteligentes de transporte. A pesar de la importancia estratégica de contar con datos de la IS, especialmente los observados por sensores de IS en superficie (los que mejor captan esta variable), estos no siempre están disponibles para los lugares de interés ni con la resolución espacial y temporal deseada. Esta limitación se une a la necesidad de disponer de predicciones a corto plazo de la IS que ayuden a la planificación y gestión de la energía. Se ha indagado y caracterizado las Redes de Estaciones Meteorológicas (REM) existentes en España que publican en internet sus observaciones, focalizando en la IS. Se han identificado 24 REM (16 gubernamentales y 8 redes voluntarios) que aglutinan 3492 estaciones, convirtiéndose éstas en las fuentes de datos meteorológicos utilizados en la tesis. Se han investigado cinco técnicas de estimación espacial de la IS en intervalos de 15 minutos para el territorio peninsular (3 técnicas geoestadísticas, una determinística y el método HelioSat2 basado en imágenes satelitales) con distintas configuraciones espaciales. Cuando el área de estudio tiene una adecuada densidad de observaciones, el mejor método identificado para estimar la IS es el Kriging con Regresión usando variables auxiliares -una de ellas la IS estimada a partir de imágenes satelitales-. De este modo es posible estimar espacialmente la IS más allá de los 25 km identificados en la bibliografía. En caso contrario, se corrobora la idoneidad de utilizar estimaciones a partir de sensores remotos cuando la densidad de observaciones no es adecuada. Se ha experimentado con el modelado de Redes Neuronales Artificiales (RNA) para la predicción a corto plazo de la IS utilizando observaciones próximas (componentes espaciales) en sus entradas y, los resultados son prometedores. Así los niveles de errores disminuyen bajo las siguientes condiciones: (1) cuando el horizonte temporal de predicción es inferior o igual a 3 horas, las estaciones vecinas que se incluyen en el modelo deben encentrarse a una distancia máxima aproximada de 55 km. Esto permite concluir que las RNA son capaces de aprender cómo afectan las condiciones meteorológicas vecinas a la predicción de la IS. ABSTRACT ABSTRACT The enrichment of knowledge about the Solar Irradiance (SI) at Earth's surface and its prediction, have a high interest for Renewable Energy (RE) - Solar Energy (SE) - and for various industrial and environmental applications. In the field of the RE, the optimal use of the SE involves having SI surface to help in the selection of sites for facilities ES, in the design stage (sizing energy production), and finally on their production. In the latter case, the observation and prediction is useful for the market, planning and management of the energy (generators and electrical system operators), especially in new contexts of smart transport networks (smartgrid). Despite the strategic importance of SI data, especially those observed by sensors of SI at surface (the ones that best measure this environmental variable), these are not always available to the sights and the spatial and temporal resolution desired. This limitation is bound to the need for short-term predictions of the SI to help planning and energy management. It has been investigated and characterized existing Networks of Weather Stations (NWS) in Spain that share its observations online, focusing on SI. 24 NWS have been identified (16 government and 8 volunteer networks) that implies 3492 stations, turning it into the sources of meteorological data used in the thesis. We have investigated five technical of spatial estimation of SI in 15 minutes to the mainland (3 geostatistical techniques and HelioSat2 a deterministic method based on satellite images) with different spatial configurations. When the study area has an adequate density of observations we identified the best method to estimate the SI is the regression kriging with auxiliary variables (one of them is the SI estimated from satellite images. Thus it is possible to spatially estimate the SI beyond the 25 km identified in the literature. Otherwise, when the density of observations is inadequate the appropriateness is using the estimates values from remote sensing. It has been experimented with Artificial Neural Networks (ANN) modeling for predicting the short-term future of the SI using observations from neighbor’s weather stations (spatial components) in their inputs, and the results are promising. The error levels decrease under the following conditions: (1) when the prediction horizon is less or equal than 3 hours the best models are the ones that include data from the neighboring stations (at a maximum distance of 55 km). It is concluded that the ANN is able to learn how weather conditions affect neighboring prediction of IS at such Spatio-temporal horizons.

