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A new cost-effective radiometer has been designed, built, and tested to measure direct normal solar irradiance (DNI). The proposed instrument for solar irradiance measurement is based on an optical fiber as the light beam collector, a semiconductor photodiode to measure the optical power, and a calibration algorithm to convert the optical power into solar irradiance. The proposed radiometer offers the advantage of separating the measurement point, where the optical fiber collects the solar irradiation, from the place where the optical power is measured. A calibration factor is mandatory because the semiconductor photodiode is only spectrally responsive to a limited part of the spectral irradiance. Experimental tests have been conducted under different conditions to evaluate the performance of the proposed device. The measurements confirm that the proposed instrument performs similarly to the expensive high-accuracy pyrheliometer used as a reference. [ABSTRACT FROM AUTHOR]

This paper compares the daily sunshine duration (SD) measured by the Campbell–Stokes sunshine recorder (CS) with three automatic instruments (Kipp & Zonen pyranometers (CM6 B), pyrheliometer (CHP1) and CSD3 sunshine recorders). The comparisons are essential, since recently, automatic sunshine duration sensors have been replacing the manual measurements, affecting, therefore, the continuity of the CS time series. The comparisons were applied to the daily SD data of three synoptic stations (Larnaca A/P, Pafos A/P and Athalassa) where parallel measurements of manual and automatic instruments are carried out during the periods 2009–2012 and 2012–2015. The CSD3 measurements were obtained from two automatic weather stations (AWSs) which are close to the synoptic station of Larnaca A/P. The comparisons referred to the daily and monthly sums of sunshine duration, as well as to their statistical distributions and their relationships. The daily differences in the pairs of sunshine recorders installed at the same location are mostly within the range of −1.0 to +1.0 h. At Larnaca, in the period 2009–2012, the pyranometric method's (Gl) values were higher than the CS in the summer months. Pafos and Athalassa showed higher values of CS compared to Gl and the pyrheliometric method (Pr), respectively, almost throughout the year. Generally, the annual average daily differences between the different pairs range between 0.3 and 0.5 h. Regression equations were determined allowing the estimation of daily SD under different sky conditions. The coefficient of determination (R2) reached the values of 0.91 and 0.96 under 'all sky' conditions, and between 0.80 and 0.91 under 'clear' sky conditions at the three locations. [ABSTRACT FROM AUTHOR]

A new cost-effective radiometer has been designed, built, and tested to measure direct normal solar irradiance (DNI). The proposed instrument for solar irradiance measurement is based on an optical fiber as the light beam collector, a semiconductor photodiode to measure the optical power, and a calibration algorithm to convert the optical power into solar irradiance. The proposed radiometer offers the advantage of separating the measurement point, where the optical fiber collects the solar irradiation, from the place where the optical power is measured. A calibration factor is mandatory because the semiconductor photodiode is only spectrally responsive to a limited part of the spectral irradiance. Experimental tests have been conducted under different conditions to evaluate the performance of the proposed device. The measurements confirm that the proposed instrument performs similarly to the expensive high-accuracy pyrheliometer used as a reference.

In this article, an adaptative two-axis solar tracker is developed. The system uses a self-calibration strategy and algorithm that, without the need for additional sensors, automates the solar pointing and tracking process for a pyrheliometer, thus avoiding a significant portion of the manual installation process and the need for error correction by an operator. • Solar trackers for pyrheliometers require high precision and are expensive. • To measure direct normal irradiance, one must adhere to the BSRN guidelines. • DNI is distributed as a revolution paraboloid around the Sun from the Earth's surface. • The algorithm enables autonomous solar localization and tracking with BSRN accuracy. • The system autonomously determines Earth's North from the Sun's sky position. [ABSTRACT FROM AUTHOR]

