Novel PV system design for green rooftops from France

From pv magazine France

France's Ecovégétal has released Héliovert, a PV system for green rooftops that combines solar power generation and urban gardening.

“The idea of ​​exploiting the synergy between these two technologies came from a study currently being carried out by Cerema, a French public agency for developing public expertise in the fields of urban planning, in Nancy,” Pierre Georgel, the CEO and founder of the green roof specialist, told pv magazine France.

The new system is the result of several years of research under the PROOF project, which aims to assess the energy contribution of a green rooftop to the electrical efficiency of a solar panel.

“The first results show an increase in electricity production yields of 8 to 10% thanks to the presence of vegetation which acts as a thermal regulator under the photovoltaic panels,” said Georgel. “But it goes much further than putting lawn on a roof. It is indeed the vegetation that serves as ballast for the solar panel fixing system.”

In the proposed system configuration, drains known as XD20, DK20, or equivalent are laid over the entire surface of the roof and serve as an anti-root sealing system and for the retention of rainwater at a rate of 7 liters per square meter. A recycled plastic slab is installed on top.

“This slab will serve both as a water drainage basin and as fixing support for the aluminum frames inclined at 15 degrees on which the photovoltaic panels are installed,” Georgel said. “It is therefore not necessary to drill into the roof structure.”

A substrate known as Saxalis 1.1,  which is non-flammable and is based on mineral aggregates enriched with organic matter, is then spread over the roof. Finally, the company plants flowering meadow-type vegetation that can change color with the seasons. The plants are chosen for their high evapotranspiration, which lowers the temperature on the roof by several degrees, especially in hot weather.

According to Georgel, the presence of certain plants on a flat rooftop can even serve as a bulwark against the risk of fire.

“Finally, we are witnessing the return of biodiversity to the urban environment. Local fauna such as birds and pollinating insects and the reintroduction of flora adapted and resistant to its environment,” he said.

Ecovégétal chooses solar modules and its partners according to customer preferences. The panels can be oriented south or east-west, depending on the geographical area and the biodiversity. Beforehand, the thickness and the dry weight of the substrate – that is to say, the weight of the earth and the vegetation in the event of severe drought – is the subject of a study to ensure the resistance of the solar panels to the wind.

The Broue-based company's first projects include the roofing of Bosch's offices in Saint-Ouen, coordinated by Metropolis Architectes Associes. An area of 990 square meters will be planted and 790 square meters will be covered with 395 solar modules.