Circumsolar Radiation Research Articles

Effect of the dimensions (width and radius) of the shading ring on the measurement of diffuse solar irradiance with the MEO shading ring measurement method

Diffuse irradiance was monitored by the Melo–Escobedo–Oliveira shading ring measurement method (MEO method) and requires the application of two corrections: one geometric and one atmospheric. The latter occurs from the interaction of radiation with atmospheric constituents and is known as circumsolar radiation, which is also obstructed by the ring. Thus, the objective of this work is to verify if geometrical aspects of the shading ring, such as width and radius, interfere in the quantification of these atmospheric corrections. Two Kipp & Zonen pyranometers were used to monitor the diffuse irradiance, one with 10/40 cm (width/radius) and one with 5/20 cm (width/radius). The values measured by the rings were compared with the diffuse irradiance obtained by the difference method. Studies were carried out under all atmospheric sky cover conditions: cloudy sky, partially cloudy sky, and clear sky. The results showed the highest atmospheric corrections were obtained for the clear sky coverage. Despite the two assemblies having the same width/radius ratio of 0.250, the assembly with the largest radius showed the lowest atmospheric correction. The application of atmospheric corrections is conditioned to the geometrical aspects of the ring, mainly the radius, to guarantee the quality of measurements carried out by the shading ring method.

A systematic bias in present models of circumsolar radiation

Light scattering by aerosol particles is known to be fairly largest modulator of circumsolar radiation, while its contribution to the direct beam irradiance progressively increases with atmospheric turbidity. Due to their forward-lobed radiation patterns, the large aerosol particles are of special importance in understanding the shaping of diffuse light distribution in the solar aureola. The mechanism of intensive small-angle scattering is the Fraunhofer diffraction which scales up linearly with the projected area (shadow) of each single particle.Theoretically founded models of circumsolar radiation usually rely on Mie scattering – a concept exclusively applicable to spherical particles. However, the particles occurring in nature typically have nonspherical shapes with aspect ratio exceeding a few times that for a sphere (for prolate particles) or smaller than unity (for oblate particles). Here we show that models based on Mie theory systematically underestimate circumsolar irradiance compared to what we can expect from nonspherical particles. The average brightening of solar corona due to nonspherical aerosols is 10–20%, but can be as large as 40–50% for needle-like or disk-like particles. We are introducing a correction factor to improve the accuracy of the present models.

Parameterization of cloud transmittance for expeditious assessment and forecasting of all-sky DNI

Radiative transfer models require vast computing resources to solve cloud transmittance and reflectance from the radiative transfer equation. As a result, models offering precise simulation in operations often acquire individual cloud transmittance or reflectance from a lookup table precomputed for practicable scenarios. To further expedite the computation of global horizontal irradiance and to reduce the storage requirements, the Fast All-sky Radiation Model for Solar applications (FARMS) parameterized the lookup table using elementary functions with specified coefficients. This study extends FARMS direct normal irradiance (DNI) computation by utilizing hyperbolic tangent functions and various polynomial functions to parameterize the cloud transmittance for scattered solar radiation in the circumsolar region. The parameterization is implemented in FARMS with DNI (FARMS-DNI) and accounts for the circumsolar radiation when assessing or forecasting DNI. The evaluation, with long-term observations at the National Renewable Energy Laboratory's, Solar Radiation Research Laboratory, and the Atmospheric Radiation Measurement, Southern Great Plains, Central Facility, shows that the parameterized DNIs are virtually identical with those computed by coupling FARMS-DNI to a lookup table of cloud transmittance. This parameterization has diverse applications in radiative transfer models and numerical weather prediction models used to assess or forecast direct solar radiation.

Improving the prediction of DNI with physics-based representation of all-sky circumsolar radiation

Direct normal irradiance (DNI) is often interpreted differently in ground measurements and forecasts using numerical weather prediction (NWP) models, leading to substantial bias in DNI forecasting especially under cloudy-sky conditions. To mitigate the bias, we use the Fast All-sky Radiation Model for Solar applications with DNI (FARMS-DNI) to conduct physics-based simulations of solar radiation in the circumsolar region. The DNI is quantified using the sum of the radiation along the sun direction and the scattered radiation within the circumsolar region. FARMS-DNI is coupled with the Weather Research and Forecasting model with solar extensions (WRF-Solar) to predict day-ahead DNI in 9-km pixels over an extended domain covering the contiguous United States. The DNI forecasts for the entire year of 2018 are validated using surface-based observations at the Atmospheric Radiation Measurement (ARM) and Surface Radiation Budget Network (SURFRAD) sites and a satellite-based solar radiation product developed by the procedure of the National Solar Radiation Data Base (NSRDB). The results show that the WRF-Solar coupled with the conventional Beer-Bouguer-Lambert law underestimes the overall DNI forecasts by 30%-60%, which is a joint effect of the uncertainties in computing the circumsolar radiation and forecasting cloud properties. This bias is significantly reduced by using FARMS-DNI.

