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Bi-Facial Module electrical characteristic with % gain


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We are seeing a lot of Bi-Facial module manufactures/products due to the cost and production gain. With that being said, how is PVsyst taking the electrical characteristic (isc and imp) into consideration since there is a percent gain due to the backside of the modules? See LG cutsheet for example http://www.lg.com/global/business/download/resources/solar/NeON%202%20BiFacial%2072cells.pdf
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PVsyst uses the one-diode model established from the usual STC data (on front side).

When using the bifacial gain, the back-side irradiance is simply added to the GlobEff value on the front side, with a coefficient named the "Bifaciality factor".

The increase of ISC current with bifacial irradiance is not relevant. This is an operating condition. When you are using a module at anothe irradiance than 1000 W/m2, you don't take care of the ISC at this irradiance: this is a result of the model.

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  • 3 years later...

I'd like to ask a question about how the Bifacial gain can be calculated from PVsyst. I've calculated the bifacial gain two different ways:

1. Toggle bifacial calculation on and off in the "bifacial system" tab of the main "system" tab; toggle from "Use unlimited trackers 2D-model" to "Don't use in the simulation"

  • I've recorded the resulting energy production for a particular project:
    • "Use unlimited trackers 2D-model" results in 44,395 [MWh/a]
    • "Don't use in the simulation" results in 43,114 [MWh/a]
    • Calculate gain: 100*(44,395 - 43,114)/43,114 = 2.97[%]

2. Calculate the Bifacial Gain from the Sankey (Waterfall) diagram:

  • (Global Irradiance on rear side[%])*(Bifaciality Factor)-(Mismatch for back irradiance[%]) = Bifacial Gain [%]
    • Global Irradiance on rear side[%] = 4.94[%]
    • Bifaciality Factor = 0.75[]
    • Mismatch for back irradiance [%] = 0.48[%]
    • 4.94*0.75-0.48 = 3.23[%]

I've done this for a few different projects and never get agreement between the 2 methods above. What am I doing wrong here and why doesn't PVsyst automatically calculate the bifacial gain?





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You are comparing an irradiance gain with an energy gain. The two are not the same, nor will ever be. It is typical to have a higher irradiance gain than your MWh gain as there is conversion still from light to electricity and other downstream losses than can be influenced (i.e. clipping). 

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There is no single frame of reference to define bifacial gain. You can look at the irradiance gain after bifaciality and consider that. Or you can look at the the output at the inverter terminals with and without bifacial included, as you have. One might compare a monofacial module to a bifacial, as there are physical (and economic) differences between the two. Using irradiance after bifaciality is common in my experience. 

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