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An algorithm for mapping Direct Normal Irradiance (DNI) in thermosolar power plants using a Mobile Robotic Sensor Network (RSN) is presented. The algorithm selects measurement spots and allocates the RSN accordingly to carry out the dynamic estimation of DNI. A generic thermosolar power plant with a fleet of vehicles is used as a simulated case study to assess the performance of the algorithm. The results show that the proposed method allows us to obtain a spatial estimation of the DNI that could improve the performance of the plant, and that outperforms estimations based on a single pyrheliometer.

This paper recounts an historical record of the origins of bifacial PV modelling in the years between 1983 and 1989. Associated to the first industrial production of bifacial PV modules, that took place in 1984, PV modelling was developed by completing the three stages of an engineering modelling path: model development, experimental validation and application in practical designs. This took place over two distinguishable stages. In the first stage, the mathematical foundations, based on the “view factor” concept and an application procedure, based on calculating the so-called albedo and sky maps were established and applied to the PV generators with only a ground reflector surface. In the second stage, an additional mathematical twist lead to solving the problem of multiple reflector surfaces, and different practical configurations were analyzed. As PV modules were expensive at that time, it was economically sensible to consider PV arrays made up of rows of modules with a module-to-module separation between the modules of the same row. That led to the development of 3D models. Experimental values of rear irradiance were compared with modeled ones, using the global horizontal irradiance and direct irradiance values given by a pyranometer and a pyrheliometer respectively as input. The theoretical ratio between the daily front and rear irradiations was within 3% of the determined experimental value. This is the same as the accuracy in standard tilted surface irradiation predictions.

The ASoRA method consists in estimating the solar resource in tower concentrated solar plants by exploiting global data sets. The solar resource parameters are not only the Direct Normal Irradiance (DNI) but also the slant path transmittance (Tsp). The column to surface-level conversion process was validated by comparing the retrieved aerosol extinction coefficient (AEC) to independent measurements performed by a ground-based nephelometer. The Aerosol Robotic NETwork (AERONET) provided column-integrated aerosol extinction properties. The European Centre for Medium-Range Weather Forecasts Reanalysis Interim boundary layer height was used to reproduce the seasonal dependence of the aerosol vertical profile. At the daily resolution, ASoRA reproduced observed AEC with a relative root mean square difference (rRMSD) of 33% and a correlation coefficient of 0.90. The spectral extrapolation from monochromatic measurements to the solar broadband was validated by comparing computations and observations of DNI. ASoRA was also able to reproduce the hourly variability of DNI observed by a pyrheliometer with a RMSD of 38 W/m2, a rRMSD of 4.6%, and a correlation coefficient larger than 0.98. Despite larger uncertainties caused by using Modern-Era Retrospective analysis for Research and Applications (version 2) instead of AERONET, the annual average of Tsp was underestimated by only 0.01, and the annual average of the irradiance loss in the slant path by only 4 W/m2. Largest impact on Tsp comes from the parameterisation of the aerosol vertical profile, and the annual average of Tsp was 0.945 at Ouarzazate (Morocco) in 2014, with an estimated uncertainty of 0.030.

A method of the scattering Phase Function Scaling (PFS) for determination of the direct normal irradiance (DNI) involving the circumsolar normal irradiance (CSNI) is developed. A fraction of the normalized phase function integrated over the field of view for a pyrheliometer is introduced in the radiative transfer equation so the scaled optical depth using this phase function is derived. The PFS method is tested in a radiative transfer scheme and a simple fast parametric scheme SUNFLUX using the observational datasets at 14 Baseline Surface Radiation Network (BSRN) stations. The evaluation results show that the modelled DNI in terms of the PFS method in the radiative transfer scheme is significantly improved compared with that using the original delta-Eddington scaling method and compared well with that determined using other two radiative transfer models of DISORT and SMARTS. The modelled DNI with the PFS method in the SUNFLUX scheme is also improved compared with that without using the PFS method and compared well with that from the SMARTS model. The CSNI is also evaluated against the results from the other three models. The results are generally poor in all cases. The poor results are mainly due to the small magnitude of CSNI and uncertainties in the input data. Uncertainty in the aerosol scattering phase function could be the main reason responsible for the poor comparisons.

A precision solar spectroradiometer (PSR) has been designed for high precision and accurate measurements of direct normal spectral solar irradiance in the wavelength range 300–1000 nm. Each PSR is radiometrically characterised and calibrated in the optical laboratory of PMOD/WRC. The calibration is traceable to SI through the primary spectral irradiance standard of the Physikalisch-Technische Bundesansalt (PTB), in Braunschweig, Germany. The expanded relative uncertainty for spectral solar irradiance measurements in the 350–1000 nm range is between 1.7% and 2.0%, with larger uncertainties below 400 nm which are mainly caused by the low signal levels and the uncertainties due to the straylight correction. A solar extraterrestrial spectrum was retrieved from solar measurements using the Langley-technique at a high altitude observatory and was found to be in excellent agreement with a literature solar spectrum (mean ratio 0.998 ± 0.019 ). On two clear sky days solar measurements from 3 PSR and one independently calibrated spectroradiometer deployed during the International Pyrheliometer Comparison agreed to within their combined expanded uncertainties of 3%. Aerosol optical depth retrieved from the same solar measurements of the three PSR showed median differences with respect to the World reference for AOD between −0.010 and +0.014 at the center wavelengths 368 nm, 412 nm, 500 nm, and 862 nm. The instrument has a second auxiliary port for connecting different entrance optics for near coincident measurements such as global solar irradiance.

The behavior of the surface solar radiation (SSR) during the first half of 20th century is an open issue due to the scarcity of SSR data during this period. This paper focuses on the re-evaluation of the pyrheliometer measurements in Madrid between September 1910 and January 1929. These records were formerly analyzed by Anton et al. (2014), reporting no statistically significant trends in the atmospheric column transparency. In the present work, metadata regarding the state of the solar disk during each pyrheliometer record have been used in order to remove measurements affected by thin clouds. The re-analysis of the atmospheric column transparency from the pyrheliometer data showed two periods with opposite sign in the trends of this variable: a decrease of (−2.9 ± 0.2)% per decade between 1910 and 1925, followed by a strong increase of (+15.0 ± 2.5)% per decade up to 1929. Both trends were statistically significant at the 95% confidence level. These results suggest a decrease of direct SSR in Madrid during the second decade of the 20th century, in agreement with the final stage of the “early dimming” which was observed at several European sites from the end of the 19th century. Additionally, the marked positive trend of the atmospheric transparency found in this work in the second half of the 1920s is in accordance with the beginning of the SSR recovery in Europe which was observed up to late 1940s (“early brightening”). Therefore, from the results of this study, the use of metadata is strongly required for an accurate assessment of the trends in early pyrheliometer measurements.