Simulation of oil productivity and quality of N-S oriented olive hedgerow orchards in response to structure and interception of radiation

Simulations of oil yield and quality are presented for N–S oriented, hedgerow olive orchards of a range of structures (viz. canopy depth, canopy width, canopy slope and row spacing) using responses of yield and quality parameters to solar irradiance on canopy walls measured in a range of orchards, cv. Arbequina, in Spain. Results reveal that orchard yield of hedgerows of rectangular shape reaches a maximum when canopy depth equals alley width (row spacing−canopy width) and decreases at wider spacing, and/or with wider canopies, as the length of productive row decreases per unit area. Maximum yields for 4-m deep canopies were 2885 kg ha−1 at 1-m width and 5-m row spacing, 2400 kg ha−1 at 2-m width and 6-m spacing, and 2050 kg ha−1 at 3-m width and 7-m spacing. Illumination of canopies can be increased by applying slopes to form rhomboidal hedgerows. Substantial yield advantage can be achieved, especially for wide hedgerows, partly by closer row spacing that increases row length per unit area. By comparison, responses to latitude in the range 30–40◦ are small and do not warrant different row spacing. Oil quality parameters also respond to orchard structure. Responses are presented for oleic and palmitic acid, stability, and maturity index. Oleic acid content declines as alley spacing increases and is smaller, shallow than in wide, deep canopies. Palmitic acid content, stability, and maturity index increase with row alley spacing and are greater in narrow, shallow than in wide, deep canopies.

Analysis of the behavior of multijunction solar cells under high irradiance Gaussian light profiles showing chromatic aberration with emphasis on tunnel junction performance

In this work, we explain the behavior of multijunction solar cells under non-uniform (spatially and in spectral content) light profiles in general and in particular when Gaussian light profiles cause a photo-generated current density, which exceeds locally the peak current density of the tunnel junction. We have analyzed the implications on the tunnel junction's limitation, that is, in the loss of efficiency due to the appearance of a dip in the I–V curve. For that, we have carried out simulations with our three-dimensional distributed model for multijunction solar cells, which contemplates a full description of the tunnel junction and also takes into account the lateral resistances in the tunnel junction. The main findings are that the current density photo-generated spreads out through the lateral resistances of the device, mainly through the tunnel junction layers and the back contact. Therefore, under non-uniform light profiles these resistances are determinant not only to avoid the tunnel junction's limitation but also for mitigating losses in the fill factor. Therefore, taking into account these lateral resistances could be the key for jointly optimizing the concentrator photovoltaic system (concentrator optics, front grid layout and semiconductor structure)

Extended Triple-Junction Solar Cell 3D Distributed Model: Application to Chromatic Aberration-Related Losses

An extended 3D distributed model based on distributed circuit units for the simulation of triple‐junction solar cells under realistic conditions for the light distribution has been developed. A special emphasis has been put in the capability of the model to accurately account for current mismatch and chromatic aberration effects. This model has been validated, as shown by the good agreement between experimental and simulation results, for different light spot characteristics including spectral mismatch and irradiance non‐uniformities. This model is then used for the prediction of the performance of a triple‐junction solar cell for a light spot corresponding to a real optical architecture in order to illustrate its suitability in assisting concentrator system analysis and design process.

Soiling and other optical losses in solar-tracking PV plants in Navarra

Field data of soiling energy losses on PV plants are scarce. Furthermore, since dirt type and accumulation vary with the location characteristics (climate, surroundings, etc.), the available data on optical losses are, necessarily, site dependent. This paper presents field measurements of dirt energy losses (dust) and irradiance incidence angle losses along 2005 on a solar-tracking PV plant located south of Navarre (Spain). The paper proposes a method to calculate these losses based on the difference between irradiance measured by calibrated cells on several trackers of the PV plant and irradiance calculated from measurements by two pyranometers (one of them incorporating a shadow ring) regularly cleaned. The equivalent optical energy losses of an installation incorporating fixed horizontal modules at the same location have been calculated as well. The effect of dirt on both types of installations will accordingly be compared.

