An Electric Sub-station Converted to Photovoltaic energy

The conversion strategy for the building seemed obvious. No longer in use for many years, it entailed installing secondary or service activities over nearly 8,000 m2 and revitalizing employment for a rather gloomy portion of the 14th arrondissement. With its gritstone abutments and its depressed arches, the work, built in the 1920s, was not subject to any heritage protection measures. Emmanuel Saadi, sensitive to the industrial nature of the monolith bearing its use and status in ceramic letters, explains that the procedure was devised without changing its image and identity. Even better: the interest of the building seems to have been glorified by limiting the operation to the creation of floor areas within the existing volume measurements, other than a small heightening of the roof and the particularly successful shaping of a simple electricity production tool: photovoltaics.

“With its unique pierced floor, the original interior did not look like much before it was completely renovated and a mezzanine was built”, explains Emmanuel Saadi. The only template modification, inspired by the existing central core continuing to the roof through an aedicule dedicated to ventilation, consisted in the creation of a vast reception area under a 20-meter glass roof, itself barded by thousands of photovoltaic cells, with a view of Paris and its suburbs. At the same time, setting up timber plank decking covered with fields of lavender has made it possible to make the entire terrace accessible.

45,000 photovoltaic cells

Altogether, no fewer than 45,000 cells were incorporated into the building, including on the terrace guardrail, for projected production of 60 to 80 kW/h, with regard to installed peak power of 123 kW. A veritable sunscreen, and treated like a material in its own right, the photovoltaic glazing, made up of 260 micron cells embedded in a 2 mm-thick resin between two pieces of hardened or tempered glass, is available in every conceivable situation. Accordingly, 80 m2 of outside glass flooring reinforced by structural laminated glass was installed on the terrace.

The irony of the story: the electricity produced by the types of in-built devices of the former Compagnie de Distribution Parisienne sub-station looks like it will be bought by Électricité de France (EDF). But beyond simple energy conservation, the architect defends the idea of sustainable development supported by architectural added value. For example, the systems implemented do away with the installation of more traditional sun-shading devices, such as shade screens, and the similarities in appearance between the porous millstone grit masonry and the picture windows with polycrystalline cells distributed almost randomly give the building its unity and plastic quality.

Two ATEx issued by CSTB

For Michel Cossavella and Jean-Louis Galéa, engineers in charge of Experimental Technical Assessments (ATEx) for CSTB, the evaluation in terms of safety, feasibility and risks of disorders focused mainly on two difficulties: the behaviour of peripheral sealing joints for the windows and the integration of electric cables into aluminium joinery rabbets.

Indeed, by making the glass surfaces opaque, the photovoltaic cells cause an increase in temperature, which must be able to remain acceptable to prevent the joints from detaching and the insulating glass air space from fogging up. As for passing cables through narrow aluminium joinery, it involved eliminating the creation of weak points detrimental to safety and maintaining the watertightness of the glass roof by preventing the drainage channels from becoming blocked. As a result, the completion of the work presupposed a very accurate control of the companies’ design and implementation tolerance.

Jean-Pierre Nauleau, in charge of façades for the Goyer Company, , confirms that a considerable amount of preparatory work turned out to be essential, including in terms of adapting profiles to the nature of the operation: “The number of photovoltaic cells varies in accordance with the sun exposure. For example, the southern façade facing the street is much more dense than the side façades, exposed to the east and the west. And their distributions are different on each block of panels.” Very detail-oriented and concerned with making modernity fit in with unity, Emmanuel Saadi explains that the pixellation of a photograph of the millstone grit guided the choices in terms of cell distribution and pattern layout plans. “The existing inscription of the pediment had to be magnified, making a building that, thanks to the beauty of the photovoltaic cells, shines forth over an area that is permanently anchored in history.”