Albedo

Definition

"Albedo" is the reflection coefficient for a ground surface. When a ground receives irradiance, it reflects part of it and absorbs the remainder. The albedo coefficient is the ratio of reflected to received irradiance.

This reflection is "Lambertian," meaning each ground point reflects irradiance uniformly in all directions. Specular reflections (like mirror reflections) are not considered in 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.

This link 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 shows that albedo contribution is maximum on vertical planes (50%) and decreases with lower tilt angles (14% at 45°, 6.7% at 30°, 3% at 20°, only 0.8% at 10°). Horizontal planes receive no albedo contribution.

Shading factor

In rows or shed arrangements, only the first row "sees" the albedo (ground level). For the entire system, the shading factor on this contribution is (n-1)/n, where n is the number of rows. For example, with 100 rows, the shading factor is 0.99.

This also applies to tracker arrays, where albedo shading loss represents a significant contribution to diffuse irradiance 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 shed or tracker systems, a counterintuitive effect occurs: albedo is part of GlobInc evaluation, which forms the basis for 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 reflections 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. :