Seeing Windows Through
Energy lost through residential and commercial windows costs U.S. consumers about $40 billion a year. Berkeley Lab pioneered the commercialization of "low-emissivity" windows and labeling systems, which reduce the energy lost through normal, double-glazed windows by 35%. Thanks to Berkeley Lab's close collaboration with window manufacturers, these advanced windows have a greater than 50- percent marketshare and save American consumers billions of dollars in energy bills each year. Berkeley Lab's continued work in this field has led to advanced double- and triple paned gas-filled windows, and transition metal switchable electrochromic windows. With 25 years of experience and a wide range of expertise, Berkeley Lab has been an important advisor to such cutting edge building projects as the new New York Times headquarters building.
In 1976, in response to the energy crisis, DOE began a program at Berkeley Lab to examine the potential of more efficient window technologies. Lab researchers faced a number of daunting challenges including developing the coatings, documenting how they could be used in a window, demonstrating their effectiveness, and finally convincing consumers that these invisible coatings were important.
Berkeley Lab's initial goal was to develop a clear scientific understanding of the heat transfer mechanisms in windows and identify the technical opportunities for reducing those gains and losses. The concept of low-emissivity, or "low-e," coatings—thin metal or metal oxide films—had been around since World War II, but no U.S. manufacturer had yet developed a product. Low-e coatings are heat mirrors; they allow visible light to pass through, preserving occupants views of the outdoors, but reflect back long-wave infrared radiation, from heat sources such as buildings' heating systems.
Berkeley Lab awarded subcontracts to several firms to develop prototype coatings and new, low-cost, thin-film deposition processes. All of the performance testing of the coatings was conducted at Berkeley Lab and lab researchers developed a new computer model (WINDOW, now in its fifth version) to determine the best use of the coatings in the overall window system.
Berkeley Lab researchers continued to develop improved energy-efficient window designs. They demonstrated that replacing air between two windowpanes with an insulating gas, such as argon, significantly decreased heat loss in cold climates. Computer simulations, as well as test data from the lab's Infrared Thermography facility (see photo) and from actual homes, helped convince manufacturers to incorporate this technique into their products and to inform purchasers that this was a reliable, cost-effective product. When leading manufacturers were interested in pushing the technology further, Berkeley Lab researchers developed a new "superwindow"—a double-paned window with two low-e coatings and a new krypton gas fill that actually preformed better than an insulating wall. A couple of years later, "superwindows" hit the market. More recent inventions include highly insulating aerogel windows and gas-filled panels, an insulation technology more effective than foam. Berkeley Lab won an R&D 100 award for this technology, and granted licenses to several companies to develop commercial products.
Forty percent of all windows are in commercial buildings, where cooling and lighting costs are of greatest concern. Building on the heat-reflecting properties of low-e coatings, Berkeley Lab scientists helped develop spectrally selective glazings for hot climates. These transparent glazings allow in visible light but filter out both long-wave and infrared radiation from the sun, reducing air conditioning demands.
Berkeley Lab has taken the lead in testing products, analyzing benefits and conducting general studies on the energy impact of windows. To encourage more homeowners and consumers to buy energy-efficient windows, Berkeley Lab developed energy efficiency labels for windows in 1997. Berkeley Lab researchers have also written a book and partnered with the University of Minnesota and the Alliance to Save Energy to create a website that contains tools to calculate the performance of different windows in each U.S. state. When approached by both small and large manufacturers, Berkeley Lab provides them with information, advice and technical assistance in producing the best windows possible.
The Berkeley Lab windows team's most recent invention is the transition metal switchable electrochromic window, which changes color when a small voltage is applied to a chamber filled with gas. This flexible and dynamic technology can be manually or automatically controlled and can be integrated with other building systems, such as lighting and heating/cooling mechanical systems, to optimize interior environmental conditions, occupant comfort, and energy-efficiency.1
Windows researchers at Berkeley Lab served as advisors on windows and daylighting technology to The New York Times Company during the development of their new, all-glass headquarters building. When the company's design team encountered difficulty finding reliable, cost-effective technologies to control daylight, glare, and electric lighting for occupant comfort and energy efficiency, they turned to Berkeley Lab researchers. They jointly created a test facility within a Times Company printing plant, inviting manufacturers to install and demonstrate their daylighting control systems. Lab researchers worked with manufacturers to simulate energy use, improve performance, and reduce costs, and with the Times Company to develop bid specifications. The net result is a high visibility building with a state-of-art daylighting control system optimized for efficiency, and a transformed market, whose players now offer commercial building systems that are less expensive and more energy-efficient.