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Advanced roofing overcomes technical hurdles to generate electricty

July 1st, 2013 / By: / Fabric Structures, Projects, Sustainability

A city carport in Munich—and a prototype for efficiency and sustainability.

It’s not always easy being green: that’s what Wika Pösl discovered when attempting to build a ground-breaking carport roof for a public works building in Munich, Germany. The project, which involved an ETFE film roof with photovoltaic cells, came under close and critical scrutiny by city officials, engineering specialists and the general public.

“For us it was a long and hard way because every step was controlled,” says Pösl, a project manager for building contractor Taiyo Europe GmbH. “It was a hard fight to get building approval, and then to construct the roof.”

The city and public had good reason for their skepticism. The new roof was meant to replace one that had partially collapsed in 2006 after a heavy snowfall—an event that gave the city and architect a lot of bad publicity. The same architectural firm, Munich-based Ackermann und Partner Architekten BDA, was hired for the re-construction. The do-over of the roof allowed designers to incorporate a new element: photovoltaic cells cushioned in EFTE film.

The end result, Pösl says, was definitely worth the extra effort. Not only does the roof serve to protect city garbage trucks, it also generates enough electricity to power the building’s operations: 140 MWh annually, roughly equivalent to the needs of two Munich apartments.

“Sometimes it was very hard for us because of the nature of this project as a prototype,” she says. “I am very glad and proud that I could be a part in such an amazing project, and I will follow the future of this roof.”

The client, Munich’s waste disposal company AWM®, is also pleased. “The new roof concept makes a significant contribution to sustainability, particularly to climate and resource protection, which AWM has declared as one of its major maxims, alongside efficiency,” Pösl says.

Technology encased

Ackermann und Partner architects came up with the idea to enclose photovoltaic cells in the EFTE film, a concept that was already being discussed in the membrane industry. “The purpose was to connect a very light architecture with some new technology with the result of the agreed gain, and to have a sustainable project,” Pösl says. “The advantages of ETFE air cushions as space-enclosing components lie in their transparency and low weight, which also affects the efficient and aesthetic quality of the primary structure.” It took some practice and patience to get it right, Pösl adds, and they did three mock-ups to get the best results.

The team asked (and answered) questions such as: Do we use a middle layer as a fixed layer for the photovoltaic cells? Where is the best placement inside the cushion? What about the temperature inside the cushion? Where is the optimal place for press cuts in the PV cells? Should we use two long press cuts, or only one? “We changed some ideas while building the mock-up, but overall we remained true to our basic idea,” she says.

The final 8,000-square-meter rooftop consists of 220 air-supported cushions, each made from three layers of translucent ETFE film.

Each layer has its own special purpose. The lower film layer is printed to give some shade to the carport deck. The middle layer contains 12 photovoltaic modules, mounted with partly moveable and flexible mechanical fasteners to resist deformation caused by heavy loads. The upper layer, which is fixed separately, acts as a protector and as an access port to the cushion center.

The biggest challenge came in mounting the photovoltaic cells. Designers considered all possible difficulties, such as avoiding any tensile loads on the photovoltaic cells under all possible conditions, including heavy snow and wind. Designers had to learn about the temperature behavior to find the right values for structural calculations for the ETFE foils. “We knew the values for the foil, but we had to think about the holes for the air circulation and if they were adequate, especially in the summer months,” Pösl says.

Cell sites

The final design had the cells fixed to the middle layer of each cushion by means of mechanical connectors, some of which can be moved, so the modules are not subjected to any bending, tensile or shearing forces even in the event of heavy snow loads, according to Pösl.

Just like a bridge support, one of the PV module attachments is always without a longitudinal hole. “In other words, it is in a permanently fixed position, preventing the PV module from floating freely,” Pösl says. The middle layer is mechanically pre-stressed to prevent creasing and is without load in the operating state, since the large ventilation openings in it lead to the same inner pressure above and below the middle layer. The cushions were heat-sealed at the seams.

The cushions are shaped to prevent water pockets. Drifting snow can collect towards the lower edge. However, the local additional loads do not endanger the roof’s structural safety and do not lead to any significant deformation, according to Pösl.

During the final assembly, each cell was placed into a load-bearing frame, built above the parking area by subcontractor steelconcept GmbH of Chemnitz, Germany. The film cushions were clamped in all-round aluminum profiles, which were then screwed to the substructure.

The air supply comes from blowers, and enters through an air inlet at each cushion’s lower layer. An air dryer prevents moisture build-up. The stability of the cushion roof structure is not dependent on air support, but efforts were made to ensure reliable air supply, Pösl says.
Air pressure is a set level, but can fluctuate under load. If snow load exceeds a pre-determined level, the cushion is compressed in a controlled way. “In this case, the upper and lower film layers bear the load together,” she adds.

Three blower units supply the air, each directed to one third of the roof area. Each station has two redundantly wired blower motors that alternate on a weekly basis and automatically replace each other if one of the blowers should fail. The air supply is also connected to an emergency power supply and a remote warning system.

Results oriented

The project, from start to finish, took two years and was completed in October 2011. It won an Award of Excellence in IFAI’s International Achievement Awards competition in 2012.

Pösl says they’ve learned a few lessons since then.

“We replaced some cells because they had minimal waves because of the movement of the cushions,” she says. “Now we try to avoid that optical problem, and we made some test series at the roof with the agreement of the client. For the gain, the waves are not relevant. The architect was concerned about the optic, but you can see the waves only from close range. This is what we had to fight in this project.”

The ETFE roof has already served as a model for similar designs. Pösl can see several ways to apply this system in all types of climates, although extremely hot weather could present special challenges due to heat build-up in the cushions.

“But you can’t say this generally, because each project is related on some special surroundings,” she says. “I think if you have a good and experienced team with good engineering, you could find a solution. This is what makes these projects so special—each building is unique in the world, because each building is its own prototype.”

Lynn Keillor is a Minneapolis-based writer and editor.

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