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Understanding bifacial loss calculations


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There is a value associated with bifacial modules whose calculation I do not understand:

1.Global incident on ground (Loss diagram)

How is this number calculated?

2. Global fraction on ground (Bifacial system > Unlimited Trackers 2D model)

There is a monthly value here. Does this number represent the fraction of radiation from that month's Global horizontal radiation that does not fall on the front surface of the bifacial modules and instead reaches the ground?

If so, I would expect that for each month: Global incident on ground = Global horizontal radiation * Global fraction on ground

The value Global incident on ground would be the sum of all 12 months. This is not the case however, even when the values are renormalized to the ground area and not the collector area.

In my example, this is what I attempted to calculate:


The value I get is 894.92 kWh/m^2 but the value in the loss diagram for Global incident on ground is 973 kWh/m^2. What is the proper way to arrive at Global incident on ground?

Please provide some clarification on how these values are calculated. Thank you!

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The global fraction on ground, that you find in Bifacial system > Unlimited Trackers 2D model, is calculated for clear sky conditions.

As such, it is not applicable to your simulation results (unless you have a clear sky meteo file).

Your intuition about how the global incident on ground is calculated is correct. However, PVsyst does the calculation hour by hour and differentiates the beam and diffuse irradiance components. The beam and diffuse components have different "fractions on ground", that will generally vary every hour with the sun / tracker positions. Therefore it is difficult (probably even impossible) to calculate it from the current outputs of PVsyst, in particular from the monthly results.

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