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PostPosted: Mon Jun 08, 2020 12:10 pm 

Joined: Thu Oct 03, 2013 9:03 am
Posts: 91
Irradiance treatment: circumsolar
The treatment of the irradiance has been improved.
We now distinguish a new irradiance contribution: the circumsolar (enhanced irradiance in a crown around the sun).
This contribution was previously included in the Diffuse component, and treated as such in the shading calculations (i.e. isotropically).
It is now evaluated from the Transposition models (it is about proportional to the beam component) so that we have now 4 irradiance contributions on the collector plane:
Beam component, circumsolar, isotropic diffuse and albedo.

NB: What is considered as Circumsolar ?
This is indeed a difficult question.
In all usual meteorologigal data, the circumsolar contribution is accounted with the diffuse component.
When the diffuse is effectively measured, the instruments may measure:
- Either the DNI in a cone of 5° around the sun (the diameter of the sun is 0.53°). The circumsolar contribution, as interpreted by the transposition models (Perez or Hay) is the irradiance outside of this cone.
- Or by a solarimeter with a rotating cache, which should correspond to this 5° cone (but very difficult to manage correctly without sophisticated tracking instruments)
- Or by a solarimeter and a shadowing band, with some uncertain corrections.
Now when establishing the diffuse with a model, the Perez model for the diffuse (DirInt) produces of course a diffuse compatible with these hypothesis. The additional "Circumsolar" contribution is evaluated in the Transposition models (either Perez or Hay).

Use in shading and IAM calculations
In PVsyst the circumsolar is now treated in the same way as the beam (i.e. coming from the direction of the sun).
This means that the Shading loss on isotropic diffuse is lower, and the shading loss on beam + circumsolar is higher.
- For the linear losses, these loss contributions approximately compensate each other.
- The Electrical shading losses are only related to the beam, and are therefore enhanced.
In the same way, we have also a transfer of the IAM losses on diffuse to IAM losses on beam component.

NB: These improvements are not very important in usual systems with low plane tilt, and negligible in tracking systems.
They become crucial in vertical bi-facial systems modelling, for the evaluation of the back side irradiance in sunny conditions. In this case the circumsolar contribution was previously included in the diffuse, and therefore highly contributed to the incident irradiance on the rear side (result of the transposition), which is obviously not correct. The circumsolar contribution is now blocked on the face opposite to the sun.
With the new simulation, rotating the East-West bifacial vertical collectors by 180° gives quasi-identical results.

This new treatment explains the differences in the simulations between V 6 and 7, concerning the contributions in the optical losses (shadings and IAM).
You can get the previous results by changing the Circumsolar treatment mode.

Activation of the Explicit treatment of circumsolar
The new calculation method was already introduced in PVsyst V6.80, but it needed to be activated explicitly by the user.
From PVsyst V7.0 onward it has become the default.
You can choose this setting in the main menu "Settings > Preferences", page "Physical models".


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