Measurements Of Global Solar Irradiance Research Articles

Processing of global solar irradiance and ground-based infrared sky images for solar nowcasting and intra-hour forecasting applications

Shadows from moving clouds in the troposphere impact the energy generated by photovoltaic systems. intra-hour solar forecast can be used to regulate solar energy dispatch. This investigation develops a data processing method for cloud dynamic feature extraction from raw sky images and Global Solar Irradiance (GSI) measurements that can be integrated into solar forecasting algorithms to reduce the operational supervision of hardware. Sky images and GSI measurements are acquired from a low-cost long-wave infrared radiometric camera and a pyranometer. This sky imager is mounted on a solar tracker that maintains the Sun in the center of sky images throughout the day. Multiple processing methods are proposed here that take advantage of a hybrid approach to approximate the optimal parameters of physical models using computationally inexpensive machine learning models. A signal processing method removes cyclostationary biases in high-resolution clear sky index values found when detrending GSI measurements using the clear sky GSI. Image processing methods are then used to remove the effects of atmospheric radiation and the Sun’s direct radiation from infrared sky images, plus the radiation effect emitted by debris on the sky imager’s germanium outdoor lens. The result is an adaptive solar forecasting algorithm that can reduce the operational cost of power grids with the high participation of solar energy in the generation mix.

Minute-Scale Models for the Diffuse Fraction of Global Solar Radiation Balanced between Accuracy and Accessibility

The separation models are tools used in solar engineering to estimate direct normal (DNI) and diffuse horizontal (DHI) solar irradiances from measurements of global solar irradiance (GHI). This paper proposes two empirical separation models that stand out owing to their simple mathematical formulation: a rational polynomial equation. Validation of the new models was carried out against data from 36 locations, covering the four major climatic zones. Five current top minute-scale separation models were considered references. The tests were performed on the final products of the estimation: DNI and DHI. The first model (M1) operates with eight predictors (evaluated from GHI post-processed measurements and clear-sky counterpart estimates) and constantly outperforms the already established models. The second model (M2) operates with three predictors based only on GHI measurements, which gives it a high degree of accessibility. Based on a statistical linear ranking method according to the models’ performance at every station, M1 leads the hierarchy, ranking first in both DNI and DHI estimation. The high accessibility of the M2 does not compromise accuracy; it is proving to be a real competitor in the race with the best-performing current models.

Characterisation and Field Test of a Simple AvaSpec Array Spectroradiometer for Solar Irradiance Measurements at an Alpine Site

Array spectroradiometers are increasingly being used to measure natural and artificial optical radiation because of their many advantages compared to traditional instruments. This study aims to thoroughly characterise a commercially available, cost-effective array device (AvaSpec ULS2048-LTEC) and compare its measurements of global solar irradiance in the 290–1100 nm wavelength range with those collected during three short-term field campaigns from more advanced, or specifically designed instruments. Moreover, the AvaSpec observations were compared with the output of a radiative transfer model. The results show that, despite its conceptually simple instrumental design, the AvaSpec can provide measurements of nearly comparable quality to those from reference instruments (e.g., UV index and global horizontal irradiance generally within ±5%) if all corrections are performed. These preliminary observations will be the basis of a long-term series at the Aosta–Saint-Christophe observatory, which can be employed to study solar energy production, biological effects and atmospheric composition changes in the Alpine environment. All procedures, including the encountered issues and proposed solutions, are described in detail.

Intra-day Variability Quantification from Ground-based Measurements of Global Solar Irradiance

Worldwide electricity production has switched from fossil fuel combustion to renewable energy sources and solar power generation has increased significantly in recent years, particularly in the form of photovoltaic (PV) power. Solar radiation is mainly influenced by the process of sunlight interaction with clouds. Being able to quantify the variability of solar irradiance due to clouds is crucial for better integration of the energy generated in the power grid by reducing the uncertainties in solar irradiance forecasts. Our objective is the characterization of a given solar site by quantifying the variability of solar irradiance caused by clouds. To do so, we provide a classification scheme of clouds conditions classes based on the daily clear sky index K c , and the hourly intraday variability I´(K c ) defined by the standard deviation of the variations of the clear sky index. As a result of irradiance classification, we obtained nine classes identified as the overcast, mixed, and clear sky conditions and subdivided into three categories: low, medium, and high variability. We used the solar irradiance data set measured at the high precision meteorological station installed in Benguerir, Morocco, for the period from 01/01/2018 to 31/12/2018.