This paper compares the daily sunshine duration (SD) measured by the Campbell–Stokes sunshine recorder (CS) with three automatic instruments (Kipp & Zonen pyranometers (CM6 B), pyrheliometer (CHP1) and CSD3 sunshine recorders). The comparisons are essential, since recently, automatic sunshine duration sensors have been replacing the manual measurements, affecting, therefore, the continuity of the CS time series. The comparisons were applied to the daily SD data of three synoptic stations (Larnaca A/P, Pafos A/P and Athalassa) where parallel measurements of manual and automatic instruments are carried out during the periods 2009–2012 and 2012–2015. The CSD3 measurements were obtained from two automatic weather stations (AWSs) which are close to the synoptic station of Larnaca A/P. The comparisons referred to the daily and monthly sums of sunshine duration, as well as to their statistical distributions and their relationships. The daily differences in the pairs of sunshine recorders installed at the same location are mostly within the range of −1.0 to +1.0 h. At Larnaca, in the period 2009–2012, the pyranometric method’s (Gl) values were higher than the CS in the summer months. Pafos and Athalassa showed higher values of CS compared to Gl and the pyrheliometric method (Pr), respectively, almost throughout the year. Generally, the annual average daily differences between the different pairs range between 0.3 and 0.5 h. Regression equations were determined allowing the estimation of daily SD under different sky conditions. The coefficient of determination (R2) reached the values of 0.91 and 0.96 under ‘all sky’ conditions, and between 0.80 and 0.91 under ‘clear’ sky conditions at the three locations.

In this work, the method for calculation of uncertainty of pyrheliometers' responsivity during their outdoor calibration process in the laboratory is exposed. It is applied first for calibration of standard pyrheliometers by comparison to cavity radiometers, and after for calibration of an end-user pyrheliometer against that standard pyrheliometer. The dissemination of the WRR irradiance scale is illustrated in practice and the increasing uncertainty in the traceability chain is quantified. The way of getting traceability to both WRR scale and to SI units in the current situation, where the shift between these radiometric scales is pending to be solved, is also explained. However, the impact of this gap between scales seems to be more important for calibrations of reference Class A pyrheliometers than in the final determination of DNI irradiance, because in this case, the cumulative uncertainty is large enough as to not significantly be affected for the difference. The way to take into account different correction terms in the measurement model function, and how to compute the corresponding uncertainty, is explained too. The influence of temperature of some pyrheliometers during calibration process and the potential impact on the DNI irradiance calculated with these instruments is exemplified. [ABSTRACT FROM AUTHOR]

We used parallel sunshine duration datasets obtained with a Jordan sunshine recorder and three automatic sunshine duration sensors to investigate the differences between these instruments. We used measurements obtained at Shangdianzi (SDZ) regional Global Atmosphere Watch (GAW) station with a Jordan sunshine recorder, a DFC2 photoelectric sunshine meter, a CHP1 pyrheliometer, and two CMP11 pyranometers from 1 January to 5 July 2019 and from 3 November 2020 to 28 February 2021. The results showed that the daily sunshine duration measurements obtained from the Jordan sunshine recorder were comparable with those from the DFC2 meter and the CMP11 pyranometers under all-sky conditions, but were considerably different from those observed by the CHP1 pyrheliometer. An analysis of potential influencing factors showed that the solar zenith angle, the spectral range of the automatic sensors, the relative humidity, and the sky conditions were the main factors affecting the measurements of sunshine duration between the Jordan sunshine recorder and three automatic sensors. We proposed a simple linear regression function—the DFC2-equivalent sunshine duration estimation (DFCESD) model—to guarantee the consistency of the long-term sunshine duration series observed by the Jordan sunshine recorder at SDZ and the measurements from the DFC2 meter. Validation of the DFCESD model showed that the mean absolute difference (MAD) between the daily sunshine duration observed by the Jordan sunshine recorder and those from the DFC2 meter improved from −0.7 to −0.2 h day−1, the relative deviation (RD) improved from −9.3% to −2.3%, and the root-mean-square deviation (RMSD) decreased from 1.0 to 0.8 h day−1. [ABSTRACT FROM AUTHOR]

