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Order of application of losses


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Is it true that the losses are applied in the order shown in the waterfall diagram? If so, why is the IAM loss applied before the soiling loss when the IAM loss only occurs when the light impinges on the module surface? By definition, light doesn't reach the panel if it encounters soiling. I don't know how much difference this would make to the overall calculation, but the current ordering doesn't make physical sense. Thanks.
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I believe it is done in order. Given the way the software calculates soiling loss, I dont think the difference is significant. Soiling takes an average monthly loss and applies it equally to each hour. In reality, soiling is more variable and non-uniform (both on module level and plant level). IAM loss also is more complex than how it is implemented in software, given as you said, the reflectivity of the light will be impacted by the material(s) present on the modules. Since the software is only accounting for the reflectivity of the glass and not accounting for reflectivity due to material on the modules, maybe that is why it takes IAM first and then takes soiling.

You can output all of the variables and determine what the difference would be if you reversed the order using the global soiling and IAM ratios. In my test (SAT in MD), there is a negligible difference. In default order, total irradiance after soiling is 2001.016 W/m^2 (shading > IAM > soiling). Swapping IAM and soiling, total irradiance is 2001.017 W/m^2 (shading > soiling > IAM). Maybe for a fixed tilt system and depending on location, you could get a higher discrepancy, but mathematically, I don't think it is significant in most cases.

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dtarin --

Thanks for the time you spent on your reply. I agree that the difference in production when you change the order of IAM and soiling losses should be small. However, applying the IAM loss first doesn't make physical sense. If sunlight is blocked by dirt, it never reaches the glass, so is not affected by IAM loss. So the incoming irradiance should first be reduced by the soiling loss before applying the IAM loss. Changing the order shows that you understand these processes (and should make the calculation ever so slightly more accurate). So I would recommend that to the PVsyst folks.

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  • 3 months later...


Sorry for intruding in an ongoing thread.

I would also consider changing the order of far shadings and Global incident in coll. plane effect. Physically, light is firstly blocked by mountains and other obstacles and then received by modules at their tilt that increases or decreases it compared to GHI.

Also, far shadings should probably be excluded from the PR calculations because, in reality, the pyranometers installed on site will only get the irradiance left after far shadings are subtracted. In some cases this is not true, so maybe make it an option.

Thanks in advance and best regards

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