Tilt to horizontal global solar irradiance conversion: An evaluation at high tilt angles and different orientations

Many transposition models have been proposed in the literature to convert solar irradiance on the horizontal plane to that on a tilted plane. The inverse process, i.e., the conversion from tilted to horizontal is investigated here based on six months of in-plane global solar irradiance measurements recorded on the roof of the Royal Meteorological Institute of Belgium's radiation tower in Uccle (Latitude 50.79°N, Longitude 4.35°E). Up to three pyranometers mounted on inclined planes of different tilts and orientations were involved in the inverse transposition process. Our results indicate that (1) the tilt to horizontal irradiance conversion is improved by using a multi-pyranometers approach and (2) the improvement from using an isotropic model to anisotropic models in the inverse transposition problem is not significant.

Evaluation of various procedures transposing global tilted irradiance to horizontal surface irradiance

Abstract. Many transposition models have been proposed in the literature to convert solar irradiance on the horizontal plane to that on a tilted plane. The inverse process, i.e. the conversion from tilted to horizontal is investigated here based upon seven months of in-plane global solar irradiance measurements recorded on the roof of the Royal Meteorological Institute of Belgium's radiation tower in Uccle (Longitude 4.35° E, Latitude 50.79° N). Up to three pyranometers mounted on inclined planes of different tilts and orientations were involved in the inverse transposition process. Our results indicate that (1) the tilt to horizontal irradiance conversion is improved when measurements from more than one tilted pyranometer are considered (i.e. by using a multi-pyranometer approach) and (2) the improvement from using an isotropic model to anisotropic models in the inverse transposition problem is not significant.

Spatio-Temporal Characteristics in the Clearness Index Derived from Global Solar Radiation Observations in Korea

The spatio-temporal characteristics of the clearness index (KT) were investigated using daily global solar irradiance measurements (290–2800 nm) for the period of 2000–2014 at 21 sites in Korea, a complex region in East Asia with a distinct monsoon season and heavy aerosol loading year-round. The annual mean KT value for all sites is 0.46, with values of 0.63 and 0.25 for clear and overcast skies, respectively. The seasonal variations in monthly average KT show a minimum of 0.37 in July at all sites except for Jeju, where the value was 0.29 in January. The maximum value (KT = 0.51) is observed in October, followed by a secondary peak (KT = 0.49) during February–April. The lowest KT value (KT = 0.42) was observed at both the Seoul and Jeju sites, and the highest (KT = 0.48) in the southeastern regions. Increases in average KT exceeding 4% per decade were observed in the middle and southeastern regions, with the maximum (+8% per decade) at the Daegu site. Decreasing trends (<−4% per decade) were observed in the southwestern regions, with the maximum (−7% per decade) at the Mokpo site. Cloud amount, relative humidity, and aerosol optical depth together explained 57% of the variance in daily mean KT values. The contributions of these three variables to variations in KT are 42%, 9% and 6%, respectively. Thus, the variations in KT in Korea can be primarily attributed to the presence of clouds and water vapor, with relatively weak aerosol effects.

The influence of sky conditions on the standardized calibration of pyranometers and on the measurement of global solar irradiation

Global solar irradiance measurement is used for applications such as verifying satellite measurements, studying the distribution and variation of the radiation received or estimating the energy yield of solar plants. This measurement is directly influenced by the reference calibration coefficient and uncertainty of the field pyranometer used. The radiometer calibration laboratory of CENER is accredited by ENAC (Spanish National Accreditation Body) since 2010 according to ISO 9847:1992 for the calibration of field pyranometers via comparison to a reference pyranometer. The accreditation’s scope covers calibrations “type Ia” (outdoor horizontal calibrations for meteorological and resource measurements, part 5.2.2, annex B.2). For the outdoor calibration, the methodology takes three different sky conditions into account: stable cloudless sky conditions, unstable sky conditions with some clouds or cloudy sky conditions. The classification criteria described by the standard have been enhanced for an easier application, establishing additional radiometric conditions, especially for unstable sky conditions with some clouds. Detailed criteria are based on CENER’s experience in solar collector testing and on the experimental CENER BSRN station (Baseline Surface Radiation Network) measurements and total sky camera images. In this study, several calibration coefficient results have been analyzed in order to validate the three different methodologies regarding the sky conditions mentioned before. The calibration coefficient can be slightly different depending on the sky conditions and the calibration methodology; therefore global solar irradiance measurements can differ too. The influence of this dependency has been evaluated in order to estimate the monthly and yearly global solar irradiation.

