Credit: Flickr/Michael Gallacher

Smart windows are getting smarter. Researchers at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (LBL) announced earlier this month the development of a window coating whose embedded nanoparticles respond to different rates of electrical charges to independently control solar-heat gain and visible-light transmittance. That’s a boon for consumers of the niche category of electrochromic window coatings, whose often-costly pioneering technology that is currently on the market regulates heat and light as a single unit.

“The conventional solutions out there always involved kind of a trade-off—if you’re going to reduce the cooling load in the building, you may have to compensate for that by increasing the lighting,” says Jason Holt, president and chief operating officer of Berkeley, Calif.-based specialty films maker Heliotrope. “Being able to separate those is key, and that’s what we can do with this technology.” Heliotrope licensed the technology and says it plans to begin sampling the product to major glass manufacturers beginning in the fourth quarter of 2013. The company received its SEED funding through private investors and grants from the Department of Energy and is headed by the team of researchers that developed the coating.

A research paper published earlier this month in the scientific journal Nature outlines the technology, which operates in three states depending on the amount of electrical charge the glass receives: A “bright” mode allows both visible light and infrared rays to pass through; a “cool” mode reflects infrared rays while allowing visible light in; and a “dark” mode reflects both infrared rays and visible light. The technology comprises solar-heat-regulating nanocrystal composite particles, which are surrounded by a matrix material that controls visible-light transmission. The researchers found that the combination of the two materials creates a space inside the glass through which electrical charges move more quickly when changing states, allowing for thinner coatings.

Different electrical charges trigger each of the new coating's "bright," "cool," and "dark" states, allowing users to block solar-heat gain and visible-light transmittance separately or together.

Different electrical charges trigger each of the new coating's "bright," "cool," and "dark" states, allowing users to block solar-heat gain and visible-light transmittance separately or together.

Credit: Lawrence Berkeley National Laboratory


By independently and dynamically controlling solar-heat gain and visible-light transmittance, the new technology lets windows serve as both a shading device and a daylighting mechanism—lowering operating costs by eliminating the need for shades and preventing extra stress on building systems’ loads. Current technologies top out at preset tints that block a fixed amount of solar heat—a metric usually determined at the point of manufacture—often affecting the views out if a building’s owner chooses glass with high reflectivity.

The team has tested dark-state visible light transmittance rates as low as 10 percent, but expects further developments to lower the rate to between 3 percent and 5 percent. “That’s where we understand you can significantly cut out glare,” Holt says. “And that’s important because if you can do away with the glare problem, then you can conceivably eliminate blinds. In a new construction scenario, that can be a pretty significant savings in capital expense.” The company says the coating’s exact infrared-blocking potential has yet to be determined, but estimates that it is capable of blocking more than 50 percent of infrared transmission. For example, Holt says, if the coating lets 75 percent of infrared rays pass through it when in the “bright” state, it would let only 25 percent of infrared rays through when in the “cool” state. The technology is designed to be synced with a building management system whose ambient sensors can detect when it is time to change the glass’ state and apply the necessary charge.

“It looks very much like a lithium-ion battery that you’d have in a cell phone or a laptop, but with the requirement that it be transparent,” Holt says. “Instead of having opaque substrates, our substrate is glass.”

Still, the biggest challenge for the new coatings is latency when transforming from the “cool” state to the “dark” state, which currently can take up to 10 minutes, Holt says, depending on the thickness of the coating and the scale of the installation. It can switch from the “bright” state to the “cool” state in less than one minute.

Heliotrope will go to market as a specialty films and coatings producer, supplying the technology to glass manufacturers later this year. Holt says the company’s capital-light business strategy includes using chemical solution deposition to apply the film, which will lower the manufacturing costs by half compared to that of first-generation products. That strategy, he hopes, will help drive down the technology’s cost and deepen its market penetration.

Windows employing electrochromic technology currently on the market cost up to three times more per square foot than standard windows, Holt says, in part due to their lower demand levels limiting manufacturing yield. “That’s important for market adoption,” he says. “Price is the biggest obstacle to the broader adoption of smart glass or smart windows.”

Image via a Creative Commons license with Flickr user Michael Gallacher.

Editor's note: An earlier version of this article misstated Jason Holt's title and the source of Heliotrope's funding. Holt is the president and chief operating officer at Heliotrope. The company received its SEED funding from private investors and grants from the Department of Energy. We regret the error.