Form and Function in Harmony - Wacker Chemie AG


Form and Function in Harmony

Bremen’s Weser Stadium is being rebuilt as a modern soccer arena featuring a new high-tech photovoltaic system. For the roof’s translucent inner ring, the stadium operator is venturing into uncharted architectural territory, and sending out a clear message about the importance of renewable energy. An indispensable component of the new roof’s attractive solar panels is a silicone gel from WACKER.

A new architectural design element: the translucent photovoltaic panels of the roof’s inner ring (in blue).

Atmospheric Stadium

To a real soccer fan, nothing tops actually being there in the stadium during a thrilling game, for no television broadcast, no matter how good, can capture the seething cauldron of suspense in the stands, nor replace the rallying and feverish excitement. In a stadium, the fans are much more than passive spectators – collectively, they are as valuable to their team as a 12th player.

But fans are no longer satisfied with the classic arena-like multipurpose stadiums, with their central playing field, surrounded by a running track, and hemmed in by an oval of austere bleachers. The trend is to build single-purpose soccer stadiums. A purpose-made soccer ground puts the fans as close as possible to the action, generating the atmosphere they’ve come to expect.

Rebuilding for the Future

This is the aim with the reconstruction of the venerable Weser Stadium – venue of major soccer league team Werder Bremen. With this project, the management is not only meeting the soccer fans’ expectations, but is also sending out a positive signal about tomorrow’s energy supplies. Bremen’s soccer stadium will receive a large photovoltaic (PV) system that is harmoniously integrated into the stadium architecture.

Silicones

Silicones, or vulcanized silicones, to be more exact, are characterized by a property profile that makes them interesting for a wide range of applications. Silicone elastomers together with silicone gels, which are less densely crosslinked, are permanently stable up to at least 180 °C. They absorb no water, have a strongly water-repellent (hydrophobic) surface, are electrical insulators and attenuate mechanical vibrations. Many applications benefit greatly from the fact that the physical and technical properties of silicone elastomers and gels remain unchanged over a wide temperature range (between -45 and +180 °C); not even continuous thermal and mechanical loads (even when exposed to oxygen, ozone and UV light) cause them to age. In this respect, they differ from all organic elastomers. In addition, silicones do not react with any other materials. These properties can be exploited in the production of photovoltaic panels, particularly with regard to their durability.

One feature of this system is the new roof’s inner ring. Its design incorporates lightweight and translucent solar panels (also called PV modules). They bestow a refined elegance on the roof, and their translucence provides a visual transition from the roofed spectator area to the open air playing field. An indispensable component of these extraordinary solar panels is a WACKER silicone gel, which durably protects the fragile solar cells within, and thus ensures that they can continue to provide electricity for years to come.

Weser Stadium: a Trendsetter in Solar-Generated Electricity

Seen from the air, the perimeter of the reconstructed stadium will keep its oval form. Also remaining are the floodlight masts, landmark features recognizable from afar due to their distinctive pairs of slender concrete pillars. The new facade will envelop the existing modern office towers at the north end, the grandstands and the roof, and give the stadium a monolithic appearance.

After reconstruction, the stands will be positioned in a rectangle around the “hallowed turf”; seating behind the goals at the curved east and west ends will be straightened up, and all the stands brought right up to the pitch. The roof will be somewhat higher than before, almost floating over the stadium bowl. Add to that an infrastructure of the highest standards and an innovative energy plan.

Solar energy will be generated from the southern and eastern sections of the facade, as well as from the entire roof of the revamped arena. The total area is nearly equal to that of two soccer fields. The approximately 200,000 solar cells that make up this installation have a combined output of over 1 megawatt, and will feed around 750,000 kilowatt hours into the grid annually. In so doing, they will save the emission of more than 400 metric tons of carbon dioxide per year. In the world of sports, this photovoltaic array is a record breaker: no other sporting arena in Germany can come close – Weser Stadium is number one.

Plastic Panels: Lightweight and Flexible in Design

After the planned three phases of construction are complete, the circa 250 to 300 kilowatt nominal output of the roof’s inner ring will deliver nearly a quarter of the installation’s total energy capacity. The solar panels installed in the inner ring do not just produce electricity; they act as conventional roof glazing and, as such, can be seen by every visitor. The architects responsible for this renovation intend these panels to serve as an innovative design element. “This project clearly demonstrates Werder Bremen’s commitment to solar energy,” says Professor Andreas Wöll, general manager of SUNOVATION GmbH in Elsenfeld (near Frankfurt a.M.). Here, in the extreme northwestern corner of Bavaria, his company produces the attractively designed panels that make up the inner roof ring.

Prof. Andreas Wöll, general manager of Sunovation GmbH, holds a panel for the inner ring of the stadium roof.

It was soon obvious to the planners and architects that conventional glass/glass panels, i.e. solar panels sandwiched between glass plates, would not work because of their weight. The new roof is supported on the existing concrete substructure, which has a limited load-bearing capacity. But it also projects much farther into the stadium bowl than the old one did, and for that reason, imposes a substantial static load. Thus, the panels for the roof’s inner ring need to weigh as little as possible. “The agency commissioned to remodel the stadium chose our SUNOVATION® panels, in which the solar cells are permanently embedded between two high-grade, transparent and permanently flexible plastic sheets,” explains Professor Wöll. “With these custom-made panels, we can fulfill not only the static requirements but also the desire for a pleasing aesthetic.” The plastic used by the manufacturer to produce the panels, Makrolon® (polycarbonate) or Plexiglas® (polymethyl methacrylate, or PMMA), depends upon the service conditions of the panels. Recently, pro K (the German Industrial Association of Semi-Finished and Consumer Plastic Products) designated the SUNOVATION® solar panel as its 2009 product of the year.

