Direct normal irradiance related definitions and applications: The circumsolar issue

•Review of existing definitions related to direct normal irradiance (DNI).•Review of applications and measurement techniques of DNI.•Expert consensus on clear definitions and terminology related to DNI.•Important role of circumsolar radiation in DNI. The direct irradiance received on a plane normal...

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Bibliographic Details
Published in:	Solar energy 2014-12, Vol.110 (C), p.561-577
Main Authors:	Blanc, P., Espinar, B., Geuder, N., Gueymard, C., Meyer, R., Pitz-Paal, R., Reinhardt, B., Renné, D., Sengupta, M., Wald, L., Wilbert, S.
Format:	Article
Language:	eng
Subjects:	Applied sciences Circumsolar irradiance Circumsolar ratio Concentrating solar technologies Definitions Direct energy conversion and energy accumulation Direct normal irradiance domain_spi.energ Earth Sciences Electrical engineering. Electrical power engineering Electrical power engineering Energy Engineering Sciences Equipments, installations and applications Exact sciences and technology Meteorology Natural energy Ocean, Atmosphere OTHER INSTRUMENTATION Photoelectric conversion Photovoltaic conversion Pyrheliometer Radiation Sciences of the Universe Solar collectors SOLAR ENERGY Solar radiation Solar system Solar thermal conversion Sun
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Summary:	•Review of existing definitions related to direct normal irradiance (DNI).•Review of applications and measurement techniques of DNI.•Expert consensus on clear definitions and terminology related to DNI.•Important role of circumsolar radiation in DNI. 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 re
ISSN:	0038-092X 1471-1257