Modification of circumsolar radiation for anisotropic diffuse radiation models: Photovoltaic collectors on sloped planes

Photovoltaic fields are deployed in multiple collector rows on horizontal and on sloped planes, therefore the second (and subsequent rows) sees the sky dome with a smaller angle than the first (front) row

A calculation method of precise direct normal irradiance based on measured CSR

Radiation from the circumsolar region called circumsolar radiation is part of direct solar radiation, but not all circumsolar radiation can be received by concentrating solar power (CSP). Circumsolar ratio (CSR) reflects the proportion of circumsolar normal irradiance in direct normal irradiance (DNI). CSR is related to the radial distribution of solar brightness (sunshape) and it is influenced by atmospheric turbidity and air mass (AM).Performance of CSP is affected by CSR, which should be considered in the CSP simulation and analysis. This work introduces a method of measuring CSR with a CCD camera and filters. Also a calculation method of DNI for different field of view based on the measured CSR is proposed.

Application of small angle approximation in circumsolar irradiance modelling

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.

Comparing View Factor modeling frameworks for the estimation of incident solar energy

View Factors are instrumental in using widely available horizontal solar radiation data for calculating the incident radiation on harvesting surfaces with arbitrary positions. This capability is notably useful for the design, optimization, and performance forecasting of solar devices. There are several main View Factor models (Liu-Jordan’s, Tian’s, and Badescu’s), which can lead to different theoretical implications and energy estimates. However, the assessments about the validity and underlying assumptions of these models are sometimes contradictory. Resolving this is important for utilizing the most appropriate framework given specific schemes and modeling goals. This work presents a comparative systematic analysis of a wide range of View Factor modeling frameworks with the purpose to gain a deeper understanding of the theoretical consistency and implications of the main View Factor models. The different sets of assumptions are evaluated through stochastic rays simulations and verified against integral models. Five frameworks for the Isotropic and Albedo View Factors were found to be consistent with Liu-Jordan’s model, two with Tian’s, and two with Badescu’s (partially); all with RMSE ⩽0.0014. Considering the most common ways to conceptualize the other components of the radiation, there was consistency with the Perez sky models (RMSE ⩽0.0055) for the View Factor of the Circumsolar radiation as a 25° cone and Horizon Brightening as a flat ring. For the View Factor of the Horizon Brightening as a 6.5° band, two regression models are introduced. By enabling a deeper insight into the sets of assumptions that are consistent with the main View Factor models, this work is valuable for the convergence and best implementation of the various theories in the modeling of incident solar radiation.

Aerosol retrievals from the EKO MS-711 spectral direct irradiance measurements and corrections of the circumsolar radiation

Abstract. Spectral direct UV–visible normal solar irradiance (DNI) has been measured with an EKO MS-711 grating spectroradiometer, which has a spectral range of 300–1100 nm, and 0.4 nm step, at the Izaña Atmospheric Observatory (IZO, Spain). It has been used to determine aerosol optical depth (AOD) at several wavelengths (340, 380, 440, 500, 675, and 870 nm) between April and September 2019, which has been compared with synchronous AOD measurements from a reference Cimel and Aerosol RObotic NETwork (AERONET) sun photometer. The EKO MS-711 has been calibrated at the Izaña Atmospheric Observatory by using the Langley plot method during the study period. Although this instrument has been designed for spectral solar DNI measurements, and therefore has a field of view (FOV) of 5∘ that is twice the recommended amount in solar photometry for AOD determination, the AOD differences compared to the AERONET–Cimel reference instrument (FOV ∼1.2∘) are fairly small. A comparison of the results from the Cimel AOD and EKO MS-711 AOD presents a root mean square (rms) of 0.013 (24.6 %) at 340 and 380 nm, and 0.029 (19.5 %) for longer wavelengths (440, 500, 675, and 870 nm). However, under relatively high AOD, near-forward aerosol scattering might be significant because of the relatively large circumsolar radiation (CSR) due to the large EKO MS-711 FOV, which results in a small but significant AOD underestimation in the UV range. The AOD differences decrease considerably when CSR corrections, estimated from libRadtran radiative transfer model simulations, are performed and obtain an rms of 0.006 (14.9 %) at 340 and 380 nm, and 0.005 (11.1 %) for longer wavelengths. The percentage of 2 min synchronous EKO AOD–Cimel AOD differences within the World Meteorological Organization (WMO) traceability limits were ≥96 % at 500, 675, and 870 nm with no CSR corrections. After applying the CSR corrections, the percentage of AOD differences within the WMO traceability limits increased to >95 % for 380, 440, 500, 675, and 870 nm, while for 340 nm the percentage of AOD differences showed a poorer increase from 67 % to a modest 86 %.