International audience; This work proposes a new empirical direct methodology to estimate both the solar flux distribution and intensity on the surface of central receivers. In solar power tower plants with deteriorated heliostats, the numerical simulations to estimate the incident solar flux are not precise. Hence the thermal behaviour of the receivers cannot be determined. In those cases, direct measurement or semi-empirical methodologies are required to characterize the radiant power on the receiver. The new methodology proposed, named "Superposition method", consists in the hourly characterization of the reflected solar beam of each individual heliostat by means of a pyrheliometer, a passive screen, a flux sensor, a camera and digital image analysis. According to the aiming strategy used during receiver operation, each individual solar flux distribution and intensity can be gathered to obtain the total incident radiant power on the solar receiver. This non-real-time method has the advantage of reproducing any solar flux distribution on the receiver at present and past time.

In this work, eighteen clear sky broadband models for estimating the direct irradiance at normal incidence (DNI) have been investigated. The used solar data has been recorded from two sites of different weather conditions; Bouzareah site (Algiers), located on Mediterranean coast and Ghardaia site, about 600 km, southern of Algiers in the desert region. Three input parameters were measured by two weather devices, namely; hygrometer and manometer systems for calculating the temperature, relative humidity and local pressure. In addition, two Pyrheliometer devices were fixed on the sun-tracker system for measuring the DNI. Moreover, three statistical indicators were used, in the goal to make comparison between the estimated and measured DNI results. Accordingly, the relative bias (rb), the relative root mean square error (rRMSE), the coefficient of determination (r2), and the relative error (re) have been calculated. Results obtained from both sites, show that the rb and rRMSE of MAC, Yang, Dogniaux, and ESRA models are typically smaller than 2%. in other hand, a further consideration has been taken to select the best model by evaluating the relative error between the measured and estimated DNI versus the solar elevation angle. The results averred that the ESRA, Dogniaux, MAC and Yang models (in decreasing of performance) performed better than the remaining models, with the rRMSE of 0.443, 1.066, 1.237 and 1.283 for Algiers, and 0.390, 1.100, 1.223 and 1.443 for Ghardaia. This fact is important to take into account, in the case where measured data are not available, for the selection of the most suitable locations for the installation of the future concentrating solar power plants in Algeria.

A comprehensive uncertainty analysis validates a Solar Spectral Irradiance Meter (SolarSIM) for accurately resolving the spectral and broadband direct normal irradiances (DNI), spectral aerosol optical depth (AOD), precipitable water vapour and atmospheric total column ozone amounts. The derivation of these parameters from four SolarSIMs were compared to reference instrumentation at the Physikalisch-Meteorologisches Observatorium Davos and World Radiation Center (PMOD/WRC) in Davos, Switzerland in September 2015. The SolarSIMs are the first instruments to ever simultaneously participate in the 12th WMO International Pyrheliometer Comparison, Fourth Filter Radiometer Comparison, and First Spectroradiometer Comparison. The SolarSIMs’ DNI data were compared to the World Standard Group’s PMO2 absolute cavity radiometer, with World Radiometric Reference factors ranging from 0.999674 to 0.994610 for the best and the worst performing devices, respectively. In addition, the SolarSIMs’ spectral DNI data was compared against PMOD’s Precision Spectral Radiometer. The mean difference of the spectral DNI was found to be less than 5% for wavelengths above 400 nm. The SolarSIMs’ measurements of AOD data were compared against PMOD’s Precision Filter Radiometer triad. The median AOD differences and their standard deviations were found to be 0.0046 ± 0.0044, 0.0016 ± 0.0034, 0.0018 ± 0.0026, and 0.0041 ± 0.0022 for 368 nm, 412 nm, 500 nm, and 865 nm, respectively. The SolarSIMs’ measurements of precipitable water vapour were compared against PMOD’s Cimel CE318 sun photometer. The median difference and the corresponding standard deviation averaged 1 ± 0.2 mm for all SolarSIMs. Furthermore, the SolarSIMs’ measurements of total column atmospheric ozone were compared against PMOD’s Brewer MkIII spectrophotometer. The median difference and the corresponding standard deviation averaged 6 ± 7 DU for all SolarSIMs.

This study presents the actual field results from a calibration procedure of reference solar radiometers using an Absolute Cavity Radiometers (ACR), which is directly traceable to the World Radiometric Reference (WRR) scale maintained by the World Radiation Center (WRC). The pyranometer and pyrheliometer traceability is established using Eppley’s AHF-AWX ACR as per the prescribed standards. The Eppley solar radiometers were exposed to solar irradiance for 3.5 years and the Hukseflux solar radiometers were exposed to solar irradiance for 1.5 years. The deviation of the new sensitivity values from the old sensitivity values is reported to be 5.42% (pyranometer, Eppley PSP), −0.21% (pyranometer, Hukseflux SR25), 0.23% (pyrheliometer, Eppley NIP) and 0.4% (pyrheliometer, Hukseflux DR03). The uncertainty values are found to be ±0.04 µV/Wm−2, ±0.05 µV/Wm−2, ±0.03 µV/Wm−2 and ±0.03 µV/Wm−2 for Eppely PSP, Hukseflux SR25, Eppley NIP and Hukseflux DR03 solar radiometers, respectively.

Circumsolar irradiance is the part of diffuse or scattered irradiance within a small angle (2.5° for typical pyrheliometers) around the sun which is measured, together with direct normal irradiance, by pyrheliometers and reported as part of the direct normal irradiance. This could create an inconsistency as much as 300 W/m2 between radiative transfer modelling and pyrheliometer measurements, having further ramifications for the validation of surface solar irradiance data derived from various sources such as sky imaging camera, satellite and numerical weather prediction models. In addition, accurate modelling of the angular pattern of solar irradiance within a very small angle from the centre of the sun is important for solar energy generation using concentrating (thermal or photovoltaic) technologies. This article presents an analytical model for the calculation of circumsolar radiation, based on the ‘small angle approximation’. Validation against full radiative transfer calculation shows that the approximate method is accurate for common pyrheliometers used operationally. The error is less than 0.5 W/m2 for opening half angle of 2.5°, and less than 2.0 W/m2 for opening half angle of 5°, for typical cloud and aerosol types and the full range of optical depths. This small angle approximation method is relatively quick to calculate (when compared to full radiative transfer modelling calculations times) and can underpin enhanced modelling of solar radiation for satellite validation, solar energy generation and numerous other applications across the globe.

