Energy Efficient Window Coatings that Please the Eye

Energy Efficient Window Coatings that Please the Eye

NREL partnerships expand market reach and provide more energy saving options.

A photo of an office space with windows that are darker in the upper portion and clear in the lower portion. Enlarge image

The dynamic electrochromic windows in this office can darken in the upper portion to block unwanted light, glare, and heat while the lower portion remains clear.
Photo from Sage Electrochromics

Start with a novel concept. Pursue related fundamental research. License intellectual property to companies. Understand technical barriers to greater market penetration of the technology. Partner with companies to overcome these barriers. Help a successful industry develop as a result and provide end-users with more options to choose from to save energy and gain other desired benefits. NREL has a long track record of following this path in the field of dynamic windows—windows with glass that changes properties based on environmental and user demands to improve the energy efficiency and aesthetics of buildings.

From Great Idea to Next-Generation Materials

In 1973, even before coming to NREL, Satyen Deb conceived of making a window that incorporated tungsten oxide thin films that had adjustable properties. Once at NREL (then, the Solar Energy Research Institute), he and his colleagues continued to pursue this technology as a device that changes color when voltage is applied—an "electrochromic" device—thus controlling the amount of light and heat transmitted through it (see sidebar).

Since the 1990s, NREL has obtained patents on key aspects of electrochromic materials, devices, and processes, and has made this intellectual property available for licensing. Various companies have entered into development partnerships and/or license agreements to explore and capitalize on NREL electrochromic innovations: SAGE Electrochromics, Inc.; Eclipse Energy Systems, Inc.; e-Chromic Technologies, Inc.; as well as for hydrogen sensor (Nuclear Filter Technology) applications.

NREL Senior Scientist Chaiwat Engtrakul, who currently conducts research in electrochromic materials, explains, "NREL and SAGE are working together to develop innovative nanocomposites that can be used in new window designs. These next-generation materials are enabling window features desired by designers and consumers and will help grow the market penetration of this technology."

One feature that will bolster market acceptance relates to aesthetics: most architects and building occupants would prefer clear windows on one extreme and dark gray glass on the other. Addressing this preference, the SAGE electrochromic coatings have no perceptible tinting in the clear state and, when combined with the standard tungsten oxide electrochromic layer, have the potential to result in the desired dark gray.

Another feature relates to performance, namely, the length of time it takes to switch between the clear and darkened states. The device switching speed depends on the ambient temperature and the area of the glass. NREL and SAGE are researching electrochromic materials that exhibit increasingly fast switching rates that consumers find desirable.

Saving Energy, Helping Companies and Consumers

Neil Sbar, vice president of Energy and Technology Applications at SAGE, concludes, "We're confident that the next-generation materials being conceived through our research and development will boost the penetration of dynamic window applications within the buildings market. The ultimate impact will be a substantial decrease in energy usage over existing window technologies."

In other electrochromic research, NREL scientists are exploring avenues for retrofitting these coatings to existing glass windows. This ability would expand the options available to remodelers for the almost 20 billion square feet of windows currently installed in U.S. commercial and residential buildings.

NREL's seminal and ongoing work in dynamic window technologies, including key collaborations with industry partners, exemplifies how a national laboratory can serve as a valuable resource to companies needing to overcome technical market barriers to bring new energy technology options to consumers.

—Written by Don Gwinner

Electrochromics: What it is, What it Does

A photo of a man wearing safety goggles and gloves working in a laboratory. Enlarge image

Reaching into a sputter deposition system, NREL scientist Chaiwat Engtrakul prepares a sample for deposition of an electrochromic film being studied in his research on next-generation materials for dynamic windows
Photo by Dennis Schroeder, NREL

Making a nanocomposite. NREL has worked with industry to develop a nanocomposite counterelectrode coating that surpasses state-of-the-art electrochromic coatings. A typical electrochromic coating is deposited on window glass and consists of five layers totaling about one micrometer thick, with transparent contact layers bookending a counterelectrode layer, ion-conducting layer, and electrochromic layer.

Low voltage applied across the stacked layers causes lithium ions to migrate out of the counterelectrode, initiating a solid-state process that tints the coating, darkening the glass. Reversing the voltage polarity reverses the lithium-ion flow, decreasing the glass tint and allowing more light to be transmitted through the window.

Controlling light and heat. Clear glass maximizes the entry of natural light into interior spaces, reducing the need for electric lighting. But at certain times, we may want darker glass to block some or most light—to reduce glare, prevent fading of carpets, or provide greater privacy. And in cold weather, we want windows to allow most solar energy to enter the building's interior to provide heat, whereas in warmer weather, we want to block the heat. This strategy reduces the load on the building's heating and cooling systems—less additional heating in the winter and less cooling required in the summer—to maintain occupant comfort while spending less money on energy.

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