Bill Marion from NREL developed a correction method that improves the ability of PVWatts to represent the nonlinear behavior at low irradiance, which is evident in x-Si module performance (Marion, 2008). Marion found that the error could be accurately characterized by the following equation: . is determined from a measurement of at 200 W/, which…
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The effect of a one-degree error in the azimuth angle for a latitude tilt system is similar throughout the year. The first figure below shows the POA irradiance error pattern on the summer solstice. At other times of the year the pattern is similar except for the changes in the length of the day. This pattern is…
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The three figures below show the effect of a one-degree error in the tilt angle on the plane of the array irradiance for key times of the year. At the equinoxes, the error is negligible. At the summer solstice the error is negative (defined as (POA36.05° – POA35.05°) and lasts longer due to the long…
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One of the most significant inputs to PV performance models is the measured irradiance on the plane of the array. If the irradiance sensor is misaligned relative to the array or the array orientation is out of specification, the prediction accuracy will suffer. This mini-study is aimed at discovering the sensitivity of predicted annual energy…
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The physical model for the incident angle modifier is based on Snell’s and Bougher’s laws, and was published by De Soto et al. (2006). Our presentation here includes correction to a few errors present in that paper. The first step is to calculate the angle of refraction () using Snell’s law. , where is the index of…
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The AM 1.5 Standard Spectrum refers to exactly two standard terrestrial solar spectral irradiance spectra. The two spectra define a standard direct normal spectral irradiance and a standard total (global, hemispherical, within steradian field of view of the tilted plane at from horizontal) spectral irradiance. The direct normal spectrum is the direct component contributing to…
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The objective of performance modeling is to predict the expected output of a solar system given 1) the system design, and 2) the environment in which it is operating, including the solar resource. To evaluate a performance model requires: simultaneously measuring the system output and the system environment, including the solar resource modeling system output,…
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Air mass is a relative measure of the optical length of the atmosphere. At sea level, when the sun is directly overhead (zenith angle = 0), the air mass is equal to 1. As the zenith angle becomes larger, the path of direct sunlight through the atmosphere grows longer and air mass increases. In contrast,…
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The angle of incidence () between the Sun’s rays and the PV array can be determined as: where and are the solar azimuth and zenith angles, respectively. and are the tilt and azimuth angles of the array, respectively. Azimuth angle convention is defined as degrees east of north (e.g. North = 0, East = 90,…
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This project is led by Joshua S. Stein, a Distinguished Member of Technical Staff at Sandia National Laboratories, Albuquerque, NM. If you would like to contribute content to this site, please email Dr. Stein at jsstein@sandia.gov with your content or ideas for content or use the form below. All contributed content will be reviewed and…
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