NREL-windows feature

 NREL-windows feature

NREL researchers are developing window films that can improve building comfort and cut energy use and greenhouse gas emissions. (Credit: P. Corkery, NREL.)

According to The Old Farmer’s Almanac, much of the US is in for a long, frigid winter. Especially in the northern tier states, forecasting a cold winter is akin to predicting widespread darkness at night, but a couple of facts remain: 1) It is going to be cold in most of the US, and 2) windows account for up to 50% of a building’s energy loss.

That second tidbit comes via a news release the US Department of Energy, which is also happy to recommend ways to cut down on energy loss from windows. A case in point is research at the National Renewable Energy Laboratory (NREL), where scientists are working to develop an insulating window film that “preserves the view while increasing occupants’ comfort and saving energy,” NREL says in its release. The film incorporates nanometer to micrometer-sized vacuum capsules that can be applied like conventional low-e window tinting films.

“Early estimates indicate that a millimeter-thick layer results in clear insulation with values equivalent to R-20, which is equal to standard wall insulations,” the release says. “By combining vacuum insulation materials and processes with low-e coated plastic films, the new technology will boost the energy efficiency of current window retrofit technologies by as much as 80% at a fraction of the cost. Best of all, building and homeowners will not need to replace their windows.”

According to the release, the window film could reduce building energy use by as much as 33%, resulting in a payback period of less than a year while saving energy and reducing greenhouse gas emissions.

Window films are one thing, and may indeed provide a relatively economical way for homeowners and commercial building owners to cut energy consumption. But how about “smart windows” that can adjust to sunlight intensity (or, in winter, the lack thereof) and other environmental conditions to help maintain a building’s set temperature?

We’ve been reporting on this technology for more than three years, and a recent article in MIT Technology Review provides a good overview of commercially available smart window technologies.

The current technology is based on layered glass composites that control transmittance of solar radiation by thermochromic, photochromic, or electrochromic means. But, according to the article, current smart windows are about twice as expensive per square foot as conventional double-pane windows and only block visible radiation.

Enter Heliotrope Technologies, a startup company working with DOE’s Lawrence Berkeley National Laboratory to commercialize a new window coating technology that can switch between three states—the company calls them Bright, Cool, and Dark—to block a wide range of solar radiation, and does so at relatively low cost. The technology recently won an R&D 100 Award.

According to a company news release (pdf), “the smart window technology that Heliotrope is bringing to market leverages a unique electrochromic effect discovered by the inventors to control light and heat transmission independently and dynamically. Heliotrope is commercializing the discovery in the form of a dynamic window coating that will deliver improved energy efficiency at a substantially lower price than smart window products currently on the market.”

The company says windows with its electrochromic coating consume minimal power during switching and “almost none” to maintain either of the two solar blocking states. The company is looking first at flat glass for commercial buildings, but also plans to investigate use of the technology for automotive glass. According to the release, Heliotrope is currently delivering prototypes and, if the technology progresses as planned, will be making small commercial windows by 2016.