Reduced energy yields with PVsyst 7.0

[ckremin]

Dear PVsyst Team,

I made a brief comparison study between PVsyst 6.8.7 and 7.0.0 with identical project settings (identical copies of the variants) to see the difference in the simulation.

The new version provides slightly reduced results for bifacial c-Si. Especially the treatment of backside irradiation but also the low irradiance loss have seen some significant changes which affect future simulations.

I used various c-Si bi-facial and thin-film modules on Fixed2P and SAT1P mounting systems for a site in the US. I noticed a significant performance reduction of up to -1.5% for the c-Si modules. Especially modules who showed an extremely good low irradiance behavior before were severely impacted. Much to my surprise there was no significant change in the result for thin-film modules!

I have attached the results overview to this post.

Can someone from PVsyst please comment if this is indeed correct and intended or if this is a potential error in the new simulation? That would be highly appreciated.

Until further clarification of these differences, I recommended to my team to stick with the previous version 6.8.7.

Thank you and kind regards,

Christoph Kremin

Director, Systems Engineering + R&D | Blueleaf Energy

Results overview v6.8.7 vs v7.0.0

[unilhexio]

I agree... as I could see, major differences between both versions are:

- IAM losses

- far shadings / Horizon losses

(same PV modules and horizon profile, of course)

Other minor changes are included in transposition model.

Would be great an explanation about this from PVsyst team.

Regards.

[André Mermoud]

Irradiance treatment: circumsolar

The treatment of the irradiance has indeed 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 (i.e. proportional to the beam component) so that we have now 4 irradiance contributions:

Beam component, circulmsolar, isotropic diffuse and albedo.

In the shading calculations and IAM, 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 + circulsolar is higher. For the linear losses, these loss contributions approximately compensate each other.

But the Electrical shading losses are only related to the beam, and are therefore increased.

NB: These improvements are not so important in usual systems. They become crucial in vertical bi-facial systems modelling, for the evaluation of the back side irradiance.

This explains the differences of the simulation, in the optical contributions in the losses (shadings and IAM).

Low-light efficiencies:

The default values of the Rserie and Rshunt PV module parameters is assumed to ensure a low-light relative efficiency of -3%@200 W/m2, with respect to the STC.

However in the Version 6, this value was not always attainable (sometimes -4 or -5 %).

This may be improved by încreasing the Rshunt, which was not done correctly in the version 6. The new version 7 ensures that the low-light performance is -3%@200 W/m2 in any case.