Albedo calculation

[Tom Bradley]

Good morning,

I have a question about how PVsyst calculates the contribution from albedo. I have created an excel based model for irradiance, which includes direct (I_(d,surface)=I_(d,horizontal)/sin(α)*cos(θ)), diffuse (Perez) and albedo (R_surface=(ρ(1-cosβ)*G_horizontal)/2). When I compare this to PVsyst, I find minor differences with the direct and diffuse, but a significant discrepancy for the albedo contribution at sunrise and sunset. This discrepancy increases as the pitch of the module increases. Does PVsyst have extra components in the albedo calculation to the basic R_surface=(ρ(1-cosβ)*G_horizontal)/2 calc?

Thanks,

Tom

[André Mermoud]

Your calculation seems to be correct.

I can't understand that this is depending on the pitch of the modules: it is completely unrelated.

However for the sunrise and sunset hours, the solar geometry is not calculated in the middle of the hour, but the middle of the interval between sunrise and the next hour. This may be the cause of your difference.