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A GlassPoint solar system.

BERKELEY, Calif. — When it comes to a sustainable future, scientists aren’t seeing clearly.

Researchers have developed a new type of tinted “smart window” that generates electricity when darkened.

The windows “can be automatically converted into a solar cell to generate electricity for us,” said Peidong Yang, a chemistry professor at UC Berkeley and researcher at Lawrence Berkeley National Laboratory in the Berkeley Hills who heads the team responsible for the discovery.

The new solar technology could potentially be used to power everything from electric vehicles to skyscrapers. Similar to solar panels currently in use, the windows would be able to generate electricity, send it to an inverter that changes that energy from a DC current to an AC current. That energy could then be used to power the building or car that houses the windows.

The smart windows are made from a mineral called perovskite. When the crystal is heated, its internal structure rearranges to darken the color of the glass, allowing it to absorb more light and create electricity. Scientists have known about the unique properties of perovskite for decades, but have only learned to harness them in the past five years.

While similar self-tinting windows on the market conserve energy passively by lowering the cost of air-conditioning, Yang’s innovation actively and reliably produces low amounts of electricity. Even after multiple transitions from light to dark, the smart windows are still able to produce electricity — something other versions have largely struggled to do.

There are several hurdles the team needs to jump through in order to transition the material from an idea to a marketable product.

The smart windows, which are in the early stages of development, switch from transparent to opaque at a very high temperature — around 212 degrees Fahrenheit (100 degrees Celsius). The team hopes to bring down the transition temperature to around 150 degrees Fahrenheit in order to make them more efficient for a wider variety of climates.

There’s still more work to be done.

“Typically technologies require at least a decade — if not more — to go from laboratory discovery to commercial product,” said Nathan Neale, a research scientist at the National Renewable Energy Laboratory. Neale is part of a team working on a similar technology that uses perovskite to create solar power-generating windows.

Both Neale and Yang’s teams are working to increase the amount of energy their perovskite-based windows produce. Commercial silicon-based solar panels currently convert about 20 percent of the sunlight they receive to energy. In order to be fully marketable, Yang insists a solar panel must be able to convert at least 10 percent of the sun’s rays to energy.

Neale’s team has developed a version of this new smart window that converts roughly 11 percent of the light it sees to energy, but loses effectiveness after each transition. The energy-generating windows from the Berkeley Lab remain effective after multiple cycles, but haven’t been able to energize more than 7 percent of the sunlight they receives. Yang said raising this number will be a critical next step in the development process.

Another hurdle? The color of the glass.

“People do not like to have colored windows.” said Michael McGehee, professor of materials science and engineering at Stanford University in Palo Alto. At the moment, the smart windows tint to shades of red and yellow.

Robert Rozbicki, Chief Technology Officer at View, a company that has installed dynamic glass at the San Francisco Airport and Kaiser Permanente agrees: “ Having the right color is important for both architects and occupants.”

Researchers at the Berkeley Lab are already working on ways to offer a more pleasing color palette.

A burgeoning trend in sustainable design and architecture, “dynamic glass” windows are created through a process called chromogenic glazing, where researchers apply a thin veneer of chemicals to sheets of glass in order to change the properties of the material. The glazes fall into three categories: thermochromic, electrochromic, and photochromic.

The smart windows that Yang and his colleagues are developing have been glazed using a thermochromic solution. Thermochromic glass reacts to heat — and in some cases, water vapor, which acts as a catalyst — causing the glass to darken and, in some cases, generate energy.

“The advantage of thermochromic (glazing) is that often you don’t need wires to the window, but at the same time, it’s not controllable” like electrochromic glazing, said Rozbicki.

Electrochromic windows, which are controllable, tint when stimulated by an electrical current. Recently, Boeing installed self-tinting electrochromic glass in its 787 Dreamliner that, at the push of a button, provide comfort to passengers and increase efficiency. Some rear-view mirrors also possess this technology.

Transition eyeglass lenses are coated in a photochromic glaze, which transitions to a darker color when lit. While earlier versions only reacted to UV light, more recent iterations transition at the first sign of any type of bright light.

Both Yang and Neale are confident that these obstacles can easily be overcome and that electricty-generating windows will gleam across skylines throughout the world. After that, the sky’s the limit.

“There’s lots of room where we can improve,” said Yang. “But it’s promising, very promising.”

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