Sunshine duration is an important variable in surface meteorological measurement. In this article, two methods for measuring sunshine duration are described, one is photovoltaic-based instrument, which ranges from 400 to 1100 nm, and the other is thermopile-based pyrheliometer, which ranges from 300 to 3000 nm. Due to the spectrum is more representative and accuracy for solar radiation, the pyrheliometer is recommended as the reference sensor for the detection of the threshold irradiance for defining the sunshine duration. The comparison between the photovoltaic- and thermopile-based instruments is carried out to uncover the spectral influence on sunshine duration measurement, analysis by the SMARTS tool is presented to calculate irradiance with different spectral ranges under different altitudes.

Highlights • ASI based nowcasting system with individual 3-D cloud objects. • Cloud transmittance probability analysis performed over 574 cloudy days. • Development of a probabilistic cloud transmittance estimation/allocation method. • Validation of transmittance allocation method and nowcast quality over 2 years. • Discussion of potential transmittance allocation improvement via cloud classification. Abstract The demand for accurate solar irradiance nowcast increases together with the rapidly growing share of solar energy within our electricity grids. Intra-hour variabilities, mainly caused by clouds, have a significant impact on solar power plant dispatch and thus on electricity grids. All sky imager (ASI) based nowcasting systems, with a high temporal and spatial resolution, can provide irradiance nowcasts that can help to optimize CSP plant operation, solar power plant dispatch and grid operation. The radiative effect of clouds is highly variable and depends on micro- and macrophysical cloud properties. Frequently, nowcasting systems have to measure/estimate the radiative effect during complex multi-layer conditions with strong variations of the optical properties between individual clouds. We present a novel approach determining cloud transmittance from measurements or from correlations of transmittance with cloud height information. The cloud transmittance is measured by a pyrheliometer when shaded, as the ratio of shaded direct normal irradiance (DNI) and clear sky DNI. However, for most clouds, direct transmittance measurements are not available, as these clouds are not shading the used pyrheliometers. These clouds receive an estimated transmittance value based on (1) their height, (2) results of a probability analysis with historical cloud height and transmittance measurements as well as (3) recent transmittance measurements and their corresponding cloud height. Cloud heights are measured by a stereoscopic approach utilizing two ASIs. We discuss site dependencies of the presented transmittance estimation method and the potential integration of automatic cloud classification approaches. We validated the cloud transmittance estimation over two years (2016 and 2017) and compare the probabilistic cloud transmittance estimation approach with four simple approaches. The overall mean-absolute deviation (MAD) and root-mean-square deviation (RMSD) are 0.11 and 0.16 respectively for transmittance. The deviations are significantly lower for optically thick or thin clouds and larger for clouds with moderate transmittance between 0.18 and 0.585. Furthermore we validated the overall DNI forecast quality of the entire nowcasting system, using this transmittance estimation method, over the same data set with three spatially distributed pyrheliometers. Overall deviations of 13% and 21% are reached for the relative MAD and RMSD with a lead time of 10 min. The effects of the chosen data set on the validation results are demonstrated by means of the skill score. [ABSTRACT FROM AUTHOR]

Abstract 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 Antón 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. Highlights • Re-evaluation of the atmospheric transparency in Madrid for the period 1910–1929 • Decrease in atmospheric transparency between 1910 and 1925 • Increase in atmospheric transparency between 1925 and 1929 • Metadata are valuable for accurate assessments of early pyrheliometer measurements. [ABSTRACT FROM AUTHOR]

Abstract 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. Graphical abstract Image Highlights • A semi-empirical methodology to estimate the incident solar flux on central receivers are proposed. • This methodology has been named Superposition method. • Superposition method is useful in deteriorated solar fields. • Superposition method characterizes each heliostat individually. • The error made in the flux estimation are comprised between 2.5 and 8%. [ABSTRACT FROM AUTHOR]

The key input quantity to climate modelling and weather forecasts is the solar beam irradiance, i.e., the primary amount of energy provided by the sun. Despite its importance the absolute accuracy of the measurements are limited—which not only affects the modelling but also ground truth tests of satellite observations. Here we focus on the problem of improving instrument calibration based on dedicated measurements. A Bayesian approach reveals that the standard approach results in inferior results. An alternative approach method based on monomial based selection of regression functions, combined with model selection is shown to yield superior estimations for a wide range of conditions. The approach is illustrated on selected data and possible further enhancements are outlined.

