Assisted by quantum physics and machine learning, researchers have developed a transparent window coating that lets in visible light but blocks heat-producing UV and infrared. The coating not only reduces room temperature but also the energy consumption related to cooling, regardless of where the sun is in the sky.
Windows are great. They provide views of the park you live across from or the bird-filled tree outside your office. But, windows can also be not-so-great. Letting in light (and the view) is one thing, but with light comes heat, especially in the hotter months.
On hot days, up to 87% of heat gain in our homes is through windows. UV radiation from sunlight passes easily through glass, heating up the room and increasing the likelihood that you need to turn on the air-con or else forgo any light (and, again, that view) by closing the curtains or lowering the blinds. However, researchers at the University of Notre Dame have developed a window coating that blocks heat-producing UV and infrared light while allowing visible light in, reducing both room temperature and cooling energy consumption.
“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 Tengfei Luo, who leads the MÖNSTER Lab (Molecular/Nano-Scale Transport and Energy Research Laboratory) at Notre Dame and is a corresponding author on the study describing the researchers’ work.
In 2022, Luo and his colleagues fabricated a glass coating using planar multilayered (PML) photonic structures. These stacked ultrathin layers have distinctive refractive indices that allow light to be selectively transmitted or reflected depending on its wavelength. Stacking silica, alumina, and titanium oxide on a glass base and topping it off with a thin layer of silicon polymer (PDMS) to reflect thermal radiation, the electromagnetic radiation emitted by a heated surface in all directions, produced a transparent coating that, they said, outperformed the other heat-reducing coatings on the market.
The researchers were determined to improve upon their previous work. Because windows are typically installed vertically, the direct sunlight hitting them throughout the day changes as the sun moves. Existing window coatings tend to be optimized for light that enters at a 90-degree angle, so their ability to block light depends on this so-called solar incident angle. At noon, the hottest part of the day, sunlight hits a window at an oblique angle, meaning that most coatings are less effective at blocking it.
Rather than addressing this problem using a trial-and-error approach, the researchers used a quantum-computing-assisted machine learning model. Specifically, they used active learning, a subset of machine learning in which a learning algorithm can query a user interactively to label data and quantum annealing, which uses quantum physics to find optimal – or near-optimal – combinations of elements.
The quantum-assisted active learning approach allowed the researchers to optimize the PML structures’ configuration and provided them with a definite advantage, Luo told New Atlas.
“It can be used to solve very complex optimization and design problems,” he said. “The complex optimization problem in this work can be hard to solve using conventional algorithms.”
Using the components they’d previously used, the researchers fabricated a transparent coating that selectively transmitted and reflected light across a wide range of incident angles. Then they tested it. Coated windows and windows with normal glass were placed vertically in identical outdoor chambers. The researchers measured the daytime temperatures in each chamber. They also tested the glass by placing the window horizontally, facing the sky, to mimic a motor vehicle’s sunroof. The coated glass demonstrated superior performance compared to normal glass, reducing the temperature by between 5.4 °C and 7.2 °C (9.7 - 12.9 °F) across a wide range of incident angles.
“The angle between the sunshine and your window is always changing,” Luo said. “Our coating maintains functionality and efficiency whatever the sun’s position in the sky.”
To estimate energy savings for cooling using their photonic structure as windows, the researchers used the EnergyPlus software to simulate consumption in standard offices in different cities. They demonstrated that all cities in the US could save up to 97.5 MJ/m2 annually. That energy savings carried over to cities worldwide, including those in tropical climates.
The researchers foresee many uses for their novel window coating, including commercial and residential buildings and cars.
“I think it can be especially useful for car windows,” Luo told New Atlas. “It can be used as sunroof/moonroof windows. It can even be used for [a] windshield, where you have to keep it transparent, but it leaks in a lot of space heating UV and IR [infrared] sunlight.”
The researchers still need to determine the window coating’s scalability.
“This is not yet known,” said Luo. “I cannot say if it is … cheaper, but as we work toward the scaling up, they can be cheap. The coating can be manufactured using industrial-scale coating processes. The materials in the coating are very common materials (no exotic materials).”
The study was published in the journal Cell Reports Physical Science.
Source: University of Notre Dame