Cloud cover and UV index estimates in Chile from satellite-derived and ground-based data

Data of Lambertian equivalent reflectivity (LER) in ultraviolet (UV)-A range recorded by Total Ozone Mapping Spectrometer (TOMS) series aboard Nimbus 7 and Earth Probe and by Ozone Monitoring Instrument (OMI) on EOS Aura have been analyzed over eight Chilean locations spanning from about 18° to 62° S (i.e. including Profesor Julio Escudero station, Antarctic peninsula), covering years 1978–2011. Generally the distribution of the reflectivity is similar for both TOMS datasets. A slightly better agreement has been found for the most southern locations while a small discordance appears for northern locations. The latter could be partly due to actual differences in the cloud cover conditions. On the other hand, OMI LER data differ from TOMS ones in almost all locations. Daily cloud modification factor (CMF) values from ground-based global solar irradiance measurements have been compared with OMI LER-based CMF data. The northernmost and southernmost locations characterized by prevalent clear sky and winter snow conditions, respectively, showed the worse agreement with a correlation coefficient r=0.63 and 0.71, while other stations showed a better correlation (i.e. r=0.83 and r=85).Clear sky ground UV index values for Santiago de Chile have been estimated for years 1979–2011 by means of an empirical reconstruction model based on data recorded by a multichannel radiometer. It allowed computing a ground-based CMF for years 1996–2011 and comparing it with satellite data. Results show that OMI CMF based on gridded cell LER data introduces significant differences with respect to equivalent TOMS CMF. On the contrary, the use of overpass LER data allows to evaluate changes in cloudiness and, by using the model, reconstructing the actual UV index. Nevertheless, LER CMF overestimates actual cloud cover conditions in winter. The trend in reconstructed satellite (ground) based UV index during summer months is +3.3±0.9% (+11.9±2.5%)/decade for years 1979–2011 (1997–2011). Further comparisons concerning the Total Cloud Fraction product of Modern-Era Retrospective analysis for Research and Applications (MERRA) suggest that it could be used a further proxy of cloudiness for UV reconstruction models; nevertheless additional analysis is necessary.

Analysis of the aerosol radiative forcing over a Mediterranean urban coastal site

The aerosol radiative forcing for the period 2003–2011 has been analyzed in Burjassot, Spain, an urban coastal site on the western Mediterranean, using measurements of global solar irradiance and aerosol properties, as well as simulated data. The aerosol radiative forcing was calculated at 5minute intervals, and then integrated over 24h to obtain daily mean values for 200 complete days under cloud-free skies. The monthly mean values of the aerosol radiative forcing showed a clear variation during the year, associated to the annual variability of the aerosol optical depth at 500nm, ranging from −6 to −29Wm−2 at the surface, and from −1.5 to −3.9Wm−2 at the top of the atmosphere, with mean values for the whole period of −17±10Wm−2 and −2.2±1.3Wm−2, respectively. The aerosol radiative forcing efficiency was found to be −139±3Wm−2 at the surface and −19.2±0.3Wm−2 at the top of the atmosphere. These negative values, which correspond to cooling effects, suggest a significant absorption of solar radiation by the atmosphere, showing the important role of atmospheric aerosols in this urban site of the Mediterranean.

Extreme total solar irradiance due to cloud enhancement at sea level of the NE Atlantic coast of Brazil

