How is the albedo handled (of the system and bifacial)

[André Mermoud]

Definition

We name "Albedo" the reflexion coefficient on a ground. When a ground receives some irradiance, it will re-emit a part of this irradiance and absorb the remaining. The albedo coefficient is the ratio of the re-emitted with respect to the received irradiance,

This re-emission is supposed "Lambertian", this means that each point of the ground re-emits the irradiance in all directions. The specular reflexions (like on a mirror) are not considered in the albedo.

PVsyst defines 2 kinds of Albedo parameters:

- The albedo of the project, which is the albedo of the far terrain in front of the PV installation. It contributes to the irradiance in the transposition model. This is defined in the Project's settings dialog.

- The albedo for the bifacial evaluation: this is the albedo of the ground, just below the installation, which is "seen" by the rear side of the collectors. It is defined in the bifacial dialog.

Both may be defined in yearly or monthly values.

The help provides usual values of the albedo coefficient, normally valid either for the far albedo and for the bifacial ground albedo.

 

Albedo of the project

When evaluating the irradiance on a tilted plane, the albedo contribution is the light "reflected" from the far terrain in front of the installation.

In all transposition models, the albedo irradiance contribution is evaluated in the same way:

AlbInc [W/m²] = ρ * GlobHor * (1 - cos (tilt) ) / 2

where GlobHor = irradiance on horizontal plane, tilt = tilt angle of the receiving plane and ρ = albedo coefficient.

The expression (1 - cos i) / 2 indicates that the albedo contribution is maximal on a vertical plane (expression value (1-0) / 2) = 50%), and diminishes when the tilt diminishes (14% at 45°, 6.7% at 30°, 3% at 20% and only 0.8% at 10° tilt). The horizontal plane doesn't see any albedo of course.

Shading factor

In a sheds (rows) arrangement, only the first row "sees" the albedo (ground level). Therefore for the whole system, we have a shading factor on this contribution equal to (n-1)/n, where n is the number of rows. As an example, if you have 100 rows the shading factor will be 0.99 !

This is also valid for tracker arrays, where to albedo shading loss is a significant contribution to the irradiance diffuse loss, even with backtracking.

If you have near obstacles in front of your system (buildings, etc) the far albedo contribution is not seen. The shading factor is an integral of the albedo contributions in all directions in front of the plane. In fact by analogy with the diffuse calculation, we integrate the contributions of the virtual portion of the sphere under the horizon, included between the horizontal plane and the plane of the collectors. This contribution is only accounted for the azimuths without near obstacle.

As for the diffuse, this shading factor on albedo is independent of the sun's position, and therefore constant over the year.

With a far horizon, you can choose the fraction of albedo (in front of the far horizon) that you will take into account.

Albedo and PR

As the albedo contribution is rather low in the global incident irradiance, the exact determination of the albedo coefficient is not very important.

However in sheds or tracker systems, there is a perverse effect: the albedo is part of the GlobInc evaluation, which is the basis for the Performance Ratio determination.

Therefore if you have a high albedo, you will have a higher GlobInc value. But as the albedo contribution is almost completely lost, the Yield will be the same. Therefore the PR will diminish !!! In other words, as the albedo loss is included in the PR, the yield will be the same whatever the albedo coefficient, but the PR will change !

As a conclusion

The albedo contribution may become significant with little systems (BIPV) without shades on the ground level, and large plane tilts.

But its contribution is low or negligible with big PV systems, so that its determination is not crucial.

Albedo for bifacial systems

This characterizes the reflexions of the ground, just below your PV system (for example your roof).

I.e. this albedo coefficient concerns the surfaces which are directly "seen" by the rear side of your bi-facial PV modules.

The bifacial irradiance contribution (and bifacial gain) will be directly proportional to this albedo value.

Albedo coefficient measurement

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. The albedo coefficient is the ratio between both measurements.

The albedo measurements on-site should be long-term measurements. The instantaneous albedo value may depend on the height of the sun, the state of the ground (wet or dry), the ageing of the ground, etc.

 

NB: Solarimeters in the plane of array, used for a reference incident irradiance in existing systems, should be positioned in a way that they "see" the albedo (on the first shed).. Otherwise the albedo coefficient has to be adapted (about null) when this POA irradiance is used in the simulation.

This is particularly true for solarimeters measuring POA irradiance in tracking systems: They should be placed on a prolongation of the axis, without neighbor trackers. :