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; Bouzaréah site (Algiers), located on Mediterranean coast and Ghardaïa 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 (r 2 ), 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 Ghardaïa. 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. [ABSTRACT FROM AUTHOR]

Abstract: Because of the cloud‐induced variability of the solar resource, the growing contributions of photovoltaic plants to the overall power generation challenges the stability of electricity grids. To avoid blackouts, administrations started to define maximum negative ramp rates. Storages can be used to reduce the occurring ramps. Their required capacity, durability, and costs can be optimized by nowcasting systems. Nowcasting systems use the input of upward‐facing cameras to predict future irradiances. Previously, many nowcasting systems were developed and validated. However, these validations did not consider aggregation effects, which are present in industrial‐sized power plants. In this paper, we present the validation of nowcasted global horizontal irradiance (GHI) and direct normal irradiance maps derived from an example system consisting of 4 all‐sky cameras (“WobaS‐4cam”). The WobaS‐4cam system is operational at 2 solar energy research centers and at a commercial 50‐MW solar power plant. Besides its validation on 30 days, the working principle is briefly explained. The forecasting deviations are investigated with a focus on temporal and spatial aggregation effects. The validation found that spatial and temporal aggregations significantly improve forecast accuracies: Spatial aggregation reduces the relative root mean square error (GHI) from 30.9% (considering field sizes of 25 m2) to 23.5% (considering a field size of 4 km2) on a day with variable conditions for 1 minute averages and a lead time of 15 minutes. Over 30 days of validation, a relative root mean square error (GHI) of 20.4% for the next 15 minutes is observed at pixel basis (25 m2). Although the deviations of nowcasting systems strongly depend on the validation period and the specific weather conditions, the WobaS‐4cam system is considered to be at least state of the art. [ABSTRACT FROM AUTHOR]

Pyrheliometer calibration must be done following strict procedures in order to ensure the required robustness and accuracy. These procedures are described in the ISO 9059:1990 and ASTM E 816 – 15 international standards. However, their application requires information that may not always be available or may be subjective, inaccurate or incomplete, as for example, the determination of “percent of cloud coverage” or “the existence of clouds 15° around the Sun”. The irradiance measurements made by the reference and test instruments involved should also be collected over wide periods after, close to and before solar noon, which might not always be the case depending on the weather conditions during calibration. When those data are not available, the standard cannot be applied properly, and the experts have to decide which data can be used for the calibration. In this study, the abovementioned two main standards for pyrheliometer calibration were thoroughly reviewed, and a harmonized protocol is proposed that uses only the main data recorded. Nineteen field pyrheliometers were calibrated to verify the proposed procedure, and the results show its robustness. After calibration, we analyzed the variability in the calibration constant and the influence of some experimental conditions on the calibration results. As in previous references, the results show that variations in solar elevation and wind speed during the day still influenced the calibration constants of most of the test devices. On the contrary, neither the angle between the wind direction and the solar azimuth nor Linke turbidity seemed to influence the calibration constants calculated. The influence of the Linke turbidity is low as the viewing geometry of all involved pyrheliometers is very similar to each other and as low turbidity prevailed. The correlation between the solar elevation and the wind speed was analyzed and calibration constants were found to vary linearly with solar elevation and wind speed, respectively. Pyrheliometer calibration measurement testing was carried out in Summer 2014 at the Plataforma Solar de Almeria (PSA) in the context of the Solar Facilities for the European Research Area 2 Project (SFERA2). [ABSTRACT FROM AUTHOR]

