I understand your bewilderment.
With the PVsyst simulations, our objective is always to approach the reality at best. But this involves models, and models are never the reality
Now PVsyst is considered by some people as a reference, and many assume that "this is the truth". This situation is quite independent on our willingness
We never pretended to become such a reference, and we cannot assume any responsibility about the full reliability of the results in any case.
In particular, this should not prevent us to do modifications in the simulation, if we judge it necessary, even if it is not in accordance with previous simulations.
A software which doesn't evolve is dead.
In PVsyst, the estimation of the shading effects on the diffuse is a model
. It is based on the hypothesis that the diffuse is isotropic, and performs an integral of the irradiance with shading factor over all directions of the sky vault "seen" by the collectors. A similar hypothesis and calculation is applied to the albedo contribution, involved in the transposition models.
This model is from my own, implemented in Pvsyst since the beginning (1993). I proposed a research project for assessing this hypothesis by an experimental work, but I couldn't obtain financial support: this question didn't have any interest for anybody!
I don't know any publication studying the effect of shadings on the diffuse part. I don't know how this question is treated in other software. However this is a crucial question as it usually represents half of the shadings loss (or more) in most cases. If someone has some references, please communicate them to me.
The calculation of the tracking systems was introduced in PVsyst more than 10 years ago. The calculation of the shading on diffuse was treated in the same way as for the fixed planes, i.e. by integrating over the shading factor table. This was indeed an erroneous assumption, but I was not aware of that until about one year ago. Correcting this represented a not megligible work, that I just performed for the version 6.1. I could'nt guess that this would imply such a high loss before doing the full calculation (see our FAQ How is calculated the shading loss factor in tracking systems
But this is quite plausible if you have a look on my publication "Optimization of row-arrangement in PV Systems, shading loss evaluations according to module positioning and connexions"
. This shows that in row arrangement the shading factor on diffuse is proportionnal to the plane tilt; then in tracking systems we have high tracker's tilts in the morning and the evening. Moreover the limit angle is usually rather high (increases with the tilt when the pitch is constant). The situation will still be worse with lower GCR, as when the distance between rows increases, the maximum tilt allowed by the backtracking will also increase.
File comment: Shading factor on diffuse in row arrangement, as computed with Pvsyst hypothesis.
It is a geometrical characteristics, independent on the meteo data and site.
Shading_Factor_On_Diffuse.png [ 17.81 KiB | Viewed 21213 times ]
The loss is also due to the albedo contribution: the transposition model yields an albedo contribution proportionnal to (1 - cosi)/2 (i = tilt angle), with about 1% loss at 30° tilt, increasing more than linearly. This contribution is completely lost in big arrays: only the first row "sees" the albedo contribution.
NB: This calculation error appears essentially in the calculation of backtracking
systems. The old calculation with "normal" tracking is much less affected: according to my first observations (to be confirmed) the result with the new calculation is rather similar to the old one: the shading factors (global of for each position) compensate each other.
This work about shadings with tracking systems was undertaken after the publication "Increased Energy Production of First Solar Horizontal Single-axis Tracking PV Systems without Backtracking", Lauren Ngan and al, First Solar, San Francisco 94105.
Now the economical impact of such uncertainties in the simulations essentially acts on the previsions
of the financial balance, not in the real yield. PVsyst has never created nor lost any kWh by itself !
This can have some implications on contracts, or technological choices like choosing Backtracking. However the backtracking choice may easily be reverted to "normal operation" without significant costs, if this appears to be better.