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 PostPosted: Thu Aug 10, 2017 12:18 pm

Joined: Mon Apr 16, 2012 7:29 pm
Posts: 1586
PVsyst now defines 2 kinds of albedo.

The albedo of the project
This is the characterization of the far terrain in front of your PV installation, and its contribution is calculated by the transposition model.
It corresponds to the reflexions of the sun's ray on the far terrain on your PV array.

The albedo irradiance contribution is proportional to the Global horizontal irradiance, with a proportionality factor = 0.5 * (1 - cos(tilt)).
This means that for an horizontal plane its contribution is null (the plane doesn't "see" the ground), and maximal for a vertical plane (factor 0.5 * (1-Cos(90°)) = 0.5).
For a plane at 25°tilt, it would be 0.5 * (1-Cos(25°)) = 0.5 * (1 - 0.906) = 0.047, i.e. 10 times less.

This albedo contribution may become significant with little systems (BIPV) without shades on the ground level, and large plane tilts.
However for a big system in sheds (rows), the effect will become very low, because only the first row "sees" the albedo. Therefore there is a shading factor of (n-1)/n on the albedo contribution. As an example if you have 100 sheds, the shading factor will be 0.99 (only 1% left for the albedo contribution) !
This means that the albedo factor determination is not very significant for the evaluation of the yield of a big system.
Now if you increase the albedo factor, you will increase the GlobInc (result of the transposition on the tilted plane), but also increase the Linear shading loss by about the same amount.
Therefore you will get about the same yield (E_grid), but diminish the Performance Ratio, as the PR is referenced to GlobInc.
This is equivalent to say that the albedo loss is included in the Performance Ratio.

The albedo for bifacial systems
This characterizes the reflexions of the ground, just below your PV system (for example your roof).
I.e. this albedo concerns the surfaces which are directly "seen" by the rear side of your bi-facial PV modules.
The bifacial irradiance contribution will be directly proportional to this albedo value.

Definition of the albedo parameters
The albedo parameter for the project is defined in the Project's settings.
The albedo for the bi-facial systems is defined in the Bifacial dialog.
Both may be specified either as Yearly value, or in Monthly values.

The albedo is measured with an albedometer, which is basically made of 2 solarimeters: one measuring the horizontal irradiance (GlobHor), and one reversed measuring any irradiance coming from below the horizontal plane.
Usual values are listed in the program and in the help.

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