WSL Institute for Snow and Avalanche Research SLF

Drone flights for photovoltaics

Knowing how much snow there is at a location in winter is important for project planners of photovoltaic systems in mountain regions. This helps them to avoid planning errors that lead to damage to the modules and substructure. SLF experts provide detailed data, which they collect from the air.

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Launch of a drone to measure snow depths. (Photo: Yves Bühler / SLF)

At the end of January 2024 on the Bernina Pass: the orange drone rises vertically into the air, then lies flat and begins its reconnaissance flight. Yves Bühler and Andreas Stoffel from the Alpine Remote Sensing research group at the SLF are measuring the snow depths here.

They are on their way on behalf of a planning company for photovoltaic systems that wants to build a solar park on the Bernina Pass. For this, it is important to know the conditions in the terrain in winter. "If you install a photovoltaic system in a place where there is seven meters of snow in winter, it will certainly break in the spring," says Bühler.

A few meters to the left or right often make the difference between suitable and unsuitable terrain - and the scientists provide the data for this. "We use drone cameras to measure the spatial distribution of snow depths. If you do this after snowfall with different wind directions, you can better estimate which locations would be suitable in terms of snow. This is particularly helpful if you can measure both before and after the construction of the facilities," explains Bühler.

Snow depths on the Bernina Pass in mid-March: The dark blue areas show a lot of snow and are not suitable for photovoltaic systems. In black at the bottom of the picture is the cleared pass road, the blue strip next to it is the snow wall that the snow blowers have thrown up during clearing.

The project planners receive the results of the aerial survey directly - or SLF colleague Stefan Margreth, head of the protective measures research group. He uses them for expert reports from which the project planners can determine whether special measures are required and, if so, which ones. "In the past, we only had rough information from maps showing the average snow depth across the whole of Switzerland, perhaps there was a measuring station nearby, which gave us an idea of the situation on the ground, but not the details," says Margreth. In practice, however, even a comparatively small depression can cause large, local snow depths. The survey flights therefore provide important information.

Snow doesn't just fall and stay there until spring. Wind moves it, creating cornices and free-blown areas, places with a lot of snow and others with little, and the differences in the mountains are enormous. "You have to adjust the ground clearance of the PV modules to the local snow depth," explains Margreth. If the substructure is too low, the modules will disappear under a white blanket and the solar park will not produce any electricity. Damage can also be caused by snow pressure.

However, the system itself also has an impact on the situation on site. "Basically, solar parks reduce the local wind speed, which means that more snow is likely to be deposited," explains Margreth. This effect can lead to a different distribution compared to the initial situation before construction. Margreth compares this to protective structures such as cross-shaped wind baffles and snow drift fences, which optimize the distribution of snow or reduce the risk of avalanches by allowing the wind to deposit the snow in certain places in a controlled manner.

Snow depths on the Bernina Pass in mid-March: The area at the top left could be suitable for photovoltaic systems. Further to the right, the situation is unfavourable due to the high amount of snow. The dark dots in the centre left show stones. The cornices that have formed on them are clearly recognisable.

The SLF expert's many years of experience show that there is generally more snow on site because of the system. He recommends taking this into account when planning the height of the substructure. "It's difficult to estimate how the construction will affect the snow height in detail," says Margreth, citing two rules of thumb: On an industrial building, a solar installation increases the snow load by up to 25 percent; on the open area, there is likely to be around 20 percent more snow. A height of three meters then becomes three and a half to four.

The first alpine solar parks in Switzerland will have the character of a test installation with regard to these consequences in a snowy winter. "It is very important that the snow depths are also monitored after the construction of a system in order to have a better basis for further solar parks and repairs," recommends Margreth.

Measured against the investment required for a solar park, the creation of snow depth maps is not expensive. A flight including evaluation costs between four and five thousand francs.

Sometimes one is not enough, for example if the local situation changes drastically over the course of the winter. In March, Bühler and Stoffel were once again on the Bernina Pass - due to extreme snowfall in the south.

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