SoDa. Info Clear-sky

Simulation of the Hourly Irradiation for one Clear Sky Day

Purpose To provide as assessment of the radiation under clear-sky, every hour in a given day. Gives the hourly irradiation on a horizontal or inclined surface as well as the daily irradiation. Outputs are global, direct, diffuse and reflected components and the irradiation at the top of atmosphere (TOA, also called the extraterrestrial irradiation). Time is the True Solar Time (zenith = 12 h). Unit is Wh.m^-2.

Inputs
Select a geographical site (see "directions of use" of the SoDa Service).

Enter a day.

For an inclined surface, enter tilt and azimuth which are angles defining the orientation of the plane surface receiving the radiation (the collector or the terrain slope). Tilt ranges from 0° (horizontal) to 90° (vertical). Azimuth ranges from -180° to 180°. Enter the value of the ground albedo (default=0.2).

Enter an altitude, in meter. If not, the value will be taken from a database.

Enter a value for the Linke turbidity factor, noted TL.

Linke Turbidity Factor
The Linke turbidity factor characterises the clearness of the sky. It sums up the effects of the aerosols and water vapour.
This factor is approximately 3 - 3.5 for Europe.
The category of services "Climatology" of the SoDa Service provides typical values of TL for each month, any site. For simulation, one may use the following table

Pure Sky Very Clear Clear Summer, with water vapour Polluted (urban or industrial areas)

TL 1.0 2.0 3.0 5.0 7.0

The lower this factor, the clearer the sky, the larger the beam irradiation, the lower the relative fraction of diffuse irradiation.
The more elevated the site, the lower the absorption - scattering of the radiation by the atmosphere and the lower the Linke turbidity factor. For very high elevations, values less than 1 are possible.
For simulations, we recommend to perform two runs, one with a low TL (e.g., 3.0), the second with a large TL (e.g., 5 or 6). The difference provides information about the sensitivity of the TL on the results.

Technical Details
The altitude of the geographical site of simulation can be either extracted from the orography resource (if set to 0) or forced by the user by inputting a value greater than or equal to 1. Unit is meter. The orography resource has a grid cell of 5' of arc angle and uses the data base TerrainBase (TerrainBase, Worldwide Digital Terrain Data. Documentation Manual, CD-ROM Release 1.0, April 1995. NOAA, National Geophysical Data Center, Boulder, Colorado, USA).

This service makes use of the clear-sky model of the European Solar Radiation Atlas (see the ESRA handbook, volume 2). More technical information on ESRA. A free software is available for the radiation on horizontal surfaces (library ESRA clear-sky model). The assessment of the radiation on inclined surfaces is made by the means of the algorithm of Muneer (1990); a library in php was written by Mrs Albane Levesque that is used in this application (2006).

References
The clear-sky model is also described in the following articles:
Rigollier C., Bauer O., Wald L., 2000. On the clear sky model of the 4th European Solar Radiation Atlas with respect to the Heliosat method. Solar Energy, 68(1), 33-48. Abstract. These authors compared the ESRA model to other clear-sky models. The diffuse clear-sky irradiation estimated by this ESRA model and by other models has been also checked against ground measurements, for different ranges of the Linke turbidity factor and solar elevation. The results show that the ESRA model is the best one with respect to robustness and accuracy. The r.m.s. error in the estimation of the hourly diffuse irradiation ranges from 11 Wh.m^-2 to 35 Wh.m^-2 for diffuse irradiation up to 250 Wh.m^-2.
Geiger M., Diabaté L., Ménard L., Wald L., 2002. A Web service for controlling the quality of global solar irradiation.Solar Energy, 73(6), 475-480. Abstract. This article reports on some corrections brought to the ESRA model in case of high terrain elevation.