Fast scheme for determination of direct normal irradiance. Part II: Parameterization of circumsolar radiation

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 Physics-Based DNI Model Assessing All-Sky Circumsolar Radiation.

SummaryBy investigating the long-term observations at Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP), we find that the routinely used Beer-Bouguer-Lambert law and the models that empirically separate direct normal irradiance (DNI) from measurements of global horizontal irradiance (GHI) have dramatic and unexpected bias in computing cloudy-sky DNI. This bias has led to tremendous uncertainty in estimating the electricity generation by solar energy conversion systems. To effectively reduce the bias, this study proposes a physical solution of all-sky DNI that computes solar radiation in the infinite-narrow beam along the sun direction and the scattered radiation falls within the circumsolar region. In sharp contrast with the other DNI models, this method uses a finite-surface integration algorithm that computes solar radiation in differential solid angles and efficiently infers its contribution to a surface perpendicular to the sun direction. The new model substantially reduces the uncertainty in DNI by a factor of 2–7.

Effects of circumsolar radiation on the optimal performance of a Stirling heat engine coupled with a parabolic dish solar collector

This paper investigates the effects of circumsolar radiation on the optimal performance of a coupled parabolic dish and a Stirling heat engine, referred to as the Dish/Stirling system. The presence of circumsolar radiation results in the widening of the solar cone in which the solar beam (direct) radiation is received by the parabolic dish. This beam radiation cone enlargement effectively degrades the collector performance, especially for the very high concentration ratio collectors such as the parabolic dish. The enlarged solar beam radiation cone angular width results in enhanced solar radiation flux spillage at the receiver due to the inability of the parabolic dish reflector surface to focus a significant fraction of incoming beam radiation. This reduces both the energy input as well as the maximum temperature of the Stirling heat engine cycle and causes a reduction in the overall thermal efficiency of the Dish/Stirling system. The present study analyzes this effect by considering various levels of circumsolar radiation, characterized by a parameter known as the circumsolar ratio. The data for the intercept factor of the parabolic dish collector is based on the circumsolar radiation measurements obtained by researchers at the Lawrence Berkeley Laboratory, and the results of analysis reported by researchers at the Solar Energy Research Institute (currently the National Renewable Energy Laboratory). For the present analysis, this intercept factor data is curve-fitted for different circumsolar ratio, mirror optical errors and the concentration ratio using a MATLAB® program. The overall thermal efficiency of the Dish/Stirling system is maximized with respect to the concentration ratio of the parabolic dish and the maximum temperature ratio of the Stirling heat engine using the first law of thermodynamics. This optimization process is performed using a MATLAB® program. Optimal values of concentration ratio for the parabolic dish collector, and the maximum temperature ratio of the Stirling cycle are obtained corresponding to the maximum overall thermal efficiency of the Dish/Stirling system. The study shows that, for a parabolic collector with low mirror optical error and rim angle of 60°, an increase in circumsolar ratio from 0.02 to 0.2 reduces the maximum overall thermal efficiency, optimal concentration ratio and optimal temperature ratio by about 13%, 18% and 7% respectively. In the case of parabolic collectors with high mirror optical error and rim angle of 60°, an increase in circumsolar ratio from 0.02 to 0.2 reduces the maximum overall thermal efficiency, optimal concentration ratio and optimal temperature ratio by about 10%, 11% and 5% respectively. Similar trends are observed for parabolic dish collectors with rim angle of 40°. Results from this study are presented in the form of performance charts that show the effects of varying circumsolar ratios, mirror optical errors, rim angles, non-dimensional radiation flux and non-dimensional convection loss parameters on the maximum overall thermal efficiency of the Dish/Stirling system.