Instrumentation for accelerated life tests of concentrator solar cells

Concentrator photovoltaic is an emergent technology that may be a good economical and efficient alternative for the generation of electricity at a competitive cost. However, the reliability of these new solar cells and systems is still an open issue due to the high-irradiation level they are subjected to as well as the electrical and thermal stresses that they are expected to endure. To evaluate the reliability in a short period of time, accelerated aging tests are essential. Thermal aging tests for concentrator photovoltaic solar cells and systems under illumination are not available because no technical solution to the problem of reaching the working concentration inside a climatic chamber has been available. This work presents an automatic instrumentation system that overcomes the aforementioned limitation. Working conditions have been simulated by forward biasing the solar cells to the current they would handle at the working concentration (in this case, 700 and 1050 times the irradiance at one standard sun). The instrumentation system has been deployed for more than 10 000 h in a thermal aging test for III-V concentrator solar cells, in which the generated power evolution at different temperatures has been monitored. As a result of this test, the acceleration factor has been calculated, thus allowing for the degradation evolution at any temperature in addition to normal working conditions to be obtained.

Advances in the modeling, characterization and reliability of concentrator multijunction solar cells

Los sistemas de concentración fotovoltaica (CPV) parecen ser una de las vías más prometedoras para generar electricidad a gran escala a precios competitivos. La investigación actual se centra en aumentar la eficiencia y la concentración de los sistemas para abaratar costes. Al mismo tiempo se investiga sobre la fiabilidad de los diferentes componentes que integran un sistema de concentración, ya que para que los sistemas de concentración sean competitivos es necesario que tengan una fiabilidad al menos similar a los sistemas basados en células de silicio. En la presente tesis doctoral se ha llevado a cabo el estudio de aspectos avanzados de células solares multi-unión diseñadas para trabajar a concentraciones ultra-altas. Para ello, se ha desarrollado un modelo circuital tridimensional distribuido con el que simular el comportamiento de las células solares triple-unión bajo distintas condiciones de funcionamiento, así mismo se ha realizado una caracterización avanzada de este tipo de células para comprender mejor su modo de operación y así poder contribuir a mejorar su eficiencia. Finalmente, se han llevado a cabo ensayos de vida acelerados en células multiunión comerciales para conocer la fiabilidad de este tipo de células solares. Para la simulación de células solares triple-unión se ha desarrollado en la presente tesis doctoral un modelo circuital tridimensinal distribuido el cuál integra una descripción completa de la unión túnel. De este modo, con el modelo desarrollado, hemos podido simular perfiles de luz sobre la célula solar que hacen que la densidad de corriente fotogenerada sea mayor a la densidad de corriente pico de la unión túnel. El modelo desarrollado también contempla la distribución lateral de corriente en las capas semiconductoras que componen y rodean la unión túnel. Por tanto, se ha podido simular y analizar el efecto que tiene sobre el funcionamiento de la célula solar que los concentradores ópticos produzcan perfiles de luz desuniformes, tanto en nivel de irradiancia como en el contenido espectral de la luz (aberración cromática). Con el objetivo de determinar cuáles son los mecanismos de recombinación que están limitando el funcionamiento de cada subcélula que integra una triple-unión, y así intentar reducirlos, se ha llevado a cabo la caracterización eléctrica de células solares monouni ón idénticas a las subcelulas de una triple-unión. También se ha determinado la curva corriente-tensión en oscuridad de las subcélulas de GaInP y GaAs de una célula dobleunión mediante la utilización de un teorema de reciprocidad electro-óptico. Finalmente, se ha analizado el impacto de los diferentes mecanismos de recombinación en el funcionamiento de la célula solar triple-unión en concentración. Por