From pv magazine France
French cooperative Céléwatt has started construction on a small-sized solar park in Carayac, a commune in the Lot department in southwestern France.
The 250 kW ground-mounted array, scheduled for completion in 2021, is built entirely on mounting structures made of raw oak wood. “Our original idea was to promote local employment and natural resources,” Bertrand Delpeuch, the president of Céléwatt, told pv magazine. “As we had no room for maneuver on the origin of our 746 monocrystalline panels, which are delivered to us from China by the company Talesun, we decided to focus on the poles.”
This region in the Lot department is indeed rich in oak forests. This straight and solid wood, about 15cm in diameter, is traditionally used as a bouchot (breeding support) for the culture of mussels in Charente-Maritime. “Replacing the galvanized steel supports of the solar park with raw wood from forests about 30km from here, this saves the extraction of ore and its transport from China, then its transformation, which is carried out in Portugal,” Delpeuch explained.
Céléwatt called on the engineering company Mécojit in Capdenac-Gare, with which it had already built another solar park. “It was a real gamble at the start,” recalls Olivier Saintignan, project manager at Mécojit. “To our knowledge, this is the first example of a solar plant with untreated, unprocessed wooden support.”
The project constraints are numerous, as the wood is not milled in a sawmill and the developer must take into account its different sections and imperfections. “Then you have to mount the structure on uneven ground,” said the engineer.
The project is being built without public support and will sell power to French energy cooperative Enercoop at a price of €0.08/kWh.
The first prototype comprises fixed connections and metal brackets at right angles. “It did not work,” said Saintignan. “The pivot links had to be able to move together to adjust the heights and create a plane.” Finally, the connections by bolting and the bores, are made directly in the field and the assembly is made by bolted threaded rod, which allows movement between the parts. The PV panels are fixed on an omega purlin, which has the ability to deform and dampen the natural movements of the wood.
To maximize the mechanical characteristics of the rough wood, the 600 oaks are cut out of the growing season, and to avoid any discontinuity in the fibers. As the trees are small in cross section, they were skidded over the shoulder by a local company, Le petit oak noir. “We have succeeded in employing several companies in the region,” Delpeuch added.
Céléwatt claims that everything has also been done to optimize the duration over time. “This is a very resistant and endogenous wood species, which will not fear weather fluctuations,” continued Saintignan. “The structure is made to adapt to the movement of the wood.”
While the risk of fungal colonization has been minimized, simple parts replacement solutions have also been provided. The stability of the panels will be checked every three years with visual inspection of the resistance of the wood, and a general overhaul will be carried out during the ten-year maintenance.
For Mécojit, what was initially a gamble has become a new outlet, marketed under the “Mécowood” brand, which the company would like to promote in other projects. “This raw wood table can have many other applications, such as the integration of solar panels in a wood storage shed or a heat pump … The possibilities are numerous,” assured Saintignan.
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