Researchers have developed a fully self-powered smart window that can instantly change from transparent to hazy and last for days without being charged.

A smart window changes the amount of sunlight let into a room by making the glass more opaque, blocking both UV rays and infrared light. In a study published March 15 in Nano Energy, researchers created a smart window that generates electricity on its own with mechanical energy and radiant energy from light. 

Combining "mechanics and radiance is a totally new paradigm," said Yunlong Zi, a co-author and corresponding author of the study and an assistant professor at The Chinese University of Hong Kong. This unique combination allowed Zi and his team to change the window's transparency "without too much external power."

Smart windows are very energy-efficient when compared with traditional panels of glass. On a bright day, sunlight passes through transparent glass and heats up the room. A lot of electricity is needed to power air conditioning or fans to cool the air back down. Smart windows "tune room temperature without external power and reduce energy consumption," Zi said. They also "help protect the privacy" of residents or employees, who can change the opacity of the windows with the touch of a button, Zi said, adding that smart windows are "highly favored in modern buildings."

However, smart windows currently on the market have a few drawbacks: They have a high level of haze, even while in a transparent state, and "the energy cost of the electric field" needed to power the switch between states is huge, Zi told The Academic Times. The members of Zi's team, who have been working on smart windows since 2019, set out to fix these problems in their new research by tailoring two important parts of the window: electricity and liquid crystals. 

Tiny materials called polymer-dispersed liquid crystals, or PDLCs, are sandwiched between two layers of glass in a window. Though these liquid crystals are only a few microns in diameter — the size of a red blood cell — they can change the appearance of entire buildings by adjusting the amount of light let in through the glass.

In smart windows, the structure of PDLCs is rearranged with an electric current. One researcher says that PDLCs can be thought of like Swiss cheese because they have tiny holes. Without an electric current, the liquid crystals are randomly arranged in these "holes" in the glass, scattering light as it passes through the window, and the window appears to be opaque. When an electric current is applied, all of the liquid crystals align in the same direction, and light can easily pass through, making the glass transparent.

Zi and his team created a new kind of liquid crystal to be used in a smart window. Their cholesteric liquid crystal has a helical structure, similar to the structure of DNA. This type of liquid crystal is very stable and allows for the window display to switch from transparent to hazy — and vice versa. Zi told The Academic Times that he was "really shocked" by how well this material performed. The cholesteric liquid crystal could sustain a hazy state for over one week after it was triggered.

The researchers powered the window with a triboelectric nanogenerator, or TENG. A nanogenerator is a relatively new type of technology that converts energy from small-scale physical changes into electricity. TENGs are appealing because of how they acquire this energy: They use triboelectric charging, in which materials are electrically charged after being separated from each other. A common example is static electricity that comes from running a plastic comb through one's hair.

Zi thinks TENG technology is very promising. He was "surprised by the remarkable optical switching performance achieved" with the TENG. "Even though the power generated by the TENG is very small," Zi said, the difference in the appearance of light after charging is huge. The TENG was able to achieve a high transparency contrast of 71.5% in the smart window. This technology can "impact our daily lives" by triggering "a big difference through a very small energy," he said.

In the future, Zi wants to "optimize this TENG-triggered smart window system to make it more energy-efficient, so that this technology can be used in large-scale windows." His group at Zi Lab also wants to "investigate a self-powered memory for our [smart window] sensors," where photons would carry energy within the window. The researchers are also looking into using this technology for wireless sensors.

Zi told The Academic Times that he "feels lucky to have wonderful collaborators" on this project. Zi is excited for the future of TENG research and tells students in his lab that "creativity can become its own motive."

The study, "A Fully Self-Powered, Ultra-Stable Cholesteric Smart Window Triggered by Instantaneous Mechanical Stimuli," published March 15 in Nano Energy, was authored by Jiaqi Wang, Sun Yat-Sen University and The Chinese University of Hong Kong; Cuiling Meng and Hoi-Sing Kwok, Hong Kong University of Science and Technology; Chun-Ta Wang, Chia-Hua Liu, Yong-Hsiang Chang, Cheng-Chang Li and Heng-Yi Teng, National Sun Yat-Sen University; and Yunlong Zi, The Chinese University of Hong Kong.