último, para determinar la fiabilidad de este tipo de células, se ha llevado a cabo un ensayo de vida acelerada en temperatura en células solares triple-unión comerciales. En la presente tesis doctoral se describe el diseño del ensayo, el progreso del mismo y los datos obtenidos tras el análisis de los resultados preliminares. Abstract Concentrator photovoltaic systems (CPV) seem to be one of the most promising ways to generate electricity at competitive prices. Nowadays, the research is focused on increasing the efficiency and the concentration of the systems in order to reduce costs. At the same time, another important area of research is the study of the reliability of the different components which make up a CPV system. In fact, in order for a CPV to be cost-effective, it should have a warranty at least similar to that of the systems based on Si solar cells. In the present thesis, we will study in depth the behavior of multijunction solar cells under ultra-high concentration. With this purpose in mind, a three-dimensional circuital distributed model which is able to simulate the behavior of triple-junction solar cells under different working conditions has been developed. Also, an advanced characterization of these solar cells has been carried out in order to better understand their behavior and thus contribute to improving efficiency. Finally, accelerated life tests have been carried out on commercial lattice-matched triple-junction solar cells in order to determine their reliability. In order to simulate triple-junction solar cells, a 3D circuital distributed model which integrates a full description of the tunnel junction has been developed. We have analyzed the behavior of the multijunction solar cell under light profiles which cause the current density photo-generated in the solar cell to be higher than the tunnel junction’s peak current density. The advanced model developed also takes into account the lateral current spreading through the semiconductor layers which constitute and surround the tunnel junction. Therefore, the effects of non-uniform light profiles, in both irradiance and the spectral content produced by the concentrators on the solar cell, have been simulated and analyzed. In order to determine which recombination mechanisms are limiting the behavior of each subcell in a triple-junction stack, and to try to reduce them when possible, an electrical characterization of single-junction solar cells that resemble the subcells in a triplejunction stack has been carried out. Also, the dark I-V curves of the GaInP and GaAs subcells in a dual-junction solar cell have been determined by using an electro-optical reciprocity theorem. Finally, the impact of the different recombination mechanisms on the behavior of the triple-junction solar cell under concentration has been analyzed. In order to determine the reliability of these solar cells, a temperature accelerated life test has been carried out on commercial triple-junction solar cells. In the present thesis, the design and the evolution of the test, as well as the data obtained from the analysis of the preliminary results, are presented.

Diffuse illuminance availability on horizontal and vertical surfaces at Madrid, Spain

The overall objective of this work is to provide diffuse illuminance availability at Madrid (Spain) through a statistical analysis of illuminance values corresponding to a long-term data series. The illuminance values are obtained from irradiance measurements by means of different empirical models for luminous efficacy. The values of diffuse illuminance on a horizontal and on vertical surfaces facing the four cardinal points are estimated and the different aspects related to daylight availability in an area with specific climatic conditions are analyzed. The experimental data consist of global and diffuse irradiance measurements on a horizontal surface provided by the National Meteorological Agency in Spain (AEMET) for Madrid. These data consist of hourly values measured in the period of 1980–2005. The statistical results derived correspond to a daylight typical year for the five surfaces considered. This information will be useful to building experts to estimate natural illumination availability when daylighting techniques are applied in building design with the main aim of electric energy savings.

