Albedo

Definition

"Albedo" is the reflection coefficient for a ground surface. When a ground surface 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 two kinds of albedo parameters:

The albedo of the project, which is the albedo of the terrain in front of the PV installation, ranging from the direct vicinity of the array to far-off distances "in sight". 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 System window.

Both may be defined in yearly or monthly values.

This page provides usual values of the albedo coefficient, normally valid for both the far albedo and 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: $$ \mathsf{AlbInc} = \rho\; \mathsf{GlobHor}\,(1 - \cos \theta) / 2\,. $$ where GlobHor is the irradiance on the horizontal plane, $\theta$ is the plane tilt, and $\rho$ is the albedo coefficient.

The expression $(1 - \cos \theta ) / 2$ is the mathematical result of the spherical integral of a constant irradiance, coming from all directions "seen" by the plane (i.e. the spherical wedge between the plane and the horizontal). It 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 for the azimuths without near obstacles.

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 decrease. 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 for small systems (BIPV) without shading at ground level, and for large plane tilts.

But its contribution is low or negligible for large PV systems, so 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).

In other words, this albedo coefficient concerns the surfaces that are directly "seen" by the rear side of your bifacial PV modules.

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

Albedo coefficient measurement

The albedo is measured with an albedometer, which is basically made of two 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 the two measurements.

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 weathering of the ground, etc.

NB: Solarimeters in the plane of array, used as 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 (close to zero) 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 an extension of the axis, without neighboring trackers.