Near shadings: Backtracking strategy |
  
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Near shadings: Backtracking strategy |
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See also Near shadings: Tracking planes.
Mutual shadings
Heliostat array layout should be carefully optimized regarding the mutual shadings. Constraints are much more critical than for the sheds disposition, as a significant yield may be waited even when the sun is very low on the horizon.
The mutual shading problem is accentuated by the electrical behavior of the strings under partial shadings. As for sheds arrangement, identical shadings appear simultaneously on each tracker and may block the yield of many strings at a time.
Backtracking strategy
The backtracking is now a widely used strategy for tracking arrays: when the mutual shadings begin, the tracking angle does no more follow the sun, but it goes back (diminishes) in order that no shading occurs.
Let's analyze the case of a simple Horizontal N/S axis system.
Beginning of backtracking Backtracking situation: sun lower
Collectors perpendicular to sun's profile angle Tracker's tilt diminished
When the backtracking is activated, the collector plane is no more perpendicular to the the sun's profile angle, we have a loss of irradiance, due to the cosine effect.
Backtracking angle calculation
We notice that the collector tilt angle is closely related to the relative parameters between two rows of trackers.
In PVsyst, the tracking angle is computed at each time step for a reference definition of the collector width and pitch, or more exactly of the width/pitch ratio. This tracking angle is applied identically to all trackers of the scene. If all the trackers of the arrays don't have an identical width/pitch ratio, the backtracking becomes inefficient:
| - | A lower pitch will induce mutual shadings, and therefore possible electrical shading losses, |
| - | A higher pitch will begin the back-tracking before it is necessary, therefore loosing some irradiance by the cosine effect. |
If you have an increased pitch between some similar arrays (passage ways between groups), you will not have mutual shadings. However the backtracking is necessary for all trackers, including the extremity ones, due to mutual shadings.
If you have adjacent arrays, and the trackers of both groups are staggered, you may have mutual shadings from one group to the other one at some seasons.
If you have altitude differences between trackers, the "pure" backtracking is not possible, nether in PVsyst, nor in the reality. See "Backtracking on Hill".
| NB: | With systems imported from other software, the trackers are usually all independent. PVsyst has to check the pitches uniformity, and to choose a pair of trackers as reference. |
| You have a tool for this in the 3D editor "Tools > Backtracking management" |
Other Tracking configurations
We have just explained the backtracking for horizontal axis.
It can also be applied to other tracking configurations. However the analytic calculation becomes sometimes very difficult.
In the present time, PVsyst has elaborated algorithms and proposes the backtracking for:
| - | Horizontal N/S axis, as explained here. |
| - | Tilted axis, as far as there is no misalignment between trackers of a group (i.e. "rectangular" arrays). |
| - | Sun-shields: when the sun is high in the sky, the sun shields become highly tilted. This is also the case for sun's east/west orientations. The compatibility with comfort conditions has to be studied. |
| - | Two-axis frames: the backtracking has been implemented only for the angle of the tables tilt within the frames, i.e. a backtracking from table to table. There is no backtracking between frames. |
| - | Two-axis heliostats: This is a very difficult problem. There may be several backtracking strategies. |
| In the present time, the backtracking mechanism between neighbor trackers is supposed to be a tilt of the plane following the sun's height, and a rotation around the vertical axis ensuring no mutual shadings. It doesn't involve passing to the north of the East-west line. It doesn't take the shadings from one row to the other into account. |
| Please don't use this option, which is not yet finalized ! |
| By the way even if we implement a strategy, nothing ensures that this will be used identically by the control system on the field ! |
Performance calculations
The backtracking strategy avoids mutual shadings between trackers, for the beam component.
However the diffuse and albedo received by the trackers are affected by the shadings of the neighbors. Namely the albedo is completely lost except for the first and last trackers. This explains that even with backtracking, you always have a Near shading loss in the final results. This is usually of the order of 2-3%.
It should be noted that the Backtracking doesn't increase the total irradiance received. It only improves the electrical loss effects of the shadings. The total irradiance reaching the modules is the same as if there were shadings: it corresponds to the total sun energy intercepted for this given sun direction, by the field area "seen" from the sun. Therefore a simulation with "Linear shadings" (not electrically realistic) and another one with backtracking should give about the same results. See Backtracking performance.