Made available in DSpace on 2018-12-11T17:04:36Z (GMT). No. of bitstreams: 0 Previous issue date: 2016-12-01 The present paper deals with atmospheric corrections factors proposed as a function of the atmospheric transmissivity in order to correct the diffuse solar irradiance measured with the Melo-Escobedo-Oliveira Shadowring Measuring Method (MEO shadowring Method). Global irradiance was measured by an Eppley-PSP pyranometer; direct normal irradiance by an Eppley-NIP pyrheliometer fitted to a ST-3 sun tracking device and diffuse irradiance by an Eppley-PSP pyranometer fitted to a MEO shadowring. The Solar Radiometric Laboratory at Sao Paulo State University provided the measurements during the years 1996–2005. Two correction models for diffuse solar irradiance were proposed: All Sky Correction Model (ASC Model) and Sky Cover Correction Model (SCC Model). The MBE and RMSE statistical indicators performed the validations. The correction models showed results in the same order of magnitude: ASC Model showed 0.81% deviation, while SCC Model showed 0.66% deviation. Therefore, the correction models proposed as a function of the sky covering (atmospheric transmissivity) were efficient to correct the isotropic diffuse irradiance, approaching the measured and reference diffuse irradiance less than 1%. Corrections show dependence on sky coverage and seasonality. The results presented that the sky cover corrections improve the MEO shadowring method, allowing the generation of a reliable global, direct and diffuse radiation database without high financial investments. College of Agricultural Sciences / FCA / UNESP Department of Bioprocess and Biotechnology College of Agricultural Sciences / FCA / UNESP Department of Rural Engineering Institute of Astronomy Geophysics and Atmospheric Sciences USP College of Agricultural Sciences / FCA / UNESP Department of Bioprocess and Biotechnology College of Agricultural Sciences / FCA / UNESP Department of Rural Engineering

Atmospheric turbidity is an important parameter in meteorology, climatology and for providing hindsight on particulate air pollution in local areas. In this work we exploit 1260 direct solar radiation measurements performed in Sfax (Center Tunisia), from March 2015 to February 2016. These measurements were made with a pyrheliometer only when clouds did not obstruct the solar disk. The atmospheric turbidity is quantified by the means of both the Linke's turbidity factor ( T LI ) and Angstrom's coefficient ( β ). Over the year, values of T LI and β are found to vary in the ranges 1–15 and 0–0.7, with the most probable values around 3.5 and 0.05, respectively. However, a marked seasonal pattern is observed for the two turbidity parameters. They achieve their maximum in the spring and summer months, their minimum in winter and autumn appears as a transitional period. The comparison of the results obtained in Sfax with those of three AERONET stations located in north (Carthage), central-north (Ben Salem), and south (Medenine) Tunisia, reveals that this seasonal pattern of the atmospheric turbidity is valid for all the Tunisian territory, and probably beyond. At shorter (hourly) time scales, the diurnal behavior of the turbidity in Sfax is different in the summer months from the one observed during the rest of the year. Indeed, an enhancement of T LI is observed during the day. This is assumedly attributed to the production of secondary aerosols by atmospheric photochemistry.

A solar spectral irradiance meter (SSIM), designed for measuring the direct normal irradiance (DNI) in six wavelength bands, has been combined with models to determine key atmospheric transmittances and the resulting spectral irradiance distribution of DNI under all sky conditions. The design principles of the SSIM, implementation of a parameterized transmittance model, and field performance comparisons of modeled solar spectra with reference radiometer measurements are presented. Two SSIMs were tested and calibrated at the National Renewable Energy Laboratory (NREL) against four spectroradiometers and an absolute cavity radiometer. The SSIMs’ DNI was on average within 1% of the DNI values reported by one of NREL’s primary absolute cavity radiometers. An additional SSIM was installed at the SUNLAB Outdoor Test Facility in September 2014, with ongoing collection of environmental and spectral data. The SSIM’s performance in Ottawa was compared against a commercial pyrheliometer and a spectroradiometer over an eight month study. The difference in integrated daily spectral irradiance between the SSIM and the ASD spectroradiometer was found to be less than 1%. The cumulative energy density collected by the SSIM over this duration agreed with that measured by an Eppley model NIP pyrheliometer to within 0.5%. No degradation was observed.

Accurate solar radiation measurements require properly installed and maintained radiometers with calibrations traceable to the World Radiometric Reference (WRR). This study analyzes the performance of 51 commercially available and prototype radiometers used for measuring global horizontal irradiances (GHI) or direct normal irradiances (DNI). These include pyranometers, pyrheliometers, rotating shadowband radiometers (RSR), 1 and a pyranometer with an internal shading mask deployed at the National Renewable Energy Laboratory’s (NREL) Solar Radiation Research Laboratory (SRRL). The radiometers in this study were deployed for one year (from April 1, 2011, through March 31, 2012) and their measurements were compared under clear sky, partly cloudy, and mostly cloudy conditions to reference values of low estimated measurement uncertainties. Mean Bias Difference (MBD) and Root Mean Square Difference (RMSD) statistics were used as metrics to compare the GHI and DNI values from individual instruments with concurrent measurements using the reference instruments over time intervals of one-minute, 10-min, and hourly averages. Deviations from the reference irradiance measurements were calculated as a percent and W/m2 of the reference value for solar zenith angles ranging from 17.5° to 85° (the range of available solar zenith angles throughout the year at SRRL, excluding data near sunrise and sunset). Under clear-sky conditions when the solar zenith angle was less than 60°, differences of less than ±5% were observed among all GHI and DNI measurements when compared to the reference radiometers. For GHI these normalized differences increased up to ±17% under mostly-cloudy and clear-sky conditions when the solar zenith angle was greater than 60°. The normalized differences were greater yet under mostly-cloudy conditions (approaching ±40%) for few DNI data sets at higher solar zenith angles. The intent of this paper is to present a general overview of each radiometer’s performance based on the instrumentation and environmental conditions available at NREL.

The Baseline Surface Radiation Network (BSRN) is a project of the Data and Assessments Panel from the Global Energy and Water Exchanges (GEWEX) under the umbrella of the World Climate Research Programme (WCRP). Currently in the archive there are data from 67 sites located in all seven continents spanning from 1992 to the present. The original BSRN records are at 1-, 2-, 3- or 5-minute intervals, and sophisticated quality-control procedures have been developed to eliminate erroneous records before hourly, daily and monthly means are computed. The resulting gaps from quality-control, however, give rise to uncertainties in computed temporal averages on various scales. There are two types of total shortwave downward fluxes: The Global 1 which is the sum of direct horizontal and diffuse irradiances, observed using the pyrheliometer and radiometer, respectively, and is recommended by the BSRN committee for its higher precision, and the Global 2 which is observed using the pyranometer. It has been found that, compared to Global 2, Global 1 is more susceptible to errors, thus resulting in more gaps in the original records. Here we examine the effect of quality-control on the monthly mean Global 1 and Global 2 and attempt to quantify the resulting uncertainties through analysis of the quality-control procedure and averaging algorithm and comparison with the NASA GEWEX Surface Radiation Budget (SRB) dataset. The SRB project is now making progress toward its Release 4.0 Integrated Products (Rel. 4.0-IP or V4.0-IP) with changes in both algorithm and inputs on the basis of its Release 3.0 (Rel. 3.0 or V3.0) products. The preliminary results from Rel. 4.0-IP span 34 years continuously from July 1983 to June 2017 on a 1° by 1° quasi-equal-area grid system in the form of 3-hourly, daily and monthly means. It is found that Global 2 generally has fewer missing records and is less affected by the quality-check procedure; on average, the monthly mean Global 1 and Global 2 differ by less than 1 W m−2; the GEWEX SRB preliminary GSW(V4.0-IP) monthly mean shortwave downward fluxes agree the best with the BSRN Global 1 monthly means computed from observational records that are nearly complete and that survive the quality-check nearly completely.

