Under heavy snow conditions the snow may accumulate on top of PV modules and cause the tracker structure to bear a weight beyond the design range, ultimately this excessive weight on top of modules is a high risk as it may cause a damage on the tracker’s structure or on PV modules themselves. Furthermore, if modules are covered in snow, they would reduce power generation, even after the snow event has finished.
Snow risk can be mitigated in advance by adjusting the tracker angle to a high steep angle, in this manner the accumulated snow will slide to a side. It is considered that an angle of 60º is sufficient to slide all the snow accumulated on top of PV modules to one side. Note this position eliminates snow but may not eliminate ice on the surface, ice forming on surface is not a problem from the structural point of view.
Arctech uses a depth snow sensor located close to NCU tower to detect and measure snow depth. This sensor is placed at an angle of approximately 20º and sends a laser pulse to measure the distance to the ground.
The snow depth sensor uses a phase shift method to precisely measure distances to objects. The light is scattered from the target and detected back with a photodiode. This returning light is compared to a reference signal to determine the distance.
After the snow depth sensor calibration is completed, the NCU can measure the snow depth and all TCU under control are protected against snow events.
Each NCU measures the “snow depth level” (𝒉𝒔𝒅) periodically and monitors the relative increment tendency using three registers:
“Snow depth steep” (𝒉𝒔𝒅𝒔): Is the snow depth level value when the tracker was close to any steep position (any at ±60º). In normal operation, this position is registered at least twice a day.
“Snow depth accumulated” (𝒉𝒔𝒅𝒂): This is the snow accumulated sincethe last “Snow depth steep” was registered. This value indicates the relative accumulation of snow and has the following formula: ℎ𝑠𝑑𝑎 = ℎ𝑠𝑑 − ℎ𝑠𝑑𝑠
“Snow depth threshold” (𝒉𝒔𝒅𝒕): This is the value to compare against 𝒉𝒔𝒅𝒂 and will trigger the snow alarm.
A snow alarm and consequent trigger of snowing mode will occur at any time the value detected in “snow depth accumulated” ” is over the “snow depth threshold” (default value of 10cm) which means 𝒉𝒔𝒅𝒂 ≥ 𝒉𝒔𝒅𝒕.
When Snow Mode is active, the tracker will rotate to a steep stow angle. Notice, as this stow position is close to ±60º, the “Snow depth steep” will be updated until the system removes the snow protection mode.
The release of snow alarm does not imply the tracker will resume automatic tracking. The system will not exit snow mode automatically and it is the responsibility of the local operator to manually resume automatic tracking mode.
This precaution is preferred to prevent damages due snow accumulation in certain parts of the plant, as the accumulated snow on one side can interfere the tracking range, or if the area where the snow sensor is located could have been cleared or melted sooner than the rest of the plant.
Therefore, a visual inspection and manual activation is required before returning to automatic mode.
