Tracking planes

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Tracking planes

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The basic definitions of tracking planes are very similar to the sheds definitions: several identical trackers, with a sensitive area and an inactive frame for mutual shadings.

There are several kinds of tracking planes, for which the disposition may be different as the sheds. This is the reason why when creating a new tracking array, your first action should be to define the Tracking Parameters. This dialog is analogous to the one of the "Orientation" general parameters, it will define the kind of tracking, the mechanical stroke limits of your trackers, and optionally parameters for backtracking or concentration.

There are two classes of tracking systems, one axis or two axis. The 2-axis systems are less common, as they require a more complex mechanics, and therefore higher initial cost and more maintenance; this is rarely compensated  by the little yield gain with respect to the one-axis systems.

In PVsyst, we can define:

One axis trackers

-Horizontal North-South axis: this is the most used layout for tracking systems. You can construct long trackers, tracking the sun's height from east to west during the day. This is particularly suited for low latitudes, where the sun is high in the sky.
-Tilted axis: this is analogous to Horizontal N/S, but with a tilted axis. Using a tilted axis may improve the yield at medium latitudes a little bit. However the mechanical realization is more complex, as you cannot construct long trackers. The axis tilt should be equal or less than the latitude.

- Vertical axis: the tracker has a fixed tilt, and rotates around a vertical axis, following the azimuth of the sun. This may be suited at very high latitudes, when the sun is low on the horizon. A particular case is the construction of a big mechanical "dish" (dozens of meters diameter), rotating towards the sun during the day, and receiving PV tables in sheds arrangement. Or equivalently floating systems following the sun's azimuth.

-Sun-shields are taking a great importance with the architectural integration. However with fixed sun-shields it is very difficult to find a good compromise between an efficient sun protection and an acceptable PV yield. The shadings from one sun-shield to the lower one when the sun is high (i.e. during the best summer hours) is difficult to overcome, especially when the facade is not exactly south. This could be partially solved by using tracking sheds with a backtracking strategy.
-Horizontal East-West axis: this will follow the height of the sun (during the day and the seasons): It is not really suited for PV systems as the diurnal or seasonal sun height variations are not very marked. In the morning or evening the sun's angle with the plane will be high. This "old" option corresponds to a disposition analogous to parabolic linear concentrators in distributed thermal plants, with trackers "one behind the other" like in fixed sheds.

Two axis trackers

-Two axis: the tracker is permanently perpendicular to the sun's rays, receiving the maximum possible irradiance. This represents a complex mechanics, therefore implemented with rather big trackers. However if the trackers are of big sizes, the problem of wind sensitivity necessitates robust mechanical supports. Usual two-axis trackers are less and less used, mainly due to high maintenance costs.
-Frame with horizontal N/S axis: some manufacturers propose rotating frames with horizontal North-South axis: the frame follows the height of the sun, and a set of tables within the frame are permanently oriented in order to stay perpendicular to the sun's rays.
-Frame with horizontal E/W axis: this is similar to the previous case, but the frame follows the height of the sun.

Stroke mechanical limits

The trackers are mechanical structures, which have obviously limits  for the rotating angles, that we will name "stroke" limits.

In PVsyst, all the dialogs defining tracking systems include the specification of the stroke limits.

The construction dialog offers a tool for testing all possible rotations, in order to check the mechanical compatibility of your array layout (using the orthogonal plane or side view). In the 3D scene, the shading animation over one day provides a powerful tool for optimizing your tracking layout by trial-and-error.

Array of trackers

Trackers are never "alone": they are organized as arrays, so that we have to take the mutual shadings into account. These mutual shadings become very important when the sun is low on the horizon, i.e. just when the tracking strategy becomes more effective.  

Therefore when defining an array of trackers, the distance between tracker's axis (pitch) is specified with the tracker's array. The mutual shadings are related to the pitch and the width of each tracker, or more specifically to the ratio  width/pitch, which is more or less represented by the "Ground Coverage Ratio"  GCR. The higher the GCR, the higher shading losses.

Two-axis trackers are usually arranged in rows and columns. We have to take care of the mutual shadings from row to row and from column to column.

Tracker's array layout should be carefully optimized regarding the mutual shadings. The Backtracking control strategy avoids mutual shadings for the beam component, and helps improving the electrical shading losses of tracking arrays.