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IAM Diffuse Component

Guest joeR

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Dear all,

a question about the IAM modifier for the diffuse component: the PVsyst help specifies that this is calculated using an integral over all possible sky directions, and depends only on the tilt of the module. How exactly is this integral calculated for an ASHRAE parametrization? Do the integration limits depend on the plane tilt, or how is the tilt accounted for in the integral?

In addition, the "detailed losses" section in PVsyst gives a diagram of the IAM loss factor depending on the plane tilt (see attached image). I am puzzled as to why the loss factor is almost the same for a tilt of 0 and 90 degrees, given that it is calculated using an integral over all sky directions - shouldn't the visible sky be only half as large for a module tilted 90 degrees?

Thank you very much in advance - all the best



IAM diffuse loss factor

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  • 4 weeks later...

The integral supposes that the diffuse is isotropic, i.e. that the irradiance is the same whatever the sky direction.

Now with a tilted plane, PVsyst executes the integral of the incident rays (for each direction "seen" by the plane", i.e. the orange slice between the plane and the horizon) affected by the IAM function for this incidence angle.

This is not the integral of the diffuse, but the integral of the IAM effect (which is a relative loss). The effective results of this integral are indeed not very intuitive. I don't have a clear explanation.

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  • 1 year later...


I have a doubt regarding IAM Losses for Diffuse and Albedo components.

As seen in the PVsyst graphs, the loss values for Diffuse and Albedo corresponding to 90º tilt do not match as I believe it should be.

What is the reason for? Are you applying any factor to Diffuse IAM loss graph such as soiling 98%, or any other...?

Thanks in advance,


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