A fast and simple model to estimate the contribution of the circumsolar irradiance to measured broadband beam irradiance under cloud-free conditions in desert environment

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 Bnsun, and that from a larger circumsolar region defined by its solid angle aperture, called the circumsolar normal irradiance CSn. 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 Bn, 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 Bn–that the modelled Bnsun and CSn are accurate. Accordingly, a library of Bnsun and CSn modelled by libRadtran for varying solid angle apertures was generated. Building on this library, a fast parametric model was developed to estimate Bnsun and CSn using G, D and Bn 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 CSn, 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 Bnsun, 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.

Determination of direct normal irradiance including circumsolar radiation in climate/NWP models

An issue in the determination of the direct component of solar radiation involving the circumsolar contribution in numerical weather prediction (NWP) and climate models is discussed. The direct solar radiation has multiple definitions in different applications, but this ambiguity has not always been clearly identified and has therefore sometimes caused confusion. The confusion arises from a fact that the photons that are scattered into the direct solar beam path by atmospheric particles are no different from the non‐scattered photons travelling in the same direction. Therefore there is a question whether these scattered photons should be included in the direct flux calculation or whether they should be excluded. Different definitions of the direct solar radiation in a wide range of applications have been discussed by Blanc et al. (2014). We further discuss it in terms of its relation to NWP and climate models. The short‐wave radiation schemes used in NWP and climate models can be classified into two groups: those that calculate the direct solar radiation without including scattering contributions and those which include the scattering contribution in terms of the delta‐Eddington approximation. We assess the two treatments by comparing modelled direct solar radiation with observations. It is found that the use of delta‐Eddington scaling results in positive errors in the direct solar flux at the surface, while neglecting the delta‐scaling leads to negative errors. The important result is that the positive error from using the delta‐Eddington scaling is at least 1 order of magnitude larger than the absolute negative error due to neglecting it. In order to include properly the scattering contribution in the direct flux calculation, a simple parametrization for dust aerosol is developed which can be used to consider the scattering contribution due to the particular case of dust aerosol to the direct solar radiation within the circumsolar region.

Numerical study on optical performances of the first central-receiver solar thermal power plant in Korea

In 2011, the first Korean central-receiver solar thermal power plant targeting the generation of 200 kWe was developed as a government research project. The project motivated us to develop an optical modeling program using the Monte Carlo ray-tracing method so that we are able to analyze performances of the solar concentration system in detailed and rigorous ways and to gain ideas to improve it. In the program, solar limb darkening without circumsolar radiation to model the sun shape was considered, and a clear-day insolation model to calculate Direct normal insolation (DNI) values was adopted. According to the breakdown of the optical efficiency, our program was developed to investigate the effects of six constituent efficiencies, i.e., cosine, shadowing, reflectivity, blocking, air transmission and spillage. On the other hand, a hierarchical screening algorithm to determine shadowing and blocking and a parallel computing technique were implemented for acceleration, which overcomes a major drawback of the Monte Carlo method and thereby enables to calculate annual performances. Comparisons with NREL’s SolTrace were made, and good agreements validated the development of our program. The hourly optical performances of the Korean solar thermal power plant on the summer and winter solstices and the autumnal equinox were simulated in terms of the solar flux and power distributions and the optical efficiencies. Basic parametric studies with the installation height and the tilt angle of the solar receiver were carried out based on annual performances. It is demonstrated that our program not only offers optical performance guidelines for operation confirms but also suggests improvement ideas that could enhance the efficiency and lead to a potential scale-up.

Solar flux distribution study of non-imaging dish concentrator using linear array of triple-junction solar cells scanning technique

A high solar flux scanner has been designed and constructed to acquire solar flux distribution profile of a non-imaging dish concentrator (NIDC). The scanner is capable of acquiring high concentrated solar flux distribution up to 1182 suns accurately and rapidly using an array of triple-junction solar cells. A numerical simulation has also been developed for modeling onsite solar flux distribution by considering all imperfection factors, i.e. circumsolar radiation, slope error and optical misalignment. A systematic technique has been proposed to analyze the amount of various imperfection factors for the NIDC prototype: mirror reflectivity of 0.93, slope error of 3 mrad, circumsolar ratio of 0.1, and optical misalignment angles of 0.2°.

Performance optimization of dense-array concentrator photovoltaic system considering effects of circumsolar radiation and slope error.

This paper presents an approach to optimize the electrical performance of dense-array concentrator photovoltaic system comprised of non-imaging dish concentrator by considering the circumsolar radiation and slope error effects. Based on the simulated flux distribution, a systematic methodology to optimize the layout configuration of solar cells interconnection circuit in dense array concentrator photovoltaic module has been proposed by minimizing the current mismatch caused by non-uniformity of concentrated sunlight. An optimized layout of interconnection solar cells circuit with minimum electrical power loss of 6.5% can be achieved by minimizing the effects of both circumsolar radiation and slope error.

