T-OPAL

Our longing for summer and sunshine has not been answered too often this year. But no sooner do the sun’s rays make their way through the clouds, complaints abound about the heat and dazzling light this generates in our modern glass buildings. Venetian blinds, shutter blinds and curtains are rolled up and down, pulled from left to right.

A markedly more elegant and less bothersome solution is provided by windows that automatically produce their own shade. Researchers at the Fraunhofer Institute for Building Physics IBP, Stuttgart, under contract to industry, are developing window panes which transform to a milky white when temperatures reach a certain level, thus serving as protection from the sun.

Dr. Holger Gödeke, an engineer at the IBP, explains, “The goal of our work was to find a thermotropic system that could be easily produced and thus compete with conventional mechanical sun-shade installations.” The outcome is T-OPAL, a cast resin glass with an integrated polymer layer. “Cast resin glass has long been used in fire and sound insulation. In order to provide protection against the sun, the production method is slightly altered,” Gödeke explains. “The polymer is poured between two panes of glass as a thin liquid mass. When exposed to UV rays the polymers turn into a solid mass.” When temperatures reach a specific point, established during production, the structure of crystals stored in the polymer layer changes, making the initially transparent window opaque.

On sunny winter days, however, the sun can shine into the building unhindered, allowing for the use of its solar energy, since it is the combination of temperature and intensity of the UV rays that triggers the transformation of the polymer layer. “Once the glass has become opaque, the temperature must drop about 5 to 10 degrees, as it does for example overnight, for the glass to return to its transparent state.” These inexpensive glass panes are also well-suited for sun protection in winter gardens, glass domes, green houses or automobiles, and serve as protection against dazzle from skylights and glazed roof panels in train stations and museums, or to prevent the overheating of solar collector panels.

Contact: The Fraunhofer Institute, Munich, Germany.