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How to explain the high loss value on diffuse with (back)tracking systems ?

André Mermoud

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Even with backtracking strategy, the simulation shows a shading loss due to the diffuse (and albedo), which is usually between 2 and 3%.

See the other post How is calculated the shading loss on diffuse with tracking systems ?

People sometimes compare this loss to the loss of row-like (sheds) systems with fixed orientation, which may be much lower, depending on the system.

How to explain this ?

Please remember that with sheds systems, the shading loss strongly increases with the tilt of the sheds.

Now with a tracker system, the tilt may obviously be very high in the morning or the evening, when the sun is low on the horizon.

The picture shows the contribution of different irradiance losses in a shed system, as function of the tilt, for a given "limit angle" of 20° (i.e. pitch increasing when the tilt increases).



Shading loss contributions as a function of tilt


First, we observe that the Albedo contribution is important.

The transposition model involves an albedo contribution proportional to (1 - cos(tilt))/2, i.e. low at usual tilts (e.g. 4.7% for tilt=25°), but significant at high tilts (25% at 60°).

Then, as only the first tracker "sees" the albedo of the ground in front of the system, the albedo is affected by a shading factor of (n-1)/n, where n = number of sheds. This means that the albedo contribution is almost completely lost in big systems

Secondly, the loss on the diffuse (sky masked by the preceding shed) is also significant, and strongly increasing with the tilt.

The diffuse shading factor is the result of an integral over the sky vault, of the shaded parts multiplied by the cosine of the incidence angle of each "ray". Therefore a hiding band of the preceding tracker has a much higher effect when the plane (tracker) is at high tilt of 60°, as the cosine of the incidence angle of the bottom of the next shed is higher (cos 30°=0.866). If your plane is 15° tilt (sheds configuration), the plane will "see" the next shed with an incidence angle of the "shaded" rays of the order of 75°, i.e. a cos(75°) = 0.25.

With backtracking strategy, the evolution of this shading loss is not varying much as function of the pitch, even for very large pitches. This is explained by the fact that with narrow pitches, the backtracking amplitude is low, when with large pitches, the trackers may take much higher tilts. And also because the loss of the albedo component is the same whatever the pitch between trackers for a given tilt (the albedo is masked as soon as the profile angle is higher than 0°).

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