Proposed reference irradiance spectra for solar energy systems testing

In a combined effort, the Centre for Solar Energy and Hydrogen Research in Germany and the Solar Energy Research Institute in the United States analyzed several thousand measurements of the solar spectral irradiance recorded at four sites. The goal was to develop a semiempirical model that describes the total solar spectral irradiance for clear and cloudy sky conditions based on readily available input data. To investigate how the spectral transmission of clouds deviates from an assumed neutral density filter, the measured spectra are compared with simulated clear-sky spectra. A correlation is established between the cloud thickness and the relatively higher transmission of clouds in the ultraviolet and blue region of the solar spectrum. Using this approach, a semiempirical model is proposed based solely on global and diffuse broadband irradiance measurements, precipitable water-vapor data, and the sun's position. The model, called SEDESI, is applied to calculate spectral irradiance in short time...

Solar spectral irradiance under clear and cloudy skies : measurements and a semiempirical model

We have analyzed the colorimetric and spectral characteristics of 2600 daylight spectra (global spectral irradiances on a horizontal surface) measured for all sky states during a 2-year period at Granada, Spain. We describe in detail the chromaticity coordinates, correlated color temperatures (CCT), luminous efficacies, and relative UV and IR contents of Granada daylight. The chromaticity coordinates of Granada daylight lie far above the CIE locus at high CCTs (>9,000 K), and a CCT of 5,700 K best typifies this daylight. Our principal-components analysis shows that Granada daylight spectra can be adequately represented by using six-dimensional linear models in the visible, whereas seven-dimensional models are required if we include the UV or near-IR. Yet on average only three-dimensional models are needed to reconstruct spectra that are colorimetrically indistinguishable from the original spectra.

/pdf/color-and-spectral-analysis-of-daylight-in-southern-europe-3nnfezw6yl.pdf

Color and spectral analysis of daylight in southern Europe.

A physically modeled method is presented to obtain accurate turbidity determinations from broadband direct irradiance measurements. The method uses parameterizations of various extinction processes affecting the transfer of shortwave radiation in a cloudless atmosphere. The integration over the shortwave solar spectrum is performed with a more realistic weighting function than is conventionally used. The calculation and properties of the broadband aerosol optical depth are discussed in detail as a function of the aerosol optical characteristics. The method is general, as it can predict any one of the four turbidity coefficients currently used in climatological studies as defined by Angstrom, Linke, Unsworth–Monteith, and Schuepp. Formal interrelationships are proposed so that climatological data based on different coefficients can be consistently intercompared without recourse to empirical formulas. The new parameterizations are more detailed than those of the literature, particularly regarding t...

https://journals.ametsoc.org/doi/pdf/10.1175/1520-0450%281998%29037%3C0414%3ATDFBIM%3E2.0.CO%3B2

Turbidity Determination from Broadband Irradiance Measurements: A Detailed Multicoefficient Approach

As part of our ongoing research into the clear daytime sky’s visible structure, we analyze over 1500 skylight spectra measured during a seven-month period in Granada, Spain. We use spectral radiances measured within 3° fields of view (FOV’s) to define colorimetric characteristics along four sky meridians: the solar meridian and three meridians at azimuths of 45°, 90°, and 315° relative to it. The resulting clear-sky chromaticities in 44 different view directions (1) are close to but do not coincide with the CIE daylight locus, (2) form V-shaped meridional chromaticity curves along it (as expected from theory), and (3) have correlated color temperatures (CCT’s) ranging from 3800 K to ‘ K. We also routinely observe that sky color and luminance are asymmetric about the solar meridian, usually perceptibly so. A principal-components analysis shows that three vectors are required for accurate clear-sky colorimetry, whereas six are needed for spectral analyses. © 2001 Optical Society of America OCIS codes: 010.1290, 330.1710, 330.1730.

/pdf/colorimetric-and-spectroradiometric-characteristics-of-3brcw5lkbb.pdf

Chuck Whitaker

Papers

Spectral solar radiation data base at SERI

Outdoor rating conditions for photovoltaic modules and systems

Photovoltaics for utility-scale applications: project overview and data analysis

Thin-film PV performance at PG&E