Some works have reported variations in direct solar irradiance at surface with solar activity one order of magnitude higher than changes in total solar irradiance at the top of the atmosphere (TSI) over the solar cycle. In the present study, daily pyrheliometer records from the Astronomical Observatory of Madrid for the period 1910–1929 have been recovered. Metadata have been used to remove measurements affected by clouds, local pollution or any other factor dimming solar radiation noticed by means of visual observations of the state of the sky near the solar disk. Furthermore, corrections for volcanic eruptions, outliers and seasonal variations have been applied. When long-term changes in atmospheric transparency are taken into account in the analysis, non-significant variations in direct solar irradiance at surface with Sunspot Number (of the same order of magnitude as the variations in TSI) are found. Otherwise, a spurious high anticorrelation is obtained. This indicates, firstly, the influence of solar activity on solar irradiance at surface is so weak that atmospheric factors mask it, and secondly, careful analyses in order to infer this relationship have to be carried out not to reach results leading to wrong interpretations.

Direct Normal Irradiance (DNI) is a critical component of solar irradiation for estimating Plane of Array (POA) irradiance on flat plate systems and for estimating photovoltaic and concentrating system power output. Current approaches to measuring or estimating DNI suffer from either high equipment costs or low precision and may require detailed environmental data. An alternative approach, using artificial neural networks to estimate DNI from the irradiance measurements of multiple pyranometers, is studied. We consider various neural network topologies and study the resulting errors. The neural network-based estimators are found to have higher accuracy than those obtained from empirical correlations of GHI measurements alone. Additionally, the use of a different GHI sensor than the one used to obtain the neural network training data does not induce significant errors. The ability of this method to be used as a quality-control instrument for pyrheliometer measurements is also discussed. We find that the proposed methodology is capable of detecting many instances of unreliable DNI measurements by considering the deviation between the predicted DNI and measured DNI. A more detailed analysis can be conducted by taking advantage of the data streams from the individual pyranometers.

Shortwave radiometers such as pyranometers, pyrheliometers, photovoltaic cells, and longwave radiometers such as pyrgeometers are calibrated with traceability to consensus References, which are maintained by Absolute Cavity Radiometers (ACRs) and the World InfraRed Standard Group (WISG), respectively. Since the ACR is an open cavity with no window, and was developed to measure the extended broadband spectrum of the terrestrial direct solar beam irradiance, then there would be discrepancy in calibrating the shortwave radiometers because of their limited spectral band. On the other hand, pyrgeometers are calibrated during the nighttime only, because no consensus reference has yet been established for the daytime longwave irradiance. This article describes a method to measure the broadband longwave irradiance in the terrestrial direct solar beam from 3 µm to 50 µm. The method might be used in developing calibration methods to address the mismatch between the broadband ACR and shortwave radiometers, and the lack of a daytime reference for pyrgeometer calibration. We used the described method to measure the irradiance from sunrise to sunset; the irradiance varied from approximately 1 W m −2 to 16 W m −2 with an estimated uncertainty of 1.46 W m −2 , for a solar zenith angle range from 80° to 16°, respectively.

Direct Normal Irradiance (DNI) is a fundamental gauge in the study of solar irradiation and the main parameter for CSP and CPV applications. However, it is not often measured due to practical reasons, and is derived instead from measurements of global irradiance or satellite-based models. DNI received at ground level depends on the extinction of solar radiation along the line to the sun, and in cloudless conditions, day to day DNI variations are mainly due to variations in aerosol loading in the atmosphere. In this paper, the use of a lidar-ceilometer is investigated for the estimation of DNI by establishing a relation between the daily variations of DNI and backscatter atmospheric extinction around solar noon, under cloud-free conditions. The experiment was carried out in Doha, Qatar for a period of 12 months, from December 2012 to November 2013. Atmospheric backscattter profiles were obtained with a Vaisala CL51 ceilometer, which is based on the elastic lidar technique, and DNI was measured at the same time and location with a Kipp and Zonen pyrheliometer. To obtain the correlation between both measurements, the hourly DNI clearness index, K n , is related to the hourly backscatter measurements integrated over a column of 5 km height. An exponential function was found to closely fit K n to backscatter coefficient; using the results of this fit, DNI is extracted with a relative bias of 1.979% and relative RMSE of 10.869%, statistical values comparable to those obtained from the derivation of DNI from satellite observations. Although elastic lidar-ceilometers operate at one wavelength, this paper shows the potential use of aerosols information as measured by lidar-ceilometer for solar radiation modeling.

Sunshine duration can be estimated using photovoltaic solar cells instead of conventional pyranometers or pyrheliometers, which are more expensive and therefore not suitable for low cost measurement applications in developing regions. A one-year meteorological dataset from Nicosia (Cyprus) including direct irradiance, global irradiance from a pyranometer and global irradiance from a reference PV cell was used to calculate sunshine duration following the WMO pyrheliometric method and three pyranometric methods by WMO Slob and Monna, Hinssen–Knap and Olivieri. Pyranometric algorithms were adapted to the tilted pyranometer and reference solar cell. Main results indicate that all the pyranometric algorithms underestimated sunshine duration over the span of a year in Cyprus in comparison with the reference pyrheliometric method; and that results between the pyranometer and the solar cell were comparable. The PV silicon solar cell is capable of measuring sunshine duration on a daily basis with an uncertainty similar to the obtained with a pyranometer when using the Olivieri algorithm.

DNI (direct normal irradiance) can be calculated from global and diffuse horizontal irradiance measurements. However, the diffuse irradiance values need to be corrected because the pyranometer's shadowband does not obstruct only the solar disk but also a larger part of the sky vault. In this study, we use four diffuse correction models (Drummond [1], LeBaron et al. [2], Batlles et al. [3], Muneer and Zhang [4]) and, considering the importance of DNI for several solar energy projects, we try to assess the performance of the models when the calculated DNI is compared to the measured one by the pyrheliometer. Based on 1-min averaged measurements covering a one-year period in Athens, Greece, it is concluded that the empirical approach of Batlles et al. performs best. It presents the lowest residuals (±1%) with no dependence from the clearness index. The estimated annual DNI, derived from the synergetic use of this model with measurements of horizontal global and diffuse irradiance, is lower by 0.4%, while the calculated values from the global and uncorrected diffuse irradiances are overestimated by 7.7%.

