Researchers at the University of Notre Dame (Indiana, US) have developed a ground-breaking window coating that effectively blocks heat-generating ultra-violet (UV) and infra-red (IR) light while allowing visible light to pass through, regardless of the sun’s angle.

This innovative coating maintains its efficiency and functionality whether the sun is high in the sky or low on the horizon.

Windows are designed to let light into interior spaces but often allow unwanted heat to enter as well. The new coating addresses this issue by preventing UV and IR light, which generate heat, from penetrating the glass while still permitting visible light to pass through.

This can lead to a significant reduction in air-conditioning cooling costs, particularly in hot climates, by more than one-third.

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

Traditional window coatings are often optimised for light coming in at a 90-degree angle, which is not always practical. For instance, at noon – the hottest part of the day – the sun’s rays enter through windows at an angle, making conventional coatings less effective.

Luo and his post-doctoral associate, Seongmin Kim, previously developed a transparent coating by layering ultra-thin films of silica, alumina and titanium-oxide on a glass base. They added a micro-metre-thick silicon polymer to enhance the coating’s cooling capabilities by reflecting thermal radiation out into space.

To perfect the coating, the researchers needed to optimise the layer arrangement to handle multiple angles of sunlight. Traditional trial-and-error methods were impractical due to the vast number of possible combinations.

Instead, the team used quantum computing, specifically quantum annealing, to find the optimal layer configuration. The result is a coating that, while similar to polarised sunglasses in reducing light intensity, remains clear and effective at various angles.

The final model of the coating-maintained transparency and reduced temperatures by up to 7.2 degrees Celsius in a test room, demonstrating its effectiveness across a broad range of light angles. The techniques developed for this project can also be applied to the design of other materials with complex properties.

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