Makrolon® for the Roof's Inner Ring

Makrolon® was chosen for the stadium roof’s inner ring. This crystal clear and rigid plastic retains its shape and is impact resistant. CDs, DVDs and airplane windows, for example, are made of this material, as is the roof glazing of Cologne’s main rail station. A lightweight yet rigid twin-wall sheeting of this highquality material serves as the carrier plate for the roof panels, whereas a solid Makrolon® plate forms a covering that shields the panels from the elements. This results in a solar panel that weighs less than half of what a comparable glass/glass panel would.

Silicone Gel

In a silicone gel, the individual polymer molecules are interconnected to form a loosely crosslinked network. The silicone chains between two crosslinking nodes retain their mobility and are able to yield whenever they encounter a hard surface. This characteristic makes silicones unusually resilient and flexible. Expressedin figures, the Young’s modulus (a modulus of elasticity measuring the resilience of an elastic material) for silicones is around 0.005 megapascal; ordinary silicone elastomers, in contrast, have a Young’s modulus between 1 and 10 megapascals. The smaller the value, the more resilient the material and the better it can alleviate mechanical stress.

Because they are elastic and resilient, create a barrier for moisture, do not react with semiconductors and display the chemical stability typical of silicones (see box on p. 11, SILICONES), silicone gels are utilized for the encapsulation of electronic chips and solar cells. Silicone gels durably protect sensitive components against mechanical damage, prevent leakage currents and short circuits, and provide a non-corrosive environment.

A silicone gel is created by the vulcanization of a silicone formulation that, unlike a conventional silicone rubber, contains absolutely no fillers. This workable, as yet unhardened form of the gel is also referred to as a silicone gel.

The Secret to Sucess: a Silicone Gel

Within the stadium roof, solar cells of monocrystalline silicon convert the sunlight into electrical energy. These cells are well protected inside the panels. The ight-permeable space between the opaque solar cells can be treated more or less exactly as the client or architect wants. In this way, the transparency of the plastic panels can be adjusted.

Painstaking accuracy and expertise are required to manufacture these panels.

The manufacturing process consists of several steps. First, the individual solar cells are fixed upon the carrier plate and electrically connected in series with thin contact strips. Then, the carrier and cover plates are sandwiched together by means of a special, permanently elastic adhesive strip that runs along the edge of the plates and seals them together. Finally, the sandwich is embedded in a liquid silicone gel – a tricky process requiring in-depth expertise. The SUNOVATION employees take special care to ensure that the encapsulant is completely free of bubbles. After the silicone has hardened, the panel is tested and packaged. It is absolutely necessary to encapsulate the solar cells, which are highly fragile. The encapsulant protects them from mechanical and chemical damage and ensures that they are electrically insulated. Because of their chemical, physical and technical characteristics, silicones are ideal for this purpose and contribute considerably to the solar panels’ high quality and durability.

Silicone Gels for High Durability

“Silicone gels provide the best protection,” says Hermann Maier, a DRAWIN sales manager who has advised his key customer, SUNOVATION, for years. DRAWIN Vertriebs-GmbH is a wholly-owned WACKER subsidiary that acts as a distributor for the WACKER SILICONES business division. “A silicone gel hardens to an elastic, highly flexible and resilient substance. For this reason, silicone gels are ideal for reducing the mechanical stress in composite materials.” As an encapsulant, silicone gel prevents the panel from being damaged by the unequal thermal expansion of the various composite components during large temperature fluctuations. And packaged within the transparent, vulcanized gel, the highly fragile solar cells can also withstand warping of the panel. Thus, it is possible to manufacture even curved panels, such as are needed for the arched roofs of bus stops and solar-powered boats.

„We tested a number of encapsulants. The best results were attained using a silicone gel from WACKER.“

Prof. Andreas Wöll general manager, SUNOVATION GmbH
Cutting edge, lightweight design: a SUNOVATION® module.

SUNOVATION uses a two-part, addition-curing silicone gel from the WACKER SilGel® line. The gel is easy to pour. Among the vulcanizing silicone products, this one is of particularly low viscosity – when freshly mixed and not yet crosslinked, its fluidity lies between that of olive oil and thin-bodied honey. At room temperature, the gel cures gradually and the process is complete after about eight hours.

The initial building phase of the roof’s inner ring is already complete and supplies solar power. Weser Stadium’s wellknown floodlights can be seen in the background.

High Quality Is Not Enough

The panel manufacturer deliberately chose the WACKER product. “Over ten years ago, we carried out feasibility studies as part of our R&D efforts, in which we also tested various encapsulants. We got the best results with the silicone gel from WACKER, which we’ve been using ever since to produce our panels,” explains Professor Wöll, who also praised the cooperation with DRAWIN and with Dr. Christian Ochs from the Industrial Solutions business team at WACKER SILICONES: “We always received prompt and unbureaucratic answers to all our questions about silicone products. Particularly helpful for us is the technical support that the customer service team at WACKER SILICONES gives us.”

With its translucent solar panels for the inner ring of the new Weser Stadium roof, the solar panel manufacturer is realizing a project that will serve as a role model for other sports arenas. In Bremen, at least, there are already two winners – the environment (the sections of the stadium roof that were completed first are already feeding electricity into the grid) and the Werder Bremen fans, who will finally get the vibrant atmosphere they dream of.