The interannual variability of the solar irradiance and meteorological conditions are often ignored in favor of single-year data sets for modeling power generation and evaluating the economic value of photovoltaic (PV) power systems. Yet interannual variability significantly impacts the generation from one year to another of renewable power systems such as wind and PV. Consequently, the interannual variability of power generation corresponds to the interannual variability of capital returns on investment. The penetration of PV systems within the Hawaiian Electric Companies’ portfolio has rapidly accelerated in recent years and is expected to continue to increase given the state’s energy objectives laid out by the Hawaii Clean Energy Initiative. We use the National Solar Radiation Database (1998–2015) to characterize the interannual variability of the solar irradiance and meteorological conditions across the State of Hawaii. These data sets are passed to the National Renewable Energy Laboratory’s System Advisory Model (SAM) to calculate an 18-year PV power generation data set to characterize the variability of PV power generation. We calculate the interannual coefficient of variability (COV) for annual average global horizontal irradiance (GHI) on the order of 2% and COV for annual capacity factor on the order of 3% across the Hawaiian archipelago. Regarding the interannual variability of seasonal trends, we calculate the COV for monthly average GHI values on the order of 5% and COV for monthly capacity factor on the order of 10%. We model residential-scale and utility-scale PV systems and calculate the economic returns of each system via the payback period and the net present value. We demonstrate that studies based on single-year data sets for economic evaluations reach conclusions that deviate from the true values realized by accounting for interannual variability. [ABSTRACT FROM AUTHOR]

The article focuses on the history of Féry pyrheliometric telescope that was re-discovered during an inventory at ESPCI Paris. Topics discussed include the measurement of the surface temperature of the Sun with the help of the instrument in 1906; its innovative design that was builds upon earlier models of pyrometers conceived by physicist Charles Féry; its traditional Newton-like optical reflecting telescope which was outfitted with a thermocouple; and how it was operated by astronomer Gaston Millochau at the top of the mont Blanc.

Routine measurements of the broadband beam irradiance at normal incidence by means of pyrheliometers or equivalent pyranometric systems include unknown contributions from the irradiance originating from within the extent of the solar disc B n sun , and that from a larger circumsolar region defined by its solid angle aperture, called the circumsolar normal irradiance CS n . This article describes a fast and simple parametric model that estimates the beam and circumsolar radiation, for opening half-angles in the interval [0.4°, 5°], under cloud-free conditions in a desert environment. 1 min measurements of the beam normal B n , global G and diffuse D horizontal irradiances at Solar Village, Saudi Arabia, and Tamanrasset, Algeria, were used for calibration and validation. Using AERONET measurements as inputs to the radiative transfer code libRadtran, it has been checked–through an ‘indirect’ validation with the ground measured B n –that the modelled B n sun and CS n are accurate. Accordingly, a library of B n sun and CS n modelled by libRadtran for varying solid angle apertures was generated. Building on this library, a fast parametric model was developed to estimate B n sun and CS n using G , D and B n as inputs. The coefficients of the model were fitted to a training set of measurements and then validated twice: once at their respective site, and once at the other site. When using the coefficients for their own site for both Solar Village and Tamanrasset for CS n , the relative bias is respectively −2.7% and −1.5%, the relative root mean square error (RMSE) is 19.9% and 19.6%, and the correlation coefficient is 0.871 and 0.935. As for B n sun , the relative bias is −2.0% and −2.2%, the RMSE is 2.7% and 3.6%, and the correlation coefficient is 0.999 and 0.998. Applying the coefficients of one site to the other site yields satisfactory results. It is recommended to use the coefficients of Tamanrasset for desert sites exhibiting frequent clear skies, and those of Solar Village for sites exhibiting frequent turbid skies. The coefficients have also been fitted using data from both sites, for a combined model. [ABSTRACT FROM AUTHOR]