Backtracking performance compared with no backtracking

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Backtracking performance compared with no backtracking

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In tracking arrays, the mutual shadings may be very important, as the gains are mainly waited when the sun is low on the horizon.

The backtracking strategy tries to suppress the mutual shadings by reorienting the modules. But it is an illusion to think that you should obtain a much better yield with Backtracking.

Comparison of backtracking with "normal" tracking

When the trackers perform a "normal" tracking (most perpendicular to sun as possible, with mutual shadings), or perform a Backtracking (deviating from the optimal orientation) they intercept about the same "Light tube" !

-With normal tracking, you have the mutual shading losses including the electrical mismatch shading losses.
-With backtracking, you have losses for mis-orientation (cosine effect), with additional losses due to shadings on diffuse and albedo, as well as higher IAM.



     No backtracking: mutual shadings + electrical losses (yellow)            Backtracking: no mutual shadings, but orientation not optimal


It is not clear which configuration receives more irradiance.

The electrical losses are penalizing the "normal" tracking, but this highly depends on the geometry.  It is rather important with one only module in the width of the tracker. It is more pronounced in portrait, except with twin half-cut cells modules. It diminishes when you have several rows of modules in the width of the trackers.  

If the trackers are not continuous (tilted axis or independent not-jointive tables) the shade will usually only concern a part of the length of each tracker.    

The main decisive advantage of the backtracking – if any – is to avoid the electrical effect of shadings (i.e. when a part of a string is shaded, the full production of the string is affected).

Yield comparison

As an example, we have done a comparison of the yield between "normal tracking" (with shades) and backtracking, for a N/S axis horizontal tracking system at Santiago (Chile).

The phi angles limits are +/- 60°, there are four rows of modules in landscape in the width of the trackers. The performance is very similar, except at very high GCR's:


Losses comparison

Now if we have a look on the different losses, we may observe that:

-With backtracking, the losses due to the mis-orientation are slightly lower than the "linear" shading losses without backtracking (irradiance deficit, including for diffuse).  
-The electrical losses + IAM without Backtracking are very similar to the losses due to albedo and diffuse shadings + IAM with backtracking.
-We can notice that with backtracking, the loss on diffuse diminish and the IAM increase with high GCR,  due to the fact that the backtracking limits significantly the tracking angles. Both effects compensate each other.


Performance ratio

Remember that the definition of the performance ratio is:

PR  =  E_Grid  /  (GlobInc * PnomPV)

It is normalized to the incident energy in the collector plane GlobInc (often named POA).

Now in backtracking situation, GlobInc is lower than in the "normal" tracking situation, as the collectors are not optimally oriented to the sun's rays. Therefore for a same yield  E_Grid, the performance ratio will be much higher with backtracking than in the normal situation.  

In other words: in normal tracking, the mutual shading losses are "included" in the PR ratio (which is a summary of the losses), when with backtracking the irradiance loss due to mis-alignment is not taken into account in the PR.

Try the no-backtracking solution !

As a conclusion, everyone wants to use the backtracking strategy, as it is a "must" in the PV industry.

However the things are not so simple, the results depend on many parameters, especially when we are not in optimal conditions.

It is worthwhile to check the "no backtracking" solution, and compare both results. This is particularly advised when you are on an uneven terrain, and the backtracking is not "perfect".