The objective of this work is to verify the attenuation caused by atmospheric constituents through the relationship between global, direct and diffuse solar radiation with respect to optical air mass. The optical air mass change has spatial and temporal dependence and influences the radiation flux incident, causing changes in the average values. Global, diffuse and direct solar irradiances were provided by the Laboratory of Solar Radiometry of Botucatu-UNESP (latitude 22.9° South, longitude 48.45° West, altitude 745 m). The period assigned for the study comprised the years 2002 to 2006. The global solar radiation was measured by an Eppley PSP pyranometer. The direct solar radiation was measured by an Eppley Nip pyrheliometer attached to a ST-3 solar tracking system. The diffuse solar radiation was calculated by the difference between global and direct solar radiations. It was observed a decrease of solar radiation with the increase of optical air mass justified by the increase in probability of collision of solar rays with atmospheric constituents. For global solar radiation and optical air mass mo = 1, it was observed a minimum at 3.1 MJ/m 2 and a maximum of 4.1 MJ/m 2 . For optical air mass mo=2, minimum at 1.5 MJ/m 2 and a maximum of 1.9 MJ/m 2 . For direct solar radiation and optical air mass mo = 1, it was observed a minimum at 2 MJ/m 2 and a maximum of 3.8 MJ/m 2 . For optical air mass mo=2, minimum at 1 MJ/m 2 and a maximum of 1.5 MJ/m 2 . For diffuse solar radiation and optical air mass mo = 1, it was observed a minimum at 0.1 MJ/m 2 and a maximum of 1.5 MJ/m 2 . For optical air mass mo=2, minimum at 0.1 MJ/m 2 and a maximum of 0.8 MJ/m 2 . The joint analysis of data from direct and diffuse solar radiation allowed better understand of the attenuation process caused by the atmosphere, establishing qualitative relationships between absorption, scattering and reflection processes.

A process to build modular solar reflectors by using plane mirrors and a sandwich structure has been developed. We applied this process to manufacture parabolic modules that can be assembled in a parabolic dish of 5 m in diameter with the focus at 2.5 m. This paper discusses the evaluation of the light collection performances of a single parabolic module. The reflectance of the mirror material was characterized by means of a UV–Vis spectrophotometer, in the 250–1200 nm wavelength range, and of a pyrheliometer, with respect to a direct solar spectrum. The illumination profile of a module mounted on a sun-tracking system was tested by power density measurements in and out of the focal plane, and spots were compared with a theoretical one. In order to evaluate the high-flux solar energy arriving at the focus of a module, a flat-plate calorimeter was built. The study was carried out by measuring the energy absorbed by the water flow and the external losses due to convection. Based on an energy balance, the intercept factor and the overall optical efficiency of the collector were estimated.

Direct solar radiation measured with a pyrheliometer installed at the meteorological station of Sidi Bou Said (Northern Tunisia) is used to investigate the atmospheric turbidity using Linke's factor. The obtained results indicate high variability of the diurnal and seasonal components of atmospheric turbidity, with a summer maximum and a winter minimum. The correlation between atmospheric turbidity and wind speed and direction shows an increase of atmospheric turbidity particularly due to the transport of particles of continental origin in the west sector. In the east sector, the correlation between atmospheric turbidity and specific humidity shows that the summer maximum is due to the heavy water vapor content of maritime air masses, carried by the north-eastern winds prevalent during this season combined with local continental particles. Correlation of atmospheric turbidity with diffuse radiation for maritime and continental air masses is derived.

Measurement of direct solar radiation has been shown to be very useful to improve control performance and disturbance rejection in solar fields by anticipating the effect of sudden changes in solar radiation due to clouds. Since direct solar radiation is measured locally by pyrheliometers, important errors in the estimation of the overall effective solar radiation can be produced when the pyrheliometer is covered by clouds while the rest of the solar field is not or viceversa. Furthermore, estimation of the overall efficiency affected by the reflectivity and absortance of metal tubes is very difficult because only local measurements can be obtained. This work proposes an algorithm for estimating overall solar radiation and efficiency at the field. The algorithm uses an unscented Kalman filter and it is validated by data obtained at the Plataforma Solar of Almeri´a (Spain).

Atmospheric turbidity is an important parameter for assessing the air pollution in local areas and controlling the attenuation of solar radiation reaching the Earth's surface under cloudless sky. An investigation of atmospheric turbidity has been undertaken in Kerkennah Island (in Tunisia). Turbidity indexes, namely, Linke factor (T L ) and Angstrom coefficient (β) were derived directly from measurements taken by pyrheliometer during a year: July (2008) to June (2009). Data obtained from these measurements in this area were compared with measurements performed in a coastal and urban region (Sidi Bou Said) located in the North of Tunisia. A comparison of the seasonal cycle of turbidity at both sites showed: *) maximum values of turbidity in the summer due to dust storms, vertical convection and breeze sea circulation; *) Values of turbidity decrease in the autumn season, due to the decreasing of days of Sirocco and to the increasing of amount of rainfall; *) low values of turbidity index in the winter, due to precipitation removal as well as relative humidity-impacted deposition; *) middle values of turbidity were observed in the spring season. Monthly, diurnal, hourly mean values and frequency of occurrence of the value of each turbidity index were used to characterize variations of atmospheric turbidity. The three-hourly standard meteorological data given by the Meteorological National Institute (Sfax) represent a valuable supplement to the data sets with the pyrheliometer measurements. The analysis of the results shows that there is a high correlation between atmospheric turbidity and local weather conditions, especially temperature and wind speed. The correlation of meteorological parameters (wind speed and temperature) with the turbidity coefficients was analyzed and discussed.

In order to investigate a potential use of concentrating solar power technologies and select an optimum site for these technologies, it is necessary to obtain information on the geographical distribution of direct normal solar irradiation over an area of interest. In this work, we have developed a method for estimating direct normal irradiation from satellite data for a tropical environment. The method starts with the estimation of global irradiation on a horizontal surface from MTSAT-1R satellite data and other ground-based ancillary data. Then a satellite-based diffuse fraction model was developed and used to estimate the diffuse component of the satellite-derived global irradiation. Based on this estimated global and diffuse irradiation and the solar radiation incident angle, the direct normal irradiation was finally calculated. To evaluate its performance, the method was used to estimate the monthly average hourly direct normal irradiation at seven pyrheliometer stations in Thailand. It was found that values of monthly average hourly direct normal irradiation from the measurements and those estimated from the proposed method are in reasonable agreement, with a root mean square difference of 16% and a mean bias of −1.6%, with respect to mean measured values. After the validation, this method was used to estimate the monthly average hourly direct normal irradiation over Thailand by using MTSAT-1R satellite data for the period from June 2005 to December 2008. Results from the calculation were displayed as hourly and yearly irradiation maps. These maps reveal that the direct normal irradiation in Thailand was strongly affected by the tropical monsoons and local topography of the country.