Triple-junction solar cell performance under Fresnel-based concentrators taking into account chromatic aberration and off-axis operation

Concentration photovoltaic (CPV) systems might produce quite uneven irradiance distributions (both on their level and on their spectral distribution) on the solar cell. This effect can be even more evident when the CPV system is slightly off-axis, since they are often designed to assure good uniformity only at normal incidence. The non-uniformities both in absolute irradiance and spectral content produced by the CPV systems, can originate electrical losses in multi-junction solar cells (MJSC). This works is focused on the integration of ray-tracing methods for simulating the irradiance and spectrum maps produced by different optic systems throughout the solar cell surface, with a 3D fully distributed circuit model which simulates the electrical behavior of a state-of-the-art triple-junction solar cell under the different light distributions obtained with ray-tracing. In this study four different CPV system (SILO, XTP, RTP, and FK) comprising Fresnel lenses concentrating sunlight onto the same solar cell are modeled when working on-axis and 0.6 degrees off-axis. In this study the impact of non-uniformities on a CPV system behavior is revealed. The FK outperforms other Fresnel-based CPV systems in both on-axis and off-axis conditions.

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.

Low-cost irradiance sensors for irradiation assessments inside tree canopies.

The solar irradiation that a crop receives is directly related to the physical and biological processes that affect the crop. However, the assessment of solar irradiation poses certain problems when it must be measured through fruit inside the canopy of a tree. In such cases, it is necessary to check many test points, which usually requires an expensive data acquisition system. The use of conventional irradiance sensors increases the cost of the experiment, making them unsuitable. Nevertheless, it is still possible to perform a precise irradiance test with a reduced price by using low-cost sensors based on the photovoltaic effect. The aim of this work is to develop a low-cost sensor that permits the measurement of the irradiance inside the tree canopy. Two different technologies of solar cells were analyzed for their use in the measurement of solar irradiation levels inside tree canopies. Two data acquisition system setups were also tested and compared. Experiments were performed in Ademuz (Valencia, Spain) in September 2011 and September 2012 to check the validity of low-cost sensors based on solar cells and their associated data acquisition systems. The observed difference between solar irradiation at high and low positions was of 18.5% ± 2.58% at a 95% confidence interval. Large differences were observed between the operations of the two tested sensors. In the case of a-Si cells based mini-modules, an effect of partial shadowing was detected due to the larger size of the devices, the use of individual c-Si cells is recommended over a-Si cells based mini-modules.

9-fold Fresnel-Köhler concentrator for increased uniform irradiance on high concentrations

Non-uniform irradiance patterns created by Concentrated Photovoltaics (CPV) concentrators over Multi-Junction Cells (MJC) can originate significant power losses, especially when there are different spectral irradiance distributions over the different MJC junctions. This fact has an increased importance considering the recent advances in 4 and 5 junction cells. The spectral irradiance distributions are especially affected with thermal effects on Silicone-on-Glass (SoG) CPV systems. This work presents a new CPV optical design, the 9-fold Fresnel Köhler concentrator, prepared to overcome these effects at high concentrations while maintaining a large acceptance angle, paving the way for a future generation of high efficiency CPV systems of 4 and 5 junction cells.

Development and experimental evaluation of a complete solar thermophotovoltaic system

We present a practical implementation of a solar thermophotovoltaic (TPV) system. The system presented in this paper comprises a sunlight concentrator system, a cylindrical cup-shaped absorber/emitter (made of tungsten coated with HfO2), and an hexagonal-shaped water-cooled TPV generator comprising 24 germanium TPV cells, which is surrounding the cylindrical absorber/emitter. This paper focuses on the development of shingled TPV cell arrays, the characterization of the sunlight concentrator system, the estimation of the temperature achieved by the cylindrical emitters operated under concentrated sunlight, and the evaluation of the full system performance under real outdoor irradiance conditions. From the system characterization, we have measured short-circuit current densities up to 0.95 A/cm2, electric power densities of 67 mW/cm2, and a global conversion efficiency of about 0.8%. To our knowledge, this is the first overall solar-to-electricity efficiency reported for a complete solar thermophotovoltaic system. The very low efficiency is mainly due to the overheating of the cells (up to 120 °C) and to the high optical concentrator losses, which prevent the achievement of the optimum emitter temperature. The loss analysis shows that by improving both aspects, efficiencies above 5% could be achievable in the very short term and efficiencies above 10% could be achieved with further improvements.