The 1970s energy crisis spawned a golden age of research that yielded low-emissivity coatings, among other advances in glazing technology. Today's eco-building movement has spawned new inquiries into the material, resulting in a next generation of performance glass with even greater efficiency.
The concept of sandwiching an insulating layer of air between two glass lites dates as far back as the early 18th century, when Daniel Defoe described the double-glazed windows of Siberian homes in his sequel to Robinson Crusoe. Insulated glass units (IGUs) proliferated in the latter half of the 20th century, and, over this time, manufacturers began to fill the space between double- and triple-pane windows with more thermally efficient gases.
Gas-filled windows tend to be thick and heavy, however, making them difficult for historic and retrofit projects. Vacuum-insulated glazing (VIG) eliminates the gas altogether. Best suited for cold-weather climates, current VIG units rival the performance of triple-pane windows at a fraction of the thickness, but at a much higher price. The Pewaukee, Wisconsin'based company V-Glass is developing a new VIG assembly whose cost should be comparable to triple glazing. It comprises two glass panes separated by a patent-pending spacer system that, CEO Peter Petit says, 'prevents panes from bowing and reduces the risk of the scratching of low-emissivity coatings.' Unlike some existing VIG products, V-Glass comprises a flexible metal-foil edge to further mitigate potential bowing, which can cause the glass surfaces to touch and transfer heat. Petit expects his system to be available within the next several years, at which point he will license it to manufacturers of residential windows for installing in their own frames.
Swiss architect Dietrich Schwarz has also set his sights on the cavity within a window assembly, in this case filling the thermodynamic glazing he created, GlassX, with calcium chloride hexahydrate. This phase-change material, with a room-temperature melting point, absorbs convective heat and solar gain, and releases it upon recrystallization. The nontoxic salt does not block visible light transmission, appearing translucent in its solid state and clear in its liquid state. GlassX assemblies can achieve a U-factor of 0.07.
Buildings featuring GlassX have been operating in Germany and Switzerland since 2005, and the company's chief technology officer, Martin Schr'cker, expects specifiers in North America (through Greenlite Glass Systems) to follow soon. Later this year, the company will offer phase-change areas of any height, which, Schr'cker says, 'gives architects more aesthetic flexibility.' Previously, GlassX assemblies could not exceed 10 feet. The maximum width remains 6' feet.
Most studies of high-performance glass manipulate solar thermal gain to reduce energy consumption. In 2010, a group of scientists at MIT'Miles Barr, Vladimir Bulovic, and Richard Lunt'began transforming surfaces like glass lites or plastic film into photovoltaic systems. They've started a company, Ubiquitous Energy, in Menlo Park, California, to market the technology they produced, ClearView Power.
ClearView Power's see-through photovoltaics are organic molecules, vapor-deposited on a clear substrate, that absorb all the energy from the sun except for wavelengths that appear to the naked eye. The transparent coatings include semiconductor materials and electrodes channeling electricity to an external circuit. While this invention does not yet produce as much energy as standard photovoltaics, ClearView Power's invisibility allows integration into greater areas of the building envelope.
Whether through generating power or saving energy, the developers of glazing products continue to explore how glass can address the increasingly complex needs of the built environment, enabling architects to maximize creative expression, building performance, and occupant comfort.