Atmospheric turbidity coefficients have been estimated in many locations, either on the basis of spectral irradiance or on broadband direct irradiance data. The Angstrom turbidity coefficient, β , linked to the absorption and diffusion of aerosols, is very important since many solar radiation models for clear days use this parameter. The Linke turbidity factor, T L , is of specific importance for the calculation of beam and diffuse solar radiation in a determined locality. Furthermore, an atmospheric turbidity index in a determined locality can be associated with the pollution level at that place. Direct solar radiation data, measured with a pyrheliometer in a coastal tourist location in Tunisia (Sidi Bou Said), are used to investigate the diurnal and monthly variations of the atmospheric turbidity using Linke's factor T L during 1999. Computed T L values vary between 2.5 and 8 with a frequency of about 53% greater than 5.0. Highest and lowest turbidity levels have been observed in August and December, respectively. Values of the turbidity coefficient β have been determined in Thala (a continental and nonindustrialized region in Tunisia), for 8 months in 2001, based on a direct fitting method of the Angstrom power low expression using sun-photometer data. Monthly averaged values of β coefficient vary between 0.15 and 0.33 with highest turbidity recorded in July and October and lowest in April. Measured meteorological parameters represent a valuable supplement to the data sets with the sun-photometer and the pyrheliometer measurements. The analysis of the results shows that there is a high correlation between atmospheric turbidity and local weather conditions, especially wind speed and wind direction.

Measurement and modeling of broadband and spectral terrestrial solar radiation is important for the evaluation and deployment of solar renewable energy systems. We discuss recent developments in the calibration of broadband solar radiometric instrumentation and improving broadband solar radiation measurement accuracy. An improved diffuse sky reference and radiometer calibration and characterization software for outdoor pyranometer calibrations are outlined. Several broadband solar radiation model approaches, including some developed at the National Renewable Energy Laboratory, for estimating direct beam, total hemispherical and diffuse sky radiation are briefly reviewed. The latter include the Bird clear sky model for global, direct beam, and diffuse terrestrial solar radiation; the Direct Insolation Simulation Code (DISC) for estimating direct beam radiation from global measurements; and the METSTAT (Meteorological and Statistical) and Climatological Solar Radiation (CSR) models that estimate solar radiation from meteorological data. We conclude that currently the best model uncertainties are representative of the uncertainty in measured data.

A sun following system was constructed and tested. The tracker gives the possibility for automatic measuring of direct solar radiation with a pyrheliometer. The mechanism is operated by a digital program in the control system, situated separately from the mechanical part. The position of the sun is calculated, and the pointing errors appearing during its daily work are stored for later analysis. Additionally, in the active operation mode, the tracker uses the signal of a sun detecting linear sensor to control the pointing. Two stepper motors move the instrument platform, keeping the sun’s beam at the center of the sensor. The mechanism was created at the Laboratory “Evaluacion Solar” of the Technical University Federico Santa Maria (UTFSM) in Valparaiso, Chile. The experiments show good results. The described sun tracker gives similar results as the Swiss sun tracker INTRA at a very much lower price.

A method is proposed to determine a so-called scaling height of the tropospheric aerosol and the exponent-type aerosol extinction coefficient profile from the pyrheliometer or sunphotometer measurements and surface visibility data. Using the method, characteristics of seasonally/yearly mean scaling heights and aerosol extinction profiles over 11 sites in China during 1994–2001 are analyzed. The yearly-mean scaling heights over the 11 sites change between 1.30 and 2.67 km, and they are very variable for different seasons, being usually larger in spring. As far as the yearly-mean scaling heights of all 11 sites are concerned, they change between 1.88 and 2.11 km during 1994–2001, having an increasing trend. The scaling heights from pyrheliometer data are compared with those from AERONET sunphotometer data, and a good agreement between them is obtained.

The “Evaluacion Solar” Laboratory of the Technical University Federico Santa Maria (UTFSM) in Valparaiso exists since 1957. Some types of sun following systems using instruments for different types of solar measurements were created during the mentioned period in this Laboratory. A solar tracking unit INTRA was recently installed in the UTFSM. It is considered a modern measuring and registering system for actual measuring of radiation in digital form, easier to store and to process. The action of the sun tracker is autonomous, which makes it a flexible tool to support direct radiation measurements. A special device was designed and constructed to support the measuring instruments. Three Eppley pyrheliometers were mounted on the unit and connected with an automatic registering system. An additional UV measuring sensor will be mounted soon. The realized measurements were compared with the results obtained manually from a K&Z pyrheliometer. The difference between both types of pyrheliometers is very small, which is a good precondition for using the INTRA sun tracker for precise measurements in the future.

An investigation of atmospheric turbidity has been undertaken for tropical Thai sky. Values of turbidity indices, namely, Linke factor (TL), Angstrom coefficient (β) and illuminance turbidity factor (Til) are derived directly from measurements taken by pyrheliometer, Volz sunphotometer and beam illuminance meter. Monthly mean values and frequency of occurrence of the value of each turbidity index are used to characterize variations of atmospheric turbidity. Simple polynomial equations are developed for computing values of Linke factor and illuminance turbidity factor as functions of solar altitude angle. Using the values of Linke factor and illuminance turbidity factor obtained from the models developed, values of beam normal irradiance and illuminance can be calculated accurately under clear sky conditions. Values of daylight illuminance are useful for daylighting application that contributes to energy conservation for buildings. Knowledge of the size of beam normal irradiance is useful for calculation of cooling load in air-conditioning buildings in tropical climate.

