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Self-shading smart window runs on sunlight

RavenBrick is making a self-tinting, heat-blocking glass coating that operates only from the sun's heat, rather than electricity.

RavenBrick's self-tinting window coating, seen here in tinted state, was used at the National Renewable Energy Laboratory's recently built energy-efficient building.

RavenBrick is developing a window that knows when it's hot enough for shade.

The Denver-based company has been working on a window coating that creates a heat-blocking tint triggered by the outdoor temperature. The company is in the process of raising $3 million in venture capital with plans to build a factory that will start operating in about a year, according to co-founder and President Wil McCarthy.

One of the trends in building design is to use large windows to bring in daylight, which creates a pleasing workspace and lowers the need for artificial lighting. One of the challenges with floor-to-ceiling windows, for example, is excessive heat from sunlight, or "solar gain."

Five-year-old RavenBrick has developed a material that's embedded as a liquid into a plastic film laminated over windows, McCarthy explained. When the window temperature reaches a certain point, the film begins to change from clear to dark to cut glare and block the sunlight's heat. How quickly the tinting occurs depends on how heat spreads through the glass, but it generally takes about 10 minutes, McCarthy said.

There are already a number of "smart glass" products available or under development. Sage Electrochromatics and Soladigm, for example, last year received funding to manufacture electrochromatic windows with glass that tints from an electrical signal.

RavenBrick's technique, which has its roots in McCarthy's research on sensors for space missions, is driven by heat rather than electricity. That makes it less cumbersome to install since no wires are needed and he expects the products to be cheaper and able to get a return on investment within a few years.

"Ours is liquid crystal based. With a TV you have a set of liquid crystals that get switched electrically. Every pixel has a transistor where you can apply a voltage across it or not to switch it to light or dark," he said. "We are doing essentially the same thing with one big pixel, but it's activated by temperature rather than an electrical signal."

The company's film has been installed on a building at the National Renewable Energy Laboratory and on a large company's offices. The temperature that starts the tinting process can be tuned during manufacture, but 94 degrees at the window's surface (not inside) works in many climates.

Rather than sell directly to consumers, RavenBrick's plan is to sell to window manufacturers, which would add the film to their products, McCarthy said. Another company called Pleotint is also testing thermally driven self-tinting windows.

RavenBrick's founders decided to focus first on self-tinting glass, but its core thermally driven nanomaterial technology can be adjusted for different purposes, McCarthy said. In the future, he expects to develop a window coating that can absorb and reflect some of the heat from sunlight and to create walls that transfer solar heat in a building.