The spectral content of sunlight is influenced by a number of atmospheric factors, including the amount of water vapor and other aerosols in the atmosphere. The figure below compares spectra at the top of the atmosphere, from a theoretical blackbody at 5,250 deg C, and for typical sunlight at sea level. Major absorption bands are…
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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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[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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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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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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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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Sandia National Laboratories has developed a function that approximates the position of the sun quite accurately. This function, along with Matlab source code, is available as part of the PV_LIB Toolbox. The function is pvl_ephemeris. The calculation is based on an internal course, Engineering Astronomy, developed by Grover Hughes, and documents in these class notes .
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