Atmospheric turbidity conditions were monitored during the period 1989–1995 in rural and urban sites in Egypt. The rural site (Aswan) is in the south region of Egypt (desert climate), while the urban site is greater Cairo (Cairo and its surroundings). The measurements were used to recover the aerosol optical depth, Linke turbidity factor, and Angstrom turbidity indices, from the total direct solar radiation measurements and knowledge of the atmospheric ozone and water vapor content. Solar direct radiation measurements were carried out with an Eppley pyrheliometer, following World Meteorological Organization recommendations, improving on previous work that made use of Voltz sun photometers. An examination of the methodologies for calculating aerosol optical depth and comparisons to observations suggests that the Davies–Hay method is superior to the method of Unsworth–Monteith. A comparison of the seasonal cycle of aerosol optical characteristics at both sites showed: aerosol of photochemical origin in the summer; a significant impact of temperature, relative humidity and dust storms in the autumn; low values of aerosol optical characteristics in the winter (due to precipitative removal as well as relative humidity-impacted deposition); and high values in the spring resulting from seasonal dust storms. An examination of the inter-annual variability of the global direct solar radiation and the anomalous aerosol optical depth at both rural and urban sites showed the signature of both the El-Chichon and Mt. Pinatubo eruptions. The Pinatubo eruption was studied in detail: anomalously high aerosol optical depth (AOD) values and low global direct radiation was observed in mid-1991 over Cairo (AOD average: 0.04) and early 1992 over Aswan (AOD average: 0.039). The annual behavior of the Angstrom turbidity coefficient and Linke turbidity factor recorded maximum values (0.27 and 7.0, respectively) over Cairo in late 1991/early 1992, with maximum values over Aswan being (0.18 and 5.57, respectively) in late 1991.

An electromechanical system to follow the position of the sun was designed and built at the Solar Evaluation Laboratory of the Technical University Federico Santa Maria (UTFSM) in Valparaiso. It allows the automatic measurement of direct solar radiation with a pyrheliometer. It operates automatically, guided by a closed loop servo system. A four-quadrant photo detector senses the position of the sun and two small DC motors move the instrument platform keeping the sun’s image at the center of the four-quadrant photo detectors. Under cloudy conditions, when the sun is not visible, a computing program calculates the position of the sun and takes control of the movement, until the detector can sense the sun again. The constructed system was tested in the climatic conditions of the city of Valparaiso, Chile. The presented tracker proves the effective work of a simple and cheap mechanism, which can be adapted to also work with larger following installations like solar cell panels, concentrators, etc.

Angstrom Turbidity Coefficients (ATC) are estimated from Aerosol Optical Depth by using spectral broadband data. These data are carried out from Pyrheliometric measurements in the period 1991–96 for two sites of different climatological and environmental view; the first one is a highly polluted urbanized site (Cairo), while the other is an unpolluted arid site (Aswan). SMARTS2 model proposed by Gueymard and SPCTRAL2 model proposed by Bird and Riordan, with two pairs of spectral broadbands, are used to select the suitable spectral broadband for estimating ATC. The turbidity levels increase during the two transition seasons, spring (due to Khamasin depressions coming from Great Sahara) and autumn (due to extend of Sudan monsoon trough), in addition to summer season. The subsidence inversion is stronger and leads to trap the pollution in the boundary layer in summer. The mean average values of ATC, over the all period, are α =0.477, 0.817 and β =0.283, 0.144 for Cairo and Aswan respectively. Turbidity level in Cairo is higher than that in Aswan because the two big industrial areas Helwan and Shoubra El-Kheima surround Cairo, in addition to traffic.

Measurements were carried out at El-Menia (28.12°N, 30.55°E) in Egypt, which lies in the middle of Egypt, 244 km south of Cairo, for a complete year (1997). Measurements were taken using the Eppley pyranometer to measure the global solar radiation (G) in the range from 280 to 2800 nm, and the Eppley pyrheliometer, which measures the normal incidence solar radiation (I) in the range from 280 to 2800 mn along with four direct spectral bands measured using three cutoff Schott glass filters in the range B1 (280‐530 mn), B2 (530‐630 nm), B3 (630‐695 nm) and B4 (695‐2800 nm). The monthly variations for all the parameters and the correlations between the different parameters have been calculated. The diffuse solar radiation and the diffuse fraction for Menia have been calculated using three different methods. Computation of the atmospheric turbidity coefficient for each spectral band was carried out using two methods, the Linke and Angstrom methods. The Linke turbidity factor is between 2.01 and 5.86, while the Angstrom turbidity coefficient is between 0.09 and 0.223. This last result shows that Menia is considered to be a highly polluted region. © 2001 Elsevier Science Ltd. All rights reserved.

The loss in the received amounts of normal incidence solar radiation at the ground level at noon time reaches 47% of that incident outside the earth’s atmosphere. The corresponding depletion figures in the amount of direct radiation received in the studied spectral band were 33% in the band less than 530 mμ, 50% in the visible range (530 λ λ >695 mμ) of the solar spectrum. The fractions of the spectral bands to the normal incidence solar radiation at the ground level are in the yearly average 30%, 20% and 49% for the above mentioned spectral bands, respectively. The extinction coefficient for normal incidence solar radiation range was 0.36–0.60 with a mean value of 0.50. Approximately, a similar trend occurs for the energy in the visible and in the infrared ranges of the solar spectrum. Extinction below 530 mμ was found to vary over 0.23–0.41 with smaller values occurring during the warmer months.

Two simple and novel pyrheliometers have been designed and constructed based on a new methodology to measure solar irradiance. They are respectively the double-sensor and the single-sensor methods. The sensor surface is made of heavily oxidized stainless steel foil, which is formed as a circular disk and thermally joined to a large stainless steel ring acting as a heat sink. For the former method, through respectively measuring the temperature differences between the two sensors' center points and the heat sink, the incident solar irradiance and the heat-loss coefficient on the sensor's surface can be determined using thermal equilibrium equations under steady-state conditions. For the latter method, the ratio of solar irradiance to the temperature difference between the single-sensor's point center and the heat sink is a constant. The constant depends on the geometric dimensions, thermophysical properties of the sensor and the heat-loss coefficient. It can be calibrated using a standard pyrheliometer. Comparing a calibrated TMI Mark-VI pyrheliometer with the double-sensor pyrheliometer, the maximum relative deviation and the standard deviation are less than 3.0% and 2.7%, respectively. For the single-sensor pyrheliometer, the corresponding deviations are 3.1% and 1.5%, respectively. The instruments are simple and inexpensive. Their accuracy can meet the need of routine measurements of solar irradiance.

Global solar radiation data obtained from actinographs of the Turkish State Meterological Service were compared with data obtained from pyrheliometers that were established recently to determine if the actinograph data were usable in practice. It has been found that the observed actinograph data have a rather high error rate with 14.7% annual and 42.1% monthly averages. It has, however, not been possible to smooth these errors as they are absolutely random. Thus, a well known quadratic model was used to produce available radiation data. A quadratic relationship between solar insolation and duration of solar radiation data has been investigated in order to estimate monthly average global irradiance for Ankara, Antalya, Samsun, Konya, Urfa and Izmir. The data from August 1993 to July 1995 have been used in the quadratic model. But, observations of Izmir differ from other stations covered for about four years. Solar insolation data used in the quadratic model were obtained from the pyrheliometer with model CR10. Duration of bright sunshine data were obtained from a Siap, Frans or Muller sunshine recorder with 60° global lens. A general quadratic formula was found that represents the whole of Turkey. The estimated monthly average global solar radiation data, then, were produced from this quadratic formula. Comparison of the estimated and measured values showed that the quadratic model was able to estimate global radiation with about a 4% annual relative error and the estimated data seemed to be more reliable than the data measured by actinograph.

