Researchers have invented a new, self-powered smart window system, made up of solar cells and a three-layered film, that can be easily installed on top of any existing glass window to block both ultraviolet and visible light.

The invention is a collaboration between University of Illinois researchers and the U.S. Department of Energy. An associated patent application was published by the U.S. Patent and Trademark Office on April 22.

The autonomous light system can be readily integrated with a building's current window systems, enabling commercial and residential businesses to retrofit their glass at a relatively low cost. Solar cells cut the energy costs of smart windows as they enable the system to run off a free and abundant power source: the sun.

"I had been vaguely familiar with energy demand in the U.S.," co-inventor Maggie Potter told The Academic Times. "Something we learned along the way, though I had ideas prior that it may be the case, was that absurdly huge amounts of electricity are just totally lost in electrical transmission, so if you can find ways to put photovoltaics directly onto your building, then you're circumventing all of those issues with losses from the transmission." 

Thirty percent of the United States' energy demand comes from either internal lighting or from heating, ventilation and air conditioning systems. Smart windows could reduce the electricity demand from fans, air conditioning and other thermal management technologies while providing a natural source of light. While it is transparent, the window allows more ambient light into a building; while it's opaque, it can block hot sunlight during the middle of the day. 

"Lighting can make office buildings more amenable to positive work environments," said another co-inventor, Mikayla A. Yoder, in an interview with The Academic Times. "[Smart] windows allow light to come through without having to shut the blinds or have fluorescent lighting. They improve mood and boost productivity."

Yoder and Potter met in Illinois' Materials Research Lab, a space well-known for its collaborative spirit. Their team wanted to upgrade the technology in electrochromic glass windows, named for the color change that occurs when electricity is applied to material inside the glass.

The existing demand for electrochromic glass is sizable, at $1.9 billion as of 2019, and the global market is projected to reach $2.6 billion by 2027. But current smart windows are still somewhat impractical, requiring a complex system of wires to supply power from an outside source. And businesses looking to upgrade to electrochromic technologies have to tear out existing windows to install new glass.

The co-inventors designed an internal solar power system to circumvent electricity issues. The solar microcells are small but mighty — they cover only about 6% of the total area of the film but can provide power to all electrochemical layers in the window, manipulating the amount of light that enters a room on demand. 

"My entire Ph.D. was centered around these solar microcells, so finally getting to a project where I could see that I was actually able to power something was really fun. Seeing a physical color change was pleasing," Yoder said.

The film is able to block or absorb light due to three layers of electrochromic material that are stacked atop one another. The first and third layers rest on transparent electrodes to make the window appear translucent in the absence of electricity. The inner layer is composed of a gel electrolyte and can facilitate a change on an atomic level when electricity is applied, making the window appear more opaque. When charged by solar electricity, the three layers work together to block light from entering a room.

As neither Yoder nor Potter is an electrochemist by training, creating these layers was a challenge. The collaborative nature of the Materials Research Lab came in handy when a third co-inventor lent his expertise to the group. Chemist Sean E. Lehman, who was working on 3D printed materials at the time, discovered the ideal material for the film.

"Finding the right electrolyte to pass the current through and finding the right properties for fast switching times … was the most challenging part" of the process, Lehman told The Academic Times. He settled on metal oxides that charge the electrolyte layer of the film, in turn changing the color of the window. Lehman notes that the system can go from transparent to opaque in under three minutes — a process that takes some conventional models hours.

Lehman was pleasantly surprised to see smart windows on a Boeing airplane recently. "I was actually pretty shocked," he said. In the next five to 10 years, Lehman suspects that more airplanes and large commercial buildings will continue to retrofit their windows with smart technology.

Today, Lehman applies his background in nanoparticles to medicine. "We take soft biological particles and think about how to produce standards for the biopharmaceutical industry," he said. His work at the National Institute of Standards and Technology's Bioprocess Measurements Group informs research on viruses and vaccines, such as COVID-19 and its associated treatments.

The other co-inventors have also charted their own courses after leaving the Materials Research Lab. Yoder now works on filtration technology at the engineering giant Donaldson, while Potter studies photons and energy at the California Institute of Technology. 

Regardless of the team members' current interests, all emphasize collaboration as a core value in research. 

"One of the strengths of having a group of people from different backgrounds — chemists, engineers, scientists — is seeing new perspectives," Lehman said.

The application for this patent, "Autonomous light management system for a window and method of controlling light transmission," was filed Oct. 6, 2020 with the U.S. Patent and Trademark Office. It was published April 22 with the application number 17/064238. The earliest priority date was Oct. 16, 2019. The inventors of the pending patent are Mikayla A. Yoder, Marjorie M. Potter, Aaron Petronico, Sean E. Lehman and Ralph G. Nuzzo, University of Illinois.

Parola Analytics provided technical research for this story.