Extraterrestrial total solar irradiance, usually called Solar Constant, is attenuated by the atmosphere in different proportions, depending mainly on solar zenith angle and altitude of the measurement point. In this work, it is presented very high and extreme horizontal plane measurements of global solar irradiance that in some days overpassed the Solar Constant corrected by the actual Sun–Earth distance (CSC). They were obtained at sea level of the intertropical Atlantic coast, in the city of Recife, Brazil, in the period February 2008–January 2009. Extreme total solar irradiance values larger than CSC were measured during 3.4% of the days of the total registered period. This percentage increases to 7.4% for global solar irradiance within 95.1–100% of the CSC and to 15.3% within 90.1–95% of the CSC. The largest extreme total solar irradiance value, 1477 ± 30 W/m 2, was registered the 28th of March 2008 at 11:34 local time (UT – 3h). It overpassed by 7.9% the CSC value for this day (1369.4 W/m 2) and by 42.3% the estimated value of the clear sky Iqbal C radiation model (1037.7 W/m 2). The observation of extreme values should be taken into account in the study of solar radiation effects related to materials exposed to the outside, UV index and biological effects, among others. Also, the detailed knowledge of this interesting effect may contribute significantly to clarify physical aspects about the interaction of global solar radiation with the ecosystem and climate change.

Is there evidence for an aerosol indirect effect during the recent aerosol optical depth decline in Europe?

Aerosol indirect effects are some of the largest uncertainties of anthropogenic climate change. To estimate the first aerosol indirect radiative effect (or cloud albedo effect), we analyzed global solar irradiance measurements under completely overcast skies during the recent period of aerosol optical depth decline in Europe. Although measurements from 15 Swiss and 8 northern German sites show clear evidence for an aerosol direct radiative effect under cloud‐free skies, trends of transmitted solar irradiance (SWtran) under overcast skies are ambiguous. Time series from 1981 to 2005 of SWtran for all overcast conditions show slightly negative, but nonsignificant trends. SWtran under overcast conditions with “thick” clouds (SWtran smaller than the long‐term mean) exhibit on average an increasing trend of +0.29 [+0.01 to +0.57] W m−2/decade. The increase of SWtran under “thick” overcast skies, however, is about nine times smaller than the increase under cloud‐free skies. Since cirrus clouds are generally excluded from and low‐level stratiform clouds are more frequently represented by “thick” overcast skies, the slight increase in SWtran may possibly result from a weak aerosol indirect effect. Alternatively, the increase in SWtran may be due to a decreasing trend in low‐level stratiform cloud amount under overcast conditions observed for these sites. We further find that solar irradiance changes caused by decreasing aerosol direct effect and increasing sunshine duration can account for most of the observed increasing all‐sky solar radiation trend. This suggests that the first aerosol indirect effect makes little contribution to surface solar radiation changes over Europe.

Detection of cloud-free skies based on sunshine duration and on the variation of global solar irradiance

Automatic detection of cloud-free skies became more and more important in recent years to evaluate and interpret surface radiation measurements in the context of the greatly debated solar dimming and brightening and to investigate possible changes of the greenhouse effect using longwave downward radiation measurements. To automatically separate cloud-free from cloudy sky situations within the MeteoSwiss automatic meteorological network (ANETZ) a new method based on sunshine duration and hourly global solar irradiance measurements, referred to as sunshine duration method (SSD-M), is presented. Results obtained with this new method are evaluated with the approved automatic partial cloud amount detection algorithm (APCADA). SSD-M identifies at the six investigated radiation sites between 88 and 98 % of cloud-free cases detected by APCADA. The probability of false detection of SSD-M does not exceed 7 % at any of the sites. The new method presents the advantage to use standard solar radiation parameters that are usually measured at advanced meteorological stations. The use of solar measurements restricts the method to detect daytime cloud-free situations only.

Improved entrance optic for global irradiance measurements with a Brewer spectrophotometer

A new entrance optic for a Brewer spectrophotometer has been designed and tested both in the laboratory and during solar measurements. The integrated cosine response deviates by 2.4% from the ideal, with an uncertainty of +/- 1%. The systematic uncertainties of global solar irradiance measurements with this new entrance optic are considerably reduced compared with measurements with the traditional design. Simultaneous solar irradiance measurements between the Brewer spectrophotometer and a spectroradiometer equipped with a state-of-the-art shaped diffuser agreed to within +/- 2% during a five-day measurement period.

Comparison of Model Calculations with Spectral UV Measurements during the SUSPEN Campaign: The Effect of Aerosols

Spectral measurements of global solar irradiance, obtained under cloud-free conditions during the SUSPEN campaign (July 1997) in Thessaloniki, Greece, are compared with radiative transfer model calculations, showing an agreement to within 65% for wavelengths higher that 305 nm. The uncertainties in the modeled spectra were analyzed with respect to the aerosol-related model input parameters (single-scattering albedo and asymmetry factor), which were not derivable from measurements. A range of single-scattering albedo values was used to investigate its impact on surface UV irradiance through comparison of measurements with model calculations. It was found that a difference in the single-scattering albedo of 0.1 changes the model‐measurement ratio by 7%‐14%, depending on solar zenith angle. Finally, an attempt was made to relate the estimated values of singlescattering albedo to wind direction and relative humidity, which control the origin and type of the aerosols in the area.

