Contribution to the diffuse radiation modelling in Évora, Portugal

Solar radiation data is crucial for the design of energy systems based on the solar resource. Since diffuse radiation measurements are not always available in the archive data series, either due to the inexistence of measuring equipment, shading device misplacement or missing data, models to generate these data are needed. In this work, one year of hourly and daily horizontal solar global and diffuse irradiation measurements in Évora are used to establish a new relation between the diffuse radiation and the clearness index. The proposed model includes a fitting parameter, which was adjusted through a simple optimization procedure to minimize the Least Square Error as compared to measurements. A comparison against several other fitting models presented in the literature was also carried out using the Root Mean Square Error as statistical indicator, and it was found that the present model is more accurate than the previous fitting models for the diffuse radiation data in Évora

Modifying a pyrheliometer to measure direct normal and circumsolar irradiance

Direct normal irradiance (DNI) is key to concentrating solar power (CSP) systems energy generation. However, pyrheliometric measurements of DNI also include a circumsolar normal irradiance (CSNI) component because the aperture angle of the instrument is larger than the sun disk. CSNI is due to the scattering of the sun rays by molecules, aerosols and some cloud types such as cirrus clouds [1]. As a result, energy is transferred from the direct solar beam to the circumsolar region, i.e., the region in the vicinity of the sun disk [2]. Typically, CSP systems have a lower aperture angle than that of pyrheliometers, which means that these systems are not able to absorb the same amount of CSNI as the amount that is present in the DNI measurements, leading to an inaccurate forecast of energy generation of CSP systems if DNI measurements are directly used in its sizing and operation. In this way, measuring CSNI for different aperture angles is important to better size, operate and optimize the energy generation of CSP systems. In this work, a conventional pyrheliometer field-of-view is modified through the variation of its collimator length and diameter in order to gather information on the circumsolar irradiance [3]. The modified pyrheliometer is composed by a revolver with four possible combinations of tube lengths and diameters and a stepper motor, which automatically enable varying the field-of-view of the pyrheliometer

Prediction of diffuse horizontal irradiance using a new climate zone model

Knowledge on the diffuse horizontal irradiance (DHI), and direct normal irradiance (DNI) is crucial for the estimation of the irradiance on tilted surfaces, which in turn is critical for photovoltaic (PV) applications and for designing and simulating concentrated solar power (CSP) plants. Since global horizontal irradiance (GHI) is the most commonly measured solar radiation variable, it is advantageous for establishing a suitable method that uses it to compute DHI and DNI. In this way, a new model for predicting the diffuse fraction (K_d) based on the climate zone is proposed, using only the clearness index (K_t) as the predictor and 1-min resolution GHI data. A review of the literature on models that use hourly and sub-hourly K_t values to compute K_d was also carried out, and an extensive performance assessment of both the proposed model and the models from the literature was conducted using ten statistical indicators and a global performance index (GPI). A set of model parameters was determined for each climate zone considered in this study (arid, high albedo, temperate and tropical) using 48 worldwide radiometric stations. It was found that the best overall performing model was the model proposed in this work

Comparison and assessment of circumsolar ratio models for Évora, Portugal

The sun rays are scattered due to molecules, aerosols and some cloud types, such as cirrus clouds, and a region near to the vicinity of the sun disk, known as circumsolar region, with relatively high diffuse sky radiance is formed [1]. Consequently, this diffuse radiation is called circumsolar normal irradiance (CSNI). Modelling the CSNI is important because pyrheliometers (the instruments that are used to measure direct normal irradiance – DNI), have an aperture angle wider than the circumsolar region, and thus DNI measurements obtained with these instruments cannot resolve the distribution of solar radiance in that region [2]. Moreover, since concentrated solar power (CSP) systems only capture solar radiation from this region but with a narrower aperture angle than that of the pyrheliometers, this leads to an overestimation of the energy reaching the receiver of the CSP systems [1]. This can result in misleading power output estimations, which can harm the bankability of CSP projects. Information on CSNI is also crucial when designing, sizing and operating CSP powerplants

Prediction of circumsolar irradiance and its impact on CSP systems under clear skies

In this work, a model to estimate circumsolar normal irradiance (CSNI) for several half-opening angles under clear skies was developed. This approach used a look-up table to determine the model parameters and estimate CSNI for half-opening angles between 0.5° and 5°. To develop and validate the proposed model, data from five locations worldwide were used. It was found that the proposed model performs better at the locations under study than the models available in the literature, with relative mean bias error ranging from −13.94% to 0.70%. The impact of CSNI for these different half-opening angles on concentrating solar power (CSP) systems was also studied. It was found that neglecting CSNI could lead to up to a 7% difference between the direct normal irradiance (DNI) measured by a field pyrheliometer and the DNI that is captured by CSP systems. Additionally, a case study for parabolic trough concentrators was performed as a way to estimate the impact of higher circumsolar ratios (CSR) on the decrease of the intercept factor for these systems. It was also concluded that if parabolic trough designers aim to reduce the impact of CSNI variation on the intercept factor, then parabolic troughs with higher rim angles are preferred

Tratamento de dados associados com a radiação solar fornecidos pelo Instituto Português do Mar e da Atmosfera, Madeira

