Weather or atmospheric observations can be made to improve the performance of some models. Some weather measurements have small variations and can be approximated based upon location (e.g. atmospheric pressure can be estimated from site elevation). Others, such as air temperature, which have greater variation and play a strong role in PV system performance should…
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[latexpage] Determining the relative position of the sun from any location on Earth at any time of the year involves a series of calculations, many of which are approximations. The first step to understanding these is to summarize a few basic quantities. Declination of the Sun () – This is the angle between the Earth’s axis…
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Extraterrestrial radiation () is the intensity (power) of the sun at the top of the Earth’s atmosphere. It is usually expressed in irradiance units (Watts per square meter) on a plane normal to the sun. It varies throughout the year because of the Earth’s elliptical orbit, which results in the Earth-Sun distance varying during the…
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Direct Normal Irradiance (DNI) may be measured directly via an absolute cavity radiometer. Absolute cavity radiometers are considered the most accurate method of solar radiation measurement and form the basis of the World Radiometric Reference (WRR). However, absolute cavity radiometers are not designed for continuous, unattended, outdoor use. Thus, the primary method of measurement of…
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Global Horizontal Irradiance (GHI) is the amount of terrestrial irradiance falling on a surface horizontal to the surface of the earth. GHI can be measured with a variety of instruments. The most common instrument used to measure GHI is called a pyranometer which has a hemispherical (180°) view angle. The hallmark of a pyranometer is…
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developed a model for determining DNI from GHI as an improvement to the DISC model. This model, “DIRINT”, improves estimation by binning zenith angle, global horizontal transmittance, and dew point temperature; or various functions of these variables, including a time derivative of the global horizontal transmittance. Each parameterization or “bin” is indicative of the sky…
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Diffuse horizontal irradiance (DHI) is the terrestrial irradiance received by a horizontal surface which has been scattered or diffused by the atmosphere. It is the component of global horizontal irradiance which does not come from the beam of the sun (where “beam” is a 5° field of view concentric around the sun). Much like global…
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The position of the sun relative to an observer on the surface of the Earth is an important input needed to model PV system performance. The convention used to describe solar position includes: Zenith angle () Azimuth angle () Solar elevation angle () is equal to The figure below shows how these angles are defined.…
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from the National Renewable Energy Laboratory published a sun position algorithm with a reported uncertainty of +/- 0.0003 degrees. The report is available from NREL at this link. NREL has developed an implementation of the algorithm in C. License information is available at this link. Sandia has implemented the algorithm as part of its PV_LIB…
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