Derivation of UV‐A irradiance from pyranometer measurements

Two statistical models have been developed from 6 years of simultaneous measurements of global radiation by pyranometers, UV‐A by a Brewer spectrophotometer and from total ozone, dew point temperature and snow cover data at Toronto. The models estimate instantaneous UV‐A irradiance at 324 nm from pyranometer data with an uncertainty as low as 3.5% (1σ) for summer sunny conditions and between 6–10% for cloudy conditions. These uncertainties are reasonably small when compared with the uncertainty of UV‐A and global solar irradiance measurements (2–3%). The uncertainty is reduced when daily and longer‐term averages are considered. The major source of error in the models is likely linked to rare occurrences of absorbing aerosols in the atmosphere. The models were also tested on a 6‐year, independent record from Edmonton. The uncertainties at Edmonton are 30–45% larger than at Toronto for the instantaneous data, approximately 20% larger for daily integrated values.

Solar radiation over Egypt: Comparison of predicted and measured meteorological data

Measurements of global solar irradiance on a horizontal surface at five meteorological stations in Egypt for three years 1987, 1988, and 1989 are compared with their corresponding values computed by two independent methods. The first method is based on the Angstrom formula, which correlates relative solar irradiance H H 0 to corresponding relative duration of bright sunshine n N . Regional regression coefficients are obtained and used for prediction of global solar irradiance. Good agreement with measurements is obtained. In the second method an empirical relation, in which sunshine duration and the noon altitude of the sun as inputs together with appropriate choice of zone parameters, is employed. This gives good agreement with the measurements. Comparison shows that the first method gives better fitting with the experimental data.

Estimation of solar radiation over Jordan—predicted tables

As measured solar radiation data for all parts of Jordan are not available, they have to be estimated using correlation relations and models. This paper presents, for the first time, values of solar radiation over Jordan as estimated from these relations. Measurements of global solar irradiance on a horizontal surface and sunshine duration at nine meteorological stations in Jordan are correlated and used for prediction of the regression coefficients of an Angstrom type correlation relation at these stations and others which only have records of sunshine duration. Regional regression coefficients are obtained and used for prediction of global solar irradiance. The agreement with measurements is better than 5% and 1% on monthly and yearly basis respectively. Estimation of diffuse solar irradiance by Page's and also Liu and Jordan's correlations, as well as the direct beam component are also performed and the results are examined and presented. The abundance of solar energy in Jordan is evident from the daily average global solar irradiance which ranges between 5 and 7 kWh/m 2. A correlation of Angstrom type of the form: H/ Ho = 0.448+0.203 S/ So is found suitable for Jordan with correlation coefficient r = 0.93.

Uncertainty estimates for global solar irradiance measurements used to evaluate PV device performance

Abstract Broadband (0.3 – 3.0 μm) global solar irradiance measurements are used in the evaluation of solar energy conversion devices. The uncertainty attached to such measurements is important in evaluating whether conclusions associated with the measurements are statistically valid. A standardized uncertainty analysis method, developed over the past 15 years in the arena of consensus standards and professional society organizations, is described and applied. The results of the uncertainty analysis for the instrument calibration and field data measurement process indicate that the total measurement uncertainty in pyranometry ( i.e. the measurement of global solar irradiance) can approach 5%. Thus comparisons of results between laboratories using different pyranometers can have a total uncertainty of up to 10%. Statistically valid conclusions on a conversion device's performance may be drawn only if such results account for known bias errors or exceed the uncertainty limits derived using this methodology.

Global solar radiation in Pais Valenciano using sunshine hours

SYNOPSIS Measurements of global solar irradiance on a horizontal surface at 5 meteorological stations in Pais Valenciano are compared with predictions made by the Barbaro et al. model. This paper describes how this model has been modified in order to determine solar irradiance from sunshine hours and minimum air mass. An appropriate parameter is determined and used to predict solar irradiance at all stations with an accuracy better than 10 per cent.