Calculation of Optical Efficiency for the First Central-receiver Solar Concentrator System in Korea

In 2011, the first Korean central-receiver solar thermal plant of 200 kWe was developed as a government research project. Its solar field consists of 450 heliostats in the northern half to a 49 m tall tower, and each heliostat is made of 4 flat mirror facets with a 1 m2 reflecting area. In this paper, we present its optical efficiency calculated with a Monte Carlo ray-tracing code. The code enables to break down the optical efficiency into its six components, that is, cosine, air transmission, shadowing, reflectivity, blocking, and spillage, while taking into account the solar limb darkening without circumsolar radiation as well as reflector surface slope error. We carried out calculations at specific times on summer and winter solstices and autumn equinox. Negligible block efficiency demonstrates that the no-blocking heliostat field layout is well realized. The shadowing efficiency, which is due mainly to the tower rather than neighboring heliostats, is also insignificant except winter solstice. Net optical efficiency is best on autumn equinox but worst on summer solstice mainly because of the cosine efficiency.

Validating surface downwelling solar irradiances estimated by the McClear model under cloud-free skies in the United Arab Emirates

McClear, a fast model based on a radiative transfer solver, exploits the atmospheric properties provided by the EU-funded MACC project (Monitoring Atmospheric Composition and Climate) to estimate the surface downwelling solar irradiances for cloud-free instances. This article presents the first validation of the McClear model for the specific climate of the United Arab Emirates where skies are frequently cloud-free but turbid. McClear accurately estimates the global horizontal irradiance measured every 10min at seven sites. The bias ranges from −9Wm−2 (−1% of the mean observed irradiance) to +35Wm−2 (+6%). The root mean square error (RMSE) ranges from 22Wm−2 (4%) to 47Wm−2 (8%) and the coefficient of determination ranges from 0.980 to 0.990. Estimates of the direct irradiance at normal incidence exhibit an underestimation that is attributed to the overestimation of the aerosol optical depth in the MACC data set and not accounting for the circumsolar radiation in McClear. The corresponding bias ranges from −57Wm−2 (−8%) to +6Wm−2 (+1%). The RMSE ranges from 62Wm−2 (9%) to 87Wm−2 (13%) and the coefficient of determination ranges from 0.830 to 0.863. When compared to two other models in the literature, McClear is better able to capture the temporal variability of the direct irradiance at normal incidence. The validation results remain comparable for the global horizontal irradiance.

Direct normal irradiance related definitions and applications: The circumsolar issue

The direct irradiance received on a plane normal to the sun, called direct normal irradiance (DNI), is of particular relevance to concentrated solar technologies, including concentrating solar thermal plants and concentrated photovoltaic systems. Following various standards from the International Organization for Standardization (ISO), the DNI definition is related to the irradiance from a small solid angle of the sky, centered on the position of the sun. Half-angle apertures of pyrheliometers measuring DNI have varied over time, up to ≈10°. The current recommendation of the World Meteorological Organization (WMO) for this half-angle is 2.5°. Solar concentrating collectors have an angular acceptance function that can be significantly narrower, especially for technologies with high concentration ratios. The disagreement between the various interpretations of DNI, from the theoretical definition used in atmospheric physics and radiative transfer modeling to practical definitions corresponding to specific measurements or conversion technologies is significant, especially in the presence of cirrus clouds or large concentration of aerosols. Under such sky conditions, the circumsolar radiation—i.e. the diffuse radiation coming from the vicinity of the sun—contributes significantly to the DNI ground measurement, although some concentrating collectors cannot utilize the bulk of it. These issues have been identified in the EU-funded projects MACC-II (Monitoring Atmospheric Composition and Climate-Interim Implementation) and SFERA (Solar Facilities for the European Research Area), and have been discussed within a panel of international experts in the framework of the Solar Heating and Cooling (SHC) program of the International Energy Agency’s (IEA’s) Task 46 “Solar Resource Assessment and Forecasting”. In accordance with these discussions, the terms of reference related to DNI are specified here. The important role of circumsolar radiation is evidenced, and its potential contribution is evaluated for typical atmospheric conditions. For thorough analysis of performance of concentrating solar systems, it is recommended that, in addition to the conventional DNI related to 2.5° half-angle of today’s pyrheliometers, solar resource data sets also report the sunshape, the circumsolar contribution or the circumsolar ratio (CSR).