Backtracking Strategy

Mutual shadings

The layout of tracker arrays should be carefully optimized to minimize mutual shading. Constraints are much more critical than in the sheds configuration, as significant energy can still be produced even when the sun is very low on the horizon.

The issue of mutual shading is intensified by the electrical behavior of strings under partial shading. As with shed arrangements, identical shading patterns can occur simultaneously on each tracker, potentially reducing the output of many strings at once.

Backtracking strategy

Backtracking is now a widely used strategy for tracking arrays: when mutual shadings begin, the trackers no longer follow the sun but move backward instead to prevent mutual shadings. The tracking angle slowly decreases. This behaviour is illustrated in the figure below.

Note that both standard tracking and backtracking intercept about the same "light tube". However, the losses to be considered vary:

• For standard tracking, mutual shading losses lead to electrical mismatch shading losses. The intensity of this loss depends on several design parameters: latitude, GCR (how densely packed the trackers are), number of rows per tracker, panel orientation, panel type (standard/half-cut).
• For backtracking, losses come from mis-orientation (cosine effect) and higher IAM. The irradiance albedo component may also be reduced by the more horizontal position of the tracker, while the diffuse irradiance reaching the panel’s front side is increased.

The main decisive advantage of backtracking is to avoid the electrical shading effect.

Backtracking angle calculation

In PVsyst, all trackers within an orientation group are operated using the same tracking angle. For backtracking, the tracker tilt angle is defined by the position of the two neighboring rows of trackers.

The tracking angle is computed at each time step based on a reference definition of the tracker width and pitch—or more precisely, the width/pitch ratio which corresponds to the Shading GCR. This tracking angle is then applied uniformly to all trackers in the scene. If not all trackers in the array share the same GCR, backtracking becomes inefficient:

• A tracker subgroup with a smaller pitch than the reference (higher GCR) will suffer from mutual shadings and therefore possible electrical shading losses.
• A tracker subgroup with a larger pitch than the reference (lower GCR) will start backtracking before it is necessary, therefore losing some irradiance due to the lower incidence angle.

If there is an occasional increase in pitch between similar tracker arrays (e.g., passageways between groups), mutual shading may not occur on the extremity tracker. However, backtracking remains necessary for all trackers, including those on the extremities, due to the mutual shading risk with the next row.

If adjacent arrays have staggered tracker positions, mutual shading from one group to the other may occur during certain seasons.

Special cases

• If there are height differences between trackers, shadings can occur between trackers even when using backtracking. See "Backtracking on Hill" for more details.

• In 3D scenes imported from other software, the trackers are usually treated as independent. PVsyst must check the uniformity of the pitches and select a pair of trackers used as a GCR reference for the backtracking computation. You can manage this GCR choice or override it using the tool in the 3D editor: "Tools > Backtracking management"

• More advanced backtracking strategies exist, such as adjusting tilt only when irradiation is improved, or AI-driven approaches that optimize individual tracker positions. These backtracking algorithms are not currently available in PVsyst. However, custom tweaks to the backtracking angle calculation are possible and described in the Backtracking management page.

Other Tracking configurations

The paragraphs above focus on Horizontal Single-Axis Trackers (HSAT), as they are the main use case for backtracking. Similar principles can be applied to other tracking configurations, but the analytical calculations can sometimes be very complex.

Currently, PVsyst supports backtracking for the following configurations:

• Horizontal N/S axis, as explained above.
• Tilted axis, as explained above, provided there is no misalignment between the trackers within a group (i.e., "rectangular" arrays).
• Sun-shields: when the sun is high in the sky, the sun shields become highly tilted. This also applies to sun positions near the east/west orientations. Compatibility with comfort conditions should be evaluated.
• Horizontal E/W axis: Several arrays of trackers should be implemented for the backtracking option to be activated.
• Two-axis frames: Backtracking is implemented only for the tilt angle of the tables within the frames (i.e., backtracking from table to table). There is no backtracking between frames.

Note that the backtracking for 2-axis trackers is not yet implemented in PVsyst.

Notes on performance

The backtracking strategy avoids mutual shadings between trackers for the beam component, and PVsyst does not optimize for diffuse irradiance.

The diffuse and albedo components received by the trackers are still affected by shading from neighboring trackers. In particular, the albedo is almost completely lost, except for the first and last trackers in a row. This explains why, even with backtracking, a near shading loss remains in the final simulation results—typically around 2–3%.

It is important to note that backtracking does not increase the total irradiance received. It only mitigates the electrical losses caused by shading. The total irradiance reaching the modules remains the same as if shading occurred: it corresponds to the sun energy intercepted, for a given sun direction, by the field area as "seen" from the sun. Therefore, a simulation using "Linear shadings" (which are not electrically realistic) and one using backtracking should produce similar energy results. See Backtracking performance.