Direct normal irradiance related definitions and applications: The circumsolar issue

Abstract

International audienceThe 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 sig-nificant, especially in the presence of cirrus clouds or large concentration of aerosols. Under such sky conditions, the circumsolar radi-ation—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 sys-tems, it is recommended that, in addition to the conventional DNI related to 2.5° half-angle of today's pyrheliometers, solar resource ScienceDirect Solar Energy 110 (2014) 561–577 data sets also report the sunshape, the circumsolar contribution or the circumsolar ratio (CSR)

Digging the METEOSAT Treasure—3 Decades of Solar Surface Radiation

“…Further, circumsolar radiation contributes to the direct irradiance. This contributes to an enhanced uncertainty of DNI due to reasons related to different measurement geometries as well as differences relative to the definition of direct irradiance in RTM or solar energy applications, please see [67] for further details.…”

A database of multi-year (2004–2010) quality-assured surface solar hourly irradiation measurements for the Egyptian territory

“…The varying aperture (or opening) halfangles of the different pyrheliometeric systems, normally between 2.5 • and 5 • , means they are also measuring the radiation surrounding the solar disc, i.e. the circumsolar radiation (Blanc et al, 2014).…”

Can AERONET data be used to accurately model the monochromatic beam and circumsolar irradiances under cloud-free conditions in desert environment?

“…This would directly contribute to the recommendation of Blanc et al (2014) to report the sunshape and circumsolar ratio with the standard DNI measurements. A further step may then be to integrate the model of Reinhardt (2013) and Reinhardt et al (2014) to devise a model which works under both cloud-free and cirrus cloudy conditions.…”

“…To tackle the ambiguity of the "small solid angle", Blanc et al (2014) recommend reporting the viewing angles of the measuring instrument along with the sunshape and contribution of the circumsolar irradiance in addition to the DNI measured by pyrheliometers or equivalent pyranometric systems. The contribution of the CSNI may be provided in terms of CSNI defined for the solid angle aperture, or in terms of the circumsolar ratio (CSR), which is the ratio of the CSNI to the sum of the CSNI and DNI S (Buie et al, 2003).…”