# Bifaciality Loss Tree

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I am finding the new bifacial irradiance loss tree doesn't appear internally consistent? See two attached reports

PVsyst v6.6.7

Test_Bifacial_Project_VC7.pdf

(1 - 0.700) * ( 1 - 0.817 ) * ( 1 + 0.238 ) * ( 1 + 0.025 ) * ( 1 - 0.05 ) = 6.6%

PVsyst reports 5.4% (1.2% discrepancy)

Test_Bifacial_Project_VC8.pdf

(1 - 0.700) * ( 1 - 0.898 ) * ( 1 + 0.230 ) * ( 1 + 0.022 ) * ( 1 - 0.05 ) = 3.7%

PVsyst reports 6.2% (2.5% discrepancy)

Test_Bifacial_Project_reports.zip

Two examples of bi-facial irradiance loss tree discrepancies

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• 2 weeks later...

Sorry, things are not so simple ...

On the ground you have 821 kWh/m2 falling on 5742 m2

Reflexion (albedo) : loss -70.0% => remains 246.3 kWh/m2 on 5742 m2, i.e 1'414 MWh total

View factor from rear side: loss -81.7% of the energy, i.e. 259 MWh total

Now, the modules area = 2909 m2 => GCR = 2909/5742 = 0.507, 1/GCR = 1.973)

=> irradiance per m2 of collectors = energy back collectors / area = 259 MWh/2909 m2 = 89.0 kWh/m2

Therefore: after applying the View factor loss in %, the irradiance has to be multiplied by 1 / GCR !

Add sky diffuse on the rear side: + 23.8% => 110.1 kWh/m2

Add Beam effective on the rear side: +2.5% => 112.8 kWh/m2

Sub Shading loss on rear side: -5% => remains 107.2 kWh/m2 on the rear side.

This is the value on the loss diagram.

Now the mentioned percentage value is not the accumulation of all these losses, but the fraction of rear irradiance with respect to GlobEff,

i.e. the bifacial relative gain 107 kWh/m2 / 1968 kWh/m2 = 5.44 % ! Loss diagram of this simulation