"Superwindow" concept, based on multiple glazing, low-emissivity coatings, and gas fills.
In 1976, in response to the energy crisis, DOE began a program at LBNL to examine the potential of new, more efficient window technologies. In 1993, after almost 20 years of an R&D partnership with industry, that effort has resulted in sizable energy savings to U.S. building operators, and the development of a new line of energy-efficient window products that are generating sales and profit opportunities for window manufacturers.
Our initial goal was to develop a clear understanding of the heat transfer mechanisms in windows and identify the technical opportunities for reducing those gains and losses. In cold climates, low-emissivity coatings allow sunlight to enter while reflecting back to the interior the long-wave infrared radiation that accounts for more than half the heat loss. Although the principle of how these coatings work was then understood, no U.S. manufacturer had yet developed a commercial product. At the time, there was no market demand (the benefits were unclear to purchasers), and it appeared impossible to produce coatings of high quality at low cost.
LBNL awarded subcontracts to several firms to develop prototype coatings and new, low-cost, thin-film deposition processes. The performance of the coatings was tested at LBNL and new computer models were developed to determine the best use of the coatings in the overall window system.
Encouraged by these efforts, by 1980 several large manufacturers were actively involved in low-emissivity window development, making major investments in manufacturing systems for new coatings. Initial product introductions in 1981- 82 by a few innovative firms stimulated major manufacturers to offer products of their own. Second-generation products emerged that had greater durability and suitability for a wider range of climates. They were tested at LBNL to demonstrate their market potential. By the mid 1980s, virtually every window manufacturer was offering low emissivity (low-e) windows. By 1987, low-e windows claimed 17% of window sales (18 million square meters per year).
Laboratory analyses at LBNL showed that the next step to improve window energy efficiency for cold climates was to eliminate the air inside the double-paned insulating unit, replacing it with low-conductivity gas (such as argon). LBNL simulation tools, as well as laboratory and field test data, helped convince manufacturers to incorporate this technique into their product and to inform purchasers that this was a reliable, cost-effective approach. Double glazings with both low-e coatings and gas fills lose only 50% of the heat lost by conventional double glazing.
"Superwindow" concept, based on multiple glazing, low-emissivity coatings, and gas fills.
Although substantial efficiency improvements had been achieved, leading manufacturers were interested in pushing the technology further. Analysis suggested that windows with specific thermal and solar gain properties would perform so well that they would have a lower winter heating load than the best insulated walls. LBNL staff developed a new "superwindow" concept for a multiple glazed window using two low-e coatings and a new krypton gas fill. LBNL teamed with five manufacturers and suppliers (Andersen, Cardinal IG, Owens-Corning Fiberglas, Pella, and Southwall Technologies) and the Bonneville Power Administration to convert this window concept into commercial prototypes. Within two years, one participating manufacturer introduced the first commercial "superwindow" to the market.
Spectrally selective glazing transmits high levels of visible light while reflecting invisible solar heat.
Spectrally selective glazings are a recent variant on low-e coatings. Designed for hot climates, they work by selectively filtering out solar heat gain while minimizing the loss of visible light transmission. This advance means potential additional savings in the Sunbelt states and in commercial buildings where cooling loads should be reduced without loss of useful daylighting. In some cases, downsizing the cooling systems (made possible by reduced cooling loads) can offset the added cost of the more efficient windows.
The cumulative energy savings attributable to advanced window coatings installed as of 1993 was $760 million. Based on energy savings "in the pipeline," i.e., for low-e-coated windows installed as of 1993, businesses and consumers will ultimately save $400 million (net of their extra capital investment), which will grow to $17 billion for technologies installed through the year 2015. These enormous savings were leveraged by a cumulative DOE investment through the early 1980s of just $3 million. The environment will also benefit from the use of advanced window coatings: In 2015, energy savings from advanced windows will allow us to avoid the emission of 71 million tons of CO2, 157,000 tons of SO2, and 142,000 tons of NOx.
Toward this end, ion-beam technology developed in LBNL's Accelerator and Fusion Research Division is being redirected by LBNL's Windows Group to improve energy- efficient window coatings. These ion-assisted processes result in coatings with superior optical properties, longer lifetime, and lower cost. These devices were previously used as sources of particles in accelerators and more recently for some semiconductor processing steps like ion implantation of dopants.
The WINDOW 4.0 software and manual were published on a CD-ROM disc for initial distribution to 15,000
building industry professionals attending the A/E/C Systems Show. The WINDOW software is the basis of
NFRC labels shown above.