Measurement and analysis of diffuse solar irradiance
Measurements of the diffuse irradiance on a horizontal surface using a continuous tracking occulting disc and a pyranometer have been examined for cloudless days. The part of the diffuse component that is blocked by the occulting disc (circumsolar component) is analyzed with respect to the aerosol optical depth at a wavelength of 500 nanometers, solar zenith angle, and precipitable water. The magnitude of the circumsolar component is determined from the direct normal irradiance measured with a pyrheliometer, aerosol optical depth and precipitable water measured with a Volz sun photometer, and direct normal irradiance calculated from a direct beam model. The pyrheliometer measures the direct beam as well as the circumsolar component. The model was used to estimate only the direct beam over the same spectral interval as measured by the pyrheliometer. The computed component is subtracted from the measured direct normal irradiance and multiplied by the cosine of the solar zenith angle. The difference is an approximate value of the circumsolar irradiance on a horizontal surface. It is found that the circumsolar irradiance is a maximum for small solar zenith angles, low aerosol optical depths, and small amounts of precipitable water. The largest change occurred for an increase in aerosol optical depth from .153 to .337, a solar zenith angle of 35/sup 0/, and 0.5 precipitable centimeters of water. The circumsolar component decreased by 17% and the measured diffuse increased by 50%.
- OSTI ID:
- 6066007
- Country of Publication:
- United States
- Language:
- English
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