Mismatch for back irradiance

[Choohuey Mei]

Hi, I am trying to calculate the value for "mismatch for back irradiance" based on the formula and explanation as following:

https://www.pvsyst.com/help/bifacial_results.htm

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"MismBak Mismatch for back irradiance

This is the result of the Mismatch parameter specified by the user (see Bifacial Procedure)

At this stage of the Array losses, on the results diagram this is referred as percntage of the full system energy.

As an example, if you have specified a loss of 10% (of the bi-facial contribution), and you have a bi-facial contribution of 15% of GlobEff, this will be 10% of GlobBakEn / (GlobBakEn + GlobEff).

NB: Here the GlobBakEn is GlobBak * Bifaciality factor"

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Lets say

My PVsyst report shown the value of:

Input for mismatch for back irradiance loss: 10%

GlobBak: 38 kWh/m2

GlobEff: 1742 kWh/m2

Bifaciality factor: 0.7

Based on the paragraph:

GlobBakEn = GlobBak x Bifaciality factor = 38 x 0.7 = 26.6 kWh/m2

"Mismatch for back irradiance" as show in the loss diagram over the whole year = 10% of GlobBakEn / (GlobBakEn + GlobEff)

= 10% of [26.6/ (26.6+1742)]

=0.1504%

I cannot get the value as per the loss diagram, as my PVsyst loss diagram show the value is 0.21%

I only get it with further divide by 0.7

= 10% of [26.6/ (26.6+1742)] /0.7

=0.2149%

Appreciate for your advice.

[S.Faulkner]

It appears that PVsyst actually calculates the effective rear mismatch loss by applying the loss (10%) to the rear irradiance BEFORE the bifacial factor is applied, ie in your example:

= 10% x [38/(38+1742)] =0.213%

It makes sense that the mismatch loss, being an electrical effect within the module, should logically be applied after applying the bifacial factor of 0.7.

However, we should also note that "The default of 10% is just a rough estimation". If the loss is applied after the bifacial factor, then a loss of about 14.4% (varying slightly with amount of bifacial uplift) will give the same result.

[Ronaldhew]

I will be glad to help. However, could you describe in more detail what the simulation look like, what need to be shown and so on.

It will be helpful if you can upload a simple diagram.

[laurahin]

I have the same question.  I have a case with the following parameters, taken from the waterfall diagram:

Effective radiation on collectors = 2051

Global irradiance on the rear side = 129

Bifaciality factor = 0.67

Rear side mismatch loss specified by user = 5%

The "mismatch for back irradiance" on the waterfall diagram is 0.3%.  Question: How is this value obtained?  Assuming that this value is applied to the total energy (from front and rear sides combined), it should be the user's input value times the fraction of the energy that comes from the rear side.  Since the module efficiency and the module area are the same on the front and rear side (once the shading loss, etc., are taken into account), this comes down to the effective irradiance on the rear side relative to the total (front and rear side) irradiance.  I calculate this as follows:

Effective rear side irradiance = rear side irradiance * bifaciality factor = 86

Total irradiance = 2137

Fraction of irradiance from rear side = 86/2137 = 4%

Thus the effective rear side mismatch loss should be 5% * 0.04 = 0.2%, but the value on the waterfall chart is 0.3%.

Clearly my thought process doesn't match PVsyst's approach.  Could someone clarify this?

Thanks,

Laura

[laurahin]

And this is on top of the front side mismatch that has already been applied to the entire energy, so it should arguably be even lower.