Sunrise to sunset, new window coating blocks heat not view

Window coating held up to sky with the Dome in the background

Windows welcome light into interior spaces, but they also bring in unwanted heat. A new window coating blocks heat-generating ultraviolet and infrared light and remains transparent to visible light, regardless of the sun’s angle.

The coating can be incorporated into existing windows or automobiles and can reduce air-conditioning cooling costs by more than one-third in hot climates.  

“The angle between the sunshine and your window is always changing,” said Tengfei Luo, Dorini Family Professor of Energy Studies at the University of Notre Dame. “Our coating maintains functionality and efficiency whatever the sun’s position in the sky.”

Tengfei Luo

Window coatings in many recent studies are optimized for light that enters a room at a 90-degree angle. Yet at noon, often the hottest time of the day, the sun’s rays enter vertically installed windows at oblique angles.

Luo and his postdoctoral associate Seongmin Kim previously fabricated a coating by stacking ultra-thin layers of silica, alumina and titanium oxide on a glass base. A micrometer-thick, silicon polymer (PDMS) was added to enhance the structure’s cooling power by reflecting thermal radiation through the atmospheric window and into outer space.

Luo's diagram showing different angles of the sun and materials used for his film
Schematic of photonic structure for energy-saving windows with spectral filter.

Additional optimization of the order of the layers was necessary to ensure the coating would accommodate multiple angles of solar light. However, a trial-and-error approach was not practical, given the immense number of possible combinations, said Luo.

To shuffle the layers into an optimal configuration—one that maximized the transmission of visible light while minimizing the passage of heat-producing wavelengths—the team used quantum computing, more specifically, quantum annealing, and validated their results experimentally.

Their model produced a coating that maintained transparency plus reduced temperature by 5.4 – 7.2 degrees Celsius in a model room, even when light was transmitted in a broad range of angles. The lab’s results were recently published in Cell Reports Physical Science.  

Video showing the visual transparency of the photonic structure with tilt.

“Like polarized sunglasses, our coating lessens the intensity of incoming light, but, unlike sunglasses, our coating remains clear and effective even when you tilt it at different angles,” said Luo.

The active learning and quantum computing scheme developed to create this coating can be used to design a broad range of materials with complex properties.

— Karla Cruise, Notre Dame Engineering