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<< Click to Display Table of Contents >> Far shadings - Horizon |
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See also Shadings - General
The horizon of far shadings part is the simplest way to define shadings in PVsyst. But this is only suited for treating shadings of objects sufficiently far, as we can consider they act on the PV field in a global way: at a given instant, the sun is or is not visible on the field. Typically the distance of these shading objects should be higher than, say, ten times the PV field size.
For nearer objects you should use the option for near shadings.
Defining a horizon profile is a very simple operation with the PVsyst graphical tool. The horizon is a broken line superimposed on the sun path diagram, which can hold any number of height/azimuth points.
| - | To modify it, simply drag the red dots with the mouse (or define the desired value in the corresponding edit box). |
| - | To add a point, click anywhere with the right button. |
| - | To delete a point, click on this point with the right button. |
Horizon measurements (list of height and azimuth of some significant points) can be obtained on-site with a compass and theodolite (clinometer), a detailed map, panoramic or fish-eye photographies, etc.
NB: an horizon profile with all heights less than 2° is considered insignificant. It is not taken into account in the simulation and is not shown on the report.
Importing a Horizon profile:
You have the possibility of importing horizon profile files from some other tools or Software.
Saving Horizon profile:
A horizon profile can be saved to reuse it in another project or meteo calculation. It is stored in the \Shadings\ subdirectory with an extension .HOR.
NB: A file with PVsyst format is not an ASCII file and cannot be exported to other software.
Treatment during the simulation process:
The effect on the beam component is of the "ON/OFF" kind: at a given instant, the sun is or is not visible on the field. As meteo is recorded in hourly time steps, the program determines the exact time when the sun crosses the horizon line and weights the beam hourly value before performing the transposition.
The effect on the diffuse component is not so clear. We can admit that radiation from the back side of the obstacles is null, and therefore the diffuse attenuation is calculated as an integral of an isotropic radiation over the portion of sphere "seen" by the plane, above the horizon line. This is independent of the sun position, and therefore constant over the year.
Albedo contribution is more difficult to estimate. For far horizons, some radiation may be reflected by the ground ahead of the collector plane. We consider the albedo to be linearly decreasing according to the horizon height (up to zero for horizon > 20°). On the other hand, if the "horizon" obstacle is rather near, albedo should be considered as null. Therefore the user has the opportunity of determining which fraction of calculated albedo he wants to take into account, according to the distance of horizon obstacle.
The reality is certainly very complex, and requires more experimental investigations to assess these hypotheses on diffuse and albedo contributions. Nevertheless, we can observe that these contributions (and their errors) are rather low for low plane tilts, since the horizon irradiation has a low cosine factor. They become more significant for very tilted or vertical planes.
Read also our note on Horizon in Meteorological data.