The Scanner for Radiation Budget (ScaRaB) is an instrument for the determination of Earth Radiation Budget parameters from space. On-board calibration is necessary during the experiment. For this purpose, various calibration sources (lamps and blackbody-simulators) are integrated in the ScaRaB instrument. Before launch, all on-board sources have to be calibrated on ground. This paper deals with the ground calibration in the solar spectral domain. The ground calibration standard for the lamps is a reference diffusor which is illuminated by the sun. The absolute radiance of the diffusor is calculated from its absolute reflectivity and the incoming solar irradiance. The latter is measured with a pyrheliometer. The total error of the ground measurements of the filtered radiance of the on-board lamp sources is estimated as 1.4 to 4% (depending of the calibration source) of the nominal value. However, the lamp sources show aging effects during ground testing from 0.5% to 4% of the nominal value.

Solar radiation spectra were measured by a ground-level pyrheliometer (Eppley NIP) equipped with a filter wheel. Three flat, circular, Schott filters were used to define three spectral bands. The filters were large band-pass filters of the types OG530, RG630 and RG695. The experiment was conducted from June 1991 to May 1992 on the roof of the new building of the National Research Institute of Astronomy and Geophysics (NRIAG) at Helwan. The Linke turbidity factor for the integrated radiation, and the spectral attentuation coefficients of solar energy caused by aerosols were computed. In comparison with earlier measurements in 1910, 1967 and 1987, it is clear that there is a continuous increase in the turbidity factor, due to an increase of industrial waste in the Helwan atmosphere. A correlation analysis between the turbidity factors and the total suspended particles, smoke and sulphur dioxide was carried out. Meteorological elements and sand storms have been taken into consideration in the correlation analysis.

A numerical scheme to correct for the delayed response of thermal solar radiation sensors is discussed. The proposed scheme employs only information that can be obtained from the response to a step input and does not require any auxiliary measurements. Furthermore, the scheme is applicable to instruments with any number of time constants and avoids the need for high order numerical derivatives. The development of such a scheme is motivated by a study of the Eppley Normal Incidence Pyrheliometer, which cannot be described accurately by a two-exponential model.

This work is based on simultaneous measurements of spectral direct solar radiation (using Eppley's pyrheliometer) and total mass particulate concentration (using a high-volume sampler) carried out in Katowice, Upper Silesian Industrial Region (USIR), Poland, in the period 1982–1987. Relationships between spectral aerosol optical depth values ( τ a λ ) expressing the solar radiation extinction and total suspended particulates concentration at near ground-level air are, in general, weak (correlation coefficient r =0.0−0.40) except after rainy (or snowy) periods in cold months [October–March (X-III)] with correlation coefficient r =0.67−0.68. In conclusion, the fundamental effect of relative humidity on variability as well as on the magnitude and range of τ a values in existing anthropogenic emission structure over USIR area was confirmed.

The directional (cosine) error of pyranometers must be eliminated if precise global radiation data are required. One possible way to do so is to measure the global radiation as a sum of the diffuse radiation and of the vertical component of the direct radiation. Due to geometrical reasons, the direct radiation excluded by the diffusometers is not equal to the vertical component of the direct radiation measured by pyrheliometers. The difference comes from the different amount of circumsolar radiation measured with the actual direct beam. With the changing solar elevation, the viewing geometry of the diffusometer composed of a shading-disk and a pyranometer also changes. In this paper the penumbra function of diffusometers is approximated by taking into account the exact slope and limit angle values. Instead of the actual circumsolar sky radiation the statistically expected one has been considered. Using these approximations the circumsolar difference values have been calculated for a cavity type absolute pyrheliometer and a Normal Incidence Pyrheliometer (NIP) as well as for the NIP and two geometrically different shading disk diffusometers. The difference is in the range of up to 5 W m−2 and should be taken into account in both meteorological and solar energy investigations.

The ground-level spectral distribution of direct solar irradiance at Reunion Island was measured for six bands covering the spectrum of solar radiation. The measurements, distributed over one year, were made under clear sky conditions with a pyrheliometer (Eppley, NIP) and six large pass-band flat filters. Good stability of spectral irradiances as a function of Solar height allows us to propose approximate relationships which significantly characterize the irradiance into each spectral band. Measurements at Reunion vary significantly from data obtained with the same apparatus in a northern hemisphere continental area (Lyon). The determination of aerosol attenuation coefficients, for different spectral bands, allows the establishment of a mean curve, for these coefficients as a function of wavelength, characteristic of marine aerosols.

Measurements of direct solar radiation intensity, using an Angstrom compensation pyrheliometer carried out over three harmattan seasons (1985–1987) at Ile-Ife (7°29′N, 4°34′E), Nigeria, have been used to determine atmospheric turbidity based on five different models of turbidity viz:: Schuepp ( B ), Angstrom (β), Kastrov ( C ), Unsworth ( τ a ) and Linke ( T ). The five parameters indicate high aerosol loading of the atmosphere during the period and high correlation is established between them: (0.919 ⩽ r ⩽ 0.999). An inverse relationship has been noticed between horizontal visibility and atmospheric turbidity: (−0.80 ⩽ r ⩽ −0.76).

The problems of how to improve the accuracy and keep the stability of the transient calorimeter pyrheliometer have been solved by the following methods: (i) a hemispheric mirror shell with a small hole at its top is situated over a copper disk sensor. The inner wall of the shell has high reflectivity, acting as a specular reflector. In this way, the effective absorptance of the sensor with the mirror can approach unity, and can be kept in long-term stability; (ii) a micro-computer is adopted as controller and data processor of the pyrheliometer. So the total error of the measuring system is less than 0.1%. Results of comparing our instrument with the cavity-type pyrheliometers, such as Eppley HF self-calibrating and TMI Mark-VI pyrheliometer, show that the maximum relative deviation, the average deviation, and the standard deviation are less than 0.6%, 0.12%, and 0.23%, respectively.

During the summer, sea breezes advect high concentrations of pollutants from the Los Angeles Basin into the Mojave Desert during the afternoon. Clear-sky pyrheliometer and pyranometer data were obtained to determine the effect of the advected contaminants on the attenuation of solar radiation and its breakdown into direct and diffuse components. Even under the most extreme conditions, the amount of global radiation received is not reduced by more than about 7 per cent. On the other hand, reductions in the direct component can be as great as 140 W m−2, which are nearly compensated for by large increases (up to almost 300 per cent) in diffuse radiation. The spatial and temporal variability in the budgeting of global radiation into the diffuse and direct components calls into question the feasibility of estimating diffuse energy based on the ratio of global to extraterrestrial radiation.