Este relatório técnico tem como objetivo descrever o modo como os registos das amostras de dados, i.e., medições das grandezas meteorológicas fornecidas pelo Instituto Português do Mar e da Atmosfera (IPMA), ilha da Madeira, foram tratadas. Foram utilizados registos de oito estações meteorológicas distribuídas pela ilha. No relatório técnico estão descritos os processos de filtragem das medições de radiação solar global e radiação solar difusa, de temperatura máxima e mínima do ar, a correção da radiação solar difusa, o preenchimento de lacunas nas medições e a dimensão da amostra de dados utilizada em cada estação. Este relatório também descreve os métodos utilizados para preencher as lacunas nas séries de dados originais, bem como os métodos utilizados na construção do ano médio e do ano meteorológico típico. O ano meteorológico típico será posteriormente usado na simulação da produção de energia solar através de sistemas fotovoltaicos instalados na ilha da Madeira

Solar resource assessment through long-term statistical analysis and typical data generation with different time resolutions using GHI measurements

This work addresses the solar resource assessment through long-term statistical analysis and typical weather data generation with different time resolutions, using measurements of Global Horizontal Irradiation (GHI) and other relevant meteorological variables from eight ground-based weather stations covering the south and north coasts and the central mountains of Madeira Island, Portugal. Typical data are generated based on the selection and concatenation of hourly data considering three different time periods (month, five-day and typical days) through a modified Sandia method. This analysis was carried out by computing the Root Mean Square Difference (RMSD) and the Normalized RMSD (NRMSD) for each time slot of the typical years taking the long-term average as reference. It was found that the datasets generated with typical days present a lower value of overall NRMSD. A comparison between the hourly values of the generated typical data and the long-term averages was also carried out using various statistical indicators. To simplify this analysis, those statistical indicators were combined into a single Global Performance Index (GPI). It was found that datasets based on typical days have the highest value of GPI, followed by the datasets based on typical five-day periods and then those based on typical months

Assessment of PV Systems Performance in the Madeira Island Using Typical Meteorological Year Data

This paper focus on the development of an algorithm using Matlab to generate Typical Meteorological Years from weather data of eight locations in the Madeira Island and to predict the energy generation of photovoltaic systems based on solar cells modelling. Solar cells model includes the effect of ambient temperature and wind speed. The analysis of the PV system performance is carried out through the Weather Corrected Performance Ratio and the PV system yield for the entire island is estimated using spatial interpolation tools

Circumsolar irradiance modelling using libRadtran and AERONET data

The aperture angle of pyrheliometers is greater than the limit angle of the solar disk, which leads to a contribution of circumsolar radiation to the measurements of direct normal irradiance (DNI). On the other hand, since Concentrating Solar Power (CSP) technologies have, generally, a lower aperture angle with respect to pyrheliometers, it is important to determine the fraction of circumsolar irradiance present in the DNI measurements as well as its angular distribution to provide accurate information for sizing and operation of CSP systems. In this work, the monochromatic circumsolar irradiance (at 675 nm) is modelled using libRadtran and AERONET data at Évora, Portugal for selected cases representing the 1st, 25th, 50th and 75th percentiles of the aerosol optical depth (AOD) measurements distribution at 675 nm and a maximum value of AOD registered during a Saharan dust outbreak in February 2017 in Portugal. The AERONET products used were the columnar values of water vapor content, aerosol optical depth (AOD) and aerosol phase function. Since there are no measurements of circumsolar irradiance at Évora, the assessment of the model output accuracy was done through a comparison of both the modelled DNI from the solar disk only and the DNI including the circumsolar contribution against measurements from a pyrheliometer included in a meteorological station installed at the same location. The mean bias error and the root mean square error were found to be lower for the DNI with the circumsolar contribution, thus validating the modelling approach of circumsolar irradiance

A 3-Year Sample of Almost 1,600 Elves Recorded Above South America by the Pierre Auger Cosmic-Ray Observatory

©2020. The Authors. Elves are a class of transient luminous events, with a radial extent typically greater than 250 km, that occur in the lower ionosphere above strong electrical storms. We report the observation of 1,598 elves, from 2014 to 2016, recorded with unprecedented time resolution (100 ns) using the fluorescence detector (FD) of the Pierre Auger Cosmic-Ray Observatory. The Auger Observatory is located in the Mendoza province of Argentina with a viewing footprint for elve observations of 3.106 km2, reaching areas above the Pacific and Atlantic Oceans, as well as the Córdoba region, which is known for severe convective thunderstorms. Primarily designed for ultrahigh energy cosmic-ray observations, the Auger FD turns out to be very sensitive to the ultraviolet emission in elves. The detector features modified Schmidt optics with large apertures resulting in a field of view that spans the horizon, and year-round operation on dark nights with low moonlight background, when the local weather is favorable. The measured light profiles of 18% of the elve events have more than one peak, compatible with intracloud activity. Within the 3-year sample, 72% of the elves correlate with the far-field radiation measurements of the World Wide Lightning Location Network. The Auger Observatory plans to continue operations until at least 2025, including elve observations and analysis. To the best of our knowledge, this observatory is the only facility on Earth that measures elves with year-round operation and full horizon coverage